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