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

The intermediate phase adduct plays a crucial role inconstructinguniform and compact tin perovskite films, thus providing an importantapproach for developing high-performance lead-free perovskite solarcells. However, the common intermediate phase adduct of SnI2 & BULL;3DMSO in tin perovskite leads to phase separation and may lackcompatibility 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-qualityFA(0.75)MA(0.25)SnI(3) films by introducinga new stable intermediate phase adduct (SnI2 & BULL;DMSO & BULL;MAFa)by using ionic liquid methylamine formate (MAFa). The resulting stableadduct suppresses the reaction rate between ammonium salts and SnI2, thereby modulating the tin perovskite crystallization andprecluding SnI2 clusters formation, and the presence ofthe SnI2 & BULL;DMSO & BULL;MAFa adduct in perovskite precursorserves as a protective barrier for Sn2+ ions, guardingthem against oxidation caused by the presence of DMSO. Moreover, theamino and carbonyl groups in residual MAFa could repair the iodinevacancy and uncoordinated Sn2+ ion defects. These featuresresult in the formation of highly uniform and pinhole-free FA(0.75)MA(0.25)SnI(3) films. The optimized devicesachieve a power conversion efficiency (PCE) of over 10%, a value of53% higher than that of the control device (6.6%). Besides, the obtainedMAFa-derived devices illustrate significantly enhanced stability ina microaerobic atmosphere, with 78% maintained initial efficiencyover 2800 h of storage under N-2 containing 50-100ppm of O-2.

Automated summary

A new stable intermediate phase adduct (SnI2•DMSO•MAFa) is introduced to fabricate high-quality FA(0.75)MA(0.25)SnI(3) films, resulting in higher power conversion efficiency and improved stability of tin perovskite solar cells.