In Situ Ligand-Exchange in Solid Quantum Dots Film Enables Stacked White Light-Emitting Diodes

Solution-processed white quantum dot light-emitting diodes (WQLEDs) hold great promise for lighting and backlight applications. Stacked blue/green/red quantum dots (QDs) films as an emitting layer through the layer-by-layer deposition offer a simple way to realize WQLEDs. However, the redissolution issue rising from the deposition of the adjacent QDs layers prevents the fabrication of high-quality multilayer emissive layers. Here, a ligand exchange strategy is developed to improve the solvent-resistance of the QDs films by exposing the QDs films in a trace acid environment. The dissociated hydrions from the acid protonate the aliphatic ligands and desorb them away from QDs' surface, leaving space for inorganic ions to anchor onto the QDs' surface. Importantly, the trace acid does not destroy the morphological properties of QDs films and reserves their initial photoluminescence efficiency. In addition, the charge transporting ability of the QDs is enhanced on account of the replaced inorganic ligands. As result, the stacked WQLEDs display pure white emission with Commission International de I'Eclairage coordinates of (0.34, 0.33), and impressive external quantum efficiency of 9.1% has been demonstrated. This solid-phase ligands exchange strategy provides an alternative way to engineer the surface of QDs for efficient optoelectronics.