What are flexible OLCDs made of?
March 25, 2020
The dominant active-matrix display technology today is LCD. It is made with amorphous silicon (a-Si) thin-film transistors (TFTs) on a glass substrate. Glass is used as the substrate due to its ability to withstand the high process temperatures required to fabricate the a-Si TFTs and its excellent optical properties. However, glass can only be bent to an extremely limited extent and is difficult to cut to non-rectangular shapes. It is also relatively thick and heavy, and can shatter, as you’ve no doubt seen on a friend’s mobile phone – or perhaps on your own.
At FlexEnable we have developed a glass-free version of LCD that uses high-performance organic transistors on flexible substrates – this innovative technology is known as Organic LCD (OLCD). The inherent flexibility of the organic transistors combined with the extreme thinness of the substrate allows OLCD to be applied to almost any surface.
The OLCD stack is similar to that of a glass LCD, but with thin TAC film in place of glass, and a complete low temperature materials set. The stack makes use of many of the materials in the existing LCD supply chain (polarisers, backlight, LC materials and others) – and as a result the flexible display cost structure is similar to a glass LCD. This makes OLCD much lower cost than other flexible displays such as OLED, and therefore cost-effectively scalable to large sizes like LCD.
Let’s take a look at some of the materials and components that are needed to make a flexible OLCD.
The hard ceramic materials used in a glass LCD (which require high-temperature inorganic TFT production) are replaced with soft, flexible organic materials that enable a low temperature manufacturing approach (below 100°C). FlexiOM™ is the name for the key organic semiconductor materials, supplied by FlexEnable. These inherently flexible materials have higher mobility values than amorphous silicon (~1.5cm2/Vs), lower off current than amorphous silicon and polysilicon and have excellent bias stress stability. Organic thin film transistor (OTFT) backplanes using FlexiOM™ materials are fabricated in standard flat panel display factories which are repurposed for the novel material set.
The ability to process a complete active matrix below 100°C allows, for the first time, the use of low cost flexible substrates. TAC film is employed for making OLCD because of its excellent optical properties. Moreover, TAC is being already used in displays as a substrate for polarisers and therefore has a ready-made and very low cost supply chain.
All transmissive LCDs require a backlight to function, and a significant part of a display’s characteristics and overall cost are determined by the choice of backlight. Broadly speaking, backlights can be divided into two categories: edge-lit and direct-lit. In an edge-lit configuration the light is provided by LEDs along one edge of a waveguide. Although an edge-lit solution can be curved, it is limited by the ability to make a bright enough backlight whilst maintaining a thin enough waveguide to keep the required level of conformability for the desired form factor. Instead of coupling from the edge, an array of LEDs can be used, emitting light directly towards the viewer. This is referred to as a direct-lit backlight. Such an array can be relatively easily curved to suit the shape of the display. Direct-lit backlights also allow local dimming to be employed, a technique which can increase the contrast of the display, and something used widely in recent implementations of HDR for LCDs. This approach is equally applicable to flexible OLCDs.
Flexible colour filters
A colour filter array (CFA) made on a flexible substrate is fabricated in the same way as a glass sample with the difference being the process temperature. Whilst conventional glass colour filters are often processed at >200°C, FlexEnable has developed a CFA process in which the highest bake temperature is below 100°C making it compatible with processing directly on low cost flexible substrates like TAC. Several materials companies are now producing colour photoresists that match the chromaticity and feature size that can be achieved on glass, but with a 100°C process temperature.
LC modes and polarisers
For conformed displays, curved to relatively tight radii (e.g. 10 mm) the user will be able to see multiple angles of a display simultaneously. This means that consistent wide-view characteristics are important for any conformed display applications as well as resistance to touch mura (for applications that are reliant on touch interaction).
In-plane switching modes are possibly the best fit for OLCDs conformed into curves and they are also most resistant to touch mura. In-plane modes benefit from the best viewing angles when they are accompanied by the appropriate wide-view polarisers.
The future of LCD is flexible
Flexible OLCDs produced in this way are lighter, thinner and more robust than their glass-based counterparts. Even in a flat state, these are desirable attributes for any display technology, but OLCDs can also be conformed to the curves of a product in ways that has not been possible until now. OLCDs are also much easier to form into complex shapes compared to the manufacturing complexities involved in cutting glass. They are also significantly cheaper and easier to manufacture than flexible OLED.
As more Gen 10.5 lines are built, there will be increasing pressure to either close or repurpose older, smaller LCD facilities. OLCD has been specifically engineered to make use of older display lines so that they can quickly make the move to flexible OLCD manufacturing, while maintaining much of the existing cost-optimized supply chain.
For more information on flexible OLCD, please get in touch with FlexEnable at firstname.lastname@example.org.