MadSci Network: Engineering

Re: What color of light will let a solar car drive the fastest?

Date: Fri Oct 13 11:42:19 2000
Posted By: Marc Breen, Post-doc/Fellow, Center Bio/Molecular Science and Engineering, U. S. Naval Research Laboratory
Area of science: Engineering
ID: 970847655.Eg


If you look at the solar spectrum, comparing intensity vs. wavelength, it
would form an arc.  In the middle, the highest intensity of light is green,
with lower amounts of red and blue light on the ends.  So a solar cell
tuned to absorb primarily green light would be able to convert a fair
portion of the total sunlight into energy.  However, it's not that
simple.  Some of the higher energy blue light can also be utilized.  You
are probably familiar with atoms and their atomic orbitals of
electrons.  These orbitals overlap and electrons are shared between atoms
to form molecules.  The electrons shared between atoms are in constant
motion.  The most stable state would be when the electrons are dead center
between the nuclei of two atoms.  When the electrons are on the opposite
sides of the nuclei, the two positively charged nuclei repel one
another.  Where as atoms have atomic orbitals, molecules have molecular
orbitals.  A large molecule could have many atoms with many overlapping
atomic orbitals, but we simply the picture by looking at net molecular
orbitals for the whole molecule.  The molecular orbitals involved in
bonding can be broken up into "bonding" and "anti-bonding" orbitals.  When 
all the electrons are in bonding orbitals, the molecule is highly
stable.  If all the electrons were to move into anti-bonding orbitals, the
molecule would blow apart into little pieces.  In our daily lives we don't
deal with atoms and molecules we deal with chunks of material composed of
zillions and zillions of atoms.  To generalize the "bulk" or net properties
of a material we talk about bands of electron orbitals.  The bonding
electrons in a material like silicon can be grouped together into valence
and conduction bands, analogous to the bonding and anti-bonding orbitals of
a molecule.  Electrons in the valence band hold the bulk material
together.  Electrons in the conduction band are free to jump from atom to
atom, thus carrying an electric charge.  This is how electrons "leap frog"
through a conducting wire, sheet of metal, or semiconductor.  Well the
energy that it takes to "excite" an electron from the valence band up into
the conduction band is very specific, and corresponds to a specific color
of light.  This is how electricity is generated in solar energy
panels.  Different compounds will absorb different colors of light, based
on their "bandgap" (the energy difference between the valence and the
conduction bands).  If a material absorbs light of a shorter
wavelength/higher energy (closer to the blue end of the spectrum), the 
energy does not exactly match the bandgap, therefore it can't excite an
electron from the valence to the conduction band to produce
electricity.  However, an electron can absorb high energy light, then loose
some of it's energy by giving off heat until it "relaxes" down to the
energy level of the conduction band.

So, this may have been much more detail than you asked for, but here is the
take home message:

Theoretically, a material designed to absorb green light, could convert all
of that light into electricity, some of the "bluer" light, and none of the
"reder" light.  It turns out that the optimum balance is a little left of
center, closer to the blue light.  In this compromise, you loose a little
of the green light, but you gain more of the light on the blue side of the

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