Science of
Solar Energy
The
The photoelectric effect
In 1905, Albert Einstein explained
how light energy travels in tiny wave packets
he called das Lichtquant, or ‘the light
quantum’. These wave packets
were later termed photons.
The photoelectric effect
explains how electrons
are released from
materials when they
are exposed to photons
of sufficient energy. In
metals, electrons are
only ejected from the
surface if the photon
has enough energy to
overcome the work
function (ο̸).
Photons
Photovoltaic cells
Photovoltaic (PV) cells use a process closely related to the
photoelectric effect to generate electricity from sunlight.
Photon energy required to free an electron
Energy
t
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Vis
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Band gap
of silicon
Conduction band
Photon
Frequency
Photons have enough energy to
free electrons from silicon
Band gap
PV cells are made from a semi-conductor material
(most commonly silicon). When a PV cell is exposed
to photons of sufficient energy, electron energy levels
are increased from the valence band to the conduction
band, rather than being ejected from the material.
Energy
Photons don’t have enough energy
to free electrons from silicon
Valence band
Once in the conduction band, the electrons can be
harnessed to generate electricity.
Silicom atom
Photovoltaic cell
detailed cross-section
Inside a Photovoltaic Cell
NEW ZEALAND CURRICULUM LEVELS 6–8. Copyright © 2010 Genesis Energy.
1 A typical PV cell is a sandwich
of two slightly different layers
of silicon. Pure silicon is a poor
conductor of electrons, so
impurities are carefully added to
each layer of silicon in a process
called doping.
2 When the layers of silicon are
placed together, a positive–
negative (p–n) junction is formed.
The p–n junction results in a
permanent potential difference,
called built-in voltage. The builtin voltage creates an electric field
across the p–n junction.
1
Fr
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to
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Electric
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3 When the energy in a photon frees an electron near the junction,
the electron is moved, in one direction only, by the force of the
electric field to the top of the PV cell.
Something
4 The electrons flow through
contacts on the surface of the
cell and around a circuit before
returning to the back of the cell.
The electrons transfer their
energy to electrical devices as
they move around the circuit.
What other materials are used to
make photovoltaic cells?
For more
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