DâA1âDâA2 Copolymer Based on Pyridine-Capped

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A D–A1–D–A2 Copolymer Based on Pyridine-Capped Diketopyrrolopyrrole with Fluorinated Benzothiadiazole for High-Performance Ambipolar Organic Thin-Film Transistors Ping Li, Long Xu, Hongguang Shen, Xianming Duan, Jianqi Zhang, Zhixiang Wei, Zhengran Yi, Chong-an Di, and Shuai Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b12050 • Publication Date (Web): 22 Mar 2016 Downloaded from http://pubs.acs.org on March 23, 2016

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ACS Applied Materials & Interfaces

A D–A1–D–A2 Copolymer Based on Pyridine-Capped Diketopyrrolopyrrole with Fluorinated Benzothiadiazole for High-Performance Ambipolar Organic Thin-Film Transistors ⊥

⊥

Ping Li, †, Long Xu, †, Hongguang Shen, § Xianming Duan, † Jianqi Zhang,& Zhixiang Wei,& Zhengran Yi, †,* Chong-an Di, § Shuai Wang †, ‡,* †

Key Laboratory for Large-Format Battery Materials and System, Ministry of Education, School of

Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, China. ‡

Flexible Electronics Research Center (FERC), State Key Laboratory of Digital Manufacturing

Equipment and Technology, School of Mechanical Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, P. R. China. §

Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of

Sciences, Beijing 100190. China. &

National Center for Nanoscience and Technology, Beijing 100190, China.

ABSTRACT:

A novel donor-acceptor-donor-acceptor (D–A1–D–A2) π-conjugated copolymer

(PDBPyDT2FBT) has been prepared by Stille coupling reaction. It is found that PDBPyDT2FBT exhibits low LUMO energy levels mainly due to multiple electron-deficient units and donor-acceptor interaction, which is favorable to obtain more efficient electron injection and transport in organic thinfilm transistors (OTFTs). Moreover, introducing two electron-deficient moieties into the thiophenecontaining copolymer increases the length of conjugated main chain and enhances the coplanarity of the backbone, which may be beneficial for promoting the molecular crystallinity and improving molecular ordering capability at low temperatures. High electron and hole mobilities up to 0.65 cm2V−1s−1 and 0.24 cm2V−1s−1 were obtained at relatively low annealing temperature of 100 ºC and 80 ºC, respectively, implying that PDBPyDT2FBT is a promising ambipolar polymer semiconductor applied in low-cost and 1 Environment ACS Paragon Plus


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large-area manufacturing of OTFTs. KEYWORDS: donor-acceptor-donor-acceptor copolymer, OTFT, ambipolar, electron mobility, hole mobility, annealing temperature

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1. INTRODUCTION Solution-processed organic thin-film transistors (OTFTs) with π-conjugated polymers as semiconductor materials have been studied widely in recent decades, owing to their unique superiority in cost-effective, large-area and mechanically flexible electronics.1−4 In the last few years, great progress has been achieved in OTFTs due to advances in molecular design and device optimization,5−9 numerous polymer-based OTFTs have achieved higher charge-carrier mobilities than 1.0 cm2V−1s−1, and even been comparable with the performance of amorphous silicon.10−15 Currently, most of reported polymers exhibit unipolar OTFT charge-carrier transport characteristics, it is generally known that the simultaneously movement of electrons and holes is important for fundamental studies of charge transport, which is also the demand for construction of commercial complementary metal-oxidesemiconductor (CMOS)-like logic circuits. In contrast to unipolar polymers, ambipolar polymers are widely considered to be as promising semiconductor materials for these logic circuits, owing to their advantages of selectively transporting electrons or holes.14,16−22 Moreover, in order to achieve highperformance transistor properties, high thermal annealed temperature was generally adopted as a method to enhance the ordered molecular packing in polymer thin films,23 which is unfavourable for the realization of the commercial application. Therefore, it still remains an urgent need to develop ambipolar OTFT polymers with high carrier mobility at relatively low annealing temperatures. Recently, conjugated polymers comprising pyridine-capped DPP block (DBPy, Scheme 1) has emerged as promising electron-donating materials for organic photovoltaic (OPV) and acceptor building block for OTFTs,22,24−26 which have lower LUMO levels than polymers based on DPP unit flanked by two five-membered rings, besides, DBPy unit possesses more electron-deficient six-membered rings and

better

coplanarity

than

phenyl-capped

DPP

unit.27−31

Moreover,

5,6-difluorobenzo-

[c][1,2,5]thiadiazole (2FBT, Scheme 1) moiety with high electron affinity is also commonly used as electron acceptor in donor-acceptor polymers for both OPV and OTFT applications,32−37 which could



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further lower the HOMO/LUMO levels, enhance simultaneously the intramolecular and intermolecular interactions of the resulting conjugated polymers.

Scheme 1. The synthetic routes to DBPyBr-24 and PDBPyDT2FBT: i) K2CO3/DMF/70 °C; ii) Pd2(dba)3/P(o-tol)3/chlorobenzene/130 °C. Herein, we reported a new donor-acceptor-donor-acceptor (D–A1–D–A2) copolymer (PDBPyDT2FBT) in which DBPy, 2FBT and thiophene are used as A1, A2 and D unit, respectively. In this system, DBPy and 2FBT are adopted as two core building blocks with strong electron-withdrawing ability to achieve a low LUMO level, which is benefited for obtaining efficient charge injection and stable transport. Moreover, to avoid steric hindrance in main chain of polymer, incorporation of 2FBT with DBPy between thiophene bridges would be benefit for promoting intramolecular interactions, facilitating the charge transport as well as adjusting the balance of hole and electronic transports. 2

EXPERIMENTAL SECTION

2.1 Materials. Catalysts (P(o-tol)3, Pd2(dba)3) and other reagents were purchased from Alfa Aesar (China) Chemicals Co., Ltd, Sigma-Aldrich, and other reagent companies. 11-(iodomethyl)-tricosane, 4 Environment ACS Paragon Plus

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DBPyBr-H, and 5,6-difluoro-4,7-bis(5-(trimethylstannyl)thiophen-2-yl)benzo-[c][1,2,5]thiadiazole were synthetized as reported in the literatures.22,38,39 The concrete synthetic pathways of PDBPyDT2FBT are shown in Scheme 1. 2.2 Synthesis 2.2.1 Synthesis of DBPyBr-24. Under argon protection, DBPyBr-H (1.5 g, 3.3 mmol), K2CO3 (2.3 g, 16.7 mmol) and 11-(iodomethyl)tricosane (3.6 g, 7.7 mmol) were added into DMF solution and stirred at 70 ºC for 24 h. Then, the mixture was put into water and extracted with CHCl3. After removing the solvent, the crude product was purified by silica column chromatography with an eluent (PE:CHCl3 = 2:1) to afford red solid. Yield: 918 mg (24.5%). 1H NMR (400 MHz, CDCl3): δ .8.93 (d, J = 8.8 Hz, 2H), 8.74(d, J = 2 Hz, 2H), 8.01(dd, J = 2.4 Hz, 2H), 4.28(d, J = 3.6 Hz, 4H), 1.53 (s, 2H), 1.25 (m, 82H), 0.89–0.85 (m, 12H);

13

C NMR (100 MHz, CDCl3): δ 162.49, 150.11, 146.05, 144.97, 139.70, 128.46,

122.58, 111.41, 46.29, 38.21, 31.93, 31.44, 30.01, 29.71, 29.68, 29.62, 29.37, 26.37, 22.70, 14.13; ESI+ HRMS m/z calcd for C64H104Br2N4O2 1121.34 [M], found 1121.65 [M]. 2.2.2 Synthesis of PDBPyDT2FBT. DBPyBr-24 (117.4 mg, 0.1047 mmol), 5,6-difluoro-4,7-bis(5(trimethyl-stannyl)thiophen-2-yl)-benzo[c][1,2,5]thiadiazole (51.74 mg, 0.1047 mmol), P(o-tol)3 (2.54 mg, 8 mol%, 0.008 mmol) and Pd2(dba)3 (1.92 mg, 2 mol%, 0.002 mmol) were added to a well dried flask. After three successive cycle of degassing and filling with argon, degassed chlorobenzene (10 mL) was added, then, the reaction solution was stirred at 130 °C for 72 h, and precipitated into a mixture solution (200 mL methanol and 8 mL hydrochloric acid). The precipitated product was purified by Soxhlet extraction successively using acetone, hexane, ethyl acetate, methanol and chloroform. The chloroform fraction was precipitated into methanol and dried to afford the target polymer in a yield of 75%. Mw/Mn (GPC) = 95700/58400. Anal. calcd for C78H108F2N6O2S3: C 73.66, H 8.50, N 6.61; found: C 73.50, H 8.57, N 6.53. Synthesis of PDBPyBT. PDBPyBT was prepared via Stille coupling polymerization of DBPyBr-24 5 Environment ACS Paragon Plus


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and α,α′-bis(trimethylstannyl)-bithiophene under similar conditions as above. Mw/Mn (GPC) = 133000/70200. Anal. calcd for C72H108N4O2S2: C 76.75, H 9.59, N 4.97; found: C 76.64, H 9.68, N 4.83. 2.3 Characterization. 1H and

13

C NMR spectra were obtained using a Bruker AVANCE III-400

spectrometer with CDCl3 as the solvent. Mass spectrum was recorded on an Ion Spec 4.7 Tesla FTMS instrument. The molecular weight and polydispersity index (PDI) were estimated by gel-permeation chromatography (GPC) using an Agilent PL-GPC220 instrument at 30 °C with THF as the eluent. Elemental analyses were conducted on a Vario EL III microanalyzer. Thermal gravimetric analysis (TGA) was conducted on a NETZSCH STA 449C, and differential scanning calorimetry (DSC) analysis was performed on a Perkin-Elmer Pyris 1. Absorption spectra were recorded on a Uv6100pc double beam spectrophotometer. Cyclic voltammetry was measured on a CHI 760E Workstation in acetonitrile containing 0.1 M Bu4NPF6 under argon atmosphere, glassy carbon electrode was used as working electrode, platinum wire as auxiliary electrode, and Ag/AgCl (saturated) as reference electrode (scan rate: 100 mV/s). Ferrocene (EHOMO = −4.8 eV) was adopted as a reference. The HOMO/LUMO levels are determined using the equations of EHOMO = − (Eox + 4.8 eV) and ELUMO = EHOMO + Egopt, where Eox is the oxidation onset potential. X-ray diffraction (XRD) studies were performed on a Rigaku-D/max-2500 X-ray diffractometer. AFM images were recorded on a Nanoscope V AFM (Digital Instruments) under room temperature. 2D grazing-incidence wide-angle X-ray scattering (2D-GIWAXS) was performed on a Xenocs-SAXS/WAXS system ( λ = 1.5418 Å). 2.4 OTFT fabrication and measurement. Bottom-gate/bottom-contact OTFT devices were built, where n+–Si layer and a thermally grown SiO2 layer (~300 nm) were adopted as the gate electrode and the gate dielectric, respectively. Gold as drain and source electrodes were thermally evaporated under vacuum (~10–6 Torr) and patterned by lithography technique. The substrates were sequentially rinsed with acetone and isopropanol. And the cleaned substrates were dried under a nitrogen flow.


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Subsequently, the dielectric surface was modified with octadecyltrichlorosilane (OTS) at 120 ºC for 3 h. Then, a polymer solution containing PDBPyDT2FBT (8 mg/mL in 1:1 (v/v) chloroform:odichlorobenzene) was spin-coated on the substrates at 2000 rpm for 120 s under ambient temperature, subsequently, the films were without annealing and annealed at different temperatures. The evaluations of OTFTs were carried out under nitrogen using a Keithley 4200 parameter analyzer. The OTFT devices had a channel length (L) of 1400 µm and a channel width (W) of 50 µm. The carrier mobilities in saturated regime were obtained according to the equation: Ids = Ciµ(W/2L)(Vgs − VT)2, where Ci is the capacitance per unit area of the SiO2 dielectric layer; Ids is the drain current in the saturated regime; VT and Vgs are threshold voltage and gate voltage, respectively. 3. RESULTS AND DISCUSSION The synthesis of DBPyBr-24 and PDBPyDT2FBT are outlined in Scheme 1. Compound DBPyBr-24 was characterized using NMR and FT-MS, copolymer was synthesized by Stille coupling polymerization

of

DBPyBr-24

and

5,6-difluoro-4,7-bis(5-(trimethyl-stannyl)thiophen-2-

yl)benzo[c][1,2,5]thiadiazole in chlorobenzene at 130 ºC, and purified by Soxhlet extraction with acetone, ethyl acetate, hexane and methanol. Finally, the target sample (PDBPyDT2FBT) was extracted using chloroform. PDBPyDT2FBT presents good solubility in tetrahydrofuran (THF), chloroform, chlorobenzene, etc. The molecular weight and PDI of PDBPyDT2FBT were evaluated by GPC analysis with THF as the eluent and linear polystyrene as the standard at room temperature. Mn and PDI of PDBPyDT2FBT were measured to be 58400 and 1.64, respectively (Figure S1). The thermal properties of PDBPyDT2FBT were analyzed using thermal gravity analysis (TGA) (Figure S2) and differential scanning calorimetry (DSC) (Figure S3) measurements. The TGA characteristic shows a high decomposition temperature of about 390 ºC at 5% weight loss, indicating the polymer possesses enough high thermal stability for device fabrication. The DSC result displays no microstructural phase transitions below 360 ºC.



The UV−vis absorption spectra of PDBPyDT2FBT in chloroform and as thin film are illustrated in Figure 1. PDBPyDT2FBT is found to exhibit two well-defined peaks both in chloroform and in film with a similarly maximum absorption (λmax) at about 700 nm, indicating that ordered structure for PDBPyDT2FBT exists both in solution and in polymer film. The thin film displays an absorption bandedge (λonsetfilm) at about 754 nm. The optical bandgap (Egopt) is 1.64 eV, which is calculated from the onset absorption (~754 nm) as thin film. The electrochemical behavior of the polymer was measured using CV to obtain its ionization potential (Figure 2). The HOMO level of PDBPyDT2FBT was determined to be −5.66 eV, versus ferrocene as the standard. The LUMO level of the polymer was estimated to be −4.02 eV, which was calculated from the HOMO value and optical band gap. The detailed optical and electrochemical properties are summarized in Table 1. Judging from the energy levels of PDBPyDT2FBT (EHOMO < −5 eV and ELUMO < −4 eV), when Au (work function in the range of 4.7–5.2 eV)40 is considered as the source and drain electrodes, both the injection barriers of electron and hole from Au should be quite small, which allows stable holes and electrons transport.41,42 Therefore, this polymer would be suitable as an ambipolar semiconductor. Thin film Solution

1.0 Normalized Intensity

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0.8 0.6 0.4 0.2 0.0 300 400 500 600 700 800 900 1000 1100 Wavelength (nm)

Figure 1. UV−vis absorption spectrum of PDBPyDT2FBT in chloroform and thin film.


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0.00004 0.00002 Current (mA)

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0.00000 -0.00002 -0.00004 -0.00006 -3

-2

-1

0

1

2

Potential, E(V) vs Ag/AgCl

Figure 2. Cyclic voltammogram of PDBPyDT2FBT thin film. Table 1. Summary of molecular weights, optical and electrochemical properties of polymers. Mn [kDa]

λmaxsol

λonsetsol

λmaxfilm

λonsetfilm

Egopt

EHOMO

ELUMO

/PDI

(nm)

(nm)

(nm)

(nm)

(eV)a

(eV)

(eV)b

699

729

701

754

1.64

−5.66

−4.02

674

715

689

726

1.71

−5.54

−3.83

Polymer

PDBPyDT2FBT 58400/1.64 PDBPyBT a

70200/1.90

Egopt= 1240/λonsetfilm; b ELUMO = EHOMO + Egopt In order to study the influences of the incorporation of 2FBT into polymer backbone on optical and

electrochemical properties of corresponding polymer, we also synthesized PDBPyBT containing DBPyBr-24 and bithiophene units. Its synthetic pathway is outlined in Scheme S1. The optical and electrochemical properties are illustrated in Figure S4-S5 and also summarized in Table 1. It is found that λmax of PDBPyDT2FBT both in solution and in film distinctly red shifted relative to those of PDBPyBT, which could be attributed that incorporating of 2FBT into polymer backbone induces the stronger intrachain push-pull electron transfer. Cyclic voltammograms of the two polymers revealed that the incorporation of one more 2FBT into polymer backbone could lower the HOMO/LUMO levels of PDBPyDT2FBT (Figure S5). Moreover, computational calculation of DBPy−BT and DBPyDT−2FBT with methyl groups at the nitrogen atoms indicated the HOMO and LUMO levels had a slightly



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decreasing trend with the incorporation of one more 2FBT (Figure S6), which is approximately in accord with the CV measurements. To investigate the device properties, bottom-gate/bottom-contact OTFT devices were built using PDBPyDT2FBT as channel semiconductor. The dielectric was modified by octadecyltrichlorosilane (OTS) at 120 ºC for 3h. Subsequently, the semiconducting layer was deposited on the treated SiO2/n+ doped Si substrate. The corresponding OTFT performance of the polymer is summarized in Table 2. Judging from the value of mobilities, the hole and electron mobilities of the polymer present decreased trend when the annealing temperature exceeds 80 ºC and 100 ºC, respectively. Figure 3 illustrates typical output and transfer characteristics in the n-channel operation mode (Figure 3a, 3c) and p-channel operation mode (Figure 3b, 3d) of OTFT devices for polymer thin film annealed at 100 ºC and 80 ºC, respectively, the OTFTs show high electron mobility and hole mobility up to 0.65 cm2V−1s−1 and 0.24 cm2V−1s−1 under controlled biases, respectively, with a high current on-to-off ratio (Ion/Ioff) of 105.

Figure 3. Transfer (a, b) and output (c, d) characteristics of PDBPyDT2FBT at optimized annealing temperatures. 10 Environment ACS Paragon Plus

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Table 2. OTFT performance of PDBPyDT2FBT at different annealed temperatures. Annealing T [ºC] rt 80 100 120 140 a

µea(avg/max) [cm2 V–1 s–1] 0.046/0.13 0.13/0.52 0.23/0.65 0.14/0.27 0.12/0.21

Ion/Ioff (avg/max) 105/106 104/106 104/105 104/105 104/104

Vth (avg/min) [V] 21.1/19.3 17.5/11.0 23.8/20.5 23.3/19.9 28.1/26.0

µha (avg/max) [cm2 V–1 s–1] 0.038/0.11 0.076/0.24 0.102/0.17 0.045/0.14 0.031/0.056

Ion/Ioff (avg/max) 104/106 104/105 104/106 104/105 104/105

Vth(avg/min) [V] −15.6/−3.5 −25.6/−3.6 −26.7/−21.7 −25.6/−22.1 −26.8/−20.7

At least 12 transistors are fabricated for the polymer. To research the effect of morphology on the OTFT performance of organic semiconductor, atomic

force microscopy (AFM) height and phase images of the polymer film (Figure 4 and Figure S7) without annealing and annealed at different temperatures were characterized. AFM height images (Figure 4) showed that the good connectivity between neighbouring domains in thin films would be benefited to establish efficient charge transport pathways, resulting in the enhancement of carrier mobility in OTFT devices. Moreover, AFM phase images (Figure S7) revealed that more apparent grain boundary existed in 80 ºC-annealing film and 100 ºC-annealing film, which might be a convincing evidence to explain why the better device performance occurred when the films were annealed at 80 ºC and 100 ºC. In order to figure out the variation tendency of charge carrier transport, the crystalline nature of the polymer film was studied by X-ray diffraction (XRD) analysis without annealing and annealed at different temperatures (Figure S8). It can be observed that there is a distinct diffraction peak (100) in the non-annealing thin film, which corresponds to a d-spacing of 21.04 Å. After annealing at 80 °C, another one peak (200) is also visible. The diffraction peak (100) becomes much stronger and sharper at annealing temperature of 100 ºC, implying that 100 ºC-annealing film exihibits more significant crystallization than 80 ºC-annealing film. However, further increasing the annealing temperatures to 120 ºC and even 140 ºC, an obvious decline of diffraction peak intension can be observed for both 120 ºCannealing film and 140 ºC-annealing film. As can be seen from XRD results, the films annealed at 120 ºC and 140 ºC have less crystalline domains as compare to those of the films annealed at 80 ºC and 100 ºC, which implies that the polymer thin film has better intermolecular packing at lower annealing temperatures. The result could be attributed to the extension of conjugated main chains. To obtain more 11 Environment ACS Paragon Plus


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exact information of molecular packing, the 2D grazing-incidence wide-angle X-ray scattering (2DGIWAXS) was measured on the 80 ºC-annealing film (Figure 5). Two distinct diffraction peaks at q = 0.28 and 0.57 Å−1 could be observed in Figure 5b, which approximately accord with the diffraction angles in the above XRD pattern. The diffraction peak (010) at q = 1.78 Å−1 in Figure 5c evidences the π−π stacking distance of 3.52 Å, which is among of the smallest π−π distances for the reported polymer semiconductors. The result also indicates that the incorporation of 2FBT and DBPy into the thiophenecontaining copolymer can induce strong intermolecular interactions. Therefore, the results from AFM and XRD are in good agreement with that of OTFT devices for the polymer.

Figure 4. AFM height images of PDBPyDT2FBT films at different annealed temperatures.


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Figure 5. 2D grazing-incidence wide-angle X-ray scattering pattern of polymer thin film annealed at 80 ºC (a), the out of-plane (b) and in-plane (c) cuts of the corresponding 2D-GIWAXS patterns. 4. CONCLUSIONS In summary, we report a novel D–A1–D–A2 copolymer (PDBPyDT2FBT) based on 2FBT and DBPy. The polymer-based OTFTs with rather low LUMO/HOMO energy levels exhibit excellent electron and hole mobilities of 0.65 cm2V−1s−1 and 0.24 cm2V−1s−1 with a high Ion/Ioff value of 105, annealed at relatively low temperatures. Its outstanding characterization, containing easy synthesis and excellent device performances at low annealing temperatures, demonstrates that PDBPyDT2FBT has a promising application prospect in low-cost and large-area OTFTs. ASSOCIATED CONTENT Supporting Information GPC chromatogram, TGA plot, DSC curve, absorption spectrum, cyclic voltammogram, computer simulation, X-ray diffraction patterns and AFM phase images of polymer, NMR spectra and HRMS Spectrum of intermediate DBPyBr-24. This information is available free of charge via the Internet at http://pubs.acs.org/. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (S. Wang); [email protected] (Z. Yi).



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Author Contributions ⊥

Two authors contributed equally.

Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This research was financially supported by the National Natural Science Foundation of China (Project No. 51173055, 21504026), the National Program on Key Basic Research Project (973 Program, Grant No. 2013CBA01600) and the China Postdoctoral Science Foundation (No. 2013M542009) for financial support. The authors gratefully thank the Analytical and Testing Centre at Huazhong University of Science and Technology for characterization assistance. REFERENCES [1] Zaumseil, J.; Sirringhaus, H. Electron and Ambipolar Transport in Organic Field-Effect Transistors. Chem. Rev. 2007, 107, 1296−1323. [2] Beaujuge, P. M.; Fréchet, J. M. J. Molecular Design and Ordering Effects in π-Functional Materials for Transistor and Solar Cell Applications. J. Am. Chem. Soc. 2011, 133, 20009−20029. [3] Jones, B. A.; Ahrens, M. J.; Yoon, M. -H., Facchetti, A.; Marks, T. J.; Wasielewski, M. R. HighMobility Air-Stable n-Type Semiconductors with Processing Versatility: Dicyanoperylene-3,4:9,10bis(dicarboximides). Angew. Chem. Int. Ed. 2004, 43, 6363−6366. [4] Wang, C.; Dong, H.; Hu, W.; Liu, Y.; Zhu D. Semiconducting π-Conjugated Systems in Field-Effect Transistors: A Material Odyssey of Organic Electronics. Chem. Rev. 2012, 112, 2208−2267. [5] Bürgi, L.; Turbiez, M.; Pfeiffer, R.; Bienewald, F.; Kirner, H. -J.; Winnewisser, C. High-Mobility Ambipolar Near-Infrared Light-Emitting Polymer Field-Effect Transistors. Adv. Mater. 2008, 20, 2217−2224. [6] Tseng, H. -R.; Phan, H.; Luo, C.; Wang, M.; Perez, L. A.; Patel, S. N.; Ying, L.; Kramer, E. J.; Nguyen, T. -Q.; Bazan G. C.; Heeger, A. J. High-Mobility Field-Effect Transistors Fabricated with


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