Abstract: of petroleum, the desire for the future

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Abstract:  Due to the high energy
demands which are followed by the crisis of petroleum, the desire for the
future lies in the renewable energy resources such as solar energy. In Photovoltaic
cells, the mainly used material is Silicon in both crystalline and also
amorphous form for the fabrication and also used in manufacturing industries.
This research paper
gives the overall overview about the materials and also the processes used for
fabricating a solar cell. The aim of this paper  is to study the solar cell fabrication
technology and also the fabrication of the solar cells. However, there are a
lot of challenges involved such as high manufacturing costs, energy conversion
efficiency, uniformity, easy handling and storage etc. In response, solutions
have been suggested in terms of both alternatives, manufacturing methods and
materials used in the photovoltaic cells. The paper further explains in detail
about the various fabrication processes utilized in the modern era. The paper
ends with contrasting the various techniques and pushes the idea of using the
most efficient solar fabrication processes.


Solar energy is the energy generated from the
atomic combination in a star, i.e. the sun. The energy is released when the
fusion process takes place. That energy goes through the layers of the sun
until the point when it achieves the surface of the sun, where the light is
emitted. Of the transmitted energy that reaches the atmosphere is known as the
solar constant. Solar panels are made up of solar cells which converts light,
energy, into electric or electricity form. The earth receives more energy from
the sun for just one hour than the world which uses in a whole year. This shows
a very high sun based radiation that should be utilized; as a substitute
strategy for the non-renewable energy sources utilized today.

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Solar panel boards which consist of cells
situated together in modules which

mean the solar cells hold a noteworthy part
in the panel’s last execution. In solar market today we use commonly three
sorts of photo voltaic cells; single and, poly-crystalline cells, thin film. Without
any difficulties the cells are separated by their appearance, thin film cells
are sometimes black and even in their shades, single-crystal cells have thick
blue color and poly-crystalline cells have various shades of blue.



The most common
semi-conductive material used in solar cells is silicon where it is important
to separate amorphous (un-structured) and crystalline (ordered) silicon.

Monocrystalline cells: Crystalline silicon solar cells represent
about 90% of the PV market today. Both crystalline cells have similar
performances; they have high durability and a high expected lifetime of about
25 years. Of the two types of crystalline solar cells, the mono-crystalline
cells tend to be a bit smaller in size per gained watt but also a bit more
expensive than the polycrystalline cells. Single-crystalline solar cells are
cut from pieces of unbroken silicon crystals. The crystals are shaped as
cylinders and sliced into circular disks of about 1mm. An advantageous property
of the single silicon crystal cell is that they are not known to ever wear out.

Polycrystalline cells: are also ordinarily made from silicon.  However, the manufacturing process is somewhat
different. Instead for the material to be grown into a single crystal it is
melted and poured into a mould. The mould forms a squared shape and the block
is then cut into thin slices. Since the discs are squared already less or no
material has to be cut off and go to waste. When the material cools down it
crystallizes in an imperfect manner which gives the polycrystalline cells a
somewhat lower energy conversion efficiency compared to the single crystalline
cells. In consequence the polycrystalline cells are slightly larger in size per
gained watt than the single crystal cells are. After the disks of crystalline
cells (mono and poly) have been made, they are carefully polished and treated
to repair any damage the slicing might have caused.

Thin Films: A more recently developed concept is the
thin film solar cell. In principle it is a microscopically thin piece of
amorphous (non-crystalline) silicon, as an alternative to the millimeter-thick
disk, which leads to less used material. Instead of the cell being a component
in itself, the thin film cells are placed directly on a sheet of glass or
metal. Therefore, the cutting and slicing steps of the production process are
removed completely. Furthermore, instead of mechanically assembling the cells
next to each other they are simply deposited as such on the material sheet.
Silicon is the material most for the thin film cells but some other materials
such as cadmium telluride may be used. Because the cells are so thin, the
panels can be made very flexible entirely dependent on how the flexible the
material is that the cells are placed onto.

Advantages can be
won from thin film modules compared to the traditional crystalline ones in both
flexibility and weight. They are also known to perform better in poor light
conditions. However, thin film technology offers lower efficiency which means that
for the same amount of output energy a larger area would be needed. Despite the
thin films lower efficiencies, the price per unit of capacity is lower than for
crystalline sells. They also tend to degrade over time because of instability
in the material structure, making the durability of the panels less certain.

From cells to modules Solar cells are built into modules or
panels because the output from a single cell is small while the combination of
many cells can provide a useful amount of energy. Design of solar panels is
reliant of the type of solar cell that is used. The crystalline cells build
stiff modules that can be integrated between some layers of material-sheets and
then cut in different shapes whereas thin film panels are very flexible, making
them applicable in other areas. Often solar panels are located on rooftops or in separate constructions where the optimal solar angle is
received. To make sure the cell loose as little light as possible in
reflection, the incident angle is kept at a minimum. The best alternative for
the panels would be perpendicular to the incoming sunlight; which is made
complicated by the earth moving. Sometimes construction alternatives on roofs
are not available or simply undesirable due to glass roofs, flat roofs, small
gardens etc. In such cases a more flexible alternative of solar panels is found
from the ones made of thin film cell modules.





                            FABRICATION TECHNIQUES


Physical Vapor
Deposition: PVD comprises of Evaporation and Sputtering

Evaporation: Used to deposit thin layers (thin films) of
metal on a substrate. Some metals films that are easily deposited by
evaporation: aluminum, chrome gold, silver, titanium.

Electron Beam Evaporation
(commonly referred to as E-beam Evaporation)
is a process in which a target material is bombarded with an electron beam
given off by a tungsten filament under high vacuum. The electron beam causes
atoms from the source material to evaporate into the gaseous phase. These atoms
then precipitate into solid form, coating everything in the vacuum chamber
(within line of sight) with a thin layer of the anode material. A clear
advantage of this process is it permits direct transfer of energy to source
during heating and very efficient in depositing pure evaporated material to
substrate. Also, deposition rate in this process can be as low as 1 nm per
minute to as high as few micrometers per minute. The efficiency of the material
is high in respect to different techniques and it offers the procedures of structural
and morphological control of the thin films. Because of the very high
deposition rate, this procedure has potential industrial applications for
thermal barrier coatings and wears resistant in the aerospace industries, hard
coatings for cutting and tool industries, and electronic and optical films for
semiconductor manufacturing factories.

SPUTTERING: Sputtering
process includes ejecting material from an “objective” that is a source onto a
“substrate” (for example, a silicon wafer) in a vacuum chamber. This impact is
caused by the bombardment of the objective by the ionized gas which often known
as an inert gas for example, argon. Sputtering is extensively utilized in the
semiconductor devices industry for the deposition of thin films of various
materials in the integrated circuits. The Anti-reflection coatings are additionally
added by sputtering on glass for optical applications. Due to the low substrate
temperatures utilized, sputtering is a perfect strategy to store metals for
thin-film transistors. Maybe the most commonplace results of sputtering are
low-emissivity coatings on glass, utilized as a part of double-pane window assemblies.
The most important advantage of sputtering is that if the materials with very
high melting points are very easily sputtered while evaporating these materials
in a Knudsen cell or resistance evaporator is very problematic and complexity.



Chemical mechanical planarization or chemical mechanical polishing CMP
is the process that removes the topography from the poly silicon, silicon oxide
and also metal surfaces. It is the most preferable planarization technique used
in the deep sub-micron IC manufacturing industries.
The smaller the requested resolution of the structure, the higher is the
request for planarity of the surface. There is a local height variation between
chip areas of different pattern densities. Chemical mechanical polishing which
is the only technique that performs global planarization of the silicon wafer.                                                          Oxide      planarization

Originally CMP is used mainly to planarize
the silicon dioxide inter level  the dielectrics of the Silicon dioxide material
deposited that is thicker than the final thickness requested and these material
are then polished back until the step heights are removed. This results in a
good flat surface for the next level. The process can be repeated for every
level of wiring that is added.

Poly-silicon planarization: Poly-silicon polished easily with almost same types of the
polishers, similar pads and slurries as they are used for the planarization of
silicon oxide. Applications are typically the polishing of poly silicon plugs,
removing the poly silicon from the inter level dielectric and leaving only the
plug filled with polysilicon. Poly-silicon planarization can also be used for
the end phase of wafer thinning or just for silicon wafer polishing.

Typical Process
Pressure: 2 to 7 psi
Temperature: 10 C to 70 C
Slurry flow rate: 100 to 200 mL/min
Typical removal rates:
Oxide CMP ~2800Å/min

Good selectivity (No lapping).
Reduced resist thickness variation.
Better resolution of photolithographic process by reducing
depth of focus.
Multilevel structures.
Improved step coverage of subsequent layer deposition





diffusion is presently the first technique for electrode fabrication in
semiconducting material (si) electric cell process. The diffusion depends on
numerous factors of that temperature and gaseous environment is most
significant .P-type semiconducting material wafers are wide utilized in star
industries and thus diffusion technologies are developed to deposit n-type
doping parts to make the contact. As a result of its low boiling temperature (105.8 ?C), at
temperature between 850-900 *c within the diffusion chamber, POCI3 is decay
into straightforward phosphorus compounds like P4, P8, P2O5, etc. The
phosphorus diffusion fabrication of crystalline semiconducting material
electric cell with electrode diffusion, surface passivation and screen printing
of conductor ends up in formation of n+ kind electrode at the highest surface
of the wafer. Phosphorus oxychloride (POCI3) could be a liquid supply that
vaporizes at temperature itself thus it ought to be unbroken in cool place. For
the diffusion method, the vapors are administrated by the N2 gas and O2 is
passed through another valve. The reaction takes place, the phosphorous
oxychloride reacts with O2 forms P2O5 and so the P2O5 reacts with the
semiconducting material to allow the silicon oxide and therefore the
phosphorus. Pre-deposition involves the formation of phosphorus oxide films on
the semiconducting material substrate .throughout installation, phosphorus
oxide film acts as an infinite supply for phosphorus diffusion into the si
substrate. Throughout pre deposition, Phosphorus oxide (P2O5) forms on the
surface of the wafers by the reaction of phosphorus with O2. The P2O5
immediately reacts with the semiconducting material by leading to diffusion of
phosphorus and formation of the phosphor silicate glass (PSG).  The
Phosphorus atoms placed at the PSG-SI interface penetrate through the SI wafer.


POCl3(liquid) + N2(bubble) ? POCl3(vapor)(predeposition)

4 POCl3+ 3O2?2P2O55 + 6Cl2
2P2O5+ 5Si ?4P + SiO2 (drive-in)
+ 3Si ? n-type doped Si

No Damage to Process.
Cost Associated is Low
Can’t be carried out at room temperature
Shallow Junctions are difficult to fabricate.

Cleaning & Texturing
Edge Isolation (Screen Printing)
POCl3 diffusion
Back metallization
Front metallization
Rapid Thermal Annealing
LIV Testing










The alternative to deposition diffusion is Ion Implantation and is utilized
to produce a region of dopant atoms deposited into a silicon wafer of shallow
surface. In this process a light emission particles of impurity ions is
accelerated to kinetic energies in the range of tens of kV and is also coordinated
to the surface of the silicon. As the impurity atoms enter the crystal, when it
is collided they passes their energy to the lattice and finally it reaches to
rest at some average penetration depth, called the projected range expressed in
terms of micro meters (um). Depending on the impurity and its implantation
energy, the range in a given semiconductor may vary from a few 100angstroms to about
1 um (micro meter). Typical distribution of impurity along the projected range
is approximately Gaussian. By performing few implantations at various energies,
it is possible to synthesize a desired impurity distribution, example:  a uniformly doped region.  A gas containing the coveted debasement is
ionized inside the particle source. The ions are produced and repulsed from
their source in a
wandering bar that is engaged earlier if goes through a mass separator that
coordinates just the particles of the desired species through a narrow
aperture. A second lens focuses this is fixed by the light emission which then
passes through an accelerator that brings the particles to their required energy
before they strike the objective and become embedded in the exposed areas of
the silicon wafers. The voltages are accelerated from 20 kV to as much as 250
kV. In some of the ion implanters, the separations of mass occur after the ions
are accelerated with the very high energy. Because the ion light emission is quite
small, which means they are provided for scanning it uniform across the wafers.
For this purpose, the focused ion light emission is scanned electro statically over the
surface of the wafer in the objective chamber. The depth of the penetration of
any particular type of ion will increase with increasing accelerating voltage.
The penetration depth will generally be in the range of 0.1 to 1.0 micro meters


Results and discussion:

Solar cells which
are characterized by their ability to convert sunlight into electricity. LIV
testing is done in the lab to observe the V-I curves of the fabricated solar
cell. And also by this testing we can obtain the efficiency of the fabricated
monocrystalline or polycrystalline cell.



The main objective of
this research is to fabricate and also study about the mono or poly crystalline
silicon solar cell in the market. Which is why the efficiency of the solar cell
is low was accepted and tried to find out the challenges and remedy to improve
the efficiency of the solar cell. The challenges are like to determine the flow
rates and timing of the gases in the diffusion chamber, doping process, doping
concentration and also the optimum temperature zones in the rapid thermal
annealing furnace. Our priority is to find the out the problems for achieving
low efficiency, and also the equipments is to be improved in order to get a
better efficiency.

Categories: Industry


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