Applications Of Zn_1-xMg_xO As Electron Transport Layer In Quantum Dot Light Emitting Diodes

Colloidal quantum-dot light-emitting diodes?QLEDs? have recently received considerable attention due to their superior features compared with the traditional organic light-emitting diodes?OLEDs?, such as high brightness, narrow emission, color tunability, solution process, etc. As a result, it has great development potential in the field of display and lighting. Especially, since the introduction of Zn O nanoparticles as an electron transport layer?ETL? material, the device performance of electrically driven colloidal quantum dot-light-emitting diodes?QLEDs? has been drastically improved. However, the research is still in the laboratory stage. In order to achieve industrial application, there are still a lot of problems to resolve, the most urgent ones are efficiency and stability.This paper mainly focused on the ZnO as electron transport layer of CdSe/CdS/ZnS and CdSe@ZnS quantum dot light emitting diodes. And then to explore the luminous intensity and efficiency of devices cased by unbalanced injection charge. In our work, Mg doped ZnO and ZnO mixed Cs₂CO₃ are used as electron transport layer, for adjusting electron injection ability device, effectively promoting recombination of electron and hole, and then improve the efficiency of QLEDs. The work in this thesis can be summarized as follows:?1? Preparation of Zn_1-xMg_xO nanoparticles at different doping concentration and investigation of performance:In this work, we prepared a series of different Zn_1-xMg_xO nanoparticles with Mg doping concentration?0?x?0.10? by the method of sol-gel, and then the Zn_1-xMg_xO nanoparticles dispersed in ethanol. It clearly shows that Zn_1-xMg_xO remains in the wurzite-type ZnO phase without any cubic MgO phase with increasing Mg content. In other words, there is no phase separation occured. And the small particle size led to a significant broadening of the characteristic diffraction pattern, which in agreement with high-resolution transmission electron microscope?HRTEM? images. In addition, the UV-VIS absorption spectra can be seen clearly typical a phenomenon with absorption edge blue shift, as well as and photoluminescence?PL? emission spectra. And the corresponding band gap will broaden of all the samples. Therefore, the Energy level is adjusted between electronic transport layer and cathode. Beyond that, Zn_1-xMg_xO nanoparticles remain the advantages of pure ZnO with high transmittance, all the samples reached over 98%, which provided safe guard for its application as electron transport materials in quantum dot light emitting diode.?2? Applications of Zn_1-xMg_xO as electron transport layer in quantum dot light emitting diodesIn this paper, we prepared CdSe/CdS/ZnS and CdSe@ZnS quantum dot light emitting diode used Zn_1-xMg_xO nanoparticles as electron transport layer and their performances were enhanced in different degree. As a result, it obviously appeared enhancement in the color purity of CdSe/CdS/ZnS quantum dot light emitting diode, because of the blue emission phenomenon is also suppresses owing to electron-hole imbalance problem. The phenomenon disappeared in quantum dots charging, caused by the accumulation of excess carrier. Thus the luminance and efficiency developed. The maximum luminance and external quantum efficiency?EQE? reached 25,272 cd m^-2 and 9.46% of optimized devices, enhanced 81% and 3.67-fold compared with conventional QLEDs, respectively. Similarly, it also obviously appeared enhancement in luminance and efficiency of CdSe@ZnS quantum dot light emitting diode when introduction ZnO as electron transport layer. The maximum luminance and external quantum efficiency?EQE? reached 52,526 cd m^-2 and 8.23% of optimized devices, enhanced 72.4% and 1.42-fold compared with conventional QLEDs, respectively. It is worth noting that the efficiency gained peak in the luminance ranging from 100-10000 cd m^-2, which can meet the needs of the business. In this structure, Zn_1-xMg_xO nanoparticles were used as electron transport layer for its higher electron mobility and tunable ability in barrier. While the direct contact between QDs and cathode can be prevented by Zn_1-xMg_xO buffer layer, therefore effectively suppressing the interface exciton quenching and improving the device efficiency.?3? Applications of Zn_1-xMg_xO:Cs₂CO₃ as electron transport layer in quantum dot light emitting diodesBased on Zn_1-xMg_xO:Cs₂CO₃ as electron transport layer in the CdSe/CdS/ZnS quantum dots red light-emitting diodes are prepared at different mixture to adjust carriers injection ability for the balance electron and hole. Since the Cs₂CO₃ was introduced in QLEDs, the luminance and efficiency have been further improved. The maximum luminance and external quantum efficiency?EQE? reached 42,910 cd m^-2 and 13.09% of optimized devices, enhanced 35.79% compared with primary QLEDs, respectively. Especially, the EQE reached 12% at 1000 cd m^-2, which keeped in high level enough to meet the demand of commercial in flat-panel displays and lighting.