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Title: X-ray absorption spectroscopy of biomimetic dye molecules for solar cells

Dye-sensitized solar cells are potentially inexpensive alternatives to traditional semiconductor solar cells. In order to optimize dyes for solar cells we systematically investigate the electronic structure of a variety of porphyrins and phthalocyanines. As a biological model system we use the heme group in cytochrome c which plays a role in biological charge transfer processes. X-ray absorption spectroscopy of the N 1s and C 1s edges reveals the unoccupied molecular orbitals and the orientation of the molecules in thin films. The transition metal 2p edges reflect the oxidation state of the central metal atom, its spin state, and the ligand field of the surrounding N atoms. The latter allows tuning of the energy position of the lowest unoccupied orbital by several tenths of an eV by tailoring the molecules and their deposition. Fe and Mn containing phthalocyanines oxidize easily from +2 to +3 in air and require vacuum deposition for obtaining a reproducible oxidation state. Chlorinated porphyrins, on the other hand, are reduced from +3 to +2 during vacuum deposition at elevated temperatures. These findings stress the importance of controlled thin film deposition for obtaining photovoltaic devices with an optimum match between the energy levels of the dye and thosemore » of the donor and acceptor electrodes, together with a molecular orientation for optimal overlap between the {pi} orbitals in the direction of the carrier transport.« less
Authors:
; ;  [1] ;  [2]
  1. Department of Physics, University of Wisconsin Madison, 1150 University Ave., Madison, Wisconsin 53706 (United States)
  2. Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720 (United States)
Publication Date:
OSTI Identifier:
21559807
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Chemical Physics; Journal Volume: 131; Journal Issue: 19; Other Information: DOI: 10.1063/1.3257621; (c) 2009 American Institute of Physics
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; 74 ATOMIC AND MOLECULAR PHYSICS; 46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND TECHNOLOGY; ABSORPTION SPECTROSCOPY; ATOMS; BIOLOGICAL MODELS; ELECTRODES; ELECTRONIC STRUCTURE; ENERGY LEVELS; HEME; LIGANDS; MOLECULES; ORIENTATION; OXIDATION; PHOTOVOLTAIC EFFECT; PHTHALOCYANINES; SEMICONDUCTOR MATERIALS; SOLAR CELLS; THIN FILMS; TRANSITION ELEMENTS; VACUUM COATING; VALENCE; X-RAY SPECTROSCOPY CARBOXYLIC ACIDS; CHEMICAL REACTIONS; DEPOSITION; DIRECT ENERGY CONVERTERS; DYES; ELEMENTS; EQUIPMENT; FILMS; HETEROCYCLIC ACIDS; HETEROCYCLIC COMPOUNDS; MATERIALS; METALS; ORGANIC ACIDS; ORGANIC COMPOUNDS; ORGANIC NITROGEN COMPOUNDS; PHOTOELECTRIC CELLS; PHOTOELECTRIC EFFECT; PHOTOVOLTAIC CELLS; PIGMENTS; PORPHYRINS; SOLAR EQUIPMENT; SPECTROSCOPY; SURFACE COATING