Guest-host LCDs

Guest-host (GH) LCDs are coloured displays, whereby the colour and changes in colour are attributable to the absorption of incident light by a dichroic guest dye dissolved in a liquid crystalline host material, usually nematic, and the co-operative reorientation of these dichroic dyes in an electric field, i.e. electro-optical devices. 

A value of i 6 is generally regarded as necessary for a GH-LCD with an acceptable contrast. 

The nematic liquid crystal mixture containing the pleochroic dye is of positive dielectric anisotropy and is aligned parallel with the director parallel to the substrate surfaces. Therefore, plane polarised light is absorbed by the dye molecules in non-addressed areas of the display and they appear coloured. 

The application of an electric field between the electrodes results in a realignment of the nematic liquid crystal mixture and the dichroic dye molecules parallel to the electric field resulting in a lower optical density (absorption) and, theoretically, the disappearance of colour assuming an ideal order parameter (S = 1) of the nematic liquid crystal director and the dye molecules. A residual absorption in this state gives rise to a display with a strongly coloured background and weakly coloured information. 

Another version of the Heilmeier and Zanoni GH-LCDs with positive contrast is essentially the inverse of the Heilmeier and Zanoni GH-LCDs with negative contrast described above. A nematic mixture of negative dielectric anisotropy incorporating a dichroic dye of negative contrast is aligned parallel 

The classical cholesteric phase materials show only a weak anisotropic interaction with electric fields and hence are of limited use in electro-optical response applications. Cholesteric phases for these outlets are consequently produced by adding chiral dopants to nematic liquid crystals. 

Interest in the use of these materials as hosts for dyes arises from the potential for making fine black LCDs, with a good angle of vision and the avoidance of the costly polarising layers used in TN LCDs. 

To be of any value in this outlet the dyes must show certain characteristics  

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Figure 3.17 Schematic representation of the transmission against voltage for a guest-host LCD.

Figure 3.19 Isomeric structures of an anthraquinone dye used in guest-host LCDs.

In the abovementioned transflective guest-host LCD, the inner quarter-wave film is put between the transflector and the guest-host layer. There are two optional positions for the trans-flector. If the transflector is located inside the LC ceU, then the quarter-wave film should also be... 

In the abovementioned two absorption-type transflective LCDs, only one polarizer is employed instead of two. Therefore, the overall image in both T- and R-modes is relatively bright. However, due to the hmited dichroic ratio of the employed dye molecules (DR 15 1), a typical contrast ratio of the guest-host LCD is around 5 1, which is inadequate for high-end full color LCD applications [29]. Thus, the absorption-type transflective LCDs only occupy a small portion of the handheld LCD market. 

T. Uchida, H. Seki, C. Shishido, and M. Wada, Bright dichroic guest-host LCDs without a polarizer, Proc. 5/D, 22,41 (1981). 

The performance of a guest-host LCD is greatly dependent on the dye parameters (such as UV stability, solubility, order parameter, absorption, etc.), the host liquid crystal properties (such as viscosity, dielectric anisotropy, birefringence, order parameter, temperature range, stability, etc.), and the compatibility of the dye and the host [12-17, 42-58]. 

Fig. 1.7 (a) Low-frequency relaxation of the parallel dielectric constant fin of a dual-liequency addressable nematic LC mixture. The dielectric anisotropy Ae=(en-fx) changes sign at the crossover frequency/c- (b) Three different color-switching guest-host LCD prototypes... 

The second example in Fig. 1.7b shows a photograph of a color-switching guest-host LCD prototype made by the author in 1979. The idea was to find a field effect... 

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