Jared Cotton
Photovoltaic cells are cells made of semiconductor material like silicon or Cadmium telluride that can
generate direct current (DC) electricity from the photons that the sun gives
off. Many photovoltaic cells can be joined together to form a solar module,
these can in turn be joined together to create a solar panel also called solar
arrays. The larger the solar array, the more electricity that can be
produced.
Silicon has a crystalline structure and is the most common material used in
photovoltaic cells. The cells are often made of two different bits of silicon;
they are normally two different types of silicon, these layers tends to be
between glass and another tough solid to help keep out moisture. On top of the glass is a special anti
reflecting material that helps to absorb light. The silicon needs this because normally it is very shiny
and doesn’t absorb light particularly well; reflected light cannot be used to make electricity. There
are different types of silicon in photovoltaic cells; the type of silicon used depends on the purpose of
the solar panel. Some photovoltaic cells use silicon that is expensive to manufacture, yet it generates
more electricity and is more efficient, while other types of silicon are cheap to make but are less
efficient at capturing the solar energy from the sun. One type of silicon, which is the best quality, is
monocrystalline silicon. This silicon is made by carefully cutting single pieces of silicon from a single
large crystal that has been grown specifically to make solar cells. This type of silicon is almost perfect,
however growing it is a very expensive procedure and a lot of silicon is wasted in saw dust when
cutting the cells. A cheap type of silicon is polycrystalline silicon which is made by pouring molten
silicon into moulds and waiting for it to set. This is easier to mass produce with and cheaper,
however the end product is not as good as monocrystalline silicon.
The solar cell works because the different layers of silicon are given different electrical charges that
cause an electric field to be made between the layers, this is called a junction. The charge is given to
the layers by ‘doping’ them with different elements. To give one layer a positive charge, it might be
doped with boron, while to give a layer a negative charge it might be doped with phosphorus. This
difference in charge is what provides the voltage. This is called the photovoltaic effect which is
defined as the creation of voltage in material that is exposed to sunlight. When the photons from
the sun hit the electrical field in the junction, they cause the electrons in the silicon crystal to move.
If the photons contain enough energy they allow the flow of electrons to move through the electric
field, this is called the current. By placing
contacts at the top and bottom of a photovoltaic
cell you can make the current go into an
external circuit. The electrons will travel around
the circuit in a certain direction because the
electric field will still affect them enough to push
them in said direction. As the electrons travel
through the circuit, their energy is used to power things such as light bulbs or it can be stored in
batteries for later use. The electrons then complete their circuit and return to the solar cell.
There is however, a small problem with solar panels. They create DC electricity which is not what is
used to power our appliances;
the electricity that we use
today is alternating current
(AC). So to change the
electricity from DC to AC a
power inverter must be used
before the electricity can be
used in our homes. You can
also use a battery to store any excess energy that is created that you are not using for a cloudy day
when not much solar power is produced. This use of the battery is also what allows for solar cells to
act as stand-alone power sources in remote areas by storing energy when you are not using much so
that when you need more than you can produce, it can be used from the battery.
In terms of the effectiveness and efficiency of solar panels, there are many variables that need to be
taken into account. For instance what the weather is like or what kind of silicon or other compound
your Photovoltaic cells are made out of. Some of the sun's energy will not be absorbed by the
photovoltaic cells and will pass through the silicon without knocking electrons free. The maximum
amount of energy a cell normally absorbs from the sun is about 25% of the energy available.
However, this figure is normally close to 15%. This happens because not all the photons from the sun
have enough energy to make the electrons move. Most of the photons that come into contact with a
cell have too much or not enough energy to affect the cell. More energy is lost when the electrons
move through the silicon or because the light hits the metal contact grid. A lot of the lost energy
becomes heat.
Another factor that affects the effectiveness of solar cells is shadows. If a shadow covers even a
small amount of the solar panel the entire panel loses productivity. The amount of productivity lost
depends on the size and darkness of the shadow. Heat is also a factor that reduces the amount of
electricity produced. If a solar panel gets too hot then the voltage and current is reduced and the
panels are at risk of damage. The panels need ventilation to prevent them from reaching the danger
zone of over 70 degrees Celsius. Scientists are always trying to increase the efficiency of solar cells,
the record for the most efficient solar panel managed to absorb 40% of the energy available which is
a significant amount. This particular solar cell structure had multiple junctions while and average
solar cell will only have one. To obtain the greatest efficiency you must position the solar panel in a
way as to attract the most amount of sunlight for the longest amount of time, this means getting the
solar panel on the right angle on a roof so that it faces the sun all day at an angle that is optimum for
absorbing photons.
While solar panels and photovoltaic cells are quite recent discoveries, they have made discoveries in
leaps and bounds in the past few years with New Zealand being highly regarded as one of the best
countries investigating the issue. There is still much progress to be made in the world of solar, they
need to be made more efficient and cheaper to make before we can mass produce them to make a
sustainable future. However, when these discoveries are made, the world will have a virtually
unlimited source of energy that has very little pollution. Solar energy could quite possibly develop
into the way of the future; we just don’t quite have the technology to be there yet.
Bibliography:
http://www.solar-facts.com/panels/panel-efficiency.php
http://science.nasa.gov/science-news/science-at-nasa/2002/solarcells/
http://www.wisegeek.com/what-are-photovoltaic-cells.htm
http://en.wikipedia.org/wiki/Photovoltaics
http://www.level.org.nz/energy/renewable-electricity-generation/photovoltaic-systems/
http://en.wikipedia.org/wiki/Solar_cell
http://science.howstuffworks.com/environmental/energy/solar-cell1.htm
www.schoolgen.com/se/pv_sunlight.aspx
www.schoolgen.co.nz/se/pv_tech.aspx
http://www.photovoltaics.org.nz/