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

Compact TiO2 is widely used as an electron transport material in planar-perov-skite solar cells. However, TiO2-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 TiO2. Herein, the planar TiO2 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 TiO2 surface leads to modification of the TiO2 band alignment, resulting in barrier-free contacts and increased open-circuit voltage. It is found that the polarity of the CuI-modified TiO2 surface can pull electrons to the interface between the perovskite and the TiO2, which improves electron extraction and reduces nonradiative recombination. The CuI solution concentration is varied to control the electron extraction of the modified TiO2 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 TiO2. These results demonstrate the hitherto unknown effect of p-type ionic salts on electron transport material.