The Transitional Heterojunction Behavior of PbS/ZnO Colloidal Quantum Dot Solar Cells

The nature of charge separation at the heterojunction interface of solution processed lead sulphide-zinc oxide colloidal quantum dot solar cells is investigated using impedance spectroscopy and external quantum efficiency measurements to examine the effect of varying the zinc oxide doping density. Without doping, the device behaves excitonically with no depletion region in the PbS layer such that only charge carriers generated within a diffusion length of the PbS/ZnO interface have a good probability of being harvested. After the ZnO is photodoped such that the doping density is near or greater than that of the PbS, a significant portion of the depletion region is found to lie within the PbS layer increasing charge extraction (p–n operation)