  Ferroelectric photovoltaics (FPVs) have drawn much attention owing to their
high stability, environmental safety, anomalously high photovoltages, coupled
with reversibly switchable photovoltaic responses. However, FPVs suffer from
extremely low photocurrents, which is primarily due to their wide band gaps.
Here, we present a new class of FPVs by demonstrating switchable ferroelectric
photovoltaic effects using hexagonal ferrite (h-RFeO3) thin films having narrow
band gaps of ~1.2 eV, where R denotes rare-earth ions. FPVs with narrow band
gaps suggests their potential applicability as photovoltaic and optoelectronic
devices. The h-RFeO3 films further exhibit reasonably large ferroelectric
polarizations, which possibly reduces a rapid recombination rate of the
photo-generated electron-hole pairs. The power conversion efficiency (PCE) of
h-RFeO3 thin-film devices is sensitive on the magnitude of polarization. In the
case of h-TmFeO3 (h-TFO) thin film, the measured PCE is twice as large as that
of the BiFeO3 thin film, a prototypic FPV. We have further shown that the
switchable photovoltaic effect dominates over the unswitchable internal field
effect arising from the net built-in potential. This work thus demonstrates a
new class of FPVs towards high-efficiency solar cell and optoelectronic
applications.
