Charge transport in quantum dot - light emitting devices

Abstract

Inorganic quantum dots have excellent optoelectronic properties. But, due in part to a lack of a suitable medium for dispersion, they have not been extensively used in optoelectronic devices. With the advent of organic semiconductors, the integration of quantum dots into optoelectronic devices has become possible. Such devices are termed as hybrid organic/inorganic quantum dot light emitting devices. In hybrid organic/inorganic quantum dot light emitting devices, the mechanisms of charge and/or energy transfer into the quantum dots include Forster energy transfer and direct charge injection. Forster energy transfer involves formation of excitons on organic semiconductors, followed by an energy transfer onto the inorganic quantum dots, where the exciton recombines resulting in emission of a photon. Direct charge injection is the mechanism in which the electrons and holes are directly injected into the quantum dots and they recombine on the quantum dots to result in a photon. Which mechanism is operating in a device has been a subject of contention. In this work, by using various device configurations, we show that both these mechanisms can operate independently to maximize the quantum dot light emission in such devices. We also propose a model for inorganic QD-LEDs, which explores the most important parameters that control their electrical characteristics. The device is divided into a hole transport layer, several quantum dot layers, and an electron transport layer. Conduction and recombination in the central quantum dot region is described by a system of coupled rate equations, and the drift-diffusion approximation is used for the hole and electron transport layers. For NiO/Si-QDs/ZnO devices with suitable design parameter the current and light output are primarily controlled by the quantum dot layers, specifically, their radiative and non-radiative recombination coefficients. Radiative recombination limits the device current only at sufficiently large bias. This model can be extended to apply to hybrid organic/inorganic QD-LEDs.