A New Passivation Route Leading to Over 8% Efficient PbSe Quantum-Dot Solar Cells via Direct Ion Exchange with Perovskite Nanocrystals

Colloidal quantum dots (QDs) are promising candidate materials for photovoltaics (PV) owing to the tunable bandgap and low-cost solution processability. Lead selenide (PbSe) QDs are particularly attractive to PV applications due to the efficient multiple-exciton generation and carrier transportation. However, surface defects arising from the oxidation of the PbSe QDs have been the major limitation for their development in PV. Here, a new passivation method for chlorinated PbSe QDs via ion exchange with cesium lead halide (Br, I) perovskite nanocrystals is reported. The surface chloride ions on the as-synthesized QDs can be partially exchanged with bromide or iodide ions from the perovskite nanocrystals, hence forming a hybrid halide passivation. Consistent with the improved photoluminescence quantum yield, the champion PV device fabricated with these PbSe QDs achieves a PCE of 8.2%, compared to 7.3% of that fabricated with the untreated QDs. This new method also leads to devices with excellent air-stability, retaining at least 93% of their initial PCEs after being stored in ambient conditions for 57 d. This is considered as the first reported PbSe QD solar cell with a PCE of over 8% to date.