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/TiO222. 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, 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 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.

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