  The bulk photovoltaic effect exhibited by the non-centrosymmetric system
gains research interest due to the observed large open-circuit voltage. The
ferroelectric systems exhibiting anomalous photovoltaic effect are mostly
crystallized with multiphase 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
Ba$_{1- x}$(Bi$_{0.5}$K$_{0.5}$)$_x$TiO$_3$ (BBKT) oxide is designed to
crystallize in single-phase tetragonal symmetry with improved polarization
characteristics, and it is found to exhibits large PV response. Both
experimental and theoretical studies on BBKT samples reveal ~18% reduction in
bandgap compared to the parent BaTiO$_3$. 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, 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 z-direction as evidenced from
the additional charge-center shift of valence band occupied by O-2p orbital and
conduction band occupied by Bi-6p orbital. Notably, the degenerate Bi-p$_z$
state at conduction band minimum in BBKT favours the large shift current
response in the z-direction.
