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Rational Design of Asymmetric Benzodithiophene based Photovoltaic Polymers for Efficient Solar Cells

Abstract

Extending π-conjugation in benzodithiophene (BDT) side chains has been proved helpful to improve the efficiencies of BDT-based polymer solar cells (PSCs). Hereon, combined with symmetry-breaking strategy of BDT unit, we further designed a new asymmetric 1D-2D (one dimensional-two dimensional) monomer asy-BDTBP with alkoxyl group as the 1D part and π-extending alkoxybiphenyl as the 2D substituted group. Medium band-gap donor-acceptor (D-A) conjugated polymer P1 was synthesized with asy-BDTBP and 4,7-di(4-(2-ethylhexyl)-2-thienyl)-5,6-difluoro-2,1,3-benzothiadiazole (DTffBT) as the donor and acceptor unit, respectively. Encouragingly, P1 blending with PC71BM exhibits obviously enhanced power conversion efficiency (PCE) compared to the reported symmetric analogue PBDTBP-DTffBT (6.70%). The PCE increased to 8.45% with open-circuit voltage (VOC) of 0.838 V, short-circuit current density (JSC) of 14.35 mA cm-2 and fill factor (FF) of 70.27%. However, P1 coupled with classical non-fullerene acceptor ITIC reveals relatively poor efficiency of 6.35% for the less complementary of absorption spectra. To match the absorption of ITIC, wide band-gap D-A polymer P2 was further designed with weak electron-withdrawing group benzo[1,2-c:4,5-c’]dithiophene-4,8-dione (BDD) instead of DTffBT as the acceptor unit. As a result, P2 possesses a complementary absorption spectra with ITIC and the resulting devices present excellent photovoltaic performance. The optimal efficiency boosts to 10.04% with VOC of 0.873 V, JSC of 17.60 mA cm-2 and FF of 65.37%. This work demonstrates the great potential of asymmetric BDTs for high efficient PSCs and the importance of rational design of polymers for different types of PSCs.

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Publication details

The article was received on 06 Nov 2017, accepted on 29 Nov 2017 and first published on 30 Nov 2017


Article type: Paper
DOI: 10.1039/C7TA09736K
Citation: J. Mater. Chem. A, 2017, Accepted Manuscript
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    Rational Design of Asymmetric Benzodithiophene based Photovoltaic Polymers for Efficient Solar Cells

    T. Zhu, D. Liu, K. Zhang, Y. Li, Z. Liu, X. Gao, X. Bao, M. Sun and R. Yang, J. Mater. Chem. A, 2017, Accepted Manuscript , DOI: 10.1039/C7TA09736K

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