Improving efficiency of solar cells

jbrehm2, February 5, 2015 | View original publication

Improving efficiency of solar cells

In collaboration with an international team of colleagues, two UNL physicists have helped reveal the atomic-level mechanisms of a specially fabricated material that could improve the efficiency of solar cells.

Peter Dowben and Xin Zhang have co-authored a study, published in the newest edition of the journal Science, that details the performance of a material featuring an organic-lead compound with a crystalline atomic structure known as perovskite.

Led by researchers from King Abdullah University of Technology, the physicists’ co-authors devised a new process to create a crack-free perovskite material exhibiting several superior qualities known to help convert light into electricity.

“Because they’re better crystals than have been previously made, we were afforded the opportunity to dig into the physics a little deeper than might otherwise be possible,” said Dowben, a Charles Bessey Professor of physics and astronomy.

The quality of the crystals, Dowben said, allowed his doctoral student Zhang and fellow researchers to conduct “perhaps the best study of the detailed quantum states of this material that’s ever been done.”

When sunlight strikes a solar cell, light particles known as photons excite the negatively charged electrons that usually occupy a quantum state of energy in the so-called valence band around an atom. These excited electrons can jump an energy gap to a previously unoccupied state known as the conduction band, leaving a positively charged “hole” and allowing photocurrent to flow.

Using a technique called inverse photoemission spectroscopy, Zhang and Dowben confirmed that the perovskite material displays a significantly narrower energy gap than numerous thin-film materials currently employed in solar cells. According to the authors, this narrow bandgap facilitates the harvesting of photons and could yield even better energy efficiency than other solar cell materials.

Zhang and Dowben further used spectroscopy to provide information about where in the conduction band the freed electrons might land after absorbing sunlight. Knowing the location of these potential landing sites, Zhang said, makes it easier to predict the performance of a device that generates electric current.

The study was also co-authored by researchers from the University of Toronto and Indiana University. Collaborating on investigations of quantum behavior has become routine at UNL, said Dowben, because the university houses one of the world’s few laboratories capable of performing both standard and inverse photoemission spectroscopies.

“In the case of something like photovoltaics, you have electrical current coming from both electrons and holes,” Dowben said. “The only way to really look at where those states are is to do this combination of techniques. We have these unique capabilities at Nebraska, and that makes us very popular.”