The discovery of a photoinduced charge transfer interaction between conjugated polymers and fullerenes has led to intensive spectroscopic investigations of solid state composites of these materials. Based on this charge transfer reaction, high efficient solar cell devices have been developed using the concept of bulk heterojunctions, i.e. blending of the conjugated polymer and fullerene. In this work, we present a spectroscopic study on such devices under working conditions. These devices were investigated by means of photoinduced absorption spectroscopy in reflection geometry at different temperatures and different applied external electric fields. Significant changes in the photoinduced signal were observed upon changing the applied voltage. These changes can be attributed to persistent charge carriers, charge carrier transport and charge carrier injection. From this the mean polaron lifetime, the built-in potential and the fraction of persistent charge carriers was estimated. (C) 2003 Elsevier B.V. All rights reserved.