Advantages and disadvantages of OLED compared to traditional LCD screens
Do “thin”, do “bend”. Since OLED does not require a large-area backlight layer and liquid crystal layer, it has a natural advantage over LCD in thickness and can be extremely thin, which is also in line with the current needs of smartphones, smart wearable devices, super TVs, and displays. . In addition, since OLED does not necessarily need a “glass substrate” as the bottom and upper layer materials, it can become bent. This technology is used in folding mobile phones in recent years, using soft PI plastic as the substrate to achieve large-angle bending fold.
“Black” is pure. Compared with the backlight technology used by LCD, it can only try to cover up when it displays black. Compared with OLED, if it displays black, it can directly cut off the voltage transmission, so that photons are no longer generated and there is no trace of it. , making the black more pure. At the same time, due to the pure blackness, the contrast ratio is qualitatively different from that of the LCD screen. The contrast ratio of the OLED desktop display is often 100,000 to 1, while the LCD screen desktop display is often only 1,000 to 1 on average.
“The color is coquettish”, the organic materials of OLED can often emit very pure three primary colors of light when they emit light to combine into different colors, while the LCD screen is subject to backlight technology and passive color display, and the color gamut is not as rich as OLED. of.
“Evenly bright”, because the LCD display needs backlight as support, and most of the backlights are “side-in”, so the uniformity of illumination is relatively general. OLED is much better in this regard. Since each pixel can emit light by itself, it is easy to control the uniformity of brightness, making the screen look more complete and unified.
“Fast action”, which is different from LCD screen display, requires time for liquid crystal molecules to deflect. Therefore, in terms of grayscale time (response time), OLED, which uses voltage to control pixels, is many times faster. In theory, The OLED screen can achieve a response delay of 0.1ms, while the response time of the fastest gaming fast IPS screen of the LCD screen is about 5ms.
So is it that OLED is invincible? Does OLED have no drawbacks at all? of course not.
“Screen burn-in” Since the color-emitting principle of OLED comes from organic matter, it has to consider the problems of organic matter loss and short lifespan. At the same time, because the service lives of red, green and blue sub-pixels of the same area are not the same, which leads to Once one of the colors (blue with the shortest lifespan) has accelerated loss, the normal display content will have a serious color shift, and even some kind of graphics will be formed by the loss area, which is the burn-in screen, which is why some OLED mobile phones and TVs are Causes of yellowing after prolonged use.
“Low resolution at the same resolution”, in order to solve the above-mentioned burn-in problem, OLED manufacturers generally control the lifespan of the three sub-pixels to be approximately equal by adjusting the size, position and quantity of the three sub-pixels of red, green and blue. In the early stage of the OLED market, Pentile arrangement will be used. Compared with the standard RGB arrangement, Pentile arrangement reduces one-third of the pixels, and the fineness is 2/3 of the same resolution LCD screen. Although with the development of the times, new changes have been made to the sub-pixel arrangement of OLEDs, such as the diamond arrangement of Samsung and the pearl arrangement of Huaxing Optoelectronics. These arrangements have increased the density and density of OLED pixels, but the highest is only 83 % or so, there is still a certain gap with the standard RGB vertical arrangement.
“High frequency flicker” PMW dimming has been well-known because of some news in the mobile phone circle in recent years, especially the new iPhone released last year, because it is equipped with high-frequency PMW dimming technology, many users have complained about watching it for a long time My eyes can’t stand it. So what is PMW dimming? PMW dimming is a kind of pulse dimming technology, the principle is more complicated, simply take the lamp as an analogy, the normal adjustment of desk lamp brightness is realized by turning the knob to adjust the size of voltage and resistance (DC dimming); while PMW dimming is It is to control the brightness by switching the light on and off in a very short period of time and using the persistence of light by the human eye. This is because the OLED screen is forced to use uneven screen display at low brightness.
OLED light-emitting principle
OLED (English name: Organic Light-Emitting Diode, Chinese literal translation: Organic Light Emitting Diode) is a light-emitting technology of organic materials. It was first developed by the French in the 1950s, and was later developed by Kodak in the United States and Cambridge University in the United Kingdom. Companies such as South Korea’s Samsung and LG started mass production in the 21st century.
The most typical structure of OLED is the “sandwich-like” type, which consists of a thin and transparent indium tin oxide (ITO) with semiconducting properties, which is connected to the positive electrode of electricity, plus another metal cathode to construct a hole transport. Layer (HTL), light-emitting layer (EL) and electron transport layer (ETL) three structures. When a certain voltage is given, the electrons of the anode and the cathode will meet and combine in the light-emitting layer to generate photons. The light-emitting layer has a special organic material (O in OLED) to turn into the three primary colors of red, green and blue together with photons.
OLED basic structure: 1. Cathode ( ); 2. Emissive Layer (EL); 3. Anode holes and cathode electrons combine in the emissive layer to generate photons; 4. Conductive Layer; 5. Anode (+), from Wikipedia.
To use an easy-to-understand analogy, the principle of OLED is like “electrocuting” organic materials. As soon as the cathode and anode are electrified, the organic material is “electrically luminous”. Since the three primary color points of red, green and blue in each pixel can be controlled by a separate voltage to emit light, there is no need for a large-area backlight as the “bright source” of the screen, so this technology is also called self-illumination technology.
From the light-emitting principle of OLED, we can see that it is much simpler in principle than LCD technology. At the same time, OLED has many advantages over traditional LCD screens. Although it has some flaws, it still does not hide its flaws. The most important definition of a good screen should be to be able to restore the true colors of the world as much as possible, and OLED can certainly do well in this regard. But is OLED the ultimate answer? You can look at the following two techniques.
Mini LED and Micro LED
Among them, we have already talked about the Mini LED technology in the last issue. The principle is to change the original LED backlight board to consist of thousands of individual LED lamp beads, of which multiple LED lamp beads form an LED backlight matrix, and each backlight The matrix can be turned into a separate light control area. Take a top Mini LED gaming screen on the market as an example, it has 4096 LED lamp beads, each of which can form a light control area, that is, there are 2048 separate light control areas.
The advantage of this is that Mini LED also has the same ultra-high contrast as OLED and more refined and adjustable local brightness. When the black area is displayed, the lamp beads in this area are in an off state, so theoretically Its contrast ratio is equal to that of OLED displays, and there is no risk of screen burn-in when OLED displays are displayed for a long time. Another major advantage of Mini LED is that its independent regional lamp beads can stimulate a large brightness in a short time. Some excellent Mini LEDs can achieve the highest local brightness of 2000 nits. Common Mini LEDs LEDs can also basically pass the HDR1000 certification, which makes Mini LED very friendly to HDR content, and high-quality Mini LED can be comparable to OLED in the display of HDR content.
However, at present, Mini However, at present, Mini LED is only a newly perfected screen type, and it cannot get rid of the problems of poor viewing angle and narrow color gamut inherent in LCD screens. If you want to solve the problem of narrow color gamut, you must add more to the Mini LED display. A layer of quantum dot film (QLED technology) is used to increase the color gamut, but doing so will greatly increase the cost of the display, which is not worth the loss. At present, high-end Mini LED displays can already achieve the level of high-end OLEDs, and at the same time, there is an advantage of about 15% in cost control.
Thanks to the aggressive attitude of domestic screen manufacturers towards the Mini LED market, in the next five years, Mini LED backlight technology will gradually become the mainstream backlight technology for mid-to-high-end displays, and its technology will continue to improve, and the number of lamp beads will increase. The technology of zoning control has also been continuously improved.
At present, although OLED has occupied most of the market of self-luminous screens, and Mini LED is ready to move, there is still a “new emperor” that has been conceived, and its “kingdom” spirit has made the former two feel Threat, it is Micro LED.
The display principle of Micro LED (English: Micro Light Emitting Diode Display, Chinese literal translation is light-emitting diode display) is to thin, miniaturize and array the LED structure design of the three primary colors of red, green and blue, so that its size is only 1~10 The micron level is around; then the micron-level LEDs are transferred to the circuit substrate in batches, and then circuits and transistors are installed under each micron LED to complete a simple Micro LED display.
Each pixel of the Micro LED contains three RGB LED sub-pixels that can be self-illuminated and independently controlled. Taking the Crystal LED, the first Micro LED product launched by Sony in 2012 as an example, the display has an area of 55 inches and a resolution of 19201080. The number of its micro-LEDs is 19201080*3=6220800. Compared with the high-end Mini LED display area with about 20,000 lamp beads, the technical difficulty of Micro LED has increased more than a little.
Introduction to Micro LED from Samsung Display official website
Because Micro LED uses a self-illuminating single micron-scale LED, it is outstanding in color performance. The FWHM of the main wavelength of the micro-LED emission spectrum is only about 20nm, which can provide extremely high color saturation. Usually greater than 120% NTSC. This is almost identical to the color gamut offered by current top-of-the-line OLED displays. At the same time, due to the stability of LED inorganic substances, the color can maintain consistency and stability no matter how long it is used, which is unmatched by OLED. At the same time, Micro LED also takes into account the characteristics of displaying pure black, and it is pure black at the pixel level, which is more direct and pure than the partitioned backlight control of Mini LED.
Introduction to Micro LED from Samsung Display official website
Another point that Micro LED can achieve is power saving and ultra-high brightness. In traditional LCD TVs, the display efficiency is about 3%, and the loss of TFT in LCD is very small because it is voltage-driven. But LCDs are inefficient due to energy losses in color filters, polarizers, and LC materials. Because of its simple structure and low energy consumption, Micro LED has higher photoelectric conversion efficiency. The power consumption can be as low as 10% of LCD and 50% of OLED, which greatly reduces unit power consumption and allows higher power consumption. The energy is used to directly emit light, allowing the highest brightness to go to nearly 2000 nits.
Introduction to Micro LED from Samsung Display official website
Micro LED combines almost all the advantages of OLED and LCD, taking into account high brightness, high color gamut, high contrast, and long life, power saving, and flexible screen. It can be said that it is the master of the future screen, so why has Micro LED not popularized with so many advantages?
It can be said that success or failure is also Xiao He. The advantage of Micro LED comes from its millions of micron LEDs, and the difficulty also appears in this. At present, Micro LED mainly has three technical difficulties and problems, quantum efficiency Droop effect (limited effective light emitting surface, low efficiency of red LED), driving capability matching problem (high current, low power consumption driving materials), mass transfer problem (High process requirements, high precision requirements and high cost). The most important problem is the mass transfer problem.
Mass implantation technology is currently the mainstream and ideal manufacturing technology for Micro LEDs. Since Micro LEDs are diodes with micron-scale units, they need to be manufactured on silicon wafers instead of directly on the screen substrate. Therefore, it is necessary to transplant the micron LEDs produced on the silicon wafer to the substrate of the screen. The transfer technology among them is called mass transplantation. Since the micron LED chips to be transferred are about 1/10 of the size of a human hair, they require highly precise and refined operations; tens of thousands or even hundreds of thousands of LEDs need to be moved at one time, which is a huge number and requires extremely high Transfer rate, which makes the realization of this technology more difficult.
At the same time, the cost of manufacturing a large number of micro-LEDs is also relatively expensive. Taking a 2K resolution Micro LED screen as an example, it requires 11.05 million micro-LEDs to achieve. Under the current manufacturing difficulty, it determines the cost of Micro LEDs. And the price is definitely not cheap. At present, in the civilian field, Micro LED has not yet officially mass-produced products. The last product that was relatively close to us was Samsung’s The Wall commercial screen. Samsung’s The Wall TV is composed of 806.4×453.6mm Micro LED panel modules. , each module has 960×540 resolution, borderless design, can be perfectly spliced. Each module has 250 – 2000 nits brightness, about 10,000:1 contrast ratio, 16bit color depth, and up to 100/120 Hz refresh rate. The screen size can be freely combined through the splicing of modules, and products up to 292 inches can be selected. The price is also over a staggering $100,000.
Although Micro LED still has many difficulties in technology, cost and manufacturing, it does not stop the desire of major screen manufacturers and big brands for it. Apple, one of the world’s most successful technology brands, will start deploying Micro LEDs in 2020. Apple has partnered with Taiwanese LED manufacturer Epistar and Taiwanese LCD panel maker AUO to build a new factory, which will be located in Hsinchu Science Park. The Longtan branch of the park, where Apple’s total investment is estimated to be NT$10 billion ($334 million). In a public report, Apple said: “Like OLED, Micro-LED is self-emissive. However, compared to OLED, Micro-LED can support higher brightness, higher dynamic range, and wider color gamut. , while enabling faster update rates, wider viewing angles, and lower power consumption, qualities Apple favors.”
After the popularity of Micro LED, it is believed that it will become the first choice of screen material in the future, and its modular assembly method allows the screen to be customized according to the user’s wishes, so that the screen can also enter the “DIY era”.
Summary: In this issue of “Hardware Chronicle”, we analyze and summarize the implementation technologies, advantages and disadvantages of the three major product lines of the current self-luminous screen camp (OLED, Mini LED, MIcro LED). The current self-luminous screen is relatively mature and the product with high market acceptance is OLED, but OLED will not dominate the self-luminous screen camp for a long time, because Mini LED will achieve corner overtaking in recent years, waiting for partition backlight technology After the control chip is mature, its advantages of long life, no attenuation, and no screen burn-in will be highlighted. Micro-LED is the general trend of screen development in the next 20 years. Modular and miniaturized product form, high brightness, wide color gamut, high contrast, power saving, and fast response features make it the last laugh.