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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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