Hard and Soft Acid and Base (HSAB) Engineering for Efficient and Stable Sn‐Pb Perovskite Solar Cells

Abstract Regulation of Lewis acid‐base adduct intermediate is more critical for the dual metal ions of Sn 2+ and Pb 2+ than for the single metal ion such as Pb 2+ in preparing high quality perovskite films. It has been reported here that the photovoltaic performance of Sn‐Pb alloyed perovskite solar cells is dependent on the interaction between metal ions and Lewis base additives. Urea and thiourea are selected as an O‐ and a S‐donor, respectively, which is used as an additive in the precursor solution including equimolar SnI 2 and PbI 2 together with organic iodides of formamidinium iodide and methylammonium iodide, forming a nominal composition of FA 0.5 MA 0.5 Pb 0.5 Sn 0.5 I 3 . Open‐circuit voltage ( V oc ) is increased while maintaining short‐circuit photocurrent density ( J sc ) after the addition of urea. On the other hand, both J sc and V oc are simultaneously increased by adding thiourea, leading to a considerable increase in power conversion efficiency from 14.58% (control) to 18.59%. A strong interaction between the relatively soft Sn 2+ , compared to Sn 4+ , and the soft sulfur in thiourea, associated with hard and soft acid and base theory, suppresses effectively a disproportionation reaction of 2Sn 2+ → Sn 4+ + Sn 0 , which results in a substantial enhancement of carrier lifetime and consequently photovoltaic performance.