The Excitation Wavelength and Solvent Dependance of the Kinetics of Electron Injection in Ru(dcbpy)2(NCS)2 Sensitized Nanocrystalline TiO2 Films

ZeitschriftfürPhysikalischeChemie, Bd. 212,S. 93-98 (1999) ©_{by Oldenbourg Wissenschaftsverlag,} München

The Excitation

_Wavelength

and

Solvent

_Dépendance

of

the

Kinetics

of

Electron

_Injection

in

_{Ru(dcbpy)2(NCS)2}

Sensitized

_{Nanocrystalline}

_Ti02

Films

By

J. R.

_Durrant1*,

Y.

_Tachibana1,

I.

_Mercer1,

J. E.

_Moser2,

M. Grâtzel

andD. R.

_Klug1

1

CentreforPhotomolecularSciences, Departmentsof

_Biochemistry

andChemistry, Imperial College,London,SW7 2AY

2 _Institut

deChimie

_Physique,

École PolytechniqueFédéraledeLausanne,

CH-1015Lausanne, Switzerland

(ReceivedAugust 14, 1998;

accepted

December 10, 1998)

Electron

_transfer

I

_Dye

sensitisation I

_{Nanocrystalline}

I Solar cells I

Interfacial

electron

_transfer

Ultrafast transient

_absorption

spectroscopy has been employed to monitor the kinetics of

_photoinduced

electron

_injection

from

_{Ru(2,2-bipyridyl-4,4'-dicarboxylate)2(NCS)2}

(Ru(dcbpy)2(NCS)2)intonanocrystallineTi02films. This process foundtoexhibit

nonex-ponential

kineticsonthe

_{femtosecond/picosecond}

timescales. Amultiexponential analysis

yielded

lifetimesof <100fs,1.3_psand 13 ps with relativeamplitudesof0.35,0.22,0.43

(±0.06)respectively. These kinetics were found to be _independent of excitation wave-length,and towhether the filmwas immersed inorganicsolventor exposedto _air.

Introduction

The

_injection

of electrons from a sensitizer

dye

into

nanocrystalline

Ti02

[1—7]

is the

_{primary charge separation}

_step

ina newclass of

dye

sensitized

photovoltaic

devices

[8],

An

_appreciation

of the

_parameters

_controlling

the kinetics of this reaction is

_likely

to be

_important

for

_development

of this

technology.

Several studies of the kinetics of electron

_injection

have

_employed

or-ganic

sensitizer

_dyes

_including

coumarin

_[1],

_perylene

_[2]

and fluorescein

*

also focussed upon ruthenium

bipyridyl

dyes,

class of

_dyes

has

_yielded

the mostefficient sensitizer

_dyes

for

_photovoltaic

cells to date

[8],

In

_particular,

solar cells

_employing

the sensitizer

_dye

Ru(dcbpy)2(NCS)2

have _{achieved energy} conversion efficiencies of —10%.

Meyer

etal.

[9],

employing

transient

_{photoluminescence}

measurements to

study

electron

_injection

in a

Ru(dcbpy)2(NCS)2

_sensitized

photoelectro-chemical cell have

_reported

distributionof

_injection

rates with a maximum

amplitude

at —109s~\

Willig

et al.

_[4]

has

_reported

an

injection

time of

<25 fs from this

_dye

into

_Ti02

electrodes under

_ultrahigh

vacuum,

although

we have

subsequently reported

that this

particular study

_{may have been}

distorted

_by

_degradation

of thesensitizer

_dye

[7].

Most

_recently,

Lian,

Nozik and co-workers

[6]

have

_reported

a sub-50 fsrateofelectron

_injection

from infrared transient

_absorption

studies of

_{Ru(dcbpy)2(NCS)2}

sensitized films

exposed

toair. Ourown studies

[3]

have

reported

that electron

injection

in

Ru(dcbpy)2(NCS)2

sensitized films covered with an inert solution

(50/50

propylene carbonate/ethylene

carbonate)

is at least

_biphasic,

with

_injection

times of <150fs and

_—1.2ps.

For this

_system,

the

_{subsequent charge}

re-combination exhibits

_{nonexponential}

kinetics which are

strongly

dependent

upon the

application

ofan _externalbias

_potential

_{tothe film}

_[10].

We have

_recently

extended our

previous

studies of electron

_injection

_to

consideration of the parameters whichcontrol therate of electron

_injection.

As

_part

of this

_study,

we

_present

here the results ofourconsideration of the

influence of the

_dye

solvent environment and excitation

_wavelength

_upon the

_injection

kinetics.

Materials and methods

Nanocrystalline

Ti02

films coated with a _10%

_monolayer

_of

Ru(dcbpy),-(NCS)2

were

prepared

as

previously

[3].

Transient

_absorption

_{data were}

collectedat room

_temperature

using

_apparatus

_describedin_detail

_elsewhere,

with a ₁₀₀

—

250 fs instrument response, a 1 kHz

repetition

_{rate, excitation}

pulse energies

of10—35 nJ

_(0.35—0.7

mJ

_cirr2)

_depending

_{upon excitation}

wavelength,

a white

light

probe

pulse,

andmultichannel detector. Transient

spectra

werecollected between 700 and 800 nmon several different

times-cales. The

_spectra

obtained were

indistinguishable

from those

reported

pre-viously

[3].

Forease of

presentation,

_transient dataare

only

shown here _at a

single

wavelength

(760 nm).

Results and

discussion

We have

_previously

demonstrated that electron

_injection

from the

The ExcitationWavelengthand Solvent _Dépendanceof the KineticsofElectron... 95 2.5 2.0 1.5 O 1.0 < 0.5 0.0 -0.5

r

-10 12 3 4 5 6 Time/ ps j_ _ _ -10 0 10 20 30 40 50 60 Time /_ps

Fig. 1. Transient

_absorption

data at 760nm

following

excitation of

Ru(dcbpy)2(NCS)2

sensitized TiO,films at560nm.

absorption

maximum from —700nmto —800

nm3.

This induced

_absorption

most

probably

results from a

ligand-to-metal charge

transfer transition

as-sociated with the NCS groups

(oxidative

degradation

of the

_{dye, causing}

a

loss of the NCS groups, results in loss of this induced

absorption

[7]).

In

Fig.

1 we show the kinetics of this red

shift,

monitored

by

the appearance

of the induced

_absorption

at760nm. These dataarein

_agreement

with those

we have

reported

previously

[3].

However the

improved signal

to noise

and extension to

longer

timescales allow us _to resolve additional kinetic

components.

A

multi-exponential analysis

revealed at least three

ex-ponential

components

with lifetimes of

<100fs,

1.3 and 13 ps. It should

also be noted this

_analysis

is

_only

intended to

provide

a

simple

quantifi-cation of the kinetics, which may result from an

underlying

distribution of

lifetimes.

A full

_{spectral analysis}

of these

_kinetics,

_allowing

consideration ofthe

contribution of excitedstate

absorption

to the

_signal

at 760nm

(as

conduct-ed

_previously

[3])

indicates that the relative

_yields

electron

_injection

associ-ated with these

_components

are

0.35, 0.22,

0.43

(±0.06)

respectively.

A

slower —

lOOps

component

could also be resolved with low

amplitude

(<10%),

however the

_{probe wavelength dépendance}

ofthis

_component

sug-gests

it is not

primarily

associated with electron

injection.

These

_{nonexponential injection}

kinetics are in contrast with

monoex-ponential

kinetics

_{recently reported}

for the

_organic

sensitizer

_dyes

coumarin

[1],

perylene

[2]

andfluorescein

_[5].

One

_possible

reasonfor thisdifference

2.0 1.5 Q O 1.0 < 0.5 0.0 -0.5 -10 0 10 20 30 ₄₀ 50 60 Time / ps

Fig.

2. Excitation

_{wavelength dependence.}

As

_Fig.

1 but with excitation

_wavelengths

of 520(+), 560(·)and 600( ) nm.

different kinetics

_resulting

from electron

_injection

from differentelectronic

states

_(recent

MO calculations

_suggest

4 different states contribute to the

ground

state

absorption

between 500 and 700nm

[11]).

In orderto address

this

_possibility,

we monitored the electron

injection

kinetics as a function

of excitation

_wavelength.

As shown in

_Fig.

2,

we find that the kinetics of

electron

_injection

are

independent

of excitation

wavelength.

We thus

con-clude that the

_{nonexponential}

_injection

kinetics are not related to the

com-plex photophysics

ofthis

_dye.

This conclusion is also consistent with our

recent observation of

_remarkably

similar

_{nonexponential injection}

kinetics with free base and Zinc

_{tetracarboxyphenyl porphyrins}

(to

be

_published

elsewhere).

Alternatively

it is

_possible

that the

_{nonexponential}

_injection

kinetics could be associated with a

dynamic

solvation of the

dye by

the _solvent

environment.

Indeed,

the recent

study by

Ellingson

et al.

_[6],

conducted with the same sensitizer

dye

buton air

exposed

films,

reported only

a

single

<50 fs

_phase

of electron

_{injection. Fig.}

3 therefore _{compares the} kinetics

we obtain for

Ru(dcbpy),(NCS)2)

films

_exposed

to air and covered in 50/50

_{propylene carbonate/ethylene}

carbonate. Datawerealso obtainedwith

ethanolic and water free acetonitrile

_{electrolytes.}

In all cases the transient

kinetics are

indistinguishable

from each

other,

indicating,

solvation of the

dye

does nothave a

significant

effect upon the

injection

kinetics. It should

also benoted that theethanol solution wasnot

dried,

and therefore contain-ed a

significant

water content.This watercontent

_evidently

has a

negligible

effect upon the kinetics of electron

injection.

Time/ ps ._I_¡_L_

The ExcitationWavelengthand Solvent Dépendanceof the Kinetics of Electron 97 2.5 2.0

_h

1.5

_h

Q O 1.0 -< S _ö 10

1

0.5 1.5 0.5 o.o 0.0 -2-10 1 2 3 4 5 6 -0.5 l'ime / ps -10 o 10 20 30 40 50 60 Time /_ps

Fig.

3. Solvent

_dependence.

As

_Fig.

1 but

_comparison

of data collected for sensitized filmsundera

propylene carbonate/ethylene

carbonate solvent(·),ordried in air( ).

We thus conclude that the kinetics of electron

injection

from

photoexcit-ed

_{Ru(dcbpy)2(NCS)2}

are

independent

oftheelectronic state

initially

popu-lated,

and the solvent environment of the sensitizer

_dye.

Consideration of the

_parameters

which do have a

_strong

influence _upon the rate of electron

injection

will be

_presented

elsewhere.

Acknowledgements

We would like to thank Dr. Chris Barnett for excellent technical

support.

J.R.D. and D.R.K.

acknowledge

support

from the

EPSRC,

the EC Joule

programmeandthe British Council. J.R.D. isa_{BBSRC Advanced Research}

Fellow. J.E.M. and M.G.

_acknowledge

the

_support

of the Swiss National Science Foundation.

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