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Electronic Energy Levels Alignment of Dye Sensitized Oxide Surfaces
Sylvie Rangan
Rutgers UniversityDepartment of Physics and Astronomy
136 Frelinghuysen Road, Piscataway, NJ 08854
-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
Calculated Structural and Electronic Interactions of the Ruthenium Dye N3 with a Titanium Dioxide Nanocrystal, Petter Persson and Maria J. Lundqvist, J. Phys. Chem. B 2005, 109, 11918-11924
Energetics: N3 molecule on TiO2
e-
TiO2
I-/I3-
Electrolyte
HOMO
LUMO
N3
h
e-
50 fs
New experimental approach of the subject
A few examples to show you it works!
Inverse Photoemission
HOMO
LUMO
LUMO + 1
Ultra-violet Photoemission
HOMO
LUMO
LUMO + 1 UV
Experimental setup
Occupied and unoccupied states
in the same UHV system
Experimental setup
VT-SPM OMICRON
Scanning Tunnel Microscope
I
Outline
N3 adsorption
Single crystals
Technologically relevant substrates
TiO2(110) rutile ZnO(11-20)
TiO2 anatase nanoparticules
ZnO nanorods
N
N
OH
O
OHO
N
N
OH
O
OHO
Ru
N
C
S
N
C
S
Direct information electronic structure
analogue to N3 linker to the substrate
Comparison N3 - INAN
OHO
Outline
Band alignment determination
Single crystals
TiO2(110) rutile ZnO(11-20)
Band alignment tuning strategies
ZnTPP derivatives
N
N N
N
Zn
O
HO
O
OH
TiO2 (110) monocrystal
UHV surface preparationAtomically resolved TiO2(110)
30 nm x 30 nm
TiO2(110)
UHV surface passivation with pivalate ions (CH3)3CCOO-
Pivalate layer
30 nm x 30 nm
Sensitization in acetonitrile solution
80 nm x 80 nm
N3/TiO2
O2p Ti3d
TiO2(110)
3.4 0.2
TiO2 (110)
UPS IPS
N3 on TiO2 (110)
3.6 eV
LUMO
LUMO + 1
N3TiO2
HOMO0.9 eV
0.5 eV
Energy diagram
3.2 eV
HOMOLUMO
Ti3d
TiO2(110)
N3 + TiO2(110)
13
Influence of intermediate steps
HOMO and LUMO due to N3
Anatase TiO2 nanoparticles
conducting glass
TiO2 100 Å
Ti3d
•Same HOMO-LUMO gap
•Anatase gap 0.2 eV larger than rutile gap
TiO2 nano
N3 + TiO2 nano
3.8 eV
LUMO
LUMO + 1
N3TiO2
HOMO 1.5 eV
0.7 eV
3.1 eV
(0.5 eV)
(0.9 eV)
(3.2 eV)
(N3 on TiO2(110))
Dunbar P. Birnie III group, Rutgers University
0.2 eV
-0.9 eV
1.5 eV
N3+nanocrystals TiO2
From Photoelectrochemical cells, Michael Grätzel, Nature, nov 2001, 338
Incident Photon to Current conversion Efficiency
Minimum photon energy that produces current: 1.6 eV
Comparison with DSSC perfomances
TiO2
nanoparticlesN3
Ec=0.6 eV
Ev=-3.2 eV
Ef=0
UPS-IPS
HOMO
LUMO
ZnO (11-20) epitaxial film
ZnO nanorods:
• Grown by Metalorganic chemical vapor deposition
•Length: 1.8 µm Diameter: 100 nm
Monoatomic step edge ~ 2 Å
100 nm x 100 nm Length (nm)
Yicheng Lu group, Rutgers University
ZnO substrates
Zn3dO2p
Zn4sp
•ZnO Gap: 3.6 eV
3.2 0.4
N3 on ZnO(11-20)
3.2 eV
0.4 eV
LUMO
LUMO + 1
N3 ZnO
HOMO
1.3 eV
2.1 eV
4.5 eV
•ZnO Gap: 3.6 eV
•Dye features clearly visible
3.4 eV
0.4 eV
LUMO
LUMO + 1
ZnO
1.3 eV
2.7 eV
N3 on ZnO - nanorod
•Smaller dye coverage
•Similar to single crystal ZnO
2.0 eV
N3
HOMO
Zn3dO2p
(2.1 eV)
(4.6 eV)4.5 eV
(1.3 eV)
(0.4 eV)
(3.2 eV)
N
N
OH
O
OHO
N
N
OH
O
OHO
Ru
N
C
S
N
C
S
Isonicotinic acid : N3 linker analogue
N
OHO
• INA analog to the N3 Dye linker
• In a simple model, compared to N3, the INA electronic structure should have:
No Ru-N=C=S like HOMO
A LUMO of similar character as the N3 LUMO
TiO2 ZnO
N3
TiO2 ZnO
INA
N3-INA comparison
INA on TiO2 model calculations
(1) DFT Study of Bare and Dye-SensitizedTiO2 Clusters and Nanocrystals;Lundqvist, Nilsing, Persson, LunellIntern. Journal of Quantum Chemistry, Vol 106, 3214–3234 (2006)
(2) Anchor group influence on molecule–metal oxide Interfaces: Periodic hybrid DFT study of pyridinebound to TiO2 via carboxylic and phosphonic acid;M. Nilsing , P. Persson, L. OjamaChemical Physics Letters 415 (2005) 375–380
(1) (2)
First Conclusion
• UPS and IPS in the same UHV system: The most direct method to characterize the ground state electronic
structure
•UV-visible absorption(exciton)/NEXAFS(core hole) typically used in the field
•First measurement of the electronic occupied/unoccupied structure of dye molecules on surfaces
•Can help improve theoretical treatment of dye/oxides systems.
• Energy level alignment of N3 on TiO2(110) and ZnO(11-20)
•N3 on TiO2 nanoparticles and ZnO nanorods
• INA vs N3: linker group good model for LUMO
Zn-TetraPhenylPorphyrin
•Appropriate HOMO/LUMO levels position in energy.
•Functional groups added to the phenyl groups have not much influence on the TPP absorption properties.
ZnTPPZinc TetraPhenylPorphyrin
HOMOsLUMOs
ZnTPP vs ZnP
N
N N
N
Zn
O
HO
O
OH
Energy levels alignment
e-
TiO2 Electrolyte
HOMO
LUMO
ZnTPP
h
e-
E. Galoppini, Rutgers
Approaches for band alignment tuning
Molecule/Molecule interaction
Adding spacer between them
Changing the linker to the surface
Adding a built-in dipole
Effect on electron transfer?
Approaches for band alignment tuning
Modifying the ring electronic properties
Ex: Electron withdrawing groupsFluorination
ZnTPP1 on TiO2
UPS IPS
TiO2(110)TiO2(110)
ZnTPP1+TiO2
ZnTPP1 ZnTPP1
N
N N
N
Zn
O
HO
O
OH
3.6 eV
LUMO
LUMO + 1
ZnTPP1 TiO2
HOMO
2.6 eV
2.1 eV
Electronic structure and molecular orientation of a Zn-tetra-phenyl porphyrin multilayer on Si(111) C. Castellarin Cudia et al.90 eV
40.8 eV
Energy level alignment
Comparison with other methods 1/2
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
N3
TiO2
0.9 eV
0.5 eV
3.2 eV
LUMO
LUMO + 1
ZnTPP1
HOMO
2.1 eV
3.5 eV
2.6 eV
2.0 eV
TiO2(110)ZnTPP
Ec=0.6 eV
Ev=-3.2 eV
Ef=0
UPS-IPS
HOMO
LUMO
Comparison with other methods 2/2
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
ZnTPP1 electronic structure
ZnPHOMOs
ZnPLUMOs
Phenyls
Phenyls
Two part electronic structure for the free molecule
ZnP HOMO+LUMO: fixed energy
Phenyls: position depends on electron withdrawing groups
Simple model for simulated DOS
DOS = overlap ZnP + Phenyls groups
Photoemission cross-section not included
Tunnel Microscopy Approach
UHV surface preparationAtomically resolved TiO2(110)
30 nm x 30 nm
TiO2(110)
UHV surface passivation with pivalate ions (CH3)3CCOO-
Pivalate layer
30 nm x 30 nm
3 nm x 2 nm
6 Å
Sensitization in ethanol solution
80 nm x 80 nm
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
?
ZnTPP3 adsorption
100 nm x 100 nm
Diameter = 20 Å
20 Å
Pivalic acid layer
Effect on the electronic structure
Adsorption modes? Aggregation?
30nmx30nm
Another conclusion
ZnTPP derivatives good candidate to study fundamental properties related to energy level alignment….
…UPS/IPS useful and direct way for measuring energy level alignment….
…Still a lot of work to do!
Jean-Patrick Theisen
Eric Bersch
THANKS !!!
Robert A. Bartynski
Senia Katalinic
Ryan Thorpe
Typical UV-visible absorption spectrum
Absorption properties
S0
Soret (or B band) at 400 nm
S2 transition
S0 S1 transition
Weaker Q band at 550 nm
•S1 and S2 first and second excited states of the molecule.
•Fast internal conversion S2 → S1
•B and Q bands both arise from to * transitions and can be explained by considering the four frontier orbitals of the porphyrin.
Goutermann Four-Orbital Model
HOMOs
LUMOs
Orbitals
Energy states
•Transitions between these orbitals gives rise to two 1Eu excited states.
•Orbital mixing splits these two states into a high energy state with a high oscillator strength and a low energy state with a low oscillator strength.
Soret (B) band
Q bands
S0
S1
S2
15 nm x 15 nm
3 nm x 2 nm
Pivalic acid covered surface
6 Å
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
N
N N
N
Zn
O
HO
O
OH
Small effect on ZnTPP elecronic structure by changing the
substrate.
ZnTPP1 on TiO2 and ZnO
Approaches for band alignment tuning 3/3…
Modifying the ring electronic properties
Metal ion