Anion Composite Engineering of Spontaneous Perovskite Passivators for Energy Alignment Modulations in Perovskite Solar Cells

Among perovskite solar cell (PSC) components, bis(trifluoromethanesulfonyl)imide (TFSI)-based additives for hole transport materials (HTMs), enhancing hole-mobility of HTMs via effective stabilization of the cationic HTM radicals, play an important role in achieving efficient PSCs. Recently emerged alkyl-primary-ammonium-TFSIs (RA-TFSIs), used as HTM additives, exhibit prominent functions as spontaneous perovskite passivators: RA cations spontaneously suppress defects over the perovskite surface during HTM deposition (viz., spontaneous perovskite passivation), while TFSI anions, remaining in the HTM bulk, enhance the hole mobility of the HTMs. Thus, RA-TFSI can enhance PSC performance without additional processes for perovskite passivation. In RA-TFSIs, their cation composites have been developed, whereas the anion composites remain unexplored. The most commonly used TFSI is a series of bis(fluorosulfonyl)imides, and the functions of other derivative anions (e.g., bis(fluorosulfonyl)imide (FSI) and bis(pentafluoroethanesulfonyl)imide (PFSI)) are of interest as HTM additives. However, the effectiveness of bis(fluorosulfonyl)imides remains controversial even in conventional Li-based ones. In this study, the functions of n-octylammonium-TFSI (OA-TFSI), the archetypal RA-TFSI, were compared with those of the newly synthesized OA-FSI and OA-PFSI. Additionally, the effects of Li-based bis(fluorosulfonyl)imides were investigated as references. The anion composites play a pivotal role in changing the ionization energies (IEs) of the HTM, thereby effectively modulating the energy alignment in PSCs. The larger C-F moieties in bis(fluorosulfonyl)imides (i.e., from FSI to TFSI and PFSI) led to deeper IEs, presumably owing to the more delocalized electrons in bis(fluorosulfonyl)imides. In particular, the FSI-based additives led to shallow IEs of the HTMs and exhibited some benefits for the photovoltaic performance of PSCs, including their stability, which have rarely been observed using the commonly used TFSI-based additives. This work serves as a guide for nascent yet potent spontaneous perovskite passivators and provides novel insights into HTM additives, which may be a bottleneck of n-i-p structured PSCs, leading to further advancements in PSCs.