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Sep 17, 2014 - Thu-Trang Do , Yasunori Takeda , Sergei Manzhos , John Bell ... José L. Zafra , Paula Mayorga Burrezo , Juan Casado , Kazuo Takimiya...
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Five-Ring Fused Tetracyanothienoquinoids as High-Performance and Solution-Processable n‑Channel Organic Semiconductors: Effect of the Branching Position of Alkyl Chains Jie Li,† Xiaolan Qiao,*,† Yu Xiong,† Hongxiang Li,*,† and Daoben Zhu‡ †

Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai 200032, China Beijing National Laboratory for Molecular Sciences, Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China

‡

S Supporting Information *

ABSTRACT: Dicyanomethylene-substituted quinoidal dithieno[2,3-d;2′,3′-d′]benzo[1,2-b;4,5-b′]dithiophene compounds (QDTBDTs) with alkyl chains branched at different positions were synthesized. Thin-film transistor characteristics showed that the type of charge carriers in QDTBDTs could be tuned by changing the branching position of the alkyl chains. QDTBDT-2C exhibited n-channel behavior, and the observed electron mobility was 0.57 cm2 V−1 s−1 without post-treatment, one of the highest values reported for spin-coated thin-film transistors with no annealing under ambient conditions. QDTBDT-4C-based transistors displayed electron-dominated ambipolar transport behavior, with electron mobilities reaching 0.2 cm2 V−1 s−1 and hole mobilities in the range of 10−3−10−4 cm2 V−1 s−1. QDTBDT-3C showed solution-concentrationdependent carrier transport characteristics, exhibiting n-type behavior at low solution concentrations and ambipolar performance at high solution concentrations with an electron mobility of 0.22 cm2 V−1 s−1 and a hole mobility of 0.034 cm2 V−1 s−1. CMOSlike inverters fabricated from QDTBDT-2C displayed high gain and high noise margins.

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INTRODUCTION Solution-processable organic semiconductors (OSCs) have attracted great interest because of their potential applications in large-area, flexible, and low-cost thin-film transistors.1−3 Until now, high mobilities of over 10 cm2 V−1 s−1 have been achieved for solution-processed p-channel OFETs,3−7 while the performance of n-channel OFETs has largely lagged behind. Though great progress on n-channel organic transistors has recently been made, with some solution-processable n-channel OSCs exhibiting mobilities higher than 1.0 cm2 V−1 s−1,8−10 most of them required high-temperature thermal annealing (>150 °C) and/or special device configurations, hindering the fabrication of devices on flexible substrates at low cost. In view of the important roles of n-channel OSCs in complementary metal−oxide−semiconductor (CMOS)-like digital integrated circuits,9,10 the development of solution-processable n-channel OSCs that can achieve high performance at low thermal annealing temperatures or even without thermal annealing is vital and required. Dicyanomethylene-substituted thiophene quinoidal compounds have low LUMO energies, making them potential candidates for high-performance n-channel OSCs.11 Currently, most of the reported dicyanomethylene-substituted thiophene quinoidal compounds are oligothiophene-based molecules, and they usually display mobilities lower than 0.2 cm2 V−1 s−1 because of the coexistence of isomers.11−13 Compared with © 2014 American Chemical Society

oligothiophene quinoidal compounds, fused thienoquinoids not only preserve the merits of quinoidal oligothiophenes but also resolve the problem of isomerism and therefore are supposed to display improved performance. As a try, we have reported a dicyanomethylene-substituted four-ring fused thienoquinoid, CMUT, which displayed an electron mobility of 0.43 cm2 V−1 s−1 under ambient conditions without thermal annealing.14 Although continuous thin films of CMUT can be formed only by the drop-casting method, which limits their practical applications, the high performance of CMUT demonstrates that fused-ring thienoquinoids are good candidates for highperformance n-channel OSCs. Moreover, it is believed that molecules with larger π-conjugated structures usually show better device performance.15 Hence, in the present work we first synthesized five-ring fused quinoidal dithieno[2,3-d;2′,3′d′]benzo[1,2-b;4,5-b′]dithiophene compounds (QDTBDTs) with alkyl chains branched at different positions (their chemical structures are shown in Scheme 1). The introduction of alkyl chains with different branching positions not only enables solution processing but also was motivated by its remarkable impact on the performance of the organic semiconductors, which has been reported previously.16−20 The spin-coated thinReceived: August 11, 2014 Revised: September 15, 2014 Published: September 17, 2014 5782

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Scheme 1. Synthesis and Chemical Structures of the Target Compounds QDTBDT-2C, QDTBDT-3C, and QDTBDT-4C

film transistor characteristics showed that all of the compounds displayed high device performance. QDTBDT-2C with the branching position one carbon away from the conjugation backbone showed the best performance, as an average electron mobility of 0.50 cm2 V−1 s−1 and a maximum mobility of up to 0.57 cm2 V−1 s−1 were achieved from the as-deposited thin films without any post-treament; the latter is one of the highest values reported for spin-coated thin-film transistors with no thermal annealing under ambient conditions. CMOS-like inverters fabricated from QDTBDT-2C exhibited a high gain of 15 and good noise margins as high as 69% of 1/2VDD. Though it is an intrinsic property of OSCs to transport both holes and electrons, most OSCs exhibit unipolar behavior in thin-film transistors since their molecular energy levels are suited to one type of charge carrier only. Thus, it is greatly important and challenging to tune the type of charge carrier (hole or electron) in OSCs. Currently, the common methods for tuning the charge carriers in OSCs are (i) modifying the conjugation backbone of the OSC, which strongly affects the ionization potential and electron affinity of the OSC,8,21−23 and (ii) optimizing the device configuration and materials processing.24−26 Interestingly, for the same device architecture and material processing conditions, we observed a conversion from unipolar semiconductor (electron) to ambipolar behavior for QDTBDTs when the branching position of the alkyl substituents was changed, suggesting that alkyl chain modification is an efficient method for the rational design of high-performance OSCs.

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step without further purification. Compounds 1-3C and 1-4C were synthesized according to the same procedure as for 1-2C. Synthesis of Compounds 2. A dry 100 mL three-neck flask was flushed with nitrogen, and then compound 1-2C (6.39 g, 9 mmol), 1,4-dibromo-2,5-bis(methylsulfinyl)benzene (1.08 g, 3 mmol), anhydrous DMF (30 mL), and Pd(PPh3)4 (0.8 g, 0.69 mmol) were added. The mixture was stirred at 100 °C for 24 h under a nitrogen atmosphere and then poured into water (100 mL). The organic material was extracted with dichloromethane (30 mL × 3). The combined organic layers were washed with water (50 mL × 3) and brine (50 mL) and then dried over anhydrous MgSO4. After evaporation of the solvent under vacuum, the residue was purified by column chromatography on silica gel (CH2Cl2/hexane = 1:1) to give 2-2C as a light-yellow oil (2.78 g, 89%). Compounds 2-3C and 24C were synthesized according to the same procedure as for 2-2C. 2-2C. 1H NMR (300 MHz, CDCl3): δ 8.16 (s, 2H), 7.10 (s, 2H), 7.03 (s, 2H), 2.55−2.58 (d, 4H), 2.54 (s, 6H), 1.62−1.64 (m, 2H), 1.26−1.35 (m, 80H), 0.85−0.90 (m, 12H). 13C NMR (100 MHz, CDCl3): δ 147.1, 143.3, 136.8, 132.5, 130.1, 126.0, 123.6, 41.5, 39.0, 34.9, 33.4, 33.3, 31.9, 30.1, 30.0, 29.7, 29.6, 29.4, 27.8, 26.8, 26.6, 22.7, 17.6, 14.1, 13.6. MS (MALDI-TOF) m/z: 1039.9 (M + H)+. HRMS (MALDI) m/z: calcd for C64H111O2S4 [M + H]+ 1039.7461, found 1039.7452. 2-3C. Yield: 55%. 1H NMR (300 MHz, CDCl3): δ 8.16 (s, 2H), 7.15 (s, 2H), 7.06 (s, 2H), 2.58−2.64 (t, 4H), 2.56 (s, 6H), 1.34−1.37 (m, 4H), 1.26−1.34 (m, 82H), 0.84−0.92 (m, 12H). 13C NMR (100 MHz, CDCl3): δ 147.1, 144.8, 136.9, 132.5, 129.7, 125.9, 122.6, 41.7, 37.1, 34.5, 33.4, 31.9, 30.1, 29.7, 29.6, 29.3, 27.7, 26.6, 22.7, 14.1. MS (MALDI-TOF) m/z: 1067.8 (M + H)+. HRMS (MALDI) m/z: calcd for C66H115O2S4 [M + H]+ 1067.7774, found 1067.7791. 2-4C. Yield: 73%. 1H NMR (300 MHz, CDCl3): δ 8.17 (s, 2H), 7.15 (s, 2H), 7.07 (s, 2H), 2.59−2.63 (t, 4H), 2.56 (s, 6H), 1.61 (m, 4H), 1.26−1.42 (m, 86H), 0.85−0.92 (m, 12H). 13C NMR (100 MHz, CDCl3): δ 147.1, 144.5, 136.9, 132.5, 130.0, 125.9, 122.8, 41.7, 37.3, 33.6, 33.4, 31.9, 30.8, 30.1, 29.7, 29.6, 29.3, 27.8, 27.6, 26.8, 26.7, 22.7, 14.1. MS (MALDI-TOF) m/z: 1095.8 (M + H)+. HRMS (MALDI) m/z: calcd for C68H119O2S4 [M + H]+ 1095.8087, found 1095.8092. Synthesis of Compounds 3. Compound 2-2C (3.12 g, 3 mmol) was dissolved in chloroform/DMF (90 mL, 2:1 v/v). To this solution was added N-bromosuccinimide (1.12 g, 6.3 mmol) in portions. The mixture was stirred for 16 h at room temperature and then poured into water (100 mL). The organic material was extracted with dichloromethane (40 mL × 3). The combined organic layers were washed with

EXPERIMENTAL SECTION

Synthesis of Compounds 1. To a solution of 3-(2decanyltetradecanyl)thiophene (0.84 g, 3 mmol) in THF (15 mL) at −78 °C was added n-BuLi (1.3 mL, 3 mmol, 2.4 M in hexane). The solution was allowed to stand at −40 to −50 °C for 1 h and then cooled to −78 °C again. (Bu)3SnCl (0.98 g, 3 mmol) was added. The reaction mixture was stirred for 1 h and then warmed to room temperature and stirred overnight. The reaction mixture was quenched with H2O and extracted with hexane. The organic layer was dried with Na2SO4 and filtered. Hexane was removed under vacuum, affording crude product 1-2c (1.41 g, 99%), which was used directly for the next 5783

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water (80 mL × 3) and brine (80 mL) and then dried over anhydrous MgSO4. After evaporation of the solvent under vacuum, the residue was purified by column chromatography on silica gel (CH2Cl2/hexane = 1:1) to give 3-2C as a yellow oil (2.84 g, 79%). Compounds 3-3C and 3-4C were synthesized according to the same procedure as for 32C. 3-2C. 1H NMR (300 MHz, CDCl3): δ 8.10 (s, 2H), 6.96 (s, 2H), 2.56 (s, 6H), 2.49−2.52 (d, 4H), 1.39−1.46 (m, 2H), 1.23 (m, 80H), 0.83−0.87 (m, 12H). 13C NMR (100 MHz, CDCl3): δ 147.4, 142.7, 136.4, 131.9, 129.9, 125.9, 112.7, 41.7, 38.6, 34.2, 33.4, 31.9, 30.1, 30.0, 29.7, 29.6, 29.4, 26.6, 22.7, 14.1. MS (MALDI-TOF) m/z: 1197.9 (M + 3H)+. HRMS (MALDI) m/z: calcd for C64H109O2S4Br2 [M + H]+ 1195.5672, found 1195.5672. 3-3C. Yield: 83%. 1H NMR (300 MHz, CDCl3): δ 8.13 (s, 2H), 7.03 (s, 2H), 2.60 (s, 6H), 2.54−2.58 (t, 4H), 1.51−1.55 (m, 4H), 1.26 (m, 82H), 0.86−0.89 (m, 12H). 13C NMR (100 MHz, CDCl3): δ 147.3, 143.8, 136.6, 131.9, 129.4, 125.8, 111.9, 41.9, 37.2, 33.6, 33.4, 31.9, 30.1, 29.7, 29.6, 29.3, 26.9, 26.7, 26.6, 22.7, 14.1. MS (MALDITOF) m/z: 1225.5 (M + 3H)+. HRMS (MALDI) m/z: calcd for C66H112O2S4Br2Na [M + Na]+ 1245.5804, found 1245.5842. 3-4C. Yield: 81%. 1H NMR (300 MHz, CDCl3): δ 8.13 (s, 2H), 7.03 (s, 2H), 2.60 (s, 6H), 2.54−2.57 (t, 4H), 1.57 (m, 4H), 1.25− 1.32 (m, 86H), 0.83−0.89 (m, 12H). 13C NMR (100 MHz, CDCl3): δ 147.4, 143.4, 136.6, 131.9, 129.5, 125.8, 112.1, 37.2, 33.5, 33.3, 31.9, 30.1, 30.0, 29.7, 29.6, 29.4, 29.3, 26.8, 26.7, 26.6, 22.8, 22.7, 14.1. MS (MALDI-TOF) m/z: 1253.8 (M + 3H)+. HRMS (MALDI) m/z: calcd for C68H116O2S4Br2Na [M + Na]+ 1273.6117, found 1273.6086. Synthesis of Compounds 4. A dry 50 mL three-neck flask was flushed with nitrogen, and then compound 3-2C (2.4 g, 2 mmol) and Eaton’s reagent (17 mL) were added. The mixture was stirred at room temperature in the dark for 3 days. The mixture was poured into ice− water, and a dark-brown solid was collected by suction filtration and dried in a vacuum oven. The solid was dissolved in pyridine (70 mL), and the solution was refluxed for 12 h. The solvent was evaporated under vacuum, and the crude product was purified by column chromatography on silica gel using hexane as an eluent to give 4-2C as an oil (1.91 g, 84%). The oil solidified after standing overnight. Compounds 4-3C and 4-4C were synthesized according to the same procedure as for 4-2C. 4-2C. 1H NMR (300 MHz, CDCl3): δ 7.97 (s, 2H), 2.62−2.63 (d, 4H), 1.94 (m, 2H), 1.24−1.32 (m, 80H), 0.86−0.89 (m, 12H). 13C NMR (100 MHz, CDCl3): δ 138.7, 137.3, 134.8, 131.2, 129.9, 114.8, 111.5, 37.2, 34.0, 33.6, 31.9, 29.9, 29.7, 29.6, 29.4, 26.5, 22.7, 14.1. MS (MALDI-TOF) m/z: 1132.7 (M + 2H) + . Anal. Calcd for C62H100Br2S4: C, 65.69; H, 8.89; Br, 14.10; S, 11.32. Found: C, 65.60; H, 8.94. 4-3C. Yield: 84%. 1H NMR (300 MHz, CDCl3): δ 8.01 (s, 2H), 2.65−2.69 (t, 4H), 1.61−1.66 (m, 4H), 1.25 (m, 82H), 0.86−0.89 (m, 12H). 13C NMR (100 MHz, CDCl3): δ 138.7, 137.4, 135.5, 131.3, 130.0, 114.9, 111.7, 37.0, 33.4, 32.0, 31.5, 30.1, 29.7, 29.4, 29.3, 26.6, 26.3, 22.7, 14.1. MS (MALDI-TOF) m/z: 1159.5 (M + H)+. 4-4C. Yield: 81%. 1H NMR (300 MHz, CDCl3): δ 8.01 (s, 2H), 2.69−2.73 (t, 4H), 1.70 (m, 4H), 1.21 (m, 86H), 0.84−0.88 (m, 12H). 13 C NMR (100 MHz, CDCl3): δ 138.7, 137.4, 135.3, 131.3, 130.0, 114.9, 111.0, 37.0, 33.5, 33.1, 31.9, 30.1, 29.7, 29.4, 29.3, 26.6, 25.1, 22.7, 14.1. MS (MALDI-TOF) m/z: 1187.5 (M + H)+. Synthesis of QDTBDTs. Malononitrile (0.106 g, 1.6 mmol) was added to an ice−salt-cooled suspension of sodium hydride (0.128 g, 3.2 mmol) in 1,2-dimethoxyethane (10 mL) under a nitrogen stream. The mixture was stirred at room temperature for another 20 min. Then a solution of 4-2C in 1,2-dimethoxyethane (10 mL) was added by a syringe, followed by tetrakis(triphenylphosphine)palladium (40 mg), and the mixture was heated to reflux for 12 h. The resulting solution was treated with saturated bromine water, and the precipitate was filtered. The crude product was washed with water, dried under vacuum, and purified by flash chromatography on silica gel (light petroleum/dichloromethane, 1:1 v/v, and then dichloromethane) to give QDTBDT-2C (0.10 g, 57%). Compounds QDTBDT-3C and QDTBDT-4C were synthesized according to the same procedure as for QDTBDT-2C.

QDTBDT-2C. Mp: 179−180 °C. 1H NMR (300 MHz, CDCl3): δ 7.54 (s, 2H), 2.91 (d, 4H), 1.93 (m, 2H), 1.23−1.33 (m, 80H), 0.85− 0.88 (m, 12H). 13C NMR (100 MHz, CDCl3): δ 165.5, 156.0, 145.3, 140.9, 136.7, 131.8, 117.3, 114.9, 113.6, 70.3, 38.8, 34.3, 33.1, 31.9, 29.9, 29.7, 29.6, 29.5, 29.3, 26.2, 22.7, 14.1. MS (MALDI-TOF) m/z: 1101.9 (M + H)+. HRMS (MALDI) m/z: calcd for C68H101N4S4 [M + H]+ 1101.6904, found 1101.6916. Anal. Calcd for C68H100N4S4: C, 74.13; H, 9.15; N, 5.08; S, 11.64. Found: C, 74.19; H, 9.18; N, 4.85. QDTBDT-3C. Yield: 57%. Mp: 166−167 °C. 1H NMR (300 MHz, CDCl3): δ 7.53 (s, 2H), 2.95 (d, 4H), 1.62−1.68 (m, 4H), 1.46−1.47 (m, 82H), 0.86−0.89 (m, 12H). 13C NMR (100 MHz, CDCl3): δ 165.1, 155.2, 145.3, 141.0, 137.4, 131.8, 117.2, 114.7, 113.2, 70.2, 37.6, 33.2, 31.9, 30.0, 29.7, 29.4, 26.5, 22.7, 14.1. MS (MALDI-TOF) m/z: 1128.6 (M)+. HRMS (MALDI) m/z: calcd for C70H105N4S4 [M + H]+ 1129.7217, found 1129.7218. Anal. Calcd for C70H104N4S4: C, 74.41; H, 9.28; N, 4.96; S, 11.35. Found: C, 74.49; H, 9.32; N, 4.92. QDTBDT-4C. Yield: 61%. Mp: 171−172 °C. 1H NMR (300 MHz, CDCl3): δ 7.54 (s, 2H), 2.59 (d, 4H), 1.70 (m, 4H), 1.29−1.40 (m, 86H), 0.86−0.89 (m, 12H). 13C NMR (100 MHz, CDCl3): δ 165.3, 155.4, 145.3, 141.1, 136.9, 131.8, 117.3, 114.8, 113.4, 70.2, 37.2, 33.4, 33.3, 31.9, 30.1, 29.7, 26.6, 22.7, 14.1. MS (MALDI-TOF) m/z: 1157.2 (M + H)+. HRMS (MALDI) m/z: calcd for C72H109N4S4 [M + H]+ 1157.7530, found 1157.7535. Anal. Calcd for C72H108N4S4: C, 74.68; H, 9.40; N, 4.84; S, 11.08. Found: C, 74.53; H, 9.38; N, 4.68. OFET Device Fabrication and Characterization. Bottom-gate/ top-contact (BGTC) OFETs were fabricated. Heavily n-doped conductive silicon wafers with a 300 nm thermally oxidized SiO2 layer (dielectric capacitance C = 10 nF/cm2) were used as the substrates. Cleaning of Si wafers and modification with noctadecyltrichlorosilane (OTS) were performed according to the reported procedure.27 Subsequently, the semiconducting layer was deposited by spin-coating of the QDTBDT-2C, QDTBDT-3C, or QDTBDT-4C solution (1−10 mg/mL in chloroform) on top of the OTS-modified SiO2 surface at 5000 rpm for 30 s under ambient conditions. Finally, 50 nm Au source and drain electrodes were deposited by vacuum evaporation on the top of the organic film through a shadow mask. The characteristics of the OFETs were measured using a Keithley 4200 semiconductor parameter analyzer under ambient conditions. The mobilities (μ) were calculated from the data in the saturated regime according to the equation ISD = (μWCi/2L)(VG − VT)2, where ISD is the drain current in the saturated regime, W (273 μm) and L (31 μm) are the semiconductor channel width and length, respectively, Ci is the capacitance per unit area of the gate dielectric layer, and VG and VT are the gate and threshold voltages, respectively.

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RESULTS AND DISCUSSION The synthesis and chemical structures of compounds QDTBDT-2C, QDTBDT-3C, and QDTBDT-4C are shown in Scheme 1. These compounds have the same conjugation backbone, but their alkyl substituents are branched one, two, and three carbons away from the conjugation backbone, respectively. Starting from the corresponding 2,7dibromodithieno[2,3-d:2′,3′-d′]benzo[1,2-b:4,5-b′]dithiophenes, the QDTBDTs were synthesized through a Pdcatalyzed Takahashi coupling reaction followed by oxidation with bromine28 and were obtained in moderate isolated yields (55−65%). The QDTBDTs display a deep-green color with a metallic luster in the solid state and have good solubilities in common organic solvents such as CH2Cl2, CHCl3, THF, and chlorobenzene. Their chemical structures were fully characterized by NMR spectroscopy, mass spectrometry (MS), highresolution MS (HRMS), and elemental analysis. The electrochemical properties of the QDTBDTs were investigated by cyclic voltammetry (CV). The redox potentials were estimated as the midpoints of the peak potentials in the forward and backward scans. As shown in Figure S1a in the 5784

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Supporting Information, all of the compounds exhibited the same CV curves, with a reversible oxidation peak (Eox 1/2 = 1.33 V vs SCE) and two reversible reduction peaks (Ered‑1 1/2 = 0.04 V vs SCE and Ered‑2 1/2 = −0.11 V vs SCE). The HOMO and LUMO energies calculated from the CV data were −5.68 and −4.39 eV, respectively, and the electrochemical energy gap (Eelec g ) was 1.29 eV. These results indicate that the changes in the alkyl branching position have no influence on the electrochemical properties of the QDTBDTs. The absorption profiles of the QDTBDTs in dilute chloroform solutions and in thin films are illustrated in Figure S1b in the Supporting Information. In solution, the absorptions of the QDTBDTs were almost identical, similar to the CV studies. The optical band gap (Eopt g ) calculated from the absorption onset in solution was 1.48 eV, which is a little higher than Eelec g . In the solid state, the maximum absorptions of QDTBDT-3C and QDTBDT-4C were blue-shifted by about 66 and 26 nm, respectively, relative to the solution spectra. On the contrary, the thin-film maximum absorption of QDTBDT2C displayed a noticeable bathochromic shift (about 14 nm) compared with that in solution. These significant absorption differences prove that the changes in the alkyl branching position strongly affect the molecular packing in the thin films. The charge transport properties of the QDTBDTs were characterized using BGTC thin-film transistors. The semiconductor layers were deposited by spin-coating of a chloroform solution (1−10 mg/mL) onto OTS-modified SiO2/Si substrates at 5000 rpm. All of the devices were fabricated and tested under ambient conditions. In each case, the mobility and threshold voltage were calculated from the saturation region. Figure 1 presents typical output and transfer curves of the transistors deposited from 10 mg/mL chloroform solutions, and the performance data for the devices are collected in Table 1. QDTBDT-2C exhibited unipolar electron transport characteristics with well-defined linear and saturation regimes, and the optimal electron motility was 0.40 cm2 V−1 s−1. QDTBDT-3C and QDTBDT-4C displayed electrondominated ambipolar transport behavior, with the typical Vshaped transfer curves. The optimal saturation mobilities of QDTBDT-3C were 0.22 cm2 V−1 s−1 for electrons and 3.4 × 10−2 cm2 V−1 s−1 for holes. For QDTBDT-4C, the saturation electron and hole mobilities were 0.15 and 8.0 × 10−3 cm2 V−1 s−1, respectively. It can be seen that changing the branching position of the alkyl substituents resulted in a conversion from unipolarity to ambipolarity. An interesting solution-concentration-dependent ambipolar transport behavior was observed for compounds QDTBDT-3C and QDTBDT-4C (Table 1). The concentrations of the spincoated solutions were varied from 5 to 2.5 to 1 mg/mL without changing any of the other experimental conditions. The transfer curves of QDTBDT devices deposited at various solution concentrations are shown in Figures S2 and S3 in the Supporting Information. With the changes in solution concentration, QDTBDT-3C-based devices displayed drastically different transport behaviors compared with that of the devices prepared from the 10 mg/mL solution. The hole transport disappeared, and the devices exhibited typical n-type characteristics with a decreasing electron mobility. For QDTBDT-4C, all of the devices showed clearly ambipolar behavior with predominant electron-transport character. The electron mobility was almost unchanged at various solution concentrations, but the hole mobility decreased by about 1 order of magnitude with the decrease in the solution

Figure 1. Output and transfer curves of thin-film transistors based on QDTBDTs: (a, b) QDTBDT-2C; (c−f) QDTBDT-3C; (g−j) QDTBDT-4C. The semiconductor layers were deposited by spincoating with a 10 mg/mL chloroform solution.

concentration. The mobility of QDTBDT-2C-based devices was slightly increased with the decrease in the concentration of the spin-coating solution. The maximum electron mobility reached 0.57 cm2 V−1 s−1, which was achieved at a solution concentration of 1 mg/mL. This mobility value is a new record for spin-coated quinoidal molecules to date and one of the highest values reported for solution-processable n-channel OSCs without post-treament under ambient conditions.14,28−37 In order to deeply understand the charge transport behaviors of QDTBDTs, atomic force microscopy (AFM) was used to investigate the surface morphologies of the thin films. Figure 2 shows AFM images of the spin-coated thin films prepared from various solution concentrations. On the whole, the films possessed distinct morphologies with different alkyl chain branching positions, and the grain size increased with the prolonging of the branching position. The films of QDTBDT2C displayed the best smoothness, and the root-mean-square (RMS) values increased with the distance of the branching site from the conjugation backbone (the RMS values of films spincoated using the same solution concentration were QDTBDT2C < QDTBDT-3C < QDTBDT-4C). All of the films of QDTBDT-2C exhibited good continuity. The morphology of the QDTBDT-3C films changed a lot at different solution concentrations, but the films were continuous. For QDTBDT4C, the morphologies changed from granular (10 mg/mL) to stripelike grains (≤5 mg/mL). On the basis of the device performance data in Table 1, the film smoothness and 5785

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Table 1. Performance of QDTBDT Transistors compd

concentration (mg/mL)

μe (cm2 V−1 s−1)a

Vth (V)

QDTBDT-2C

10 5 2.5 1 10 5 2.5 1 10 5 2.5 1

0.30 (0.40) 0.36 (0.43) 0.47 (0.56) 0.50 (0.57) 0.18 (0.22) 0.093 (0.1) 0.033 (0.043) 0.018 (0.019) 0.13 (0.15) 0.11 (0.13) 0.16 (0.17) 0.13 (0.14)

−6 to 12 −2 to 10 2−7 5−14 −3 to 9 −8 to −5 14−21 9−10 2−12 7−16 9−15 20−21

QDTBDT-3C

QDTBDT-4C

μh (cm2 V−1 s−1)a NA NA NA NA 2.2 × NA NA NA 5.9 × 1.3 × 1.1 × 7.6 ×

10−2 (3.4 × 10−2)

10−3 10−3 10−3 10−4

(8.0 (1.6 (1.3 (8.3

× × × ×

10−3) 10−3) 10−3) 10−4)

Vth (V) NA NA NA NA −58 NA NA NA −57 −68 −65 −70

to −68

to to to to

−70 −72 −73 −73

Average mobilities, with the maximum mobilities shown in parentheses. The average film thickness of QDTBDT films cast from 10, 5, 2.5, and 1 mg/mL solutions were 73, 35, 16, and 7 nm, respectively. More than 20 devices were characterized at each concentration of QDTBDT-2C, QDTBDT-3C, and QDTBDT-4C. a

Figure 3. (a−c) 1D-GIXD patterns in the 2θ range of 2.5−5° for thin films of (a) QDTBDT-2C, (b) QDTBDT-3C, and (c) QDTBDT-4C spin-coated from solutions with different concentrations. (d−f) 2DGIXD patterns for thin films of (d) QDTBDT-2C, (e) QDTBDT-3C, and (f) QDTBDT-4C spin-coated from 10 mg/mL solutions.

Figure 2. AFM images (4 μm × 4 μm) of thin films of (a−d) QDTBDT-2C (a, 10 mg/mL; b, 5 mg/mL; c, 2.5 mg/mL; d, 1 mg/ mL), (e−h) QDTBDT-3C (e, 10 mg/mL; f, 5 mg/mL; g, 2.5 mg/mL; h, 1 mg/mL), and (i−l) QDTBDT-4C (i, 10 mg/mL; j, 5 mg/mL; k, 2.5 mg/mL; l, 1 mg/mL).

QDTBDT-4C, two closely neighboring peaks (corresponding to d spacings of 32.7 and 31.4 Å) were observed at various solution concentrations, indicating the coexistence of two phases. Since single crystals of QDTBDTs are difficult to grow because of the long branched alkyl chains, 2D-GIXD measurements, which provide information on both the outof-plane (qz) and in-plane (qxy) directions, were performed to further study the orientation and crystallinity of the thin films. Figure S5 in the Supporting Information shows the 2D-GIXD images of QDTBDTs films spin-coated using different solution concentrations. All of the films exhibited a series of out-of-plane peaks along qz, consistent with the peaks observed by 1DGIXD. It is necessary to point out that the neighboring double peaks of QDTBDT-3C and QDTBDT-4C films were nearly invisible (Figure S5e for QDTBDT-3C and Figure S5i−l for QDTBDT-4C). This is ascribed to the inferior resolution of the 2D detector compared with the 1D detector. Under these experimental conditions, the two closely neighboring peaks overlapped. The resolution of 2D-GIXD can be improved by moving the 2D detector farther away from the sample. The high-resolution images of the QDTBDT films spin-coated from 10 mg/mL solutions (the same films as used in Figure S5a,e,i) are shown in Figure 3(d−f). Neighboring peaks can be clearly seen in Figure

continuity and the size of grains and grain boundaries impacted the transistor performance. Out-of-plane grazing-incidence X-ray diffraction (1D-GIXD) measurements are an effective way to identify the molecular packing in organic thin films, which is closely related to the device performance. Figure 3a−c shows 1D-GIXD patterns of the QDTBDT thin films deposited at different solution concentrations with 2θ in the range of 2.5−5° (1D-GIXD diffraction results with 2θ = 2.5−25° are shown in Figure S4 in the Supporting Information). All of the thin films exhibited clear Bragg diffractions peaks, suggesting a high degree of crystallinity. The films of QDTBDT-2C exhibited good uniformity and a symmetrical peak with a d spacing of 32.4 Å independent of the solution concentration, consistent with the device performance. QDTBDT-3C films also showed one diffraction peak at low solution concentrations (≤5 mg/mL) and the d spacing estimated from this peak was 36.2 Å, which is much larger than that of QDTBDT-2C. Interestingly, an additional new peak with a d spacing of 33.2 Å appeared for the films spin-coated from 10 mg/mL solutions, indicating that a new phase had arisen in the thin films. For the films of 5786

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range of 10−3−10−4 cm2 V−1 s−1. QDTBDT-3C showed solution-concentration-dependent carrier transport characteristics, exhibiting n-type behavior at low solution concentrations and ambipolar performance with an electron mobility of 0.22 cm2 V−1 s−1 and a hole mobility of 0.034 cm2 V−1 s−1 at high solution concentrations. CMOS-like inverters fabricated from QDTBDT-2C displayed high gain and high noise margins. All of these results suggest the potential practical applications of QDTBDTs as n- channel OSCs and demonstrat that alkyl chain modification is an efficient method for the rational design of high-performance OSCs.

3e,f, consistent with the 1D diffraction peaks. Unfortunately, for all of the thin films, there were scarcely any diffraction peaks in the in-plane direction (qxy) except for the weak stripelike spots in Figure S5i (marked by the white arrow) with a distance of 3.57 Å, which probably derived from the π−π stacking of QDTBDT-4C. Although we could not obtain the exact molecular packing states of the QDTBDTs, it was particularly interesting that the QDTBDT thin films with two-phase structures (QDTBDT-3C films deposited from 10 mg/mL solution and all of the QDTBDT-4C films) displayed ambipolar transport character, while the films with single-phase structures [all of the QDTBDT-2C films and the QDTBDT-3C films prepared using low solution concentrations (≤5 mg/mL)] showed unipolar behavior. QDTBDT-3C was the critical point for the conversion from one to two thin-film phases, which led to the conversion from unipolar to ambipolar transport. There is no doubt that the newly emergent phase in the QDTBDT-3C films facilitates hole transport. We suppose that the holes and electrons might be transported in segregated paths in QDTBDT-based ambipolar transistors, though more evidence to support this assumption is required. The molecular packing of QDTBDTs in the thin films is determined by several factors, including substrate−molecule interactions, molecule−molecule interactions, and solvent− molecule interactions. Since all of the films were deposited from chloroform solutions on OTS-modified SiO2 substrates and the QDTBDTs have very similar chemical structures, the changes in molecule−molecule interactions should be responsible for the different molecular packings of QDTBDTs in the thin films, demonstrating the important role of the branching position of the side chains in determining the molecular aggragation state. To inspect the practicality of the new quinoidal molecules, CMOS-like inverters were constructed. QDTBDT-2C films were selected for fabrication of the n-channel thin-film transistors because of its high electron mobility. We selected our previously reported nine-ring fused linear thienoacene with C6H13 side chains38 (for the chemical structure, see Figure S6a in the Supporting Information), which has a hole mobility of 0.25−0.5 cm2 V−1 s−1, as the p-channel organic semiconductor. The inverters exhibited clear inverting functionality with a gain of 15 at VDD = +100 V (Figure S6c), which is comparable to that of the state-of-the-art CMOS-like inverters based on twocomponent organic semiconductors. Moreover, high noise margins of up to ∼69% of 1/2VDD were achieved (Figure S6d). These results demonstrate the potential applications of quinoidal molecules in complementary circuits.

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ASSOCIATED CONTENT

S Supporting Information *

Absorption spectra, CV data, electric characteristics of thin-film transistors, GIXD characterization, and the complementary inverter details. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21190031, 51273212, and 51303201) and the “Strategic Priority Research Program” of the Chinese Academy of Sciences (XDB12010100). H.L. thanks the Shanghai Synchrotron Radiation Facility (beamline BL14B1) for providing the beam time.

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REFERENCES

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CONCLUSIONS In conclusion, dicyanomethylene-substituted quinoidal dithieno[2,3-d;2′,3′-d′]benzo[1,2-b;4,5-b′]dithiophenes with alkyl side chains branched at different positions were strategically designed and successfully synthesized. Thin-film transistor characteristics showed that a conversion from a unipolar organic semiconductor to an ambipolar one was achieved by changing the branching position of the alkyl chains. QDTBDT-2C with the branching position one carbon away from the conjugation backbone exhibited the best performance, with an electron mobility of 0.57 cm2 V−1 s−1 without annealing. QDTBDT-4C-based transistors displayed electrondominated ambipolar transport behavior, with the electron mobility reaching 0.2 cm2 V−1 s−1 and the hole mobility in the 5787

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