Effects of C₇₀derivative in low band gap polymer photovoltaic devices: Spectral complementation and morphology optimization

Efficient polymer solar cells based on a low band gap copolymer poly{(9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(3-decyloxythien-2-yl)-2,1, 3-benzothiadiazole]- 5′, 5″ -diyl} and (6,6)-phenyl- C71 -butyric acid methyl ester (C70 -PCBM) were demonstrated with 2.4% power conversion efficiency under air mass 1.5 G, 100 mW cm2 illumination. The broad absorption peak of C70 -PCBM in 440-530 nm complements the absorption valley (regions between two absorption peaks at 416 and 584 nm) of the polymer. The external quantum efficiency measurement further demonstrates that this increased absorption contributes significantly to the generation of photocurrent. Morphology studies on the blend films indicated that excellent miscibility between polymer and C70 -PCBM favors exciton separation. The linear relationship between light intensity and short circuit current density shows efficient and balanced charge transport resulting in increased photocurrent and fill factor.