Bipolar-shell resurfacing for blue LEDs based on strongly confined perovskite quantum dots

Colloidal quantum dot (QD) solids are emerging semiconductors that have been actively explored in fundamental studies of charge transport¹ and for applications in optoelectronics². Forming high-quality QD solids-necessary for device fabrication-requires substitution of the long organic ligands used for synthesis with short ligands that provide increased QD coupling and improved charge transport³. However, in perovskite QDs, the polar solvents used to carry out the ligand exchange decompose the highly ionic perovskites⁴. Here we report perovskite QD resurfacing to achieve a bipolar shell consisting of an inner anion shell, and an outer shell comprised of cations and polar solvent molecules. The outer shell is electrostatically adsorbed to the negatively charged inner shell. This approach produces strongly confined perovskite QD solids that feature improved carrier mobility (≥0.01 cm² V^-1 s^-1) and reduced trap density relative to previously reported low-dimensional perovskites. Blue-emitting QD films exhibit photoluminescence quantum yields exceeding 90%. By exploiting the improved mobility, we have been able to fabricate CsPbBr₃ QD-based efficient blue and green light-emitting diodes. Blue devices with reduced trap density have an external quantum efficiency of 12.3%; the green devices achieve an external quantum efficiency of 22%.