Electric Field Inhomogeneity in Colloidal QD‐LEDs

Abstract It is demonstrated that the electroluminescent layer in a colloidal quantum dot light emitting diode (QD‐LED), formed by stochastic methods such as spin‐coating, incorporates morphological thickness inhomogeneities, resulting in local electric field variations. These inhomogeneities can be directly visualized and quantified using confocal micro‐photoluminescence (PL) and micro‐electroluminescence (EL), as showed in QD‐LEDs with stochastically processed InP/ZnSe/ZnS colloidal quantum dots (QDs). Around 5% of the device shows EL darkspots under forward bias and PL hotspots under photoexcitation, with a strong spatial correlation between these features. The PL hotspots (EL darkspots) correspond to thicker regions in the stochastically‐processed QD film. This thickness variation leads to two distinct QD sub‐populations responding differently to optical excitation. Time and energy‐resolved spectral diffusion measurements reveal that most excitons belong to a “more‐mobile” sub‐population with fast energy transfer and short, electric field‐dependent lifetimes, while a smaller fraction belongs to a “less‐mobile” sub‐population with slower energy transfer and longer, electric field‐independent lifetimes. The “less‐mobile” excitons correlate with thicker QD regions. These findings shed light on the local electric field inhomogeneity in QD‐LEDs, offering insights into device operation, possible degradation mechanisms, and strategies for developing stochastically‐processed micro‐QD‐LEDs.