Ferroelectrics have recently attracted attention as a candidate class of materials for use in photovoltaic devices, and for the coupling of light absorption with other functional properties. In these materials, the strong inversion symmetry breaking that is due to spontaneous electric polarization promotes the desirable separation of photo-excited carriers and allows voltages higher than the bandgap, which may enable efficiencies beyond the maximum possible in a conventional p-n junction solar cell. Ferroelectric oxides are also stable in a wide range of mechanical, chemical and thermal conditions and can be fabricated using low-cost methods such as sol-gel thin-film deposition and sputtering. Recent work has shown how a decrease in ferroelectric layer thickness and judicious engineering of domain structures and ferroelectric-electrode interfaces can greatly increase the current harvested from ferroelectric absorber materials, increasing the power conversion efficiency from about 10(-4) to about 0.5 per cent. Further improvements in photovoltaic efficiency have been inhibited by the wide bandgaps (2.7-4 electronvolts) of ferroelectric oxides, which allow the use of only 8-20 per cent of the solar spectrum. Here we describe a family of single-phase solid oxide solutions made from low-cost and non-toxic elements using conventional solid-state methods: [KNbO3]1 - x[BaNi1/2Nb1/2O3 - δ]x (KBNNO). These oxides exhibit both ferroelectricity and a wide variation of direct bandgaps in the range 1.1-3.8 electronvolts. In particular, the x = 0.1 composition is polar at room temperature, has a direct bandgap of 1.39 electronvolts and has a photocurrent density approximately 50 times larger than that of the classic ferroelectric (Pb,La)(Zr,Ti)O3 material. The ability of KBNNO to absorb three to six times more solar energy than the current ferroelectric materials suggests a route to viable ferroelectric semiconductor-based cells for solar energy conversion and other applications.

We use density functional theory (DFT) calculations to study the lattice vibrations and electronic properties of the correlated metal LaNiO3. To characterize the rhombohedral to cubic structural phase transition of perovskite LaNiO3, we examine the evolution of the Raman-active phonon modes with temperature. We find that the A1g Raman mode, whose frequency is sensitive to the electronic band structure, is a useful signature to characterize the octahedral rotations in rhombohedral LaNiO3. We also study the importance of electron-electron correlation effects on the electronic structure with two approaches which go beyond the conventional band theory (local spin density approximation): the local spin density+Hubbard U method (LSDA+U ) and hybrid exchange-correlation density functionals which include portions of exact Fock-exchange. We find the conventional LSDA accurately reproduces the delocalized nature of the valence states in LaNiO3 and gives the best agreement to the available experimental data on the electronic structure of LaNiO3. Based on our calculations, we show that the electronic screening effect from the delocalized Ni 3d and O-2p states mitigate the electronic correlations of the d 7 Ni cations, making LaNiO3 a weakly correlated metal.