Enhanced Switchable Ferroelectric Photovoltaic Effects in Hexagonal Ferrite Thin Films via Strain Engineering.

Ferroelectric photovoltaics (FPVs) are being extensively investigated by virtue of switchable photovoltaic responses and anomalously high photovoltages of ∼104 V. However, FPVs suffer from extremely low photocurrents due to their wide band gaps (Eg). Here, we present a promising FPV based on hexagonal YbFeO3 (h-YbFO) thin-film heterostructure by exploiting its narrow Eg. More importantly, we demonstrate enhanced FPV effects by suitably exploiting the substrate-induced film strain in these h-YbFO-based photovoltaics. A compressive-strained h-YbFO/Pt/MgO heterojunction device shows ∼3 times enhanced photovoltaic efficiency than that of a tensile-strained h-YbFO/Pt/Al2O3 device. We have shown that the enhanced photovoltaic efficiency mainly stems from the enhanced photon absorption over a wide range of the photon energy, coupled with the enhanced polarization under a compressive strain. Density functional theory studies indicate that the compressive strain reduces Eg substantially and enhances the strength of d-d transitions. This study will set a new standard for determining substrates toward thin-film photovoltaics and optoelectronic devices.