Bulk photovoltaic effect in BaTiO₃-based ferroelectric oxides: An experimental and theoretical study

The bulk photovoltaic (PV) effect exhibited by non-centrosymmetric systems gained research interest due to the observed large open-circuit voltage. Ferroelectric systems exhibiting anomalous photovoltaic effects are mostly crystallized with multi-phase coexistence. Hence, the computational difficulty in building a multi-phase system restricts the detailed photovoltaic studies through phenomenological and shift current theory. In this work, ferroelectric Ba1−x(Bi0.5K0.5)xTiO3 (BBKT) oxide is designed to crystallize in single-phase tetragonal symmetry with improved polarization characteristics, and it is found to exhibit large PV response. Both experimental and theoretical studies on BBKT samples reveal ∼18% reduction in bandgap compared to the parent BaTiO3. Short-circuit current measured as a function of light intensity and light polarization angle reveal linear and sinusoidal response, respectively. The observed features are in accordance with phenomenological theory. Remarkably, the x = 0.125 sample displays ∼8 times higher open-circuit voltage (7.39 V) than the parent compound. The enhanced PV effect is attributed to the large shift current along the z direction as evidenced by the additional charge-center shift of the valence band occupied by the O-2p orbital and the conduction band occupied by the Bi-6p orbital. Notably, the degenerate Bi-pz state at the conduction band minimum in BBKT favors the large shift current response in the z direction.