Enhanced photocurrent spectral response in low-bandgap polyfluorene and C70-derivative-based solar cells

Plastic solar cells have been fabricated using a low-bandgap alternating copolymer of fluorene and a donor-acceptor-donor moiety (APFO-Green1), blended with 3'-(3,5-bis-trifluoromethylphenyl)-1'-(4-nitrophenyl)pyrazolino[70]fullerene (BTPF70) as electron acceptor. The polymer shows optical absorption in two wavelength ranges, lambda < 500 nm and 600 < lambda < 1.000 nm. The BTPF70 absorbs light at lambda < 700 urn. A broad photocurrent spectral response in the wavelength range 300 < lambda < 1000 nm is obtained in solar cells. A photocurrent density of 3.4 mA cm(-2), open-circuit voltage of 0.58 V, and power-conversion efficiency of 0.7 % are achieved under illumination of AM1.5 (1000 W m(-2)) from a solar simulator. Synthesis of BTPF70 is presented. Photoluminescence quenching and electrochemical studies are used to discuss photoinduced charge transfer.