Formation and decay of charge carriers in hybrid MDMO-PPV:ZnO bulk heterojunctions produced from a ZnO precursor

Recently, there has been growing interest in photovoltaic cells based on bulk heterojunctions of ZnO and a conjugated polymer. In this work, the photophysical processes in a hybrid film of ZnO with MDMO-PPV were studied by means of optical absorption spectroscopy and microwave photoconductivity measurements. Films were prepared from a blend of a ZnO precursor and MDMO-PPV in which the volume fraction of ZnO (VZnO) was varied from 0.14 to 0.33. Efficient charge separation, resulting from excitation of the polymer in the visible, is observed in samples with VZnO=0.14 and 0.25. The photoconductance decays on a timescale of tens of microseconds. For a sample with VZnO=0.33, a much lower photoconductivity is observed, possibly due to a lower charge separation efficiency. Annealing of the samples for two hours at  100 ∘C leads to a dramatic increase of the photoconductivity, especially for VZnO=0.14. This is possibly related to the formation of ZnO rich domains into which electrons can escape from geminate recombination.