Modification of Zinc Oxide Electron Transport Layer for Highly Efficient and Stable Quantum-Dots Light Emitting Devices

Due to its superior optoelectronic properties, colloidal quantum dots (QD) have been receiving enormous attention from the research community as a strong candidate for a next-generation electroluminescent devices. Quantum mechanical spatial confinement allows tuning the optical bandgap of the material relatively easily, making them front runners for full-color flat panel displays, i.e. light emitting device (LED). At the present, the external quantum efficiency (EQE) of QDLED is on par with that of state-of-the-art organic LED. Although the EQE progressively approaches to the requirement for commercialization, device lifetime is still below what is required for commercial application. QD emission mechanism has mainly been focused to understand degradation mechanisms because QD is vulnerable to carrier-loss via Auger recombination process from unbalanced charge in QD. Recently, charge transport layers have been found to also play a major role in device degradation. More specifically, the organic material-based hole transport layer has been found to degrade by excitons or charges during electrical stress. On the other hand, the influence of electron transport layer (ETL), typically made of ZnO and thus assumed to play a little role in device degradation, has been overlooked. Recently, some reports suggested that ZnO ETL could deteriorate charge imbalance in adjacent QD layer due to its relatively higher carrier mobility than organic material-based hole transport layer. In addition, intrinsic defect states in ZnO can act as charge trapping sites. In this regard, some reports suggested that modification of ZnO strongly affect device performance, both efficiency and stability. This modification usually includes the use of a bilayer form of ZnO or a functional layer or metallic anion doping, e.g. aluminum, magnesium, in ZnO layer. These approaches somehow effectively control device efficiency but there are still many challenges in terms of stability in device operation. In this report, we will discuss different unique strategies for ZnO modification with a focus on improving device stability. Various doping techniques in ZnO effectively modify electrical and optical properties of ZnO and improve both device efficiency and stability significantly. Especially, device stability improves more than 10 times than a control device. Various electrical, optoelectrical, structural and spectroscopic observations in the experiments will be discussed to shed light on new strategies for improving device stability and performance.