DEPARTMENT OF PHYSICS AND ASTRONOMY

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       Basics: Dye Sensitized Solar Cells; Organic Dyes on Oxide Surfaces

Dye Sensitized Solar Cells (DSSC) have emerged as a potentially cost-effective alternative to silicon based cells.  Typical cells are comprised of a volume bounded by a transparent conducting electrode (that admits light) on one side and a counter electrode coated with an oxide  (usually TiO2) nanoparticle film infiltrated with a chromophore dye on the other. The volume between the electrodes is filled with a redox couple electrolyte.  Key to the function of the cell is a dye molecule whose HOMO and LUMO straddle the oxide conduction band minimum. Absorption of a photon by a dye molecule excites a HOMO-LUMO transition (1) followed by a rapid (sub-100 fs) charge transfer from the molecule to the oxide nanoparticle surface (2) and ultimately to the counter electrode. The ionized dye molecule is neutralized by electron donation from the redox couple electrolyte (for example I-/I3- ), and finally the oxidized part of the couple in solution is restored by reduction at a counter electrode completing the circuit. The performance of DSSCs depends strongly on the relative alignment of the dye molecular levels with respect to the substrate band edges. Using UPS and InvPE, we can determine the HOMO-LUMO positions relative to the substrate band structure.  The geometry of the dye molecule at the surface is also studied using scanning tunneling microscopy and spectroscopy.

       Recent Highilghts: Organic Dyes on Oxide Surfaces

HOMO and LUMO energies of N3 dye on TiO2(110) Functional DSSCs typically employ anataze TiO2 nanoparticles. To obtain an atomistic view of the dye molecule-TiO2 surface interaction, we are studying the adsorption of N3 dye on the single crystal TiO2(110) surface.  A scanning tunneling microscope (STM) images of the atomically clean and well-ordered TiO2(110) surface, shown at the upper left, demonstrates that large, well-ordered, atomically flat terraces can be obtained.  The lower image illustrates how the surface can be passivated by exposure to pivolic acid in UHV forming a pivalate layer that retains the surface morphology.  Upon removal from the UHV chamber, the surface can be sensitized to N3 dye in solution, where in an exchange reaction adsorbed pivolate is replace by dye molecules. After re-insertion to the UHV chamber, we perform UPS and InvPE measurements to determine the relative alignment of the HOMO and LUMO levels of the molecule, and the conduction and valence band edges of the substrate. The figure below shows photoemission and inverse photoemission of the occupied and unoccupied states, respectively of the clean and N3-dye covered TiO2(110) surface. The HOMO and LUMO energies are 1.1 eV above the TiO2 valence band edge and 0.3 eV below the conduction band edge, respectively. A systematic study of various dye/substrate combinations would enable tailoring DSSC properties. Read more: Direct determination of HOMO and LUMO band alignment for N3 dye and isonicotinic acid on TiO2(110) and ZnO(11-20) S. Rangan, E.J. Bersch, J.-P. Theisen, and R.A. Bartynski, Science, (submitted)

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