Ionic Liquid-Mediated Intermediate Phase Adduct Constructing for Highly Stable Lead-Free Perovskite Solar Cells

The intermediate phase adduct plays a crucial role in constructing uniform and compact tin perovskite films, thus providing an important approach for developing high-performance lead-free perovskite solar cells. However, the common intermediate phase adduct of SnI₂·3DMSO in tin perovskite leads to phase separation and may lack compatibility with mixed cation tin perovskites composed of formamidinium (FA) and methylamine (MA), impeding the further device stability. Here, a facile and reproducible method is developed to fabricate high-quality FA_0.75MA_0.25SnI₃ films by introducing a new stable intermediate phase adduct (SnI₂·DMSO·MAFa) by using ionic liquid methylamine formate (MAFa). The resulting stable adduct suppresses the reaction rate between ammonium salts and SnI₂, thereby modulating the tin perovskite crystallization and precluding SnI₂ clusters formation, and the presence of the SnI₂·DMSO·MAFa adduct in perovskite precursor serves as a protective barrier for Sn²⁺ ions, guarding them against oxidation caused by the presence of DMSO. Moreover, the amino and carbonyl groups in residual MAFa could repair the iodine vacancy and uncoordinated Sn²⁺ ion defects. These features result in the formation of highly uniform and pinhole-free FA_0.75MA_0.25SnI₃ films. The optimized devices achieve a power conversion efficiency (PCE) of over 10%, a value of 53% higher than that of the control device (6.6%).