Flexible, bendable, roll-up displays covering large areas at low cost

Wearable information terminals (left) and large rollable-screen TV (right) are made using super-flexible LC technology covering large areas, with high-resolution at low-cost.

Researchers at Tohoku University have developed a super flexible liquid crystal (LC) device, in which two ultra-thin plastic substrates are firmly bonded by polymer wall spacers.

The team, led by Professor Hideo Fujikake and Associate Professor Takahiro Ishinabe of the School of Engineering, hopes the new organic materials will help make electronic displays and devices more flexible, increasing their portability and all round versatility. New usage concepts with flexibility and high quality display could offer endless possibilities in near-future information services.

Previous attempts to create a flexible display using an organic light-emitting diode (OLED) device with a thin plastic substrate were said to be promising, but unstable. The plastic substrates are poor gas-barriers for oxygen and water vapor, and the OLED materials can seriously be damaged by their gasses. As for flexible OLEDs, there has also been no device fabrication technology established so far for large-area, high-resolution and low-cost displays.

To overcome these challenges, Fujikake’s research team decided to try making existing LC displays flexible by replacing the conventional thick glass substrates, which are both rigid and heavy, with the plastic substrates, because LC materials do not deteriorate even for poor gas barrier of flexible substrates.

Flexible LC displays have many advantages, such as established production methods for large-area displays. The material itself, which is inexpensive, can be mass produced and shows little quality degradation over time.

However, in conventional flexible LC displays, one important problem remains. The gap of plastic substrates (100 ?m thick) sandwiching an LC layer becomes non-uniformed when the LC device is bent, causing the display image to be distorted.

In their study, Fujikake’s team developed a super-flexible LC device by bonding two ultra-thin transparent polyimide substrates (10 ?m thick approximately) together, using robust polymer wall spacers.

The structure of super-flexible LC device is created by ultra-thin plastic substrates bonded by polymer wall spacers.

The ultra-thin transparent substrate is made using the coating and debonding processes of a polyimide solution supplied by Mitsui Chemicals. The result is a flexible sheet, similar to food-wrapping cling film.

The ultra-thin polyimide film (left) was formed by coating and debonding processes, and the roll-up resistance (right) was tested for developing super-flexible LC devices.

The substrate has the attractive features of heat resistance, and the ability to form fine pixel structures, including transparent electrodes and colour filters. The refractive index anisotropy is extremely small, making wide viewing angles and high contrast ratio possible.

The polymer wall spacers bonding substrates are formed by irradiating a twisted-alignment LC layer including monomer component with patterned ultra-violet light through single thin substrate. While the substrate gap is more variable as the substrate thickness is decreased, the stabilization of ultra-thin substrates becomes possible by small pitch polymer walls.

The research team also demonstrated that the device uniformity is kept without breaking spacers even after a roll-up test to a curvature radius of 3mm for rollable and foldable applications.

The above research results show that LC displays with large-area, high-resolution and excellent stability can be as flexible as OLED displays. The super-flexible LC technology is applicable to mobile information terminals, wearable devices, in-vehicle displays and large digital signage.

Moving forward, the team plans to form image pixels and soften the peripheral components of polarizing films, and a thin light-guide sheet for backlight.

Learn more: Super-Flexible Liquid Crystal Device for Bendable and Rollable Displays



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‘Greener,’ low-cost transistor heralds advance in flexible electronics

This transparent transistor, which functions even when wrapped around a thin pen, could help make flexible electronics widely accessible. Credit: American Chemical Society

This transparent transistor, which functions even when wrapped around a thin pen, could help make flexible electronics widely accessible.
Credit: American Chemical Society

As tech company LG demonstrated this summer with the unveiling of its 18-inch flexible screen, the next generation of roll-up displays is tantalizingly close.

Researchers are now reporting in the journal ACS Nano a new, inexpensive and simple way to make transparent, flexible transistors — the building blocks of electronics — that could help bring roll-up smartphones with see-through displays and other bendable gadgets to consumers in just a few years.

Yang Yang and colleagues note that transistors are traditionally made in a multi-step photolithography process, which uses light to print a pattern onto a glass or wafer. Not only is this approach costly, it also involves a number of toxic substances. Finding a greener, less-expensive alternative has been a challenge. Recently, new processing techniques using metal oxide semiconductors have attracted attention, but the resulting devices are lacking in flexibility or other essential traits. Yang’s team wanted to address these challenges.

The researchers developed inks that create patterns on ultrathin, transparent devices when exposed to light. This light sensitivity precludes the need for harsh substances or high temperatures. “The main application of our transistors is for next-generation displays, like OLED or LCD displays,” said Yang. “Our transistors are designed for simple manufacturing. We believe this is an important step toward making flexible electronics widely accessible.”

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‘Nano-pixels’ promise thin, flexible high-res displays

Still images drawn with the technology: at around 70 micrometres across each image is smaller than the width of a human hair.

A new discovery will make it possible to create pixels just a few hundred nanometres across that could pave the way for extremely high-resolution and low-energy thin, flexible displays for applications such as ‘smart’ glasses, synthetic retinas, and foldable screens.

A team led by Oxford University scientists explored the link between the electrical and optical properties of phase change materials (materials that can change from an amorphous to a crystalline state). They found that by sandwiching a seven nanometre thick layer of a phase change material (GST) between two layers of a transparent electrode they could use a tiny current to ‘draw’ images within the sandwich ‘stack’.

Initially still images were created using an atomic force microscope but the team went on to demonstrate that such tiny ‘stacks’ can be turned into prototype pixel-like devices. These ‘nano-pixels’ – just 300 by 300 nanometres in size – can be electrically switched ‘on and off’ at will, creating the coloured dots that would form the building blocks of an extremely high-resolution display technology.

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