Conformally deposited ZnS nanoparticle capping layers for precise emission control in red/green/blue top-emitting quantum dot light-emitting diodes

Colloidal quantum dots are promising emitters for quantum dot light-emitting diodes (QLEDs), driving the development of optical strategies to enhance light outcoupling. In top-emitting architectures, capping layers (CPLs) enable precise microcavity control; however, conventional CPLs are typically vacuum-deposited, increasing fabrication complexity. Here, we demonstrate a solution-processed ZnS nanoparticle CPL that forms conformal films, enabling well-defined microcavity interference. Red, green, and blue (RGB) top-emitting QLEDs incorporating ZnS CPLs are systematically optimized, showing strong agreement with optical simulations. As a result, the devices achieve 2.08-, 1.52-, and 1.66-fold enhancements in external quantum efficiency for RGB QLEDs, along with improved angular color stability.