Electronic Bucket Brigade Could Boost Solar Cell Voltages
If solar cells could generate higher voltages when sunlight falls on them, they'd produce more electrical power more efficiently. For over half a century scientists have known that ferroelectrics, materials whose atomic structure allows them to have an overall electrical polarization, can develop very high photovoltages under illumination. Until now, no one has figured out exactly how this photovoltaic process occurs.
Now a team of researchers at the U.S. Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab) and the 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.
"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 characteristics 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 300 nanometers (billionths of a meter) across, separated by domain walls a mere two nanometers thick. In each of these stripes the electrical polarization is opposite 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 material.
By contrast, says Ager, "We knew very precisely the location and the magnitude of the built-in electric 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.
_____________
bottle opener Auto Warranty
You cannot post new topics in this forum You cannot reply to topics in this forum You cannot edit your posts in this forum You cannot delete your posts in this forum You cannot vote in polls in this forum
