Substituent Effects on Physical and Photovoltaic Properties of 5,6

(9-12) To overcome this problem, considerable studies have been directed .... Calcd for C44H58N2S5: C, 68.17; H, 7.54; N, 3.61; S, 20.68. ... The acti...
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Substituent Effects on Physical and Photovoltaic Properties of 5,6Difluorobenzo[c][1,2,5]thiadiazole-Based D−A Polymers: Toward a Donor Design for High Performance Polymer Solar Cells Yan Wang,†,∥ Xin Xin,†,∥ Yong Lu,†,∥ Ting Xiao,⊥ Xiaofeng Xu,† Ni Zhao,⊥ Xiao Hu,‡,* Beng S. Ong,§,* and Siu Choon Ng†,* †

School of Chemical and Biomedical Engineering, Nanyang Technological University, Nanyang Drive, Singapore 637459 School of Materials Science and Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798 § Department of Chemistry and Institute of Creativity, Hong Kong Baptist University, Kowloon, Hong Kong SAR, China ⊥ Department of Electronic Engineering, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong SAR, China ‡

S Supporting Information *

ABSTRACT: 5,6-Difluorobenzo[c][1,2,5]thiadiazole (FBT)-based conjugated donor−acceptor (D-A) polymers with straight and branched side chains were synthesized via Stille-coupling copolymerization to study their physical, optoelectronic and photovoltaic properties. The results show that both the nature of pendant side chains and the electron acceptor strength of the acceptor moiety of D−A polymers have critical impacts on material and photovoltaic properties. Better π−π stacking of polymer backbones enabled by appropriate substituents such as fluorine atoms and branched alkyl chains leads to a reasonably high power conversion efficiency of over 6% when the polymer is utilized as a donor material with PC71BM as an active layer in bulk heterojunction solar cells.

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INTRODUCTION Over the past decade, polymer solar cells (PSCs) have attracted considerable attention owing to their unique advantages which include potentially low manufacturing cost, lightweight characteristics, and device structural flexibility,1−7 to name a few. Currently the bulk heterojunction (BHJ) architecture is commonly adopted in PSC devices for its easier fabrication, more efficient charge separation, and potentially higher power conversion efficiency (PCE). In a typical BHJ architecture, the active layer generally consists of an electron-accepting fullerene derivative dispersed in an electron-donating conjugated polymer matrix.8 An illustrative example is an active layer composed of a blend of poly(3-hexylthiophene) (P3HT) and methanofullerene-6,6-phenyl-C61-butyric acid methyl ester (PCBM). This system has been extensively studied in BHJ PSC with a PCE of about 4−5%. However, the narrow absorption spectral properties and high HOMO level of P3HT are the major drawbacks for further improvement of device efficiency.9−12 To overcome this problem, considerable studies have been directed toward development of low-band gap polymers and device optimization.13 Common design considerations for efficient low-band gap polymers for BHJ PSCs can be generalized as follows: (1) reasonable solution-processability to enable excellent thin-film formation with electron accepting materials (e.g., PCBM); (2) good matching of lowest unoccupied molecular orbital (LUMO) energy levels between the polymer and fullerene derivatives to facilitate efficient © XXXX American Chemical Society

excition dissociation and charge transfer; (3) a low-band gap (1.3−1.9 eV) to harvest more solar energy. One common strategy for low-band gap polymer design is use of alternating electron donor and acceptor (D−A) polymer structure, which provides flexibility in tuning the energy levels of the polymer through its constituent donor and acceptor strengths. Several D−A polymers of this nature have been reported to achieve PCE of over 8% in BHJs.14 Among various D−A polymers, benzo[c][1,2,5]thiadiazole(BT-) based polymers have been actively studied as donor materials for BHJ PSCs.15−18 Although the energy levels of these D−A polymers can be adjusted by their BT moieties, the resulting band-gaps are generally in the 1.7−1.9 eV regime, which adversely limits solar energy harvesting. Significant amount of work has subsequently been conducted on BT modification to improve polymer’s solar energy harvesting capability. These modified structures include, for example, benzobisthiadiazole,19,20 [1,2,5]thiadiazolo[3,4-g]quinoxaline21 and naphtho[1,2-c:5,6-c]bis[1,2,5]thiadiazole.22 Most recently, fluorinated benzothiadiazole-based D−A polymers have been demonstrated to be promising electron-donor polymers for BHJ PSCs, providing PCE ranging from about 4% to over 7%.23−26 The incorporation of fluorine atom appears to be Received: August 14, 2013 Revised: November 18, 2013

A

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Scheme 1. Synthesis Routes of PFBT-T12TT, PFBT-T20TT, and PBT-T12TT Polymers

as δ values (ppm) relative to internal tetramethylsilane standard. Gel permeation chromatography (GPC) was measured on a Shimadzu HPLC system using THF as eluent after calibration with standard polystyrene. Elemental Analysis (EA) was determined on a vario EL elemental analysis instrument. UV−visible absorption spectra were collected on a Shimadzu UV 3101 spectrophotometer. DSC measurements were performed under a nitrogen atmosphere at a heating rate of 10 °C with a TA 2920 analyzer. TGA analyses were performed on Perkin-Elmer Diamond TG/DTA analyzer. Cyclic voltammetry (CV) measurements were conducted on a threeelectrode electrochemistry workstation (Model CHI006B) in 0.1 M anhydrous acetonitrile solution of Bu4NBF4 under a nitrogen atmosphere. A platinum stick electrode coated with a thin film layer of polymer was used as the working electrode. A silver wire electrode was used as the reference electrode. A platinum wire was used as the counter electrode. Materials. All chemicals and reagents were purchased from Aldrich and used without further purification unless stated otherwise. 5,6difluoro-4,7-diiodobenzo[c][1,2,5]thiadiazole,23 2-tributylstannyl-4dodecylthiophene,28 2-tributylstannyl-4-(2-octyldodecyl)thiophene,28 4,7-bis(5-bromo-4-dodecylthiophen-2-yl)benzo[c][1,2,5]thiadiazole,27 and 2,5-bis(trimethylstannyl)thieno[3,2 -b]thiophene,29 were prepared according reported procedures. All the reactions were carried out under nitrogen atmosphere. 5,6-Difluoro-4,7-bis(4-dodecylthiophen-2-yl)benzo[c][1,2,5]thiadiazole (1). 2-Tributylstannyl-4-dodecylthiophene (5.38 g, 11.0 mmol), 5,6-difluoro-4,7-diiodobenzo[c][1,2,5]thiadiazole (2.12 g, 5.0 mmol) and Pd(PPh3)4 (60 mg) were dissolved in 25 mL of anhydrous toluene. The solution was degassed and refluxed 2 days. After cooling to room temperature, the mixture was extracted three times with dichloromethane. The combined dichloromethane extract was dried over anhydrous magnesium sulfate. The solvent was removed under reduced pressure and the residue was purified by column chromatography on silica gel using hexane/ethyl acetate as eluent. The desired product was obtained as an orange solid (2.21 g, 65.6%). 1 H NMR (300 MHz, CDCl3): δ (ppm) 8.15 (s, 2H), 7.22 (s, 2H), 2.71 (t, 4H), 1.68 (m, 4H), 1.2−1.5 (m, 36H), 0.91 (t, 6H). 5,6-Difluoro-4,7-bis[4-(2-octyldodecyl)thiophen-2-yl]benzo[c][1,2,5]thiadiazole (2). Compound 2 was prepared by the same procedure for compound 1 with 2-tributyl-4-(2-octyldodecyl)thiophene instead of 2-tributylstannyl-4-dodecylthiophene. Yield:

advantageous in two respects: (1) it lowers the highest occupied molecular orbital (HOMO) energy level, resulting in an increased open voltage (Voc); (2) it provides inter or intramolecular F−H bonding, which help facilitate better π−π stacking leading to a greater extent of backbone ordering. In this article, we report the synthesis, characterization, and photovoltaic studies of novel 5,6-difluorobenzo[c][1,2,5]thiadiazole-based D−A polymers PFBT-T12TT and PFBTT20TT (see Scheme 1), respectively with pendant straight and branched long alkyl chain substitutions. An analogous polymer based on nonfluorinated benzo[c][1,2,5]thiadiazole has been reported to give a PCE of 0.42%.27 Our investigation shows that both branched alkyl chain and fluorine substitutions are important for achieving high PCEs. Branched alkyl substituents are critical in improving polymer processability for fabrication of good thin films while the fluorine substitution enhances the intermolecular interaction. On the other hand, with straight alkyl chain substitution, the polymer suffers from limited solubility and thus difficult processability. Specifically, PFBTT20TT exhibits strong broad absorptions and enhanced intermolecular polymer chain interaction in its thin film state. When used as the donor material and blended with PC71BM to form an active layer in BHJ devices, high PCE of over 6% was obtained. For comparison, BHJ devices with various different donor systems were fabricated and evaluated under the same condition. The results clearly revealed that much higher photovoltaic performance can be achieved by devices based on PFBT-T20TT than those from PBT-T12TT and PFBT-T12TT. Consequently, the incorporation of fluorine atom and long branched side chains in polymer backbone would be a viable strategy for designing desirable polymer donors for BHJ PSC application.

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EXPERIMENTAL SECTION

General Measurement and Characterization. 1H NMR spectra were recorded on a 300 MHz, Bruker Advanced DPX 300 apparatus in deuterated solution of CDCl3 at 25 °C. Chemical shifts were reported B

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3.14 g, 70%. 1H NMR (300 MHz, CDCl3): δ (ppm) 7.95 (s, 2H), 7.18 (s, 2H), 2.65 (t, 4H), 1.75 (m, 2H), 1.2−1.5 (m, 64H), 0.88 (t, 12H). 5,6-Difluoro-4,7-bis(5-bromo-4-dodecylthiophen-2-yl)benzo[c][1,2,5]thiadiazole (3). NBS (0.76 g, 4.25 mmol) was added in small portions to a solution of compound 1 (1.75 g, 2 mmol) in 60 mL of THF. The reaction mixture was stirred in the dark at room temperature overnight. The mixture was poured into water and extracted three times with dichloromethane. The combined dichloromethane extract was dried over anhydrous magnesium sulfate. Solvent was removed under reduced pressure and the residue was purified by chromatography. The product was obtained as an orange solid after recrystallization from ethanol. Yield: 1.30 g, 78%. 1H NMR (300 MHz, CDCl3): δ (ppm) 8.0 (s, 2H), 2.68 (3, 4H), 1.72 (m, 4H), 1.2−1.5 (m, 36H), 0.89 (m, 6H). 5,6-Difluoro-4,7-bis[5-bromo-4-(2-octyldodecyl)thiophene-2-yl]benzo[c][1,2,5]thiadiazole (4). Compound 4 was prepared according to the procedure for compound 3 using compound 2 instead of compound 1. Yield: 1.73 g, 82%. 1H NMR (300 MHz, CDCl3): δ (ppm) 7.95 (s, 2H), 2.60 (t, 4H), 1.78 (m, 2H), 1.2−1.4 (m, 64H), 0.87 (t, 12H). General Procedure for Polymerization. Tris(dibenzylideneacetone)dipalladium (0.004 mmol) and tri(o-tolyl)phosphine (0.032 mmol) was added to a stirred solution of a mixture of 2,5-bis(trimethylstannyl) thieno[3,2-b]thiophene (0.2 mmol) and compound 3 or 4 monomer (0.2 mmol) in anhydrous chlorobenzene (12 mL) under nitrogen atmosphere. The mixture was stirred at 130 °C for 72 h. Subsequently, the reaction mixture was cooled to room temperature and poured into methanol with vigorous stirring. A powdery solid was separated and filtered. The resulting polymer was subjected to Soxhlet extraction sequentially with methanol and hexane for 24 h. The polymer product was finally obtained by extraction with chlorobenzene. The chlorobenzene extract was concentrated under reduced pressure and the polymer product was precipitated from methanol and collected by filtration. Polymer PFBT-T12TT. Yield: 47 mg (28%). 1H NMR (300 MHz, CDCl3): δ (ppm) 8.15 (br, 2H), 7.42 (br, 2H), 2.66 (br, 4H), 1.25 (br, 40H), 0.86 (br, 6H). Anal. Calcd for C44H56F2N2S5: C, 65.14; H, 6.96; N, 3.45; S, 19.76. Found: C, 66.68; H, 7.552; N, 3.13; S, 18.27. Molecular weight: Mn = 7.6 kDa; PDI = 1.48. Polymer PFBT-T20TT. Yield: 138 mg (65%). 1H NMR (300 MHz, CDCl3): δ (ppm) 7.83 (br, 2H), 7.11 (br, 2H), 2.62 (br, 4H), 1.25 (br, 66H), 0.88 (br, 12H). Anal. Calcd for C60H88F2N2S5: C, 69.58; H, 8.56; N, 2.70.44; S, 15.48. Found: C, 70.17; H, 9.523; N, 2.682. S, 15.52. Molecular weight: Mn = 9.8 kDa; PDI = 1.69. Polymer PBT-T12TT. Yield: 120 mg (75%). 1H NMR (300 MHz, CDCl3): δ (ppm) 7.88 (br, 2H), 7.77 (br, 2H), 7.31 (br, 2H), 2.58 (br, 4H), 1.25 (br, 40H), 0.85 (br, 6H). Anal. Calcd for C44H58N2S5: C, 68.17; H, 7.54; N, 3.61; S, 20.68. Found: C, 69.78; H, 8.348; N, 3.096; S, 19.17. Molecular weight: Mn = 12.0 kDa; PDI = 2.08. BHJ PSC Device Fabrication. The donor polymer (PBT-T12TT, PFBT-T12TT, or PFBT-T20TT) was codissolved with PC71BM in 1,2-dichlorobenzene (DCB) in different weight ratios from 1:1 to 1:4, with a concentration of 7.5 mg/mL. ITO-coated glass substrate were cleaned stepwise in detergent, water, acetone, and isopropyl alcohol under ultrasonication for 10 min each and subsequently dried by N2 blowing and then pretreated by oxygen plasma for 1.5 min. The PEDOT:PSS (VP Al 4083 from H. C. Stack, filtered through 0.45 μm PVDF syringe filter) buffer layer was spin-coated at 3000 rpm for 60s. After transferring into a nitrogen-filled glovebox (0.3 eV) with PC71BM well, D

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Figure 4. Current density−voltage (a) and EQE curves (b) of photovoltaic devices using blends of donor polymer/PC71BM (w:w = 1:3) as active layers. Figure 3. Current density−voltage (a) and EQE curves (b) of photovoltaic devices using the blend of PFBT-T20TT/PC71BM (w:w = 1:1, 1:2, 1:3 and 1:4) as active layers.

Table 3. Photovoltaic Parameters of Photovoltaic Devices Based on Different Donor Polymer/PC71BM under Illumination (100 mW/cm2, AM 1.5)

Table 2. Photovoltaic Parametersa of PFBT-T20TT/ PC71BM-Based Photovoltaic Devices with Different Ratios under Illumination (100 mW/cm2, AM 1.5) weight ratios (PFBTT20TT: PC71BM)

Voc (V)

Jsc (mA/cm2)

FF (%)

PCE (%)

1:1 1:2 1:3 1:4

0.8 (0.78) 0.82 (0.8) 0.77 (0.76) 0.77 (0.72)

10.3 (9.56) 12.93 (12) 12.78 (10.1) 8.35 (6.8)

45 60.9 33.7 46.9

3.61 (3.2) 6.3 (5.55) 3.3 (2.7) 3.1 (2.3)

a

polymer blends

Voc (V)

Jsc (mA/cm2)

FF (%)

PCE (%)

PBT-T12TT:PC71BM PFBT-T12TT:PC71BM PFBT-T20TT:PC71BM

0.75 0.55 0.76

2.73 0.29 12.78

28 11.98 33.7

0.57 0.02 3.3

pendant straight long alkyl chains and PFBT-T20TT with pendant branched long alkyl chains, have been synthesized to study the effects of pendant substituents on their physical and photovoltaic properties as compared to their nonfluorinated analogue, PBT-T12TT. Both fluorinated polymers exhibit broader absorptions and deeper HOMO levels as compared to the nonfluorinated polymer, a direct consequence of fluorine incorporation. Spectral evidence has revealed enhanced interchain interaction in the thin films of the fluorinated polymers, which may facilitate charge transfer and explain in part for the observed high photovoltaic efficiency in PFBTT20TT-based devices. The PFBT-T12TT/PC71BM devices have generally afforded poor photovoltaic performance primarily due to the polymer’s poor solubility and thus the difficulty in fabricating good-quality active layer thin films. The optimized PFBT-T20TT/PC71BM devices have demonstrated

Average values in parentheses.

poor and accordingly only low EQE could be obtained by the PFBT-T12TT based device, leading to a followed low Voc. The incorporation of bulk branched pendant alkyl chains in PFBTT20TT not only improved polymer solubility for fabrication, but also enhanced its miscibility with PC71BM, affording a higher-quality active layer thin film with ensuing significantly improved device performance.

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CONCLUSIONS In conclusion, two novel 5,6-difluorinated benzo[c][1,2,5]thiadiazole-based D−A polymers, namely, PFBT-T12TT with E

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a high PCE of 6.3% with an EQE of ∼60% from 400−700 nm. These are very encouraging results, which demonstrate the substituent effects on polymer ordering, charge transport and photovoltaic efficiency. Both the incorporation of fluorine and branched long side-chain substituents into the benzo[c][1,2,5]thiadiazole-based D−A polymers have been shown to be effective for improving polymer processability, device fabrication, and finally photovoltaic activity in BHJ PSCs. We believe that with optimized donor moiety and substituent design, the benzo[c][1,2,5]-thiadiazole-based D−A polymers can provide significantly much higher photovoltaic efficiency.

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ASSOCIATED CONTENT

S Supporting Information *

1

H NMR spectra of compounds 1−4, polymer PFBT-T12TT, PFBT-T20TT, PBT-T12TT, and the UV−vis absorption of polymer PFBT-T20TT at different annealing temperature. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (S.C.N.). *E-mail: [email protected] (B.S.O.). *E-mail: [email protected] (X.H.). Author Contributions ∥

Y.W., X.X., and Y.L. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to thank A*STAR, Singapore for providing financial support (Grant No. 102 170 0135). REFERENCES

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