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

Emerging alkyl-primary-ammonium-bis(trifluoromethanesulfonyl)imides (RA-TFSIs), used as additives for hole-transport materials (HTMs) in perovskite solar cells (PSCs), exhibit prominent functions as spontaneous perovskite passivators; RA cations spontaneously suppress defects over the perovskite surface during HTM deposition, while TFSI anions, remaining in the HTM bulk, enhance the hole mobility of the HTMs. In RA-TFSIs, their cation components have been developed, whereas the anion components remain unexplored. The most commonly used TFSI is a series of bis(fluorosulfonyl)imides (e.g., bis(fluorosulfonyl)imide (FSI) and bis(pentafluoroethanesulfonyl)imide (PFSI)), yet the effectiveness of bis(fluorosulfonyl)imides remains controversial. In this study, the functions of n-octylammonium (OA)-bis(fluorosulfonyl)imides (i.e., OA-FSI, OA-TFSI, and OA-PFSI) are verified. The anion components manipulate ionization energies (IEs) of the HTM, thereby effectively modulating the energy alignment in PSCs. The larger C-F moieties in bis(fluorosulfonyl)imides, from FSI to TFSI and PFSI, led to deeper ionization energies (IEs), presumably owing to their spatially more expanded electron delocalization. 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 discussed. This work provides novel insights into HTM additives, which may be a bottleneck of n-i-p structured PSCs, leading to further advancements in PSCs.