Charge carrier dynamics in tantalum oxide overlayered and tantalum doped hematite photoanodesElectronic supplementary information (ESI) available: XPS elemental analysis table and transient absorption fitting details, decay component spectra, and additional decays. See DOI: 10.1039/c8ta09501a

• Main Authors: Ruoko, Tero-Petri, Hiltunen, Arto, Iivonen, Tomi, Ulkuniemi, Riina, Lahtonen, Kimmo, Ali-Löytty, Harri, Mizohata, Kenichiro, Valden, Mika, Leskelä, Markku, Tkachenko, Nikolai V
• Format: Article
• Language: English
• ISSN: 2050-7488; 2050-7496
• DOI: 10.1039/c8ta09501a

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. The effects that Ta 2 O 5 -overlayer and Ta-doping have on the photoelectrochemical performance and surface state capacitance of hematite photoanodes.