The material itself – bismuth ferrite (BFO) – isn’t going to change the course of photovoltaic solar. But new insights into how BFO produces very high photovoltages under illumination could do just that, say scientists at the Lawrence Berkeley National Laboratory (Berkeley Lab) and UC Berkeley.
That’s because the researchers believe the structural principle they discovered at work with BFO is true for all such ferroelectric materials, holding out the possibility for more efficient solar photovoltaic power production.
The Berkeley team, led by Joel Ager (pictured above with fellow researcher Esther Alarcón-Lladó), worked with super-thin BFO films grown by a colleague. In these BFO films, domains with opposite electrical polarization, averaging about 140 nanometers wide and separated by walls 2 nanometers thick, form a well-aligned array. The researchers found that when the BFO is illuminated, electrons collect on one side of the walls and holes on the other, driving the current at right angles to the walls. A solar cell loses efficiency if electrons and holes immediately recombine, but with BFO that doesn’t happen because of the strong fields at the domain walls created by the oppositely polarized charges of the domains.
Ager’s metaphor: “It’s like a bucket brigade, with each bucket of electrons passed from domain to domain.” Ager calls this increasing voltage “a sawtooth potential,” and while BFO makes for a poor solar cell material because it responds to a very narrow range of light, Ager said they’re sure “this effect will occur in any system with a sawtooth potential, and perhaps in other geometries as well. We are already beginning to investigate new candidates.”
“Efficient photovoltaic current generation at ferroelectric domain walls,” by Jan Seidel, Deyi Fu, Seung-Yeul Yang, Esther Alarcón-Lladó, Junqiao Wu, Ramamoorthy Ramesh, and Joel W. Ager III, appears in Physical Review Letters.