Solar Cells

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Solar Cells

Typical Solar Cell I-V Curve

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.
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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

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Variations of Sunlight Intensity


Variations of Temperature


File translated from TEX by TTH, version 4.03.
On 2 Dec 2012, 07:04.