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