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nanotimes News in Brief

11-10 :: October 2011

Solar Cells // Berkeley Lab Scientists Uncover the Secret of Remarkable Photovoltages in Ferroelectrics

University of California at Berkeley has resolved the high-voltage mystery for one ferroelectric material and determined that the same principle should be at work in all similar materials. The team’s results are published in Physical Review Letters.

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“We worked with very thin films of bismuth ferrite, or BFO, grown in the laboratory of our colleague Ramamoorthy Ramesh,” says Joel Ager of Berkeley Lab’s Materials Sciences Division (MSD), who led the research effort. “These thin films have regions – called domains – where the electrical polarization points in different directions. Ramesh’s group is able to make film with exquisite control over this domain structure.”

Because BFO has a range of unusual properties, the group led by Ramesh, who is a member of MSD and a professor of materials sciences, engineering, and physics at UC Berkeley, has long studied its cha- racteristics by building custom devices made from the material. The BFO films studied by Ager and his colleagues have a unique periodic domain pattern extending over distances of hundreds of micrometers (millionths of a meter). The domains form in stripes, each measuring 50 to 300nm across, separated by domain walls a mere two nanometers thick. In each of these stripes the electrical polarization is opposite

team of researchers at Lawrence Berkeley National Laboratory (Berkeley Lab) and the

from that of its neighbors. Because of the wide extent and highly periodic domain structure of the BFO thin films, the research team avoided the problems faced by groups who had tried to understand photovoltaic effects in other ferroelectrics, whose differences in polarity were thought to surround impurity atoms or to occur in different grains of a polycrystalline mate- rial. By contrast, says Ager, “We knew very precisely the location and the magnitude of the built-in elec- tric fields in BFO.” Thus Ager and Jan Seidel of MSD were able to gain “full microscopic understanding” of 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 vol- tage of the material itself,” says 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 bet- ween the regions of opposite electrical polarization