Improving Internal Exciton Confinement for Efficient CdZnSeS-Based Blue Quantum Dot Light-Emitting Diodes

Quantum-dot light-emitting diodes (QLEDs) are thought to be the base for next-generation display technology. However, the performance of blue-emitting QLEDs still falls behind those of green and red ones, which can be attributed to the energy loss from Auger recombination and the strong coupling of excitons with surface states. Blue quantum dots (QDs) with giant CdZnSeS alloy cores are expected to improve the internal confinement of excitons due to the nonmonotonical energy landscape of their conduction band, while they haven't shown high performance in QLEDs due to insufficient optimization of their shell structures. In this work, giant CdZnSeS alloy cores were synthesized by diffusing Zn atoms into CdSeS cores, so that the core/shell lattice stress was released due to the optimized gradient compositions. As a result, exciton transfer and Auger recombination are both suppressed, leading to a breakthrough external quantum efficiency (EQE) of 24% in blue QLEDs with giant CdZnSeS alloy cores. Compared to the more extensively studied blue quantum dots with CdSeZn alloy cores, blue QLEDs with giant CdZnSeS alloy cores also benefit from the suppressed Fermi level and nonmonotonical energy landscape of the conduction band minimum (CBM), which are crucial for confining the wavefunctions of the excitons. The improved exciton confinement explained the superior performances of giant CdZnSeS alloy cores over CdSeZn cores in blue QLEDs.