Efficiencies of organic solar cells based on an interpenetrating network of a conjugated polymer and a fullerene as donor and acceptor materials still need to be improved for commercial use. We have developed a postproduction treatment that improves the performance of solar cells based on poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C-61-butyric acid methyl ester (PCBM) by means of a tempering cycle at elevated temperatures in which an external voltage is simultaneously applied, resulting in a significant increase of the short-circuit current. Using this postproduction treatment, an enhancement of the short-circuit cur rent density, I-sc, to 8.5 mA cm(-2) under illumination with white light at an illumination intensity of 800 W m(2) and an increase in external quantum efficiency (IPCE, incident photon to collected electron efficiency) to 70 % are demonstrated.