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Quantum Dot Micro-LEDs
A traditional CRT television creates an image by exciting a fluorescent screen with an electron beam. It accelerates and controls electrons fired from an electron gun inside a vacuum tube, and is designed to emit the three primary colors of R (red), G (green), and B (blue) from a single phosphor pixel. LCDs and plasma displays subsequently appeared. Efforts to reduce costs and improve performance led to the demise of plasma displays, and LCDs came to monopolize the market. However, LCDs have are known for problems including slow reaction speed, poor conversion efficiency, and poor color saturation. Recently, organic light-emitting diode (OLED) displays and light-emitting diode (LED) displays have been developed and put into practical use. Compared with LCDs, OLED displays offer advantages including self-luminescence, wide field of view, high contrast, fast response speed, and low energy usage. However, they also exhibit problems including difficulty of mass production and high cost of materials. LED-based displays enable low power consumption and excel in high color intensity and high brightness, features that lend them primarily to use in screens for outdoor applications. Research and development is addressing smaller mini-LEDs and micro-LEDs to bring LEDs closer to desired screen resolution, creating a current need for chips with a size of 200 μm or smaller. Research and development of micro-LEDs at 100 μm or smaller size is still at the laboratory stage.
The application of micro-LEDs to wearable watches, mobile phones, smartphones, automobile head-up displays, micro-projectors, high-quality televisions, and other devices is also under study. Micro-LEDs can be used in flexible displays, and they have the potential to surpass OLED displays in performance because of advantages including higher contrast, higher brightness, longer life, and lower power consumption. However, micro-LEDs still face a number of technical issues, including transfer-printing issues at the mass production level and the need to represent all colors in display applications.
The principle behind RGB full-color displays is the production of all colors through the combination of three primary colors. The application of different currents to each of the RGB LEDs is a method of controlling RGB intensity. Another method is the use of ultraviolet rays, blue LEDs, or other illuminants to convert colors by excitation of light (photons). Color-conversion materials used in this method include phosphors and quantum dots.
Phosphors are able to emit specific wavelengths of light in response to excitation light, with each phosphor having its own fluorescence wavelength. Synthesis is also relatively simple. After LEDs are mounted on a circuit board, phosphors are deposited on the LEDs using methods such as spin coating or pulse spray coating, which also enables relatively easy preparation procedure as a color conversion material. However, problems exist as well, such as lower conversion efficiency caused by the absorption of excitation-light energy by the phosphors themselves. Moreover, while phosphors exhibit fluorescence effect at sizes of 1 to 10 μm, these sizes are large relative to the even-smaller micro-LED pixel sizes that are in increasing demand. Reduced efficiency caused by uneven surfaces is another problem. Attention has focused on quantum-dot-based micro-LEDs as a way to make up for these drawbacks, with the result that quantum dots have been called the only material capable of creating good micro-LEDs.
Quantum dots are characterized by high quantum yield, size-dependent emission wavelength, and a narrow full width at half maximum. Research is underway on micro-LEDs that combine UV micro-LEDs and RGB-emitting quantum dots to create full-color images using aerosol jet (AJ) technology.