1. Overview
Polysilazane is a highly active polymer with Si-N bonds as the main chain, which can react strongly with water, oxygen and a variety of polar substances. This material has a wide range of applications in the ceramic, aviation, aerospace and coating industries. According to their structure, polysilazanes can be divided into organic and inorganic two categories. Organic polysilazanes carry organic groups on their side chains, while inorganic polysilazanes, also known as perhydropolysilazanes or PHPS, contain only three elements, silicon, nitrogen and hydrogen. Because of its simple structure and high market value, PHPS is mainly used to make ceramic precursors and thermal insulation materials. PHPS does not contain organic groups and therefore can be converted by a variety of methods at lower temperatures and has good adhesion to the substrate. The characteristics of the converted coating include corrosion resistance, high and low temperature resistance, gas isolation, long-term durability, transparency and scratch resistance, so it has been widely used in the preparation of coatings. In photoelectric technology, an important branch of modern science, the development of coating technology is a challenge, and PHPS coating technology plays a vital role in improving the performance of photoelectric equipment and solving key technical problems in the photoelectric field.
2. Formation mechanism
In an oxygen or water environment, PHPS can be achieved by high temperature treatment or light irradiation to the conversion of the silicon oxide coating, whether or not a catalyst is present. Many researchers have explored the mechanism of coating formation of PHPS under different conditions, including the chemical reaction and phase transition of PHPS to silicon oxide at high temperature. The illustration shows the phase separation phenomenon in the PHPS conversion process, showing the transition from the PHPS phase to the silica phase, specifically including the continuous phase and the sea-island structure of PHPS, and the sea-island structure of silica. The chemical reaction diagram illustrates the hydrolysis, condensation and oxidation reactions in the conversion process. It is found that when the conversion temperature is lower than 180°C, the hydrolysis and condensation reaction of Si-H and Si-N mainly occurs, and the conversion is insufficient, and the structure of silicon oxide as the dispersed phase is formed. At this time, the refractive index is higher, but the modulus and hardness are lower. In the temperature range of 180°C to 300°C, the transformation is mainly the oxidation reaction of Si-H and Si-N, the silicon oxide phase gradually grows to form a bicontinuous phase structure, and when it exceeds 200°C, the silicon oxide phase becomes dominant, which significantly improves the mechanical properties of the material. In the temperature range of 300 ° C. to 600 ° C., a network structure of silicon oxide is substantially formed and further densified at high temperature.
3. Application of PHPS coating
3.1 as a dielectric layer
The silicon dioxide dielectric layer prepared by the liquid phase method of PHPS can solve the shortcomings of traditional methods such as thermal oxidation, chemical vapor deposition (CVD) and plasma enhanced chemical vapor deposition (PECVD), so it is very popular.
The molecular structure of PHPS has a significant effect on the properties of the dielectric layer. One study prepared coating compositions using PHPS with molecular masses in the range of 800 to 2500 and 3000 to 8000, with a ratio of weight average molecular weight to number average molecular weight between 6 and 12. The composition is coated on a substrate having a gap and heated at a temperature of 1000 ° C. or lower to form a siliceous film deep into the gap. In addition to focusing on molecular mass, more research has focused on the effect of specific element or group content in PHPS on coating performance. For example, PHPS compositions that are free of N-H and C, but rich in Si, contain units [-N(SiH3)x(SiH2-)y], where the sum of x and y is 2 or 3, respectively, with different values. These PHPS in combination with different catalysts can produce oxide films with low shrinkage, which are particularly suitable for filling semiconductor gaps. Using PHPS with specific 1H NMR spectral characteristics, with a specific ratio of peak 1 and peak 2, a silica layer with excellent layer thickness uniformity can be prepared. Finally, PHPS having a molecular weight of 8000 to 15000 and a nitrogen content of 25% to about 30% of the total weight were prepared, and silica layers prepared from these PHPS exhibited excellent etch resistance.
3.2 as barrier layer
Barrier layers, especially those that have a barrier effect on gases such as water vapor, are a common type of coating on the surfaces of electronic and optical devices. In one study, a barrier layer was developed using PHPS, which together with an adhesive layer formed an adhesive sheet. The surface density of the barrier layer ranges from 2.4 to 4.0g · cm³, in which the proportion of oxygen, nitrogen and silicon accounts for 60% to 75%, 0% to 10% and 25% to 35% respectively. This is one of the early patents for the development of barrier layer using PHPS. Another study explored the effect of PHPS structure on gas barrier properties, and found that by adjusting the ratio of SiH3 to SiH and SiH2 to 1 :( 10 to 30), a gas barrier film with excellent stability under high temperature and high humidity conditions can be prepared.
In addition to using PHPS alone, it is often used in combination with modifying materials to make barrier layers. For example, a silicon-containing film having a structural formula of SiOxNyMz was prepared using PHPS and a metal compound such as aluminum tri-sec-butoxide together, showing excellent stability under high temperature and high humidity conditions. Other studies have described several additives for use in combination with PHPS, including hydrocarbyl-substituted guanidines, oxygen-nitrogen-containing crown ether amines, amino-substituted polycyclic cycloalkyls, hydrocarbyl-substituted oximes, the use of which significantly improves the gas barrier properties of the resulting films.
The composition of the solution also has an effect on the performance of the PHPS barrier layer. By limiting the specific structural units and the ratio of Si-R bonds to Si-H bonds of PHPS, it can be dissolved in aliphatic hydrocarbon solvents to prepare silica-like glass barrier layers with low water vapor transmission rate.
In addition, the effect of the preparation method on the performance of the barrier layer is also the focus of the study. A barrier film with a specific refractive index range was prepared using heating and plasma treatment, while a film with excellent gas barrier property and retardation film function was prepared on a variety of polymer substrates using PHPS under vacuum ultraviolet irradiation. Finally, SaSaki et al. studied the effects of vacuum ultraviolet (VUV)-induced Si-N bond number, PHPS film composition and free volume on the film densification process. It was found that VUV irradiation can promote the rapid release of hydrogen and the densification of the film. It provides valuable guidance for the development of nano-SiN films with high density and excellent gas barrier properties.
3.3 as an optical film
PHPS is commonly used in the manufacture of optical films by combining with modifying raw materials to form composite materials that can be used in the manufacture of optical films. A method of preparing a low refractive index film using a combination of PHPS and at least one organic polymer selected from the group consisting of silazanes, silazanes, and ureosilazanes. The solution containing PHPS was mixed with the fluoropolymer solution and coated to obtain a silica optical film layer with high strength, oleic acid resistance and easy sliding. The xylene solution of PHPS was used as a precursor to prepare a silica coating doped with spiropyran (SP). With the conversion of PHPS to silica, the film changed from transparent light yellow to red, and the absorbance at 500 nm increased. After exposure treatment, the color of the film deepens and shows reversible photochromic properties, which proves its potential in optical film applications.
In addition to the type of raw materials, the preparation method also has a significant impact on the performance of the optical film. Nakagawa et al. prepared PHPS-converted organic-inorganic hybrid films by sol-gel method, which were applied to the active layer of OLED and showed excellent electroluminescent properties. Lee et al. spin-coated an HPS layer on Si(100) by dibutyl ether solution and prepared a dense silica crosslinked layer in water or hydrogen peroxide using 405 nm UV light irradiation, demonstrating the influence of the preparation process on the stoichiometric ratio and refractive index. Baek et al. used intense pulsed ultraviolet light (IPL) to treat HPS in air environment and low temperature to prepare SiOx layer, which showed similar conversion rate and refractive index with heat-treated silica layer, proving the application potential of IPL process in optical thin film industry. These studies show that by adjusting the raw materials and preparation methods, the performance of optical films can be significantly affected, which provides a diversified way for the preparation of optical films.
3.4 other applications
The application of PHPS in the field of solar cell coatings has increased significantly in recent years, and it plays a variety of key roles in solar cell devices. For example, PHPS is used to fabricate a dielectric barrier layer for solar cells, which is disposed between a metal or glass substrate and a CIS (copper indium sulfide) or CIGSe (copper indium gallium selenide) photovoltaic structure. The thin film solar cell encapsulation layer made of PHPS, which allows the chalcopyrite-based solar cell to have an average reflectance of less than 95% in the optical band of 300 to 900nm and a reflectance of more than 200 in the band of 1100 to 1500nm, shows excellent aging resistance. PHPS is used to prepare an anti-glare film for solar cells, which has an appropriate anti-glare surface texture and can effectively remove surface contamination. Vacuum ultraviolet light is used to convert PHPS into silica to encapsulate flexible perovskite solar cells (PSC). In order to prevent PSC degradation caused by PHPS solution and VUV(λ = 172nm) light, CdSe/ZnS quantum dots are used as barrier layers distributed on polydimethylsiloxane substrates. The water vapor transmission rate of the encapsulation layer is extremely low, increasing the room temperature service life of flexible solar cells by more than 400 hours.
In addition, by dissolving PHPS in xylene and hydrolyzing it by ammonia, the perovskite film is further adhered to the dense layer of titanium oxide, which provides a new idea for the large-scale production of perovskite photovoltaics.
In addition to conventional dielectric, barrier and optical layers, PHPS is also used to prepare other functional layers. PHPS is used to form a compound layer on a substrate, in which a part of the silazane compound is converted to a compound containing a siloxane bond, and a metal layer containing silver as a main component is formed thereon to produce a transparent conductive film. Studies have shown that a wavelength conversion film prepared using a solution containing PHPS and a wavelength conversion agent has an improved visible light transmittance of 50% or more compared to an aqueous solution. An oxide layer and a silica coating formed by curing PHPS, which are sequentially stacked on a metal substrate, are used for a thermally conductive insulating sheet for an electronic component, and such an insulating sheet exhibits good thermal conductivity and insulating properties. PHPs block copolymers comprising a linear or cyclic block A and a silicon-rich polysilazane backbone block B were prepared by light crosslinking reaction with the help of a crosslinking agent. These block copolymers have a unique structure that enables the preparation of thick, high-density sacrificial films with good adhesion to the substrate, providing additional functional layers for solar cells. These studies show that PHPS, as a multifunctional material, has a wide range of applications in the field of solar cells, not only to improve the efficiency and life of the battery, but also to enhance the environmental stability and reliability of the battery. Through the different treatment and application of PHPs, high-performance solar cell modules can be prepared to meet specific needs, which further promotes the development of solar energy technology and the progress of photovoltaic industry.
4. Outlook
There is still a big gap between China's comprehensive competitiveness in the field of optoelectronics and developed countries, and the excellent processing performance and product performance of PHPS coating make it have broad application prospects in the field of optoelectronics. In the preparation of PHPS, China's comprehensive strength is weak, and for the application of PHPS coating, AZ electronic materials, Samsung Co., Ltd., Konica Minolta Co., Ltd., Lindeke Co., Ltd. and other developed countries in China to carry out patent layout early, the number of patents. Compared with foreign research on PHPS coating, there are few domestic research reports, and the application of PHPS in the field of optoelectronics is rarely reported, which undoubtedly poses a challenge to the development of China's optoelectronic industry. China should strengthen the research on the preparation and application methods of PHPS, break through the difficulties in the preparation and application of PHPS and strengthen the protection of intellectual property rights, so as to build a competitive PHPS coating industry chain.