Highly Reproducible and Efficient FASnI₃ Perovskite Solar Cells Fabricated with Volatilizable Reducing Solvent

Lead-free tin halide perovskite solar cells (PSCs) have attracted great attention because of their low toxicity, ideal band gap, and high carrier mobilities. However, the efficiency and reproducibility of tin halide PSCs has been limited because of the facile oxidation of Sn²⁺ to Sn⁴⁺. Herein, liquid formic acid (LFA) was introduced as a reducing solvent in the FASnI₃ (FA: formamidinium) perovskite precursor solution. Unlike solid reducing additives, the LFA solvent is volatile, so no residual LFA remained in the FASnI₃ perovskite film. Use of the LFA solvent resulted in production of the FASnI₃ perovskite film with high crystallinity, low Sn⁴⁺ content, reduced background doping, and low electronic trap density. As a result, an efficiency of over 10% was obtained for lead-free tin halide PSCs with improved reproducibility.