Thin blend polymer films made of poly(3-hexylthiophene-2 5-diyl) (electron donor) and fullerene derivatives as electron acceptors ([6 6]-thienylC61 butyric acid methyl ester and [6 6] thienylC71 butyric acid methyl ester) are prepared by the spin-coating technique on indium tin oxide covered glass substrates Time-resolved photo-Induced changes of surface potentials are detected by Kelvin force microscopy (KFM) Changes of surface potentials by 10-150 mV reveal different quality and kinetics of charge generation in the two blends in short (minutes) and long (hours) time periods This is attributed to a combination of electron accumulation trapping and organic material degradation under ambient conditions As KFM characterizes the blend films directly without metal contact layer it reveals differences in the opto-electronic behavior of the blends which are not detected by common photovoltaic cell characterization (c) 2010 Elsevier B V All rights reserved