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LBNL Oct. 1, 2010

Oleg Prezhdo

U. Rochester

Photoinduced Electron Transfer

at Molecule-Semiconductor Interfaces:

A Time-Domain Ab Initio Perspective

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Adiabatic vs. Nonadiabatic MD

electrons treated quantum-mechanically

nuclei treated classically

e e

ee

e

e

Nonadiabatic MD: Couplingbetween potential surfacesopens channels for system tochange electronic states.

transition allowed

weak coupling strong coupling

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Time-Domain DFT for

Nonadiabatic Molecular Dynamics

p pxx 2)(

SDNvqptxtxtx ,,, 21

Electron density derives from Kohn-Sham orbitals

H tRDFT functional depends on nuclear evolution

txHt

txi pp ,,

2,1pVariational principle gives

Ricci R

Orbitals are expanded in adiabatic KS basis

xtctx pp ,

tRxtRtRxtRxH ;;;

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How to think of bulk-molecule interface:Cluster models or MOs weakly coupled to bands?

Electron-vibrational relaxation (heating):

Which phonons are involved and why?

General Questions

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Dye-Sensitized Semiconductor

Solar Cell

Oregan, Gratzel Nature 353 6346 (1991)

Photovoltaics:

optimize voltage, current,

photo yield

electron transfer mechanism

and its properties

Molecule-bulk interface:

little understood in

molecular-electronicsand other fields

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The electron transfer did not involve redistribution of vibrational excitation energy and wascompletely different from the well known Marcus-Levich-Jortner-Gersicher mechanism.

Burfeindt, Hannappel, Storck, Willig JPC101 6799 (1997)

A new mechanism, not Marcus-Levich-Jortner-Gerischer ?

Reaction Mechanism ?

One possibility for the observed fast injection is a strong coupling of the dcbpy * orbital withTiO2, leading to an adiabatic electron transfer from dcbpy to TiO2.

However, it is unclear whether strong coupling is necessary a large accepting state density

in TiO2 would also give rise to an ultrafast injection time even when the coupling is weak.

Asbury, Ellingson, Ghosh, Ferrere, Nozik, Lian JPC103 3110 (1999)

Adiabatic orNon-adiabatic ?

dye

dye

TiO2

TiO2

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Types of Photoexcitation

TiO2-Alizarin TiO2-Catechol, Ru-dyes

VB VB

CB CB

dye*

dye*

dye

dye

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Electronic State Densities

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Alizarin Motion

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Vibrationally Induced Dynamics

of Electronic Energy Levels

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Dynamics of Electronic

Energy and Coupling

Energy

Localization

(dye

/semicond

.coupling

)

surface OH

Photoexcited state oscillates around the CB edgeSurface OH contribute to state localization, but not energy

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Photoexcited

State Energy and Localization

Distribution of Excite State Energies

~40 vibrations give Gaussian distribution of excited state energyalizarin excited state delocalizes into CB at high energy

Localization of Photoexcited

State on Alizarin

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Alizarin ET, simulationDuncan, Stier, PrezhdoAdv. Mater. 16 240 (2004)

JACS 127 7941 (2005)

total

adiab.

NA

Experiment 6 fs, theory 8 fs

Adiabatic ET dominates over non-adiabatic ETPhotoexcited state is 30% delocalized onto TiO2

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Inside

conduction band

initial state dynamics final state

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Multiple entrances

into conduction band

initial state dynamics final state

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Single entrance

into conduction band

initial state dynamics final state

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Adiabatic ET vs. Temperature

14 12.8 10x s 13 1

2 10dye x s

KG 50

kTGkET /exp

low barrier, hard to see T-dependence experimentallymultiple acceptor states speed up transfer

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Electron Relaxation Inside TiO2

relaxation time is nearly independent of initial energy

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Role of Electrolyte

Fisher, Peter, Ponomarev, Walker, Wijayantha, J. Phys. Chem. B. 2000, 104, 949.

One of the main reasons for efficiency loss

I2 dissociates off TiO2

I2-binds to TiO2

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Electron-Hole

Recombination

1.5-2.6 ps

in experiment

(very fast!)

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Complete Sequence of Events

(based on alizarin simulation)

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VOLTAGE

Injection at CB edge

CURRENT

Anneal surface traps Keep electrolyte away from surface

METAL-LIGAND vs. ORGANIC DYE

Excited state towards surface Ground state away from surface Slower vibrational modes

Practical Considerations

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a short-lived ( 15 fs) state

2.4 eV above the Fermi level

Defect States

O2- VB

Ti4+ CB

EF

Evac

e-

TiO2 H2O

Wet-Electrons on TiO2

Petek: Science 308 1154 (2005); 311 1436 (2006)

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Charge density of wet-electron state supported via

dangling H atoms on the surface H2O more important than OH

High level of charge delocalization over TiO2 substrate

Fischer, Duncan, PrezhdoJ. Am. Chem. Soc. 131 15483 (2009)

Wet-e: Electronic States

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Thermal fluctuations distort geometry, affecting electronic dynamics

Changes in the Ti-O bond lengths/angles affect the electrostatic interactions

between the H2O and the substrate surface

Wet-e: Surface Motions

Fischer, Duncan, PrezhdoJ. Am. Chem. Soc. 131 15483 (2009)

Surface layers shift sideways

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Energy is affected by movement of surface hydrogen atoms,

while localization depends mostly on motion of heavier atoms

Energy Localization(donoracceptor

coupling)

Active Phonon Modes

Fischer, Duncan, PrezhdoJ. Am. Chem. Soc. 131 15483 (2009)

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Strong coupling & high TiO2 DOS favor ultrafast ET

ET starts from a state with significant TiO2 contribution

NA & adiabatic ET mechanisms play equal roles

Electron Transfer Dynamics

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Displays similar characteristics to dye-semiconductor systems

Ultrafast ET driven by relatively low frequency vibrational modes

Both adiabatic and NA mechanisms are important

Wet-electron differs in being a true interfacial state

Results have practical implications TiO2 is a popular electrode material and the wet-electron state is likely to

play role in electro-chemical photolysis, e.g. water splitting

Provides fundamental understanding for the role of electrolyte in dye-

sensitized semiconductor solar cells

Wet-e: Conclusions

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How to think of bulk-molecule interface:Cluster models or MOs weakly coupled to bands?

Electron-vibrational relaxation (heating):Which phonons are involved and why?

General Questions

Ri R

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TiO2particle size effect on ET dynamics Polymer on TiO2 Reverse process: CdSe to a dye

Quantum dot on TiO2 Realistic models of electrolyte

Role of triplet, etc. states in Ru-dyes

Chromophore diads Anatase-Rutile interface

Current & Future Projects

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Annu. Rev. Phys.Chem. 58 143 (2007)

Acc. Chem. Res. 41 339 (2008)Prog. Surf. Sci. 84 39 (2009)

J. Phys. Chem.B 106 8047 (2002)

J. Mol. Struct630

33-43 (2003)Isr. J. Chem. 42 213-224 (2003)

Adv. Mater. 16 240 (2004)

J. Am. Chem. Soc. 127 7941 (2005)

J. Phys. Chem.B 109 365 (2005)

J. Phys. Chem.B 109 17998 (2005)

Phys. Rev. Lett. 95 163001 (2005)

J. Am. Chem. Soc. 129 8528 (2007)

J. Am. Chem. Soc. 130 9756 (2008)

J. Am. Chem. Soc. 131 15483 (2009)

Book Chapters:Elsevier(2006), Springer(2007)

Publications

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Bill Stier (UT-Austin)

Walter Duncan (Schrodinger Inc.)

Zhenyu Guo (visitor from China)

Sean Fischer (current student)

DOE, NSF

Acknowledgments