Charge generation in PbS quantum dot solar cells characterized by temperature-dependent steady-state...

Charge-carrier generation and transport within PbS quantum dot (QD) solar cells is investigated by measuring the temperature-dependent steady-state photoluminescence (PL) concurrently during in situ current-voltage characterization. We first compare the temperature-dependent PL quenching for PbS QD films where the PbS QDs retain their original oleate ligand to that of PbS QDs treated with 1,2-ethanedithiol (EDT), producing a conductive QD layer, either on top of glass or on a ZnO nanocrystal film. We then measure and analyze the temperature-dependent PL in a completed QD-PV architecture with the structure Al/MoO3/EDT-PbS/ZnO/ITO/glass, collecting the PL and the current simultaneously. We find that at low temperatures excitons diffuse to the ZnO interface, where PL is quenched via interfacial charge transfer. At high temperatures, excitons dissociate in the bulk of the PbS QD film via phonon-assisted tunneling to nearby QDs, and that dissociation is in competition with the intrinsic radiative and nonradiative rates of the individual QDs. The activation energy for exciton dissociation in the QD-PV devices is found to be ∼40 meV, which is considerably lower than that of the electrodeless samples, and suggests unique interactions between injected and photogenerated carriers in devices.