Organic transistors in the new decade: Toward n-channel, printed, and stabilized devices

Authors

  • Srinivas Kola,

    1. Department of Materials Science and Engineering, Johns Hopkins University, 206 Maryland Hall, 3400 North Charles Street, Baltimore, Maryland 21218
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    • Srinivas Kola and Jasmine Sinha contributed equally to this work.

  • Jasmine Sinha,

    1. Department of Materials Science and Engineering, Johns Hopkins University, 206 Maryland Hall, 3400 North Charles Street, Baltimore, Maryland 21218
    Search for more papers by this author
    • Srinivas Kola and Jasmine Sinha contributed equally to this work.

  • Howard E. Katz

    Corresponding author
    1. Department of Materials Science and Engineering, Johns Hopkins University, 206 Maryland Hall, 3400 North Charles Street, Baltimore, Maryland 21218
    2. Department of Chemistry, Johns Hopkins University, 206 Maryland Hall, 3400 North Charles Street, Baltimore, Maryland 21218
    • Department of Materials Science and Engineering, Johns Hopkins University, 206 Maryland Hall, 3400 North Charles Street, Baltimore, Maryland 21218
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Abstract

Significant progress has been made in designing organic semiconducting materials (OSCs) for the past few decades for organic field-effect transistors (OFETs). Much attention has been paid to the development of p-channel OSCs, with less but highly significant progress on n-channel OSCs. In this review, we focus on the advances made with OFETs in the last few years to achieve high performance in n-channel modes, air stability, and solution processability, leading to printable active electronics. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012

INTRODUCTION

Organic semiconductors (OSCs) have emerged as a subject with intensive academic and commercial interest over the past few decades. Organic field-effect transistors (OFETs) stand out as a key elements of organic integrated circuits for flexible smart cards, low-cost radio frequency identification (RFID) tags, and organic active matrix displays. The wide variety of semiconducting small molecules or polymers for OFETs results in low cost, low-power consumption organic complementary circuits (organic-CMOS) with sufficient operating speed.1–22 Despite significant progress in the p-channel OFETs, development of n-channel OFETs, while also considerable, remains more challenging, with fewer reported n-OSCs that are sufficiently easy to synthesize, soluble, and stable under ambient conditions.

OFETs essentially consist of conductors, which includes, source, drain, and gate electrodes, an insulator that is the gate dielectric, and an OSC as an active element. The basics of OFETs have been reported in many reviews.23–27 Among the most investigated organic materials for n-channel use are those based on naphthalene and perylene tetracarboxylic diimides whose performance under ambient condition is comparable with that of p-channel semiconductors.28 Developing new high-mobility polymeric semiconductors with good processability and excellent device environmental stability is essential for organic electronics. Apart from this, the electron trapping has been an issue resulting in air-unstable materials, hence researchers have made an attempt to design air-stable small molecules or polymers or introducing dopants into the primary system resulting in OFETs which would be operational at ambient condition. Also, compounds with LUMO energy level less than −4.0 eV have been recommended as a possible and promising material for air-stable n-type OFET materials but designing such molecules is still a challenge.29–34 In fact, ambipolar device architectures or materials have been demonstrated as interesting model, as they would provide both n- and p-channel performance enabling complementary inverters. Blending with n- and p-channel materials would also result in ambipolar transport in a single layer.22

Further, many attempts have been made to obtain soluble polymeric semiconducting material because they could be readily processed and easily printable, removing the conventional photolithography for patterning.35, 36

Many review articles have been published recently that cover developments in the OFET field.37, 38 This review article focuses on the recent developments in ambipolar and then n-channel OFETs. Then, the discussion is directed to means of stabilizing OFETs, including additives being used as a blending system or dopant. Finally, polymeric gate dielectrics, solution processable materials and applications of the same to inkjet printing are discussed.

n-CHANNEL ACTIVITY FROM p-CHANNEL POLYMERS: AMBIPOLAR POLYMERIC ORGANIC SEMICONDUCTORS

Ambipolar organic semiconductors have gained much attention because of their potential use in CMOS-like digital integrated circuits and ambipolar light-emitting field-effect transistors (LFETs).39, 40 In particular, there is interest in the application of ambipolar polymer FETs, because of the hole and electron transport within the same material, which can simplify the circuit design and offers processing simplicity and flexibility to fabricate low cost, flexible, and portable devices.41, 42 Most organic semiconductors are known to be unipolar, but their intrinsic ambipolar nature can be realized under suitable biasing conditions, device configurations and by understanding the crucial role played by the gate dielectric.43 Although ambipolar OFETs can be achieved by the use of blending systems as an active layer in the channel, or of bilayers separately consisting of hole transporting and electron transporting materials, the achievement of the ambipolarity from a single-component material would be the most ideal strategy.44

One of the best examples of ambipolar charge transport is poly(9,9-di-n-octylfluorene-alt-benzothiadiazole (P1), which has OFET mobilities around 0.5–1 × 10−2 cm2/(Vs) for holes and electrons.45 Recently, Sirringhaus et al. systematically controlled the electron and hole contact resistance of P1 in an ambipolar OFET, by using 1-decanethiol (1DT) as self-assembled monolayers (SAMs), and very recently, a US-Korea collaboration reported controlled carrier injection into ambipolar polymers by applying inorganic charge injection layers.46 Sirringhaus observed a thin P1 film on 1DT-treated gold electrodes and O2 plasma treated gold electrodes and improved charge injection properties for the former. In this work they also observed a slight shift in the ring stretching frequency of the polymer backbone in the Raman spectra of P1 films on top of O2 plasma-treated gold and SAM-treated gold electrode. This could be due to the SAM-induced changes of the bulk molecular packing and orientation of the semiconducting polymer (Scheme 1).

Scheme 1.

The same group has also studied the ambipolar charge transport in a regioregular poly(3,3″-di-n-decylselenophene) (P2) and poly(3-octyl)selenophene (P3) by using top-gate, bottom contact (TGBC) configuration with gold electrodes.39 P2-based OFETs showed high electron and hole mobilities up to 0.03 cm2/(Vs). Another selenophene-based polymer, poly(3-decyl-2,5-selenylenevinylene) (P4) exhibited balanced hole and electron mobilities of 1 × 10−4 cm2/(Vs).47 The observation of electron mobility and electron transport in these polyselenophenes is due to their smaller band gaps and lower LUMO levels. By using P2, Sirringhaus et al. verified that the electrons that remain mobile and contribute to FET current have a localized wave function limited to single polymer chain when compared with holes (localized over several polymer chains) as evidenced by the charge modulation spectroscopy (CMS). This is useful to understand the effect of deep electron traps on transporting parameters48 (Scheme 2).

Scheme 2.

Incorporating donor–acceptor (D–A) copolymers is a promising approach to achieve ambipolar OFETs.22 The first example of an air-stable, ambipolar D–A conjugated polymer was reported by Marks and coworkers.49 They used indenofluorenebis(dicyanovinylene) core as an acceptor and the resulting polymer P5 exhibited similar electron and hole mobilities near to 2 × 10−4 cm2/(Vs). Here, the D–A backbone enhanced the core rigidity and π-conjugation. Bao et al. used acenaphthyl-based fused thienopyrazine moiety as an acceptor in a thiophene and fluorene copolymer P6 and obtained charge carrier mobilities of 0.07 cm2/(Vs) and 0.046 cm2/(Vs) for holes and electrons, respectively.50 Here, acenaphthyl-based fused thienopyrazine was strategically used to improve the π–π stacking between the polymeric chains. Recently, thiophene and 1,3,4-thiadiazole-based copolymer P7 showed carrier mobilities of about 5 × 10−3 cm2/(Vs) for electrons and 3.4 × 10−4 cm2/(Vs) for holes, when incorporated in OFETs with Al source and drain electrodes, which can be attributed to its ordered face-to-face π-stacked solid structure with an intermolecular distance of ∼3.7 Å.51 Planar polymers will also have good π–π stacking and hence Baumgarten et al. connected thiadiazoloquinoxaline and thiophene moieties with a ethynylene π-spacer and obtained high FET mobilities up to 0.028 (for holes) and 0.042 cm2/(Vs) (for electrons) in a series of semiconducting polymers, P8 and P952 (Scheme 3).

Scheme 3.

Although many D–A copolymers can exhibit ambipolar behavior, up to now the types of acceptor units are few, these include diketopyrrolopyrrole, rylene diimides,53 and benzobisthiadiazole.41 A soluble, spray-processable, donor–acceptor polymer P10 having dithieno[3,2-b:2′,3′-d]pyrrole (DTP) functionalized with a trialkoxyphenyl group (donor) and benzo[1,2-c;4,5-c′]bis[1,2,5]thiadiazole (BBT) (acceptor) was reported by Reynolds and coworkers and it has shown ambipolar charge transport mobilities of 1.2 × 10−3 cm2/(Vs) and 5.8 × 10−4 cm2/(Vs) for holes and electrons, respectively.54 Interestingly, this polymer has a very low optical band gap of 0.5–0.6 eV. Another polymer P11, with this acceptor also exhibited charge carrier mobilities of 7.1 × 10−4 cm2/(Vs) and 3.3 × 10−3 cm2/(Vs) for holes and electrons, respectively, in OFETs.55 Recently, Wudl et al. reported a series of semiconducting polymers with the above mentioned acceptor BBT, namely, polybenzobisthiadiazole-dithienopyrrole (P12), polybenzobisthiadiazole-dithienocyclopentane (PBBTCD or P13), and polybenzobisthiadiazole-dithienosilole (PBBTSiD or P14), and polybenzobisthiadiazole-fluorene (PBBTFL or P15).41 These polymers exhibited balanced electron and hole mobilities in the range of 10−4 cm2/(Vs) to 10−1 cm2/(Vs), when employed in BGBC FET configuration with photolithographically defined gold source-drain electrodes (Fig. 1; Scheme 4).

Figure 1.

Typical transistor output characteristics of (a) PBBTCD, (b) PBBTSiD, and (c) PBBTFL. Transfer characteristics are shown in the inset. Mobilities were determined to be 0.076 cm2 V−1 s−1 for electrons and 0.10 cm2 V−1 s−1 for holes for PBBTCD, 0.016 cm2 V−1 s−1 for electrons and 0.007 cm2 V−1 s−1 for holes for PBBTSiD, and 0.00015 cm2 V−1 s−1 for electrons and 0.004 cm2 V−1 s−1 for holes for PBBTFL. (d) Voltage transfer characteristics for a complementary inverter with PBBTCD as the active layer, annealed at 240 °C. The maximum gain was determined to be 20.41

Scheme 4.

Ambipolar polymer P16, based on 2,1,3-benzothiadiazole (BTZ) and 1,2,3-benzotriazole, was reported by Ober et al., showing FET mobilities of 3 × 10−3 cm2/(Vs) and 4 × 10−5 cm2/(Vs) for electrons and holes, respectively with PMMA gate dielectric.56 When a bisthiophene moiety was introduced to these moieties the resulted random copolymer P17 exhibited respected mobilities of 7 × 10−5 cm2/(Vs) and 3 × 10−4 cm2/(Vs) for holes and electrons respectively with PMMA gate dielectric. Enhanced electron mobilities (0.02 cm2/(Vs) for P16 and 8 × 10−4 cm2/(Vs) for P17) were observed when polystyrene was used as a gate dielectric material (Scheme 5).

Scheme 5.

Another acceptor moiety that has been extensively used in D–A copolymers is diketopyrrolopyrrole (DPP). Yang et al. reported a DPP and BTZ-based polymeric semiconductor, namely, poly[3,6-dithiene-2-yl-2,5-di(2-decyltetradecyl)-pyrrolo[3,4-c]pyrrole-1,4-dione-5′,5″-diyl-alt-benzo-2,1,3-thiadiazol-4,7-diyl] (P18).44 For this polymer, the hole mobility was slightly higher than electron mobility with Au-based source/drain electrodes and the vice versa with the Al-based source/drain electrodes. However, equivalent charge carrier mobilities (0.09 cm2/(Vs) and 0.1 cm2/(Vs) for electrons and holes respectively) were obtained in the FET constructed with Au as source and Al as drain electrodes. A similar polymer P19 having different alkyl chains was also reported to have high and balanced ambipolar charge carrier mobilities of 0.35 cm2/(Vs) (for holes) and 0.40 cm2/(Vs) (for electrons) with Au as source/drain electrodes.57 Ting et al. reported two ambipolar semiconducting copolymers with this acceptor and BTZ.58 Because of its crystalline nature, P20 showed comparatively good ambipolar characteristics relative to P21, with the hole and electron mobilities of 0.115 cm2/(Vs) (on/off ratio: 2.49 × 104) and 3.08 × 10−3 cm2/(Vs) (on/off ratio: 7.34 × 102), respectively (Schemes 6 and 7).

Scheme 6.
Scheme 7.

Janssen et al. reported ambipolar transport in OFETs by using a DPP-based polymer (P22) with unsubstituted terthiophene as a spacer between DPPs and obtained hole mobility of 0.04 cm2/(Vs) and electron mobility of 0.01 cm2/(Vs), which are nearly independent of molecular weight of the polymer.59 By using this ambipolar semiconducting polymer, CMOS-like inverters and ring oscillators were fabricated and showed high frequency of 42 kHz in ring oscillators, which made it fastest organic CMOS-like circuit reported to date.60 They also used electron rich trimers containing furan and benzene or thiophene as donors (P23P27) and their hole and electron mobilities were found to be higher than 0.01 cm2/(Vs) in FET devices.61 In this series, although furan based conjugated polymers have shown good OFET performance, their performance is somewhat less than that for the corresponding thiophene based materials. Similarly, DPPs with varying thiophene units were copolymerized with thieno[3,2-b]thiophene (TT) unit and the resulting semiconducting polymers (P28P32) have shown ambipolar charge transport properties up to 0.03 cm2/(Vs) and 0.009 cm2/(Vs), for holes and electrons, respectively62 (Scheme 8).

Scheme 8.

After that, a series of DPP and thiophene-based copolymers with electron-rich 9,9-dioctylfluorene (F) and 4,4-dioctylcyclopenta[2,1-b:3,4-b0]dithiophene (CPDT) were synthesized by the same group. These polymers (P33–P38) exhibited ambipolar charge transfer mobilities up to 2.1 × 10−3 cm2/(Vs) and 1.6 × 10−4 cm2/(Vs), for holes and electrons respectively.63 Recently, another electron rich moiety, silaindacenodithiophene has also been copolymerized with DPP and the resultant low band gap (1.4 eV) copolymer P39 showed high ambipolar charge transport properties of 0.65/0.1 cm2/(Vs) for holes/electrons in bottom contact/top gate OFETs64 (Scheme 9).

Scheme 9.

With a view that “benzodipyrrolidone has similar structure to DPP,” Wudl et al. synthesized two benzodipyrrolidone-based low-band-gap polymers (P40 and P41). When the OFETs were fabricated in bottom gate/bottom contact geometry, P40 showed electron mobility of 2.4 × 10−3 cm2/(Vs) and P41 exhibited ambipolar properties with hole mobility of 10−3 cm2/(Vs) and electron mobility of 10−3 cm2/(Vs).65

A polyradical (P42) consisting of alternating triarylamine and perchlorotriphenylmethyl radical moieties was reported by Lambert et al. and the charge mobilities were found to be about 3 × 10−5 cm2/(Vs) for electrons and holes in OFET devices by using a top contact/bottom gate (TC/BG) device structure with an additional organic insulating layer of polypropylene-co-1-butene (PPcB) placed upon the SiO2 surface and Au source/drain contacts.66 Here, a low permittivity material PPcB was used to suppress the electron trapping at SiO2 surface and to reduce the energetic disorder at the semiconductor/insulator interface.

As a small molecule, pentacene has one of the highest thin film hole mobilities and smallest HOMO–LUMO gaps and thus is attractive for low band gap polymers.67, 68 Recently, Bao et al. synthesized two regioregular and regiorandom pentacene-based copolymers (P43P45).69 Among them regioregular copolymers has shown better ambipolar charge transport properties than regiorandom copolymers. The regioregular copolymer having highest crystallinity has shown comparatively high charge transport properties of 2 × 10−4 cm2/(Vs) (for electrons, in nitrogen) and 5 × 10−5 cm2/(Vs) (for holes, in air).

n-CHANNEL AND AMBIPOLAR BLENDS

Blending of OSCs is a fruitful approach to improve the performance of the individual components or to achieve required properties like processability or ambipolarity. Blending of conjugated polymers and other multicomponent organic semiconductors offers advantages like easy fabrication of semiconducting layers and use of single metals as source/drain electrodes. Bilayer thin films or coevaporated blend thin films of p- and n-type semiconductors have been used in bulk heterojunction solar cells,70, 71 organic light emitting diodes,72, 73 and ambipolar OFETs.23, 74, 75 Efficient blending not only improves mobilities but also gives stable Ion/Ioff ratios, realized for p-type polymer OFETs.76, 77 However, very few attempts have been made to study blending on n-type materials in OFETS. Zhu et al. reported that the mobilities of the solution-processed n-type organic semiconductor, N,N-di((Z)-9-octadecene)-3,4,9,10-perylene tetracarboxylic diimide (PTCDI-e) can be improved up to 30 times by blending with electron donors such as (tetramethyltetraselenafulvalene (TMTSF) and tetrathiafulvalene (TTF) (Fig. 2).78 In this study, they also found that blending can decrease the grain boundaries (Fig. 3; Scheme 10).

Figure 2.

Field-effect mobility (μ) versus blending ratio.78

Figure 3.

AFM images of the solution-processed PTCDI-e films blended with TMTSF at different ratios: (a) 0, (b) 9.05, (c) 16.59, (d) 28.46, (e) 44.31, and (f) 54.41 mol % (height scale: 200 nm).78

Scheme 10.

Most of the solution-processable ambipolar bulk heterojunction OFETs were fabricated by blending fullerenes80 or n-channel small molecular organic semiconducting materials (OSCs)81 with p-type polymers.82 Solution-processable ambipolar OFETs were fabricated by using a blend of thieno[2,3-b]thiophene terthiophene polymer and PCBM (Fig. 4).79 Balanced and high-field effect mobilities of 0.009 cm2/(Vs) (for holes) and 0.004 cm2/(Vs) (for electrons) were observed for these devices and a maximum gain of 65 has been observed when complementary-like inverters were fabricated. Recently, The Northwestern group reported a series of solution-processable oligomers and copolymers based on phenacyl-thiophene core.83 Among them vacuum deposited small molecular organic semiconductor 1 exhibited electron mobility up to 0.45 cm2/(Vs) with on/off ratio of 108 and high crystallinity as per the X-ray diffraction studies. On the other hand, polymer P46 exhibited low electron mobilities around 1.7 × 10−6 cm2/(Vs) and solution-processed thin films of P46 had poor crystallinity. They used a blend of oligomer 1 (1) and polymer P46 (3) (1:1 wt % ratio) and obtained high electron mobilities of 0.02 cm2/(Vs) (Fig. 5) and succeeded in combining favorable rheological properties of the polymer and good semiconducting properties of the small molecule to achieve a more processable organic semiconductor (Schemes 11 and 12).

Figure 4.

(a) Schematic cross-section of an OFET: (1) highly doped Si; (2) SiO2 insulating layer; (3) organosilane SAM; (4) gold source and drain electrodes; (5) polymer–PCBM blend. (b) Chemical structure of thieno[2,3-b]thiophene terthiophene polymer and (c) chemical structure of PCBM.79

Scheme 11.
Scheme 12.

However, very few attempts have been made towards all polymer ambipolar OFETs because of the small and inadequate amount, and insufficient solubility, of n-channel polymers. One such example is a blend of poly(benzobisimidazobenzophenanthroline) BBL and 98% by weight of poly[(thiophene-2,5-diyl)-alt-(2,3-diheptylquinoxaline-5,8-diyl)] (PTHQX).84 Among the several blend compositions 98 wt % PTHQX blend has a two-phase bicontinuous network structure (Fig. 6) and exhibited electron mobility of 1.4 × 10−5 cm2/(Vs) and a hole mobility of 1 × 10−4 cm2/(Vs) in ambipolar OFETs. For the blend compositions between 10 and 80 wt % PTHQX unipolar electron mobilities were 1 × 10−3 cm2/(Vs). Recently, Loi and coworkers reported ambipolar OFETs with balanced charge carrier mobilities of 4 × 10−3 cm2/(Vs) for electrons and 2 × 10−3 cm2/(Vs) for holes, by using a blend of PNDI2OD-T2 and P3HT (1:1 wt %).85 Thin film morphology and phase separation will play a major role during the fabrication of polymer–polymer blend-based OFET and this discussion was recently reviewed by Anthopolous et al.355

Figure 5.

FET transfer plots measured under vacuum for 1 drop-cast (a), 3 spin-cast (b), and a 1:3 blend drop-cast (c).83

Figure 6.

AFM topographic images of BBL/PTHQx blend thin films on a SiO2 surface on a silicon substrate: 10 (a), 30 (b), 50 (c), 80 (d), and 98 wt % (e).84

Apart from the technological interest, fabricating OFETs with blends is an excellent test-bed to understand the inner lying mechanisms and charge transport between the components. The blends of π-conjugated polymers are extensively used for the devices like electroluminescent diodes,86, 87 photovoltaic cells,88 photodetectors,89, 90 and electrophotographic imaging devices.90 Understanding the charge transport, microstructural and morphological changes with respect to processing and composition is also useful to develop blend-based devices and to realize new device architectures.

Recently, OFETs of neat P3HT, neat PCBM and P3HT:PCBM (1:1 wt % ratio) were constructed and the effect of annealing temperature on microstructural and morphological state of a binary blend was studied by Anthopolous et al.91 Bottom-contact, bottom-gate OFETs were fabricated in this study. Amorphous films observed for the blend annealed below 140 °C and phase segregation of PCBM-rich crystallites were observed for annealing temperatures between 140 and 220 °C. The charge carrier mobilities of the OFETs altered with respect to the annealing temperature of the device as shown in the Figure 7. These changes are found to be relevant to the eutectic phase behavior of the binary blend as evidenced by the polarized optical micrographs of the annealed films. These in situ mobility measurements of the OFET devices gave clear picture of the kinetics of microstructure formation.

Figure 7.

(a) Average field-effect mobility of holes and electrons measured at room temperature from several 1:1 (wt %) P3HT:PC61BM blend ambipolar BG-BC transistors as function of annealing temperature. (b) Differential scanning calorimetry heating thermogram of 1:1 (wt %) P3HT:PC61BM blend films.91

Yeates et al. demonstrated the effect of solution processing on microstructural and morphological changes by fabricating an OFET with a blend of poly(4,4-diphenyl-(4-methoxy-2-methyl-phenyl)-amine) (P50) and polystyrene (1:1 wt %) through drop casting and inkjet printing techniques.92 Inkjet-printed thin films exhibited greater phase segregation than drop-casted films. This is possible because of the very fast drying rate in inkjet printing. On the other hand, relatively low OFET mobilities were obtained for inkjet-printed OFET when compared with OFETs having continuous semiconducting layers (processed through drop-casting). In this work authors employed noncontact mode scanning probe microscopy to observe the phase segregation (Fig. 8) and bottom-gate, top-contact geometry for OFETs to evaluate the mobilities. This study is useful to explore the device characteristics, reliability, and uniformity with respect to various solution processing techniques.

Figure 8.

Film morphology by drop casting and inkjet printing of PTAA–PS 50:50 w/w ratio [optical micrograph (200 × 200 mm2) and SFM true noncontact mode, 5 × 5 mm2, phase image).92

SOLUTION-PROCESSABLE n-TYPE SMALL MOLECULES AND POLYMERS

Solution processable organic and polymeric semiconducting materials are attractive to realize low-cost, high-volume, large-area electronic circuits on flexible substrates. Several solution-based processes, such as spin-coating, drop-casting, and inkjet printing can be used to form semiconducting thin-films. However, solution-casted thin films usually suffer from the less crystallinity and poor molecular grain sizes which are necessary to give high charge carrier mobilities. Unlike vacuum deposition, the time given to organic semiconductors in solution processing is relatively less to form crystalline films and hence OFETs fabricated by vacuum deposition generally exhibit relatively higher performance. Tremendous efforts are being made by the researchers in this area to overcome these problems and hence higher mobilities have been realized by solution processing techniques. This became possible by tuning the conjugated cores and side chains at molecular level to control molecular packing and solubility, and by careful engineering of semiconductor/electrode and semiconductor/dielectric interfaces at the device level (Scheme 13).

Scheme 13.

In general, semiconducting polymers suffer from the insufficient purity, low molecular weight and high polydispersity index and hence sometimes usage of small molecular OSCs is also a viable approach for practical applications to avoid the adverse effects caused by impurities. With careful design it is easy to achieve strong intermolecular π–π overlap in small molecules to achieve high charge carrier mobilities in OFETs at the expense of solubility. Recently, Holmes et al. reported three solution-processable fluorenyl hexa-peri-hexabenzocoronenes (24), having hole mobilities in OFETs.93 In these OSCs, incorporation of bulky 9,9-dioctylfluorene unit is inevitable to impart solubility. But this affected the intermolecular packing of 2. In contrast to 2, bis-substituted compounds 3 and 4 showed strong aggregation properties in solid state and thus they exhibited higher charge carrier mobilities than 2 in OFETs. Among these three compounds, 4 exhibited high charge carrier mobilities of 2.8 × 10−3 cm2/(Vs) (for holes, neat active layer) and 1.2 × 10−4 cm2/(Vs) (for electrons, blended with PCBM, 1:2 w/w as an active layer) in OFET devices. Several approaches were considered by researchers to achieve solubility in n-channel materials. These include incorporation of soluble alkyl chains, fluorocarbon substituents,94 and dicyanated core substituents95, 96 (Scheme 14).

Scheme 14.

Marks and coworkers reported that an electron-deficient and ladder type indenofluorenebis(dicyanovinylene) core based small molecular semiconductor 5 can exhibit electron mobility as high as 0.16 cm2/(Vs) with an on/off ratio between 107 and 108 in solution-processed OFETs and excellent stability in air.49, 97 Solution-casted thin films of 5 showed well-organized lamellar microstructures and large film grain sizes in XRD and AFM studies and hence higher electron mobilities observed for 5-based OFETs. Aso et al. used π-conjugated compounds with dicyanomethylene-substituted cyclopenta[b]thiophenes as an active layer and an electron mobility of 3.5 × 10−3 cm2/(Vs) was obtained for 25 in vacuum when the OFETs were fabricated by spin-coating technique.98 Meredith and coworkers used a solution processable small molecule [2-({7-(9,9-di-n-propyl-9H-fluoren-2-yl}benzo[c][1,2,5] thiadiazol-4-yl]methylene]malononitrile (6) as an active layer in OFETs and obtained a maximum electron mobility of 2.4 × 10−3 cm2/(Vs).99 Surprisingly, the spin-coated 6 thin films have larger grain sizes than vacuum deposited films and thus high performance in spin-coated OFETs. Similar observations were made by Marks et al. in the case of 7.100 But they explained that this could be due to the degradation of 7 upon sublimation and to the creation of partial grain alignment in the solution-based film (Scheme 15).

Scheme 15.

The polymeric organic semiconductors have several advantages like smooth and uniform film formation for better control over the large-area devices, excellent rheological properties, which are advantageous for printing techniques, and good mechanical strength to realize flexible substrates. Hence, they are the main focus in the area of solution-processed devices.101 One of the first examples of a solution-processed n-channel polymer was poly(benzobisimidazobenzophenanthroline) BBL. This ladder-type polymer exhibited high electron mobility of 0.1 cm2/(Vs) with on/off ratio 103 in OFET.102 Several solution-processed polymers based on rylene diimides also exhibited high electron mobilities in OFETs. Thelakkat et al. reported n-channel mobility of 1.2 × 10−3 cm2/(Vs) by using solution-processable polymer P48.103 Zhan et al. used perylene diimide-based polymer P49 and obtained a saturation electron mobility of 1.3 × 10−2 cm2/(Vs) under nitrogen atmosphere.104 Recently, Fachetti and coworkers reported electron mobilities of 0.002 cm2/(Vs) and 0.08 cm2/(Vs) for polymers P50 and PNDI2OD-T2, respectively.105 Recently, solution-processable naphthalene-1,8:4,5-bis(dicarboximide) (NDI)-based ladderized polymer (P51) was also prepared and found to have electron mobility of 0.0026 cm2/(Vs). Unlike BBL, this ladderized polymer is soluble in common organic solvents. Ambient stable OFETs fabricated with NDI bithiophene copolymer PNDI2OD-T2 as an active layer, exhibited electron mobility of 0.85 cm2/(Vs) in the year 2009; so far, this is the highest mobility from a solution-processable polymer106 (Scheme 16).

Scheme 16.

The potential of NDI-based materials is further demonstrated by Zhu et al. A novel core-expanded NDI small molecule 11 has been used and air-operable OFETs were constructed by careful processing. Electron mobilities as high as 1.2 cm2/(Vs) have been obtained.108 Recently, Marder and coworkers reported three solution-processed small molecular organic semiconductors based on bis(naphthalene diimide) derivatives (810).107 By using NDI2-DTP (10) as an active layer, excellent charge carrier mobility of 1.5 cm2/(Vs) was obtained in top-gate OFETs with a CYTOP/Al2O3 bilayer gate dielectric (Fig. 9). This is the highest OFET electron mobility value reported to date for a solution-processed small molecule.

Figure 9.

Output (left) and transfer (right) characteristics for n-channel operation of a particular top-gate OFET with NDI2-DTP as semiconductor and CYTOP/Al2O3 gate dielectric layer with Au source/drain electrodes (W/L = 6050 μm/180 μm).107

AIR-STABLE OFETs

Stability of a material in an OFET refers to performance parameters such as VT, μ, and Ion/Ioff over time. A principal issue, particularly for n-channel conductors, is air stability and a major challenge for researchers. Most of the n-channel OTFTs when operated in air show significant degradation, leading to limited lifetime.20 Air-stable n-channel semiconducting materials are promising candidates for p–n junction diodes, bipolar transistors, and complementary circuits.109, 110 A majority of the reported OFETs use p-channel organic and polymeric materials, and only few examples exist for n-channel semiconductors. Among them, C60 and C70 are reported to have the highest field-effect mobilities, about 0.08 cm2/(Vs).111, 112 However, the devices are highly air-sensitive, making application difficult. One of the classes of OSCs for n-OFETs known to operate at ambient conditions is perylene tetracarboxylic diimides, which are important due to their high carrier mobility (between 0.1 and 1 cm2/(Vs) and their flexibility to be used in combination with gold electrodes for efficient electron injection.33 The introduction of electron-withdrawing diimide substituents to the π-conjugated aromatic units of naphthalene, anthracene, and perylene cores have afforded attractive candidates for n-channel semiconductors. Also, oligothiophene-based n-type OFETs have emerged recently with electron-withdrawing groups such as fluorine, perfluoroalkyl, carbonyl, and cyano causing lowering of LUMO level.113 One of the early example reported on arylenediimide semiconductors was based on fluoroalkylated naphthalenediimides, which exhibited air-stable electron mobilities of ∼0.1 cm2/(Vs) with Ion/Ioff = 105, whereas the nonfluorinated arylenediimide OFETs could only be operated under inert atmosphere due to the absence of fluorine functional groups which act as a kinetic barrier to O2/H2O trap penetration.94 Some groups had observed similar behavior with copper perfluorophthalocyanine and fluorocarbon-substituted oligothiophenes.114, 115 These reportedly air-stable n-type compounds were observed to be less affected by the chemical structure of dielectric surfaces. All these fluorinated n-OSCs though exhibited air-stability depending on the O2 partial pressure and film methodology but the OFET performance degrades over periods of hours to days.116 Facchetti and group have demonstrated a series of perylene molecules substituted with strong electron withdrawing substituents (cyano and/or fluoro-alkyl groups), which resulted in high electron mobilities of 0.1–3 cm2/(Vs) in air.24, 95, 117 Also, it is well reported now that the cyanated core of perylenes has lower LUMO in comparison to unsubstituted by 0.4–0.6 eV.118

Based on the same methodology, Zhu et al. expanded the core of naphthalene diimides by fusing them with 2-(1,3-dithiol-2-ylidene)malonitrile groups having long branched N-alkyl chains (11) resulting in high performance, ambient-stable, solution-processed n-channel organic thin film transistors with high electron mobilities of up to 0.51 cm2/(Vs) and Ion/off ratios of 105 to 107 with threshold voltages below 10 V under ambient conditions.119 Similarly, when the core was expanded by fusing it with 1,4-dithiine-2,3-dicarbonitrile, alkyl 2-(1,3-dithiol-2-ylidene) cyanoacetate and 2,3-dicyanothiophene with the modification of branched N-alkyl chains (12–15) for better solubility, excellent operation in air was realized. The solution processed bottom-gate OFETs for all these compounds showed mobility in the range of ∼10−6 to 0.26 cm2/(Vs). The device not only showed air stability but also showed high performance (Fig. 10). The studies based on these materials demonstrated that the device performance not only depends on the nature of the π-backbone but also is influenced by the branched N-alkyl substituent, which is the first such report to date (Fig. 11).120

Figure 10.

Output and transfer characteristics of OTFT devices based on 11, 12, and (14/15), respectively, spin coated on OTS-treated SiO2/Si substrate and annealed at 180 °C (11 and 12) or 120 °C (14/15); IDS was obtained at drain-source voltage VDS = 60 V.120

Figure 11.

Correlation between device performance and alkyl chain length for NDI-DTYM2 derivatives (15), where the average electron mobility acts as a function of carbon atom number of 2-branched N-alkyl chain (from 12 to 24).120

However, the incorporation of sulfur in the NDI moiety also enables the reduction in the LUMO level, enabling the molecules to form air-stable n-type semiconductors.121 Not only have naphthalene, anthracene and perylene been dominating n-type semiconductors, but quinone derivatives have also emerged due to their high electron affinity. A dicyanomethylene-substituted terthienoquinoid derivative, an extended thienoquinoidal that has linear and rigid benzodithienoquinoidal structure, results in lowering of LUMO, making it an interesting core. The solution-processed n-channel OFET using an extended thienoquinoidal gave electron mobility of 0.16 cm2/(Vs) under ambient conditions.122 Further the same group extended its work by synthesizing a series of soluble (alkyloxy)carbonyl)cyanomethylene-substituted thienoquinoidal compounds (16–19) (Schemes 17181917–19).

Scheme 17.
Scheme 18.
Scheme 19.

The electron withdrawing nature of the alkyloxycarbonyl cyanomethylene terminal group resulted in low-lying LUMO (4.0–4.2 eV), an important parameter for air-stable n-channel organic semiconductors (Fig. 12). Thienoquinoidals are found to be more chemically stable than benzoquinoidals due to the stability caused by intramolecular interaction of sulfur atom in thiophene ring to that of the carbonyl oxygen in the ester moiety. The solution processed OFETs fabricated out of a terthienoquinoidal showed good n-channel mobilities of up to 0.015 cm2/(Vs) and Ion/Ioff values of ∼105 under ambient conditions123–125 (Schemes 202122232420–24).

Figure 12.

Calculated HOMO and LUMO of thienoquinoidal compounds.123

Scheme 20.
Scheme 21.
Scheme 22.
Scheme 23.
Scheme 24.

Recently heteroarene-based organic semiconductors based on benzo[1,2-b:4,5-b′] dichalcogenophenes, benzochalcogenopheno-[3,2-b][1] benzochalcogenophenes, and dinaphtho[2,3-b:2′,3′-f]-chalcogenopheno[3,2-b]chalcogenophenes were evaluated. The vapor-processable dinaphtho[2,3-b:2′,3′-f]-chalcogenopheno[3,2-b]chalcogenophenes and solution processable benzochalcogenopheno-[3,2-b][1] benzochalcogenophenes were found to exhibit mobility as high as 3.0 cm2/(Vs) and 2.8 cm2/(Vs), respectively, which is among the best new materials recently developed.126

Solution processable n-type oligothiophenes containing dicyanomethylene-substituted cyclopenta[b]thiophene (20–23) as an active material for OFETs showed electron mobility of 0.016 cm2/(Vs) (Scheme 25).

Scheme 25.

By varying the number of dicyanomethylene groups, the LUMO level was found to be in the range of −4.1 to −4.2 eV (Fig. 13). Compound 20 did not show n-type characteristics whereas, 21air = 3.2 × 10−7, μvac = 1.6 × 10−2) and 23air = 2.4 × 10−3, μvac = 5.0 × 10−3) showed improvement in mobility in comparison to 22air =4.6 × 10−4, μvac = 5.4 × 10−4) under vacuum.127

Figure 13.

Chemical structures, calculated HOMO and LUMO, and their energy levels.127

Further, the group extended their work with the incorporation of fluorine and hexylbridged bithiophene as inner conjugation units. The compounds were found to be active materials in hole-free-n-type OFETs. All the newly designed materials BCNHH-BCN (23), BCN-2Tb-BCN (24), and BCN-2Pb-BCN (25), showed electron mobilities of 5.4 × 10−4, 4.0 × 10−4, and 3.5 × 10−3 cm2/(Vs), respectively, with Ion/Ioff ratios ranging from 104 to 105. Interestingly, the electron mobility of BCN-2Pb-BCN (25) was one order of magnitude higher than those of BCN-HH-BCN (23) and BCN-2Tb-BCN (24). The ordered orientation of BCN-2Ph-BCN (25) molecule in the solid state confirmed from X-ray crystallography was concluded to be the cause of the increase in electron mobility. On exposing the device to air, good n-type characteristics were retained, with an electron mobility of 1.0 × 10−3 cm2/(Vs) and Ion/Ioff ratio of 105 for ten days but the threshold voltage shifted in the positive direction from 27 V with increasing storage period (Fig. 14)98, 128 (Scheme 26).

Figure 14.

(a) Electron mobility of BCN-2Pb-BCN-based OFETs versus storage period. (b) Transfer characteristics of OFET based on BCN-2Pb-BCN, measured in air after 1 day (red), 2 days (orange), 3 days (green), 7 days (blue), and 10 days (black) at a drain voltage of 80 V.98

Scheme 26.

Very few reports on n-type semiconducting polymers that could operate under ambient conditions are present in the literature; most of the reports are based on p-type semiconducting materials.

The donor–acceptor strategy for designing oligomers/polymers has enabled engineering of HOMO and LUMO energy levels, which would indeed tailor the energy band gap and hence alter the charge transport properties. We highlight here some of the recently developed donor–acceptor copolymers. Pei et al. designed OSCs following the donor–acceptor strategy having isoindigo core incorporated into the backbone of polythiophene in high yield. On introducing bithiophene as the repeat unit into the backbone, the polymer exhibited high mobility of 0.79 cm2/(Vs) with Ion/Ioff ratio of ∼107, which is known to be the highest field effect performance polymer till date. As the introduction of thiophene decreases the HOMO level; the resulting polymer has good stability in ambient and high humidity conditions129 (Scheme 27).

Scheme 27.

However, Jenekhe et al. reported copolymer (P54–P59) based on benzobisthiazole and donor moieties (dithienosilole, dithienopyrrole, cyclopentadithiophene, carbazole, benzodithiophene, and bithiophene) having crystalline nature with high-field effect carrier mobility [up to 0.011 cm2/(Vs)] and highly stable under ambient condition for 2 years (Fig. 15), a most promising result130 (Schemes 28 and 29).

Figure 15.

Performance of P59 OFETs as a function of air-exposure time: (a) mobility of holes, (b) threshold voltage, and (c) on/off current ratio. An overlay of the (d) output and (e) transfer characteristics of P59 devices tested for almost 2 years. Devices first measured in N2-filled drybox were brought out and stored and periodically tested in ambient conditions.130

Scheme 28.
Scheme 29.

Also, the same group has recently investigated the stability and durability of p- and n-channel polymer thin film transistors in air for a period of 4 years based on n-channel poly (benzobisimidazobenzophenthroline) (BBL) and a p-channel P3HT (Fig. 16). The complementary polymer inverter designed exhibited excellent switching characteristics along with large voltage gain. The electron mobility of BBL was observed to remain constant (∼1–6 × 10−4 cm2/Vs) though the electron mobility of P3HT dropped by more than four order of magnitude. The excellent stability of BBL devices was contributed to its rigid and planar ladder structure, which facilitates π–π stacking and strong intermolecular interactions, and the BBL device is the longest air-stable and durable nonencapsulated organic electronic device so far.131

Figure 16.

Air-stability analysis of P3HT and BBL transistors. Plot of (a) mobility, (b) current on/off ratio, and (c) threshold voltage as a function of time for both BBL and P3HT TFTs. Measurements were carried out in ambient laboratory air without humidity control.131

Further, Marks et al. designed bithiophene-imide based conjugated copolymers by increasing the conjugation length of the donor comonomer unit from mono to tetrathiophene, which resulted in decreasing the electron transport capacity and increasing the hole transport capacity. Bottom-gate/top-contact and top-gate/bottom-contact OFETs based on copolymer P61 and P62 exhibited excellent solubility with stability under ambient condition, and hole mobility of ∼10−3 and ∼10−1 cm2/(Vs) respectively, which remained unchanged for 200 days (Fig. 17). P60 was found to be unstable in air whereas, air-stability of P61 and P62 is attributed to the low lying HOMOs (>0.2 eV lower than that of P3HT. The strong electron withdrawing nature of bithiophene-imide essentially induces low-lying HOMOs29 (Scheme 30).

Figure 17.

(a) Transfer plot (VD = 100 V) for a fresh P3b-based device (solid line) and after 9 months storage in air (dashed line). Temporal evolution of OFET performance in air: (b) carrier mobility, (c) threshold voltage, and (d) Ion/Ioff ratio for the present BTI-based polymers.29

Scheme 30.

Further, the group designed an OSC based on thieno[3,4-c ]pyrrole-4,6-dione (TPD) as a building block with good processability as well as good, stable mobility for p-type semiconductors. Depending upon the comonomer in the polymeric backbone, the designed polymer showed lower HOMOs than P3HT by 0.24–0.57 eV. A series of polymers synthesized had varied conjugation length, by introducing alkyl/alkoxy-functional thiophene with solubilizing side chains. As the conjugation length was increased by incorporation of oligothiophene, enhanced p-type OTFTs response was obtained with high mobility of ∼0.6 cm2/(Vs) for P64 having three thiophene units, but with increase in the conjugation length of bithiophene to terthiophene to tetrathiophene, decreased Ion/Ioff and increased off-current were obtained (Fig. 17; Scheme 31).

Scheme 31.

However, the increase in conjugation length also caused decrease in ionization potential resulting in the high lying HOMOs and making the material more p-channel in nature. OFETs fabricated out of P63, P64, P65, P66, and P67 showed stability up to 5 months under ambient condition without any significant degradation (Fig. 18). Hence, it was concluded that OFET electrical characterization is not only sensitive to the oligothiophene conjugation length but also to the solubilizing side chain substituents (length, positional pattern). In fact, the polymers based on TPD are found to be superior to BDI due to increased solubility, processability, substantial hole mobility, and good device stability.132

Figure 18.

On_off cycles at VDS = −50 V (top) under ambient conditions for OTFTs annealed at optimized temperatures, fabricated with polymers P63 (a), P64 (b), and P65 (c) and measured at the indicated gate bias. Overlapping of transfer characteristics (bottom) before (black) and after (red) 1000 on_off cycles.132

A commercially available amorphous fluoropolymer, CYTOP, which is highly hydrophobic, has provided an attractive approach for top-gate OFETs as it exhibits excellent chemical stability with its ability to solubilize in fluorinated solvents.77, 133–137 However, CYTOP-based OFETs generally operate at high voltages because of its low dielectric constant and because it is difficult to reduce its thickness while maintaining good device performance.138 OFETs with a top-gate geometry are relatively rare due to the limited choice of gate dielectric materials and its deposition can potentially damage the organic semiconductor layer underneath.

Earlier, Sirringhaus and Facchetti observed that with the decrease in the thickness of CYTOP dielectric layer below 200 nm, the device performance suffers due to pinhole formation. They introduced crosslinked CYTOP formulation which was spin-coated to form a uniform film on the top of various organic semiconductors with low gate leakage current densities (<10 nA mm−2) and high dielectric breakdown strengths (>2 MV cm−1). Furthermore, C-Cytop-based OFETs exhibited reduced bias stress and better air stability with respect to C-PMMA because of the inert perfluorinated chemical structure of the polymer (Fig. 19).138

Figure 19.

Comparison of F8T2 FET stability for devices with C-Cytop and C-PMMA as gate dielectrics (W/L = 1 mm/10 μm). (a) Corresponding bias stress measurements for both devices in the glove box (Vd = −1 V and Vg = −10 V). (b) Corresponding stability data for both devices in ambient (Vd = −10 V and Vg = −10 V).138

Further, Kippelen and coworkers fabricated highly stable, solution-processable, small molecule blend OFETs with top gate geometry on a flexible polyethersulfone substrate (Fig. 20). The group demonstrated TIPs-pentacene and PTAA blend OFETs with the CYTOP/Al2O3 bilayer gate dielectric which showed an average saturation mobility value of 0.24 ± 0.08 cm2/(Vs) without hysteresis at operation voltages below 8 V. Similarly, an effort was made in fabricating top-gate bilayer gate insulator comprised of CYTOP and a high-k metal-oxide layer fabricated by atomic layer deposition (ALD) in OFETs based on 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene) and poly(triarylamine) (PTAA) blends as the organic semiconductor. The leakage current densities of the Al2O3 and CYTOP/Al2O3 films were found to be below 3 × 10−7 A/cm2 at applied fields with a magnitude up to 3 MV/cm (Fig. 21). The leakage current of a 780 nm-thick CYTOP film reached a value of 2 × 10−7 A/cm2 at an applied field of 1.2 MV/cm. However, the OFETs demonstrated using the bilayer gate dielectric showed environmental stability with no significant degradation in the mobility and threshold voltage even after multiple scans up to 20,000 under a constant dc bias stress for 24 h. The choice of proper thickness would allow operational stability and the author commented that it would also allow the compensation of effects arising from filling of deep traps and simultaneously dipolar orientation at the CYTOP/Al2O3 interface and/or charge injection from the gate electrode.139, 140

Figure 20.

Schematic cross-sectional view and (b) a photograph of the OFET array fabricated on a flexible PES substrate.139

Figure 21.

Representative hysteresis-free (a) transfer and (b) output characteristics of OFETs (W/L = 2550 μm/180 μm) before and after bias stress test. (c) Temporal evolution of the normalized drain current for the stress condition of VGS = VDS = −8 V.139

DOPANTS INCORPORATED INTO ORGANIC SEMICONDUCTORS

Doping of OSC has recently gained attention as earlier studies have soon that doping results in increase in the conductivity by many order of magnitude.141 Controlled and stable doping allows the reduction of Ohmic loss in transport layers, easier carrier injections from contacts, and tunable built-in potentials of Schottky or p–n junctions.142 Recently, the concept of doping has resulted in high performance organic light-emitting diodes143 and organic photovoltaics.144 Doping of p-type molecules have been investigated every now and then but the doping of n-type molecule is more challenging due to the need of higher HOMO than LUMO of the matrix material, which makes them susceptible to ambient conditions. The present approaches to n-type doping include: (i) use of alkali metal atoms,145, 146 (ii) new compounds with extremely high-lying HOMOs,147, 148 (iii) utilization of cationic salts which act as stable precursors for strong molecular donors,149–151 and (iv) molecular design of a reduced analog of the host-molecule, so that dopants do not interrupt the crystalline order.141 In earlier reports, pyronin B (PyB) chloride acted as an excellent dopant for 1,4,5,8-napthalene tetracarboxylic dianhydride (NTCDA).141, 149 Bao et al. developed for the first time a new, solution processable n-type dopant, 4-(1,3-dimethyl-2,3-dihydro-1H-benzoimidazole-2-yl)-phenyl-dimethyl-amine (N-DMBI). [6,6]-phenyl C61 butyric acid methyl ester (PCBM) could be effectively doped with N-DMBI by solution processing and the conductivity of the doped film was found to increase significantly and the film also showed improved air-stability (Fig. 22). The mobility of 2 wt % N-DMBI doped PCBM OTFTs remained higher than 10−2 cm2/(Vs) even after 20 days (Fig. 23).152

Figure 22.

(a) Chemical structures of the materials. (b) Proposed mechanism of hydrogen and electron transfer for n-type doping.152

Figure 23.

(a) Conductivities of undoped and N-DMBI-doped PCBM films at varying doping concentrations. Inset: AFM image of 2 wt % N-DMBI-doped PCBM film. (b) Transfer characteristics of N-DMBI-doped PCBM OTFTs at varying doping concentrations in a glove box. (c) Output characteristics of 0.5 wt % N-DMBI-doped PCBM OTFTs in a glove box. (d) Changes of field-effect mobilities in air as a function of time for undoped and 0.5 and 2 wt % N-DMBI doped PCBM OTFTs.152

Further the group has investigated the effect of n-type doping on the air-stability of vacuum-processed n-channel OTFTs by changing the location of doping (Scheme 32).

Scheme 32.

The group used two materials as the active layer, N,N′-dibutyl-1,7-difluoroperylene-3,4:9,10-tetracarboxylic diimide (F-PTCDI-C4) and N,N′-ditridecyl-perylene-3,4:9,10-tetracarboxylic diimide (PTCDI-C13), n-channel semiconductors, which exhibits high electron mobility under inert atmosphere and also have LUMO levels just above the value as observed for air-stability of arylene diimide-based n-channel OFETs. The investigation showed that the n-type doping of the bulk active layer or channel region improved air-stability of PTCDI-based n-channel OTFTs while maintaining a high Ion/Ioff (∼107). Though the air-stable ambient operational device was not obtained, this provides a direction for achieving air-stable n-channel OFETs (Fig. 24).153 Although the perylene core itself has been widely used as the dopant, there are very few reports of its use to date.30, 154

Figure 24.

Current–voltage characteristics and air-stability measurements for PyB-doped F-PTCDI-C4 OTFTs. (a, b) Transfer characteristics at VDS = +100 V (a) in a N2-filled glove box and (b) in air 1 h exposure. (c) Comparison of the normalized field-effect mobilities as a function of time for the pristine and doped devices.153

Recently a PDI derivative, N,N′-didodecylperylene-3,4,9,10-bis-(dicarboximide) was used as dopant in the hole transport layer of a green-emitting OLED. The luminous and power efficiency values increased significantly by a factor of 15 with respect to undoped device. Also, the increase in the concentration of dopant increased the efficiency of the devices; using more soluble PDI derivatives still allowed higher ratios without aggregation. At constant voltage of 12 V, the luminance of undoped device was 7544 cd/m2, whereas the luminance for the different PDI doped devices, 0.2, 0.4, and 0.8 wt % were 8700, 10,520, and 11,500 cd/m2, respectively and the turn-on voltages were slightly decreased from 6 to 4.5 V.155

POLYMER GATE DIELECTRICS

Extensive studies have been done on semiconducting materials to understand and improve charge transport, but very little attention has been paid to dielectric materials. The use of polymers as gate dielectric has become essential in OFETs as it reduces the operating voltage and simplifies fabrication of organic circuits for practical application.156–158 The different crosslinked polymers used as a dielectric layer are poly(4-vinylphenol),159 polyimides,160 glass resins,161 poly (methyl methacrylate) (PMMA),162, 163 poly(vinyl alcohol),164 and photoalignment layers,165 whereas crosslinked cyanoethylated poly(vinyl alcohol)166 has been used to obtain high-gate dielectric constant, which resulted in good insulator behavior and transistor characteristics for operating biases below 3 V. The main limitations of the polymer gate dielectrics investigated includes high annealing temperature, which further limits the compatibility with the plastic substrates or the use of moisture sensitive crosslinking agents.167 The use of commercially available polymers as gate dielectric materials, which could be solution-processed, would reduce the cost of fabrication. Gate dielectric material also strongly influences the molecular packing and electron mobility for variety of OSCs, which includes pentacene, perfluoro-copperphthalocyanine, oligothiophenes, and fullerene derivatives.168–172

Park et al. have recently demonstrated photocurable and patternable polymer blend gate dielectrics wherein poly (4-dimethylsilyl styrene) (PDMSS) was blended with poly (melamine-co-formaldehyde) (PMFA) for vacuum deposited pentacene and N,N-ditridecyl-3,4,9,10-perylenetetracarboxylic diimide (PTCDI-C13) and solution processed triethylsilylethynyl anthradithiophene (TES-ADT) OFETs. The group observed that the dielectric constant for 400-nm thick PDMSS/PMFA blend films decreased from 3.7 to 3.1 with the increase in the content of PDMSS. For PDMSS, loading of 20 wt % in the blend system showed low leakage current level of <10−6 A cm−2 at 2MV/cm. A good linear and saturation behavior was obtained for both p-type pentacene and n-type PTCDI-C13 based OFETs using a dielectric blend of PDMSS/PMFA (10/90), with field effect mobility of 0.93 and 0.2 cm2/(Vs) respectively. However, TES-ADT OFETs on PDMSS/PMFA blend gate dielectric operated at low voltage and exhibited good linear and saturation behavior along with air-stability even after operating 500 times (Fig. 25).173 Also, pentacene-based OFETs with different common polymer gate dielectrics, poly(vinyl alcohol), poly(4-vinyl phenol), and polystyrene, were fabricated for n-type conduction channels by Wen et al. The hydroxyl group based dielectrics were observed to hinder the active layer because of the electron traps at the contact, but on employing Ca as the source-drain electrodes and PS as the gate dielectrics, the electron mobility could reach up to 0.077 cm2/(Vs) (Fig. 26).174

Figure 25.

(a) Scheme of the top-contact OFETs employing different 50-nm thick semiconductors on PDMSS/PMFA blend gate dielectrics. (b–d) IDVD output curves (left) and IDVG transfer curves (right) of (b) pentacene, (c) PTCDT-C13, and (d) TES-ADT OFETs based on PDMSS/PMFA gate dielectrics. In (c) and (d), 400-nm thick PDMSS/PMFA (10/90) blend films were used as gate dielectrics.173

Figure 26.

(a) The structure of a pentacene-based OFET with metal electrodes, and PS, PVA, or PVP polymeric dielectrics spin-cast on crosslinked PVP as double-layer gate dielectrics. The chemical structures of PS, PVA, and PVP polymer are illustrated. (b) The output characteristics of IDS versus VDS plot of pentacene-based OFETs with Ca source-drain electrodes, and three polymers as top gate dielectrics at VG of 0 to +100 V. (c) The transfer characteristics of IDS versus VG plot of pentacene-based OFETs at a constant VDS of +100 V with PS (○), PVA (▵), and PVP (□) as the top gate dielectrics.174

Ribierre and Aoyama with his coworker fabricated a solution processable ambipolar OFETs based on dicyanomethylene-substituted quinoidal quaterthiophene [QQT(CN)4], using three different soluble gate dielectrics: polyimide (CT4112), crosslinked poly-4-vinylphenol (PVP), and a fluorinated polymer with low dielectric constant (AL-X501) with gold top-contact electrodes. The group observed that OFETs with the substrate treated with OTS showed better field-effect mobilities but had larger hysteresis, whereas when fluorinated polymer AL-X501 having low dielectric (k = 2.8) reduced hysteresis and improved both hole and electron field effect mobilities of QQT(CN)4 thin films to 0.04 and 0.002 cm2/(Vs). Additionally, among QQT(CN)4 OFETs having different source-drain electrodes (gold, silver, chromium, and aluminum) when fabricated, gold electrodes showed highest field-effect mobilities, independent of the gate dielectric material, whereas more balanced hole and electron mobilities were achieved in case of chromium electrodes. The fabricated devices demonstrate that device structure also plays a major role for improvement in electrical performance.175 Cho et al. introduced interpenetrating polymer network dielectrics by blending commercially available polymer (PMMA, PtBMA, and PS) with a crosslinkable polymeric SSQZ (silsesquiazane) for pentacene OFETs. They observed that the leakage current for the PMMA and PtBMA gate dielectric blend with SSQZ decreased significantly by an order of two in magnitude as compared with pristine. The effect is as a result from the decrease in free volume and thermal dynamic motions available to the polymer chains due to the formation of the polysiloxane network.176 Recently, Sariciftci et al. demonstrated water soluble poly(1-vinyl-1,2,4-triazole) as a dielectric layer for pentacene as p-channel and C60 as n-channel semiconducting layers. The device showed hysteresis- free transfer characteristics with relatively high Ion/Ioff ratios (>1000) and low threshold voltages.177

Also, the performance of tetracene thin film transistors with polystyrene and parylene C (PARY C) as polymer dielectric was improved by Santato et al., with increase in hole mobility and low permittivity, PS (k = 2.5) and PARY C (k = 3.12), compared with SiO2 (k = 3.9).178 Recen focus has also been on SAMs wherein functionalization of SiO2 is carried with a SAM of a silane with more or less electronegative substituents. Following the same approach, Klauk et al. have successfully demonstrated the use of mixed SAMs (alkyl/fluoroalkyl phosphonic acid) which allowed the device to be operated at low threshold voltage (≤3V) for organic p-channel (pentacene) and n-channel (F16CuPc) OFETs. The threshold voltage of the OFETs was found to be a linear function of the atomic fluorine concentration in the mixed SAM dielectric with a modulation coefficient of 40 mV/%. Ultra-thin dielectrics based on mixed SAMs provided a powerful method to continuously tune the threshold voltage of organic transistors. TFT with 100% alkyl SAM dielectric for n- and p-OFETs showed negative threshold voltage whereas 100% fluoroalkyl SAM dielectric showed positive threshold voltage for both n- and p-type semiconductors. An appropriate mixture of the two dielectrics, 50% alkyl + 50% fluoroalkyl, resulting in symmetrical TFT threshold voltages provided an inverter switching voltage of exactly half the supply (1V) (Fig. 27).179

Figure 27.

(a–c) Water contact angles measured on a SAM of 100% alkyl phosphonic acid (a), on a mixed SAM of 50% alkyl and 50% fluoroalkyl phosphonic acid (b), and on a SAM of 100% fluoroalkyl phosphonic acid (c). (d–f) Schematic cross-sections and transfer characteristics of pentacene p-channel TFTs and F16CuPc n-channel TFTs with three different SAMs as the gate dielectric: SAM of 100% alkyl phosphonic acid, mixed SAM of 50% alkyl and 50% fluoroalkyl phosphonic acid, and SAM of 100% fluoroalkyl phosphonic acid. (g–h) Transfer characteristics of complementary inverters (each composed on one pentacene and one F16CuPc TFT) with the same SAMs. Using the mixed SAM of 50% alkyl and 50% fluoroalkyl phosphonic acid, the inverter has a switching voltage of exactly half the supply voltage (1 V) and a low-power consumption of 20 pW (the product of the supply voltage, 2 V, and the maximum supply current, 10 pA).179

In fact, Hill and coworkers have demonstrated phosphonic acid SAM modified SiO2 and bare SiO2 dielectric to be unacceptable for use in n-channel PTCDI-C13 OTFTs whereas, CYTOP and parylene-c as dielectric possesses better properties resulting in excellent device performance with high stability and electron mobilities.180

INKJET METHODS

Inkjet-printing has recently emerged as a promising patterning method for forming active layers in OFETs and integrated circuits for the manufacture of microelectronic devices, such as formation of the color filter of a liquid crystal display and patterning of a luminescent polymer for an organic light-emitting display.106, 181–185 The advantage of inkjet printing is its ability to be processed at low temperature, low cost, and with low material waste with noncontact patterning.186 The direct-write ability of the inkjet printing process removes the need for masks, leading to decreased device manufacturing complexity, material waste, and crosscontamination. To have a high performance device by inkjet printing, both n-type and p-type printable organic semiconductors should have high and balanced electron and hole mobilities using the same FET architecture, gate dielectrics, and S/D electrode to reduce costs.185 Devices like organic emitting diodes (OLEDs), thin-film transistors, and microlenses have been fabricated by inkjet printing. For successful FET fabrication by printing, it is necessary to achieve smooth and uniform semiconductor film morphology to obtain a film with free pinhole gate dielectric. Several groups have demonstrated inkjet-printed OFETs and inverters using conjugated small molecules or copolymers. OFETs fabricated by inkjet-printing based on small molecules are found to be superior to those from polymers as the former readily form highly crystalline films. The important step for the fabrication of OFETs by inkjet-printing is the formation of stable and uniform droplets with no satellite drops.187–190

Facchetti et al. reported a highly soluble and printable n-channel polymer (P(NDI2OD-T2)) exhibiting OFET characteristics with electron mobility of ∼0.45–0.85 cm2/(Vs) under ambient condition in combination with Au contact and various other polymer dielectric layers like CYTOP (poly (perfluoroalkenylvinyl ether)), PTBS (poly(t-butylsytrene)), PS (polystyrene), ActivInk D2200 (polyolefin-polyacrylate) and PMMA (poly(methylmethacrylate)) (Fig. 28). The polymer P(NDI2OD-T2) has been demonstrated to exhibit high solubility and comparable carrier mobilities on rigid and flexible substrate. Top-gate bottom-contact OFETs made by flexographic and inkjet printing of the semiconducting layer afforded electron mobility of 0.1 cm2/(Vs). The group then further obtained excellent performance of OFETs based on gravure-deposited semiconducting and dielectric layer with a sharp turn on, good saturation and negligible I–V hysteresis with electron mobility of 0.1–0.65 cm2/(Vs) and threshold voltage ratio of >106 under ambient condition (Fig. 29). Fabrication of a complementary inverter using gravure printing with P3HT (p-channel) and P(NDI2OD-T2) (n-channel) resulted in remarkably small hysteresis which indeed reflects the stability of the threshold voltage (Fig. 30).106

Figure 28.

(a) Illustration of the TFT material components (left), chemical structure of P(NDI2OD-T2) and P3HT semiconducting polymers, and of the top-gate bottom-contact (TGBC) TFT architecture (right) used in this study.106

Figure 29.

(a) Current–voltage output plot as a function of VSG for a PMMA-based device on glass. Inset, low-voltage scan (axes as main plot) highlighting the linear IV characteristics and the line intersection through the axes origin. (b) Current–voltage output plot as a function of VSG for a PTBS-based device on glass. Inset, scan as (a). (c) TFT transfer plots of current versus VSG for representative TGBC device on glass using the indicated polymeric dielectrics. (d) TFT transfer plot of current versus VSG for representative devices on PET using the indicated polymeric dielectrics.106

Figure 30.

P3HT (p-channel)-P(NDI2OD-T2) (n-channel) complementary inverters. Static switching characteristics of a spin-coated inverter based on PMMA.106

Noh et al. have recently demonstrated highly uniform OFETs that were inkjet-printed using an n-channel small molecule N,N′-bis(n-octyl)-(1,7&1,6)-dicyanoperylene-3,4:9,10-bis(dicarboximide) (PDI8-CN2) (Fig. 31). The performance and uniformity were improved by inkjet-printing of PDI8-CN2 solution on a 60 °C heated substrate. The crystallinity of the film was improved from room temperature to 60 °C and the PDI8-CN2 showed a high-field effect mobility of 0.05–0.06 cm2/(Vs) with high on/off ratio of ∼106. On constructing inkjet-printed organic complementary inverters using n-channel (PDI8-CN2) and p-channel (6,13-bis(triisopropyl-silylethynyl)-pentacene) or poly(3-hexylthiophene) semiconducting material onto silicon dioxide gate dielectric, the inverter exhibited a high-voltage gain of more than 15 and small standard deviation of inverting voltage with the gain of ±0.95 V and ±0.56 V, respectively.191

Figure 31.

(A) Molecular structure of PDI8-CN2 and a schematic diagram of a bottom-gate/bottom-contact OFET. (B) High-speed CCD camera images of inkjet droplet formation at a delay time (sd) of 80 ls for solution of PDI8-CN2 in CB. (a) Optical microscopy images of PDI8-CN2 film inkjet-printed on a SiO2/Si(n+) substrate at room temperature (RT = 25 °C) or 60 °C. (b) A single drop of PDI8-CN2 in DCB at RT, (c) multidrop printing of PDI8-CN2 in DCB at RT, and (d) line printing of PDI8-CN2 in CB at 60 °C. Inset shows magnified images of selected parts of the figures.191

Further the group has extended the work and demonstrated for the first time the fabrication of high speed polymeric complementary circuits using n-channel ([poly{[N,N′-bis(2-octyldodecyl)-napthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl] alt-5,5′-(2,2′-dithiophene)} [P(NDI2OD-T2), Polyera ActivInk N2200] and two p-channel polymers [poly(3-hexylthiophene] and dithiophene based polymer (Polyera ActivInk P2100). The top-gate/bottom-contact OFETs exhibited high hole and electron mobility of 0.2–0.5 cm2/(Vs). PMMA dielectric solution in 2-ethoxyethanol optimized the performance of both transistor types. The inverters exhibited high-voltage gains (>30) and a fast average propagation delay of 0.22 ms both in air and under inert ambient conditions.185 Recently, n-channel organic transistor based PDI-8CN2 was deposited by inkjet printing on Si (gate)/bare SiO2 (dielectric)/Au (contact) substrate using dichlorobenzene, chloroform and mixture of the two solvent. On investigating the influence of mixtures of solvent on printing quality it was observed that a mixture of 3:2 of dichlorobenzene:chloroform resulted in a uniform film (Fig. 32). The L = 40 μm, 3:2 DCB:CF OFETs exhibited higher mobility than pure solvent devices and also this mixture ensured the limited degradation in the device performance when larger area was printed. So, the choice of an optimized ink composition induces a specific morphology of PDI-8CN2 where larger grain domains are intercalated with smaller crystallites mostly oriented along the printing direction. Though the device showed reduction in mobility (μ ∼ 0.0035 cm2/(Vs)) from that obtained from vapor deposition of PDI-8CN2 (μ = ∼0.02 and ∼0.03 cm2/(Vs)), this decrease of one order could be ascribed due to different methodology used for deposition (Fig. 33). Hence, solvent mixture influences the surface morphology, an important issue in the inkjet printing to develop organic electronic circuitry.192

Figure 32.

Atomic force microscopy (AFM) images of the PDI-8CN2 film printed by 3:2 DCB:CF mixture. The image size is 30 m × 30 m for (a) and 10 m × 10 m for (b).192

Figure 33.

Electrical response of inkjet printed PDI-8CN2 OTFTs: (a) 3:2 DCB:CF output curves; (b) transfer-curves in saturation regime (VDS = 50 V) of OTFTs printed by employing different DCB:CF mixtures (in the inset the same curves are reported in semilog scale).192

Moving away from traditional Au electrodes, Zhu and Xu fabricated both p- and n-type bottom contact OFETs based on pentacene and fluorinated copper phthalocyanine (F16CuPc) using graphitic electrodes (PPOD, pyrolyzed poly(1,3,4-oxadiazole)), an easily patternable using solution process followed by converting to conducting electrode (102 S/cm) through simple pyrolysis. The graphitic electrode induced regular orientation of the organic molecules alone and near the edges with large grains. CuPc based device with PPOD electrodes exhibited the highest mobility of 1.45 × 10−2 cm2/(Vs) at 130 °C, 10 times higher than that obtained using Au electrodes (μ = 1.19 × 10−3 cm2/(Vs)) The advantage of this organic electrode lies in its abundance, simple fabrication process, and generally low expense. This organic electrode also exhibited unique properties of (1) ultrasmooth surface with moderate thickness (rms roughness of about 0.3 nm), (2) high chemical and thermal stability, and (3) excellent compatibility with organic semiconductors and suitable work function.193

CONCLUSIONS

There is now a wide selection of materials and approaches for n- and p-channel organic semiconductors, and means of stabilizing them, printing them, and integrating them with electrodes and dielectrics. Undoubtedly, with the power of organic synthesis, still more such materials will be designed and studied. At the same time, the field would likely benefit from the wider application of a smaller set of more standardized materials, so that consistent performance parameters can be obtained from accepted device building blocks across multiple laboratories.

Acknowledgements

This work was supported by AFOSR (Grant number: FA9550-09-1-0259) and Department of Energy, Basic Energy Sciences (Grant number: DE-FG02-07ER46465).

Biographical Information

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Jasmine Sinha is a postdoctoral fellow at Department of Materials Science and Engineering, Johns Hopkins University, under the direction of Prof. Howard Katz. She received her Ph.D. in chemistry in 2011 from the Indian Institute of Technology, Bombay, India. Her interests lie in the field of semiconducting polymer synthesis and its application in OFETs, sensors.

Biographical Information

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Srinivas Kola is a postdoctoral fellow in the Department of Materials Science and Engineering, Johns Hopkins University, under the direction of Prof. Howard Katz. He did his Ph.D. research at the Indian Institute of Chemical Technology, Hyderabad, India. His research interests include synthesis of organic polymeric semiconductors and their application in OFETs.

Biographical Information

original image

Howard Katz is a Professor and Chair of Materials Science and Engineering at Johns Hopkins University, and Professor of Chemistry. He earned his PhD from UCLA, and worked at Bell Laboratories from 1982 to 2004, becoming “Distinguished Member of Technical Staff” in 1998. He has over 200 papers and 40 patents and is a fellow of four professional societies. His research interests are organic, hybrid, and interfacial materials for electronic and optoelectronic devices, including OFETs, sensors, and diodes.