Charge and exciton dynamics in quantum dot light-emitting diodes

Abstract

Colloidal quantum dot based light-emitting diodes (QD-LEDs) offer the possibility of bright, saturated, and tunable emission for the next generation of display and solid state lighting technologies. In this thesis, we study how the interplay of charges and excitons in a QD-LED affect its operational behavior. In order to construct a physical model of a QD-LED, we start by developing quantitative characterization methods that directly measure charge accumulation and electric field in an operating device. Comparison of measured internal device variables with observed electroluminescence and current density allows us to disentangle the deleterious effects of charge imbalance, electric field, and Joule heating on the external quantum efficiency. We also find that the magnitude of electron accumulation on the QD film is sensitive to its interface with the neighboring hole transport layer (HTL) and can reach nearly one electron per QD even in the best performing device. We next investigate how exciton formation is affected by the high charge density. Since the degree of electron charging of a nanocrystal shifts the energy barrier for hole injection, the kinetics of exciton formation are dependent on electron occupation statistics on the QD film. Using kinetic Monte Carlo simulations that explicitly incorporate both long and short range Coulomb interactions, we find that energetic disorder of the QD film strongly enhances the formation of negatively charged excitons by increasing the population of two-electron occupied QDs. Finally, we demonstrate that the photoluminescence yield of a QD film can be intentionally quenched by up to 99.5% in a QD-LED under reverse bias. This paves the way for a voltage-tuned optical down-conversion device using colloidal QDs.