"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.
Solar Cell Absorbs Invisible Light
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.