Time-resolved opto-electronic properties of poly(3-hexylthiophene-2,5-diyl): Fullerene heterostructures detected by Kelvin force microscopy

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.