Improving the optical absorption of BiFeO3 for photovoltaic applications via uniaxial compression or biaxial tension

First-principles computations are employed to investigate the electronic structures and optical absorption of rhombohedral BiFeO3 under uniaxial compression and biaxial tension. We find that the bandgap of BiFeO3 is reduced under uniaxial compression, and it can be tuned to the ideal value for photovoltaic applications; furthermore, the indirect-to-direct bandgap transition occurs, which would lead to much enhanced optical absorption near the band edge. Similar results are found for biaxial tensile strain. Strong optical absorption is critical to build efficient solar cells based on ferroelectric thin films; strain engineering is thus a practical route towards realizing this scheme, in which no junction is needed to separate charge carriers.