Synthesis and Characterization of Silicon Nanowire for Solar Cell Application

Syed Mudassir Rehman, Mohammad Wasim Akram Khatri, Jahangir Ahmed Nisar MohammedDepartment of Electrical Engineering, University of Missouri, Kansas City, MO 64112, USA

srdmb@mail.umkc.edu

Abstract- In this paper, we will be discussing the science and scope of nanowire technology in the field of Solar Energy, which elaborates on how the technological innovation will improve upon the current methods in cost, durability and efficiency. The value that this technology holds and the ability to make solar power a feasible worldwide option will also be discussed. As it was estimated, the world’s supply of fossil fuels is getting eradicated and reduced to a bare minimum this century. Due to their cost and efficiency, Wind and Solar energy have yet to replace the major energy supply contributors. The reason why Nanotechnology is being dragged into the field of solar energy is to combat the fault and thereby improve cost and efficiency. Solar Cells based on nanotechnology that have already been made are mostly based on the organic, inorganic and hybrid materials or made of semiconductors. Earlier the efficiency produced by the crystalline Silicon based solar cells was less and now with the use of Nanowires in solar cells, it has increased evidently. The Silicon Nanowires are relatively easy to synthesize and can be used with technologies of low cost substrates like foil and glass. These substrates will not only allow nanowires to be durable but also much easier to produce than the present Si based solar cells. The overall developments in these fields are important and instrumental if we commit to secure the stability of our economy.

Keywords- Photovoltaic cells; Solar Cells; Solar Energy; Nanowires; Nanotechnology

I. INTRODUCTIONEnergy is one of the great challenges of this century. It is of great importance to generate electricity for the renewable energy. To get a large audience, it has to be cost-effective. Solar energy is one of the few most promising route for producing renewable energy. At this time the price per unit of energy produced by the solar cell is higher than the electrical energy produced by fossil or nuclear power plants. Practically, the efficiency needs Rise and simultaneously costs must reduced. To realize this nanowires is the best material. Because of the small size of the metallic nanowires inside, different materials can be more easily combined compared to large systems, and other more sophisticated tandem cells could be manufactured. In addition, light can be absorbed more efficiently by using conical shapes and radial nanowire dimensions of optical absorption path length can be released from the charge separation distance, which allows for more design freedom. This all may increase the efficiency of solar cells. The cost of the nano cable solar cells can be reduced in the manufacturing methods less expensive and by the fact that less of rare metals used in these nanostructured solar cells.

II. EXPERIMENTALImagine a solar panel can more efficiently than today's best solar panels, but with 10 000 times less material. This is what researchers expect in the light of the recent results on these small filaments called nanowires. Solar energy technologies integrate all put large amount of light and the production efficiency you an incredible energy a much lower cost. This technology is a possible future for better microchips and forms the basis for new generations solar panels. Despite their size, nanowires have great potential for energy production. All of them are extremely small filaments . In this case able to intercept light with diameter that measures tens or hundreds nanometers, where nanometer is one millionth millimeter. The miniscule wire is up to 1000 times smaller than the diameter of human hair, or comparable in diameter to the size of the virus. When assembled with proper electronic properties, the nanowire becomes a tiny solar cell, transforming sunlight into electric current

Researchers built a nanowire solar cell made up of Gallium Arsenide which has a capacity to convert light into power twelve (12) times more than normal solar cell. In addition, optimization dimension nanowire, improving quality gallium arsenide and with better electrical contacts tonne extract current prototype could increase efficiency. Arrays of nanowire solar cells offer new prospects for energy production. This study suggests that a set of nanowires can reach 33% efficiency in practice, whereas commercial solar panels (flat) are now only 20% efficiency. Also, arrays of nanowires will be used at least 10,000 times less gallium arsenide, allowing for the industrial use of expensive materials. Translating it into dollars for gallium arsenide, the price will be only $ 10 per square foot, instead of $ 100,000.

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For the engineer free imagination to mount all of these to a wide range substrate panels, whether it's a flat, flexible or withstand toughest conditions. In a world in which energy consumption is on the rise these nanowires may one day supply all of your favorite gadget to space missions to Mars.

III. SYNTHESISNanowire solar cell design consists of four different steps such as nanowire synthesis, junction formation, contacting, characterizing.A large collection of literature is now earmarked for nanowire synthesis, and excellent reviews describe the growth mechanisms. Here we focus on the two techniques most commonly used in nanowire solar cells: chemical vapor deposition and chemical etching and frayed. The proposal chemical vapor deposition, they are synthesized in all flowing chemical precursors vapors into hot zone furnaces to respond to substrate, often with the help catalyst nano particle metal. Par source can be any gas, liquid, solid or heated. The precursor vapors are transported the substrate in an inert carrier gas, is often combined with other reactant gas along the way. Substrate is placed in the deposition zone of the furnace, where chemical decomposition is desirable. A number of mechanisms nanowire promote development rather uniform thin-film deposition. The most commonly cited is the vapor-liquid-solid mechanism, which uses a metal catalyst to form a eutectic liquid at the desired nanowire material. Upon chemical decomposition and dissolution into the liquid eutectic more quickly, the solution becomes supersaturated and overcomes the nucleation barrier two begin filter microplate. Additional flux of dissolved species leads to further filter micro plate and nano wire growth. With the appropriate substrate, precursor, temperature, catalytic converter and concentration, vertical nanowire growth is possible, that is favorable for solar cells, as has already been said. Catalyst joyous patterning approaches allow for ordered nanowire array synthesis. Dopants may be engaged in a period of growth or in a separate diffusion step. In Situ doping has the advantage that it can be done at lower temperatures because it is not based on exclusively on diffusion but can be combined with the catalytic converter. Unlike in situ doping, like situ doping do not affect the nanowire or thin film growth kinetics and growth decouples doping time and temperature. Measurement of dopant concentration and distribution systems in nanowire is much harder than the bulk wafer or thin film. Though VLS (vapour-liquid-solid) mechanism is common in nanowire which developed by chemical vapor deposition, there are other mechanisms also which are possible, they are vapor-solid-solid (VSS), vapor-solid (VS) etc. VSS is similar mechanism as of VLS for launch on growth, but the role of the lighting, instead of forming a eutectic fluid. This phase difference means that chemical concentration and not dominant driving forces 1-D growth VSS-mechanism, but the catalytic converter accelerates precursor decomposition. The dislocation medium by mechanism growth uses high energy damaged areas to take up atoms along a decay screws, which leads to it, 1-D growth likewise stairs to spiral leads upward.

The chemical etching is a top-down, bottom-up attempt which involves lithography is followed by single-single step. The fabrication step for hybrid scheme and images of resulting nanowire array can be seen in Figure

Nanowire solar cell fabrication. (Top row) Schematic of the fabrication process. HF denotes hydrofluoric acid. (a) Scanning electron microscopy (SEM) image of a close-packed monolayer of silica beads assembled on a silicon wafer using dip coating. (b) Plan-view SEM image of a completed ordered silicon nanowire radial p-n junction array solar cell made by bead assembly and deep reactive-ion etching. The inset shows the edge of a top contact finger, demonstrating that the metal completely fills in between the nanowires. (c) Tilted cross-sectional SEM of the solar cell in panel b. (d) Tilted optical image of 36 silicon nanowire radial p-n junction solar cell arrays from panels a–c. The color dispersion demonstrates the excellent periodicity present over the entire substrate.

Silica acid and polymer balls to be synthesized can be able to be built up for roll methods of the lacquer finish with a wide range by different diameters (usually 100-1,000 Nm) and over large surfaces with the help of so-called Langmuir Blodgett , dip Coating, and roll-coating techniques. Anodic alumina & block co polymer models can access much more small model dimensions (typically 10-100 nm) and have been used to manufacture metallic nanowires inside with diameters of∼10 nm .The chemical etching technique gives the advantage of making wafer on thin film sets doping level and material composition, which allows for accurate control from material parameters and simples te characterization of material.

Junction FormationAfter nanowire is synthesized, there must be a introduction of junction which separates the charges and collects it, This junction can occur along the radial diameter or the axial length of the nanowire or to the substrate interface .The only requirement for the connection it brings a chemical potential difference of electrons and holes caused by traveling in opposite directions in order to collect an air carrier. The first type of interface used nanowire of solar cells is known as interface p-n homo junction. In this case, only one material used semiconductor doping, and tasks are spent properties of atoms or defects in the different regions of the nanowire creation of the chemical Potential difference. In dissemination, if the time is too long or if the temperature is too high, the metal can be converted nanowires completely in contrast to Haus-Satellite n type, in this case the interface benefits of radial will be lost (48- 50). The connection between an increase in the field can also benefits coils very pure if the beginning. in thin layers deposits case shell must be endowed with a kind of support against the nanowire and can only if grown crystalline

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powder obtained by epitaxial growth on silicon but usually polycrystalline.

a)Radial Junction b)Axial Junction c)Substrate Junction

(a) Periodic arrays of nanowires with radial junctions maintain all the advantages, including reducedreflection, extreme light trapping, radial charge separation, relaxed interfacial strain, and single-crystalline synthesis on nonepitaxialsubstrates. (b) Axial junctions lose the radial charge separation benefit but keep the others. (c) Substrate junctions lack the radial chargeseparation benefit and cannot be removed from the substrate to be tested as single-nanowire solar cells.

The easily available solar cells including silicon (both crystalline and amorphous) and high efficiency multij-unction cells use homo junction to separate the transporters. The second type of solar cell uses a Schottky junction to separate charges. High performance Schottky-cells with a highly doped semiconductor with a large or small work function in contact with an n-type (or a p-type semiconductors) to induce a depletion region or even a reversal in the semiconductor surface. This leads to a barrier for the carrier flow in only one direction. From a volume figure perspective, this case is very similar to the p-n-boundary layer. However, the semiconductor must have a low-density defective, as otherwise the Fermi level pinning, and the solar cell is on a much smaller output voltage than would theoretically be possible. The conductive polymers are used in the conjunction with silicon nanowires to form Schottky junction solar cells. The semiconductor electrolyte solar cell uses electrolyte as conductor and are the common nanowire Schottky cells.

Silicon nanowire solar cell structure. (a) Schematic cell design with the single-crystalline n-Si nanowire corein brown, the polycrystalline p-Si shell in blue, and the back contact in black. (b) Cross-sectional scanning electron microscopy image of a completed device demonstrating excellent vertical Salignment and dense wire packing. (c) Transmission electron microscopy (TEM) image showing the single-crystallin Sicorean the polycrystalline p-Si shell. The inset is the selected area electron diffraction pattern. (d) TEM image from the edge of the core-shell nanowire showing nanocrystalline domains.

The third type of solar cell is named as heterojunction. It uses type II band offset between two different conductors to separate carriers. The Type II offset offset is a type I preferred (in the latter, a material combines both bands of

the other material), because it helps ensure that transfer electrons and holes is especially in the desired direction. Many of the common thin-film solar materials, such as Cadmium telluride, Copper Indium Gallium (CIGS), copper zinc tin sulfide/zinc selenide (CZTS), organic substances, as a general rule, use heterojunctions to separate charge carriers. Dye-sensitized solar cells are a special type of heterojunction cell as so-called heterostructures, the require a redox couple to regenerate the surface adsorbed dye after photoexcitation and electron injection. Further key heterojunctions in the area are usually produced by the deposit of a thin film on top of the another key in the array of standard methods such as chemical vapor deposition, pulsed laser deposition, electrode, or chemical bath deposition of inorganic materials and spin coating or solution dye adsorption for organic substances and the dye cells. The inorganic nanoparticles have also been used for private to infiltrate nanowire arrays; the subsequent annealing or ligand exchange allows improved charge transport between the nanoparticles.

ContactsAfter the metallic nanowires inside are cultivated and the junction is formed, the contacts must be removed to extract the electrons and holes. As planar solar cells, ohmic contacts optimize the open circuit voltage (Voc), short-circuit current density (JSC), fill factor (FF), and the overall energy conversion effectiveness (η). Methods of ohmic contacts of planar solar cells, such as the heavy doping and interfacial layers, also apply to nanowire system. Clear schottky junction solar s require a schottky and ohmic contact.A single nanowire solar cells are usually contacted by using e-beam or photolithography and metal evaporation. Radial junctions (core-shell) require multiple lithography and etch steps so that the electrons and holes can be retrieved separately. Nanowire array contact plans are generally similar to those of planar layers thin solar cells. The lower or upper section of contact must be transparent which allows light to pass through, and the other contact is usually made of metal reflects. Obtaining Conformal (or at least continuous) metal identifies sieves or transparent conductive coatings on high-aspect-ratio structures can be difficult and often requires a large layer thicker and more uniform deposition techniques (such as sputtering or by electroplating) that in the planar cells. If the junction is made by filing, the nanowire diameters can develop to the point that they can begin to touch or perhaps even the fuse assembly completely, simplifying the regime contact. Semiconductor-electrolyte junctions provide the most simple uniform contact.

IV. CHARACTERIZATIONNanowire solar cells can be made from metallic nanowires inside or individual arrays. A single nanowire devices enable a thorough study of the fundamental processes such as load transfer, surface recombination and minority carrier diffusion. They also simplify the analysis of doping density, surface condition, and conductivity measurements by removing all average and non-uniform contact effects. However, they cannot be used to compare nanowire devices

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with standard planar technology or to investigate these phenomena which depend on a matrix or vertical geometry as photonic crystal light trapping or absorption/separation orthogonalization load effects. For single nanowire mobility, the concentration is not clear like bulk wafer.

Diagram of the silicon nanowire solar cell. Each individual nanowire is a tiny p-n junction. The darker outer shell is n-type silicon. The lighter inner core is p-type silicon.

Techniques like secondary ion mass spectrometry (SIMS), Auger electron spectroscopy (AES), cannot be used to obtain meaningful results, so the usual transport measure is the basic tool for getting these properties. The usual measuring system is back gated or top gated field effect transistor (FET) used for transport and carrier concentration in thin films. In this nanowire is grown epitaxially between the doped silicon source and drain electrodes. Similar atom can be prepared by casting nanowires and by using e-beam lithography to define metal source and drain electrode contacts. The figure shows a corresponding schema and scanning electron microscopy (SEM) images of a single of nanowire FET which has both a top of door and a rear door. In this case, the nanowire was cultivated obtained by growth between epilayer should degenerately doped silicon source and drain electrodes, but a similar structure can be prepared by drop casting metallic nanowires inside on a substrate and by the use of e-beam lithography to define metal source and drain electrode contacts.

Device structure, interface state density, and radial dopant profile. (a) Schematic of the capacitance-voltage (C-V) device structure with p+-Si source and drain pads (gray), SiO2 buried oxide (blue), p+-Si back gate (gray), p-Si nanowire (orange), atomic layer deposition Al2O3 surround gate oxide (green), and chromium surround gate metal ( yellow). (Inset) Cross-sectional schematic view with a hexagonally faceted silicon nanowire. (b) Scanning electron microscopy image of an actual nanowire device. (Inset) A cross-sectional view of the device taken after focused ion beam milling.

Because of the uncertainty of the ability of the grid, surface depletion effects and non-uniform dopant distribution, extract the mobility and the carrier concentration values can have significant errors when standard assumptions are used. Khanal & Wu used with finite element method (FEM) simulations show that the mobility measures can have an error between approximately a factor of two and ten when the infinite cylinder on a plan template is used for the capacitance. The largest error occurred in low-doped metallic nanowires inside with small diameters and thin back-gate oxides.

V. BENEFITS Nano wires are able to produce power same as thin film

of similar material if they cover only 10% compared to 100%.

Solar cells using nano wires can produce more energy at low cost and also reduces cost by reducing amount of material needed.

Nano wires also provide new charge separation technique which was problem in previous solar cells.

Nano wires are best technique for charge collection through band conduction.

Nano wires charge collection efficiency is very good compared to trap limited diffusion mechanism because it has faster band conduction.

VI. REFERENCES[ 1]www.tue.nl/en/university/departments/applied-physics/research/functional-materials/photonics-and-semiconductor-nanophysics/research/research-areas/nanowires/nanowire-s

[ 2]http://www.rdmag.com/news/2013/04/nanowires-power-transform-solar-energy

[ 3]http://pubs.acs.org/doi/abs/10.1021/nl100161z

[ 4]Garnett EC, Yang P. 2008. Silicon nanowire radial p-n junction solar cells. J. Am. Chem. Soc. 130(29):9224–25

[ 5]Garnett E, Yang P. 2010. Light trapping in silicon nanowire solar cells. Nano Lett. 10(3):1082–87

[ 6]Garnett EC, Tseng Y, Khanal DR, Wu J, Bokor J, Yang P. 2009. Dopant profiling and surface analysi of silicon nanowires using capacitance-voltage measurements. Nat. Nanotechnol. 4(5):311–14

[ 7]Khanal DR, Wu J. 2007. Gate coupling and charge distribution in nanowire field effect transisor Lett. 7(9):2778–83

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