Design of high performance indium phosphide (InP) - based quantum dot light emitting diodes (QLEDs)

Abstract

This dissertation is dedicated to the development and understanding of fundamental device mechanism and engineering of the device architecture of quantum dot light-emitting diodes (QLEDs) based on indium phosphide (InP) quantum dots. QLEDs have emerged as a next-generation flat panel display (FPD) technology with promising properties. Their device performance and fabrication methods are developed extensively through the assistance of the highly developed organic light-emitting diode (OLED) technology. However, the operation mechanism and the optimized device architecture, which both control the device performance, remain unclear especially for devices with cadmium (Cd)-free materials. Moreover specifically, the device efficiency and luminance are relatively low compared to the Cd-based QLEDs. The important QD material design aspects such as outer shell thickness of the InP/ZnSe/ZnS, core/multishell structure and the ligand chain length of the QDs are investigated with conventional QLEDs to improve device performance. Through the QD design, maximum external quantum efficiency (EQE) and luminance of the conventional QLEDs reach 2.5% and 3164 cd/m², respectively. Since an inverted device structure is more favorable for commercialization of QLED displays due to recent advances in the well-developed active matrix (AM) OLED technology, this architecture is more thoroughly investigated considering a charge carrier balance in the multilayered QLEDs. In addition, the inverted architecture offers other advantages such as an improved device stability and enhanced efficiency. The maximum EQE of the inverted QLEDs of 3.1% was achieved by controlling electron transport with an adopted multi-spin-coated zinc oxide (ZnO) nanoparticle electron transport layer (ETL); however, the maximum luminance was less than 3000 cd/m². Further enhancement of device performance (i.e. maximum EQE and luminance of 3.3% and 8449 cd/m², respectively) and stability was accomplished through well-balanced charge carriers and a charge neutralization effect in the QD emission layer. Moreover, a recombination-zone (RZ) shift model which depends on the different thicknesses of QD film, and a charge neutralization model consisted of electric field-assisted Auger electron injection via the mid-gap states of ZnO nanoparticles were developed based on the experimental results and theoretical hypotheses. These two theoretical models provide a broad scope to understand the optimization process of InP QD-based QLEDs, not only for this dissertation but also the further investigations. Consequently, the developments in this dissertation can provide the experimental guidelines and theoretical insights for designing efficient and stable Cd-free QLEDs.