We report a transient absorption and luminescence study addressing the charge separation, recombination, and regeneration reactions atnanostructured metal oxide/PbS quantum dot/organic hole conductorheterojunctions. We show that yields of charge separation aresignificantly higher at PbS/SnO2 interfaces relative to PbS/TiO222. We also report a correlation between the PbS particle size and the yield of charge separation at PbS/SnO2interfaces, with a smaller PbS particle radius resulting a higher yieldof charge separation. Finally we investigated the regeneration of thephotooxidized PbS by an organic hole transporting material, namely, spiro-OMeTAD. The overall spiro-OMeTAD+ yield is found to be 27% at a SnO2/PbS (3 nm diameter)/spiro-OMeTAD heterojunction. The addition of a lithium salt was found to raise the overall spiro-OMeTAD+ yield from its initial value of 27% (where no Li+is present) to 50%. We believe this to be a result of an increase inthe primary charge injection yield to near unity as the SnO2 conduction band is lowered (with increasing [Li+]),increasing the driving force for electron injection. The presentfindings are discussed with relevance to the design of PbS-sensitizedmetal oxide layers for DSSCs.
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