Doped Bilayer Tin(IV) Oxide Electron Transport Layer for High Open-Circuit Voltage Planar Perovskite Solar Cells with Reduced Hysteresis

Tin oxide (SnO2) is an emerging electron transport layer (ETL) material in halide perovskite solar cells (PSCs). Among current limitations, open-circuit voltage (V-OC) loss is one of the major factors to be addressed for further improvement. Here a bilayer ETL consisting of two SnO2 nanoparticle layers doped with different amounts of ammonium chloride is proposed. As demonstrated by photoelectron spectroscopy and photophysical studies, the main effect of the novel ETL is to modify the energy level alignment at the SnO2/perovskite interface, which leads to decreased carrier recombination, enhanced electron transfer, and reduced voltage loss. Moreover, X-ray diffraction reveals reduced strain in perovskite layers grown on bilayer ETLs with respect to single-layer ETLs, further contributing to a decrease of carrier recombination processes. Finally, the bilayer approach enables the more reproducible preparation of smooth and pinhole-free ETLs as compared to single-step deposition ETLs. PSCs with the doped bilayer SnO2 ETL demonstrate strongly increased V-OC values of up to 1.21 V with a power conversion efficiency of 21.75% while showing negligible hysteresis and enhanced stability. Moreover, the SnO2 bilayer can be processed at low temperature (70 degrees C), and has therefore a high potential for use in tandem devices or flexible PSCs.