JORDAN UNIVERSITY OF SCIENCE AND
TECHNOLOGY
SCIENCE AND ART COLLAGE
APPLIED PHYSICS DEPARTMENT
Physics 791
SEMINAR
SOLAR CELLS FABRICATION
SUPERVISION:
Dr: ADNAN SHARIAH
ENAS.M.HAMASHA
20053092008
1
TABLE OF CONENT:
page
Abstract…………………………..…………….3
1. Introduction …………………………………4
Single-crystal wafers technology………………6
2.1 Reduction of sand to metallurgical-grade
silicon………………………………..…………6
2.1.1 Arc furnace……………….………………7
2.2 Metallurgical-Grade silicon to semiconductor
grade silicon …………..……....………………9
2.3 Conversion of Semiconductor-grade silicon to
single-crystal silicon wafers………..……………11
2.3.1 Czocharalski process…..…………………...11
2.3.2 Czocharalski procedure..…………………...11
2.3.2.1 Crucible………………………………….12
2.3.3 Silicon wafers……………………………..14
3. Processing of single crystal wafers to solar
cells…………………………….………………..16
3.1 Doping……………………….……………...16
3.2 Contact wafers with metal..............................18
3.3 Antireflection coating..............................…...20
3.4 Texturing surface ………………..………….22
4. Solar cell to solar module..........................…...24
5. Discussion and result………………………….26
6. References……………………………………..27
2
TABLE OF FIGURE:
PAGE
Figure 1: schematic representation of the cost distribution for solar cell
and module……………………………………………………………..6
Figure 2: the schematic explain the procedure fabrication solar cell……7
Figure 3: arc furnace and the inside reaction………………………8
Figure 4: electric arc furnace……………………………………………..9
Figure 5: CZ process……………………………………………………13
Figure 6: crucible use in CZ process before use………………………..13
Figure 7: crucible during annealing……………………………………14
Figure 8: A crucible after used………………………………………….14
Figure 9: single crystal ingot……………………………………………15
Figure 10: silicon wafer a) after cutting b) after polishing…………..….16
Figure 11: phosphourus diffusion process………………………..…….18
Figure 12: distribution of phosphorus impurities………………….19
Figure 13: Solar wafer with metal contact………………………………20
Figure 14: a metal shadow mask………………………………..21
Figure 15: Major feature of solar cell…………………………………...22
Figure 16: texturing surfaces silicon by SEM…………..………………24
Figure 17: Tracked PV Array containing 16 panels……………………..25
Figure 18: Photovoltaic cells, modules, panels and arrays……………...26
3
Abstract
The life cycle of single crystal solar cell start with silica or
silicon dioxide SiO2 also known as quartz sand, in order to
obtain 99% pure metallurgical-grade silicon (MG-Si) the
oxygen must first be extracted from the silicon dioxide by
carbon reduction in an arc furnace then use siemens process to
produce high purity silicon (99.9999%) semiconductor-grade
silicon (SeG-Si) which is used for making the silicon wafers.
These SeG-Si are melted in a crucible with boron dopant and
either pulled/grown as cylinder this is czocharalski process to
produce single crystalline cylindrical ingots, after solidification
ingots are sawn into thin slices called wafers and doping this
wafers by phosphorous impurities to form p-n junction in silicon
wafers and will contact with metal grid in n-type side and metal
layer in p-type side to accumulate the current carriers and to
pass the electrical current outside the cells.
The last step is contact many wafers with series or parallel to
formation solar module and encapsulation solar module with
glass to protection.
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1. Introduction
Beside other renewable energy sources photovoltaic (PV)
present a prime source of non polluting energy, basically it is
silicon based today, in particular, silicon is used in PV for single
crystalline and poly crystalline wafers production.
More than 90% of the annual solar silicon production is based
on single crystalline silicon wafers, [1].
Therefore the silicon wafers fabrication is the most important
technology for PV today.
single c-Si cells are the most common in the PV industry and
has a uniform molecular structure (which is the arrangement of
atom in the material) compared to non crystal materials because
the entire structure is grown from the same crystal, single c-Si
high uniformly result in higher energy conversion efficiency, the
higher a PV cells conversion efficiency the more electricity it
generates for a given area of expose to the sunlight.
The average price for a single-crystal solar module is 50$ in
1970 and dropped to 5$ in 1988 and 3.97$ in 1996, [2] the
reason for the reduction cost is the improvement of the
technology PV cell production.
The cost of a module-typically (1-2) m² in size can be broken
down into three main components:
 35% for the solar Si material (ready for cell production)
 30% for the solar cell technology (making a solar cell)
 30% for module manufacture
If one uses the conventional process to make single
crystalline wafers as the starting material for solar cells,
the cost can be broken down as follows:

30% starting material (poly-Si)

35% Crystal growth

30% sawing the crystal into wafers

5% for refining (etching, polishing, cleaning), [3]
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One can shows this distribution of the cost in the Figure
below:
Starting
material30%
Sawing30%
Crystal
growth35%
Refining 5%
Figure 1: schematic representation of the cost distribution for
solar cell and module.
The objective in this project is studying the production of
single-crystal wafers of silicon.
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2-Single-crystal wafers technology
The processing steps for the fabrication of a silicon
wafers, which is the starting material for the fabrication of
electronic devices and solar cells.
Many process steps are conducted to bring Si from its
native to crystalline substrate, one uses for solar cell
fabrication,
These steps show below:
Figure 2: the schematic explain the procedure fabrication
solar cell.
2.1 Reduction of sand to metallurgicalgrade silicon
Silicon is the second most abundant element in the crust of
the earth 27%, but it doesn't occur native element because
SiO2 in the form of quartz, quartzite, and other component
[4].
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The starting silicon for PV application is 99% pure
metallurgical-grade silicon (MG-Si) obtained from
reduction of SiO2 in an electric arc furnace, by carbon (in
the form of a mixture of wood chips, coke and coals) [5]
although the overall reaction can be considered:
SiO2 + C
CO2 + Si
There are a complex series of reaction that take place in
different temperature region of the furnace show in Figure
3:
Figure 3: arc furnace and the inside reaction.
2.1.1 arc furnace
The first electric arc furnace was development by Paul
He'roult of furnace (1899) to make steel.
An electric arc furnace is a system that heats charged
material; the furnace is primarily split into three sections:
1) The shell, which consist of the sidewall and lower steel
and bowel
2) The hearth, which consist of the refractory that lines
the lower bowel
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3) The roof, which may be refractory-lined or watercooled, and can be shaped as a section of a sphere or
conical section[6],
We can see the real arc furnace in Figure 5 below:
Figure 4: electric arc furnace
As shows in the Figure 3 the liquid silicon finally forming
from SiC, Then the silicon liquid is periodically tapped
from the furnace and typically allowed to solidify in
shallow mold about 1.5× 1 m² in size, and the major
impurities exist in silicon liquid is Al and Fe as table as
below:
Table 1: concentration impurity in M-G Si
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So silicon liquid solidifies with impurities 99% and is
subsequently broken into chunks.
Characteristic of this stage is:
 1 million metric tons of this MG-Si is produce globally
each year (large production).
 Use MG-Si in the steel and aluminum industries.
 The total processing energy requirement is acceptable.
 The result the MG-Si is inexpensive (low cost) in 1997
MG-Si cost about 5$ per Kg.
 99% pure with the major impurities being iron and
aluminum.
2.2 Metallurgical-Grade silicon to
semiconductor grade silicon
In this stage one can get a semiconductor grade silicon by
purification metallurgical–Grade silicon in an (a siemens
process) which included multiple steps.
The following refining procedures use the conversion
MG-Si into volatile component such as silane (SiH4) by
reaction MG-Si with HCl.
The volatile component are purified by chemical method
which is condensed and fractional distillation
Back to the arc furnace reaction to take phase product:
Solidificatio
n
SiO2 (s) + C (s)
arc furnace
CO2 (g) + Si(l)
Quartzite
Coke, Coal,
Pebbles
Wood chips
MG-Si (metallurgical
grade silicon)
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The second step is:
MG-Si(s) + 3HCl(g)
SiHCl3 (g) + H2 (g)
Condenser and
multiple
fractional
distillations
SeG-SiHCl3 (l)
S
Semiconductor-grade
purity trichlorosilane
And reaction with a hydrogen gas in a chamber containing
silicon rod heated electrically to 1000-1200 °C results in
deposition a fine grained poly crystalline form onto an
electrically heated silicon rod.
SeG-SiHCl3 (g) + H2 (g)
3HCl(g) + SeG-Si(s)
Polycrystalline
SemiconductorGrade Silicon [7]
The net result from Siemens process to produce poly-crystalline
semiconductor-grade silicon with purity is 99.9999%.
Characteristics:
 Requirement a lot of energy.
 Low yield ~37%
 The high cost of this steps.
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2.3 Conversion of Semiconductor-grade silicon
to single-crystal silicon wafers
To make silicon in a single-crystal state, one must first melt high-purity
silicon this then cause it to reform or solidify very slowly in contact with
a single crystal "seed".
The silicon adapts to the pattern of the single-crystal seed as it cools and
gradually solidifies, not surprisingly, because one starts from a seed say
that this process is growing a new rod (often called boule) of a single
crystal.
Several different processes can be used to grow a boule of single crystal
silicon, the most established and dependable process is the czocharalski
processes (CZ method).
2.3.1 Czocharalski process
The czocharalski process is a technique for growing crystal in order to
obtain solitary crystals of semiconductor like silicon, metal like silver
and gold as well as several salts.
This process is named after Jan czocharalski who first demonstrate it in
1916; he was studying the rate of crystallization of different material.
The most significant applications of the czocharalski process are the
growing of large use ingots of single crystal silicon [8].
2.3.2 Czocharalski procedure
High-purity, semiconductor-grade silicon is melted down in a crucible.
Dopant impurities atom such as a boron can be added to the molten
intrinsic silicon in precise a mount in order to dope the silicon, thus
12
changing it into p-type extrinsic silicon, this influence the electrical
conductivity of the silicon.
A seed crystal mounted on a rod is dipped into the molten silicon, the
seed crystal rods is pulled upwards and rotates at the same time[9] as
shown in the figure below:
Figure 5: CZ process
By precisely controlled the temperature gradient, rate of pulling and
speed of rotation, its possible to extract a large single crystal cylindrical
ingot from the melt, this process is normally performed in an inert
atmosphere such as argon and an inert chamber such as quartz crucible.
2.3.2.1 Crucible
A crucible is a cup-shaped piece of laboratory equipment used to contain
chemical compound when heated them to very high temperature, the
receptacle is usual made of porcelain, the crucible used in CZ molten
shown below:
Figure 6: crucible use in CZ process before use.
13
These crucibles are very durable and resist temperatures to
over 1600 °C. A crucible is placed into a furnace and, after
the melting, the liquid metal is taken out of the furnace and
poured into the mold [10],
Figure 7: crucible during annealing
And the other case:
Figure 8: A crucible after used
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2.3.3 Silicon wafers
The solidification the silicon ingots as shown in figure:
Figure 9: single crystal ingot
Silicon wafers are obtained from large silicon ingots by slicing
and polishing to achieve a high degree of flatness. These chips,
normally about 0.75 mm in thickness are thus used for the
manufacture of integrated circuits. The ingots from which they
are cut may range from anywhere between 1 and 2 metres in
length with diameters up to 400 mm [11].
15
Figure 10: silicon wafer a) after cutting b) after bolishing
Finally, the cylindrical ingot is cut with saws (wire or diamond
saws) into round waferes about 250 μm thick.
Characteristics:
 The present wafering technology it's difficult to cut wafers
from the large crystals which are any thinner than 300μm
and still retain reasonable yields.
 More than half the silicon is wasted as kerfs or cutting loss
in the process
 The low overall yields of single-crystal.
16
3. Processing of single crystal wafers to solar
cells
Wafers are clean with industrial soap and then etched using hot
sodium hydroxide to remove saw damage.
3.1 Doping
The objective is to create planer region having different
concentration of impurities so as to form a p-n junction, thus in
n-on p silicon cells.
One may start with a p-type wafers containing boron as its chief
impurity, and produce a very thin n-type layer near its surface
by introducing phosphorous impurity into the wafers.
Impurity may be added intentionally to a molten semiconductor
before solidification. The traditional method of forming a layer
of different conductivity on the surface of such of a wafer has
been thermal diffusion of dopant atoms present in a gaseous
molecule in a high temperature furnace in which the carefully
cleaned wafers.
Usually two-steps diffusion process is employed, in the
predepostion step the desired impurity, transport by carriers gas
to the hot semiconductor surface, and is diffused to a depth of a
few tenths of a micron.
To dope Si with P, one may bubble nitrogen gas through liquid
POCl3 into a furnace held at from 800 to 1100°C for anywhere
from minutes to an hour or so. Alternatively, a solid oxide such
as P2O5 might be heated for producing gaseous P2O5 which
will react at the hot Si surface to form P and solid SiO2.
In drive-in step, the semiconductor is simply heated and the
predeposited dopant atoms diffuse into the semiconductor to a
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greater depth, up to about 1μm as and formation oxide layer on
the surface shown in the Figure below [12]:
a)
Figure 11:phosphourus
diffusion process:
a) Schematic
b) Real
In asubsequence processing the oxide layer is removed as are
the junction at the side and back of the cell to give the structure
below:
18
Figure 12: distribution of phosphorus impurities:
a) Immediately after diffusion process.
b) After etching of back and side of wafers [13].
3.2 Contact wafers with metal
In a p-n junction solar cells ohmic contact are typically made
with a uniform metallic electrode over the back surface and
fingerlike front electrode.
They should be highly conducting and should bond well by
soldering to contact wires or conducting supports.
So the contacts contain three separate layers of metal:
1. Titanium
2. Silver, Al or copper.
3. Palladium.
A thin layer of titanium is used as the button layer for good
adherence to silicon; the top layer is silver for low resistance
and solder ability, sandwiches between these two is layer of
palladium which prevent an undesirable reaction between Ti and
Ag layer in the presence of moisture.
19
Figure 13: Solar wafer with metal contact [14].
Metal contacts are then attached to both the n-type and the ptype region. In the standard technology, the process used is
known as vacuum evaporation. The metal to be deposited is
heated in a vacuum to a high-enough temperature to cause it to
melt and vaporize. It will then condense on any cooler part of
the vacuum system in direct line of sight, including the solar
cells. The back contact is normally deposited over the entire
back surface, while the top contact is required in the form of a
grid two techniques are available for defining such pattern, one
is to use a metal shadow mask shown in Figure 14.
Alternatively, the metal can be deposited over the entire front
surface of the cell and subsequently etched a way from
unwanted region using a photographic technique known as
photolithography.
20
Figure 14: a metal shadow mask [15].
3.3 Antireflection coating
The one type of the losses in solar cells of an optical nature that
bare silicon is quite reflective. The antireflection losses to about
10%.
Antireflection coating consisting of layers indices of refraction
between that of the semiconductor and air, the simplest AR is a
single layer whose thickness is one-quarter wavelength of light
in the coating and whose index of refraction nc is the
geometrical mean of the indices of the two media containing it
so nc =1.84 for silicon.
21
Figure 15: Major feature of solar cell
But no material having exactly this index is known, some oxide
and fluorides approximating it do exist see table below:
Table 2: Indices of refraction of material [16].
Material
Al2O3
Glasses
MgO
SiO
SiO2
Ta2O5
TiO2
Index of refraction
1.77
1.5-1.7
1.74
1.5-1.6
1.46
2.2
2.5-2.6
90% of light is transmitted through 600Å coating of Ta2O5 on
silicon.
Antireflection coating are applied primarily by vacuum
evaporation is deposited on the top of the cell.
22
The AR material to be deposited is heated in a vacuum to a
high – enough temperature to cause it to melt and vaporize if
will condense on cell cooler parts of vacuum system in direct
line of sight.
Characteristics:
• Yield of about 90% from starting wafers to completed
terrestrial cells can be obtained
• This make the processing very labor-intensive
• The vacuum evaporation equipment is expensive
compared to its throughput
• the material expensive such Ag.
3.4 Texturing surface
The other technique to reduce reflection from the surface is
texturing.
To minimize reflection from the flat surface solar cell wafers are
textured, this means a creating a roughened surface, so that
incident light will have a larger probability of being absorbed
into the solar cell. This is performed by etching in a week
alkaline solution such as Hf.
23
Figure 16: texturing surfaces silicon by SEM [17].
The textured surface results when a single –crystal
semiconductor is treated with an orientation –dependent etch
that attack some crystal planes faster than it does other.
For etching a Si wafers whose normal orientation is in the (100)
or equivalent direction in heated (90 to 95°C) hydrazine (36
percent N2H4) produces small pyramid whose faces are (111)
planes shown in figure 16.
Light incident on these pyramids is partially transmitted and
partially reflected at each contact with their surface as the light
bounces its way down toward the bottom of the pyramids.
As with a properly designed AR coating, the surface of a
textured cell appear dark when viewed in white light.
24
4. solar cell to solar module
After the three steps to obtain solar wafers prepared to contact
together to formation a solar module by metal wire with high
conductivity to accumulate the electric current from every cell.
After interconnecting between cells solar cells require
encapsulation by glass to:
1. Mechanical protection
2. Electrical isolation
3. Chemical protection
4. Mechanical rigidity to support the prattle cells and their
flexible interconnection
Figure 17: Tracked PV Array containing 16 panels [18].
The most common material for the structural back has been
anodized Aluminum, porcelainized steel, epoxy board or
window glass.
The structural front configuration glass is the obvious choice for
the structural layer, it combine excellent weather ability with
low cost and good self- cleaning properties.
So we will connect many cells to make module and connect
more than two modules to make panel and the many panel to
make array show below:
25
Figure 18: Photovoltaic cells, modules, panels and arrays.
26
5. Discussion and result
The result show that dedicated process of solar cell module from
the sand in nature to solar cell system can produce a new a
renewable energy sources.
And open wide field to improvement the fabrication solar cell
by modern technique to reduction the time and the money.
27
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science and Engineering B 134,257-262,(2006).
[2]Department of Energy, Renewable energy annual,Vol
1,Chapter 2, Table 28, 1997.
[3] http://WWW.SILICON.COM
[4] J.Gee, J.Van Den Avyle, J.slepanek, B.L.Sopori, 520552,(1998).
[5] T.Wang, F.Ciszek, Journal. Crystal Growth 174, 176,(1997).
[6] http://en.Wikipedia.org/Wiki/Electric arc furnace
[7] C.Hu, R.M.White, Solar Cell from basic to advance system,
First Edition,MC Grow Hill, (1983).
[8] http://en .Wiki/Czocharalski process
[9]http://WWW.article world.org/index.php/czocharalski
process
[10] http://WWW.en.Wikipedia.org/Wiki/Crucible
[11] http://WWW.article
world.org/index.php/Monokrristallines_Silizium
[12] M.Green, Solar Cell operation principal Technology and
system Application, Prentice-Hall.Inc,firstedition,(1982).
[13] http://WWW. Solar Photovoltic Cells.com
[14] M.Green, Solar Cell operation principal Technology and
system Application, Prentice-Hall.Inc,firstedition,(1982).
[15] H.Möller,Semiconductor for Solar Cells,Artech House,first
edition,(1993).
[16] http://WWW. IMEC/Crystal Growth, Crystalline Si Solar
Cells.htm
[17] Bp Solar web page "An Introduce to BpSolar in Australia"
(online),http://WWW.bp.com.au/Solar/Default.asp Accessed on
16 Feb 2007.
[18] http://WWW.Wikipedia.org/Wiki/panel
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