Surface in situ reconstruction of inorganic perovskite films enabling long carrier lifetimes and solar cells with 21% efficiency

Inorganic perovskite solar cells suffer from charge carrier losses. Chu et al. treat the perovskite surface with caesium fluoride, forming a wider-bandgap perovskite that increases the carrier lifetime and hole extraction efficiency. All-inorganic perovskites are emerging as excellent photovoltaic candidates for single-junction or tandem solar cells. However, large energy loss due to non-radiative recombination is the main constraint for performance enhancement. Accordingly, we developed a surface in situ reconstruction (SISR) strategy for inorganic perovskite by CsF treatment, which can suppress non-radiative recombination and promote hole extraction simultaneously. Surface defects can be effectively passivated by the introduced fluorine, and carrier lifetime was prolonged from 11.5 ns to 737.2 ns. In addition, a wider-bandgap perovskite layer can be generated as a graded heterojunction to facilitate hole extraction. The SISR reaction mechanism was also verified from both kinetic calculations and experiments. As a result, CsPbIxBr3-x solar cell with SISR achieved an efficiency of 21.02% with a high open-circuit voltage of 1.27 V and fill factor of 85.3%. This work provides an effective approach to modulate inorganic perovskite surfaces for the design of efficient solar cells.