Electronic Material Properties - und Geowissenschaften ...

Synchrotron Induced Photoelectron Spectroscopy at the Solid-
Liquid Interface of Dye Sensitized Solar Cells
Konrad Schwanitz, Eric Mankel, Ralf Hunger, Thomas Mayer,
and Wolfram Jaegermann
At BESSY we run the experimental station SoLiAS, dedicated to solid-liquid interface
analysis. SoLiAS allows for the transfer of wet chemically prepared surfaces to the ultra
high vacuum without contact to ambient air. In addition in situ (co)adsorption of volatile
solvent species onto liquid nitrogen cooled samples is possible. SoLiAS proves to be very
useful in analyzing the chemical and electronic structure of solid-liquid interfaces e.g. of
dye sensitized solar cells. A monolayer of Ru(N3)-dye was adsorbed from ethanol solution
under clean N2 atmosphere in an UHV-integrated electrochemical cell (EC). Acetonitrile or
benzene were adsorbed from the liquid in the EC or in situ from the gas phase. Ex situ
sintered nanocrystalline anatase substrates as well as in situ deposited polycrystalline
TiO2 samples were used. Distinct reversible changes occur in synchrotron induced
photoelectron valence band and core level spectra when the solvent is adsorbed to
pristine and dye covered TiO2 substrates. Based on the experimental results the alignment
of electronic states and a model on the dye-solvent interaction have been deduced.
The valence band maximum of nc-TiO2 is found at EB = 3.6 eV binding energy while the
fundamental gap is 3.2 eV only. Surface gap states related to Ti 3+ 3d 1 orbitals are found
with a maximum at EB = 1.3 eV and in addition just below the Fermi level. In the rigid band
model these states are assigned to occupied conduction band states but may be due to
substochiometric TiO2-x. Adsorption of acetonitrile is accompanied by quenching of the
surface gap states (Fig. 1). This finding is confirmed in Ti2p core orbital spectra by the
quenching of the Ti 3+ low binding energy shoulder of the Ti 4+ bulk emission. Coadsorption
of acetonitrile (Fig. 2) shifts the dye HOMO by 150 meV from 2.0 eV binding energy to
2.15 eV.
Fig. 1: Intensity normalized (O2p valence
band) SXPS spectra of the gap states of
nanocrystalline anatase TiO2 in the course of
acetonitrile adsorption and desorption.
Fig. 2: SXPS spectra of the gap states of
nanocrystalline anatase TiO2 in the course of
Ru(N3)-dye adsorption and acetonitrile
coadsorption and desorption.
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