Highly orientated asymmetrically strained CdZnSeS/ZnSeS/ZnS/CdZnS quantum dots for efficient green light-emitting diodes

Abstract

Quantum dot light-emitting diodes (QLEDs) have significantly progressed in recent years. However, the random isotropic emission of photons within the device influenced by the interfacial effect of multilayer structures results in mediocre photon out-coupling efficiency, posing a challenge for the continued development of QLEDs. This study introduces asymmetrically strained green CdZnSeS/ZnSeS/ZnS/CdZnS quantum dots (QDs) with a substantial size of 16.0 nm, synthesized through secondary nucleation and thick multi-shells growth strategy, aimed at enhancing the directivity of photon emission in QLEDs. Specifically, the irregular growth of the continuous gradient ZnSeS/ZnS/CdZnS shells imposes asymmetric strains on the CdZnSeS cores, promoting the formation of well-aligned QD films with a horizontal dipole orientation factor exceeding 80 %. This orientation is advantageous for enhancing the photon out-coupling efficiency of the devices. As a result, the QD films exhibit stable emission spectra with an impressive near-unity photoluminescence quantum yield (PLQY) of 98.7 %. Based on these findings, highly efficient green QLEDs were fabricated, achieving a maximum external quantum efficiency (EQE_max) of 28.8 %, a luminance over 230,000 cd m⁻², and exceptional operational stability (T₉₅, tested at 1000 cd m⁻²) of 6350 hours. These results underscore the effectiveness of the proposed strategy in realizing high-performance QLEDs.