Time-Resolved Photoemission to Unveil Electronic Coupling between Absorbing and Transport Layers in a Quantum Dot-Based Solar Cell

Abstract

Lead sulfide (PbS) colloidal quantum dot-based photodiodes are remarkable structures because of their outstanding optoelectronic performances obtained via colloidal engineering. They combine surface ligand engineering to design a p–n junction with all-solution processability. Here, we investigate the electronic structure of PbS diode combining static and dynamic photoemissions with transport measurements. We show that the n-type nature of the I– capped PbS CQDs shifts the valence band away from the Fermi level compared to the thiol-capped nanocrystals. This change in majority carriers can be probed by time-resolved X-ray photoemission spectroscopy (TRXPS). We also prove that the photoinduced binding energy shift depends on the nanoparticle surface chemistry. Finally, we demonstrate the ability of TRXPS to selectively probe the electronic structure of each side of an interface. We explore the PbS/MoO3 interface used as a hole extractor in a PbS solar cell using this method. We demonstrate that the PbS layer photosensitizes the MoO3 layer and that the two layers have a quasi-rigid electrostatic coupling. We identify the band bending occurring on the PbS (EDT)/MoO3 to be a limiting factor for the device performance and suggest strategies to overcome this limitation.