As the core component of optoelectronic devices, transparent electrodes are widely used in light emitting diodes (LEDs), liquid crystal displays (LCDs), and organic solar cells. Generally, the transparent light source is required to have a visible light transmittance of more than 80% at 550 nm. The impedance is below 1000Ω / sq or the conductivity is 1000S / m.
Transparent electrodes are used in many areas, including touch screens, solar cells, smart window glass, liquid crystal displays, organic light emitting diodes, and more. With the rapid development of various industries, the performance of transparent electrodes is also facing higher and higher challenges, which require both high light transmittance and low resistance. At the same time, there are extremely high requirements for the mechanical strength, chemical resistance, heat resistance and work function of the material itself. As an excellent conductive material, graphene can meet the needs of the development of the electronics industry. Therefore, its application in the field of transparent electrodes is bound to have broad development prospects.
1 Graphene Overview
British scientists used a simple tape micromechanical peeling method in 2004 to successfully obtain a perfect single layer of graphene from graphite, and tested excellent electrical properties.
The unique structure of the two-dimensional hexagonal honeycomb lattice makes graphene possess the characteristics of room temperature quantum Hall effect. The reason why graphene is called an excellent conductive material is, on the one hand, because its conductivity reaches 106s / m, this new two-dimensional carbon nanomaterial has extremely fast electron transport speed, which can even reach three times the speed of light. One percent, this speed is much higher than other semiconductor materials. At the same time, graphene also has a mobility 100 times higher than that of semiconductor silicon, up to 2 × 105 cm 2 / V · s.
2 Application status of graphene in transparent electrodes
Graphene, as a typical carbon family material, has ultra-high electronic conductivity, ideal capacitor energy storage and light transparency characteristics. It has great advantages in constructing high-performance transparent conductive films (TCE) and flexible transparent supercapacitors. potential.
2.1 Application in solar cells
In 2009, Li and others developed a new type of solar cell structure that uses graphene as the anode of the electrode and combines it with a silicon semiconductor to form a graphene-silicon Schottky junction solar cell structure. As shown in Figure 3. On the Si / SiO2 substrate, a thin layer of graphene is covered, and there is a silicon layer window with an area of about 0.1-0.5 cm2 above the graphene film, and gold wires are used as gates all around.
In recent years, a new technology has appeared in the field of silicon-based solar cells, namely, doping graphene with polytrifluoromethanesulfonate (TFSA) as a dopant. This type of cell is a graphite doped with TFSA. The ene is transferred to the Si substrate, and the technology improves the cell efficiency from 1.9% to 8.6%, which greatly improves the conversion efficiency of photovoltaic cells.
Later, Enzheng Shi et al. Used titanium dioxide as an anti-reflective coating to make the battery reduce the light reflection and enhance the light absorption effect, which further improved the photoelectric conversion efficiency to 14.1%. Despite this, there is still a gap in efficiency compared to traditional ITO.
2.2 Application in display
At present, the transmittance of ITO commonly used in liquid crystal displays on the market is about 90%. In contrast, the advantage of single-layer graphene is that it has a visible light absorption as low as 2.3%, and its transparency is 7.7% higher than 90% of ITO. Although the increase of 7.7% transmittance will not have a great impact on human vision, but due to the limitations of ITO mentioned above, the development of graphene in the field of transparent electrodes is also possible.
Peter Blake and others successfully prepared graphene as a transparent electrode liquid crystal display. First, a graphene film was prepared on a glass sheet by mechanical peeling. 5 mm of chromium and 50 nm copper were sprayed around the graphene film. Then, a 40 nm alignment film was added on the surface, 20μm liquid crystal, 40nm alignment film, ITO and glass sheet. Adding an electric field across the liquid crystal layer disrupts its alignment, thereby changing the effective birefringence and light transmission intensity of the display. The contrast between the strongest and weakest output light is greater than 100. The results of this study also provide a basis for the study of graphene in liquid crystal displays.
2.3 Application in touch screen
Graphene in the field of touch screen application research countries include China, Japan, South Korea, Britain and the United States. In Europe and the United States, the United States of America as the representative of the sharp science and technology, has entered the large-area graphene flexible version of the touch screen market, and plans to apply to mobile phones, tablets and portable device displays within the next 3 years. In South Korea, the applied research of graphene has also received great attention from the government. In 2010, the well-known Samsung Group in South Korea cooperated with researchers from a domestic research institute to successfully develop pure graphene based on 63mm flexible transparent glass fiber polyester board. Its size is similar to that of a television and is flexible. Touch screens have also come out successfully on this basis. In Japan, the Industrial Technology Research Institute released a graphene film device with a width of 594mm synthesized in a roll-to-roll manner. This institute uses microwave plasma technology to synthesize graphene by low-temperature CVD at 300-400 ° C. In addition, Toshiba and Panasonic have also prepared large-area graphene films and graphene heat dissipation films with a thickness of only 10 μm. In China, the Changzhou 2D carbon research and development team has broken through the touch process of graphene films used in small and medium-sized mobile phones, and realized thin film materials.
Docking with ITO module process line. Industry experts said that if the graphene film process line is connected to the existing ITO module process line, the industrialization of graphene film materials in the field of touch display will be accelerated.
2.4 Application in OLED
Tae–Hee Han et al. Used a combination of chemical vapor deposition and AuCl3 doping to produce high-performance CVD graphene, whose performance is comparable to ITO. Through doping, the resistivity of the graphene surface is significantly reduced, and the working energy is also increased from 4.4eV to 5.95eV, thereby solving the hole injection barrier between graphene and the organic semiconductor film layer [. Graphene was patterned by cationic etching, and then an organic semiconductor film layer and a metal electrode were evaporated on the surface to successfully prepare an OLED. The research also made possible the application of graphene in the field of flexible OLEDs.
ZDNet and Korea Herald reported on April 11, 2017 that the Korea Institute of Electronics and Communications cooperated with Hanwha Techwin to produce a transparent electrode with a thickness of less than 5 nanometers from graphene and developed a 370mm × 470mm (equivalent) (19-inch screen) OLED panel, the industry's first. This also makes it possible to promote the use of graphene transparent electrodes in the field of organic light emitting.
3 conclusions
With the rapid development of the electronics industry and the increasing global energy crisis, the research and development of graphene conductive materials is of great significance. In recent years, great progress has been made in the research and use of graphene in transparent electrodes, but there are also shortcomings: (1) insufficient understanding of the material's micro-theory, which leads to discrepancies between theoretical and actual values; (2) adoption When chemical methods are used to prepare transparent materials, due to the limitation of the substrate and reaction conditions, it is impossible to achieve a high reduction of graphene oxide; (3) the preparation method of the material is not perfect, and the preparation cost is too high; (4) the composite material is prepared using PET as the substrate At present, its economic and environmental protection type needs to be explored. Therefore, there is still a long way to go before the industrialization of graphene in this field.
In the future, in the study of graphene conductive materials in transparent electrodes, the following points will become the focus of research: (1) how to improve the flexible substrate material, on the one hand, to solve environmental protection issues, and on the other hand, reduce the restrictions on the reduction conditions due to substrate performance To increase the reduction ratio of graphene oxide; (2) how to reduce production costs and improve production efficiency; (3) how to improve the flexibility of graphene conductive materials.