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Photovoltaic Cells (Solar Cells), How They Work

The photovoltaic cell (PV cell) offers a  limitless and environmentally friendly source of electricity. Also called a solar cell,  the PV cell is able to create electricity directly from photons.



Solar Cell Structure:


Photovoltaic Cell Diagram




The path of the photon.

After a photon makes it's way through the encapsulate it encounters the antireflective layer. The antireflective layer channels the photon into the lower layers of the solar cell. Click on the following link if you would like to learn about our novel room temperature wet chemical growth antireflective layer (RTWCG - AR).


Once the photon passes the AR coating, it will either hit the silicon surface or the contact grid metallization. The metallization, being opaque, lowers the number of photons reaching the Si surface. The contact grid must be large enough to collect electrons yet cover as little of the solar cell's surface, allowing more photons to penetrate.


A photon causes the photoelectric effect.

The photon's energy transfers to the valance electron of an atom in the n-type Si layer. That energy allows the valance electron to escape its orbit leaving behind a hole. In the n-type silicon layer, the free electrons are called majority carriers whereas the holes are called minority carriers. As the term "carrier" implies, both are able to move throughout the silicon layer, and so are said to be mobile. Inversely, in the p-type Si layer, electrons are termed  minority carriers and holes are termed majority carriers, and of course are also mobile.


The pn-junction.

The region in the solar cell where the n-type and p-type Si layers meet is called the pn-junction.


As you may have already guessed, the p-type Si layer contains more positive charges, called holes, and the n-type Si layer contains more negative charges, or electrons. When p-type and n-type materials are placed in contact with each other, current will flow readily in one direction (forward biased) but not in the other (reverse biased).


An interesting interaction occurs at the pn-junction of a darkened photovoltaic cell. Extra valance electrons in the n-type layer move into the p-type layer filling the holes in the p-type layer forming what is called a depletion zone. The depletion zone does not contain any mobile positive or negative charges. Moreover, this zone keeps other charges from the p and n-type layers from moving across it.


So, to recap, a region depleted of carriers is left around the junction, and a small electrical imbalance exists inside the solar cell. This electrical imbalance amounts to about 0.6 to 0.7 volts. So due to the pn-junction, a built in electric field is always present across the solar cell.


The pn-junction of a solar cell.



P = V I

When photons hit the solar cell, freed electrons (-) attempt to unite with holes on the p-type layer. The pn-junction, a one-way road, only allows the electrons to move in one direction. If we provide an external conductive path, electrons will flow through this path to their original (p-type) side to unite with holes.


The electron flow provides the current ( I ), and the cell's electric field causes a voltage ( V ). With both current and voltage, we have power ( P ), which is just the product of the two. Therefore, when an external load (such as an electric bulb) is connected between the front and back contacts, electricity flows in the cell, working for us along the way.


Solar Cell Diagram






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