Graphene is a two-dimensional material with a honeycomb structure, renowned for its exceptional electrical, mechanical, and physical properties. Due to its flexibility, it is used as a transparent electrode in organic photovoltaic cells (OPV). 

Since 2008, Chemical Vapor Deposition (CVD) has become one of the most significant methods for graphene production. CVD graphene offers substantial advantages over ITO in flexible electronic applications, particularly in terms of transparency, cost-efficiency, conductivity, and scalability. Discover Nanografi’s cutting-edge graphene products and elevate your next project with unmatched flexibility and performance.

Introduction

Graphene, with its two-dimensional honeycomb structure, is widely studied in optoelectronic devices due to its exceptional electrical, physical, and mechanical properties. Although various production methods such as chemical and mechanical exfoliation exist, the Chemical Vapor Deposition (CVD) method, reported in 2008-2009, is considered the most effective for producing single-layer graphene. CVD graphene stands out as a promising material, particularly as flexible and transparent electrodes in organic photovoltaic cells (OPV) and field-effect transistors (FET).

Graphene Synthesis via Chemical Vapor Deposition (CVD): Method and Process

There are two primary methods in graphene synthesis. The first is bottom-up methods (e.g., epitaxial growth on different surfaces, arc discharge methods, and Chemical Vapor Deposition—CVD), and the second is top-down methods (e.g., exfoliation, chemical oxidation, and mechanical exfoliation).

Graphene production using the CVD method consists of two main stages: in the first stage, carbon atoms are formed through the pyrolysis of the precursor material; in the second stage, the dissociated carbon atoms form graphene's regular carbon structure. During this process, pyrolysis ensures the accumulation of carbon atoms on the surface and prevents the precipitation of carbon clusters in the gas phase. For carbon atoms to form an orderly graphene structure, high temperatures (above 2500 °C without a catalyst) are required. However, the use of catalysts reduces this temperature to around 1000 °C, improving the efficiency of the reaction. As a result, metals like copper, due to their low carbon solubility, stand out as effective catalysts.

Graphene produced via the CVD method is suitable for large-scale applications, offering properties such as high transparency, conductivity, and flexibility. Thus, it is widely used in optoelectronic devices and flexible electronics.

Properties of CVD Graphene and Its Performance in Organic Photovoltaic Cells

Graphene films produced via CVD offer features such as high transparency, low cost, excellent conductivity, and scalability. Their flexible nature makes them an ideal choice for applications requiring flexibility, such as organic photovoltaic cells (OPV). To achieve optimal performance in OPV cells, a suitable balance between optical transparency and sheet resistance must be maintained.

In performance comparisons with OPV cells using indium tin oxide (ITO), tests conducted on PET substrates have shown that CVD graphene delivers 93% of the power output density achieved by ITO. However, CVD graphene films tend to have higher sheet resistance and lower optical transparency. In terms of flexibility, bending tests have revealed that CVD graphene preserves the flexibility of OPV cells, unlike ITO, which is known for its brittleness. CVD graphene also prevents the formation of micro-cracks, enhancing the durability of flexible devices.

Overall, due to its high flexibility, transparency, and conductivity, CVD graphene demonstrates significant advantages over ITO in OPV cells.

Applications of CVD Graphene

Graphene films produced by the CVD method, due to their superior surface and electrical properties, are considered a more promising material for optoelectronic devices compared to reduced graphene oxide (rGO). The ability to transfer CVD graphene onto various surfaces has made this method a versatile technology for a wide range of applications.

One of the key application areas of graphene is its use as an electrode material in both inorganic (e.g., silicon) and organic solar cells. The electrons in graphene’s two-dimensional lattice structure move only between carbon atoms, effectively making graphene a nearly two-dimensional material. These interactions create a single charge carrier system, and these carriers are known to move ballistically across the graphene surface, enabling excellent electrical conductivity.

In this context, two important studies are examined in this blog post: (i) the design and electrical characterization of thermo-electrochromic sensors using CVD graphene, and (ii) the synthesis and characterization of graphene produced via the CVD method. These studies shed light on graphene's potential applications in optoelectronics and sensor technologies.

Preparation of Thermoelectrochromic Devices

In 1^st attempt, from copper foil, Graphene was transferred by the method of high-speed electrochemical delamination onto a flexible substrate. Then the CVD-graphene substrate, which is flexible, was covered with different colors of four strips. As both the electrodes, thermally deposited gold (Au), was used in CVD-graphene's created devices.

Thermoelectrochromic Behavior of Devices with CVD Graphene and Performance of Thermochromic Strips

The potential for color change in thermochromic strips used in devices was tested by modulating the thermal properties of CVD graphene through electro-modulation. For this purpose, the device's voltage was gradually increased from 0 to 30V, and the color change was observed as the temperature increased. The results confirmed that the thermal properties of CVD graphene can be effectively utilized in sensor technologies. Thermal camera data clearly showed that as the temperature of the graphene layer increased, the selected strips changed color. It was demonstrated that the device with CVD graphene exhibited a reversible effect and was stable under varying environmental conditions. The thermochromic strips continued to change color reversibly with temperature even after prolonged use.

To further explain these effects, a commercially available Peltier device was used to control the temperature changes of the strips. In this application, graphene’s electrical properties offered a significant advantage: the voltage applied to the graphene layer through electrodes increased the layer’s temperature, causing the electrochromic layer to switch from its initial colored state to a transparent state. These transparent layers help sensors adapt to the surface of the object being monitored.

This innovative design holds potential for use in sensors capable of detecting the presence and location of damage—for example, in automotive windows, store displays, helicopter ballistic coatings, or bulletproof vests. These sensors can significantly enhance safety by enabling the monitoring of protection levels, benefiting individuals such as police officers, civilians, and soldiers. The combination of graphene’s electrical properties and the optical properties of thermochromic materials provides an advanced solution for applications such as ballistic coatings and the inner layers of glass panels.

Transfer of Graphene Films

To make graphene produced for photovoltaic and nanoelectronic applications function effectively, it must be carefully removed from catalytic metal substrates and transferred to desired substrates. This transfer process is a critical step to enable the use of graphene in various application areas.

In the first step, the graphene is coated with a thin layer of polymethyl methacrylate (PMMA) and baked at 102°C to allow the solvent to evaporate. Then, catalytic metal substrates, such as Cu or Ni, are removed through an appropriate etching process, leaving only the graphene/PMMA film. This film is cleaned with deionized water and then transferred to the target substrate. After the water evaporates, the PMMA layer is removed using acetone, leaving the graphene film on the desired substrate.

Through this method, graphene that covers the entire surface obtained from Cu foil or Ni film can be successfully transferred onto substrates such as glass, Si/SiO2, and polyethylene terephthalate (PET) films. This process is a crucial step for effectively utilizing graphene films in photovoltaic and nanoelectronic devices.

Graphene Films in Photovoltaic Cells

Due to its flexibility, high conductivity, and transparency, graphene films are considered a promising candidate for transparent conductive electrodes in photovoltaic cells. In conventional organic photovoltaic (OPV) cells, indium tin oxide (ITO) is widely used as the anode material, offering high transparency and low sheet resistance (~25 Ω/sq). However, the limited availability of indium reserves and the brittle nature of metal oxides restrict ITO’s use in applications where flexibility and physical adaptability are critical. Additionally, ITO's low cost-efficiency makes it unsustainable, particularly for flexible electronic devices.

At this point, graphene films, with their advantages in flexibility and conductivity, have the potential to replace ITO and are regarded as the material of the future for photovoltaic cells.

Large-Grain Graphene and Graphene Transistors

CVD graphene films, continuously synthesized on Cu foils and Ni films, typically have a polycrystalline structure with very small grains (a few micrometers). The grain boundaries between graphene crystals negatively affect both the mechanical and electrical transport properties, thereby limiting graphene's overall performance. Therefore, the production of single-crystal, large-grain graphene is critical for enabling the effective use of graphene in various applications.

To produce single-crystal large-grain graphene, a vapor trapping method has been developed. In this technique, a CH₄/H₂ gas mixture is introduced into a large quartz tube (2 inches in diameter), while a Cu foil is placed inside a smaller quartz tube. The growth process occurs on the Cu foil outside the tube, and the vapor trapping technique alters the environmental conditions to promote the formation of large, flower-shaped graphene grains. This method allows for the production of large-grain graphene, essential for high-performance electronic and optoelectronic devices..

Conclusion

The synthesis of CVD graphene on Cu and Ni has provided significant findings for the production of large-grain graphene as a conductive anode material for OPV cells, particularly with flexible and transparent multilayer graphene films. Low-temperature graphene production also offers a cost advantage.

A better understanding of graphene growth processes and the development of single-crystal, large-grain graphene play a critical role, especially in flexible electronics and photovoltaic applications. CVD graphene has vast potential for industrial applications.

For more information on the wide range of graphene applications, visit Blografi!

References

Chemical Vapor Deposition CVD Graphene - Nanografi Nano Technology. (n.d.). Retrieved October 15, 2024, from https://nanografi.com/blog/chemical-vapor-deposition-cvd-graphene/

CVD Graphene - Creating Graphene Via Chemical Vapour Deposition – Graphenea. (n.d.). Retrieved October 15, 2024, from https://www.graphenea.com/pages/cvd-graphene

Explained: Graphene, Graphene Oxide, and Reduced Graphene Oxide and Applications- Nanografi Nano Technology. (n.d.). Retrieved October 15, 2024, from https://nanografi.com/blog/explained-graphene-graphene-oxide-and-reduced-graphene-oxide-and-applications/

Graphene Sheet Films Applications- Nanografi Nano Technology. (n.d.). Retrieved October 15, 2024, from https://nanografi.com/blog/graphene-sheet-films-applications/

Januszko, A., Iwan, A., Maleczek, S., Przybyl, W., Pasternak, I., Krajewska, A., & Strupinski, W. (2017). CVD-Graphene-Based Flexible, Thermoelectrochromic Sensor. Journal of Nanomaterials, 2017(1), 2757590. https://doi.org/10.1155/2017/2757590

Kalita, G., Tanemura, M., Kalita, G., & Tanemura, M. (2017). Fundamentals of Chemical Vapor Deposited Graphene and Emerging Applications. Graphene Materials - Advanced Applications. https://doi.org/10.5772/67514