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Artur Braun1 Yelin Hu1 2 Michael Grätzel2

1, Empa. Swiss Federal Laboratories for Materials Science and Technology, Duebendorf, , Switzerland
2, EPFL, Lausanne, , Switzerland

Photovoltaic cells are increasingly taking advantage of heterostructure deposition technology. A similar trend is observed in the development of photoelectrodes for solar water splitting in electrochemical cells (PEC). One complification in the PEC is that the photoelectrodes are exposed to humidity or liquid aqueous eectrolytes, which, in a polarizing electrical field, can cause corrosion of the electrode material or passivation of important electronic states at the surface. For the optimization of PEC electrodes it is therefore necessary to build up complex heterostructures where the light absorption is optimized independent from the optimization of the electrocatalytic electrode surface and potential buffer layers between absorber and current collector.
We have addressed this analytical problem by employing synchrotron based photoemission spectroscopy in the resonant mode. This allows us to decompose the x-ray spectra into element specifiic and orbital specific components for the electrode surface, the bulk and and underlying buffer layer. Moreover, we have done this study under photoelectrochemical conditions using an ambient pressure spectroscopy instrument.
We find that the oxygen 2p specific component in the valence band spectra scales analog with the DC current in dark and in lght conditions, highlighting the role of the O2p holes in the water oxidation processes. More over we find that the interface between absorber and current collector forms an electronic structure with electronic states that prevent light induced photoelectrons from passing with ease to the metal oxide current collector. Engineering a buffer layer between current collector and absorber seems to "eliminate" these states with the effect that the photocurrent is significantly increased.

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