Azodye Aligning Layers for Liquid Crystal Cells

The photo-induced alignment of liquid crystal onto a photochemical stable azo dye film is studied for the liquid crystal display (LCD) applications. The photo-aligning of azo dye takes place due to the pure reorientation of the molecular absorption oscillators perpendicular to the UV-light polarization. The order parameters S= -0.4 (80% of the maximum absolute value S_m= -0.5) was measured at the wavelength of 372 nm from the polarized absorption spectra. The temperature stable pretilt angle of 5.3⁰ was obtained by a two-step exposure of azo- dye film using normally incident polarized light followed by oblique non-polarized light. The azimuthal anchoring energy of the photo-aligned substrate was A_φ> 10^-4 J/m², which is of the same magnitude as the anchoring of the rubbed PI layer. VHR value of the photo-aligned LC cell was also found to be very high (98-99%) at room temperature and more than 95% at T=80⁰C. The thermal stability of the photo-aligned azo-dye layers is sufficiently high, while UV-stability has to be improved e.g. by polymerization. The bright perspectives for the new LCD aligning technology are envisaged.