The development history, function, utility and market forecast of polarizers for OLED. OLED is also called organic light-emitting diode, and the biggest difference from the mainstream LCD display technology in the current market is the information displayed by self-luminous control of organic materials.
OLED has all the characteristics of solid state, active illumination, ultra-high contrast, ultra-thin, low power consumption, no viewing angle limitation, easy to realize flexible display and 3D display, etc. It 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 utility of key material polarizers in panel production, and the development trend of polarizers due to the development of OLED research and development technology and applications.
Development history of polarizers for OLED
At present, the largest raw material used in the production of polarizers on the market is TAC film, and in order to achieve specific optical effects, comprehensive cost and other factors, the market has used PET, COP, PMMA and other materials to replace a part of TAC materials.
Among the products used in OLEDs, for its core PVA part, Nitto has begun to use PVA coating to realize 5 micron polarizers in the iPhone.
At present, the shortest lifetime of OLED luminescent materials is the blue light part. Japan's Masaya Adachi et al. proposed the concept of BECP, and by adding a layer of cholesteric liquid crystal to the inner layer of the circular polarizer, the efficiencies of the blue portion of the OLED are improved by nearly 50%. The overall OLED energy consumption can be reduced by 17%.
In 2012, researchers such as Norio Koma suggested that adding photochromism to the outer layer of the polarizer can be used to enhance the contrast of the product in outdoor sunlight.
The organic functional layers and electrodes in the OLED panel are sensitive to oxygen and water in the air, and the OLED panel is easily corroded after contact to reduce the life.
In 2014, Hefei Optoelectronics applied for a patent on polarizers for flexible OLED screens: in order to further reduce the reflected light of the polarizer and enhance the barrier to moisture and oxygen.
In 2014, Peng Meizhi and others from the Taiwan Industrial Technology Research Institute jointly published the research results on the compensation film technology for AMOLED. Among them, there are two kinds of compensation films produced in the industry, one is the extension process, the other is It is the way of liquid crystal coating.
At present, most of the extension processes are compensation in a narrow wavelength range, and the compensation of a wide wavelength range is required to be realized by a multilayer film stack. The optical properties of liquid crystal coated products are easy to adjust, and thinner compensation methods can be realized. Nitto Denko, Fuji and DNP have been developed.
In August 2016, Taiwan Industrial Technology Institute released a circular polarizer made by full coating at the touch panel and optical film process, equipment and materials exhibition. The total thickness is only 30 microns, and it is more resistant to processes up to 100 °C. It has passed the 100,000-degree deflection test with a curvature of 3 mm and can be used first in flexible OLED products.
At the IDW Display International Conference, which opened in Fukuoka, Japan from December 7th to 9th, 2016, AUO presented a two-way foldable AMOLED display with both internal and external displays bent 180 degrees.
Principle of polarizer for OLED
The basic structure of the OLED polarizer is divided into a polarizing portion (polarizer) and a 1/4λ functional compensation portion (1/4λ wave plate). The most ideal state of the polarizer requires a degree of polarization >99.9%, a transmittance of 45% or more, and a 1/4λ compensation portion requires compensation of the full wavelength of the visible light region.
At present, the optical parameters of the highly polarized polarizers that have been routinely achieved in the industry are polarization degrees >99.9%, and transmittances of about 43%.
Recently, the luminescence lifetime of OLEDs has been improved, reaching 50,000 hours from the past 5,000 hours, but from the perspective of energy saving of OLEDs, higher transmittance is required, considering the balance between luminous efficiency and lifetime of OLEDs. Transmittance needs to be as high as possible in the case of basically satisfying one black. At present, there is a successful precedent in the industry to adjust the polarization to achieve higher transmittance.
Functional requirements for polarizers for OLEDs
The functional requirements of polarizers for OLEDs are divided into three areas:
Bending requirements: In order to match the rewound performance of the OLED panel, the circular polarizer for OLED needs to be thin enough and has certain bendability. The current demand for bendable parts in the industry is basically between 60-70 microns, and it is necessary to be able to perform 100,000 tests under 2 mm curvature in terms of bending performance.
Reliability requirements: The application of OLED display panels has covered everything from consumer products to industrial vehicle products. LG and automotive manufacturers have cooperated to introduce OLED panels into automotive products.
Since the polarizer is in the outermost part, it is required to meet the standards of the vehicle product for its high temperature resistance and temperature and humidity resistance, such as high temperature 95 ° C × 500 hrs, temperature and humidity 65 ° C × 93% × 500 hrs and the like. After such severe testing, the product needs to ensure that its optical change is less than 3%, and no bubbles, delamination or peeling may occur.
Scratch resistance 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 affects the screen display, so the surface needs to be hardened, and at the same time, a certain anti-friction requirement is required.
The utility of polarizers for OLED
The OLED display panel itself is a self-luminous display mode, but when the external light source is reflected on the metal electrode of the OLED, it will cause reflected light interference on the display surface of the OLED to reduce the contrast.
Therefore, in the structural design of the OLED (Fig. 1), a polarizer with a 1/4 lambda plate is placed on the outer layer to block the reflection of external light to ensure a high contrast of the screen.