  Quantum dot light-emitting diodes (QLEDs) are promising building blocks for
prospective lighting and display applications. Despite the significant
advancements achieved towards increasing the efficiency and brightness levels
of QLEDs, the unavoidable demand for an inert atmosphere during the fabrication
process restrains their potential for large-scale manufacturing. Here, we
demonstrate ZnCdSe/ZnSe/ZnSeS/ZnS core/multi-shell QDs with ultra-thick shell
(14 monolayers), significantly suppressing the diffusion of moisture and
oxidative species to realize efficient all solution-processed QLED devices
fully fabricated under ambient air conditions. The lattice-mismatch-engineered
shell growth in the synthesized QDs leads to the strong confinement of charge
carriers inside the core and near-unity photoluminescence quantum yield (PLQY)
in the amber-to-red wavelength region (595-625 nm). The best amber-emitting
QLED devices exhibit an external quantum efficiency (EQE) of 14.8%, a current
efficiency (CE) of 41.1 cd/A, and a luminance of 5.6 x 105 cd/m2 at 12 V. To
the best of our knowledge, these are the highest efficiency levels achieved in
air-fabricated QLEDs. Importantly, the demonstrated devices show significant
suppression of non-radiative Auger recombination and optimal charge balance,
leading to negligible EQE roll-off (i.e., droop-free behavior) at the maximum
luminance level. Furthermore, employing a refractive-index-matched optical
compound to enhance light out-coupling increases the EQE to 26.6%, which is
comparable to the best devices fabricated in an inert atmosphere. Collectively,
these advances will greatly facilitate the large-scale industrial fabrication
of air-stable QLEDs.
