QUANTUM DOT LIGHT-EMITTING DIODES WITH CHARGE BALANCED DEVICE ARCHITECTURE

Abstract

Quantum dot light-emitting diodes (QLEDs) have attracted great attention due to their various merits such as ease of color tunability by size control, excellence in color purity and capability of cost-effective fabrication process. These outstanding merits make QLEDs as a powerful candidate for the next-generation display and solid-state lighting, so intensive research studies on QLEDs have been conducted in both academia and industry. High performance and stability are prerequisite for commercialization of QLEDs. For that purpose, understanding the luminescence mechanism of QD as well as operating mechanisms should be headed for successful realization of QLEDs. Especially, device structure of the balanced charge carrier injection into the QD emissive layer is necessary for achieving high efficiency and relieving the fast degradation of QLEDs. In this thesis, I construct effective approaches to enhance the electron and hole carrier injection balance for realization of high performance QLEDs. The chemically grafted QD-semiconductor hole transporting polymer hybrid system was introduced in order to enhance hole and electron carrier balance in QD emission layer. The systematic analysis on the relationship between the morphology of emission layers and device performance was conducted. Moreover, the modification Zinc oxide (ZnO) electron transport layer by adopting Yttrium (Y) doping with sol-gel method or inserting self-assembled monolayer (SAM) was studied from the perspective of suppression of excessive electron injection to QD emission layers. First of all, chemically grafted QD-semiconducting polymer hybrids are fabricated by the ligand exchange between QDs and synthesized block copolymer consisting of a carbazole-based elctroactive block with a low highest occupied molecular orbital level and a disulfide-based anchor block. The QD-polymer hybrids are evenly distributed throughout the semiconducting polymer matrix, and they also provides the extend of the distance between QDs, so hybrids lead to the improved charge balance and suppressed photoluminescence quenching of QDs. As a result, hybrid QLEDs with the peak external quantum efficiency (EQE) of 5.6% which outperform the conventional devices with QD-only emission layer are fabricated. Secondly, systematic studies for enhancing the charge balance of device by modifying electron transport layer (ETL) of QLEDs were performed. ZnO is the best candidate for electron-transport layer (ETL) in QLEDs because of its superior performance compared to other metal oxides. However, nearly barrier-free electron injection into the QD emission layer can lead to the spontaneous charge transfer and the imbalance of charge carriers, resulting in reduced device performance and the fast degradation. Thus we introduced rare-earth metal yttrium (Y)-doped ZnO (YZO) sol-gel ETLs into QLEDs to adjust charge balance and improve the performance and lifetime of QLEDs. Yttrium doped ZnO film exhibited not only the limited electron mobility compared to that of pure ZnO film, but also the smooth surface morphologies, resulting in the improvement of device efficiency and lifetime. As a result, by adopting the YZO ETL into the inverted structure QLED enables us to achieve color-saturated red emission, an improved EQE of up to 8.6%, and an 8 times longer lifetime compared to the device with undoped ZnO. Finally, the effect of the SAM treatment on ZnO electron transport layer on device performance was investigated. It is observed that the self-assembled molecule, 4-methoxybenzoic acid (MBA), has effects on blocking electron injection by its intrinsic insulativity and leads to better charge injection balance. Furthermore, SAM treated ZnO layer provides smooth surface morphology than that of ZnO nanoparticles. As a result, the efficiency of QLEDs was considerably increased, reaching a maximum EQE of 9.7% and prolonged lifetime without any changes in turn-on voltage. This thesis proposes practical approaches to control the balance of electron and hole carriers at QD emission layer for achieving highly efficient QLEDs. By modifying the morphology of QD emission layers for the applications of semiconducting hole-transporting polymer hybrids, improvement of the charge balance and suppression of photoluminescence quenching between QDs will be achieved. Furthermore, we believe that systematic studies on modifying electron transport layer of inverted structure QLEDs with metal doping method or SAM treatment will offer a useful platform for designing other charge-balanced optoelectronic devices.