Eliminated Interfacial Side Reactions in Perovskite Solar Cells by Sterically Protected Ammonium Passivation

Surface passivation utilization of ammonium salts has emerged as an effective strategy to reduce interfacial defects at the surface and improve device performance. However, ammonium salts are prone to deprotonation under high temperature, which can induce side reactions with organic cations (FA⁺ and MA⁺) at the perovskite surface and further influence the device stability. Herein, we systematically investigate the passivation effects of cyclohexylammonium iodide (CHAI), methylcyclohexylammonium iodide (MCHAI), and N,N-dimethylcyclohexylammonium iodide (DMCHAI) on PSCs. DMCHAI exhibits a significantly suppressed deprotonation tendency due to the sterically hindered ammonium group, effectively inhibiting the reaction with FA⁺ at the interface. As a result, the champion device with DMCHAI modification achieves a PCE of 26.06% with V_oc of 1.20 V and retains over 98% of the initial PCE after 7200 h under N₂. Moreover, the device with DMCHAI modification remains over 86% of its initial efficiency after 550 h of thermal stress at 85 °C. This work highlights the importance of deprotonation-resistant ammonium salts in constructing thermally stable perovskite interfaces and provides a rational design guideline for passivation materials in PSCs.