Self-alignment technique of liquid crystal using a novel thiol additive for LCD and smart window applications

국문 초록 (Abstract)

LCD(Liquid Crystal Display)의 배향 막 공정이 필요 하지 않은 SA-VA(Self-Aligned Vertical Alignment) 기술은 비용 효율성과 LC 정렬 제어 및 LCD 제조의 유용성으로 인해 상당한 주목을 받아왔다. 그러나, 표면에 자가 배향 첨가제 부착이 균일하지 않아 LC 정렬 불량이 발생한다는 치명적인 단점이 있다. 결과적으로 이 기술은 아직 상용 어플리케이션에 채택되지 않았다. 본 연구에서 우리는 thiol 그룹을 포함하는 새로운 자가 배향 첨가제와 표면에 첨가제의 열 유도로 물리화학적 흡착을 사용하는 고급 SA-VA 기술을 제안한다. 새로운 재료와 방법을 사용하여 불 균일한 정렬 결함 없이 LC의 완벽한 수직 정렬을 성공적으로 입증했다. 또한, 테스트 셀의 전기광학 성능은 기존의 상용 소자와 대등함을 확인하였다. 따라서 우리의 방법은 기존 SA-VA 기술의 한계를 극복할 수 있으며 상용 SA-VA 모드 디스플레이에 적용할 수 있다. 또한 자체 정렬 기능을 통해 향후 초고해상도 디스플레이에 적용될 LC를 사용하는 비용 효율적인 스마트 윈도우와 같은 미래 디스플레이 응용 분야에 유용할 수 있다.
Guest-Host Liquid Crystal(GHLC)으로 구성된 스마트 윈도우는 유망한 기술이지만 몇 가지 한계가 있다. 광 투과율의 상대적으로 좁은 조정 범위와 폴리이미드(Poly Imid) 배향 막 공정으로 인한 비용 효율적인 roll-to-roll 공정 채택의 어려움 등이다.
이 논문 3장에서는 GHLC에 티올기(thiol) 첨가제를 사용하는 SA(self-alignment) 기술을 소개하고 PI 층 코팅 단계를 건너뛰어 PI-less GHLC 장치를 성공적으로 소개하였다. SA 첨가제 분자는 소자의 내부 표면에 단단히 부착되어 자기조립 단층을 형성하고 액정의 우수한 수직 배향을 유도한다. 또한, SA 첨가제 부착 과정에서 세포 내 이온 농도가 증가하고 이온은 높은 인가 전압에서 유체역학적 불안정성 (hydrodynamic instability)을 유발하는 역할을 한다. 결과적으로 SA-GHLC 디바이스는 낮은 인가 전압에서 투과율 제어모드(Transmittance Control Mode)와 높은 인가 전압에서 헤이즈 제어모드(Haze Control Mode)의 이중 모드(Dural Mode)로 작동할 수 있다. 따라서 PI-less SA-GHLC는 고온의 PI 코팅 공정 없이 간단한 공정으로 제작할 수 있어 우수한 스마트 윈도우 특성을 나타낸다.
본 논문에서 소개하는 SA 기술은 roll-to-roll공정이 가능한 저렴한 비용으로 LCD 어플리케이션 뿐만 아니라 스마트 윈도우용으로도 적용 가능 하리라 기대된다.

다국어 초록 (Multilingual Abstract)

Self-aligned vertical alignment (SA-VA) technique in liquid crystal display (LCD), in which the alignment layer coating process is not necessary, has attracted significant attention owing to its cost-effectiveness and usefulness in the control of LC alignment and the fabrication of LCDs. However, it presents a critical drawback pertaining to the occurrence of LC alignment defects caused by non-uniform additive attachment on the surface; as a result, the technology has not yet been adopted for commercial applications. Here, we propose an advanced SA-VA technology using novel additives containing thiol groups and thermally induced physicochemical adsorption of additives on the surface. Using the new materials as well as method, we successfully demonstrate a perfect vertical alignment of LC without non-uniform alignment defects. In addition, it is confirmed that the electro-optical performance of the cell is comparable to that of a conventional commercial device. Thus, our method can overcome the limitation of existing SA-VA technology, and may be applied to commercial SA-VA mode displays. Further, with its self-alignment feature, the technology may be useful for the future display applications such as cost-effective smart window using LC to be applied to ultra-high-resolution displays in the future.
A smart window composed of guest–host liquid crystals (GHLCs) is a promising technology but has several limitations, such as a relatively narrow tuning range of optical transmittance and the difficulty in adopting a cost-effective roll-to-roll process due to the polyimide (PI) alignment layer process.
In this dissertation Chapter 3, we introduce a self-alignment (SA) technology that uses a thiol-group additive into GHLC and successfully demonstrate a PI-less GHLC device by skipping the PI layer coating step. The SA additive molecules attach firmly to the inner surfaces of the devices, forming a self-assembled monolayer, and induce an excellent vertical alignment of the liquid crystals. Additionally, during the SA additive attachment process, the ionic concentration increases within the cell, and the ions play a role in inducing hydrodynamic instability at high applied voltages. Consequently, the SA-GHLC device can be operated in dual modes: a transmittance control mode at low applied voltages and a haze control mode at high applied voltages. Thus, the PI-less SA-GHLC exhibits excellent smart-window properties as can be fabricated using a simple process without a high-temperature PI coating process.
The SA technology introduced in this paper is expected to be applicable for not only LCD applications but also LC smart window applications at a low cost that allows roll-to-roll processes.

목차 (Table of Contents)

• Chapter 1. Introduction 1
• 1.1 Introduction of Liquid Crystals 1
• 1.1.1 What are the Liquid Crystals 1
• 1.1.2 Basic Requirement and physical Properties 3
• 1.1.3 Liquid Crystal Display Technical Modes 4
• 1.2 Motivation 7
• 1.3 Dissertation Overview 9
• Chapter 2. Self-alignment Technique of Liquid Crystal using a Novel Additive containing Thiol Group for LCD Applications 10
• 2.1 Introduction of Self-Aligned Liquid Crystal 10
• 2.1.1 Current Technical Status and Advantages of Self-Aligned Liquid Crystal Technology 10
• 2.1.2 General Process of SA-VA Technology 12
• 2.1.3 A Novel SA Additive containing Thiol Group 14
• 2.2 Experimental 16
• 2.2.1 Material Preparation 16
• 2.2.2 Fabrication of LC Cells 17
• 2.2.3 Experimental Method 18
• 2.3 Results and Discussions 20
• 2.3.1 LC Alignment 20
• 2.3.2 LC Alignment Confirmation after Disassembling and Refabrication 25
• 2.3.3 Contact Angle Measurement of Each Cell 27
• 2.3.4 Electrooptical Performance of the cell injected with SA- LCB Mixture 32
• 2.4 Conclusion 34
• Chapter 3. Self-alignment Technique of Liquid Crystal using a Novel Thiol Additive for Smart Window Applications with Dual Modes 36
• 3.1 Introduction 36
• 3.2 Experimental 41
• 3.2.1 Materials and LC cell Conditions 41
• 3.2.2 Experimental Method and Characterization 44
• 3.3 Results and Discussion 48
• 3.3.1 Result of First Experimental Set Self-alignment DDLC 48
• 3.3.2 Result of First Experimental Set Transmittance SA-DDLC 51
• 3.3.3 Result of Second Experimental Set Self-Alignment DDLC 56
• 3.3.4 Result of Second Experimental Set Study of Surface energy for SA-DDLC 59
• 3.3.5 Result of Second Experimental Set Electro Optical Property of SA-DDLC Devices 62
• 3.3.6 Result of Second Experimental Set Conductivity and Power Consumption of SA-DDLC Devices 66
• 3.3.7 Result of Second Experimental Set Dual Modes: Transmittance Control Mode and Haze Control Mode 68
• 3.3.8 Result of Second Experimental Set Investigation of William Domain and Electrohydrodynamic Instability (EHDI) 70
• 3.3.9 Result of Second Experimental Set Repeatability test of SA 5 Device after 6 Months and Viewing Property 72
• 3.4 Conclusion 75
• Chapter 4. Summary 76
• References 78
• Korean Abstract 84

참고문헌 (Reference)

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4. `` Vertical alignment of liquid crystals without alignment layers, K.-H. Kim et al., vol . 40 , no . 3 , pp . 391-395, , 2013

5. `` Phase separation of monomer in liquid crystal mixtures and surface morphology in polymer-stabilized vertical alignment liquid crystal displays, J. J. Lyu et al. ,, vol . 44 , no . 32 , p. 325104, , 2011