"It has been shown that in hybrid polymer-inorganic photovoltaic devices, not all the photo-generated excitons dissociate at the interface immediately, but can instead exist temporarily as bound charge pairs (BCPs). Many of these BCPs do not contribute to the photocurrent as their long lifetime as a bound species promotes various charge carrier recombination channels. Fast and efficient dissociation of BCPs is therefore considered a key challenge in improving the performance of polymer-inorganic cells. Here we investigate the influence of an inorganic energy cascading Nb2O5 interlayer on the charge carrier recombination channels in poly(3-hexylthiophene-2,5-diyl) (P3HT)-TiO2 and PbSe colloidal quantum dot-TiO2 photovoltaic devices. We demonstrate that the additional Nb2O5 film leads to a suppression of BCP-formation at the heterojunction of the P3HT cells and also a reduction in the non-geminate recombination mechanisms in both types of cells. Furthermore we provide evidence that the reduction in non-geminate recombination in the P3HT-TiO2 devices is due in part to the passivation of deep mid-gap trap states in the TiO2, which prevents trap assisted Shockley-Read-Hall recombination. Consequently a significant increase in both the open-circuit voltage and the short-circuit current was achieved, in particular for P3HT-based solar cells where the power conversion efficiency increased by 39%."
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