OLED Development history, functions, utility, and market forecast of polarizers. OLEDs are also known as organic light-emitting diodes, which are currently mainstream in the market. LCD Display technology The biggest difference is through the self-luminous control of organic materials. display Information.
OLED has all the solid state, active light, ultra-high contrast, ultra-thin, low power consumption, no viewing angle restrictions, easy to implement flexible display and 3D display and many other features, will become the most 'money scene' display technology in the next 20 years.
This paper briefly describes the progress of polarizer materials for OLEDs, the effectiveness of polarizers for key materials in panel production, and discusses the demand trends of polarizers with the development of OLED R&D technologies and applications.
The development of OLED polarizers
At present, the largest raw material for polarizer production on the market is the TAC film. In order to achieve specific optical effects, comprehensive costs and other factors, the market has used PET, COP, PMMA and other materials to replace a part of TAC materials.
In the products used in OLEDs, Nitto has begun to use the PVA coating method to achieve 5 micron polarization in the iPhone for its core PVA part.
At present, the shortest lifetime of the luminescent material of OLEDs is the blue light part. Japan's Masaya Adachi proposed the concept of BECP by adding a layer of cholesteric liquid crystal in the inner layer of the circular polarizer to achieve a nearly 50% improvement in the efficiency of blue light transmittance of the OLED. The overall OLED energy consumption can be reduced by 17%.
In 2012, researchers such as Norio Koma proposed that adding photochromism to the outer layer of polarizers can be used to enhance the contrast of products displayed in outdoor sunlight.
The organic functional layer and electrode in the OLED panel are sensitive to oxygen and water in the air. After contact, the OLED panel may be easily corroded to reduce the life.
Year 2014, Hehui Optoelectronics One of the patented applications for 'polarizer patents for flexible OLED screens' mentioned above is: In order to further reduce the reflected light of polarizers and to increase the barrier to moisture and oxygen.
In 2014, Peng Meizhi and others from the Taiwan Industrial Technology Research Institute jointly published AMOLED The use of compensation film technology research results, which mentions the current industry has two processes to produce the compensation film, one is the extension process, the other is the liquid crystal coating.
At present, most of the stretching processes are compensated in a narrow wavelength range. To achieve wide wavelength range compensation, multilayer film stacking is required. The optical characteristics of liquid crystal coating products are easy to adjust, and thinner compensation methods can be realized. Nitto Denko, Fuji and DNP have all been put into development.
In August 2016, Taiwan’s Industrial Technology Research Institute released a circular polarizer manufactured using the full coating method at the touch panel, optical film process, equipment, and materials exhibition. The total thickness is only 30 μm, and it is more resistant to processes up to 100°C. , Through 3mm curvature 100,000 deflection test, can be used in flexible OLED products.
From the 7th to the 9th of December 2016, at the IDW International Conference held in Fukuoka, Japan, AUO demonstrated bidirectional folding AMO. LED display , inside and outside monitor Can be bent 180 degrees.
OLED polarizer principle
The basic structure of the OLED polarizer is divided into polarizers (polarizers) and 1/4λ functional compensation (1/4λ waveplates). Polarizers need to have a degree of polarization > 99.9%, and a transmission rate of 45. % or more, and the 1/4λ compensation part requires the full wavelength of the visible light region to compensate.
At present, the optical parameters of the high-polarization polarizers in the industry, which normally achieve ideal conditions, are polarization degree > 99.9%, and the transmittance is about 43%.
Recently, the luminescence lifetime of OLEDs has been improved, reaching 50,000 hours in the past 5000 hours. However, from the perspective of OLED energy saving, higher transmittance is still required. Considering the balance between luminous efficiency and lifetime of OLEDs, Transmittance needs to be improved as much as possible when it basically meets the need of integration. At present, there are successful precedents in the industry for adjusting the degree of polarization in order to realize higher transmittance.
Functional requirements of polarizers for OLEDs
The functional requirements of polarizers for OLEDs are divided into three aspects:
Flexibility requirements: In order to match the flexability of OLED panels, circular polarizers for OLEDs need to be thin enough and have a certain degree of bendability. The current demand for bendable parts in the industry is basically required to reach 60-70 microns. In terms of bending performance, it needs to be able to perform 100,000 tests under 2 mm curvature conditions.
Reliability requirements: O LED display The application of the panel now covers everything from consumer products to industrial vehicle-mounted products. LG and automotive manufacturers have collaborated to import OLED panels into automotive products.
Since the polarizer is in the outermost part, its high temperature resistance and temperature and humidity resistance are required to meet the standards of automotive products, such as high temperature 95 °C × 500 hrs, temperature and humidity 65 °C × 93% × 500 hrs and other conditions. After undergoing such rigorous testing, the product needs to ensure that its optical change is less than 3%, and no bubbles, delamination or peeling can occur.
Anti-scratch performance requirements: Considering that the user directly touches the surface of the polarizer, if there is no hardening treatment, the surface is prone to scratches and affect the screen display. Therefore, the surface needs to be hardened, and at the same time, it must achieve certain friction resistance requirements.
Effectiveness of polarizers for OLEDs
The OLED display panel itself is a self-illumination display mode, but when the external light source is reflected on the metal electrode of the OLED, it will cause reflection light interference on the OLED display surface to reduce the contrast.
Therefore, in the OLED structure design (Figure 1), a layer of polarizer with 1/4λ wave plate is placed on the outer layer to block the reflection of external light to ensure that the screen maintains high contrast.
Figure 1 Polarizer structure and working principle for OLED
The early PMOLED products had only one color, two colors, etc. The requirement for polarizers was simply to reduce the external reflected light, and did not require the overall black state to be proposed.
The polarizer used at the time needed only a general Polarizer with 1/4λ wave plate to meet the demand. At the present stage of AMOLED, the product has been full-color, and the contrast ratio has reached 10,000:1 or more, which requires that the polarizer can be fully realized. Cut off the visible spectrum of the outside world, so as to achieve one black effect.
From Figure 2, we can see the difference between the dark state effects of the OLEDs before and after the polarizers.
Figure 2 OLED panel black effect diagram: 1 one black effect; 2 ordinary film 1 / 4λ polarizer anti-reflection effect; 3 no polarizer effect
On the other hand, in order to obtain a better one-piece black effect, the total visible spectrum reflectance needs to be sufficiently low and there is no special color display, and it is necessary to use an ideal full-visible spectrum while using a polarizer with a sufficient degree of polarization. 1 / 4λ materials. According to the industry's early analysis of the phase difference spectra of different materials, most of the materials are positive wavelength distributions.
Figure 3 shows the phase difference distribution of a single optical axis material, in which the distribution of APO material is the most ideal, and the 1/4λ material that is closer to the ideal state can be obtained by further superimposing the A-PO materials of 1/4λ and 1/2λ. - - That is inverse wavelength distribution.
Fig. 3 Phase difference map of a single optical axis
Figure 4 shows the phase difference distribution of superposition of multi-layer A-PO materials. In this paper, we use PC, COP and liquid crystal, which are the most commonly used compensation materials with polarizers, to make a practical comparison test. In theory, the lower the reflectivity, the The black effect is better. From the data comparison results, the COP compensating film has the best black effect. From the actual effect, it is also the best one of the COP compensating film.
Fig. 4 Phase difference map of superimposed A-PO materials
Figure 5 Comparison of reflectance of different materials
Figure 6 actual renderings
However, if considering the transmittance (using the same polarizer, the transmittance is 45.2%), the liquid crystal compensation film, the PC compensation film and the COP compensation film are bonded to each other, and the reflectivity Rt of different compensation materials is obtained in FIG. 7. The transmittance contrast Tt can be seen as follows: The liquid crystal compensation film basically has no effect on the overall transmittance after lamination, and the PC compensation film and the COP compensation film have an influence on the overall transmittance after lamination.
With the development of OLED thinning and flexibility, liquid crystal compensation film with a thickness of only 2 to 10 μm will become its biggest advantage.
Fig. 7 Comparison of reflectivity R t and transmittance T t for different materials
Therefore, based on the above evaluation, the liquid crystal compensation film will become the main development direction of the OLED polarizer in the future.
The OLED display device is rapidly developing toward the curved surface and the flexible direction. In order to satisfy the requirements of the surface and flexibility, the polarizers for the OLEDs used with them will also be developed in the direction of thinning and flexibility.
OLED polarizer market forecast
UBI Research, a market research organization, said that in the first quarter of 2017, AMOLED revenue reached 4.31 billion U.S. dollars, an increase of 15% over the same period of last year.
In terms of quantity, the number of AMOLED panels shipped in the first quarter was 99.1 million units, a year-on-year increase of 9%. It can be seen that the unit price of the panel has increased. This may be because most of the growth is from the high-end flexible AMOLED display and large size. OLED TV panel.
UBI expects that in the coming quarters, there will be more than 100 million AMOLED panel shipments, and the number of OLED TV panels will exceed 300,000. The AMOLED market will grow at a compound annual growth rate of 33%, and it will reach 59.3 billion US dollars by 2020. .
The flexible curved display device market is expected to grow strongly in the next few years. According to market analysts of industry research firm Touch Display, flexible and curved displays will account for 16% of the global equipment market in 2023.
