Synthesis of a Conjugated Polymer with Broad Absorption and Its

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Synthesis of a Conjugated Polymer with Broad Absorption and Its Application in High-Performance Phototransistors Yao Liu,†,‡ Haifeng Wang,† Huanli Dong,† Jiahui Tan,†,‡ Wenping Hu,*,† and Xiaowei Zhan*,† †

Beijing National Laboratory for Molecular Sciences and CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China ‡ Graduate School of Chinese Academy of Sciences, Beijing 100049, China S Supporting Information *

ABSTRACT: An amorphous copolymer (PBDT−BBT) of 4,8-bis(2thienyl)benzo[1,2-b:4,5-b′]dithiophene and 5,5′-bibenzo[c][1,2,5]thiadiazole was synthesized by Stille coupling polymerization. PBDT−BBT exhibited good solution processability, excellent thermal stability with decomposition temperature of 437 °C, broad absorption (300−800 nm), deep HOMO level (−5.7 eV), and LUMO level (−3.7 eV). The microstructure order of PBDT− BBT thin films is not susceptible to thermal annealing temperature (80−200 °C). Field-effect transistors based on this polymer exhibited a charge-carrier mobility of 6 × 10−3 cm2 V−1 s−1, threshold voltage of −1 V, and on/off current ratio of 106 without any post-treatments. Thin film phototransistors of PBDT−BBT exhibited a photoresponsivity of 3200 mA W−1 and photocurrent/dark current ratio of 4 × 105.

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A W−1.38 The highest photoresponsivity of conjugated polymers (250 A W−1) was achieved from poly(3-hexylthiophene) (P3HT)-based devices.39 We reported a phototransistor based on a polycarbazole derivative with quite high on/off ratio of 4.6 × 104.4 Recently, the large planar benzo[1,2-b:4,5-b′]dithiophene (BDT) unit has emerged as an attractive building block for conjugated polymers. Copolymers of BDT and bithiophene exhibited a high mobility of 0.25 cm2 V−1 s−1 in OFETs.34,40 The polymer solar cells (PSCs) based on blends of BDT-based copolymers and soluble fullerene derivatives PC61BM or PC71BM afforded high power conversion efficiencies (4− 8%).41−52 In particular, 4,8-bis(2-thienyl)benzo[1,2-b:4,5-b′] dithiophene-based copolymers exhibited broad absorption and high performance as donor materials in PSCs,48−50 However, the application of BDT-based copolymers in phototransistors has not been reported. Very recently, we reported a new n-type building block 5,5′-bibenzo[c][1,2,5]thiadiazole (BBT) and a BBT-based polymer for high-performance PSCs.53 Here, we report a low band gap copolymer (PBDT−BBT, Scheme 1) of 4,8-bis(2-thienyl)benzo[1,2-b:4,5-b′]dithiophene and 5,5′bibenzo[c][1,2,5]thiadiazole. PBDT−BBT has a deep highest occupied molecular orbital (HOMO) level (−5.7 eV) and a deep lowest unoccupied molecular orbital (LUMO) level (−3.7 eV) due to the stronger electron-withdrawing ability of BBT unit. In addition, BBT tends to adopt a twisted geometry, which can improve solubility of the polymer. Therefore,

INTRODUCTION Phototransistors, allowing additional light as the “fourth terminal” compared to common organic field-effect transistors (OFETs), are more promising as photodetectors than conventional photodiodes due to their high photosensitivity deriving from an internal amplification of the photocurrent.1−6 Phototransistors have been widely employed for light induced switches, light triggered amplification, detection circuits, and highly sensitive image sensors.7−9 Critical processes, which confine the realization of high-performance phototransistors, include the generation of charge carriers under light irradiation, the transfer of charge carriers, and the collection of charge carriers for signal magnification.4,10−15 Conjugated polymers have emerged as a class of solutionprocessable semiconductors, and their optical and electronic properties can be tuned by chemical modification.16−20 On one hand, the donor (D)−acceptor (A) strategy is widely used in synthetic design of narrow band gap conjugated polymers to harvest more incident photons for efficient charge-carrier generation.17,21−23 On the other hand, to improve chargecarrier mobility and ensure broad absorption, fused-ring building blocks have been introduced into conjugated polymer backbone.24−31 This approach can lead to more rigid and planar polymer backbone, thereby extending π-conjugated length, lowering band gap, and facilitating intermolecular charge carrier hopping and transporting.29,32−34 Therefore, conjugated polymers are promising semiconductors for high-performance phototransistors.11,35−37 Recently, several groups have reported their great progress in polymer thin film phototransistors. For example, Wang et al. reported phototransistors based on a polyfluorene derivative F8T2 with a photoresponsivity of 18.5 © 2012 American Chemical Society

Received: November 27, 2011 Revised: January 10, 2012 Published: February 1, 2012 1296

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Scheme 1. Synthetic Route of Polymer PBDT−BBT

(GPC) measurements were performed on a Waters 515 chromatograph connected to a Waters 2414 refractive index detector, using tetrahydrofuran (THF) as eluent and polystyrene standards as calibrants; three Waters Styragel columns (HT2, 3, 4) connected in series were used. The film morphology was analyzed in air using a Nanoscope III atomic force microscope (Digital Instruments) operated in tapping mode. X-ray diffraction (XRD) of thin films was performed in the reflection mode at 40 kV and 200 mA with Cu Kα radiation using a 2 kW Rigaku D/max-2500 X-ray diffractometer. Device Fabrication and Characterization. All the devices based on PBDT−BBT thin films were fabricated in the bottom gate, top contact configuration under ambient atmosphere. Highly n-doped silicon and thermally grown silicon dioxide (500 nm) were used as back gate and gate dielectric, respectively. The substrates were cleaned with pure water, hot concentrated sulfuric acid−hydrogen peroxide solution (concentrated sulfuric acid/hydrogen peroxide water = 2:1), pure water, and pure isopropyl alcohol. Then vaporized octadecyltrichlorosilane (OTS) was used for surface modification of the gate dielectric layer. PBDT−BBT in o-dichlorobenzene (about 2 mg mL−1) was drop-casted onto OTS treated substrates to form polymer films (about 100 nm). After the evaporation of solvent under dark for 48 h, polymer films were formed. Prior to thermal evaporation of top contact electrodes, the films were baked in a vacuum chamber located in the glovebox for 30 min to completely remove the residual solvent or anneal the samples. Gold contacts (25 nm) for source and drain electrodes were vacuum-deposited at a rate of 0.1 Å s−1 through a metal shadow mask that defined a series of transistor devices with a channel length (L) of 27 μm and a channel width (W) of 210 μm. The characterization was accomplished by a Keithley 4200 SCS with a micromanipulator 6150 probe station in a clean shielded box at ambient atmosphere. Then field-effect mobility was calculated from the standard equation for saturation region in metal−dioxide−

PBDT−BBT could be an air-stable and solution-processable semiconducting polymer. PBDT−BBT exhibited strong absorption across the entire visible region and a relatively high hole mobility of 6 × 10−3 cm2 V−1 s−1. The phototransistors based on PBDT−BBT thin films exhibited a photoresponsivity of 3200 mA W−1 and a very high photocurrent/dark current ratio of 4 × 105.

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EXPERIMENTAL SECTION

Measurements and Characterization. The 1H NMR and 13C NMR spectra were measured on a Bruker AVANCE 400 MHz spectrometer using tetramethylsilane (TMS; δ = 0 ppm) as an internal standard. Mass spectra were measured on a GCT-MS micromass spectrometer using the electron impact (EI) mode or on a Bruker Daltonics BIFLEX III MALDI-TOF analyzer using MALDI mode. Elemental analyses were carried out using a FLASH EA1112 elemental analyzer. Thermogravimetric analysis (TGA) measurements were performed using a DTG-60 thermal analysis system under N2 at a heating rate of 10 °C min−1. Differential scanning calorimetry (DSC) measurements were performed using an SII EXSAR6000 (DSC 6220) under a nitrogen atmosphere with a heating and cooling rate of 10 °C min−1. Solution (chloroform) and thin-film (on quartz substrate) UV− vis spectra were recorded on a Jasco V-570 spectrophotometer. The electrochemical measurements were carried out under nitrogen on a deoxygenated solution of tetra-n-butylammonium hexafluorophosphate (0.1 M) in acetonitrile with a computer-controlled CHI660C electrochemical workstation, a glassy-carbon working electrode coated with the polymer film, a platinum-wire auxiliary electrode, and an Ag wire anodized with AgCl as a pseudoreference electrode. Potentials were referenced to the ferrocenium/ferrocene (FeCp2+/0) couple by using ferrocene as a standard. The gel permeation chromatography 1297

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semiconductor field effect transistors: IDS = (W/2L)μCi(VG − VT)2, where IDS is drain-source current, μ is field-effect mobility, W and L are the channel width and length, Ci is the capacitance per unit area of the gate insulator (Ci = 7.5 nF cm−2), VG is the gate voltage, and VT is the threshold voltage.54 As for the test of phototransistors, white light was irradiated directly on polymer films during the operation of field-effect transistors. To evaluate the performance of thin film phototransistors, photoresponsivity (R) and photocurrent/dark current ratio (P) are defined by the following equations:

R = (IDS,i − IDS,d)/(SPi)

(a)

P = (IDS,i − IDS,d)/IDS,d

(b)

n-hexane) was added dropwise at room temperature. The reaction was stirred at 50 °C for 2 h. After the reaction solution was cooled to room temperature, 88 mg of trimethyltin chloride (0.44 mmol, 1 M in nhexane) (caution! very toxic!) was added. After stirring at room temperature for 6 h, the mixture was poured into 200 mL of cool water and extracted with diethyl ether. The organic layer was washed with water and then dried over anhydrous MgSO4. After the solvent was removed under vacuum, the residue was yellow oil (240 mg, 90%) and used for the next reaction without further purification due to instability of this compound. 1H NMR (400 MHz, CDCl3): δ 7.71 (s, 2H), 7.34 (d, J = 3.4 Hz, 2H), 6.91 (d, J = 3.4 Hz, 2H), 2.89 (d, J = 6.4 Hz, 4H), 1.75 (m, 2H), 1.38−1.22 (m, 80H), 0.90 (t, J = 6.5 Hz, 12H), 0.41 (s, 18H). Anal. Calcd for C72H122S4Sn2: C, 63.90; H, 9.09. Found: C, 65.55; H, 9.30%. 4,8-Bis(5-(2-decyltetradecyl)thiophen-2-yl)-2,6-bis(thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene (4). Compound 3 (853 mg, 0.63 mmol) and 2-bromothiophene (250 mg, 1.5 mmol) were dissolved in dry toluene (10 mL) and deoxygenated with nitrogen for 15 min. Then, Pd(PPh3)4 (60 mg, 0.05 mmol) was added under nitrogen. The mixture was stirred at reflux for 48 h. After the reaction solution was cooled to room temperature, KF (5 g) in 10 mL of water was added, and the reaction mixture was stirred for another 2 h. The mixture was poured into 200 mL of cool water and extracted with CH2Cl2. The organic layer was washed with 10% hydrochloric acid, saturated NaHCO3 aqueous solution and then dried over anhydrous MgSO4. After the solvent was removed under vacuum, the residue was purified by column chromatography (silica gel, hexane) to afford pale yellow solid (230 mg, 31%). 1H NMR (400 MHz, CDCl3): δ 7.65 (s, 2H), 7.31 (d, J = 3.3 Hz, 2H), 7.28 (d, J = 3.4 Hz, 2H), 7.26 (d, J = 4.8 Hz, 2H), 7.04 (dd, J = 4.8 Hz, J = 3.4 Hz, 2H), 6.91 (d, J = 3.3 Hz, 2H), 2.88 (d, J = 6.5 Hz, 4H), 1.75 (m, 2H), 1.30−1.22 (m, 80H), 0.87 (t, J = 5.2 Hz, 12H). 13C NMR (100 MHz, CDCl3): δ 145.93, 138.69, 137.96, 137.65, 137.32, 136.99, 127.93, 127.82, 125.59, 125.54, 125.22, 123.47, 119.17, 40.16, 34.81, 33.55, 32.06, 30.18, 29.87, 29.52, 26.85, 22.82, 14.23. MS (MALDI): m/z 1191 (M+). Anal. Calcd for C74H110S6: C, 74.56; H, 9.30. Found: C, 74.42; H, 9.42%. (5,5′-(4,8-Bis(5-(2-decyltetradecyl)thiophen-2-yl)benzo[1,2b:4,5-b′]dithiophene-2,6-diyl)bis(thiophene-2-yl))bis(trimethylstannane) (5). A solution of 4 (170 mg, 0.14 mmol) in dry THF (4 mL) was deoxygenated with nitrogen for 15 min. 0.13 mL of n-butyllithium (0.33 mmol, 2.5 M in n-hexane) was added dropwise at −78 °C. After the mixture was stirred at −78 °C for 1 h, 62.6 mg of trimethyltin chloride (0.31 mmol, 1 M in n-hexane) (caution! very toxic!) was added. The reaction mixture was stirred at room temperature for 12 h. Then, the mixture was poured into 200 mL of cool water and extracted with diethyl ether. The organic layer was washed with water and then dried over anhydrous MgSO4. After the solvent was removed under vacuum, a yellow oil was obtained (200 mg, 95%) and was used for polymerization without further purification due to instability of this compound. 1H NMR (400 MHz, CDCl3): δ 7.65 (s, 2H), 7.38 (d, J = 3.2 Hz, 2H), 7.31 (d, J = 3.3 Hz, 2H), 7.11 (d, J = 3.3 Hz, 2H), 6.91 (d, J = 3.2 Hz, 2H), 2.88 (d, J = 6.5 Hz, 4H), 1.75 (m, 2H), 1.38−1.24 (m, 80H), 0.89 (t, J = 4.7 Hz, 12H), 0.39 (s, 18H). Anal. Calcd for C80H126S6Sn2: C, 63.31; H, 8.37. Found: C, 62.72; H, 8.52%. Poly{[2,6-bis(thiophene-2-yl)-4,8-bis(5-(2-decyltetradecyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene-5,5′-diyl]-alt(5,5′-bibenzo[c][1,2,5]thiadiazole-4,7-diyl)} (PBDT−BBT). 4,7Dibromo-5,5′-bibenzo[c][1,2,5]thiadiazole (52 mg, 0.12 mmol) and compound 5 (180 mg, 0.12 mmol) were dissolved in dry toluene (4 mL) and deoxygenated with nitrogen for 15 min. Then, Pd(PPh3)4 (12 mg, 0.01 mmol) was added under nitrogen. The mixture was stirred at reflux for 3 days. After the reaction mixture was cooled to room temperature, KF (5 g) in 10 mL of water was added, and the mixture was stirred at room temperature for another 2 h. The organic phase was extracted with chloroform, washed with water, and dried over anhydrous MgSO4. The polymer was precipitated by addition of 200 mL of methanol. The precipitate was filtered. Finally, the polymer was purified by size exclusion column chromatography over Bio-Rad Bio-Beads S-X1 eluting with chloroform. The polymer was recovered

where IDS,i and IDS,d are the drain-source current under illumination and in dark, Pi is the power of the incident light per unit area, and S is the effective device area.55 Materials. 4,7-Dibromo-5,5′-bibenzo[c][1,2,5]thiadiazole was synthesized according to the procedure reported by our group.53 Toluene and THF were distilled from sodium and benzophenone under nitrogen before use. Bio-Rad Bio-Beads S-X1 is a kind of porous crosslinked polystyrene polymers used for gel permeation separations of lipophilic polymers and low molecular weight, hydrophobic materials in the presence of organic solvents. Unless stated otherwise, the other reagents were purchased from commercial sources and used without further purification. 2-(2-Decyltetradecyl)thiophene (1). A solution of thiophene (378 mg, 4.5 mmol) in dry THF (10 mL) was deoxygenated with nitrogen for 15 min, and 2 mL of n-butyllithium (5 mmol, 2.5 M in nhexane) was added dropwise at 0 °C. After the solution was stirred at 0 °C for 30 min, 11-(bromomethyl)tricosane (2.08 g, 5 mmol) was added. Then the mixture was heated to 60 °C and was stirred for 16 h. After the reaction mixture was cooled to room temperature, it was poured into 200 mL of cool water and extracted with hexane. The organic layer was washed with water and then dried over anhydrous MgSO4. After the solvent was removed under vacuum, the residue was purified by column chromatography (silica gel, hexane/CH2Cl2 = 1:1) to afford pale yellow oil (1.02 g, 54%). 1H NMR (400 MHz, CDCl3): δ 7.11 (d, J = 5.1 Hz, 1H), 6.92 (dd, J = 5.0 Hz, J = 3.1 Hz, 1H), 6.75 (d, J = 3.1 Hz), 2.77 (d, J = 6.7 Hz, 2H), 1.62 (m, 1H), 1.32−1.25 (m, 40H), 0.90 (t, J = 6.8 Hz, 6H). 13C NMR (100 MHz, CDCl3): δ 144.38, 126.58, 125.03, 122.97, 40.12, 34.32, 33.27, 32.07, 30.10, 30.02, 29.81, 29.51, 26.70, 22.83, 14.25. HRMS (EI): 420.3796 (calcd for C28H52S, 420.3790). Anal. Calcd for C28H52S: C, 79.92; H, 12.46. Found: C, 79.62; H, 12.42%. 4,8-Bis(5-(2-decyltetradecyl)thiophen-2-yl)benzo[1,2-b:4,5b′]dithiophene (2). A solution of 1 (838 mg, 2 mmol) in dry THF (5 mL) was deoxygenated with nitrogen for 15 min, and 0.88 mL of nbutyllithium (2.2 mmol, 2.5 M in n-hexane) was added dropwise at 0 °C. Then the solution was allowed to warm up to 50 °C and stirred for 1.5 h; benzo[1,2-b:4,5-b′]dithiophene-4,8-dione (176 mg, 0.8 mmol) was added. The mixture was stirred at 50 °C for another 1.5 h. After the reaction mixture was cooled to room temperature, SnCl2·2H2O (1.44 g, 6.4 mmol) in 2.56 mL of 10% hydrochloric acid) was added, and the reaction mixture was stirred for 1.5 h. The mixture was poured into 200 mL of cool water and extracted with CH2Cl2. The organic layer was washed with water and then dried over anhydrous MgSO4. After the solvent was removed under vacuum, the residue was purified by column chromatography (silica gel, hexane) to afford yellow oil (467 mg, 57%). 1H NMR (400 MHz, CDCl3): δ 7.65 (d, J = 5.7 Hz, 2H), 7.45 (d, J = 5.7 Hz, 2H), 7.30 (d, J = 3.4 Hz, 2H), 6.89 (d, J = 3.4 Hz, 2H), 2.86 (d, J = 6.7 Hz, 4H), 1.73 (m, 2H), 1.38−1.24 (m, 80H), 0.89 (t, J = 6.7 Hz, 12H). 13C NMR (100 MHz, CDCl3): δ 145.72, 145.05, 139.17, 137.46, 136.65, 127.81, 127.47, 125.50, 124.22, 123.54, 40.21, 34.98, 33.57, 32.17, 30.22, 29.95, 29.62, 26.88, 22.92, 14.33. MS (MALDI): m/z 1027 (M+). Anal. Calcd for C66H106S4: C, 77.13; H, 10.40. Found: C, 77.42; H, 10.42%. (4,8-Bis(5-(2-decyltetradecyl)thiophen-2-yl)benzo[1,2-b:4,5b′]dithiophene-2,6-diyl)bis(trimethylstannane) (3). A solution of 2 (207.4 mg, 0.2 mmol) in dry THF (5 mL) was deoxygenated with nitrogen for 15 min. 0.18 mL of n-butyllithium (0.45 mmol, 2.5 M in 1298

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Figure 1. (A) TGA curve of PBDT−BBT under nitrogen. (B) DSC traces of PBDT−BBT under nitrogen. (C) Absorption spectra of PBDT−BBT in solution and thin film. (D) Cyclic voltammogram of PBDT−BBT. as a red brown solid (80 mg, 47%). 1H NMR (400 MHz, CDCl3): δ 8.10−6.91 (br, 14H), 2.89 (br, 4H), 1.74−1.22 (br, 82H), 0.83 (br, 12H). Anal. Calcd for (C86H112N4S8)n: C, 70.83; H, 7.74; N, 3.84. Found: C, 69.22; H, 7.57; N, 3.68%. Mn, 16 600, Mw, 77 700, Mw/Mn, 4.69.

Optical Properties. Figure 1C provides intensity-normalized UV−vis absorption spectra of PBDT−BBT in chloroform solution (ca. 10−6 M) and in thin film. The polymer film exhibited a broad and strong absorption plateau across the entire UV−vis region (300−800 nm). In film, the absorbance maximum of PBDT−BBT is 591 nm, 28 nm red shift relative to that in solution, suggesting the strong interchain interaction. The optical bandgap of PBDT−BBT estimated from the absorption edge in film is 1.72 eV. Electrochemical Properties. To investigate the electrochemical properties of the polymer and estimate its HOMO and LUMO energy levels, cyclic voltammetry (CV) was carried out using a glassy-carbon coated with polymer film, a Pt wire, and an Ag wire coated with AgCl as the working electrode, counter electrode, and quasi-reference electrode, respectively. CV measurements were conducted in a 0.1 M Bu4NPF6− acetonitrile solution under N2 at a scan rate of 50 mV s−1. PBDT−BBT displays irreversible oxidation wave and reversible reduction wave. The onset oxidation and reduction potentials versus Ag are 1.4 and −0.6 V, respectively. The onset oxidation and reduction potentials versus FeCp2+/0 (half-wave potential of 0.5 eV) are 0.9 and −1.1 V, respectively. PBDT−BBT has a HOMO level of −5.7 eV and a LUMO level of −3.7 eV, estimated from the onset oxidation and reduction potentials, assuming the absolute energy level of ferrocene/ferrocenium to be 4.8 eV below vacuum. The deep HOMO and LUMO levels of PBDT−BBT are attributed to the strong electron-accepting ability of BBT. The HOMO level of −5.7 eV is about 0.9 eV deeper than that of P3HT (−4.8 eV), indicating the excellent ambient stability of PBDT−BBT.56 The HOMO−LUMO gap obtained from electrochemistry is 2.0 eV, larger than the optical

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RESULTS AND DISCUSSION Synthesis and Characterization. The synthetic route of the polymer is outlined in Scheme 1. PBDT−BBT was synthesized by Stille coupling copolymerization of BBT dibromide and tetrathiophene-substituted BBT ditin. To ensure good solution processability of the resulting copolymer, long branched alky chains were symmetrically attached to thiophenes. PBDT−BBT is soluble in common organic solvents such as chloroform, THF, and dichlorobenzene. The incorporation of thiophene spacers into polymer backbone can reduce steric hindrance between BBT and BDT units, consequently leading to an extended π-conjugation. Molecular weights of PBDT−BBT were determined by gel permeation chromatography (GPC) using polystyrene standards as calibrants. PBDT−BBT has a number-average molecular weight (Mn) of 16 600, a weight-average molecular weight (Mw) of 77 700, and a polydispersity index (Mw/Mn) of 4.69. Thermal Properties. Thermogravimetric analysis (TGA, heating ramp rate of 10 °C min−1 under N2) was used to determine the thermal stability of PBDT−BBT (Figure 1A). The thermolysis onset temperature at 5% mass loss for PBDT− BBT is 437 °C, indicating that PBDT−BBT has excellent thermal stability. The thermal property of PBDT−BBT was also investigated by differential scanning calorimetry (DSC) at a scanning rate of 10 °C min−1 (Figure 1B). No endotherm/ exotherm transition was found across the entire scanning range, suggesting that PBDT−BBT is amorphous. 1299

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Figure 2. XRD patterns of PBDT−BBT thin films drop casted onto OTS-treated SiO2/Si substrate after annealing at (A) 80, (B) 120, (C) 160, and (D) 200 °C for 30 min.

band gap (1.72 eV), which is a common phenomenon reported in the literature.57−59 Thin Film Morphology. Thin films of PBDT−BBT used in the characterization of X-ray diffraction (XRD) and atomic force microscopy (AFM) were prepared by the same procedure as device fabrication. Figure 2 shows the XRD patterns of PBDT−BBT thin films after thermal annealing at 80, 120, 160, and 200 °C. For all the XRD patterns of thin films, a broad peak accompanied by three sharp peaks appeared in 2θ = 15°− 30° region corresponding to a d-spacing of 4.1 Å for 2θ = 21.2°, 3.7 Å for 2θ = 23.5°, and 3.3 Å for 2θ = 26.4°. No lamellar organization feature was found for the samples. These XRD patterns indicate that the films are amorphous collections of randomly oriented chains with a possible π−π stacking distance of 3−4 Å. Figure 3 shows AFM height images of the films annealed at 80, 120, 160, and 200 °C. For all the cases, the films exhibited typical amorphous state without any crystalline domains. The root-mean-square (rms) roughness of these films is in the range of 0.6−0.9 nm. These XRD and AFM results indicate that PBDT−BBT is a thermal stable organic semiconductor, and thermal annealing has little effect on film morphology and organization of PBDT−BBT. Thus, we can expect that PBDT−BBT is a potential isotropic material in the application of OFETs and phototransistors without the use of complicated post-treatments such as thermal annealing for performance improvement. Organic Field-Effect Transistors. Since microstructure order of PBDT−BBT thin films is not susceptible to thermal annealing temperature, we only baked the films at 80 °C to completely remove the residual solvent. Then, charge transport property of PBDT−BBT was investigated by the abovementioned device configuration. The representative output and transfer characteristics of the devices are shown in Figures 4A and 4B. The devices exhibited typical p-type behavior with a charge-carrier mobility of 6 × 10−3 cm2 V−1 s−1, threshold voltage of −1 V, and on/off current ratio of 106. Because of the deep HOMO level of PBDT−BBT, the operation of these devices is quite stable under ambient atmosphere. Under a

Figure 3. Tapping-mode AFM height images of PBDT−BBT dropcasted onto OTS-treated SiO2/Si substrate after annealing at (A) 80, (B) 120, (C) 160, and (D) 200 °C for 30 min.

continuous sweep (10 cycles) of gate-source voltage (VGS) with constant drain-source voltage (VDS), no obvious degradation in performance was found as shown in Figure 5A. Long-term stability of the devices was investigated over one month. Figure 5B shows the transfer characteristics of the devices versus the storage time under ambient atmosphere. No obvious changes in mobility and on/off ratio were found over the period. Thin Film Phototransistors. Thin film phototransistors based on PBDT−BBT were fabricated with a similar device configuration to that of field-effect transistors, except for a top illumination applied on the devices. Figure 6A shows the 1300

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Figure 4. (A) Transfer and (B) output characteristics of drop-casted PBDT−BBT thin film transistors based on OTS-treated SiO2/Si substrate.

Figure 5. (A) Transfer characteristics of the devices after continuing measurement for 10 cycles in air. (B) Transfer characteristics of the devices after storage in air for 30 days.

Figure 6. (A) Transfer characteristics of the polymer thin film phototransistors under dark and light irradiation. (B) Output characteristics of the polymer thin film phototransistors at different light power.

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“transfer” characteristics of the phototransistor in the dark and under illumination. Under a white light irradiation of 5.51 mW cm−2, these devices exhibited strong photodependence with a photoresponsivity (R) = 3200 mA W−1 and photocurrent/dark current ratio (P) = 4 × 105. The efficient charge carrier generation benefited from the broad light absorption of PBDT−BBT is the major factor for the high performance of the phototransistors. To the best of our knowledge, this is the highest on/off ratio for polymer phototransistors. The device can also work very well at low operation voltage (