1
SECTION ONE
1.1 INTRODUCTION
Formation of solar cell using semi-conductor material is the act of using a semiconductor to build a cell that can generate electricity when light (photons) falls on it. In
the field of electrical and electronics engineering a semi-conductor simply means those
materials whose conductivity lies somewhere between that of pure conductors and
insulators. They are neither good conductors nor good insulators at normal room
temperature (250C). They are insulators at very low temperatures and good conductors at
high temperature (Mahajan and Kimerling, 2012).
The Germanium (Ge) and silicon (Si) elements is the most important semi-conductor
used in electronics (Macdonald and Geerligs, 2014).
A pure type of semi-conductor without doping is called an intrinsic semi-conductor,
while extrinsic semi-conductor is the one with doping. Doping is the intentional addition
of impurities to semi-conductors so as to increase its conductivity. For instance using
silicon, when a silicon is doped with boron the resulting type is P-type, while if with
phosphorous the resulting type is N-type. Hence solar cell has two parts the N-type and
P-type side. And without this electric field, the cell would not work and this field forms
when the N-type and P-type silicon are in contact (Macdonald and Geerligs, 2014).
Hence the electrons form the N-type side will rush to fill up the hole in the P-type side.
Then when light in form of photons falls on the solar cell the energy frees electrons-hole
pairs, with the presence of the electric field the electrons are caused to flow in a particular
direction. Hence electric current is provided and the cells electric filed causes a voltage
flow. Therefore the product of this voltage and the current gives the power.
In most of today solar cells the absorption of photons, which results in the generation of
the charge carriers, and the subsequent separation of the photo-generated charge carriers
take place in semiconductor materials. Therefore, the semiconductor layers are the most
important parts of a solar cell; they form the hart of the solar cell. There are a number of
different semiconductor materials that are suitable for the conversion of energy of
2
photons into electrical energy, each having advantages and drawbacks (Mahajan and
Kimerling, 2012).
The crystalline silicon (c-Si) solar cell, which dominates the PV market at present, has a
simple structure, and provides a good example of a typical solar cell structure.
An absorber material is typically a moderately doped p-type square wafer having
thickness around 300μm and an area of 10 × 10 cm2 or 12.5 × 12.5 cm2. On both sides of
the c-Si wafer a highly doped layer is formed, n+-type on the top side and p+-type on the
back side, respectively. These highly doped layers help to separate the photo-generated
charge carriers from the bulk of the c-Si wafer. The trend in the photovoltaic industry is
to reduce the thickness of wafers up to 250μm and to increase the area to 20 × 20 cm2.
Fig. 1.1 Symbol of a Photovoltaic cell (Macdonald and Geerligs, 2014)
3
SECTION TWO
LITERATURE REVIEW
2.1 History of solar cell
Photovoltaic solar cells are thin silicon disks that convert sunlight into electricity. These
disks act as energy sources for a wide variety of uses, including: calculators and other
small devices; telecommunications; rooftop panels on individual houses; and for lighting,
pumping, and medical refrigeration for villages in developing countries. Solar cells in the
form of large arrays are used to power satellites and, in rare cases, to provide electricity
for power plants (Goetzberger et al., 2013).
When research into electricity began and simple batteries were being made and studied,
research into solar electricity followed amazingly quickly (Zhuiykov, 2014). As early as
1839, Antoine-Cesar Becquerel exposed a chemical battery to the sun to see it produce
voltage. This first conversion of sunlight to electricity was one percent efficient. That is,
one percent of the incoming sunlight was converted into electricity. Willoughby Smith in
1873 discovered that selenium was sensitive to light; in 1877 Adams and Day noted that
selenium, when exposed to light, produced an electrical current. Charles Fritts, in the
1880s, also used gold-coated selenium to make the first solar cell, again only one percent
efficient. Nevertheless, Fritts considered his cells to be revolutionary. He envisioned free
solar energy to be a means of decentralization, predicting that solar cells would replace
power plants with individually powered residences (Goetzberger et al., 2013).
With Albert Einstein's explanation in 1905 of the photoelectric effect—metal absorbs
energy from light and will retain that energy until too much light hits it—hope soared
anew that solar electricity at higher efficiencies would become feasible. Little progress
was made, however, until research into diodes and transistors yielded the knowledge
necessary for Bell scientists Gordon Pearson, Darryl Chapin, and Cal Fuller to produce a
silicon solar cell of four percent efficiency in 1954.
Further work brought the cell's efficiency up to 15 percent. Solar cells were first used in
the rural and isolated city of Americus, Georgia as a power source for a telephone relay
system, where it was used successfully for many years (Zhuiykov, 2014).
4
A type of solar cell to fully meet domestic energy needs has not as yet been developed,
but solar cells have become successful in providing energy for artificial satellites. Fuel
systems and regular batteries were too heavy in a program where every ounce mattered.
Solar cells provide more energy per ounce of weight than all other conventional energy
sources, and they are cost-effective (Goetzberger et al., 2013).
Only a few large scale photovoltaic power systems have been set up. Most efforts lean
toward providing solar cell technology to remote places that have no other means of
sophisticated power (Smith et al., 2018). About 50 megawatts are installed each year, yet
solar cells provide only about. 1 percent of all electricity now being produced. Supporters
of solar energy claim that the amount of solar radiation reaching the Earth's surface each
year could easily provide all our energy needs several times over, yet solar cells have a
long way to go before they fulfill Charles Fritts's dream of free, fully accessible solar
electricity.
The photovoltaic
effect was
experimentally
demonstrated
first
by
French
physicist Edmond Becquerel. In 1839, at age 19, he built the world's first photovoltaic
cell in his father's laboratory. Willoughby Smith first described the "Effect of Light on
Selenium during the passage of an Electric Current" in a 20 February 1873 issue
of Nature. In 1883 Charles Fritts built the first solid state photovoltaic cell by coating
the semiconductor selenium with a thin layer of gold to form the junctions; the device
was only around 1% efficient. Other milestones include (Zhuiykov, 2014):
a) 1888 – Russian physicist Aleksandr Stoletov built the first cell based on the
outer photoelectric effect discovered by Heinrich Hertz in 1887.
b) 1905 – Albert Einstein proposed a new quantum theory of light and explained
the photoelectric effect in a landmark paper, for which he received the Nobel Prize in
Physics in 1921.
c) 1941 – Vadim Lashkaryov discovered p-n-junctions in Cu2O and Ag2S protocells.
d) 1946 – Russell Ohl patented the modern junction semiconductor solar cell, while
working on the series of advances that would lead to the transistor.
5
e) 1948 - Introduction to the World of Semiconductors states Kurt Lehovec may have
been the first to explain the photo-voltaic effect in the peer reviewed journal Physical
Review.
f) 1954 – The first practical photovoltaic cell was publicly demonstrated at Bell
Laboratories. The inventors were Calvin Souther Fuller, Daryl Chapin and Gerald
Pearson.
g) 1957 – Egyptian engineer Mohamed M. Atalla develops the process of silicon surface
passivation by thermal oxidation at Bell Laboratories. The surface passivation process
has since been critical to solar cell efficiency.
h) 1958 – Solar cells gained prominence with their incorporation onto the Vanguard
I satellite.
2.2 Application of Solar Cells
Fig. 2.1 Possible components of a photovoltaic system (Willson, 2015)
Assemblies of solar cells are used to make solar modules that generate electrical power
from sunlight, as distinguished from a "solar thermal module" or "solar hot water panel".
A solar array generates solar power using solar energy (Willson, 2015).
6
2.3 Cells, modules, panels and systems
Multiple solar cells in an integrated group, all oriented in one plane, constitute a solar
photovoltaic panel or module. Photovoltaic modules often have a sheet of glass on the
sun-facing side, allowing light to pass while protecting the semiconductor wafers. Solar
cells are usually connected in series creating additive voltage. Connecting cells in parallel
yields a higher current.
However, problems in paralleled cells such as shadow effects can shut down the weaker
(less illuminated) parallel string (a number of series connected cells) causing substantial
power loss and possible damage because of the reverse bias applied to the shadowed cells
by their illuminated partners (Smith et al., 2018).
Although modules can be interconnected to create an array with the desired peak DC
voltage and loading current capacity, which can be done with or without using
independent MPPTs (maximum power point trackers) or, specific to each module, with or
without module level power electronic (MLPE) units such as micro inverters or DC-DC
optimizers. Shunt diodes can reduce shadowing power loss in arrays with series/parallel
connected cells (Willson, 2015).
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SECTION THREE
FORMATION OF SOLAR CELLS USING SEMICONDUCTOR MATERIAL
3.1 How a Solar Cell Works Using Semiconductor Material
A solar cell is made of two types of semiconductors, called p-type and n-type silicon. The
p-type silicon is produced by adding atoms—such as boron or gallium—that have one
less electron in their outer energy level than does silicon. Because boron has one less
electron than is required to form the bonds with the surrounding silicon atoms, an
electron vacancy or “hole” is created (Aberle, 2011).
The n-type silicon is made by including atoms that have one more electron in their outer
level than does silicon, such as phosphorus. Phosphorus has five electrons in its outer
energy level, not four. It bonds with its silicon neighbor atoms, but one electron is not
involved in bonding. Instead, it is free to move inside the silicon structure (Fahey et al.,
2019).
A solar cell consists of a layer of p-type silicon placed next to a layer of n-type silicon
(Fig. 3.1). In the n-type layer, there is an excess of electrons, and in the p-type layer,
there is an excess of positively charged holes (which are vacancies due to the lack of
valence electrons). Near the junction of the two layers, the electrons on one side of the
junction (n-type layer) move into the holes on the other side of the junction (p-type
layer). This creates an area around the junction, called the depletion zone, in which the
electrons fill the holes (Fig. 3.1, close-up) (Aberle, 2011).
8
Figure 3.1. Schematic representation of a solar cell, showing the n-type and p-type layers,
with a close-up view of the depletion zone around the junction between the n-type and ptype layers. (Aberle, 2011)
When all the holes are filled with electrons in the depletion zone, the p-type side of the
depletion zone (where holes were initially present) now contains negatively charged ions,
and the n-type side of the depletion zone (where electrons were present) now contains
positively charged ions (Fahey et al., 2019). The presence of these oppositely charged
ions creates an internal electric field that prevents electrons in the n-type layer to fill
holes in the p-type layer.
When sunlight strikes a solar cell, electrons in the silicon are ejected, which results in the
formation of “holes”—the vacancies left behind by the escaping electrons. If this happens
in the electric field, the field will move electrons to the n-type layer and holes to the ptype layer. If you connect the n-type and p-type layers with a metallic wire, the electrons
will travel from the n-type layer to the p-type layer by crossing the depletion zone and
then go through the external wire back of the n-type layer, creating a flow of electricity
(Fahey et al., 2019).
9
3.2 Raw Materials
The basic component of a solar cell is pure silicon, which is not pure in its natural state.
Fig. 3.2 Quartz gravel (Lécuyer and Brock, 2016)
To make solar cells, the raw materials—silicon dioxide of either quartzite gravel or
crushed quartz—are first placed into an electric arc furnace, where a carbon arc is applied
to release the oxygen. The products are carbon dioxide and molten silicon. At this point,
the silicon is still not pure enough to be used for solar cells and requires further
purification (Lécuyer and Brock, 2016).
Pure silicon is derived from such silicon dioxides as quartzite gravel (the purest silica) or
crushed quartz. The resulting pure silicon is then doped (treated with) with phosphorous
and boron to produce an excess of electrons and a deficiency of electrons respectively to
make a semiconductor capable of conducting electricity. The silicon disks are shiny and
require an anti-reflective coating, usually titanium dioxide (Nakamura, 2014).
The solar module consists of the silicon semiconductor surrounded by protective material
in a metal frame. The protective material consists of an encapsulant of transparent silicon
rubber or butyryl plastic (commonly used in automobile windshields) bonded around the
cells, which are then embedded in ethylene vinyl acetate. A polyester film (such as mylar
or tedlar) makes up the backing. A glass cover is found on terrestrial arrays, a lightweight
plastic cover on satellite arrays. The electronic parts are standard and consist mostly of
copper (Lécuyer and Brock, 2016). The frame is either steel or aluminum. Silicon is used
as the cement to put it all together (Smith et al., 2018).
10
3.3 The Manufacturing Process
Purifying the silicon
a) The silicon dioxide of either quartzite gravel or crushed quartz is placed into an
electric arc furnace. A carbon arc is then applied to release the oxygen. The
products are carbon dioxide and molten silicon. This simple process yields silicon
with one percent impurity, useful in many industries but not the solar cell industry.
b) The 99 percent pure silicon is purified even further using the floating zone
technique. A rod of impure silicon is passed through a heated zone several times in
the same direction. This procedure "drags" the impurities toward one end with
each pass. At a specific point, the silicon is deemed pure, and the impure end is
removed.
Making single crystal silicon
a) Solar cells are made from silicon boules, polycrystalline structures that have the
atomic structure of a single crystal. The most commonly used process for creating
the boule is called the Czochralski method. In this process, a seed crystal of silicon
is dipped into melted polycrystalline silicon. As the seed crystal is withdrawn and
rotated, a cylindrical ingot or "boule" of silicon is formed. The ingot withdrawn is
unusually pure, because impurities tend to remain in the liquid (Nakamura, 2014).
Making silicon wafers
a) From the boule, silicon wafers are sliced one at a time using a circular saw whose
inner diameter cuts into the rod, or many at once with a multiwire saw. (A
diamond saw produces cuts that are as wide as the wafer—. 5 millimeter thick.)
Only about one-half of the silicon is lost from the boule to the finished circular
wafer—more if the wafer is then cut to be rectangular or hexagonal. Rectangular
or hexagonal wafers are sometimes used in solar cells because they can be fitted
together perfectly, thereby utilizing all available space on the front surface of the
solar cell (Nakamura, 2014).
11
Fig. 3.3 Making silicon wafers (Ayers, 2017)
After the initial purification, the silicon is further refined in a floating zone
process. In this process, a silicon rod is passed through a heated zone several
times, which serves to 'drag" the impurities toward one end of the rod. The impure
end can then be removed (Ayers, 2017).
Next, a silicon seed crystal is put into a Czochralski growth apparatus, where it is
dipped into melted polycrystalline silicon. The seed crystal rotates as it is
withdrawn, forming a cylindrical ingot of very pure silicon. Wafers are then sliced
out of the ingot.
b) The wafers are then polished to remove saw marks. (It has recently been found
that rougher cells absorb light more effectively, therefore some manufacturers
have chosen not to polish the wafer.)
Doping
a) The traditional way of doping (adding impurities to) silicon wafers with boron and
phosphorous is to introduce a small amount of boron during the Czochralski
process in making single crystal silicon as described above. The wafers are then
sealed back to back and placed in a furnace to be heated to slightly below the
melting point of silicon (2,570 degrees Fahrenheit or 1,410 degrees Celsius) in the
12
presence of phosphorous gas. The phosphorous atoms "burrow" into the silicon,
which is more porous because it is close to becoming a liquid. The temperature
and time given to the process is carefully controlled to ensure a uniform junction
of proper depth (Ayers, 2017).
A more recent way of doping silicon with phosphorous is to use a small particle
accelerator to shoot phosphorous ions into the ingot. By controlling the speed of
the ions, it is possible to control their penetrating depth. This new process,
however, has generally not been accepted by commercial manufacturers (Frosch
and Derrick, 2017).
Placing electrical contacts
a) Electrical contacts connect each solar cell to another and to the receiver of
produced current. The contacts must be very thin (at least in the front) so as not to
block sunlight to the cell. Metals such as palladium/silver, nickel, or copper are
vacuum-evaporated through a photoresist, silkscreened, or merely deposited on the
exposed portion of cells that have been partially covered with wax.
Fig. 3.4 Makeup of a typical solar cell (Ayers, 2017)
13
All three methods involve a system in which the part of the cell on which a contact
is not desired is protected, while the rest of the cell is exposed to the metal (Frosch
and Derrick, 2017).
b) After the contacts are in place, thin strips ("fingers") are placed between cells. The
most commonly used strips are tin-coated copper (Smith et al., 2018).
The anti-reflective coating
a) Because pure silicon is shiny, it can reflect up to 35 percent of the sunlight. To
reduce the amount of sunlight lost, an anti-reflective coating is put on the silicon
wafer. The most commonly used coatings are titanium dioxide and silicon oxide,
though others are used. The material used for coating is either heated until its
molecules boil off and travel to the silicon and condense, or the material
undergoes sputtering. In this process, a high voltage knocks molecules off the
material and deposits them onto the silicon at the opposite electrode. Yet another
method is to allow the silicon itself to react with oxygen- or nitrogen-containing
gases to form silicon dioxide or silicon nitride. Commercial solar cell
manufacturers use silicon nitride (Frosch and Derrick, 2017).
Encapsulating the cell
a) The finished solar cells are then encapsulated; that is, sealed into silicon rubber or
ethylene vinyl acetate. The encapsulated solar cells are then placed into an
aluminum frame that has a mylar or tedlar backsheet and a glass or plastic cover
(Smith et al., 2018).
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3.4 Conclusion
The overwhelming majority of solar cells are fabricated from silicon with
increasing efficiency and lowering cost as the materials range from amorphous (noncrystalline)
to
polycrystalline
to
crystalline
(single crystal)
silicon
forms.
Unlike batteries or fuel cells, solar cells do not utilize reactions or require fuel to
produce electric power, and, unlike electric generators, they do not have any moving
parts.
Solar cells can be arranged into large groupings called arrays. These arrays, composed of
many thousands of individual cells, can function as central electric power stations,
converting sunlight into electrical energy for distribution to industrial, commercial, and
residential users. Solar cells in much smaller configurations, commonly referred to as
solar cell panels or simply solar panels, have been installed by homeowners on their
rooftops to replace or augment their conventional electric supply. Solar cell panels also
are used to provide electric power in many remote terrestrial locations where
conventional electric power sources are either unavailable or prohibitively expensive to
install.
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