Breaking the Open-Circuit Voltage Deficit Floor in PbS Quantum Dot Solar Cells through Synergistic Ligand and Architecture Engineering

To realize the full potential of colloidal quantum dot (CQD) based solar cells, it is important to address the issue of large open-circuit voltage (VOC) deficit which is a major roadblock in reaching higher efficiencies. The origin of the VOC deficit in these solar cells lies primarily in the presence of sub-bandgap trap states of the QDs. Here, we present a synergistic engineering framework to passivate these sub-bandgap states in PbS QDs through chemical surface passivation and remote passivation exploiting ligand and architecture engineering. In particular, we form bulk nanoheterojunctions (BNH) by mixing PbS QDs with ZnO nanocrystals in conjunction with mixed ligand treatments to passivate surface traps. We employ the mixed ligand system of zinc iodide and 3-mercatopropyonic acid to leverage the benefits of both organic and inorganic ligands for surface passivation and improved charge transport. This mixed ligand treatment in BNH architectures leads to record low VOC deficit for PbS QDs of 0.4–0.55 V compared to previously reported 0.6–0.8 V for the range of 1.1–1.35 eV bandgap PbS QDs.