Solar power is becoming more common, but solar cells still aren't great at harvesting all of the available spectrum of sunlight and turning it into electricity, and they can be expensive. So researchers are continually looking for ways to improve the efficiency of solar cells and find better ways to make solar cells cheaply.
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Among them are post doctoral research student Pralay Santra and professor Prashant Kamat, of the University of Notre Dame, who are building solar cells with layers of quantum dots, instead of silicon, which is typically used. Quantum dots are nanometer-sized crystals that glow when stimulated by an external source such as ultraviolet light. They are simpler and cheaper to turn into solar cells than silicon because quantum dots don't require an expensive chip manufacturing plant. Dots can also be tuned to generate electrons when hit with specific wavelengths of light, ranging across the whole spectrum. Silicon cells only respond to reddish and near-infrared light waves.
"In typical photovoltaics blue light has more energy (than red light) which gets converted to heat," Kamat told Discovery News. That heat is useless for generating power.
Kamat and Santra wanted to improve the efficiency of quantum dots and make them more competitive with silicon. A solar cell that uses light from several wavelengths would do that.
They layered three types of quantum dots made of a mix of cadmium, sulfur and selenium and tuned them to respond to green, orange, and red light.
First, they built a two-layered cell with orange and red quantum dots and calculated the efficiency would be 2.27 percent. But it was 3.2 percent. Next they added a layer of green dots. They calculated an efficiency of 1.87 percent but got 3.0.
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The strange thing is that the scientists still don't understand where the power boost came from and why it was so high. It was a total mystery. What was happening? Santra and Kamat write in their research results that they aren't sure. It could be that some electrons from the dots tuned to the shorter wavelengths of light are cascading down to the substrate the dots are sitting on, which generates extra current. It could also be a transfer of light energy rather than the electrons, with some dots sending energy to others around them. It's possible that both mechanisms are involved.
What's important is that the solar cells made of quantum dots could be much cheaper than those made of silicon. Quantum dots can also be made into shapes and spread on surfaces - Kamat and a group of scientists at the University of Waterloo in Canada made a solar paint of the dots in 2011. Kamat said coupled with the lower cost of manufacturing, a boost in efficiency to the five or six percent range would make them a good alternative. That's why quantum dots are touted as the next generation of solar cells.
The researchers reported their findings in the Journal of the American Chemical Society.
Credit: University of Utah / Technology Venture Development