dithiophene-Based Donor–Acceptor Polymers for ... - ACS Publications

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New Alkylfuranyl-Substituted Benzo[1,2‑b:4,5‑b′]dithiophene-Based Donor−Acceptor Polymers for Highly Efficient Solar Cells Yang Wang,†,‡ Feng Yang,† Ying Liu,† Ruixiang Peng,† Shaojie Chen,† and Ziyi Ge*,† †

Macromolecules 2013.46:1368-1375. Downloaded from pubs.acs.org by UNIV PIERRE ET MARIE CURIE on 08/24/18. For personal use only.

Ningbo Institute of Material Technology and Engineering (NIMTE), Chinese Academy of Sciences (CAS), Zhuangshi Road 519, Ningbo, Zhejiang 315201, P. R. China ‡ University of Chinese Academy of Sciences, Beijing 100049, P. R. China S Supporting Information *

ABSTRACT: Two new alkylfuranyl-substituted conjugated donor−acceptor polymersPBDTF-DPP and PBDTFDPPFwere designed and synthesized. To compare the properties of the new polymers, PBDT-DPP and PBDT-DPPF with alkoxy side chains were also synthesized. The photophysical and electrochemical measurement demonstrated that the alkylfuranyl-substituted polymers had smaller optical band gaps, broader absorption range, and lower HOMO energy levels, thus leading to a larger short current density (Jsc) and higher open circuit voltage (Voc) in photovoltaic devices. Under the same fabricating conditions, the efficiency of the polymer solar cells (PSCs) based on PBDTF-DPP and PBDTF-DPPF reached 3.5% and 5.1%, respectively, whereas PSCs based on PBDT-DPP and PBDT-DPPF only showed an efficiency of 1.0% and 2.9%. After thermal annealing, the efficiency of the PSCs based on PBDTF-DPPF:PC71BM further achieved as high as 6.1%. The results indicate a great potential for largely improving the efficiency of the PSCs by replacing alkoxy with alkyfuranyl group and the building block BDTF in creating exceptional performance materials for PSCs.

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electron-deficient moieties (such as fluorine) into the polymer backbone: providing polymers with a relatively low-lying HOMO energy level, which offers enhanced Voc;5 and (iii) diverse film processing approaches to achieve optimal nanoscale phase-separated morphology, such as thermal annealing,6 solvent annealing,7 the use of a processing additive,8 and mixture solvent treatment.9 Therefore, polymer solar cells (PSCs) with a PCE in the range of 6−8% have been achieved by a combination of these strategies.4b,d,5,7a,8a 5-Alkylthiophene-2-yl-substituted benzo[1,2-b:4,5-b′]dithiophene (BDTT) units have been flourishing as electron donor units in past 2 years. Several copolymers of BDTT with different conjugated units, such as 4,7-dithiophene-2-yl-2,1,3benzothiadiazole (DTBT),5c thieno[3,4-b]thiophene (TT),10 quinoxaline (Qx),11 etc., were synthesized, and the 2D copolymers showed better thermal stabilities, lower HOMO and LUMO energy levels, higher hole mobility, and greatly improved photovoltaic performance in comparison with their corresponding alkoxy-substituted counterparts.10,11 And very recently, Yang et al. and Ma et al. had reported a series of new LBG polymers based on the BDTT and diketopyrrolopyrrole (DPP) units, which showed excellent performance in both solar cells (PCE > 5%) and organic field effect transistors (μh = 0.16 cm2 V−1 s−1)12 (structures shown in Figure 1).

INTRODUCTION State-of-the-art polymer bulk heterojunction (BHJ) solar cells have the potential for achieving large-area, low-cost, highthroughput energy generation photovoltaic devices and have received enormous scientific and industrial interest in recent years.1 So far much of the research effort has been devoted to the designing and synthesizing multifused aromatic molecules, such as benzodithiophene (BDT) and its derivatives, which are excellent building blocks to synthesize novel advanced p-type conjugated polymers.2 Meanwhile, we gain mechanism understandings and set design rules relevant to organic photovoltaic devices to achieve a high power conversion efficiency (PCE) in BHJ solar cells, such as (i) efficient light harvesting to increase the short circuit current (Jsc); (ii) sufficient energy offset between the highest occupied molecular orbital (HOMO) level of the polymer semiconductor and the lowest unoccupied molecular orbital (LUMO) level of the fullerene derivative to facilitate a high open circuit voltage (Voc); and (iii) existence of an interpenetrating network of the BHJ polymer blend to enable efficient charge separation and high photocurrent.3 In this regard, many strategies have been developed to fulfill the above requirements, including (i) the incorporation of the “donor−acceptor” or “push−pull” architecture to the polymer backbone to form the low band gap (LBG) polymers, thus extending sunlight absorption at longer wavelengths to increase the photocurrent density;4 (ii) the replacement of strong electron-donating group with a weaker one (such as replacing an alkoxy chain with an alkyl chain) or the introduction of © 2013 American Chemical Society

Received: December 15, 2012 Revised: January 27, 2013 Published: February 13, 2013 1368

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on both donor and acceptor units to ensure enough solubility and optimal performance of PSCs. Finally, two new promising polymers, reported here, namely PBDTF-DPP and PBDTFDPPF, were obtained via Pd(0)-catalyzed Stille-coupling polymerization. In order to compare the properties of the new alkylfuranyl-substitued polymers, PBDT-DPP and PBDTDPPF with alkoxy side chains were also synthesized. Their thermal, UV−vis absorption, and electrochemical properties were well characterized. Under the same device fabrication conditions, the efficiency of the PSCs based on PBDTF-DPP and PBDTF-DPPF was 3.5% and 5.1%, respectively, much higher than the PSCs based on PBDT-DPP and PBDT-DPPF. After thermal annealing, the efficiency of the PSCs based on PBDTF-DPPF:PC71BM reaches as high as 6.1%. These results indicate a great potential for largely improving the efficency of the PSCs by replacement alkoxy with alkyfuranyl side chains and the building block BDTF in creating exceptional performance materials for PSCs.

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RESULTS AND DISCUSSION Synthesis. The synthetic routes of new monomer (4,8bis(5-(2-ethylhexyl)furan-2-yl)benzo[1,2-b:4,5-b′]dithiophene2,6-diyl)bis(tributylstannane) (BDTF) and four copolymers are shown in Scheme 1. The new monomer (BDTF) was successfully synthesized via a butyllithium method,19 as shown in Scheme 1. The synthesis of 2,5-diethylhexyl-3,6bis(5-bromothiophene-2-yl)pyrrolo[3,4-c]pyrrole-1,4-dione (DPP), 2,5-diethylhexyl-3,6-bis(5-bromofuran-2-yl)pyrrolo[3,4c]pyrrole-1,4-dione (DPPF), (4,8-bis((2-ethylhexyl)oxy)benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl)bis(tributylstannane)

Figure 1. Chemical structures of BDT-DPP-based copolymers in the literature (top) and in this work (bottom).

While alkylthienyl-based 2D conjugated polymers have been well explored during recent years, the research field of developing aromatic side chains still remains underexplored. Therefore, other kinds of aromatic side chains should be further explored to create promising polymer donor materials for highly efficient solar cells. In this contribution, we reported two new alkylfuranyl-based 2D conjugated polymers which had exceptional performance than their alkoxy analogues (see Figure 1). It was worth mentioning that most furan-containing polymers for solar cells were based on the DPP unit which was developed several decades ago for dyes but now appeared to be a favorable building block for applications in solar cells and transistors.14 For example, Janssen et al. reported several LBG copolymers based on furan-containing DPP, which had optical band gap around 1.4 eV and demonstrated ambipolar charge transport in field-effect transistors with hole and electron mobilities higher than 10−2 cm2 V−1 s−1. In combination with PC71BM, the PSCs gave PCE up to 3.7% in BHJ solar cells.15 In the meantime Frechet et al. reported two similar furan-containing polymers, PDPP2FT and PDPP3F, with higher PCE reaching 5.0% and 4.1%, respectively.8b All these studies have demonstrated that furan-containing conjugated polymers can provide good performance in solar cells. In addition, it has been shown that the substitution of furan for thiophene in the backbone of the polymer will improve polymer solubility, allowing for the use of shorter side chains while preserving high performance of the PSCs.8b Importantly, furan derivatives can be available from biorenewable sources; hence, they can be a step toward the “green” electronic materials.16 In this work, we attached alkylfuran to the 4- and 8-positions of the BDT unit to form 5-alkylfuran-2-yl-substituted benzo[1,2-b:4,5-b′]dithiophene (BDTF) units. The incorporation of alkylfuranyl side chain was to extend the conjugation of the molecule. It was significant to note that the aliphatic side chains of the polymers played a critical role in determining the performance of the PSCs.17 As shown in previous literature, shorter alkyl chains led to poor solubility (poor ability of solution processing), but too much longer alkyl chains led to low Jsc.18 Therefore, we grafted the moderate bulky 2-ethylhexyl

Scheme 1. Synthesis and Molecular Structures of the Monomer BDTF and Copolymers

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(BDT), 2-(2-ethylhexyl)furan, and 4,8-dehydrobenzo[1,2-b:4,5b′]dithiophene-4,8-dione was performed similarly to previous literature (refs 5d, 8b, and 19). 2-Alkylfuran was treated with n-butyllithium in tetrahydrofuran (THF) under argon, and then the reaction mixture was heated. After several hours, benzodithiophenedione was added quickly into the flask under argon. The nucleophilic reaction completed upon heating for several hours. After reduction by SnCl2, compound 2 was obtained as a yellow oil. Finally, the above compound could be efficiently lithiated by n-butyllithium followed by quenching with tributyltin chloride to afford BDTF in a moderate yield of 55%. Consequently, four LBG polymers, namely PBDTF-DPP, PBDTF-DPPF, PBDT-DPP, and PBDT-DPPF, were synthesized via Stille-coupling reaction (Scheme 1). From 1H NMR, the number of aromatic and aliphatic protons estimated from integration of the peaks was generally consistent with the expected repeating unit of the copolymers. Gel permeation chromatography (GPC) studies (using polystyrene as the standard and chloroform as the eluent) showed that these polymers purified by successive reprecipitation and Soxhlet extraction had similar number-average molecular weight (Mn) between 26.4 and 30.5 kDa as listed in Table 1. Because of the

Figure 2. TGA curves of the copolymers with a heating rate of 10 °C min−1 under a N2 atmosphere.

can be ascribed to the enhanced intermolecular π−π interaction22 that is originated from the extended conjugation by replacing alkoxyl group with an alkylfuranyl unit. From the onset of the thin film absorptions, we could estimate the optical bandgaps of the copolymers. The band gap of PBDTF-DPP was 1.44 eV, which was reduced by 0.12 eV in comparison with that of PBDT-DPP (1.56 eV), and the band gap of PBDTFDPPF was 1.47 eV, which was also reduced by 0.12 eV in comparison with that of PBDT-DPPF (1.59 eV). Cyclic voltammetry (CV) was employed to examine the electrochemical properties and evaluate HOMO and LUMO energy levels of the polymers. Figure 4 showed the CV curves of the copolymer films on platinum plate working electrodes in 0.1 mol/L tetrabutylammonium hexafluorophosphate (Bu4NPF6)−acetonitrile solution. For calibration, the redox potential of ferrocene/ferrocenium (Fc/Fc+) was measured under the same conditions, and it is located at 0.40 V to the Ag/AgCl electrode. It is assumed that the redox potential of Fc/Fc+ has an absolute energy level of −4.80 eV to vacuum.12a The HOMO and LUMO energy levels were then calculated according to the following equations:23

Table 1. Molecular Weight and Thermal Properties of the Polymers polymer

Mn [kDa]

PDI

Td [°C]

PBDTF-DPP PBDTF-DPPF PBDT-DPP PBDT-DPPF

26.4 30.5 28.9 29.5

5.37 5.55 5.26 5.23

388 380 341 338

aggregation of DPP-based copolymers in solution at room temperature, the weight-average molecular (Mw) was often overestimated, which led to a relatively high polydispersity indexes (PDI).20 The resulting copolymers showed excellent solubility in common organic solvents, such as chloroform, toluene, chlorobenzene, and 1,2-dichlorobenzene. Thermal Properties. Thermal properties of the copolymers were determined by differential scanning calorimetry (DSC). As shown in Figure S1 (Supporting Information), DSC analysis of all the polymers demonstrated no apparent thermal transition up to 280 °C. This indicated that all polymers possessed the amorphous nature. Thermogravimetric analysis (TGA) of these copolymers under a nitrogen flow showed their thermal stabilities (Figure 2). The 5% weight-loss temperature (Td) value of alkylfuranylsubstituted polymers PBDTF-DPP and PBDTF-DPPF was 388 and 380 °C, respectively, ∼40 °C higher than alkoxysubstituted polymers (see Table 1), exhibiting better thermal stability. This result was similar to the alkylthienyl-based polymers.10,11 Optical and Electrochemical Properties. The UV−vis absorption spectra of all the copolymers both in a chloroform solution and as a thin film are shown in Figure 3, and the photophysical data of polymers are summarized in Table 2. A shoulder at a shorter wavelength was observed in all copolymers, and it could be a vibronic shoulder arising from polymer aggregates in solution.20b,21 More importantly, in both solution and thin film, BDTF-based polymers (PBDTF-DPP and PBDTF-DPPF) showed red-shifted absorption onset (about 70 nm), compared with BDT-based polymers, which

HOMO = −e(Eox + 4.4) (eV)

(1)

LUMO = −e(Ered + 4.4) (eV)

(2)

Eg ec = e(Eox − Ered) (eV)

(3)

where Eox is the onset oxidation potential vs Ag/AgCl and Ered is the onset reduction potential vs Ag/AgCl. The results of the electrochemical measurements are summarized in Table 2. When the alkoxy side chain was replaced by the alkylfuranyl conjugated side chain, the HOMO and LUMO levels of the polymers were shifted to lower energy levels. For example, the HOMO and the LUMO of PBDTFDPP were 0.11 and 0.24 eV lower than those of PBDT-DPP, respectively. And this phenomenon could also be found in thiophene-substituted analogues.10,12 The lower HOMO energy level of the 2D conjugated BDT copolymers should be beneficial for a higher Voc value for the PSCs having the polymers as donors because the Voc was limited by the difference between HOMO of the donor and LUMO of the acceptor.24 Electrochemical bandgaps were usually larger than optical bandgaps, and this discrepancy might be induced by the presence of an energy barrier at the interface between the polymer film and the electrode surface.25 1370

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Figure 3. UV−vis absorption spectra of the copolymers: (a) in dilute choloroform solution; (b) in solid film cast from chloroform solution.

Table 2. Optical and Electrochemical Properties of the Polymers filmb

solutiona polymer PBDTF-DPP PBDTF-DPPF PBDT-DPP PBDT-DPPF

λmax (nm) 696, 690, 653, 652,

762 766 721 721

λonset (nm) 851 836 775 754

λmax (nm) 702, 691, 650, 636,

780 766 722 711

λonset (nm)

Egopt (eV)

HOMO (eV)

LUMO (eV)

Egec (eV)

863 844 793 778

1.44c 1.47 1.56 1.59

−5.16 −5.24 −5.05 −5.15

−3.64 −3.74 −3.40 −3.44

1.52 1.50 1.65 1.71

a

Measured in chloroform solution. bCast from chloroform solution. cBandgap estimated from the onset wavelength of the optical absorption of films.

CN mixed solvents exhibited the performance with PCE of 3.5% for PBDTF-DPP, 5.1% for PBDTF-DPPF, 1.0% for PBDT-DPP, and 2.9% for PBDT-DPPF. The Voc of the alkylfuranyl-substituted polymer PBDTF-DPP exhibited 0.69 V with 0.07 V higher than the alkoxyl analogue PBDT-DPP, which benefited from their lower-lying HOMO energy level. The same trend was also observed in PBDTFDPPF and PBDT-DPPF. Meanwhile, the Jsc of PBDTF-DPP and PBDTF-DPPF showed 10.46 and 12.64 mA cm−2, respectively, much higher than that of the alkoxyl-substituted copolymers. The higher Jsc obtained by the PSCs based on alkylfuranyl-substituted polymers might be originated from the lower bandgaps and broader absorption range. Considering the promising photovoltaic properties of the polymer PBDTF-DPPF, device optimization was further performed by prethermal annealing (thermal annealing before the deposition of the cathode materials), in which PBDTFDPPF/PC71BM blend films were thermal annealed at different temperatures between 90 and 140 °C. Figure 5b showed the J− V curves of the PSCs at different annealing temperatures under the illumination of AM 1.5G, 100 mW cm−2. The photovoltaic performance data of the PSCs, including Voc, Jsc, FF, and PCE values, are summarized in Table S1. As shown in Figure 5b and Table S1, the thermal treatment had obvious effects on the photovoltaic performances of these PSCs. The PSCs by annealing the active layer at 90 °C for 10 min delivered a higher PCE of 5.9% with a largely increased FF of 62.6%. By annealing the active layer at 100 °C for 5 min, the device demonstrated the best performance with Voc of 0.73 V, Jsc of 13.32 mA cm−2, FF of 62.4%, and PCE of 6.1%. This value is one of a few DPP-based PSCs which exhibited PCE exceeding 6%.26 Notably, by annealing the active layer at 100 °C for 10 min, the PSCs gave a pretty high Jsc of 14.0 mA cm−2, but with a slightly decreased FF of 59.1%. Further increasing the annealing temperature to 140 °C, the PCEs of the PSCs decreased due to the reduction of Jsc and FF. We attributed the

Figure 4. Cyclic voltammograms of PBDTF-DPP, PBDTFDPPF,PBDT-DPP, and PBDT-DPPF films cast on platinum wire in 0.1 M Bu4NPF6/CH3CN at a scan rate of 100 mV s−1.

Photovoltaic Characteristics. Bulk heterojunction PSCs were fabricated with a general device structure of indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)/polymer: PC71BM/LiF/Al, and their performances were measured under 100 mW/cm2 AM 1.5 G illumination. The ratio of polymer to PC71BM was adjusted from 1:1 to 1:3 (by weight), and the optimized condition was 1:2 for all of them. For further improving the device performance, a small amount (4 vol %) of the high boiling point additive 1-chloronaphthalene (CN) was added to optimize blend morphology, since adding a certain amount of CN can facilitate the formation of nanoscale phase separation of furan-containing polymers: PCBM blends during spincoating and thus improve the performance of PSCs.8b The current density−voltage (J−V) curves of polymer:PC71BM devices with and without CN as a processing additive are shown in Figure 5a and Figure S2, respectively, and the photovoltaic parameters of the devices are summarized in Table 3. The polymer:PC71BM devices spin-coated from chloroform/ 1371

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Figure 5. (a) Current density−voltage curves of the PSCs based on polymer/PC71BM (1:2, w/w) with CN as a processing additive, under the illumination of AM1.5G, 100 mW cm−2. (b) Current density−voltage curves of the PSCs based on PBDTF-DPPF/PC71BM (1:2, w/w) with different annealing temperatures, under the illumination of AM1.5G, 100 mW cm−2. (c) EQE for the BHJ solar cells derived from PBDTF-DPPF with (circle) or without (square) CN additive and with CN and then thermal annealing (triangle).

and Table S1), and the difference between the Jsc and the integral of the EQE by the solar irradiation spectrum, AM 1.5G, 100 mW, was ∼4%, which provided the solid evidence for the reliability of the photovoltaic results. Besides the above parameters such as thermal stability, absorption, and energy levels, the morphology of the photoactive layer played a key role in the photovoltaic performance of PSCs.28 We employed a tapping-mode atomic force microscopy (AFM) to investigate the morphology of the PBDTF-DPPF:PC71BM (1:2, w/w) blend films without or with additive and with additive plus thermal treatment. The AFM topography and phase images are shown in Figure 6. It could be observed that the blend film without CN additive exhibited relatively weak phase separation (Figure 6A). The nonoptimized morphology may be due to fast drying of the CF solvent.12b Upon adding a little amount of CN additive, nanoscaled interpenetrating network of PBDTF-DPPF and PC71BM was achieved (see Figure 6B), which was beneficial to the charge transportation, thus leading to an increase in Jsc as well as the device efficiency (see the photovoltaic performance in Table 3 and EQE curves in Figure 5c). After annealing at 100 °C for 10 min, the surface of the blend film demonstrated a more uniform distribution of PBDTF-DPPF and PC71BM (see Figure 6C). The thermal annealing optimized and controlled the film morphology into more ordered structure, achieving both a relative low Rs and a high Rsh. Thus, it improved FF of the device.

Table 3. Comparison of the Photovoltaic Properties of the PSCs Based on Polymer/PC71BM (1:2, w/w) without or with CN as an Additive under the Illumination of AM1.5G, 100 mW cm−2 polymer PBDTF-DPP PBDTF-DPPF PBDT-DPP PBDT-DPPF

CN (vol %)

Voc (V)

Jsc (mA cm−2)

FF (%)

PCE (%)

0 4 0 4 0 4 0 4

0.66 0.69 0.69 0.72 0.58 0.62 0.57 0.63

5.27 10.46 11.03 12.64 2.21 3.87 7.39 8.78

43.6 48.9 48.1 56.5 43.8 40.0 42.7 53.3

1.5 3.5 3.7 5.1 0.6 1.0 1.8 2.9

enhanced performance, especially the increased FF after thermal annealing, to the higher shunt resistance (Rsh) and lower series resistance (Rs) of the devices.27 The Rsh increased from 0.48 to 0.62 kΩ and the Rs decreased from 5.5 to 3.9 Ω cm2 after thermal annealing at 90 °C for 10 min. The external quantum efficiency (EQE) curves of the devices based on PDPPF-BDTF/PC71BM without CN, with CN, and with CN plus thermal treatment are shown in Figure 5c. All devices exhibited a very broad response range from 400 to 800 nm, and relatively low EQE values were observed in the devices without CN additive, suggesting that the photon−electron conversion processes were not highly efficient. Upon adding 4 vol % CN, the EQE values increased dramatically, and the device with additive plus thermal treatment showed the highest EQE value with the peak values reached over 50%. This result was consistent with the data in J−V measurements (see Table 3

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CONCLUSION In summary, we synthesized two new alkylfuranyl-substituted D−A copolymers, PBDTF-DPP and PBDTF-DPPF, and 1372

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Figure 6. Tapping-mode AFM topography image (top) and phase image (bottom) of the blend film of PBDTF-DPPF/PC71BM (1:2, w/w): (A) without CN; (B) with CN; (C) with CN and thermal annealing at 100 °C for 10 min (all images size: 2.0 μm × 2.0 μm). and counter electrode, respectively. The energy level of the Ag/AgCl reference electrode(calibrated against the Fc/Fc+ redox system) was 4.40 eV below the vacuum level. The cell was purged with pure argon prior to each scan. Surface images were measured by a Veeco Dimension 3100 V atomic force microscope. Materials. n-BuLi, Pd(PPh3)4, and Sn(C4H9)3Cl were obtained from Acros Organics and used as received. Tetrahydrofuran (THF) was dried over Na/benzophenone ketyl and freshly distilled prior to use. Other reagents and solvents were purchased commercially as analytical-grade quality and used without further purification. The synthesis of 2,5-diethylhexyl-3,6-bis(5-bromothiophene-2-yl)pyrrolo[3,4-c]-pyrrole-1,4-dione (DPP), 2,5- diethylhexyl-3,6-bis(5-bromofuran-2-yl)pyrrolo[3,4-c]-pyrrole-1,4-dione (DPPF), (4,8-bis((2ethylhexyl)oxy)benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl)bis(tributylstannane) (BDT), 2-(2-ethylhexyl)furan, and 4,8dehydrobenzo[1,2-b:4,5-b′]dithiophene-4,8-dione was performed similarly to previous literature (refs 5d, 8b, and 19). All the other compounds were synthesized following the procedures described herein. Synthesis of 4,8-Bis(5-(2-ethylhexyl)furan-2-yl)benzo[1,2-b:4,5b′]dithiophene (2). In a dried three-neck flask with a condenser, nbutyllithium (18.5 mL, 44 mmol, 2.4 M in hexane) was added dropwise to the mixture of 2-(2-ethylhexyl)furan (7.20 g, 40 mmol) in THF (52 mL) at 0 °C under an argon atmosphere. The reactant mixture was heated up to 50 °C for 3 h. After that, 4,8dehydrobenzo[l,2-b:4,5-b′]dithiophene-4,8-dione (2.2 g, 10 mmol) was then added quickly, and the mixture was stirred for 3 h at 50 °C. After the mixture cooled down to ambient temperature, SnCl2·2H2O (18g, 80 mmol) in HCl (10%, 32 mL) was added, and the mixture was stirred for another 3 h. The mixture was subsequently poured into ice water and extracted by dichloromethane three times. The combined extracts were dried with anhydrous MgSO4 and evaporated. The crude product was purified by column chromatography on silica gel eluting with hexane to give compound 5 as a yellow liquid (1.82 g, 35%). 1H NMR (CDCl3, 400 Hz, δ/ppm): 7.99 (d, 2H), 7.56 (d, 2H), 7.00 (d, 2H), 6.28 (s, 2H), 2.74 (t, 4H), 1.87 (t, 2H), 1.47−1.30 (br, 16H), 0.90−0.88 (t, 12H). Synthesis of (4,8-Bis(5-(2-ethylhexyl)furan-2-yl)benzo[1,2-b:4,5b′]dithiophene-2,6-diyl)bis(tributylstannane) (BDTF). Under a nitrogen atmosphere, compound 2 (1.05 g, 1.92 mmol) was dissolved in dry THF (80 mL) and cooled to −78 °C, and then n-butyllithium (3.00 mL,4.80 mmol, 1.6 M in hexane) was added dropwise. The reaction mixture was stirred at−78 °C for 1 h and then brought to

studied the relationship between the molecular structure and photovoltaic properties. The investigation of electrochemical properties demonstrated that PBDTF-DPP and PBDTF-DPPF had deeper HOMO energy level than that of their alkoxy counterparts, thus leading to a higher Voc in PSCs. More importantly, the alkylfuran side moiety of PBDTF-DPP and PBDTF-DPPF aroused ∼70 nm red-shifted absorption in comparison with its alkoxyl analogues, leading to a higher Jsc in fabricated devices. Under the same fabricating conditions, the efficiency of the PSCs based on PBDTF-DPP and PBDTFDPPF was 3.5% and 5.1%, respectively, whereas PSCs based on PBDT-DPP and PBDT-DPPF only exhibited an efficiency of 1.0% and 2.9%. After further device optimization by thermal annealing, the efficiency of the PSCs based on PBDTFDPPF:PC71BM reached as high as 6.1%. This result definitely demonstrated that the alkylfuranyl group was an excellent alternative to the alkoxyl group to form high-performance polymers employed in PSCs. In addition, due to the proper optical band gap (∼1.5 eV) and the broad absorption matching with P3HT, we envision that PBDTF-DPPF is also a promising candidate to be used in tandem solar cells to realize even higher efficiency.

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EXPERIMENTAL SECTION

General Measurement and Characterization. 1H and 13C NMR spectra were measured using a Bruker DMX-400 spectrometer. Chemical shifts of NMR were reported in ppm relative to the singlet of CDCl3 at 7.26 ppm for 1H NMR spectroscopy and 77.6 ppm for 13C NMR spectroscopy. Differential scanning calorimeter (DSC) was measured on a Metler Toledo DSC822 Instrument, and thermogravimetric analysis (TGA) was recorded on a PerkinElmer Pyris under a nitrogen atmosphere at a heating rate of 10 °C/min. Absorption spectra were collected on a PerkinElmer Lambda 950. The molecular weights of the polymers were measured by the GPC method on a Waters 1515, and polystyrene was used as the standard (room temperature, chloroform as the eluent). The electrochemical cyclic voltammetry (CV) of the polymer film was conducted in acetonitrile with 0.1 M of tetrabutylammonium hexafluorophosphate using a scan rate of 100 mV s−1 at room temperature. Platinum plate, Ag/AgCl, and platinum wire were used as the working electrode, reference electrode, 1373

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Macromolecules

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room temperature. The stirring was continued for 60 min, and then the reaction mixture was cooled to −78 °C again. A solution of tributylstannyl chloride (1.60 mL, 5.76 mmol) was added dropwise. The mixture was slowly brought to room temperature and stirred overnight. Water was subsequently added to the reaction mixture, and the aqueous phase was extracted with petroleum three times. The combined organic layers were dried over Na2SO4 and concentrated to afford a yellow viscous oil. The crude product was purified by column chromatography on silica gel eluting with hexane (containing a small amount triethylamine) to give title compound as a yellow viscous oil (1.19 g, 55%). 1H NMR (CDCl3, 400 MHz, δ/ppm): 8.05−7.99 (t, 2H), 7.00−7.01 (d, 2H), 6.28−6.29 (d, 2H), 2.74−2.76 (d, 4H), 1.88 (m, 2H), 1.67−1.59 (m, 12 H), 1.40−1.32 (m, 28H), 1.22−1.18 (t, 12H), 0.95−0.88 (m, 36H). 13C NMR (CDCl3, 100 MHz, δ/ppm), d = 150.73, 146.18, 136.35, 135.54, 130.58, 126.91, 113.21, 106.36, 103.36, 34.15, 28.13, 27.80, 24.95, 24.26, 22.82, 22.53,22.24, 21.26, 18.34, 9.40, 8.93, 6.14, 6.04. Polymerization for PBDTF-DPP. DPP (137 mg, 0.20 mmol) and BDTF (225 mg, 0.20 mmol) were dissolved into toluene (12 mL) and DMF (3 mL). The solution was flushed with argon for 10 min, and then Pd(PPh3)4 (10 mg) was added into the flask. The flask was purged three times with successive vacuum and argon filling cycles. The polymerization reaction was heated to 120 °C, and the mixture was stirred for 36 h under an argon atmosphere. Then, the reactant was cooled to room temperature and poured slowly into methanol (200 mL). The precipitate was filtered and washed with methanol and hexane in a Soxhlet apparatus to remove the oligomers and catalyst residue. Finally, the polymer was extracted with chloroform. The solution was condensed by evaporation and precipitated into methanol. The title polymer was collected as a green-purple solid (200 mg, 60%, Mn = 26.4 kg/mol, PDI = 5.37). 1H NMR (400 MHz, CDCl3): d = 8.93−7.37 (br, 4H), 7.06−6.07 (br, 6H), 4.53−2.71 (br, 8H), 2.03−0.62 (br, 60H). Polymerization for PBDTF-DPPF. PDPPF-BDTF was prepared using the same procedure as PDPP-BDTF. The resulting copolymer PDPPF-BDTF was obtained as a green-purple solid with a yield of 62% (Mn = 30.5 kg/mol, PDI = 5.55) 1H NMR (400 MHz, CDCl3): d = 8.89−7.71 (br, 4H), 7.18−6.04 (br, 6H), 4.91−2.41 (br, 8H), 2.18− 0.60 (br, 60H). Fabrication and Characterization of BHJ Devices. All organic photovoltaic devices had a conventional device architecture, ITO/ PEDOT:PSS/polymer:PC71BM/LiF/Al. ITO/glass substrates were ultrasonically cleaned sequentially in detergent, water, acetone, and isopropanol (IPA), followed by treating in an ultraviolet ozone chamber (Ultraviolet Ozone Cleaner, Jelight Company, Irvine, CA) for 25 min. The cleaned substrates were covered by a 30 nm thick layer of PEDOT:PSS (Baytron PV PAI 4083, Germany) by spin-coating. After annealing in a glovebox at 150 °C for 20 min, the samples were cooled to room temperature. Polymers were dissolved in chloroform (CF) or adding 4 vol % 1-chloronaphthalene (CN) in CF solution, and then PC71BM (purchased from Nano-C) was added. The solution was then heated at 55 °C and stirred overnight at the same temperature. The solution of polymer:PC71BM was then spin-coated to form the active layer (∼100 nm). The cathode made of LiF (1 nm thick) and aluminum (100 nm thick) was sequentially evaporated through a shadow mask under high vacuum (