Due to their outstanding properties, including transparency, barrier effect and conductivity, graphene layers promise major advances and efficiency increases in solar applications, energy storage and smart glass applications. The lack of scalable deposition processes with consistently high layer quality at cost-efficient throughputs has so far prevented the breakthrough of this promising material. As part of the funded EU project NewSkin, the Fraunhofer Institute for Electron Beam and Plasma Technology (FEP) has developed an innovative PECVD process that enables the deposition of graphene at high process speeds and offers higher production throughputs and a wider range of substrates at lower process temperatures
Graphene has the potential for exceptional performance in applications ranging from solar cells, organic light-emitting diodes (OLEDs), electrical energy storage devices or smart windows to membranes, e.g. for seawater desalination. Its outstanding properties are impressive: the material is transparent, light and stronger than steel. Graphene also has a high conductivity for heat and electricity and is very flexible. Researchers have been working for several years on the integration of graphene as a transparent electrode in photovoltaics or in OLEDs, e.g. for curved touchscreens or to increase the efficiency of solar cells. Thanks to its high electrical conductivity, graphene can contribute to faster charging and discharging cycles in energy storage devices such as batteries or supercapacitors. Its large surface area also enables a higher capacity and energy density, while the mechanical stability of the material contributes to the service life and reliability of such storage devices.
Until now, the scalability of depositing graphene with consistently high quality has limited its use. In addition, the integration of graphene into existing manufacturing processes poses a technical challenge.
The Fraunhofer FEP has more than 30 years of expertise in the development of customized layers and layer systems. With its core competencies in electron beam and plasma technology as well as in process development for the deposition of homogeneous, thin layers, e.g. in roll-to-roll processes, the researchers are working on a new technology for the synthesis of graphene using PECVD (Plasma Enhanced Chemical Vapor Deposition). The first important foundations for future cost-efficient graphene deposition processes have now been laid as part of the funded EU project NewSkin (FKZ 862100).
Schematic of the PECVD synthesis process for graphene
Plasma process for more efficient production of graphene
Current synthesis processes for graphene require the application of high temperatures and the use of catalysts. The scientists at Fraunhofer FEP, on the other hand, use plasma-assisted processes. This allows the parameter window for the production of graphene to be significantly expanded so that deposition is also possible at lower substrate temperatures and higher throughputs at the same time. The researchers at Fraunhofer FEP used the versatility of the MAXI inline coating system to develop the new process. Depending on the development maturity of the process, the multifunctional vacuum system offers the possibility of carrying out processes both in sheet-to-sheet mode and in roll-to-roll mode. The variety of processes extends to pilot production, making the system a good basis for the development and scaling of graphene deposition processes.
Dr. Stefan Saager, Group Manager for Coating Metal and Energy Technology, explains the technology: "Graphene can be deposited on metallic strips using the developed PECVD process. The roll-to-roll mode of the MAXI system is used for this. In the first step, the metal strip is coated with a thin layer of a catalyst material such as copper in a vacuum.
This allows the desired substrate material to be selected independently of the suitable catalyst material. Immediately afterwards, the coated metal strip is moved into a process unit with an argon plasma discharge. The argon ions generated there collide with the substrate and heat it highly efficiently in a very short time. By adding a suitable precursor gas such as methane or acetylene in the immediate vicinity of the plasma discharge, the respective molecules can be broken down into their individual components and partially ionized at the same time.
Ideally, the carbon atoms and ions generated in this process are deposited on the substrate in a single-layer, ordered 2D structure, thus creating the desired layer of graphene."
By supporting the ions present in the plasma, the formation process can be realized at comparatively lower substrate temperatures than was previously possible with other state-of-the-art processes.
Using the newly developed PECVD process, the researchers at Fraunhofer FEP have already been able to synthesize graphene over a width of 280 mm at process speeds of one meter per minute. The process thus enables high production throughputs and the associated cost savings for future production processes. In addition, the technology allows an expansion of the substrate materials that can be used and therefore a broader range of applications.
Reproducibility and optimization on the agenda
In the next step, the scientists at the Fraunhofer FEP are working on the reproducibility of the results and on further improving the layer properties achieved, e.g. with regard to the number of graphene layers. A further challenge of the new technology development lies in the precise control of plasma and temperature conditions for uniform layer qualities and morphologies. Future research will also focus on improving the winding process of the hot ribbon and further scaling up the existing process parameters.
With the results achieved, the existing know-how and the plant equipment, the Fraunhofer FEP offers manufacturers from the electronics and renewable energy sectors as well as other technology companies and research institutions a basis for projects for the further development and scaling of processes for the synthesis of graphene.
