The operation of solar cells is based on the formation of a junction. The important feature of all junctions is that they contain a strong electric field. To illustrate how this field comes about, let us imagine the hypothetical situation where the p-n junction is formed by joining together two pieces of semiconductor, one p-type and the other n-type. In separation, there is electron surplus in the n-type material and hole surplus in the p-type. When the two pieces are brought into contact, electrons from the n region near the interface diffuse into the p side, leaving behind a layer which is positively charged by the donors. Similarly, holes diffuse in the opposite direction, leaving behind a negatively charged layer stripped of holes. The resulting junction region then contains practically no mobile charge carriers, and the fixed charges of the dopant atoms create a potential barrier acting against a further flow of electrons and holes. Note that the electric field in the junction pulls the electrons and holes in opposite directions.
The potential barrier of a junction permits the flow of electric current in only one direction - the junction acts as a rectifier, or diode. This can be seen in our example where electrons can only flow from the p region to the n region, and holes can only flow in the opposite direction. Electric current, which is the sum of the two, can therefore flow only from the p-side to the n-side of the junction (remember that it is defined as the direction of flow of the positive carriers!). The I-V characteristic of a diode is shown below:
To see how a diode under illumination becomes a solar cell, we must first consider how light is absorbed by a semiconductor.
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