Transient Optical Studies of Interfacial Charge Transfer at Nanostructured Metal Oxide/PbS Quantum Dot/Organic Hole Conductor Heterojunctions

We report a transient absorption and luminescence study addressing the charge separation, recombination, and regeneration reactions at nanostructured metal oxide/PbS quantum dot/organic hole conductor heterojunctions. We show that yields of charge separation are significantly higher at PbS/SnO2 interfaces relative to PbS/TiO2 interfaces, and conclude that this behavior is a result of the ca. 300-500 meV lower conduction band edge in SnO2 as compared to TiO2. We also report a correlation between the PbS particle size and the yield of charge separation at PbS/SnO2 interfaces, with a smaller PbS particle radius resulting a higher yield of charge separation. Finally we investigated the regeneration of the photooxidized PbS by an organic hole transporting material, namely, spira-OMeTAD. The overall spiro-OMeTAD(+) yield is found to be 27% at a SnO2/PbS (similar to 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 in the 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 present findings are discussed with relevance to the design of PbS-sensitized metal oxide layers for DSSCs.