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 SnI 2 ·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 0.75 MA 0.25 SnI 3 films by introducing a new stable intermediate phase adduct (SnI 2 ·DMSO·MAFa) by using ionic liquid methylamine formate (MAFa). The resulting stable adduct suppresses the reaction rate between ammonium salts and SnI 2, thereby modulating the tin perovskite crystallization and precluding SnI 2 clusters formation, and the presence of the SnI 2 ·DMSO·MAFa adduct in perovskite precursor serves as a protective barrier for Sn 2+ 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 Sn 2+ ion defects. These features result in the formation of highly uniform and pinhole-free FA 0.75 MA 0.25 SnI 3 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 N 2 containing 50–100 ppm of O 2 .