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Energy Conversion and Storage; Plasmonics and Optoelectronics
Charge Separation, Band-Bending and Recombination in WO3 Photoanodes Sacha Corby, Ernest Pastor, Yifan Dong, Xijia Zheng, Laia Francàs, Michael Sachs, Shababa Selim, Andreas Kafizas, Artem A. Bakulin, and James R. Durrant J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.9b01935 • Publication Date (Web): 16 Aug 2019 Downloaded from pubs.acs.org on August 16, 2019
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The Journal of Physical Chemistry Letters
Charge Separation, Band-Bending and Recombination in WO3 Photoanodes Sacha Corby1, Ernest Pastor1, Yifan Dong1, Xijia Zheng1, Laia Francàs1, Michael Sachs1, Shababa Selim1, Andreas Kafizas1,2, Artem A. Bakulin1 and James R. Durrant2* 1Department
of Chemistry, Imperial College London, White City Campus, 80 Wood Lane, London, W12 0BZ, UK 2Grantham Institute for Climate Change, Imperial College London, South Kensington, London, SW7 2AZ, UK *corresponding:
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Abstract In metal oxide-based photoelectrochemical devices, the spatial separation of photogenerated electrons and holes is typically attributed to band-bending at the oxide / electrolyte interface. However, direct evidence of such band-bending impacting upon charge carrier lifetimes has been very limited to date. Herein we use ultrafast spectroscopy to track the rapid relaxation of holes in the space-charge layer and their recombination with trapped electrons in WO3 photoanodes. We observe that applied bias can significantly increase carrier lifetimes on all timescales from picoseconds to seconds, and attribute this to enhanced band-bending correlated with changes in oxygen vacancy state occupancy. We show that analogous enhancements in carrier lifetimes can be obtained by changes in electrolyte composition, even in the absence of applied bias, highlighting routes to improve photoconversion yields/performance, through changes in band-bending. This study thus demonstrates the direct connection between carrier lifetime enhancement, increased band-bending and oxygen vacancy defect state occupancy.
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The Journal of Physical Chemistry Letters
Light-driven water oxidation is a kinetically challenging reaction, and as such requires holes to be long lived.1-4 Metal oxides are the most frequently studied material class for both photoelectrochemical and photocatalytic water oxidation. Most models of oxide photoelectrode function assume that charge carrier longevity, and the ability of slow surface chemistry to compete against faster recombination processes, is the product of surface bandbending.5-7 However, actual studies of the role of band-bending and concomitant spacecharge layer (SCL) formation on the dynamics of photogenerated charge carriers at early times are very limited. Long-lived holes (micro-second to second timescales) with lifetimes capable of driving water oxidation catalysis have been observed under anodic bias in several systems.8-10 However, the yield of these long-lived holes, and thus the final quantum yield for a device, is primarily determined by the efficiency of charge separation that occurs on much faster timescales. Despite many studies of charge carrier dynamics on ultrafast timescales in such photoanodes, there is, to the best of our knowledge, only one report demonstrating any relation between applied field and carrier lifetime,11 even with the widely accepted importance of band-bending induced charge separation in most models of photoelectrochemical function. The role of band-bending is also of particular importance, and poorly understood, for particulate photocatalyst systems, which may evolve oxygen without bias and sometimes without scavenger,12,13 suggesting that electrolyte-induced band-bending alone may be adequate to inhibit recombination processes and enable water oxidation. However, the extent to which either bias- or electrolyte-induced band-bending impacts upon early carrier dynamics, and ultimately device quantum yields, remains unclear. In this work, we examine the ultrafast kinetics of electrons and holes in nanostructured WO 3 photoanodes, a promising, robust material for solar water oxidation. WO3, with a band gap of ~2.6 eV, is generally reported to have a high intrinsic doping density as a result of readily formed oxygen vacancy defects.14,15 These defects manifest as reduced tungsten centres neighbouring sites absent in oxygen, and thus give rise to a characteristic broad W5+ absorption at red wavelengths, rising into the near IR.16,17 There is ongoing discussion in the literature regarding the effects of these states on charge transport and catalysis, with proponents claiming improved conductivity and critics proposing increased trap-mediated recombination.18-23 These states lie energetically below the conduction band edge, with a fairly large energetic distribution.17,24 Recent studies of ultrafast kinetics in WO3 have shown that the presence of large numbers of such occupied vacancy states results in rapid trapping of photogenerated holes (
