OPTO−ELECTRONICS REVIEW 18(2), 121–136

DOI: 10.2478/s11772−010−0008−9

Organic field-effect transistors

M.J. MAŁACHOWSKI1 and J. ŻMIJA*2

1Department of Technical Education, Technical University of Radom, 29 Malczewskiego Str., 26−600 Radom, Poland 2Institute of Technical Physics, Military University of Technology, 2 Kaliskiego Str., 00−908 Warsaw, Poland

The paper reviews the recent year publications concerning organic field−effect transistors (OFETs). A lot of works have been performed to help understanding the structural and electrical properties of materials used to construct OFETs. It has been established that in partially ordered systems, the charge transport mechanism is thermally activated and field−assisted hopping transport and the hopping transport between disorder−induced localized states dominate over intrinsic polaronic hopping transport seen in organic single crystals. Many research attempts have been carried out on the design of air−stable organic semiconductors with a solution process which is capable of producing OFETs with excellent properties and good stability when subjected to multiple testing cycles and under continuous electrical bias. Recent experiments have demon− strated ambipolar channel conduction and light emission in conjugated polymer FETs. These achievements are the basis for construction of OLED based displays driven by active matrix consisting of OFETs.

Keywords: organic electronics, organic field−effect transistors, opto−electronic materials, information displays.

1. Introduction systems and organic field−effect transistors (OFETs) are in active development (in order to indicate the transistor struc− In the inorganic semiconductors such as Si and Ge, the at− ture, through this paper we shall use also OTFT, OFET or oms are held together with very strong covalent bonds, as OTFFET abbreviations for organic thin film transistor, high as 300 kJ/mol for Si. In these semiconductors, charge however, all transistors considered here operate as the field carriers move as highly delocalized plane waves in wide bands and indicate a high mobility of charge carriers μ >> 1 effect devices). The processing characteristics and demon− cm2/Vs. In organic semiconductors, intramolecular interac− strated performance of OFETs suggest that they can replace tions are also mainly covalent, while intermolecular interac− existing or be useful for novel thin−film−transistor applica− tions are usually induced by much weaker van der Waals tions requiring large−area coverage, low−temperature pro− and London forces resulting in energies smaller than 40 cessing, mechanical flexibility and, especially, low cost. kJ/mol. Thus, the transport bands in organic crystals are Such applications include switching devices for active−ma− much narrower than those of inorganic ones, and the band trix flat−panel displays (AMFPDs) based on either liquid structure is easily broken by disorders in such systems, crystal pixels (AMLCDs) or active matrix with organic light− leading to creation of states in the energy gap. −emitting diodes (AMOLEDs). At present, hydrogenated Almost all organic solids are insulators. However, in or− amorphous silicon (a−Si:H) is the most commonly used as ganic crystals that consist of molecules that have the p−con− active layer in TFTs backplanes of AMLCDs. The higher jugate system, electrons can move via p−electron cloud performance of polycrystalline silicon TFTs is usually re− overlaps. That is why these crystals exhibit electrical con− quired for well−performing AMOLEDs. Improvements in ductivity. Polycyclic hydrocarbons and phthalocyanide salt the efficiency of both the OLEDs and the OTFTs could crystals are examples of this type of organic semiconductor. broaden applications. OTFTs could also be used in active− Among the conjugated polymers there exist conductive and −matrix backplanes for “electronic paper” displays based on semiconductive polymers. pixels comprising either electrophoretic ink−containing Organic semiconductors, such as conjugated oligomers microcapsules or “twisting balls”. Other applications of and polymers, have become materials for a wide range of OTFTs include low−end smart cards and electronic identifi− not only electronic but also optoelectronic devices. Organic cation tags. light−emitting diodes (OLEDs) inserted into commercial Most of the investigated OFETs are not all organic as well as not flexible ones. The devices are usually con− structed on no flexible silicon substrate. In order to fabri− *e−mail: [email protected] cate the flexible AMOLED structure, rather all−organic

Opto−Electron. Rev., 18, no. 2, 2010 M.J. Małachowski Organic field−effect transistors

FETs should be used. The three−dimension bottom struc− patterned anode. Source and drain contacts can be poly− ture of this device is shown in Fig. 1(a). Figures 1(b) and meric. The active semiconducting layer can be deposited by 1(c) present top and bottom structures, respectively, cross spin coating of an appropriate solution. The gate dielectric sections of not all organic TFTs because heavily doped layer can be prepared by spin coating a solution of insulat− silicon and metal electrodes are used to fabricate them. ing polymer. The aim of numerous studies on OTFTs of Both types of construction were extensively studied and configurations shown in Fig. 1 or similar are to improve the their advantages as well as disadvantages have been transistor performance, which includes almost all elements shown. of the device as well as the charge transport mechanism The investigation results of various soluble organic se− between source and drain. a, w miconductors show that, −dihexylsexithiophene Progress in plastic electronics is achieved mainly by (DH−6T) has become the most promising candidate for rising in the mobilities of organic semiconductors, increase all−organic semiconductor integration because of its air in the capacitance of suitable gate dielectrics, and reduc− stability, good solubility, and easy synthesis. For these tions in the channel dimensions of the devices. Both poly− purposes, the polymethylmethacrylate (PMMA) is used as mer and small molecule−based organic thin films may be a dielectric layer to substitute SiO2 in order to confer the used in OFET technology. Mobile charge carriers of either transfer characteristics of OTFTs with PMMA as polarity can be injected into organic semiconductors from a dielectric layer. contacts of suitable materials. The mobilities of these All−organic TFTs on flexible polyimide substrates can charge carriers, however, are low in comparison to inor− be produced indicating electrical performance similar to that ganic semiconductors, and the device performance is often of TFTs fabricated with inorganic gate dielectrics and metal strongly lowered by carrier trapping. The transport mecha− contacts. For production the OFET devices, a technique can nism is best described as a hopping process and the charge be based on area−selective electropolymerization on a pre− carriers are polarons. For brevity, a conventional semicon− ductor language is used to refer to the negative charge carriers as electrons and to the positive charge carriers as holes. The purpose of this article is to review the recent most important papers concerning the investigations on the ef− fects governing the operation of OFETs, which have a thin film as well as crystalline form. There are known some reviews focused at different problems connected with OFETs, just to mention the papers [1–6] and others [7,8].

2. Charge transport in organic semiconductors and OFETs

As it was mentioned above, organic semiconductors have major drawbacks in relatively low mobility of charge car− riers arising from weak intermolecular interaction in solid state and in the fact that doping is problematic because it is interstitial and not substitutional as in inorganic semi− conductors. An organic semiconductor is considered with the VB or the highest occupied molecular orbital (HOMO) and the CB or the lowest unoccupied molecular orbital (LUMO) which are separated from each other by the gap which is usually rather large, EG > 2eV.Fororganic semiconductors, the manifolds of both the LUMOs and the HOMOs are characterized by random positional and energetic disorder. To model the current flow in organic semiconductors, the charge transfer complex (CT complex) idea is intro− duced. CT complex is defined as a pair of molecular groups, where one is electron−donating (donor) and the other is elec− Fig. 1. Three−dimensional view of an all−organic bottom structure tron−accepting (acceptor) and where there is a partial trans− thin film field effect transistor (a), top electrode structure of OTFT fer of charge from the acceptor to the donor in an excited (b), and bottom electrode structure of OTFT (c). molecular state. CT complexes have a transition between

122 Opto−Electron. Rev., 18, no. 2, 2010 © 2010 SEP, Warsaw the excited molecular state and a ground state and almost all 3. Principles of operation of MOSFETs CT complexes have absorption bands in the ultraviolet or and TFFETs visible region. The interaction between donor and acceptor is not only a charge transfer interaction but also electrostatic 3.1. Silicon single crystal MOSFET force. This kind of interaction is usually much weaker than interactions of the hydrogen bond and the covalent bond, We shortly discuss some well understood principal effects oc− but it is essential for constructing the crystal structures. curring in commonly used silicon single crystal (metal oxide Since the HOMO level in most organic semiconductors is semiconductor field effect transistors – MOSFETs). deep, the gap separating LUMO and HOMO states, as men− A MOSFET structure consists of a p−type silicon single crystal + tioned above, is wide, energies of donor and acceptor mo− semiconductor on which two n electrodes have been formed creating the source and the drain contacts. The third electrode, lecules are normally well below LUMO and above HOMO the gate, which is used to modulate the conductivity of the levels. source−drain channel, is isolated from the semiconductor sub− In 1962, an important acceptor, tetracyanoquinodimetha− strate by an insulating (oxide) layer. A depletion region in ne (TCNQ), was exploited and in 1970, an important donor, which the concentration of charge carriers is lower than in the tetrathiafulvalene (TTF), was synthesized. In 1973, a CT bulk is generated near the source and the drain electrodes. This complex composed of the TTF and TCNQ was discovered, it kind of transistor appears to be most often used in the elec− was the organic conductor. In a crystal of TTF−TCNQ, the tronic circuits, however, there are another transistor structures TTF and TCNQ are stacked independently and an electron differing from MOSFET device. transfer from donor (TTF) to acceptor (TCNQ) occurs. When a sufficiently high positive voltage is applied to Hence, electrons and holes can transfer in the TCNQ and the gate of a MOSFET, an inversion layer forms interface TTF columns, respectively. It is possible to interfere in the between the insulator and semiconductor, providing a con− columns ordering. ducting channel between the source and the drain. This A well known theory by Mott [9] assumes that the con− turns the device on. One of main advantages of the duction process in organic materials is entirely determined MOSFET structure is that the depletion region between the by the tunnelling transitions of carriers between localized p−type substrate and both the n−channel and the n+ regions states, provided that the electronic wave functions of the lo− below the source and drain contacts provides isolation from calized states have sufficient overlap. The transport proper− any other device fabricated on the same substrate. Very low ties of many organic semiconductors can be well described off currents are also achieved because the both n+ regions by the variable range hopping theory VRH [10,11]. Despite act as reverse−biased diodes. If we assume that the channel decades of researching progress, however, there are still length L is much larger than the insulator thickness t and some effects of the charge transport in organic semi− that the charge mobility m is constant, the drain current ID is conductors to be explained. related to the source−drain voltage VD and the source−gate An interesting approach in the understanding of the voltage VG in the folowing way transport of charge carriers in organic semiconductors WCìæ ö has been elaborated in the theoretical and experimental =--m i j VD I D í ç VGB2 ÷ VD study by Li et al. [12]. In that work, it is presented an ana− L îè 2 ø lytical model for hopping transport in doped, disordered (1) 2e qN ïü organic semiconductors based on the VRH and the perco− -+2 SA j 32 - j 32 []()VDB2 ()2 B ý lation theory. It was assumed in Ref. 12 that localized 3 Ci þï states are randomly distributed in both the energy and the coordinate space, and that they form a discrete array where W is the channel width, Ci is the insulator capacitance of sites. Conduction proceeds via hopping between these e (per unit area), s is the semiconductor permittivity, and NA sites. is the doping level of the p−type substrate. Equation (1) pre− The mobile carrier density and its mobility are the main dicts that, for a given gate voltage, the drain current first in− parameters determining the charge transport in organic creases linearly with the drain voltage (linear regime), then semiconductors. The mobile electron and hole densities in gradually levels off to a constant value (saturation regime). a semiconductor, inorganic or organic one, are in any case It also predicts that the drain current increases when the gate determined by the distribution of transport states, which is voltage increases. For a small VD, Eq. (1) is reduced to Eq. characterized by their density of states (DOS). For thermo− (2), where VT is the threshold voltage, which corresponds to dynamic equilibrium it is not of interest whether the states the onset of the strong inversion regime, is given by Eq. (3) are extended or localized and whether one has correspond− WC ingly band conduction or hopping conduction. In the case of =-m i I D ()VVVGTD, (2) a conjugated polymer, where the carriers are polarons, one L has to consider just these polaron states. In any case, the dis− 4ejqN =+j SB A tribution of the transport states is characterized by their where VTB2 . (3) DOS. Ci

Opto−Electron. Rev., 18, no. 2, 2010 M.J. Małachowski 123 Organic field−effect transistors

Two important technological parameters are the channel ration may compete or totally replace the previous one if the conductance gD and the transconductance gm, which, in the molecular material is deposited onto a thin dielectric layer linear regime, are given by Eqs. (4) and (5), respectively. In covering a conductive gate. By applying a potential between the saturation regime, the drain current and transconduc− the source and the gate, charges are created on the two sides tance are given by Eqs. (6) and (7), respectively of the insulating layer following the equation

¶I WCm i Q = CV, g ==-D ()VV, (4) D ¶ GT VD = L VG const where Q is the electric charge, C is the capacitance, and V is the voltage. The drain−to−source current can therefore be ¶I WCVm iD g ==D , (5) modulated by the gate−to−source voltage. At sufficiently m ¶ VG V= const L high VDS voltages, injection of charge carriers from the elec− D trodes is possible. The probability of electron transfer from WCm i (or to) the electrode to (or from) the molecular material de− I =-()VV2 , (6) Dsat, 2L GT pends on the energy difference between the Fermi level of the metal and the HOMO and LUMO levels. WC =-m i The results described so far demonstrate that the mole− gm ()VVGT. (7) L cular material based field effect transistors are original from a fundamental point of view, they differ totally from sili− 3.2. Thin film transistor con−based devices for both the charge transport mechanisms (hopping model against band theory) and physicochemical In order to apply the accumulation effect in the structure of reasons which afford the modulation of the drain−to−source thin film field effect transistors (TFFETs), the low conduc− current by a gate voltage. The molecular FET may show tivity materials are used and amorphous silicon layer is an fairly good mobilities, high ION/IOFF ratios and respectable example of such a material. This remark can be adapted, in stabilities in air. When rapid switching times are not re− most cases, for the organic materials. However, it should be quired, the molecular FET could be used in practical appli− mentioned that the attempts have been made to achieve the cations. For this reason, care has to be taken when transfer− field effect transistor behaviour in inorganic semiconduc− ring the equations of the drain current from the MOSFET to tors of moderate conductivity [13]. To achieve this, a very the TFFET. The absence of a depletion region leads to thin film of active material should be used to develop the de− a simplification of Eq. (1), which can now be written as pletion region. The above described type of a process taking WCm i æ V ö place in MOSFETs, does not occur for organic materials I =--ç VV D ÷ V . (8) D è GT ø D which are insulators (rather than semiconductors) when L 2 undoped (with the exceptions of Pc2Lu and PcLi). In most Here, the threshold voltage is the gate voltage for which conductivity measurements of organic materials, the inten− the channel conductance (at low drain voltages) is equal to sities of current measured are, in most cases, very small. that of the whole semiconducting layer. It is given by Eq. The use of field effect could permit us to alleviate this diffi− (9), where N is the density of doping centres (donors or ac− culty. To achieve this effect, an electron−active molecular ceptors, depending on whether the semiconductor is n or material thin film is deposited on a dielectric (SiO ,SiN , 2 3 4 p−type) and d is the layer thickness. Equation (9) assumes polymers) of very small thickness (100–200 nm) covering that all doping centres are ionized, which is far from being a metallic electrode (the gate). Two gold electrodes (source the case in organic semiconductors, as will be seen later. and drain) separated by the distance L are then deposited on the molecular material. qNd V = . (9) The source and drain electrodes form ohmic contacts di− T Ci rectly to the conducting channel. Unlike the MOSFET structures described above, there is no depletion region to In the saturation regime, the current is given by Eq. (6). isolate the device from the substrate. Low off currents are We must be aware that this equation was derived under as− only guaranteed by the low conductivity of the semi− sumptions that are not always fulfilled in organic semicon− conductor. ductors, particularly that of a constant mobility. A second crucial difference to the MOSFET is that, the The current characteristics of OTFTS often show a dis− OFET operates in the accumulation regime and not in the advantageous nonlinearity at low drain voltages. In top con− inversion regime. Let us consider that the molecular mate− tact OTFTs this effect can be caused by trap recharging if rial does not possess a significant density of intrinsic charge the contacts are of Schottky type. For bottom contact carriers. When a drain−to−source voltage is applied, no cur− OTFTs, Schottky contacts as origin of the nonlinearity are rent (or a negligible one) can flow from one electrode to the often stated. A Schottky contact only at drain leads to such other. The molecular material behaves as an insulator if the a nonlinearity. However, with the same Schottky contacts at quantity of charges injected from the electrodes (injected drain and source, the effect is covered by the high resistance current) is small. Another mechanism of charge carrier gene− of the contact at a source. Further, it was demonstrated with

124 Opto−Electron. Rev., 18, no. 2, 2010 © 2010 SEP, Warsaw simulations [14] that the combination of the presence of and c + Eg for holes where c is the electron affinity acts as Schottky contacts with a field dependence of the mobility barrier at the contacts. can cause the nonlinearity. Since the p−type organic layers are most common, such electrodes as ITO or CuxSe as the hole carrier sources 4. Examples of energy bands in organic should be more intensively studied. semiconductor devices The structure of energy level scheme is shown in Fig. 3 (after Refs. 16 and 17) for a p−doped semiconductor. The The shape of the energy bands appearing in organic semi− contact formation leads to a common Fermi level. The ener− conductor devices, especially at the contact area, is essential gy zero is chosen at the Fermi level eF. The contact forma− in understanding the transport processes in a particular de− tion is connected with the position dependent potential ener− vice. Almost all OFETs have the structure with low work− gy V(x) which approaches in the bulk semiconductor the −function electron and high work−function hole injecting bulk intrinsic energy level which is denoted by Vbulk. The to− source/drain contacts to a conjugated polymer organic semi− tal drop in V(x) has to compensate the built−in potential Vbi, conductor. On the basis of Ref. 15 we show the energy band i.e., the work function difference between the semi− shape through a channel. It was considered the source−semi− conductor and the metal. conductor−drain structure in the form of metal−pentacen− −P13−metal (Fig. 2). Here, source and drain are made from two different metals (Au and Mg), and as a consequence of the unequal metals for source and drain contacts and the layer arrangement, two cases have to be discussed in which either Au or Mg is used as source contact. The corresponding band scheme is depicted in Fig. 2. The bilayer is formed from pentacene as electron−transport material and NN −ditridecylperylene−3, 4, 9, 10−tetracar− boxylic diimide (P13), as hole transport material. The two different contact metals for source and drain, respectively, were chosen to provide efficient electron and hole injection. SiO2 serves as gate insulator. The work functions of the me− tallic source and drain contacts are crucial quantities and Fig. 3. Energy scheme of a contact between metal (left) and semi− should be chosen individually to enable efficient injection conductor layer of the thickness d (right), with valence and conduc− of holes from one and electrons from the other contact. tion band edges, or centers of the HOMO and the LUMO distribu− Thus, for the Au contact it was chosen Au = 5.0 eV, which tions ev and ec with the common Fermi energy eF. The position de− is close to the valence band (highest occupied molecular or− pendent potential energy V(x) and the built−in potential Vbi (after bital – HOMO) of pentacene (Fig. 2). The work function for Ref. 17). F the Mg contact Mg has been varied; 3.635 or 3.66 eV is used. The difference between these metal work functions 5. Charge transport along an OTFT channel and the energy of the transport states; i.e., c for electrons Charge injection phenomena and the impact of source and drain contact resistances have been established to play a cru− cial role in the OTFTs performance. The physics of these ef− fects is still a subject of active investigation. In OTFTs a low conductivity in the bulk layer is required for a large on−off cur− rent ratio. When the bulk conductivity is not negligible, there appears crossover effect from field−effect dominated current to the bulk dominated current in the transfer characteristics of a doped accumulation mode disordered organic transistor. Conventional devices with an unshielded drain electrode suffer from parasitic currents outside the channel area, which may obscure the crossover from field−effect dominated current to bulk dominated current. Fig. 2. Band scheme for the structure of metal−pentacen−P13−metal transistor along the channel (x axis) and perpendicular (y axis) to− ward the Mg top contact. Affinities, gaps and work functions are 5.1. p-type channel OFETs given in eV. Correspondingly, the hole (electron) injection barrier (bold lines) at the Au (Mg) contact is 0.07 eV (0.26 eV), and the elec− Most of the OFETs that have been described to date have tron barrier from P13 to pentacene (hole barrier from pentacene to a p−type channel. A typical output characteristic of the pen− P13) is 0.18 eV (0.33 eV) (after Ref. 15). tacene based on the thin film OFET is shown in Fig. 4 [18].

Opto−Electron. Rev., 18, no. 2, 2010 M.J. Małachowski 125 Organic field−effect transistors

−work function metals as source−drain electrodes exhibit n−type characteristics under oxygen−free conditions [21,22]. Recently, Chua et al. [23] stated that the trapping of elec− trons by the hydroxyl groups on the surface of SiO2 gate di− electric layer affected the n−channel conduction of OFETs. Most conjugated polymers yield n−type characteristics when a suitable hydroxyl−free polymer, such as a divinyltetra− methylsiloxane− bis(benzocyclobutene) derivative, is used as the gate dielectric. Accordingly, the properties of the in− terface between the gate dielectric and the organic active layer are critical to the transport and accumulation of posi− tive or negative type of charge carriers. Pentacene is fre− quently used as an active layer for p−type OFETs, but it was found that it exhibits the n−type characteristics as well [24,25]. The n−type transport in OFETs incorporating pentacene on a SiO2 dielectric has been demonstrated [17]. The beneficial effect of a thin layer of oxidized Ca between the SiO2 dielectric and pentacene on the electron transport properties in OFETs has been analyzed in detail [23]. The poly(4−vinyl phenol) (PVP) polymer layer is a hydroxyl− −group−rich polymer dielectric, which is considered to inter− Fig. 4. Output characteristics of polymer film formed on SiO2 gate fere with the conduction of electrons for n−type OFETs, as insulating layer treated with OTS (after Ref. 16) immersed in proposed by Chua et al. [23]. Presumably, the hydroxyl or a 50−mM solution of a fresh organosilane, octyltrichlorosilane the polar groups on the PVA dielectric favour the formation (OTS). of n−channel conduction in the pentacene active layer, and these groups have different functionalities from those on the The characteristic is typical for transistors having the p−type SiO2 gate dielectric. accumulation channel. Typical IDS vs VDS characteristics in the case of n−type The studies of Wang et al. [19] gave us progress in un− channel pentacene−based OFET fabricated on the PVA derstanding of the charge transport mechanisms in the chan− cross−linked PVP dielectric, operated at a positive VG, are nel of polycrystalline organic semiconductors. The group shown in Fig. 5 [26]. has fabricated OTFTs, using pentacene as the active layer. Experimental data suggested that thermally activated and 6. Gate insulators for OTFTs field−assisted hopping transport between disorder−induced localized states, dominates the intrinsic polaronic transport A remarkable amount of papers have been devoted to study seen in organic single crystals. The experimental results different dielectric materials that can play the role of the have been found to exhibit a Frenkel−Poole−type depend− gate insulators in OTFTs. In addition to the conventional in− ence consistently over a wide range of channel lengths, fields, and temperatures. Hamadani et al. [18,20] have reported the results of in− teresting investigation of a field dependence of the longitu− dinal mobility in OFETs based on poly(2,5−bis)3−tetra− decylthiophene−2−yl(thieno)3,2−b(thiophene) (pBTTT−C14) as the active polymer layer. Devices were fabricated in a bottom−contact configuration on a degenerately doped n+ silicon wafer as a gate. The gate dielectric is 200 nm of ther− mally grown SiO2. The electrodes are deposited by thermal evaporation of Ti and Au. The devices operated as standard p−type OFETs in accumulation mode, the output charac− teristics are shown in Fig. 4.

5.2. n-channel OFETs As mentioned above, most papers that have appeared to date are devoted to the p−type channel OFETs but few have an Fig. 5. IDS vs. VDS plots of pentacene−based OFET fabricated on n−type channel, Ref. 20 is an example. OFETs with high− PVA/ cross−linked PVP dielectric, operated at the positive VG in the −electron−affinity organic materials as active layers or low− n−channel conduction regime (after Ref. 26).

126 Opto−Electron. Rev., 18, no. 2, 2010 © 2010 SEP, Warsaw organic materials such as SiO2,Al2O3, and HfO2, various or− ganic materials were also tried for the gate dielectric in OTFTs. Insulating polymers including poly−4−vinylphenol, polymide, parylene, and polyaniline have been successfully employed for the gate dielectric and have shown to enhance device performance. In addition, polymethyl methacrylate (PMMA) derivative polymers are one of the new materials for the gate dielectric or buffer layer, exhibiting high field− −effect mobility and low threshold voltages. A wide used SiO2 dielectric as a gate insulator can play a different role in OFETs depending on its surface treating, even change in the channel type can be achieved. Mis− kiewicz et al. have demonstrated that the surface energy of the dielectric can have large impact on the performance of FETs based on TTF−4SC18 [27]. Average charge carrier mobility was 30 times increased as a result of proper treat− ing. The surface energy of the employed SiO2 dielectric de− creased. The authors pointed out that the performance of the OFETs produced on SiO2 depends also on such factors as surface energy and method of organic semiconductor depo− sition, in addition to the surface roughness. The achieved 2 μFET of 0.2 cm /Vs is among the best results published for large area solution−processed OFETs prepared and mea− sured in air, especially taking into account the reprodu− cibility and uniformity of zone−cast layers.

6.1. Influence of higher dielectric constant Some studies are devoted to obtain the gate insulator of high dielectric constant and strength and proper interaction with Fig. 6. Drain current vs. drain voltage characteristics of a p−type the active semiconductor leading to improvements in OFET pentacene OFET fabricated on the Ta2O5/Ta gate as a function of gate voltage (after Ref. 29): (a) high voltage and (b) low voltage op− performance. SiO2 film appears to be a good insulator and has a very high dielectric strength, approximately 10 erations. MV/cm. However, the dielectric constant of SiO2 is not very high, approximately e = 4. Thus, many research groups have been trying to fabricate OFETs on a high dielectric constant 6.2. Gate dielectrics on flexible substrates gate insulator to inject more carriers. Another advantage of It should be also indicated that the SiO film is grown onto high−e materials is a lower operating voltage. There has 2 been shown very clearly the beneficial role that plays the unflexible silicon substrate. The organic gate dielectric is a key ingredient for the realization of all−organic transistors, high dielectric constant tantalium oxide (Ta2O5) as a gate insulator [28,29] in OFETs. which is important for flexible display and other low−cost In order to improve the carrier injection, Ueno et al. [29] device applications. As an example, in order to make possi− ble application of OFETs on flexible substrates, Parashkov have studied carrier injection into OFETs with Ta2O5 as a gate insulator. Figure 6(a) shows the drain current vs. drain et al. [30] have fabricated fully patterned all−organic TFTs voltage characteristics of a p−type pentacene OFET as with a variety of organic polymer insulators and the gates a function of the gate voltage. As shown in Fig. 6(b), this printed on top of the gate dielectric layer. The group has OFET operated also at low gate/drain voltages due to the produced OTFTs on flexible polyimide substrates and ob− high dielectric constant of the Ta2O5 gate material. tained devices with electrical performance similar to that of For the p−type OFET, the reverse negative bias voltage OTFTs fabricated with inorganic gate dielectrics and metal was applied between the gate and source electrodes, corre− contacts. sponding to the reverse voltage to the anodized Ta2O5/Ta Majewski et al. [31] have investigated bottom−gate gate. The investigation result shows that the anodized OFETs using a commercially metallized Mylar films coated Ta2O5/Ta gate can also work as the gate capacitor for an with an ultra−thin (3.5 nm) SiO2 layer as a flexible substrate. n−type OFET. It has been shown that using high−e gate The OFETs have been fabricated using this substrate, regio− structure provides possibility of injection a higher amount regular poly(3−hexylthiophene) (rr−P3HT) as a p−type semi− of charge carriers into organic active layers than on the conductor, and gold source and drain contacts. This resulted SiO2/Si gate conventionally used in most OFETs. in OFETs that operated with voltages of the order of 1 V.

Opto−Electron. Rev., 18, no. 2, 2010 M.J. Małachowski 127 Organic field−effect transistors

7. Doping

The doping process of organic semiconductors can largely be depicted by a standard model used for crystalline inor− ganic semiconductors, a general doping model still remains a challenge for organic semiconductors. In particular, for some experiments, a strong superlinear increase in the con− ductivity with doping is observed. One of the limiting mechanisms in OFETs performance is an increase in p−type doping due to a charge−transfer reac− tion with ambient atmosphere. An increase in doping leads to an increase in the conductivity of the bulk semiconductor, which reduces the current modulation, or on−off ratio, of the transistor. The polymers P3HT and PTV, exhibit a dopant density increase upon air exposure in the dark, which re− quires a reevaluation of the doping mechanism in terms of Fig. 7. Temperature dependence of conductivity in a disordered a charge−transfer reaction with oxygen under light exposure. hopping system at different doping concentrations (after Ref. 12). Chen et al. [32] have studied the impact of FeCl3 doping on polymer−based TFTs (PTFTs) performance. It has been ob− Figure 7 illustrates the temperature dependence of the served that field effect mobility of rr−P3HT can be increased conductivity for different doping concentrations [12]. The by two orders of magnitude from 7.2×10–4 cm2/Vs to Arrhenius−type temperature dependence, Eq. (10), can be 7.4×10–2 cm2/Vs by doping with FeCl . The results suggest 3 observed clearly in Fig. 7. It was concluded that at higher that FeCl doping may induce changes in the interface states 3 temperatures, almost all the carriers occupy the intrinsic between P3HT and Au. Since the interface states may vary states, therefore the dopants do not change the trap−free hop− from material to material, dopants other than FeCl3 may be needed to reduce the contact resistance with Au electrode. ping process. The doping process is quite efficient for ZnPc with dopant F4−TCNQ. Interesting results have been reported by Li et al. [12]. Their experimental and theoretical studies indicated that conductivity of the organic semiconductors strongly de− 8. Traps pends on doping and temperature. The temperature depend− ence of the carrier conductivity for different doping concen− Organic semiconductors often contain significant densities trations the research group illustrated experimentally in the of charge carrier traps, and carrier trapping at the organic graphical form. The Arrhenius−type temperature depen− semiconductor/gate insulator interface can be important in dence OFETs operation. The importance of trapping decreases æ -E ö with increasing charge carrier densities because traps are s µ expç A ÷ , (10) filled and thus become inactive. When the carrier densities è ø kTB are small the devices are particularly sensitive to carrier trapping. Some of the injected electrons and holes occupy could be observed clearly in the presented figure by the localized trap states and are not mobile. This effect is mo− group. In the figure (we do not show these figures) they delled by treating the carrier mobilities as density depen− plotted the graph dent, with low carrier densities implying low mobilities. s -2 ln vsT , (11) Goldmann et al. [35] have investigated charge trapping which was observed to deviate slightly from a straight line. in rubrene single−crystal OFETs with two different typical It was proved that the conductivity increases with both the and frequently employed interfaces, an untreated SiO2 di− temperature and the doping ratio. Assuming a simple electric/semiconductor interface and a chemically treated Arrhenius law Eq. (10) it was obtained the relation between SiO2/semiconductor interface. A reversible and reproduc− ible shift of the I−V characteristics is observed upon both the activation energy EA and doping ratio. It was found that, EA decreases with the doping ratio, indicating that less and negative and positive gate bias stress. The physical origin of less energy will be required for a carrier activated jump to these results is identified as charge trapping and detrapping neighbouring sites when the doping ratio increases. at the crystal/SiO2 insulator interface on a time scale of 1 h. The superlinear dependence of conductivity on the dop− Figure 8 presents energy distribution of the DOS at an ing concentration has been investigated extensively by sev− organic single−crystal/dielectric interface [35]. s µ g eral groups [33,34], where the empirical formula N D is Et denotes the demarcation energy between the occupied used to describe this dependence. Using the described above and the unoccupied traps. Part of the unoccupied traps be− model, such superlinear increase in the conductivity upon tween EF and Et will capture the charge carriers relatively doping has been predicted successfully. The model gives quickly, i.e., on a time comparable to the OFET measure− g = 4.9 for T = 250 K, and g = 3.9 for T = 200 K. ment time 10 s to 1 min, and will appear as hysteresis in the

128 Opto−Electron. Rev., 18, no. 2, 2010 © 2010 SEP, Warsaw transport occurs by hopping of charges between localized states. A main difference between the delocalized and loca− lized transport is that, in the former, the transport is limited by phonon scattering, whereas in the latter, it is phonon as− sisted. Accordingly, the charge mobility decreases with temperature in conventional semiconductors, the reverse be− ing true in most organic materials. Several models have been developed to rationalize the hopping transport. In most cases, the temperature dependence of the mobility follows m = m 1/a a a law of the form 0exp[–(T0/T) ], where is the inte− ger ranging from 1 to 4. The boundary between the localized and delocalized processes is usually taken at the mobility between 0.1 and 1 cm2/Vs. The mobility in highly ordered molecular crystals was found to be close to that limit.

9.2. Small polaron model

Fig. 8. Sketch of trap density at an organic molecular crystal/dielec− Localization in conjugated organic materials occurs via the tric interface (after Ref. 35). Due to long−term relaxation processes, formation of polarons. A polaron results from the deforma− the demarcation energy El that separates occupied from unoccupied tion of the conjugated chain under the action of the charge. traps is not equal to the Fermi energy EF. In other words, in a conjugated molecule, a charge is self− −trapped by the deformation it induces in the chain. This I−V characteristics. Other traps will be filled and emptied mechanism of self−trapping is often described through the considerably more slowly. creation of localized states in the gap between the valence Balakrishnan et al. [36] have developed a model to cal− and the conduction bands. The existence of such levels in culate the current−voltage characteristics of OTFTs includ− doped conjugated polymers and oligomers has indeed been ing the effect of field dependent trap occupancy. The model identified by UV−visible spectroscopy. A useful model to takes into account the influence of high electric field on the describe the charge transport in organic materials is that of trap occupancy. Relevant equations have been solved nu− the small polaron, developed by Holstein [37]. It is a one−di− merically using an iteration method. mensional, one−electron model. An important parameter is the polaron binding energy EB, which is defined as the en− ergy gain of an infinitely slow carrier due to the polarization 9. Mobility of charge carriers in organic and deformation of the lattice. In Holstein’s model,EB = materials 2 2Mw 2 A /( o ), where M is the reduced mass of each molecular w The low charge carrier mobility of organic semiconductors site, 0 is the unique frequency of the harmonic oscillators compared to silicon is a significant problem for their practi− vibration. The mobility of small polaron is calculated by cal use. In order to improve their performance, it is impor− solving the time−dependent Schroedinger equation. Its tant to clarify various working mechanisms of OTFTs, e.g., high−temperature limit is given by Eq. (12). carrier injection by electric fields and the transport process. 2p 32qa 2 J 2 æ -E ö The essential physical point in the voltage dependence of m = expç B ÷ . (12) 32 è 2kTø the mobility for these structures is that for voltages where hEkT2 BB() B the carrier density is small the carriers are trapped and the mobility is low, whereas at voltages where the carrier den− It is worth pointing out that the term qa2/h has the di− sity is large the trapping sites are saturated and the mobility mension of the mobility, and is close to 1 cm2/Vs in most is higher. molecular crystals, a is the parameter of distance dimension.

9.1. Hopping model 9.3. Field-dependent mobility In metals and conventional inorganic single crystals semi− A general feature of charge transport in organic materials is conductors, charge transport occurs in delocalized states, that the mobility becomes field dependent at high electric and is limited by the scattering the carriers, mainly on pho− field (namely, at fields in excess of ~105 V/cm). This phe− nons, that is, thermally induced lattice deformations. Such nomenon occurs through a Poole−Frenkel mechanism [38], a model is no longer valid in low conductivity materials in which the coulombic potential near the localized levels is such as organic semiconductors, where a simple estimate modified by the applied field in such a way as to increase shows that the mean free path of carriers would become the tunnel transfer rate between sites. The general depend− lower than the mean atomic distance. In these materials, ence of the mobility is given by Eq. (13).

Opto−Electron. Rev., 18, no. 2, 2010 M.J. Małachowski 129 Organic field−effect transistors

æ b ö D mm= ç F ÷ where is the zero−field hopping barrier or low−field activa− 0 exp . (13) m è kTB ø tion energy and i is the intrinsic mobility at zero hopping barrier. m b pee 1/2 Here, 0 is the mobility at zero field, = (q/ 0) the The critical field F0 making the hopping barrier effec− Poole−Frenkel factor, and F is the magnitude of the electric tively zero is field. æ Dö2 Of particular interest is the dependence of mobility on = F0 ç ÷ . (15) the longitudinal (source−drain) electric field, commonly re− è b ø ferred to as the Poole−Frenkel (PF)−like effect. A variety of models have been proposed to account for this field−de− For long channel, mobility significantly decreases with pendent mobility, a comparison of the analysis in terms of lowering temperature Eq. (13), while for short channel, mo− V these different models can be found in Ref. 39. Although the bility at high DS does not change much since the longitudi− nal fields imposed on the channel are approaching the criti− observation of a field−dependent mobility is expected in cal field F to make the hopping barrier effectively zero. OFETs, reports of such behaviour in the literature are not 0 Bässler’s model also predicts a field dependence of mobi− common. lity similar to the Poole−Frenkel model. Empirically, it is so− The performance of pentacene−based OFETs has been metimes found that both the Bässler and Holstein models fit steadily improved as a consequence of achieving field−effect the m(T) data well for T in the range from RT to RT–100 K. mobilities and on/off current ratios as high as 0.5–1 cm2/Vs and 106–108, respectively [1–8]. These values are compara− ble to those of amorphous silicon thin−film transistors. 9.5. Experimental results connected with carrier mobility 9.4. Mobility models used most often Hamadani et al. [20] have reported the observation of a pro− nounced field dependence of the longitudinal mobility in Charge−carrier transport in almost all organic semiconduc− tors, polymeric or low molecular weight, amorphous or OFETs based on poly(2,5−bis(3−tetradecylthiophene−2−yl) (semi)crystalline, has one feature in common, carrier mobil− thieno [3,2− b]thiophene) (PBTTT−C14) as the active poly− ity will generally increase with increasing temperature, at mer layer. The author’s results suggest that the presence of least for temperatures not too far below room temperature disorder at the grain boundaries can pose a problem in (RT). In a few exceptional cases, notably pentacene, a low charge transport in organic field−effect devices. temperature mobility minimum with subsequent increase Guo et al. [26] have fabricated pentacene OTFTs by the toward an even lower temperature is observed, but for prac− organic molecular beam deposition (OMBD) method. It has tical OFETs, this is not relevant. The field mobility m can be been found that the drain current and the mobility increased with film thickness and always achieved a saturation stage extracted from the well known relation for the saturated after a critical thickness of around 25–35 nm. Therefore drain current Eq. (6), by plotting and extracting the slope of a thickness of 40 nm has been used for all the devices, so I as a function of the gate voltage V . Dsat G that the thickness dependent effect has been excluded and The physical effect of longitudinal electrical field is to a stable performance was guaranteed. The structure im− effectively reduce the hopping barrier. From this concise provement indicated that annealing at 45°C caused impro− concept and the reasonable assumption of Coulomb poten− ved mobility. tial type for hopping barrier, the hopping probability, and Wang et al. [19] reported the experimental study of the therefore the mobility, will demonstrate a dependence on electric−field−dependent charge transport mechanisms in po− electrical field which follows a Frenkel−Poole relationship. lycrystalline pentacene thin−film field−effect transistors with Field dependence of mobility becomes more severe at pentacene layer average grain size about 100 nm, smaller lower temperature, as Frenkel−Poole’s law predicts. When than the channel lengths investigated in that article. Their temperature decreases, the field dependence of mobility be− measurements were connected with the temperature and comes stronger as indicated in Eq. (13). It is interesting and electric−field dependences of the mobility in organic thin− important to notice that at the field sufficiently high (in Ref. −film transistors with scaled device geometry when carrier 40 about 7.3×105 V/cm), mobilities at all temperatures fall densities are at levels of practical importance. By combining onto the same value (about 0.15 cm2/Vs in Ref. 40), corre− field and temperature dependence studies, a physical picture sponding to a zero hopping barrier at such a high field. This of charge transport in OFETs was drawn, which was consis− is also predicted by Frenkel− Poole’s model. Recalling that tent over a range of scaled channel lengths. The true behav− m iour of field−dependent mobility was extracted by minimiz− zero−field mobility 0 in Eq. (13) can be expressed as ing contact effects consistently over a range of channel æ D ö lengths. In their partially ordered systems, experimental mm=-expç ÷ , (14) data suggest that the charge transport mechanism is ther− 0 i è ø kTB mally activated and field−assisted hopping transport and the

130 Opto−Electron. Rev., 18, no. 2, 2010 © 2010 SEP, Warsaw hopping transport between disorder−induced localized states and makes the analysis of electrical measurements a com− dominate over the intrinsic polaronic hopping transport seen plex task because contact effects need to be disentangled in organic single crystals. The experimental results were from transistor properties. found to exhibit a Frenkel−Poole−type dependence con− The method usually applied in inorganic semiconduc− sistently over a wide range of channel lengths, fields, and tors of enhancing the current injection by means of heavy temperatures. local doping of the semiconductor interfacial regions to pro− In disordered organic semiconductor system, charge vide a tunnelling contact, is not viable. Therefore one has to transport occurs mainly by hopping between adjacent or rely on proper alignment of metal Fermi level with HOMO nearby localized states which are induced by disorder. (LUMO) level in order to achieve an Ohmic contact for holes (electrons). This poses some restrictions because the 10. Threshold voltage work function range of easily processible metals is some− what limited and also surface dipoles can develop at the in− In addition to the charge carrier mobility, the threshold volt− terface modifying the expected energy level alignment. age VT is an important parameter that needs to be controlled When examining transport properties of OFETs, it is im− to ensure proper operation of the circuits. VT can depend on portant to determine whether operation of the devices is the time a gate voltage has been applied (bias stress), on the dominated by the bulk (channel) or the contacts. Develop− exposure of the device to light, it can be shifted using ments in organic semiconductors and engineering have led a polarizable gate insulator or it depends on thickness of the to a situation where the performance of state−of−the−art semiconductor layer. Furthermore, a dependence on the OFETs often is limited by the resistance at the metal/semi− work function of the gate electrode has been reported. VT conductor contacts, rather than the semiconductor channel. voltage additionally has been found to depend strongly on In the case of non−Ohmic contacts, externally applied volta− the preparation of the surface on which the organic material ges partly drop on the channel and partly drop on the contact is deposited. Threshold voltage shifts in thin film as well as regions. Because of the voltage drops across the contact single−crystal OFETs have been found to be induced by di− resistances, the current magnitude diminishes and its func− pole monolayers on the gate insulator [41]. Thickness of the tional dependence on the externally applied gate and drain semiconductor layer influences the values of the threshold voltages is generally altered. As a consequence, it is not voltage and, to a lesser extent, the saturation current. The possible to determine real carrier mobility from OTFT cur− thickness−dependent part of the threshold voltage results rent measurements. It underestimates the real one and does from the presence of an injection barrier at the gold−penta− not reflect the real material properties. This effect is more cene contact. It has been demonstrated controllable shift of serious with OTFTs with shorter channels, since their the threshold voltage and the turn−on voltage in pentacene smaller resistivity makes the voltage drop on the contact TFTs and rubrene single crystal OFETs by the use of nine resistances more significant. organosilanes with different functional groups. Fukuda et al. [42] examined the role of cesium fluoride 11.1. Some recent experimental observations (CsF) layer inserted at the active layer/electrode interface. The layer induces a large threshold voltage shift, and reali− Zaumseil et al. [44] have presented a detailed study of the zes low−gate−voltage operation of n−type OFETs with [6,6]− electrical properties of OTFTs, with an emphasis on the na− −phenyl C61−butyric acid methyl ester as active layer. The ture of the laminated contacts with the p− and n−type semi− threshold voltage shift is mainly caused by the efficient conductors pentacene and copper hexadecafluorophthalo− electron injection due to the coexistence of CsF. cyanine, respectively. The results of the study demonstrated Fabrication of pentacene OTFTs with a dual−gate struc− that the resistances related to the laminated contacts and ture has been described [43], in which 300−nm−thick ther− their coupling to the transistor channel are considerably mally grown SiO2 and 500−nm−thick parylene were used as lower than those associated with conventional contacts a bottom−gate and a top−gate dielectric, respectively. The formed by evaporation of gold electrodes directly on top of threshold voltage VT of the dual−gate OTFT changed system− the organic semiconductors. atically with the application of voltage bias to the top−gate Majewski et al. [45] have showed that organic transistor electrode. This shift of VT due to the body effect allows the contacts can be improved by the most important industrial change from enhancement to a depletion−mode transistor, process for the deposition of noble metals, electroplating. which is beneficial for the fabrication of organic circuits. The group discussed the advantages of electroplating over vacuum−based techniques such as evaporating and sputter− 11. Contact and bulk resistance ing, in particular for the deposition of platinum (Pt). Chesterfield et al. [46] have reported on OTFTs based FETs require the Ohmic source and drain contacts for ideal on N,N8−dipentyl−3,4,9,10−perylene tetracarboxylic dimide operation. In many real situations, however, and specifically (PTCDI–C5) and specifically focused on characterizing the in OFETs, the injection of charge carriers from metals into contact resistance, conduction mechanism (by variable tem− semiconductors can be an inefficient non−Ohmic process. perature measurements), and threshold voltage instability. This has an adverse impact on the performance of OTFTs The contact resistance has been found to be of similar mag−

Opto−Electron. Rev., 18, no. 2, 2010 M.J. Małachowski 131 Organic field−effect transistors nitude for all three contact metals studied (Au, Ag, and Ca), 12.1. Some results of recent studies therefore the dominant contact resistance in these devices is Park et al. [49] reported the improvements in electrical not the Schottky barrier at the metal/PTCDI–C5 interface, but more likely an access resistance due to the staggered properties of OTFTs with a C60 buffer layer between penta− OTFT structure. To explain the observed effects, the model cene and Au electrodes. The mobility increased from 0.147 2 based on the formation of a metastable complex between to 0.364 cm /Vs for the device with a 1.5−nm−thick C60 layer. They also found that the improved hole injection PTCDI–C and oxygen has been applied, which creates 5 arose from the reduction in contact resistance owing to the a deep acceptor−like trap state. presence of the C60 buffer layer. In addition, the threshold Hamadani et al. [18] have reported OTFTs based on voltage decreased from –5 to almost 0 V. poly(2,5−bis(3−tetradecylthiophene−2−yl)thieno[3,2−b]thio− Park et al. [50] have examined the pentacene−based phene) (PBTTT−C ) as the active polymer layer. The au− 14 OTFTs and organic circuits fabricated using a plasma−en− thors have found that, at RT and below, the contact resis− hanced chemical vapour deposition (PECVD) SiO2/cross− tances were minimal and that contact resistances were negli− −linked poly(vinyl alcohol) (PVA) double−layer insulator. gible for the annealed device set for all the channel lengths A considerable reduction in hysteresis is achieved by opti− considered in that experiment. The as cast set analysis from mizing the double−layer insulator thickness. On the basis of RT down to around 100 K also revealed that contacts do not the proposed OTFTs, an electrically stable organic inverter adversely affect the transport. and a buffer working at a frequency as high as 1 kHz have been fabricated. 12. Stability of transistors and circuits An interesting solution have been reported concerning fabrication of the vertical−channel OTFTs with a channel Operation of OFET in an ambient atmosphere usually re− length smaller than 100 nm showing low the gate and drain sults in a significant increase in off−state conduction and, driving voltages (<10 V) and improved the saturation charac− to some extent, in a deficiency of the saturation behaviour teristics and reduced the leakage current [51]. It was found in the output current. It has been suspected that molecular that Fowler−Nordheim tunnelling is the dominant mechanism oxygen (O2) and/or water can influence the device opera− determining the ultra short−channel device behaviour. tion of these OFETs. Hoshino et al. [47] have investi− Sekitani and Someya [52] have achieved a significant gated an OFET based on a P3HT to determine the influ− improvement in the stability of OFETs under ambient con− ence of moisture on device characteristics and thus gain ditions by employing organic/metallic hybrid passivation a deep understanding of the mechanism underlying the layers. Pentacene FETs were fabricated on plastic films and susceptibility to air of the operation of OFETs of this encapsulated in organic/metallic hybrid passivation layers. kind. Analyses of the fundamental output characteristics, Ficker et al. [53] have presented results showing that which include effective field−effect modulation and satu− PTFTs are generally suitable for integrated circuits or plas− ration behaviour in the output current, have shown that tic chips. For the use in high humidity, an encapsulation is they remained almost the same for every current−voltage necessary. The output characteristic of two representative profile in a vacuum, N2 and O2. By contrast, operation in PTFTs, one measured directly after production, the other a N2 humidified with water caused enlarged off−state con− year later, exhibited a good saturation, with field−effect mo− duction and deterioration in the saturation behaviour, in bilities of 0.05 and 0.045 cm2/Vs, respectively. the same manner as that experienced with exposure to room air. The authors have concluded that atmospheric water had a greater effect on the susceptibility of the de− 13. Schottky contact vice operation to air than O2, whose p−type doping activ− Schottky non−Ohmic contacts have been extensively used in ity as regards P3HT resulted in a small increase in the organic semiconductor technology to build various elec− conductivity of the active layer and a slight decrease tronic devices including OLEDs and organic solar cells. in the field−effect mobility with exposure at ambient Also, organic Schottky diodes were used as rectifying ele− pressure. ments in prototype organic circuits. An interesting observation have been made by Ashimine OFETs with non−Ohmic contacts, e.g., pentacene with et al. [48] which suggest that the OTFTs stability may be gold electrodes, exhibit a linearly growing threshold volt− connected with the HOMO energy level height. They have age with increased film thickness due to the tunnel injection. found that organic semiconductor with the high HOMO level Schroeder et al. [54] have achieved OFET of improved per− shows a good stability and the one with the low HOMO en− formance with Schottky contacts. The group has applied ergy level shows a poor stability when stored in air. gold/pentacene contact and obtained low threshold voltage Finally, it should be mentioned a noticeable improve− independent of pentacene thickness. By doping the ment in stability in the high−mobility thin−film transistors pentacene in the contact area with FeCl3 (iron−III−chloride), with highly ordered semiconducting polymer films [16]. the metal−insulator−type tunneling barrier was changed to The further investigations in this direction seem to be highly a metal−semiconductor Schottky barrier. Since the injection promising. through a Schottky barrier depends on the potential and not

132 Opto−Electron. Rev., 18, no. 2, 2010 © 2010 SEP, Warsaw on the electric field, the threshold voltage appears to be no them we know at present memory and logic elements, LCD longer a function of the semiconductor thickness. panel driven OTFT and OFETs as chemical sensors. Although the capacitance measurement is a common Anthopoulos et al. [59] have shown that organic transistors method to obtain the depletion width in a Schottky contact, based on the solution processible methanofullerene [6,6]− the method is challenging in an organic Schottky junction −phenyl−C71−butyric acid methyl ester exhibit promising since the capacitance is a combination of the capacitances ambipolar charge transport characteristics. This structure associated with the trapped charges, bulk semiconductor, was found to allow the realization of logic circuits with and the depletion region. Takshi et al. [55] have proposed good noise immunity. a metal−semiconductor−FET structure to measure the deple− Pinto et al. [60] have shown that a spin coated rr−P3HT tion width in an organic Schottky contact. To study the de− single channel split gate OFET functioned as a dual input pletion width in an organic Schottky contact, rr−P3HT, a logic ‘AND’ gate with an order of magnitude higher mobil− well characterized and relatively stable p−type organic semi− ity. A significant advantage of this architecture is that conductor, has been chosen as the semiconductor. The de− ‘AND’ logic devices with multiple inputs can be fabricated pletion width is calculated from the drain current in the tran− using a single rr−P3HT channel with multiple gates. The re− sistor when a small VDS (–0.3 V) is applied. versible memory effect due to extrinsic trap states and the bias stress effect originate from totally different mecha− 14. Printing of circuits nisms and can be separated in data analysis [61]. Write−once−read−many−times (WORM) memory devices In order to facilitate the production process of OFETs or the have been demonstrated using a material system of polyethy− circuits with organic transistors, many printing technologies lene dioxythiophene:polystyrene sulphonic acid and a con− have been reported. Just to mention Knobloch et al. [56], jugated copolymer containing fluorene and a chelated euro− which have designed structural and electrical properties of pium complex. Some rewritable devices are characterized printed polymeric thin films and multilayers to set up PFETs. by embedding nanoparticles, while others are characterized A fully printed functional integrated circuit in the form of by the intrinsic memory effects of the materials. The former a seven−stage ring oscillator has been presented. The layers devices are nanoparticle−based organic memory units in have been based on solution processible polymers and filled which the organic materials themselves have no memory ef− polymers, which were processed by traditional graphic art fects. This flexibility provides, in a single class of material, printing techniques such as pad printing and blade coating. The the ability to achieve complementary logic and even devices circuit layout has been designed to match the characteristics of such as p−n diodes and more complex systems. Complemen− the applied printing and coating process, so that polymer tran− tary inverter based on interface doped pentacene has been sistors, inverters, and ring−oscillator circuits could be achieved. shown to be promising device, which has been demonstra− When applied to OTFT fabrication, all or parts of the transis− ted in Ref. 24. tor’s constituents, i.e., electrodes, gate dielectrics, and Hur at al. [62] have studied p−channel, n−channel, and semiconductors, can be solution processible. ambipolar SWNT FETs with electrodes defined by a Kim et al. [57] have demonstrated OTFTs based on the high−resolution printing process. Complementary logic ink−jet printed electrodes in which a reduced channel length gates with these types of devices illustrate their suitability was accomplished by laser ablation. The carrier mobility for complex circuits. values of the fabricated OTFTs with channel lengths of 10, The successfully operating ChemOFET devices with 20, and 50 μm were 4.8×10–3,4.6×10–3, and 3.4×10–3 cm2/Vs sensitive, for some vapours, the organic films as the channel in the saturation regime, respectively. These mobility values materials have been demonstrated [63]. ChemOFETs with were comparable to those of the device involving the ink−jet channels of small organic molecules or conducting poly− printed silver electrodes with much larger channel length of mers show high sensitivity and selectivity to a range of va− 100 μm [1.3×10–3 cm2/Vs]. pours. The details can be found in the review paper [64]. Hines et al. [58] have developed a method of transfer printing for fabricating organic electronics onto flexible 16. Conclusions substrates. The process has been demonstrated for a model system consisting of a pentacene TFT on a polyethylene A considerable amount of research has been recently carried terephthalate (PET) substrate with Au gate and S/D elec− out to help understanding the influence of structural and trodes separated by a PMMA dielectric layer. The result al− electrical properties of the materials used to construct lows the device components to be sequentially assembled OFETs on their performance. The efforts have been under− onto the plastic substrate via transfer printing. taken to achieve high−performance especially n−channel OFETs with large electron mobility, low threshold voltage, 15. Some applications of OFETs low subthreshold slope, and large on/off current ratio. A re− markable amount of papers have been devoted to study dif− In spite of research upon the OFETs are still in its develop− ferent dielectric materials that can play the role of the gate ment and improvement stage there appeared some interest− insulators in OTFFETs. Many groups demonstrated the im− ing applications of these devices. Just to mention three of portant functionalities of organic gate dielectrics and their

Opto−Electron. Rev., 18, no. 2, 2010 M.J. Małachowski 133 Organic field−effect transistors effect on the output performance of OFETs. For instance the 7. J. Żmija, M.J. Małachowski, M. Wacławek, and K. Ścieżka, polymer dielectric that was directly attached to the penta− “Engineering of organic materials in electronics”, Chemia. cene layer governed the transport and accumulation of Dydaktyka. Ekologia. Metrologia XI, No. 3/4, 15-30 (2006). charge carriers. The mechanisms by which PVA facilitates (in Polish) the formation of the n channel in the pentacene active layer 8. J. Żmija, M.J. Małachowski, and J. Zieliński, “Progress in th biased in the accumulation at the positive V regime are not application of organic semiconductors in displays”, 5 Sci− G entific Symposium, Proc. SPIE 82 7207−664−2, 41–59 completely established as yet. There has been shown very (2006). (in Polish) clearly the beneficial role which plays high dielectric con− 9. N.F. Mott, Introductory talk; Conduction in non−crystalline stant Ta2O5 as a gate insulator in OFETs. Significant prog− materials, Cavendish Laboratory, Cambridge, available on− ress has been made to reduce the defect density in organic line 13 May (2003). semiconductor devices and thus, to improve their electrical 10. C. Godet, “Variable range hopping revisited: the case of an performance. exponential distribution of localized states”, J. Non−Cryst. Despite decades of researching progress, some rather Solids 299−302, 333–338 (2002). ubiquitous features of the charge transport in organic semi− 11. M. Pope, “Electronic processes in organic solids”, Annu. conductors are still far from being well understood. One Rev. Phys. Chem. 35, 613–655 (1984). such example is the relation between conductivity and dop− 12. L. Li, G. Meller, and H. Kosina, “Analytical conductivity model for doped organic semiconductors”, J. Appl. Phys. ing. A general doping model still remains a challenge for or− 101, 033716−4 (2007). ganic semiconductors. For instance, it is not clear whether 13. M.J. Małachowski, “HgSe thin−film transistors”, Phys. Stat. the dopants are homogeneously distributed in the material. Sol. 14, K35–K37 (1966). In particular, it is beneficial to produce an electrical conduc− 14. S. Scheinert and G. Paasch, “Interdependence of contact tor by a direct writing technique, such as ink−jet printing, be− properties and field− and density−dependent mobility in or− cause such a technique would allow for fully data−driven de− ganic field−effect transistors”, J. Appl. Phys. 105, 014509 position of expensive materials. However, in order to con− (2009). duct research on physical effects occurring in organic semi− 15. T. Lindner, G. Paasch, and S. Scheinert, “Operation and conductors, most recent works involving OTFTs have typi− properties of ambipolar organic heterostructure field−effect cally employed the selective deposition of metallic conduc− transistors”, J. Appl. Phys. 101, 014502 (2007). tors through a shadow mask or vacuum deposition followed 16. T. Umeda, S. Tokito, and D. Kumaki, “High−mobility and by photolithographic patterning, an approach that is both air−stable organic thin−film transistors with highly ordered semiconducting polymer films”, J. Appl. Phys. 101, 054517 time consuming and costly. Hence, it is highly desirable to (2007). achieve high−performance especially n−channel OFETs that 17. G. Paasch and S. Scheinert, „Space charge layers in organic can be processed at room temperature using standard field−effect transistors with Gaussian or exponential semi− physical vapour deposition. conductor density of states”, J. Appl. Phys., 101, 024514 Light emission from an OFET requires ambipolar trans− (2007). port, as well as efficient radiative decay. Recent experi− 18. B.H. Hamadani and D. Natelson, “Gated nonlinear transport ments have demonstrated ambipolar channel conduction in organic polymer field effect transistors”, J. Appl. Phys. 95, and light emission in conjugated polymer FETs. These 1227 (2004) achievements make realistic fabrication of suitable in com− 19. L. Wang, L.D. Fine, D. Basu, and A. Dodabalapur, “Elec− mercial applications the complex active matrix with organic tric−field−dependent charge transport in organic thin−film light−emitting diodes (AMOLEDs). transistors”, J. Appl. Phys. 101, 054515 (2007). 20. B.H. Hamadani, C.A. Richter, D.J. Gundlach, R.J. Kline, I. McCulloch, and M. Heeney, “Influence of source−drain elec− tric field on mobility and charge transport in organic field−ef− References fect transistors”, J. Appl. Phys. 102, 044503 (2007). 21. Y. Inoue, Sh. Tokito, K. Ito, and T. Suzuki, “Organic thin− 1. G. Horowitz, “Organic field−effect transistors”, Adv. Mater. −film transistors based on anthracene oligomers”, J. Appl. 10, 365–377 (1998). Phys. 95, 5795–5799 (2004). 2. G. Guillaud, J. Simon, and J.P. Germain, “Metallophthalo− 22. A. Dodabalapur, H.E. Katz, L. Torsi, and R.C. Haddon, “Or− cyanines gas sensors, resistors and field effect transistors”, ganic heterostructure field−effect transistors”, Science 269, Coordin. Chem. Rev. 178–180, 1433–1484 (1998). 1560–1562 (1995). 3. A.R. Brown, C.P. Jarrett, D.M. de Leeuw, and M. Matters, 23. L.L. Chua, J. Zaumseil, J.F. Chang, E.C.W. Ou, P.K.H. Ho, “Field effect transistors made from solution −processed or− H. Sirringhaus, and R.H. Friend, “General observation of ganic transistors”, Synthetic Met. 88, 37–55 (1997). n−type field−effect behavior in organic semiconductors”, Na− 4. A. Facchetti, “Semiconductors for organic transistors”, Ma− ture (London) 434, 194 (2005). ter. Today 10, 28–37 (2007). 24. T.B. Singh, P. Senkarabacak, N.S. Sariciftci, A. Tanda, C. 5. A. Salleo, “Charge transport in polymeric transistors”, Ma− Lackner, R. Hagelauer, and G. Horowitz, “Organic inverter ter. Today 10, 38–45 (2007). circuits employing ambipolar pentacene field−effect transis− 6. Y.D. Park, J.A. Lim, H.S. Lee, and K. Cho, “Interface engi− tors”, Appl. Phys. Lett. 89, 033512−4 (2006). neering in organic transistors”, Mater. Today 10, 46–54 25. N. Benson, A. Gassmann, E. Mankel, T. Mayer, C. Melzer, (2007). R. Schmechel, and H. von Seggern, “The role of Ca traces in

134 Opto−Electron. Rev., 18, no. 2, 2010 © 2010 SEP, Warsaw the passivation of silicon dioxide dielectrics for electron “Threshold voltage shift in organic field effect transistors by transport in pentacene organic field effect transistors”, J. dipole monolayers on the gate insulator”, J. Appl. Phys. 96, Appl. Phys. 104, 054505−10 (2008). 6431–6438 (2004). 26. T.F. Guo, Z.J. Tsai, S.Y. Chen, T.C. Wen, and C.T. Chung, 42. S. Fukuda, H. Kajii, H. Okuya, T. Ogata, M. Takahashi1, and “Influence of polymer gate −dielectrics on n−channel conduc− Y. Ohmori, “Investigation of interfaces between insulator tion of pentacene−based organic field−effect transistors”, J. and active layer, and between active layer and electrodes in Appl. Phys. 101, 124505−11 (2007). n−type organic field−effect transistors”, Jpn. J. Appl. Phys. 27. P. Miskiewicz, S. Kotarba, J. Jung, T. Marszalek, M. 47, 1307–1310 (2008). Mas−Torrent, E. Gomar−Nadal, D. B. Amabilino, C. Rovira, 43. J.B. Koo, K.S. Suh, I.K. You, and S.H. Kim, “Device charac− J. Veciana, W. Maniukiewicz, and J. Ulanski, “Influence of teristics of pentacene dual−gate organic thin−film transistor”, SiO2 surface energy on the performance of organic field ef− Jpn. J. Appl. Phys. 46, 5062–5066 (2007). fect transistors based on highly oriented, zone−cast layers of 44. J. Zaumseil, K.W. Baldwin, and J.A. Rogers, “Contact resis− a tetrathiafulvalene derivative”, J. Appl. Phys. 104, tance in organic transistors that use source and drain elec− 054509−16 (2008). trodes formed by soft contact lamination”, J. Appl. Phys. 93, 28. L.A. Majewski, R. Schroeder, M. Grell, P.A. Glarvey, and 6117 (2003). M.L. Turner, “High capacitance organic field−effect transis− 45. L.A. Majewski, R. Schroeder and M. Grell, “Organic field− tors with modified gate insulator surface”, J. Appl. Phys. 96, −effect transistors with electroplated platinum contacts”, 5781–5787 (2004). Appl. Phys. Lett. 85, 3620–3622 (2004). 29. K. Ueno, Sh. Abe, R. Onoki, and K. Saiki, “Anodization of 46. R.J. Chesterfield, J.C. McKeen, C.R. Newman, C.D. Frisbie, electrolytically polished Ta surfaces for enhancement of car− P.C. Ewbank, K.R. Mann, and L.L. Miller, “Variable tem− rier injection into organic field−effect transistors”, J. Appl. perature film and contact resistance measurements on operat− Phys. 98, 114503–114511 (2005). ing n−channel organic thin film transistors”, J. Appl. Phys. 30. R. Parashkov, E. Becker, G. Ginev, T. Riedl, H.H. Johannes, 95, 6396–6405 (2004). and W. Kowalsky, “All−organic thin−film transistors made of 47. S. Hoshino, M. Yoshida, S. Uemura, T. Kodzasa, N. Takada, poly(3−butylthiophene) semiconducting and various polymeric T. Kamata, and K. Yase, “Influence of moisture on device insulating layers”, J. Appl. Phys. 95, 1594–1596 (2004). characteristics of polythiophene−based field−effect transis− 31. L.A. Majewski, R. Schroeder, and M. Grell, “Organic field− tors”, J. Appl. Phys. 95, 5088–5093 (2004). −effect transistors with ultrathin gate insulator”, Synth. Met. 48. T. Ahimine, T. Yasuda, M. Saito, H. Nakamura, and T. 144, 97–100 (2004). Tsutsui, “Air stability of p−channel organic field−effect tran− 32. Y. Chen, I. Shih, and S. Xiao, “Effects of FeCl3 doping on sistors based on oligo−p−phenylenevinylene derivatives”, polymer−based thin film transistors”, J. Appl. Phys. 96, Jpn. J. Appl. Phys. 47, 1760–1762 (2008). 454–458 (2004). 49. J. Park, J. Park, N. Kim, H.J. Lee, and M. Yi, “Performance 33. Semiconducting Polymers−Chemistry: Physics, Engineer− enhancement of organic thin−film transistors with C60/Au ing, edited by G. Hodziioannou and P.F. van Hotten, bilayer electrode”, Jpn. J. Appl. Phys. 47, 5668–5671 (2008). Wiley−VCH Verlag GmbH, 2000. 50. D.W. Park, C.A. Lee, K.D. Jung, B.J. Kim, B.G. Park, H. 34. D.J. Gundlach, K.P. Pernstich, G. Wilckens, M. Grüter, S. Shin, and J.D. Lee “Electrically stable organic thin−film tran− Haas, and B. Batlogg, “High mobility n−channel organic sistors and circuits using organic/inorganicdouble−layer in− thin−film transistors and complementary inverters”, J. Appl. sulator”, Jpn. J. Appl. Phys. 46, 2640–2644 (2007). Phys. 98, 064502 (2005). 51. H.W. Zan and K.H. Yen, “Vertical−channel organic thin−film 35. C. Goldmann, C. Krellner, K.P. Pernstich, S. Haas, D.J. transistors with meshed electrode and low leakage current”, Gundlach, and B. Batlogg, “Determination of the interface Jpn. J. Appl. Phys. 46, 3315–3318 (2007). trap density of rubrene single−crystal field−effect transistors 52. T. Sekitani and T. Someya, “Air−stable operation of organic and comparison to the bulk trap density”, J. Appl. Phys. 99, field−effect transistors on plastic films using organic/metallic 034507 (2006). hybrid passivation layers”, Jpn. J. Appl. Phys. 46, 4300– 36. R.V.R. Balakrishnan, A.K. Kapoor, V. Kumar, S.C. Jain, and 4306 (2007). R. Mertens, “Effect of field dependent trap occupancy on or− 53. J. Ficker, A. Ullmann, W. Fix, H. Rost, and W. Clemens, ganic thin film transistor characteristics”, J. Appl. Phys. 94, “Stability of polythiophene−based transistors and circuits”, J. 6302–6306 (2003). Appl. Phys. 94, 2638–2641 (2003). 37. T. Holstein, “Studies of polaron motion: part II. The small 54. R. Schroeder, L.A. Majewski, and M. Grell, “Improving or− polaron” Ann. Phys. 8, 343–389 (1959). ganic transistor performance with Schottky contacts”, Appl. 38. J. Frenkel, “On breakdown phenomena of insulators and Phys. Lett. 84, 1004–1006 (2004). electronic semiconductors” Phys. Rev. 54, 647–648 (1938). 55. A. Takshi, A. Dimopoulos, and J.D. Madden, “Depletion 39. S. Scheinert and G. Paasch, “Interdependence of contact width measurement in an organic Schottky contact using a properties and field− and density−dependent mobility in or− metal−semiconductor field−effect transistor”, Appl. Phys. ganic field−effect transistors”, J. Appl. Phys. 105, 014509 Lett. 91, 083513 (2007). (2009). 56. A. Knobloch, A. Manuelli, A. Bernds, and W. Clemens, 40. T. Minari, T. Nemoto, and S. Isoda, “Temperature and elec− “Fully printed integrated circuits from solution processable tric−field dependence of the mobility, of a single−grain penta− polymers”, J. Appl. Phys. 96, 2286–2291 (2004). cene field−effect transistor”, J. Appl. Phys. 99, 034506 57. D. Kim, S. Jeong, J. Moon, S. Han, and J. Chung, “Organic (2006). thin film transistors with ink−jet printed metal nanoparticle 41. K.P. Pernstich, S. Haas, D. Oberhoff, C. Goldmann, D.J. electrodes of a reduced channel length by laser ablation”, Gundlach, B. Batlogg, A.N. Rashid, and G. Schitter, Appl. Phys. Lett. 91, 071114 (2007).

Opto−Electron. Rev., 18, no. 2, 2010 M.J. Małachowski 135 Organic field−effect transistors

58. D.R. Hines, V.W. Ballarotto, E.D. Williams, Y. Shao, and 62. S.H. Hur, C. Kocabas, A. Gaur, O. Ok Park, M. Shima, and S.A. Solin, “Transfer printing methods for the fabrication of J.A. Rogers, “Printed thin−film transistors and complemen− flexible organic electronics”, J. Appl. Phys. 101, 024503 tary logic gates that use polymer−coated single−walled car− (2007). bon nanotube networks”, J. Appl. Phys. 98, 114302 (2005). 59. T.D. Anthopoulos, D.M. de Leeuw, E. Cantatore, P. van’t 63. R.D. Yang, J. Park, C.N. Colesniuc, I.K. Schuller, W.C. Hof, J. Alma, and J.C. Hummelen, “Solution processible or− Trogler, and A.C. Kummel, “Ultralow drift in organic thin− ganic transistors and circuits based on a C70 methanofullere− −film transistor chemical sensors by pulsed gating”, J. Appl. ne”, J. Appl. Phys. 98, 054503 (2005). Phys. 102, 034515 (2007). 60. N.J. Pinto, R. Pérez, C.H. Mueller, N. Theofylaktos, and F.A. 64. G. Guillaud, J. Simon, and J.P. Germain, “Metallophthalo− Miranda, “Dual input AND gate fabricated from a single cyanines gas sensors, resistors and field effect transistors”, channel poly 3−hexylthiophene thin film field effect transis− Coordin. Chem. Rev. 178/180, 1433–1484 (1998). tor”, J. Appl. Phys. 99, 084504 (2006). 61. G. Gu, M.G. Kane, and S.C. Mau, “Reversible memory ef− fects and acceptor states in pentacene−based organic thin− −film transistors”, J. Appl. Phys. 101, 014504 (2007).

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