Wide-Bandgap Conjugated Polymers Based on Alkylthiofuran

Mar 4, 2018 - in Figure 1, and the corresponding data are summarized in Table 1. As displayed in Figure 1a, the absorption spectra of both polymers in...
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Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

Wide-Bandgap Conjugated Polymers Based on Alkylthiofuran-Substituted Benzo[1,2‑b:4,5‑b′]difuran for Efficient Fullerene-Free Polymer Solar Cells Yueyue Gao,†,‡ Zhen Wang,§ Jianqi Zhang,§ Hong Zhang,† Kun Lu,*,§ Fengyun Guo,† Zhixiang Wei,§ Yulin Yang,‡ Liancheng Zhao,† and Yong Zhang*,† †

School of Materials Science and Engineering and ‡School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin 150001, China § CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology, Beijing 100190, China S Supporting Information *

ABSTRACT: The alkythiofuran-based benzo[1,2-b:4,5-b′]difuran (BDFS) unit presents the planar and rigid structure with favorable inter/intramolecular interactions and is expected to be a promising building block to construct the highly efficient photovoltaic polymers. Herein, we reported two novel wide-bandgap conjugated polymers, PBDFS-Bz and PBDFS-fBz, which were based on BDFS and benzo[d][1,2,3]triazole (Bz) units, and investigated their photovoltaic performances in fullerene-free polymer solar cells (PSCs). PBDFS-Bz and PBDFS-fBz show similar optical properties with the wide bandgaps of ∼1.88 eV. The HOMO energy level of PBDFS-Bz is at −5.30 eV, whereas it is downshifted to −5.45 eV for PBDFS-fBz due to the strong electron-withdrawing fluorine substitutes. The optimized fullerene-free PSC based on PBDFS-Bz:ITIC achieved a power conversion efficiency (PCE) of 8.07% with a Voc of 0.82 V, a Jsc of 15.14 mA cm−2, and a FF of 65%. Under the same conditions, PBDFS-fBz:ITIC device won a promising PCE of 9.00% with an enhanced Voc of 0.88 V, a Jsc of 15.26 mA cm−2, and a FF of 67.0%. It is found that the improved photovoltaic performance of PBDFS-fBz:ITIC device is mainly due to the enhanced Voc in comparison with that of PBDFS-Bz:ITIC device. To the best of our knowledge, the photovoltaic performances are among the best devices reported for fullerene-free PSCs with BDF polymers as donors. These results demonstrate that the rational design of BDF building block is highly important in obtaining the state-of-the-art photovoltaic performances.

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INTRODUCTION Polymer solar cells (PSCs) have received continuous attention over the past 20 years because of their intrinsic advantages, i.e., lightweight, cost-effectiveness, the potential for fabricating large-area flexible devices via roll-to-roll technology, etc.1−3 With the great efforts on the developments of conjugated polymer donors and acceptors, and the innovations of device engineering, the photovoltaic performances of the current PSCs have reached the power conversion efficiencies (PCEs) of over 13%.4−6 Among various types of conjugated polymers, the most successful design is based on the donor−acceptor (D−A) strategy, where the conjugated polymer contains an alternative donor and acceptor unit along the polymer backbone.7−9 Up to now, numerous conjugated polymers based on the donor units of such as fluorene,10−12 carbazole,13−15 indacenodithiophene (IDT),16−20 and benzo[1,2-b:4,5-b′]dithiophene (BDT)21−23 and different acceptor units have been developed. As one of the most promising building blocks of conjugated polymers for PSCs, the rigid and planar structure of BDT provides the great potentials on tuning the energy levels, bandgaps, and the charge carrier mobility with the desired chemical structure modifications.24−26 © XXXX American Chemical Society

In comparison with BDT, however, its furan analogue benzo[1,2b:4,5-b′]difuran (BDF) is considerably less developed as an efficient building block for the photovoltaic materials. It is known that the furan unit possesses a smaller size than that of thiophene unit, which may form more planar and rigid structures with favorable inter/intramolecular interactions in such as BDF-based conjugated polymers.27,28 In addition, the stronger electronegativity of furan will also induce BDF-based polymers possessing the lower highest occupied molecular orbital (HOMO) energy level in comparison with that of BDT-based polymers.29,30 Moreover, furan can be obtained easily and cheaply from the extensive renewable resources, i.e., vegetables, leaves, and crops.31,32 Even with these advantages over BDT, it is unfortunate that the current developments of BDF-based polymers for PSCs still lag far behind BDT-based polymers in either the photovoltaic performance or the kinds of available highly efficient polymers. For examples, Stefan and Biewer et al. synthesized two BDFbased polymers PBDF-FTT and PBDF-FDPP and achieved the Received: December 18, 2017 Revised: March 4, 2018

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backbone to form a more planar structure.27,28 Moreover, it is well-known that the introduction of alkylthio substituent could also enhance the extinction coefficients of the polymers so as to benefit for efficient light-harvesting.38 As shown in Scheme 1, the compound 1 was synthesized through the simple lithiation and quenched by sulfur and 2-ethylhexyl bromide (see the Supporting Information).39 Compound 1 was then deprotonated by n-BuLi, then to attack 4,8-dehydrobenzo[l,2-b:4,5-b′]difuran-4,8-dione (2) to generate the BDFS unit (3). The distannyl compound BDFS-T was prepared from compound 3 by lithiation with n-BuLi followed by quenching using trimethyltin chloride. The 4,7-bis(5-bromothiophen-2-yl)-2-(2-hexyldecyl)-4,5-dihydro-2Hbenzo[d][1,2,3]triazole (Bz-Br) and 4,7-bis(5-bromothiophen2-yl)-5,6-difluoro-2-(2-hexyldecyl)-4,5-dihydro-2H-benzo[d][1,2,3]triazole (fBz-Br) were prepared according to the reported methods.40,41 The Stille polycondensations between BDFS-T and Bz-Br or fBz-Br gave the BDF polymers PBDFS-Bz and PBDFSfBz, respectively, with chlorobenzene/DMF as the solvent and Pd2(dba)3/P(o-tol)3 as the catalysts. The resulted polymers were purified by Soxhlet extraction to remove catalysts and oligomers. The number-averaged molecular weights (Mn) were measured by gel permeation chromatography (GPC) with polystyrene as standard and 1,2,4-trichlorobenzene as the eluent and were 16.8 and 14.2 kDa for PBDFS-Bz and PBDFS-fBz, respectively, with the polydispersity index (PDI) of 1.75 and 1.87 (Table 1).

PCEs of 5.23% and 5.55%, respectively, in fullerene-based PSCs.33,34 Zou et al. developed a polymer BDFPS-HFQx based on BDF and quinoxaline units and obtained a PCE of 5.16% with PC71BM as the electron acceptor.35 Recently, Li et al. also developed the efficient BDF-based polymers with benzothiadiazole (BT) or fluorobenzothiazole as the comonomers, and the PCEs of up to 11.05% could be achieved for the fullerene-free PSCs.36,37 To further improve the photovoltaic performances of BDF-based conjugated polymers in either fullerene or fullerene-free PSCs, therefore the design and preparation of the novel BDF-based conjugated polymers with the aforementioned advantages are urgently needed. In this work, we designed and synthesized two wide-bandgap donor−acceptor conjugated polymers PBDFS-Bz and PBDFSfBz with a novel BDF building block, 4,8-bis(5-((2-ethylhexyl)thio)furan-2-yl)benzo[1,2-b:4,5-b′]difuran (BDFS), as the donor unit and 4,7-bis(5-thiophen-2-yl)-2-(2-hexyldecyl)-2H-benzo[d][1,2,3]triazole (Bz) and 4,7-bis(5-thiophen-2-yl)-5,6-difluoro-2(2-hexyldecyl)-2H-benzo[d][1,2,3]triazole (fBz) as the acceptor unit, respectively. The optical and electrochemical properties as well as the polymer chain stacking behaviors were investigated systematically. The fullerene-free PSCs of both polymers were fabricated with ITIC as the electron acceptor. The PSC based on PBDFS-Bz:ITIC achieved a PCE of 8.07% with a Voc of 0.82 V, a Jsc of 15.14 mA cm−2, and a FF of 65%. With two fluorine substitutes, PBDFS-fBz possessed lower-lying HOMO energy level, denser π−π stacking spacing, and higher hole mobility than that of PBDFS-Bz. As result, the PSC based on PBDFS-fBz:ITIC won a promising PCE of 9.00% with an increased Voc of 0.88 V, a Jsc of 15.26 mA cm−2, and a FF of 67.0%, which is among the best reported photovoltaic performance for fullerene-free PSCs based on BDF polymers. These results demonstrate that the desirable design of BDF building block is highly important to obtain the state-of-the-art photovoltaic performances and show that the BDFS unit is a promising building block for constructing highly efficient PSCs.

Table 1. Optical and Electrochemical Properties of PBDFS-Bz and PBDFS-fBz polymer PBDFS-Bz PBDFS-fBz a

absorption (nm) solution film 553, 601 549, 595

558, 606 556, 604

Egopt a (eV)

HOMOb (eV)

LUMOb (eV)

Egcv (eV)

1.88 1.89

−5.30 −5.45

−3.33 −3.44

1.97 2.01

From absorption edge of thin film. bFrom the CV measurement.

Both polymers possess good solubility in chloroform, chlorobenzene, dichlorobenzene, etc. The decomposition temperatures (5% weight loss) of both polymers were found at ∼360 °C (Figure S1), indicating a good thermal stabilities. Optical Properties. The absorption spectra of PBDFS-Bz and PBDFS-fBz in chloroform solution and film state are shown in Figure 1, and the corresponding data are summarized in Table 1. As displayed in Figure 1a, the absorption spectra of both polymers in solution showed the similar structured absorption bands with the predominant peaks at ∼500−600 nm, which are ascribed to the intramolecular charge transfer (ICT) peak between BDFS and Bz units. The absorption peaks for the 0−0 transition for PBDFS-Bz and PBDFS-fBz are 601 and 595 nm in solution, respectively. The slight blue-shift absorption spectrum of PBDFS-fBz compared to that of PBDFS-Bz is believed to the strong electronwithdrawing fluorine substitutes on PBDFS-fBz.42 The weak absorption bands at ∼300−400 nm could be ascribed to the π−π* transitions of the monomers.43−48 As shown in Figure 1b, the absorption spectra of both polymers in film show similar structured characteristic but present the red-shifted absorptions in comparison with that in solution. The 0−0 absorption peaks in film for PBDFS-Bz and PBDFS-fBz were red-shifted to 606 and 604 nm, respectively. The absorption edges of PBDFS-Bz and PBDFS-fBz were at 661 and 657 nm, respectively, which correlates to the optical band gap of 1.88 and 1.89 eV. Figure 1b also shows the absorption spectra of ITIC in film state, and it can find that the absorption of PBDFS-Bz and PBDFS-fBz are complementary well with that of ITIC, which will be beneficial for the current density by absorbing more sunlight in the fullerene-free PSCs.

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RESULTS AND DISCUSSION Synthesis. The synthetic routes of monomers and polymers are shown in Scheme 1. Herein, 2-((2-ethylhexyl)thio)furan as Scheme 1. Synthetic Routes of Monomers and Polymers

the side chains were introduced onto BDF since the furan possesses smaller size, which may present smaller torsion angle with B

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Figure 1. Absorption spectra of PBDFS-Bz and PBDFS-fBz in chloroform (a) and film state (b). The absorption spectrum of ITIC in film was also shown in (b).

Figure 2. Cyclic voltammetry (CV) curves (a) and the energy level diagram of materials used in the fullerene-free PSCs (b).

Electrochemical Properties. Cyclic voltammetry (CV) measurements were used to investigate the electrochemical properties of PBDFS-Bz and PBDFS-fBz. The HOMO and lowest unoccupied molecular orbital (LUMO) energy levels for polymers were calculated from the equation EHOMO = −e(Vox + 4.80 − Vferro) and ELUMO = −e(Vred + 4.80 − Vferro), where Vox and Vred are the onsets of oxidation and reduction peaks of polymers with Ag/AgCl reference electrode and ferrocene (FeCp2) as the internal reference, which is 4.80 eV below the vacuum level, and the Vferro (0.21 V) is the deviation potential of FeCp2 in the measurement.49 As shown in Figure 2a, the onsets of oxidation and reduction potentials (Vox/Vred) were found at 0.71 and −1.26 V for PBDFS-Bz and 0.86 and −1.15 V for PBDFS-fBz, respectively. Then, the HOMO and LUMO energy levels of PBDFS-Bz are calculated to be −5.30 and −3.33 eV, respectively. Because of the strong electron-withdrawing properties of fluorine atoms, the HOMO and LUMO energy levels of PBDFS-fBz are downshifted to −5.45 and −3.44 eV, respectively,50 in which the lower HOMO energy level is beneficial for obtaining higher Voc in PSCs. Figure 2b shows the energy level diagram of materials used in the fullerenefree PSCs. It can find that the LUMO energy offsets between PBDFS-Bz or PBDFS-fBz and ITIC are 0.51 and 0.40 eV, which exceed the empirical threshold of 0.3 eV of excition dissociation energy.51 It is interesting that the HOMO energy offsets between PBDFS-Bz or PBDFS-fBz and ITIC are found to be only 0.24 and 0.09 eV, which is regarded not enough for the efficient exciton dissociation in the fullerene-based PSCs. However, recent studies have proved that the smaller or even zero HOMO energy offset is still able to produce the efficient exciton dissociation in the fullerene-free PSCs.52,53

Density Functional Theory Calculations. The groundstate geometric and electronic structures of the dimers of PBDFS-Bz and PBDFS-fBz were calculated with density functional theory at the B3LYP/6-31G(d) level.54 To simplify the calculations, the 2-ethylhexylthio side chains were replaced with methylthio, which has no effect on the results since the side chains play little role on the electronic structures of the polymers. Figure 3 shows the optimized geometries and frontier molecular orbitals of both polymers. It can find that both polymers adopt a nearly flat backbone configuration with dihedral angles of ∼0.33°−1.18° between BDFS and acceptor (Bz or fBz) unit, which is in part due to the planar structure of BDF unit. In addition, dihedral angles between Bz unit and thiophene spacer in PBDFS-fBz are smaller than that of BDFS-Bz, which is believed to be due to the “molecule lock” effect induced by F···H and F···S in PBDFS-fBz.42 The more planar structure in PBDFS-fBz would be beneficial for charge carrier transport, which may contribute to a high Jsc and FF in PSCs. As seen from Figure 3, the HOMO orbital distributions of both polymers are mainly delocalized on BDFS unit but also have some distributions on Bz unit. For the LUMO energy levels, the orbital distributions for both polymers, however, are mostly localized on Bz units with a small extension on BDFS unit.55,56 The calculated HOMO and LUMO energy levels are −4.52 and −2.35 eV for PBDFS-Bz and −4.80 and −2.57 eV for PBDFS-fBz, respectively. It shows a similar trend with that observed from CV measurements. X-ray Diffraction. The polymer chain packing behaviors in thin film state were investigated by X-ray diffraction (XRD). Figure 4 displays the XRD patterns of PBDFS-Bz and PBDFSfBz in film states, and the inset presents the diagram of the C

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Figure 3. Optimized geometries and HOMO and LUMO electron density distributions of PBDFS-Bz and PBDFS-fBz by density functional theory with the B3LYP/6-31G* basis set.

density−voltage (J−V) curves of the PSCs under the illumination of AM 1.5G, 100 mW cm−2 are shown in Figure 5a. The photovoltaic performances are summarized in Table 2. The best device based on PBDFS-Bz:ITIC was achieved from thermal annealing at 130 °C for 2 min following by the solvent annealing under THF atmosphere for 1 min. Under the best condition, the PSC based on PBDFS-Bz:ITIC obtained a PCE of 8.07% with a Voc of 0.82 V, a Jsc of 15.14 mA cm−2, and a FF of 65.0%. As discussed above, PBDFS-fBz possesses a lower HOMO energy level; therefore, the PBDFS-fBz-based device will be expected to exhibit a higher Voc. As expected, the Voc of PBDFS-fBz:ITIC device under the same conditions was improved to 0.88 V. As a result, PBDFS-fBz:ITIC device delivered a promising PCE of 9.00% with a Jsc of 15.26 mA cm−2 and a FF of 67.0%, which is among the best device performances based on BDF polymers in fullerenefree PSCs. The external quantum efficiencies (EQEs) curves of the optimal PSCs are shown in Figure 5b. Owing to the complementary absorption of polymer donor with acceptor ITIC, it can find both devices show broad photoresponses from 300 to 800 nm with the maximum value of 70.5−72.9%, which demonstrates the efficient photon harvesting and charge collection. As seen from Figure 5b, both polymers and ITIC make contributions to the overall current, in which the photoresponses below 600 nm are mainly from PBDFS-Bz or PBDFS-fBz, and the photoresponses in the range of 600−800 nm are dominant by ITIC. The calculated Jsc by integrating the EQE curve with an AM 1.5 G reference spectrum are 14.99 and 15.06 mA cm−2 for PBDFS-Bz:ITIC and PBDFSfBz:ITIC, respectively. The calculated Jsc by integrating the EQE curve is agreement well with the Jsc values obtained from the J−V measurements within 5% mismatch (Table 1). Film Morphology. Atomic force microscopy (AFM) and transmission electron microscopy (TEM) were used to investigate the surface morphology of PBDFS-Bz:ITIC and PBDFS-fBz:ITIC blend films. As displayed in Figures 6a and 6b, the blend film of PBDFS-Bz:ITIC presents an uniform and smooth surface with

Figure 4. XRD patterns of PBDFS-Bz and PBDFS-fBz in film state (inset: diagram of the dlamellar and dπ−π between polymer chains).

lamellar d-spacing (dlamellar) and π−π stacking d-spacing (dπ−π) between polymer chains. As displayed in Figure 4, it can find that there is a sharp (100) diffraction peak at 3.79° observed for PBDFS-Bz, corresponding to the dlamellar of 23.31 Å. For PBDFSfBz, the (100) diffraction peak is shifted to 4.11° with the increased intensity, and the corresponding dlamellar decreases to 21.48 Å, in which the denser polymer chain packing may be induced by the enhanced intermolecular interactions through F···H, F···S, and F···πF interactions. On the other hand, a blunt diffraction (010) peak at 22.56° is observed for PBDFS-Bz, which is resulted from π−π stacking with a dπ−π of 3.94 Å. Similarly, the (010) peak of PBDFS-fBz shifts to the large angle of 23.16° with a decreased dπ−π of 3.84 Å. The smaller π−π stacking distance in PBDFS-fBz polymer chains may be beneficial for the charge carrier transport. Photovoltaic Properties. The fullerene-free polymer solar cells with the conventional structure of ITO/PEDOT:PSS/polymer:ITIC/PFN/Al were fabricated to investigate the photovoltaic properties of PBDFS-Bz and PBDFS-fBz. The optimized weight ratio of polymer:ITIC (D:A) was 1:1.5 with the polymer concentration of 10 mg/mL in chlorobenzene. The current D

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Figure 5. (a) J−V curves and (b) external quantum efficiencies (EQEs) of PSCs based on polymer:ITIC under the optimal condition.

Table 2. Photovoltaic Parameters of PSCs Based on Polymer/ITIC Measured under the Illumination of Simulated AM 1.5G Conditions (100 mW cm−2) donor/acceptor

Voc (V)

Jsc (mA cm−2)

FF (%)

PCE (%)

Jsc (mA cm−2)

μh (cm2 V−1 s−1)

PBDFS-Bz:ITIC PBDFS-fBz:ITIC

0.82 0.88

15.14 15.26

65 67

8.07 9.00

14.99 15.06

2.26 × 10−4 5.60 × 10−4

blend film adopts a mixed orientation (edge-on and face-on) to the substrate. However, as shown in Figures 7b and 7d, the blend film of PBDFS-fBz:ITIC presents a stronger diffraction peak for π−π stacking at 1.85 Å−1 (d = 3.39 Å), and two lamellar peaks at 0.25 Å−1 (d = 25.1 Å) and 1.36 Å−1 (d = 4.62 Å) related to (100) and (200) peaks in the out-of plane were also observed. The result indicates the PBDFS-fBz:ITIC blend film shows better intermolecular orientations and nanostructure orders with a preferred face-on orientation, which would be beneficial for charge carrier transport and is consistent with the observed higher hole mobility of PBDFS-fBz:ITIC (5.60 × 10−4 cm2 V−1 s−1) than that of PBDFS-Bz:ITIC (2.26 × 10−4 cm2 V−1 s−1) (Figure S2).51 Conclusions. We have designed and synthesized two novel wide-bandgap polymers PBDFS-Bz and PBDFS-fBz with BDFS and Bz units. The optical and electrochemical properties as well as the polymer chain stacking behaviors of both polymers were investigated systematically. PBDFS-Bz and PBDFS-fBz show very similar absorption spectra in both solution and films with the main peaks at ∼595−612 nm and the wide bandgaps of ∼1.88 eV. The HOMO and LUMO energy levels for PBDFS-Bz are at −5.30 and −3.33 eV, respectively. For PBDFS-fBz, the HOMO and LUMO energy levels are downshifted to −5.45 and −3.44 eV, respectively, due to the strong electron-withdrawing fluorine substitutes. Because of the “molecule lock” effect of fluorine, PBDFS-fBz shows denser π−π distance than that of PBDFS-Bz. The photovoltaic performances of fullerene-free polymer solar cells were also investigated with ITIC as the electron acceptor. The PBDFSBz:ITIC based device achieved a PCE of 8.07% with a Voc of 0.82 V, a Jsc of 15.14 mA cm−2, and a FF of 65%. Under the same device condition, PBDFS-fBz:ITIC based device showed a promising PCE of 9.00% with an enhanced Voc of 0.88 V, a Jsc of 15.26 mA cm−2, and a FF of 67.0%. To the best of our knowledge, the photovoltaic performances of the PBDFS-fBz:ITIC device is among the best in fullerene-free PSCs with BDF polymers as the donors. The results demonstrate that the appropriate molecular design of BDF building block is highly of importance in achieving highly efficient photovoltaic performances.

Figure 6. AFM height images, phase images, and TEM images of PBDFS-Bz:ITIC (a−c) and PBDFS-fBz:ITIC (d−f) blend films. The inset in (a, d) is the 3d AFM height image.

a root-mean square roughness (RMS) of 4.24 nm, where the PBDFS-fBz:ITIC blend film shows a more uniform and smoother surface with a smaller RMS of 2.81 nm (Figures 6d and 6e). The TEM images (Figures 6c and 6f) of both blend films show the similar well-distributed interpenetrated nanofibrillar structures with a domain size of ∼15−20 nm. The similar morphologies of PBDFSBz:ITIC and PBDFS-fBz:ITIC blend films are consistent with the similar measured Jsc and FF in both devices. Therefore, it can be concluded that the enhanced photovoltaic performance of PBDFSfBz:ITIC device compared with that of PBDFS-Bz:ITIC is mainly due to the increased Voc as a result of the lower-lying HOMO energy level in PBDFS-fBz. The intermolecular orientations and nanostructure orders of the polymer:ITIC blend films were investigated by two-dimensional grazing incident wide-angle X-ray scattering (2D GIWAXS). Figure 7 shows the 2D GIWAXS patterns (a and b) and corresponding 1D line cuts (c and d) along in-plane and out-of-plane directions of polymer:ITIC blend films relative to the substrate. As displayed in Figures 7a and 7c, the blend film of PBDFS-Bz:ITIC shows a diffraction peak for π−π stacking at 1.75 Å−1 (d = 3.59 Å), and there are two stronger lamellar peaks at 0.24 Å−1 (d = 26.2 Å) and 1.26 Å−1 (d = 4.98 Å) which are related to (100) and (200) peaks in the out-of plane. The result demonstrates that PBDFS-Bz:ITIC

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EXPERIMENTAL SECTION

Synthesis of BDFS-T. To a solution of compound 3 (1.28 g, 2.21 mmol) in dry THF (15 mL) at −78 °C was added n-BuLi (6.63 mmol, 2.65 mL, 2.5 M in hexane). After the mixture was E

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Figure 7. 2D GIWAXS patterns and profiles of active layers based on (a, c) PBDFS-Bz:ITIC and PBDFS-fBz:ITIC (b, d) prepared under the optimal condition. maintained at −78 °C for 1 h, trimethyltin chloride (6.63 mmol, 6.63 mL, 1 M in hexane) was added. Then the solution was stirred at room temperature overnight. The mixture was extracted with 3 × 50 mL hexane after pouring into water. The organic phase was washed with deionized water and dried with Na2SO4. After removing solvent, the crude product was recrystallized in ethanol and dried under vacuum. Finally, we obtained a pale yellow solid (1.30 g, 63.7% yield). 1H NMR (400 MHz, CDCl3, δ/ppm): 7.70 (s, 2H), 7.29 (d, J = 3.2 Hz, 2H), 6.69 (d, J = 3.2 Hz, 2H), 2.93 (d, J = 5.6 Hz, 4H), 1.80 (m, 2H), 1.27 (m, 16H), 0.89 (t, J = 7.2 Hz, 12H), 0.49(s, 18H). 13C NMR (CDCl3, ppm): 166.15, 152.83, 151.11, 146.02, 121.99, 118.09, 112.20, 104.49, 40.53, 39.35, 32.03, 28.66, 25.16, 22.95, 14.07, 10.74. Synthesis of PBDFS-fBz. BDFS-T (226 mg, 0.250 mmol) and fBz-Br (175 mg, 0.250 mmol) were charged into a 25 mL round-bottom flask with a condenser under N2 protection. After degassed twice, dry chlorobenzene (10 mL), dry DMF (2 mL), Pd2(dba)3 (5 mg), and P(o-tol)3 (10 mg) were added into the flask consequently. The resulting mixture was further degassed twice and heated to 135 °C for 48 h. The reaction was end-capped with 1.00 mmol of 2-bromothiophene and 1.05 mmol of 2-tributylstannylthiophene in order. The mixture was poured into methanol, and the precipitate was collected. Then the mixture was dissolved in chloroform (100 mL) and got a flash silica column with chloroform as eluent. A majority of the solvent was removed under pressure, and the residue, which was about 8−10 mL, was deposited into acetone (100 mL). The crude product was purified by Soxhlet extraction with hexane and chloroform for 12 h in order. The chloroform fraction was precipitated in methanol (100 mL). Finally, the crude product was collected by filtration and dried under vacuum at 50 °C overnight. Finally, we obtained the polymer as a brown powder (230 mg, 82% yield). GPC: Mn = 14.2 kDa; Mw = 26.6 kDa; PDI = 1.87. 1H NMR (400 MHz, CDCl3, δ/ppm): 8.05−7.7 (m, 4H), 7.29−7.20 (m, 4H), 6.69 (m, 2H), 4.74 (m, 2H), 2.87−2.26 (m, 5H), 1.31−1.24(br, 42H), 0.89−0.81 (br, 18H). Synthesis of PBDFS-Bz. PBDFS-Bz was synthesized following the similar procedure of PBDFS-fBz. A brown powder was obtained (234 mg, 87% yield). GPC: Mn = 16.8 kDa; Mw = 29.4 kDa; PDI = 1.75. 1H NMR (400 MHz, CDCl3, δ/ppm): 7.80−7.54 (m, 6H),

7.29−7.12 (m, 4H), 6.64 (m, 2H), 4.75 (m, 2H), 2.93−2.36 (m, 5H), 1.31−1.22(br, 42H), 0.90−0.81 (br, 18H).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b02676. Characterization methods, device fabrication, synthesis, and thermogravimetric figure (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (K.L.). *E-mail: [email protected] (Y.Z.). ORCID

Zhixiang Wei: 0000-0001-6188-3634 Yulin Yang: 0000-0002-2108-662X Yong Zhang: 0000-0002-9587-4039 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the support from the National Natural Science Foundation of China (51403044 and 21644006). Y. Zhang thanks the support of the Fundamental Research Funds for the Central Universities (Harbin Institute of Technology).

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