Deconstructing the basic principles of LCD, OLED and quantum dots

LCD TV is already out of breath! OLED TVs are not yet on the market, how come a quantum dot TV! What is OLED? What is a quantum dot?

Since 1990, the world's first TV leap forward has been launched. The cathode cathode ray tube (CRT) TV, which has been accompanying us for more than half a century, has been replaced by a smaller liquid crystal display ( LCD ). It has become the protagonist in the living room. However, the goal of scientists to pursue the color quality and thickness reduction of TV screens has not stopped. After several years of development, the "Organic Light Emitting Diode" (OLED) and "Quantum Dots" (the "Quantum Point") Quantum Dot, QD) TV matures, the possibility of a rising star gradually replace LCD TVs higher and higher, we look at the future of these two new displays in the end what is the difference now.

The three primary colors of light Before we understand the principle of display , let us first understand how color is achieved in various display technologies . They are all red (Red), green (Green), blue (Blue, B) 3 The color "different brightness" can be combined into almost all visible light colors in the continuous spectrum, so we call the three colors of red, green and blue "the three primary colors of light". As shown in Figure 1, suppose there is a square to display a certain color. Such a square is called a "pixel". The square is cut vertically into three small squares, representing three kinds of RGB. Color, such a small square is called "Sub-pixel".

â–² Figure 1: Red, green, and blue colors can be combined into almost all colors in the continuous spectrum with different brightness.

The display of color has an infinite variety of colors. The second picture shows the Chromaticity diagram, which represents all the colors that the naked eye can see. The wavelength of light is the color, how many wavelengths of light, and how many colors there are. The red, green, and blue colors mentioned above are actually only a general visual experience. The wavelengths of light generated by different light-emitting elements are somewhat different, so the colors produced by different light-emitting elements are different and different.

The traditional LCD TV (LCD-TV) with white LED (WLED) as the backlight is called "LED-TV". Note that although the manufacturer calls it LED-TV, it is actually "LED" The backlight LCD-TV" uses the "Color filter" to generate RGB 3 colors of light. The triangle vertices in the figure are RGB 3 colors, and the color inside the triangle is such a TV. All the colors that can be combined (the larger the triangle, the more colors can be combined, the more realistic the color). It obviously cannot be combined into all the colors that the naked eye can see, but this is enough to make a TV to let us see the real image. Later, some manufacturers developed RGB 3 colors produced by different white light-emitting diodes (LEDs) and color filters, which can be combined into more colors (red triangles), called "wide color gamut LED-TV". However, both types of TVs use color filters to filter white light to produce RGB 3 colors, and currently the white light-emitting diodes (LEDs) used in industrial backlights are limited in color, so they can be combined into The color is still not enough; it can be seen from the figure that the RGB 3 colors produced by the organic light-emitting diode TV (OLED-TV) can be combined into more colors (green triangles); and the quantum dots (QD-TV) are produced. RGB three colors can be combined into more colors (blue triangles).

â–² Figure: Chromaticity diagram.

The organic light-emitting diode organic light-emitting diode ( OLED ) is also called "Organic Electrical Luminescence" (OEL). Its structure is shown in Figure 3. It will emit red, green and blue light. An organic light-emitting semiconductor (a polymer that emits light) is coated on a conductive glass, and then a metal electrode is vapor-deposited, and a voltage is applied to an organic light-emitting semiconductor of different colors to inject electrons and holes, and electrons and holes are formed in the organic light-emitting semiconductor. Internally combined with luminescence. Its structure is simple, the brightness is high enough, but unfortunately, the organic light-emitting semiconductor is actually a kind of "polymer" (plastic). Due to the unstable nature of the material of the plastic light-emitting, the yield is low during production, the cost can not be lowered, and the use is It has a short life span and will fade after a long time, so it has been used in small amounts on monochrome displays . After more than a decade of efforts, South Korea's Samsung and LG have successfully improved the yield. Now they have already produced more than 60-inch color TVs, leaving only the problem of over-priced. It seems that it has slowly seen its commercial value.

â–² Figure 3: Organic Light Emitting Diode (OLED) construction. (Source: LG)

Quantum dot television (QD-TV) scientists have discovered that when the size of a material is as small as 100 nanometers or less, a "quantum co nfinement effect" occurs, in which electrons and holes are confined within the nanomaterial to form an ad hoc group. The steady state causes a change in the luminescent properties, and the smaller the size of the nanomaterial, the stronger the luminescence intensity of the material, and the shorter the illuminating wavelength (blue), this phenomenon is called "blue shift", as shown in Figure 4. It is shown that the wavelengths of light of different colors are different, so the wavelength of light is the color. In the visible light, the wavelength of red light is the longest, the second is green light, and the blue light is the shortest. In other words, when the size of the nano material is large, the intensity of the light is weak. The color is red light (the longest wavelength); when the size of the nano material becomes smaller, the luminous intensity becomes stronger, the color is green light (the wavelength is second); when the size of the nano material is smaller, the luminous intensity is stronger, and the color is blue light ( The wavelength is the shortest).

â–² Figure 4: The "blue shift" of quantum confinement effects. (Source: Tae-Ho Kim et al., Heterogeneous stacking of nanodot mo nolayers by dry pick-and-place transfer and its applications in quantum dot light-emitting diodes, Nature Communications, Article number: 2637, Published 06 November 2013)

The principle of traditional white light-emitting diode (WLED) illumination is that blue light emitting diode (BLED) crystal grains emit blue light to irradiate YAG fluorescent powder, and fluorescent powder absorbs blue light to produce white light, as shown in Figure 5(a). As mentioned above, the RGB color of such components is limited, and the colors that can be combined are still not enough, so we can directly use the quantum dot (QD) instead of the fluorescent powder called "quantum dot light-emitting diode (QD-LED). ) ", as shown in FIG five (b), by using such elements is a liquid crystal display (LCD) backlight RGB color can be increased, as shown in FIG five (c), and therefore can be combined into the color increased greatly, and You do not need to modify the structure too much liquid crystal display (LCD), and the lowest cost, but still have to use a color filter.

If you want to add RGB color, you can use the blue LED (BLED) as the backlight, as shown in Figure 5 (d), through the LCD panel, and print the quantum dots directly on the panel to form a different The sub-pixel replaces the color filter and is illuminated by the blue backlight module. The red quantum dot absorbs blue light and emits red light. The green quantum dot absorbs blue light and emits green light. The blue quantum dot absorbs blue light and emits blue light. The blue pixel is the same color as the backlight module, so the blue quantum dot can be omitted. The structure of the final quantum dot television (QD-TV) is shown in Figure 6.

â–² Figure 5: Schematic diagram of white light-emitting diode and quantum dot light-emitting diode (QD-LED) TV.

â–² Figure 6: Schematic diagram of the construction of quantum dot television (QD-TV).

Conclusion The traditional LCD TV ( LCD- TV) market with white light-emitting diode (WLED) as the backlight has matured, and there is not much room for improvement in technology; although organic light-emitting diode TV ( OLED- TV) The resulting RGB 3 colors can be combined into more colors, but the lower yield results in higher prices, which seems to be not fully promoted in a short period of time; and the RGB 3 colors produced by QD-TV It can be combined into the most color, and does not need to modify the structure too much liquid crystal display (LCD), and indeed is one of the possible ways, but remember, in fact, is a kind of quantum dot TV LCD TV, it still depends on Liquid crystal imagery, so it is still a kind of LCD TV. The term "quantum dot", which is quoted by the marketing market, is full of "technical color". Most of the terms are also advertising marketing techniques.

In the foreseeable future, the TVs in our homes will become thinner and thinner, and the colors of the pictures will become more and more abundant, allowing us to see pictures that are as real as nature on TV.