A modified two-step sequential spin-coating method for perovskite solar cells using CsI containing organic salts in mixed ethanol/methanol solvent

Two-step sequential spin-coating method has been widely used to prepare mixed FA/Cs and FA/MA/Cs perovskite solar cells. However, due to the extremely low solubility of Cs salts in the commonly used isopropanol solvent, which is used to dissolve the organic salts, the Cs salts can only be dissolved into the PbI2 precursors with mixed DMF/DMSO solvent. Herein, we have proposed a modified two-step sequential spin-coating method to solve the above problem. A mixed solvent consisting of ethanol and methanol is used to dissolve the CsI, FAI, and MACl at the same time. The solubility of CsI in the mixed ethanol/methanol solvent can be tuned by varying the ethanol/methanol ratio. After optimizing the ethanol/methanol ratio, high quality FA1-xCsxPbI3 perovskite film with large perovskite crystal, low defect density and long carrier life time can be obtained using our modified two-step sequential spin-coating method, which can result in a perovskite solar cell with champion power conversion efficiencies of 21.17% (cell area 0.1 cm2) and 14.65% (cell area 0.25 cm2) respectively. Our study provides a new method for fabricating mixed FA/Cs perovskite solar cells using CsI containing organic salts solution, which can be further optimized to fabricate highly efficient perovskite solar cells.