This document summarizes research on using silicon nanowires for solar cell applications. It discusses how silicon nanowires can improve the efficiency and reduce the cost of solar cells compared to traditional crystalline silicon solar cells. The document describes methods for synthesizing silicon nanowires using chemical vapor deposition or chemical etching. It also discusses approaches for forming p-n junctions in the nanowires to separate charges and techniques for contacting nanowire arrays to extract the charges. Characterization of individual nanowires and nanowire arrays is also outlined to evaluate device performance. Overall, the document reviews the potential for silicon nanowire solar cells to advance solar energy technologies through lower material usage and higher conversion efficiencies.

1. 1
Synthesis and Characterization of Silicon Nanowire
for Solar Cell Application
Syed Mudassir Rehman, Mohammad Wasim Akram Khatri, Jahangir Ahmed Nisar Mohammed
Department 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. INTRODUCTION
Energy 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 solarcells.
II. EXPERIMENTAL
Imagine 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.

2. 2
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. SYNTHESIS
Nanowire 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
periodicitypresent overthe 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 Formation
After 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

3. 3
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
testedas single-nanowire solarcells.
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.
Contacts
After 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. CHARACTERIZATION
Nanowire 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

4. 4
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.Eachindividual nanowire is a
tiny p-njunction. 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, YangP. 2008. Silicon nanowire radial p-njunction solar
cells. J. Am. Chem. Soc. 130(29):9224–25
[ 5]Garnett E, YangP. 2010. Light trappingin silicon nanowire solar cells.
Nano Lett. 10(3):1082–87
[ 6]Garnett EC, TsengY, Khanal DR, WuJ, BokorJ, YangP. 2009. Dopant
profilingandsurface analysi of siliconnanowires usingcapacitance-voltage
measurements. Nat.Nanotechnol.4(5):311–14
[ 7]Khanal DR, WuJ. 2007.Gate couplingandcharge distributionin
nanowire fieldeffect transisor Lett.7(9):2778–83