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

Ruoko, Tero-Petri, Hiltunen, Arto, Iivonen, Tomi, Ulkuniemi, Riina, Lahtonen, Kimmo, Ali-Löytty, Harri, Mizohata, Kenichiro, Valden, Mika, Leskelä, Markku, Tkachenko, Nikolai V.
Journal of materials chemistry A 2019 v.7 no.7 pp. 3206-3215
capacitance, chemistry, dielectric spectroscopy, electric current, electric potential difference, hematite, irradiation, light, tantalum oxide
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 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 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.