Tailorable PC₇₁ BM Isomers: Using the Most Prevalent Electron Acceptor to Obtain High-Performance Polymer Solar Cells

Despite being widely used as electron acceptor in polymer solar cells, commercially available PC₇₁ BM (phenyl-C₇₁ -butyric acid methyl ester) usually has a "random" composition of mixed regioisomers or stereoisomers. Here PC₇₁ BM has been isolated into three typical isomers, α-, β₁ - and β₂ -PC₇₁ BM, to establish the isomer-dependent photovoltaic performance on changing the ternary composition of α-, β₁ - and β₂ -PC₇₁ BM. Mixing the isomers in a ratio of α/β₁ /β₂ =8:1:1 resulted in the best power conversion efficiency (PCE) of 7.67 % for the polymer solar cells with PTB7:PC₇₁ BM as photoactive layer (PTB7=poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]]). The three typical PC₇₁ BM isomers, even though sharing similar LUMO energy levels and light absorption, render starkly different photovoltaic performances with average-performing PCE of 1.28-7.44 % due to diverse self-aggregation of individual or mixed PC₇₁ BM isomers in the otherwise same polymer solar cells.