Image distortion-free transparent elastic substrates show the potential of next generation displays

Stretchable display materials gaining traction in the next generation of display markets have the advantage of being able to stretch and bend freely, but limitations on existing materials have resulted in poor distortion screens and fit.

Common elastomeric substrates tend to cause distortion due to the “Poisson ratio” phenomenon, and when extended in one direction, the screen contracts vertically. In particular, electronic devices that are in close contact with the skin, such as wearable devices, run the risk of wrinkles and pulling skin during stretching or shrinking, resulting in poor fit and performance.

The research team, led by Dr. John Gong, son of the Korea Institute of Science and Technology (KIST), and Professor Yongtaek Hong, of Seoul National University, has developed a stretchable substrate that matches the nanostructures that dramatically reduce the Poisson ratio. This work is featured in Journal Advanced Materials.

This study is worth noting for its ability to reduce Poisson’s ratio while maintaining transparency and simultaneously solve problems with screen distortion and light scattering.

Researchers achieved this by combining two important ideas. First, they used block copolymers, which are polymer blocks linked to align internal nanostructures. Block copolymer (SIBS) is made up of hard polystyrene (PS) and soft polybutylene (PIB) that are placed in one direction to maximize differences in elasticity between parallel and vertical directions and reduce shrinkage.

The Poisson ratio of traditional elastomers is 0.4-0.5, but researchers reduced this to a Poisson ratio below 0.07. In other words, even in the stretch direction, there is little contraction perpendicular to the substrate, and screening distortion is significantly reduced.

The second idea was to introduce a shear transfer process to ensure even alignment of the nanostructures across the substrate. Apply uniform shear forces at high temperatures using the difference in roller and stage speed. This process allowed the nanostructures to be reliably aligned on thick substrates without compromising transparency.

In the experiment, the researchers found that even when the substrate extends more than 50% vertically, there was little longitudinal contraction.

The researchers applied the developed substrate to actual devices and observed changes in pixel placement. Traditional elastomeric substrates showed distortion at 50% elongation at juggled intervals between pixels or stacked vertical pixels.

On the other hand, the substrates suited to the nanostructures had even pixel arrangements, resulting in unbroken images and transparency without wrinkles or rough surfaces.

The new stretchable substrate is expected to be used as a core material in a variety of fields, including next-generation displays, wearable electronics, and solar cells. Furthermore, the shear rotation process used in this study can be applied to other block copolymers and polymer films, making it a suitable technique for processing large areas in a simple way.

“This study proposes a new method for developing a completely transparent stretchable substrate with no strain, by precisely controlling the nanostructures, and the shear rotation process to implement it can be easily applied to mass production and industrialization.”

“We are currently researching to achieve a real display device that is free from distortion even when pulling by using this board to transfer the light-emitting device of the display.”