Research at CMMPEDevices & applications — Liquid crystal displays

Liquid crystal display technology is now commonplace in a wide variety of commercially available devices. However, research continues to further improve performance parameters. In particular, research is being carried out at CMMPE into a variety of new liquid crystal technologes, which will hopefully give rise to displays that have some or all of the following characteristics:

    • Low power consumption (for portable devices)
    • Fast switching (for hight frame-rate video applications)
    • Polarisation independent (for brighter and more efficient displays)
    • High brightness, contrast and colour resolution

A number of liquid crystal technologies are being researched and developed by CMMPE towards achieving these aims. These are detailed below:

Jump directly to the following sections: Bistable - Flexoelectro-optic - LC laser - Blue phase - PDLC - Dye guest-host - Hybrid


Bistable liquid crystal displays

Some liquid crystals, including Smectic A and Ferroelectric, have two possible stable molecular orientations, which do not require an electrical field to be applied to maintain the liquid crystal orientation state. However, if a suitable electrical signal pulse is applied to the cell, the liquid crystal can be switched from one stable state to another. Such materials are extremely useful if one wishes to make a binary (on or off) display. Furthermore, the lack of requirement for a maintaining electric field offers the potential of very low power consumption, making bistable displays a highly attractive technology for mobile devices, such as laptops, mobile phones, etc.

Smectic A devices

Electro-optic devices using the smectic A liquid crystal phase are low cost, simple and highly efficient. This liquid crystal phase has orientational order, as in nematic liquid crystals, but also has a degree of positional ordering. The positional order gives rise to unusual properties, such as negative conductivity anisotropy, that form the basis of useful electro-optic effects.

The devices work in the following manner: for low frequency electric fields (e.g. < 100 Hz), the flow of ionic dopant in the liquid crystal induces a highly scattering opaque state – ‘write’. As the frequency is increased (e.g > 1 kHz), the ionic motion is damped out and the device becomes transparent by dielectric reorientation of the liquid crystal molecules – ‘erase’. The resultant electro-optic effect is similar to the PDLC device described above; in the smectic A case, however, both the write and erase modes are stable at zero field. This is termed bistability and allows production of highly efficient devices since power is only required when the image updates.

One of the key differences between these materials and previous Smectic A liquid crystals is due to their unprecedented conductivity properties, which allow for low-voltage driving; in addition to the wide temperature range performance and ease of fabrication. In addition, these materials allow for much increased flexibility in the driving schemes, since frequency or voltage (or both) can be varied to induce the two modes. This is achieved by careful design of the material properties. For example, it would be possible to drive a ‘smart window’ e.g. for architectural use, that can be driven off mains voltage at 50 Hz. This would allow considerable simplification of driving circuitry compared to the standard designs of electrochromic and Polymer dispersed liquid crystal (PDLC) smart windows, for example. Power consumption estimates suggest that, for one hour’s use, the amount used in the Smectic A window is 0.8% that of the PDLC type. Further work is in progress to optimise material performance and extend the technology to full motion frame rates to open up new application areas.

Further reading:

More information about the phenomenon of bistable switching can be found here.

D. Coates, W. A. Crossland, J. H. Morrissy, B. Needham
J. Phys. D Appl. Phys. 11, 2025 (1978)

Organosiloxane LCs for Fast-Switching Bistable Scattering Devices
Gardiner D.J. and Coles H.J.
J. Phys. D: Appl. Phys. 39(23), 4948-4955, (2006)

Highly Anisotropic Conductivity in Organosiloxane LCs
Gardiner D.J. and Coles H.J.
J. Appl. Phys. 100, 124903, (2006)

Traditionally, smectic A devices used standard cyanobiphenyl liquid crystal materials. Recently, however, a novel class of liquid crystals containing siloxane units have been shown to offer much potential for use in smectic A based devices. In these compounds, although the siloxane group is bonded to the rest of the molecule, the three components of the liquid crystal molecule - siloxane, alkyl chain and aromatic core- are chemically distinct. This distinction results in self-assembly of the components on a molecular level – a phenomenon termed microphase separation. By virtue of this structure, organosiloxane smectic A materials possess:

1. Highly anisotropic conductivity – up to 3 orders of magnitude higher than standard smectic A materials.

2. Inherent wide-range temperature operation – the neat materials possess smA phases in excess of 100 deg.

3.
Solubility. It is possible to significantly increase solubility of additives e.g. dyes, which ordinarily possess limited solubility in the host, by chemically bonding siloxane units to such additives the solubility can be significantly increased.

In summary the advantages of smectic A technology are:

1. Simplicity. No polarisers, alignment layers or colour filters are required. Dyes can be readily incorporated into these systems leading to colourful displays with wide viewing angle.

2.
Bistability. The electrically induced textures can be stored long-term; images have been maintained for longer than 10 years. Additionally, any intermediate, greyscale value can be selected and stored in the same manner.

3.
Readable. Devices using this electro-optic effect show excellent daylight readability and 180° viewing angle – unlike standard nematic liquid crystal displays, for example.

4.
Formulation. Using organosiloxane materials as the host material, it is possible to generate smectic A mixtures with wide temperature ranges, low driving voltages and ruggedness.

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Flexoelectro-optic liquid crystal displays

[Info to be added on devices using flexo effect]

More information about the phenomenon of the flexoelectro-optic effect can be found here.

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Liquid crystal laser displays

Most colour displays commonly utilise pixels that are independantly capable of displaying three colours: red, green and blue. By combining red, green and blue in different combinations and ratios, it is possible to create any desirable colour output at any position on the screen. In LCDs, this is achieved using a white back-light, which is then filtered by electrically driven LC elements to leave only red, green and blue light remaining. Other technologies, such as LED (including OLED and PLED technologies) are more light efficient, and instead use separate red, green and blue light emitters, which can be rapidly switched and their intensities varied.

Laser displays are soon set to reach the market, which utilise three independant red, green and blue lasers as light sources. Like LED-based technology, this offers improved light efficiency over back-light filtered systems. Laser displays can also be far brighter than other display technologies, which is of particular interest for the projector TV/monitor markets. More importantly however, the narrow spectral linewidth (single wavelength) characteristic of lasers, gives the opportunity to manufacture displays with far higher colour resolutions than any other competing technolgies (including LED, OLED and PLED). The coherent nature of lasers also opens the door to new opportunities in diffractive or holographic displays.

One of the problems with laser displays is the complexity and high cost of utilising three independant red, green and blue lasers. Furthermore, if an extremely high colour resolution display is desired, then the emission wavelengths of these three lasers must be at very specific values, lying at the extreme corners of the visible colour gamut. Sourcing cheap lasers that emit efficiently at all these specific wavelengths is very difficult.

Recent research into photonic band-gap materials has led to the development of the liquid crystal laser. CMMPE has carried out extensive research in this field. It is hoped that LC lasers might be suitable for use in future laser display systems, as they offer the significant advantages of being mechanically simple, cheap, self-assembly construction, low-speckle, highly efficient and have the ability to be be tuned to virtually any visible wavelength that is desired.

Further reading:

Click here for more information about CMMPE's research on liquid crystal lasers.

Polychromatic liquid crystal laser arrays towards display applications
S.M. Morris, P.J.W. Hands, S. Findeisen-Tandel, R.H. Cole, T.D. Wilkinson, H.J. Coles
Optics Express, 16 (23), 18827-18837, (2008)

Simultaneous polychromatic emission from a
gradient pitch liquid crystal laser

Funding and collaborators:

This work forms part of the EPSRC basic technology project, COSMOS (Coherent Optical Sources using Micromolecular Ordered Structures), which is in collaboration with Prof. Sir Richard Friend (Optoelectronics, Department of Physics), Prof. Eugene Terentjev (Biological Soft Sytems, Department of Physics), Prof. Willhelm Huck (Melville Lab, Department of Chemsitry) and Prof. Ian White (Centre for Photonic Systems, Department of Engineering).

Parallel and complimentary work in this area is also been carried out by the CAPE-funded project, Chiralase.

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Blue phase liquid crystal displays

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Polymer-dispersed liquid crystal (PDLC) displays

One liquid crystal switching mode that we are interested in is the polymer dispersed liquid crystal (PDLC). In this case, the LC is contained within polymer vesicles. Due to the mismatch of the refractive indices between the LC and the polymer, light is scattered when no electric field is applied across the cell and the sample appears opaque. Specifically, the average refractive index of the LC and that of the polymer are different. With the application of an electric field the vesicles reorient so that the ordinary refractive is aligned with the polarisation of the propagating electromagnetic wave. If the refractive index of the polymer matches that of the ordinary refractive of the LC then light is transmitted and the sample appears transparent. The principle of the LC mode is shown in Figure 1. Our research includes dye-doped PDLCs, photopolymerized PDLCs and emulsion-based PDLCs.

For a more detailed introduction to PDLC materials, please click here.

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Dye guest-host systems

Info to be added here soon.

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Hybrid displays

For the past few years CMMPE has been working in collaboration with the Printed Segment Electroluminescent display manufacturer, Pelikon, to develop a novel electroluminescent PDLC hybrid display. This display combines the merits of both EL and LC technology providing high visibility in both low light level and high light level conditions.

Click here for more information on the hybrid displays research project with Pelikon.

 

 

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