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 are promising owing to their thermal stability. However, they suffer from the absence of perovskite passivation techniques suitably combined with PTAA HTMs. Herein, phenylalkylammonium bis(trifluoromethylsulfonyl)imides (PRA-TFSIs) are presented as spontaneous perovskite passivators specifically tailored for PTAA HTMs. The influence of the alkyl chain length with 1-4 carbons (C = 1-4) of PRA cations is systematically explored using the newly synthesized PRA-TFSIs. PTAA solutions containing PRA-TFSIs deposited on perovskite photoabsorbers spontaneously form monolayer-like passivation overlayers comprising PRAs, avoiding the formation of crystalline overlayers that hamper carrier transfer and providing thermal stability at 85 °C. In addition to suppression of the defects over the perovskite surfaces, the resulting passivation layers enhance the interfacial affinity between PTAA and the perovskite layer, likely owing to π-π interactions induced by the phenyl moiety in PRA cations, except for phenylbutylammonium-TFSI, because its long alkyl chain weakens the affinity. Among the PRA-TFSIs, phenylpropylammonium-TFSI (C = 3) is the optimal passivator, with a power conversion efficiency as high as 23.2%. This value is highest among PTAA-based PSCs with n-i-p structures that are Li-free, not subjected to post-passivation treatments, and with PTAA substantially attached to the perovskite layers.