A Bifunctional Organic Salt as p-Type Dopants of Spiro-OMeTAD for Perovskite Solar Cells with Enhanced Stability

2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenyl-amine)9,9′-spirobifluorene (spiro-OMeTAD) has been a commonly employed in hole transport layer (HTL) for normal-structured perovskite solar cells (PSCs). Lithium bis(trifluoromethane)sulfonimide (Li-TFSI) is introduced into the spiro-OMeTAD as a <i>p</i>-type dopant to improve conductivity and band alignment to facilitate hole extraction. However, the lithium salt causes undesired side effects such as enhanced hygroscopicity, migration and accumulation of Li cations, and formation of by-products, which can lead to degradation of PSC performance and stability. Herein, we propose an alternative organic salt dopant, tetramethylammonium bis(trifluoro-methane)sulfonimide (TMA-TFSI). The TMA-TFSI reacts with the top surface of formamidinium lead iodide perovskite, forming a tetramethylammonium lead iodide film acting as a surface passivation layer of underlying perovskite thin film, contributing to an enhancement of open-circuit voltage of the devices. Furthermore, TMA-TFSI enhances resistance of the HTL against harsh environmental conditions, such as humidity and high temperature, owing to suppressed ion migration and hydrophobic nature. Resultingly, the TMA-TFSI-based devices demonstrate superior stability under high temperature (~85 degrees Celsius) and maximum-power-point tracking compared to those based on Li-TFSI.