The role of molecular structure andconformation in polymer opto-electronicsCharge separation: Molecular structure

Enrico Da Como

Conjugated polymers

polythiophene

Conjugated polymers

polythiophene

Bottom – up design for electronics

Optical, electrical & ordering properties arise at the molecular scale

Structure-function relationships

Polymer solar cells: transport, recombination & efficiency

-0.75 -0.50 -0.25 0.00 0.25 0.50 0.75-12

-8

-4

0

4

8

Cu

rre

nt d

en

sity (

mA

/cm

2)

Voltage (V)

Solar cells

1. Absorption of light and photogeneration of excitons

Mott-Wannier

~ 0.1 eV

Large radius

Charge transfer excitons

~ 0.1 – 1.0 eV

Localised between molecules

Frenkel excitons

~ 0.5 – 1.0 eV

Localised on molecule

Solar cells

E

e-

e-

e-

h+

h+

h+

h+

h+ DriftDiffusion

e-

e-

eV

Built-in Potential

pn junction, heterojunction

2. Exciton dissociation & 3. Transport of charge

Solar cells

2. Exciton dissociation & 3. Transport of charge

Donor-Acceptor system

S

-

+

G. Yu, … A. J. Heeger, Science 207, 1789 (1995)

Y. Yang & Solarmer Nature Photonics 3, 649 (2009)

Polymer/fullerene photovoltaics

> 8% efficiency on lab cells

Substrate

Anode

Transport layer

Active layer: Polymer/fullerene

Metal contact

Polymer/fullerene photovoltaics

Polymer/fullerene photovoltaics

Polymer/fullerene photovoltaics

Polymer/fullerene photovoltaics

The race for 10 %

Konarka's Power Plastic

Achieves World Record 8.3%

Substrate

Anode

Transport layer

Active layer: Polymer/fullerene

Metal contact

Physics on different length scales

Efficiency

Layers & interfaces

Substrate

Anode

Transport layer

Active layer: Polymer/fullerene

Metal contact

Mesoscopic scalebulk heterojunction

Physics on different length scales

Charge transport

Morphology & molecular ordering

Substrate

Anode

Transport layer

Active layer: Polymer/fullerene

Metal contact

Molecular scaleDonor-Acceptor

Mesoscopic scalebulk heterojunction

Physics on different length scales

Exciton generation & dissociation

molecular ordering & mobility

Substrate

Anode

Transport layer

Active layer: Polymer/fullerene

Metal contact

Molecular scaleDonor-Acceptor

Charge transport

Charge separation

Physics on different length scales

Mesoscopic scalebulk heterojunction

Exciton generation & dissociation

molecular ordering & mobility

Photovoltaic action: competing mechanisms?E

ne

rgy

Charge separation

absorption

Polymer Fullerene

HOMO

HOMO

LUMO

LUMO

En

erg

y

Charge separation

absorption

Large donor-acceptor interface

Polymer Fullerene

morphology & mobility

HOMO

LUMO

HOMO

LUMO

Photovoltaic action: competing mechanisms?

Efficiency = JscVocFF

Pin

Polmyer solar cell: device parameters

-0.75 -0.50 -0.25 0.00 0.25 0.50 0.75-12

-8

-4

0

4

8

Curr

en

t density (

mA

/cm

2)

Voltage (V)

= 2.9 %

P3HT:PCBM

Short circuit current Jsclight absorption

transport

Fill factor FFcharge collection

Open circuit voltage VocHOMO-LUMO offset

Charge transfer @ polymer:fullerene interface

structure

conformation

ordering

Donor-acceptor distance

Excitons in polymers

Frenkel exciton (~ 0.5 eV – 1 eV)Intra (inter) chain excitationLifetime ~ns, diffusion length ~ 10 – 20 nm

Polymer structure, conformation & excitons

Excitons in polymers

Frenkel exciton (~ 0.5 eV – 1 eV)Inter or intrachain excitationLifetime ~ns, diffusion length ~ 10 – 20 nm

Chemical structure, excitons, long range ordering

Excitons in polymersE

ne

rgy

HOMO

LUMO

En

erg

y

S0

S1

S2

T1

absorption

Excitons in Polymer:fullerene systems

Charge transfer excitonCoulomb bound electron-hole pair @ the donor-acceptor interface

Excitons in Polymer:fullerene systems

Polymer Fullerene

HOMO

HOMO

LUMO

LUMO

En

erg

y

S0

S1

S2

T1CTE?

Excitons in Polymer:fullerene systems

Polymer Fullerene

HOMO

HOMO

LUMO

LUMO

En

erg

y

S0

S1

S2

T1CTE?

Where is the CTE energetically?What role does it play in charge transfer/recombination?CTE vs molecular structure, conformation and ordering?

Excitons in Polymer:fullerene systems

Why do CTEs dissociate?Field dependence

Only 60 % of CTEs dissociate in polymer fullerene solar cells at room temperature

V. Mihailetchi, L. Koster, J. Hummelen, P. Blom, Phys. Rev. Lett. 93, 216601 (2004)

Excitons in Polymer:fullerene systems

Are CTEs a necessary step for charge separation?

Voc limited by CTE

Polymer Fullerene

HOMO

LUMO

LUMO

Excitons in Polymer:fullerene systems

Are CTEs a necessary step for charge separation?

Polymer Fullerene

HOMO

LUMO

LUMO

Veldman et al., JACS 2008

Change molecular ordering, interface states

Excitons in Polymer:fullerene systems

Mixed amorphous & crystalline polymer regions enhance charge separation

Higher charge separation efficiency with engineered heterojunctions

Bulk properties influence CTE dissociation

Charge transfer @ polymer:fullerene interface

Acceptor concentration

En

erg

y(e

V)

-6.1

-5.4

-3.2

-4.2

HOMO

LUMO

HOMO

LUMO

MDMO-PPV/PCBM

0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.20.0

0.4

0.8

1.2 PCBM

PL

(a.u

.)

0.0

0.4

0.8

1.2 MDMO-PPV pristine

PL

(a.u

.)

Probing recombination with PL spectroscopy

Energy (eV)

Adv. Funct. Mater. 19, 3662 (2009)

En

erg

y(e

V)

-6.1

-5.4

-3.2

-4.2

HOMO

LUMO

HOMO

LUMO

MDMO-PPV/PCBM 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.20.0

0.4

0.8

1.2 MDMO-PPV/PCBM blendP

L (

a.u

.)

0.0

0.4

0.8

1.2 PCBM pristine

PL

(a.u

.)

0.0

0.4

0.8

1.2 MDMO-PPV pristine

PL

(a.u

.)

CTE

Energy (eV)

Probing recombination with PL spectroscopy

Adv. Funct. Mater. 19, 3662 (2009)

0.8 1.2 1.6 2.00

1x105

2x105

3x105

4x105

PL

(a

.u.)

Energy (eV)0.8 1.2 1.6 2.0

Energy (eV)

80 wt % PCBM60 wt % PCBM

0.8 1.2 1.6 2.0

Energy (eV)

20 wt % PCBM

Vary the donor-acceptor interface

Adv. Funct. Mater. 19, 3662 (2009)

0.8 1.2 1.6 2.00

1x105

2x105

3x105

4x105

PL

(a

.u.)

Energy (eV)0.8 1.2 1.6 2.0

Energy (eV)

80 wt % PCBM60 wt % PCBM

0.8 1.2 1.6 2.0

Energy (eV)

20 wt % PCBM

Vary the donor-acceptor interface

Adv. Funct. Mater. 19, 3662 (2009)

CTE dissociation depends on acceptor concentration

Increased probability of exciton dissociation

Arkhipov et al., Appl. Phys. Lett. 2003 82, 4605.

Charge transfer @ polymer:fullerene interface

Donor/Acceptor structure

The role of the fullerene acceptor

En

erg

y (

eV

)

HOMO

LUMO

HOMO

LUMO

Donor/acceptor

PCBM

bis-PCBM

DPM

MDMO-PPV

VOC

Appl. Phys. Lett . 97 023301 (2010)

CTE recombination

Appl. Phys. Lett . 97 023301 (2010)

Anti-Correlation of PLCTE intensity and JSC

0 20 40 60 80 100 120 140 160 180 2000.0

0.2

0.4

0.6

0.8

1.0

1.2

PL

CT

E (

arb

. u

.)

JSC

µA/cm2

Appl. Phys. Lett . 97 023301 (2010)

Anticorrelation Jsc and CTE

Morphology and transport

bis-PCBM PCBM

me=2 10-4 cm2/Vs me=8 10-3 cm2/Vsme= 1 10-3cm2/Vs

Appl. Phys. Lett . 97 023301 (2010)

Long range ordering? Transport?

Changing morphology with chain regioregularityRegiorandom P3HT Regioregular P3HT

Amorphous vs. Polycrystalline

Adv. Funct. Mater. 19, 3662 (2009)

1.0 1.5 2.0

ra-P3HT

PCBM

Energy (eV)

X10

RE-P3HT

RE-P3HT/PCBM

Changing morphology with chain regioregularity

100 nm

= 2.1%

PL I

nte

nsity

PL I

nte

nsity

= 0.9%

1.0 1.5 2.0

ra-P3HT

ra-P3HT/PCBM

Energy (eV)

EnergyEnergy

100 nm

Regiorandom P3HT Regioregular P3HT

What is the role of donor-acceptor distance?

Model system: „low band gap“ polymers

PCPDT-BT

M. Svensson, F. Zhang, O. Inganas, & M. R. Andersson, Synth. Met. 135, 137 (2003)

N. Blouin, A. Michaud, M. & Leclerc Adv. Mater. 19, (2007)

Z. Zhu, D. Waller, R. Gaudiana, M. Morana, D. Muhlbacher, M. Scharber, C. Brabec,

Macromolecules 40, 1981 (2007).

„Low bandgap“ co-polymers for better light absorption

dithiophene

benzodiathiazole

LUMO

HOMO

Increasing solar cell efficiency

PCPDT-BT

M. Svensson, F. Zhang, O. Inganas, & M. R. Andersson, Synth. Met. 135, 137 (2003)

N. Blouin, A. Michaud, M. & Leclerc Adv. Mater. 19, (2007)

Z. Zhu, D. Waller, R. Gaudiana, M. Morana, D. Muhlbacher, M. Scharber, C. Brabec,

Macromolecules 40, 1981 (2007).

„Low bandgap“ co-polymers for better light absorption

dithiophene

benzodiathiazole

Low-bandgap copolymers

500 1000 1500 2000

Abs

orpt

ion

(arb

. uni

ts)

Wavelength (nm)

800 nm

PCPDT-2TBT

PCPDT-BDT

PCPDT-2TTP

PCPDT-BT

800 nm

660 nm

800 nm

Low-bandgap copolymers

Tautz et al submitted

Stronger vs weaker acceptor

Shifting the donor-acceptor centre of mass

IR Absorption

HOMO -1

LUMO +1

Measuring IR absorption of chemically

induced polarons

HOMO

LUMO

e-P1

Where are the polarons?

-0.1

0.0

0.1

500 1000 1500 2000 2500 3000 3500

-0.1

0.0

0.1

-0.1

0.0

0.1

P1

Ch

emic

ally

in

du

ced

O

D (

arb

. u

.)

GB

P1

GB

Probe

Wavelength [nm]

Probe

P2

P1

GB

Ex

Ex P2

GB

P1

P2

-5

0

5

0

5

-10

0

10

-0.1

0.0

0.1

-5

0

5

P2

Probe

P2

IR Absorption

HOMO -1

LUMO +1

Measuring IR absorption of chemically

induced polarons

HOMO

LUMO

e-P1

Where are the polarons?

-0.1

0.0

0.1

500 1000 1500 2000 2500 3000 3500

-0.1

0.0

0.1

-0.1

0.0

0.1

P1

Ch

emic

ally

in

du

ced

O

D (

arb

. u

.)

GB

P1

GB

Wavelength [nm]

P2

P1

GB

P2

GB

P1

P2

-0.1

0.0

0.1

P2

IR Absorption

HOMO -1

LUMO +1

Measuring IR absorption of chemically

induced polarons

HOMO

LUMO

e-P1

Where are the polarons?

-0.1

0.0

0.1

500 1000 1500 2000 2500 3000 3500

-0.1

0.0

0.1

-0.1

0.0

0.1

P1

Ch

emic

ally

in

du

ced

O

D (

arb

. u

.)

GB

P1

GB

Probe

Probe

Wavelength [nm]

Probe

Ex P2

P1

GB

Ex

Ex P2

GB

P1

P2

-5

0

5

0

5

-10

0

10

Op

tica

lly

in

du

ced

(1

0-4)

-0.1

0.0

0.1

-5

0

5

P2

Probe

Polaron formation in realtime

10

20

30

10

20

10

20

Time delay (fs)

PCPDT-BT

Po

laro

n p

air

yie

ld (

%)

P3HT

PCPDT-BDT

PCPDT-2TBT

PCPDT-2TTP

IRF

-1000 -750 -500 -250 0 250 500 7500

10

20

10

20

D A

D A

D A

D A

UU U U-

= 15.9%

= 21.4%

= 13.9%

= 7.9%

= 23.6%

Polaron formation in realtime

10

20

30

10

20

10

20

Time delay (fs)

PCPDT-BT

Po

laro

n p

air

yie

ld (

%)

P3HT

PCPDT-BDT

PCPDT-2TBT

PCPDT-2TTP

IRF

-1000 -750 -500 -250 0 250 500 7500

10

20

10

20

D A

D A

D A

D A

UU U U-

= 15.9%

= 21.4%

= 13.9%

= 7.9%

= 23.6%

Polaron formation in realtime

10

20

30

10

20

10

20

Time delay (fs)

PCPDT-BT

Po

laro

n p

air

yie

ld (

%)

P3HT

PCPDT-BDT

PCPDT-2TBT

PCPDT-2TTP

IRF

-1000 -750 -500 -250 0 250 500 7500

10

20

10

20

D A

D A

D A

D A

UU U U-

= 15.9%

= 21.4%

= 13.9%

= 7.9%

= 23.6%

Acceptor strength only slightly influencing efficiency

Important role of spatial separation

Charge transfer @ polymer:fullerene interface

structure

conformation

ordering

Donor-acceptor distance

Substrate

Anode

Transport layer

Active layer: Polymer/fullerene

Metal contact

Mesoscopic scalebulk heterojunction

Physics on different length scales

Charge transport

Morphology & molecular ordering

Substrate

Anode

Transport layer

Active layer: Polymer/fullerene

Metal contact

Molecular scaleDonor-Acceptor

Mesoscopic scalebulk heterojunction

Physics on different length scales

Exciton generation & dissociation

molecular ordering & mobility

How to improve efficiency at every length scale?

Conformation & structure

Long range ordering

Charge transfer

Adv. Funct. Mater. 19, 3662 (2009)

1.0 1.5 2.0

ra-P3HT

PCBM

Energy (eV)

X10

RE-P3HT

RE-P3HT/PCBM

Changing morphology with chain regioregularity

100 nm

= 2.1%

PL I

nte

nsity

PL I

nte

nsity

= 0.9%

1.0 1.5 2.0

ra-P3HT

ra-P3HT/PCBM

Energy (eV)

EnergyEnergy

100 nm

Regiorandom P3HT Regioregular P3HT

100 nm

The effect of long range ordering

AnnealedNot Annealed

= 2.1% = 4.0%

100 nm

AnnealedNot Annealed

= 2.1% = 4.0%

1.0 1.5 2.0

PL

in

ten

sity

X10

RE-P3HT/PCBM

RE-P3HT/PCBM

(annealed)

Energy (eV)Adv. Funct. Mater. 19, 3662 (2009)

The effect of long range ordering

100 nm

AnnealedNot Annealed

= 2.1% = 4.0%

Adv. Funct. Mater. 19, 3662 (2009) J. App. Phys.100, 043702 (2006)

Ambipolar transportUnipolar (hole) transport

The effect of long range ordering

How to induce long range ordering?

Substrate

Anode

Transport layer

Active layer: Polymer/fullerene

Metal contact

Conformation & structure

Long range orderingDoping

Charge transfer

Increase mobility without changing morphology?

Increasing mobility by molecular doping

P doping by electron transfer in the

ground state

F4TCNQ

Yim et al., Adv Mater, 2008, 20Zhang et al., Phys Rev B, 2010, 81

Zhang et al., Adv Func Mater, 2009, 19

Increasing mobility by molecular doping

P doping by electron transfer in the

ground state

PCPDTBT:PCBM

F4TCNQ

SPP1355

Fill tail states with excess

charge carriers

Increase Mobility

+

Energ

y (

eV

)

Disordered film

Increasing mobility by molecular doping

g(E)

Substrate

Anode

Transport layer

Active layer: Polymer/fullerene

Metal contact

Charge transport

Charge separation

Improvement in charge separation, mobility, efficiency

Photocurrent & Efficiency?

0.9 1.0 1.1 1.2 1.3 1.4

PCBM

PCPDTBT

1.0 1.2 1.4 1.6 1.8

PL inte

nsity (

arb

.units)

Energy (eV)

PCPDTBT/PCBM

Energy (eV)

PL inte

nsity (

arb

. units)

Doping & Charge separation

+

-

0.9 1.0 1.1 1.2 1.3 1.4

PCBM

PCPDTBT

1.0 1.2 1.4 1.6 1.8

PL inte

nsity (

arb

.units)

Energy (eV)

PCPDTBT/PCBM

Energy (eV)

PL inte

nsity (

arb

. units)

+

-

0%

1%

3%

4%

Doping & CTE recombination

0 200 400 600 800

Time (ps)

0%

2%

4%

5%

No

rm.

PL

inte

nsity

Doping & CTE recombination

0 100 200 300

0%

2%

4%

5%

PL inte

nsity (

arb

. u

nits)

Time (ps)

Lower density of CTE or very fast dissociation with doping?

Doping & CTE recombination

Doping & Polaron formation

Janssen et al. Adv. Mater

(2010)-T/

T x

10

4

0 50 100 150 200 250 3000

2

Time delay (ps)

-

T/T

(x10

-3)

0%

EProbe

Doping & Polaron formation

Janssen et al. Adv. Mater

(2010)-T/

T x

10

4

0

2

0 50 100 150 200 250 3000

22%

-

T/T

(x10

-3)

0%

Time delay (ps)

EProbe

Doping & Polaron formation

Janssen et al. Adv. Mater

(2010)-T/

T x

10

4

0

2

0

2

0 50 100 150 200 250 3000

24%

2%

-

T/T

(x10

-3)

0%

Time delay (ps)

EProbe

Doping & Polaron formation

Janssen et al. Adv. Mater

(2010)-T/

T x

10

4

0

2

0

2

0

2

0 50 100 150 200 250 3000

25%

4%

2%

-

T/T

(x10

-3)

0%

Time delay (ps)

EProbe

PCPDTBT PCBM

En

erg

y

tCT-r

tCT-ftFR-r

tP-f

tP-r

0 100 200 300

0%

2%

4%

5%

PL

in

ten

sity (

arb

. u

nits)

Time (ps)

Time Delay (ps)

0

2

0.0

1.6

0

2

0.0

1.6

0

2

0.0

1.6

0

2

0.0

1.6

0 50 100 150 200 250 300 Po

laro

n d

en

sity (

x1

01

7/c

m3)

0%

-T

/T (

x1

0-4)

2%

4%

5%

Rate equation model

PCPDTBT PCBM

En

erg

y

tCT-r

tCT-ftFR-r

tP-f

tP-r

0 100 200 300

0%

2%

4%

5%

PL

in

ten

sity (

arb

. u

nits)

Time (ps)

Time Delay (ps)

0

2

0.0

1.6

0

2

0.0

1.6

0

2

0.0

1.6

0

2

0.0

1.6

0 50 100 150 200 250 300 Po

laro

n d

en

sity (

x1

01

7/c

m3)

0%

-T

/T (

x1

0-4)

2%

4%

5%

Doping

[%] tFR-r tCT-f tP-f tCT-r tP-r[ps] [ps] [ps] [ps] [ps]

0 125 0.2 0.2 300 1400

2 125 0.5 0.2 300 1000

4 125 0.95 0.2 300 400

5 0.15 0.95 0.2 250 300

Rate equation model

Decrease in CTE emission and larger density of polarons with doping

Conclusion: doping helps!

Phys. Rev. Lett. 107, 127402 (2011)

Substrate

Anode

Transport layer

Active layer: Polymer/fullerene

Metal contact

Charge transport

Charge separation

Improvement in charge separation, mobility, efficiency

Efficiency?

09.03.2015 Präsentationstitel 80

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