Recently, there has been considerable interest in flexible displays, as they facilitate the fabrication of displays that are thin, lightweight, robust, conformable and flexible. To enable a flexible display, a flexible substrate must be used as the fundamental starting component in place of a conventional glass substrate. Generally metal foils, ultra-thin glasses and plastic films are considered as candidates for a flexible substrate. Particularly, flexible displays using plastic substrates based on organic polymers have been a part of the main subject not only because these show outstanding flexibility and optical clarity at the same time, but also because they offer the possibility of a reduction in cost, coupled with a roll-to-roll process and ink-jet printing technology. Common examples of commercially available polymers are polyethylene terephthalate (PET), polyether sulfone (PES), polyethylene naphthalate (PEN), polycarbonate (PC) and polyimide (PI). To replace the conventionally used glass substrate, a plastic substrate must be equipped with properties of glass, such as optical transparency, thermal/chemical stability, $O_2/H_2O$ permeability, low birefringence (or retardation), dimensional stability and a low coefficient of thermal expansion (CTE). Among these properties, the low CTE of plastic substrates coupled with dimensional stability is arguably the most important requirement, as it is directly related to compatibility with all other necessary display layers to be integrated onto them. Although there have been extensive studies of flexible devices built on polymer substrates, such as electrophoretic displays and OTFTs, little progress have been made even with high-temperature processed devices such as active-matrix liquid crystal displays (AMLCDs).[5] Major obstacles include the high CTEs of polymers insufficient for the display layer integrations. Moreover, polymers usually have low glass transition temperatures ($T_g$) where abrupt CTE changes is accompanied. This greatly limits their practical application in terms of the process temperature. Even polymers with high $T_g$, e.g. PES, are known to have a CTE of ~50 ppm/℃, much higher than the typically required level (less than 20 ppm/℃). Other high-Tg polymers such as polytetrafluoroethylene (PTFE), poly(ether ether ketone) (PEEK) also have a significant drawback for implementation into a large-area plastic electronics, as they are unfavorable in terms of cost. PI has a low CTE of ~20 ppm/℃ and a high $T_g$, but its practical use is nonetheless problematic due to the high birefringence and yellowness. As the fabrication of high-performance electronic devices such as a-Si TFT and oxide TFT usually requires a high process temperature and high dimensional stability for layer integration, a flexible substrate must be equipped at least with a low CTE and high thermal stability for the implementation of a stable flexible display. Chemical stability is another important requirement, as a flexible substrate should undergo chemically harsh environments such as wet-etching during a series of lithography processes. High optical transparency in a flexible substrate is also essential for typical bottom emissive displays. Unfortunately, none of the conventional plastic substrates currently available can satisfy all of these requirements. This has been a major obstacle for the straightforward implementation of a stable flexible display. In the present study, a novel high-performance transparent glass-fabric reinforced composite film (GFRHybrimer) which can be used as a substrate for flexible devices was fabricated. The GFRHybrimer film was fabricated by impregnation and/or lamination of a woven glass-fabric (or glass cloth) with a photo-curable, oligosiloxane-based hybrid material (hybrimer) as the matrix. In terms of material, the use of such an organic/inorganic hybrid-type matrix offers versatile properties such as chemical inertness, thermal stability, optical transparency, and ease of functionalization. These properties originate from the desirable nature of organic/inorganic phases which are synergistically combined to form a homogeneous hybrid network. Additionally, the glass-fabric reinforcement enables not only a significant reduction in the CTE, but also a remarkable increase in the modulus and flexibility.
플렉시블 디스플레이 (flexible display)는 평판 디스플레이 (FPD) 단계를 넘어서는 차세대 디스플레이의 형태로서, 종이처럼 얇고 유연한 기판을 통해 손상없이 휘거나, 구부리거나, 말 수 있는 디스플레이를 말하며, 최근 플렉시블 디스플레이를 포함하는 플렉시블 소자 개발에 대한 연구가 활발히 진행되고 있다. 플렉시블 디스플레이를 구현하기 위해서는 여러 가지 요소 기술들이 필요한데 그 중 플렉시블 기판 (flexible substrate) 기술은 플렉시블 디스플레이 구현을 위한 가장 기본적이고 중용한 요소 기술이다.[1, 2] 플렉시블 기판으로서는 크게 메탈 호일 (metal foil), 박형 유리 (thin glass), 플라스틱 기판 (plastic substrate)이 사용될 수 있다.[1] 메탈 호일의 경우 기계적인 내구성이 좋고 열팽창계수 (CTE)가 작으며 비교적 가격이 저렴하다는 장점이 있지만 투명하지 않다는 단점이 있다. 박형 유리의 경우, 내열 및 내화학성이 우수하고 광투과도가 우수하지만 유연성이 부족해 깨지기 쉽다는 단점이 있다. 최근에는 투명성과 유연성을 동시에 가지며 비교적 가볍고 내구성이 좋은 플라스틱 소재를 이용한 기판 연구가 활발히 이루어지고 있다. 그러나 플라스틱 기판이 여러 가지 장점을 가짐에도 불구하고 열팽창계수 (CTE)가 크고 유리전이거동 (glass transition)을 가짐으로 인해 플라스틱 기판을 이용한 a-Si 및 산화물 TFT와 같은 소자의 제작이 쉽지 않은 실정이다. 본 연구에서는 솔-젤 유/무기 하이브리드 재료 (하이브리머)를 이용한 낮은 CTE와높은 광투과도를 가지는 유리-섬유 강화 투명 복합체 기판 (GFRHybrimer)을 제작하고, 이러한 투명 복합체 기판을 사용하여 산화물 TFT, 비정질 실리콘 기반 태양전지 및 유기발광다이오드 등의 플렉시블 소자를 제작하는 것은 목표로 한다. 하이브리머는 투명성이 우수하고 내열 및 내화학성이 높으며 복굴절이 낮다는 장점 이외에 특히 굴절률의 조절이 용이하여 유리섬유와의 복합체 형성 시 복합체 기판의 높은 광투과도를 얻는데 유리하다. 또한 CTE가 낮은 유리섬유를 강화재로 사용함에 의해 복합체 기판의 CTE를 크게 낮출 수 있다.
