Air-processed mixed-cation Cs_0.15FA_0.85PbI₃ planar perovskite solar cells derived from a PbI₂-CsI-FAI intermediate complex

Developing facile strategies that enable high-performance perovskite solar cell (PSC) fabrication in ambient air is appealing yet challenging for low-cost and large-scale industrial deployment. However, attempts in this regard are still limited to methylammonium lead triiodide (CH₃NH₃PbI₃), which lags significantly behind the rapid development of the state-of-the-art formamidinium (FA) based mixed perovskites. Here, a PbI₂-(CsI)_0.15-(FAI)_x intermediate complex is proposed to unlock the processing limitation and efficiently facilitate the growth of high-quality mixed-cation Cs_0.15FA_0.85PbI₃ perovskite films in ambient air with relative humidity (RH) as high as 70 ± 10%. It is found that the PbI₂-(CsI)_0.15-(FAI)_x (x = 0.2 to 0.4) intermediate complex consisting of a mixture of composites is favorable for the conversion of PbI₂ to high-quality Cs_0.15FA_0.85PbI₃ perovskite. Eventually, it reduces the required FAI concentration deposited on the intermediate for the conversion of PbI₂ to perovskite, which allows the strong hygroscopic FAI to be processed in ambient air. With the PbI₂-(CsI)_0.15-(FAI)_0.3 intermediate complex, a low-temperature air-processed planar Cs_0.15FA_0.85PbI₃ PSC achieves a champion power conversion efficiency (PCE) of 15.56% with superior thermal-stability and photo-durability, demonstrating its great potential for the large scale development of PSCs.