Stoichiometric Dissolution of Defective CsPbI 2 Br Surfaces for Inorganic Solar Cells with 17.5% Efficiency

The existence of a defective area composed of nanocrystals and amorphous phases on a perovskite film inevitably causes nonradiative charge recombination and structural degradation in perovskite photovoltaics. In this study, a stoichiometric etching strategy for the top surface of a defective cesium lead halide perovskite is developed by using ionic liquids. The dissolution of the original defective area substantially exposes the underlying perovskite, which is a high‐quality surface with retained stoichiometry and lattice continuity. The ionic liquid molecules are adsorbed on the perovskite surface via Coulombic interactions and passivate the undercoordinated surface lead centers. Such a structural modulation considerably reduces the trap density of the perovskite devices and enables a record power conversion efficiency of 17.51% and an open‐circuit voltage of 1.37 V of the CsPbI 2 Br cell with a perovskite bandgap of 1.88 eV. This work provides a novel technical route to improve the efficiency and environmental resilience of perovskite‐based optoelectronic devices.