Researchers from Berkeley Lab and UC Berkeley have contructed a highly controlled ferroelectric film that allows them to observe and draw conclusions about how ferroelectric materials generate photovoltaic energy.
September 20, 2011 — Lawrence Berkeley National Laboratory (Berkeley Lab) reseachers are working with the University of California at Berkeley (UC Berkeley) on understanding the photovoltaic properties of ferroelectrics under illumination. While the scientific community was aware of this process, the Berkeley work is the first to propose a principle for what is happening.
Researchers worked with very thin bismuth ferrite (BFO) films grown by Ramamoorthy Ramesh’s lab, explained Joel Ager of Berkeley Lab?s Materials Sciences Division (MSD), who led the project. Thin BFO films have regions (domains) where the “electrical polarization points in different directions.” The internal BFO generation gave researchers “exquisite control” over the domains. Ramesh is a professor of materials sciences, engineering, and physics at UC Berkeley and member of MSD.
The BFO films’ unique periodic domain pattern extends over hundreds of micrometers in stripes, each measuring 50-300nm wide, separated by domain walls 2nm thick. In each of these stripes the electrical polarization is opposite from that of its neighbors.
|Figure. Domains with opposite electrical polarization, averaging about 140nm wide and separated by 2nm-thick walls, form an array in a BFO thin film. SOURCE: Berkeley Lab.|
This structure eliminated impurity effects, as have been observed in other research on ferroelectric photovoltaics. The team “knew very precisely the location and the magnitude of the built-in electric fields in BFO,” Ager notes. Ager and Jan Seidel of MSD were able to observe what went on within each separate domain, and across many domains.
“When we illuminated the BFO thin films, we got very large voltages, many times the band gap voltage of the material itself,” said Ager. “The incoming photons free electrons and create corresponding holes, and a current begins to flow perpendicular to the domain walls — even though there?s no junction, as there would be in a solar cell with negatively and positively doped semiconductors.”
In an open circuit the current flows at right angles to the domain walls, and to measure it the researchers attached platinum electrical contacts to the BFO film. Says Ager, ?The farther apart the contacts, the more domain walls the current had to cross, and the higher the voltage.?
It was clear that the domain walls between the regions of opposite electrical polarization were playing a key role in the increasing voltage. These experimental observations turned out to be the clue to constructing a detailed charge-transport model of BFO, a job undertaken by Junqiao Wu of MSD and UC Berkeley, and UCB graduate student Deyi Fu.
Each oppositely oriented domain creates excess charge and then passes it along to its neighbor. The opposite charges on each side of the domain wall create an electric field that drives the charge carriers apart. On one side of the wall, electrons accumulate and holes are repelled. On the other side of the wall, holes accumulate and electrons are repelled.
Electrons and holes cannot immediately recombine, because of the strong fields at the domain walls created by the oppositely polarized charges of the domains. The electrons and holes still “go in search of one another,” Ager explained, moving away from domain walls in opposite directions toward the center of the domain where the field is weaker. Because there?s an excess of electrons over holes, the extra electrons are pumped from one domain to the next in the same direction, as determined by the overall current (see figure).
Ager says the stepwise voltage increases have a sawtooth potential. As the charge contributions from each domain add up, the voltage increases dramatically.
BFO is not a good candidate for a commercial solar cell material: it responds to blue and near ultraviolet light. However, the photovoltaic efficiency observed in this ferroelectric material reveals useful information: efficiency is best near the domain walls. While very high voltages can be produced, the other necessary element of a powerful solar cell, high current, is lacking.
Ager’s group is now investigating other candidate materials. The same principle should be at work in all similar materials.
?Efficient photovoltaic current generation at ferroelectric domain walls,? by Jan Seidel, Deyi Fu, Seung-Yeul Yang, Esther Alarcón-Lladó, Junqiao Wu, Ramamoorthy Ramesh, and Joel W. Ager III, appears in Physical Review Letters and is available online at http://prl.aps.org/abstract/PRL/v107/i12/e126805.
|Photo. Joel Ager with Esther Alarcon-Llado, an author of the report on the origin of BFO’s photovoltaic response. Courtesy of Roy Kaltschmidt, Lawrence Berkeley National Laboratory Public Affairs.|
This work was supported by the U.S. Department of Energy?s Office of Science through the Helios Solar Energy Research Center, and by Berkeley Lab?s Laboratory Directed Research and Development program.
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