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 properties1,2,3,4,5,6,7. 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 cell2,6,8,9,10. 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 sputtering3,5. Recent work3,5,11 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: [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.
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Members of the Davies group—D.V.W., D.M.S., L.W. and P.K.D.—were supported by the Energy Commercialization Institute of BFTP. We also thank M. R. Suchomel for assistance with collection of the synchrotron X-ray data. Use of the Advanced Photon Source at Argonne National Laboratory was supported by the US Department of Energy, Office of Basic Sciences, under contract number DE-AC02-06CH11357. Members of the Spanier group—M.T. and J.E.S.—were supported by the Army Research Office, under grant number W911NF-08-1-0067. G.C. was supported by NSF grant DMR 0907381. A.R.A. was supported by the Energy Commercialization Institute of BFTP and by NSF grant DMR 1124696. E.M.G. was supported by an ASEE Postdoctoral Fellowship. Support for instrumentation used in this project was provided by the ARO DURIP programme and the NSF under grant DMR 0722845. J.E.S. also acknowledges C. L. Schauer for permitting access to the spectroscopic ellipsometer and the Drexel Centralized Research Facilities for access to instrumentation. We thank F. Yan, M. A. Islam and C. L. Johnson for assistance in transparent electrode thin-film deposition, Raman scattering, and sample thinning and polishing, respectively. Of the Rappe group, I.G. was supported by the Department of Energy, Office of Basic Energy Sciences, under grant number DE-FG02-07ER46431, G.G. was supported by the Energy Commercialization Institute and A.M.R. was supported by the Office of Naval Research, under grant number N00014-12-1-1033. Computational support was provided by a Challenge Grant from the High Performance Computing Modernization Office of the US Department of Defense and the National Energy Research Scientific Computing Center of the US Department of Energy.
The authors declare no competing financial interests.
Extended data figures and tables
a, Dielectric data for x = 0.1–0.4 KBNNO. Two dielectric anomalies (arrows) at about 450 K and about 600 K are present (solid lines indicate heating; dotted lines indicate cooling). b, Ferroelectric hysteresis loops at 77 K, showing the effect of increasing the maximum poling voltage. c, Ferroelectric hysteresis loop for approximately 20-μm-thick x = 0.1 KBNNO film at 170–200 K. d, Ferroelectric hysteresis loop for approximately 20-μm-thick x = 0.1 KBNNO film at 300 K.
Band structures (top) and orbital-projected density of states (PDOS, bottom) for KBNNO Ni–VO–Ni and Ni–VO–Nb solid solutions near the Fermi level. The high-symmetry points in the Brillouin zone are Γ (0, 0, 0) A (−0.5, 0.5, 0), B (−0.5, 0, 0), D (−0.5, 0, 0.5), E (−0.5, 0.5, 0.5), Z (0, 0, 0.5), C (0, 0.5, 0.5) and Y (0, 0.5, 0). k is the wavevector. The more stable Ni–VO–Nb structure provides a smaller bandgap. As Ni concentration rises, Ni–VO–Ni becomes more common and the bandgap energy rises.
Extended Data Figure 3 Switchable bulk photovoltaic effect in KBNNO and the dependence of photocurrent on poling.
Ferroelectric photovoltaic effect for approximately 20-μm-thick x = 0.1 KBNNO film in ambient conditions under 4 mW cm−2 of above-bandgap illumination following poling by an 80-V pulse applied for 300 s (a), a 50-V pulse applied for 300 s (b), a 50-V pulse applied for 180 s (c), a 50-V pulse applied for 30 s (d) and a 50-V pulse applied for 10 s under 4 mW cm−2 of above-bandgap illumination (e). Black denotes collected dark current; blue and red traces indicate photocurrent following poling under positive and negative voltages, respectively. f, Short-circuit photocurrent Isc for different product of duration and magnitude of poling voltage. The current is collected through 200 µm × 200 µm ITO and Cr–Au electrodes on the top and bottom of the sample, respectively. The height of each error bar is two standard deviations in the measured short-circuit current. As the applied voltage and poling time are increased, the difference between the photocurrents for the up- and down-polarized sample increases and the photocurrent magnitude rises by two orders of magnitude until saturation caused by leakage. This indicates that the sample is not yet fully poled even for the highest voltage possible in our set-up. Therefore, our results are the lower limit for the photocurrent for a fully poled material that can be achieved by application of larger electric fields in thinner films.
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Grinberg, I., West, D., Torres, M. et al. Perovskite oxides for visible-light-absorbing ferroelectric and photovoltaic materials. Nature 503, 509–512 (2013). https://doi.org/10.1038/nature12622
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