Proceedings of nanoGe Fall Meeting 2018 (NFM18)
DOI: https://doi.org/10.29363/nanoge.nfm.2018.061
Publication date: 6th July 2018
One of the critical challenges for efficient solar water splitting is the identification of a stable photoelectrode material with a bandgap of 1.6 - 1.9 eV that can be used as a top absorber in an efficient D4 tandem device. Recently, n-type α-SnWO4 was identified as a potential photoanode candidate due to the combination of its ideal bandgap (~1.9 eV) and a very negative photocurrent onset potential (~0 V vs. RHE) [1-3]. However, up to now, α-SnWO4 photoelectrodes have shown very low photoconversion efficiencies. The reason for this is not fully understood, and many of the essential material parameters are still elusive.
In this study, we identify the major limiting parameters of pulsed laser-deposited α-SnWO4 photoanodes: (I) the oxidation of Sn2+ to Sn4+ at the surface creating a hole blocking layer and pushing the photoconversion efficiency to almost zero, and (II) the relatively poor charge carrier dynamics resulting in a mismatch between the charge carrier diffusion length and the light penetration depth. To address these challenges, we explored several strategies such as the deposition of a hole-conducting NiOx protection layer resulting in a new benchmark sulfite oxidation photocurrent of 0.75 mA cm-2 at 1.23 V vs. RHE. We furthermore show that a high-temperature treatment enhances the charge carrier transport by improving the film crystallinity, resulting in a charge carrier diffusion length comparable to state-of-the-art BiVO4 photoanodes. Finally, the remaining challenges for oxygen evolution using our α-SnWO4 films will be discussed.
Our findings provide important insights into the limitations and key properties of α-SnWO4 photoelectrodes and will help to further improve the performance of this promising photoanode material.
[1] Ziani et al., APL Materials, 3 (2015) 096101
[2] Zhu et al., ACS Appl. Mater. Interfaces, 9 (2017) 1459-1470
[3] Pyper et al., Chin. Chem. Lett., 26 (2015) 474-478
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