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.
The path of the
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
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 region in the solar
cell where the n-type and p-type Si layers meet is called the
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
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
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
P = V ×
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.
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.