Class 12 Physics Semiconductor Electronics Solar cell

Solar cell

Principle–These photo voltaic devices convert the optical radiation into electricity

Circuit

  • When solar light falls on a p-n junction, it generates emf
  • As the solar radiation is incident at the junction, the junction area is kept much larger for more power generation
  • A p Si layer of about 300 * 10-6m is taken. About this a still thin layer of about 0.3 * 10-6 m n Si layer is grown on one side by the process of diffusion
  • The other side of the p-layer is coated with a metal. This servers as a back contact
  • On the top of n Si layer, metallic grid is deposited. This is called front contact
  • The light is incident on the grid from the top

Working

  • The generation of emf by the solar cell, when light falls on, is due to the following three basic processes – (a) generation (b) separation and (c) collection
  • Generation
    • The generation of electron-hole paid due to light with energy hט> Eg close to the junction
  • Separation
    • The separation of electrons and holes due to the electric field of the depletion region
    • The electrons are swept to the n-side and the holes to the p-side
  • Collection

 

  • The electrons reaching the n-side are collected by the front contact and holes reaching the p-side are collected by the back contact
  • Thus, the p-side becomes positive and the n-side becomes negative giving rise to photo voltage
  • When external load is connected, a photo current IL flows through the load

Graph

  • The graph showing the VI characteristics, with V along the X-axis and I along the Y-axis is as given above
  • The graph is indicated in the fourth quadrant as solar cell does not draw current but supplies the same to the load

Application

  • Solar cells are used in power electronic devices in satellites and space vehicles
  • They are also used as power supply in calculators

Criteria for material selection of material for solar cell

  • Band gap between 1.0 and 1.8 eV
  • High optical absorption
  • Electrical conductivity
  • Availability of raw material
  • Cost effective

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