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

Tero-Petri Ruoko, Arto Hiltunen, Tomi Iivonen, Riina Ulkuniemi, Kimmo Lahtonen, Harri Ali-Löytty, Kenichiro Mizohata, Mika Valden, Markku Leskelä, Nikolai V. Tkachenko

Research output: Contribution to journalArticleScientificpeer-review

25 Citations (Scopus)
18 Downloads (Pure)

Abstract

We employ atomic layer deposition to prepare 50 nm thick hematite photoanodes followed by passivating them with a 0.5 nm thick Ta 2 O 5 -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 Ta-doped 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 Ta 2 O 5 -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 Ta 2 O 5 -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.

Original languageEnglish
Pages (from-to)3206-3215
Number of pages10
JournalJournal of Materials Chemistry A
Volume7
Issue number7
DOIs
Publication statusPublished - Jan 2019
Publication typeA1 Journal article-refereed

Publication forum classification

  • Publication forum level 2

ASJC Scopus subject areas

  • General Chemistry
  • Renewable Energy, Sustainability and the Environment
  • General Materials Science

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