Multi-functional L-histidine self-assembled monolayers on SnO₂ electron transport layer to boost photovoltaic performance of perovskite solar cells

The photovoltaic performance of pemvskite solar cells (PSCs) is sensitive to the interface properties between perovskite and electron transport layer (ETL). An appropriate interface engineering on SnO2 ETL is highly desired for achieving efficient PSCs. Herein, we introduced L-histidine (L-His) self-assembly layer on SnO2 ETL to realize three following functions. The crystal quality and conductivity of SnO2 ETL have been improved since the carboxyl groups in L-His coordinate with the tin ion in SnO2 and fill the oxygen vacancy. The improved wettability caused by L-His deposition induces the formation of pemvskite films with larger grain size and better crystallization quality. The hydrogen bond formed between perovskite and L-His proved by proton nuclear magnetic resonance (H-1 NMR) enhances the interface interaction, and the L-His also modulates the energy level structure of the ETL/perovskite interface, which not only enhances the charge transport properties but also leads to a higher open-circuit voltage. Based on these benign optimizations, the CH3NH3PbI3-based PSCs with the optimized SnO2/L-His ETL have achieved a power conversion efficiency of 21.04%. Such biological friendly modification technique is highly compatibility to the technique of large-scale production of PSCs.