| Technology |
What are Solar Cells?
Solar cells are devices which convert solar energy directly into electricity, either by
photovoltic effect, or by converting the solar energy to heat or chemical energy.
The most common form of solar cells is based on the photovoltic (PV) effect in which light
falling on a two layer semi-conductor device produces a photovoltage or potential
difference between the layers. This voltage is capable of driving a current through an
external circuit and thereby producing useful energy. Silicon solar cells are made using
either single crystal wafers, polycrystalline wafers or thin films. Single crystal wafers
are sliced, (approx. 1/3 to ½ of a millimeter thick), from a large single crystal ingot
at around 1400 °C. (see figure 1 (a)). |
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Figure
1 (a) |
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| Polycrystalline wafers are made by casting where molten
silicon is poured into a mould and allowed to set and then sliced into wafers (see
figure 1 (b)). |
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Figure
1 (b) |
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| Amorphous silicon, one of the thin film technologies,
is made by depositing silicon onto a glass substrate from a reactive gas such as Silage
(SiH4) (see figure 1 (c)) |
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Figure
1 (c) |
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Working of Solar Cells
Solar cells consist of two types of material—p-type silicon and n-type silicon. Light
of certain wavelengths is able to ionize the atoms in the silicon and the internal field
produced by the junction and separate some of the positive charges ("holes")
from the negative charges (electrons) within the photovoltic device. The holes are swept
into the positive or p-layer and the electrons are swept into the negative or n-layer.
Although these opposite charges are attracted to each other, most of them can only
recombine by passing through an external circuit outside the material because of the
internal potential energy barrier. Therefore if a circuit is made, (see figure 3)
power can be produced from the cells under illumination, since the free electrons have to
pass through the load to recombine with the positive holes. |
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Figure
3 The Photovoltic Effect in a Solar Cell |
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The amount of power available from a PV device is
determined by the type and area of the material; the intensity of the sunlight; and the
wavelength of the sunlight.
Single crystal silicon solar cells, for example cannot currently convert more than 25% of
the solar energy into electricity, because the radiation in the infrared region of the
electromagnetic spectrum does not have enough energy to separate the positive and negative
charges in the material.
Polycrystalline silicon solar cells have an efficiency of less than 20% at this time and
amorphous silicon cells are presently about 10% efficient, due to higher internal energy
losses than single crystal silicon.
A typical single crystal silicon PV cell of 100 cm2 will produce about 1.5
watts of power at 0.5 volts DC and 3 amps under full summer sunlight (1000Wm2).
The power output of the cell is almost directly proportional to the intensity of the
sunlight. (For example, if the intensity of the sunlight is halved the power will also be
halved). (see figure 4) |
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| Figure 4 Graph showing
current and voltage output of a solar cell at different light intensities. |
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The voltage of a PV cell does not depend on its size,
and remains fairly constant with changing light intensity. However, the current in a
device is almost directly proportional to light intensity and size.
The power output of a solar cell can be increased quite effectively by using a tracking
mechanism to keep the PV device directly facing the sun, or by concentrating the sunlight
using lenses or mirrors. However, there are limits to this process, due to the complexity
of the mechanisms, and the need to cool the cells. The current output is relatively stable
at higher temperatures, but the voltage is reduced, leading to a drop in power as the cell
temperature is increased.
If an application requires more power than can be provided by a s |