Identifying the optimum thickness of electron transport layers for highly efficient perovskite planar solar cells

The fabrication of pinhole-free and compact electron transport layers is crucial for achieving high power conversion efficiency of perovskite solar cells. In this work, we report efficient perovskite solar cells using ultrathin TiO2 films (5-20 nm) as high-quality electron transport layers deposited by the atomic layer deposition technique. The as-prepared solar cells on FTO substrates show a high efficiency of 13.6%, employing the optimum 10 nm thick TiO2 layer. Furthermore, the flexible cells on PET substrates exhibit an efficiency of 7.2% with low-temperature-processed TiO2 layers at 80 degrees C. The effects of layer thicknesses on the cell performance are investigated to reveal the mechanism of high-performance, which is mainly attributed to high transmittance, low leakage current, and low charge transfer resistance and recombination rate. Our major findings are expected to provide a guide to design ultrathin compact electron transport layers for efficient perovskite solar cells.