Spatially-Resolved Exciton Dynamics Under an Electric Field in Colloidal QD-LEDs

Colloidal quantum dots (QDs) are solution-processable emitters with high quantum efficiencies and tunable emission wavelengths. They have emerged as promising materials for displays, solid-state lighting, and optical communication. The demand for QD-based display technologies has necessitated the development of environmentally-benign QD compositions with similar optical properties to high-performing, but toxic, heavy-metal-containing QDs. Using time-resolved confocal micro-photoluminescence mapping under an applied electric field and transmission electron microscopy (TEM), we observe direct evidence of inhomogeneity in exciton response to the electric field for InP/ZnSe/ZnS quantum dot light emitting diodes (QD-LEDs). By applying a negative bias across the diode while photo-exciting the QDs, the steady-state and time-resolved exciton dynamics are modulated, resulting in PL quenching and a decrease in average exciton lifetime. Under confocal micro-PL mapping, intensity hotspots imaged in the QD thin film are found to be less-reactive to the applied electric field, resulting in spatial inhomogeneity in PL quenching. Finally, cross-sectional TEM images of the QD-LED demonstrate distinct regions of inhomogeneous layer height in the spin-coated QD thin film. We hypothesize that thicker regions of the QD film screen the effective electric field experienced by the dots in that region, leading to limited field-induced modulation. Subsequently, we posit that under positive bias, the regions that are unreactive under negative bias do not contribute to charge injection and electroluminescence, leading to increased field exposure and aging in the surrounding QDs. This work provides a possible mechanism for the accelerated operational lifetime decays observed for InP/ZnSe/ZnS QD-LEDs.