We compared the performances of polymer-based photovoltaic devices prepared from different formulations of poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS). The PEDOT:PSS buffer layer is incorporated between the indium tin oxide (ITO) electrode and the active layer, which is composed of a blend of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). A highest efficiency of 3.86% (under AM1.5 solar illumination) was achieved for device prepared from a PEDOT:PSS trade-named high conductivity grade. However, annealing devices at a temperature over 120 °C results in decreased photovoltaic performance. This study shows that attention has to be paid to chemicals used to formulate high conductive PEDOT:PSS to become compatible with the production of solar cells involving thermal processing.