One-Step Solvent-Free Mechanochemical Incorporation of Insoluble Cesium Salt into Perovskites for Wide Band-Gap Solar Cells

The preparation of high-quality perovskite thin films with a low concentration of defects has recently been achieved through cation engineering using, for example, Cs halide salts. However, many Cs salts cannot be adopted readily due to their frequent insolubility in typical N,N-dimethylformamide (DMF) or dimethyl sulfoxide (DMSO) solvent systems. Herein, we report the application of green, rapid, and solvent-free mechanosynthetic ball-milling for the incorporation of the otherwise insoluble CsBr to realize wide band-gap perovskite solar cells (PSCs). We mechanically synthesize triple-cation (cesium (Cs)/formamidinium (FA)/methylammonium (MA)) wide band-gap perovskites, resulting in subsequent powders that were soluble in mixed DMF/DMSO (4:1, V/V) solvents. Otherwise, the preparation of triple cations for wide band-gap perovskites through conventional solution processing could not be realized. The use of mechanosynthesis perovskites for thin-film formation allows for the growth of relatively large crystalline grains with grains diameter in the range of 500–700 nm. The champion device achieved a maximum PCE of 7.3% (7.03% stabilized), with JSC of 7.08 mA cm–2, VOC of 1.48 V, and a fill factor (FF) of 70%. This performance and voltage are among the highest reported for wide band-gap PSC devices incorporating triple-cation Csx(FAyMA(1–y))(1–x)PbBr3 perovskites. These results show that the use of a mechanosynthetic strategy to add insoluble dopants to wide band-gap perovskites provides a promising strategy for the formation of high-quality films. Furthermore, mechanoperovskite showed higher phase purity, VOC, and efficiency as compared to the conventional solution-processed devices.