Some factors influencing charge generation

and recombination in organic solar cells

James Durrant Centre for Plastic Electronics, Imperial College London

www.imperial.ac.uk/people/j.durrant, j.durrant@imperial.ac.uk

Imperial

• Safa Shoaee, Fiona Jamieson, Tracey Clarke,

Yvonne Soon, Stoichko Dimitrov, Dan Credgington,

George Dibbs, Chris Shuttle, Andrea Maurano, Rick

Hamilton, Florent Deladelle, Pabitra Tulhadar

• Jenny Nelson, Donal Bradley, Thomas Kirchatz,

Thomas Anthopoulos et al., Dept. of Physics,

• Natalie Stingelin, et al. Dept. of Materials

• Iain McCulloch, Martin Heeney, Saif Haque, John de

Mello, Brian O’Regan et al. Dept of Chemistry,

Plus

• Sam Jenekhe et al., (U. Washington), Quyen

Ngyuen et al. (UCSB), Seth Marder et al. (Georgia

Tech) Christoph Brabec et al. (Konarka), Dairn Laird

et al. (Plextronics),

Acknowledgements

Topics to be addressed

Key themes:

• Energetics

• Quantifying the impact of Film Microstructure

How do these impact upon:

• Exciton diffusion to the donor / acceptor interface

• Charge dissociation and the role of charge transfer states

• Non-geminate recombination

• Electric field dependence of charge separation

• Role of material crystallinity in modulating energetics

JSC

FF &

VOC

Charge photogeneration and recombination

- +

+ +

- -

Charge separation

Role of geminate recombination

Short circuit current

Charge collection

Role of non-geminate recombination

Device FF and voltage

Materials design: Energy Level Based Device Models

2.00

3.00

4.00

5.00

6.00

7.00

8.00

2.00

1.00

9.00

10.00

11.00

2.8 2.4 2.0 1.6 1.2-3.0

-3.2

-3.4

-3.6

-3.8

-4.0

T

Band Gap [ eV ]

LU

MO

Level D

onor

[ eV

]

S

Donor Acceptor

An

od

e

Cath

od

e

~VOC

DELUMO

Photocurrent:

IQE assumed 100% if DELUMO > 0.3eV

Strategy: reduce optical band gap to

increase overlap with solar spectrum

Voltage:

VOC~ IPdon- EAacc - 0.3

Strategy: increase IPdon or lower EAacc

BUT:

In practice most (but not all) new materials

significantly underperform relative to this

simple energetic model

Why? Scharber, Brabec and many others....

Polymer energy level design

Structure : Device Function Relationships

-0.5 0.0 0.5 1.0-8

-6

-4

-2

0

2

4

6

J (

mA

/cm

2)

Voltage (V)

P3HT PDI

O

O

High sensitivity transient

optical and optoelectronic

studies 2

4

6

810

-5

2

4

6

810

-4

2

D

D (

FIL

M)

10-7

10-6

10-5

10-4

Time [s]

810

-5

2

4

6

810

-4

2

4

6

810

-3

D

D (D

EV

ICE

)

60J cm-2

36 24 6 4

150nm

Film Microstructure

150nm

HRTEM micrograph

of P3HT/PCBM

• BUT:generally only see well defined polymer domains for very crystalline

polymers such P3HT

• Even for P3HT, probably significant fraction of more amorphous P3HT and

molecularly mixed P3HT/PCBM regions

• Most low bandgap polymers are more amorphous, with less defined phase

segregation

Polymer photoluminescence quenching

Strong emission quenching suggests for many polymers there is intimate mixing

of polymer and fullerene.

0 10 20 30 400.0

0.2

0.4

0.6

0.8

1.0 0

0.5

1

2

5

10

20

30

40

50

Time /ps

Flu

ore

scen

ce in

tensity

Concentration of acceptor

Residual emission of 1:1 blend films for range

of donor polymers blended with PCBM

Ultrafast kinetics of exciton quenching in

polyfluorene / PCBM blend films

More Quantitatively: Distance exciton moves given be L ~ Lex (1-PLQ)1/2

Lex typically 6-10 nm – so 98% PL quenching suggests exciton moves < 1 nm

Annalisa Bruno

Saif Haque et al

Jamieson et al Chem Sci 2011

A more realistic image of film microstructure?

•With the exception of very crystalline

polymers such as P3HT, most polymers

when blended with PCBM form rather

intimately mixed polymer / PCBM domains.

•No need for polymer exciton diffusion.

Questions:

• PCBM domains?

• Amorphous versus crystalline?

• Impact of intimate mixing on charge

separation / recombination

Jamieson et al Chem Sci 2011

Exciton Diffusion and Separation

Overall conclusion: For polymer : PCBM films, exciton diffusion to the interface is

often not limiting.as PCBM highly miscible with many donor polymers.

Similarly for most polymers, exciton separation at the interface is not limiting (as

evidenced by high polymer PL quenching).

Exceptions: Diffusion of PCBM excitons, polymer to PCBM energy transfer for blue

emissive donor polymers (Soon et al. Chem Sci 2011)

Charge dissociation and geminate recombination

e -

Polymer C60

Exciton separation

h + h +

LUMO

HOMO

e - e -

h + h +

Geminate recombination of bound

polaron pairs (charge transfer

states)

•Key consideration:

How do initially generated charge transfer states overcome their

coulomb attraction and dissociate into free charges?

r

eV

r 0

2

4

Coulomb Attraction:

r = 3-4 for organics

EBCT ~ 0.1-0.5 eV

e - e - Charge Dissociation

Charge Transfer States in OPV

a)

c)

b) CT state

electroluminescence Tvingstedt JACS 2009

CT state

absorption Goris JMS2005

a)

c)

b)

CT state

Photoluminesce Loi AFM 2007

Clarke and Durrant

Chem. Rev. 2010

CT state

Coulomb attraction Huang Nat Mat 2008

CT state

Donor Acceptor

HOMO

LUMO

HOMO

LUMO

+

h

+

COLL

ABS

DISS(EX)

DISS(CT)

Spectroscopic probes of charge photogeneration

DGCS

600 650 700 750 800 850

QPL

= 90%

PL

In

ten

sity

(nm)

P3HT

P3HT:PDID

PL studies of exciton quenching

High sensitivity transient

absorption studies of

dissociated polaron yields

measured after pulsed

laser excitation

2

4

6

810

-5

2

4

6

810

-4

2

D

D (

FIL

M)

10-7

10-6

10-5

10-4

Time [s]

810

-5

2

4

6

810

-4

2

4

6

810

-3

D

D (D

EV

ICE

)

60J cm-2

36 24 6 4

Dissociated polaron yield as determinant of device

photocurrent

Correlation indicates

optical measurement of

charge separation yield in

blend films with PCBM is

remarkably good

predictor of photocurrent

density in devices

Clarke et al. Adv Mat 2010

Low mobility

polymers

h < 10-4

But only for ‘crystalline’

polymers

0.0 0.2 0.4 0.6 0.8 1.0

1E-6

1E-5

1E-4

1E-3

polymer:PCBM (19:1)

polymer:PCBM (1:1)

polymer:PDI2 (1:1)

P3HT:PDIx (1:1)

CT polyemrs:PCBM (1:1)

UoW polymer:PCBM

DO

D

-DGCS

rel (eV)

Energetics of charge photogeneration

• Observe a strong correlation between yield of dissociated charges and DGCS for several materials series: DGCS is a key determinant of ηdiss(CT)

• Differences between series: other factors influencing charge generation

• Not all series show energetic correlation – due to other factors

CT state

Donor Acceptor

HOMO

LUMO

HOMO

LUMO

+

h

+

COLL

ABS

DISS(EX)

DISS(CT)

DGCS Energy loss driving charge separation (eV)

Yie

ld o

f dis

socia

ted c

harg

es (

a.u

.)

Ohkita et al. JACS 2008, Shoaee et al. JACS 2010

DGCS

Kinetic Model for charge photogeneration

S0

S1

T1

CT*

1CT 3CT

CS*

CS

+

-

Fre

e E

nerg

y

h

kCT

ktriplet

kGR

kCS*

kCS

kBR

kISCDGCS

DGCT

ktherm kthermCSCT

Clarke and Durrant Chem. Rev. 2010

Bredas et al. Acc. Chem Res. 2009

Follows on from Morteani et al PRL 2004

Key competition:

Thermalisation versus Dissociation of the

‘hot CT’ states

kBT

rc a

e-h

distance

V

Recombination

Dissociation

e-

h

Onsager

Theory

The long jump analogy

Energy

Spatial separation of P+ & PCBM-

Nearest neighbour

separation (1-3 nm?)

Coulomb attraction of

P+ and PCBM-

100-400 meV

1P3HT*

e-

1Pol*

e-

e-

e-

Other factors: film microstructure:

Geminate recombination in a finely mixed blend

• Without phase segregation to enable spatial separation of charges –

observe fast (~100 ns) geminate recombination

0.01 0.1 1 10 100

0.01

0.1

1

10

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

0.01

0.1

1

P3HT:1

Time (s)

mD

OD

Time (s)

mD

OD

NNOHC

NO2

NNOHC

NO2

NNOHC

NO2

Shoaee et al. En. & Environ. 2010.

Geminate recombination Non-geminate

recombination

JSC~ 10 mAcm-2

JSC< 0.1 mAcm-2

What about the role of mixed polymer / fullerene

domains in mediating recombination?

P3HT

Some lessons for materials design for charge

separation from polymer excitons

Charge dissociation yield relatively independent of:

• Polymer hole mobility,

• polymer crystallinity,

• alkyl side chain length,

• Energetics relative to PCBM singlet (except for blue

luminescent polymers)

• Energetics relative to PCBM/Polymer triplets?

Charge dissociation yield relatively dependent upon:

• DGCS,

• acceptor electron mobility (?),

• charge transfer character/ polarizability of polymer,

• nanomorphology

Non-geminate recombination:

Impact on cell voltage and Fill Factor

V

e -

Polymer C60

e -

h+

EFpol

EFPCBM

• Device voltage corresponds to splitting of electron and hole Fermi levels

• This corresponds to an increase in electron and hole densities and can

result in electron / hole recombination.

• Rate constant for this non-geminate recombination should depend upon

microstructure and mobility

LUMOs

HOMOs

EFpol

EFPCBM

h+

e -

Light

PEDOT

:PSS

+ -

PCBMPolymer

Exciton

generation

and diffusion

Charge

separation+

+

+-

-

-

Non-geminate recombination

Energy level model for VOC

Donor Acceptor

Anode

Cath

ode

~VOC

Scharber et al. Adv. Mat. 2006

Also analyses based on CT state energies:

Vandewal et al. Nat Mat 2009

Veldman et al. Adv Func Mat 2009

VOC~ IPdon- EAacc - 0.3

But these energy level based models donot take account microstructure......

Only agrees with experiment to +/- 100 mV

• Simplest model: VOC when JGen = Jrecom

• JGen from short circuit current (assumes generation independent of cell voltage)

• Jrecom = krecom n p = krecom n2

• Need to measure n(V, Int) and krecom

• For charge as function of Voltage – need in situ measurement including intraband trap states

Polymer C60

h+

e - e -

ITO/

PEDOT Al

Efh

Efe

Vint

Non-geminate recombination analysis of V)C

Generation flux JGen

hu

Non-gem

Recombination

flux JRecom

Recombination to

traps:

Shockley-Reed-Hall

recombination

Recombination

probably via CT

states

Measuring charge densities, lifetimes & krecom

1.6x1017

1.4

1.2

1.0

0.8

0.6

0.4

0.2

n [

cm

-3]

0.600.550.500.450.40Voc [V]

Differential Charging

Charge extraction

10

2

3

4

56

100

2

3

Dn [

s]

2 3 4 5 6 7 8 9

1017

n [cm-3

]

D nn

Charge density in

photoactive layer

increases

exponential with

voltage

Transient photovoltage measurements

of carrier lifetime

Shuttle et al. APL 2008a, 2008b, PRB 2008, Maurano et al. J. Phys Chem C 2011

2

4

6

810

-13

2

4

6

810

-12

2

k /

cm

3s

-1

2 3 4 5 6

1017

2 3

n /cm-3

TAS PV

Small perturbation analyses of

P3HT/PCBM cells held at open circuit

as function of bias light intensity.

2

0

)( nnk

nkdt

dn

recom

recom

Impact of film microstructure on kRecom

10-14

10-13

10-12

10-11

10-10

Bim

ole

cu

lar

Reco

mbin

atio

n,

k /

cm

-3 s

-1

4 5 6 7 8 9

1016

2 3 4 5 6 7 8 9

1017

2

Charge Carrier Density, n / cm-3

Non-Annealed

Non-Annealed Langevin Rate Annealed

Annealed Langevin Rate

• Annealing P3HT/PCBM films drives formation of relatively pure polymer

and fullerene domains

• This reduces kRecom (even though mobility increases)

Non-Annealed

Annealed

(a)

(b)

Numerical Model Calcs Measured krecom

Hamilton et al. JPC Lett. 2010,

Non-annealed

Annealed

104

105

106

107

108

Ca

lcu

late

d B

imo

lecu

lar

Re

co

mb

ina

tio

n R

ate

, k (

arb

un

its)

1014

2 3 4 5 6 7 8 9

1015

2 3 4 5 6 7 8 9

1016

Charge Carrier Density, n / cm-3

12 nm 40 nm

24 nm

48 nm

P3HT/PCBM

Charge carrier lifetimes for different blend films

Maurano et al, Adv. Mater. (2010)

Maurano et al. J.Phys Chem C (2011)

Credgington et al, Adv Func Mat (2011)

P3HT/PCBM has

the slowest

recombination –

indicative of

microstructure

enhancing spatial

separation of

charges

Calculating VOC

At VOC: recombination flux =

genertion flux

JGEN = JSC = JRecom = -dekrecomn2

Maurano et al. Adv Mat 2010

Measure n(Int) and krecom(n)

Allows us to calculate Voc correctly

to +/- 5 mV

Works for a very broad range of

cells

Comparison against energy dependence alone

BI

rec

SCOC

J

JAEAIPV ln

Recombination model for VOC

Energy Level Model: (Scharber et al.)

1.2

1.0

0.8

0.6

0.4

0.2

0.0

Ca

lcu

late

d V

OC/V

1.21.00.80.60.40.20.0

Measured VOC/V

PMCDT

PEOT

PEOT-DCBT

P3BT

P3HT

PCPT

PCPDT

MDMO-PPV

PDPDT

P3H4NT

PDOFDTBT

PCPDTBT

Squares:

Scharber et al. 3.0 EAIPVOC

Effective electrical

bandgap from n(Voc) Charge generation

from Jsc

Recombination flux

from n and krecom

Quantifies impact of film microstructure

on Voc. Ten fold increase in krecom

decreases VOC by ~ 80 mV

4.02.0 EAIPVOC

PCPDTBT: large krecom

Less defined phase

segregation

P3HT: small krecom

Strong phase

segregation

Ballantyne et al, Macromolecules (2010)

Maurano et al. Adv Mat (2010)

Se

R

n RMS:20.62nm S

R

n

Impact of phase purity: P3HT versus P3HS

-1

0

1

2

3

4

5

6

-50 0 50 100 150 200 250 300

t (s)

V (

mV

)

P3HT:PCBM

P3HS:PCBM

n ~ 3x1016

cm-3

•VOC lower for P3HS even though IP ~

100 mV larger.

•Structural analyses give larger domain

sizes for P3HS/PCBM relative to

P3HT/PCBM

• Recombination rate coefficient 30

times faster for P3HS/PCBM

• Origin appears to be lower phase

purity – consistent with PL and Raman

analyses

• Results in ~140 mV loss of voltage

relative to P3HT.. This cancels out

voltage gain from higher IP..

Influence of processing conditions

P3HT / PCBM solar cells

Voc for P3HT / PCBM cells varied

from 0.4 – 0.6 V depending upon

film/device processing.

Transient analyses allow us to

understand how this results from

variations in:

• Jgen (mainly due to film

thickness)

• IP – EA (P3HT’s IP decreases

with increased crystallinity)

• krecom , assigned primarily to

changes in microstructure

Credgington et al. Adv Func Mat (2011)

10

5

0

-5

-10

J [

mA

cm

-2]

-0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8

Applied Voltage [V]

Jlosses

JGEN

JLight

Jdark

RECOMDark JJ RECOMGENLight JJJ

ndekJ recomRECOM

Model assumes:

• JGEN indep of voltage

• n(x)=p(x)=n

• Dark and light charge

behave the same

Requires determination of:

• JGEN (equated to JSC)

• krecom and

• n(V, Int)

Non-geminate Recombination Model of J/V curve

Model can be generalized to JLOSS including ‘shunt’ or

leakage losses resulting from non-selective contacts

JRECOM

is ‘order’ of the recombination

For ideal bimolecular: = 2

nJJJ recomscLight

2

recom )()( nndeknJ eff

Simulation includes Rseries:

Vapp = Vcell - IRseries

• No fitting parameters!

• Suggests device FF based on competition between transport and nongeminate

recombination.

• As approach VOC, charge density increases, increasing nongeminate

recombination losses

-12

-10

-8

-6

-4

-2

0

2

J [

mA

cm

-2]

0.60.50.40.30.20.10.0Vapp [V]

0.5 Suns 0.75 Suns 1 Suns 1.4 Suns

◊: simulations

Simulating J-V curve for P3HT / PCBM solar cells

Shuttle et al. PNAS 2010

Analysis of Bilayer organic solar cells

Ca:Al

Pentacene

C60

ITO/Glass

BCP

PEDOT:PSS

Calculated (points) vs expt (lines) J/V curve

'ln

1 0

C

JV SC

OC

Rshunt = 30kΩ

VOC vs Light Intensity

Expt ideality = 1.69

Calc ideality = 1.62

AR

V

A

eCJ

shunt

Cq

LOSS

0

' 1'

•Simple model based on measured decrease in charge carrier lifetime with cell

voltage/charge carrier density (100 s – 100 ns) is in good agreement with experimental

J / V data

•Suggests charge photogeneration not strongly dependent upon electric field

•Note differs from most BHJ cells in that most charge on electrodes not in active layer.

Credgington J. Phys Chem. Lett 2011

Role of PCBM crystallisation in stablising charge

separation?

PL data indicate often no significant pure

polymer domain

But if PCBM present throughout film, why isn’t

this causing fast electron / hole recombinaton?

Why is PCBM so hard to replace /

improve upon?

Jamieson et al Chem Sci 2011

Study of PBTTT: PCBM blends

PCBM melting peak

McGehee & co-workers

Stanford

Threshold

Formaton of ‘crystalline’ PCBM domains at PCBM

compositions >50%

Effect of fullerene concentration on PL quenching

Appearance of PCBM emission for

compositions above 1:1.

Includes 525 nm emission band

specific to aggregated PCBM (Cook et al Chem Phys Lett 2008)

Evidence for formation of relative

pure, ‘crystalline’ PCBM domains.

PBTTT:PCBM blends

Effect of fullerene concentration on function

Threshold of PCBM

composition for long lived

charge separation

matches appearance of

‘crystalline’ PCBM

domains.

450 500 550 600 650 700 750 8000.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

no

rmalis

ed

PL

nm)

PCBM

PCBM:PS (1:1)

PCBM aggregation and redunction potential

-1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0-1.0

-0.8

-0.6

-0.4

-0.2

0.0

no

rma

lise

d c

urr

en

t (A

)

Voltage (V)

ICBA

ICBA:PS

450 500 550 600 650 700 750 8000.0

0.2

0.4

0.6

0.8

1.0

no

rma

lise

d P

L

(nm)

ICBA

ICBA:PS (1:1)

500 nm PL of aggregated C60

Cook et al. CPL 2008

~ 80 mV

shift of

PCBM EA

with

aggregation -1.50 -1.25 -1.00 -0.75 -0.50 -0.25 0.00

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

No

rma

lise

d C

urr

en

t (A

)

Voltage (V)

PCBM

PCBM + PS (1:1)

PCBM + PS (1:4)

Role of PCBM crystallisation in providing energy

offset to stabilise charge separation

•PCBM crystallites exhibit higher EA than mixed regions

•Serves an energy offset to stabilise charge separation

•Maybe why other fullerenes (e.g. ICBA) do not work with amorphous polymers

•See similar shifts in HOMO with crystalline polymers (P3HT) – crystalline polymer

domains also function as hole sinks to stabilise spatial separation of charge

Jamieson et al Chem Sci 2011, Clarke et al. Adv Func Mat 2010

• Crystalline polymers such as P3HT work with ICBA.

• Formation of a relatively pure, molecularly ordered phase of the polymer

or the fullerene component is important for energetically stabilising spatial

charge separation

•Ability of PCBM to both mix with polymer and form crystalline domains

may be key reason for its success

Intimately mixed

Polymer:

fullerene phase

+

- -

~100 ns

-

-

Crystalline

fullerene

phase

-

<10 ns

~s

Intimately mixed

Polymer:

fullerene phase

+

- -

~100 ns

-

Crystalline

polymer

phase

-

<10 ns

~s

+

Intimately mixed

Polymer:

fullerene phase

+

- -

~100 ns

- -

<10 ns

Crystalline Polymer Both amorphous Crystalline Fullerene

Lessons for Materials Design

Some factors influencing charge generation and

recombination: Exciton Dissociation

e -

Polymer C60

h u h u

Exciton separation

h + h +

LUMO

HOMO

e - e -

Polymer excitation:

•Little requirement for exciton diffusion.

•Relatively facile exciton separation

•Can compete with energy transfer to C60 for blue, luminescent polymers

Some factors influencing charge generation and

recombination: Exciton Dissociation

Polymer C60

h u h u

Exciton separation

h + h +

LUMO

HOMO

e - e -

Fullerene excitation:

Significant requirement for exciton diffusion.

- and therefore PCBM domain size

h + h +

Excit

on

Dif

fusio

n

Some factors influencing charge generation and

recombination: Geminate recombination of CT states

e -

Polymer C60

h + h +

LUMO

HOMO h + h +

Geminate recombination of bound

polaron pairs (charge transfer

states)

e - e - Charge Dissociation

Geminate recombination of bound CT states:

•Dependent upon DGCS, interface structure, electron mobility?

• Timescale < 10 ns

Some factors influencing charge generation and

recombination: Geminate recombination (2)

e -

Polymer C60

h + h +

LUMO

HOMO h + h +

Geminate recombination of loosely

bound polarons (not charge

transfer states?)

e - e - Charge Dissociation

Geminate recombination of loosely bound polarons???:

• Dependent upon film microstructure and crystallinity, device electric fields

• Timescale ~100 ns

Some factors influencing charge generation and

recombination: Non-geminate recombination

Polymer C60

LUMO

HOMO h + h +

Geminate recombination of loosely

bound polaron pairs (not charge

transfer states?)

e - e -

Charge Collection

Non-geminate recombination of dissociated polarons:

• Dependent upon charge density, film microstructure, mobility

• Timescale ~100 ns – 100 s

Conclusions

• Simple energy level models are insufficient to give reliable materials

design principles

• For most blend films, simple two pure phase structural models are

inappropriate.

• Starting to unravel some of the factors influencing how different

materials perform in OPV devices

• Charge transfer state recombination – energetics, charge transfer character,

electron/hole mobility.....

• Interfacial energy transfer as well as electron transfer

• Material crystallisation to drive phase segregation and modulate energetics

between phases

• Role of film microstructure in influencing non-geminate recombination, and

thus FF and voltage

• Role, for some materials, of device electric fields in driving charge separation

0.5 0.6 0.7 0.8 0.9 1.0

1E-6

1E-5

1E-4

DO

D (

1

s)

-DGCS

rel / eV

annealed

unannealed

Transient absorption data Increase in EQE correlates with

increase in polaron absorption

Clarke et al., Adv Func Mat., 2009

annealed

unannealed

Annealing of P3HT / PCBM blends

Increase in polaron yield consistent with

decrease in P3HT IP.

Drives P3HT polaron localisation onto

crystalline P3HTdomains

100 mV shift

Electron versus energy transfer across the polymer /

fullerene interface

Fast energy transfer competes efficiently with

charge separation.

*PCBM1 too low in energy to drive charge

separation (small HOMO/HOMO offset)

May explain why good OLED materials often

make poor OPV materials

Soon et al, Chem Sci 2011

1PCBM*(1.8eV)

3PCBM*(1.5eV)

CSS (2.2eV)3IF8BT*

1IF8BT*(2.5eV)

ISC

Energy

Transfer

1CT 3CT

1P*

3P*

P

1BRP 3BRP

CSS

hhνν

CSS (2.2eV)

IF8BT

1PCBM*(1.8eV)

3PCBM*(1.5eV)

CSS (1.9eV)3IF8TBTT*

1IF8TBTT*(2.0eV)

1CT 3CT

1P*

3P*

P

1BRP 3BRP

CSS

h

hνν

CSS (1.9eV)

Electron

Transfer

IF8TBTT

Dissociated polaron yield as determinant of device

photocurrent

Correlation indicates

optical measurement of

charge separation yield in

blend films with PCBM is

remarkably good

predictor of photocurrent

density in devices

Clarke et al. Adv Mat 2010

Low mobility

polymers

h < 10-4

But only for ‘crystalline’

polymers

6

7

8

910

-13

2

3

4

k [c

m3s-1

]

2 4 6 8

1015

2 4 6 8

1016

2 4

n [cm-3

]

6

7

8

910

-4

2

3

4

[cm

2V-1s

-1]

k(n(V=Voc))

(n(V=0))

Shuttle et al. AFM 2010

Why is krecom varying with charge density?

Optoelectronic analysis of devices Transient optical analysis of films

Eng et al. J.Phys Chem Lett 2010

n

• krecom is function of both film microstructure and carrier mobility

• krecom increase with n correlates with increase of mobility with n

• Effect originates from trap filling – saturates when all traps filled.

Data for

P3HT/PCBM

Charge densities across the J/V curve

• Charge extraction used to

measure charge at any

operating condition relative

to dark, short circuit.

• Charge >> electrode

capacitance charge – in

photoactive layer.

• Charge function of both

voltage and light intensity

• Charge density increases

with cell voltage as less

charge swept out by device

electric fields – large

increase in non-geminate

recombination losses (~ n3)

10-10

2

3

4

5

6

78

10-9

2

3

4

5

Ch

arg

e/C

0.80.60.40.2

Voltage/V

@VOC

Dark

50%-Sun

100%-Sun

Electrode charge

Data for P3HS/PCBM solar cells

Photon energy dependency?

S 0

S 1

T 1

CT*

1 CT D 3 CT

CS*

CS

+

-

Energ

y

h n

k CT

k triplet

k GR

k CS*

k CS

k BR

k ISC D G CS

D G CT k therm k therm

CS CT therm

k exc

See Baldo et al.

JACS 2010,

Also Silva, Friend ...

But difficult to have conclusive results:

• For hot/cold CT excitation – ‘cold’ CT very deep

in bandgap with very weak optical density

• For hot/cold excitons – only see effect if no

exciton diffusion and if cold excitons have low yield

of charge separation

Screening materials for influence of hot excitons

Ground state absorption spectra and transient absorption excitation spectra for

dissociated polarons (normalised per absorbed photon) for 1:1 blends with PCBM.

No effect as in both cases ‘cold’ excitons can drive charge separation efficiently (and

exciton diffusion for P3HT).

S

S S

Excitation wavelength dependence of charge

generation in BTT-DPP:PCBM films andsolar cells

BTT-DPP:PC60BM = 1:1 IQE (black line) and absorption corrected

DOD excitation spectrum (red squares)

Polymer Fullerene

HOMO

LUMO

Both photocurrent IQE and polaron quantum yield

drop off when exciting red side of absorption band

Charge photogeneration in polymer / fullerene solar

cells – hot excitons

Consistent with excess thermal energy aiding charge separation

Hot exciton driven aided charge separation.

Hot exciton driven charge separation

Almost doubling of initial polaron quantum

yield and photocurrent if excitation

wavelength shifted from 850 to 750 nm S 0

S 1

T 1

CT*

1 CT D 3 CT

CS*

+

-

Energ

y

h n

k CT

k triplet

k GR

k CS*

k CS

k BR

k ISC

k therm CT

therm k

exc