Phosphate-Passivated SnO₂ Electron Transport Layer for High-Performance Perovskite Solar Cells

Tin oxide (SnO₂) is widely used in perovskite solar cells (PSCs) as an electron transport layer (ETL) material. However, its high surface trap density has already become a strong factor limiting PSC development. In this work, phosphoric acid is adopted to eliminate the SnO₂ surface dangling bonds to increase electron collection efficiency. The phosphorus mainly exists at the boundaries in the form of chained phosphate groups, bonding with which more than 47.9% of Sn dangling bonds are eliminated. The reduction of surface trap states depresses the electron transport barriers, thus the electron mobility increases about 3 times when the concentration of phosphoric acid is optimized with 7.4 atom % in the SnO₂ precursor. Furthermore, the stability of the perovskite layer deposited on the phosphate-passivated SnO₂ (P-SnO₂) ETL is gradually improved with an increase of the concentration. Due to the higher electron collection efficiency, the P-SnO₂ ETLs can dramatically promote the power conversion efficiency (PCE) of the PSCs. As a result, the champion PSC has a PCE of 21.02%. Therefore, it has been proved that this simple method is efficient to improve the quality of ETL for high-performance PSCs.