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Illumination & Displays

Novel polarizer-free flexible liquid-crystal displays

Dye-doped liquid-crystal gels that provide a stable mode promise new paper-like high-efficiency display designs.
4 August 2008, SPIE Newsroom. DOI: 10.1117/2.1200807.1229

Most liquid-crystal displays (LCDs), including monitors, televisions, and laptop panels, require at least one polarizer. The resulting optical efficiency (∼4%) and viewing angle are therefore limited.1 Polarizer-free displays can achieve higher optical efficiencies and simultaneously reduce the amount of energy required for backlighting. In addition, paper-like flexible LCDs facilitate novel developments in the field of electronic paper, electronic tags, and decorative display devices.2 We recently developed a new polarizer-free reflective, flexible, and trimmable paper-like LCD based on a dye-doped liquid-crystal (LC) gel.3,4 Our new device combines the effects of scattering and absorption of light. Without the need for polarizers, the optical efficiency is high and the viewing angle wide. The contrast ratio (determined using laser measurements) reaches ∼450:1, with a response time of ∼6.5ms and using a driving voltage (Vrms) of ∼30V. This development provides a new LC mode for the operation of polarizer-free flexible displays.

Structure and the operating principles

The structure of the dye-doped LC gels is illustrated in Figure 1. The cell consists of a conductive Zn-doped indium-oxide film deposited on a plastic substrate, an alignment layer, LCs, polymer networks, and dichroic dye molecules. Dichroic dyes are materials whose light-absorption capability depends on the polarization of the incident light. At zero voltage, the LCs, dye molecules, and polymer networks are aligned perpendicularly to the substrate. The entire structure is operating in the bright state. There is no significant scattering but instead a little absorption for all polarizations of the incident light. If the voltage is increased to a sufficiently high level, the LC molecules are tilted away from the z direction. Meanwhile, they cause dye molecules to rotate in tandem with them. As a result, the cell's structure is changed to a multidomain operation. The tilt angles of LC and dye molecules are the same, but the orientations are random. Increasing both scattering and absorption causes a decrease of the material's reflectance, and the cell will consequently look darker. If the voltage is increased further, the LC and dye molecules are aligned randomly along the (x, y) plane, leading to all polarizations of the incident light experiencing the strongest possible scattering and absorption. The cell is now polarizer-free and has been forced into its dark state.


Figure 1.Structure and operating principles of the newly developed dye-doped liquid-crystal (LC) cell. The cell consists of a conductive Zn-doped indium-oxide (IZO) film deposited on a plastic substrate, an alignment layer, LCs, polymer networks, and dichroic dye molecules. The operation of the LC cell at both zero and a nominal operating voltage is illustrated schematically.
Electro-optical properties

Figure 2 shows the cell's electro-optical properties. When we apply an AC voltage to a single pixel of the polarizer-free LCD, its reflectance decreases with increasing potential difference due to the increase of both scattering and absorption. Based on laser measurements, the contrast ratio (CR), i.e., the reflectance ratio of 30 to 0VRms potential difference, is ∼450:1 with a corresponding response time of around 6.5ms. The maximum reflectance is about 55%. Compared with typical ‘guest-host’ LCDs (for which the CR is ∼5:1, the maximum reflectance below 50%, and the response time around 50ms), our design is not only polarizer-free but also characterized by a higher CR and faster response.


Figure 2.Voltage-dependent reflectance.
A single pixel of the polarizer-free flexible LCD

Figure 3 shows a single pixel of our polarizer-free flexible LCD under free bending at 0 and 30Vrms, where white paper was used as a diffusive reflector. The transmission of this single pixel is almost constant for radii of curvature greater than 21mm, as the vertically aligned polymer networks help to maintain the thickness between two plastic substrates under bending distortion. In addition, the devices are trimmable because of the gel-like material used for their construction (see Figure 4). Their performance remains similar after cutting: the gel-like materials do not leak from the cut. Since no polarizer is needed, the bending-induced birefringence of the plastic substrates does not at all affect the performance of our flexible display. The color of the novel LCD can be adjusted by changing the type of dye molecules, as illustrated in Figure 5.


Figure 3. Single pixel of the polarizer-free LCD under free bending at 0 and 30Vrms.

Figure 4. Single pixel of the polarizer-free LCD (a, b) before, (c) during, and (d, e) after trimming.

Figure 5. Single pixel of the polarizer-free LCD using different dye molecules.
Summary

We have demonstrated a polarizer-free and flexible LCD design using dye-doped LC gels with high CR, fast response, and high reflectance. In addition, the gel-like display material is bendable and trimmable yet provides a stable performance. However, the issues we have yet to overcome are the high driving voltage and red color of the dye-doped LC gels. Improving the LC materials and dye could be a way forward. Our dye-doped LC gels provide a stable LC mode and open a new window to paper-like flexible-display-technology developments.

The authors are indebted to the Electronics and Optoelectronics Research Laboratories at the Industrial Technology Research Institute, Taiwan, for technical support and to the National Science Council of Taiwan for financial support.


Yi-Hsin Lin, Chih-Ming Yang 
Department of Photonics
National Chiao-Tung University
Hsinchu, Taiwan

Yi-Hsin Lin is an assistant professor. Her research interests focus on LCDs, their underlying physics, and applications of LC/polymer composite systems. She was awarded the Glenn H. Brown Prize by the International Liquid Crystal Society in 2008, the 2006 New Focus/Bookham award by the Optical Society of America, a 2005 Newport research excellence award, and a SPIE educational scholarship in optical science and engineering in 2005.

Chih-Ming Yang is a graduate student. His research interests focus on flexible displays and LC/polymer composite systems.