Explain how a voltage difference is generated in a p-n junction when it is used in a photovoltaic solar cell.

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written 3.5 years ago by

teamques10 ★ 68k

The working principle of all today’s solar cells is essentially the same. It is based on thephotovoltaic effect.
In general, the photovoltaic effect means the generation of a potential difference at the junction of two different materials in response to visible light or other radiation.
The basic processes behind the photovoltaic effect are:
1. Generation of the charge carriers due to the absorption of photons in the materials that form the junction.
2. Subsequent separation of thephoto-generated charge carriers in the junction.
3. Collection of the photo-generated charge carriers at the terminals of the junction.
Thus, the main condition for a p-n junction to generate a potential difference in illumination. And it’s important to know how thep-n junction acts under illumination.
When ap-n junction is illuminated the additional electron-hole pairs are generated in the semiconductor due to the absorption of photons and heat. The concentration of minority carriers (electrons in thep-type region and holes in then-type region) strongly increases.
This increase in the concentration of minority carriers leads to the flow of the minority carriers across the depletion region into the qausi-neutral regions. Electrons flow from thep-type into then-type region and holes from then-type into thep-type region.
The flow of the photo-generated carriers causes the so-calledphotogeneration current,Jph, which adds to the thermal-generation current,Jge.
When no external contact between then-type and thep-type regions is established, which means that the junction is in the open-circuit condition, no net current can flow inside thep-n junction.
When a load is connected between the electrodes of the illuminatedp-n junction, only a fraction of the photo-generated current will flow through the external circuit.
The electrochemical potential difference between then-type andp-type regions will be lowered by a voltage drop over the load. This in turn lowers the electrostatic potential over the depletion region which results in an increase of the recombination current.
The net current flowing through the load is determined as the sum of the photo and thermal generation currents and the recombination current.

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