Perovskite oxides for visible-light-absorbing ferroelectric and photovoltaic materials

• Published in: Nature (London) 2013-11, Vol.503 (7477), p.509-512
• Main Authors: Grinberg, Ilya, West, D. Vincent, Torres, Maria, Gou, Gaoyang, Stein, David M., Wu, Liyan, Chen, Guannan, Gallo, Eric M., Akbashev, Andrew R., Davies, Peter K., Spanier, Jonathan E., Rappe, Andrew M.
• Format: Article
• Language: English
• Subjects: 639/301/119/996; 639/301/299/946; 639/624/1075/524; Condensed matter: electronic structure, electrical, magnetic, and optical properties; Design and construction; Dielectrics, piezoelectrics, and ferroelectrics and their properties; Energy conversion; Exact sciences and technology; Ferroelectric devices; Ferroelectricity and antiferroelectricity; Ferroelectrics; Humanities and Social Sciences; letter; multidisciplinary; Oxides; Perovskite; Photovoltaics; Physics; Properties; Science; Solar cells; Solar energy; Solid solutions; Spectrum analysis; Switching phenomena; Temperature; Thin films
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature12622

Most known ferroelectric photovoltaic materials have very wide electronic bandgaps (that is, they absorb only high-energy photons) but here a family of perovskite oxides is described that have tunable bandgaps, allowing their use across the whole visible-light spectrum. Making the most of the Sun The spontaneous electrical polarization that characterizes a ferroelectric material is attractive for solar-cell applications as the positive and negative charges generated by light absorption have a natural tendency to separate, making them easier to harvest efficiently. Unfortunately most known ferroelectrics have wide electronic bandgaps — that is they absorb only higher energy photons that make up a small fraction of the solar spectrum. Ilya Grinberg and colleagues now show that a classic ferroelectric can be chemically engineered to tune the bandgap over a broad range, achieving strong absorption and photocurrent generation across the solar spectrum. 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 1 , 2 , 3 , 4 , 5 , 6 , 7 . 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 2 , 6 , 8 , 9 , 10 . 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 3 , 5 . Recent work 3 , 5 , 11 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: [KNbO 3 ] 1 −   x [BaNi 1/2 Nb 1/2 O 3 −   δ ] x (KBNNO). These oxides exhibit both ferroelectricity a