Charge carrier dynamics in tantalum oxide overlayered and tantalum doped hematite photoanodes

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Ruoko , T-P , Hiltunen , A , Iivonen , T , Ulkuniemi , R , Lahtonen , K , Ali-Löytty , H , Mizohata , K , Valden , M , Leskelä , M & Tkachenko , N V 2019 , ' Charge carrier dynamics in tantalum oxide overlayered and tantalum doped hematite photoanodes ' , Journal of Materials Chemistry. A , vol. 7 , no. 7 , pp. 3206-3215 . https://doi.org/10.1039/c8ta09501a

Title: Charge carrier dynamics in tantalum oxide overlayered and tantalum doped hematite photoanodes
Author: Ruoko, Tero-Petri; Hiltunen, Arto; Iivonen, Tomi; Ulkuniemi, Riina; Lahtonen, Kimmo; Ali-Löytty, Harri; Mizohata, Kenichiro; Valden, Mika; Leskelä, Markku; Tkachenko, Nikolai V.
Contributor: University of Helsinki, Department of Chemistry
University of Helsinki, Department of Physics
University of Helsinki, Department of Chemistry
Date: 2019-02-21
Language: eng
Number of pages: 10
Belongs to series: Journal of Materials Chemistry. A
ISSN: 2050-7488
URI: http://hdl.handle.net/10138/300055
Abstract: We employ atomic layer deposition to prepare 50 nm thick hematite photoanodes followed by passivating them with a 0.5 nm thick Ta2O5-overlayer and compare them with samples uniformly doped with the same amount of tantalum. We observe a three-fold improvement in photocurrent with the same onset voltage using Ta-overlayer hematite photoanodes, while electrochemical impedance spectroscopy under visible light irradiation shows a decreased amount of surface states under water splitting conditions. The Tadoped samples have an even higher increase in photocurrent along with a 0.15 V cathodic shift in the onset voltage and decreased resistivity. However, the surface state capacitance for the Ta-doped sample is twice that of the reference photoanode, which implies a larger amount of surface hole accumulation. We further utilize transient absorption spectroscopy in the sub-millisecond to second timescale under operating conditions to show that electron trapping in both Ta2O5-passivated and Ta-doped samples is markedly reduced. Ultrafast transient absorption spectroscopy in the sub-picosecond to nanosecond timescale shows faster charge carrier dynamics and reduced recombination in the Ta-doped hematite photoanode resulting in the increased photoelectrochemical performance when compared with the Ta2O5-overlayer sample. Our results show that passivation does not affect the poor charge carrier dynamics intrinsic to hematite based photoanodes. The Ta-doping strategy results in more efficient electron extraction, solving the electron trapping issue and leading to increased performance over the surface passivation strategy.
Subject: ELECTRON-HOLE RECOMBINATION
WATER OXIDATION
THIN-FILMS
SURFACE PASSIVATION
ALPHA-FE2O3
PHOTOELECTRODES
LAYER
TA2O5
TIO2
PHOTOOXIDATION
221 Nano-technology
116 Chemical sciences
114 Physical sciences
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