표면제어를 통한 수직배향 구조 나선변형 강유전성 액정 모드의 전기광학 특성에 대한 연구

Abstract

액정 디스플레이는 다양한 방면으로 연구되고 발전해 왔다. 특히 경량• 박형, 고대비비, 고해상도 저전력소비 등의 뛰어난 특성으로 모바일 제품에서부터 대화면 TV에 이르기 까지 폭넓은 시장을 형성하고 있다. 최근 시장의 흐름은 자연스런3D 입체영상 구현과 잔상 없는 고품질 영상을 위한 고속응답 특성을 요구하고 있다. 네마틱 액정(Nematic Liquid Crystal; NLC)은 고속응답 구현에 대하여 유전율 이방성과 전기장의 결합에 의한 반응속도 에 대한 근본적인 한계를 갖고 있어 고속응답 특성을 구현하는데 어려움이 있다. 그러나 강유전성 액정(Ferroelectric Liquid Crystal; FLC)은 자발분극(spontaneous polarization; P)이라는 고유한 특성 때문에 네마틱 액정에 비하여 매우 빠른 응답 특성을 갖는다. 이러한 강유전성 액정의 특성은 시장이 요구하는 고속응답에 구현에 부흥할 수 있는 차세대 디스플레이에 적용 가능한 물질로 재조명 받고 있다. 강유전성 액정 모드는 크게 표면 안정화 강유전성 액정 (surface stabilized FLC; SSFLC)모드 와 나선 변형 강유전성 액정(deformed helix FLC; DHFLC)모드로 크게 나눌 수 있는데 수평으로 배향된 모드는 연속적인 계조 표현이 어렵거나, 복잡한 표면 효과에 의해 균일한 배향 특성을 얻기 힘들다. 하지만 수직 배향 DHFLC(vertical alignment-DHFLC)모드는 기판에 평행하게 스멕틱 층을 형성하여 표면 효과에 대한 영향을 줄이고, 러빙공정을 생략하여 대면적에서 균일한 배향특성을 확보하였다. 또한 액정의 평균 광축이 기판에 수직하고 나선피치가 가시광의 파장보다 짧아 완전한 off상태를 얻을 수 있고, in-plane 전극 구조를 사용하여 광시야각을 갖는 연속적인 계조 표현 기능과 함께 높은 대비비를 실현하였다. 하지만 VA-DHFLC 모드에서 강한 전기장을 인가하게 되면 전극 가장자리 부분에서 먼저 스멕틱 층들이 휘어지며 빛이 세는 현상이 시편 전체 영역 전파 되어 나간다. 심한 경우 전기장이 제거되어도 다시 원상태로 회복이 되지 않는 이 현상은 VA-DHFLC시편에 일정 이상의 전기장 인가를 어렵게 하고 일정한 투과율 특성을 유지하는데 매우 큰 단점이 되었다. 본 논문에서는 VA-DHFLC 시편에 광경화성 단분자(reactive monomer)를 적용하여 UV에 의한 광경화 후 시편에 형성된 고분자 구조의 형태와 특성을 이해하고 연구한 결과, 액정 초기 배향 상태 돌아가게 하는 특성을 강화시키고 스멕틱 층의 변형을 제어하는 기능을 알 수 있었다. 그리고 전기광학적 실험 결과 이 구조가 적용된 VA-DHFLC 시편의 우수한 특성을 확인할 수 있었다. 고분자 구조가 형성된 VA-DHFLC 특성을 조사한 결과 높은 전압 구동에서 발생하는 투과율의 변동 문제를 최소화하였고 안정적이면서 연속적인 계조 표현이 가능하였다. 또한 장시간 시편의 구동에서도 일관된 특성을 보이는 등, VA-DHFLC모드가 갖지 못했던 전기광학적 구동 안정성을 확보 할 수 있었다.|Liquid crystal displays (LCDs) have been extensively studied and used for a wide range of display applications because of their several advantages such as light weight, low power consumption and high resolution and so on. In particular, display technologies have led to the development of fast response time for reducing picture image blur are commercially available. But, due to the intrinsic nature of a dielectric coupling with an external electric field in the Nematic liquid crystals (NLCs), the dynamic response is limited. Since the discovery of ferroelectricity in a tilted chiral smectic phase (Sm C*), ferroelectric liquid crystals (FLCs) have attracted great attention due to the fast electrooptic (EO) response, resulting from a direct coupling between the spontaneous polarization of the FLCs with an electric field. Thus far, most of the FLC devices have been realized in a planar alignment (PA) configuration including a surface stabilized FLC (SSFLC) mode. The SSFLC structure can be produced by removing the energy degeneracy of the Sm C* state in a thin cell gap compared to the helical pitch of the FLC. However, for practical applications, this SSFLC have several problems such as non-uniform alignment due to zigzag defects, associated with chevron structures and lack of gray-scale capability because of intrinsic bistability. In contrast, a deformed-helix FLC (DHFLC) mode with a short helical pitch compared to the wavelength of vision light is expected to possess many desirable properties such as fast response, analog gray scale, and wide viewing properties. However, the PA-DHFLC configuration has a low contrast ratio resulting from the existence of striped domains. The vertically aligned (VA) structure of DHFLC has attracted interest in display since its excellent features such as fast response time and complete dark state. In the VA-DHFLC, the in-plane switching electrodes were used for electro-optic switching. In such configuration, however, the light leakage near the electrode edge was observed due to the irrecoverable layer deformation by the strong vertical field. I n this paper, we proposed a VA-DHFLC device with the stabilized smectic layer against the field-induced deformation of the VA-DHFLCs with introducing the polymer structure. After exposure of ultra-violet (UV) light, the polymer structure enhanced the stability of the smectic layer against the vertical electric field on the in-planed electrode edges. As a result, we could obtain the VA-DHFLC device with the stable operating characteristic despite the strong electrical field. and This opens a possible investigation of the polymer/DHFLC mixed structure to understand the underlying stabilization mechanism of the smectic layers.; Liquid crystal displays (LCDs) have been extensively studied and used for a wide range of display applications because of their several advantages such as light weight, low power consumption and high resolution and so on. In particular, display technologies have led to the development of fast response time for reducing picture image blur are commercially available. But, due to the intrinsic nature of a dielectric coupling with an external electric field in the Nematic liquid crystals (NLCs), the dynamic response is limited. Since the discovery of ferroelectricity in a tilted chiral smectic phase (Sm C*), ferroelectric liquid crystals (FLCs) have attracted great attention due to the fast electrooptic (EO) response, resulting from a direct coupling between the spontaneous polarization of the FLCs with an electric field. Thus far, most of the FLC devices have been realized in a planar alignment (PA) configuration including a surface stabilized FLC (SSFLC) mode. The SSFLC structure can be produced by removing the energy degeneracy of the Sm C* state in a thin cell gap compared to the helical pitch of the FLC. However, for practical applications, this SSFLC have several problems such as non-uniform alignment due to zigzag defects, associated with chevron structures and lack of gray-scale capability because of intrinsic bistability. In contrast, a deformed-helix FLC (DHFLC) mode with a short helical pitch compared to the wavelength of vision light is expected to possess many desirable properties such as fast response, analog gray scale, and wide viewing properties. However, the PA-DHFLC configuration has a low contrast ratio resulting from the existence of striped domains. The vertically aligned (VA) structure of DHFLC has attracted interest in display since its excellent features such as fast response time and complete dark state. In the VA-DHFLC, the in-plane switching electrodes were used for electro-optic switching. In such configuration, however, the light leakage near the electrode edge was observed due to the irrecoverable layer deformation by the strong vertical field. I n this paper, we proposed a VA-DHFLC device with the stabilized smectic layer against the field-induced deformation of the VA-DHFLCs with introducing the polymer structure. After exposure of ultra-violet (UV) light, the polymer structure enhanced the stability of the smectic layer against the vertical electric field on the in-planed electrode edges. As a result, we could obtain the VA-DHFLC device with the stable operating characteristic despite the strong electrical field. and This opens a possible investigation of the polymer/DHFLC mixed structure to understand the underlying stabilization mechanism of the smectic layers.