Review Paper: Emerging vertical‐alignment liquid‐crystal technology associated with surface modification using UV‐curable monomer

Abstract

-The recent development of polymer-induced pretilt angle in multi-domain vertical-alignment liquid-crystal (LC) structures is reviewed. To create a small but well-defined pretilt angle, 0.1 wt.% of a photo-curable monomer was mixed in an LC host and a bias voltage was applied to reorient the LC directors within each domain. The monomers are polymerized near the substrate surfaces by UV exposure. The formed polymer layers change the surface pretilt angle of the LC from 90°t o about 89°with a defined azimuthal orientation. Consequently, within each domain the LC reorientation direction responding to the external field is well-defined which leads to faster rise time and higher transmittance. This new technology overcomes the long standing problems of conventional MVA devices and is therefore expected to play a dominant role in the future.Keywords -Multi-domain vertical alignment, liquid crystal, polymerization, surface tilt angle. DOI # 10.1889/JSID17.7.551 1 IntroductionAt present, two major wide-view liquid-crystal (LC) approaches are commonly practiced: homogenous alignment (HA) and vertical alignment (VA) in the initial state. The HA camp includes in-plane switching (IPS) 1,2 and fringe-field switching (FFS), 3,4 in which the LC directors essentially rotate in plane so that wide view is a built-in feature. While in the VA camp, multi-domain VA (MVA) using protrusion 5-8 and patterned VA (PVA) 9,10 are the two major approaches. In both types of VA devices, the LCs are vertically aligned with respect to the substrate with the help of a polymer alignment layer under crossed polarizers and their normalized transmittance T/T o can be described aswhere ψ is the angle between one of the transmission axes of the crossed polarizers and the LC director, ∆n eff is the voltage-dependent effective birefringence of the LC medium, and λ and T o are the wavelength and intensity of the incident light, respectively. In the absence of an electric field, ψ = 0°and the device appears black. In the presence of an electric field, the effective retardation (d∆n eff ) starts to occur. Ideally, the transmittance will reach a maximum at ψ = 45°. In both MVA and PVA devices, the LC directors are reoriented by the oblique fields. In MVA, the oblique field is generated by a protrusion and patterned pixel electrode, whereas in PVA the oblique field originates from a patterned common and pixel electrode, as depicted in Fig. 1. Both MVA and PVA devices have been successfully commercialized; however, their performance and cost still need to be improved.Recently, a new type of VA technology which enables the control of the surface pretilt angle of the LC by using a UV curable monomer was introduced. This technique improves the rise time and transmittance and has become a strong contender for future wide-view VA technology. Major issues in VA technologiesLet us first describe the general problems associated with VA devices and the demerits of conventional MVA and PVA devices.In the initial state, the LC directors are perfectly aligned...

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