Bandgap-tunable Zn_xCd_1-xS electron transport layers for high-performance inverted quantum dot light-emitting diodes

Metal oxide nanoparticles (NPs), such as ZnO, SnO₂, and TiO₂ NPs, are widely used as electron transport layers (ETLs) in quantum dot light-emitting diodes (QLEDs). In comparison, the application of metal sulfides in QLED ETLs remains relatively unexplored. Herein, we report a bandgap-tunable Zn_xCd_1-xS alloyed thin film as a promising ETL material for QLEDs. Owing to its suitable electron mobility and high conductivity, the Zn_xCd_1-xS ETL enables highly efficient charge transport. Furthermore, the bandgap of the Zn_xCd_1-xS ETL can be continuously tuned from 2.48 eV to 3.83 eV by adjusting the Zn/Cd ratio, thereby allowing favorable energy-level alignment with adjacent functional layers. As a result, the red inverted QLED device fabricated with a Zn_0.1Cd_0.9S ETL achieves a maximum external quantum efficiency (EQE) of 16.0%, a peak current efficiency of 22.1 cd A^-1, and a maximum luminance of 103 910 cd m^-2. These findings demonstrate that Zn_xCd_1-xS is a highly promising ETL candidate for high-performance QLEDs and may open a new pathway for designing efficient QLEDs based on metal-sulfide ETLs.