The open-circuit voltage V-oc of polymer:fullerene bulk heterojunction solar cells is investigated as a function of light intensity for different temperatures. Devices consisted of a blend of a poly(p-phenylene vinylene) derivative as the hole conductor and 6,6-phenyl C-61-butyric acid methyl ester as the electron conductor. The observed photogenerated current and V-oc are at variance with classical p-n junction-based models. The influence of light intensity and recombination strength on V-oc is consistently explained by a model based on the notion that the quasi-Fermi levels are constant throughout the device, including both drift and diffusion of charge carriers. (C) 2005 American Institute of Physics.