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

Abstract

AbstractRegulation of Lewis acid‐base adduct intermediate is more critical for the dual metal ions of Sn2+ and Pb2+ than for the single metal ion such as Pb2+ 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 SnI2 and PbI2 together with organic iodides of formamidinium iodide and methylammonium iodide, forming a nominal composition of FA0.5MA0.5Pb0.5Sn0.5I3. Open‐circuit voltage (Voc) is increased while maintaining short‐circuit photocurrent density (Jsc) after the addition of urea. On the other hand, both Jsc and Voc 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 Sn2+, compared to Sn4+, and the soft sulfur in thiourea, associated with hard and soft acid and base theory, suppresses effectively a disproportionation reaction of 2Sn2+→ Sn4+ + Sn0, which results in a substantial enhancement of carrier lifetime and consequently photovoltaic performance.