Solution Processed ZnO-based Electron Injection Layer for Stable OLEDs

With the introduction of zinc oxide nanoparticles as the most preferred electron transport layer (ETL), solution-based light-emitting diodes (LEDs) technology was significantly improved because of high transparency, high electron mobility, and low production cost in ZnO and a favorable alignment of its energy bands. Using ZnO as the ETL, we have developed highly efficient and stable Pd(II) 7-(3-(pyridine-2-yl-κN)phenoxy-κC)(benzo-κC)([c]benzo[4,5]imidazo-κN)[1,2-a][1,5]naphthyridine (named Pd3O8-Py5) inverted inorganic-organic hybrid OLEDs. By applying ZnO in various device architectures, we carefully examined the electrical properties and explored how to optimize device performance. According to our studies, a Pd3O8-Py5-based OLED achieved a peak EQE of 23.9%, retained high EQEs of 23.5%, and 18.7% at 1,000 and 10, 000 cd m^-2. As a result of optical outcoupling enhancement, its peak EQE reached 48.6%. Furthermore, the device demonstrated outstanding operational stability with a measured LT₉₀ of 207 h at 10,435 cd m^-2, which corresponds to an estimated LT₉₀ of 11,180 h at 1,000 cd m⁻². It is expected that such hybrid inorganic-organic OLEDs will emerge as a potential competitor to conventional OLEDs, since they have intrinsically stable electrodes and can be partially solution-processed, resulting in low-cost, large-area, and shadow mark-free OLEDs for future display and lighting applications.