Stanford research may up solar cell efficiency

Quantum dots might be small, but they could have a big impact on the efficiency of photovoltaics.

Research presented Feb. 20 by Stanford University chemical engineering Professor Stacey Bent at the annual American Association for the Advancement of Science meeting in Washington, D.C. showed that a single layer—less than a nanometer thick—of quantum dots tripled the solar efficiency of the tested cell.

Quantum dots, in principle, can reach higher efficiencies than traditional photovoltaics, which are limited by the fixed energy level they can absorb. Quantum dots, however, can be fine-tuned to absorb different energy levels, Bent said.

“The thing that’s nice about the quantum dots is you can choose what spectrum by the size,” she said. Different sizes of quantum dots correlate to different areas of the solar spectrum, she added. Smaller quantum dots respond to higher-energy light levels—bluer lights—while larger quantum dots respond to lower-energy light levels.

For the subset of molecules studied, the research group found that they had the same effect on the solar cell, she said.

“[The research] is pretty specific to these sensitized solar cells. It could be used in dye-sensitized solar as well,” she said. “Their efficiencies have been quite low. The hope is they can get to higher levels.”

At present, the sensitized solar cells developed by Bent and her researchers are only 0.4 percent efficient. But, according to a Stanford press release, the tested cells were not optimized. Bent and her research group can fine-tune the cells to further improve the efficiency.

The cadmium sulfide quantum dots used in the experiments are not ideal for solar cells, Bent said in a press release. Going forward, the group will try using other quantum dot materials. The group will also attempt using other molecules for the organic layer, and could redesign the photovoltaic cell in an attempt to absorb more light and produce more electrical charge.

The research could have further implications for other forms of photovoltaics.

“We are very interested in looking at modifying interfaces in a variety of solar cells,” she said. “We are looking at doing interface engineering to improve the efficiency.” However, that research would likely require different strategies to implement.

Pictured: Professor Bent’s research team at Stanford.