Renewables - Beyond Smoke and Mirrors - page 269

Technical Note . : Photovoltaic cells

In the semiconducting material from which PV cells are
made, electrons are stuck onto atoms and cannot move freely
in the material; they are stuck in what is called the valence
band of the material. To get them to move they have to be
shifted up in energy to the conduction band where they can
move through the material and out onto a wire to deliver
electricity and then return to the semiconductor. Where
they leave and where they return to the PV cell is deter-
mined by speci c impurities that are introduced in the

manufacturing process.
There is a minimum amount of energy called the band gap
that has to be delivered to move an electron up into the
conduction band. If the quantum of light (photon) has less
energy than the band gap, the electrons stay stuck. If it has
much more, the electron is excited above the band gap and
the excess energy is lost to heat rather than being converted
to electricity. The band gap in silicon is about electron

volts. The band gap for a single material system that gives
the highest conversion ef

.

electron volts
when the solar spectrum is considered. With silicon only,
the maximum possible ef

ciency is about .

ciency is about %, but nothing

gets near that.
Silicon solar cells made from single crystal material like
that used for computer chips have typical ef ciencies of about

% and are the most expensive. Polycrystalline materials
have ef

ciencies of about % and cost less to make.

Amorphous thin %and

are the least costly of all the silicon-based cells. In the overall
economics the thin

film cells have ef ciencies of about

film cells are the front runners. Other
materials include gallium arsenide, copper indium selenide,
copper indium gallium diselenide, and cadmium telluride.
They have different band gaps, and different ef ciencies.

ciency solar cells are laminates of different
material whose band gaps match different parts of the solar

The highest ef