Effect of (CH₃)₂Sn(COOH)₂ Electron Transport Layer Thickness on Device Performance in n-i-p Planar Heterojunction Perovskite Solar Cells

The electron transport layer (ETL) plays an important role as a buffer layer in the efficient n-i-p planar heterojunction perovskite solar cells (PSCs). Recently, a new ETL, (CH₃)₂Sn(COOH)₂ (CSCO), with excellent conductivity and defect passivation of perovskites at the ETL/perovskite interface is synthesized by our group, which leads to a high-performance n-i-p PSC. Nevertheless, the effect of CSCO film thickness on the power conversion efficiency (PCE) of PSCs is unclear. In this work, we systematically explore the influence of CSCO film thickness on photovoltaic performance of Cs₀.₀₅FA₀.₈₁MA₀.₁₄PbI₂.₅₅Br₀.₄₅ (CsFAMA)-based PSCs. When the thickness of the CSCO film is less than 42 nm, the coverage of CSCO on an indium tin oxide (ITO) substrate becomes more complete as the CSCO film thickness increases, which reduces the leakage current and increases the charge-transfer efficiency at the CSCO/CsFAMA interface and therefore improves the PCE of the PSCs. However, for a thicker CSCO film with a thickness >42 nm, the ultraviolet–visible absorption intensity of CSCO increases, which is harmful to light absorption of the perovskite layer and therefore decreases the short-circuit current density of PSCs. Furthermore, a thicker CSCO film (>42 nm) leads to a sharply decreased shunt resistance, lowering the fill factor and open-circuit voltage. As a consequence, the maximum PCE of 20.42% is achieved by the PSCs based on the CSCO film with a thickness of 42 nm. In addition, the optimal PSC without encapsulation shows remarkable stability, retaining more than 85% of its initial conversion efficiency after 3000 h of storage in an ambient environment.