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