Some ferroelectric materials can develop extremely high voltages when exposed to light, which could improve solar cells, but scientists haven’t been able to figure out how the process works. Now, researchers at the US Department of Energy's Lawrence Berkeley National Laboratory and the University of California in Berkeley have apparently solved the mystery of one ferroelectric material, bismuth ferrite (BFO), which they say should determine that the same principle can be applied to similar materials. They say the secret is an electronic “bucket brigade” that passes electrons stepwise from one electrically polarized region to the next. The team worked with thin films of BFO, which has a range of unusual properties, including a unique periodic domain pattern extending over distances of hundreds of microns. The domains form in stripes, each measuring 50 to 300 nm across, separated by domain walls a mere 2 nm 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. At top, domains with opposite electrical polarization, averaging about 140 nm wide and separated by walls 2 nm thick, form a well-aligned array in a thin film of bismuth ferrite. When illuminated, electrons collect on one side of the walls and holes on the other, driving the current at right angles to the walls. Voltage increases as excess electrons accumulate stepwise from domain to domain. (Images: Lawrence Berkeley National Laboratory) “We knew very precisely the location and the magnitude of the built-in electric fields in BFO,” said Joel Ager of Berkeley Lab's Materials Sciences Division, who led the research effort. Thus Ager and colleague Jan Seidel were able to gain a 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 bandgap 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; to measure it, the researchers attached platinum electrical contacts to the BFO film. “The farther apart the contacts, the more domain walls the current had to cross, and the higher the voltage,” Ager said. The researchers measured current and voltage with platinum contacts set at right angles to the current. In a typical setup, the film was 100 nm thick, its width from back to front was 1 mm, and the distance between contacts was 200 µm. Voltage and current increase with the number of domain walls; with contacts 200 µm apart, voltage reached 16 volts, several times the 2.7-eV bandgap of bismuth ferrite. 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. The model presented a surprising — and yet surprisingly simple — picture of how each of the oppositely oriented domains creates excess charge and 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. While a solar cell loses efficiency if electrons and holes immediately recombine, that can’t happen here because of the strong fields at the domain walls created by the oppositely polarized charges of the domains. “Still, electrons and holes need each other, so they go in search of one another," said Ager. Holes and electrons move away from the 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 — all in the same direction, as determined by the overall current. "It's like a bucket brigade, with each bucket of electrons passed from domain to domain," Ager said. “As the charge contributions from each domain add up, the voltage increases dramatically. BFO itself is not a good candidate for a solar cell material. For one thing, it responds only to blue and near-ultraviolet light, which eliminates most of the solar spectrum. "So we need something that absorbs more light.” The efficiency of BFO's response to light 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. The team's results are published in Physical Review Letters. For more information, visit: www.lbl.gov
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