Preparation, structure and gas barrier characteristics of poly silazane-derived silica thin film formed on PET by simultaneously applying ultraviolet-irradiation and heat- treatment

Preparation and gas barrier characteristics of polysilazane-derived silica thin films formed on PET by simultaneously applying ultraviolet-irradiation and heattreatment have been investigated. Water-vapor-permeability of the thin film formed by UV irradiation at 100°C was 0.03 g/m2・day, indicating the good water-vapor resistance. The values of gas barrier characteristics depended on the substrate temperature at the time of photo irradiation. The TEM cross-sectional observation revealed that the gas barrier films were dense and uniform, and the reactive layer existed at the interface of the thin film and PET. The PET film with gas barrier thin film has good transparency in the region of visible light, flexibility as well as high gas barrier characteristics. This film-formation method is an effective way of forming a gas-barrier film at low temperature by using a film coating via an inorganic precursor solution. Introduction In recent times, as next-generation electronics products, devices featuring flexibility—such as flexible displays and electronic paper (characterized by their thinness, lightness, and bendability), flexible solar cells, and wearable devices—are gaining considerable attention [1-4]. Moreover, the so-called “flexible electronics” used to construct these products represent a field that is expected to grow rapidly from now onwards [5,6]. As essential elements required for creating flexible electronics, flexible substrates are indispensable component and make it possible to form various flexible devices. As for a flexible substrate, an organic-resin film with flexibility is generally used. A typical organic film, however, faces a major problem: its “gas-barrier” property (in regards to water vapor and oxygen) is poor, leading to oxidation of various device elements formed on the flexible substrate and degradation of device performance. As a result, it is necessary to develop a flexible substrate with an improved gas-barrier property. One means to improve the gas-barrier property of an organic film is to form a dense inorganic thin film on top of it. However, the low thermal resistance of an organic film means that it is a problem to form an inorganic film with good gas-barrier performance. The most-common material used for organic films is polyethylene terephthalate (PET), and forming an inorganic thin film on a PET film improves the gas-barrier property of the PET. The thermal resistance of PET is limited to 100°C, so it has become mainstream to form metal oxide thin film like silicon-dioxide (SiO2) thin films on PET by using high-vacuum deposition apparatuses using methods like sputtering, vacuum deposition and atomic layer deposition (which restrain the heating temperature of the substrate) [7-12]. Unfortunately, the gasbarrier property of a SiO2 thin film is still insufficient, and using such complex high-vacuum equipment is one cause of increased costs. An alternative convenient method of forming an inorganic film is the application of an inorganic precursor solution. However, forming a dense inorganic thin film in this manner requires heating to several hundred degrees Celsius [13-17]. Applying this inorganic-precursor method in the case of organic films with low thermal resistance is thus difficult. Recently, a method for forming a dense inorganic thin film on a PET film at low temperature—by excimer-light irradiation and ultraviolet (UV) irradiation of a polysilazane coating film—is being investigated [18-26], and an organic film (with an inorganic coating formed by that method) with a comparatively good gas-barrier property is being developed. Since this method uses a coating film applied by “printing” of an inorganic precursor solution, it is drawing considerable attention as a means of creating low-cost “printable electronics”. By irradiating a polysilazane coating film with UV light (by using a low-pressure mercury lamp) while heating the film, the authors have succeeded in developing an alicyclic polyimide film with high heat resistance and high transparency as well as a good gas-barrier property [27]. In the present study, a gas-barrier film on a PET film (with low heat resistance) was formed by using the same method using photo-irradiation and heat-treatment simultaneously, and the properties of the film were investigated. Correspondence to: Tomoji Ohishi, Department of Applied Chemistry, Faculty of Engineering, Shibaura Institute of Technology, 3-7-5 Toyosu, Koto-ku, Tokyo 135-8548, Japan; Tel: +81-3-5859-8154; Fax: +81-3-5859-8101; E-mail: [email protected]