  We propose a device model that elucidates the role of domain walls in the
photovoltaic effect in multi-domain ferroelectric perovskites. The model
accounts for the intricate interplay between ferroelectric polarization, space
charges, photo-generation and electronic transport. When applied to bismuth
ferrite, results show a significant electric potential step across both
71-degree and 109-degree domain walls, which in turn contributes to the
photovoltaic (PV) effect. We also find a strong correlation between
polarization and oxygen octahedra tilts, which indicates the nontrivial role of
the latter in the PV effect. The domain wall-based PV effect is further shown
to be additive in nature, allowing for the possibility of generating
above-bandgap voltage
