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Infrared Solar Cells
Name: Charlie W.
Status: student
Grade: 9-12
Location: N/A
Country: N/A
Date: 1/17/2005
Question:
Would it be possible to make a solar cell the could
absorb in the infrared range? Like, say, the frequency that iron makes
when it is heated to 400 degrees? I know they have things that can
detect such temperatures but can they actually receive energy from it?
Solar cells can easily detect objects as cool as human beings, so detecting
iron at 400 F is easy. In order to detect an object, the cell must absorb
some energy emitted by the body. The problem with using such cells for the
production of electricity are economic and practical. Such cells are quite
expensive. They are also rather inefficient; currently around 10% of the
solar energy striking them is converted into electricity.
The efficiency is even lower in the infrared. This is because the
individual photons which actually interact with the solar cell each have an
energy proportional to the frequency of the light. Infrared light has a
long wavelength and a lower frequency so the photons are less energetic and
so less effective in knocking out electrons in the semiconductor.
They do, however, receive energy, even from relatively cool objects.
Best, Dick Plano...
Yes, it is possible to make an infrared solar cell, but it is difficult and
impracticable so far.
It is technologically not easy to get electric power from light. Good
quality practical cells are made of silicon materials engineered in rather
sophisticated ways. Ordinarily, materials simply absorb light and get warm.
It is quite a deal to "trick" the light to generate voltage.
Now, infrared radiation is even lower in energy and conventional materials
simply do not work. There has been some progress at using "quantum dots" to
extract energy from infrared light. On the other hand, some researchers try
using more even exotic semiconductor materials (requiring many layers) to
make use of the lower energy photons.
Bob Erck
Charlie-
400 degrees C + (0degreesC = 273degreesK) = 673degreesK
room temperature = 300degreesK
ratio = 673/300 = 2.24.
Thermodynamically, you could make some kind of energy source from this
temperature difference.
But for solar cells, the ratio usually needs to be bigger.
Normal solar cells have
visible_light / room-temp-IR
band-gap 1 eV / 0.025eV = 40
color-temp 6000degK / 300degK = 20
Anyway, something over 10:1.
673K/300K * 0.025eV = 0.056eV
It is possible to find a semiconductor with 0.050 eV band-gap that would
absorb the light and make photo-excited carriers.
But this semiconductor would also be awash in thermally excited carriers,
both electrons and holes, at room temperature.
These carriers would short out the photo-current.
Your cell would need to be cooled to somewhere below liquid nitrogen
temperature to be able to work.
In space, where you often have a dark, cold, starry sky to passively
radiate heat into, it might be an option.
But there are other kinds of heat engine which I think would be easier to
try there.
Jim Swenson
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Update: February 2012
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