Effective Passivation of Perovskite Solar Cells Involving a Unique Secondary Ammonium Halide Modulator

Aromatic ammonium salts have been regarded as the promising passivators in perovskite solar cell (PSC) fabrications. However, the complicated passivation procedure and inevitable formation of undesirable low‐dimensional (LD) perovskite layers limit further development. Furthermore, how the steric and electronic properties of different ammonium cations would influence the passivation is not well understood. Herein, two carefully engineered passivators based on the unique benzothiophene moiety involving the primary and secondary ammonium terminals, BTMA‐1 and BTMA‐2, respectively, are developed. It is shown that defects and, thus, nonradiative recombination reactions are effectively suppressed by simple posttreatments without the formation of LD perovskite. Interestingly, the champion efficiency of the BTMA‐2‐treated device increases to 23.10% from ≈20%, along with great stabilities and negligible hysteresis. An in‐depth understanding of the passivation effect influenced by steric and electronic properties is explored. The extra electron‐donating methyl on the ammonium nitrogen (BTMA‐2) increases the electron density on the N atom and the N–H+ ionic bond is, thus, boosted, which helps the positive terminal to anchor more tightly to the [PbI6]4− structure of the perovskite resulting in improved passivation effects. This novel and promising design strategy for ammonium passivators can promote PSCs to achieve further breakthroughs in both efficiency and stabilities. Two judiciously engineered passivators for use in perovskite solar cells based on the unique benzothiophene moiety involving the primary and secondary ammonium terminals, BTMA‐1 and BTMA‐2, respectively, are developed for the first time. Interestingly, the champion efficiency of the BTMA‐2‐treated device without the formation of low‐dimensional perovskite increases to 23.10% from ≈20%, along with great stabilities and negligible hysteresis.