Effects of C70 derivative 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]-$5^{\prime },5^{″}$ -diyl} and (6,6)-phenyl-${\mathrm{C}}_{71}$ -butyric acid methyl ester (${\mathrm{C}}_{70}$ -PCBM) were demonstrated with 2.4% power conversion efficiency under air mass $1.5\mathrm{G}$ , $100\mathrm{mW}∕{\mathrm{cm}}^2$ illumination. The broad absorption peak of ${\mathrm{C}}_{70}\text{-}\mathrm{PCBM}$ in $440–530\mathrm{nm}$ complements the absorption valley (regions between two absorption peaks at 416 and $584\mathrm{nm}$ ) 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 ${\mathrm{C}}_{70}\text{-}\mathrm{PCBM}$ 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.