Compensational Effects of 4- <i>tert</i> -Butylpyridine Additive for PTAA Hole Transport Material Using a Spontaneous Perovskite Passivator in Perovskite Solar Cells

Development of hole transport materials (HTMs) plays an important role in the advancement of perovskite solar cells (PSCs). Typical HTMs are organic semiconductors; dopants are required to enhance the hole collection properties of PSCs. One typical set of dopants used is lithium bis(trifluoromethylsulfonyl)imide (Li-TFSI) combined with 4-tert-butylpyridine (TBP). The primary role of TBP is to dissolve Li-TFSI, yet the effects of TBP independent of those of Li-TFSI remain elusive, especially when combined with poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA), a promising HTM possessing the advantage of thermal stability. To investigate the effects of TBP, we employed n-octylammonium TFSI (OA-TFSI) room-temperature ionic liquid (RTIL) additive for PTAA with a calibrated TBP concentration. The recently emerged OA-TFSI additive functions as a spontaneous perovskite passivator; during the deposition of the HTM solution containing the OA-TFSI additive, the OA cations spontaneously passivate the perovskite, effectively suppressing defects on the perovskite surface. As OA-TFSI is an RTIL, the necessity of TBP for dissolving OA-TFSI in the HTM solution is potentially negated, unlike Li-TFSI. Indeed, the OA-TFSI without TBP exhibited relatively high-power conversion efficiencies (PCEs) in PTAA-based n–i–p-structured PSCs of up to 22.0%, exploiting the effective spontaneous perovskite passivation. However, TBP addition to the HTM solution resulted in compensational effects on the photovoltaic (PV) performance. TBP addition improved the uniformity of the PTAA HTM layer, contributing to an increased fill factor, and TBP addition presumably hampered spontaneous perovskite passivation and caused energy loss between the perovskite and PTAA, which decreased the open-circuit voltage. Consequently, the PSCs with optimal TBP addition exhibited a PV performance similar to that without TBP addition (e.g., highest PCE of 21.6%). The obtained insights provide valuable guidance regarding the use of additives for PTAA HTMs, which can be a bottleneck for PTAA-based PSC development, thus contributing to the further development of PSCs.