Designing nanostructured ferroelectric films: geometry, polarization and electronic properties of the BaTiO₃/hematite interface from first principles

In solar energy harvesting, current photovoltaic (PV) devices are limited to a maximum theoretical efficiency of ~34%, referred to as the Shockley-Queisser (S-Q) limit. Therefore, innovative technological approaches are needed to identify mechanisms that can entirely overcome this efficiency limit. Some ferroelectric materials can produce a photovoltaic effect without the need of a semiconductor junction, known as the bulk photovoltaic (BPV) effect which originates from the internal crystal asymmetry that gives rise to their permanent electrical polarization. Most importantly, they are not subject to the S-Q limit,¹ offering potential new routes to exceed current PV efficiencies. However, low photoconductivity is required for high voltage generation via the BPV effect, something that is impossible in a narrow bandgap semiconductor with efficient light harvesting.

By coupling together a junction-based PV system and a BPV effect-based material in a nanocomposite thin film device, limitations of both technologies could be overcome. This idea relies on the proven ability of ferroelectrics to influence coupled materials, such as photocatalysts and organic photovoltaics. By using the recently-developed Electronic Lattice Strain (ELS) procedure,² we identified the BaTiO₃/hematite interface as a promising candidate for the proposed device. Screening was performed in terms of epitaxially-compatible interfaces (to minimize defects) and appropriate band alignment (to minimize charge transfer). To gain insights into the geometry, polarization and electronic properties of the aforementioned system, Density Functional Theory (DFT) modelling is employed. To this purpose, we assume the BaTiO₃ (110) surface as substrate on which the hematite (100) surface grows epitaxially strained to it. Within the supercell approach, we are required to employ 5×5 surface unit cells of BaTiO₃ (110) and 4×2 units of hematite (100) to reproduce the minimally strained interface (~2.6%). Knowledge of such an epitaxially-compatible interface will enable us to develop a model to describe and predict behavior of novel devices, validated by experiment.

(1) Spanier et al, Nat. Photonics, 2016, 10, 611-616.

(2) Butler et al., J. Mater. Chem. C, 2016, 4, 1149-1158.