Solution-processed optoelectronic and electronic devices are attractive owing to the potential for low-cost fabrication of large-area devices and the compatibility with lightweight, flexible plastic substrates. Solution-processed light-emitting diodes (LEDs) using conjugated polymers or quantum dots as emitters have attracted great interest over the past two decades1, 2. However, the overall performance of solution-processed LEDs2, 3, 4, 5—including their efficiency, efficiency roll-off at high current densities, turn-on voltage and lifetime under operational conditions—remains inferior to that of the best vacuum-deposited organic LEDs6, 7, 8. Here we report a solution-processed, multilayer quantum-dot-based LED with excellent performance and reproducibility. It exhibits colour-saturated deep-red emission, sub-bandgap turn-on at 1.7 volts, high external quantum efficiencies of up to 20.5 per cent, low efficiency roll-off (up to 15.1 per cent of the external quantum efficiency at 100 mA cm−2), and a long operational lifetime of more than 100,000 hours at 100 cd m−2, making this device the best-performing solution-processed red LED so far, comparable to state-of-the-art vacuum-deposited organic LEDs2, 3, 4, 5, 6, 7, 8. This optoelectronic performance is achieved by inserting an insulating layer between the quantum dot layer and the oxide electron-transport layer to optimize charge balance in the device and preserve the superior emissive properties of the quantum dots. We anticipate that our results will be a starting point for further research, leading to high-performance, all-solution-processed quantum-dot-based LEDs ideal for next-generation display and solid-state lighting technologies.