액정 재료가 Fringe Field Switching Mode의 투과율 및 신뢰성에 미치는 영향

Abstract

Liquid crystal displays (LCDs) has been widely used from a small size such as mobile phone and notebook PC to large size such as LCD TV in the modern society in recent years due to the high performance and the image quality and transmittance. Based on numerous researches and studies, researchers have developed different types of LC mode, such as polymer-sustained alignment (PSA), in-plane switching (IPS) and fringe field switching. Recently, the fringe-field switching (FFS) mode has been widely using in liquid crystal displays (LCDs) because of high image quality and high-resolution. In the market, the FFS mode using a liquid crystal (LC) with positive dielectric anisotropy (+LC) is dominant due to low operating voltage and fast response time. However, in the recent ultra-high resolution over 600 ppi region, the aperture ratio drop caused by increasing occupation of the non-transmitting elements, such as TFTs and contacts become a emerging concern. This drop of the transmittance and results in degradation of increase of back light power dissipation. Therefore, many researchers have worked to improve the transmittance of FFS mode devices by various approaches such as the optimization of panel design, the development of advanced material, the improvement of mode efficiency. Recently, a liquid crystal (LC) with negative dielectric anisotropy (-LC) is applied to high resolution mobile LCDs because the light efficiency of the FFS mode using +LC is lower than that when a LC with negative dielectric anisotropy (-LC) is used. But the response time is slower than for a device using a +LC because of the greater viscosity of liquid crystal. Thus, recently, an FFS device with a +LC has been commercialized. In this dissertation, the theoretical and experimental researches to improve transmittance characteristics of the FFS mode using positive LC with negative singles were investigated from physical properties and applications. At first, we shows several kinds of the method for the improvement of transmittance in the FFS mode such as the optimization of panel design, the development of advanced material, the improvement of mode efficiency. And their merits and demerits are introduced briefly. After that, we mention physical principle of the transmittance in the IPS mode and theoretically discuss the relationship between transmittance and perpendicular dielectric constants. From this results, we will propose the ApLC by introducing +LC with negative single (ApLC) to improve the transmittance in the FFS mode. Next, the enhancement of the transmittance and decreasing of tilt angle in the electrodes for the ApLC system are discussed. At first, we simulated that the transmittance was improved and the tilt angle was decreased on the electrodes by increasing perpendicular dielectric constant controlled the concentration of negative singles in the FFS mode. After that, we made test FFS cells with ApLC mixture, and we achieved high transmittance and fast response time in the FFS mode. In addition, based on experimental and phenomenological results, we study the dependence of perpendicular dielectric constant for the transmittance and theoretically discuss the relationship between transmittance and perpendicular dielectric Constant. Continually, to confirm that our proposed ApLC system is simple and available in real device, we made the 23.8 inch FHD (1920 _ RGB _1080) FFS panel and measured electro-optic (EO) characteristics. We could achieve not only the improvement of transmittance but fast response time.In addition, we checked the reliability characteristic for real panel. However, the ApLC system has the weakness for reliability such as line image sticking. From reliability results, we have to consider improvement of line image sticking for the ApLC system to apply TFT-LCD applications. In addition, ApLC system has much more vulnerable than normal positive LC for line image sticking, and we introduce the causes of the line image sticking in the ApLC system, and we show solutions to eliminate line image sticking fundamentally. We used the Electron spin resonance(ESR) and time of ight-secondary ion mass spectrometry(TOF-SIMS) to find out the source of impurity for the ApLC system, and the electrical measurement such as voltage holding ratio (VHR) was measured to investigate both ions trap and transport. Also, we simulated electrostatic potential and ion affinities based on the density functional theory (DFT) to study the relationship between the ions trap and LC structure. From these results, we proposed the ApLC system mixed UV radical scavenger mixed to achieve fundamental solution for line image sticking. Eventually, we achieved not only the improvement of transmittance but high reliability in the real panel In conclusion, throughout this dissertation, we discussed the enhancement of the transmittance and decreasing of tilt angle on the electrodes under the ApLC system, and we achieved high transmittance and fast response time in the FFS mode. What is the role of the perpendicular dielectric constants to contribute to the transmittance in the FFS mode are discussed and analyzed. In addition, based on experimental and phenomenological results, we studied the dependence of perpendicular dielectric constant for transmittance and theoretically discussed the relationship between transmittance and perpendicular dielectric constants. Furthermore, we confirmed what is the cause of both the impurities in the cell and line image sticking, the solution based on experimental and theoretical results lead to the elimination of line image sticking basically. Additionally, we made 23.8-inch real FFS panel and could achieved not only the improvement of the transmittance but also reliability without any severe degradation of performances. Ultimately, we believe that our proposal method is useful for improving the transmittance and reliability for the applications based on FFS mode and will be helpful in the further development for the FFS mode.