The information
presented in these two lectures is accurate.
The lecture by John O'Donnell covers solar
thermal electricity.
The other lecture, delivered by Scott Elrod,
is about concentrating photovoltaics (CPV), which is a
technology that is similar to light-emitting diodes (LEDs).
Solar Thermal
Click here
to view the solar thermal video lecture without ITunes.
That page provides the lecture slides in PDF format.
The slides for the solar thermal lecture can also be downloaded
in slide show format
here
(can be opened with
Open Office).
III-V CPV
Regarding the question that was asked
35 minutes and 50 seconds (35:50) into the CPV video,
the answer from the audience is correct.
The question was: Why does silicon degrade so much with heat?
Scott Elrod, who works with III-V materials instead of silicon,
said he did not know why silicon is such
an inferior material. He wondered if it had to do with
“kT versus band gap”. Someone from the audience then mentioned
it was simply the materials science property of silicon
to do that.
The relevance of kT versus band gap is as follows:
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“If the photon energy is much larger
than the bandgap energy…then the
excited electron…has to lose the
extra energy…to reach thermal equilibrium.
The extra energy…is lost to lattice vibrations
as heat” [ 3 ]
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By generating more heat than other materials,
the capacity to generate even more heat is diminished.
A bigger problem for silicon is that it is an
indirect
semiconductor instead of a
direct
semiconductor. That is a critical materials science
property. Direct semiconductors can use photons
directly, while indirect semiconductors can only use
photons that are first converted to quantum heat (phonons).
The indirect materials like silicon are only suitable for cooler temperatures.
The III-V semiconductors, on the other hand,
are direct semiconductors, suitable for more uses
including higher temperatures.
In photovoltaics, sunlight separates electrons from atoms,
and your external wire helps “recombine” them.
Some electrons recombine prematurely (internal absorption),
not flowing through your external circuit (not generating
electricity). In terms of LEDs, which work in reverse
but with these same characteristics, the premature recombination
is called radiative recombination:
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“In indirect-gap
semiconductors…radiative transitions are mediated
by phonons…
Since phonons are more abundant at high temperatures,
radiative recombination (mediated by the absorption
of a phonon) can increase with temperature.” [ 4 ]
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1.
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Sidney Soclof,
"Optoelectronic Devices",
in Richard C. Dorf, ed.,
The Engineering Handbook,
2nd ed.,
p. 119-17.
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2.
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Winston, Minano and Benitez,
Nonimaging Optics,
p. 363-364.
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3.
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S. O. Kasap,
Principles of Electronic Materials and Devices,
3rd ed.,
p. 427.
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4.
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E. Fred Schubert,
Light-Emitting Diodes,
2nd ed.,
p. 38.
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