Micro-LED display colorization of three major technical means

LED coloring and environmental protection network Micro- LED display colorization is an important research direction. In today's rigorous trend of pursuing colorization and its high resolution and high contrast ratio, the world's major companies and research institutes propose various solutions and are expanding. This paper will implement the main micro-LED colorization. The method is discussed, including RGB three-color LED method, UV/blue LED+ luminescence medium method, and optical lens synthesis method.

First, RGB three-color LED method

The principle of RGB-LED full color display is mainly based on the basic principles of three primary colors (red, green, blue). It is well known that RGB three primary colors can synthesize most of the colors in nature through a certain ratio. In the same way, for the red-, green-, and blue-LEDs, different brightness can be applied to control the brightness value, thereby realizing the combination of the three primary colors to achieve the full-color display effect, which is commonly used in the current LED large screen. method 1].

In the RGB colorized display method, each pixel contains three RGB three-color LEDs. Generally, the P and N electrodes of the three-color LED are connected to the circuit substrate by means of bonding or flip-chip. The specific layout and connection mode are as shown in Fig. 1 [2].

After that, each LED is pulse-width modulated (PWM) current driven by a dedicated LED full-color driver chip. The PWM current driving mode can realize digital dimming by setting the current effective period and duty ratio. For example, an 8-bit PWM full-color LED driver chip can realize 28=256 dimming effects of a single-color LED, so a pixel with three-color LED can theoretically realize 256*256*256=16,777,216 kinds. Dimming effect, ie 16,777,216 colors display. The driving principle of the specific full color display is shown in Figure 2 [2].

But in fact, because the actual output current of the driver chip will be inaccurate with the theoretical current, each LED in a single pixel has a certain half-wave width (the narrower the half-width, the better the color rendering of the LED) and the light decay phenomenon, and then Produce the deviation of LED pixel full color display.

â–² Figure 1 Schematic diagram of single pixel layout of RGB full color display

â–² Figure 2 RGB full color display drive principle diagram

Second, UV / blue LED + illuminating medium method

A method of UV LED (ultraviolet LED) or blue LED + luminescent medium can be used to achieve full colorization. If UV micro-LED is used, it is necessary to activate red, green and blue three-color illuminating medium to achieve RGB three-color ratio; if using blue micro-LED, it needs to be matched with red and green illuminating medium, and so on. The technology was patented and awarded by Professor Liu Jimei and Professor Liu Zhaojun of the Hong Kong University of Science and Technology in 2009 (Patent No.: US 13/466,660, US 14/098,103).

Luminescent media are generally classified into phosphors and quantum dots (QD: Quantum Dots). Nano-material phosphors emit light of a specific wavelength under the excitation of blue or ultraviolet LEDs. The color of the light is determined by the phosphor material and is easy to use. This makes the phosphor coating method widely used in LED lighting and can be used as a kind of Traditional micro-LED colorization method.

Phosphor coating is typically applied to the surface of the sample by spin coating or dispensing after the micro-LED is integrated with the driver circuit. Figure 3 is an application of a phosphor coating method, in which (a) shows that one pixel unit contains four sub-pixels of red, green and blue, and (b) shows the color effect of the micro-LED after lighting [3] ].

The method is intuitive and easy to understand, but there is a deficiency. A phosphor coating will absorb part of the energy and reduce the conversion rate. The second is that the size of the phosphor particles is larger, about 1-10 microns. - LED pixel size continues to decrease, phosphor coating becomes more uneven and affects display quality. And this gives quantum dot technology a chance to shine.

(a)(b)

â–² Figure 3 Pixel design and display effect of phosphor colorized micro-LED

Quantum dots, also known as nanocrystals, are nanoparticles composed of Group II-VI or Group III-V elements. Quantum dots generally have a particle size between 1 and 10 nm and are suitable for smaller size micro-displays. Quantum dots also have the effect of electroluminescence and photoluminescence. After excitation, they can emit fluorescence. The color of the luminescence is determined by the material and size. Therefore, the wavelength of the quantum dots can be adjusted to change the wavelength of different luminescence.

When the quantum dot particle size is smaller, the illuminating color is more blue; the larger the equivalent sub-point, the more reddish the illuminating color. Quantum dots have a variety of chemical compositions, and the luminescent color can cover the entire visible region from blue to red. Moreover, it has high-capacity absorption-light-emitting efficiency, narrow half-height width, and wide absorption spectrum, so it has high color purity and saturation. The structure is simple, thin, and can be curled, which is very suitable for micro-display applications [4].

At present, rotary coating and spray coating technology are often used to develop quantum dot technology, that is, sprayer and airflow control are used to spray uniform and controllable quantum dots. The device and principle diagram are shown in Fig. 4 [5]. It is coated on the UV/blue LED to be excited by RGB three-color light, and then fully colorized by color matching, as shown in Figure 5 [5].

However, the main problem of the above technology is the interaction between each color uniformity and each color, so solving the three-color separation of red, green and blue and the uniformity of each color become one of the important problems of quantum dot light-emitting diodes for use in microdisplays.

In addition, the current quantum dot technology is not mature enough, and there are also shortcomings such as poor material stability, high heat dissipation requirements, and need to be sealed and have a short life. This greatly limits the scope of its application, but as technology advances and matures, we expect quantum dots to have a chance to play a more important role.

â–² Figure 4 (a) Aerosol jet technology and its (b) schematic.

â–² Figure 5 Schematic diagram of three primary color arrays of red, green and blue using high-precision spraying technology

Third, optical lens synthesis

Lens optical synthesis refers to the synthesis of RGB three-color micro-LEDs in full color by optical prisms (Trichroic Prism). The specific method is to package three red, green and blue micro-LED arrays on three package boards, and connect a control board and a three-color prism.

The image signal can then be transmitted through the drive panel, the brightness of the three-color micro-LED array can be adjusted for colorization, and the optical projection lens can be used for micro-projection. The physical map and schematic diagram of the whole system are shown in Figure 6, and the display effect is shown in Figure 7 [6].

â–² Figure 6 prism optical synthesis method a), b) physical map, c) schematic diagram

â–² Figure 7 shows the effect of prismatic optical synthesis

references:

[1] WC Chong, et al, SID 13 Digest, 44(1): 838-841.

[2] D. Peng, et al, IEEE J. Display Technol., Vol. 12, Issue 7, pp. 742-746, 2016.

[3] ZJ Liu, et al, SID 11 Digest, 42(1): 1215-1218.

[4] K, J. Chen, et al, SPIE Opto, 2013, 8641(1): 115-125.

[5] HV Han, HC Kuo, et. al, OSA, 23(25), 2015.

[6] ZJ Liu, WC Chong, et al, IEEE Photon. Technol. Lett., Vol.25, no.23, 2013.