HOW SOLAR CELLS WORK?
The last two decades the contribution of solar energy to worlds is the total energy of supply grown up significantly.
Here we are going to see how solar cell or photo-voltaic cell produces electricity.
Energy from the sun is the most abundant and absolutely freely available energy on planet earth. In order to utilize the energy we need help from second most abundant on earth is sand. The sand is converted 99.999% pure silicon crystal to use in solar cells. To achieve this sand has to go through complex purification process.
Sand + carbon — (2000°) – Raw silicon.
The raw silicon lets converted into gases silicon compound form. Next with the hydrogen to get highly purified to poly-crystalline silicon this silicon engage U-shaped and converted into very thin spices called silicon wafers. Silicon wafers is the heart for silicon photo-voltaic cell.
When we analyse the structure of silicon atoms they are bonded together. The electron in the silicon structure also has no freedom of movements.
Assume phosphorous atoms with valance electrons are injected into it. Here one electron is free to move.
In this structure the when the electrons gets sufficient energy they will move freely. They are highly simplified solar cells only using this type of material. When light strikes then electrons will gain photon energy and free to move. This movement of electron is random this results in no current to the load.
To make electron flow unidirectional driving force is needed easy and produces force in PN junction.
Let’s see how PN junction producing driving forces. Similarly to N-type doping you inject boron with three valance electron into silicon. They will be one hole for each atom.
This is called P-type Doping. These two types of doped materials joined together some electrons from N side migrate to the P-region fills the holes available there this is the way the depletion region is formed.
There are no free electrons and holes due to electron migration. N side boundary becomes slightly positive charged P side becomes negative charged. The electric field definitely formed these charges. This electric field produces necessary driving force.
When light strikes PN junction when light strikes the N region of PV cell and penetrates and reaches up to depletion region. This photon energy is sufficient to produce electron hole pair in the depletion region. The electric field in the depletion region drives the electrons and holes out of the depletion region.
The concentration of the N region and holes in the P region is so high the potential difference between will develop between them. As soon as if we connect the load between them electrons will start flowing through the load.
The electrons will recombine with the holes P region after completing the path. In this way solar cell continuously give direct current.
In practical solar cells we can see that top layer N-type is very thin is heavily doped. The P-type is thick lightly doped. This is used to increase the performance of the cell. Here the thickness of the depletion region is much higher.
This means the light striking the electron holes pairs are generated wider area. These results more current generation PU Cell
The advantage is due to thin top layer more light energy can reach the depletion region.
Let’s analyse the structure of solar panel. The solar panel has different layers one of them is the layer of cells.
Let’s see how PU cells are interconnected.
After passing through the fingers the electrons are collected in the bars. Top negative side of cell is connected to the back side of the next cell through copper strips it forms a series connection. When you connect the series connected cells parallel to another cell series we will get the solar panel
Single PV cells produce around 0.5 voltage combination of series and parallel connection the current and voltage values to the usable range. The layer of EVA sheet at both sides of the cell is to protect them from the shocks, vibrations and humidity.
Why are two different appearances of the solar panels?
This is because of the internal structure.
In poly crystalline solar panel multi crystals are randomly oriented. If the chemical process of silicon crystals is taken one step further poly crystalline cells will become mono crystalline cells.
Even though the principles of operation both are the same mono crystalline cells are costlier not widely used. Solar panels in the home have the option to store electricity with the help of battery and solar cell controller.