Proceedings of September Meeting 2016 (NFM16)
Publication date: 14th June 2016
Charge carrier dynamics near the surface of a semiconductor crucially determine the efficiency of photoelectrochemical (PEC) hydrogen production and, in the most basic picture, results from the competition of carrier recombination and carrier injection into the electrolyte. To increase the carrier lifetime at the surface of the absorber an appropriate surface functionalization can be applied. Such a suitable layer can both passivate interfacial surface defects, which act as recombination states, and the created heterojunction can prevent the majority carriers from reaching the surface, thus also reducing surface recombination. We have employed femtosecond-resolved two-photon photoemission spectroscopy (fs-2PPE) as an explicitly surface-sensitive method to study the dynamics of surface electrons in Cu2O single crystals for different crystal facets and surface reconstructions. By measuring the number of photoemitted electrons as a function of pump-probe time delay, we determined the corresponding quantity of surface electrons. For the well-defined surface reconstructions of the Cu2O(111) facet obtained by sputtering and annealing cycles, a significant increase of surface electron lifetime was found compared to the untreated surface. Simultaneous analysis of the kinetic energy spectra of photoemitted electrons indicated the presence of a long-lived surface state. Subsequent growth of thin TiO2 layers on the reconstructed Cu2O surface by atomic layer deposition (ALD) decreased the surface electron lifetime by several orders of magnitude, most probably due to the introduction of defect states during the ALD process.
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