Flexoelectric effect in an in-plane switching (IPS) liquid crystal cell for low-power consumption display devices.

Min Su Kim,Seung Hee Lee,Joong Hee Lee,Philip J Bos,Deng-Ke Yang,Dong-Woo Kim

doi:10.1038/srep35254

Abstract

Technology of displaying static images in portable displays, advertising panels and price tags pursues significant reduction in power consumption and in product cost. Driving at a low-frequency electric field in fringe-field switching (FFS) mode can be one of the efficient ways to save powers of the recent portable devices, but a serious drop of image-quality, so-called image-flickering, has been found in terms of the coupling of elastic deformation to not only quadratic dielectric effect but linear flexoelectric effect. Despite of the urgent requirement of solving the issue, understanding of such a phenomenon is yet vague. Here, we thoroughly analyze and firstly report the flexoelectric effect in in-plane switching (IPS) liquid crystal cell. The effect takes place on the area above electrodes due to splay and bend deformations of nematic liquid crystal along oblique electric fields, so that the obvious spatial shift of the optical transmittance is experimentally observed and is clearly demonstrated based on the relation between direction of flexoelectric polarization and electric field polarity. In addition, we report that the IPS mode has inherent characteristics to solve the image-flickering issue in the low-power consumption display in terms of the physical property of liquid crystal material and the electrode structure.

Highlights

Technology of displaying static images in portable displays, advertising panels and price tags pursues significant reduction in power consumption and in product cost
Considering the symmetry arguments, nematic liquid crystals are commonly described as a rod-like body with indistinguishable head and tail; the asymmetric shape and the electrical polarity in single molecule become important where the non-polar nematic symmetry is broken, i.e., if the head of a molecule is arranged in one direction
We prepare a unit cell with the in-plane switching (IPS) mode as schematically illustrated in Fig. 1(a) which the blue and red arrows indicate the direction regarding the polarity of the applied electric fields

Summary

Introduction

Technology of displaying static images in portable displays, advertising panels and price tags pursues significant reduction in power consumption and in product cost. The name flexoelectricity points out a connection between a geometric deformation of the director field and the electric polarization on liquid crystals Understanding of this phenomenon is crucial to explain some effects, which are not as usual as the dielectric effect[1]. Reports on the flexoelectric coefficient with bent-core molecules interestingly exhibits the order of eb ~ 50 nC/m7,9, while it is relatively small based on the calamitic liquid crystal molecule: the order of 10 pC/m3,4,6,8,10,11 Such measurements are executed by generating the director deformation by both mechanical strain and biased external electric fields. The result shows that the dielectric and flexoelectric effects of calamitic liquid crystals are optically distinguishable under the context of the lateral electric field and vertical optic path Such a system consists of three energetic terms: elastic free energy density felas, quadratic dielectric coupling fdielec, and linear flexoelectric coupling fflexo. Even at lower frequencies, where the flexoelectric coefficients are non-zero, the viscosity-limited director field response can cause the flexoelectric effect to be unobservable

Methods

Results

Conclusion