New polymeric material for transparent and flexible displays

Thursday, 31 January, 2019

New polymeric material for transparent and flexible displays

The next generation of flexible and transparent displays will require a high-performing and flexible polymeric material that has the optical and thermal properties of glass. The material must also be transparent to visible light and have a low coefficient of thermal expansion (CTE). But such a material has not yet been available — until now.

Researchers from the Korea Advanced Institute of Science and Technology (KAIST) have succeeded in making a new polymeric material with an exceptionally low CTE value while retaining high transparency and excellent thermal and mechanical properties. The method, developed for amorphous polymers with a controlled CTE, can be applied to control the thermal expansion of organic materials as well.

Most objects expand upon heating and shrink upon cooling, and organic polymers have a relatively large CTE compared to that of ceramics or metals. Thin, lightweight planar substrates for semiconductor devices should have a similar CTE to ceramics, otherwise the device may crack due to the stress caused by thermal expansion and contraction. Therefore, matching the CTE of the semiconductor device and the substrate is crucial for successful manufacturing of display devices.

Forming a network structure by connecting polymer chains is a well-known method of reducing the CTE of amorphous polymers; however, polymers with a network structure eventually lose their flexibility and become brittle. To avoid this, Professor Sang Youl Kim and his team decided to adjust the distance and interaction between polymer chains. Thermal expansion and contraction of polymer films can be minimised by introducing interactive forces between the polymer chains and by arranging the direction of the force to be perpendicular.

The team successfully implemented this approach by appropriately designing the chemical structure of a transparent polymeric material. It is called poly(amide-imide) film, which is a transparent, flexible and high-performing polymeric material. It is thermally stable enough to be used in the AMOLED (active-matrix organic light-emitting diode) fabrication process (stable at >400°C) with a low CTE (4 ppm/°C).

The team made IGZO TFT (indium gallium zinc oxide thin film transistor) devices on the newly synthesised transparent poly(amide-imide) film and confirmed that the device could indeed operate normally even when it is folded down to a radius of 1 mm. Their results have been published in the journal Science Advances.

“Our results suggest a way of controlling the thermal expansion of amorphous polymers similar to a level of glass without chemical cross-linking, which has long been regarded as a challenging problem,” Prof Kim said. “At the same time, we succeeded in making the polymer transparent and flexible. We expect that it can be applied to controlling the thermal expansion of various organic materials.”

Image caption: Photo of PAI(s-u) freestanding film. Image credit: Byungyong Lee.

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