For the second time this week, research reveals path to solar cells with 65% efficiency

Minneapolis-St. Paul, MN--A team of University of Minnesota (U of M)-led researchers has cleared a major hurdle in the drive to build solar cells with potential efficiencies up to twice as high as current levels, which rarely exceed 30%.¹ The stated potential of the research parallels that of a release earlier this week by the Technische Universiteit Eindhoven in The Netherlands.

The U of M-led team has shown how energy that is now being lost from semiconductors in solar cells can be captured and transferred to electric circuits. Fabrication methods based on the research could also slash the cost of manufacturing solar cells by removing the need to process them at very high temperatures.

The achievement crowns six years of work begun at the university Institute of Technology (College of Science and Engineering) chemical engineering and materials science professors Eray Aydil and David Norris and chemistry professor Xiaoyang Zhu (now at the university of Texas-Austin) and spearheaded by U of M graduate student William Tisdale.

In most solar cells now in use, rays from the sun strike the uppermost layer of the cells, which consists of a crystalline semiconductor--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.

Getting the hot electrons
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 made into quantum dots (QDs) their properties change.

"Theory says that QDs should slow the loss of energy as heat," said Tisdale. "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. "

In the current work, Tisdale and his colleagues demonstrated that lead selenide QDs 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% exists, according to Aydil.

Next: actual solar cells
The next step is to construct solar cells with QDs 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, the researchers said.

The research was funded primarily by the U.S. Department of Energy and partially by the National Science Foundation. Other authors of the paper were Brooke Timp from the U of M and Kenrick Williams from UT-Austin.

REFERENCE:

1. William A. Tisdale et al., Science, Vol. 328, p. 1543, 18 June 2010.