  Bulk photovoltaic (BPVE) effect in crystals lacking inversion symmetry offers
great potential for optoelectronic applications due to its unique properties
such as above bandgap photovoltage and switchable photocurrent. Because of
their large spontaneous polarizations, ferroelectric materials are ideal
platforms for studying BPVE. However, identifying the origin of experimentally
observed photovoltaic response is often challenging due to the entanglement
between bulk and interface effects, leading to much debate in the field. This
issue is particularly pronounced in vertical heterostructures, where the two
effects are comparable. Here we report a crossover between bulk- and
interface-dominant response in vertical BiFeO3 heterostructures when changing
the photon energy. We show that well above-bandgap excitation leads to bulk
photovoltaic response, but band-edge excitation requires interface band bending
to separate the photocarriers. Our findings not only help to clarify
contradicting reports in the literature, but also lay the ground for a deeper
understanding of ferroelectric photovoltaic effect and its applications in
various devices.
