Electrochromic glass, as the name suggests, is a glass that changes color when energized. The commonly used electrochromic materials that use glass as the carrier for electrochromic materials are called electrochromic glass (EC glass for short), and if a polymer film is used as the carrier for electrochromic materials, it is called electrochromic film. Electrochromism is a change in the chromaticity of an object caused by the electrochemical redox reaction that occurs in electrochromic materials. The color changing process is the decomposition and reduction process of electrochromic materials. The materials and structures used for electrochromic glass depend on the specific application. In the 1930s, Kobosew and Nekrassow first studied the electrochemical coloring of bulk tungsten oxide. T. Kraus began detailed research on electrochemical coloring in tungsten trioxide (WO3) thin films in 1953. In 1969, S K. Deb discovered electrochromism in WO 3 thin films. Deb observed electrochromism by applying an electric field to WO3 thin films. S. K. Deb disclosed electrochromic technology in 1973.
At present, electrochromic technology has been widely applied in many fields such as smart windows, anti glare interior and exterior rearview mirrors, and camera dimmer plates in automobiles. Taking EC glass as an example, its working principle and structure are shown in Figure 1
The basic structure of EC glass is composed of two pieces of glass, the conductive layer on the inside of the two pieces of glass and the frame glue connecting the two pieces of glass. The space between the two pieces of glass enclosed by the frame glue is filled with electrochromic materials. The intermediate carrier of the color changing materials can be liquid, colloidal (gel) or solid. Regardless of the intermediate carrier state of EC glass color changing materials, there are three main different layered materials: EC layer and ion storage layer, conductive ion and electron layer. Electrolytes are pure ionic conductors that separate two EC layers. Transparent conductors are pure electronic conductors. When electrons enter the EC layer from a transparent conductor and charge balancing ions enter from the electrolyte, light absorption occurs. Simply put, when EC glass is not powered on, the ions inside the two pieces of glass are in an irregular state. (Figure 2)
In this state, EC glass is transparent, and after being coated with a reflective film, the glass becomes a high reflectivity lens. When EC glass is electrified, the ions inside the two pieces of glass are in a regular state. In this state, EC glass appears as a color changing substance, and after being coated with a reflective film, the glass becomes a low reflectivity lens. (Figure 3)
In the color changing materials of EC mirrors, the commonly used EC base layer is the main layer of EC glass, with tungsten trioxide (WO3) as a typical representative. The ion storage layer (usually an anode color changing material) is a key layer for improving the performance of electrochromic devices and achieving technological application. Its main function is to store and supply the ions required for color changing reactions, maintaining the charge balance of the entire electrochromic process. Currently, the most typical anode material is nickel oxide (NiO), which is widely used due to its low price, high coloring efficiency, and wide range of light modulation. The intermediate carrier electrolyte of EC materials generally uses liquid gel or solid electrolyte, and solid inorganic or organic materials such as Ta2O5 and ZrO2 are used in solid electrolyte EC devices. In terms of the application of EC color changing materials, in addition to inorganic color changing materials, organic electrochromic materials can also be used to achieve electrochromism, mainly including polythiophene and its derivatives, viologen, tetrathiofulvalene, metal phthalocyanine compounds, etc.
EC glass can theoretically achieve infinite size, but due to the distance of current penetration, The maximum size of EC glass can only be within 50cm x 50cm, so currently EC glass is only used in the automotive field for electronic anti glare. External rearview mirror and camera sunshade. If the problem of EC's current penetration distance is solved, its application can be extended to car sunroofs, front and rear windows, and side window glass. So the application prospects of EC glass are quite objective.
The electronic anti glare interior and exterior rearview mirrors for automobiles are the most mature products of EC glass application. When the car's electronic anti glare interior and exterior rearview mirrors are driven at night, when the headlights behind the vehicle illuminate the interior and exterior rearview mirrors, the electronic anti glare interior and exterior rearview mirrors will instantly change the reflectivity of the lenses to prevent glare caused by the rear headlights to the driver and avoid safety accidents caused by glare.
At present, domestic scholars have newly developed a new type of electrochromic coated glass, which has a different structure from the EC glass mentioned above. The EC glass mentioned above is a sandwich structure similar to that in food, which is filled with organic or inorganic color changing materials in a box surrounded by two pieces of glass. The color changing materials are decomposed or reduced by the positive and negative electrodes of a DC power supply, causing the color changing of the EC glass. However, the production process of EC glass with this sandwich structure is complex, with a low yield rate, and it is still difficult to manufacture glass with an area exceeding 50cm. The new type of electrochromic coated glass can effectively avoid these drawbacks. The electrochromic coated glass adopts a vacuum overlay coating process, which layers and overlays improved inorganic composite materials on the glass surface using a vacuum coating process (see Figure 5).
The base layer material can be tungsten trioxide (WO3) or organic molecule viologen (RV2+) based on double quaternization. The active material in the middle layer can help the base layer material and the auxiliary color changing material in the third layer achieve electrochromism and adjust the chromaticity of the color changing. Moreover, a conductive layer (ITO) is coated on the outermost layer of the electrochromic coated glass color changing film. After being charged, the various coatings of the electrochromic coated glass will undergo color changes under the action of charges. Electrochromic coated glass is then coated with a reflective layer to form electrochromic lenses (see Figure 6), which can be used for automotive electronic anti glare interior and exterior rearview mirrors.
At present, the power supply mode of electrochromic coated glass is different from that of EC glass, EC glass exists as an electrical terminal in the entire closed-loop power supply system, and is excited to change color by the positive and negative electrodes of the DC power supply. Electrochromic coated glass must have a direct current passing through, which causes the layers of materials stacked on the glass to work and change color. Therefore, in the working circuit of electrochromic coated glass, there must be a series of resistive (capacitive) loads.
There is also a solution, which is to first coat a conductive layer (ITO) on the surface of the glass, then sequentially overlay each layer of material, and then coat the outermost layer with a conductive layer (ITO) to form a virtual sandwich structure (see Figure 8). After the positive and negative electrodes of the DC power supply excite the colored materials stacked on the glass, the charge changes, causing the glass to change color.
Compared with double-layer EC glass, electrochromic coated glass has a simple processing technology and a relatively higher yield. Especially when used as a single (double) curved car exterior rearview mirror, it will not produce uneven color change due to the inconsistent curvature of the two glass sheets. The biggest feature of electrochromic coated glass is that it can be used to make large-sized panoramic car skylights, as well as front, rear, and side window glass. However, there are still unresolved defects in electrochromic coated glass at present, such as low surface hardness of the film layer. When making sunroof and window glass, it is necessary to use a laminated glass structure to prevent scratches on the film surface., Another issue that needs to be addressed is the slow speed of color change. The research and development personnel are screening suitable intermediate active materials to improve the color changing speed due to the problem of slow color changing speed.
In summary, as the technology of electrochromic coated glass becomes increasingly mature, its application in people's daily lives will become more and more widespread. It can not only be used for automotive components, but also for the development of information display devices, electrochromic intelligent dimming windows, non glare mirrors, and electrochromic information storage. In addition, some recent technological products, such as color changing mirrors, high-resolution photoelectric camera equipment, photoelectrochemical energy conversion and storage devices, and electron beam metal plate printing technology, have also been applied. It can be said that high technology will bring high-quality life to humanity.
The article is reprinted from Wang Zhao's discussion on cars
