Electron transfer optimisation in Electron transfer optimisation in organic solar cellsorganic solar cells

James DurrantCentre for Electronic Materials and Devices

Departments of ChemistryImperial College London

• Introductory remarks• Charge recombination vs. charge separation and transport• Interface engineering• Inhomogeneity

Why organic PV now?• Political: global warming• Commercial: perception that Si based PV

may not have the potential for mass PV production

• Scientific: building up recent advances in– Organic electronics – LED’s and FET’s– Molecular electronics: supermolecular

photochemistry– Materials control and measurement on the

nanometer scale

Organic photovoltaic technologiesGlass substrate

ITOMixed Layer

Glass substrateITO

Mixed Layer

h+e-

Dense TiO2 (~ 40 nm)(Hole blocking layer)

Porous TiO2 (~100 nm)

Polymer (~50nm)

Device structure

ITO substrate

+ -

LightTiO2 nanoparticles

PEDOT

Au electrode

Silver paint

Dense TiO2 (~ 40 nm)(Hole blocking layer)Dense TiO2 (~ 40 nm)(Hole blocking layer)

Porous TiO2 (~100 nm)Porous TiO2 (~100 nm)

Polymer (~50nm)Polymer (~50nm)

Device structure

ITO substrate

+ -

LightTiO2 nanoparticles

PEDOT

Au electrode

Silver paint

Device structure

ITO substrateITO substrate

+ -+ -

LightLightTiO2 nanoparticlesTiO2 nanoparticles

PEDOTPEDOT

Au electrodeAu electrode

Silver paint Silver paint

dye sensitisedphotoelectrochemical Molecular thin film

Polymer/C60 blend

Organic/inorganic hybrid

Challenges• Stability

– liquid versus solid state, O2, water….

• Processibility– low temp processing on flexible substrates

• Efficiency– Improved red spectral response– Improved voltage and FF whilst maintaining high IQE– Efficiency versus processibility / stability issues

Haque et al. Chem Comm 2003

Molecular donor/acceptor dyads

0.1 1.0 10.0 100.0 1000.00.0

1.0

2.0

3.0

4.0

0.0 20.0 40.0 60.0 80.00.0

1.0

2.0

3.0

4.0

m∆

OD

Time (µs)

m∆

OD

Time (µs)

SS

SS

OC12H25

C12H25ON

C8H17

τ50% = 20 µs

SS

S SOC6H13

C6H13O

SS

NC8H17

τ50% = 0.8 µs

Kinetics in organic solar cellsPolymer

hυ

e-

Charge separation

Charge recombination

h+

C60

AlITO

electron collection

holecollection

e-

h+

transport

e- transport

Light driven charge separation

Time / ps-5 0 5 10 15 20 25

Elec

tron

inje

ctio

n yi

eld

0.0

0.5

1.0

10-6 10-5 10-4 10-3 10-2 10-1

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

m∆O

D

Log10time / seconds

Ultrafastinjection Millisecond

recombination

Tachibana et al. J. Phys Chem 1996

TiO2

hν

e-

Electroninjection

Charge recombination

Model of Reaction dynamics

CB

Transport S / S+

Injection

Trapping

S* / S+

Charge recombination

TiO2 Dye

Charge recombination dynamics controlled upon electron transport and interfacial electron transfer kinetics depending

upon metal oxide and sensitiser dye employed.

Molecular Control of Recombination

k ∝ exp(-βr) where β = 0.95 ± 0.2 Å-1

Ti

Ti

RuN

N

C

C

O

OO

O

r

Clifford et al JACS 2004

Signatures of transport in recombination dynamics

2.2%50

−∝ ntn = A t-0.25

1

10

100

1000

0.001 1 1000 1000000

t50% / nse

per D

ye+

Ethanol triflate

Non-linear dependence on electron density:

t50% ∝ n-1/α

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.E+01 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 1.E+07 1.E+08

T ime (ns)

Rel

ativ

e de

nsity

of e

xcite

d dy

es S

(t)/S

0

0 mV

100mV

200mV

300mV 400mV

Dispersive (Stretched exponential) decaysStrong dependence on TiO2 EF

Haque et al. J Phys Chem B 1998, 2000, Nelson et al. Phys Rev B 1999, 2001

Recombination in MDMO-PPV/PCBM blends

polymer PCBM

π

Charge separation

Charge recombination

π∗

• Recombination kinetics dominated by slow, thermally activated power law decay resulting from

10-6 10-5 10-4 10-3 10-2 10-1

10-7

10-6

10-5

positive polaron trapping in polymer

Montanari et al. APL 2002 Nogueira et al. J Phys Chem B 2003

T = 220 K T = 298 K

∆O

D

time (s)

Recombination versus Transport in polymer / C60 devices

g(E)

10-8 10-7 10-6 10-5 10-4 10-3 10-210-7

10-6

10-5

10-4

10-3

∆ A

bsor

banc

e

75µJ data 4µJ data 0.22µJ data

0.25µJ

4µJ

75µJ

Time (s)

10-6 10-5 10-4 10-3

10-7

10-6

10-5

10-4

10-3

30V

60V

Cur

rent

Den

sity

/ ar

b. u

nits

Time / s

TAS studies of recombination TOF studies of transport

• Smooth lines from trapping/detrapping model with same dos• Same microscopic model explains both recombination and

transport• Open question of benefit of traps

Recombination versus transport in dye sensitised solar cells

Transport dynamics

Recombination to redox couple

Recombination to dye cations

TiO2 200µs 10ms 600µs

SnO2 300ns 9µs ~ 600ns

Cur

rent

Den

sity

/ A

cm-2

0.0 0.2 0.4 0.6 0.8-0.002

0.000

0.002

0.004

0.006

J sc

VocVoltage/V

TiO2

SnO2

SnO2/MgO

Cur

rent

Den

sity

/ A

cm-2

0.0 0.2 0.4 0.6 0.8-0.002

0.000

0.002

0.004

0.006

J sc

VocVoltage/V

TiO2

SnO2

SnO2/MgO

CB

Transport S / S+

Injection

Trapping

S* / S+

Recombination

TiO2 Dye

I-/I3-

Regeneration

Charge separation versus recombination

102 104 106 108 1010 10120.0

0.2

0.4

0.6

0.8

Mon

ochr

omat

ic E

ffic

ienc

yCharge separation rate / s-1

Two level system numerical model of organic solar cellBased on assumption that electronic coupling for charge separation and recombination scale proportionally.

J.Nelson et al.Phys.Rev.B 2004, Appl.Phys.A 2004

J

J

V/2 Jav

Jav

Jca

V/2

Charge separation in dye sensitised solar cells

100 101 102 103 104 105

0.0

0.5

1.0

(ii)

(i)

(ii)

(i)

Inje

ctio

n Yi

eld

time / picoseconds

Dye sensitised film

Solar cell

Haque et al. JACS 2004

Dynamics versus Device function

Electrolyte Jsc/mAcm-2 Voc /Volts η / % τ50%(inj) τinit(rec)+Li+ 16.8 0.51 5.5 ~10 ps 20 ms+Li+/tBP 16.3 0.63 7.25 ~150 ps 100 ms+tBP 7 0.73 3.75 ~ 1 ns 400 ms

Optimised device:• Injection just sufficient to compete with excited state decay to ground• Allows minimisation of recombination losses

CB /

trap

states

2

1

TiO2 Dye

I- / I3-

hv

3

D*/D+

D/D+

2

1

TiO2 Dye

I- / I3-

hv

3

D*/D+

D/D+

Influence of electrolyte composition upon density of conduction band / trap states in TiO2

Electrolyte B: No Li+

• Slow Electron Injection (1)

• Slow Charge Recombinationrates (2) & (3)

Electrolyte A: Both Li+ and 4-tert-butyl pyridine

• IntermediateElectron Injection rate (1)

• Intermediate Charge Recombination rates (2) & (3)

E

D/D

2

1

TiO2

CB /

trap

states

Dye

hv

3

I- / I3-

D*/D+

D/D++

Electrolyte C: No 4-tert-butyl pyridine

• Fast Electron Injection rate (1)

• Fast Charge Recombination rates (2) & (3)

Electrolyte control of interfacial dynamics

Optimum device performance: injection half-time ~ 150 ps

Electrolyte B+ tert-butyl pyridine

Electrolyte A+ tert-butyl pyridine and Li+

Electrolyte C+ Li+

Materials approaches to control of interfacial electron transfer dynamics

10-6 10-5 10-4 10-3 10-2 10-1

0.00

0.05

0.10

0.15

0.20

0.25

m∆O

.D.

Time / Seconds

N NN N

N NN N

CH3

CH3

H3C

H3C

N NH3CO

SO3Na

HOA

B

a b

c d

a b

c d

Al2O3coated

Uncoated

10-6 10-5 10-4 10-3 10-2

0

1

2

3

m∆O

D

Time / Seconds

TiO 2 MFHTM

DFHTM

Dye

TiO 2 MFHTM

Li + - DFHTM

Dye

+ Li+- Li+

10-6 10-5 10-4 10-3 10-2

0

1

2

3

m∆O

D

Time / Seconds

TiO 2 MFHTM

DFHTM

DyeTiO 2 MFHTM

DFHTM

Dye

TiO 2 MFHTM

Li + - DFHTM

Dye

+ Li+- Li+ N N

OCH3 OCH3

n

O

OO

O

O

OO

O

Li+ Li+

Li+- DFHTM

Haque et al.Adv Mat 2004

Haque et al. Adv Func Mat2004

Palomares et al. JACS 2003

HeterosupramolecularPhotochemistry

TiO2

hν

picoseconds

nanoseconds Supramolecularcontrol of recombination dynamics

~ 1 s

Distance control: supersensitiserfunction

1E-6 1E-5 1E-4 1E-3 0.01 0.1 10

1

N719 Pump:550nm, Probe:800nm N845 Pump:516nm, Probe:850nm

∆O.D

. (no

rmal

ized

)

Time [s]

N

N

N

NRu

NN

CSC

S

HOOC

HOOC

COOH

COOH

NN

N

NRu

NN

CSC

S

HOOC

HOOC

O

N

OCH3

OCH3

HOMO calcs:Increase in distance ~ 4 Å

e-

Hirata et al.Chem. Eur. J.2004

Influence of inhomogeneityWide bandgapsemiconductor

Adsorbed Sensitiser Dye

Electrolyte

S0 / S +

S* / S +

e-

Charge recombination

Electron injection

I-

e-

Dye re-reduction

/ I 3-

Inhomogeneous energetics result in non-exponential dynamics and make device optimisation much harder

∆inhomo

1D* / D+

E

g(E)

<di>=0

d1

d2

g1g0g2

TiO2 Dye

Modelling electron injection energetics

Monte Carlo Simulation as detailed in:Tachibana et al. (2002) J. Photochem Photobiol A: ChemistryOnly fit parameters k(0) and ratio ∆/E0

100 101 102 103 104 105

0.0

0.5

1.0

(ii)

(i)

(ii)

(i)

Inje

ctio

n Yi

eld

time / picoseconds

( ) ( ) ( )( ) ( ) ⎟⎟

⎠

⎞⎜⎜⎝

⎛==

02

2 2exp00

0Edk

VdV

kdk iii Inhomogeneous broadening

∆inhomo ~ 0.15 eV film∆inhomo ~ 0.3 eV DSSC

g(E)∝ exp(E/E0)

Excited state decay

to ground

Hole transfer in solid state DSSC’s:

Valence Band

Conduction Band

S0 / S+

S* / S+

e-

Wide bandgap semiconductor

Adsorbed Sensitiser Dye

HTM/HTM+

e-

Hole transporting material

Dye re-reduction

Hole transfer ~ 300 ps(Another example of kinetic redundency!)Hole transfer controlled by thermodynamics not kinetics

N N

OCH3

H3CO

OCH3

OCH3

NN

OCH3

OCH3

OCH3

H3CO

Hole transfer yield as function of mean reaction free energy

-0.4 -0.2 0.0 0.2 0.40

20

40

60

80

100

Yie

ld o

f hol

e tra

nsfe

r / %

∆G(Dye-HTM) / eV

Homogeneous Model

Inhomogeneous Model

Distribution of D/D+ states

Vacuum Level

IP

+Em(HTM/HTM+)

Em(D/D+)

∆G(Dye-HTM) = Em(HTM+/ HTM) – Em(D+/ D)

Experimental data Inhomogeneous Model

N N

R2

R3

R1

R4

Haque et al . Chem Phys Chem (2003)

Minimisation of energetic inhomogeneity

-0.4 -0.2 0.0 0.2 0.40

20

40

60

80

100

Dye

rege

nera

tion

effic

ienc

y / %

∆G(dye-HTM)

ITO

/ eV

TiO2 Dye HTM / Li+

TiO2 Dye HTM

Li+

Li+

Li+Li+

Li+

Li+

Li+ Li+

Li+

Li+

ITO

- Li+

+ Li+

-0.4 -0.2 0.0 0.2 0.40

20

40

60

80

100

Dye

rege

nera

tion

effic

ienc

y / %

∆G(dye-HTM)

ITO

/ eV

TiO2 Dye HTM / Li+

TiO2 Dye HTM

Li+

Li+

Li+Li+

Li+

Li+

Li+ Li+

Li+

Li+

ITO

- Li+

+ Li+

ITO

/ eV

TiO2 Dye HTM / Li+TiO2 Dye HTM / Li+

TiO2 Dye HTM

Li+Li+

Li+Li+

Li+Li+Li+Li+

Li+Li+

Li+Li+

Li+Li+ Li+Li+

Li+Li+

Li+Li+

ITO

- Li+

+ Li+

Ionic screening by Li+ ions reduces inhomogeneity of hole transfer energetics

N N

O

OO

OO

OO

O

R2

R3

R1

R4Li+ Li+

2[(CF3SO2)2]-

N N

R2

R3

R1

R4

Conclusions

• Exciting times for organic PV• Optimisation of electron transfer dynamics

in organic PV requires consideration of:– Recombination versus transport, and the role of

traps – Charge separation versus recombination and the

potential for interface engineering– Energetic inhomogeneities

AcknowledgementsColleagues at Imperial College:Colleagues at Imperial College:Jenny Nelson, Jenny Nelson, DonalDonal Bradley, David Bradley, David KlugKlugSteffan Cook, Ana Flavia Nogueira, Ivan Montanari, Samantha HandaEmilio Palomares, Saif Haque, Narukuni Hirata, Alex Green, Hari Upadahyaya, John Clifford

Collaborations:Michael Gratzel (EPFL), Jan Kroon (ECN)Andrew Holmes (Cambridge / ICL), Serdar Sariciftci (Linz)Christoph Brabec (Siemens/Konarka), Nazario Martin (Madrid), Kees Hummelen (Groningen), Merck Chemicals, Dow Chemicals, Covion GmbH, Johnson Matthey Ltd.

FundingFunding::EPSRC, DTI, EU, BP