Interface Contact‐Dominated Thermal Degradation of Mixed‐Halide Blue Perovskite LEDs

ABSTRACT The operational instability of perovskite light‐emitting diodes (PeLEDs), particularly under thermal stress, remains a major obstacle to commercialization. Here, we combine device characterization with in situ photoelectron spectroscopy (PES) to elucidate thermal degradation mechanisms in mixed‐halide blue PeLEDs, revealing a strong dependence on electron transport layers (ETLs). Devices employing 1,3,5‐tris(N‐phenylbenzimidazole‐2‐yl) benzene (TPBi) exhibit rapid failure due to weak physical interfacial contact, which leads to thermal detachment and severe morphological changes. In contrast, devices based on 2,4,6‐tris[3‐(diphenylphosphinyl)phenyl]‐1,3,5‐triazine (POT2T) retain 90% of their initial efficiency at 80°C, owing to strong chemical bonding at the interface and high thermal stability. Importantly, PES reveals universal Cl ion migration from the perovskite into both ETLs as the primary origin of electroluminescence spectral shifts. These findings provide new insights into the dual role of interfacial stability and ion migration, highlighting the critical need to engineer robust chemical interfaces to suppress degradation pathways and achieve stable PeLEDs.