Unusually strong bulk photovoltaic effect in planar BaTiO3 films and its scaling with polarization

The bulk photovoltaic effect (BPVE) leads to the generation of a photocurrent from the bulk of an symmetric material. Recently, strong photocurrents and high power conversion efficiencies (PCEs) have been obtained in 1D and 2D nanoscale systems and also by using nanoscale tip electrodes. By contrast, photovotaic response of standard bulk ferroelectrics and planar thin films with planar electrodes has typically been considered weak. We studied a series of SrRuO3/BaTiO3/ITO film systems grown on substrate with different lattice parameters, allowing adjustment of the BaTiO3 c/a and polarization while preserving the chemistry of the film and the interfaces. We found that surprising, smaller polarization achieved by growing on the KTaO3 substrate and by growing on (110) and (111) oriented substrates led to a dramatic increase in the photocurrent. We ascribe this to the effect of the Schottky barriers that prevent efficient collection of the hot carriers generated through the BPVE. Analysis of the results in the literature shows that elimination of such Schottky barriers through interface engineering led to strong enhancement of the BPVE response. The photocurrents obtained for our BaTiO3 films are much larger than the magnitudes predicted by first-principles calculations for both shift-current and ballistic-current mechanisms of BPVE. Therefore, we suggest that the deviation from periodicity induced in thin BaTiO3 films in the vicinity of the interface are responsible for the strong asymmetric scattering of the generated hot carriers and the high BPVE observed photocurrent. This suggest a path forward to achieving high photocurrents and PCE values approaching those of standard PV devices.