Examination Committee

Prof Brahim BENSAOU, CSE/HKUST (Chairperson)
Prof Hoi Sing KWOK, ECE/HKUST (Thesis Supervisor)
Prof Ru-Shi LIU, Department of Chemistry, National Taiwan University (External Examiner)
Prof Man WONG, ECE/HKUST
Prof Zhiyong FAN, ECE/HKUST
Prof Shihe YANG, CHEM/HKUST

Abstract

Over the past years, QLEDs technology has experienced a huge development. With the EQEs of RGB QLEDs based on cadmium compound II-VI QDs all exceeding 10 % and even approaching that of state-of-the-art phosphorescent OLEDs, QLEDs technology becomes a promising technology for future thin, flexible and vivid displays. This thesis focuses on overcoming three challenges facing QLEDs technology from the aspects of device structure, fabrication process and materials.
 
Firstly, the challenge of maintaining balanced carrier injection into QDs is addressed by device structure design, fabrication condition optimization and material engineering. Specially, a newly developed material, namely Mg-doped ZnO (Zn1-xMgxO) NPs, is employed as the ETL for green QLEDs. Thanks to its excellent electron injection and low defect concentration, the green devices can realize a more than triple efficiency enhancement from 7.84cd/A to 36.40cd/A, comparing to the devices using traditional ZnO NPs as the ETL.
 
Then, liquid metal EGaIn is applied as the top electrodes for QLEDs to solve the challenge of vacuum-free-processed devices. With the patterned EGaIn electrodes, efficient vacuum-free-fabricated QLEDs are demonstrated with high EQEs of >10%, >15% and >5% for the red, green and blue devices, respectively. Particularly, the best green QLED achieves an over 80cd/A current efficiency and over 18% EQE, which is among state-of-the-art QLEDs. The vacuum-free-deposited EGaIn electrodes developed in this thesis can eliminate the vacuum processes, thus significantly reducing the production cost, allowing for fast fabrication of devices.
 
Finally, sputtered NiOx is investigated for its application as HIL or HTL for QLEDs to circumvent the organic p-type materials. NiOx is used as both HIL and HTL to replace all the p-type organic layers so that all inorganic QLEDs with low turn-on voltage and high current density are constructed. By engineering band structure and setting separation layer, the efficiency of these QLEDs can be enhanced.