Unanticipated Interfacial Redox Reaction in the NiO_x Transport Layer of Perovskite Light-Emitting Diodes

This study reveals an unanticipated interfacial redox reaction between the nickel oxide (NiO_x) hole transport layer and perovskite active layer in organolead halide perovskite light-emitting diodes (PeLEDs). Specifically, metallic nickel (Ni⁰) present in the NiO_x layer undergoes oxidation while lead ions (Pb²⁺) from the perovskite precursor are reduced, forming nickel (II) ions (Ni²⁺) and metallic lead (Pb⁰), as confirmed by X-ray photoelectron spectroscopy (XPS). The formation of Pb⁰, a well-known luminescence quenching center, is correlated with the significantly suppressed photoluminescence (PL) before biasing and the pronounced electroluminescence (EL) overshoot observed at the onset of electrical excitation. Introduction of an electrode interlayer, such as a polyvinyl carbazole (PVK), prevents direct contact between NiO_x and the perovskite, thereby effectively suppressing the detrimental redox reaction between them. This electrode interfacial modification eliminates Pb° formation, mitigates luminescence quenching, and suppresses EL overshoot. Moreover, the trap density at the perovskite interface is substantially reduced, and the deep trap distribution remains stable under bias. The findings offer critical insights into bias-induced luminescence enhancement phenomena and present a reliable, scalable strategy to mitigate these phenomena, contributing to the development of advanced PeLEDs.