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