At top, domains with opposite electrical polarization, averaging about 140 nanometers wide and separated by walls 2 nanometers 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. Credit: LBL
A news release out of Lawrence Berkeley National Lab reports that “the average installed cost of residential and commercial PV systems completed in 2010 fell by roughly 17 percent from the year before, and by an additional 11 percent within the first six months of 2011.” The information is from a recent LBL report (pdf), “Tracking the Sun IV: An Historical Summary of the Installed Cost of Photovoltaics in the United States from 1998 to 2010.”
The cost reductions are attributed to reductions in the cost of PV modules and to nonmodule costs, like labor, marketing, business overhead and nonmodule system components. The report authors note that the drop in nonmodule costs is significant because they can be influenced easily by policies intended to reduce market barriers and expedite deployment. Module cost reduction, however, is R&D dependent and more difficult to influence timewise.
One aspect of module cost reduction is to develop more efficient photovoltaic materials. High efficiency in solar cells is a function of voltage and current, and more (of both) is better.
Ferroelectric materials have very high photovoltaic responses to illumination, but the mechanism has been unknown. Some new research, also out of LBL, describes a model for the high voltages seen in thin-film bismuth ferrites (BFOs), which may provide some insight to developing more efficient PV materials. BFOs themselves are not candidate PV materials because they respond only to a very small slice of the solar spectrum (blue and near ultraviolet).
BFO thin films have a highly periodic domain structure (regions where the electrical polarization orients in different directions). Joel Ager, the lead researcher said in a press release, “When we illuminated the BFO thin films, we got very large voltages, many times the band gap of the material itself.”
The question is, why? The voltage measured across the film increased as the number of domains between electrodes increased, showing researchers that somehow the domain walls were involved.
The press release describes the charge-transport model that was developed:
The model presented a surprising, and surprisingly simple, picture of how each of the oppositely oriented domains 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.
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,” says Ager, “so they go in search of one another.” 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 says, who describes the stepwise voltage increases as “a sawtooth potential. As the charge contributions from each domain add up, the voltage increases dramatically.”
Ager expects that the mechanism will apply to all materials with “sawtooth” potentials, which opens the possibility of developing new PV materials with high voltage and high current.