Archetype-Cation-Based Room-Temperature Ionic Liquid: Aliphatic Primary Ammonium Bis(trifluoromethylsulfonyl)imide as a Highly Functional Additive for a Hole Transport Material in Perovskite Solar Cells

Room-temperature ionic liquids (RTILs) have attracted significant attention owing to their unique nature and a variety of potential applications. The archetypal RTIL comprising an aliphatic primary ammonium was discovered over a century ago, but this cation is seldom used in modern RTILs because other bulky cations (e.g., quaternary ammonium-, pyridine-, and imidazole-based cations) are prominent in current major applications, such as electrolytes and solvents, which require low and/or reversible reactivities. However, although the design of materials should change according to the intended application, RTIL designs remain conventional even when applied in unexplored fields, limiting their functions. Herein, RTIL consisting of an archetypal aliphatic primary ammonium (i.e., n-octylammonium: OA) cation and a modern bis(trifluoromethylsulfonyl)imide (TFSI) anion is proposed and demonstrated as a highly functional additive for a 2,2′,7,7′-tetrakis(N,N-di-4-methoxyphenylamino)-9,9′-spirobifluorene (Spiro-OMeTAD), which is the most common hole transport material (HTM), in perovskite solar cells (PSCs). The OA-TFSI additive exhibits prominent functions via permanent reactions of the component ions with the PSC components, thus providing several advantages. The OA cations spontaneously and densely passivate the perovskite layer during the HTM deposition process, leading to both suppression of carrier recombination at the HTM/perovskite interface and hydrophobic perovskite surfaces. Meanwhile, the TFSI anions effectively improve the HTM function most likely via efficient stabilization of the Spiro-OMeTAD radical, enhancing hole collection properties in the PSCs. Consequently, PSC performances involving long-term stability were significantly improved using the OA-TFSI additive. Based on the present results, this study advocates that reconsidering the RTIL design, even when it differs from the current major designs yet is suitable for a target application, can provide functions superior to conventional ones. The insights obtained in this work will spur further study of RTIL designs and aid the development of the broad materials science field including PSCs.