A REVIEW ON GRAPHENE BASED SOLAR CELLS

Over the past few decades, the photovoltaic industry has grown rapidly following the improvements in the efficiency and the demand for alternative energy resources. Commercial silicon photovoltaic with a power conversion efficiency of more than 20% plays a dominate role, but the high manufacturing cost is still a major issue for large-scale implementation. Many efforts have been made to reduce the cost of photovoltaic devices. There are huge demands for developing new electrode materials with lower cost and comparable performance. To date Graphene and Graphenederived materials have created an immense research interests due to its extraordinary physical and chemical properties, which delineated Graphene as an outstanding material for future electronics, optics, and energy-harvesting devices. This paper analyzes some of these properties of graphene & investigates whether graphene or graphene based materials can be a potential replacement for conventional materials used in solar cells or not. Graphene is a single atom thick two-dimensional material thus exhibit ~97.7% transmittance during the entire visible light spectrum. Graphene has unusual electronic transfer properties which follows the characteristic of 2D Dirac fermions, quantum hall effects. Electro catalytic actions of grapheme play a key role enhancing the efficiency of electro chemical solar cell like dye-sensitized solar cell (DSSCs), where the liquid/solid interface acts as a pathway for transfer electrons.


INTRODUCTION
Two of the most serious issues that we face today are Energy Crisis and Global Warming. The supply for each of these originates from fossil fuels. Fossil fuels are unit fuels shaped by decomposition of dead organisms like coal, fossil oil or gases. Generally, they have supplied almost the whole world energy needs for the last two centuries because of their high energy density. But, this development of fossil fuels gave rise to the two afore-mentioned issues. The harmful gases discharged into the atmosphere owing to the burning of fossil fuels which harmed the atmosphere for many years that gave rise to global warming. There are completely different suggestions on what will replace fossil fuels as a source of energy etc. The most promising solution to each of the aforementioned issues is alternative energy. Most of the renewable sources of energy trace their origin to the sun, like solar or photovoltaic cells, wind energy; bio fuels etc. [1] Out of these energy sources solar cells are the best source of energy because it directly converts daylight to electricity without polluting the environment. A comparison has been given between different solar cell designs utilizing graphene to examine which design of solar cell can be best replacement to current Silicon based solar cell in future.

Background on Existing Solar Energy
The use of Solar/Photovoltaic cell to convert sunlight to electricity has been in practice for over a century now. Photovoltaic effect was first observed by French physicist Edmond Becquerel in 1839 when he discovered that when two brass plates were exposed to light, after immersing in a liquid, they produced current. In 1883 Charles Frits first developed a Selenium based solar cell with 1% efficiency. In 1954, Bell Labs in USA invented the first Silicon based solar cells with 6% efficiency. Till now, it is the most popular material used to fabricate Solar Cells. In 1980-81 Multifunction Solar Cells first came into use. [2] Some of the constraints of using solar energy via solar cells are:  Solar energy is not predictable as Sunlight is not predictable throughout the day.  The installation of solar or PV modules is very expensive.  Till now only around 30% efficiency has been achieved by solar cells.
The basic principle of a solar cell is photons in electrons out. Electromagnetic waves or light waves are absorbed by the solar cells which excites the electrons in the valence band to the This layer permits sunshine to enter the electric cell, protects the cell & is capsulated by a metal frame for support. The cell core is formed of n-doped semiconductor electrons that mix with the holes generated within the p semiconductor region. [3] They're separated by a p-n junction which is that the region of the core & together wherever the electrons from the n region meet with the holes from the p-region. [3]

Multi Junction Solar Cell
To increase the efficiencies of the traditional cells bicycle or Multi junction solar cells are developed. The essential plan of the Multi junction is that multiple sub cells composed of materials having Fargab Ahmed et al., A Review on Graphene Based Solar Cells conduction band leaving a hole in the valence band & which in turn results in charge flow. This charge is collected at the two contacts & electricity is generated. This is generally how onventional solar cell works. However, over the years different types of solar cells have been developed namely Multijunction solar cells, organic solar cells, organic-inorganic hybrid solar cells, Dye sensitized solar cells etc. [2] the highest tip layer is plastic as shown in fig 1. This electric cell, protects the cell electric cell, protects by a metal frame for support. The cell doped semiconductor that produces within the p-doped n junction which is that the absorption the electrons from the n region. [3] the traditional solar are beginning to be of the Multi junction solar cells is that multiple sub cells composed of materials having completely different band gaps one another connected along by heavily doped tunnel junctions as shown in fig 2. As a result, the solar spectrum is being absorbed & This will increase the potency 1. As a result large portion of the solar spectrum is being absorbed and utilized to generate electricity. 2. In these, photovoltage is neith created as in single junction cells.

Dye Sensitized Solar Cell
This is another variety of organic cell, however, whose style is somewhat totally different from the standard solar cell as shown in fig 3. It typically consists of a clear cathode, an extremely porous semiconductor layer with a soaked layer of dye, A N solution answer containing oxidation reduction pairs and a counter conductor. The essential solar cells is dye molecule harvests order that it injects negatron phenomenon band of TiO semiconductor. The injected anode & through the external circuit to the cathode. The dye molecule takes one negatron from iodine oxidizing it to electron tube missing negatron from external disseminative to the counter conductor reported by such a solar cell is around 12.3% which is much lower than the silicon solar cells but due to their low cos easy fabrication these are a popular subject of research. [5] 1. The highest efficiency reported by such a solar cell is around 12.3% which is much lower than the silicon solar cells but due to their low cost & easy fabrication these are a popular subjec 2. Parameters that affect the performance of a solar cell which will be discussed in this paper are band gap, conductivity of electrodes & transparency of the electrodes. As a result large portion of the solar spectrum is being absorbed and utilized to generate electricity. In these, photovoltage is neither sacrifice nor losses created as in single junction cells.
This is another variety of organic cell, however, whose style is somewhat totally different from the standard solar cell as shown in fig 3. It typically consists of a clear cathode, an extremely porous semiconductor layer with a soaked layer of lution answer containing oxidation reduction pairs The essential working rule of those cells is dye molecule harvests daylight and is happy in negatron directly within the physical band of TiO 2 that is that the porous negatron then moves to the clear anode & through the external circuit to the cathode. The dye from iodine within the solution by electron tube. The electron tube recovers its from external disseminative circuit by conductor. The highest efficiency reported by such a solar cell is around 12.3% which is much lower than the silicon solar cells but due to their low cost & easy fabrication these are a popular subject of research. [5] The highest efficiency reported by such a solar cell is around 12.3% which is much lower than the silicon solar cells but due to their low cost & easy fabrication these are a popular subject of research. Parameters that affect the performance of a solar cell which will be discussed in this paper are band gap, conductivity of electrodes & transparency of the

International Journal of Recent Scientific Research
GaAs. Also, Si is easily found in nature as from the extraction by the reduction of carbon Whereas Ge and GaAs are not so easily available in nature that's why Si is better semiconductor material used as a substrate in solar cell.

Why graphene?
Graphene has different chemical, physical properties from most 3-d materials. Intrinsic Graphene is also known as a zero semiconductor or semi-metal. Si has electron mobility almost two times less than of Graphene thereby making it super conducting, since it has a direct band gap it can absorb more photons in a much slender area as compared to indirect band gap semiconductors like Si. Graphene also has excellent optical properties & therefore can be used as translucent electrodes & interconnects between two sub cells in tandem solar cells.
Graphene is a C-sheet one atom thick comprises of condensed six member rings. The C-atoms are sp 2 bonded in create a hexagonal 2D lattice. An ideal Graphene sheet has a very high carrier mobility on the order of 10 room temperature. This goes over the carrier mobility of Si by one or two orders suggesting that Graphene can be utilized a replacement for Si in nanotechnology. Besides its outstanding electrical properties, Graphene also has very high mechanical strength. Graphene also has a tunable band gap. 3D Graphene is known to be Graphite. It is the slimmest material with maximum strength; it is also highly crystal clear & highly conducting. This makes Graphene a smart choice to make translucent electrodes in solar cells. Graphene can be modified in each and every dimension e.g., 1-D, 2-D and 3 in fig 4. [8] The following properties of graphene are:

Strength
 Graphene is super strong-even stronger than diamond.  We can stretch 20-25% of its original length without it breaking.

Electronic Property
 Graphene is more interesting to manipulate the flow of electron that carries electricity.  Electron move through graphene a bit like photon, at speed close enough to the speed of light.

Optical Properties
 Graphene transmit about 97-98% light.  Graphene has a flat transmittance spectrum from the ultra violet (UV) region to the long wavelength infrared region (IR)

Fig 4 Structure of Graphene Synthesis [8]
International Journal of Recent Scientific Research Vol. 8, Issue, 5, pp. 16893-16896, May, 2017 GaAs. Also, Si is easily found in nature as from the extraction by the reduction of carbon Whereas Ge and GaAs are not so e in nature that's why Si is better semiconductor Graphene has different chemical, physical properties from most d materials. Intrinsic Graphene is also known as a zero-gap metal. Si has electron mobility almost two times less than of Graphene thereby making it super conducting, since it has a direct band gap it can absorb more photons in a much slender area as compared to indirect band lso has excellent optical properties & therefore can be used as translucent electrodes & interconnects between two sub cells in tandem solar cells.
sheet one atom thick comprises of condensed bonded in Graphene & create a hexagonal 2D lattice. An ideal Graphene sheet has a very high carrier mobility on the order of 10 5 cm 2 / (V s) at room temperature. This goes over the carrier mobility of Si by one or two orders suggesting that Graphene can be utilized as a replacement for Si in nanotechnology. Besides its outstanding electrical properties, Graphene also has very high mechanical strength. Graphene also has a tunable band gap. 3D Graphene is known to be Graphite. It is the slimmest material with rength; it is also highly crystal clear & highly conducting. This makes Graphene a smart choice to make translucent electrodes in solar cells. Graphene can be modified D and 3-D. as shown even stronger than diamond. 25% of its original length without it more interesting to manipulate the flow of Electron move through graphene a bit like photon, at speed close enough to the speed of light. 98% light. transmittance spectrum from the ultra violet (UV) region to the long wavelength

Graphene Based Solar Cell Design Graphene Based Electrodes
Graphene is used to create flexible, translucent, conductive electrodes by taking the benefit its tremendously high optical clearness & electrical conductivity. Graphene has a superior transmission coefficient than other translucent oxides used like ITO (Indium Tin Oxide) in the high incident photons. Also, since grap decreases the losses due to outline of the grid electrodes. Chemical doping of Carbon nanotubes (CNTs) or graphene outcomes in significant increase in its conductivity, promoting charge transfer. Both Single walled Carbon Nano (SWCNT) & Multi walled Carbon Nano being seen as basic factors to make the electrodes of the solar cells & research is being carried out on how to implement them to better enhance the efficiency of the solar cells. A solar cell with electrodes based on CNT is discussed in fig 5.

SWCNT Based Photoactive Device
A p-n junction carbon nanotube (CNT) due to its def diode structure shows ideal diode behavior. In this design SWCNTs were nanowelded across two asymmetric metal electrodes with high & low work functions which established a strong built in electric field along the length of the tubes as shown in fig 6. The SWCNTs are vertically aligned which enhances the photo absorption properties of the device as photons not absorbed by a tube is likely to be reflected or transmitted towards another tube having a different bandgap. Also, the photo absorption is ma incident light is parallel polarized to the tube axis. The tubes are arranged such that higher band gap tubes absorb the shorter wavelength photons. [10] Structure of Graphene Synthesis [8]

Graphene Based Solar Cell Design
Graphene is used to create flexible, translucent, conductive benefit its tremendously high optical clearness & electrical conductivity. Graphene has a superior transmission coefficient than other translucent oxides used like ITO (Indium Tin Oxide) in the high-wavelength area of the incident photons. Also, since graphene is transparent, it decreases the losses due to outline of the grid electrodes. Chemical doping of Carbon nanotubes (CNTs) or graphene outcomes in significant increase in its conductivity, promoting charge transfer. Both Single walled Carbon Nano-tubes (SWCNT) & Multi walled Carbon Nano-tubes (MWCNT) are being seen as basic factors to make the electrodes of the solar cells & research is being carried out on how to implement them to better enhance the efficiency of the solar cells. A solar cell trodes based on CNT is discussed in fig 5. Si heterojunction and multifunctionalities of the DWNTs. a) Illustration of coating a patterned Si/SiO 2 substrate (Si window: 7mm _7mm) with a DWNT film in cell fabrication. The DWNT film serves as a transparent electrode for light illumination (AM 1.5) and charge collection. b) Band scheme diagram of the DWNT-Si heterojunction. c) Illustration e separation occurred at the interface between a DWNT and the Si substrate (cross-section view). d) Illustration of charge transport through a percolated DWNT network. [9] SWCNT Based Photoactive Device n junction carbon nanotube (CNT) due to its defect free diode structure shows ideal diode behavior. In this design SWCNTs were nanowelded across two asymmetric metal electrodes with high & low work functions which established a strong built in electric field along the length of the tubes as g 6. The SWCNTs are vertically aligned which enhances the photo absorption properties of the device as photons not absorbed by a tube is likely to be reflected or transmitted towards another tube having a different bandgap. Also, the photo absorption is maximized when electric field of incident light is parallel polarized to the tube axis. The tubes are arranged such that higher band gap tubes absorb the shorter

CONCLUSION
These solar cells can be built up to replace conventional solar cells which would provide analogous efficiencies. A combination of all the applications of graphene in a solar cell might offer a better solution to this problem & provide better efficiencies.