Degradation of Methylammonium Lead Iodide Perovskite Structures through Light and Electron Beam Driven Ion Migration

Abstract

Organometal halide perovskites show promising features for cost-effective application in photovoltaics. The material instability remains a major obstacle to broad application because of the poorly understood degradation pathways. Here, we apply simultaneous luminescence and electron microscopy on perovskites for the first time, allowing us to monitor in situ morphology evolution and optical properties upon perovskite degradation. Interestingly, morphology, photoluminescence (PL), and cathodoluminescence of perovskite samples evolve differently upon degradation driven by electron beam (e-beam) or by light. A transversal electric current generated by a scanning electron beam leads to dramatic changes in PL and tunes the energy band gaps continuously alongside film thinning. In contrast, light-induced degradation results in material decomposition to scattered particles and shows little PL spectral shifts. The differences in degradation can be ascribed to different electric currents that drive ion migration. Moreover, solution-processed perovskite cuboids show heterogeneity in stability which is likely related to crystallinity and morphology. Our results reveal the essential role of ion migration in perovskite degradation and provide potential avenues to rationally enhance the stability of perovskite materials by reducing ion migration while improving morphology and crystallinity. It is worth noting that even moderate e-beam currents (86 pA) and acceleration voltages (10 kV) readily induce significant perovskite degradation and alter their optical properties. Therefore, attention has to be paid while characterizing such materials using scanning electron microscopy or transmission electron microscopy techniques.

Figure 1

Scanning e-beam altering the perovskite PL. (a–c) PL images taken at different stages of the intensity changes (0, 22, and 220 s). The excitation power density was 14 W/cm². The scanning e-beam current was 86 pA. (d) PL time traces of regions with (red) and without (blue) the scanning e-beam. The PL intensity trace under e-beam scanning (red) is averaged on a randomly selected region highlighted with a solid white square in panel a. The inset shows both traces in the first 10 s. (e, f) PL intensity time trace and the corresponding spectral evolution under laser illumination (30 W/cm²) and e-beam scanning (86 pA). (g) Emission spectra (normalized) at different time points, which are highlighted with corresponding colors in panel f.

Figure 2

Structural changes induced by e-beam treatment and the corresponding changes in PL. (a) SEM micrograph with the minimum electron beam dose acquired on the original perovskite film. (b) SEM micrograph acquired on the identical area after drawing the “T” pattern highlighted with solid lines (guide to the eye). (c) White-light transmission image acquired on the same area after the e-beam treatment. (d) AFM image measured on the same area after the e-beam treatment. (e) Height profiles along the three white solid lines in panel d. (f) SEM micrograph acquired before laser illumination. The left half of the structure, highlighted with a cyan square, is subjected to e-beam scanning (86 pA, 10 kV) for 5 min. (g) PL image acquired after the 500 W/cm² laser illumination is applied for 12 s. (h) SEM micrograph on the same area in panels f and g after laser illumination.

Figure 3

Structural and PL evolution of two cuboid clusters upon degradation. (a) SEM micrograph of the two clusters before the optical measurement. The large cluster is outlined with red solid lines. The small cluster is highlighted with a black circle. (b) PL image on the identical area at the beginning of the optical measurement. The excitation power density was 140 W/cm². (c) PL time traces of the large cluster (in red color) and the small cluster (in black color). (d–j) Overlapped SEM and PL images on the identical area at 10, 20, 100, 140, 220, 300, and 380 s, respectively. (k) SEM micrograph on the small cluster before the optical measurement. The orange solid line outlines the shape of the cluster. (l) The SEM image on the small cluster when its PL intensity reaches its maximum at 380 s. Compared with the original shape outlined with the orange solid lines, the dimensions of the cluster shrank almost evenly at each direction for about 40 nm.