Sub-Monolayer Sn/Cu(100)

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Zinc Tetra Phenyl Porphyrin Chromophores
February 2009
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Introduction
A Dye Sensitized Solar Cells (DSSCs) a potential
low cost alternative to Si solar cells.

• -Sunlight enters the structure,
• excites electron-hole pairs
• in dye.
• -Electron is transferred into
• TiO2 conduction band, and
• hole is filled by I- ions
• (3I- ? 2e- I3-).
• -Electron travels through circuit,
• reduces I3- (2e- I3- ? 3I-)

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Zn-TetraPhenylPorphyrin

• Zn-TPP exhibit a long-lived (gt1ns) p excited
  state
• Appropriate HOMO/LUMO levels position in energy.
• Functional groups have not much influence on the
  TPP electronic structure.

ZnTPP Zinc TetraPhenylPorphyrin
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Energy levels alignment
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Experimental setup
Occupied and unoccupied states in the same UHV
system
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First results
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Zn-based dyes
ZnTPP2
ZnTPP1
These dye are believed to adsorb perpendicular to
the substrate and might form clusters of parallel
molecules.
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Zn-based dyes
ZnTPP3
This dye is believed to adsorb flat on the
substrate.
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ZnTPP1 on TiO2
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ZnTPP1 on TiO2
No apparent band shift in XPS
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ZnTPP1 on TiO2
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• Compared to a multilayer DOS, same main
  characteristics with slightly different weight.
• Broadening of the principal features could be
  related to
• the lack of resolution
• molecular interaction
• substrate influence
• photon energy

40.8 eV
Electronic structure and molecular orientation of
a Zn-tetra-phenyl porphyrin multilayer on Si(111)
C. Castellarin Cudia et al.
90 eV
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Comparison with other methods
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Reproducibility of results

• Variability between samples even when using the
  same dying procedure.
• Broadening of the principal features could be
  related to order on the surface.

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ZnTPP1 on ZnO(11-20)
Normalization to the substrate chosen to obtain
similar molecular DOS after subtraction of the
substrate contribution to the total DOS.
TiO2(110)
ZnO(11-20)
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Comparison Zn-TPP derivatives
UPS He II
ZnTPP1
ZnTPP2
HOMO
ZnTPP3

• Roughly the same features are found for the three
  dyes. In particular, HOMO at the same position in
  the three cases.
• Effect of adsorption mode difficult to interpret
  in terms of electronic structure. No obvious
  dependence.

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Comparison Zn-TPP derivatives
ZnTPP1
IPS
ZnTPP2
ZnTPP3

• Very different results from one dye to another.
  Need to be checked one more time.
• Effect of adsorption mode difficult to interpret
  in terms of electronic structure. No obvious
  dependence.

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Next steps

• Quantitative analysis of all the XPS data for
  coverage determination.
• Better understanding of the free ZnTPP electronic
  structure to anticipate the modifications of the
  DOS as a function of the adsorption geometry.
• Ab-initio electronic structure calculation
  (GAMESS)
• Literature?...
• Using the low cross section of Zn sp states in
  ZnO CB would allow a better characterization of
  the unoccupied molecular states.

ZnTPP1
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Next steps

• STM imaging of individual molecules on a pivalic
  acid passivated surface.
• Spectroscopic measurements (XPS-UPS-IPS) with
  prior pivalic acid passivation of the surface.

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DOS from calculated MO
Ph
Zn 3d
Ph
Ph
R
R
HOMO
LUMO
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ZnP molecular orbitals
HOMO1
LUMO
HOMO
LUMO1
Zn 3d
dyz
dzx
dz2
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ZnTPP molecular orbitals
HOMO1
LUMO
LUMO1
HOMO
LUMO3
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ZnTPP vs ZnP molecular orbitals
ZnTPP
ZnP
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Pivalic acid covered surface
6 Å
3 nm x 2 nm
15 nm x 15 nm
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ZnTPP3 covered surface
15 Å
50 nm x 50 nm
30 nm x 30 nm
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Porphyrin as a model dye
eg
LUMOs
Energy states
HOMOs
Soret (B) band
S2
Q bands
S1
a2u
a1u
S0
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Zn Tetra Phenyl Porphyrin derivatives
ZnTPP1
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ZnTPP vs ZnP molecular orbitals
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Approaches for band alignment tuning 1/3
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Approaches for band alignment tuning 2/3
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Approaches for band alignment tuning 2/3
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