Slow halide exchange in CsPbIBr2 films for high-efficiency, carbon-based, all-inorganic perovskite solar cells

Halide exchange offers a versatile way to modify the properties of halide perovskites, but it is particularly challenging to slow the reaction rate to restrain defect growth in the products. Herein, we propose a slow halide exchange strategy to simultaneously fine-tune the optical and microstructural characteristics of CsPbIBr2 films by physically pairing CsPbIBr2 and CH3NH3PbI3 films. Once a proper heating treatment is applied, halide exchange of Br- and I- ions between the films is activated, and the reaction rate can be well controlled by the heating recipe, in which a high temperature can accelerate the exchange reaction, while a low temperature slows or stops it. By using an optimal halide exchange temperature (110 degrees C) and time (2 h), the parent CsPbIBr2 film was transformed into high-quality CsPbI1+xBr2-x film, featuring an extended absorption onset from 590 to 625 nm, coarsened grains, improved crystallinity, reduced surface roughness, suppressed halide phase segregation, and identical stability to the pristine film. Accordingly, the efficiency of a carbon-based, all-inorganic perovskite solar cell (PSC) was boosted to 10.94%, which was much higher than that of the pristine CsPbIBr2 film (8.21%). The CsPbI1+xBr2-x PSC also possessed excellent tolerance against heat and moisture stresses.