p‐Type CuI Islands on TiO₂ Electron Transport Layer for a Highly Efficient Planar‐Perovskite Solar Cell with Negligible Hysteresis

Compact TiO 2 is widely used as an electron transport material in planar‐perovskite solar cells. However, TiO 2 ‐based planar‐perovskite solar cells exhibit low efficiencies due to intrinsic problems such as the unsuitable conduction band energy and low electron extraction ability of TiO 2 . Herein, the planar TiO 2 electron transport layer (ETL) of perovskite solar cells is modified with ionic salt CuI via a simple one‐step spin‐coating process. The p‐type nature of the CuI islands on the TiO 2 surface leads to modification of the TiO 2 band alignment, resulting in barrier‐free contacts and increased open‐circuit voltage. It is found that the polarity of the CuI‐modified TiO 2 surface can pull electrons to the interface between the perovskite and the TiO 2 , which improves electron extraction and reduces nonradiative recombination. The CuI solution concentration is varied to control the electron extraction of the modified TiO 2 ETL, and the optimized device shows a high efficiency of 19.0%. In addition, the optimized device shows negligible hysteresis, which is believed to be due to the removal of trap sites and effective electron extraction by CuI‐modified TiO 2 . These results demonstrate the hitherto unknown effect of p‐type ionic salts on electron transport material.