Metal halide perovskites are promising as the emitter for efficient light-emitting diodes (LEDs). However, the operational stability of perovskite LEDs (Pe-LEDs) is still very low and, therefore, needs to be improved for future practical applications through the clarification of their basic degradation mechanisms. In this study, we investigated the stability of quasi-two-dimensional (q2D) perovskite films under three different conditions, i.e., carrier injection only, photoexcitation only, and photoexcitation in combination with carrier injection. As a result of the aforementioned comparison, we found that excited states (Wannier excitons) formed in q2D perovskite films by photoexcitation are relatively unstable. On the other hand, no degradation of q2D perovskite films was observed when electrons or holes were injected without the formation of excited states. The unstable excited states were associated with the change from the q2D phase to the three-dimensional phase and the formation of metallic lead working as an excited-state quencher in films. Furthermore, the excited-state stability became worse when the excited states and the charge carriers coexisted in films. This would be one of the reasons for the low stability of q2D Pe-LEDs under continuous electrical operation.
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