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 FA0.75MA0.25SnI3 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 FA0.75MA0.25SnI3 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%). Besides, the obtained MAFa-derived devices illustrate significantly enhanced stability in a microaerobic atmosphere, with 78% maintained initial efficiency over 2800 h of storage under N2 containing 50–100 ppm of O2.