Electric Field Inhomogeneity in Colloidal QD‐LEDs

Abstract

It is demonstrated that the electroluminescent layer in a colloidal quantum dotlight emitting diode (QD-LED), formed by stochastic methods such as spin-coating, incorporates morphological thickness inhomogeneities, resulting inlocal electric field variations. These inhomogeneities can be directly visualizedand quantified using confocal micro-photoluminescence (PL) and micro-electroluminescence (EL), as showed in QD-LEDs with stochastically processedInP/ZnSe/ZnS colloidal quantum dots (QDs). Around 5% of the device showsEL 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-processedQD film. This thickness variation leads to two distinct QD sub-populationsresponding differently to optical excitation. Time and energy-resolved spectraldiffusion measurements reveal that most excitons belong to a “more-mobile”sub-population with fast energy transfer and short, electric field-dependentlifetimes, while a smaller fraction belongs to a “less-mobile” sub-populationwith slower energy transfer and longer, electric field-independentlifetimes. The “less-mobile” excitons correlate with thicker QD regions. Thesefindings 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.