Efficient small bandgap polymer solar cells with high fill factors for 300 nm thick films.

A high-molecular-weight conjugated polymer based on alternating electron-rich and electron-deficient fused ring systems provides efficient polymer solar cells when blended with C60 and C70 fullerene derivatives. The morphology of the new polymer/fullerene blend reduces bimolecular recombination and allows reaching high fill factors and power conversion efficiencies for films up to 300 nm thickness.