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Building a Solar Cell
This project was constructed by a group of students from Liceo M.A. Vassalli, Qormi together
with their physics teacher.
Initial observations
During a lesson in the lab, our Physics
teachers showed us a solar panel and
described how these objects could change
the energy given by the sun to electrical
energy. We were very interested and
asked the teacher a lot of questions about
these objects. How could these small black
panels change the rays of the sun to
electricity in order to drive a motor? Why
didn’t we use these things to generate
electricity and avoid using fossil fuels that
create some much pollution?
By using these panels we could do away
with using fossil fuels and may be generate
electricity in a cheaper way! We could also
reduce
pollution
by
reducing our
dependency on
the
power
station! Our teacher however told us that
these panels were very expressive and we
needed bigger solar panels even to light a
bulb. Therefore we decided to find out if we
could actually construct our own solar cell
with material that could be easily found
and eventually try to improve our project in
order to generate as much electrical
energy as possible.
Purpose of the Project
In this project we want to find out if we can actually construct our own solar cell and improve
its design to make it work as efficiently as possible. We would also like to find out how real
solar panels work and why electrical energy is generated when these are placed in the sun.
The project wants to find an answer to the following questions;
1.
2.
3.
4.
Can a solar panel be constructed from the material we find at home?
How do actually solar cells work?
What is the best design for the solar cell to be more effective?
What is the highest voltage and current that can be generated by our solar cell? How
can these values be increased?
5. What is the highest power we can generate using the solar cell?
6. How shall our cell compare with a commercial cell?
7. Would it be possible to generate enough energy to light a 100W bulb through our cell?
Building a solar cell
1
Our Hypothesis
From our initial research on the internet we found out the solar cell can
be constructed using simple material such as copper plates and other
material found at home and at school. The descriptions found also
suggested that the battery could not generate a lot of electrical energy.
When considering the questions we want to answer, we think that;
1. The solar cell could be constructed using simple and cheap material.
2. The solar panel changes energy from the sun to electrical energy but there must be
some chemical reaction taking place in the solar panel. We needed to find more about
this.
3. We believed that the solar cell should actually have a flat shape so that when facing
the sun the same amount of energy falls on each part of the solar panel.
4. We also believed that the voltage and current must be low but from our Physics
lessons we think that by connecting the batteries in series and/or parallel we can have
higher values.
5. The power of the cell shall depend on the amount of sunlight; the more the power of
the sun, the higher the power of our battery. Our teacher also suggested that the
power can be calculated form the equation Power = Current x Voltage.
6. We believe that our cell will generate less power than a commercial cell.
7. We think that more panels would be needed to light a 100W bulb but a less powerful
bulb might light.
Identify Variables
When constricting our solar cell we needed to have something that remained fixed through
the experiment. Therefore we discussed that the better thing to do is to have the same
surface area (0.12m x 0.14m = 0.0168m2) for our solar panel and this had to be kept the
same way each time a cell had to be constructed. This was our control. The variables in our
experiment were;
i)
the shape of the cell.
ii)
the amount of sunlight on the cell.
iii)
the number of cells used to generate electrical energy.
Designing the experiments to test our hypothesis
Through our research we noticed that we could actually design a solar cell using copper
plates and cuprous oxide plates (Cu2O). The research also suggested that we could actually
design two types of solar panels we called ‘Bottle type solar cell’ and ‘Flat panel solar cell’.
As already suggested, we decided to keep the same area for the solar panel which was 12
cm by 14cm (168cm3) and try to check which solar panel could produce the higher values of
Building a solar cell
2
current and voltage. After selecting the best design, we could perform further activities on
that particular set up. Before we, decided to find out how the solar cell worked.
Theory behind the study
In 1839 Edmund Bequeral, a French physicist, documented the
effect of sunlight on two electrodes in a weak conducting
solution. A small voltage was measured when the setup was
exposed to sunlight. Bequeral and other scientists began to
build more powerful systems using the sun's energy as a power
source for creating electricity, but the knowledge of how to
reliably harness the sun's energy was not understood. Further studies where carried out (See
Appendix 1) and much later efforts were concentrated on the use of semi-conductors. We
tried to follow Bequeral’s steps in constructing our own solar cell.
How do typical solar cells work? Different solar cells use different types of semi-conducting
materials to work (See Appendix 2). In our solar cell the semi-conductor is the cuprous oxide.
In the cuprous oxide there is an energy gap between the electrons that are tightly bound to
the atom, unavailable for conduction, and the electrons that are farther from the atom and
free to move and conduct. Energy from the sun gives the tightly bound electrons enough
energy to bridge the energy gap and move in to the conduction band where they are free to
conduct electricity.
This takes place because sunlight is composed of
photons, which can be thought of as "packets" of
energy (the amount of energy in a photon being
proportional to the frequency of its light). When
photons strike a solar cell, the vast majority are
either reflected or absorbed (some really highenergy photons will blow right through, but they're of
no concern here). When a photon is absorbed, its
energy is transferred to the semiconductor - in
particular, to an electron in an atom of the cell. If
enough energy is transferred, the electron can escape from its normal position associated
with that atom. In the process, the electron causes a hole (i.e., an empty spot where the
electron used to be) to form. Each photon with enough energy will normally free exactly one
electron, and one hole. Note that both electrons and holes are mobile, and as such can be
current carriers.
The electron flow provides the current, and a
voltage. With both current and voltage, we have
power, which is just the product of the two.
To find the efficiency of the solar cell we shall
calculate the incident power of the sun. To find the
incident power from the sun we have to multiply
Building a solar cell
3
the solar constant (1000W/m2) by the area of the solar cell in square meters.
Typically a photovoltaic cell is composed of a thin wafer consisting of an ultra-thin layer of
phosphorus-doped (N-type) silicon on top of a thicker layer of boron-doped (P-type) silicon.
An electrical field is created near the top surface of the cell where these two materials are in
contact, called the P-N junction. Regardless of size, a typical silicon PV cell produces about
0.5 – 0.6 volt DC under open-circuit, no-load conditions. The current (and power) output of a
PV cell depends on its efficiency and size (surface area), and is proportional the intensity of
sunlight striking the surface of the cell. For example, under peak sunlight conditions a typical
commercial PV cell with a surface area of 160 cm2 will produce about 2 watts peak power. If
the sunlight intensity were 40 percent of peak, this cell would produce about 0.8 watts.
Therefore we shall also try to compare our results with those of a commercial cell.
Constructing our project
Solar panel Type 1: Bottle type solar cell
Materials
A sheet of Copper
Crocodile clips
Copper wire
Large plastic bottles (Picture 5)
Some table salt
Tap water
Sheet metal shears
Tools
Picture 1: Plastic bottles used
An electric stove
Ruler
A sand paper or a wire brush
Scissors
Method
1. The first step was to cut the copper in pieces
which were about the size of the burner on the
stove (Picture 2). Then wash the copper plates
with soap to get any oil or grease off of it. Use the
sandpaper or wire brush to thoroughly clean the
copper plates, so that any sulphide or other light
corrosion is removed.
Building a solar cell
Picture 2: Cutting the copper plates to be
burnt
4
2. Place a plate of the cleaned and dried copper on
the burner and turn the burner to its highest
setting. As the copper gets hotter, it will take a
black coating of cupric oxide.
3. When the burner is glowing red-hot, the sheet of
copper will be coated with a black cupric oxide
coat (Picture 3). Let it cook for another half an
hour, so the black coating will be thick. This is
important. Since a thick coating will flake off
nicely, while a thin coat will stay stuck to the
copper. After the half hour cooking, turn off the
Picture 3: The black cupric oxide coat
burner.
4. Leave the hot copper on the burner to cool slowly. As the
copper cools, it shrinks. The black cupric acid also shrinks.
But they shrink at different rates, which make the black cupric
oxide flack off. When the copper has cooled to room
temperature (this takes about 20 minutes), most of the black
oxide will be gone. A light scrubbing with your hands under
running water will remove most of the small bits. Don’t remove
all the black spots by hard scrubbing or by flexing the soft
copper. This might damage the delicate red cuprous oxide
layer we need to make the solar cell work.
5. Cut in the plastic bottle (Picture 4) and inside put a burnt
copper and a new piece of copper of the same size without
touching one another. Attach the alligator clips, to the new
copper plates and to the cuprous oxide coated plates in a
Picture 4: Cut plastic bottle
series circuit. Connect the lead from the last clean copper
plate to the positive terminal of the meter. Connect the lead
from the last cuprous oxide plate to the negative terminal of the meter.
6. Now mix a couple teaspoons of salt into some hot water. Stir the saltwater until all the salt
is dissolve. Then carefully pour the saltwater in the bottles being careful not to get the clip
leads wet. The saltwater should not completely cover the plates-you should live about
2/3cm of plate above the water, so you can move the solar cell around without getting the
clip leads wet.
Building a solar cell
5
Solar panel Type 2: Flat panel solar
cell
Materials (Picture 5)
Cd case
Silicone rubber glue
Copper plates
Copper wire
Salt
Water
Large eyedropper
Crocodile clips
Picture 5: Materials used
Tools
Ruler
Screwdriver
Scissors
Hot plate
Method
Picture 6: Cutting the copper plates
1. The first step to make is a cuprous oxide plate. This is
done with the hot-plate by bringing one side of the
copper plate red-hot as described above in the ‘Bottle
Type solar cell’. This time we sanded one corner,
clean it all the way down the shiny copper using fresh
water. As the copper plates were cut, an end was left
as the negative lead to which the crocodile clips and
wires could be connected.
2. We made the positive plate by cutting a copper
sheeting, a little bit larger than the cuprous oxide
plate. Again as the copper plates were cut, ends were
left to connect the crocodile clips (Picture 6).
Picture 7: Removing some ends of the
case
3. The cd cases were opened and some of casing was removed from the middle using a
screwdriver (Picture 7).
Building a solar cell
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4. The next thing to do is to glue the copper plate to
the cd case. We used plenty of silicone glue so the
saltwater won’t leak.
5. We noticed that the silicone glue doesn’t have to
completely cover all the copper because some of
the copper must eventually have to make contact
with the saltwater (Picture 8).
6. The preceding step was to lay a good size bead of
glue onto the clean copper plate. This layer will be
as an insulator between the copper plate and the
cuprous oxide plate. It must be thick to fill it with
saltwater.
Picture 8: Gluing the copper plates
7. Now the cuprous plate was gently pressed onto the glue. You should press hard to make
sure that the glue seals off and there aren’t any gapes. One has to be careful when
pressing; make sure that the two plates don’t touch.
8. Note that we left a hole so than we can add the saltwater. To make sure that no saltwater
will leak out; make another bead of glue all around the plates.
9. The next step is to add the saltwater with the large eyedropper and fill the cell to the top
of the copper plate. Then make another bead of glue to seal the hole and leave the glue
to dry.
10. Now the flat panel solar cell was ready to be tried out!
Trying out our project: Results and Calculations
When trying out our two solar cells, we found out that the ‘Bottle type solar cell” the voltage in
the shade was 0.105V while in the sun the voltage was 0.134 and a current of approx. 89
micro amps was reached.
Building a solar cell
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This means that the power generated by the ‘Bottle type solar cell’ in the sun was,
P = I . V = 0.000089A x 0.134V = 11.9 micro Watts
Where the power Input = 1000W/m2 x 0.0168m2 = 16.8W and power output = 11.9 micro
Watts
The efficiency of our cell is therefore very low!
When trying out the ‘Flat panel solar cell” the voltage in the shade was 0.023V while in the
sun the voltage was 0.03V and a current of 34 micro amps was reached.
This means that the power generated by the ‘Flat panel type solar cell’ was,
P=
=
=
I.V
0.000034A x 0.03V
1.02 micro Watts
One notes that the ‘Bottle type solar panel’ generates a
higher power output and is more efficient. Therefore we
decided to construct a number of ‘Bottle type solar cells’ (9
cells, see picture aside) to increase the power generated.
We have also decided to connect the cells in series with
each other to try maximize the voltage generated. In the
shade the 9 cells generated 0.825V
Building a solar cell
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When connecting the cells in
series and placed in the sun,
voltage increased to 0.917V
(at times it was greater than
1V!) but the current didn’t
change much. The current
obtained was about 84 micro
amps. This generated a
power of 77 micro Watts in
sunlight.
When connecting the cells in parallel the voltage was very low and the current didn’t increase
much. Actually the experiment had to be repeated various times to obtain any readings.
Conclusions and consideration of results
When thinking of our questions we can say that not all our
hypotheses were correct.
1. We constructed the solar cell and it worked using simple
material found at school and at home.
2. We learnt how solar cells work because they are made of a semi-conductor.
3. The best solar cell was not the one with flat surface but the ‘Bottle type solar cell. It
generated a power output of 77micro Watts. It is not a high value but we believe that if
the experiment was to be repeated in summer the values can be 2 and even 3 times
higher.
4. We noticed that the voltage can be increased when connecting the cells in series and
it increased each time we added a cell but the current was still very low. When
connecting the cells in parallel the current didn’t increase a lot although when
Building a solar cell
9
discussing with the teacher we think it should have increased. Our main problem was
to increase the current – We generated only a very low current.
5. The power of the cell depended on the amount of sun because with no sun, the
voltage and current were reduced. This really reduced the power generated. For this
reason we think that that if the experiment was repeated in summer, when the rays of
the sun are stronger, the readings would be better and also our value for efficiency.
6. Our cell generated less power than a commercial cell because while our cell gave
12microWatts of power (in January), we found that a commercial cell would generate
0.8 Watts in peak sunlight. However we are happy with our results because
commercial cells need a lot of work and manufacturing to be done and our cell was
made up from simple material you find at school and at home.
7. To light a 100W battery we would actually need about 8,000,000 similar! The number
will certainly be reduced if the experiment is performed in summer.
Another problem with this type of cells is that the copper plates tend to become corroded
in the salty water and the performance of the cell reduces with time. We have found out
that after a month the cell generates almost no voltage. However we are happy with our
results since we learnt a lot about solar cells and how to make one that works.
References
Breithaupt, J. (2001). Key science. Physics. Cheltenham: Stanley Thornes
Lister, T and Rensham, J (1991). Understanding Chemistry. Cheltenham: Stanley Thornes
http://www.fsec.ucf.edu/pvt/pvbasics/
http://www.greenenergy.org.uk/pvuk2/technology/types.html
http://www.solarenergy.com/info_history.html
http://www.solarbotics.net/starting/200202_solar_cells/200202_solar_cells.html
http://www.thesolarplan.com/articles/your-own-solar-panel-collector.html
http://sci-toys.com/scitoys/scitoys/echem/echem3.html
http://sci-toys.com/scitoys/scitoys/echem/echem2.html#solarcell
http://www.howstuffworks.com/solar-cell1.htm
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Appendix 1: History of Solar cells.
After Bequerel, much later in 1877, Charles Fritts constructed the first true solar cells (at
least, the first resembling modern cells in that it was made from only solid materials) by using
junctions formed by coating the semiconductor selenium with an ultrathin, nearly transparent
layer of gold. Fritts's devices were very inefficient, transforming less than 1 percent of the
absorbed light into electrical energy, but they were a start.
Substantial improvements in solar cell efficiency had to wait
for a better understanding of the physical principles involved
in their design, provided by Einstein in 1905 and Schottky in
1930. By 1927 another metal semiconductor-junction solar
cell, in this case made of copper and the semiconductor
copper oxide, had been demonstrated. By the 1930s both
the selenium cell and the copper oxide cell were being
employed in light-sensitive devices, such as photometers,
for use in photography. These early solar cells, however, still
had energy conversion efficiencies of less than 1 percent (so
they made fine light sensors, but lousy energy converters).
PV cell by Bell Laboratories
Solar cell efficiency finally saw substantial progress with the development of the first silicon
cell by Russell Ohl in 1941. In the early 1950's, Bell Laboratories was able to build the
world's first PV cell. The small circuit, similar to that of a transistor battery, harnessed and
utilized solar energy for a human advantage-electricity. Scientists were able to capture
energy from the sun's photons and transfer the energy into electricity. The early system used
layers of pure silicon to serve as semiconductors within the photovoltaic cell. Because of the
high expense of these early solar cells, experimentation and use mainly occurred in
laboratory settings. Bell tried to advertise the use of photovoltaics for the "everyday" family,
but using pure silicon as the semiconductor made this feat monetarily difficult for most
people.
In 1954, three other American researchers, G.L. Pearson, Daryl Chapin, and Calvin Fuller,
demonstrated a further-refined silicon solar cell capable of a 6% energy conversion efficiency
(in direct sunlight). By the late 1980s silicon cells, as well as those made of gallium arsenide,
with efficiencies of more than 20% had been fabricated. In 1989 a concentrator solar cell, a
type of device in which sunlight is concentrated onto the cell surface by means of lenses,
achieved an efficiency of 37% thanks to the increased intensity of the collected energy.
Building a solar cell
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Appendix 2: Types of PV cells
Monocrystalline Silicon Cells:
Made using cells saw-cut from a single cylindrical crystal of
silicon, this is the most efficient of the photovoltaic (PV)
technologies. The principle advantage of monocrystalline cells
are their high efficiencies, typically around 15%, although the
manufacturing process required to produce monocrystalline
silicon is complicated, resulting in slightly higher costs than other technologies.
Multicrystalline Silicon Cells:
Made from cells cut from an ingot of melted and recrystallised
silicon. In the manufacturing process, molten silicon is cast into
ingots of polycrystalline silicon, these ingots are then saw-cut into
very thin wafers and assembled into complete cells.
Multicrystalline cells are cheaper to produce than monocrystalline
ones, due to the simpler manufacturing process. However, they
tend to be slightly less efficient, with average efficiencies of
around 12%., creating a granular texture.
Thick-film Silicon:
Another multicrystalline technology where the silicon is deposited in
a continuous process onto a base material giving a fine grained,
sparkling appearance. Like all crystalline PV, this is encapsulated
in a transparent insulating polymer with a tempered glass cover
and usually bound into a strong aluminium frame.
Amorphous Silicon:
Amorphous silicon cells are composed of silicon atoms in a thin
homogenous layer rather than a crystal structure. Amorphous
silicon absorbs light more effectively than crystalline silicon, so the
cells can be thinner. For this reason, amorphous silicon is also
known as a "thin film" PV technology. Amorphous silicon can be
deposited on a wide range of substrates, both rigid and flexible,
Building a solar cell
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which makes it ideal for curved surfaces and "fold-away" modules. Amorphous cells are,
however, less efficient than crystalline based cells, with typical efficiencies of around 6%, but
they are easier and therefore cheaper to produce. Their low cost makes them ideally suited
for many applications where high efficiency is not required and low cost is important.
Other Thin Films:
A number of other promising materials such as cadmium telluride (CdTe) and copper indium
diselenide (CIS) are now being used for PV modules. The attraction of these technologies is
that they can be manufactured by relatively inexpensive industrial processes, certainly in
comparison to crystalline silicon technologies, yet they typically offer higher module
efficiencies than amorphous silicon. New technologies based on the photosynthesis process
are not yet on the market.
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