Solution mediated halide exchange engineering for the fabrication of a thick CsPbCl 3 film and its application in photovoltaics with outstanding performance.

It is a big challenge to prepare thick CsPbCl 3 films using traditional solution processed approaches owing to the low solubility of precursors of PbCl 2 and CsCl in common solvents. Here, we propose an indirect solution process to prepare thick CsPbCl 3 films. In this new approach, a mother film of CsPbBr 3 is first prepared through a solution process, and then it is dipped into a diluted HCl/methanol solution. During the dipping process, it triggers a halide exchange reaction between Br - and Cl - , and it eventually produces a thick CsPbCl 3 film (∼400 nm) with high quality and purity. Afterwards, a carbon based hole transportation layer (HTL) free solar cell with a configuration of FTO/TiO 2 /CsPbCl 3 /carbon is constructed, and it delivers an average PCE of 1.23% and an outstanding PCE of 1.39% in a batch of PSCs. Meanwhile, the solar cell maintains its 82% initial PCE after storage in open air for 31 days. This work overcomes the obstacle of the traditional solution approach for the preparation of CsPbCl 3 films, which makes it promising for preparing various CsPbCl 3 film-based devices via a solution process.