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Solar Cells Rawa’a Fatayer
Introduction
Energy is very important in our life and can be found in a number of different forms. It can be chemical energy, electrical energy, heat (thermal energy), light (radiant energy), mechanical energy, and nuclear energy,Fossil Fuels - Coal, Oil and Natural Gas
energy.
Energy is defined as: "the ability to do work."
The most common kind of these energies is fossil fuel and oil energy but we also know that this kind of energy going to disappear by 22th century so the scientist started to look about some alternative sources of energy.So they started to think about the solar energy because we all know that the sun is our nearest star and no life will be exist on our planet with out it.
in 1950s they developed a device called solar cell which is depend on solar energy.They first use it on U.S. space satellites.
How they used the solar energy?!
Solar cells are also called photovoltaic cells – or PV cells for short – and can be found on many small appliances, like calculators.
What is the solar cells?
Photovoltaic solar cells are thin silicon disks that convert sunlight into electricity.
When the light incident on the surface of the solar cell as the photoelectric effect there will be an electron which is freely moves to external circuit this external circuit will convert the solar energy to electric energy.
How does it work?
Solar cells are made from kind of material called semiconductors
In these semiconductors there is to bands filled band (valence band) and the band in which electrons are free
to move (conduction band) are separated by a potential difference of about 1 volt.
Hence,light coming in can push an electron from the
valence band into the conduction band if ithas an energy of about 1 electron volt (1 eV).
The electron in the conduction band is freeto move. If it is kept from recombining, it can
give up its energy in an external circuitbefore coming back to the material
Light consists from photons
so the incident energy is equal:
E=h C/λ
To prevent recombination, two different types of doped semiconductor are grown
together to make the solar cell. Pure silicon is grown in a furnace in the presence of silicon
vapor. The silicon vapor is doped with acceptors or donors (p-type and n-type
semiconductors)
Doping of semiconductors
to deposit layers of p-type or n-type material. On occasional lattice sites in p-type semiconductors are atoms or compounds with fewer electrons in the outermost
shell than in the rest of the atom. Thus, there are occasionally “vacancies,” or holes for
electrons, within the lattice of a p-type semiconductor. An n-type semiconductor has
occasional lattice sites occupied by an atom or compound with more electrons in the
outermost shells than in the rest of the atoms.
There are occasionally excess electrons inthe lattice in n-type semiconductors. The
extra electrons can move around in the n-type
material in response to an external potential; in the p-type material, the holes move
around in response to an external potential (an external electric field that is applied). Such materials therefore conduct electricity better than would be guessed from the band gap
Solar cells act in a way similar to the diode, so that current can flow
in only one direction when the cell is exposed to light.
Cells are assembled into modules, which are further assembled into arrays.
A. Material used in solar cells The most popular choice for solar cells is
silicon (Si), with a band gap of 1.1 eV, production cell efficiencies of about 12%,(110-113) and a maximum efficiency of about
15%, and gallium arsenide, with a band gap of 1.4 eV and a maximum efficiency of about
22%.
Types of solar cell
The maximum theoretical efficiency for a single cell is 33%. For multiple cells, the theoretical maximum is 68%. Both of these materials must be grown as single crystals
under very precisely controlled conditions to minimize imperfections, which can cause recombination.
The material gallium arsenide (GaAs) is also very popular for solar cells. Gallium and
arsenic are exactly one atomic higher and lower than silicon, so the system has many
similarities to a silicon-based semiconductor.
Additionally, only very thin films of gallium arsenide need be used since it is so
effective at absorbing light
Silicon, gallium arsenide, and aluminum gallium arsenide have different band gaps. They therefore absorb light of different energies
Crystalline silicon cells are manufactured very carefully. In the original setup, a starter
was dipped into a vat of molten silicon (~ 1400 °C), and a single crystal slowly formed as
the crystal was drawn out over a long period of time. It was essential to the process that
uniformity be maintained.
B. Crystalline silicon cells
Nowadays, the process is more automated and requires less care.
The cells need to be at least 100 µm thick because of problems with absorption; the thickness helps allow the light to be absorbed.
Typical thickness is about 300 µm forsawn silicon, but it can be made as thin as
about 170 µm using wire-cutting techniques. Both silicon and gallium arsenide are easily eaten away by chemical reactions with the holes.
Most current photovoltaic materials are made of a single layer of light absorber. However,
given the differences among solar cells in terms of the energy they absorb, it can be
advantageous to “stage” or layer them. Cells of different band gaps stacked atop one
another are known as multijunction cells.
C. Single and multijunction cells
A multijunction cell functions by absorbing sequentially lower energy light from the incident light. Because there are several band gaps, more energy from the light is absorbed. Here, the band gap for material 1 is greater than that for material 2, which in turn is greater than that for material 3.
Materials that have no crystal structure are classed as amorphous, from the Greek,
meaning “lack of structure.” Amorphous silicon has no crystal structure, and its atoms are
ordered over only a very short distance; small pieces of silicon crystal abut one another at
random orientations in such a way that no long-distance structure exists.
D. Amorphous cells
Amorphous silicon solar cells in thin films exhibit better absorption than pure silicon (40 times as efficiently as crystalline silicon),(104) but because of the many structural defects, they are only about 11% efficient at maximum, and most cells are about 4% to 8% efficient.
Amorphous silicon cells can degrade on exposure to sunlight
Amorphous silicon is much easier to make than grown silicon crystals, and by using
several layers, each set for a different band gap “tuned” to a different part of the spectrum, a greater part of the visible spectrum can be used
However,solar cell efficiency falls if too much material is added
The point-contact solar cell has a textured surface that reduces the reflection of incident
light and a rear-surface mirror. It absorbs 90% of the incident light. Such cells need be
only 100 µm thick
Point-contact cells
Organic solar cells
Gallium indium nitride
Plastic solar cells
Other types of solar cells
Thank you