Charge Recombination in P3HT/PC70BM Composite Studied by Light-Induced EPR

A composite of conductive polythiophene P3HT and soluble fullerene derivative PC70BM (the material widely used as the active layer in organic photovoltaics devices) was studied by light-induced EPR (LEPR). In contrast to P3HT/PC60BM composite, LEPR signal in P3HT/PC70BM can be detected in a wide temperature range up to room temperature. This signal was attributed to charge carriers P3HT(+) and PC70BM-. The dependence of the intensity of LEPR signal on light intensity and the decay of LEPR signal upon switching light off are interpreted in frame of trap-limited bimolecular recombination model with finite rate of back electron transfer in [P3HT(+) PC70BM-] encounter complex at polymer-fullerene interface. The apparent recombination order was found to be close to p = 3.5 for the temperature range from 100 K to room temperature. For temperatures above 150 K Arrhenius behavior of effective recombination rate constant was obtained with the activation energy E-a = 0.16 +/- 0.01 eV, which is larger than analogous values for P3HT/PC60BM reported previously. This difference is attributed to the change of polymer/fullerene interface induced by the replacement of PC60BM by PC70BM.