Investigation of Interactions between Carbazole-Based Quaternary Ammonium Salts and Lead-Based Perovskites

Over a relatively short period of time, the power conversion efficiency of perovskite solar cells (PSCs) has increased from 3.8 to 27%. Despite this rapid progress, stability issues still remain a major challenge for widespread commercial application, thus motivating extensive research on perovskite surface defect passivation that would improve the longevity of PSCs. Although many reported passivation strategies are demonstrated under specific deposition conditions and device architectures, the reasons behind their effectiveness are not fully understood. In this study, we systematically examine quaternary ammonium and thiouronium organic salts as surface-passivating agents for triple-cation PSCs and compare their behavior with that of the commercially available sulfonium-based salt DMPESI. By combining photoluminescence (PL), time-resolved photoluminescence (trPL), X-ray photoelectron spectroscopy (XPS), liquid-state nuclear magnetic resonance (NMR), and device measurements, we show that different characterization techniques probe different features of organic–perovskite interactions. Although some treatments improve radiative recombination, as indicated by PL and trPL data, these results do not directly relate to the photovoltaic performance. Only the iodide-containing ammonium salt CzEAI shows device improvement consistent with XPS analysis. This work reveals the complex nature of defect passivation in PSCs.