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Extraction of mobile charge carrier photogeneration yield spectrum of ultrathin-film metal oxide photoanodes for solar water splitting

An Author Correction to this article was published on 09 September 2021

Abstract

Light absorption in strongly correlated electron materials can excite electrons and holes into a variety of different states. Some of these excitations yield mobile charge carriers, whereas others result in localized states that cannot contribute to photocurrent. The photogeneration yield spectrum, ξ(λ), represents the wavelength-dependent ratio between the contributing absorption that ultimately generates mobile charge carriers and the overall absorption. Despite being a vital material property, it is not trivial to characterize. Here, we present an empirical method to extract ξ(λ) through optical and external quantum efficiency measurements of ultrathin films. We applied this method to haematite photoanodes for water photo-oxidation, and observed that it is self-consistent for different illumination conditions and applied potentials. We found agreement between the extracted ξ(λ) spectrum and the photoconductivity spectrum measured by time-resolved microwave conductivity. These measurements revealed that mobile charge carrier generation increases with increasing energy across haematite’s absorption spectrum. Low-energy non-contributing absorption fundamentally limits the photoconversion efficiency of haematite photoanodes and provides an upper limit to the achievable photocurrent that is substantially lower than that predicted based solely on absorption above the bandgap. We extended our analysis to TiO2 and BiVO4 photoanodes, demonstrating the broader utility of the method for determining ξ(λ).

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Fig. 1: Extraction of the photogeneration yield spectrum from optical and photoelectrochemical EQE measurements of a 7-nm-thick haematite film.
Fig. 2: TRMC measurements of a 150-nm-thick haematite film.
Fig. 3: Comparison of TRMC and photoelectrochemical EQE analysis.
Fig. 4: Contributing and non-contributing components of the absorption spectra.

Data availability

The authors declare that the data supporting the findings of this study are available within the paper and its Supplementary Information files.

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Acknowledgements

D.S.E., D.A.G. and Y.P. acknowledge G. Ankonina for generously assisting on technical matters whenever needed in the Technion’s Photovoltaics Laboratory, and also A. Inbar for assisting in the EQE measurements in this work. We thank G. Atiya for the TEM measurements, L. Popilevsky from the FIB Lab at the Technion’s Russell Berrie Nanotechnology Institute (RBNI) for preparing the TEM sample, and J. N. Hilfiker from J. A. Woollam Co. for helpful correspondence regarding ellipsometry analysis. The research leading to these results received funding from the PAT Center of Research Excellence supported by the Israel Science Foundation (grant no. 1867/17). The EQE and optical measurements were carried out at the Technion’s Photovoltaics Laboratory (HTRL), supported by the RBNI, the Nancy and Stephen Grand Technion Energy Program (GTEP) and the Adelis Foundation. Part of this research was carried out within the Helmholtz International Research School ‘Hybrid Integrated Systems for Conversion of Solar Energy’ (HI-SCORE), an initiative co-funded by the Initiative and Networking Fund of the Helmholtz Association. Part of the work was funded by the Volkswagen Foundation. D.A.G. acknowledges support from the Center for Absorption in Science of the Ministry of Aliyah and Immigrant Absorption in Israel. Y.P. acknowledges support by GTEP and for a Levi Eshkol scholarship from the Ministry of Science and Technology of Israel. A.R. acknowledges the support of the L. Shirley Tark Chair in Science.

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D.A.G. and H.D. conceived the idea and initiated the research. D.A.G., D.S.E. and Y.P. developed the methodology for the ultrathin-film spatial collection efficiency analysis. D.A.G., Y.P. and P.S. fabricated the haematite, BiVO4 and TiO2 photoanodes. Y.P. performed the ellipsometry analysis. D.S.E. designed the EQE experiment. M.K., D.F., F.F.A. and D.A.G. designed the TRMC experiments with the help of R.v.d.K., D.A.G., D.S.E., Y.P., M.K., D.F. and A.K. performed the characterizations and data analysis. D.A.G. and D.S.E. wrote the first draft of the manuscript. A.R. supervised the project. All authors contributed to the scientific discussion and editing of the manuscript.

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Correspondence to Daniel A. Grave or Avner Rothschild.

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Grave, D.A., Ellis, D.S., Piekner, Y. et al. Extraction of mobile charge carrier photogeneration yield spectrum of ultrathin-film metal oxide photoanodes for solar water splitting. Nat. Mater. 20, 833–840 (2021). https://doi.org/10.1038/s41563-021-00955-y

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