Inverted structural quantum dot light-emitting diodes based on Al-doped ZnO electrode.

As an indium-free transparent conducting film, Al-doped zinc oxide (AZO) was prepared by magnetron sputtering technique, exhibiting good electrical, optical and surface characteristics. UPS/XPS measurements show that AZO and zinc oxide nanoparticles (ZnO NPs) have matched energy level that can facilitate the electron injection from AZO to ZnO NPs. Inverted structural green quantum dot light-emitting diodes based on AZO cathode were fabricated, which exhibits a maximum luminance up to 178 000 cd m-2, and a maximum current efficiency of 10.1 cd A-1. Therewith, combined with the simulated space-charge-limited current (SCLC) theory, the measured current density-voltage characteristics of charge-only devices were analyzed. It demonstrated that AZO and ZnO NPs had much better electron injection efficiency than ITO, showing a electron injection efficiency close to 100%. By studying the relationship between the electric field and the current density, the measured curve of AZO-based devices nearly fits the theoretical curve of SCLC and the AZO electrode has a better ohmic contact with ZnO NPs than ITO.