Switchable Photovoltaic Effects in Hexagonal Manganite Thin Films Having Narrow Band Gaps

Ferroelectric photovoltaics (FPVs) are being extensively studied owing to their anomalously high photovoltages, coupled with reversibly switchable photocurrents. However, FPVs suffer from their extremely low photocurrents, which is primarily due to their wide band gaps. Herein, we present a new class of FPV by demonstrating (i) a nearly optimum band gap of ∼1.55 eV and (ii) the ferroelectric polarization switching in the epitaxial hexagonal manganite thin films, <i>h</i>-RMnO₃, where R = Lu and Y. According to the thickness-dependent photovoltaic measurements, the ITO/<i>h</i>-LuMnO₃/Pt solar cell shows a power conversion efficiency of ∼0.11% when the thickness of the <i>h</i>-LuMnO₃ layer is ∼150 nm. We have shown that the PCE is 1–3 orders higher than those of classical FPVs such as undoped Pb­(Zr,Ti)­O₃ and BiFeO₃ under the standard AM 1.5G illumination. We have further elucidated that the switchable photovoltaic effect dominates over the nonferroelectric internal field effect