Tuning of Charge Polarity in Thin Film Transistors ... - ACS Publications

Jun 11, 2015 - Department of Organic and Polymeric Materials, Tokyo Institute of Technology, 2-12-1 .... simple but effective way to tune energy level...
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New Semiconducting Polymers Based on Benzobisthiadiazole Analogues: Tuning of Charge Polarity in Thin Film Transistors via Heteroatom Substitution Yang Wang,† Hiroyasu Masunaga,‡ Takaaki Hikima,§ Hidetoshi Matsumoto,† Takehiko Mori,† and Tsuyoshi Michinobu*,† †

Department of Organic and Polymeric Materials, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8552, Japan ‡ Japan Synchrotron Radiation Research Institute (JASRI)/SPring-8, 1-1-1 Kouto, Sayo, Sayo 679-5198, Japan § RIKEN SPring-8 Center, 1-1-1 Kouto, Sayo, Sayo 679-5148, Japan S Supporting Information *

ABSTRACT: As one of the most effective molecular design strategies in organic electronics, heteroatom substitution was employed for the first time to study the acceptor variation effects on the optical, electrochemical, molecular assembling, and chargetransport properties of novel semiconducting polymers containing benzobisthiadiazole (BBT)-related heterocycles, namely, poly(dithiazolfluorene-alt-thiadiazolobenzotriazole) (PSN), poly(dithiazolfluorene-alt-selenadiazolobenzotriazole) (PSeN), and poly(dithiazolfluorene-alt-selenadiazolobenzothiadiazole) (PSeS). The effect of the heteroatom substitution was clearly shown in the UV−vis−NIR absorption spectra in which the substitution of the sulfur (S) and/or nitrogen (N) atoms in PSN with the selenium (Se) and sulfur (S) atoms led to a red-shift in the absorption profile. In addition, the energy levels of these polymers, determined from cyclic voltammetry (CV) measurements and density functional theory (DFT) calculations, also varied due to the hetroatom substitution effect. Accordingly, thin film transistors (TFTs) based on these polymers showed different charge transport properties. For example, PSN displayed p-type unipolar performances with a high hole mobility up to 0.65 cm2 V−1 s−1. In contrast, PSeS showed n-type dominant charge transport properties with an electron mobility up to 0.087 cm2 V−1 s−1. Intriguingly, PSeN exhibited ambipolar charge transport properties with balanced μh and μe values. These different charge polarities in the TFTs were correlated to the energy levels, π−π stacking distances, and polymer crystallinities evaluated by their grazing-incidence wide-angle X-ray scattering (GIWAXS) patterns and atomic force microscopy (AFM) images. We believe that this simple and effective approach will guide the way to developing high-performance ambipolar and/or n-channel semiconducting polymers for TFTs.

1. INTRODUCTION π-Conjugated semiconducting polymers are promising materials for the next-generation electronic devices, which are implemented in mobile displays, radio-frequency identification tags, electronic papers, and skins due to their versatile molecular synthesis, solution processability at low temperature, high thermal stability, and mechanical flexibility and/or ruggedness.1 In the past 5 years, significant achievements have been made in polymer thin film transistors (TFTs) with high carrier mobilities over 10 cm2 V−1 s−1values that are competitive with or even surpassing those of the crystalline films of small molecular weight organic semiconductors and amorphous silicon semiconductors.2 Generally speaking, such a vigorous boost in polymer TFTs is ascribed to three important polymer design principles. The first one is to develop new conjugated coplanar backbones with a low conformational disorder, resulting in short π−π stacking distances and strong intermolecular interactions in the thin film © XXXX American Chemical Society

states. For example, Yang et al. recently reported a thienoisoindigo (TIIG)-based semiconducting polymer (PTIIG-NP, see Chart 1) exhibiting an ultrahigh hole mobility of 14.4 cm2 V−1 s−1.3 As compared to the isoindigo (IIG) unit, the TIIG unit is more coplanar due to the intramolecular S···O interactions and the enhanced contribution by the quinoid structure.4 Thus, PTIIG-NP showed a better molecular ordering, longer effective π-conjugation, and higher charge carrier mobilities than the counter IIG-based polymers.5 Second, side chain engineering, especially adopting branched alkyl chains with a greater distance of the bifurcation point, has recently become very popular. The introduction of suitable alkyl side chains onto the π-conjugated backbone not only improves the polymer solubilities but also induces a high Received: April 17, 2015 Revised: May 21, 2015

A

DOI: 10.1021/acs.macromol.5b00802 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

Chart 1. Chemical Structures of Some High-Performance Semiconducting Polymers in the Literature (Top) and Three BBT Analogue-Based Copolymers in This Study (Bottom)

crystallinity in the thin film states.6 For example, Pei et al. recently reported the effects of the side chain branching positions on the electron mobilities of BDPPV-C (Chart 1). It was shown that different side chain branching positions result in different π−π stacking distances, polymer crystallinities, and solid-state packing conformations, thus leading to different electron mobilities.6a Last, but not the least, the incorporation of “heteroatoms”, such as chalcogen, nitrogen, and halogen atoms, into the π-conjugated backbone has been found to be a simple but effective way to tune energy levels and intermolecular interactions.7 Benzobisthiadiazole (BBT) is a planar triple-fused-ring with 14-π electrons that shows a strong electron-accepting feature and excellent intermolecular π−π interactions when it is incorporated into narrow band gap semiconducting polymers.8 Wudl et al. reported very high hole mobilities up to 2.5 cm2 V−1 s−1 and electron mobilities up to 1.36 cm2 V−1 s−1 for the TFTs based on the BBT-containing polymers.8d,e Very recently, we also reported three new BBT-based polymers, whose charge polarity could be controlled by the π-spacer structures.9 In order to develop more advanced organic semiconductors and increase the family of BBT derivatives, heteroatom-substituted BBT analogous monomer structures, namely thiadiazolobenzotriazole, selenadiazolobenzotriazole, and selenadiazolobenzothiadiazole derivatives, were previously synthesized by Grimsdale et al.10 However, the corresponding polymers have not been described. In this study, we, for the first time, report the synthesis and TFT performances of the heteroatomsubstituted BBT analogue-containing semiconducting polymers, namely poly(dithiazolfluorene-alt-thiadiazolobenzotriazole) (PSN), poly(dithiazolfluorene-alt-selenadiazolobenzotriazole) (PSeN), and poly(dithiazolfluorene-altselenadiazolobenzothiadiazole) (PSeS). The dithiazolfluorene unit was employed as an electron-donating comonomer unit due to the excellent TFT performances of PBBT-Tz-FT, which was one of the BBT-based polymers in our recent study.9 It should be noted that most of the previous semiconducting polymer studies of the heteroatom substitution have been investigated for the donor units, e.g., furan, thiophene, selenophene, and tellurophene; thus, it is rare to vary the acceptor unit.

2. EXPERIMENTAL SECTION 2.1. General Measurements. Nuclear magnetic resonance (NMR) spectra were recorded using a JEOL model AL300 (300 MHz) at room temperature. Deuterated chloroform was used as a solvent. Chemical shifts of NMR were reported in ppm (parts per million) relative to the residual solvent peak at 7.26 ppm for 1H NMR spectroscopy and 77.6 ppm for 13C NMR spectroscopy. Coupling constants (J) were given in hertz. The resonance multiplicity was described as s (singlet), d (doublet), t (triplet), and m (multiplet). Fourier transform infrared (FT-IR) spectra were recorded on a JASCO FT/IR-4100 spectrometer in the range from 4000 to 600 cm−1. MALDI−TOF mass spectra were measured on a Shimadzu/Kratos AXIMACFR mass spectrometer equipped with a nitrogen laser (λ = 337 nm) and pulsed ion extraction, which was operated in a linearpositive ion mode at an accelerating potential of 20 kV. Tetrahydrofuran (THF) solutions containing 1 g L−1 of a sample, 10 g L−1 of dithranol, and 1 g L−1 of sodium trifluoroacetate were mixed to a ratio of 1:1:1; and then 1 μL aliquot of this mixture was deposited onto a sample plate. Size exclusion chromatography (SEC) was measured on a JASCO GULLIVER 1500 equipped with a pump (PU-2080 Plus), an absorbance detector (RI-2031 Plus), and two Shodex GPC KF-803 columns (8.0 mm i.d. × 300 mm L) based on a conventional calibration curve using polystyrene standards. 1,2Dichlorobenzene (40 °C) was used as a carrier solvent at the flow rate of 0.5 mL min−1. UV−vis−NIR spectra were recorded on a JASCO V-670 spectrophotometer. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) measurements were carried out on a Rigaku TG8120 and a Rigaku DSC8230, respectively, under nitrogen flow at the scan rate of 10 °C min−1. Electrochemistry measurements were carried out on a BAS electrochemical analyzer model 612C at 25 °C in a classical three-electrode cell. The working, reference, and auxiliary electrodes were a glassy carbon electrode, Ag/ AgCl/CH3CN/(nC4H9)4NPF6, and a Pt wire, respectively. The polymer films for electrochemical measurements were coated from a chloroform solution (ca. 5 g L−1). For calibration, the redox potential of ferrocene/ferrocenium (Fc/Fc+) was measured under the same conditions, and it was located at 0.09 V vs the Ag/AgCl electrode. It was assumed that the redox potential of Fc/Fc+ has an absolute energy level of −4.80 eV to vacuum. The HOMO and LUMO energy levels were then calculated according to the equations

E HOMO = − (φox + 4.71) (eV)

(1)

E LUMO = − (φre + 4.71) (eV)

(2)

where φox is the onset oxidation potential vs Ag/AgCl and φre is the onset reduction potential vs Ag/AgCl. 2.2. Fabrication and Characterization of Polymer Thin Film Transistors. Top-contact/bottom-gate TFT devices were fabricated B

DOI: 10.1021/acs.macromol.5b00802 Macromolecules XXXX, XXX, XXX−XXX

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to 70 °C for 4 h, it was cooled to room temperature, diluted with water (100 mL), and extracted with CH2Cl2 (3 × 100 mL). The combined organic layers were washed with aqueous NaHCO3 solution (2 × 100 mL) and dried over MgSO4. Filtration followed by solvent removal under reduced pressure gave yellowish-brown oil (2-(2-ethylhexyl)4,7-bis(4-octylthiophen-2-yl)-2H-benzo[d][1,2,3]triazole-5,6-diamine) that was relatively pure and used for the next step without further purification. 2-(2-Ethylhexyl)-4,7-bis(4-octylthiophen-2-yl)-2H-benzo[d][1,2,3]triazole-5,6-diamine (0.680 g, 1.05 mmol) was dissolved in dry pyridine (10 mL) under argon. To this solution, N-thionylaniline (0.290 g, 2.10 mmol) followed by trimethylsilyl chloride (TMS-Cl, 0.210 g, 1.89 mmol) was added. The mixture was stirred for 36 h at 80 °C and was then evaporated to dryness. The residue was dissolved in hot toluene (20 mL) and filtered through a silica gel plug to give dark blue powder. Column chromatography (SiO2, hexane/CH2Cl2 5:1) afforded a dark bluish purple solid (0.450 g, two-step yield of 57%). 1H NMR (300 MHz, CDCl3): δ = 8.58 (s, 2H), 7.19 (s, 2H), 4.81 (d, J = 6.3 Hz, 2H), 2.77 (t, J = 8.4 Hz, 4H), 2.35−2.27 (m, 1H), 1.83−1.73 (m, 4H), 1.45−1.32 (m, 28H), 1.09−1.04 (t, J = 6.9 Hz, 3H), 0.97− 0.90 (m, 9H) ppm. 13C NMR (75 MHz, CDCl3): δ = 149.7, 143.7, 142.3, 136.8, 132.1, 124.4, 111.6, 60.4, 40.5, 31.9, 30.7, 30.6, 30.5, 29.5, 29.4, 29.3, 28.5, 24.1, 22.9, 22.7, 14.1, 14.0, 10.6 ppm. IR (neat): ν = 2955, 2926, 1716, 1654, 1459, 1365, 1340, 1298, 1184, 1117, 1055, 1032, 984, 918, 850, 724, 648, 621, 608 cm−1. MALDI-TOF MS (Mw = 678.1): m/z = 677.9 [M+]. Synthesis of 4,8-Bis(5-bromo4-octylthiophen-2-yl)-6-(2ethylhexyl)[1,2,5]thiadiazolo[3,4-f]benzotriazole (SN). 4,8-Bis(4-octylthiophen-2-yl)-6-(2-ethylhexyl)[1,2,5]thiadiazolo[3,4-f ]benzotriazole (0.319 g, 0.471 mmol) was dissolved in dry THF (40 mL) under a N2 atmosphere. NBS (0.176 g, 0.988 mmol) was added several portions over the course of 20 min at room temperature in the dark. The mixture was stirred overnight, poured into water (150 mL), and extracted with CH2Cl2. After evaporation of the solvent, the crude product was purified by column chromatography (SiO2, hexane/ CH2Cl2 5:1) to afford a sticky dark blue solid (0.346 g, 88%). 1H NMR (300 MHz, CDCl3): δ = 8.20 (s, 2H), 4.70 (d, J = 6.0 Hz, 2H), 2.62 (t, J = 6.9 Hz, 4H), 2.26−2.18 (m, 1H), 1.77−1.67 (m, 4H), 1.41−1.33 (m, 28H), 1.06 (t, J = 6.9 Hz, 3H), 0.98−0.90 (m, 9H) ppm. 13C NMR (75 MHz, CDCl3): δ = 148.9, 142.4, 141.6, 136.5, 131.3, 115.0, 110.4, 60.4, 40.5, 31.9, 30.7, 29.8, 29.6, 29.5, 29.4, 29.3, 28.5, 24.1, 23.0, 22.7, 14.2, 14.1, 10.6 ppm. IR (neat): ν = 2955, 2923, 2853, 1566, 1477, 1465, 1435, 1385, 1329, 1256, 1196, 1107, 1006, 928, 875, 846, 819, 801, 764, 723, 713, 643, 610 cm−1. MALDI-TOF MS (Mw = 835.9): m/z = 837.1[M + H]+. Synthesis of 4,8-Bis(4-octylthiophen-2-yl)-6-(2-ethylhexyl)[1,2,5]selenadiazolo[3,4-f ]benzotriazole (4). A 100 mL three-necked flask was charged with 2-(2-ethylhexyl)-5,6-dinitro-4,7-bis(4-octylthiophen2-yl)-2H-benzo[d][1,2,3]triazole (0.730 g, 1.03 mmol), iron dust (1.31 g, 23.4 mmol), and acetic acid (25 mL). After the mixture was heated to 70 °C for 5 h, it was cooled to room temperature, diluted with water (150 mL), and extracted with CH2Cl2 (3 × 100 mL). The combined organic layers were washed with aqueous NaHCO3 solution (2 × 150 mL) and dried over MgSO4. Filtration and solvent removal under reduced pressure gave yellowish-brown oil (2-(2-ethylhexyl)-4,7-bis(4octylthiophen-2-yl)-2H-benzo[d][1,2,3]triazole-5,6-diamine) that was relatively pure and used for the next step without further purification. 2-(2-Ethylhexyl)-4,7-bis(4-octylthiophen-2-yl)-2H-benzo[d][1,2,3]triazole-5,6-diamine (0.603 g, 0.927 mmol) was dissolved in dry ethanol (10 mL) under argon, and SeO2 (0.141 g, 1.23 mmol) was added under argon protection. After the mixture was refluxed for 24 h, it was cooled down to room temperature, poured into water (50 mL), and extracted with CH2Cl2 (3 × 50 mL). Evaporation and column chromatography (SiO2, hexane/CH2Cl2 3:1) afforded a dark green solid (0.440 g, two-step yield of 60%). 1H NMR (300 MHz, CDCl3): δ = 8.59 (s, 2H), 7.21 (s, 2H), 4.77 (d, J = 6.3 Hz, 2H), 2.77 (t, J = 7.2 Hz, 4H), 2.30 (s, 1H), 1.80−1.76 (m, 4H), 1.44−1.33 (m, 28H), 1.06 (t, J = 6.9 Hz, 3H), 0.94−0.91 (m, 9H); 13C NMR (75 MHz, CDCl3): δ = 155.9, 143.6, 143.0, 137.5, 132.4, 125.1, 111.8, 60.5, 40.4, 31.9, 30.7, 30.6, 30.5, 29.5, 29.4, 29.3, 28.5, 24.1, 22.9, 22.7, 14.1, 14.0, 10.6 ppm. IR (neat): ν = 2955, 2923, 2851, 1717, 1560, 1439, 1378, 1194,

on n+-Si/SiO2 substrates where n+-Si and SiO2 were used as the gate electrode and gate dielectric, respectively. The substrates were subjected to cleaning by piranha solution (7:3 mixtures of H2SO4 and H2O2) and then rinsed with deionized water. The cleaned substrates were modified with octadecylsilanes (octadecyltrimethylsilane (OTMS)) to form a self-assembled monolayer (SAM). Thin films of the polymers were deposited on the treated substrate by spincoating the polymer solutions (2−4 g L−1) in chloroform or 1,2dichlorobenzene in a glovebox or under ambient conditions (air, RH = 40−60%), optionally followed by thermal annealing at 100, 130, 150, 170, 190, 210, or 230 °C in the glovebox. After the polymer thin film deposition, ∼50 nm thick gold was deposited as source and drain contacts using a shadow mask. The TFT devices had a channel length (L) of 100 μm and a channel width (W) of 1 mm. The TFT performances were measured under air or vacuum (10−4−10−5 mbar) using a Keithley 4200 parameter analyzer on a probe stage. The carrier mobilities, μ, were calculated from the data in the saturated regime according to the equation

ISD = (W /2L)C iμ(VG − VT)2

(3)

where ISD is the drain current in the saturated regime, W and L are the semiconductor channel width and length, respectively, Ci (Ci = 13.7 nF cm−2) is the capacitance per unit area of the gate dielectric layer, and VG and VT are the gate voltage and threshold voltage, respectively. VG − VT of the devices was determined from the square root values of ISD at the saturated regime. For p-type characterization, current on/off ratios (Ion/Ioff) were determined from the minimum current at around VGS = 0 to −40 V (Ioff) and the current at VGS = −100 V (Ion). The sweep direction in the transfer characteristics was from VGS = 20 to −100 V. For n-type characterization, Ion/Ioff were determined from the minimum current at around VGS = 0 to 40 V (Ioff) and the current at VGS = 100 V (Ion). The sweep direction in the transfer characteristics was from VGS = −20 to 100 V. 2.3. Atomic Force Microscopy (AFM) Measurements. AFM samples were prepared as follows: PSN and PSeN were spin-cast from the chloroform solutions, and PSeS was spin-cast from 1,2dichlorobenzene on OTMS-treated Si/SiO2 substrates. Both pristine and thermally treated films were examined by a Seiko Instruments SPA-400 with a stiff cantilever of Seiko Instruments DF-20. 2.4. Grazing-Incidence Wide-Angle X-ray Scattering (GIWAXS) Measurements. Grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements were carried out at BL45XU in SPring8 (Hyogo, Japan). The wavelength of the X-ray beam was 0.1 nm, and camera length was 395 mm. The 2D-scattering image was acquired using a photon counting detector (Pilatus3 2M, Dectris Ltd.). The samples were mounted in a helium cell to reduce radiation damage. A data acquisition time was 10 s. GIWAXS data were measured at the incident angle of 0.10°, which was lower than the critical angle of total external reflection at silicon surface and was close to those of samples. The components of the scattering vector, q, parallel and perpendicular to the sample surface were defined as qxy and qz, respectively. Thin film samples for GIWAXS measurements were prepared by spin-coating of the polymer solutions onto an OTMStreated Si/SiO2 substrate. 2.5. Materials and Synthetic Details. All chemicals were purchased from Tokyo Chemical Industry (TCI), Kanto, and SigmaAldrich and used as received unless otherwise stated. 4,7-Dibromo-2(2-ethylhexyl)-5,6-dinitro-2H-benzo[d][1,2,3]triazole (1), 2-(2-ethylhexyl)-5,6-dinitro-4,7-bis(4-octylthiophen-2-yl)-2H-benzo[d][1,2,3]triazole (2), 4,7-dibromo-5,6-dinitro-2,1,3-benzothiadiazole (5), 4,7bis(4-(2-decyltetradecyl)thiophen-2-yl)-5,6-dinitrobenzo[c][1,2,5]thiadiazole (6), (5,5′-(9,9-dioctyl-9H-fluorene-2,7-diyl)bis(thiazole5,2-diyl))bis(tributylstannane) (8), 2-(tri-n-butylstannyl)-4-decyltetradecylthiophene, and 2-(tri-n-butylstannyl)-4-octylthiophene were prepared according to a literature method.9,10,12 Synthesis of 4,8-Bis(4-octylthiophen-2-yl)-6-(2-ethylhexyl)[1,2,5]thiadiazolo[3,4-f ]benzotriazole (3). A 100 mL three-necked flask was charged with 2-(2-ethylhexyl)-5,6-dinitro-4,7-bis(4-octylthiophen-2yl)-2H-benzo[d][1,2,3]triazole (0.830 g, 1.17 mmol), iron dust (1.31 g, 23.4 mmol), and acetic acid (25 mL). After the mixture was heated C

DOI: 10.1021/acs.macromol.5b00802 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. Synthesis of BBT Analogous Acceptor Monomers, SN, SeN, and SeS

1117, 1033, 985, 918, 849, 753, 723, 713, 643, 606 cm−1. MALDI-TOF MS (Mw = 725.0): m/z = 724.6 [M+]. Synthesis of 4,8-Bis(5-bromo-4-octylthiophen-2-yl)-6-(2ethylhexyl)[1,2,5]selenadiazolo[3,4-f ]benzotriazole (SeN). 4,8-Bis(4-octylthiophen-2-yl)-6-(2-ethylhexyl)[1,2,5]selenadiazolo[3,4-f ]benzotriazole (0.261 g, 0.360 mmol) was dissolved in dry THF (40 mL) under a N2 atmosphere. To this solution, NBS (0.135 g, 0.756 mmol) was added several portions over the course of 10 min at room temperature in the dark. The mixture was stirred overnight, poured into water (150 mL), and extracted with CH2Cl2. After evaporation of the solvent, the crude product was purified by column chromatography (SiO2, hexane/CH2Cl2 5:1) to afford a sticky dark green solid (0.280 g, 88%). 1H NMR (300 MHz, CDCl3): δ = 8.30 (s, 2H), 4.69 (d, J = 7.80 Hz, 2H), 2.67 (t, J = 7.5 Hz, 4H), 2.27−2.18 (m, 1H), 1.78−1.68 (m, 4H), 1.45−1.32 (m, 28H), 1.06 (t, J = 7.5 Hz, 3H), 0.96−0.88 (m, 9H) ppm. 13C NMR (75 MHz, CDCl3): δ = 155.1, 142.4, 142.3, 137.3, 131.6, 115.9, 110.8, 60.5, 40.5, 31.9, 30.7, 29.9, 29.7, 29.6, 29.5, 29.4, 28.6, 24.1, 22.9, 22.7, 14.2, 14.1, 10.6 ppm. IR (neat): ν = 2955, 2925, 2854, 1559, 1438, 1386, 1194, 1108, 1055, 844, 751, 723, 713, 665, 635, 622, 610 cm−1. MALDI-TOF MS (Mw = 882.8): m/z = 882.6 [M+]. Synthesis of 4,7-Bis(4-(2-decyltetradecyl)thiophen-2-yl)[1,2,5]selenadiazolo[3,4-f ]benzothiadiazole (7). A 50 mL round-bottom flask was charged with 4,7-bis(4-(2-decyltetradecyl)thiophen-2-yl)-5,6dinitrobenzo[c][1,2,5]thiadiazole (0.510 g, 0.480 mmol), iron dust (0.520 g, 9.40 mmol), and acetic acid (25 mL). The mixture was heated to 100 °C for 5 h, then cooled to room temperature, and diluted with water (100 mL). The product was extracted with CH2Cl2 (3 × 100 mL). The combined organic phase was washed with water (2 × 100 mL) and aqueous NaHCO3 (2 × 100 mL) and dried over MgSO4. The solvent was removed under reduced pressure to give yellowish-brown oil (4,7-bis(4-(2-decyltetradecyl)thiophen-2-yl)benzothiadiazole-5,6-diamine) that was relatively pure and used for the next step without further purification. 4,7-Bis(4-(2decyltetradecyl)thiophen-2-yl)benzothiadiazole-5,6-diamine (0.433 g, 0.432 mmol) was dissolved in dry ethanol (5 mL) under argon. SeO2 (0.096 g, 0.860 mmol) was added quickly under argon protection. The mixture was refluxed for 24 h and then cooled down to room temperature. After the mixture was poured into water (50 mL), the organic fraction was extracted with CH2Cl2 (3 × 50 mL). The combined organic layer was then evaporated to yield a crude product. It was further purified by column chromatography (SiO2, hexane/ CH2Cl2 5:1) to afford a dark green solid (0.160 g, two-step yield of

31.1%). 1H NMR (300 MHz, CDCl3): δ = 8.62 (s, 2H), 7.21 (s, 2H), 2.71 (d, J = 6.6 Hz, 4H), 1.78 (s, 2H), 1.38−1.26 (m, 80H), 0.91−0.86 (t, J = 7.2 Hz, 12H) ppm. 13C NMR (75 MHz, CDCl3): δ = 156.3, 150.7, 142.5, 138.0, 135.1, 127.7, 113.0, 39.0, 35.0, 33.4, 32.0, 30.2, 29.8, 29.8, 29.8, 29.73, 29.70, 29.68, 29.39, 29.37, 26.7, 22.7, 14.1 ppm. IR (neat): ν = 2953, 2923, 2853, 2369, 2344, 2319, 1507, 1457, 1427, 1389, 1275, 1222, 1178, 1001, 930, 917, 885, 860, 836, 802, 771, 725, 714, 703, 686, 669, 660, 640, 630, 615, 607 cm−1. MALDI-TOF MS (Mw = 1078.7): m/z = 1078.8 ([M+]). Synthesis of 4,7-Bis(5-bromo-4-(2-decyltetradecyl)thiophen-2-yl)[1,2,5]selenadiazolo[3,4-f ]benzothiadiazole (SeS). 4,7-Bis(4-(2decyltetradecyl)thiophen-2-yl)[1,2,5]selenadiazolo[3,4-f ]benzothiadiazole (0.160 g, 0.148 mmol) was dissolved in THF (40 mL). NBromosuccinimide (NBS, 0.053 g, 0.297 mmol) was added at room temperature in the dark. After the mixture was stirred overnight at room temperature, the solution was poured into water and extracted with CH2Cl2. After evaporation of the solvent, the crude product was purified by column chromatography (SiO2, hexane/CH2Cl2 5:1) to afford a dark green solid (0.132 g, 73%). 1H NMR (300 MHz, CDCl3): δ = 8.34 (s, 2H), 2.59 (d, J = 6.0 Hz, 4H), 1.80 (s, 2H), 1.38−1.31 (m, 80H), 0.91−0.86 (t, J = 6.3 Hz, 12H) ppm. 13C NMR (75 MHz, CDCl3): δ = 155.7, 150.2, 142.0, 137.8, 134.4, 118.8, 111.8, 38.6, 34.2, 33.4, 31.9, 30.2, 29.81, 29.77, 29.75, 29.74, 29.73, 29.69, 29.41, 29.39, 26.6, 22.7, 14.1 ppm. IR (neat): ν = 2953, 2923, 2854, 2360, 2342, 1507, 1457, 1424, 1376, 1172, 1080, 1011, 933, 854, 804, 781, 727, 718, 671, 652, 638, 631, 608 cm−1. MALDI-TOF MS (Mw = 1236.5): m/z = 1236.7 ([M+]). Stille Polycondensation to Synthesize PSN. A mixture of 4,8-bis(5bromo4-octylthiophen-2-yl)-6-(2-ethylhexyl)[1,2,5]thiadiazolo[3,4-f ]benzotriazole (0.173 g, 0.207 mmol) and (5,5′-(9,9-dioctyl-9Hfluorene-2,7-diyl)bis(thiazole-5,2-diyl))bis(tributylstannane) (0.235 mg, 0.207 mmol), Pd2(dba)3 (0.015 g, 0.016 mmol), P(o-tolyl)3 (0.015 g, 0.048 mmol) in toluene (8 mL) was refluxed for 48 h under N2. After cooling down to room temperature, the reaction mixture was poured into methanol (200 mL). The precipitate was collected by filtration and purified with Soxhlet extraction using methanol, acetone, hexane, and chloroform. The chloroform-soluble fraction was concentrated and reprecipitated into methanol, yielding a green solid (0.130 g, 51%). GPC (eluent: o-dichlorobenzene): Mn = 30.0 kg mol−1, PDI (Mw/Mn) = 2.38. 1H NMR (CDCl3, 300 MHz): δ = 8.72−8.10 (br, Th−H), 7.65−7.63 (br, Ar−H), 7.61−7.58 (br, TzH), 7.43−7.34 (br, Ar−H), 4.94−4.81 (br, N−CH2), 2.98−2.95 (br, CH2), 2.36−2.31 (br, CH), 2.10−2.08 (br, fluorene−CH2), 1.85−1.80 D

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Macromolecules Scheme 2. Synthesis of Copolymers by Palladium-Catalyzed Stille Polycondensation

(br, CH2), 1.55−1.13 (br, CH2), 0.91−0.81 (br, CH3) ppm. IR (neat): ν = 2956, 2924, 2852, 2363, 2341, 1558, 1541, 1521, 1507, 1466, 1435, 1397, 1319, 1260, 1189, 1089, 1018, 865, 792, 748, 727, 717, 670, 659, 612 cm−1. Stille Polycondensation to Synthesize PSeN. A mixture of 4,8bis(5-bromo4-octylthiophen-2-yl)-6-(2-ethylhexyl)[1,2,5]selenadiazolo[3,4-f ]benzotriazole (0.115 g, 0.130 mmol) and (5,5′(9,9-dioctyl-9H-fluorene-2,7-diyl)bis(thiazole-5,2-diyl))bis(tributylstannane) (0.148 g, 0.130 mmol), Pd2(dba)3 (0.0010 g, 0.011 mmol), and P(o-tolyl)3 (0.0010 g, 0.033 mmol) in toluene (6 mL) was refluxed for 48 h under N2. After cooling down to room temperature, the reaction mixture was poured into methanol (200 mL). The precipitate was collected by filtration and purified with Soxhlet extraction using methanol, acetone, hexane, and chloroform. The chloroform-soluble fraction was concentrated and reprecipitated into methanol, yielding a dark brown solid (0.126 g, 76%). GPC (eluent: odichlorobenzene): Mn = 12.6 kg mol−1, PDI = 2.21. 1H NMR (CDCl3, 300 MHz): δ = 8.71−8.10 (br, Th−H), 8.07−7.95 (br, Ar−H), 7.81− 7.75 (br, Tz-H), 7.46−7.43 (br, Ar−H), 4.89−4.80 (br, N−CH2), 2.90−2.73 (br, CH2), 2.34−2.30 (br, CH), 2.13−2.10 (br, fluorene− CH2), 1.78−1.70 (br, CH2), 1.56−1.17 (br, CH2), 0.92−0.80 (br, CH3) ppm. IR (neat): ν = 2954, 2921, 2852, 2364, 2342, 1558, 1541, 1507, 1456, 1436, 1397, 1378, 1312, 1261, 1186, 1103, 1018, 812, 756, 728, 718, 673, 651, 613 cm−1. Stille Polycondensation to Synthesize PSeS. A mixture of 4,7bis(5-bromo-4-(2-decyltetradecyl)thiophen-2-yl)[1,2,5]selenadiazolo[3,4-f ]benzothiadiazole (0.112 g, 0.0906 mmol), (5,5′-(9,9-dioctyl9H-fluorene-2,7-diyl)bis(thiazole-5,2-diyl))bis(tributylstannane) (0.103 g, 0.0906 mmol), Pd2(dba)3 (0.0064 g. 0.0072 mmol), and P(otolyl)3 (0.0064 g, 0.022 mmol) in dry toluene (6 mL) was refluxed for 48 h under N2. After cooling down to room temperature, the highly viscous black solution was poured into methanol (200 mL), and the mixture was stirred for 1 h. The crude polymer precipitate was collected by filtration and washed with methanol and hexane. The resulting dark solid was further purified by Soxhlet extraction for 12 h with methanol, 12 h with acetone, and 12 h with hexane to remove the low-molecular-weight fractions and the residual catalysts. The residue was finally extracted with chlorobenzene and reprecipitated into methanol, yielding a dark brown solid (0.097 g, 67%). GPC (eluent: odichlorobenzene): Mn = 20.3 kg mol−1, PDI = 3.53. 1H NMR (CDCl3, 300 MHz): δ = 8.94−8.50 (br, Th−H), 8.09−7.90 (br, Ar−H), 7.77− 7.60 (br, Tz−H), 7.47−7.31 (br, Ar−H), 2.84−2.78 (br, CH2), 2.17− 2.15 (br, fluorene−CH2), 1.83−1.80 (br, CH), 1.56−1.22 (br, CH2), 0.84−0.80 (br, CH3) ppm. IR (neat): ν = 2960, 2920, 2850, 2364,

2342, 2321, 1800, 1698, 1684, 1653, 1635, 1558, 1541, 1521, 1507, 1457, 1419, 1392, 1259, 1081, 1016, 863, 796, 719, 692, 616 cm−1.

3. RESULTS AND DISCUSSION 3.1. Synthesis and Characterization. The synthetic routes of the BBT analogous acceptor units and three new donor−acceptor (D−A) copolymers are shown in Schemes 1 and 2, respectively. The acceptor unit, 4,8-bis(5-bromo4octylthiophen-2-yl)-6-(2-ethylhexyl)[1,2,5]thiadiazolo[3,4-f]benzotriazole (SeN), was prepared from 4,7-dibromo-2-(2ethylhexyl)-2H-benzo[d][1,2,3]triazole. Nitration followed by the Stille coupling reaction with tributyl(4-octylthiophen-2yl)stannane afforded 2 in 74%. Reduction of the nitro groups of 2 into amino units was performed in the presence of iron dust. Because of the poor air stability of the diamine compound, this crude product was directly employed in the ring-closure reaction with selenium dioxide (SeO2) in refluxing ethanol, producing compound 3 in 57% yield (two steps). Finally, compound 3 was subjected to a bromination reaction, affording the acceptor monomer SN in 88% yield. Similar to SN, SeN and SeS were prepared from 1 and 5 by the Stille coupling, reduction, and ring-closure procedure in the total yields of 39 and 14%, respectively (Scheme 1). It should be noted that a SeS derivative without any alkyl chains was reported to be unstable in air.10 In contrast, SeS could be stored several months in a refrigerator. This fact suggested that the long and branched decyltetradecyl chains not only possess a solubility in the common organic solvents but also have a remarkable air stability. Three new semiconducting polymers, namely PSN, PSeN, and PSeS, were synthesized via the palladium-catalyzed Stille polycondensation at 115 °C for 48 h (Scheme 2). As described above, the dithiazolfluorene monomer 8 was employed as a comonomer. All three polymers were purified by Soxhlet extraction with methanol for 12 h, acetone for 12 h, and hexane for 12 h to remove the low-molecular-weight fractions and residual catalysts. This was followed by reprecipitation from chloroform or chlorobenzene into methanol. The resulting three copolymers showed sufficient solubilities in the common organic solvents, such as chloroform, chlorobenzene, and 1,2dichlorobenzene. Gel permeation chromatography (GPC) E

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vis−NIR absorption profiles of the three copolymers (Figure 1). All the polymers showed the two following absorption bands: band I from 400 to 600 nm and band II from 600 to 1500 nm (Figure 1 and Table 2). The former band could be ascribed to the π−π* transition and the latter attributable to the intramolecular charge transfer (ICT) which was characteristic of donor−acceptor systems.13 It was found that PSeN showed a red-shift in the absorption profile and a narrower optical band gap (Egopt) as compared to PSN. For example, the absorption maximum (λmax,sol) in chloroform shifted from 746 nm for PSN to 791 nm for PSeN. This was also the case for the film absorption maximum (λmax,film); λmax,film shifted from 779 nm for PSN to 845 nm for PSeN. The same trend was observed when PBBT-Tz-FT and PSeS were compared (Table 2). Among the polymers explored in this study, PSeS exhibited the most bathochromically shifted λmax,sol of 968 nm and λmax,film of 1017 nm, resulting in the narrowest band gap of 0.98 eV (Table 2). Since the triazole unit is a weaker electron-withdrawing group than the thiadiazole unit, PSN and PSeN had larger band gaps than PSeS. A comparison between the solution and thin film absorption peaks, |λmax,sol − λmax,film|, revealed the considerable difference in band II by 30−55 nm (Table 2), indicating the occurrence of a strong aggregation in the solid states. The extent of the shift or aggregation tendency was more significant for the selenium-containing polymers, PSeN and PSeS, than the corresponding suflur-containing polymers, PSN and PBBT-TzFT. The frontier molecular orbital energies of the polymers were estimated using cyclic voltammetry (CV). Figure 2a shows the CV curves of the polymer films on a glassy carbon working electrode measured in CH3CN with 0.1 M (nC4H9)4NClO4. The film of PSN showed the reversible oxidation and reduction peaks with the onset oxidation potential (Eox) of 0.50 V (vs Fc/ Fc+) and onset reduction potential (Ered) of −1.23 V (vs Fc/ Fc+). The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels (EHOMO and ELUMO, respectively) of PSN, calculated from the Eox and Ered values, were −5.21 and −3.48 eV, respectively (Figure 2b and Table 3). Substitution of the sulfur atom (S) with the selenium atom (Se) led to a certain degree of EHOMO elevation and ELUMO decrease due to the selenodiazole ring being less aromatic than the thiadiazole ring. Thus, the EHOMO and ELUMO of PSeN were −5.17 and −3.53 eV, respectively (Table 3). Accordingly, the narrower band gap was produced.

studies (using polystyrene as the standard samples and 1,2dichlorobenzene as the eluent at 40 °C) suggested that these polymers have the following number-average molecular weight (Mn) and polydispersity (PDI, Mw/Mn): PSN (Mn = 30.0 kg mol−1, PDI = 2.38), PSeN (Mn = 12.6 kg mol−1, PDI = 2.21), and PSeS (Mn = 20.3 kg mol−1, PDI = 3.53) (Table 1). It Table 1. Molecular Weights and Decomposition Temperatures of Copolymers PSN PSeN PSeS a

Mn (kg mol−1)

PDI

Tda (°C)

30.0 12.6 20.3

2.38 2.21 3.53

418 372 371

Temperature at which 5% weight loss occurred.

should be noted that Stille polycondensation between SeS with short branched side-chains (2-butyloctyl) and 8 afforded slightly soluble PSeS even in 1,2-dichlorobenzene, which prevents the fabrication of the spin-cast films. 3.2. Thermal Properties. In order to understand the thermal stability of the copolymers, a thermogravimetric analysis (TGA) was carried out under flowing nitrogen at the heating rate of 10 °C min−1 (Table 1 and Figure S19). The 5% weight loss temperature (Td) of PSN was 418 °C, which was higher than those of PSeN (372 °C) and PSeS (371 °C). This might reflect the different intermolecular interactions caused by the chalcogen atoms. The high thermal stability of these polymers ensured their applications in organic electronic devices. Subsequently, the thermal transition properties of the polymers were investigated by differential scanning calorimetry (DSC). All the polymers showed some endothermic transitions upon heating and exothermic transitions upon cooling, implying the presence of a crystalline temperature range. For example, PSN had a melting peak at 263 °C during the second heating process and the corresponding crystallization peak at 241 °C during the second cooling process, indicating that PSN is a semicrystalline polymer. Moreover, it should be noted that the glass transition temperature (Tg) of PSN (130 °C) was significantly higher than that of PSeN (50 °C) probably due to the higher weight-average molecular weight. 3.3. Effects of Heteroatom Substitution on the Optoelectronic Properties of the Copolymers. The effect of heteroatom substitution was clearly observed in the UV−

Figure 1. UV−vis−NIR absorption profiles of PSN, PSeN, and PSeS (a) in dilute chloroform solutions and (b) in thin film states spin-cast from chloroform solutions (5 mg mL−1) on glass substrates. F

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Macromolecules Table 2. Optical Properties of the Polymers λmax,sola (nm)

polymer PSN PSeN PBBT-Tz-FTd PSeS a

448, 459, 472, 480,

746 791 882 968

λmax,filmb (nm) 468, 489, 477, 499,

779 845 914 1017

|λmax,sol  λmax,film| (nm)

λonset,film (nm)

Egopt c (eV)

20, 33 30, 54 5, 32 19, 49

957 1043 1166 1270

1.30 1.19 1.06 0.98

Measured in chloroform solution. bCast from chloroform solution. cBand gap estimated from the onset wavelength of the films. dReference 9.

Figure 2. (a) Cyclic voltammograms of PSN, PSeN, and PSeS films drop-cast on a glassy carbon electrode, measured in CH3CN with 0.1 M (nC4H9)4NClO4 at the scan rate of 0.1 V s−1. (b) HOMO/LUMO energy levels of PSN, PSeN, and PSeS. Note that those of PBBT-Tz-FT were included for comparison.

methyl group. The molecular orbitals (MOs) and frontier orbital energies (FOEs) of the acceptor building blocks and the repeat units of the copolymers are shown in Figure 3. The LUMOs of the acceptor units suggested that the contribution to the wave function decreases in the order of selenium atom > sulfur atom > nitrogen atom (Figure 3a). This result was consistent with the experimental LUMO energy levels and strongly supported the fact that the selenium atom was more effective than the sulfur and nitrogen atoms in the stabilization of the LUMOs. The FOEs of the repeat units also showed a linear correlation with the experimental energy levels (Figure 3b). For instance, the replacement of the sulfur atom with the selenium atom (PSN → PSeN and PBBT-Tz-FT → PSeS) raised the EHOMO and lowered the ELUMO for both the experimental and calculated energy levels. On the contrary, the replacement of the sulfur atom with the nitrogen atom (PBBTTz-FT → PSN and PSeS → PSeN) significantly raised both the EHOMO and ELUMO levels. Accordingly, the calculated band gaps were also in good agreement with the experimental results. In addition to the DFT calculations, time-dependent-DFT (TD-DFT) calculations were applied to investigate the S0 → Sn vertical ground-to-excited state transitions.13b−d After the optimization of the geometric structures of the polymer repeat units, TD-DFT calculations were performed by using the longrange-corrected CAM-B3LYP hybrid density functionals. The results are summarized in Figure S21 and Table S1. In general, the UV−vis−NIR absorption profiles simulated by the TDDFT agree well with the experiment results. Thus, the highenergy bands could be attributed to the π−π* transitions, i.e., HOMO → LUMO+1. On the other hand, it was clearly shown that the low-energy bands are largely associated with the

Table 3. Electrochemical Properties and Energy Levels of the Copolymersa copolymer

Eoxa (V)

Ereda (V)

HOMO (eV)

LUMO (eV)

Egec (eV)

PSN PSeN PBBT-Tz-FTb PSeS

0.50 0.46 0.71 0.63

−1.23 −1.18 −0.91 −0.85

−5.21 −5.17 −5.42 −5.34

−3.48 −3.53 −3.80 −3.86

1.73 1.64 1.62 1.48

a Thin films measured in CH3CN with 0.1 M (nC4H9)4NClO4 at the scan rate of 0.1 V s−1. bReference 9.

This trend was also observed for PBBT-Tz-FT and PSeS (Figure 2b and Table 3). Furthermore, it was shown that the triazole-containing polymers, PSN and PSeN, possess higher EHOMO and ELUMO values than the corresponding thiadiazole analogues. For example, PSeN displayed the EHOMO and ELUMO of −5.17 and −3.53 eV, respectively, which were 0.2−0.3 eV higher than those of PSeS (Figure 2b and Table 3). In addition, one could observe that the electrochemical band gaps (Egec), estimated from the difference between the Eox and Ered values, were slightly larger than the corresponding optical band gaps (Egopt). However, there was a linear correlation between these two band gaps, suggesting that both band gaps originated from the same orbitals (Figure S20). In order to further understand the effects of the substituted heteroatoms (nitrogen, sulfur, and selenium atoms) on the optoelectronic properties of the polymers, density functional theory (DFT) calculations were performed on the acceptor units. The repeat units of the copolymers were also calculated in the vacuum state using a B3LYP/6-31G(d,p) basis set. The N-alkyl substituents and alkyl side chains were simplified as a G

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Figure 3. Molecular orbitals (MOs) and correlation diagram of Frontier orbital energies (FOEs) of (a) four acceptor units and (b) four oligomers (repeat units of the copolymer).

HOMO → LUMO electronic transition. All these results again support the experimental absorption spectra. 3.4. Tuning of Charge Polarity in Polymer Thin Film Transistors via Heteroatom Substitution. The type of majority carriers of the D−A semiconducting polymers is generally determined by the relative strength of their donor and acceptor groups.14 In order to estimate the charge transport properties, bottom-gate/top-contact type polymer TFTs were fabricated. The polymer solutions (PSN and PSeN: 3 g L−1 in CHCl3; PSeS: 2 g L−1 in 1,2-dichlorobenzene) were spincoated onto an octadecyltrimethylsilane (OTMS)-treated SiO2 (300 nm)/n++-Si substrate to form the organic semiconducting layers. After thermal annealing at a selected temperature (100, 130, 150, 170, 190, 210, or 230 °C), ∼50 nm thick gold was deposited as the source and drain contacts using a shadow mask. The detailed fabrication procedure was described in the Experimental Section. As shown in Figure 4 and Table 4, it is noted that the charge transport properties of the copolymers

dramatically differed upon the heteroatoms substitution on the acceptor units. The spin-coated film of PSN without annealing exhibited a unipolar hole mobility (μh) of 0.11 cm2 V−1 s−1 and on−off ratio (Ion/Ioff) of 2.3 × 104 (Figure 4a). Annealing the polymer thin films led to a dramatic increase in μh, and the maximum μh of 0.47 cm2 V−1 s−1 and Ion/Ioff of 4.8 × 105 were achieved when heated to 170 °C (Figure 4b). The enhancement of the on/off ratios and a shift in the threshold voltages to a lower value were probably caused by the release of the residual oxygen and moisture (H2O) trapped at the interfaces, which formed a better ohmic contact between the polymer and Au electrode after thermal annealing. Interestingly, spin-coating the polymer thin film in air followed by a thermal annealing at 170 °C resulted in a higher μh of 0.65 cm2 V−1 s−1 probably due to the oxygen doping effect (Figure 4b).15 However, a further increase in the annealing temperature (>170 °C) decreased the TFT performances (Table 4). In the case of PSeN in which only the sulfur atom of PSN was replaced by the selenium H

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Figure 4. Current−voltage (I−V) characteristics of TFTs fabricated by spin-coating: (a) transfer characteristics for PSN films as-cast and annealed at 170 °C with VDS = −80 V; (b) transfer characteristics for PSN films spin-coated in a glovebox and spin-coated in air; (c) output characteristics for PSN pristine films; (d) output characteristics for PSN films after annealing at 170 °C; (e) transfer characteristics for PSeN films as-cast and annealed at 150 °C at (left) hole- and (right) electron-enhancement operation with VDS = −80 and +80 V, respectively; (f) output characteristics for PSeN films after annealing at 150 °C; (g) transfer characteristics for PSeS films as-cast and annealed at 190 °C at (left) hole- and (right) electronenhancement operation with VDS = −80 and +80 V, respectively; (h) output characteristics for PSeS films after annealing at 190 °C (L = 100 μm and W = 1 mm, the PSN-based TFTs were measured under air whereas the PSeN- and PSeS-based TFTs were measured under vacuum (10−4−10−5 mbar)).

was dramatically changed from the unipolar p-type of PSN to the balanced ambipolar PSeN and finally to the n-type dominant PSeS. All the TFT performances were consistent with the energy levels of the copolymers estimated by both the electrochemistry and computational calculations. It is remarkable that switching the charge polarity in the TFT devices can be achieved through such a subtle change in the chemical structure, namely, heteroatom substitution. It is obvious that the selenium atom was more effective in stabilizing the LUMO than the sulfur atom, and this was also the case for the sulfur atom vs the nitrogen atom. Moreover, the lone pair electrons of the selenium atom with the outer shell electron distribution of 4s24p4 are more labile than those of the sulfur atom with 3s23p4.16 Consequently, PSeS with the deeper LUMO and higher electron affinity exhibited the highest electron mobility among the three copolymers. The electron mobility would be further improved when the top-gate/ bottom-contact (TG/BC)-type device configuration is adopted

atom, ambipolar performances were observed. The pristine ascast PSeN-film displayed a μh of 1.0 × 10−3 cm2 V−1 s−1 and a μe of 2.9 × 10−3 cm2 V−1 s−1, which amounted to a balanced μh/μe ratio of 0.34. Annealing the PSeN-films also led to an increase in the mobilities and Ion/Ioff. The optimized annealing temperature of 150 °C resulted in the highest μh value of 1.7 × 10−3 cm2 V−1 s−1 and μe of 7.8 × 10−3 cm2 V−1 s−1 (Figure 4e). Note that the annealing had a positive effect on the n-type Ion/ Ioff (from 1.0 × 102 to 1.6 × 104). Interestingly, the replacement of the nitrogen atom of PSeN with the sulfur atom eventually realized the n-type dominant semiconducting polymer of PSeS. The TFT performances of this polymer were almost inverted from those of PSN. The PSeS films exhibited the maximum μh and μe of 7.3 × 10−3 and 8.7 × 10−2 cm2 V−1 s−1, respectively, with a high n-type Ion/Ioff of 1.7 × 104 after annealing at the optimized temperature of 190 °C (Figure 4g). The significantly enhanced μe as compared to the μh produced a μh/μe = 0.084. Overall, the charge polarity of these copolymers-based TFTs I

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Macromolecules Table 4. Summary of Polymer TFT Device Performances polymer/annealing temp (°C)

μha (cm2 V−1 s−1)

PSN (N/A) 100 130 170c 170d 190 230 PSeN (N/A) 100 150c 170 190 PSeS (N/A) 100 150 190c 210

0.11 (0.060) 0.18 (0.075) 0.31 (0.20) 0.47 (0.33) 0.65 (0.40) 0.14 (0.079) 0.060 (0.030) 1.0 × 10−3 (8.2 × 10−4) 1.3 × 10−3 (9.6 × 10−4) 1.7 × 10−3 (1.2 × 10−3) 1.3 × 10−3 (8.0 × 10−4) 8.4 × 10−4 (6.3 × 10−4) 2.0 × 10−3 (7.8 × 10−4) 2.4 × 10−3 (1.6 × 10−3) 4.7 × 10−3 (2.5 × 10−3) 7.3 × 10−3 (4.5 × 10−3) 5.5 × 10−3 (3.1 × 10−3)

μeb (cm2 V−1 s−1)

2.9 × 10−3 (2.4 × 10−3) 4.4 × 10−3 (3.1 × 10−3) 7.8 × 10−3 (6.8 × 10−3) 6.3 × 10−3 (4.5 × 10−3) 3.5 × 10−3 (2.4 × 10−3) 1.5 × 10−2 (6.5 × 10−3) 3.5 × 10−2 (9.3 × 10−3) 6.6 × 10−2 (3.5 × 10−2) 8.7 × 10−2 (5.8 × 10−2) 5.4 × 10−2 (2.1 × 10−2)

Ion/Ioff p: p: p: p: p: p: p: p: p: p: p: p: p: p: p: p: p:

104−105 104−105 104−105 105−106 104−105 104−105 103−104 102−103; 102−103; 102−103; 102−103; 102−103; 101−102; 101−102; 101−102; 101−102; 101−102;

n: 101−102 n: 102−103 n: 104−105 n: 102−103 n:102−103 n: 103−104 n: 103−104 n: 103−104 n: 104−105 n: 103−104

a

Annealing temperature dependent maximum values of the hole mobilities of PSN-based devices measured under air, PSeN, and PSeS-based devices measured under vacuum (10−4−10−5 mbar). bElectron mobilities measured under vacuum (10−4−10−5 mbar). The average values are depicted in parentheses (from 5 to 10 devices). cThe hole/electron mobilities under optimized annealing conditions are indicated in boldface. dPolymer active layer was spin-cast in air.

Figure 5. 2D-GIWAXS patterns of the thin films of (a) PSN, (b) PSeN, and (c) PSeS after the treatment of optimized annealing conditions.

plane diffraction observed near qxy = 0.3 Å−1 corresponded to the crystalline structure. The small-angle out-of-plane (qz) reflections indicated the existence of lamellar textures. The outof-plane (010) diffraction near qz = 1.6 Å−1 corresponded to the π−π stacking distances of each polymer. Especially, the PSeN film displayed a very strong peak. In contrast, a very weak inplane (010) diffraction peak was detected for the films of PSN and PSeS. These results suggested that PSeN tended to form face-on dominant packing textures, whereas PSN and PSeS had dual textures (both face-on and edge-on packing). In addition, the π−π stacking distances were in the order of PSN (3.49 Å) < PSeS (3.51 Å) < PSeN (3.55 Å). All these results could reasonably explain the TFT results. Collectively, the GIWAXS

because it is known that this device structure had better injection characteristics.1a,17 3.5. Characterization of Molecular Organization and Film Morphologies. The crystalline structures of the semiconducting polymer films are one of the influential factors in governing the charge carrier mobilities. To gain an insight into the polymer orientation within the thin films, wide-angle scattering in the grazing-incidence mode was examined.18 Similar to the TFT devices, the polymer films were spin-coated on an OTMS-treated SiO2/Si substrate and subjected to treatment under the optimized annealing conditions. All three polymer films showed well-defined grazing-incidence wideangle X-ray scattering (GIWAXS) patterns (Figure 5). The inJ

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Figure 6. Tapping-mode AFM phase images of the polymer films prepared by spin-coating of a chloroform solution of PSN and PSeN and a 1,2dichlobenzene solution of PSeS. Images from left to right: PSN, PSeN, and PSeS. Top: as-cast pristine films; bottom: films after annealing at the optimized temperatures for 10 min. AFM size: 2 × 2 μm2.

unipolar TFT performances with the high hole mobility up to 0.65 cm2 V−1 s−1. Remarkably, this polymer TFT was stable in air as shown by the mobility increase in terms of the oxygen doping effect. PSeN and PSeS displayed ambipolar charge transport properties with the mobility balance being tuned by the heteroatoms (nitrogen vs sulfur). Thus, PSeS exhibited ntype dominant charge properties with electron mobilities up to 0.087 cm2 V−1 s−1. In the future, PSeS could be potentially used as an n-type semiconducting material in other organic electronic devices, e.g., all-polymer solar cells.

patterns revealed that PSeN had a face-on packing with the longest π−π stacking distance, resulting in the lowest μh and μe among the three polymers. On the other hand, PSN and PSeS had mixed face-on and edge-on packing textures with shorter π−π stacking distancesthe parameters that are preferable for achieving high charge transport properties utilizing 3-D conduction channels.6a,19 To further elucidate the film quality, the surface morphology of the polymer thin films was investigated using tapping-mode atomic force microscopy (AFM). The AFM phase and height images are shown in Figure 6 and Figure S22, respectively. The as-cast thin film of PSN showed a root-mean-square (rms) roughness of 1.78 nm. When the PSN film was annealed at the optimized temperature of 170 °C for 10 min, it showed crystalline fiber-like intercalating networks with the rms deviation of 2.36 nm, indicating enhanced interchain interactions after thermal annealing. This result would support the observed dramatic increase in the mobilities upon thermal annealing (vide supra). Similar to PSN, thermal annealing of the PSeS film (spin-cast from a 1,2-dichlorobenzene solution) also exhibited fiber-like intercalating networks with a larger domain size and higher crystallinity, as represented by the increase in the rms roughness from 0.27 to 0.48 nm. In contrast, the PSeN film showed a slight increase in the rms from 0.45 to 0.49 nm upon thermal annealing at the optimized temperature of 150 °C for 10 min. This might be due to the different polymer orientations (face-on for PSeN vs mixed face-on and edge-on for PSN and PSeS), as revealed by the GIWAXS measurements.

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ASSOCIATED CONTENT

S Supporting Information *

Synthetic details of compounds 1, 2, 5, 6, and 8, 1H NMR and 13 C NMR spectra of the monomers and polymers, GPC curves of the polymers, TGA and DSC curves of the polymers, a summary of the band gaps, detailed computational calculations, and AFM height images of the polymer films. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b00802.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (T.M.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by JSPS KAKENHI Grant 26620173, the Tokuyama Science Foundation, the Mayekawa Houonkai Foudation, and the Support for Tokyotech Advanced Researchers (STAR). Y.W. is grateful to the Japanese government (MEXT: Monbukagakusho) scholarship for the financial support to study at Tokyo Institute of Technology. The synchrotron radiation experiments were performed at BL45XU in SPring-8 with the approval of JASRI (Proposal No. 2014B1103). We thank Mr. Tsukasa Hasegawa (Department of

4. CONCLUSIONS In summary, new semiconducting polymers composed of the BBT-related heterocycles were synthesized, and the heteroatom substitution effects on the charge polarity in their TFTs were comprehensively investigated. Taking into account the fact that the heteroatom substitution of the acceptor units is rare, this study offers a new opportunity to tailor the optoelectronic properties, molecular organizations, and charge carrier transport properties in polymeric TFTs. PSN showed p-type K

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Yoon, Y.; Park, W.-T.; Kwak, K.; Son, H. J.; Kim, B.; Noh, Y.-Y. Adv. Mater. 2015, 27, 3045. (14) Li, J.; Zhao, Y.; Tan, H. S.; Guo, Y.; Di, C.-A.; Yu, G.; Liu, Y.; Lin, M.; Lim, S. H.; Zhou, Y.; Su, H.; Ong, B. S. Sci. Rep. 2012, 2, 754. (15) (a) Okamoto, H.; Kawasaki, N.; Kaji, Y.; Kubozono, Y.; Fujiwara, A.; Yamaji, M. J. Am. Chem. Soc. 2008, 130, 10470. (b) Pietro, R. D.; Fazzi, D.; Kehoe, T. B.; Sirringhaus, H. J. Am. Chem. Soc. 2012, 134, 14877. (c) Lei, T.; Dou, J.-H.; Cao, X.-Y.; Wang, J.-Y.; Pei, J. Adv. Mater. 2013, 25, 6589. (16) (a) Cho, S.; Lee, J.; Tong, M.; Seo, J. H.; Yang, C. Adv. Funct. Mater. 2011, 21, 1910. (b) Kanimozhi, C.; Gross, N. Y.; Chou, K. W.; Amassian, A.; Anthopoulos, T. D.; Patil, S. J. Am. Chem. Soc. 2012, 134, 16532. (c) Lee, J.; Han, A.-R.; Yu, H.; Shin, T. J.; Yang, C.; Oh, J. H. J. Am. Chem. Soc. 2013, 135, 9540. (d) Kang, I.; An, T. K.; Hong, J.A.; Yun, H.-J.; Kim, R.; Chung, D. S.; Park, C. E.; Kim, Y.-H.; Kwon, S.-K. Adv. Mater. 2013, 25, 524. (17) (a) Di, C.-A.; Liu, Y.; Yu, G.; Zhu, D. Acc. Chem. Res. 2009, 42, 1573. (b) Usta, H.; Newman, C.; Chen, Z.; Facchetti, A. Adv. Mater. 2012, 24, 3678. (18) (a) Fujisawa, T.; Inoue, K.; Oka, T.; Iwamoto, H.; Uruga, T.; Kumasaka, T.; Inoko, Y.; Yagi, N.; Yamamoto, M.; Ueki, T. J. Appl. Crystallogr. 2000, 33, 797. (b) Renauda, G.; Lazzari, R.; Leroy, F. Surf. Sci. Rep. 2009, 64, 255. (19) Lei, T.; Dou, J.-H.; Ma, Z.-J.; Yao, C.-H.; Liu, C.-J.; Wang, J.-Y.; Pei, J. J. Am. Chem. Soc. 2012, 134, 20025.

Organic and Polymeric Materials, Tokyo Institute of Technology) for the assistance in the GIWAXS analysis.

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