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Introduction to Solar energy

An introduction to basic concepts of solar energy is presented. The presentation consist of the basic concepts including but not limited to fermilevel,valence and conduction band,energy level etc. will be introduced to reduce the fear factor from the students.

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Introduction to Solar energy

  1. 1. Dr. Mrinmoy Majumder Topic of Renewable Energy Course of MTech in Hydroinformatics Engg. National Institute of Technology Agartala More such tutorial in http://www.baipatra.ws
  2. 2. Photon is a packet of energy from sun This extra energy can dislodge an electron from the valence band and transfer it to the conduction band. This free electron can create a current when connected by an external circuit.
  3. 3. Valence band is where the number of electrons in the orbit is sufficient as per the level of energy associated with the band Conduction band is where the number of electrons are loosely bound and can be thrown out or if connected create electricity if connected to a circuit. There are free energy or spaces at the conduction band whereas in valence band there is no scope of free energy.
  4. 4. Semi-conductor materials have four bonds where the outer energy-level stores four electrons. Example of semiconductor materials : Silicon(Si), Cadmium Sulphide(CdS), Gallium Arsenide(GaAs). Semiconductor can be of two types :Extrinsic and Intrinsic.
  5. 5. • Pure Semiconductor no doping at low temperature • Impure Semiconductor with doping at high temperature • Extrinsic semiconductors are semiconductors that are doped with specific impurities. • The impurity modifies the electrical properties of the semiconductor • While adding impurities, a small amount of the suitable impurity is added to pure material, increasing its conductivity by many times. • Extrinsic semiconductors are also called impurity semiconductors or doped semiconductors. The process of adding impurities deliberately is termed as doping and the atoms that are used as an impurity are termed as dopants.
  6. 6. While doping tetravalent atoms such as Si or Ge, two types of dopants are used, and they are: • Pentavalent atoms: Atoms with valency 5; such as Arsenic (As), Phosphorous (Pi), Antimony (Sb), etc. • Trivalent atoms: Atoms with valency 3; such as Indium (In), Aluminium (Al), Boron (B), etc. • The reason behind using these dopants is to have similar-sized atoms as the pure semiconductor. Both Silicon and Germanium atoms belong to the fourth group in the periodic table. Hence, the choice of dopants from the third and fifth group is more viable. This ensures that the size of the atoms is not very different from the fourth group. Therefore, the trivalent and pentavalent choices. These dopants give rise to two types of semiconductors as follows: • n-type semiconductors • p-type semiconductors
  7. 7. Photons : tiny capsule of energy Sunlight consist of photons Number of photons in solar radiation depends on : 1)Intensity of Solar Radiation 2)Energy content on the wavelength band
  8. 8. Solar Spectrum constitutes : 1)Ultraviolet(UV) : Wavelength < 0.4µm : 9% irradiance(i) 2)Visible (V) : 0.4< Wavelength <0.7µm : 45% i 3)Infrared (IR) Region >0.7µm : 46% i 98% of solar energy is confined within 0.25 to 2.5µm
  9. 9. • Far end of the infrared region has major part of irradiance(greater than 1.15µm) • This part remain unutilized by solar cell as it resides in the invisible range of sunlight • Resulting in low efficiency of the solar cells
  10. 10.  Level of solar intensity before entering the earth’s atmosphere : 1367 W/𝑚2  This is known as Solar Constant or Air Mass Zero (AM-0) condition  AM-1 condition is known as after crossing earth’s atmosphere :1070 W/𝑚2  AM-2 condition is known as after crossing earth’s atmosphere :750 W/𝑚2  Distribution of Extra Terrestrial Solar Energy(AM-0) within UV,V and IR region of solar spectrum is 88,656,623 W/𝑚2 i.e. 6.4,48,45.6% of percentage carries the energy respectively  When photon impinge a solar cell made of silicon it absorb the energy in the visible region and dislodge an electron in the outer or conductance orbit.  But for silicon, band gap is 1.1eV whereas photon energies vary in the visible region, from 1.8 to 3 eV. As a result, high energy photons are not suitable for photovoltaic current production.
  11. 11. The level from which carriers can move to the next orbit becoming “charge carriers”. Fermi level exist in the equi-distant level from Ec and Ev for intrinsic semiconductor For extrinsic semiconductor fermi level exist nearer to either Ec or Ev depending upon the type of impurities with which it was doped. For donor impurities(n-type) Ef lies nearer to Ec and for acceptor(p-type ) impurities Ef is positioned near to Ev
  12. 12.  Ed represents level of electrons from donor impurities  Ea represents the level of excess holes from acceptor impurities  kT or Thermal energy defines the energy difference between Ec and Ed and Ea and Ev to excite the electrons  k is the Boltzmann’s constant = 1.38x10-23 J/K  T is the absolute temperature in kelvin  At thermal equilibrium,  𝑛 = 𝑁𝑐 exp 𝐸 𝑓−𝐸 𝑐 𝑘𝑇 (n = number of electrons per unit volume of crystal in conduction band and Nc is effective density of states in conduction band)  𝑛 = 𝑁𝑣 exp 𝐸 𝑣−𝐸 𝑓 𝑘𝑇 (p = number of holes per unit volume of crystal in valence band and Nv is effective density of holes in valence band)

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