Alleviating Electron Over-Injection for Efficient Cadmium-Free Quantum Dot Light-Emitting Diodes toward Deep-Blue Emission

Developing high-quality and cadmium-free blue quantum dots (QDs) and their corresponding efficient light-emitting diodes (LEDs) is essential for facilitating their industrialization. ZnSe-based QDs as the prospective blue alternative material for cadmium-based QDs have attracted great attention. However, realizing efficient blue-emitting, especially deep-blue-emitting, devices is seriously limited by the deep valence band and excessive defect states in the wide bandgap QDs. Although the common electron transport layer, that is, ZnO nanoparticles (NPs) can provide effective electron injection, the large hole injection barrier usually causes unbalanced charge injection. Here, we report deep-blue cadmium-free QLEDs at 443 nm with improved efficiency and operational lifetime employing ZnO with Sn doping for mitigating electron over-injection. Theoretical and experimental results reveal that Sn doping causes an upshifted ZnO conduction band and reduces its electron mobility and defect sites. Thus, the electron over-injection in devices is inhibited to achieve charge balance, and the exciton quenching in QDs is reduced to improve radiation recombination. Resultantly, the external quantum efficiency of devices is improved to 13.6 from 5.1%, and the device lifetime (T50@100 cd m–2) is enhanced 21-fold, reaching 305 h, representing the best among ZnSe-based QLEDs so far. These results offer an effective pathway for deep-blue QLEDs toward commercialization.