Influence of electronic transport properties of polymer-fullerene blends on the performance of bulk heterojunction photovoltaic devices

The photovoltaic properties of polymer-fullerene bulk-heterojunction solar cells with light absorbers based on the conjugated polymers poly (2-methoxy-5-(3-,7-dimethyl-octyloxy)-1,4-phenylene vinylene (OC1C10-PPV) and regioregular poly(3-hexyl thiophene) (P3HT), each blended with the acceptor-type methanofullerene [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), are discussed. Although devices with both material combinations demonstrate good performance in terms of power conversion efficiency, they are very different in their electronic transport behavior. The profiling of the photocurrent in solar cells based on OC1C10-PPV shows a strong temperature dependence reflecting that the current is limited by the recombination of photogenerated charge carriers in the absorber layer. In contrast, devices with P3HT:PCBM active layers display a temperature independent photocurrent above some temperature, indicating that the transport and extraction of charge carriers are not significantly attenuated by losses, i.e., the mean drift length of photogenerated charge carriers exceeds the active layer thickness under device operating conditions. Therefore the active layer thickness could be increased resulting in high values of short-circuit current due to improved light absorption. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)