Highly-Crystalline SnO₂ Thin Films for Efficient Planar Perovskite Solar Cells

Due to the excellent properties of wide band gap, high bulk carrier mobility, and suitable band alignment with a perovskite layer, tin dioxide SnO2 is an outstanding candidate as an electron transport layer (ETL) for perovskite solar cells (PSCs). However, it is still desirable and challenging to deposit crystalline SnO2 thin films by the low-temperature solution process. By adjusting the metal-halide chemical bonding strength of a tin precursor, thus controlling the hydrolysis intermediate species, we demonstrated a simple method for preparing high-quality SnO2 thin films. This method is free of organic surfactants and the formation of tin-alkoxide intermediates. Consequently, SnO2 thin films with high crystallinity and high-carrier-mobility can be obtained under low temperature. This crystalline SnO2 ETL enables efficient charge separation/transport and reduces charge recombination in PSCs. Moreover, the Br-rich perovskite region is established near the SnO2/perovskite interface, resulting in a beneficial gradient band gap profile in the device. PSCs with this SnO2 ETL show a PCE of over 22% with an outstandingly high fill factor of 83.32%.