Small-molecule engineering of a PbS QD/ZnO nanowire interface and its impact on infrared PbS QD solar cell performance

Lead sulfide (PbS) quantum dots (QDs) are promising photovoltaic absorbers owing to their tunable absorption range from the visible to infrared regions. Realizing high efficiency in PbS QD/zinc oxide (ZnO) heterojunction solar cells requires precise energy-level alignment between the PbS QDs and ZnO, which significantly affects carrier transport and recombination processes, particularly when employing infrared-absorbing PbS QDs. In this study, we conducted systematic interfacial engineering through a small-molecule treatment to tailor the PbS QD/ZnO nanowire (NW) heterojunction for enhanced infrared solar cell performance. Five molecules featuring hydroxy (-OH), thiol (-SH), and methyl (-CH₃) functional groups were strategically selected to tune the interfacial energetics based on their molecular dipoles, their electron-withdrawing abilities, and the surface coverage on ZnO, among other factors. These molecular modifications revealed the key parameters that influenced the energy levels of the conduction band minimum, the valence band maximum, and the Fermi level, thereby shaping the overall band structure of the PbS QD/ZnO NW heterojunction. Controlled interface engineering enables the transformation of spike-shaped heterojunctions, which impede carrier transport from the PbS QD region to the ZnO region, into cliff-shaped junctions, which are more favourable for carrier extraction. Solar cells with cliff-shaped heterojunctions exhibit increased short-circuit current densities and external quantum efficiencies. Importantly, the carrier-recombination frequency at the interface depended significantly on the type of functional group introduced by the modifying molecules. This study provides valuable insights into the selection and design of modifying molecules for controlling the properties of metal oxide/infrared QD heterojunction-based solar cells.