Photogeneration and decay of charge carriers in hybrid bulk heterojunctions of ZnO nanoparticles and conjugated polymers

The photogeneration and decay of charge carriers in blend films of ZnO nanoparticles ( diameter 5 nm) and poly[2-methoxy-5-(3', 7'-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) or poly(3-hexylthiophene) (P3HT) were studied by means of microwave-photoconductance measurements. Excitation of the polymer in the visible spectrum was found to lead to a transient photoconductance due to dissociation of excitons at the interface between ZnO and the conjugated polymer. From the similar decay kinetics of the photoconductance and the effects of UV illumination, it is concluded that the measured photoconductance is due to electrons in ZnO. Increasing the weight fraction of ZnO in the blend films leads to a higher photoconductance. This is attributed to enhanced formation of mobile electrons by interfacial dissociation of excitons at clusters of ZnO nanoparticles rather than at individual nanoparticles. The dependence of the photoconductance on the weight fraction of ZnO is found to be different for ZnO: MDMO-PPV and ZnO: P3HT blends. This is most likely due to the presence of a smaller number of relatively large ZnO clusters in ZnO: P3HT blends and a shorter exciton diffusion length, as compared with ZnO: MDMO-PPV blends. After exposure of the blend films to UV light, a significant increase in the magnitude and the lifetime of the photoconductance is observed. This is explained in terms of the filling of electron traps in ZnO by UV exposure.