Spontaneous Perovskite Passivators Effectively Combined with PTAA Hole Transport Materials in Perovskite Solar Cells

Perovskite solar cells (PSCs) utilizing poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine (PTAA) as hole-transport materials (HTMs) in n-i-p structures have promising thermal stability compared with those employing Spiro-OMeTAD—the most widely used HTM. However, PTAA-based PSCs often exhibit lower efficiencies than their Spiro-OMeTAD counterparts, presumably due to the absence of perovskite passivation techniques suitably employed to PTAA HTMs. In this study, phenylalkylammonium bis(trifluoromethylsulfonyl)imide (PRA-TFSI) was developed as a spontaneous perovskite passivator specifically tailored for PTAA HTMs. The influence of alkyl chain lengths in PRA cations was systematically explored using newly synthesized PRA-TFSI with carbon chain lengths ranging from 1 to 4: benzylammonium-TFSI (Bn-TFSI: C1), phenylethylammonium-TFSI (PEA-TFSI: C2), phenylpropylammonium-TFSI (PPA-TFSI: C3), and phenylbutylammonium-TFSI (PBA-TFSI: C4). During the deposition of PTAA solutions containing PRA-TFSIs on perovskite photoabsorbers, spontaneous perovskite passivation formed monolayer-like overlayers, offering thermal stability at 85 ℃. This approach contrasts with conventional iodide salt-based passivation techniques, which mostly generate two-dimensional perovskites (e.g., (PEA)2PbI4) under thermal stress, potentially degrading photovoltaic performance. Notably, longer alkyl chains in PRA cations more effectively suppressed defects on perovskite surfaces owing to their accessibility to the physically deeper defects, which is a unique trend exhibited by PRA-TFSIs. Additionally, the resulting passivation layer enhanced the interfacial affinity between PTAA and the perovskite layer, likely owing to π-π interactions facilitated by the phenyl moiety in PRA cations, except for PBA-TFSI, because its long alkyl chain weakened the affinity. Among the PRA-TFSIs, PPA-TFSI (C3) emerged as the optimal passivator, achieving a power-conversion efficiency of up to 23.2%, which is high among PTAA-based PSCs with n-i-p structures—particularly among those that are Li-free, not subjected to post-passivation, and possesses substantial PTAA attachment on perovskite layers —with retention of the prominent thermal stability at 85 ℃. This study advances monolayer-like perovskite passivation technics, allowed by the spontaneous passivators, and highlights the potential of PTAA-based PSCs.