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

It is a big challenge to prepare thick CsPbCl films using traditional solution processed approaches owing to the low solubility of precursors of PbCl and CsCl in common solvents. Here, we propose an indirect solution process to prepare thick CsPbCl films. In this new approach, a mother film of CsPbBr 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 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/CsPbCl/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 films, which makes it promising for preparing various CsPbCl film-based devices a solution process.