In Situ Electron Transport Layers by a Carboxyl Ionic Liquid-Assisted Microwave Technique for a 20.1% Perovskite Solar Cell

A high-quality electron transport layer (ETL) plays an indispensable role in perovskite solar cells (PSCs). An ideal ETL should possess the features of low-temperature fabrication, simple preparation process, high quality, fast electron transport ability, and so forth. However, the traditional preparation methods of the commonly used ETLs always involve a tedious procedure, high temperature, long time, and so forth. In this work, a simple in situ carboxyl ionic liquid (IL)-assisted microwave technique was successfully developed for TiO₂-based hybrid high-quality ETLs of PSCs. The in situ IL-assisted ETLs were processed directly by the Ti-source solution at a low temperature and a short time. The TiO₂-based hybrid ETL finally achieved a high efficiency exceeding 20%, which was greatly improved by 20.8% compared with the pristine device. The systematic mechanism investigations were clarified, such as the positive effects of ILs in the in situ microwave technique, the related properties and photovoltaic performances of the in situ TiO₂, ETLs/IL-TiO₂, and ETLs/IL-TiO₂-based hybrid ETL, the beneficial influences of the hybrid structure, the charge extraction, transfer, recombination, and so forth. These results provide a simple, green synthesis technique and effective strategy for a high-quality ETL and highly efficient PSCs.