Photostability of liquid crystals and alignment layers

Abstract

-Photostability of liquid-crystal (LC) materials and surface alignment layers was evaluated using a UV lamp and a blue laser beam. Both organic polyimide (PI) and inorganic silicon-dioxide (SiO 2 ) alignment layers were studied under nitrogen environment. Two commercial TFT-grade LC mixtures (low-birefringence MLC-9200-000 and high-birefringence TL-216) were used for comparisons. Results indicate that SiO 2 alignment layers are much more robust than PI layers, and low birefringence LCs are more stable than the high-birefringence ones. At the He-Cd laser wavelength (λ = 442 nm), both LC mixtures and SiO 2 alignment layers are hardly damaged. To lengthen the lifetime of an LCD projector, inorganic SiO 2 alignment layers, high-optical-density UV filter, long cutoff-wavelength blue filter, and short-conjugation (low birefringence) LC materials should be considered.Keywords -Liquid crystals, photostability, lifetime, alignment layers, inorganic SiO 2 , LCD projector. IntroductionPhotostability of liquid-crystal (LC) materials and alignment layers play a crucial role in affecting the lifetime of liquid-crystal-display (LCD) devices. [1][2][3][4] In direct-view displays, ultraviolet (UV) light is often used to seal the LC panels. In projection displays using liquid-crystal-on-silicon (LCOS), 5 the employed arc lamp is relatively bright. Although a cold mirror and a hot mirror are used to filter out the unwanted UV and infrared contents of the lamp, residual UV light could still penetrate into the LC panels and cause performance degradation. Even if the UV contents are completely filtered out, the blue wavelength might still be harmful to the LCD panels. These high-energy photons could cause gradual degradation of the organic alignment layers and the LC molecules. Once the LC medium is decomposed or the alignment layers deteriorated, the pretilt angle is changed and the consequent electro-optic effects are altered. 6 In this paper, we report the photostability of LC alignment layers and LC materials in the UV and blue wavelengths. Regarding the alignment layers stability, we studied the buffed polyimide (PI) and sputtered inorganic silicondioxide (SiO 2 ) alignment layers under nitrogen environment. On the LC-material stability studies, we selected two Merck LC mixtures: TL-216 (∆n = 0.20) and MLC-9200-000 (∆n = 0.08). A high birefringence LC mixture, such as TL-216, enables a thinner cell gap to be used. The advantages are twofold: (1) faster response time and (2) weaker fringing-field effect. For instance, the Philips color-sequential LCOS projector 7 uses a~1-µm cell gap in order to achieve fast response time and minimize the fringing-field effect. On the other hand, a low-birefringence LC mixture, such as MLC-9200-000, is commonly used for three-panel LCOS projectors. UV stability of LC alignment layersThe Hamamatsu UV light was delivered to the sample through a fiber bundle and collimated by a lens. The output light passed through a narrow-band interference filter centered at λ~365 nm. The light intensity ...

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