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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 investment.

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.

  

 

PV-Photovoltaic Principles

What Is a Photovoltaic Effect?

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 300,000 kilometers/sec.

 

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 cells.

 

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.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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Nucleus = +14 Protons

-14 Electrons

Basic Solar Cell Structure

Photon

e-

Sun or other applicable light source

Type n

Type p

 

L

Current

Junction

Voltage

Cell electrode

L= Electrical Load

0.4 to 0.7 volts

Potential barrier

Material type p is made with Boron atoms

Material n is made with

Phosphorus atoms

Energy Bands in a Silicon Atom

Valence Band and Third Band : This band has 4 electrons plus 14 vacant electron positions available

Energy Bands

Second Band:  8 electrons, this is a full band, no vacant electron positions are available

The yellow bands are the prohibited barriers where electrons can not orbit.

First Band: 2 electrons in this band

Conduction Band: when the electrons absorb sufficient energy they jump to the conduction

band