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On August 12, Samsung brought the Galaxy Z Fold3 at the press conference, and then Samsung Display Weibo said that the phone used ECO2 OLED™, which replaces the traditional core material polarizer through the screen lamination structure to improve light transmittance and reduce panel power consumption. This is the first application of polarizer-free AMOLED products in consumer electronics, why can this technology reduce panel power consumption? How is the polarizer removed? This article will provide a brief analysis of this technique.
ECO2 OLED™
<b>What is a polarizer? </b>
Schematic diagram of common liquid crystal panel structure
Polarizers are used to turn light into fluctuations in a certain direction, known as polarization. The polarizers that can often be seen in daily life include 3D glasses in cinemas, polarizers for photography enthusiasts, sunglasses with polarization functions, etc. 3D glasses in cinemas are polarizers that are perpendicular to each other in the direction of transmission, so that the images seen by the two eyes are slightly different, and the stereoscopic sense of viewing is realized through brain supplementation.
Simple schematic diagram of common LIQUID crystal panel structure
In digital products, LCD screens are a large consumer of polarizers. Polarizers are used on LCD screens to form polarized light, which in turn transmits light to form color and chiaroscuro contrasts. An LCD screen will use two polarizers, the lower one is used to convert the ordinary backlight into polarized light, and the upper one is used to analyze the polarized light formed after liquid crystal modulation, and then form a chiaroscuro and color contrast. The specific principle we will not repeat here, interested readers can search for relevant science. <b>Application of polarizers in AMOLED screens</b>
AMOLED screen and LCD screen display principle is not the same, AMOLED screen equivalent to millions of small LED lights, by controlling the brightness of each LED light, shade and flicker frequency to adjust the color and brightness, so theoretically AMOLED screen display does not need polarizer.
OlED structure diagram
But the problem is that the AMOLED substrate will reflect light, it looks like a silver reflective tin foil, the kind that will not be wrinkled, resulting in a slightly stronger ambient light, the screen because of the reflection, the display effect and contrast will be greatly reduced. Therefore, the AMOLED screen uses a relatively special polarizer - circular polarizer - to reduce reflections. As shown in the following figure, the light enters the polarizer and becomes a polarized light in the vertical direction, and after reflection, it becomes a horizontal polarized light, so it cannot pass through the polarizer and reflect it into the human eye. That is to say, on AMOLED screens, the purpose of the polarizer is to reduce the reflection.
The role of OLED polarizers
The absence of polarizers can increase theoretical brightness and reduce the bend radius of the screen, so it is the first to be used on folding screens that are relatively insensitive to cost.
OlED display technology requirements for polarizing devices
<b>The biggest problem with polarizers is that the screen brightness is reduced. </b>This problem is the same as LCD backlight, the polarizer can theoretically only let half of the light through, Samsung Display's Weibo said that "when the light passes through the polarizer, the brightness drops by more than 50%"; Huaxing Optoelectronics' patent also mentions that "in general, the transmittance of the polarizer is only about 43%, so the attachment of the polarizer affects the display efficiency of the OLED display device and increases the cost." In order to provide visibility under strong light in the daily use of mobile phones, AMOLED panels have to continue to increase brightness, resulting in increased power consumption and reduced life.
<b>Another problem with polarizers is the increase in screen thickness. </b>The thickness of the polarizer is about 80-100μm, in the past few years, AMOLED polarizer is not a big problem, the cost is not expensive, the application is also mature, the thickness is much lower than the LCD screen, until the folding screen comes. In the design of the folding screen, the mobile phone screen must be as thin as possible, so as to reduce the bending radius during the bending process, prevent the plastic part of the folding area from stretching for a long time, resulting in a change in length, and finally form a fold, reduce the screen life, and the thickness of the polarizer becomes one of the obstacles to the application of the folding screen. The Galaxy Fold 3 is designed with a stylus, so a layer of Wacom electromagnetic film is added, with a thickness of about 80μm, and the application of polarizer-free technology is also to reduce the thickness of the screen.
<b>How is AMOLED screen polarizer achieved? </b>
From the name of BOE's non-polarizer technology, BOE calls this technology COE, Color filter On Encapsulation, etc.
Polarizer-free technology is to complete the traditional AMOLED package, and then coated with a layer of color filter (Color Filter), each pixel on the corresponding pixel color film, pixel gap through the black matrix (BM, Black Matrix) separated.
<b>This layer of color film can solve the problem of reflection and light transmission. </b>As shown below, the unwanted part of the external incident light is absorbed by the black matrix (BM) region, and through the incident part of the green color film, it either becomes the green pixel and is blocked by the black matrix (BM) area in the process of reflection, or absorbed in other color film regions. Not to mention the problem of light transmission, the main wavelength transmittance of color film for RGB is higher, about 70-90%, at least much higher than the polarizer.
OLED polarizer-free product technology
As a result, polarizer-free technology can consume less power on the screen at the same display brightness, or at the same power consumption, the screen brightness is brighter. The Samsung ECO2 OLED™ reduces power consumption by 25%, which is how it comes.
In addition, because this layer of color film is steamed up, the thickness is only a few microns, so it can greatly reduce the thickness of the screen, more conducive to extending the life of the folding screen, while saving the cost of polarizers.
Therefore, in addition to Samsung, major domestic manufacturers are stockpiling related technologies. This technology is called CFOT in Pegasus, Color Filter On TFE; COE in BOE. According to an article by Omdia in December last year, almost every OLED factory in the country has its own COE plan.
Tianma sent a message on March 29 this year: Hubei Yangtze River New Display Industry Innovation Center Co., Ltd. (hereinafter referred to as "Innovation Center"), a wholly-owned subsidiary of Tianma, held the first product lighting ceremony of the experimental line. The illuminated folding screen integrates the industry's most advanced CFOT technology, which is significantly lower in power consumption than the existing folding screen, and the thickness of the polarizer is omitted, which is more suitable for folding products and can achieve the ultimate bending radius. <b>Polarizer-free structure AMOLED popularity? Cost and yield issues also need to be addressed. </b>
As mentioned above, the polarizer-free structure needs to be steamed on the already prepared AMOLED screen with a layer of color film, and the alignment of this process and the prevention of deformation are the key influencing factors of yield and price.
To achieve a polarizer-free structure, it is necessary to increase the investment of four or five reticles, and these four or five reticles are the key to the problem. As can be seen from the above figure, the filter on the color film and the pixels below must be aligned, otherwise the display will be biased, and the accuracy requirements are very high. Secondly, the current polarizer-free structure AMOLED screens are based on thin film packaging, in other words, plastic. The current mask processing and materials mostly use high-temperature processes, so the need to use low-temperature photoresist and low-temperature processing technology, even the current low-temperature process, the temperature is also 90 degrees Celsius, the film may still appear deformation, resulting in a decline in the final yield. Judging from the Samsung Galaxy Fold 3, the two technologies behind Samsung have reached the standard of mass production.
<b>summary</b>
In a sense, the polarizer on AMOLED is a compromise in the development of the AMOLED industry, but this compromise has been compromised for more than ten years, until the folding screen technology requires every effort to reduce the thickness of the screen. AMOLED polarizer-free technology can effectively increase brightness, reduce power consumption, and compress thickness, but this technology wants to be applied on a large scale, and there are still many difficulties in the manufacturing process. The Galaxy Fold 3 is off to a good start, and I hope other screen makers will follow suit as soon as possible.