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.

 

INTRODUCTION:

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 which 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.

 

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 .

 

SOLAR
CELL CONCEPTS:

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 Mechanisms.

 

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 material utilization efficiency is high
relative to other methods and the process offers structural and morphological
control of films. Due to the very high deposition rate, this process has
potential industrial application for wear resistant and thermal barrier
coatings aerospace industries, hard coatings for cutting and tool industries,
and electronic and optical films for semiconductor industries.

SPUTTERING: Sputtering
process involves ejecting material from a “target” that is a source onto a
“substrate” (such as a silicon wafer) in a vacuum chamber. This effect is
caused by the bombardment of the target by ionized gas which often is an inert
gas such as argon. Sputtering is used extensively in the semiconductor industry
to deposit thin films of various materials in integrated circuit processing.
Anti-reflection coatings on glass for optical applications are also deposited
by sputtering. Because of the low substrate temperatures used, sputtering is an
ideal method to deposit metals for thin-film transistors. Perhaps the most
familiar products of sputtering are low-emissivity coatings on glass, used in
double-pane window assemblies. An important advantage of sputtering is that
even materials with very high melting points are easily sputtered while
evaporation of these materials in a resistance evaporator or Knudsen cell is
difficult and problematic.

 

CHEMICAL MECHANICAL
POLISHING:

Chemical mechanical planarization or chemical
mechanical polishing CMP is a process that can remove topography from silicon
oxide, poly silicon and metal surfaces. It is the preferred planarization technique
utilized in deep sub-micron IC manufacturing.
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. CMP is the only technique that
performs global planarization of the wafer.                                           Oxide planarization

Originally CMP was
used mainly to planarize silicon dioxide interlevel dielectrics
Silicon dioxide deposit thicker than the final thickness requested and the
material is 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 can be polished easily with almost the same
types of 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
Conditions
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
 

                  ADVANTAGES:
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

 

 

 

 

 

PHOSPHOROUS
DIFFUSION:

Phosphorus (P) diffusion is currently the
primary method for emitter fabrication in silicon (Si) solar cell processing.
The diffusion depends on various factors of which temperature and gaseous
environment is most important. P-type silicon wafers are widely used in solar
industries and therefore diffusion technologies have been developed to deposit
n-type doping elements to create the p-n junction. Due to its low boiling
temperature (105.8 ?C), at temperatures between 850-900 ?C in the diffusion
chamber, POCl3 is decomposed into simple phosphorus
compounds like P4, P8, P2O5, etc. The phosphorus diffusion fabrication
of crystalline silicon solar cell with emitter diffusion, surface passivation
and screen printing of electrode leads to formation of n+ type emitter at the
top surface of the wafer. Phosphorus oxychloride (POCl3) is a liquid source which vaporizes at room temperature
itself hence it should be kept in cool place. For the diffusion process, the
vapors are carried out by the carrier nitrogen and oxygen is passed through
another valve. The reaction takes place, the phosphorus oxychloride reacts with
oxygen forms phosphorus pentoxide and then the phosphorus pen oxide reacts with
the silicon to give the silicon dioxide and the phosphorus. Pre-deposition
involves the formation of phosphorous-rich oxide films on the silicon
substrate. During drive-in, the phosphorous-rich oxide film acts as an infinite
source for phosphorous diffusion into the Si substrate. During pre-deposition,
phosphorus pentoxide (P2O5) forms on the
surface of the wafers by the reaction of phosphorous with oxygen. The P2O5 immediately reacts with the silicon, by
resulting in diffusion of phosphorus and formation of the phosphosilicate
glass(PSG). The phosphorus atoms formed at the PSG-Si interface penetrate
through the silicon wafer.

 

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

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

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

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

 

 

 

 

 

 

 

 

ION
IMPLANTATION:

Ion Implantation is an alternative to a deposition
diffusion and is used to produce a shallow surface region of dopant atoms
deposited into a silicon wafer. In this process a beam of impurity ions is
accelerated to kinetic energies in the range of several tens of kV and is
directed to the surface of the silicon. As the impurity atoms enter the
crystal, they give up their energy to the lattice in collisions and finally
come to rest at some average penetration depth, called the projected range
expressed in micro meters. Depending on the impurity and its implantation
energy, the range in a given semiconductor may vary from a few hundred
angstroms to about 1micro meter. Typical distribution of impurity along the
projected range is approximately Gaussian. By performing several implantations
at different energies, it is possible to synthesize a desired impurity
distribution, for example a uniformly doped region. A gas containing the
desired impurity is ionized within the ion source. The ions are generated and
repelled from their source in a diverging beam that is focused before if passes
through a mass separator that directs only the ions of the desired species
through a narrow aperture. A second lens focuses this resolved beam which then
passes through an accelerator that brings the ions to their required energy
before they strike the target and become implanted in the exposed areas of the
silicon wafers. The accelerating voltages may be from 20 kV to as much as 250
kV. In some ion implanters, the mass separation occurs after the ions are
accelerated to high energy. Because the ion beam is small, means are provided
for scanning it uniformly across the wafers. For this purpose, the focused ion
beam is scanned electrostatically over the surface of the wafer in the target
chamber. The depth of 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.