Over the past years, QLEDs technology has experienced a huge development in aspects of fabrication process, device structure, QDs and CTLs materials, and patterning method. 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 competitive rival to OLEDs technology for future thin, flexible and vivid displays.

However, there still exist a few challenges needed to be overcome so that efficient QLEDs can be really applied in commercial displays. The first challenge is to maintain balanced carrier injection into QDs since Cd-based QDs’ intrinsic deep conduction band and valence band give rise to easy electron injection but relatively hard hole injection, which...[ Read more ]

Over the past years, QLEDs technology has experienced a huge development in aspects of fabrication process, device structure, QDs and CTLs materials, and patterning method. 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 competitive rival to OLEDs technology for future thin, flexible and vivid displays.

However, there still exist a few challenges needed to be overcome so that efficient QLEDs can be really applied in commercial displays. The first challenge is to maintain balanced carrier injection into QDs since Cd-based QDs’ intrinsic deep conduction band and valence band give rise to easy electron injection but relatively hard hole injection, which brings about QDs charging and thus excitons quenching by Auger recombination. The second one is to extend the concept of all-solution-processed devices to solution-deposited top electrodes in order to make QLEDs fully compatible with the roll-to-roll printing process for manufacturing flexible and large area displays. The last issue is to develop new HTLs with good stability, high hole mobility and excellent hole injection, as the currently used HTLs in QLEDs are mostly organic materials, which possess low hole mobility and shallow HOMO level, and thus the poor hole injection problem has not yet been perfectly solved. Therefore, this thesis focuses on overcoming these three challenges from the aspects of device structure, fabrication process and materials.

For solving the unbalanced carrier injection problem, a device structure of inserting EBL in ETL is firstly proposed for red QLEDs to partially block the excessive electron injection. With a thin PVK layer as EBL, the efficiency of the red devices can achieve a 2.2-fold enhancement as a result of improved charge balance. Moreover, the fabrication ambience is optimized by conducting the spin-coating process and the annealing treatment in N₂-filled glovebox to reduce the electron mobility of the ZnO NPs layer. With this optimization, balanced carrier injection can be simply obtained for red QLEDs without any EBL, and a similar double efficiency improvement from 5.63 cd/A to 11.09 cd/A can also be achieved. At last, a newly developed material, namely Mg-doped ZnO (Zn_1-xMg_xO) 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 7.84 cd/A to 28.17 cd/A, comparing to the devices using traditional ZnO NPs as the ETL. Further optimization with the HTLs and the QDs layer can push the performance of the green devices to a new level. The optimized green QLED can achieve a maximum current efficiency of 36.4 cd/A, a maximum EQE of 8.52 % and a maximum brightness over 200000 cd/m² at 12V.

The second challenge is overcome by adopting the solvent-free liquid metal EGaIn as the top electrodes. In order to controllably pattern the EGaIn, a fast, low-cost, high resolution and large-scale patterning method is proposed and developed by utilizing the wetting/dewetting behaviors of EGaIn on different surfaces. This patterning method makes it possible to paint a lot of fine EGaIn patterns, whose line width is smaller than 100 μm, on both solid and flexible substrates. With the patterned EGaIn electrodes, efficient vacuum-free-fabricated QLEDs are demonstrated with high EQEs of 11.51 %, 12.85 % and 5.03 % for the red, green and blue devices, respectively, which are about 1.7-, 1.5- and 1.1-fold higher than that of the corresponding devices using thermally evaporated Al cathodes. The vacuum-free-deposited EGaIn electrodes developed in this thesis can eliminate the vacuum and high-temperature processes, thus significantly reducing the production cost, allowing for fast fabrication of devices and paving the way to industrial roll-to-roll manufacturing of large area QLEDs-based displays. Besides, due to the liquid nature of EGaIn, the patterned EGaIn electrodes can be stretched and compressed without cracking and thus find application in stretchable circuits and various soft wearable or configurable electronic devices.

At last, the sputtered NiO_x is investigated for its application as HIL or HTL for QLEDs. By optimizing the sputtering conditions and the annealing temperature of the NiOx film, the efficiency of the red & green devices using NiO_x as HIL can approach that of the standard devices with PEDOT:PSS as HIL. Furthermore, NiO_x is used as both HIL and HTL to replace all the p-type organic layers so that an all inorganic QLED with low turn-on voltage and high current density is constructed. To improve the efficiency of this all inorganic QLED, two strategies are adopted. The first strategy is to engineer the band of NiO_x by deepening its valence band through alloying MgO. As a result, the η_A of the device can be enhanced from around 3.4 cd/A to over 4 cd/A. The second one is to set an inorganic layer between NiO_x or NiMgO and QDs to separate the quenchers and the excitons. Thin MgO and LiF are tried as such separation layer. With a 6 Å MgO layer, the device can achieve a maximum luminance of over 40000 cd/m² and a peak η_EQE of 1.47 % (η_A: 6.08 cd/A), which is two order of magnitude higher than the EQEs (<0.1 %) reported in literature for all inorganic QLEDs. Also, a 9 Å LiF layer can help increase the efficiency by passivating the defects in NiO_x using released Li atoms.