Most low bandgap metal oxide materials with water splitting potential are not well explored with respect to charge carrier transport properties and carrier lifetimes. A variety of promising oxide based photoelectrocatalyst materials have been identified that satisfy energetic requirements for photocatalysis but almost all suffer from low mobility and small carrier diffusion lengths limiting the photocatalytic performance [1]. This substantially depends on the individual preparation techniques applied for thin film deposition of photoelectroactive layers. For an advanced development of metal oxide photoelectrodes it is crucial to understand how material composition is related to carrier transport and capture dynamics in a wide time range.

We investigate bulk and interface reactions in metal oxides such as BiVO4 and Fe2O3 with time resolved spectroscopy from femtoseconds to milliseconds applying transient absorption, THz and microwave conductivity in transmission and reflection mode. The conductivity measurements reveal the photo-excited carrier lifetime in dependence of (oxygen)vacancies, interstitials, impurities or doping while optical absorption supplies information on the dynamics and energetics of bandgap states. Also, THz mobility spectra recorded at different times after photoexcitation yield profiles of the real and imaginary parts of the conductivity that allow for microscopic understanding of the transport mechanisms.

Sub-picosecond THz photoconductivity is demonstrated for the first time for Fe2O3 prepared by pulsed laser deposition exhibiting ultrafast decay on the order of a picosecond in agreement with transient absorption measurements [2]. Non-Drude conductivity is demonstrated for BiVO4 doped/undoped spray pyrolysed film and single crystal samples. The spectra suggest weakly-bound carriers in agreement with recent reports [3].

[1] A. J. Cowan, J. R. Durrant, Chem. Soc. Rev. 42, 2281 (2013)

[2] A. G. Joly, J. R. Williams, S. A. Chambers, G. Xiong, W. P. Hess, D. M. Laman, J. Appl. Phys. 99, 053521 (2006)

[3] F. F. Abdi, T. J. Savenije, M. May, B. Dam, R. van de Krol, J. Phys. Chem. Lett. 4, 2752 (2013)

Rainer Eichberger

Helmholtz-Zentrum Berlin

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