A combined flexible blue optically rewritable LCD. [Image: Zhang et al. / Donghua University and HKUST]

For decades, inventors have dreamed of electronic displays that flex like the paper they would replace. Researchers in China and Hong Kong have now created a thin, lightweight liquid-crystal display (LCD) that bends easily and could be inexpensive to manufacture (Appl. Phys. Lett., doi:10.1063/1.5021619).

The new work at Donghua University in Shanghai and the Hong Kong University of Science and Technology (HKUST) builds upon the recent development of optically rewritable LCDs. In these liquid-crystal panels, the crystals and supportive structures called spacers are sandwiched between an optically active substrate—in this case, a sulfonic azo dye—and an optically passive substrate. Polarized light, rather than the electrical currents of conventional LCDs, modulates the intensity of the pixels, and the device can retain the image without consuming electrical power. The researchers’ device used polarized light with a wavelength of 450 nm.

The right spacer

The key to making optically rewritable LCDs flexible, according to the Donghua-HKUST team, is making spacers that do not allow the liquid crystals to flow between the two substrates when the display bends, which results in blank spots. Keeping the thickness of the liquid crystals constant is also necessary for good contrast and fast response times.

The researchers tested three different methods for fabricating the proper spacers:

  • Stamp-printing of regularly distributed spacers onto the polyethersulfone substrate before it is coated with the sulfonic azo dye. This technique is suitable for mass production, according to the team.
  • Spraying tiny ball-shaped spacers onto an epoxy-coated substrate and securing them by curing the substrate under ultraviolet light.
  • Stamp-printing the spacers as a mesh of small rods arranged as square cells with gaps at the corners. These squares measured 200 μm on each side.

The first two methods cannot prevent some flow of the liquid crystals between the gaps in the spacers, but the third makes the cells insensitive to mechanical forces, according to finite-element modeling.

In addition to the spacer technology, the research team made its optically rewritable LCD a true three-color display by giving it a reflective backboard made of flexible cholesteric liquid-crystal mirrors, a few microns thick. Previous optically rewritable LCDs were limited to two primary colors.