A few days ago an important paper has been published
Nechache, C. Harnagea, S. Li, L. Cardenas, W. Huang, J. Chakrabartty &F. Rosei
Nature Photonics (2014) doi:10.1038/nphoton.2014.255
The paper demonstrates 8.1% power conversion efficiency (PCE) at 1 sun light intensity, using a double perovskite thin film Bi2FeCrO6. This means that the paper reports the first serious efficiency of “ferroic photovoltaics”, a possible new class of photovoltaic (PV) solar energy converters that was incipiently developed so far and now shows itself with great force.
Ferroic photovoltaics came to general notice by the appearance of ferroic effects in hybrid halide perovskites (that reached officially 20% PCE also last week).1-3 In the organic-lead-iodide perovskite CH3NH3PbI3, however, ferroic behavior has been associated more to a nuisance than to a real advantage, although some, including this writer, think that there are new phenomena in there that may show great potential for development, and indeed this has been shown by Rosei and coworkers, albeit not with hybrid but with inorganic oxide perovskites.
Ferroelectric materials are materials with a low symmetry phase below a certain phase transition temperature, that are able to maintain spontaneous electric polarization. The polarization, may be not unique, as often there are equivalent types of distortions of the lattice, that allows the formation regions with different orientation of polarization. These are called domains, and are separated by a very thin domain wall. When an external electric field is applied, domains are oriented preferentially along with the applied field producing a macroscopic polarization. It can be switched if the field is applied in the opposite direction. In general, materials are called ferroic if they have the ability to experience substantial polarization even if it is not stable over a long time. It is also very important to take into account the properties of contacts, since they hold free electronic charge that cancels out the ionic bound charge. Tiny spatial distribution of the charge at contacts may create a large depolarizing field that counteracts the permanent polarization and finally removes it.
The PV effect in ferroelectric materials depends on the polarization-induced internal electric field, but the principles that cause the observed response are unclear yet. Regular inorganic perovskites, mainly oxides ABO3, often show internal enhanced electrical fields by ferroelectric effect that can be exploited for PV applications. It is known since 50 years that ferroelectric perovskites may produce a large photovoltage exceeding by far the bandgap on single crystal materials.4,5 However, most metal oxide perovskites with ferroelectric properties ABO3 are unsuitable to harvest of the solar spectral photons, as they have a large bandgap of 3-4 eV. This property is due to the fact that the main optical transition occurs between the oxygen 2p states and the B transition metal d states, that involves a large difference of electronegativity. Recently there has been success in the synthesis of materials closer to the required 1.5 eV by using elements that form a less ionic bond with O, showing promising PV properties.6,7 In general these materials are single crystal oxides with very low PV performance due to very small current and low fill factor, as the mechanism of photovoltage generation is ohmic and only works well in high resistance cells.
The larger than bandgap voltage cannot be explained by the regular mechanism of separation of Fermi levels in a homogeneous absorber layer. One assumption to explain these phenomena relies on a local disposition of electric fields formed by spontaneous polarization over short distances of a few nm. This structure causes local steps of built-in potential as suggested by recent observations of domain wall structure at the nanoscale.8 However other experimental results indicate that the large voltage observed in epitaxial BiFeO3 films may be attributed to the “Bulk photovoltaic effect”, that is created by an asymmetric generation of current, or photogalvanic effect.9
At present it is still under discussion, if a ferroic PV material can form a directional current by local microscopic reasons so that the current might not be driven by the gradient of electrochemical potential. This would be a convincing reason for distinction with respect to all the rest of solar cells that operate with thermalized carriers.
The singular result of the paper by Rosei et al. is to develop a ferroic oxide Bi2FeCrO6 that for the first time provides a non-negligible photocurrent and PCE. The result is based on two combined breakthroughs. The first is a control of long range ordering of the cationic components of the perovskite, Fe and Cr. In this way they reduce the absortpion edge from aprox. 2.5 eV of the parent compositions with Fe or Cr, to an impressive 1.5 eV, thanks to charge transfer excitations between Fe and Cr. Nevertheless, the absorption of each layer is too low to provide sufficient photocurrent.
Having shown that one may tune the bandgap by controlling the cationic ordering, the second important development is to fabricate multilayer structures with a graded bandgap. This realization enhances the current to 20 mA cm-2, giving the PCE of 8.1%. However, the device needs to be under very strong polarization, and the energy cost of maintaining such polarization is unclear from the report.
The results of this paper show the enormous potential for this new class of solar cells. Of course many new questions are open about how it works, and what it may achieve. It seems very likely that a community will be formed soon to investigate these interesting ferroic photovoltaics.
(1) Frost, J. M.; Butler, K. T.; Brivio, F.; Hendon, C. H.; van Schilfgaarde, M.; Walsh, A. Atomistic Origins of High-Performance in Hybrid Halide Perovskite Solar Cells, Nano Letters 2014, 14, 2584-2590.
(2) Sanchez, R. S.; Gonzalez-Pedro, V.; Lee, J.-W.; Park, N.-G.; Kang, Y. S.; Mora-Sero, I.; Bisquert, J. Slow dynamic processes in lead halide perovskite solar cells. Characteristic times and hysteresis, The Journal of Physical Chemistry Letters 2014, 5, 2357−2363.
(3) Kutes, Y.; Ye, L.; Zhou, Y.; Pang, S.; Huey, B. D.; Padture, N. P. Direct Observation of Ferroelectric Domains in Solution-Processed CH3NH3PbI3 Perovskite Thin Films, The Journal of Physical Chemistry Letters 2014, 5, 3335-3339.
(4) Chynoweth, A. G. Surface Space-Charge Layers in Barium Titanate, Physical Review 1956, 102, 705-714.
(5) Neumark, G. F. Theory of the Anomalous Photovoltaic Effect of ZnS, Physical Review 1962, 125, 838-845.
(6) Grinberg, I.; West, D. V.; Torres, M.; Gou, G.; Stein, D. M.; Wu, L.; Chen, G.; Gallo, E. M.; Akbashev, A. R.; Davies, P. K.; Spanier, J. E.; Rappe, A. M. Perovskite oxides for visible-light-absorbing ferroelectric and photovoltaic materials, Nature 2013, 503, 509-512.
(7) K. Rezaie, F.; Panjwani, D.; Nath, J.; Fredricksen, C. J.; Oladeji, I. O.; Peale, R. E. “Junctionless thin-film ferroelectric oxides for photovoltaic energy production”; Energy Harvesting and Storage: Materials, Devices, and Applications V, Baltimore, Maryland, USA | May 05, 2014, 2014.
(8) Yang, S. Y.; Seidel, J.; Byrnes, S. J.; Shafer, P.; Yang, C. H.; Rossell, M. D.; Yu, P.; Chu, Y. H.; Scott, J. F.; Ager, J. W.; Martin, L. W.; Ramesh, R. Above-bandgap voltages from ferroelectric photovoltaic devices, Nat Nano 2010, 5, 143-147.
(9) Bhatnagar, A.; Roy Chaudhuri, A.; Heon Kim, Y.; Hesse, D.; Alexe, M. Role of domain walls in the abnormal photovoltaic effect in BiFeO3, Nat Commun 2013, 4.