Quantum dot light-emitting diodes (QLEDs) are expected to be the basis of next-generation displays and have consequently been extensively investigated with the aim of commercialization. Herein, QLED brightness, efficiency, and lifetime are significantly improved by insertion of an Al₂O₃ barrier layer via atomic layer deposition (ALD), which effectively suppresses the etching reaction with poly(3,4-ethylenedioxythiophene):polystyrenesulfonate and prevents metal ion diffusion from indium tin oxide (ITO) into the emission layer, thereby effectively reducing the effect of exciton quenching. The above-mentioned suppression of exciton quenching is verified using time-resolved photoluminescence spectroscopy/energy-dispersive X-ray spectroscopy, and a device prepared using four ALD cycles is shown to exhibit increased maximal luminance (39 410 cd/m²; two times the value achieved without the Al₂O₃ layer), current efficiency (47.89 cd/A; eight times the value achieved without the Al₂O₃ layer), and external quantum efficiency (12.89%). In addition, all Al₂O₃-containing QLEDs feature longer lifetimes than the QLED without Al₂O₃.