Simple process makes flexible LCDs

17 Jun 2002

A method to fabricate flexible liquid-crystal displays on any substrate is described in today's Nature.

Flexible liquid-crystal displays (LCDs) could be made as thin as 150 µm thanks to a technique developed at Philips Research Laboratories and Eindhoven Technology University, both in the Netherlands.

Traditionally, LCDs are produced by sandwiching all the required components between two layers of glass substrate. Here, the team builds the device up from a single layer. They say that displays can be formed on a variety of substrates, an important step towards 'display-on-anything' and 'paintable displays'. (Nature 417 55)

A process known as photo-enforced stratification (PES) forms the basis of this research. PES is a two-step photopolymerization process that leads to the formation of micrometer-sized boxes filled with a switchable liquid-crystal phase.

In the first step, the team deposits a mixture containing a liquid crystal, a polymer-forming monomer, an ultraviolet-absorbing dye and a photoinitiator onto a glass substrate using standard techniques. Step two involves subjecting the mixture to two doses of ultraviolet (UV) light.

Exposing the sample to high-intensity UV light at 400 nm through a mask creates a series of polymer walls. Dirk Broer of Philips told Optics.org: "We use a mercury lamp with a bandpass filter in a continuous-wave mode. This light is not absorbed by the dye. Polymerization in the exposed areas leads to horizontal stratification and forms polymer walls."

The second exposure uses low-intensity UV light at 365 nm, which is absorbed by the dye, to form a 10µm-thick polymer lid on top of the liquid crystal. Broer explained that "the dye has a large absorption coefficient at 365 nm. It ensures that the polymer is formed at the top and the liquid crystal phase separates near the bottom".

The result is a grid of polymer boxes that are 500 µm by 500 µm square and 10 µm deep, separated by walls about 100 µm thick. Each box contains a small amount of liquid crystal. To achieve this, the exposures of UV light occur on a scale of minutes.

"We are overexposing to avoid insufficient phase separation, "said Broer. "Later in production this needs to optimized to suppress production costs. Smaller dimensions of the polymer boxes would make faster manufacturing possible."

Both of the electrodes are in the substrate so lateral fields drive the liquid crystal. The consequence of this in-plane switching is a superior viewing angle at the expense of brightness.

Broer said: "Our R&D program is directed towards improved brightness and contrast. The reliability under extreme conditions such as temperature, humidity and pressure also needs to be improved."

Author
Jacqueline Hewett is news reporter on Optics.org and Opto & Laser Europe magazine.