A solar cell is essentially a semiconductor diode. The entire wafer of
silicone material is used to produce just one solar cell. This is why
the solar cell has the shape of a wafer, which is a paper-thin square
with rounded edges. As all diodes, solar cells have two contacts:
anode and cathode. Most cells have those contacts made out of
solderable ribbons running accross the cell. If a good ammeter is
connected between the cell contacts, and the cell is exposed to
sunlight, one will observe a current reading. This reading is called
the short-circuit current. Typical cell might give 7A of current in
daylight. It is important to use a good ammmeter, which has a low
internal resistance (here maximum 30mOhm) to properly measure this
current.
When exposed to light, solar cells behave like current
sources. Therefore, short-circuiting the cell with an ammeter allows
for the current to flow between the contacts undisturbed. However,
when the connection is removed, the current cannot flow out of the
cell. In such a case, the inherent solar cell diode turns on and
internally shunts the flow of current. As with every silicon-based
diode, the voltage between anode and cathode of forward-biased diode
is around 0.7V. This voltage, called the open-circuit voltage, can be
measured with a voltmeter connected between the contacts of the cell.
The open-cricuit voltage and the short-circuit current are important
parameters of a solar cell. In fact, the entire I-V characteristic is
normally scanned to describe the electrical properties of the
cell. This is normally done by loading the cell exposed to light with
varying resistive loads and recording the current and voltage
reading. This produces the graph, such as the one shown in the figure.
Size of Solar Cell
The short-circuit current is proportional to the surface area of the
cell. If a cell is cut in half, the current it produces will also be
half of the original value. It is, in fact, possible to cut cells as
long as the integrity of electrical contacts is preserved. Silicon
wafer normally offers the best efficiency in its middle. Therefore in
some applications, where cost is not the main factor (racing solar
cars is one example), solar cells are trimmed to keep the middle
only. More efficient solar array can be built out of cell centers.
The open-circuit voltage does not depend on the surface-area of the
cell. More precisely speaking, two cells of different sizes, made out
of the same material using the same technological process will produce
the same open-circuit voltage when exposed to the same amount of
light.
String of Solar Cells in a Panel
Variations of Sunlight Intensity
Variations of Temperature
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