Windows that Act Like an LCD Screen

A dye-doped PNLC cell in the transparent and opaque states, placed on a printed sheet of paper. In the transparent state, the clear background image can be seen because of the high transmittance of this cell (left). In the opaque state, black color is provided and the background image is completely blocked, because the incident light is simultaneously scattered and absorbed. (Source: T.-H. Yoon, Pusan NU / AIP)

A dye-doped PNLC cell in the transparent and opaque states, placed on a printed sheet of paper. In the transparent state, the clear background image can be seen because of the high transmittance of this cell (left). In the opaque state, black color is provided and the background image is completely blocked, because the incident light is simultaneously scattered and absorbed. (Source: T.-H. Yoon, Pusan NU / AIP)

The idea of transparent displays has been around for a few years, but creating them from conventional organic light-emitting diodes has proven difficult. “The transparent part is continuously open to the background,” said Tae-Hoon Yoon, primary investigator of a group at Pusan National University, South Korea. “As a result, they exhibit poor visibility.”
Light shutters, which use liquid crystals that can be switched between transparent and opaque states by scattering or absorbing the incident light, are one proposed solution to these obstacles, but they come with their own set of problems. While they do increase the visibility of the displays, light shutters based on scattering can’t provide black color, and light shutters based on absorption can’t completely block the background. They aren’t particularly energy-efficient either, requiring a continuous flow of power in order to maintain their transparent ‘window’ state when not in use.
Yoon’s group’s new design remedies all of these problems by using scattering and absorption simultaneously. To do this, they fabricated polymer-networked liquid crystals cells doped with dichroic dyes. Their polymer network structure scatters incident, or oncoming light, which is then absorbed by the dichroic dyes. The light shutters use a parallel pattern of electrodes located above and below the vertically aligned liquid crystals.
When an electric field is applied, the axes of the dye molecules are aligned with that of oncoming light, allowing them to absorb and scatter it. This effectively negates the light coming at the screen from its backside, rendering the display opaque – and the screen’s images fully visible.
In its resting state, this setup lets light pass through, so power need only be applied to switch from transparent to opaque view. And because the display’s on-off switch is an electric field, it has a response time of less than one millisecond – far faster than that of contemporary light shutters, which rely on the slow relaxation of liquid crystals.
Future work for Yoon’s group includes respectively increasing and decreasing the device’s transmittance at the transparent and opaque states, as well as developing a bi-stable light shutter which consumes power only when states are being switched. (Source: AIP)

References: J. Heo et al.: Fast-switching initially-transparent liquid crystal light shutter with crossed patterned electrodes, AIP Advances 5 (2015) 047118; DOI: 10.1063/1.4918277
Links: American Institute of Physics

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