Reactivity Manipulation of Ionic Liquid Based on Alkyl Primary Ammonium: Protonation Control Using Pyridine Additive for Effective Spontaneous Passivation of Perovskite via Hole Transport Material Deposition

Alkyl-primary-ammonium-based room-temperature ionic liquids (RTILs) designed to exhibit specific reactivities allowing the functions that cannot be achieved by the current major RTILs (e.g., pyridine-based RTILs) have recently emerged. The archetype of the reactive RTILs is n-octylammonium bis(trifluoromethanesulfonyl)imide (OA-TFSI), which has promising functions as an additive for the hole transport material (HTM) in perovskite solar cells (PCSs); the high reactivity of the OA cations on the perovskite surface allows effective spontaneous perovskite passivation via HTM deposition, significantly improving the PV performance of the PSC. However, although the reactivity manipulation of the reactive RTILs is instrumental for exploiting their potential functions and exploring their application scope, methods for reactivity control have not been developed. In this study, we propose and demonstrate that the co-addition of a pyridine moiety can effectively manipulate the reactivity of OA-TFSI by controlling the protonation between OA and the 2,2',7,7'-tetrakis-(N,N-di-4-methoxyphenylamino)-9,9'-spirobifluorene (Spiro-OMeTAD) HTM. The pyridine prevented OA deprotonation presumably via stabilization of the OA cation, thus retaining its ammonium form, which allowed effective spontaneous perovskite passivation. Although the proton being with OA owing to pyridine addition is disadvantageous for Spiro-OMeTAD radical formation via its protonation, which is crucial when conventional RTILs are used, a supportive function of the spontaneous perovskite passivation (i.e., the absence of cationic species in the HTM core) likely facilitated Spiro-OMeTAD radical formation, mitigating the requirement of Spiro-OMeTAD protonation. Therefore, overall, optimal pyridine addition significantly enhanced the PV performance, revealing the preference of protonation in the OA-TFSI system used in this study, which is opposite to that in conventional RTILs and represents the specificity of the reactive RTILs. This study provides valuable guidance for developing spontaneous perovskite passivation techniques, which can lead to further advancement of PCSs. Furthermore, this first proposal of a means in manipulating reactivity of the reactive RTILs will develop the nascent RTILs and contribute to further development of material science.