Flexo-Photovoltaic Effect in a Bandgap-Engineered Ferroelectric BaTiO₃‑Based Epitaxial Thin Film

The ferroelectric photovoltaic (PV) effect has several advantages over other PV systems because of the reported above-bandgap photovoltage, switchable characteristics, ability to be free from the thermodynamic Shockley–Queisser limit, environmental stability, etc. Several methods have been adopted to improve the PV performance of the ferroelectric oxides. In this work, the strain gradient-induced flexoelectric effect is employed to enhance and extend the PV effect to a high temperature. This novel mechanism, called the flexo-photovoltaic effect, is demonstrated on the bandgap-tuned 0.95BaTiO₃–0.05Bi(Ni_1/2Nb_1/2)O_3+δ film grown epitaxially under the compressively strained condition on the SrRuO₃-buffered SrTiO₃(001) substrate. The enhanced and nonswitchable PV response with a short-circuit current density (J_SC) of 11.2 μA/cm² and an open-circuit voltage (V_OC) of 0.3 V, which are larger than the value reported for the parent BaTiO₃ films, is attributed to the calculated strain gradient-induced flexoelectric field (E_f = 120 kV/cm). The observed switchable but low value of PV response (J_SC = 1 nA/cm² and V_OC = 0.15 V) measured along the direction (in-plane direction) perpendicular to the flexoelectric field direction (out-of-plane direction) reiterates the effect of the flexoelectric field on the PV response. In addition, the observed PV response at 135 °C, which is well above the Curie temperature (T_C) of the bulk sample, gives additional evidence for the flexo-photovoltaic effect in epitaxial oxide thin films. Overall, the demonstrated flexo-photovoltaic effect opens up a wide range of oxide systems, including centrosymmetric oxides for PV applications.