Passivation Strategy of Reducing Both Electron and Hole Trap States for Achieving High-Efficiency PbS Quantum-Dot Solar Cells with Power Conversion Efficiency over 12%

For the current passivation strategy, pure iodine passivation during solid-state ligand exchange (SSE) cannot completely passivate the entire surface of PbS colloidal quantum dots (CQDs). Here, a simple stepwise passivation strategy is proposed based on the postpassivation of PbS CQD films with a halogen (Cl, Br, or I) after iodine passivation through the SSE. This postpassivation could compensate for the missing ligands caused by the polar environment during the SSE. Thus, both electron- and hole-trapping states are greatly reduced, and the charge transport in the CQD film is significantly improved. The PbS CQD films post-treated with chlorine exhibit a carrier diffusion length increased by 70% when compared with that of control samples. We demonstrate the highest power conversion efficiency of 12.4% among the reported PbS CQD solar cells prepared with the SSE method to date. In addition, the unencapsulated device shows good stability in air.