High‐Efficiency Infrared Sulfide Lead Quantum Dot Solar Cells via Mixed Halide Ions Ligand Engineering

Tandem solar cells are regarded as an effective way to break through the theoretical efficiency of the Shockley–Queisser limit, and large‐size lead sulfide quantum dots (PbS QDs) are considered ideal infrared (IR) photovoltaic materials for absorbing low‐energy IR photons in the bottom subcells of tandem solar cells due to their tunable bandgaps and multiexciton generation effects. However, the exposed (100) surface of large‐size PbS QDs leads to poor passivation during the ligand‐exchange process, resulting in degraded device performance. Herein, a mixed halide ions ligand passivation strategy is developed to synergistically passivate the (111) and (100) facets of large‐size QDs. Particularly, a gradual dropwise addition of lead chloride salts dissolved in dimethyl sulfoxide for achieving precisely proportional surface passivation of Cl ions in the ligand exchange is proposed, which enhances the quality of the QD solids and obtains excellent control of the surface trap states. Additionally, optical and electrical characterization results show that the passivated IR PbS QD solids have higher carrier mobility, longer carrier lifetime, and lower defect density, which increases the IR efficiency of IR PbS QD solar cells to 1.25%.