Humidity-Induced Phase Transition Stages in FAPbI₃ Revealed by Ultralow-Dose 4D-STEM

Metal halide perovskites, particularly formamidinium lead triiodide (FAPbI₃), are promising for solar cells due to their thermal stability and near-optimal bandgap. However, a significant challenge with FAPbI₃ films is their tendency to transition from the desirable α-phase to a photoinactive δ-phase at room temperature [1]. This transition is believed to involve several intermediate stages, including hidden polytypes, and identifying them is crucial for improving the stability of the material system [2]. However, tracking these stages at the single-grain level is difficult due to the sensitivity of the materials to electron beams and mechanical tips [3].<br/>To address this challenge, we established ultralow-dose four-dimensional scanning transmission electron microscopy with nano-beam diffraction (4D STEM-NBD) using direct detection electron detectors (DDEC). This approach maximizes the signal-to-noise ratio for tracking phase changes in FAPbI3 at the nanoscale under humidity exposure.<br/>We first determined a critical electron dose of approximately 30-50 e/Å2, below which the electron beam does not induce changes in the diffraction pattern. For the 4D STEM-NBD measurements, we significantly reduced the electron dose to less than 2 e/Å2 per scan, substantially lower than the critical electron dose, while still maintaining identifiable diffraction signals. We confirmed that the pristine diffraction patterns were preserved even after 10 subsequent scans, enabling us to track sequential phase changes through multiple scans of the same area.<br/>Using the established 4D STEM-NBD technique, we investigated the degradation processes of FAPbI₃ under controlled conditions. The sample was exposed to 60% relative humidity outside the microscope and systematically analyzed at the same spot every hour. We observed that the phase transitions proceeded sequentially from 3C to 6H, 4H, and 2H, ultimately leading to amorphization initiating from the grain boundary. The 3C phase refers to the cubic α-FAPbI₃, and the 6H, 4H, and 2H phases are different polymorphs of the δ-FAPbI₃[2]. The two distinct lattice orientation relations were observed in the 3C to 6H transition, while a single coherent orientation was observed in transition in between δ -phases (6H-4H-2H). The transition rate of 3C-6H was significantly faster than that of the 4H-2H transition, implying that the transitions between polytypes of the δ-phase are the rate-determining steps, possibly due to differences in the atomic displacement steps involved. The exact roles of previously suggested approaches to suppress the α-δ transition, including compositional and additive engineering, on each stage remain unclear. Nevertheless, these findings shed light on the hidden mechanisms of the α-δ phase transition and provide insights into developing effective strategies to suppress this transition.<br/>[1] Science 374,1598-1605 (2021)<br/>[2] ACS Energy Lett. 2, 12, 2686–2693 (2017)<br/>[3] Adv. Energy Mater. 10, 1903191 (2020)