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U of M researchers build more efficient solar cells

A team of University of Minnesota researchers has built solar cells with potential efficiencies better than twice as high as current levels.

A major obstacle for developers of solar energy has been the fact that most of the energy — 70 percent or more — captured in solar cells escapes before it can be harnessed and used.

A team of University of Minnesota researchers has cleared that hurdle and built solar cells with potential efficiencies better than twice as high as current levels.

Their breakthrough, published in the latest issue of Science magazine, greatly improves chances of building cheaper, more efficient solar energy devices. It also could slash the cost of manufacturing solar cells by removing the need to process them at very high temperatures.

The research team — including U of M graduate student William Tisdale, chemical engineering and materials science professors Eray Aydil and David Norris and chemistry professor Xiaoyang Zhu (now at the university of Texas-Austin) — worked for six years on the project, which was funded primarily by the U.S. Department of Energy and partially by the National Science Foundation.

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In most solar cells now in use, rays from the sun strike the uppermost layer of the cells, which is made of a crystalline semiconductor substance — usually silicon. The problem is that many electrons in the silicon absorb excess amounts of solar energy and radiate that energy away as heat before it can be harnessed.

An early step in harnessing that energy is to transfer these “hot” electrons out of the semiconductor and into a wire, or electric circuit, before they can cool off. But efforts to extract hot electrons from traditional silicon semiconductors have not succeeded.

However, when semiconductors are constructed in small pieces only a few nanometers wide — “quantum dots” — their properties change.

“Theory says that quantum dots should slow the loss of energy as heat,” Tisdale said in a statement. “And a 2008 paper from the University of Chicago showed this to be true. The big question for us was whether we could also speed up the extraction and transfer of hot electrons enough to grab them before they cooled. ”

Tisdale and his colleagues demonstrated that quantum dots — made not of silicon but of another semiconductor called lead selenide — could indeed be made to surrender their “hot” electrons before they cooled. The electrons were pulled away by titanium dioxide, another common inexpensive and abundant semiconductor material that behaves like a wire.

“This is a very promising result,” said Tisdale. “We’ve shown that you can pull hot electrons out very quickly – before they lose their energy. This is exciting fundamental science.”

The work shows that the potential for building solar cells with efficiencies approaching 66 percent exists. But that new level needs to improve too, Aydil said.

“This work is a necessary but not sufficient step for building very high-efficiency solar cells,” he said. “It provides a motivation for researchers to work on quantum dots and solar cells based on quantum dots.”

The next step is to construct solar cells with quantum dots and study them. But one big problem still remains: “Hot” electrons also lose their energy in titanium dioxide.  New solar cell designs will be needed to eliminate this loss as well, the researchers said.

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Still, “I’m comfortable saying that electricity from solar cells is going to be a large fraction of our energy supply in the future,” Aydil noted.