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Operando electrochemical study of charge carrier processes in water splitting photoanodes protected by atomic layer deposited TiO2


Cui, Wei; Moehl, Thomas; Siol, Sebastian; Tilley, S David (2019). Operando electrochemical study of charge carrier processes in water splitting photoanodes protected by atomic layer deposited TiO2. Sustainable Energy Fuels, 3(11):3085-3092.

Abstract

Semiconductor-based solar energy conversion devices are often multilayer structures with each layer serving a distinct purpose towards generating an efficient and stable device. In water splitting, the use of atomic layer deposited TiO2 (ALD-TiO2) layers enables the stable operation of materials that would normally photocorrode in the aqueous electrolyte. Interestingly, thick ALD-TiO2 (>50 nm) has been successfully used to protect high performance photoanodes, despite an apparent band mismatch that should preclude charge transfer. The understanding of the charge transfer through the relatively thick TiO2 layer remains controversial and warrants further study. Here, we introduce an operando methodology to study charge carrier processes in the ALD-TiO2 protected photoanode by utilizing photoelectrochemical impedance spectroscopy (PEIS) combined with the dual-working-electrode (DWE) technique to resolve if the charge transport through the TiO2 is a conduction band process or involves a hopping through defect states. Two silicon-based systems were evaluated, one featuring a buried homojunction (np+Si/TiO2/Ni) and the other a purely n-type Si directly interfaced with TiO2 (nSi/TiO2/Ni). The additional series resistance imparted by the TiO2 layer (RTiO2) was extracted from the PEIS measurements. Both the potential and thickness dependence of RTiO2 were analyzed, and the DWE technique enabled the sensing of the potential of the TiO2 layer under operation, indicating a strong band bending with the conduction band even more positive than the oxygen evolution potential. Together, these data suggest a conduction band-based transport mechanism, in spite of the presence of defect states in the bandgap of ALD-TiO2, and a detailed picture of the charge transfer through the multilayer structured photoanodes was obtained.

Abstract

Semiconductor-based solar energy conversion devices are often multilayer structures with each layer serving a distinct purpose towards generating an efficient and stable device. In water splitting, the use of atomic layer deposited TiO2 (ALD-TiO2) layers enables the stable operation of materials that would normally photocorrode in the aqueous electrolyte. Interestingly, thick ALD-TiO2 (>50 nm) has been successfully used to protect high performance photoanodes, despite an apparent band mismatch that should preclude charge transfer. The understanding of the charge transfer through the relatively thick TiO2 layer remains controversial and warrants further study. Here, we introduce an operando methodology to study charge carrier processes in the ALD-TiO2 protected photoanode by utilizing photoelectrochemical impedance spectroscopy (PEIS) combined with the dual-working-electrode (DWE) technique to resolve if the charge transport through the TiO2 is a conduction band process or involves a hopping through defect states. Two silicon-based systems were evaluated, one featuring a buried homojunction (np+Si/TiO2/Ni) and the other a purely n-type Si directly interfaced with TiO2 (nSi/TiO2/Ni). The additional series resistance imparted by the TiO2 layer (RTiO2) was extracted from the PEIS measurements. Both the potential and thickness dependence of RTiO2 were analyzed, and the DWE technique enabled the sensing of the potential of the TiO2 layer under operation, indicating a strong band bending with the conduction band even more positive than the oxygen evolution potential. Together, these data suggest a conduction band-based transport mechanism, in spite of the presence of defect states in the bandgap of ALD-TiO2, and a detailed picture of the charge transfer through the multilayer structured photoanodes was obtained.

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Additional indexing

Item Type:Journal Article, refereed, original work
Communities & Collections:07 Faculty of Science > Department of Chemistry
08 Research Priority Programs > Solar Light to Chemical Energy Conversion
Dewey Decimal Classification:540 Chemistry
Scopus Subject Areas:Physical Sciences > Renewable Energy, Sustainability and the Environment
Physical Sciences > Fuel Technology
Physical Sciences > Energy Engineering and Power Technology
Language:English
Date:1 January 2019
Deposited On:07 Feb 2020 16:02
Last Modified:29 Jul 2020 14:08
Publisher:Royal Society of Chemistry
ISSN:2398-4902
OA Status:Hybrid
Free access at:Publisher DOI. An embargo period may apply.
Publisher DOI:https://doi.org/10.1039/c9se00399a
Project Information:
  • : FunderSNSF
  • : Grant IDPYAPP2_160586
  • : Project TitleSolar Water Splitting: Photovoltage, Surface Dipole, and Catalysis Strategies

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