Abstract
Titanium oxide materials have multiple functions such as photocatalytic and photovoltaic effects. Ferroelectrics provide access to light energy conversion that delivers above-bandgap voltages arising from spatial inversion symmetry breaking, whereas their wide bandgap leads to poor absorption of visible light. Bandgap narrowing offers a potential solution, but this material modification suppresses spontaneous polarization and, hence, sacrifices photovoltages. Here, we report successive-redox mediated ferrophotovoltaics that exhibit a robust visible-light response. Our single-crystal experiments and ab initio calculations, along with photo-luminescence analysis, demonstrate that divalent Fe2+ and trivalent Fe3+ coexisted in a prototypical ferroelectric barium titanate BaTiO3 introduce donor and acceptor levels, respectively, and that two sequential Fe3+/Fe2+ redox reactions enhance the photogenerated power not only under visible light but also at photon energies greater than the bandgap. Our approach opens a promising route to the visible-light activation of photovoltaics and, potentially, of photocatalysts.
Highlights
Titanium oxide materials have multiple functions such as photocatalytic and photovoltaic effects
Defect levels inside the bandgap, called gap states, of Cr3+ are located above the valence band maximum (VBM), and the donor states play a crucial role in visible-light activation[9]
The ferroelectric PV effect has been extensively studied for wide-bandgap oxides such as LiNbO311,16,17, BaTiO318–22, PbTiO3-based perovskites[23,24,25], and BiFeO314,15,26
Summary
Titanium oxide materials have multiple functions such as photocatalytic and photovoltaic effects. A single transition-metal dopant with two different valence states introduces these gap states, thereby providing successive redox cycles under illumination In principle, it can induce PV currents at small hv without being restricted by the material bandgap. Our experimental and theoretical study on a prototypical ferroelectric BaTiO3 demonstrates that the 3d orbitals of iron derive donor and acceptor levels in the Fe2+-Fe3+ coexisting state and that e’-h pairs injected by two sequential Fe3+/Fe2+ reactions deliver a robust PV response under visible-light and at hv greater than Eg
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