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Resonant light trapping in ultrathin films for water splitting

Nature Materials volume 12, pages 158–164 (2013)Cite this article

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Abstract

Semiconductor photoelectrodes for solar hydrogen production by water photoelectrolysis must employ stable, non-toxic, abundant and inexpensive visible-light absorbers. Iron oxide (α-Fe2O3) is one of few materials meeting these requirements, but its poor transport properties present challenges for efficient charge-carrier generation, separation, collection and injection. Here we show that these challenges can be addressed by means of resonant light trapping in ultrathin films designed as optical cavities. Interference between forward- and backward-propagating waves enhances the light absorption in quarter-wave or, in some cases, deeper subwavelength films, amplifying the intensity close to the surface wherein photogenerated minority charge carriers (holes) can reach the surface and oxidize water before recombination takes place. Combining this effect with photon retrapping schemes, such as using V-shaped cells, provides efficient light harvesting in ultrathin films of high internal quantum efficiency, overcoming the trade-off between light absorption and charge collection. A water photo-oxidation current density of 4 mA cm−2 was achieved using a V-shaped cell comprising ∼ 26-nm-thick Ti-doped α-Fe2O3 films on back-reflector substrates coated with silver–gold alloy.

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Figure 1: Resonant light trapping in quarter-wave films.
Figure 2: Light intensity maps.
Figure 3: Sunlight absorption in α-Fe2O3 films on back-reflector substrates versus their counterparts on transparent substrates.
Figure 4: Internal quantum efficiency map.
Figure 5: Photocurrent distribution maps.
Figure 6: Photocurrent as a function of film thickness.
Figure 7: Water photo-oxidation current densities obtained with ultrathin film α-Fe2O3 photoanodes on silver-based back reflectors.

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Acknowledgements

This work was supported by the European Commission Seventh Framework Programme (NanoPEC, project 227179), by the I-CORE Program of the Planning and Budgeting Committee and The Israel Science Foundation (grant No. 152/11) and by the KAMIN project from the Office of the Chief Scientist (OCS) in the Ministry of Industry, Trade and Labor (MOITAL). The authors thank S. C. Warren, G. Bartal and N. Tessler for reading the manuscript and providing useful comments and suggestions for improving it.

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Authors and Affiliations

  1. Department of Materials Science and Engineering, Technion—Israel Institute of Technology, Haifa 32000, Israel

    Hen Dotan, Elad Sharlin, Oshri Blank, Moran Gross, Irina Dumchin & Avner Rothschild

  2. Physics Department, Technion—Israel Institute of Technology, Haifa 32000, Israel

    Ofer Kfir

  3. Photovoltaics Laboratory, Technion—Israel Institute of Technology, Haifa 32000, Israel

    Guy Ankonina

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  1. Hen Dotan
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Contributions

H.D. and O.K. developed the optical simulation model, and H.D. and A.R. added to it the charge transport model. H.D. designed and fabricated the photoelectrodes and carried out most of the optical and photoelectrochemical measurements. E.S. and O.B. developed the stable silver–gold alloy back reflectors. H.D. and M.G. developed the selective electron transport hole blocking under layers. I.D. carried out the cross-section transmission electron microscope measurements. G.A. carried out the spectroscopic ellipsometry measurements and analysis. A.R. supervised the project and wrote the manuscript. All authors discussed the manuscript and agreed on its final content.

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Correspondence to Avner Rothschild.

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Dotan, H., Kfir, O., Sharlin, E. et al. Resonant light trapping in ultrathin films for water splitting. Nature Mater 12, 158–164 (2013). https://doi.org/10.1038/nmat3477

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