We report the influence of thickness and thermal annealing of the poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) layer on the performance of bulk heterojunction solar cells made from blends of regioregular poly(3-hexylthiophene) and 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C61. Results show that the power conversion efficiency was significantly improved by inserting the PEDOT:PSS layer but was not strongly sensitive to either the layer thickness or the annealing temperature. Although the short circuit current density was enhanced slightly by annealing the PEDOT:PSS layer at high temperatures, the fill factor was slightly decreased. The trend in device performance could not be explained by the observed changes in the work function of the PEDOT:PSS layer. © 2008 Elsevier Ltd. All rights reserved.