Origins of Efficient Perovskite Solar Cells with Low-Temperature Processed SnO₂ Electron Transport Layer

Recently, SnO2 has been noticed as a promising material for electron-transport layer of planar perovskite solar cells. SnO2 layer presents advantages of low-temperature processability and high power-conversion efficiency, and understanding the correlations between the SnO2 properties and device performance will provide a key to realize more efficient perovskite solar cells. Herein, uniform electron-transport layer using SnO2 nanoparticles is fabricated, and the effect of annealing on the solar-cell performance is discussed. Solar cells with low-temperature processed SnO2-nanoparticle layer (below 120 °C or even at room temperature) exhibit desirable short-circuit current, open-circuit voltage, and fill factor with the highest efficiency of 19.0%. Using atomic force microscopy and ultraviolet photoelectron spectroscopy, both great surface uniformity and favorable band alignment of low-temperature processed SnO2 layer have been observed, which are responsible for the device performance. Furthermore, deep electronic-trap states at the SnO2/perovskite interface are investigated via impedance analysis. Compared to the cells processed over 160 °C, low-temperature processed cells exhibit trap states shifting toward the bandedge and reduced trap density, verifying that controlling the interfacial trap states holds a dominance on the open-circuit voltage and is a critical requisite to enable efficient perovskite solar cells. These less-defective solar cells fabricated below 120 °C show high thermal stability, suggesting further commercial applications.