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Solar Energy, Solar Cells Photovoltaic Principles One
Solar Photovoltaic Energy
Solar energy, together with wind and geothermal are the most economically exploited
alternative energies, although bio diesel fuels are also getting more attention and
Solar energy is used in two main modes, photovoltaic and heating that uses the infrared
thermal energy of the sunlight, this section will deal only with photovoltaic effect
of the sun produced when sun light falls over certain materials called photovoltaic
materials. Photovoltaic materials are used for the manufacture of photovoltaic solar
cells or for short solar cells, exist several types of solar cells with different
properties, efficiencies and cost.
The first known practical photovoltaic cell was developed in 1954 in the USA Bell
Telephone laboratories, it was a silicon cell produced from a silicon wafer, it was
observed that exposing it to light produce a voltage. Since this discovery tremendous
improvements in materials and manufacturing efficiencies have been achieved, this
has resulted in great cost reductions not imaginable only few years ago.
A photovoltaic effect is produced when a photon hits some kind of materials called
photovoltaic materials producing the displacement of one corresponding electron.
When electrodes are connected between the negative and positive semiconductor materials
and these two materials are connected via a resistor a current is produced, in a
short circuit condition maximum current is obtained but it will damage the cell.
What Is a Photon?
Photon is the minimum unit of light, it can exhibit both wave radiation or mass characteristics
under certain conditions, this is the reason why a photon is normally represented
by a wave line. The photons have no mass but they can transfer a high level of energy
in the order of 4×10–19 joules, because they are a unit of light they travel at the
known speed of light, the speed of light in space is 299,792,458 m/sec or approximately
What Is a Electron?
An electron is an atomic particle that is present in atoms in all materials and elements,
it is the unit of the electrical negative charge equal to approximately1.602×10−19
coulombs. The electron mass at rest is very small and is equal to 9.1096 X 10-31
kg. The electrons rotate in orbits around the nucleus in a similar manner as planets
around the Sun. In accordance with quantum physics the electrons around the nucleus
are distributed in specific numbers in specific concentric rings or layers.
A silicon atom is depicted below, showing 14 electrons (negative charges) and a nucleus
with 14 protons (positive charges)
The distribution of electrons in the atomic layers obey this rule:
X electrons in orbit N = 2 X N2
In the first orbit = 2
In the second orbit = 8
In the third orbit = 18
In the fourth orbit = 32 electrons
The orbits are subdivided in more levels but we are not going to expand into it,
The total number of electrons in an atom determines his atomic number which is specific
for each element, as example the silicon has only 14 electrons with 2 in the first
orbit, 8 on the second and only 4 in the third, this left the third orbit with a
deficit of 14 electrons, in other words this third orbit should be completed with
18 electrons in accordance with atomic physics, but instead it has only 4. Thanks
to these uncompleted orbits, elements like silicon exhibit special properties as
conductors and semiconductors making possible the solid state electronics and solar
The most outer electrons orbit with respect to the nucleus is called the valence
band and it is the band or orbit where the electrons can be more easily removed or
displaced if an electrical field, heat or light is applied, when the electrons receive
sufficient energy from any of these energy sources they can move to a higher energy
band where they can more easily escape their atom influence, this high energy band
is now called the conduction band. The valence band is the responsible to determine
if an element will behave as conductor, semiconductor or insulator. In it pure refined
state the silicon is a crystal, meaning that his molecular structure follows a repetitive
pattern as a lattice, due to this disposition each atom will have a neighbor above,
down and on both sides.
The figure shows that the electron orbits are secluded by the energy bands, they
can not occupy arbitrary energy levels, between each energy levels are the prohibited
bands, in these layers electrons are not permitted.