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

Regulation 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 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 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 2Sn(2+)-> Sn4+ + Sn-0, which results in a substantial enhancement of carrier lifetime and consequently photovoltaic performance.

Automated summary

The regulation of Lewis acid-base adduct intermediate plays a critical role in the preparation of high quality perovskite films with dual metal ions of Sn2+ and Pb2+. The interaction between metal ions and Lewis base additives, such as urea and thiourea, significantly affects the photovoltaic performance of Sn-Pb alloyed perovskite solar cells.