2, University of Pennsylvania, Philadelphia, Pennsylvania, United States
3, University of Pennsylvania, Philadelphia, Pennsylvania, United States
Arrays of electronically coupled colloidal quantum dots (QDs) present a unique model system for exploring charge transport mechanisms and energy conversion in strongly disordered inorganic films. In this work, we present the results of dynamic junction capacitance spectroscopies (drive level capacitance profiling (DLCP) and thermal admittance spectroscopy (TAS)) on PbS QD solar cells. We study properties critical to the charge transport of both dark and photo-excited carriers in these devices, such the spatial uniformity of bulk defects and their energetic depth within the semiconductor band gap, carrier densities, and illumination-induced meta-stabilities.
Our results indicate a deep bulk trapping response present throughout the PbS film, regardless of whether halide (I-) or thiol (MPA) ligand treatments are used. Fermi level pinning at the heterojunction interface is also observed, and both the density of bulk and interface states is determined using novel DLCP analysis. The conductivity of these deep states is also calculated and suggests weakly conductive activated transport channels within a deep defect band. Upon oxygen exposure, films of halide-capped QDs exhibit a pronounced shift in activation energy not observed in thiol treated films, which we use to explore the chemical origins of the deep state response. Finally, capacitance spectroscopy is also performed under infrared illumination of the solar cells, and indicates that photo-excited charge in the QDs is preferentially trapped in deep bulk defects, providing direct insight into the role of defects in device operation.