Lead-Free Copper Halide LEDs: Leapfrogging in Performance with Device Engineering Employing Coevaporation of Precursors with Nanoscale Process Control

Lead-free copper halide light-emitting diodes (LEDs) have emerged as a promising alternative to perovskite LEDs, particularly in the context of environmental challenges. This study investigates the performance enhancement of cesium copper iodide (CsCu₂I₃) LEDs through device engineering techniques, including precursor coevaporation, cohost engineering, and process optimization. Coevaporation of cesium iodide (CsI) and copper iodide (CuI) offers better control over film composition compared with conventional techniques such as wet chemical synthesis or solution processing, thereby simplifying the device fabrication. This dry deposition method minimizes issues related to solvent residues and simplifies the fabrication process. Incorporating the cohosts, 1,3,5-tri(m-pyridin-3-ylphenyl)benzene (TmPyPB) and 4,4’,4-tris(carbazol-9-yl)triphenylamine (TcTa), in the emissive layer improves charge balancing and film formation, enhancing overall performance. The optimal results were achieved with a 6:1 cohost ratio and a 2.5% CsCu₂I₃ doping ratio, resulting in a maximum luminance of 6278 cd/m², a current efficiency (CE) of 4.14 cd/A, a power efficiency (PE) of 1.22 lm/W, and an external quantum efficiency (EQE) of 1.44%. The substrate temperature of 60 °C further influenced device performance, with almost 50% improvement in EQE, reaching 2.14%. The device improvements are a result of the nanoscale control over film morphology, composition, and interface quality enabled using controlled coevaporation. Overall, this study highlights the potential of coevaporation of precursors with cohosts and the benefits of substrate temperature in fabricating high-performance and stable CsCu₂I₃-based LEDs.