Phenyltrimethylammonium‐Alloying Strategy for Efficient and Durable Formamidinium‐Based Perovskite Solar Cells

As for the commonly used formamidinium lead iodide (FAPbI 3 ) perovskite, adding small amount of larger organic cations is a viable strategy to overcome its instability caused by formamidinium (FA) or iodide (I). Herein, FAPbI 3 perovskite is modified by addition of quaternary cation phenyltrimethylammonium (PTMA), which forms 2D perovskite phase. With the presence of PTMA cations and small‐sized Br anions, the local strain of perovskite layer is mitigated. Also, X‐ray analyses reveal that PTMA cation directs FAPbI 3 to have more vertically oriented structure, which aids the enhanced photocarrier generation and extraction. Defect analyses suggest that PTMA bromide alloying also greatly reduces the trap sites. Due to these combined effects, the PTMABr‐modified device results in the improved open‐circuit voltage plus stability under 60 °C or 1 sun illumination, maintaining 80% of its initial efficiency after 1800 and 1400 h, respectively.