Ferroelectric oxides have recently attracted much attention as photovoltaics because they can separate the photo-excited charge carriers well due to the strong inversion symmetry breaking. They can generate above band gap photovoltages which may enable power conversion efficiencies beyond the maximum possible in a conventional p-njunction solar cell. However, further improvements in photovoltaic efficiency have been inhibited by their wide band gaps (>2.7 eV).We design single-phase solid oxide solutions: [KNbO3]1-x [Ba(Ni1/2Nb1/2)O3-d]x (KBNNO). The x=0.1 composition is polar at room temperature, has a direct band gap of 1.39 eV, and has a photocurrent density approximately 50 times larger than that of the classic ferroelectric (Pb,La)(Zr,Ti)O3material.We also study the effects of vacancy, composition and cation arrangement on the shift current response using a simpler material Pb(NixTi1-x)O3-x. As the electronic transitions at the band edges involve more of the Ti 3d and Pb 6p orbitals, the shift vector direction flips, allowing the shift current from different k regions to propagate along the same direction, enabling stronger shift current response. The involvement of more delocalized orbitals, together with certain structures that have larger polar distortions, enhance the shift current. These findings will improve the applicability of shift current in photovoltaic devices.In addition to ferroelectric perovskites, we also report the properties of polar rocksalt structure materials. Heteroepitaxial superlattice materials can induce interlayer strain, leading to symmetry breaking. We therefore study sulfide rocksalt superlattices, since binary sulfides form rocksalt structures more readily than oxides and since the band gaps of sulfides are lower than those of oxides. We calculate various combinations of (Ag,Bi,Pb)S superlayers using DFT. (Ag,Bi,Pb,Sr)S compounds show band gaps from 1.0 to 1.2 eV in PBE0 calculations, exhibit nearly direct band gaps, and show shift current response that is much larger than BiFeO3. These are promising candidates as photovoltaic materials.

Ferroelectric oxides for visible-light photovoltaics and engineering of shift current

248th National Meeting (2014)

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