Quantum Dots Give Solar Cells Mystery Boost in Power

Research shows newly developed solar powered cells may soon outperform conventional solar technology.

Solar power is becoming more typical, but solar cells still aren’t great at collecting most of the available spectrum of sunlight and turning it into electricity, and they can be quite expensive. So experts are regularly looking for ways to improve the efficiency of solar cells and find better ways to make solar cells cheaper.

Magnetic Bacteria Could Build ‘Bio-Computers’

Among them are post doctoral research student Pralay Santra and quantum dotsprofessor Prashant Kamat, of the University of Notre Dame, who are building solar cells with sheets of quantum dots, instead of silicon, which is generally used. Quantum dots are nanometer-sized crystals that glow when excited by an external supply such as ultraviolet light. They are simpler and cheaper to turn into solar cells than silicon because quantum dots don’t require a costly chip producing plant. Dots can also be tuned to create electrons when hit with specific wavelengths of light, ranging across the whole spectrum. Silicon cells only reply 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 producing energy.

Kamat and Santra wanted to improve the effectiveness of quantum dots and make them far more competative with silicon. A solar cell that uses light from a number of wavelengths would do that.

They layered three varieties 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 surprising thing is that the experts still don’t grasp exactly where the power boost came from and why it was so high. It was a complete mystery. What was happening? Santra and Kamat record in their study results that they aren’t sure. It may be that some electrons from the dots tuned to the shorter wavelengths of light are cascading directly to the substrate the dots are sitting on, which stimulates extra current. It could also be a transfer of light energy rather than the electrons, with some dots providing energy to others close to them. It’s possible that both mechanisms are involved.

What’s significant is the fact that the solar cells made of quantum dots could be much cheaper than those manufactured from silicon. Quantum dots may also be fashioned into shapes and spread on surfaces — Kamat and a group of scientists at the University of Waterloo in Canada designed a solar paint of the dots in 2011. Kamat said in addition to the lower cost of production, a boost in effectiveness to the five or six percent range would make them a good option. That’s why quantum dots are touted as the next generation of solar cells. The report can be found in the Journal of the American Chemical Society.

Comments are closed.