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Electronic Dynamics in Photocatalytic and Energy Transporting Systems

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Electronic Dynamics in Photocatalytic and Energy Transporting Systems


S. Lochbrunner1*, F. Fennel1, S. Wolter1, A. Friedrich1, C. Merschjann1, S. Tschierlei1

1Institute of Physics, University of Rostock, Universitätsplatz 3, 18055 Rostock, Germany

Femtosecond pump-probe experiments on photocatalytic and organic model systems reveal detailed insights into the behavior of their electronic excitations. This is helpful in designing materials, composite structures and interfaces for photonic applications like water splitting devices or organic solar cells. In the case of light harvesting, long living mobile excitons are crucial. To characterize their migration properties the dynamics caused by the bimolecular exciton-exciton annihilation process is analyzed. For J-aggregates, which represent chain like molecular nanostructures, a reasonable high and strictly one dimensional mobility was found [1]. In the case of disordered systems the impact of inhomogenous broadening has to be considered [2]. We developed a formulation of the Förster theory which takes this explicitly into account and tested it for a guest-host system with well-defined spectral properties. The predicted diffusion constants agree well with those derived from the observed exciton dynamics. Most photocatalytic systems rely on efficient charge separation processes. Iridium photosensitizers have proven to be good absorbers for light driven water splitting. Their molecular properties can be well studied in homogeneous systems. Time resolved photoluminescence shows that the electron transfer from a donor substrate to the sensitizer, which is the first step in the photocatalytic reaction path, is surprisingly improbable [3]. However, the long triplet lifetime of the Ir-complex in combination with a high substrate concentration leads nevertheless to an efficient system. The situation is different in a heterogenous system when the sensitizer is at the surface of a semiconductor like TiO2. Then we observe very fast electron injection into the conduction band. A different route is taken with new semiconductor materials like carbon nitride. Here the semiconductor itself absorbs the light and the charge carriers have to get to the surface where the catalytic active sites are. By analyzing the time dependent absorption and fluorescence of carbon nitride we find indications for an anisotropic mobility of the carriers.

References:

[1] S. Wolter, J. Aizezers, F. Fennel, M. Seidel, F. Würthner, O. Kühn, and S. Lochbrunner, New J. Phys. 14, 105027 (2012).

[2] F. Fennel and S. Lochbrunner, Phys. Rev. B 85, 094203 (2012).

[3] A. Neubauer, G. Grell, A. Friedrich, S. Bokarev, P. Schwarzbach, F. Gärtner, A.-E. Surkus, H. Junge, M. Beller, O. Kühn, and S. Lochbrunner, J. Phys. Chem. Lett. 5, 1355 (2014).