Effective Addition of Cation and halides in FAPbI₃ Perovskite Solar Cells by Multiple Thermal Co-EvaporatioH

Metal halide perovskite solar cells (PSCs) have rapidly advanced as a next-generation solar cells with high efficiency over 25%. Among the various fabrication methods for the perovskite, thermal evaporation stands out as a suitable option for scaling up commercialization. However, achieving precise stoichiometry with multiple sources is challenging when using a co-evaporation method due to their varying volatility. In this study, multiple co-evaporation method was designed to fabricate the stoichiometrically mixed cation and halides PSCs using two sources. Formamidinium iodide (FAI) and lead iodide (PbI₂) were co-evaporated followed by the co-evaporation of methylammonium iodide (MAI) and PbI₂ or methylammonium bromide (MABr) and lead bromide (PbBr₂). The deposition rates and thickness of secondly co-evaporated layer were controlled to achieve stoichiometry of mixed cations and halides precursors. After that, samples were annealed to form FAPbI₃, (FA,MA)PbI₃, and (FA,MA)Pb(I,Br)₃ films. The completed PSCs were semi fully-vacuum-deposited, except for the hole transport layer. Among the samples, (FA,MA)Pb(I,Br)₃ solar cells showed the highest performance of 11.38% efficiency with improved open-circuit voltage of 25 mV and current-voltage hysteresis. Herein, band gap of MA and Br added FAPbI₃ slightly increased indicating only small amount of MAPbI₃ and MAPbBr₃ phases formed. Still, mixed cations samples showed improved absorbance compared to FA cation only sample. In conclusion, multiple co-evaporation is effective method for mixing cations and halides in PSCs.