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Name: Hyethug
Status: Student
Age: 6-8
Location: CA 
Country: United States
Date: April 2006


Question:
I did an experiment on how the temperature affects the current, voltage and power of a solar cell and my results showed me that the voltage and wattage went down and higher temperatures and the current increased at the higher temperatures. So my question is why did the current increase?



Replies:
Not sure. But an idea: your solar cells were silicon and you were illuminating with an incandescent lamp.

Silicon, unfortuantely for solar-cell performance, has an indirect band-gap. That means it cannot simply absorb all available photons greater than its band-gap energy.

It only absorbs a photon when there is a phonon, a quantum of heat and lattice vibration, in the right place to help (indirect absorbtion), or when the photon is of considerably higher energy and can activate a higher-energy direct transition (direct absorbtion). Direct absorbtion is fast, occuring within 1 micron or less, but it only works for higher-energy light such as blue. Indirect absorbtion is slower and is variable, so an 0.9eV IR photon can reach 10,100, or 1000 microns into the silicon, depending on temperature.

Incandescent lamps are known for producing more IR than visible light. Some of this IR sails right thru the solar cell layers, so deep into the chip that, when it is finally absorbed, the resulting electron-hole pair might decay before wandering over to the diode junction near the front surface. When that happens it only makes heat, no electric current. Raising the temperature can increase photocurrent at least two ways:

1) more phonons, so more indirect absorbtion of far-IR photons occurs within working range of the junction.
2) reduced energy of direct transitions along with the reduced indirect band-gap, so the strong fast direct absorbtion reaches farther towards the intense IR end of the lamp's spectrum. There might be other ways, too.

Solar cells in a direct-gap semiconductor such as GaAs (Gallium Arsenide) should have less of these effects, have nearer to 100% quantum efficiency, and behave more like constant-current sources proportional to the illumination intensity and independent of temperature. In GaAs the absorbed visible & near-IR light all produces carrier-pairs close to the surface, and the transmitted far-IR light makes none, and the transition range between them is very narrow. However most of us will not have GaAs cells available.

Changing your lamp to get a narrow band of shorter wavelengths, such as an ultrabright LED of green, should help too. Even a red LED will work better, if it is a very bright one up close. Using several should help too.

Jim Swenson



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