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Design principles for maximizing photovoltage in metal-oxide-protected water-splitting photoanodes

Nature Materials volume 15, pages 99–105 (2016)Cite this article

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Abstract

Metal oxide protection layers for photoanodes may enable the development of large-scale solar fuel and solar chemical synthesis, but the poor photovoltages often reported so far will severely limit their performance. Here we report a novel observation of photovoltage loss associated with a charge extraction barrier imposed by the protection layer, and, by eliminating it, achieve photovoltages as high as 630 mV, the maximum reported so far for water-splitting silicon photoanodes. The loss mechanism is systematically probed in metal–insulator–semiconductor Schottky junction cells compared to buried junction p+n cells, revealing the need to maintain a characteristic hole density at the semiconductor/insulator interface. A leaky-capacitor model related to the dielectric properties of the protective oxide explains this loss, achieving excellent agreement with the data. From these findings, we formulate design principles for simultaneous optimization of built-in field, interface quality, and hole extraction to maximize the photovoltage of oxide-protected water-splitting anodes.

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Figure 1: Three junction types showing the extraction barrier imposed by the protection layer illustrated with silicon water-oxidation photoanodes.
Figure 2: Photovoltage loss in ALD-TiO2-protected nSi MIS anodes.
Figure 3: Photovoltage loss observed with increasing thickness of the SiO2 interlayer with and without TiO2.
Figure 4: Simulated band diagrams reveal differences between Type I Schottky junction structures and Type 2 p+n buried junction structures.
Figure 5: Charge extraction photovoltage loss eliminated and record photovoltages achieved with Type 2 p+n buried junction cells.

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Acknowledgements

We thank T. Carver for metal e-beam evaporation and all the members of the RENEW collaboration—in addition to the authors: M. Pemble, A. Mills, I. Povey, J. Kegel, K. Cherkaoui, S. Monaghan and D. Hazafy–as well as A. Talin from Sandia for their insightful discussions. T. Burke is also acknowledged for insightful discussions on solar cell physics. A.G.S. would like to thank R. Long, E. Newton, P. F. Satterthwaite, D. Q. Lu and O. Hendricks from the McIntyre and Chidsey groups for their support and insights throughout this work. We also thank K. Tang, L. Zhang and M. Kitano for their help in building and maintaining the ALD chambers. This work was partially supported by the Stanford Global Climate and Energy Project and National Science Foundation programme CBET-1336844. A.G.S. graciously acknowledges financial support from a Stanford Graduate Fellowship and a National Science Foundation Graduate Fellowship. The authors from Tyndall National Institute acknowledge the financial support of Science Foundation (SFI) under the US-Ireland R&D Partnership Programme—Grant Number SFI/13/US/I2543. The Tyndall silicon fabrication facility is acknowledged for the p+n silicon junctions used in this study.

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Authors and Affiliations

  1. Department of Materials Science and Engineering, Stanford University, Stanford, California 94305, USA

    Andrew G. Scheuermann, John P. Lawrence, Kyle W. Kemp, T. Ito & Paul C. McIntyre

  2. Tokyo Electron Limited, Technology Development Center, 650, Hosaka-cho Mitsuzawa, Nirasaki, Yamanashi 407-0192, Japan

    T. Ito

  3. Tyndall National Institute, University College Cork, Cork, Ireland

    Adrian Walsh & Paul K. Hurley

  4. Department of Chemistry, Stanford University, Stanford, California 94305, USA

    Christopher E. D. Chidsey

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  1. Andrew G. Scheuermann
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Contributions

A.G.S. and J.P.L. prepared all samples and performed all measurements for the initial TiO2 photovoltage series first observing the photovoltage loss. A.G.S. prepared all samples and performed all measurements for the SiO2 photovoltage series, capacitance voltage measurements, and buried junction p+nSi experiments. A.W. prepared all p+n buried junction substrates and performed all physical characterization of these junctions. K.W.K. performed Sentaurus modelling for light absorption in Type I and Type II cells, as well as simulating band diagrams. T.I. performed the SPA plasma oxidation for the experiments varying SiO2 thickness. A.G.S. and K.W.K. maintained the ALD chamber for TiO2 depositions and A.G.S. qualified and performed the runs. A.G.S., C.E.D.C. P.K.H. and P.C.M. designed the experiments and developed the solid-state capacitor model to explain the loss. All authors helped in the preparation of the manuscript.

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Correspondence to Paul C. McIntyre.

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Scheuermann, A., Lawrence, J., Kemp, K. et al. Design principles for maximizing photovoltage in metal-oxide-protected water-splitting photoanodes. Nature Mater 15, 99–105 (2016). https://doi.org/10.1038/nmat4451

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