Realizing Efficient Lead-Free Formamidinium Tin Triiodide Perovskite Solar Cells via a Sequential Deposition Route

Recently, the evolved intermediate phase based on iodoplumbate anions that mediates perovskite crystallization has been embodied as the Lewis acid-base adduct formed by metal halides (serve as Lewis acid) and polar aprotic solvents (serve as Lewis base). Based on this principle, it is proposed to constitute efficient Lewis acid-base adduct in the SnI2 deposition step to modulate its volume expansion and fast reaction with methylammonium iodide (MAI)/formamidinium iodide (FAI) (FAI is studied hereafter). Herein, trimethylamine (TMA) is employed as the additional Lewis base in the tin halide solution to form SnY2-TMA complexes (Y = I-, F-) in the first-step deposition, followed by intercalating with FAI to convert into FASnI. It is shown that TMA can facilitate homogeneous film formation of a SnI2 (+SnF2) layer by effectively forming intermediate SnY2-TMA complexes. Meanwhile, its relatively larger size and weaker affinity with SnI2 than FA+ ions will facilitate the intramolecular exchange with FA+ ions, thereby enabling the formation of dense and compact FASnI(3) film with large crystalline domain (>1 mu m). As a result, high power conversion efficiencies of 4.34% and 7.09% with decent stability are successfully accomplished in both conventional and inverted perovskite solar cells, respectively.