Exploiting an Interfacial Reaction in Perovskite Solar Cells: Quaternary Alkylammonium Ionic Salt Dopants for Spiro-OMeTAD

The ionic salt dopants devised to replace lithium bis(trifluoromethane)sulfonimide (Li-TFSI) used to dope widely used spiro-OMeTAD hole transporting material in perovskite solar cells (PSCs) has been reported to effectively suppress undesired side effects caused by the Li-TFSI. Nevertheless, roles of cationic molecules and its potential interactions with underlying perovskite film surface is largely unexplored. Here, it is unraveled that ionic salts introduced into the spiro-OMeTAD layer can chemically interact with underlying perovskite to induce an interfacial reaction. Inspiring from such interaction, a strategy is proposed to benefit from the interfacial reaction between the cation used for the dopant and underlying perovskite film surface to passivate surface defects. A simple quaternary alkylammonium salt, tetramethylammonium bis(trifluoromethanesulfonyl)imide (TMA-TFSI) is strategically adopted to demonstrate proof-of-concept devices. The TMA-TFSI not just functioned as an effective dopant for oxidizing spiro-OMeTAD with enhanced environmental stability, but also formed a 1D passivation layer through an interfacial reaction with the underlying perovskite film surface. Consequently, operational and environmental stability of the PSC are significantly enhanced with improved power output. This work should provide an important insight into the design of effective ionic salt dopants for PSCs to simultaneously improve performance and operational stability.