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Zinc Tetra Phenyl Porphyrin Chromophores
February 2009
-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-)
A Dye Sensitized Solar Cells (DSSCs): a potential low cost alternative to Si solar cells.
Introduction
Zn-TetraPhenylPorphyrin
ZnTPPZinc TetraPhenylPorphyrin
N
N N
N
Zn
•Zn-TPP exhibit a long-lived (>1ns) * excited state
•Appropriate HOMO/LUMO levels position in energy.
•Functional groups have not much influence on the TPP electronic structure.
N
N N
N
Zn
O
HO
O
OH
Energy levels alignment
e-
TiO2 Electrolyte
HOMO
LUMO
ZnTPP
h
e-
Ultra-violet Photoemission
HOMO
LUMO
LUMO + 1 UV
Inverse Photoemission
HOMO
LUMO
LUMO + 1
Experimental setup
Occupied and unoccupied states
in the same UHV system
First results
N
N N
N
Zn
O
HO
O
OH
N
N N
N
Zn
O
HO
O
OH
Zn-based dyes
These dye are believed to adsorb perpendicular to the substrate and might form clusters of parallel molecules.
ZnTPP1 ZnTPP2
Zn-based dyes
N
N N
N
Zn
O
O
O
O
O
O
O
O
H
N+
Et EtEt
H
N+
Et EtEt
H
N+
Et EtEt
H
N+
Et EtEt
This dye is believed to adsorb flat on the substrate.
ZnTPP3
ZnTPP1 on TiO2
N
N N
N
Zn
O
HO
O
OH
No apparent band shift in XPS
ZnTPP1 on TiO2
3.4 eV
0.2 eV
LUMO
LUMO + 1
ZnTPP1 TiO2
HOMO
2.6 eV
2.1 eV
ZnTPP1 on TiO2
Normalization established by comparison with ZnO
Electronic structure and molecular orientation of a Zn-tetra-phenyl porphyrin multilayer on Si(111) C. Castellarin Cudia et al.
•Compared to a multilayer DOS, same main characteristics with slightly different weight.
•Broadening of the principal features could be related to :
the lack of resolutionmolecular interactionsubstrate influencephoton energy
90 eV
40.8 eV
Electron Injection and Recombination in Dye Sensitized Nanocrystalline Titanium Dioxide Films: A Comparison of Ruthenium Bipyridyl and Porphyrin Sensitizer DyesYasuhiro Tachibana, Saif A. Haque, Ian P. Mercer, James R. Durrant and David R. Klug, J. Phys. Chem. B, Vol. 104, No. 6, 2000
2.0 eV
TiO2(110)ZnTPP
Ec=0.6 eV
Ev=-3.2 eV
Ef=0
UPS-IPS
HOMO
LUMO
Tetrachelate Porphyrin Chromophores for Metal Oxide Semiconductor Sensitization: Effect of the Spacer Length and Anchoring Group PositionJonathan Rochford, Dorothy Chu, Anders Hagfeldt, and Elena GaloppiniJACS 129 (2007) 4655
650–700 nm
Comparison with other methods
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.
ZnTPP1 on ZnO(11-20)
TiO2(110)
ZnO(11-20)
Normalization to the substrate chosen to obtain similar molecular DOS after subtraction of the substrate contribution to the total DOS.
Comparison Zn-TPP derivatives
ZnTPP1
ZnTPP2
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.
HOMO
UPS He II
Comparison Zn-TPP derivatives
•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.
ZnTPP1
ZnTPP2
ZnTPP3
IPS
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
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.
30 nm x 30 nm
TiO2(110) Pivalate layer
30 nm x 30 nm
HOMO LUMO
RR
Ph
Ph
Ph
Zn 3d
DOS from calculated MO
HOMO
HOMO+1
ZnP molecular orbitals
Zn 3d
dz2
dzx
dyz
LUMO
LUMO+1
ZnTPP molecular orbitals
HOMO
HOMO+1 LUMO
LUMO+1
LUMO+3
ZnTPP vs ZnP molecular orbitals
ZnTPP
ZnP
15 nm x 15 nm
3 nm x 2 nm
Pivalic acid covered surface
6 Å
ZnTPP3 covered surface
30 nm x 30 nm
15 Å50 nm x 50 nm
Porphyrin as a model dye
LUMOs
HOMOs
eg
a2u a1u
Energy states
Soret (B) band
Q bands
S0
S1
S2
Zn Tetra Phenyl Porphyrin derivatives
N
N N
N
Zn
O
HO
O
OH
ZnTPP1
HOMOsLUMOs
ZnTPP vs ZnP molecular orbitals
Approaches for band alignment tuning 1/3
Molecule/Molecule interaction
Adding spacer between them
Changing the linker to the surface
Adding a built-in dipole
6.5 Debeye dipole
Effect on electron transfer?
Approaches for band alignment tuning 2/3
Modifying the ring electronic properties
Ex: Electron withdrawing groupsFluorination
Approaches for band alignment tuning 2/3
Modifying the ring electronic properties
Metal ion