An Architype-Cation-Based Room Temperature Ionic Liquid: Aliphatic-Primary-Ammonium Bis(Trifluoromethylsulfonyl)Imide as An Additive for Hole Transport Layers in Perovskite Solar Cells Allowing Spontaneous Passivation of The Perovskite Layer

While the architype room temperature ionic liquid (RTIL) based on aliphatic-primary-ammonium was discovered in 1914, it has been undeveloped until 1990s. In this three decades, RTIL composition (i.e., cations and anions) have been developed aiming at electrochemical application and green chemistry. Accordingly, bulky quaternary-ammonium, pyridine, and imidazole have nowadays been the major cations of RTILs. Currently, novel applications using the matured series of RTILs are being highly explored, and one of the promising targets is an additive for hole transport material (HTM) in perovskite solar cells (PSCs). However, although design of RTIL should be modified along each application, RTILs for the PSCs application so far has remained conventional, which limits the function. Herein, a novel RTIL comprising the architype aliphatic-primary-ammonium (i.e., n-octyl-ammonium: OA) cation and a modern Bis(Trifluoromethylsulfonyl)Imide (TFSI) anion, which are designed as an additive for hole transport layer in PSCs, was successfully synthesized for the first time. The OA cation spontaneously and densely passivated perovskite layer taking advantage of the accessibility to its cationic moiety, and simultaneously TFSI anion most likely effectively stabilized cationic radical of HTM owing to the absence of the counter cation in HTM core regime, leading to effective hole correction and thereby power conversion efficiency of 22.9% in the best cell. This work sheds light on importance of RTIL design along each application, even out of the current trend, and will help further development of various material science fields including PSCs.