Soft X-Ray Spectroscopy for Solar Energy Conversion

CFM Seminars

Ioannis Zegkinoglou.Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA
Auditorium of the Centro de Fisica de Materiales, Paseo Manuel de Lardizabal 5, Donostia-San Sebastián
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Soft X-Ray Spectroscopy for Solar Energy Conversion **Ioannis Zegkinoglou** Chemical Sciences Division, Lawrence Berkeley National Laboratory Berkeley, CA **Soft X-Ray Spectroscopy for Solar Energy Conversion** **Abstract** The development of more efficient photovoltaic and photoelectrochemical devices for solar-to-electric and solar-to-chemical energy conversion can be accelerated by understanding and systematically tailoring the electronic properties of their constituent materials, rather than relying exclusively on intuition and empirical approaches. The energy positions of the band edges of the light-harvesting substance with respect to the valence band maximum of the electron donor and the conduction band minimum of the electron acceptor play a key role in the efficiency of the charge generation and separation processes. In water splitting applications additional constraints are imposed by the oxidation and reduction potentials of the aqueous electrolyte. The optical properties of the light absorber, the morphology of the electrode materials and the interface properties in the boundary regions between different materials further determine the overall energy conversion efficiency. Element- specific, synchrotron-based, core-level soft x-ray spectroscopy techniques were used to probe selected electron states in a manifold of both organic and metal oxide semiconductors, as well as in combinations of them. Two example cases will be presented in more detail: the study of custom porphyrin-based dye molecules characterized by the highly efficient ‘donor-π-acceptor’ architecture; and the investigations on hematite-based nanostructures, aimed to be employed as photoanodes in artificial photosynthetic applications. Backed by optical spectroscopy as well as density functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations, we were able to establish a feedback loop between synthesis, spectroscopy and theory, which provided microscopic understanding of the mechanisms that determine the efficiency of the investigated materials. The results demonstrate the great potential of the combined use of core level spectroscopy and first principles calculations for achieving advances in the field of solar energy conversion.