Studies on the next-generation quantum dots light-emitting diodes for full-colour display and smart lighting application by printing technology
The recent research trends on the advanced display and smart lighting system are focused on the wide colour gamut, high colour purity, higher brightness, stability and flexibility with respect to the performance and form factors. Considering the above factors on the basis of the self-emitting light source candidates, a quantum dot light-emitting device (QD-LED) is strongly recommended for the next-generation display and lighting application.
Herein, this thesis touches the studies on the science and technologies of QD-LEDs from the material, the process technology, the device design and architecture to the system integration and its characterization for the specified applications. Especially by scrutinizing QD material and its characteristics, unique fabrication technologies, various design of device architecture, and system level innovation, the QD-LEDs devices and systems are fabricated and intensively studied for the display and smart lighting applications.
On the materials, Cd-embedded and Cd-free QD materials are used for QD-LED, and the electrical and optical analyses are studied from the materials level to the specific system level of QD-LEDs. With respect to the process level for the colour pixel patterning of QD-LED, unique printing technology called as transfer printing is used and its process technologies are intensively studied with respect to various materials, devices architecture, and system integration, in addition to the conventional process technology such as spin-coating.
On the device perspective, the framework of design of device and structuring of various device architecture based upon passive-matrix (PM) based QD-LEDs and active-matrix (AM) based QD-LEDs with thin-film transistor (TFT) backplane is deeply studied for the desired full-colour display and smart lighting system. The control parameters for higher performance devices are extracted and studied by way of modelling the desired architecture and discovering unique and innovative fabrication.
On the system, various QD-LED device architectures are embedded into the displays and smart lighting systems and analysed with respect to the PM and AM based systems. The transparent display system, the smart QD lightings system with mixed mode, stacked mode, and patterned mode pixel architecture, and the displays with PM and AM based mono and full-colour displays are integrated and deeply studied with electro-optical analysis. Especially, the transparent model of displays is considered to be a strong candidate to replace the top emission displays which require more difficult fabrication processing steps. The PM based smart QD lighting with the emission areas of 2 by 2-inch is studied with respect to the various pixel architectures. This smart QD lighting is also first demonstrated as textile lighting by conventional weaving technology, and this is considered to be a major engineering breakthrough in the area of systems with free form factor, free size-ability, and offering full flexibility such as rollable, bendable, and foldable applications. Finally, the various AM QD-LED display systems with Cd- or Cd-free red (R), green (G), and blue (B) pixels, are fabricated by way of spin coating and transfer printing and studied deeply.
In order to enhance the functional properties of the display and lighting system, in this study, the TFT technology is implemented and optimised for large and higher pixelated display system using in-house research facilities. To achieve improved AM TFT backplane performance, In-Ga-Zn-O (IGZO) is used for thin-film transistor (TFT) channel material due to its ultra-thin channel and high electron mobility leading to fast turn-on current and low-temperature processability. Vacuum-deposited IGZO TFT demonstrates ON/OFF current ratio of higher than 109 for pixel control and a stiff subthreshold swing (SS) of 0.2 V decade-1 for fast switching behaviour. The developed TFT, low gate leakage current (<10 pA) with high mobility of 15 cm2 V-1 s-1 delivers sufficient current of hundredth of micro ampere to manipulate pixel colour brightness.
Having this TFT backplane, multi-purpose AM QD-LEDs are integrated for the stable and scalable QD-LED. A unique transfer printing technology is implemented for RGB pixels with various device architecture on the TFT backplane. Finally, the 1.5-inch diagonal display composed of 120 x 90 x 3 colour pixels (120: horizon, 90: vertical, and 3 colours) with Cd and Cd-free based QDs and the IGZO TFT array backplane has been successfully integrated as full-colour displays. The world’s first display with Cd-free InP (Red), InP (Green) and ZnTeSe (Blue) QDs on AM QD-LED display system is demonstrated in this thesis. Work conducted in this research thus, sets out the route from the QD materials, process, and device architecture and design to system integration for QD-LEDs as a framework for developing cutting-edge QD-based display and smart white lighting applications.