Flexible liquid crystal using "no substrate" challenge organic El

1. Preface

This article will cover the topic of the organic El display, the mainstream of the LCD panel challenge of the flexible display, "22th International display workshops (IDW ' 15)" in Asia's largest display international conference. At the center of the research results of the rattan hanging stone pot in northeastern University of Japan, together with the basic technology.

　　2. The LCD can also be bent to maintain the substrate spacing

In a variety of flat panel display (FPD), the organic El display is able to achieve high contrast, high color performance range, and easy to achieve ultra-thin, so as the mainstream technology of flexible display is concerned about, the author in the previous article also introduced this point (see this site).

The representative electronic paper-electrophoresis display (EPD) has long been put into practice as a flexible display. Furthermore, the display is a reflective type with memory and no backlight, and it does not require a polarizer, so it can be displayed in a bright, non-colored color. However, there are problems in color display and video display. This has also been introduced before (see this site).

and LCD display can achieve large-screen, high-precision, with the ability to achieve a large range of color performance, as a high-quality display has been widely used. Moreover, the LCD screen manufacturing technology is very mature, before long, China's production will become the first. However, liquid crystals are always fluid, must be ordered, so, with the development of extremely thin and flexible, LCD screen in the display quality of the stability of the problem.

Figure 1 is the basic structure of a flexible liquid crystal display. To achieve a practical flexible LCD display, the focus is to use the micro-polymer formed in the liquid crystal box (polymer spacer wall) network structure, to maintain a certain gap in the LCD box, and maintain a stable orientation. The laboratory of rattan hanging stone pot of Northeastern University of Japan, through the dissolution of molecular orientation polymer material (resin) in the liquid crystal, UV pattern exposure, developed a non-destructive liquid crystal orientation, can make 2 substrates at fixed intervals bonded polymer spacer wall, to "invited advanced polymer and LC Technologies for high quality flexible displays "addressed the topic (Thesis Number: flx2/lct5-1). Figure 1, "Bonding polymer Wall Spacer", is the polymer they developed next door.

Figure 1: Basic structure of the flexible liquid crystal display (information provided by Tohoku University)

Also, flexible LCD displays require a flexible backlight. Figure 2 is a thin-type flexible backlight light guide plate with the use of liquid crystal polymer composite film, to help achieve high-contrast, power-saving local dimming backlight system. The liquid crystals and polymers in the composite membrane have molecular orientations, which can be expressed as light scattering or transparent state by switching voltage.

Figure 2: Flexible thin local dimming backlight system (information provided by Tohoku University)

In addition, the local dimming backlight lamp automatically controls the brightness of the backlight according to the image, and has the function of improving the contrast of the image while reducing the power consumption. It can also suppress the "white" phenomenon of the black part of the image. In Figure 2, the image shows the area of the "moon" with the voltage "on", the "Light scattering state", and the voltage in the other areas as "OFF" and "Transparent state".

In the speech, Northeastern University also introduced the optical compensation of plastic substrates and liquid crystals, and announced the trial production and display quality of the VA mode and the IPS mode flexible LCD display. Throughout the 2015 IDW all theses, Northeastern University rattan hanging stone Pot Laboratory published this flexible LCD screen related papers stand out. Below, I will introduce my special interest in the content.

　　3. Support Technology of flexible liquid crystal display

　　3.1 Stainless steel foil is expected to become a substrate material

Tohoku University developed a VA-mode reflective flexible liquid crystal display using ultra-thin stainless steel foil with heat resistance and polycarbonate film (Teijin-made) as substrate material to "flexible reflective LCDs using stainless Steel Substrate and Optical compensation technology "gave a lecture on the topic (thesis Number: flxp1-4l).

The prototype of the reflective flexible LCD screen is shown in section structure 3. The surface of the two substrates is coated with polyimide film ("SE-4811" of the Nissan Chemical Industry) and is heated at a temperature of 120 ℃ and used as the orientation membrane for liquid crystals. The thickness of the LCD box is 2μm, vacuum filled with liquid crystal material (Germany Merck's "MLC-2038"). In order to achieve high contrast and wide viewing angle, biaxial tensile film, polarizer and light diffusion plate are also used. Figure 4 shows an example of a prototype flexible reflective LCD display (pictured when the stainless steel substrate is bent).

Figure 3: Sectional structure of a flexible reflective VA-mode LCD with stainless steel substrates (information provided by Tohoku University)

Figure 4: Display example of a flexible reflective VA mode LCD display using stainless steel substrates (information provided by Tohoku University)

The liquid crystal display has achieved a good contrast ratio of more than 20:1. It is proved that stainless steel foil is a substrate technology which is expected to realize high quality and low power consumption of reflective flexible liquid crystal display panel.

　　3.2 Solving the problem of flexible liquid crystal using In-cell polarizer

For the In-cell-type polarizer using a dye-like polarizing film and the results of its application in the TN LCD display, Tohoku University published the fabrication of Thin flexible Liquid crystal display using Dye-type In-cell polarizer "speech (Thesis No.: FLXP1-5L). Plastic substrates lead to poor display quality and narrow viewing angle, which is a major issue for flexible liquid crystal display panel. Moreover, 5 shows that the thickness of the polarizer and the phase difference plate caused the problem of poor flexibility. As a way to solve these problems, Northeastern University has come up with the idea of using In-cell polarizer.

Figure 5: Structure comparison of the usual flexible LCD display with the display using the In-cell polarizer

On the left is the usual flexible liquid crystal display, and on the right is a flexible LCD screen using the In-cell polarizer. (Information provided by Tohoku University)

The steps for making the panel are as follows. First, the Ito film (film Resistance is 100ω/-) is generated at low temperature on the dye-type polarizer, and the PVA-type adhesive is used to engage with the TAC (Triacetylcellulose) film and the Polarizer as the base plate. The TAC film is then bonded to the glass substrate with the release membrane (made in San Tin, Japan). Re-use of 250nm UV, irradiation polarizer surface for 6-12 minutes, so that after the surface degeneration, the use of spin coating to apply the light-oriented film. Using the spacer wall of the 5μm, the LCD box thickness remains fixed, using UV hardening resin to create a seal pattern, and then inject liquid crystals.

The voltage transmission characteristic of trial panel is basically equal to that of external polarizer. As shown on the right side of the 5, the display thickness is reduced to 0.2mm, achieving a thin profile. And the contrast ratio reached 260:1. Its characteristics are comparable to the TN LCD screen using the usual glass substrates (Figure 6).

Figure 6: TN LCD display with In-cell polarizer

(a) voltage on, (b) voltage off. (Information provided by Tohoku University)

3.3 extendable Non-substrate LCD display

In the future, the market needs not only flexibility, but also an extendable "telescopic display". To achieve this type of display, it is necessary to develop a component structure that eliminates the need for a substrate and enables high-contrast display. To this end, the researchers hold in the independent liquid crystal polymer composite membrane, the free control of the molecular orientation of the liquid crystal dispersion of the purpose of the UV pattern exposure based on the liquid crystal polymer phase separation.

Japan Northeastern University in a speech entitled "Morphological Control of the Liquid Crystal droplets in molecular-aligned polymer for Substrate-free LCDs" (thesis Item No: lctp5-9l) In this paper, the "non-substrate liquid crystal display" is introduced by controlling the molecular orientation of liquid crystals and polymers and using birefringence to achieve high contrast display.

Specifically, in the nematic liquid crystal (JNC manufacturing "Td-1013la") mixed with 50wt% two functional liquid crystalline monomer C (DIC), injected with the use of friction method to distort the orientation of the liquid crystal box (LCD box thickness of 10μm). Using orthogonal mesh-like masks (spaced 120μm or 60μm), the LCD box is exposed to ultraviolet pattern. Ultraviolet intensity is 3~100mw/cm2 and temperature is controlled in 25~50℃. The results show that the dispersed shape of the orientation liquid crystal can be controlled by changing the condition of phase separation, so that the polymer contains more liquid crystals which control the orientation.

Figure 7 is a polarizing microscope photograph of the voltage being loaded into the prototype device (TN liquid crystal droplet). Figure 8 is a comparison of the voltage transmission characteristics of the device with the typical TN LCD display. The characteristics of the TN liquid crystal droplets produced by the trial are slightly higher than the usual TN liquid crystal display, and the transmittance of the closed state is relatively high, and there is room for improvement. But through this development, obtained the use of polymer orientation to maintain the liquid crystal orientation of the non-substrate LCD display of useful knowledge, the future development is worth looking forward to. Listening to the speech of Northeastern University, I can not help but think: "Through the use of multilayer coatings must rely on thin films to play a role in the flexible organic El Technology, is it possible to achieve no substrate?" ”

Figure 7:tn Mode liquid crystal droplet action (information provided by Tohoku University)

　　3.4 Verifying stainless steel foil with flat film

In order to study the possibility of metal foil as a flexible substrate, the use of a flat layer of stainless steel foil ("#190SB" made of Nippon Gold Housing). Nippon Steel (planarized stainless Steel Foil for flexible substrate) made a speech on stainless foil substrates (paper number: fmc3-1). The plate thickness of the substrate is 8~100μm, the surface state is super bright (SB), and the CTE of thermal expansion coefficient is 11x10-6/℃. A flat film is formed on the surface using spin-coating and roll-to-roll (R2R) coating methods respectively. By applying a flat film, the surface roughness Ra reduced from 6.2nm to 0.6nm,rmax from 78.2nm to 8.9nm.

When the organic El screen was produced, the company tested the gas released by the flat layer (3μm), and the data was small to be negligible. The results of the organic El illumination samples on the two substrates show that both of them can work properly. In addition, the thickness of stainless steel foil using R2R coating method is 50μm and the width is 300mm. The above results show that the stainless steel foil can be used as a flexible base plate.

3.5 Organic TFT Drive Plastic LCD display

The mobility of organic thin film transistors (OTFT) is 1~10cm2/vs, exceeding the 0.5cm2/vs of amorphous Si (A-SI) TFT. In this IDW, the British flexenable Company "invited Plastic Liquid Crystal displays Enabled by Organic transistor technology" as the title, introduced the use of TAC film , the results of using OTFT IPs LCD display for active-drive use. The 40μm-thickness TAC is a low birefringence, rth<1nm, ro<1nm with the same optical properties as the glass substrate and is suitable for use as a substrate for a liquid crystal display panel. The orientation control of liquid crystal adopts the light orientation, and the thickness of the liquid crystal box is controlled by using lithography to form spacers.

Through the use of newly developed liquid crystal materials with self-organizing polymer wall, it improves the robustness. Trial-produced Otft drive IPs LCD screen size of 4.7 inches, the screen width to height ratio of 16:9, the thickness of 300μm, remove the backlight after the weight of 10g. The device can be bent to a radius of 50mm degrees. The OTFT, which can be produced at low temperatures below 100 ℃, opens up a new application field for the active matrix backplane.

The Otft bend radius is 0.5mm,1 after the test of the threshold voltage change is very small, very stable. The change of the on-state current is equally stable.

In addition, as previously reported, the British plastic Logic company has begun to produce OTFT-driven electronic paper (see the Site report). In addition, the production of gas sensors, image sensors and X-ray detectors using OTFT has also been achieved.

　　4. Conclusion

This time, together with the basic technology, introduced a lot of content, I think that reading should be very enjoyable, I do not know how you feel?

The local dimming backlight, In-cell polarizer (unfortunately not coated type), non-substrate LCD screen, etc., are excellent research results. The systematic research and development for the realization of the flexible liquid crystal display is an important condition for Japan to win international competition in this field in the future. However, as previously reported, Flex Enable has developed the OTFT-drive IPs LCD display by working with Merck. Japan also has to speed up.

But now it is said that "the university does not teach display-related courses", can only say that such a view of lack of predictability. From a long ago, "lighting Engineering" from the electrical disciplines, "heat transfer Engineering" from the course of mechanical subjects disappeared. The people who set up the course have absolutely no idea how important these technologies are when developing a smartphone. I hope to tell the students and young technical staff, if the sighted of the words of the people seriously, Japan will have no future.

Flexible liquid crystal using "no substrate" challenge organic El