Investigating photovoltage in Fe 2 O 3 photoanodes Louise I. Oldham 1 , Daniele Benetti 1 , Tianying Liu 2 , Dunwei Wang 2 and James R. Durrant 1 1 Department of Chemistry, Imperial College London, 82 Wood Lane, London, W12 0BZ, UK, 2 Department of Chemistry, Merkert Center, Boston College, 2609 Beacon Street, Chestnut Hill, MA 02467-3860, USA Photovoltage describes the additional driving force generated in a semiconductor under illumination, where generation of photoexcited electrons and holes result in quasi-Fermi level splitting (QFLS). It is widely used to characterise the performance of solar cell devices but proves challenging to define in photoelectrochemical (PEC) devices for three main reasons: (1) photoelectrodes only have one electrical contact, making operando measurements of photovoltage difficult; (2) QFLS occurs at the semiconductor/electrolyte interface where it is important to consider surface chemistry and (3) PEC devices often operate under external applied bias rather than open circuit. Previous work has shown that the open circuit potential matches the internal QFLS for fast redox couples, but water oxidation is a slow multi-hole process. 1 Others have highlighted the difference in open circuit and applied bias conditions. 2 Previously we have demonstrated that it is possible to measure the QFLS in metal oxide photoanodes by directly probing photoinduced hole populations under operando conditions. 3 Here, we use Fe 2 O 3 as a model photoanode material to investigate the light intensity dependence of QFLS measured from spectroscopy and understand its relationship to the photovoltage measured from the current-voltage curves. Our results show that the QFLS matches the external photovoltage, and these differ from the change in open circuit potential under light illumination. Under conditions of applied bias, the band bending is fixed and the photovoltage depends on the accumulation of electrochemical populations rather than electrostatics. As the anodic applied bias increases, the amount of photovoltage decreases and its light intensity-dependence increases. At low light intensity, we observe pinning of the hole quasi-Fermi level (E F,p ) to states ~200 meV above the valence band edge. Pinning to these states can be correlated with first order water oxidation behaviour and is not directly associated with oxygen evolution. Only when the applied bias causes sufficient band bending is it possible to accumulate enough holes to fill these states and start accumulating valence band holes at higher light intensities. Under such conditions, the E F,p is no longer pinned to states above the valence band and water is oxidised to oxygen via a third order mechanism. References 1. S. Daemi, A. Kundmann, K. Becker, P. Cendula and F. E. Osterloh, Energy Environ Sci , 2023, 16 , 4530–4538. 2. R. Chen, D. Zhang, Z. Wang, D. Li, L. Zhang, X. Wang, F. Fan and C. Li, J Am Chem Soc , 2023, 145 , 4667–4674. 3. B.Li, L. I. Oldham, L. Tian, G. Zhou, S. Selim, L. Steier and J. R. Durrant, J Am Chem Soc , 2024, 146 , 12324–12328.
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