11-10 :: October 2011

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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 sur- prisingly simple, picture of how each of the oppositely oriented domains creates excess charge and then passes it along to its neigh- bor. 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 recom- bine, that can’t happen here because of the strong fields at the domain walls created by the oppositely polarized charges of the domains.

J. X. Zhang, Q. He, M. Trassin, W. Luo, D. Yi, M. D. Rossell, P. Yu, L. You, C. H. Wang, C. Y. Kuo, J. T. Heron, Z. Hu, R. J. Zeches, H. J. Lin, A. Ta- naka, C. T. Chen, L. H. Tjeng, Y.-H. Chu, and R. Ramesh: Microscopic Origin of the Giant Ferroe- lectric Polarization in Tetragonal-like BiFeO3

, In:

Physical Review Letters, Volume 107, Issue 14, September 30, 2011, Article 147602 [5 pages], DOI:10.1103/PhysRevLett.107.147602: http://10.1103/PhysRevLett.107.147602

At top, domains with opposite electrical polarization, averaging about 140nm wide and separated by walls 2nm 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. © UC Berkeley / University of California at Berkeley

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