Graphene For The Solar Cells Of The Future

Energy and resources of energy are one of the most studied topics in the world due to the depletion and scarcity of conventional energy resources. Energy is the basic necessity of life. Without energy, no one can imagine life. Due to this reason, researchers are interested in this field. Renewable energy resources, unlike fossil fuels, have many advantages, for example, sustainability, environmentally friendly, and some of the renewable energy sources can be harvested endlessly, for instance, sun. Renewable energy sources consist of solar, wind, tidal, hydro, geothermal energy, and biofuels (derived from biomass of various types).

For decades new technologies and devices have been emerging for the production, storage, and effective utilization of solar energy. Sun is a safe, clean, cheap, renewable, and sustainable source of energy through which light energy can be directly converted into electricity without the production of any pollutant or environmental problem. To harvest light from the sun, solar cells are used. Solar cells are also called Photovoltaic cells. Photovoltaic materials and devices convert the sunlight into electric power by the photoelectric or photovoltaic effect.

Among different types of solar cells, thin-film solar cells (CZTS and CIGS) and organic solar cells (Perovskite, Dye Sensitized) conventional silicon based solar cells still dominate in the market. Silicon based solar cells have advantages like high efficiency, long lifetime, and low cost. Researchers have been working for more than 25 years to enhance the efficiency of photovoltaic modules by up to 80%. But the maximum achieved valency of silicon based solar cells is 26.7% only. The maximum theoretical silicon-based photovoltaic module has an efficiency of around 33.7% that means the maximum solar energy that can be converted to electrical energy is limited to 33.7%.

The efficiency of solar cells can be increased by various methods. Doping, Fabrication of heterostructures, and surface texturing are the most common techniques available today. But these methods also have some demerits such as difficult fabrication/production process, limited increase in overall efficiency, and high cost.

This has diverted the attention of researchers towards two dimensional materials to increase efficiency due to their remarkable properties. Graphene is one of the isolated two dimensional crystals and has been widely investigated for its properties in the 2D material family. These properties include high thermal conductivity, high surface area, and high optical transmission ability of charge carriers. These properties make graphene a superior material to be used in an extensive range of applications such as PV cells, transistors, batteries, and supercapacitors. 

Extensive research in this field has paved the way in making graphene-based materials to enhance the efficiency of PV cells. Researchers have used graphene and graphene based materials in different types of solar cells. For example, in thin film solar cells, there exist different layers in which it can be used as a catalytic counter electrode, transparent conductive electrode, active layer, electron charge transport layer, etc. Due to its electrical properties and variability in the use/application in thin film solar cells, it is beneficial to be used in this technology.

On the other hand, dye-sensitized PV cells decorated with graphene have attracted researchers and investors. The use of graphene and graphene derived materials in dye-sensitized PV cells is comparable to other kinds of PV cells available in the market.  In the early days, Platinum (Pt) was extensively used as a counter electrode for dye sensitized PV cells.

The role of the counter electrode is the collection of electrons from photoanode and works as a catalyst for the regeneration of the redox couple in the electrolyte. But the high cost of platinum metal and limited resources subjected the researchers to find a substitute for platinum as a catalyst.  Therefore, high carrier density, charge mobility, conductivity, and electrochemical catalytic activities, high surface area distance make graphene based dye sensitized solar cells to achieve efficiency between 5 to 20%.

It has also been studied that Schottky Junction solar panels have advantages compared to ITO-Si (Indium tin oxide Junction) due to the following reasons:

  1. To achieve the desirable efficiency of the PV cells, the properties of the device can be tuned through graphene’s work function
  2. Graphene electrodes are  easy to synthesize and are inexpensive
  3. graphene has superior optical and electrical properties
  4. Wide range of photon energy (UV to IR)
  5. Provides better heat dissipation 

Along with high optical transparency and higher charge mobility, zero bandgap and high mechanical strength make graphene suitable for application in solar cells. The most studied graphene derivatives are graphene oxide, GO (single or multilayered graphene in oxidized form) and reduced graphene oxide (rGO). 

Among all properties of graphene, its electrical properties are the most significant. Charge careers have high mobility (120,000 cm2/Vs), high velocities (4×107 cm/s) at temperature 240 K, which is the highest value reported for any semiconductor. This property is due to the reason that charge careers are restricted to a layer with a thickness equal to only one atom. The optical and optoelectronic characteristics of graphene have received considerable attention. A single layer has transferred 97.7% in the visible region with a very low reflectance of < 0.1%. Though, the transparency decreases with the increase in the layers.

The application of graphene into different types of solar cells has been discussed. Graphene and silicon based solar cells have promising features. Silicon based solar cells are the most common type of photovoltaic cells, that their efficiency and lifetime are limited when fabricated by conventional methods. As mentioned earlier, the remarkable properties of graphene provide better efficiency and durability to silicone based solar modules. Graphene is a near zero bandgap substance, but owing to high electrical conductivity and mobility of therapy, the junction of n-type semiconductor/graphene heterojunctions are considered to be an efficient Schottky Junction.

The work function difference between n-type semiconductor and graphene creates sufficient built in potential.

Graphene can work as both an electron and hole transport layer for a solar cell.  Silicon based solar cells have the advantage of easy production and low cost. Therefore using silicon based solar cells with graphene is beneficial. The advanced electronic coupling, doping, junction formation and surface passivation, also contribute to making it a suitable material for solar cells. The studies have revealed that deficiency of silicon based photovoltaic cells has increased with graphene because it aids to increase open-circuit voltage and also fits into the factor of solar cell efficiency parameter.

The efficiency parameters can be explained through the high mobility and carrier density of graphene film, leading to an increase in the electron transfer rate and thus decreasing the recombination losses.  However, the increase in cell parameters can be attributed to the high light transmittance and hyperfunction of graphene. Especially graphene provides an extensive range of solar spectrum (visible to the infrared range) harvesting in a solar cell. Hence it is predicted that graphene as a transparent electrode will bring for the improvement in the quantum efficiency of photovoltaic cells, which will consequently increase cell efficiency. Some of the issues that are needed to be are the selection between conductivity and transparency graphene, modulating the work function and simultaneously controlling the sheet resistance upgrade fees, and lowering the recombination probability.

When considering graphene and silicon based solar cells, these hybrid structures have the potential to be commercialized in the near future. In addition, the electronic and optical characteristics of graphene will open new research areas, increasing the attention towards solar cells. This material has the potential to take over the industrial cell market.

References

Choi, H., Kim, H., Hwang, S., Choi, W., & Jeon, M. (2011). Dye-sensitized solar cells using graphene-based carbon nano composite as counter electrode. Solar Energy Materials and Solar Cells, 95(1), 323-325.

Das, S., Sudhagar, P., Kang, Y. S., & Choi, W. (2014). Graphene synthesis and application for solar cells. Journal of Materials Research, 29(3), 299-319.

Mahmoudi, T., Wang, Y., & Hahn, Y. B. (2018). Graphene and its derivatives for solar cells application. Nano Energy, 47, 51-65.

Miao, X., Tongay, S., Petterson, M. K., Berke, K., Rinzler, A. G., Appleton, B. R., & Hebard, A. F. (2012). High efficiency graphene solar cells by chemical doping. Nano letters, 12(6), 2745-2750.