Electrical and Electronic Measurements& Instrumentation
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Photovoltaic Cell
Solar cells are running on junction effect principle. To understand
junction effect, we should understand n-type and p-type material. Doping process is needed
to obtain n-type or p-type material. Doping means inserting another atom into the bulk
crystal. Consider silicon crystal: each silicon atom has four electrons in its valance band and
these electrons make bonds with other Silicon atom. You can see the silicon crystal in the left
side with valance electrons of each Si atom. Note that we call that structure as crystal since
all Si atoms are perfectly aligned. We can convert this structure in to n-type or p-type by
doping different atoms. For example let’s dope it by boron. Boron atom has 3 electrons in its
valance band. When we insert B atom instead of a Si atom, one bond between B atom and a
Si atom will be very weak. To complete the prefect symettry in this structer, crystal willbe
aimed to catch an external electron. As you can see an electron is missing since B atom has 3
electron in its
valence band. This missing bond can be treaeted a
positively charged particle called ‘hole’. This material is called p-type material. What if we
dope Phosphoroous atom instead of Boron atom? Phosphorous atom has 5 electrons in its
valance
band.
When P atom is inserted into the Si lattice, 4 electrons will be able
make bond with neighbour Si atoms. However 5th electron wil be hanged on. So, it will be in
an energy level that very close to conduction band since it will be nearly free. This nearly free
electron can easly leave P atom with a small thermal energy. Note that there is an extra
electron in this new structure. So we call this new material n-type material. In contrast to ptype material, n-type material has a tendency to give electrons. Consequaently we have two
types of maerials. One wants to give electrons and the other wants to receive electrons. We
can create a p-n junction by bringing them together.
Electrical and Electronic Measurements& Instrumentation
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p-n junction
When we bring p-type and n-type material together, a diffusion occurs on the surface
between them. Electrons starts to diffuse from n-type to p-type. Similarly, holes diffuses from
p-type region to n-type region. This diffusion creates aelectron-hole free region in a very
short distance at the interface region. This thin layer is called depletion region.
You can see the diffusion in depleti
on in figure. Blue
line indicates the electron concentration while red line indicate hole concentration trough
semiconductor material. As you see there is an electric field from the n-side to the p-side of
the depletion region. Since the electrons are negative charges this electric field applies a force
to an electron entering the depletion region. Any electron generated by sun light in the
vicinity of the depletion region may pass to the n-side of the junction very easily. If we
connect a wire or any load between the ends of n-type and p-type region with metal contacts,
this electron will flow to the p-type through this external load. So we need an external energy
to create this current: something should energize the electrons in the p-type region to enter
Electrical and Electronic Measurements& Instrumentation
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depletion region. Solar radiation is an excellent energy source to do this job.
The solar cell type explained above is the example of first generation, wafer based Crystal
Silicon solar cells. There are some differences in the structure of other solar cell types. But
the basic principle is the same: some kind of p-n junction (or similar potential energy profile)
has to be used to conver the solar radiation to electrical energy