Slight Structural Disorder in Bithiophene-based Random Terpolymers with Improved Power Conversion Efficiency for Polymer Solar Cells
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A series of random terpolymers P2-P5 were designed and synthesized by randomly embedding 5 mol%, 10 mol%, 15 mol% and 25 mol% feed ratios of low cost 2,2-bithiophene as the third monomer to the famous donor-acceptor (D-A) type copolymer PTB7-Th (P1). All polymers showed similar molecular weight with number-average molecular weight (Mn) and weight-average molecular weight (Mw) in the range of (59-74) and (93-114) kg·mol-1, respectively, to ensure a fair comparison on the structure-property relationships. Compared with the control copolymer PTB7-Th, the random terpolymers exhibited enhanced absorption intensity in a wide range from 400 nm to 650 nm in both solution and film as well as in polymer/PC71BM blends. From grazing incident wide-angle X-ray diffraction (GIWAXS), compared with the regularly alternated copolymer PTB7-Th, the random terpolymers demonstrated mild structural disorder with reduced (100) lamellar stacking and slightly weakened (010) π-π stacking for the polymers as well as slightly reduced PC71BM aggregation in polymer/PC71BM blends. However, the measured hole mobility for terpolymers ((1.20-3.73) × 10-4 cm2·V-1·s-1) was evaluated to be comparable or even higher than 1.35 × 10-4 cm2·V-1·s-1 of the alternative copolymer. Enhanced average power conversion efficiency (PCE) from 7.35% to 8.11% and 7.79% to 8.37% was observed in both conventional and inverted device architectures from copolymer P1 to terpolymers P4, while further increasing the 2,2-bithiophene feed ratio decreased the PCE.
KeywordsBithiophene-based random terpolymers Structural disorder Polymer solar cells Photovoltaic properties
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This work was financially supported by the the National Natural Science Foundation of China (No. 61761136013), the Natural Science Foundation of Jiangsu Province (Nos. BK20160042 and BK20160990). S.H. is thankful to DFG (392306670) and Y.Z. is thankful to the CSC for financial support. Parts of this research were undertaken at the SAXS/WAXS beamline of the Australian Synchrotron.
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