HIGHLY EFFICIENT QUANTUM DOT LIGHT-EMITTING DIODES BASED ON THE OPTIMIZATION OF QUANTUM DOT STRUCTURE AND DEVICE ARCHITECTURE

Abstract

Semiconductor nanocrystals or colloidal quantum dots have many advantages because their superb optical and electrical properties can be tuned via shape- and size-control. To utilize these properties, substantial research has been devoted to enable semiconductor nanoparticles to be applied in next-generation optoelectronics (e.g., light-emitting diodes (LEDs), solar cells, and photo catalysts). For many years, the device fabrication processes and performances of quantum dot light-emitting diodes (QLEDs) have undergone substantial development because of efforts addressing material synthesis, electrophysical analysis and device design. Thus, the tailoring of the structure of semiconductor nanocrystals to control optical or electrical properties and their applications in optoelectronic devices must be discussed. In this thesis, high-performance colloidal QLEDs were studied from the perspective of device mechanism and device structure engineering. We developed and demonstrated highly efficient red, green, and blue (RGB) QLEDs with improved group II-VI QDs and environmentally benign Cd-free QDs. First, we investigated the influence of the shell thickness of group II-VI type-I heterostructure QDs on the QLED performance. We found that thick-shell QDs exhibited reduced Auger-type decay rates and suppressed energy transfer (ET) within QD films. In addition, we characterized the device performance and found high efficiency (peak EQE ~ 7.4 %) and record brightness (105,870 cd/m2). The operation stability of the devices is presented along with the improved device performance. Our suggestions for QLED design offer simple results and approaches but propose novel structural designs of core/shell heterostructure QDs to allow engineering of the optical properties of QD solids and the performances of corresponding devices

in addition, they provide rational guidelines for the practical use of QLEDs in high-power light sources. To improve the electron transport layers (ETLs), we demonstrate that the efficiency of inverted QLEDs is enhanced by using a double electron transport layer (ETL) consisting of ZnO nanoparticles and 1,3,5-tris(N-phenylbenzimidizol-2-yl)benzene (TPBI) as an organic electron transport material. TPBI, as a soluble organic electron transport material, fills the voids in the ZnO nanoparticle film and thereby reduces the leakage current path. As a result, the efficiency of blue QLEDs was considerably increased, reaching a maximum EQE of 3.4 %. For highly efficient InP QLEDs, the inverted device structure with a ZnO electron transporting layer (ETL) was adopted because of its numerous advantages in process and integration. However, the large difference in the conduction band (CB) between InP QDs and ZnO impedes efficient electron injection from ZnO to InP QDs. To solve the injection issue, a solution-processable poly[(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-ioctylfluorene (PFN) layer was selected as an interfacial dipole layer. Because of differences in the solubility of ZnO, PFN, and QDs, stacking the three different layers substantially improved device performance in terms of maximum external quantum efficiency (EQE) of 3.46 % and a maximum luminance of 3900 cd/m2. In our forthcoming research, we believe that an in-depth investigation of nonradiative multicarrier decay during device operation and minute engineering of the core@shell heterostructure to minimize such processes will guide us one step further toward producing high–performance InP QLEDs. This thesis demonstrates a novel approach to increase the efficiency and carrier injection of inverted QLEDs. Furthermore, the physical properties of QDs were systematically studied to establish a method to maximize device performance. In addition, the novel InP QLED structure may be applied to a variety of optoelectronic devices, such as thin-film solar cells, LEDs and transistors.