Influence of crystal structure on charge carrier effective masses in BiFeO3

Ferroelectric-based photovoltaics have shown great promise as a source of renewable energy, thanks to their in-built charge separation capability, yet their efficiency is often limited by low charge-carrier mobilities. In this paper, we compare the photovoltaic prospects of various phases of the multiferroic material BiFeO3 by evaluating their charge-carrier effective masses from first-principles simulations. We identify a tetragonal phase with the promising combination of a large spontaneous polarization and relatively light charge carriers. From a systematic investigation of the octahedral distortions present in BiFeO3, we clarify the relationship between structure and effective masses. This relationship is explained in terms of changes to the orbital character and overlap at the band edges that result from changes in the geometry. Our findings suggest some design principles for how to tune effective masses in BiFeO3 and similar materials through the manipulation of their crystal structures in experimentally accessible ways.