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.

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. 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. 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. 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. • 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. 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. 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. 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. • 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.  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. 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.  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)