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Display Materials and Components

Dye-Free Production of LCDs

13 November 2017
Row A: Schematic illustration of desired patterns of alignment; row B: various irradiated light patterns; row C: polarized optical microscopy images. Image credit: Atsushi Shishido, Tokyo Institute of Technology.

Liquid crystal displays (LCDs) are created by irradiation of a liquid crystal film containing added photoresponsive dye molecules, with uniform polarized light. The interaction of the dye dipole and the polarization axis of light, controls the net liquid crystal alignment. The problem? The strong dyes needed can discolor or degrade optical and stability properties. What we really need in the industry is a dye-free method.

So far, two approaches to dye-free methods have been explored:

  1. Two-step alignment: The liquid crystal materials are coated over a very thin dye-containing photoalignment layer, then aligned or fixed by polymerization. This method has proven very successful for achieving stimuli-responsive, 2D-aligned liquid crystals and elastomers used for photonics, solar energy harvesting, microfluidics and soft-robotic devices. But it is expensive, time-consuming and unsuitable for aligning patterns on the nanoscale over large areas.
  2. Surface topography: The liquid crystals are aligned over a surface topography template through methods such as lithography, nanoimprinting or inkjet techniques. This method allows for 2D micropatterning of molecular alignment, but still requires multi-step processing and, again, is expensive and time-consuming. The surface roughness of the topographic templates also proves challenging for the production of thin films.

But now there’s something new. As published in the journal Science Advances, a research group at Tokyo Institute of Technology has reported the development of a new method of scanning wave photopolymerization utilizing spatial and temporal scanning of focused guided light. As the polymerization reaction proceeds, a mass flow in the film is triggered, resulting in alignment of the liquid crystals with the incident light patterns. In short, the light-triggered mass flow allows the desired alignment to be achieved in a single step.

The new method also generates arbitrary alignment patterns with fine control over larger areas in a wide variety of liquid crystal materials -- without the need for strong dyes or additional processing steps. Additionally, the 2D patterns could have unlimited complexity, restricted in principle only by the light diffraction limits. The method is readily introducible into existing photoproduction facilities.

Currently, the new method is limited to photopolymerizable liquid crystal systems with a thickness below tens of micrometers. But further investigation could expand useable material systems into the area of nanorods, nanocarbons and proteins. The researchers see the method as a powerful pathway for the simple creation of highly functional organic materials on the nanoscale over large areas, with arbitrary, fine molecular alignment patterns.

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