Hi, and thanks for the question
The bulk of this is condensed from the US Department of Energy page on
Photovoltaic physics at
http://www.eere.energy.gov/solar/pv_physics.html
Photovoltaic (PV) materials and devices convert light energy into
electrical energy (Photoelectric Effect), as French physicist Edmond
Becquerel discovered as early as 1839.
Initially, the energy a photovoltaic cell uses comes from the sun.
There, hydrogen nuclei fuse with each other to form helium nuclei and
energy.
Photons
_______
Photons are the energy byproducts of this nuclear reaction in the sun.
They are essentially "packets of energy."
Electrons Absorb Photons
________________________
When light shines on a PV cell, it may be reflected, absorbed, or pass
right through. But only the absorbed light generates electricity. When
photons from the sun hit a photovoltaic cell, they may be absorbed by
an electron. With this extra energy, the electron may become excited
and escape from their normal positions in the atoms of the
semiconductor PV material and become part of the electrical flow, or
current, in an electrical circuit.
The Photovoltaic cell
_____________________
A special electrical property of the PV cell, a "built-in electric
field"?provides the force, or voltage, needed to drive the current
through an external "load," such as a light bulb.
The DOE site describes the process of creating the electric field as follows:
"To induce the built-in electric field within a PV cell, two layers of
somewhat differing semiconductor materials are placed in contact with
one another. One layer is an "n-type" semiconductor with an abundance
of electrons, which have a negative electrical charge. The other layer
is a "p-type" semiconductor with an abundance of "holes," which have a
positive electrical charge.
"Although both materials are electrically neutral, n-type silicon has
excess electrons and p-type silicon has excess holes. Sandwiching
these together creates a p/n junction at their interface, thereby
creating an electric field.
"When n- and p-type silicon come into contact, excess electrons move
from the n-type side to the p-type side. The result is a buildup of
positive charge along the n-type side of the interface and a buildup
of negative charge along the p-type side.
"Because of the flow of electrons and holes, the two semiconductors
behave like a battery, creating an electric field at the surface where
they meet?what we call the p/n junction. The electrical field causes
the electrons to move from the semiconductor toward the negative
surface, where they become available to the electrical circuit. At the
same time, the holes move in the opposite direction, toward the
positive surface, where they await incoming electrons.
The DOE site above has much more detail, including details of the
structure of the silicon atom and how photvoltaic devices and systems
are puit together.
There is another good site with pictures and a concise explanation at
the NASA page :
How do Photovoltaics Work? by Gil Knier
http://science.nasa.gov/headlines/y2002/solarcells.htm
The page at "Solar Cell Principles and Applications" from the
Australian CRC for Renewable Energy at
http://acre.murdoch.edu.au/refiles/pv/text.html
has a well laid out explanation of how the cells work, and also a
detailed look at power output levels from PV cells.
Hope that answers your question
willie-ga
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