Graphene transparent conductive film with smart dimming coating

Large-Area, Low-Cost Production of Graphene Transparent Conductive Films

Graphene, as a material with unique properties, is currently being applied in various fields such as transparent conductive electrodes, touchscreens, solar cells, hammer batteries, supercapacitors, thermal films, seawater desalination, and environmental pollution control, arousing high expectations.

In the massive graphene "flagship" research program initiated by the European Union in early 2013, the study of transparent conductive films became a significant focus. Simultaneously, numerous world-class companies, including IBM from the United States, BASF from Germany, and Samsung from South Korea, have invested heavily in the research and development of graphene transparent conductive film products, aiming to secure advantageous positions in this emerging material development field. Some companies have even announced plans to commercialize graphene transparent conductive films in the near future.

The global enthusiasm for graphene development has gradually influenced academic and business communities in China. Especially since the Nobel Prize was awarded to the discoverers of graphene in 2010, various levels of government departments in China have increased their support for graphene research. The National Natural Science Foundation of China, the Ministry of Science and Technology, the Chinese Academy of Sciences, and other departments have initiated several significant research projects related to graphene. Many regions, including Beijing, Shanghai, Jiangsu, Zhejiang, Guangdong, and others, have proposed strategies and plans to support graphene research and development. Numerous enterprises have also entered graphene product development through research, collaborative development, and pilot investments, leading to the gradual initiation of graphene research and application studies in China.

However, the core challenge faced by graphene transparent conductive film development, both domestically and internationally, remains the large-area, low-cost, high-quality preparation technology of graphene films. Currently, the widely adopted technology for graphene film preparation is chemical vapor deposition (CVD). While this technology can yield large, high-quality monolayer graphene sheets with performance advantages in transparency and surface conductivity, it still suffers from fatal drawbacks such as complex process routes, high costs, low yield rates, and limitations on film area. Another preparation technology for graphene, the reduction of graphene oxide method, uses a solution process, enabling large-area continuous preparation at a lower cost. However, due to certain defects and a higher number of boundaries in graphene sheets, it falls slightly short in performance.

To achieve the large-scale and commercialized production of graphene transparent conductive films, further technological innovation and substantial investment in manpower and resources are necessary. The research group led by Dr. Zhilin at the National Center for Nanoscience and Technology has conducted extensive research in the preparation of graphene transparent conductive films using the reduction of graphene oxide method. The developed production technology for graphene transparent conductive films possesses unique technical advantages compared to similar international technologies. The application of rod-coating technology for the first time achieved large-area, continuous, and rapid coating of oxidized graphene films. Additionally, the first use of room-temperature catalytic reduction technology successfully produced composite graphene films, forming the first continuous production line for graphene transparent conductive films with a silver-to-silver base. Through the application of graphene transparent conductive films, the cost of smart dimming film products has been significantly reduced, and production efficiency has greatly increased. This is crucial for the rapid and comprehensive promotion of graphene film materials in practical applications, aligning quickly with diverse end products and markets. Beijing Sheng Mei Hongye Technology Co., Ltd. has established a close cooperation with the National Center for Nanoscience and Technology. Currently, it has developed large-scale, low-cost production capabilities for graphene transparent conductive films, and the performance of its products has reached an advanced level compared to similar products. It is expected to launch a prototype product of graphene smart dimming film by the end of 2014, filling the gap in relevant fields both domestically and internationally.

Intelligent Dimming Film Based on Graphene Transparent Conductive Films

With the rapid development of China's economy and society, remarkable achievements have been made in various fields. However, the accompanying high energy consumption and pollution have become significant factors restricting the sustainable development of the Chinese economy. In the "Twelfth Five-Year Plan," the State Council proposed vigorous promotion of energy conservation, emission reduction, and the development of green buildings. China's per-unit building area energy consumption is more than three times that of developed countries, with windows being the main energy-consuming component of buildings. To reduce building energy consumption, there is an urgent need for an intelligent window product that is highly effective, practical, and meets the demands of energy conservation. The intelligent dimming film emerges in response to this trend.

Intelligent Dimming Film Based on Graphene Transparent Conductive Films

With the rapid development of China's economy and society, the country has achieved impressive accomplishments in various fields. However, the accompanying high energy consumption and pollution have become significant factors limiting China's sustainable economic development. In the "Twelfth Five-Year Plan," the State Council emphasized the importance of promoting energy conservation, emission reduction, and the development of green buildings. China's per-unit building area energy consumption is more than three times that of developed countries, with windows being the primary energy-consuming component of buildings. To reduce building energy consumption, there is an urgent need for an intelligent window product that is highly effective, practical, and meets the demands of energy conservation. The intelligent dimming film emerges in response to this trend.

1.Intelligent Dimming Film as a Key Solution for Building Energy Efficiency

China's building energy consumption accounts for 35% of the total energy consumption, and issues such as delayed action on building energy efficiency, high energy consumption, and heavy pollution have become prominent problems restricting China's sustainable economic development. The thermal loss from the exterior walls of Chinese buildings is three to five times that of similar buildings in North America, with window thermal loss being over two times higher. The commercial area of doors accounts for 20% to 30% of the building area, with glass making up 70% to 80% of the door area. About 70% of building energy consumption is lost through doors, and one-third of that is lost through glass. Radiative heat transfer is the primary mode of heat conduction, accounting for over 60%.

Since 2006, the national government has actively promoted building energy efficiency, implementing a series of policy measures. For example, the Ministry of Housing and Urban-Rural Development issued the "Green Building Evaluation Standard," which assesses new and existing buildings using three grades (stars). It includes requirements for the layout of residential buildings to ensure indoor and outdoor sunlight, lighting, and ventilation, meeting the standards outlined in the "Urban Residential Area Planning and Design Code" (GB 50180). In January 2013, the State Council forwarded the "Action Plan for Green Building" by the National Development and Reform Commission and the Ministry of Housing and Urban-Rural Development. It urged the transformation of urban and rural construction models, improving resource utilization efficiency, reasonably enhancing building comfort and livability, and advancing green building action comprehensively through policies, regulations, institutional mechanisms, planning and design, standards, technical promotion, construction and operation, and industry support. The goal of implementing building energy efficiency by 2020 was also clearly defined.

In the "Twelfth Five-Year Plan for the Development of New Materials Industry" released in 2012, graphene was explicitly listed as a key frontier new material. The plan aimed to advance research and application of nanomaterials in energy conservation and other fields. China leads in graphene research internationally, with the highest number of related patents globally, making graphene industrialization widely recognized. China's graphene industry is still in the exploration phase, determining the most suitable forms for graphene products. The intelligent dimming film is a newly developed energy-saving product for doors and windows in this context. The intelligent dimming film is a new type of optoelectronic product that integrates sun shading and heat insulation inside and outside doors and windows, providing unique light transmission and thermal insulation functions. In summer, it dynamically adjusts the solar light transmittance, controlling the transmitted light and heat effectively, peaking to lower indoor temperatures. In winter, it restores the ability of light and heat to pass through, blocking indoor heat loss. Compared to existing building doors and windows, the comprehensive energy-saving effect of the intelligent dimming film is increased by more than 30%. It can effectively overcome the single deficiency of existing low-emissivity films, which can only reduce heat radiation but lack light recovery functions. The intelligent dimming film can also change the glass color and hue according to user needs, enhancing the comfort of living or working spaces. When combined with sensors, the intelligent dimming film can achieve automatic adjustment, providing users with the optimal experience and energy-saving effects.

2.Graphene Transparent Conductive Films as the Ideal Material for Intelligent Dimming Film

Currently, the transparent conductive electrode in intelligent dimming film products mainly uses indium tin oxide (ITO), known for its good transparency, low surface resistance, and mature processing technology. However, ITO is not resistant to bending and is prone to breakage during production, transportation, and use, leading to scrap. Moreover, indium resources are scarce and scattered, making mining and recycling difficult. With continuously depleting resources, the cost of indium will continue to rise. Due to these reasons, alternatives to ITO are continuously sought after. Currently, potential materials include silver nanowires, metal grids, carbon nanotubes, and graphene. Graphene, as a new type of two-dimensional carbon-based material, possesses ultra-thin, super-flexible, and high specific surface area characteristics. With a single atomic layer thickness and two-dimensional properties, coupled with graphene's excellent electron transport performance and wide-band transparency, graphene films have intrinsic transparent conductive properties, making them an ideal material for transparent conductive films. Compared to traditional ITO-based transparent conductive materials, graphene has the advantage of flexibility, making it easy to apply on flexible substrates such as PET, expanding the application range of products. Flexible materials are suitable for integration with roll-to-roll processes, enabling full continuous production, reducing processing costs. Additionally, the raw material cost of graphene itself is lower, providing a significant cost advantage. Therefore, the international focus on the research and development of graphene transparent conductive films is one of the key areas of interest.

3.Features and Applications of Intelligent Dimming Film

The intelligent dimming film finds widespread applications in modern architecture, including glass walls, window glass, interior partitions, various vehicles, and spacecraft side windows. China adds 2 billion square meters of new construction annually, with existing buildings totaling 40 billion square meters, over 80% of which are high-consumption structures. Building exterior windows and transparent glass walls are the most active areas for heat conduction, thermal diffusion, and heat loss in the peripheral structure.

The intelligent dimming film can be used as a sandwich or attached film for building sloped glass. By adjusting the incident light and blocking thermal energy, it provides a solution to control heat entering and exiting through windows. It can automatically change color as needed, eliminating the need for interior blinds or external sunshades for shading or heat insulation. It integrates indoor and outdoor light seamlessly, controlling glare and heat while effectively reducing energy consumption. According to Nanomarkets, an authoritative organization, the market size of smart window products, including intelligent dimming films, is expected to reach $40 billion by 2018.

According to estimates by the U.S. Department of Energy, electrochromic shading systems can save at least 20% in operating costs, reduce peak electricity demand by 24%, decrease HVAC system loads by 25%, and cut lighting expenses by 60%. This contributes to lowering building costs from both electricity demand and equipment investment perspectives. A project funded by the California Energy Commission and completed by Lawrence Berkeley National Laboratory pointed out that controllable electrochromic windows (EC windows) can adjust incident light intensity, glare, and heat transfer while maintaining visibility. This effectively reduces energy consumption and peak electricity demand for cooling, heating, and lighting. The research, including simulation calculations, laboratory tests, and a two-and-a-half-year on-site test in a large office building, examined the performance of large-area EC window prototypes in actual commercial environments. It covered aspects such as light transmission range, color uniformity, color change speed, and control accuracy. Additionally, it integrated smart windows with lighting control systems, optimizing efficiency and user comfort through sensors and algorithms. Through intelligent design and control methods, the smart window became an integral part of the building control system, presenting a complete solution. Results showed that the peak cooling load of the electrochromic window decreased by 19% to 25%, and self-consumption for lighting decreased by 48% to 70%. Electrochromic windows also contribute to better user subjective experiences, including reduced glare, lower computer screen reflections, reduced window brightness, and improved visibility. With improved employee comfort, employees tend to be more engaged, leading to increased work efficiency.

The application of intelligent dimming products in the transportation sector has gained traction. In 2008, Ferrari installed electronically controlled dimming panoramic windows on the flagship model 612 Scaglietti. Covering the entire top section of the cabin, these windows, made of electrochromic glass, allow passengers to adjust the intensity of incoming light with the press of a button. On December 15, 2009, the Boeing 787 Dreamliner, equipped with intelligent dimming windows, successfully completed its maiden flight. By automatically adjusting the brightness of the windows, passengers can enjoy a more comfortable travel experience, and the control effect on light and heat transfer contributes to energy conservation. Rearview mirrors with anti-glare functionality using intelligent dimming film have also been widely used in motor vehicles. When the vehicles behind use high beams, the anti-glare rearview mirror automatically reduces light reflection, eliminating glare for the driver and ensuring driving safety.

Dimming films based on polymer-dispersed liquid crystal technology (PDLC) have the characteristic of appearing misty (opaque) when powered off and becoming transparent when powered on. This dual nature allows dimming glass to function as both regular glass and electronically controlled curtains. In the zero state, it can effectively protect privacy and isolate private spaces. In the transparent state, it maintains clear visibility, making it widely used in places such as meeting rooms, control rooms, and hotels that require privacy. In the zero state, the liquid crystal dimming film can also be used as a projection screen, and when combined with touch functionality, it forms a composite product, primarily used in places like shopping malls for product displays.

4. Working Principle of Intelligent Dimming Film

Intelligent dimming film can be primarily categorized into electrochromic and polymer-dispersed liquid crystal types based on their working principles.

• Electrochromic Type: This type utilizes the electrochromic effect, which involves the transition between the colored and transparent states of electrochromic materials when ions and electrons are injected or withdrawn. Electrochromic devices mainly consist of transparent electrode layers, electrochromic layers, counter electrode layers, and ion-conductive layers. Currently commercialized electrochromic materials include inorganic materials such as tungsten oxide and nickel oxide, as well as organic materials like polyaniline and its derivatives, polypyrrole and its derivatives, polythiophene and its derivatives, phthalocyanine derivatives, tetrathiafulvalene, and metallophthalocyanine compounds.

• Polymer-Dispersed Liquid Crystal Type (PDLC): This type utilizes a polymer-dispersed liquid crystal layer as the functional layer. Liquid crystal droplets are uniformly dispersed in a polymer matrix. When no electric field is applied, the liquid crystal molecules are randomly oriented, causing incident light to scatter, resulting in a foggy state. When an electric field is applied, liquid crystal molecules align uniformly along the electric field direction. As the refractive index of liquid crystal is nearly the same as that of the polymer, light can pass through the film without obstruction, rendering the film transparent. This type of film possesses the dual characteristics of regular glass and electronically controlled curtains, offering effective privacy protection in the opaque state and clear visibility in the transparent state.

• Trends in the Development of Intelligent Dimming Film

• For intelligent dimming film to achieve significant energy savings and enhance visual comfort, an accurate control system is crucial. Connected sensors and intermediate controllers regulate the light transmission of the dimming film based on external light conditions. This ensures blocking excessive sunlight to reduce cooling energy consumption while meeting visual comfort requirements. Simultaneously, it strives to introduce sunlight to minimize lighting energy consumption, achieving optimal energy efficiency and user experience. In 2006, the initial commercialized versions of intelligent dimming film had relatively simple functionalities, limited to on-off control, and were costly. Only with expanded production scales did costs significantly decrease. Currently, the value of intelligent dimming film products lies not only in reducing energy expenditures but also in load management and user comfort, critical factors for attracting early consumers.

  The future development direction of intelligent dimming film involves improved performance and lower costs. This includes the need for precise controllers, faster light transmission conversion speeds, avoiding coloration in opaque states, expanding the dimming range, reducing production costs, and designing more universal and reliable lighting control algorithms to adapt to diverse users, climates, and HVAC systems. Establishing product rating systems and industry standards, as well as developing design tools, are essential. If personalized, the application scope of these products will significantly expand, addressing various surface shapes and even incorporating wearable applications, such as color-changing camouflage clothing. Intelligent dimming film, with its on-demand light transmission adjustment capabilities, can create energy-efficient, comfortable, and personalized work and living environments. It is a necessary component for green smart buildings and modern high-tech transportation. However, strict processing conditions and high production costs limit its widespread application. Introducing graphene transparent conductive films to replace traditional ITO materials in intelligent dimming film products fully utilizes graphene's characteristics—high light transmission, high conductivity, and flexibility. This not only maintains product performance but also reduces costs, providing momentum for the widespread application of such products.

  China holds an international leadership position in graphene research, and graphite resources are abundant. Transforming graphene, this remarkable material, into an industrial advantage has become a crucial topic influencing the entire new materials industry. Considering factors such as product form, graphene's current development status, and market expectations, intelligent dimming film represents an excellent application point for graphene under current performance conditions. Developing this industry will greatly boost graphene's research, production, and application, generating a driving effect on the entire industry chain and benefiting the public with the miraculous material, graphene.