DOI 10.1515/polyeng-2013-0267 J Polym Eng 2014; 34(4): 279–338

Review

Shamim Ahmad* Organic semiconductors for device applications: current trends and future prospects

Abstract: With the rich experience of developing silicon 1 Introduction devices over a period of the last six decades, it is easy to assess the suitability of a new material for device appli- A wide range of organic semiconductors (OS) are cur- cations by examining charge carrier injection, transport, rently being explored extensively for their applications and extraction across a practically realizable architecture; in organic electronics (OE) primarily due to the growing surface passivation; and packaging and reliability issues need of substituting silicon (Si) with some alternative besides the feasibility of preparing mechanically robust cost-effective materials offering relatively simpler and wafer/substrate of single-crystal or polycrystalline/ commercially viable technologies [1–7] at least in some amorphous thin films. For material preparation, param- niche areas. Based on the recent developments in the area eters such as purification of constituent materials, crystal of OS, processes involving printing of semiconductor, con- growth, and thin-film deposition with minimum defects/ ductor, and dielectric patterns on a variety of hard as well disorders are equally important. Further, it is relevant to as flexible substrates are fast emerging as part of a com- know whether conventional semiconductor processes, mercially viable technology for the upcoming OE devices. already known, would be useable directly or would require In contrast to OS (primarily Si), the additional possibility completely new technologies. Having found a likely can- of chemically modifying the organic molecules by incor- didate after such a screening, it would be necessary to porating a number of functional groups during synthesis identify a specific area of application against an existing resulting in a number of chemical functionalities along list of materials available with special reference to cost with significant influence on the charge carrier transport reduction considerations in large-scale production. Vari- properties [8] is adding further impetus to make this kind ous families of organic semiconductors are reviewed here, of search more attractive and meaningful. The examples, especially with the objective of using them in niche areas cited later, highlight this concept further. For instance, it of large-area electronic displays, flexible organic electron- is now easy to design a molecule to have the desired fea- ics, and organic photovoltaic solar cells. While doing so, tures in terms of its solubility in a specific solvent, color it appears feasible to improve mobility and stability by of the light emission and a typical crystalline molecular adjusting π-conjugation and modifying the energy band- packing, to name a few. Some such features realized from gap. Higher conductivity nanocomposites, formed by modifying the molecular designs have already been put to blending with chemically conjugated C-allotropes and use in a number of newer applications as mentioned here. metal nanoparticles, open exciting methods of design- A good example is that of nonvolatile memory elements ing flexible contact/interconnects for organic and flexible [9], wherein polyvinylidene fluoride copolymers and tri/ electronics as can be seen from the discussion included tetra-fluoroethylene radicals [10] were used in preparing here. useful devices that were found especially appropriate for flexible electronic circuits. In another case, the end groups Keywords: conjugated polymers; crystalline/polycrystal- of an organic sensor array were modified such that they line organic compounds; hopping conduction; organic not only responded to chemical and biological species field-effect transistors; organic semiconductors. but also measured pH levels [11], food freshness, toxic compounds, stress, and pressure in apparels [12]. Rapid *Corresponding author: Shamim Ahmad, Center of Excellence in growth witnessed in OE is offering several newer areas of Nanotechnology, Confederation of Indian Industry Western Region, applications with better performances, improved reliabil- Ahmedabad, Gujarat 380006, India, e-mail: [email protected] ity and stability, long lifetime, good control, and repro- ducibility as demanded by the different industry sectors. For instance, organic light-emitting device (OLED)-based 280 S. Ahmad: Organic semiconductor devices displays, marketed by Philips and Organic Electronic A number of OS-based devices, especially OLEDs Components, in cell phones [13] and other mobile devices [2, 47–49], OPV solar cells [50–59], and organic field-effect along with car radios and digital cameras are some of transistors (OFETs) [1, 60–64], are fastly being developed the applications that are expected to multiply in the near involving conjugated organic molecules that offer special future. features in terms of tunable energy band-gap, redox poten- The quest for exploring the electronic applications of tials, and charge carrier transport properties, besides OS has already grown manifold in recent past [8, 14–23] being easy to process. These materials and processes are while attempting to replace the use of inorganic materials certainly going to offer the lightweight, low-cost, thin- in some relevant areas. In addition, efforts are also being film, large-area, and flexible electronics of tomorrow. made to explore newer concepts and theoretical models including molecular-level designs for controlling the structural, physical, and chemical properties of organic 1.1 Organic molecules for device applications molecules to meet the system requirements better in the future. This kind of approach is currently more relevant, From the point of view of device applications, the organic as the earlier attempts made in realizing OS devices faced molecules are conveniently classified into groups accord- unavoidable problems of material instabilities during ing to the nature of the charge carrier transport they processing besides the large density of structural defects support, for instance, hole and electron-transporting (HT/ introduced during material growth that prevented the ET) materials, besides their architecture involving small- fuller exploitation of the intrinsic charge carrier transport molecule oligomers or macromolecular polymers and properties. dendrimers. OS are currently getting almost practically poised for Commonly used definition of p- or n-type material supplementing and/or replacing the conventional OS, does not necessarily reflect the intrinsic ability of an especially in some niche areas, because of the associated organic material to transport holes or electrons as men- distinct advantages. From the 1980s onward, OS have gone tioned in the published literature. Rather, it only speci- through various stages of development meant for improv- fies how easily holes and electrons are injected from their ing the material quality that led to and even surpassed the contact electrodes. This subtle deviation from the classic performance of amorphous Si (α-Si) [24] in terms of the definition was further elaborated in subsequent studies charge carrier mobilities in organic thin-film transistors [65, 66], where it was shown that, in many organic mate- (OTFTs). Easier solution printing of the organic materials rials, although the intrinsic electron and hole mobilities at low temperatures on even fairly larger area substrates may be comparable, their drastically reduced values, without using a high vacuum system [24–28] is well worth measured experimentally, may be the result of external considering, where various processes including screen influences of the traps or instabilities introduced by water, [29], ink-jet [30–33], and microcontact [34, 35] printings hydroxyl groups, or oxygen exposures [67, 68]. This exter- have been especially advanced for flexible and transparent nal influence on intrinsic charge carrier mobilities was device fabrications [36–39] employing plastic substrates further illustrated in the case of SiO2 gate dielectric that offering practically feasible integration of LEDs and organic carried a large density of hydroxyl groups on its surface, photovoltaic (OPV) solar cells on the same substrate. which acted as traps for injected electrons into the Knowing that a single-crystal semiconductor is an channel. However, the process of covering the dielectric ideal platform for studying and exploiting the intrin- with BCB resulted in good electron transport in materials sic charge carrier transport properties, extensive efforts such as polythiophene, polyfluorene, and polyparaphe- were put in to grow crystalline OS [40–43] and succes- nylene-vinylene with charge carrier mobilities in the range sive improvements achieved in material crystallinity did of 10-2 to 10-3 cm2/Vs [23]. show significant improvement in the charge carrier mobil- Small organic molecules for device applications ity. For example, very low values of the mobilities, meas- include polycyclic aromatic hydrocarbons, fused heterocy- ured in earlier samples, in the range of 0.001 to 0.1 cm2/Vs clic aromatic compounds, oligothiophenes, oligoarylenes, [44] were pushed to as high as 20 cm2/Vs in crystalline and macrocycles such as phthalocyanines, fullerenes, ruberene and pentacene samples at room temperature. and perylene pigments along with perylene and vio- Still higher mobility of 400 cm2/Vs was measured in naph- lanthrone as electron donor and conducting materials, thalene samples at 10 K [45]. This kind of improvement respectively [69, 70]. Crystalline pentacene, tetracene, and in charge carrier mobility provided a real push for using ruberene are, so far, the best molecules for OE. Ever since them in device fabrications [8, 46]. the realization of sexithienyl-based organic transistors, S. Ahmad: Organic semiconductor devices 281 the interest in oligothiophenes has been growing quickly monodisperse macromolecules with well-defined func- [71]. Although most oligothiophenes and oligoacenes are tionalities according to their generation number, size, and p-type materials, their fluorinated backbones do show molecular weight. Additionally, in dendrimers, a regular n-type behavior [71]. Vacuum-deposited TPD from triph- correlation is found to exist between the structure and enylamines family were extensively used as hole transport the charge carrier transport properties. For instance, the layers (HTLs) in OLEDs. Perylene exhibited a herringbone- dependence of charge carrier transport properties on the like molecular packing, where attaching perylenetetracar- nature of the core, the total generation number, and the boxylic dianhydride (PTCDA) or perylenetetracarboxylic presence of functional groups in each layer of a dendrimer diimide (PTCDI) moieties produced n-type materials [71]. was carefully studied [89] using spin-coated amorphous Pentacene and numerous other aromatic hydrocarbons dendrimer thin films. The experimental results indicated are frequently used in fabricating OFETs [72]. Well-ordered that, in the case of a phenylene-vinylene encapsulated and crystalline oligothiophenes are currently being inves- triphenylamine core [89], there is a drastic reduction in tigated for optoelectronic applications especially in the mobility from 5.1 × 10-6 to 5.5 × 10-9 cm2/Vs after changing form of molecular wires [73–76] using raw as well as alkyl- the generation number from 0 to 3 [89]. The PAMAM deco- + substituted forms including 16-, 20-, and 27-mers. Besides rated [Ru(bpy)3] 2 dendrimers exhibited similar trend of crystalline oligothiophenes, amorphous oligothiophenes mobility reductions with increasing generation number [76, 77] have also been used in several devices as reported [90]. In contrast, iridium core dendrimers with carbazole in published literature [73, 75, 78–83]. in each layer [91] exhibited twofold increase in the mobil- Although the significant influence of the material crys- ity from (3–6) × 10-5 cm2/Vs for generation 1 to (7.3–12) × 10-5 tallinity on device performance was already demonstrated­ cm2/Vs for generation 3. while comparing the mobility of 35 and 1.5 cm2/Vs [72] in Conjugated polymers have been studied extensively devices prepared from crystalline and thin-film penta- for their applications in flexible electronics [54, 59, 92–99] cene, respectively, still it is interesting to note that the and optoelectronic devices [51, 93, 100] as reported in the polycrystalline organic materials make better OFETs and published literature. Several methods were subsequently OPV solar cells despite having lower mobilities compared developed to take care of the polydispersion-induced to their crystalline counterparts [72]. Nevertheless, using disorders in such thin films. For example, the inherent amorphous materials in device fabrication has its own nature of P3HT leading to self-assembly was found to merits over the crystalline ones by considering the ease in be helpful in enhancing the mobility in the range of 10-4 processing, transparency, and homogeneously isotropic to 10-1 cm2/Vs depending on the processing conditions material properties. including solvent, annealing time, and temperature [60, During thin-film deposition, smaller organic mol- 62, 101]. The lamellar assembly of P3HT molecules along ecules easily convert into glasses [84] possessing no grain the plane perpendicular to the substrate with π-π interac- boundaries but are otherwise replete with disorders in tions among the 3-hexylthiophene rings [3] showed higher intermolecular distances and orientations, which turn mobility measured in a plane parallel to the substrate, out to be helpful in preparing uniform films by vacuum while that along the plane normal to the substrate showed evaporation or spin coating with well-defined ordered reduced values [102] due to poor carrier hopping. molecular structures. Besides, nonplanar molecules such Because of higher degree of disorders in organic mate- as triarylamines, spiro-compounds, and 1,3,5-triphenylb- rials especially in thin films, significant contributions are enzene or 2,4,6-triphenyltriazine exhibit mobilities in required from other considerations than the charge carrier the range of 10-2 to 10-5 cm2/Vs [84]. Compounds such as hopping to show moderate mobilities in the range of 10-6 quinoxaline, benzimidazole, pyridine, and oxadiazole to 10-1 cm2/Vs. For example, poly(thienylenevinylene) having electron-withdrawing properties [85, 86] are found molecules with lower band-gap of 1.6 to 1.8 eV exhibited to be useful for device applications, for instance, materi- higher hole mobilities of 10-1 cm2/Vs, making them useful als such as 1,3,5-tris(9,9-dimethylfluoren-2-yl) benzene in OPV applications. [87] and 2,4,6-tris[di(2-pyridyl)amino]-1,3,5-triazine offer Mobility measurement in naphthalene and anthra- better form of electron transport layers (ETLs) for OLED cene crystals [103] clearly established that the intrinsic applications [88]. behavior of charge carrier transport was realizable only In contrast to general polymers, where significant in the bulk crystals. It further demonstrated enhanced variations are seen in the average molecular weight and in mobility at lower temperatures with pronounced anisot- their batch-to-batch distributions causing varying device ropy [104, 105]. However, there are applications where properties accordingly, well-structured dendrimers are surface transport is more relevant, for instance, in the case 282 S. Ahmad: Organic semiconductor devices of OFETs. Field-induced charge carriers, moving along carriers in an organic device “hop” from one π-orbital to the interface between the OS and the gate, are essentially another on the molecular backbone, for which several confined to only a few monolayers [1, 106, 107], where ways have been developed to maximize the mobility and the charge carrier density is especially several orders of on/off ratio as briefly discussed below. For meeting the magnitude higher than that encountered in the time-of- above target, it is, first of all, necessary to ensure that flight (TOF) mobility measurements besides facing strong the energy levels of the contact electrodes and the active electron-polaron interactions [108, 109]. Additionally, layers align well. In the case of a gap between the two movement of the charge carriers in the channel is not energy levels, there would be an equally large potential only affected by the polarization of the gate dielectric barrier responsible for reduced charge carrier injection. [110] but also by the molecular arrangements present at For having proper energy-level alignment, it is necessary the organic surfaces, which may be very different from to have either the right kind of electrode material with the that in the bulk. It is thus clear that the study of polaronic desired value of its work function or chemically function- transport on organic surfaces is necessary to understand alize the electrode surfaces to shift its energy level to the the processes that determine the overall performances of matching position with respect to the selected electrode these organic devices. Despite having limited knowledge material as discussed later. Next, the organic molecules of transport properties of OS, the family of all-organic in the channel region must be appropriately organized to devices of the active matrix displays involving OLEDs and ensure the optimal charge carrier transport. Here, proper OFETs is already entering into commercialization. This is, molecular design, improved self-alignment through incidentally, very different from the situation that existed surface modification, and thermal annealing all help to in the case of the inorganic electronics in the mid-1960s, enhance the orbital overlap leading to improved charge when the first Si transistor was developed [111]. carrier hopping [112–116]. Subsequently, the dielectric film used must provide the maximum capacitance, which is possible either using high dielectric constant material 1.2 Material selection criteria for OS devices or reducing the dielectric film thickness. The thinner the dielectric layer, the lesser is the operating voltage required Based on the concept of a conventional FET, simple to turn the device “on”. While depositing thin dielectric replacement of the inorganic channel by an organic one layer, it is essential to ensure the film uniformity so that was adapted in early developments of OFETs. In actual the leakages through relatively thinner regions or defects device fabrication, it begins with the coating of the organic left within are minimized. In addition, the dielectric layer layer onto the patterned source and drain contacts on a should also facilitate promoting self-organization of the

SiO2-coated heavily doped Si substrate, wherein the Si gate semiconducting layer deposited on top. Finally, the device induces charge carriers in the channel region according dimensions should be decided carefully in optimizing the to the applied gate bias by modulating the channel con- OFET performance. For instance, reducing the channel ductivity. Although the OFET channel is certainly differ- length results in not only higher mobility but also pro- ent from that of conventional Si, still the basic transistor vides lesser chances of meeting defects in the path of architecture and the functioning remain identical to that charge carriers or amorphous regions inhibiting charge of an inorganic counterpart. In subsequent developments, carrier flow. all organic devices were also developed using organic Keeping these factors in mind is certainly helpful in gate electrode and dielectric layers, but such devices were improving the OFET performance as briefly mentioned found to be comparatively inferior. above, it is useful to know what all have been attempted The charge carrier mobility in the channel region in this context as mentioned here. In recent years, OFETs is an important FET parameter as the higher the charge were explored for flexible display backplane applications. carrier mobility, the faster it switches the device. Thus, Charge carrier mobilities in the evaporated- and solution- the primary target during device design and fabrication processed materials, in the mean time, have already is to get as high mobility and on/off ratio as it is practi- exceeded the best value of the α-Si devices. In a recent cally possible. For example, a typical mobility and on/off review [117], it was noted that, by mid-2011, there were current ratio for an α-Si FET is approximately 1 cm2/Vs and about 40 different OS possessing mobilities above 1 cm2/ 108, respectively, as a reference for comparing the emerg- Vs. However, despite having access to such a large number ing OS devices. of OS, there were only a few that could go up to the stage Despite sharing the same device architecture and of large-scale integration necessary for display backplane electrical operations of an inorganic device, the charge arrays possibly due to serious reliability issues. S. Ahmad: Organic semiconductor devices 283

OS are either aromatic ring-based small molecules or molecular conjugation as a result. For example, in PQT-12, the conjugated polymers. Generally, lower solubility of the presence of solubilizing side chains on the first and small molecules makes it difficult to use them in low-cost fourth thiophene rings [121–123] leaving two thiophenes fabrication processes involving solution-coating tech- without side chains created a slight twist in the molecu- niques. Instead, they are better vacuum evaporated to get lar chain that did not disturb the conjugation much but the crystalline films. Taking pentacene as an example of otherwise created a larger band-gap, making it very stable a small-molecule, better-quality pentacene film, OFETs under ambient conditions while still retaining higher have already surpassed the performance of the best α-Si mobility. Such stable PQT-12 films exhibited mobility of counterpart after exhibiting mobilities in the range of ∼3 0.07 to 0.18 cm2/Vs. cm2/Vs, three times that of the highest mobility of α-Si A dielectric layer with structural perfection and low [118, 119]. However, due to a higher degree of conjuga- trap density is required for the OFET gate insulator [124– tion and low ionization potential (IP), pentacene mol- 126]. For easier movement of the charge carriers across ecules oxidize very fast in normal ambience degrading the channel surface along the dielectric semiconductor the device performance leading to serious reliability prob- interface, it needs lower surface energy as well as polar- lems. Because of such a drawback, the search for a soluble ity. Moreover, the dielectric layer deposition must ensure precursor for pentacene deposition after fabrication has a very smooth surface, especially in the case of top contact been a subject to investigate in the future. On the contrary, devices where the surface roughness is noted to affect the the conjugated polymers exhibit better environmental sta- molecular ordering adversely. In the case of very rough bility and tunable electronic properties due to extensive surfaces, molecules are not able to orient into lamel- π-conjugation in addition to better solubility due to appro- lar structures resulting in crystal domain defects, which priate side chains. Although these materials are easy to behave as traps, reducing the overall device performance. coat as thin films, perhaps a simple coating of complex For instance, a surface roughness of even a few nano­ molecules generally results in inferior molecular order meters disturbs the charge carrier conduction across the showing relatively poor performance compared to small- channel in the proximity; consequently, a much larger molecule films as observed in the case of P3HT, which electric field is required to create a conduction channel has the mobility of 10-3 cm2/Vs for an unannealed film on to overcome such a disturbance [127], which defeats the a plain substrate. However, by activating the di­electric basic purpose of achieving the target of the low-voltage surface to promote molecular self-organization [3, 120] applications of OFETs. and thermal annealing, it could improve the molecular Dielectrics have been chosen from both inorganic and 2 ordering leading to the mobility up to 0.1 cm /Vs. It is thus organic materials, where the most common ones are SiO2 concluded that although polymers are poor semiconduc- and SiN used in test devices with a heavily n-doped Si as tors from a conductivity point of view, they are otherwise the gate electrode. Besides being expensive to deposit, better in stability and ease of processing than small-mol- using complex methods such as physical and chemi- ecule materials. It was further noted that not only is the cal vapor deposition, most of the inorganic dielectric π-conjugation along the main chain important, but so are films are brittle and as such are not appropriate for flex- the nature of the side chains and narrow molecular weight ible substrates. To avail of the solution-based process- distribution which both help in improving the crystallin- ing methods, polymeric dielectrics were thus preferred, ity and reducing the band gap leading to higher mobilities which were usually insulating, having a large band-gap [3]. Knowing well the importance of reducing the intermo- that made them good dielectrics. Defect-free and a few lecular spacing through improved crystallinity, methods tens of nanometer thin dielectric films are possible now based on surface modification to promote “edge-on” ori- to deposit easily from solution. Electrophoresis and in situ entation and regioregularity-based molecular designs to poly­merization methods have also been used to deposit promote lamellar ordering were successfully developed. very smooth and very thin dielectric films. For instance, While trying to improve the charge carrier transport by the common organic dielectric materials include polym- conjugation enhancement using closely packed π-orbitals, ethyl-methacrylate (PMMA) [126], polyimide [128], and it was also noted that the presence of higher energy levels polyvinyl alcohol (PVA) [129] for their device applications. and smaller band-gaps imparted a better mobility but oth- The performance comparison of OFETs using Si/SiO2 erwise made these materials more susceptible to the oxi- and flexible polymer substrates was made [130] in terms of dation and photodegradation under ultraviolet (UV) light mobility and VT. PEN substrate, polyimide dielectric along exposure. To alleviate these problems, side chains were, with pentacene and PTCDI-C13 as p and n-type semicon- very often, added to improve solubility by distorting the ductors were used in these experimental OFET devices. It 284 S. Ahmad: Organic semiconductor devices was noted in this study that the n-type devices on flexible on the size and capping agents of the metal NPs. Using substrates exhibited better mobility and lower VT in con- this method, electrodes are printed with reasonable reso- trast to those on Si/SiO2. On the contrary, p-type devices lution and high conductivities, for instance, using Au and on Si/SiO2 substrates were better than those on the flex- silver (Ag) NP suspensions, where conductivities have ible substrates [130]. been realized in the range of 4–10 × 106 and 2–4 × 106 S/m, While studying the influence of annealing on struc- respectively. On the contrary, an organic conductor was ture and electronic properties of pentacene OFETs with reported using poly(3,4- ethylenedioxythiophene):poly(st polyimide dielectric [131], it was noted that 140°C anneal- yrene sulfonate) (PEDOT:PSS) combination as a transpar- ing improved the mobility by almost doubling from 0.07 ent conducting film with conductivity up to 103 S/m [136]. to 0.12 cm2/Vs along with substantial reduction in trap Although this value is enough for organic conductors, it is density. Further measurements confirmed that the crystal still low compared to the solution-processed metal elec- structure of the pentacene film on polyimide did not trodes, and for this reason, PEDOT:PSS films are generally change with annealing up to 140°C, whereas pentacene on preferred where a transparent electrode is required, such

Si/SiO2 substrate exhibited transitions from the (001) thin- as a cathode for OPV solar cells. film phase to the bulk phase [131]. Gate dielectrics pre- In order to modify the conductivity of polymeric mate- pared from methylated poly(melamine-coformaldehyde) rials beyond the limit set by crystallization and annealing, (MMF) and polyvinyl phenol (PVP) for flexible pentacene possibilities of preparing composites were examined. For OFETs [132] on polyestersulfone (PES) substrate exhibited example, a low band-gap semiconductor [137] PDPP/TNT mobility of 0.88 cm2/Vs along with reduced hysteresis and was prepared by combining naphthalene and diketopyr- leakage current [133]. rolopyrrole (DPP) as the acceptor and donor component, For OFET contact electrodes, the foremost important respectively, in a polymer backbone that exhibited hole parameter is the proper matching of the work functions of mobility of 0.65 and 0.98 cm2/Vs in the bottom gate and the electrode and the active region materials as mentioned dual gate OTFT device configurations, respectively, besides already. Gold (Au) is the preferred electrode material for being well suited for OPV solar cell applications due to its organic devices, as it has a work function of 5.1 eV, which band gap of 1.5 eV and HOMO of 5.29 eV. While using this is fairly close to the highest occupied molecular orbitals semiconductor with PC71BM as D-A pair in OPV solar cells, (HOMOs) of many conjugated polymers in the range of ∼5 an efficiency of 4.7% was reported [137]. In a more recent eV. In case a conducting material is not available with a publication [138], the structural design, optoelectronic work function close to the energy level of the semiconduc- properties, molecular organization, morphology, as well as tor, surface treatments are available to modify the work the device performance in OFETs and OPV solar cells using function for minimizing the detrimental effect of contact high-mobility DPP-based materials were examined in detail. resistance as discussed later separately. Having chosen By extending the basic concept of forming highly ordered the material with aligned work functions, the next param- crystalline domains while associating the DPP unit with eter is the material conductivity, which should be as high an electron donating polymer, the charge carrier transport as possible. For these reasons, the natural choice falls was further improved due to intermolecule and interdomain mostly on metals. Unfortunately, in the case of patterned interactions besides energy-level adjustments, resulting in metal contacts, prepared by vacuum deposition and p/n-type/ambipolar OS. In contrast to the observed mobil- shadow masking, it is rather difficult to keep the cost low. ity of 1 cm2/Vs and efficiency of 4.7%, as noted earlier [137], Consequently, the recent developments have been looking more recent semiconductor formulations exhibited still for alternate options such as either using metal precursors higher hole, electron, and ambipolar (hole/electron) mobili- in solution form to deposit continuous metal films after ties of 10.5, 3, and 1.18/1.86 cm2/Vs, respectively [138], and printing followed by reduction and annealing [126, 134] or efficiency of 6.05% in the OPV solar cells [138]. employing metal nanoparticle (NP) suspensions to form a The influence of the starting purity of the polymer on continuous film after printing and sintering [126, 135]. The the performance of OFETs was specifically studied using solution-processed metal films do not possess as high con- in situ measurement of electrical characteristics. Start- ductivity as that from the vacuum-deposited parent con- ing with purified and nonpurified pentacene samples, it ductor but are otherwise reasonably adequate for organic was noted [139] that the field effect started appearing at devices. The metal NP films require sintering to render the 1.5 nm thickness and above in purified samples in contrast printed electrodes conducting, but this step can be easily to that starting at 3.0 nm and above for the nonpurified converted into a roll-to-roll manufacturing, as the sinter- counterpart. Moreover, the hole mobility improved from ing temperature is only approx. 120°C to 200°C depending 0.13 to 0.23 cm2/Vs after sublimation purification of the S. Ahmad: Organic semiconductor devices 285 pentacene, which also exhibited larger grain size and film same extensive network of π-orbitals is present there on coverage, resulting in better crystallinity of the thin-film its surface leading to similar aggregation as observed in structure due to the absence of the impurities [139]. CNT bundle formations, and a large amount of energy is required to break such aggregations. The process of physically reducing the channel length 1.3 Relevance of fullerenes, nanotubes, for increasing mobility by bringing source and drain con- and graphene in OS devices tacts closer is useful in device fabrication, but it has its own limitations. Alternatively, blending conductive addi- Despite considerable advances made in developing high- tives, CNTs, and graphenes with the host polymer or stabi- quality organic materials, the mobility has still been rela- lizer molecules also reduces the effective channel length tively lower as the most polymeric materials exhibited causing mobility improvements without asking for physi- mobilities in the range of 0.1 to 0.6 cm2/Vs. One way to cally bringing the electrodes closer. In addition, CNT and improve the mobility without synthesizing new materials graphene composites exhibit higher mobility by enhanc- was to introduce an additive within the polymeric matrix ing the charge carrier transfer between two disordered that resulted in something imbibing the properties of regions of the amorphous phase that are invariably there either of the components involved. From a large number in polymeric films mixed with the regions of ordered crys- of additives available for OS-based composites, carbon talline structures. Proper dispersion of conducting addi- allotropes were found to be the best in this context. This tives helps in reducing the influence of barriers created originally started with carbon fullerenes and nanotubes by disordered regions by providing additional bridging (CNT) with P3HT in OPV solar cells [140], where fuller- regions [144]. For instance, a small amount of CNT/gra- ene became an obvious choice due to its better electrical phene helps in linking the ordered regions and improves properties and the ease with which it could be dispersed the charge carrier mobility by reducing the possibility of in the organic matrix in bulk heterojunction solar cells. charge carrier transport across the disordered regions. Although CNTs did follow as the next additive their one- However, there is a typical percolation problem associated dimensional (1D) structure did not improve the solar cell with CNTs, as an additive concentration is in the range of performance; rather, their applications in OFETs appeared 10 to 20 wt% causing shorts in OFETs. more promising [141, 142]. Reducing the energy band-gap [145] is yet another pos- The advantages of using CNTs include ultrahigh sibility of increasing the polymer conductivity. In general, conductivity/mobility values, arising from very efficient most OS have an energy level above -5 eV. The SWCNTs have conjugation present along the CNT axis that imparts a work function in the range of -5 to -5.2 eV, which places very efficient charge carrier transport along the length them somewhere between the semiconductor and Au elec- as well as low band-gap and very low threshold voltages. trode. Thus, a CNT composite, with such a semiconductor, Unlike multiwalled CNTs (MWCNTs), where each unit is would have an overall energy level slightly lower than the always conducting, single-walled CNTs (SWCNTs) behave pure polymer, which would reduce the energy barrier for both as semiconducting and conducting entities, and efficient injection of charge carriers. In this context, P3HT due to this nature, they are not used alone as they will has been widely used as the matrix polymer. From among short circuit the transistor units. It is therefore advisable several methods developed to prepare CNTs and graphene to disperse mixed CNTs in an organic matrix for device composites in stabilized forms, the one based on chemical applications. The large surface-to-volume ratio and an functionalization is preferred over the others [146–155]. extended π-conjugation make MWCNTs form bundles very MWCNTs are better mobility-enhancing additives com- easily, which can be checked by using stabilizer having pared to SWCNTs, as the outer layer of MWCNTs can be functional groups that can covalently bond to the outer- easily functionalized without disturbing the conjugations most layers. In one way, this appears helpful, but finally of the inner shells. Such is not the case in a single sheet of it disrupts the electron delocalization causing formation graphene, but using few stacked graphene layers known of charge trap sites leading to impaired electrical conduc- as graphene nanoplatelets (GNPs) can be used as an alter- tion [143]. Further, these functional groups also introduce native in the same context. additional energy barriers within CNT composites by Because carbon fullerenes, CNTs, and graphenes have interrupting the interaction between CNT and the poly- all been extremely useful additives for preparing nano- meric matrix. This is also true for graphene sheets, which composites, it will be helpful to quickly look into the ways are SWCNTs cut open to form a monolayer thin sheet. The of preparing CNTs and graphenes for realizing the right graphene being a two-dimensional (2D) C-allotrope, the kind of composites for device applications. 286 S. Ahmad: Organic semiconductor devices

For preparing a CNT-polymer nanocomposite, the Although a number of covalent functionalization nature of the interactions between CNTs and the matrix and schemes were successfully developed to attach func- the degree of dispersion would decide the overall behavior tional groups onto the surface of CNTs, in general, these of the end product. The CNTs are, as such, inert additives techniques suffered from two major drawbacks. First, facilitating load transfer across the CNT-matrix inter- the ultrasonic treatment required for initiating function- face through van der Waals interactions with chemically alization reactions created a large number of defects on stable constituent carbon atoms. Therefore, for realizing a the sidewalls, and in some extreme cases, even CNTs good composite with an objective of improving the charge were fragmented into smaller pieces, which not only carrier transport, methods would be needed to modify the degraded the mechanical properties but also disrupted surface properties of the CNTs for their improved disper- the π-electrons of the nanotubes (NTs). Such disturbances sions besides their active participation in carrier transport. ultimately affected the transport properties of CNTs, as CNT functionalization employs either chemical or they caused additional electron and phonon scatterings physical methods based on the interactions between the impeding the electrical and thermal conductions, respec- active organic molecules and carbon atoms of the CNTs. tively. Second, the concentrated acids or strong oxidants, In the chemical method, the functional groups attached to often used in CNT functionalization, were highly toxic in the CNT sidewall change sp2 to sp3 hybridization, resulting nature, and due to these reasons, efforts were made to in reduction of π-conjugation. This was clearly evidenced develop methods that were easy to use at low costs besides in reversible fluorination of SWCNT fluorination, where causing less damage to the CNT structure. anhydrous hydrazine could easily remove the fluorine Keeping these above-mentioned problems in view, one atoms [156]. Additionally, C-F bonds, involved in fluorina- process of noncovalent functionalization was alternatively tion, were relatively weaker than those of alkyl fluorides developed using suspension of CNTs in poly(phenylene- [157]. They, in turn, offered better substitution sites for vinylene) [177] or polystyrene [178] resulting in polymer additional functionalization [158] involving amino, alkyl, wrapping around the CNTs to form supermolecular com- and hydroxyl groups [159, 160]. Besides, other conjuga- plexes employing van der Waals interactions and π-π tion methods including the Diels-Alder reaction, carbene stacking between CNTs and polymer chains containing and nitrene addition [161–163], chlorination, bromination aromatic rings. [164], hydrogenation [165], and azomethineylides [166] Besides polymers, appropriate surfactant molecules reactions were also explored successfully in this context. were also found useful in functionalizing CNTs by study- The defect sites present on the opening ends and/or ing the influence of the surfactant on dispersion and other holes in the sidewalls as well as the pentagons/heptagons properties of CNTs [143, 179–187]. Lowering of surface in the hexagon network including oxygenated sites on tension due to physical adsorption of the surfactant on CNTs all were used in the “defect functionalization”. For the CNT surfaces very effectively prevented the aggregate instance, oxidation induced defects created on the side- formations. Further, the surfactant-treated CNTs over- walls and the open ends of CNTs by treating with strong came the van der Waals attractions by electrostatic/steric acids such as HNO3, H2SO4, or mixture [167] or with a strong repulsive forces as a function of the surfactant, medium oxidizing agent as KMnO4 [168], ozone [169, 170], and reac- chemistry, and polymer matrix. It was also noted that, tive plasma [171, 172]. It was possible to implement the in water-soluble polymers such as polyethylene glycol, defect functionalization. In addition, attaching (-COOH) cationic surfactants have some advantages, whereas, in or (-OH) groups stabilized the defects on CNTs created by water-insoluble polymers such as polypropylene, CNT the above-said oxidants, as these functional groups, once dispersion is promoted by a nonionic surfactant [184, attached, imparted a rich chemistry to the CNTs that could 185]. The nonionic surfactants treatment deploys a strong also be used as a precursor for further chemical reactions hydrophobic attraction between the solid surface and such as silanation [170], polymer grafting [173], esterifica- the tail group of surfactant, and once the surfactant is tion [174], thiolation [175], and alkylation and arylation adsorbed onto the additive surface, the surfactant mol- [176]. These functionalized CNTs dissolve easily in organic ecules self-assemble into micelle forms [143]. solvents because the attached polar groups change them In addition, endohedral confinement of atomic and from hydrophobic to hydrophilic forms. Thus, it is antici- molecular species inside the hollow cavity of the NTs is pated that the chemically functionalized CNTs participate another noncovalent method for CNT functionalization, well in forming interfacial bonds with many polymers, where they enter via defect sites, localized at the ends enabling CNT nanocomposites to possess improved or on the sidewalls involving capillary action. The inser- mechanical and functional properties. tion of inorganic NPs such as Ag, Au, Pt, and C60 [188] is S. Ahmad: Organic semiconductor devices 287 a typical example in this category. Even small biomol- melt blending has limited scope to handle only low addi- ecules such as proteins and DNA were easily entrapped tive concentrations in thermoplastic matrices [195]. in the inner cavity of CNTs by adsorption [189, 190]. Such On the contrary, in situ polymerization offers uniform a combination of CNTs and the guest molecules has been CNT dispersion in a thermosetting polymer by blending examined for their uses in catalysis, energy storage, nano- with monomers either in the presence/absence of a solvent technology, and molecular-scale devices [188]. followed by its polymerization via addition/condensation Compared to the traditional polymer microcompos- reactions with hardener and curing agents at an elevated ites, improved CNT dispersion leads to shorter interpar- temperature. The possibility of covalent bond formation ticle distances, which influence the properties even at between the functionalized CNTs and the matrix offers an low additive concentrations. For example, the electrical additional advantage for improving mechanical proper- conductivity of CNT/epoxy composite was enhanced by ties of the composites through strong interfacial bonds. several orders of magnitude with < 0.5 wt% of CNTs in one In latex method, SWCNTs/MWCNTs are blended with study [191]. The mechanical properties such as strength [185, 194, 195] colloidal aqueous dispersions of polymer and stiffness, combined with other physical properties of particles either produced by emulsion polymerization CNTs, offer many potential applications as already seen in or converted into the emulsion form. Compared to the in the current experimental studies, although the commer- situ polymerization, the CNTs are added after the polymer cial-level success is still awaited for its realization in the synthesis is complete. Thus, the first step of the process near future. begins with the exfoliation of SWCNT bundles or disper- Following the first report [192] of preparing a CNT/ sion/stabilization of entangled MWCNTs in an aqueous polymer nanocomposite in 1994, numerous efforts were surfactant solution followed by mixing the stable disper- already made to understand their structure-property rela- sion of surfactant-treated CNTs with polymer latex. After tionship to explore their applications in different fields freeze-drying and subsequent processing, CNT nanocom- and such efforts became more pronounced after the posite dispersed in a polymer matrix is obtained. This industrial-scale production of CNTs at lower costs at the technique [185, 194] enjoys flexibility, reproducibility, and beginning of the 21st century [193]. Application-wise, CNT/ reliability in processing the two aqueous components into polymer nanocomposites are classified as “structural” or a viscous polymer matrix at low cost and in environment- “functional composites” [194]. In “structural composites”, friendly manner. the mechanical properties of CNTs, including the high For explaining the conduction properties of the values of modulus, tensile strength, and strain to fracture, nanocomposites, the “percolation theory” has invariably are put to use for much improved mechanical properties. been used. In this context, it is generally noted that while Similarly, in “functional composites”, the CNT features slowly increasing the additive concentration, the compos- such as electrical, thermal, optical, and damping prop- ite undergoes a transition from insulator-to-conductor erties along with their excellent mechanical strength are state. The additive concentration at which this transition invoked in multifunctional composites for thermal insu- occurs is termed as the “percolation threshold” when the lation, chemical sensing, electrical and thermal manage- electrical conductivity sharply increases by several orders ment, photoemission, electromagnetic absorbing, and of magnitude due to the formation of continuous electron energy storage, to name a few [194]. conducting pathways. Prior to this transition, electron It has been experienced that solution mixing is the pathways are not there and the matrix material dominates commonest method of preparing CNT/polymer nanocom- the electrical properties while above the transition, and posites [185, 194, 195] involving processes such as solvent multiple electron pathways facilitate conduction, which dispersion by mechanical, magnetic, or ultrasonic treat- finally ends up in saturation [191, 197, 198]. ment at room/elevated temperature followed by precipita- In conventional conducting additives, micron-sized tion or casting at the end. metal/carbonaceous materials, such as carbon black (CB), Melt blending is another method, where thermoplas- exfoliated graphite, and carbon fibers were used in high tics such as polypropylene [160], polystyrene [178], and concentrations from 10 to 50 wt% [191, 197–200], result- poly(ethylene 2,6-naphthalate) [196] are used without any ing in poor mechanical strength. In contrast to these, the solvent to disperse CNTs. In melt blending, a high shear blending of CNTs into polymer minimized such problems force at high temperature disperses CNTs in a polymer by forming conducting pathways in the insulating polymer that is, as such, very compatible with current industrial by converting them into conducting composites at very processes employing high-temperature extruder/injection low additive concentrations. For most polymers, they machines. Compared to the solution mixing, however, the change from an insulator to a conductor as the conducting 288 S. Ahmad: Organic semiconductor devices additive concentration reaches approximately 5 wt%, but carbon fibers puts serious constraints on the large-scale there is no fixed percolation threshold for CNTs, as this use of CNT composites. Therefore, nanocomposites of may spread over a range of 0.002 to 7 wt% [191, 201–204] hybrid additives were purposely developed as a compro- primarily due to the variations introduced by the type of mise solution [209–215]. In such composites, the CNT the CNTs used and the preparation methods involved. This network serves as backbone and the other conducting large spread in percolation threshold confirms that the additives facilitate the charge carrier transport enhanc- CNT dispersion is affected by functionalization along with ing the overall conduction. For example, nanocomposites the processing conditions in determining the ultimate [211] blended with hybrid additives of 1% Au NPs and 1% conduction property of the nanocomposites. For instance, CNTs showed a conductivity of 4.7 × 10-3 S/cm, which was 2 in one of the related studies [205], a 50 times increase in orders of magnitude higher than that with 2 wt% Au NPs conductivity was measured in well-dispersed CNTs com- alone. In yet another study [215], hybrid CNTs/CB addi- pared to that of the entangled ones. In another experiment tives were mixed for enhancing the electrical conductiv- [191], CNT/epoxy nanocomposites were prepared exhibit- ity with balanced mechanical strength while reducing the ing percolation thresholds in the range from 0.1 to 1 wt% cost, which not only reduced the percolation threshold decided by the dispersion state and aspect ratio of CNTs. It but also enhanced the ductility and fracture toughness is thus concluded that aspect ratio, disentanglement, and of the nanocomposites. Hybrid additives with varying uniform distribution of individual/agglomerates of CNTs geometrical shapes and dispersion characteristics offer finally decided the percolation threshold. unique ways to lower the final cost of CNT-based nano- Despite experiencing all the advantages mentioned composites with multifunctional properties. earlier, however, such functionalizations do disturb the The next category of conducting additive involves π-electrons and degrade the intrinsic electrical proper- functionalized graphene with numerous functional groups ties of CNTs besides the additional damages introduced including dispersion in organic solvents after attaching during processing as discussed already. It is interesting to certain organic groups, which is necessary for preparing note that silane functionalized CNTs exhibited wrapping nanocomposites. In addition, organic functional groups of insulating material onto CNTs [204], which showed no offering new properties could also be combined with the percolation behavior. On the contrary, amino-functional- conduction properties of the graphene. In most cases, ized CNT nanocomposites [206] did show a typical perco- when organic molecules are covalently attached to the lation behavior but with conductivities lower than those graphene surface, although the π-conjugation present containing pristine or untreated CNTs. there is disturbed, still it enables the manipulation of its The electrical conductivity of CNT/polymer compos- conduction properties. For instance, arriving at a suitable ites is typically in the range of 10-5 to 10-3 S/cm above the band-gap through chemical doping is a powerful method percolation threshold [201–204], but further increase in for using graphene in nanoelectronic devices [216]. additive concentration increases the conductivity only The covalent functionalization of graphene follows very marginally. Additionally, the solution viscosity two routes: the one forming bonds between free radicals/ becomes too high to produce void-free composites for the dienophiles and C = C bonds of pristine graphene and the CNT content higher than 1.0 wt%. other forming bonds between functional groups and the It is noted from the above discussions that the pro- oxygen groups of graphene oxide (GO). The most useful cessing techniques for improving the electrical con- radicals for reaction with sp2-hybridized C-atoms are ductivity of nanocomposites around the percolation organic free radicals/dienophiles, which are implemented threshold becomes critical to produce highly conducting by heating a diazonium salt to produce free radical that composites. The critical role of contact resistance of CNTs attacks the sp2-hybridized C-atoms, which in turn is used in enhancing the conductivity of nanocomposites was for attaching nitrophenyls [217, 218]. In situ measurement further substantiated by the simulation studies [207, 208]. of the conductivity of a graphene sheet during chemical This was experimentally confirmed [206], where the use functionalization with diazonium salts showed notewor- of silver-decorated CNTs exhibited a significantly higher thy decrease in conductivity due to disruption caused conductivity above the percolation threshold than those by the transformation from sp2 to sp3 hybridization as containing pristine CNTs alone; for instance, a conductiv- also observed in the case of CNTs. Similarly, the cova- ity of 0.81 S/cm was measured with 0.5 wt% of Ag-deco- lent attachment of nitrophenyls to graphene sheets [216] rated CNTs. introduces a band gap, which can be controlled, making Higher cost of CNTs, especially of SWCNTs, com- the functionalized graphenes a semiconducting nano- pared with other additives such as graphite, CB, and material. The reaction with diazonium salts is used for S. Ahmad: Organic semiconductor devices 289 functionalizing different types of graphenes including excellent dispersion and processing features in various chemically/thermally converted graphenes, single sheets solvents. Covalent bond formation between thermally from cleavage of bulk graphite, as well as epitaxial gra- generated nitrene and epitaxial graphene was success- phenes [218–223]. Another alternative method involving fully explored [116] for controlling the band gap of the the reaction of benzoyl peroxide with graphene sheets functionalized graphene. Similarly, aryne cycloaddition [224], where Ar-ion laser irradiation initiated reaction on [238] to the graphene surface using 2-(trimethylsilyl) aryl graphene deposited on a Si substrate put in a benzoyl triate as a precursor resulted in enhanced dispersion in peroxide/toluene solution, is used to modify a graphene DMF, o-DCB, ethanol, chloroform, and water. sheet placed on an OFET device, where, apart from sig- Functionalized graphene-based flexible OFETs were nificant decrease in conductivity due to the increase of sp3 fabricated for the first time [239] by preparing conducting carbon atoms after the covalent addition of phenyl groups, source/drain/gate electrodes, with device characteristics an increase in the level of hole doping is also observed. very much similar to those with Au source/drain elec-

Azomethineylide is one of the commonest dienophiles trodes on SiO2/Si substrates with Si gate, with excellent that is used in the functionalization of fullerenes, NTs, flexibility without performance degradation over severe nano-onions, and nanohorns, offering numerous appli- bending cycles. Furthermore, successful demonstration of cations in areas including polymer composites, biotech- inverter circuits using all-graphene-electrode OFETs con- nology, nanoelectronic devices, drug delivery, and solar firmed that the dream of all-carbon flexible electronics is cells [225–230]. Graphene sheets dispersed in organic fairly close to a practical reality [239]. solvents [231, 232] are substituted with pyrrolidine rings Another useful variant of graphene is GO, a single via azomethineylide cycloaddition. The introduction of monolayer with randomly distributed sp2-hybridized hydroxyl groups increases the dispersion in ethanol and C-atoms and oxygenated aliphatic regions of sp3-hybrid- dimethylformamide (DMF). This procedure allows choos- ized C-atoms containing hydroxyl, epoxy, carbonyl, and ing from several aldehydes or substituted α-amino acids carboxyl functional groups. It was noted that the epoxy as precursors, resulting in a variety of desirable functional and hydroxyl groups were placed above and below each groups. For example, conjugating graphene with tetra­ graphene layer and the carboxylic groups were usually phenylporphyrin (TPP) or palladium-TPP, TPP aldehyde, attached along the edges. The oxygen groups existing Pd analogue, and sarcosin are used as useful precursors on GO surface provided hydrophilicity and analogous [233] in this context. chemical reactivity. GO were prepared either by oxida- In another attempt, for selective binding of Au tion of graphite with strong acidic media [240] or ozone nanorods (NRs) to graphene sheets [234], paraformalde- [241] or the chemical/thermal exfoliation of graphite hyde precursors with NH2-terminated α-amino acid are oxide [242–247]. Further, it was noted that GO formed used, where phenyl and alkyl azides react with the C-C unstable dispersions in water and ethylene glycol, DMF, bonds via intermediate formation of nitrene resulting in a N-methylpyrrolidone (NMP), and tetrahydrofuran (THF) variety of graphene derivatives. The aziridine ring is used solvents, having tendency to aggregate through π-π [235] to attach several functionalities resulting in combina- stacking forming large particles known as GO platelets. tions of varying solubility-dispersion and surface energy Compounds such as octadecylamine [248], 1-octyl-3-me- combinations. Nitrene addition is also used to function- thyl-imidazolium [142], large aromatic molecules [249], alize graphene sheets with phenylalanine. A variety of didodecyldimethyl-ammonium bromide [250], polysty- hexyl, dodecyl, hydroxyl-undecanyl, and carboxy-unde- rene [251], and poly(sodium 4-styrenesulfonate) [252] canyl radicals were also attached to graphene [236]. For and also elastomeric Si foams [253] and DNA [254] were instance, attaching carboxylate group facilitated attach- explored for stabilizing GO nanoplatelets in the solution. ing Au NPs, which were used as markers to investigate the It was rather difficult to remove all the oxygen reactive site distribution. groups and defects even after full conversion of sp3 into A novel structure comprising a 2D macromolecu- sp2-hybridized C-atoms during reduction of GO. In this lar brush of a large number of chemical moieties on a context, several methods of GO reduction were attempted graphene sheet was prepared [237] via nitrene cycload- from hydrazine [242, 243] to bacterial treatments [255] dition possessing enhanced thermal and chemical stabili- besides a number of other methods [171, 247, 256–258]. The ties. The functionalized graphene was further modified conductivity of the reduced GO was invariably influenced by different chemical reactions, including amidation, by the residual oxygen and defects [211]. surface-initiated polymerization, and reduction of metal Superior optical limiting property was shown ions resulting in electrically conducting materials with by amine-terminated oligothiophenes attached GO 290 S. Ahmad: Organic semiconductor devices

nanoplatelets compared to standard C60 and control vacuum-evaporated organic thin films [272] and OFETs sample consisting of GO and oligothiophene [228]. The employing conjugated polymers [48]. Consequent upon solubility of GO functionalized with -CH2OH-terminated the consistently improving performances achieved in regioregular P3HT [259] was improved facilitating its use preparing better-quality materials and fabricating high- in photovoltaic devices along with C60, which recorded performance devices over the last few decades, OLEDs twice the power conversion efficiency of a pure P3HT/C60 have progressed to the level of a commercial product system. incorporating OLED displays. GNPs grafted with porphyrins, phthalocyanines, and Having gone through various stages of the process azobenzene [224, 260–262] exhibited higher visible light development including synthesis of altogether new organic extinction coefficients necessary for OPV applications materials, optimizing thin-film depositions to have best- with better dispersion solvents. Similarly, GO grafted suited ordered structures and morphologies and optimizing with polymeric chains of hydroxyls and amines including the device architectures for improving the overall perfor- polyethylene glycol, polylysine, polyallylamine, and PVA mances, it finally led to a very impressive device develop- showed improved dispersion solvents along with mechan- ment involving OS for their specific ­applications in various ical strength, electrical conductivity, chemical reactivity, fields [24]. It started with a very low mobility of 1.5 × 10-5 cm 2/Vs and reinforcement of the mechanical properties. in mercocyanine, which was reported in 1984; the mobility figures for various organic materials remained confined to the range of 10-5 to 10-3 cm2/Vs up to 1992; it started improving slowly and reached up to 4 × 10-2 cm2/Vs over another 3 years; 2 Historical development and in 1996, the technology almost matured to a level where perspectives mobility went up to 0.6 cm2/Vs, but thereafter the improve- ment has been rather slow and only incremental [24]. Although the photoconductivity of anthracene crystals was The inspection of the compiled highest mobility data studied during the early 20th century, the phenomenon of for the most widely used OS such as pentacene, thiophene electroluminescence (EL) in molecular crystals, observed oligomers, and regioregular poly(3-alkyl-thiophene) in the 1960s, triggered renewed interest by identifying points toward a performance maturity attained by now and studying the basic processes involved in optical exci- as individual improvements vs. time has reached almost tation and charge carrier transport [263–265]. Even after saturation [24]. It is interesting to note that room tempera- successfully demonstrating organic electroluminescent ture hole mobility of 2.4 cm2/Vs of vapor-grown pentacene diodes, there were few drawbacks that prevented the use films [273–277] containing the grain sizes larger than the of these devices. For example, it was difficult to sustain channel length reached fairly close to the mobility of 3.2 sufficient current to produce adequate light output mainly and 2.7 cm2/Vs [274, 276, 277] reported in vapor-grown due to material stability problems. Using relatively thicker pentacene single crystals. This also explains the upper materials in the range of microns to millimeters resulted limit of charge carrier mobility in OS crystals determined in very high operating voltages besides poor-quality con- by room temperature TOF experiment [278] falling in the tacts. However, this situation changed considerably over a range of 1 to 10 cm2/Vs. It could thus be concluded that the period of another decade, resulting in improved material molecular vibrational energies being fairly close to that synthesis and controlled doping of conjugated polymers of the intermolecular bonding energies at or above room that produced a very important class of OS. Simultane- temperature ultimately decide the highest mobility values ously, with the availability of the organic photoconduc- instead of other scattering mechanisms [24]. In the case of tors and conducting polymers, the first few applications phonon-assisted charge carrier hopping among the local- of organic materials started in the form of conductive coat- ized states causing scatterings in the disordered OS, the ings [266] or photoreceptors in electrophotography [267]. mobility was found to be temperature dependent. It is also In the 1980s, undoped OS started drawing more interesting to note that the mobility of ∼1 cm2/Vs coincides attention of the researchers with the demonstration with the boundary between band-assisted and hopping of two important classes of devices, namely, photo- charge carrier-based transports [61, 265, 279], which voltaic solar cells involving p- and n-type materials in were measured in ordered OS such as pentacene and a heterojunction configuration [268] and OFETs from other derivatives [279–281]. The observation of tempera- conjugated polymers and oligomers [269–271]. The pro- ture-independent mobility in some cases [265] including gress was thus accelerated much faster after fabricat- polycrystalline pentacene films [279] could be explained ing high-performance electroluminescent devices from by taking into account the charge carrier trapping at the S. Ahmad: Organic semiconductor devices 291 grain boundaries and the dependence of trap concentra- vibrations at higher temperatures. Employing this kind tion on the film deposition conditions [277]. of strategy might improve the room temperature mobility At low temperatures, coherent band-like transport of either very comparable to that observed at low tempera- delocalized charge carriers in single-crystal pentacene, tures in acene series crystals or otherwise much higher tetracene, and other related materials was confirmed by than the room temperature values. The second strategy measuring TOF hole mobility of 400 cm2/Vs in naphtha- may involve an array of single molecules such as NTs or lene at 4.2 K [45, 282] and field-effect hole mobility of polymer chains bridging between the source and drain 105 cm2/Vs in tetracene and pentacene at 1.7 K [277]. The contacts to augment the conduction mechanism. For avail- mobility increase from its room temperature value of 3 to ing of the intramolecular transport of the charge carriers, 105 cm2/Vs at 1.7 K followed a power law type of tempera- the channel length should be reduced from microns to ture dependence [103, 277, 282, 283]. Furthermore, band- nanometers, so that it is less than the length of the single assisted charge carrier transport at low temperatures was molecule. Based on these considerations, CNTs were used confirmed by quantum Hall and cyclotron resonance in OFETs, showing impressive mobility of 100 cm2/Vs [287, experiments in acene single crystals [277, 284, 285]. The 288]. The successful realization of these strategies would temperature dependence of the electron mobility in pen- prove that the experimentally observed device perfor- tacene and tetracene single crystals was shown to follow mance limitations were due to nonoptimal design archi- the same power law that described the hole mobility tem- tecture but certainly not due to the intrinsic nature of the perature dependence in the same materials from 1.7 to organic materials involved. 300 K. Similarly, the temperature dependence of the elec- tron mobility in single-crystal naphthalene, below 100 K, also followed a power law type of temperature depend- ence along the three principal directions, consistent with 3 High-mobility OS thin films the band model transport [45, 103, 286]. However, in the temperature range of 100 to 300 K, the electron mobil- Due to inherent technological problems with the use of ity along the c-axis was found almost temperature inde- monocrystalline organic materials in device fabrications, pendent [103, 263, 286], which could be interpreted as it is natural to consider thin films prepared out of such a result of the superposition of two independent carrier materials as a second option even at the cost of sacrific- transport mechanisms involving (i) molecular polarons ing the associated mobilities to a certain extent. There is [286] that are formed due to interaction of the carriers roughly an order of magnitude reduction in the mobility with intramolecular vibrations of the local lattice environ- in employing thin films in place of crystals, which still ment [286] and (ii) involving polarons that participate in appears a reasonable price to pay for availing the addi- thermally activated hopping resulting in an exponential tional advantages of the flexible devices on plastic sub- dependence of mobility with temperature. The superpo- strates covering larger areas of applications. sition of these two mechanisms could thus explain the High-quality organic films are deposited in a vacuum temperature-independent mobility from just a few Kelvin system, where the base pressure decides the mean free to room temperature [286]. All these experimental results, path of the target molecules in the presence of the com- measured in the case of the different materials, proved bined population of impurity atoms and target molecules that ultrapure single-crystal OS were essential for higher present at the substrate during film formation, especially mobilities, as it was not possible in the polycrystalline at the initial stages of the film deposition. In this context, films where the charge carrier transport through two various kinds of pumps are used to produce ultrahigh or more grains, in terms of their associated traps due to vacuum (UHV), high vacuum (HV), and low vacuum (LV) structural defects, dominated the transport. in the range of 10-9, 10-6, and 10-3 torr, respectively, during Based on above-mentioned observations, it seems thin-film deposition using molecular beams [289], bell jar possible to remove the upper limit of ∼1 to 10 cm2/Vs of the deposition, and glass-wall vacuum sublimation systems room temperature mobility for OFETs by further strength- [290], respectively. The efficacy of removing unwanted ening the nearest-neighbor interactions without breaking impurities is accordingly very high, medium, and low in the molecular conjugation and reducing the intermolecu- UHV, HV, and LV environments, respectively. Besides, lar overlaps. These stronger interactions would ultimately carrier gas is also used in transporting the organic mol- push the organic molecular arrangements toward rigid ecules from the source to the substrate [273, 275, 277], crystalline structures, which would generate substan- where the substrate temperature and deposition rate are tial scattering of highly delocalized carriers by lattice the two parameters that critically influence the thin-film 292 S. Ahmad: Organic semiconductor devices morphology and the transport characteristics of OFETs and the absence of heterogeneous nucleation on the clean

[291, 292] prepared from the material so grown. Ultrahigh Si substrates [300]. The nucleation density on SiO2 surface purity precursors and ultraclean substrates ultimately was 100 times more than that on Si (001) and cyclohex- ensure the overall quality of an OFET, as they directly ene-modified Si (001) surfaces under the same deposition affect the charge carrier accumulation, which are gen- conditions [300]. It was further noted that polycrystalline erally confined to first few monolayers of the OS at the pentacene films were grown on polyimide substrates with interface with the insulator [1, 278]. The impurities at grain sizes of 100 μm using a substrate temperature below the growing interface thus affect the mobility, the on/off 200°C [277]. ratio, and sometimes even the polarity of the OFETs. For example, iodine-doped pentacene is, as such, a p-type material [293], whereas alkaline metal-doped pentacene shows n-type behavior [294]. During the early 1990s, 4 Organic single crystals vapor-grown polycrystalline 6T and α-ω-di-hexyl-sexi- thiophene [271, 295, 296] film-based OFETs played a key Most polymers possess a mixture of small crystals and role in further evolutions of these devices by not only amorphous material melting over a range of tempera- proving that relatively higher mobilities were feasible in ture than at a single temperature. In crystalline poly- polycrystalline organic films but also helped in setting mers, some sort of molecular order is frequently formed guidelines for choosing the right kind of methodologies involving folding and stacking of the polymer chains in that turned out to be necessary for improving the device contrast to the amorphous or glass-like structures, where performance. For instance, in the case of chain or rod-like the chains are very randomly tangled. Although there are molecules, such as thiophene oligomers, π-conjugations polymers that are completely amorphous, most of them along the long axis of the molecule as well as the close have a combination of the disordered regions enclosing molecular packing along at least one of the short molecu- the crystalline areas. lar axes (π-stacking) were noted as two critical conditions In a polymer crystallization process, it has been for achieving high carrier mobility even valid in the case observed that relatively short-chain molecules organize of OFETs based on vapor-deposited polycrystalline penta- into crystalline structures more readily than larger mol- cene thin films [277, 291, 295, 297]. ecules. Polymers with a higher degree of polymerization Growing amorphous pentacene films at the rela- get tangled very easily. During slow cooling of molten tively much lower temperature of -196°C produced insu- polymer, the process of reorganization of molecular struc- lating films, as the overlap of the molecular orbitals of tures gets sufficient time to turn into a better crystalline nearest-neighbor molecules was insufficient because of structure, whereas rapid quenching produces highly the prevailing disorders frozen in the solid at such a low amorphous materials. In addition, subsequent annealing temperature. In contrast, the room temperature deposi- produces a significant improvement in the crystallinity of tions resulted in highly ordered films [291, 292, 298] with most polymers primarily due to substantial stress relief reasonably high mobility of 0.6 cm2/Vs. When a mixture of resulting in reorientations. amorphous and monocrystalline phases was employed to Small-molecule polymers have very weak intermo- grow thin films [291], the associated carrier mobility was lecular bond strength due to van der Waals forces holding found to be very low possibly due to the presence of high them together, which allow the crystalline layers to slip defect concentrations resulting from the coexistence of past one another causing a break in the material. Large- the two phases. molecule polymers, in contrast, are held together relatively The detailed examination of room temperature- more strongly because the molecular species are fairly grown pentacene films on SiO2 using shadow masks [299] tangled between the layers. In most polymers, the com- revealed single-crystal island formations during the initial bination of crystalline and amorphous structures offers stage of the growth followed by the subsequent layers a material composition with useful features of strength growing on top of these islands that were smaller in size and stiffness. Besides, the size and shape of the monomer leading to a terrace-and-step type of morphology. Poly- groups affect the polymer properties, as the larger and crystalline pentacene films with grain sizes approaching irregularly shaped monomers do not allow the molecular 100 μm were fabricated on clean Si (001) surfaces pas- chains to arrange themselves in an ordered form resulting sivated with a cyclohexene layer [300], where the larger- in amorphous solids in contrast to the smaller monomers sized grain growth was attributed to the relatively low possessing rod-like structures that easily form crystalline nucleation density of pentacene grains on such surfaces compounds. S. Ahmad: Organic semiconductor devices 293

For more than two decades in the recent past, con- superconducting. Organic thin films are currently used in siderable efforts were made to improve OFETs by carrying fabricating OFETs [310, 311], LEDs [312], and photovoltaic out extensive theoretical investigations and technological organic solar cells (PVOSCs) [313] because of their light developments [301] to the extent that the organic OFETs weight, flexibility, solubility, low-temperature process- outperformed the best α-Si (α-Si:H) transistors. However, ing, higher yields, and low cost of production based on even in the best OFETs, the charge carrier transport was various kinds of printing technologies that are especially still found to be dominated by the presence of structural tailored for large-scale productions [309]. In this context, defects and chemical impurities, which indicated that the high purity and low defect density organic single crys- current OFET structures were not appropriate for studying tals offer an ideal platform [40, 43, 314, 315] for studying the basic transport mechanisms in organic materials [280]. structure-performance relationships [316], intrinsic [306] In contrast, the recently developed single-crystal organic and anisotropic [21] charge carrier transport, and pho- transistors with significantly reduced disorders [18, 19, 21, toconductivities [317, 318] that are put to use in device 40, 302–305] provided sufficient push to explore the fun- simulations and modeling studies [104, 319]. Weak inter- damental processes that ultimately control the operation molecular interactions in OS make them very different and reliability of the organic devices. For the first time, the from conventional in OS, as they possess relatively lower intrinsic transport properties of the field-induced charge melting points and low sublimation temperatures, which carriers at organic surfaces were observed in these single- make their crystal growth substantially different from crystal OFETs [21, 306, 307] in the form of an order of magni- those of inorganic semiconductors [308, 309]. tude higher carrier mobility compared to the best observed Despite having synthesized a number of semiconduct- in conventional OFETs [40]. Furthermore, the fabrication ing organic crystals so far [309], only a few of them were of single-crystal FETs turned out to be highly reproducible, successfully processed into samples fit for transport meas- that is, the devices fabricated even in different laboratories urements and still fewer were available into few micron- exhibited almost identical characteristics. This reproduci- sized freestanding single crystals [320–327]. This clearly bility, which is essential for the fundamental investigations explains why there is a need for going into thin-film-based of electronic properties of OS, has never been achieved device development and not on large-sized wafers as in in thin-film OFET devices, whose electrical characteris- the case of Si and other inorganic semiconductors. tics were strongly dependent on the details of fabrication processes and handling environment. These observations necessitated exploring the single-crystal devices in more 4.1 Melt-grown OS crystals detail as described in the following paragraphs. Single-crystal OS can be, in principle, prepared involv- After having access to a rich expertise in growing zero- ing the transport of the constituent species from any one defect large size crystal ingots of inorganic elemental of the solid, liquid, or vapor phases. Based on these trans- and compound semiconductors [308], it was only natural formations, the organic crystal growth is put into solid, to follow the same way in the case of organic materials. melt/solution, or the vapor growth category. It is by now However, the organic crystal growth involving Czochral- well established that, during crystal growth, a polycrystal- ski, Bridgman, or float zone methods has not been that line/amorphous material is converted into a single crystal easy to extend due to high vapor pressure and the mate- by pushing the defects in the form of grain boundaries out rial instabilities present near the melting point of the of the sample during the solid-phase growth of the crystal starting organic samples [309]. Of course, only in the case [308]. A current survey of the crystal growth technology of some OS, available in quantity such as naphthalene observed that almost 80% of the single crystals are melt [328], anthracene [329], phenanthrene [330], pyrene [331], grown compared to approx. 5% vapor grown, 5% each tetracene [332], and stilbene [333], it was possible to melt- of low- and high-temperature solution grown, 3% of the grow them, as these molecules were relatively stable at solid, and only 2% of hydrothermal grown. their melting points, but still they were noted to be suscep- Organic materials are, in general, high-resistivity tible to polymerization and/or decomposition during long insulators at room temperature [309], but the molecules heat treatments or under intense light irradiation. Mostly, with sp2 hybridization and delocalized δ-electrons, such the melt growth of OS is a complex phenomenon [309], as as in conjugated hydrocarbons, metal phthalocyanines, the material having concurrent sublimation/evaporation and oligothiophenes, exhibit semiconducting properties; tendency during melting requires elaborate modifications besides, the organic charge-transfer compounds based to have a useful crystal puller. For instance, fast evapora- on donors and acceptors [307] become conducting or tion from the melt in the Czochralski method [308] during 294 S. Ahmad: Organic semiconductor devices crystal pulling restricts the applicability of this method to use their solubility for growing crystals of large molecule only a few materials such as benzophenone and benzyl compounds, which otherwise decompose easily into [334, 335], where organic powders form melt in a crucible smaller molecules during high-temperature sublimation and a rotating crystal seed is slowly immersed and then process. Based on the properties of a given organic mol- pulled out of the melt at slow rate, making the crystal ecule, several solution-grown methods [309] were devel- grow along the orientation of the seed. However, the oped in the past, as discussed very briefly in the following Czochralski method has not been sufficiently investigated paragraphs. for growing a large variety of OS; therefore, knowing the The simplest but most effective method of growing precise behavior and purity of the grown crystals needs organic single crystal is that of solvent evaporation further studies in the related areas. employing solvents such as dichloromethane, chloro- In the Bridgman method, large-sized single crystals form, toluene, benzene, and chlorobenzene [309]. The are grown inside quartz ampoules [336] sealed under solvent evaporation from such a saturated solution causes vacuum or inert gas environment and by moving the supersaturation, where spontaneously formed nuclei ampoule through a properly designed temperature gradi- grow finally into larger crystals. In the case of higher solu- ent [308] furnace. Once the crystal nucleation is induced bility of organic material in a solvent, the drop casting is at the tip of the ampoule, the crystallization front moves another better option widely used for the fabrication of progressively through the melt. During the slow move- single-crystal films. For instance, single-crystal TTF films ment of the ampoule across the temperature gradient, and its derivatives were prepared by pouring n-heptane or the impurities in the melt having different solubilities chlorobenzene saturated solution onto prepatterned sub- segregate from the growing crystal [308, 337]. Further, as strates leading to crystallization after the solvent evapora- the Bridgman method uses a sealed ampoule, adequate tion, where higher values of mobilities (∼1 cm2/Vs) were chemical purification is not possible; therefore, the puri- measured [322, 325, 327] in such samples. The drop-cast fication needs to be carried out separately before starting millimeter-sized single-crystal monolayers of HTEB with the crystal growth. Following rigorous material purifica- mobilities up to 1 cm2/Vs [326] were found very useful tion and setting optimal growth conditions, the electri- besides P3HT crystals [341]. cal properties of the large-sized Bridgman-grown organic For moderately soluble semiconductors, a slow crystals matched well with those of the gas-phase-grown cooling method was found better, especially when solubil- small crystals. The quality of Bridgman-grown DPA single ity changed with temperature [309]. In such cases, raising crystals exhibited room temperature electron and hole the solution temperature dissolved more material leading mobility of 13 and 3.7 cm2/Vs, respectively [338], which to saturation, which made the solution spontaneously were extremely useful for semiconductor devices. The form nuclei, leading to large-sized crystals by consum- Bridgman method produced large-sized crystals limited ing additional material to deposit onto the existing seeds. to the ampoule dimensions used, but the strain at the This process was repeated few times, raising and lower- boundary between the crystal and the quartz walls of the ing the temperature around the saturation temperature in ampoule did induce cracks, stress, or small-angle grain such a way that only a small portion of material dissolved boundaries in the crystals. and crystallized in the following temperature cycle, In a zone melting process, a series of short-heating resulting in selective dissolution of the smallest seeds and elements produce melt in several narrow zones and force further growth of the larger seeds leading to millimeter- purer crystalline material to accumulate outside the sized organic crystals. For instance, rubrene crystals were molten zone [308]. This method is useful for purifying grown using solvent evaporation method using 1-propa- large amount of OS, such as anthracene, naphthalene, nol, demonstrating a mobility of 1.6 cm2/Vs [342]. and perylene [339, 340]. Although very high purity crystals The vapor diffusion method [309] is yet another are prepared using this technique, because of the repeated variant of growing OS, where a saturated solution is pre- melting of the material, this time-consuming method has pared by dissolving the molecules in a “good” solvent. been used only for few materials [339, 340]. Here, a container with such saturated solution is placed inside another larger container with a volatile solvent that dissolves the organic compound only slightly. When the 4.2 Solution-grown OS crystals larger container is sealed, the volatile solvent diffuses into the solution of the OS, making it supersaturated causing Most OS are soluble in a number of solvents over a wide nucleation and crystallization [309]. Although this temperature and pressure ranges, making it feasible to method was initially used to grow smaller-sized crystals, S. Ahmad: Organic semiconductor devices 295 by selecting only a few seeds, millimeter-sized tetrathi- form good crystals in the presence of an inert gas environ- afulvalene (TTF)-tetracyanoquinodimethane (TCNQ) ment. For instance, during high vacuum, evaporation of crystals were grown by adjusting the growth conditions rubrene produces quasi-amorphous films involving free appropriately [343]. noninteracting twisted rubrene molecules [343] resulting A combination of two distinct solvent layers, having in low mobility [344]. In contrast, under inert gas flow, different solubility of the compound, was used in a liquid- the same rubrene molecules form several centimeter- liquid diffusion method [309], where the low-solubility sized single crystals with maximum reproducible mobility layer diffused into the high-solubility layer, causing ranging from 5 to 20 cm2/Vs depending on the dielectric saturation at the interface between the two layers. For constant of the material used as a gate insulator in OFETs instance, triisopropylsilylethynyl pentacene (TIPS-PEN) [345]. Hydrogen, argon, and nitrogen are generally used as microribbons were grown using toluene and acetonitrile carrier gas along with precisely controlled temperature for as the “good” solvent and the low-solubility solvent, growing single crystals [326]. respectively [187]. High-quality micron-sized crystals were In the next variant [309] of the closed PVT system, grown at the interface without any stress from the sub- the source material is sealed in a quartz ampoule under strate [309]. a vacuum/inert gas environment, where neither the reac- In the process of growing crystals from those com- tants nor the products escape during crystal growth. pounds, which are insoluble in most common solvents Anthracene, pyrene, naphthalene, and C60 crystals and have melting points so high that they start dissociat- were grown successfully in a closed PVT system [347]. ing [309], a solid solvent is found helpful. For instance, However, despite using very pure source materials, some when a mixture of anthracene and CuPc was heated, the new molecules got mixed up because of decomposition, anthracene melt dissolved the CuPc [309], and after slowly photoreaction, polymerization, or chemical reactions reducing the melt temperature, black needle-shaped crys- that unavoidably occurred during growth in a closed tals were separated from the solid anthracene by adding PVT system. To remove such contaminants, a semi- toluene [344]. closed system was thus advanced in which one end of the furnace tube was completely sealed and the other end was connected to a HV pump to continuously pump out the 4.3 Vapor-grown OS crystals volatile impurities. For instance, urea crystals were suc- cessfully prepared in a semiclosed PVT system [348]. Most organic compounds, having relatively lower melting points and low sublimation temperatures, are purified by high-vacuum evaporation [309], where even light- 4.4 Supercritical fluid-grown OS crystals weight impurities sublime and condense together with the desired material on the substrate. The method of Supercritical fluids (SCFs) are excellent solvents for pre- physical vapor transport (PVT) helps in separating the cipitating large organic molecules. For example, SC-CO2 organic compounds from such impurities [274, 276, 344] behaves like an antisolvent to control the precipitation of by combining crystal growth and material purification C60 crystals from the solution. In one of the experiments, in different configurations including open, closed, and when a saturated solution of C60 in toluene was injected semiclosed systems, where open and closed systems into SC-CO2, it reduced the solvent power of toluene pre- employ vacuum or inert gas environment. For instance, in cipitating C60 [274]. Various combinations of these kinds an open PVT system, the inert carrier gas flow not only of interactions were possible to work out for preparing controls the sublimation speed, deposition, and crystal organic crystals by adjusting the operating parameters growth of the organic molecules but also prevents oxida- and selecting suitable solvents. tion. However, the impurities deposit in a location other than that of the semiconductor because of their different molecular weights. 4.5 Binary OS crystals For starting the process of crystallization in the pres- ence of weak intermolecular interaction forces operat- A binary mixture of acceptor-donor-type charge-transfer ing on the molecules, the involved individual molecules compounds is certainly a useful combination for device must get extra energy to reconstruct, link to each other, fabrications. Fortunately, crystal growth of a binary com- and form crystal surfaces. It is an interesting observa- pound is noted to be very much similar to that of the tion that some OS do not crystallize in high vacuum but individual ones. For instance, a combination of parylene 296 S. Ahmad: Organic semiconductor devices

and TCNQ was used for preparing three sets of binary growing C60 film [323]. In this experiment, the nucleation compounds, namely, P1T1, P2T1, and P3T1 [349, 350], was specifically confined to OTS-stamped regions, leaving from the gas-phase PVT or from solvents, but the stoi- the rest of the SiO2 background unaffected. The single- chiometry control in such compounds was not clearly crystal growth in patterned regions was thus realized by understood. choosing the pattern size, such that it supported only one nucleation site at the most [323] and for determining this optimum size; a detailed study was carried to correlate 4.6 Patterned deposition OS crystals the number of nucleation sites vs. OTS domain sizes. An average of one nucleus per 7 μm2 area was observed in this OS device fabrication, in general, differs from those of study [323]. Site-selective pentacene thin-film growth on the conventional microelectronic devices particularly OTS-treated Au surface was attributed to different types because the OS surfaces are easily damaged during pho- of surface interactions [356]. For instance, selective pen- tolithography, patterning, wet and dry etching, sputter- tacene growth on SiO2-coated substrate with OTS and ing, and similar others processes [40] involved therein. perfluorodecyltrichlorosilane (FDTS) stamps resulted Instead, the organic crystals are manually placed or ran- [357] only within a small window of growth conditions. domly cast on a prepatterned metallized substrate for However, the selectivity of the above-referred growth was realizing the device structure. This kind of handling is due more to the roughness of the thick OTS-stamped film. more liable to contaminate the device surfaces and cause Further, the OTS films were directly stamped onto the damages resulting in impaired device performance with OFET source-drain electrodes for fabricating large arrays lower yields [351]. In actual practice, it is known that of high-performance single-crystal devices demonstrat- manually placing a micron-sized organic crystal is an ing, for instance, a high mobility of 2.4 cm2/Vs in rubrene, extremely difficult task with very low yield. On the con- comparing well with those of the best α-Si transistors and trary, unwanted deposition of smaller-sized debris during other high-performance devices reported in the literature solution and/or solvent-based processing is equally trou- [118, 358, 359]. Pentacene, C60, and TCNQ single-crystal blesome. In order to mitigate these problems, a number OFETs were also fabricated using the same technique of alternative schemes were developed including fabrica- exhibiting mobilities of 0.3, 0.03, and 10-4 cm2/Vs, respec- tion of free-standing single-crystal devices [18, 352], elec- tively. In addition to using SiO2/Si substrates, this tech- trostatic bonding of a single crystal onto prefabricated nique was also tried on a flexible kapton sheet with Au device electrodes [19, 302, 323, 353], formation of single gate and a spin-coated poly-4-vinylphenol film as the die- crystals by drop-casting from organic solutions [320, 354], lectric. In these experiments, flexible rubrene and penta- and solvent-grown organic single-crystal nanowires [355] cene devices showed mobilities as high as 0.9 and 0.1 cm2/ as reported in the published literature. Keeping various Vs, respectively, besides showing no change in perfor- types of the associated problems in view, several alter- mance even with a 6-mm-diameter bend confirming the nate strategies were developed to deposit organic single superior endurance of the patterned single-crystal devices crystals directly onto the substrate: either using PVT or to sufficiently large bending stress without performance solution-based processing techniques [224]. degradation [323]. In further extended studies [354], it Most of the organic crystals used in OFETs are grown was noted that the stamped OTS films were particularly using PVT [274], where the starting material is placed in the not as smooth as a self-assembled OTS monolayers, as the high-temperature reactor zone producing gaseous precur- stamped domains were populated with large numbers of sors for growing crystals somewhere downstream, within approx. 100-nm-tall OTS pillars that facilitated the selec- a narrow temperature range in the furnace [40, 41, 274]. In tive nucleation [356, 357]. This, in fact, was further clari- 2006, patterned growth of large-area organic single crystal fied by observing that nucleation started from the base of was successfully attempted for OFET applications [323], the rough OTS surface and the crystal grew on the surface where octadecyltriethoxysilane (OTS) film was microcon- of OTS pillars [354]. Having identified this mode of crystal tact printed onto a clean SiO2/Si substrate using a poly- growth, patterning of OS on other rough surfaces was sub- dimethylsiloxane (PDMS) stamp prior to deposition [274]. sequently realized, as expected. For example, a variety This scheme was equally applicable in the case of a large of OS exhibited preferential nucleation on CNT bundles, variety of materials, including rubrene, pentacene, tet- which contained many surface steps wherein it was noted racene, C60, F16CuPc, and TCNQ. It was noted that material that pentacene molecules nucleated directly from the sublimation and crystal growth took only 5 min to com- surface of CNT bundles, and by stamping CNT bundles on plete in the case of pentacene; in contrast, it took 2 h for the device channel region, pentacene single-crystal FETs S. Ahmad: Organic semiconductor devices 297 with mobilities as high as 0.4 cm2/Vs were realized [360] pregrown organic single crystals. In one case, an organic successfully. crystal suspension was first cast on a substrate with pat- Another route for patterned growth of organic crys- terned wettability and then removed. Upon receding of tals involved selective growth from nanocrystalline seeds the three-phase contact line of the suspension, organic [351] prepared separately in the form of a suspension, crystals were transported to the wetting regions and were where dipping of a SiO2/Si substrate led to predeposition deposited. Using this process, CuPc crystals with different of the nanocrystals (NCs). Such predeposited substrates sizes and shapes were prepared successfully. One advan- transferred to a PVT system exhibited selective deposi- tage of this approach is its suitability to any OS crystal, tion of CuPc nanoribbons in the case of predeposited CuPc soluble or insoluble in organic solvents. Another notable NCs. It was further noted that the predeposited NCs were advantage of this method is in growing 1D crystals, which aligned during dip coating, which resulted in parallel had otherwise been a challenge in most patterning tech- nanoribbons. OFETs fabricated with patterned CuPc [351] niques. In the other method, crystals are patterned into and F16CuPc [348] ribbons showed mobilities of 0.1 to 0.5 solvent dewetting regions instead of wetting regions and and 0.2 cm2/Vs, respectively, performing much better than the organic single crystals are first deposited uniformly those devices fabricated by the manual process of crystal on surfaces with patterned wettability followed by rinsing placement described earlier [351]. with a solvent. The crystals on the solvent wetting regions Patterned deposition techniques based on other are selectively removed by lift-off process, resulting in mechanisms were also developed, for example, CuTCNQ patterned crystals on the dewetting regions. OFET arrays nanoribbons were grown via the selective reaction are thus fabricated by patterning organic single crystals between TCNQ vapor and the patterned Cu surfaces [125, directly onto transistor source-drain electrodes. However, 361, 362] by exposing to TCNQ vapor for device fabrication. these devices show relatively poor performance possibly CuTCNQ nanoribbons were found to grow from two neigh- due to weak lamination of the pregrown crystals to the boring electrodes to stretch toward each other and finally dielectric interface [360]. bridge the interelectrode gap [125]. In another example, In the solvent dewetting method of patterned crys- using Au NPs as template, F16CuPc-based 1D nanostruc- tals growth, the device containing substrate, with lyo- tures were grown using HV vapor deposition resulting in philic electrode regions and the lyophobic background, controlled assembly of F16CuPc into uniform 15- to 30-nm- is dipped in an emulsion of a saturated aqueous semicon- diameter 1D nanostructures [363]. ductor solution. While withdrawing the substrate, drop- For availing of the low-cost processing advantages lets of the molecular solution adhere to the Au electrode of OE, solution-processed organic single crystals directly pads exclusively. After evaporation of the solvent, pat- deposited onto the device structures would be preferred terned crystals are formed only on the electrode regions. ultimately. In the past few years, a variety of materials It was noted that the presence of a liquid vortex generated such as inorganic crystals [364], organic-inorganic hybrid during continuous stirring significantly improved the materials [4], conjugated polymers [319, 365, 366], and oli- selectivity of the molecular solution droplets. The shear- gomers [319, 367] were pattern deposited using solution ing force exerted on the substrate surface by the vortex processing with self-assembled monolayers (SAMs) as stripped off the small droplets that were adsorbed on the templates. It is noted that SAMs provide selective nuclea- lyophobic surface regions and trim larger droplets that tion sites for patterned growth of organic single crystals covered the individual electrode pads. Using this tech- by serving as a template [367] or wetting/dewetting sites nique, large arrays of CH-4T and TMS-4T single-crystal [319, 360]. For instance, in a study of nucleation activity OFETs were fabricated, exhibiting the highest mobil- of anthracene on Au surfaces functionalized with differ- ity of 0.11 cm2/Vs in TMS-4T and 0.02 cm2/Vs in CH-4T ent SAMs [368], it was noted that the nucleation density transistors. Besides excellent device performance, this was highest on SAMs of terphenylthiol (-3P) and lowest technique also offered several advantages over existing on those of alkanethiols (-CH3). By dipping the patterned techniques, such as the cleanliness and selectivity of the substrates into a saturated anthracene solution contain- patterns, the patterning speed, and the extremely small ing these two types of SAMs and allowing the solvent to amount of material needed. This method is useful in evaporate, highly localized crystals were found onto -3P depositing crystals of small OS molecules that are oth- patterned regions [368]. With SAMs serving as wetting/ erwise difficult to form films. Furthermore, it is possible dewetting patterns, two strategies for preparing pre- to have fine control over the crystallization conditions by grown organic single crystals were demonstrated sub- choosing the initial concentration and host-liquid tem- sequently [360], employing suspensions prepared from perature [360, 369]. 298 S. Ahmad: Organic semiconductor devices

4.7 OS epitaxy phthalocyanines [380, 383, 395, 396] and polycyclic aro- matic compounds using naphthalene and parylene have Molecular beam epitaxy (MBE) along with its several vari- prominently been explored. In particular, the planar ants has been extremely successful in producing high- stacking of PTCDA molecules has attracted attention of quality epitaxial inorganic semiconductor thin films for various research groups [397–404]. quantum-well heterojunctions along with clarifying a The MBE system comprises an UHV chamber fitted number of epitaxial growth-related fundamental issues with a number of temperature-controlled effusion cells to employing ultraclean and UHV environment loaded with produce collimated beams of the desired species using a high-precision diagnostic tools for in/post-processing series of orifices, after passing through which they fall on material characterizations. Having access to such ultra- a perpendicularly oriented substrate held at 10 to 20 cm high precision material growth technology for elemental away from where epitaxial growth takes place at a back- and compound semiconductor thin films for device appli- ground pressure of 10-9 to 10-11 torr [308]. The molecular cations with monolayer precision [308, 370], it was very beam flux is controlled by effusion cell temperature and natural to consider a similar route for the OS after witness- mechanical shutters, which switch the specific beam “on/ ing the successful development of a wide variety of mul- off” as desired. Using a number of effusion cells along tiple quantum well (MQW) devices, including low current with sequential shuttering of the beams, it is possible to threshold laser diodes, low-noise avalanche photodetec- grow multilayer structures of alternate layers of differ- tors, and high-bandwidth optical modulators [371–375]. ent compounds, required for the device applications, as The unique success of these MQW devices stemmed from reported [386, 387] in growing MQW structures. two factors: ultrahigh precision-controlled growth of the Although the typical deposition rates in MBE system epitaxial layers with required density of states in conduc- may be somewhere in the range of 0.0001 to 10 nm/s, tion and valence bands that lowered the laser threshold it is, however, necessary to note that lower deposition currents [376] and the quantum-confined stark effect in rates allow contaminant absorption onto the substrate, optical modulators and the heterojunction energy band whereas, during higher rate depositions, the precise offsets between two materials of different band structures. control of layer thickness is rather difficult. Thus, the Encouraged by the success of MBE in the past, work most useful growth rates were experimentally found in started to grow epitaxial organic films aiming for similar the range of 0.01 to 0.5 nm/s as a compromise between goals by tailoring the material properties in terms of con- two extremes. Maintaining the effusion cell below subli- trolling the density of states and the energy band offsets mation temperature during the idle periods keeps them as mentioned above. Organic materials were especially continuously degassed for storing high-purity source examined for their varied applications in nonlinear optics materials over long durations by eliminating the impuri- [377–381] and optoelectronic devices [382–385]. Indeed, ties and moisture completely. the progress of growing organic thin-film nanostructures The substrate temperature in organic thin-film deposi- by MBE using MQWs [386, 387] evoked theoretical inter- tions is chosen anywhere in the range of 80 to 400 K [386, ests in predicting completely newer optical phenomena 405–408], as the lower substrate temperature absorbs in organic/organic and organic/inorganic MQW com- the impurities fast, which may be particularly critical in binations [388–392]. Thus, the successful realization of the case of films grown at modest background pressures. entirely new class of materials, offering unique opportu- For the large-sized organic molecules, the sticking coef- nity for extending the understanding of the basic proper- ficients are almost in unity throughout the above range of ties of a large class of organic crystals, is undoubtedly to substrate temperature, provided that it is maintained well be explored in the very near future. At this early stage of below the desorption temperature. However, substrate development, MBE-deposited “small-molecule”-based temperature control can also be put to use with the advan- organic LEDs are being commercially produced as light tage of changing the desorption rates during controlled sources for flat-panel displays [272, 393, 394] with several growth of monolayers of organic materials [409], where advantages. Exploring the areas of applications of such loosely bound species are eliminated automatically due MBE-deposited organic materials is only limited by the to their easy desorption resulting in more compact films. ultimate ability to control composition and structure of The most significant advantage of MBE system is to the resulting thin films. avail of several in situ thin-film diagnostics tools such as From among a fairly large variety of organic mole­ residual gas analysis (RGA) meant for detecting the pres- cular systems studied so far, the optoelectronic proper- ence of all species inside the chamber prior to as well ties arising due to planar stacking of molecules such as as during growth, reflection high/low-energy electron S. Ahmad: Organic semiconductor devices 299 diffraction (LEED/RHEED) for determining crystal struc- single-crystal ZnPc grains were observed on p-6P surfaces ture [410, 411], and thermal desorption spectroscopy and the fabricated OFETs exhibited reasonably higher (TDS) for identifying absorbed species by measuring mobilities of 0.32 cm2/Vs, which was similar to that in the their binding energies during as well as immediately after case of single-crystal OFETs with metal phthalocyanines. growth [308], besides many other analytical tools such as Organic molecules having orientational and vibra- scanning tunneling microscopy (STM) [412, 413], Auger tional degrees of freedom are certainly different [429] from electron spectroscopy, and in situ ellipsometry that are atomic species while considering their participation in very helpful for material characterization [308]. epitaxial growth. For instance, the orientational degree of The crystalline order and symmetry of the inorganic freedom may change the molecular orientations such as single-crystal substrates can be used as a template for the films with “lying-down” and “standing-up” molecules. the growth of organic crystals, especially when the elec- Similarly, the vibrational degrees of freedom may influ- trostatic interaction between the ionic lattice of halide ence the interaction with the surface as well as the ther- crystals and π-electronic molecular orbitals of organic malization upon adsorption and the diffusion behavior of molecules promoted the epitaxial growth of organic crys- the molecules arriving upon the surface. tals on inorganic substrates [414]. This kind of study was The organic molecules, belonging to soft materials, extended further, for instance, in several cases including behave differently during intermolecular and molecule- 6P on mica [415], CuPc on indium tin oxide (ITO) [416], α-6P substrate interactions compared to the atomic adsorbates. on TiO2 [417], α-6T on TiO2 [418], α-quinquethiophene on This certainly modifies the capacity of the growing thin Cu [419], hexabenzocoronene (HBC) on Au [420], α-6P on film in accommodating larger strains, in terms of very KCl [421], and PTCDA on KCl [422]. However, many limited large critical thickness, above which the growth mode is investigations were conducted on the fabrication of OFETs compelled to change to contain the strain in heteroepitaxy. based on these crystals or thin films, which showed oth- Further, due to the presence of van der Waals interactions, erwise interesting morphologies and molecular align- the temperature needed for crucible evaporation as well ments. Currently, aligned crystals/thin-film-based OFETs as diffusion on the substrate is much lower. Although the are mostly fabricated using a wet transfer method [423], integrated molecular interaction energy over the “contact where the organic single-crystal/thin film is transferred area” may be comparable to that of strongly interacting from the inorganic template to the device substrate by atomic adsorbates, still the interaction energy per atom of dissolving the inorganic template. A family of BP2T single the organic molecules is usually weaker. It is also useful to crystals, grown on KCl substrates in hot-wall epitaxy [424] note that the closed-shell structure of organic molecules with aligned crystalline needles, showed high mobility of has no dangling bonds, and as a consequence, the surface 0.66 cm2/Vs [425] as well as biphenyl-capped thiophene energies are usually lower than those in the case of the oligomers crystals realized in the same way [414]. Besides, inorganic substrates. epitaxially grown thin-film OFETs were also reported Fairly larger size of the molecules and the unit cells [426], for instance, in the case of PtOEP deposited on KBr practically smear out the lateral potential variations substrates exhibiting mobilities of 2.2 × 10-4 cm2/Vs [427]. resulting in weaker surface interactions than for atomic One problem of using epitaxially grown crystals/films for adsorbates. Moreover, the size difference between the OFETs is that the in-plane mobilities are usually very low. adsorbates unit cells and the substrate provides more Due to the strong interaction between organic molecules translational domains. Organic materials frequently crys- and the inorganic single crystals, organic molecules tend tallize in low-symmetry structures, which further lead to to lie/adsorb flat to these substrates; consequently, the multiple translational and orientational domains behav- direction of higher conductivity between molecules is not ing like disorders, in addition to vacancies known in inor- parallel to the film plane, leading to low in-plane mobil- ganic systems. ity [428]. To overcome this problem, a method known as Although a fairly large number of organic thin films weak epitaxy growth (WEG) was advanced, which allowed were studied and employed in device fabrication, only a the molecules to stand up on the substrate by reducing few of them are included here to illustrate their features the interaction between organic molecules and inor- responsible for their frequent uses. ganic substrates. In this manner, a thin layer of rod-like Several research groups have used PTCDA, a parylene p-6P oligomer was deposited on a substrate forming large derivative-based red dye as a model system for studying domains followed by deposition of ZnPc, a disk-like mole- the molecular beam deposition [430–439]. Although the cule on the p-6P surface that resulted in oriented domains bulk has layered molecular planes, even films grown on of ZnPc. Compared with ZnPc films on SiO2, much larger SiO2 and many other substrates are similar. In one of the 300 S. Ahmad: Organic semiconductor devices

studies, it was observed that PTCDA on SiO2 exhibited a 2D structure in a phthalocyanine monolayer, whereas, smooth surfaces only for low-temperature growth at < 50°C in thicker films, there is a competition between the lying- although with very poor crystal quality [440]. In contrast, down orientations in the first layer followed by the ten- higher-temperature films exhibited better crystallinity, but dency to having stand-up configuration in the following with a tendency toward island growth leading to surface layers onward. The pentacene has attracted considerable roughness. Epitaxial growth of PTCDA was carried out attention due to excellent charge transport properties as [431, 436–438] on Ag(111) with a herringbone structure of reported [24, 103]. It is noted that pentacene grows well in the flat-lying molecules with a vertical PTCDA-Ag spacing thin films in spite of the competition between “bulk” and of 0.285 ± 0.01 nm [439], whereas the growth extending “thin film” as the dominant structure. Pentacene on SiO2 beyond a monolayer resulted in a complex azimuthal was studied for OFETs [24]. The pentacene grains as large distribution as a function of temperature. The compari- as 0.1 mm [451] were grown successfully. In thicker films, son of PTCDA on Au(111) showed a similar behavior [432, complications were noticed in terms of simultaneous exist- 434, 441], although the epitaxy details differed consider- ence of “thin film” and “bulk structure” depending on the ably. For instance, on Ag(110) surface, an entirely differ- growth conditions. The effect of film morphology and die- ent structure comprising a “brick-stone” kind of layout lectric surface preparation was examined by measuring was observed [436]. Growth on Cu(110) differed from those the OFET characteristics including surface modification known from other substrates; for instance, thicker films using SAMs [452, 453]. Pentacene monolayers on Cu(110) were similar to the case on Ag(111). exhibited long-range order having molecules lying flat on A red dye, named diindenoperylene (DIP) of perylene the surface, but in thicker films an orientational transition family, exhibited out-of-plane ordering [103, 442, 443] from a lying-down configuration to a standing-up configu- that gave good charge carrier transport properties. DIP ration was clearly seen [429]. It was also noted that films films, prepared on SiO2/Si substrates at 145 ± 5°C, had the grown at low temperatures (200 K) were ordered because upright-standing molecules on well-ordered film through- of hyperthermal deposition, whereas thermal deposition out [443], displaying rapid roughening that increases resulted in disordered films. with thickness more than the random deposition limit. It is very possible that when certain regions of the surface grew faster than others, the surface turned out to be 4.8 Special OS crystals uneven. It was speculated that such spatial inhomogenei- ties cropped up from the different tilt domains of the film While attempting oriented growth of organic crystals, a and inevitably present grain boundaries in between [442]. process known as polymer-assisted solvent vapor anneal- In a DIP monolayer, the molecules are noted lying flat on ing (PASVA) was studied and developed further for pre- the substrate [444] with a physisorptive [442] interaction paring organic crystals in special forms [235, 454]. For with Au. In contrast to film on SiO2, due to the stronger instance, C8-BTBT was prepared as a rod-like structure interaction with the Au substrate, the lying-down configu- with good air-stability and high mobility [235, 454, 455] ration tends to prevail upon in multilayers. As the stand- using PASVA, an all-solution process for growing quality ing-up configuration appeared to have the more favorable organic crystals at room temperature. The PASVA process surface energy, there is obviously a competition between helps in reorganizing the molecules into several hundred the two configurations as they are found to coexist [442]. micron-sized single crystals in length. This method was Phthalocyanines have been very popular materials carried out on a patterned substrate, limiting the growth [383, 445–449] as they exhibit tunability due to the central direction of the crystals and allowing the fine control of metal ion, which can be changed within a broad range in the crystal location and orientation over a wide area. In addition to having a number of side groups [383, 447]. For a subsequent experiment [457], an SiO2-coated Si sub- instance, F16CuPc is a good conducting polymer [450] with strate was patterned into wetting/dewetting regions using interesting optoelectronic applications as a blue dye [383, OS solution [456–459] as mentioned earlier. The wetting 447, 449]. Phthalocyanines grow in a stand-up configu- regions were defined by standard photolithography fol- ration in thick films on “inert” substrates. F16CuPc films lowed by Cytop™ spin-coating and annealing at 90°C. exhibit very good out-of-plane crystalline order [429]. The The photoresist lift-off thus defined the dewetting Cytop™ in-plane structure is, of course, azimuthally disordered regions on the substrate enclosing a pattern of wetting because of the isotropic substrate. However, due to these trenches. An anisole solution of C8-BTBT and PMMA was growth complications, F16CuPc and H16CuPc [429] both end then applied to the substrate, resulting in trace amounts of up in needle-like crystals as a result. The molecules form the semiconductor solution entering the wetting trenches. S. Ahmad: Organic semiconductor devices 301

While the anisole evaporated, a polycrystalline C8-BTBT favorably. Some of these concepts tried out in OS devices thin film was formed on an underlying PMMA film by are discussed below. vertical-phase separation in the trenches. The chloroform PASVA was carried out for 10 h, causing the polycrystal- line C8-BTBT films in the trenches to recrystallize into rod- 5.1 Impurity-doped OS p-n junctions like single crystals oriented along the trench directions. Besides, without PMMA, the crystals were not grown in The phenomena of doping OS means that the n-type the trenches, confirming the role of PMMA to absorb the dopants should donate electrons to the lowest unoccupied chloroform, enabling C8-BTBT molecules to reorient easily molecular orbital (LUMO) states, while the p-type dopants for crystallization. An organic single-crystal array with sin- extract electrons from the HOMO states, creating holes. gle-crystal orientation over a wide area was thus achieved Although species such as iodine, bromine, lithium, by using PASVA [456], with the evidence that the crystals cesium, strontium, and Lewis acids were used earlier in were formed only in the trenches and they were uniformly phthalocyanine exhibiting high conductivities, they did similar, indicating good control of both the crystal posi- not produce stable layers due to their fast diffusing nature tions and orientations. The X-ray diffraction (XRD) analy- arising from their smaller sizes. In contrast, aromatic mol- sis confirmed that the C8-BTBT molecules were stacked in ecules such as ortho-chloranil, TCNQ, and dicyano-p-ben- the same direction [456]. A batch of 32 OFETs were success- zoquinone (DDQ) were used in doping phthalocyanines, fully fabricated with bottom-gate top-contact configura- stacked phthalocyanines, and oligothiophenes [463–467] tion using highly doped Si substrate as a gate and SiO2 and but less effectively. In the last few years, systematic inves- PMMA layers underneath the organic crystals as gate insu- tigations were carried out to understand the underlying lator. With C8-BTBT HOMO of 5.7 eV, an efficient carrier doping physics, which led to the development of OLEDs injection was achieved for Au source/drain electrodes, [468–481]. while an amorphous FeCl3 layer was inserted at the metal/ Although doping in inorganic semiconductors is organic interface that improved the charge injection effi- explained well by the hydrogen model [482], where the ciency still further. The average mobility assessed from the electron is released from a dopant having the binding measurements of 32 FETs was 1.1 cm2/Vs with maximum energy equal to that of hydrogen renormalized by the mobility of 3.8 cm2/Vs. These devices showed good device dielectric constant and the electron/hole effective masses, action with an average “on/off” ratio of 104, indicating an this is not applicable in organic hosts, as lower dielectric attractive performance for plastic electronics. This simple constant and higher effective masses of holes and elec- method is possible to extend to other solution-processed trons result in an order of magnitude higher binding and materials having 1D structure such as zinc oxide [460, 461] two orders of magnitude higher Coulomb binding ener- and lanthanide hydroxide NRs [462]. gies, respectively, besides lacking the symmetry of a single atom. Consequently, attempts were made to study doping of organic hosts using UV photoelectron spectroscopy (UPS) and theoretical modeling [482]. It was noted that the 5 OS homo, hetero, and Schottky energy difference between hybridized and transport states junctions of the host was found to reduce the doping efficiency. In another study, the interaction of the dopants was found to Two dissimilar materials used in an electronic device reduce the energy barrier of separating dopant and charge fabrication communicate through the interface formed carrier [482]. between the two. In order to have minimum influence of The p-type doping of polycrystalline and amorphous such an interface on device functioning, it is ideal to have hosts exhibited conductivities above 10-4 S/cm sufficient minimum hindrance to the flow of charge carriers across for OSC and OLED applications [431]. Knowing the fact such an interface. In actual practice, it is almost impos- that increase in free charge carrier concentration shifts sible to get rid of this interface effect as stipulated above. the Fermi level, it was possible to detect by Seebeck Due to this unavoidable situation of interfaces, it is better effect measurement. Further, using UPS, it was possible to modify them electrically such that they start aiding the to resolve the transport state (HOMO) distributions and flow of charge carriers as far as it is possible. Therefore, their positions with respect to the Fermi level in a metal/p- instead of depending on the basic limitations alone posed doped OS junction. by the real-life interfaces formed during device fabrica- The alignment of the Fermi level (EF) and metal work tion, it is better to modify them electronically to behave function (Wfm) during the metal-semiconductor contact 302 S. Ahmad: Organic semiconductor devices formation, along with IP and interface dipole, form injec- The n-type doping by K, Na, and Li was reported way tion barrier at the interface supported by a depletion back [482] in the 1970s and 1990s. Li is still used in OLEDs region, causing HOMO states bend upward resulting in based on controlled diffusion from Li-containing inter- gradual decrease in the gap between HOMO and EF, which face layer inside the bulk of BCP, CuPc, and Alq3 layers finally merges with the bulk Fermi level at the end of that are commonly used in OLEDs [482]. High doping ratio depletion layer [482]. coevaporation­ of Li with the host is another method of bulk In intrinsic MeO-TPD, with increase in doping level, doping, where the thickness of the initially doped layer is there is a gradual decrease in EF until it saturates at more controllable. Efficient OLEDs were produced with approximately EFmin∼0.35 eV. In addition, F4-TCNQ doping a Li-doped Alq3 or BPhen as an electron injection layer of MeO-TPD, ZnPc, and PV-TPD shows that the host has a [483]. Both methods of interface and bulk doping with distinct influence on the saturation behavior; for instance, alkali metals are frequently used for OLEDs and the ben- molecules such as MeO-TPD, PV-TPD, and ZnPc saturate at eficial effect of LiF interlayer between the organic material 0.35, 0.74, and 0.2 eV, respectively [482]. and the Al electrode is also useful in OSCs [484]. Cs and Taking the absorption due to C-N stretching mode of their salts/alloy compounds are used along with organic

F4-TCNQ depending on the dopant charging state as sig- ETLs leading to highly efficient OLEDs [482]. nature, the efficiency of charge carrier transfer between The first attempt [485] of organic molecule-based the dopant and the host is estimated by Fourier transform n-type doping of NTCDA involved electron-donating mol- infrared (FTIR) spectroscopy [482]. ecule BEDT-TTF. In this attempt, it was noted that the

The efficient charge transfer during doping of TTN molecules doped F16ZnPc but not the Alq3 [486]. In phthalocyanines and TDATA derivatives was explained by another attempt, a compound CoCp2 was found to be a the electron affinity (EA) being larger than the ionization good dopant for tris(thieno)hex-aazatriphenylene deriva- energy of the host, whereas, in a higher IP host like TPD, tive [487], where a shift of 0.5 eV in the position of EF was the charge transfer is less efficient [482]. Charge trans- noted, confirming the phenomena of n-type doping. fer in TPD derivatives can be enhanced in case the IP is The electron-donating nature of metal complexes lowered by electron-pushing methoxy groups at the outer [482] such as [Ru(terpy)2]0, [Cr(bpy)3]0, and [Cr(TMB)3]0 benzene rings. However, the unity charge-transfer ratio was explored for n-doping [488] for their applications in does not necessarily mean that one free electron is gen- OSCs but was not useful as n-dopant in OLEDs because of erated per dopant molecule [482]. For instance, the hole the higher lying LUMO values of the electron-transporting transferred to the host still experiences the attractive force materials. of the negatively charged dopant due to a lower dielec- Chromium and tungsten complexes such as Cr2(hpp)4 tric constant producing stronger attractive force, making and W2(hpp)4 [482] formed with 1,3,4,6,7,8-hexahydro- the charge carrier not completely free [482] as mentioned 2H-pyrimido[1,2-a]pyrimidine (hpp) were found to be earlier. The doping efficiency, defined as the ratio of the effective in doping C60 with conductivities well above 1 S/ number of free charge carriers to the number of dopants, cm. Both dopants were found to produce stable doping in can be effectively below unity, even if all dopants transfer C60 up to 100°C. However, one general drawback of this charge to the host. The method of UPS [482] estimates the approach was the increased instability of the dopants free charge carrier density in m-i-p junctions with the Ag with respect to oxygen for higher HOMO values, requiring anode, an intrinsic layer of MeO-TPD, and p-doped layer handling of the materials under an inert environment. of MeO-TPD. Another alternative suggested was to use precursors Although it is difficult to find suitable materials for that donated an electron to the matrix by being activated n-type doping, it is essential for realizing p-i-n devices by heat or illumination such as pyronin B chloride, which needed at times. In addition, these dopants must be large was found to be a strong molecular donor [489, 490]. enough to discourage their migrations in thin films and Doping can also be used to optimize injection and to not to act as traps; they must add electrons to the LUMO of reduce the losses at the metal-semiconductor interface. a wide variety of hosts. For a direct electron transfer from Ohmic contacts are thus realized for a wide range of elec- a dopant, its HOMO has to lie above the LUMO of the host, trode/OS combinations [491]. making it fundamentally more difficult, because a high For various reasons, already mentioned in brief HOMO reduces the stability against oxidation [482]. earlier, it took several decades until 2005 to realize the There are three ways [482] of n-type doping involving (i) first stable and reproducible p-i-n homojunction with alkali metals, (ii) compounds with very high HOMO levels, organic materials [492]. The material choice for organic and (iii) air-stable electron-donating precursor molecules. p-n homojunction [482] very restricted by energy-level S. Ahmad: Organic semiconductor devices 303 requirements. For instance, p-type dopant molecules actual device having both Schottky contacts, the voltage must have LUMO similar to or even below the HOMO of drop at the source is found to be somewhat higher than that the host, and even more challenging is finding suitable at the drain in “bad” contacts, whereas “good” contacts n-type dopants. Given that organic molecules are typically show comparable drops and this difference was assigned wide-gap materials, doping the same matrix with p- and [495] to the material quality in the regions just beneath the n-type dopants requires dopants with an extraordinaryly electrodes from that of the rest of the conducting channel. low LUMO and with a very high HOMO, respectively. Thus, It was further observed that top contacts in OFETs usually in many cases, formation of heterojunctions was preferred offered lower contact resistance than bottom contacts. for device applications [482]. The asymmetry of the organic-metal contact, depending Further, employing doped wide-gap transport layers, on whether the organic film was deposited on the metal the donor/acceptor layer thickness could be reduced or the metal on the organic layer, was studied from both without shorts and recombination at the organic-metal the theoretical [496] and experimental [497] angles. In interfaces. The corresponding layer structure for a typical detailed experimental investigations, UPS/X-ray photo- p-i-n stack would thus contain a transparent hole contact, electron spectroscopy (XPS) measurements clarified that a p-doped wide-gap HTL, a photovoltaic active layer, an the deposition of Au film on pentacene showed signs of n-doped wide-gap ETL, and an electron contact layer metal penetration coupled with the formation of metal [482]. This typical p-i-n structure turns out to be flexible clusters, leading to a substantial reduction of the interface for optimizing the absorption of the active layers, choos- barrier from 1 to 0.3 eV. ing the right kind of contact materials as well as the order Despite improving the charge carrier transport in of the layers. In recent years, this kind of p-i-n configura- each layer used in an organic device, the interface proper- tion is proving to be a versatile concept for investigating ties of the metal-semiconductor [498–512] and insulator-­ the fundamental processes as well as optimization of OSC semiconductor [8, 513–521] are not only important from devices [481]. the point of view of influencing the device performance significantly but also assuming the role of a limiting factor while trying to extract the best out of a given configura- 5.2 Metal-OS Schottky junctions tion. For example, the quality of the interfaces between the OS on one side and source/drain electrode and the The source and drain contacts in an FET structure are insulator on the other ultimately limits the overall perfor- modeled as a Mott-Schottky (MS) junction [493], which mance of the OFETs realized experimentally. states that such interfaces would behave as ohmic when In a FET structure, just next to the insulator-semicon- the work function of the metal is close to the HOMO/LUMO ductor interface, a conduction channel is formed on the level of the p- or n-type semiconductor, respectively. In semiconductor side through which the charge carriers the absence of this condition, formation of a barrier layer flow supported by the source drain bias [24, 280, 522–524]. is inevitable at the interface, leading to a significantly The associated parameters of such an interface in terms reduced charge injection. From this point of view, Au- of roughness [515–517] and the presence of defects and pentacene interface behaves as a low-resistance contact, charges [518–521] influence the charge carrier lifetime as the IP of pentacene is close to the Au work function, but and the mobility significantly. Similarly, in the case of the in actual practice the contact resistance of these devices is source/drain electrode-semiconductor interfaces, actual noted to be somewhat higher than the expected. Employ- interfacial contact resistance affects the carrier move- ing UPS to study the locations of the energy levels on both ments, and for easier flow, naturally the lowest barrier sides of the interface [494], it was clearly resolved that height is essential [494, 501–503, 506, 509, 510, 525–536]. these interfaces strongly deviated from the MS model, con- Especially, in case the device dimensions are reduced, the firming the presence of an additional “dipole” barrier that contact resistance ultimately limits the total device resist- shifted the vacuum level upward by more than 1 eV, hence ance as it starts dominating over the channel resistance; increasing the barrier height by the same amount. Such therefore, for deciding the final speed of the organic inte- large interface dipoles were considered arising from the grated circuits, the intrinsic carrier mobility becomes sec- electron density tail extending from the metal free surface ondary in importance [527]. into vacuum, thus forming a dipole pointing at the metal It is a fact that improving the source and drain con- bulk. Molecules, deposited on such a surface, tried to tacts in OFETs by the conventional processes such as push back this tail, thus reducing the surface dipole and semiconductor doping or metal alloying at elevated tem- decreasing the metal work function. Additionally, in an peratures is not that easy. In this context, it is much better 304 S. Ahmad: Organic semiconductor devices to search for a metal having the right kind of work func- the OS directly on this pattern. Total contact resistance tion and low contact resistance from the group of different was thus determined from the measured I-V character- possible electrode materials. In addition, making use of istics of such OFETs. In the case of experimentally real- the profound impact of surface functionalizations on the ized Al and Ni contacts, rectifying features were noted at interface characteristics appears more promising in this low currents due to larger barrier heights; in contrast, Au context. electrode exhibited the lowest ohmic contact resistance. Observation of low current nonlinear I-V characteristics as mentioned above, especially at low voltages, was possibly 5.3 Interface engineering for OS devices due to the presence of native oxide formed on the metal surface. Taking the example of p-type OS pentacene, the barrier Besides being relatively time-consuming and inac- height between the metal and the pentacene layer is fixed curate due to the large number of samples involved, the by the difference between the metal work function and serious problem of the TLM method is its inability to dis- the HOMO of pentacene [528, 534, 537]. In this context, it tinguish between source and drain contacts. For char- is more helpful to have an idea of the work functions of acterizing each contact separately, an alternate gated various contact materials estimated by UPS as given here four-probe measurement was developed using thin ohmic in each bracket as follows: Al (3.7 eV), Ni (4.6 eV), Mo (4.6 electrodes extended into the source-drain channel region eV), Cu (4.6 eV), ITO (4.7 eV), NiOx (5.2 eV), and Au (4.85 to monitor the potential drop along the channel and relat- eV). These values may differ from those reported in the lit- ing that to the potential drop in the contact regions. For erature possibly due to the fact that these measurements an externally applied source-drain voltage, the effective were carried out on thin films instead of the bulk materi- voltage appearing across channel and the losses due to als; besides, there might have been some surface contami- contact resistances are possible to estimate and correct nations [538–541], considering the approximate nature of for using the channel electrodes. The gated four-probe UPS values for the metal work function [538, 542, 543]. method was already used in pentacene OFETs [509], The contact resistances offered by the source and drain where it was noted that, for suitable device geometry, contacts influence the experimental values of the charge the channel probe fingers did not influence the potential carrier field-effect mobility in FET devices [544, 545], as distribution in the channel region. While using the four- there is a finite difference between externally applied probe geometry for P3HT devices, the entire contact geom- source-drain voltage and that effectively appearing across etry of source/drain contacts and the channel electrodes the channel. Under such conditions, a reduced channel was patterned directly onto the gate insulator for minimiz- current reflects lower mobility rather than the actual ing process-induced damages to the active layer. The field- value. The metal-semiconductor interface studies gener- effect mobilities in P3HT devices, without correcting for ally attribute the contact resistance, mentioned above, to the effects of contact resistances, were in the lower part of the presence of interface dipoles [546, 547] and the phe- the 10-3 cm2/Vs range, whereas, after taking into account nomena of Fermi-level pinning [548] examined separately. the contact resistances, the mobility was found to be in In order to remove the influence of contact resistance the higher part of the 10-3 cm2/Vs range [545]. on the channel characteristics, several methods were Because of the matching Au work function with HOMO developed as reported in the published literature. The of pentacene (for instance, 5.1 vs. 5.2 eV, respectively), Au most popular method is of transmission line measurement is the most widely used S/D contact material in pentacene (TLM), in which a number of test structures are fabricated [537, 538]. Further, during deposition, the Au surface is under the same conditions only varying the channel length found to be relatively cleaner and stable without a native in a regular step. As the incremental change in the total oxide. The only drawback with the usage of Au is its higher device resistance is proportional to the additional channel cost; therefore, other contact materials such as Ag, Ca, Pd, length, included in each case, the contact resistance con- and ITO were also explored [553, 555–557] for similar uses. tribution remains the same throughout. It is thus very easy Invented in 1991, Kelvin probe force microscopy to estimate the contact resistance from the current-voltage (KPFM) is essentially a non-contact-type of scanning characteristics of such a device configuration [15, 544]. For probe method, where the local surface work function is instance, a TLM scheme [106, 549–553] was implemented measured and mapped at an atomic/molecular level for using an electrode material pattern deposited on a glass analyzing the composition and electronic states of the substrate with a gap between adjacent electrodes from 50 local structures of the solid. This method measures poten- to 90 μm with an increment of 5 μm [553] and depositing tial offset between a probe tip and a surface using the S. Ahmad: Organic semiconductor devices 305 same principle as the Kelvin probe, where the cantilever is of the devices by comparing the surface potential to the a reference electrode forming a capacitor with the surface, topographic maps. It is necessary to go for a right kind of over which it is scanned laterally at a constant separation. contact metal, as metal-semiconductor contact resistance KPFM, being a noncontact technique, has been exten- is highly sensitive to the choice of the metal electrode. For sively used in exploring the electronic and morphological instance, in pentacene devices, Pt contacts showed a very behavior of working devices based on molecular organic small contact resistance compared with Pd and Ni [560] materials or polymers. Understanding and controlling the and a similar behavior was noted in the case of methyl- relation between the electronic transport properties and quinquethiophene [561] FETs. the morphology of conducting layers is important for opti- The electrical characterization of NT-based devices is mizing the uses of these materials. In this context, suitable very complex, as the semiconducting NTs behave as a p- or modification of the metal-organic interface for improving n-type when they are measured in air or a vacuum, respec- the device performance is a basic issue involved in the tively. KPFM measurements on Au-NT-Au systems, using optimization of the working devices such as LEDs, FETs, only semiconducting single-walled NTs, clarified that the and OPVSCs. Investigations using KPFM revealed that the p-type behavior in air was due to oxygen adsorption on contact resistance at the metal-organic interface depends the Au surface, creating a dipole layer and thus changing on many factors such as metal work function, IP of the the Au work function. It was easy to suppress the dipole organic surface, the effect of the diffusion layer, and the formation by simple “passivation” of the Au surface using built-in potential at the interface [502]. Moreover, the H2S or alkanethiols [180]. metal-semiconductor interface also involves many con- In order to adjust the electrical conditions at differ- troversial parameters such as vacuum-level shift, band- ent interfaces encountered in practice, various types of bending, and interface-dipole formations; therefore, SAMs were developed in the past and used in different interpretation of the results should be made with proper contexts such as patterning of nanoscale devices, corro- care. KPFM measures the potential difference map of the sion prevention, and controlling surface properties [518, potential drops due to low-conductivity areas, short cir- 552–554, 562–574]. SAM modification of a metal surface cuits, electrical defects, and phase separations providing offers a better possibility to improve the performance of valuable analytical information. Especially, in situ KPFM OE devices as well [518, 562–574]. The formation of SAMs measurements are carried out for quantitative determi- on a metal surface, in the present context of OFETs, is basi- nation of the potential drop along source/channel/drain cally meant for improving the contact properties across a interfaces in different organic OFETs, while current is semiconductor-metal interface. In reality, it is not only flowing across the operating device [502, 558, 559]. The the work function of a metal that is modified with SAMs measured potential decay in P3HT FETs allows the estima- on its surface [537, 570, 574–576] but also the morphol- tion of the resistances of the source and drain contacts, the ogy of the pentacene layer subsequently grown on SAMs charge carrier mobility and the resistance of the polymer, is influenced compared to the layer grown directly on a as well as the linear and nonlinear transistor operating metal surface [577–580]. The formation of SAMs employs ranges [558, 559]. the chemical reaction between the functional groups of A detailed study of the importance of source-drain the molecule and the surface, such as in chemisorption of contacts in OFET devices was recently reported [501] by alkanethiols on Au, hydrolysis of alkyl-trichlorosilanes on mapping the surface potentials of the operating pentacene hydrated surfaces, and adsorption of carboxylic acids on OFET with two different contact geometries, deposited metal oxides [556]. above or beneath the active organic layer. The surface- Consequent upon using the above criteria of interface potential distribution enabled quantitative estimations engineering in OS device fabrications, the influence of of the potential drops at the source and drain contacts; SAMs on oxides such as SiO2, Al2O3, and TiO2 [563, 565– in this study, the bottom-contact devices were observed 569], metals such as Au and Pt [562, 570, 572, 573, 579], to be contact limited at large gate voltages, while the top- and native oxides of metals such as Cu, Ag, Ti, Al, and Fe contact FETs were not. In both geometries, the contact were investigated in detail [562, 571]. For instance, in the and the channel resistances decreased notably with case of Mo thin films, although it has a low resistivity of the increasing (negative) gate bias but did not depend 5 μΩ cm, its work function is in the range of 4.6 to 4.9 eV strongly on the drain bias. This study highlighted the use [581], resulting in a larger barrier height to hole injection of KPFM in monitoring the charge carrier-transport bot- in pentacene than that with Au. With the formation of tlenecks in operating pentacene devices and more gener- SAMs on its surface, work function of Mo was increased ally to correlate the electrical behavior with the structure resulting in improved organic-metal interface accordingly 306 S. Ahmad: Organic semiconductor devices

[575, 576]. In addition, annealing of as-deposited Mo thin functionalized thiols supported on metal films were suc- film also increased the work function by another 0.2 eV cessful in promoting nucleation and growth of inorganic [582], which was probably due to defect density reduction crystals with finely tuned crystal sizes, crystallographic caused by heating. Following on the same lines, introduc- orientation, and crystal micropatterning [362, 597–599], ing OTS SAMs on Mo surface increased its work function their uses in organic crystallization were rather limited by 0.1 eV. With ITO, the work function was modified using due to prevailing van der Waals interactions against the polar molecules such as 4-bromopropyl-trichlorosilane ionic ones that were critical at the organic-inorganic inter- and trichloro(4-chlorophenyl) silane [553, 578, 583, 584] faces. It was found that patterned SAM templates could resulting in reduced contact resistance. produce site-specific crystallization of organic charge transferring salts [600] and pentacene films [356, 357]. Successful growth of 100 μm size pentacene single crys- 5.4 SAM-modified OS devices tals was induced by Si surface modified with a monolayer of cyclohexene molecules [451]. For furthering this quest, SAMs have been extensively used for surface modifica- oligoacene molecules were put to a detailed investigation tions of metals and oxides for diverse applications in in this direction. Substrates modified with alkanethiols the areas of molecular electronics [557, 585–587] and OE such as methyl, amine, and carboxylic acid end-groups [23, 321, 588, 589] as partly described in brief above. In as well as with monolayers of thiophenol, biphenylthiol, one such study, the growth of organosilane SAMs at the and terphenylthiol SAMs were immersed in a saturated surface of OS was found to significantly increase the anthracene/THF solution and allowed to evaporate [368]. surface conductivity of organic materials approaching Detailed analysis of the nucleation density and surface 10-5 S/square, which was close to the highest conductivity coverage of the OS crystals clearly indicated that the realized in OFETs at ultrahigh densities of charge carriers nucleation density of anthracene crystals on different

[108, 109, 590]. This kind of SAM-functionalized organic SAMs increased in the following order: -CH3 < -NH2 < -COO surface was found very useful for sensing applications, as H < Au < -P < -2P < -3P [368]. Next, micropatterned substrates the interaction between polar molecules in gaseous form with regions of -3P and -CH3 having different geometries and SAM-functionalized semiconductor resulted in a fast and relative sizes were microcontact printed [368] and and reversible change of the conductivity, proportional to placed in a saturated anthracene/THF solution, where the analyte’s vapor pressure. subsequent crystal growth occurred by solvent evapora- The concept of using single crystals in polymeric tion [368]. Highly localized crystal growth was observed device fabrication was primarily motivated by success- onto oligophenylene thiol-patterned regions. Similarly, fully assessing the intrinsic charge carrier transport prop- dip coating was used [368] for selective crystallization of erties in the absence of grain boundaries and molecular OS molecules, where the coverage of the -3P region was disorders [21], which are invariably there in polycrystal- three times that on the nonpatterned -3P substrates result- line thin films [591, 592], degrading mobility and impair- ing in 100 × 100 μm size single crystals occupying the ing device performance. Although improvements made whole -3P region. in this direction could produce ruberene single-crystal OFETs exhibiting charge carrier mobilities as high as 20 cm2/Vs [303], there were problems such as poor-quality electrical contacts and difficulty in handling the fragile 6 OS-NPs and NP thin films crystals that limited their uses in device fabrications. In the absence of a suitable replacement of a manual method Quantum confinement of charge carriers in nanosized of placing devices, which was highly impractical for fab- entities including NCs, NPs, NRs, NTs, superlattices, ricating devices over a large area, a method for induc- nanofilms, and many other variants gives rise to addi- ing site-specific growth of large organic crystals using tional energy levels superimposed upon the conduction micropatterned SAMs as nucleation templates was defi- and valence band edges of the parent material imparting nitely needed [366]. tunable optical and electronic transport properties based Although a number of challenging problems were on the size and shape of the sample. Various nanosized faced in controlling nucleation sites, sizes, and orienta- metal, semiconductor, and insulator species are currently tion of the OS single crystals, some recent developments being investigated and developed for their use as funda- have been found helpful in controlling such crystalli- mental building blocks for realizing new type of meta- zations [593–596] as presented here. Although SAMs of materials in which the required physical, chemical, and S. Ahmad: Organic semiconductor devices 307 biological features are taken as input parameters to their evaporated to obtain a stable dispersion of polymer NPs design and subsequent synthesis. As OS, in their present in water. The size of the NPs, in this process, can be varied form, are poised for their diverse applications in the area in the range of 30 to 500 nm depending on the concentra- of flexible electronics of tomorrow, it is equally important tion of the polymer solution used. However, the processes to know about the changes brought out in their properties such as Ostwald ripening or flocculation by coalescence due to quantum confinement, especially from the point of start destabilizing such droplets, which can easily be view of their use as material building blocks for a newer prevented by adding either hydrophobic agent to the dis- class of materials. The prevailing situation in this context persed phase or suitable surfactant accordingly. is examined in brief in the following paragraphs. In the earlier two methods of precipitation or emul- sion, a millimolar concentration of target material is added to a large volume of nonsolvent resulting in very 6.1 OS-NPs dilute OS-NP dispersions, which are not applicable in the case of organic materials that are poorly soluble in organic Solubility-based methods of preparing OS-NPs are partic- solvents. A method employing condensation of the vapor ularly important because of their simplicity and meeting of the target material into a liquid dispersion medium was low-cost requirement of resources involved besides pro- therefore developed next [603] to overcome the problems ducing materials somewhat closer to those prepared by associated with the methods of precipitation or emulsion. sophisticated self-controlled growth. In this context, the For concentrated dispersions of OS-NPs, temperatures in procedures developed such as precipitation, emulsion, different zones of a properly designed tube furnace are and condensation are discussed here in brief by pointing adjusted according to the target material ensuring that out their positive and negative aspects for comparative no condensation of the organic vapor occurs in the tube. evaluation of each method. Rather, the evaporated target material is carried out by an The precipitation method [601] of producing poly- inert gas flowing to the vapor injection tube, which guides meric NPs, first introduced in 1992, employs higher and the organic vapor into a liquid condensation medium con- lower solubility of the target compound in two different sisting of an aqueous solution of surfactants or polymeric solvents that are used in realizing a hydrophobic inter- stabilizers, where the rapid cooling finally leads to con- action while mixing the solution of the OS material dis- densation of the organic vapor into OS-NPs. These NPs solved in a “good” solvent and pouring it into a “poor” are subsequently in situ stabilized by the surfactant or solvent such as water and stirring the mixture vigor- polymeric additives at the bubble/liquid interface to form ously. After the NPs are formed, the organic solvent is a stable dispersion. The size of OS-NPs prepared by this removed either by vacuum evaporation or by repeated method is, in general, in the range of 100 to 200 nm for dialysis to produce water-dispersed OS-NPs. The driving fused aromatic hydrocarbons such as pentacene, rubrene, force behind this process is the hydrophobic effect real- and tetracene. ized during mixing of the two solutions, where the subject In an entirely new concept of material preparation, molecules try to avoid contact with water. Forced by this laser ablation-based preparation of OS-NPs, several tendency of hydrophobic interactions, they try to fold or micron-sized organic crystals are pulse laser irradiated pack into spherical shapes to achieve minimum exposure while suspended in a liquid medium [604, 605]. For without involving any surfactant; especially, this process instance, the target material is added and sonicated to a is equally applicable in a wide variety of OS including sodium dodecyl sulfate aqueous solution, which is put both polymers and small molecules given that they are into a quartz cuvette, stirred vigorously, and simultane- soluble in some “good” organic solvents. Moreover, it is ously exposed to the second harmonic of a nanosecond very clear that there is ample scope of varying the size of YAG laser with a fairly large spot size. The organic crys- NPs by adjusting the solution concentration and the tem- tals, present there in the solution, absorb the laser radia- perature appropriately. tion leading to a local increase in temperature resulting Alternatively, in the emulsion method of preparing in the evaporation of the material from the crystal sur- OS-NPs [602], the target compound is first dissolved in faces, which is immediately cooled by the surrounding an organic solvent that is immiscible with water and then liquid forming OS-NPs. The competition between tran- the solution is injected into an aqueous solution of an sient heating and liquid-assisted cooling that gives rise to appropriate surfactant. The mixture is stirred vigorously a finite transient temperature rise ultimately determines to form a stable emulsion comprising smaller droplets of the OS-NP size. For instance, higher fluency gives higher the polymer solution. The organic solvent is subsequently effective transient temperature rise, leading to efficient 308 S. Ahmad: Organic semiconductor devices fragmentation to smaller particles. One advantage of this polymers are either polymerized in situ from the mono- method is its superior control of size and phase of NPs by mers absorbed on the surface of the particle templates or tuning laser pulse width, wavelength, fluency, and the deposited from a layer-by-layer technique through electro- shot number. However, this method is limited to fabricat- static interactions [609]. ing OS-NPs based on small molecules only. It is further Growing awareness to the environmental concerns in noted that NP formation occurs within the narrow laser current times has been compelling to reorient the inves- beam resulting in only a small volume of dispersions. tigations toward developing NP syntheses without using Possibly, the intense laser light may cause severe photo- organic solvents. In this context, SCF-based processes chemical damages especially in the case of rather sensi- offer the possibility to design and prepare NPs without the tive organic materials. limitations of traditional methods [610–615]. Working on While solidifying the target material from a fluid to a these lines, two procedures were developed for NP prepa- solid phase, if the space available is confined to nanosize, rations using SCF. The first one is based on a rapid expan- it is very possible to have nanosized solid particles as a sion of a supercritical solution (RESS) and the second one result. While confining the overall volume of the vacant uses an extension of the first one, where RESS is released space by some means, as explained later, it is practically into a liquid solvent (RESSLS). possible to assign certain shapes and sizes to such a tem- In a RESS process, organic macromolecules are dis- plate. It is interesting to note that, in template growth, solved in an SCF solution, which subsequently undergoes a there is an automatic control of morphology provided rapid expansion through a nozzle into ambient air forming by the template involved. There are a number of ways in well-dispersed particles from a homogeneous nucleation which these templates are realized as discussed below in imposed by the high supersaturation conditions com- brief. bined with the rapid pressure reduction [616]. Generally,

Micelles have been used as the soft templates for SCF-CO2 is used in the majority of cases. The polymer is polymerization in an aqueous heterophase system by dis- first dissolved in a CO2 solution at ambient temperature in persing the appropriate monomers, surfactant, solvent, the mixing cell. Subsequently, the solution moves in the and catalysts in an aqueous medium, where the coupling pre-expansion unit with the help of the syringe pump and reaction takes place exclusively within the confines of the heated to the pre-expansion temperature until it expands hydrophobic interior of the surfactant-controlled micelles through the nozzle at ambient pressure. Poly(hepta-deca- to produce, for instance, the poly(arylene diethynylenes) fluoro-decyl-acrylate) [617] or poly(L-lactic acid) [618] NPs [606] and poly(p-phenyleneethynylene) (PPE) NPs using were prepared by this technique, wherein the factors such this method [607]. The molecular structure of the sur- as concentration and degree of saturation of the polymer, factant used in the aqueous heterophase has significant the processing conditions, the molecular mass, and the influence on the shape of the NPs so formed, as using melting point of the polymer mattered the most. Although dodecyl-benzene sulfonic acid surfactant with PEDOT the method is performed without organic solvents and doping agent results in amorphous and polydisperse NPs produces a majority of nanosized particles, the major having diameter dispersed in the 35 to 100 nm range [608]. drawback is the simultaneous generation of micron-sized In a similar experiment, short-chain alcohol ethoxylate particles or agglomerates. surfactants yielded more spherical NPs, but significant To overcome the above-said problem of agglomera- amounts of surfactant residue were also found trapped tion encountered in the RESS process, another modified on the PEDOT latex causing secondary nucleation. These process was developed, where the SCF solution is allowed examples clearly show that the soft template approach is to expand into a liquid solvent instead of ambient air a versatile method for preparing conjugated polymer NPs. [619]. The liquid solvent suppresses the agglomeration of However, control of parameters such as diameter and primary NPs. For instance, PHDFDA [620] NPs were pro- polydispersion is often not so simple. duced using water as the solvent in which the SCF solu- Shape retaining features of hard templates, such as tion expanded and precipitated the polymer. It was shown monodisperse silica and polystyrene NPs, offer a more that the particle formation resulted from the aggrega- appropriate method to control the morphology of the tion of initially formed NPs. In addition, the presence of conjugated polymer NPs, especially in core-shell con- NaCl in the water phase helped in stabilizing the NPs due figurations, which are finding valuable applications in to an increase in the ionic strength. PMMA and poly(L- optoelectronic devices. Conjugated polymers such as lactic acid) NPs were synthesized by this method using polypyrrole, polyaniline (PANI), PEDOT, and PPE are a CO2 cosolvent as the SCF. The cosolvent allows a better attached to the surface of colloidal NPs and the conjugated dissolution of the polymers in the SCF solution and the S. Ahmad: Organic semiconductor devices 309 presence of NaCl in the water solution generates only NPs individual nanostructured species still retain their identi- [621]. Despite the availability of a number of fluids such ties while participating in offering the typical properties as CO2, n-pentane, ammonia, and many others, the poor of the assembly as such. In this kind of arrangement, it is solubility of polymers in these SCFs remains a major limi- easy to realize a large variety of superlattices for having tation of this process. access to the band structure engineering aspects of such Incidentally, the spray-drying process has been in assemblies for using their electronic and optoelectronic use, for the past several years, for preparing micron-sized properties especially for realizing useful devices accord- organic particles or converting NP suspensions in dry ingly. How to assemble these nanostructured species in powder form mainly for biomedical and pharmaceutical an ordered structure in two and three dimensions is cur- applications, especially with reference to drug delivery rently an area of considerable interest current. It has been [622–624]. A typical spray-drying consists of first atom- observed that, by controlling the interparticle separations izing a liquid into a spray of fine droplets, which is sub- in such assemblies, the delocalization of electron states sequently brought in contact with a hot gas to remove are possible to maneuver the conductivity of the material the moisture and help in forming the solid product that so synthesized from being an insulator to conducting via is recovered via a cyclone unit. The basic form of spray- the semiconducting state. It is therefore very important to drying technology has gone through several stages of explore the phenomena of thin-film formation using NP improvements continuously in the past and the synthesis dispersion in different solvents as can be seen from the of OS-NPs was recently realized in a one-step procedure brief descriptions of the processes involved and their pos- by spray drying of a polymer solution. For example, prep- sible applications. arations of NPs out of gum Arabic, whey protein, PVA, As OS-NPs are usually synthesized in solution with modified starch, and maltodextrin [32] were successfully a low concentration, thin film-forming processes such as carried out using a “nanospray dryer”. A similar tech- spin coating or dip casting are not suitable for preparing nique was extended to produce bovine serum albumin OS-NP thin films. Alternate methods were therefore devel- NPs [625], where the “nanospray dryer” was modified to a oped as discussed here briefly. vibrating mesh spray that created still finer droplets. The In one of the methods of electrophoretic deposition of generation of these tiny droplets uses a 60 kHz piezoelec- NP thin films, it involves field-assisted separation of small tric actuator-fed perforated membrane with micron-sized charged particles dispersed in dielectric liquids such as in holes, varying from 4 to 7 μm in diameter. The membrane producing phosphors for cathode ray tubes, oxide super- vibration causes ejection of millions of nanodroplets per conductors [626], and carbon NTs for cold cathodes [627]. second with a very narrow size distribution. The final NP Colloidal solution of OS-NPs carry charged entities accord- size and its standard deviation depends on parameters ing to Coehn’s empirical rule, which states that the elec- such as the nature and concentration of the polymer, trostatic charge separations occur when two dielectrics the spray mesh size, the operating conditions including are put in intimate contact. The substance with the higher drying temperature, feed rate, drying gas flow rate, or the dielectric constant acquires the positive charges, while concentration of surfactant, in case used in the formula- the other one receives the negative ones. A DC bias of a few tion. Finally, another advantage of this novel technology hundreds of volts applied between two ITO-coated glass is its very high yield of NP production in the 70% to 95% electrodes soaked in the NP suspension forces the parti- range. cles to move toward the corresponding electrodes under the influence of the electric field [628]. The SCF-based technique of preparing OS-NP film is

6.2 OS-NP thin films based on a rapid expansion of SCF-CO2 solution contain- ing dissolved OS [629], where the dissolved compound Despite preparing, synthesizing, and characterizing a dispersed in SCF is sprayed on the substrate through a large variety of NPs, NCs, quantum dots (QDs), quantum capillary. After rapid evaporation of CO2, OS-NPs precipi- wires (QWs), and many more nanostructured species in tate on the substrate surface, and by adjusting the process colloidal form or in isolation, so far, their further applica- parameters, it is easy to produce OS-NPs with tunable tions in the form of a useable piece of material is not that sizes and optical properties opening up newer avenues straightforward. For exploiting the unique properties of to create functional films and devices using OS-NPs as these nanostructured species, it is necessary to put them building blocks with additional possibility of mixing dif- together in the form of a useable thin film or a piece of bulk ferent building blocks of OS-NPs or combining different material on a substrate or in free-standing form, where molecules within each building block. However, the only 310 S. Ahmad: Organic semiconductor devices limitation of this technique is that it is primarily applicable The advantage of this method is to form OS-NP films in in the case of smaller molecule-based OS-NP films. Adding situ on the substrate. Nevertheless, this method is, once Fluorolink 7004 surfactant to the SCF solution helps in again, only appropriate for small molecules with certain adjusting the size of the OS-NPs [629] experimentally. structures and is not applicable in the case of most of the In the solvent evaporation-based technique of NP polymer semiconducting materials. thin-film preparation, the phenomenon of evaporation Besides preparing NP thin films based on manipula- during the drying of a solution can be advantageously put tion of NPs in liquid and solid phases, another area has to use in controlling the film morphology and the distribu- emerged in this context using jet printing technology, tion of the solute in the final films so produced. It is known which is especially well suited for cost-effective prepa- that, when a liquid drop, containing dispersed solid con- ration of OS devices. In this context, it is useful to recol- tents, evaporates on a flat surface, it leaves a ring-like lect that when a drop of OS-NP solution is placed on the deposit along the perimeter as the contact line is pinned surface of a substrate, the OS-NPs form a coffee-stain-like during the drying process, leading to a fixed contact area structure after evaporation of the solvent as described on the substrate. In this process, a capillary flow of the above; therefore, simple inkjet printing of OS-NP solutions solvent takes place from the center to the contact line of cannot provide good film morphology. To take care of this the drop to replenish the evaporation loss, and this flow problem, an aqueous dispersion of OS-NPs is deposited by thus transports the solute to its periphery [630]. In the inkjet printing onto a polymer surface patterned by soft case of OS-NP solution, such a phenomenon results in an embossing [632], wherein, due to interaction between the uneven distribution across the deposited films. However, OS-NPs and the undulated surface, self-assembly is trig- in case another counterbalancing flow is introduced in gered, resulting in the formation of OS-NP-based nano- the direction opposing the above-said capillary flow, structures determined by the embossed template. This it will be very possible to balance the transportation of method is very appropriate for incorporating the OS-NPs NPs toward the contact line by the said capillary flow. into a device structure. In this context, a process based on the Marangoni effect Yet, in another alternative, even spin coating of thin can be put to use where the mass transfer along an inter- films of NPs was attempted to simplify the processing face between two fluids due to surface tension gradient, further. Although without using an appropriate additive, usually observed in a solution containing two solvents it is rather difficult to spin coat very low concentration of with different surface tensions and boiling points, gives OS-NP solutions in a thin-film form. Consequently, addi- rise to the liquid flow induced by the surface tension gra- tives such as surfactants or polymer matrices are added dient in the solution during solvent evaporation. Such a to assist the deposition of NP films. Using the charge-car- flow and its direction can be adjusted in the form of either rying properties of NPs on their surfaces when they are outward or inward spreading of a drop on a solid surface dispersed in solutions, negatively charged NPs are col- depending on the boiling points and surface tensions of lected on polycationic films with the help of electrostatic the two solvents mixed together. Consequently, by proper interactions via spin coating and vice versa. For instance, mixing of a second solvent into the NP solution, a Maran- layers of LPPP NPs were spin coated on polycyclic aro- goni flow with an opposite direction to the capillary flow matic hydrocarbons (PAH) exhibiting a homogeneous is realized accordingly. The solvent evaporation-induced fluorescence over large areas [602]. Similarly, conjugated self-assembly method for preparing the NP films from polymer NPs such as polyfluorene derivatives and LPPP their OS-NP solutions uses ethylene glycol as the second spin-coated on PEDOT:PSS films also exhibited good solvent having a higher boiling point but a low surface film morphology [633]. Besides auxiliary underlayers, a tension, and in this manner, the capillary flow in the solu- polymer matrix was also used as a binder to improve the tion is counterbalanced by the Marangoni flow. The final film quality deposited from OS-NP solutions, as it assisted self-assembly of NPs on the substrate is achieved through in reducing the electric field singularities around the NPs the NP-substrate and NP-NP van der Waals interactions. that might result in pinholes in electronic devices. PVA Alternatively, the vapor-driven self-assembly process [634], hexadecyl-modified poly(ethylene oxide) [632], employs selective phase demixing and self-assembled and PEDOT:PSS [635–637] polymer matrices were used, in aggregate formation from a molecularly dispersed solid this context, successfully. Although, in this way, the film solution of a specific molecule in a polymer matrix when quality is improved, the additives staying in the NP films it is exposed to volatile organic solvent vapors [631]. The are not at all good for the optical and optoelectronic prop- supramolecular self-assembly of OS material finally leads erties of OS-NPs. For example, while using PEDOT:PSS to spherical NP formation after solvent evaporation. additive to the OS-NP aqueous solutions for preparing thin S. Ahmad: Organic semiconductor devices 311 films by spin coating, the acidity of PEDOT:PSS degraded device fabrications of OFETs, the following sequences the luminescent properties of the conjugated compounds were followed as described here. OFETs were fabricated significantly. with bottom-contact geometry involving a highly doped

n-type Si wafer with 300 nm thermally grown SiO2 for electrical measurements. The wafer was piranha cleaned 6.3 SAMs in OS devices [306] before deposition of Cr and Au films for source and drain contacts using shadow mask resulting in 115 × 10 μm

Carbon fullerene (C60) [638] is a material that is being tar- and 2 mm × 85 μm channels for C60 and HBC transistors, geted for its device applications such as in PVSCs [639– respectively. After the deposition of monolayers accord-

641] and FETs [642–644] especially because of its higher ing to the procedure mentioned above, C60 monolayer- electron mobilities [645, 646]. C60 SAMs [557] offer unique based FETs were fabricated by thermal evaporation of and attractive features for their use in advanced electronic 50 nm C60 on to the substrate. HBC transistors were fabri- device applications [647], for example, monolayers of C60 cated by spin coating from a solution of tetradodecyloxy derivatives on Au, ITO, or other substrates [648–655] have HBC in CHCl3 or (CH2Cl)2. The devices were tested at room been investigated extensively in the recent past. Besides temperature, in Ar atmosphere in the case of C60 transis-

C60 monolayer on Au, there are only a few reports about tors, or in normal ambient atmosphere in the case of HBC them on SiO2 surfaces [346, 656–663] that are especially transistors. important for OFETs. While conducting these experiments, Field-effect mobility of 0.02 cm2/Vs was measured it would have been very possible that reactions with a in CPC monolayer-based devices, which was very typical nitrogen functional groups on the surface [346, 657, 658] for C60 FETs [644], whereas, in CPCFe-based devices, the 2 resulted in incomplete reactions with C60 itself or prefunc- mobility was 0.04 cm /Vs and this difference was attrib- tionalized C60 with the surface using a coupling reagent uted to the presence of ferrocene moiety. Compared to

[656, 660–663], leading to significant surface contamina- the mobility of a normal C60 transistor on bare Si as 0.01 2 tion, meaning it would be rather difficult to have a SiO2 cm /Vs, both C60 transistors with CPC and CPCFe showed surface with a uniform electronic structure under these cir- higher current possibly because of shielding of the surface cumstances. Such problems of unwanted reactions on SiO2 hydroxyl group and higher crystalline packing of C60. An were taken care of at a later stage by growing monolayers aligned dipole layer, generated by the salt of amine and of C60 derivatives on SiO2 surfaces by using C60 SAMs func- carboxylic acid at the interface of CPC or CPCFe and tionalized with carboxylic acids. Experimentally, C60 SAMs APTES monolayer, could be the cause of the experimen- were deposited by soaking SiO2 substrate into CH2Cl2 at tally observed shift in VT [588, 665]. While investigating room temperature for 15 min followed by 20 min boiling the effect of CPC and CPCFe monolayers in HBC OFETs, in a 1:1:5 = NH4OH/30% H2O2/deionized water (DI H2O) at good performance of spin-coated HBC devices were

70°C. Immediately after rinsing in DI H2O and drying in reported from which it is concluded that the intermo-

N2 gas, the samples were immersed in a 2% (v/v) solution lecular interactions from these compounds are not only of 3-aminopropyltriethoxysilane (APTES) in ethanol for useful because C60 and HBC are electron acceptor and 25 min at room temperature. Post-baking at 120°C for 5 min donors, respectively, but also their shapes are comple- gave monolayer of APTES and then these covered samples mentary [666]. These two devices showed similar charac- were immersed in 0.1 mM solution of CPC or CPCFe [655, teristics, but when the measurements were carried out in 664] in THF for 24 h and 30 min, respectively, at room the ambient light, the CPC transistor had higher current temperature. After the monolayer assembly, the samples than the CPCFe one. OFETs employing an acetic acid layer were soaked in clean THF for 30 min at room temperature [667] instead of CPC or CPCFe showed similar character- for washing out the unbound molecules. These CPC and istics. The behavior of the monolayer/HBC devices was CPCFe monolayers were very stable, as the UV-visible spec- explained by the photo-induced charge transfer between trum did not show any significant changes over several C60 moiety of the monolayer and the HBC [346, 668]. These weeks under air or more than 15 h under argon at 300°C. experiments demonstrated that reliable CPC and CPCFe

Compared to monolayer of C60, pentabiphenyl derivative monolayers on SiO2 surface could be successfully used for

C60 (C6H4C6H4-COOH)5Me on Au [655] is very reasonable, as the surface modification of the insulating layer of OFETs these derivatives stood upright on bare Au surface without [669]. When C60 is used as a semiconducting layer in an any adhesion layer under in situ STM conditions. OFET, the mobility is 0.02 cm2/Vs and this value doubles in Fullerene-based OS devices have been explored using the presence of CPCFe monolayer, indicating the efficient SAM-based technique. In one of the typical experimental channel formation by the electron-donating ferrocene 312 S. Ahmad: Organic semiconductor devices moiety. The generation of photocurrent in the presence of noted that the implantation damage necessary to encap- contorted HBC is possibly due to donor-acceptor and the sulate the OFETs did not penetrate deeper but remained geometrical ball-socket interactions responsible for the within a superficial layer between 50 and 300 nm of photocurrent generation [666]. thickness. N+ and Ne+ implantations [670] resulted in harder superficial pentacene layers with higher conduc- tivity due to the damage created by ion implantation in both cases. Subsequently, working pentacene OFETs were 7 Ion implanted OS device implanted where a controlled damage induced by the ions stabilization in the superficial part of the pentacene layer created an encapsulation layer; under that layer, a fresh layer of pen- Improvement in the performance of OFETs [670] was real- tacene was available for device action [672]. The superfi- ized with ion-implanted passivation of the superficial cial implanted layer of 50 nm was sufficient to keep the layer by eliminating the instability due to the atmospheric electrical parameters of the OFET, including mobility and interactions while leaving a fresh layer at the dielectric VT stable, while protecting them from the atmospheric interface besides enhancing the adhesion of the film to interactions for at least 2000 h [672]. From among several the substrate. In the case of inkjet printed organic OFETs combinations of ion species studied, the coimplantation or the organic electrochemical transistors, it is necessary N2+ and Ne+ was found to be the most suited without dam- to have good adhesion at the substrate, as the OS starts aging the good features of OS as transparency, flexibility, as a liquid drop that solidifies on touching the substrate and biocompatibility. and is then in contact with a liquid electrolyte [670]. Ion Plasma source-based ion implantation [670] was implantation did render the interface strong with stable noted as having the potential for commercial production adhesion by modifying the interfacial atomic structures as of large-area surfaces due to immersion of the sample reported [671]. inside the plasma of accelerated ions. This technique Ion implantation-induced variations in the transport ensured the lower costs of the OS, employed in large-area properties of OFETs, such as charge carrier mobility and configurations such as in large-area displays and photo-

VT, were measured in several samples implanted with dif- voltaic solar cells. ferent types of ions, doses, and beam energies including single, double, and coimplant. For instance, in the case of F+ and S+ implanted devices, it was noted that F+ implants not only improved the mobility but also stabilized the 8 Printed OS devices + device performance, whereas S implants stabilized VT but with a huge initial shift at low voltage compared to VT0 Producing a variety of electronic components and devices before implantation [670]. using different kinds of raw materials employing a set of The changes taking place in the transport properties printing technologies [673] falls under the printed elec- with time in ion-implanted devices were recorded [670] to tronics (PE) category. The printing process consists of pat- know the influence of the irradiation process on aging and terning of thin/thick film structures on flexible or rigid device degradation. These measurements [670] showed substrates, depositing single/multiple ink layers. The clear stabilization of carrier mobility and/or VT for at least printing technologies, including flexography, soft lithog- up to 2000 h after the implantation. Low-dose implant of raphy, screen, gravure, and inkjet printing, have all been 35 keV Ne+ ions stabilized the carrier mobility, whereas N+ growing very fast, especially keeping in view their appli- ions induced VT stabilization. For instance, VT stabiliza- cations in electronic manufacturing, which is strongly tion was optimized for 25 keV ions up to 1 × 1015 cm-2 dose motivated by various factors such as reduced cost derived and it was further noted that higher doses and energies from high-throughput/volume production of lightweight/ also induced stabilization of VT, although it occurred at small, flexible, and disposable, inexpensive electronic values that were largely shifted with respect to the refer- components and devices. ence of the nonimplanted device [670]. In today’s printing technology, both inorganic and The optimal way [670] to have stable electrical param- organics inks are used. The inorganic inks are prepared eters of ion-implanted pentacene OFETs was to coimplant generally from metallic dispersions of Cu, Au, Ag, and Al N2+ ions with a combination of beam energy/dose of 25 NPs in a matrix primarily used for passive components keV/5 × 1014 ions/cm2 and then Ne+ ions with beam energy/ and interconnects, while liquid-phase organic conduc- dose combination of 25 keV/2 × 1014 ions/cm2. It was further tors, semiconductors, and dielectrics are especially used S. Ahmad: Organic semiconductor devices 313 in fabricating active devices of OE. For instance, conduct- During printing, the plate rolls over the substrate and the ing polymer inks find their applications in fabricating bat- ink is transferred to the substrate in the desired pattern by teries, electromagnetic shields, capacitors, resistors, and applying pressure on the impression cylinder [674, 675]. inductors and OS inks are used in printing active layers This process is suitable for printing on plastics and foils of organic photodiodes, LEDs, FETs, PVSCs, sensors, and [674, 676], employing a large variety of fast-drying inks electrochromic displays [673]. and soft-printed plates, despite low precision and resolu- It is significant to note that PE is not considered as tion resulting from the ink spread outside the image area, a substitute for conventional Si electronics that has been caused by the compressive pressure between the printing offering ultra-large-scale integration of ultrafast switch- plate and the impression cylinder. ing devices and systems; rather, it is especially poised for In gravure printing [673], laser or photolithographi- developing low-cost/high-volume production markets, cally etched target pattern on the rotogravure holds the which do not require the high-performance conventional ink inside the grooves from the ink supply before trans- electronic components and devices. Particularly, PE is ferring to the substrate through the pressure between the supposed to offer viable options of low cost of produc- printing plate and the impression cylinder enabling very tion based on mostly low-temperature processes, which high printing speed and quality using wide ranges of are carried out on flexible substrates such as plastics materials and inks. foils or paper with the capability of handling larger area The contact electrodes of flexible 5 μm channel substrates. The minimization of the material waste by OFETs on PEN substrates were reported to be fabricated employing an additive process, where the inks are site [677] using a h-PDMS stamp and Ag ink resulting in a selectively deposited consuming only a few picoliters of field-effect mobility of 0.06 cm2/Vs. Using PDMS stamp in material in each drop, is the most significant feature of PE gravure printing, roll-printed OFETs were fabricated [678] against the subtractive processes, which involve blanket using Ag paste, flexible 150 × 150 mm2 substrates, PVP depositions of the material and photoresist over the whole dielectric and TIPS-PEN exhibiting mobility approx. 0.08 substrate for patterning features using conventional pho- to 0.1 cm2/Vs. This PDMS-based process produced 12 to 74 tolithography and etching followed by removing the mate- μm channel OFETs otherwise not possible in traditional rial from the unwanted areas and the residual photoresist printing techniques. layer – all adding to cost [663]. PE is generally divided into contact and noncontact types, in which contact printing includes processes such 8.2 Noncontact printing as flexography, offset lithography, gravure, and screen printing, where the printing plate comes in direct contact In a noncontact printing method, process-induced damage with the substrate, whereas in the noncontact printing, as and contamination are practically nonexistent. The addi- in inkjet printing, the substrate receives the drops of inks tional capability of having accurate alignment with the form of printheads located at a distance. existing patterns on the substrate offers a unique opportu- nity of fabricating multilayered component structures and devices [673]. Furthermore, no direct contact between the 8.1 Contact printing printhead and the substrate at low temperatures makes this technique print patterns on surfaces such as glass, Various types of contact printings have been dominating Si, metals, rubber, plastics, and many other contact-sen- the PE industries [673] because of their associated high sitive materials. The process of noncontact printing, being throughput and low production cost capability employ- digital in nature, does not require a physical master of the ing roll-to-roll printing configurations that are suitable for target image, as the information regarding the image is in mass customization. The direct contact with the surface digital form where a variety of modifications as well as a in such printing has, indeed, limitations in terms of the number of prints can be prepared without an additional pattern resolution and the range of materials used includ- cost [675]. ing the substrates, inks, and solvents. Direct laser writing [673]-based printing combines a In flexography, a cylinder transfers the positive pattern number of noncontact printing techniques, which permits onto the substrate surface after applying an adequate the realization of 1D to three-dimensional (3D) structures quantity of ink on the printing plate from an engraved by laser-induced deposition of metals, semiconductors, cylinder that releases the ink according to the engravings polymers, and ceramics, without using lithography. and a doctor blade removes the excess ink before printing. Materials are deposited straight from the corresponding 314 S. Ahmad: Organic semiconductor devices precursors. For instance, laser chemical vapor deposi- and air resistance forming a shape and pattern. The solid- tion uses focused argon ion laser with a micron-sized ification of the liquid occurs through evaporation of the spot onto a substrate placed in a reaction chamber filled solvent and the chemical changes such as the cross-link- with a gaseous precursor of the target element where the ing of polymers or crystallization. The printed patterns are heat generated by laser absorption dissociates the pre- annealed or sintered in case they are needed [679, 681]. cursor leading to the deposition of a thin solid layer onto Inkjet printers work in two modes, namely, continuous the substrate surface. Herein, the repetitive scans enable or drop-on-demand (DOD). It is noted that the material the deposition of multilayered structures. This technique throughput in a single nozzle is different from that in a is expensive and limited by the specific requirement of continuous inkjet system with a higher printing speed. On volatile metal-organic and inorganic precursor materials the contrary, DOD technology allows higher placement not suitable for organic substrates. Similarly, in laser- accuracy and smaller drop size leading to better resolu- enhanced electroless plating, the substrate is submerged tions of the printed patterns. in a chemical solution of the metallic ions required for the In a continuous-mode inkjet [673], the ink is pumped deposition, while a laser beam irradiates the substrate through a nozzle forming a continuous jet where a piezo- surface causing instantaneous temperature rise, which electric transducer causes the breakdown of the ink flow decomposes the liquid leading to the deposition of a in single drops at regular intervals, but a stream of liquid metallic layer on the substrate. A subsequent electroplat- emerging from an orifice is intrinsically unstable with a ing can be used to further increase the thickness of the tendency to break into drops also under surface tension deposited layer. This technique does not allow the crea- alone [679]. Once formed, the drops cross an electro- tion of 3D structures in the real sense. In another variant, static field acquiring charges as they leave the stream. termed as laser-induced forward transfer, a laser beam These charged drops are then directed to the desired causes controlled evaporation of the target material put location on the substrate by an electrostatic deflection on an optically transparent support plate placed about system and neutral drops are collected in an ink recircu- 100 μm below the substrate, wherein the laser beam lation system [682, 683]. The pattern is thus created on a evaporated material on the support recondenses on the moving substrate. In contrast, in a DOD system, an ink substrate. Although a variety of materials such as metals, droplet is ejected from a reservoir through a nozzle only oxides, and polymers can be used for forming both 2D and when a voltage pulse is applied to the transducer. There 3D structures, the limitation of printing is only onto a flat are thermal and piezoelectric transducers employed in substrate kept parallel to the material support. forming droplets. In a thermal DOD inkjet printer, the ink The process of aerosol/spray printing [673] employs droplet is ejected by means of a heating resistor located the following steps: first the ink is transformed into inside the nozzle [684]. In piezoelectric DOD systems, the liquid particles with a diameter ranging from 20 nm to mechanical force generated in a piezoelectric material 5 μm based on the viscosity using an atomizer. The inks due to electric field ejects the droplets. Before the print- contain NP suspensions of metals, alloys, ceramics, poly- ing starts, the ink chamber is influenced by a bias to the mers, adhesives, or even biomaterials. This dense aerosol piezoelectric transducer to prevent the ink from falling from the atomizer is transported into the deposition head from the nozzle. A zero voltage is next applied to the piezo by a nitrogen flow. The aerosol is further focused by flow transducer to bring back the chamber in its relaxed posi- guidance in the deposition head before depositing on the tion, leading to a flow of fluid in the ink chamber from substrate. This is a versatile technique capable of writing the reservoir. Subsequently, the chamber is strongly com- features with dimensions varying over three orders of pressed causing the drop to be ejected from the nozzle. magnitude. This technique has a wide range of applica- Finally, the chamber is brought back to the initial decom- tions in displays, TFTs, dielectric passivation layers, and pressed condition to pull back the ink in the chamber and solar cells [679, 680]. to prepare the system for the next ejection (DMP-2800, 2008). As thermal DOD involves the vaporization of the ink inside the chamber, it requires the volatile solvent- 8.3 Inkjet printing based inks. This restriction is not there in piezoelectric DOD that is therefore more versatile than the thermal one. The process of inkjet printing [673] involves the transfer The inkjet technology has recently proliferated into of a fixed quantity of liquid-phase ink in the form of drop- the area of mass production [673] primarily because of lets from a chamber through a nozzle. The ejected drops increasing printing speeds by increasing both jetting fre- fall onto the substrate surface under the force of gravity quency higher than 50 kHz and the number of nozzles on S. Ahmad: Organic semiconductor devices 315 a printing head. High nozzle density of 200 nozzles per pentacene derivatives, TIPS-PEN is the most sought after inch has been achieved due to the development of micro- for OFETs [687], possessing good solubility in solvents electro-mechanical systems (MEMS) [38, 685]. Further- such as toluene, chlorobenzene, THF, and chloroform. more, it embodies all the advantages, already mentioned Additionally, the bulky functional groups in TIPS-PEN for noncontact printing techniques, with the additional maximize π-orbital overlap resulting in enhanced carrier plus point of lower cost of the equipment compared to mobility. those involving other direct writing processes. For these TIPS-PEN can be spin coated, drop cast, and inkjet reasons, it is certainly considered as the most promising printed for the active layer of OFETs, in which the electri- technique in PE technology. Nevertheless, like every kind cal characteristics are especially linked to the processing of microfabrication technique, the inkjet printing does conditions including the solvent used, the postprocessing have some limitations and critical issues, which must be treatment, and the deposition method, as they determine taken care of to achieve the best performance. The poor the morphology of the deposited film. The highest-perfor- resolution of conventional inkjet printers, which is lower mance devices have been fabricated with single-crystal than the resolution required for high-performance elec- TIPS-PEN semiconductor [670] because they are free of tronic devices such as OFET, is the first limitation. The grain boundaries and molecular disorders, which oth- minimum achievable resolution is primarily a function erwise degrade the charge carrier transport through the of the droplet size that is limited by the nozzle diameter material. Generally, using high boiling point solvent and and ink surface tension. In most of the commercial inkjet drop casting allowed slower solvent evaporation leading printers for electronic applications, it is difficult to reduce to highly ordered films. Mobilities up to 1.8 cm2/Vs in the droplet volume below 1 pL [38]. Another feature that drop-cast TIPS-PEN OTFTs were reported in literature strongly affects the print resolution is the interaction [673]. Although the process optimization of inkjet printed between the ink and the substrate, especially the spread- TIPS-PEN OFETs is still in progress [670], trial runs dem- ing phenomena leading to the formation of coffee-ring- onstrated good device mobility by varying the parameters like structures as a result of the surface-tension-driven such as changing the TIPS-PEN concentration, the drop solution transport along a surface with an evaporating spacing, the channel length, the frequency of injection, solvent [341, 681]. The choice of the ink is therefore criti- the material used for the electrodes, the chemical treat- cal, as its physical properties such as density, viscosity, ments of the substrate, and the real number of drops that surface tension, volatility, and shelf life strongly affect the cover the channel [673]. quality of printed films. Additionally, there is a practical There are two methods of inkjet printing in practice, problem of nozzle clogging, which needs frequent clean- namely, single-droplet printing and multiple-drop print- ing causing material and time waste besides the damage ing [459, 688, 689]. In single-drop printing, each droplet of the nozzles occasionally. In addition, different inks acts as an individual functional deposit, whereas, in the require different postprocessing treatments as sintering, multiple droplets, a continuous film covering the whole annealing, or simply drying in air, which could change area between the source and the drain electrodes forms the morphology and the uniformity of the printed pattern the active channel. With the first method, high ordering and extend the manufacturing time [686]. The optimiza- is easily achievable controlling the hydrodynamic flow tion of the inks and of the substrate treatment processes in the drying droplet, which forms the channel using a thus constitutes the main research challenge in order to mixed solvent system or varying the surface energy of the achieve improvements in resolution and repeatability of substrate, leading to hole mobilities up to 0.2 cm2/Vs for inkjet printed patterns and devices. pure TIPS-PEN inks and up to 1 cm2/Vs for inks containing Pentacene, being poorly soluble in many solvents, TIPS-PEN blended with other polymers. On the contrary, poses severe limitations in terms of its inkjet printed printing multiple droplets is more effective in forming depositions. On the contrary, the pentacene transport a uniform thin film over a large area, especially in case properties are strongly dependent on its crystal packing. the spot produced by one single droplet does not cover Thus, in order to overcome the insolubility problem for the channel region. In the case of printing using several preparing more ordered thin films, several pentacene separate single drops, it is very difficult to avoid overlaps derivatives were developed by substitution with dif- between droplets affecting the uniformity of the printed ferent functional groups with the additional possibil- active layer [670]. Using multiple-drop printing, device ity of having access to electronic properties tuning, for mobilities up to 0.24 cm2/Vs were estimated in the case instance, in terms of charge carrier injection, HOMO- of TIPS-PEN [689] and up to 0.11 cm2/Vs in devices where LUMO gaps, and charge carrier transfer rates. Among the TIPS blended with polystyrene were used [688]. 316 S. Ahmad: Organic semiconductor devices

For preparing inkjet printing experiment [670], it C10-DNTT-based OFETs involving polyimide dielectric on starts with 5 wt% TIPS-PEN solution in toluene, which is polycarbonate substrate were developed with very high stirred for 1 h at 90°C and subjected to ultrasonic treat- mobility of 2.4 cm2/Vs, which turned out to be five times of ment for 15 min before filling a DMP-11610 cartridge and the parent DNTT-based devices placed on the same sub- printing on parylene C substrate kept at 60°C for fast strate [696]. solvent evaporation to obtain a better crystallization Compared to pentacene, a new pyrene-based organic [690]. The single-drop patterns were printed with 150 liquid crystal (LC) semiconductor was prepared by varying μm drop spacing using single nozzle at 1 kHz frequency, the substrate temperature and SAM-modified gate dielec- as increasing frequency led to chaotic and uncontrolled tric deposition that resulted in the mobility of 2.1 cm2/Vs in droplet depositions. The characterization of inkjet printed the case of BOBTP [697] with enhanced long-term device TIPS-PEN devices [670] showed that the multiple droplet stability. approach did not result in high-mobility devices, whereas An OS compound was prepared [698] having mobil- the single drop in a single row showed the highest mobil- ity of 4 cm2/Vs at < 10 V of operation with added advan- ity. Based on the experimental observations, the mobility tage of being appropriate for solution coating process. The trend seems strictly related to the inkjet process; in partic- devices prepared from this compound especially appear ular, a more ordered deposition of the droplets gives better to be of much use in flexible display backplane or low- performance [346, 691]. cost, low-frequency printed logic applications. Practical realization of flexible high-gain inverters and functional ring oscillators was demonstrated [699] recently to represent a significant progress leading to 8.4 Soft lithography dispel the prevalent uncertainty in connection with the practical realization of high-performance microelectronic The soft lithography [673] is based on a different princi- devices from OS, opening up the newer possibilities hith- ple utilizing an elastomeric stamp, molded from a master erto unknown in Si domain. Microcircuits produced by template, to transfer the desired pattern to the substrate. printing instead of photolithography offered the long Mixing a polymeric material out of polyurethane, polyim- sought after opportunity of having low-cost and large-area ide, or PDMS with a catalyst/curing agent and pouring the flexible electronics using solution-processed high-perfor- mixture over the master for preparing the stamp. Heating mance OTFTs and other devices and components. of mixture on master helps in cross-linking the compound Vapor-phase deposited ZnO nanowires and tetra- in solid form in few hours. Thereafter, the stamp is pealed pods [700] derived from zinc compound reduction in a off from the master in the form of a flexible and transpar- novel approach were used for preparing an n-type OS – ent elastic negative of the master pattern [692, 693], which [6,6]-phenyl-acid methyl ester-based nanocomposite is used for transferring the material of interest on the sub- showing electron mobility in the range of 0.3 to 0.6 cm2/Vs strate. The success of preparing high-definition micro- representing a factor of 40 enhancement from the pristine structures and nanostructures in this process depends on state, with a promising enhancement in the case of n-type the intrinsic property of the elastomer to form a conformal OFETs. contact with the master surface [694]. Soft lithography Using soluble polyacene in a proprietary binder, The printing has provided a cost-effective solution for pat- Center for Process Innovation UK developed a compound terning over large areas with a resolution of about 20 nm. named FlexOS™ for OFET applications. The devices However, this process cannot be converted into a large prepared using FlexOS™ in top-gate-bottom-contact throughput of the roll-to-roll type of printing as in flexog- configuration with Au source/drain contacts defined raphy and gravure printing described earlier. by photolithography and chemical etching along with CytopCTL-809M fluoropolymer dielectric showed high mobility with < 5% standard deviation across a 100-mm- 9 Recent OS device developments diameter substrate. In contrast to earlier use of low permittivity binders in preparing small-molecule semi- Recently developed materials such as BDXs, BXBXs and conductor thin films [701], in the present work, a series DNXXs [695] produced air-stable and high-performance of binders having permittivity in the range of 2.8 to 5.8 OFETs for different kinds of applications. The DNTT and were used [702], which enabled easy control of the phase alkylated BTBT-based devices showed very high mobility separation of small-molecule polymer and binder in the of 3 and 2.8 cm2/Vs, respectively [695]. In continuation, deposited films. S. Ahmad: Organic semiconductor devices 317

10 Encapsulation issues of OE brief, it is still a challenging task to find a satisfactory encapsulation solution, which can be deployed using Various reliability issues of OS devices were systemati- commercially available resources. Most of the commercial cally studied in a doctoral program [703] from where some formulations typically require specialized equipment and of the salient observations and experimental results are intellectual property licenses. taken in the following discussion. Besides sensitivity to moisture and oxygen, delami- OS are generally sensitive to moisture and oxygen and nation is another phenomenon [703], which may crop up therefore need encapsulation for proper protection [704– due to environmentally induced oxidation in which mois- 706] from further deterioration during their use in devices. ture permeates through defects into the interface between Consequently, device reliability issues arise due to the cathode and the active layer, where the ensuing chemical environmental instability of both the active materials and reactions give rise to outgassing or volumetric expansion low work function electrodes. Low work function cathode culminating into delamination. In addition, exposure metals oxidize very fast after exposure to oxygen and of any active layer during device fabrication is equally moisture resulting in insulating oxide barrier layers that harmful [703]. For example, C60 in OFETs undergo rapid impair charge carrier injections. For protecting devices degradation after exposure to the ambient air when com- fabricated on transparent flexible substrates, which are pared to a transistor with alumina [703]. not impermeable to moisture and oxygen, it is necessary From among various flexible encapsulation that the barrier also meets the device requirements such approaches, thin films have attracted the most attention as in the case of an OLED, the barrier should be flexible due to their light weight, transparency, and high level of and transparent to give the full light output. For devices mechanical flexibility. Besides offering good barrier fea- fabricated on glass substrates, combinations of epoxy/ tures to the encapsulant under consideration, other criti- getter are found effective but are not useable for trans- cal aspects such as compatibility with the OE, changes parent flexible organic devices. The purpose of a barrier introduced into the active layers and substrates having layer in encapsulation is to improve storage lifetime and low glass transition temperatures and thermal stability, enhance device-operating lifetime. The half-life is esti- and the limitations due to barrier layer deposition and mated by measuring the time after which, for instance, processing temperatures are equally important to take initial brightness of an OLED is reduced to 50%. This is care of [703]. Consequently, all inorganic layers involved a common method of determining the OLED life that has in encapsulation must be deposited at low temperature been used to evaluate the barrier properties [707, 708]. for being compatible with the device. However, the pro- Several methods including epoxy and glass seal [707] cessing at low temperature is invariably associated with were proposed for encapsulating OLEDs and other organic more defects, limiting the barrier performance. Thus, the semiconductor-based devices that use a glass substrate. entire process of developing and integrating high-bar- In this method, the adhesive is applied using a syringe, rier encapsulation films with OE remains a compromise which may cause device damage in case the epoxy between two extremes [703]. touches the semiconductor layers. Alternatively, Al-Li and The barrier performances of inorganic materials are high-density polyethylene (HDPE) multilayers were used highly promising for thin-film encapsulation for improv- with a half-life of 63 h on glass substrate [709], while else- ing the lifetime and reliability of OE, as they simplify where a combination of oxide, polymer, and epoxy seals the manufacturing process compared to other multilayer [710] was employed for OLEDs on glass with a half-life of encapsulation methods. However, inorganic films have a over 1000 h. Accelerated testing at 65°C and 85% relative serious drawback due to large defect density in the films, humidity (RH) of PECVD SiOx/Si combinations on glass which enable moisture and oxygen to permeate through [708] gave a half-life of 7500 h. For devices prepared on the barrier layers. Methods such as physical vapor deposi- flexible substrates, chemical vapor deposition (CVD) par- tion (PVD), plasma-enhanced CVD (PECVD), atomic layer ylene-N/C, individually or in combination, showed four deposition (ALD), and plasma-enhanced ALD are used times enhanced lifetime over that of unencapsulated [711] in depositing thin films with improved barrier properties devices. A proprietary barrier formulation on PET gave [703]. Currently, the deposition of inorganic oxides such as over 2000 h of half-life [712], whereas a lamination-based Si-O-N and Al-O-N using PECVD and ALD are being consid- encapsulation of OLEDs on plastic substrate showed a ered most favorably in this context. Comparatively, PECVD half-life of 230 h [713]. is more viable for manufacturing due to higher deposition Despite attempting several formulations for improv- rates [714, 715] compared to ALD, whereas ALD provides ing the reliability of OS devices as mentioned above in defect-free compact thin films of very high quality. PECVD 318 S. Ahmad: Organic semiconductor devices

SiOx and SiNx films show useful barrier properties, except cannot be used in flexible device configurations and containing large defect densities, which may severely limit the existing thin-film encapsulation techniques lack the the utility of these films in meeting the target requirement high impermeability requirement of glass encapsulation, of OE. The encapsulation quality of the thin film involved encapsulation of flexible OLEDs still remains a challenge. is directly correlated with improved lifetimes by integrat- In catastrophic degradation [721], a sudden decrease ing it with the actual device. For instance, the lifetime of or total loss of the luminance is observed due to shorts encapsulated OLEDs with PECVD SiNx was 600 h com- appearing across the device [722] caused by preexisting pared to 6 h for a bare device [716]. Recently, PECVD SiOx:H defects in the active layers or the electrodes of the OLED. films exhibited better protection of OLEDs [708]. Using an It is possible to contain this kind of degradation by mini- appropriate combination of HDMSO and O2 precursors, it mizing the defects during material deposition resulting in was possible to deposit a single composite layer consist- very uniform and homogenous films. In intrinsic degra- ing of SiO2 and Si, which protected OLEDs up to 7500 h, dation [721], a progressive decrease in the luminance of when stored at 65°C in 85% RH. Alternatively, ALD-based the OLED occurs during device operation [722], leading to

Al2O3 has shown promising encapsulation properties as an intrinsic decrease in the EL quantum efficiency of the reported [710, 717, 718] due to high density, low number of OLED. defects, and conformal coating on the uneven surfaces at Dark spots are generally attributed [721] to the exter- relatively low temperatures. nal contaminants, pinholes, bubble formation, gas evolu- ALD is a layer-by-layer deposition of materials based tion, and crystallizations following causative phenomena on chemisorption of molecular precursors, which are in an OLED. introduced into the deposition chamber with an inert Substrate cleaning and removal of the loosely attached carrier gas to form monolayer coverage on the surface material chunk from the previous run sticking inside the of the sample. Excess precursor is purging with an inert chamber before starting thin-film deposition are very criti- gas followed by the introduction of the next precursor. cal, as the foreign material contaminants either present on These processes are repeated to build high-quality films the substrate or getting dislodged from the chamber inner with featureless microstructure and conformal coatings. surface, during deposition, are noted to create direct path-

In addition, laminated structure of Al2O3 and ZrO2 and ways in the active layer and cathode-organic interface for dual-layer structure of SiOx and Al2O3 using ALD have permeation of oxygen/moisture [704]. In the case of the shown excellent barrier performance [719, 720]. In another foreign particles that are larger in size than the thickness attempt [720], nanolaminate comprising alternate layers of the organic layers, it may lead to incomplete coverage of 2.6 nm Al2O3 and 3.6 nm ZrO2 film with total thickness of during organic film deposition offering easier conduits for

100 nm deposited at 80°C was compared with single Al2O3 moisture/oxygen propagation. layer with same thickness. Rigor of the substrate cleaning involved in OLED fab- In order to address various reliability issues related rication can be appreciated from the brief description of to OS device developments, a detailed study was carried a typical fabrication [723] sequence, where ITO-coated out recently [721] to study the degradation mechanisms substrate is spin cleaned first with acetone, isopropanol, involved in OLEDs. Some of the salient points arrived at and methanol followed by HMDS and AZ4620 photore- during this study have been included in the present dis- sist coating. The patterned resist is postbaked at 110°C cussion to highlight the importance of this kind of reli- before etching in aqua regia. The cleaning of ITO pattern ability investigations, which are necessary for improving in acetone, isopropanol, and methanol is finally flushed the performance of these as well as other semiconductor with O2 plasma for 2 min. Just after the plasma treatment, devices needed in OE. the ITO pattern is coated with PEDOT:PSS and baked at Degradation of the performance of an OLED over time 110°C inside a glove box with access to the evaporation can be attributed to three major categories, namely, dark and CVD chambers. The OS comprises 50 nm spiro-TPD spots formation, catastrophic degradation, and intrinsic and 50 nm Alq3 and cathode of 50 nm of 10:1 Mg:Ag alloy degradation [722], which are briefly described here. along with 50 nm Ag. Although glass substrate is an excel- Formation of nonemissive spots within the emissive lent moisture/oxygen barrier, the cathode area, however, area of a device causing device degradation is termed as is permeable, and to protect the organics against attack a dark spot in OLEDs, wherein the total device luminance from the cathode side, the sample is coated with 2 μm Par- is reduced [721]. This phenomenon is accelerated by the ylene C and then 100 nm Al with proper masking so that presence of moisture and oxygen, which can be controlled the device is not shorted. This Parylene-Al stack is repeated via proper device encapsulation. As rigid encapsulation twice. The measured half-life of 600 h at 196 cd/m2 [723] S. Ahmad: Organic semiconductor devices 319 was measured for devices prepared using the above-said intergrain gaps or pinholes. Hence, understanding and sequence. In the case of OLEDs on PEN substrate [723], it controlling the thermal effect on the ETL/Alq3 is crucial uses ITO-coated PEN, which is cleaned using the same sol- for keeping the cathode-organic interface integrity within vent-cleaning procedure as in the case of glass substrate, acceptable limits, which in turn controls device lifetime UV/ozone treatment for 5 min, and aqua regia etching of and performance. For instance, for good cathode-organic pattern. The OLED fabrication is also identical to that in interface quality, Alq3 films should not exhibit small glass substrate-based devices. grain morphology as seen in amorphous organic materi- On the contrary, pinholes are formed during film als; however, neither should they exhibit highly irregular depositions especially when deposition rates are faster, crystalline morphology. When Alq3 is deposited on top of causing not only uneven layers but also trap gases. Such the HTL at room temperature, it is amorphous exhibiting defects in the cathode and/or protective layers easily rough surface morphology, comprising small grains with a provide pathways for oxygen and/or moisture to permeate high grain boundary density providing for a high number through the sensitive metal layers and reach to the active of percolation paths for moisture and oxygen. Studies layers. Cathode oxidation, for instance, leads to reduced have shown [732] that annealing Alq3 deposited on NPB electron injection and the degradation of the active layers at 100°C renders the surface of Alq3 very smooth and fea- causes overall device performance impairment. The tureless [733], exhibiting fewer grain boundaries, which growth rate of the dark spots is related to the pinhole size mean fewer percolation paths for moisture and oxygen. [724]. The larger the pinhole size, the faster the growth Further studies have shown [733] that Alq3 deposited at rate, causing still shorter device lifespans [724]. 100°C on top of ITO/NPB leads to improved device lifetime Gas evolution produced by electrochemical processes than annealed ITO/NPB/Alq3 stack. Atomic force micros- going on at the cathode/organic interface in the presence copy (AFM) inspection revealed that substrate heating, of moisture/oxygen results in bubble formation at the while depositing Alq3 at 100°C, led to a slightly rougher interface, which ultimately causes cathode delamination surface when compared to the annealed Alq3/NPB/ITO from the organic layers underneath [111, 725–729]. This stack [733]. This minimal roughness might be beneficial causes disruptions in the electron injection at the cath- in regards to increasing cathode adhesion by increasing ode-organic interface; consequently, dark spots are seen surface-adhesion area. Moreover, larger-scale surface at such sites. roughness due to larger grains might result in lesser mois- Organic coordination compound tris(8-hydroxyqui- ture permeation. Hence, substrate heating while deposit- nolinato) aluminum, commonly abbreviated as Alq3, ing Alq3 or annealing Alq3 at a temperature below glass is employed in fabricating OLEDs and its exposure to temperature can result in large grain films more useful for humidity through pinholes induces the formation of Alq3 improving device lifetime. crystallites in the originally amorphous films [728], result- Traditionally, many OE devices employ glass/metal ing in protruding structures that are several times thicker lids as barrier layers to protect against moisture/oxygen. An than the original film with water content higher than that inert metal or glass layer is used as a “lid” that is sealed in the amorphous Alq3 regions [728]. The moisture dif- with an UV-cured epoxy resin. Getter materials are also fusion through the microscopic defects in the cathode used in such structures besides using an inert gas that fills causes crystallization of Alq3 in the OLED structure itself, the voids to make the device quite stable. Although this forming Alq3 crystals, which ultimately cause surface approach does provide protection against moisture/oxygen irregularities affecting the adhesion between the organic permeability, it has flexibility as well as robustness issues and metal layers. Delamination occurs due to the crys- that need to addressed. A device encapsulated with glass talline Alq3 clusters, being thicker than the surrounding is prone to accidental breakage compromising the barrier amorphous regions, lifting the cathode, leading to loss of features causing overall device deterioration. Further, glass contact between Alq3 and the cathode. The areas with no encapsulation is impossible to integrate into roll-to-roll contact appear as nonemissive dark spots [728]. OLED fabrication. Rollable display screens, foldable maps, Either heating of the HTL/ETL directly or Joule’s and flexible lighting options are just few of the attractive heating due to current flow is noted to induce crystal- market applications for OLEDs. However, flexible OLEDs lization in amorphous organic materials. Although the require flexible encapsulation structures. Furthermore, crystallites, produced in this manner, are comparatively flexible OLEDs also require low-temperature encapsulation less affected by moisture and hence do not cause chemi- procedures so as not to damage the flexible substrate and cal decomposition [730, 731], a higher degree of crystal- the organic layers. Thus, for roll-to-roll manufacturing of lization indeed induces irregular surfaces that lead to OLED, it is essential to go for flexible encapsulation. 320 S. Ahmad: Organic semiconductor devices

On the contrary, using flexible polymer lids may Longer processing time associated with multilayer seem to be better than glass in the aspect of flexibility, thin-film encapsulation makes it prone to complications/ but polymer lids can be subjected to delamination while contamination. Multiple constituent materials may also folded leading to device degradation. Hence, the only increase the cost of fabrication and thus make the fabrica- option for reliable encapsulation for flexible OLEDs is tion of the encapsulated OLED potentially expensive. thin-film encapsulation. Besides lengthy processing, it was also shown that Thin-film encapsulation layers provide easy integra- Barix thin-film encapsulation does not acquire the integ- tion into flexible OLEDs. In the case of thin-film encap- rity of glass barrier layers for flexible or rigid substrate sulation utilizing low-temperature deposition processes, OLEDs [712]. Therefore, a need exists for alternative means it can overcome all the obstacles that glass encapsulation of suppressing dark spots growth – means that would not encounter for flexible OLEDs. Moreover, thin-film barrier introduce complexity or potential for cost increase. We layers are extremely thin and are transparent unlike metal know that the cathode-organic interface is the main dark lids, and they are not as bulky as glass substrates and spots formation site as a result of moisture/O2 propagation are easy to integrate into roll-to-roll production. Despite through the pinholes in the metal cathode in the OLED. many advantages, thin-film encapsulation layers have to In case the interfacial adhesion between the cathode and maintain a high level of impermeability for being a viable the organic layers is strong enough, the facile formation option due to the sensitive nature of OLEDs. Certain mul- of metal hydroxide sites at the interface is potentially hin- tilayer thin-film encapsulations utilizing different inor- dered. By finding ways to strengthen the cathode-organic ganic and/or organic layers have been shown to possess interface, and consequently the inherent resistance of the high impermeability to moisture/oxygen. OLED structure to moisture and oxygen, suppression of In spite of possessing flexibility and transparency, dark spots can potentially be achieved. thin-film barriers still lag behind the stringent require- Despite superior thin-film encapsulation method ments for moisture and oxygen impermeability require- available such as Barix, it produced OLEDs with higher ment of OE. A water vapor transmission rate (WVTR) of dark spots growth when compared to glass encapsulated 10-6 g/m2/day and an oxygen transmission rate (OTR) in devices [734]. Another study conducted in this context the range of 10-3 to 10-5 cm3/m2/day are desirable for OLEDs. [712] demonstrated that the use of Barix encapsulation on Multilayer encapsulations offer promising features plastic substrates showed one fourth of the half-life of a for protection from ambient conditions for flexible OLEDs. glass encapsulated and glass substrate devices. For com- Oxygen permeation rates of 10-6 cm3/m2/day were achieved parison, Barix encapsulation resulted in 3700 h of half-life via Barix encapsulation on a plastic substrate [734]. Barix for a glass substrate OLED vs. 2500 h for a plastic substrate multilayer encapsulation, comprising alternating organic OLED [712]. This study clarified that even superior com- and inorganic layers, prolongs the permeation path of mercial thin-film barrier layers did not achieve the high- oxygen and/or moisture through the barrier layers wherein quality encapsulation, which was seen with glass barriers. the organic layers act as a planarization layer that helps in The nonencapsulation approach of dark spots sup- a smoother surface, whereas the inorganic layers such as pression were studied by examining the influence of aluminum oxide act like a permeation barrier to moisture thermal annealing and heating flexible encapsulated and oxygen [734]. Barix encapsulation is also transpar- OLEDs in terms of their lifetime and controlling dark spots ent and hence is suitable for top-emitter OLEDs too. Most growth [63, 730, 731]. Furthermore, using graded cathode, importantly, the encapsulation deposition process is a low a mixed organic metal layer (MOML) was also noted to temperature one, making the process extremely suitable prolong device lifetime [735]. for flexible OLEDs. A study was conducted [731] to examine the influ- The organic materials in Barix method are deposited ence of annealing of OLEDs especially in terms of device using a nonconformal deposition technique – the organic lifetimes under carefully selected temperature ranges material starts as a liquid, which is then vaporized and by heating the individual layers. For instance, HTL and UV-cured to produce a solid layer [734]. The inorganic ETL were deposited at 140°C and the Al cathode at 60°C,

Al2O3 is deposited via DC sputtering. Four to five polymer which enabled OLEDs to show 30% better luminance at (organic)/inorganic dyads are used for encapsulation 20 mA/cm2 current density when compared to a non-heat- [734]. The drawbacks of multilayer structures such as treated devices with the same organic and cathode layers Barix are that they require complex post-OLED fabrication [731]. This improvement was mainly attributed to the deposition with the need to have separate deposition tech- crystallization of HTL, such that further operation and/or niques for different layers. storage would not affect the device adversely. S. Ahmad: Organic semiconductor devices 321

When these devices were stored for 2 weeks in 40% α-Si, though lacking in long-range order, contains RH environment, the heat-treated devices continued to abundant atomic species with unattached bonds, which possess EL at 9 V [731], and on the contrary, the non-heat- give rise to localized trap energy states situated below the treated devices exhibited no emission at all. Another set of conduction band edge [737]. Although a large density of OLEDs with only the HTL deposited at 140°C was also pre- such defects lowers the conductivity, it can be mitigated pared, which showed EL at 9 V after 2 weeks of exposure to some extent by hydrogenation [738] resulting in a to 40% RH conditions, but they only possessed a slight hydrogenated amorphous state termed as α-Si:H. A low- increase in luminance over the non-heat-treated OLEDs temperature PECVD is available to deposit films with con- when compared to the luminance increase seen with the trolled hydrogen out-diffusion. Carrier transport in α-Si:H individually heated layer OLEDs [731]. involves thermally assisted hopping between trap states In another subsequent study, conducted later in this [737], leading to a mobility approx. 1 cm2/Vs [738]. Even context [736], it was concluded that 160°C deposition of this low mobility is acceptable having a number of other the organic layer retarded dark spots growth, and after advantageous features such as low defect density result- 20 h storage in air, almost 80% of the emissive area of a ing in good turn-on, subthreshold slope under 1 V/decade non-heat-treated devices became nonemissive. On the and threshold voltage VT < 1 V [739]. High-quality PECVD contrary, dark spots on the 160°C deposited organic layer tools are available for depositing extremely uniform films OLEDs were still smaller in size [736]. on large-area glass substrate with very uniform device In yet another subsequent study [63], it was shown that characteristics [740]. 120°C annealing of OLED structure comprising ITO/NPB/ Material reliability is a major issue [739] with α-Si:H, Alq3/Al for 1 h resulted in marked improvement in lumi- where large density weak Si-Si bonds, present in a ran- nescent efficiency, brightness, and stability when com- domly distributed Si network, are prone to disruption pared to nonannealed devices. For instance, maximum under the influence of applied electric field. Besides, there luminance of a 120°C annealed device was 6240 Cd/m2 is sufficient charge trapping at the poor-quality interface 2 compared to a mere 3650 cd/m for an unannealed device between α-Si:H and Si3N4. Both these effects lead to tem- [63]. Furthermore, OLEDs were selectively annealed layer poral shifts in device threshold voltage [741]. Although by layer to focus on the effect of heating on progressively n-type metal oxide semiconductor (NMOS) α-Si:H devices deposited layers. It was noted that, by annealing a device, are good enough for TFTs in displays, they are not all which includes the Al cathode at 120°C, the best lumi- appropriate for logic applications. nance (670 cd/m2) and efficiency (3.5 cd/A) were reported In polycrystalline Si (poly-Si), the next alternative, [63]. Furthermore, new OLEDs with the configuration of there are randomly distributed crystalline grains ranging ITO/NPB/Alq3/LiF/Al were fabricated with individual from tens of nanometer to several microns in size having layers selectively annealed just like done before [63]. Once crystalline quality [739] surrounded by grain boundaries again, luminance and luminance efficiency improve- rich in defects. Here, the larger the grains, the better it ments were observed for the OLEDs with all the layers is for device applications. For instance, if the grains are annealed. Moreover, improved half-life from 14 to 52 h was larger than the transistor channel length, device perfor- noted with all the layers annealed. These results show mance approaches that of single-crystal Si. Poly-Si films that annealing the cathode leads to enhanced electron are deposited [739] either as a polycrystalline film as such injection. AFM scan of the Al cathode with all the layers or as an amorphous film followed by recrystallization, annealed showed that annealing led to leveling out of the where larger grains are possible to have. Si deposition, in undulation of the Al film [63], which could possibly be the this context, uses any one of the methods involving low- reason behind better operational stability. pressure CVD (LPCVD), PECVD, or sputter deposition. In LPCVD, a deposition above 580°C produces poly-Si films and below it produces α-Si films. PECVD processes are typically run between 250°C and 400°C, whereas sputter 11 Discussion deposition is possible between room temperature to as high as permitted by the tool. The recrystallization of Before addressing the various salient features of OS mate- as-deposited film [739] is performed either by thermal rials presently covered in this review, a quick look into the annealing at > 600°C or laser annealing. Laser annealing status of amorphous and poly-Si devices for large-area produces large-grain poly-Si and is compatible with low- display and flexible electronics applications will be useful temperature substrates, despite raising the temperature as a benchmark for frequent comparisons. above Si melting point. For instance, 30 ns excimer laser 322 S. Ahmad: Organic semiconductor devices pulse take < 1 ms [742] for heating/cooling cycle for each oxidation that is harmful for device applications. With the pulse. Thus, having an appropriate barrier layer beneath observation of similar instabilities in higher oligoacenes the Si film, the molten Si cools back to room temperature such as naphthacene and hexacene, it was suggested that before enough heat diffuses into the substrate causing perhaps further improvements in mobility were not pos- damage. sible using acene structures. However, in this context, Mobility in poly-Si depends on the grain size. For the enhancement of intermolecular interactions using instance, LPCVD poly-Si may have electron mobility heteroaromatic compounds of S and Se was worth explor- approximately 10 cm2/Vs compared to the thermally ing [677], as it was noted to provide conduction paths in annealed one having 30 cm2/Vs [743]. Excimer laser crystal­ organic charge-transfer salts and organic superconduc- lized poly-Si may have electron mobility as high as 100 to tors. However, using this approach in oligothiophene 200 cm2/Vs [744]. Using special annealing methods such and oligoselenophene-based OFETs, the mobilities still as sequential lateral solidification (SLS) or grain engineer- remained very low [749]. Further study of such unexpected ing, electron mobility has gone up to 300 to 500 cm2/Vs results not only explained the observed behavior but also [745, 746] approaching the single-crystal Si value. indicated the importance of the geometry of the fron- Comparing OS with the inorganic ones, few points tier molecular orbitals in improving­ the mobility. Based emerge clearly. Despite the best efforts put in to grow on this experience of designing better molecules, it is organic crystals and deposit thin films, the mobilities are worth noting the following results in this context. Vapor- still an order of magnitude or two lower than in inorganic deposited DPh-BTBT and DPh-BSBS thin films on Si/SiO2 crystals, respectively. However, the organic thin-film exhibited mobility of 2.0 and 0.3 cm2/Vs in the respec- devices have outperformed the α-Si devices because of tive devices [750, 751] accompanied by extremely good nonexistent grain boundaries with excess defects. air-­stability and more than 6 months of shelf-life under

Compared to polymers, small molecules give better ambient conditions. Cn-BXBXs OFETs with a top contact crystals with improved mobility but produce poor thin configuration exhibited mobility better than 0.5 cm2/Vs; films. Polymers form more ordered thin films but with in particular, C13-BTBT showed the best performances­ reduced mobility due to a larger number of disorders. Thus, with the maximum mobility of 2.8 cm2/Vs [455]. In con- polymer thin films are better for OE applications where trast, the Cn-BSBS-based devices showed two orders of even lower mobilities may be useable. MBE has further magnitude lower mobility than that of C13-BTBT [751]. Air- promises for smaller molecules but needs more studies stable DNTT and DNSS-based OFETs showed mobility of following a process possibly similar to gas source MBE 3.0 and 2.0 cm2/Vs, respectively [752, 753], with negligible [306], where appropriate precursors can be developed for degradation and low hysteresis in the device character- their use in high-quality epitaxial films. Of course, ALD istics over long duration. When compared to pentacene, [306] is highly attractive for depositing high-quality metal DNTT may be taken as standard OS in the next generation and dielectric thin films for device fabrications, where of OFET developments [677, 754]. large-scale fabrication tools are already available. The role of SAMs in modifying the interfaces between In polymers, inserting appropriate functional groups organic-organic, organic-inorganic, and metal-organic leads to better mobility via improved π-conjugation besides combinations is possibly more profound than those creating mechanical torsion in the backbone leading to observed in inorganic semiconductors possibly due to higher band-gap resulting in better stability. Pentacene very strong covalent bonds present there compared to was investigated in detail by examining two basic para­ van der Waals forces in OS. The influence of OS NPs with meters that characterize the hopping conduction, namely, immense scope of changing their functionalities using reorganization energy of carrier exchange and inter­ core-shell configuration is another area worth examining molecular orbital coupling transfer integral [747]. Having with reference to their use as SAMs in this context. five benzene rings and a delocalized π-system with a rigid Combination of metal NPs with CNTs is another structure, pentacene molecules need smaller reorganiza- area of great interest for their use in contact electrodes tion energy with larger orbit coupling transfer integral and interconnects. One example of rubber-like intercon- between neighboring molecules. It packs in a herringbone nect developed recently [755, 756] is a useful example in structure in solid state that provides 2D isotropic electro­ this context. Bundles of CNTs with BMITFSI [38] ionic nic structure on the substrates, which is also advanta- liquid and rubber-like fluorinated copolymer were stirred geous in the application of TFTs [748]. Although pentacene together by sonication to prepare stretchable intercon- appears as an ideal OS for OFETs, its chemical instability nect films after drying in air. These flexible intercon- and higher oxidation potential make it susceptible to air nects could be stretched by 40% due to CNTs embedded S. Ahmad: Organic semiconductor devices 323 in the rubber-like fluorinated copolymer. A new printing transport in OS in the near future. In this context, an SAM technology [757] was reported in this context involving of pentathiophene derivative deposited on an electron microporous base coated with polyethylene, PVA, boracic beam transparent substrate was successfully character- acid, or other materials, which absorbed the solvent of the ized using TEM, where high-precision crystallographic Ag ink. The ink included Ag NPs, alcohol, surface-active structural maps were produced [758]. These maps were agent, and dispersant, which reacted with the base on the used to decipher size, symmetry, and orientation of unit paper at room temperature without causing any damage to cell as well as the orientation, structure, and the degree the OFETs. When the printed interconnect was stored, the of crystallinity inside the domains, which are essential for lifetime was estimated approximately 6 months, with 10% understanding the transport properties of the charge car- increase in interconnect resistance after 1 month [756]. riers in such semiconductor films [758]. This kind of char- The extensive use of impurity dopings and ion implan- acterization is expected to experimentally determine the tation [306] is well known in microelectronics, while influence of various molecular functionalization schemes mass-producing devices and circuits with numerous mod- but also help in optimizing the same in a convenient way. ifications were introduced in the tool designs to handle OS devices such as OLEDs, OPVSCs, memory ele- larger-sized Si wafers in sufficiently large-sized batches ments, and OFETs are predicted to reduce fabrication aided by process automations and robotics. In the case of costs and enable new functions. Besides optimizing the OS, the status of diffusion doping and ion implantation performances of these individual devices where the phe- is at the beginning of understanding the basic processes nomena of electronic and optoelectronic material prop- involved before exploring their applications further. Nev- erties are explored for best possible performance, the ertheless, organic devices are already there in a number of feasibility of combining features of two different devices mobile device displays, in which the doping has enhanced in one may be explored for some unique applications. In the efficiency of optoelectronic devices and improved this context, combining the FET action with light genera- process reproducibility for commercial productions [482]. tion and detection has been already explored in recent Despite having succeeded in improving the performance past. The development of newer compounds designed at of optoelectronic devices even with less than the com- molecular levels is worth considering in this direction as plete knowledge of the precise mechanism of doping, it shown in the examples mentioned here. is yet to be put to use in organic solar cells and organic The concept of organic light-emitting transistor transistors, opening newer opportunities for research in (OLET), having the FET structure with the capability of times to come. Similarly, ion implantation has entered the light generation, is seriously being explored for bright OS device processing area in the form of surface passiva- multicolor electroluminescent displays with simpler tion technique [670] for improving the device reliability, driving circuits besides possessing technological potential a welcome start. Like impurity doping, the process of ion for realizing intense nanoscale light sources and highly implantation may also take some more time to find even integrated optoelectronic systems, including electrically better applications in the future. pumped organic laser [759]. Although OLEDs are the most Based on the recent findings of various research developed OS devices as the active matrix OLED displays groups involved in studying OS for their uses in device are already in the market, the major associated problems applications, considerable improvements in basic under- of the exciton-charge interactions and the photon losses standing of the processes involved accompanied with at the electrodes need further consideration for improv- more profound technological applications are anticipated ing their reliability and life. High-density electrons and as illustrated here with few specific examples. holes, present in the light-emitting layer, cause significant Understanding the basic mechanism of charge carrier exciton-charge carrier quenching in the close proximity transport in OS is the key to improve the performance of of the excitons [759]. For enhancing efficiency, bright- the explored device structures after realizing the optimum ness, and stability, the exciton-charge carrier quenching conditions necessary for material growth and processes is necessary to be minimized. With this background, the involved therein. In this context, it is necessary to have development of OLET is primarily motivated by the pos- the right kind of diagnostic tool to verify the theoretical sibility of having an alternate display/light source by predictions based on the proposed models proposed by adapting a geometry to suppress the photon losses and several groups. In this context, the recent development exciton quenching inherent in the OLED architecture. In of using transmission electron microscopy (TEM) to map this context, the phenomenon of exciton-metal interac- the material structure very precisely at atomic level is tion has been, so far, minimized in ambipolar single-layer going to boost the level of understanding of charge carrier OLETs [759] by separating the light-emitting area from 324 S. Ahmad: Organic semiconductor devices the metal contact. These single-layer devices still have was nonlinearly dependent on the amplitude of the AC the charge carrier accumulation and the exciton forma- gate voltage. tion zones coinciding together, leading to severe exciton- In another study [763], a single-crystal light-emitting charge quenching resulting in poor external quantum transistor (SCLET) was realized by employing single-crys- efficiency (EQE) of 0.2% but demonstrated remarkable tal optical waveguide, coupler, and resonator prepared results in terms of brightness [759]. An alternate structure from as-grown organic single crystals. Using parallel of horizontal p-n heterojunction OLET was also fabricated edges of the crystal as Fabry-Perot cavity, effective optical having light emission area far from the contacts; however, coupling between single-crystal waveguides and optical exciton-charge quenching could not be avoided [759]. feedback resonator was realized by simple crystal lami- Alternatively, two types of bilayer configurations involv- nation. Practical realization of the concept demonstrated ing a highly efficient luminescent layer superimposed spectral narrowing behavior as a clear evidence of the over a unipolar conducting layer or putting p- and n-type cavity effect. These results open a route to the develop- transport layers directly in contact with each other were ment of color tunable and highly efficient SCLETs as well attempted, which did not provide any control over exciton as electro-optical interconnecting devices [763]. quenching or photon losses as such. Yet, in another work Successful fabrication of air-stable ambipolar hetero- [759], a trilayer heterostructure OLET enabled simultane- junction-based organic light-emitting FETs (OLEFETs) was ous control of electrode-induced photon losses and exci- demonstrated using multidigitated top-contact device ton-metal and exciton-charge interactions. OLET devices geometry [764]. Active layers of p-type pentacene and with EQEs of 5% were demonstrated, which exceeded the n-type N,N′-ditrydecylperylene-3,4,9,10-tetracarbocyclic best OLEDs based on the same emitting layer and opti- diimide (P13) with a protecting layer of 2,5-bis(4-biphenyl mized transport layers [759]. thiophene) (BP1T) were deposited using cluster beam dep- In a recent attempt, a basic OFET device was modified osition technique for well-ordered thin-film deposition. to produce light emission by employing self-assembled These results open up the newer possibilities of explor- oligophenylene and phenylene-thiophene nanofibers ing the light generation capability of OLEFETs in the near [760] on FET platform appropriate for producing EL and future, where many problems of conventional device photoluminescence (PL) simultaneously. The phenom- structures would be taken care of appropriately. enon of localized, polarized, and wave-guided EL was A real hybrid combination of LC-on-OFET was realized in aligned nanofibers on OFETs by applying AC reported in another recent publication [765] to sense bias on the gate electrodes [761], which caused sequential ultra-low-level gas flows. These devices were realized by injection of holes and electrons resulting in charge carrier mounting 4-cyano-49-pentylbiphenyl-5CB LC-on-OFET recombination leading to light emission from a small channel. The LC molecules on the channel layer enhanced area near the metal-nanofiber interface. In this configu- the source-drain current due to dipoles of LC molecules. ration, the nanofibers acted as optical waveguides and When low-intensity nitrogen gas flow was employed, the part of the generated light was therefore guided along drain current increased depending on the intensity and the nanofibers, which finally radiated from the fiber end. time of the gas flows. These LC-on-OFET devices detected Besides hexaphenylene nanofibers, this scheme was extremely low level of nitrogen flows (0.7 sccm–11 ml/s), found equally applicable [762] in the case of nanofib- which could not be felt by human skins. The successful ers made from a variety of different molecules or mate- demonstration of similar behavior of the LC-on-OFET rial with altered spectral characteristics. The realization devices onto a polymer film skin suggests a viable appli- of an electrically biased organic nanoscale light emitter cation in tactile sensing [765]. demonstrates the ability to fabricate on-chip light sources With the emergence of nanoscience and technol- with a tunable emission spectrum via synthesis of appro- ogy disciplines, the concept of “material by design” (S. priate molecular building blocks. Extending this concept Ahmad. Nanocrystals, superlattices and nanomaterials, further, PL and EL of two devices made from naphthyl communicated for publication) appears to be a reality in end-capped oligothiophenes were investigated based the near future. This concept has been primarily consid- on an organic light-emitting FET (OLEFET) configura- ered as an extension of the well-known principle of mate- tion. Observing the similarity between EL and PL spectra rials being the manifestations of the constituent atomic from both materials, it indicates that the light emission is and molecular species arranged in specific orders. In caused by the same electronic transitions. It was further case atoms and molecules of conventional materials are observed that the intensity of the EL emission increased replaced by nanosize building blocks of numerous mate- with increasing frequency of the AC gate voltage, and it rial species already known to date, still a bigger family of S. Ahmad: Organic semiconductor devices 325 mesomaterials is possible to design and realize. As physi- made in this context in the present review may be used cal, chemical, and biological properties of nanosized accordingly hopefully. material species are strong functions of size and shapes, it gives enormous latitude in designing materials by Acknowledgments: The author gratefully acknowledges choosing from large varieties of material species, which the support and encouragement extended by the Confed- is possible to further extend to another level of sophisti- eration of Indian Industry (CII) and the Center of Excel- cation. In this context, even materials would be possible lence in Green Nanotechnology, CII Western Region, to design with preassigned features by choosing the right Ahmedabad, Gujarat, India. The challenging R&D envi- kind of building blocks resulting in the desired combina- ronment, created and supported by Mr. Anjan Das, Mr. tion of physical, chemical, and biological features. Such G.K. Moinudeen, and colleagues at the Center of Excel- synthetic materials are also named as mesomaterials or lence in Green Nanotechnology, was especially enjoyed metamaterials belonging to the category of materials by by the author while conducting this study and the positive design. In this context, the role of organic molecules are encouragement received is sincerely acknowledged. In certainly going to play very significant as can be seen from connection with preparing the present manuscript, vari- the discussions presented here in this review. Combining ous concepts, developed over past few decades in attempt- the strength of organic molecules with more complex bio- ing to improve the material characteristics for device molecules, the possibilities of realizing newer families applications, which have been elaborately discussed in of synthetic materials are immense as can be verified by various excellent review articles and especially the pre- the marvels of nature where such complex modifications sent status of the device-related developments reported are playing their roles in evolving species over millions of in recent publications, have been used with due acknowl- years in the past. edgements. The references provided here may not be that Besides intensive efforts made by material scientists exhaustive as in most review papers mentioned above, and chemists to design and synthesize newer molecules but the contributions made by numerous researchers in with immense possibilities of tailoring the properties this area are all duly acknowledged, directly or indirectly, appropriate for their use in device fabrication, it is equally through the references currently included in the text as important for the device scientists and process engineers well as those mentioned in the referred review articles. to examine the gap areas for improving the device behav- ior and manufacturability at low cost especially required Received October 16, 2013; accepted January 14, 2014; previously in niche areas as mentioned earlier. Various observations published online March 13, 2014

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