Scientists describe how unusually high photo-induced voltages are generated in thin films of bismuth ferrite.
Joel Ager, Esther Alarcon Llado and their colleagues demonstrated a new photovoltaic mechanism.
The photovoltaic mechanism by which a ferroelectric material generates high voltages could help create better solar cells. Lead author Jan Seidel and other researchers at Lawrence Berkeley National Laboratory (U.S.A.) and the University of California at Berkeley described how unusually high photo-induced voltages are generated in thin films of bismuth ferrite (Phys. Rev. Lett. 107, 126845). If this mechanism can be applied to other materials, we may be able to design more efficient photovoltaic materials.
Bismuth ferrite forms domains with opposite electrical polarizations. Instead of studying the material in bulk, researcher Ramamoorthy Ramesh grew very thin films. By using such thin films, lead researcher Joel Ager and the other authors were able to understand how photovoltage accumulates across many domains.
The films have a periodic domain pattern of stripes 50 to 300 nm across. Each stripe has an opposite polarization from its neighbors. The stripes are separated by domain walls a mere 2 nm thick. When light creates free charge carriers, these domain walls push one type of charge away. This reduces the recombination rate. (The researchers estimate that the internal quantum efficiency near the walls is around 10 percent. The movement leads to a net current in the material perpendicular to the walls. Excess charge is passed on to the neighboring domain in a direction determined by the net photocurrent.) The voltage generated in the periodically ordered thin film adds up as charges are passed from domain to domain, so the structure can generate voltages much larger than bismuth ferrite’s bandgap.
Although bismuth ferrite is unlikely to be useful for solar cells, the researchers expect that the additive photovoltage mechanism applies to other systems with similar periodic potential structure.
Yvonne Carts-Powell is a freelance science writer who specializes in optics and photonics.