Optimization of Conjugated-Polymer-Based

Bulk Heterojunctions

J.C. (Kees) HummelenMolecular Electronics

Materials Science Centre Plus

University of Groningen, The Netherlands

GCEP Solar Energy WorkshopOct. 18 – 18, 2004 Stanford University

GCEP2004

TCOSUBSTRATE

CPC BLENDAl

~ 20 nm ~ 500 nm

Introduction, Morphology, Mobilities, Improving Voc , Improving Isc,

- N.S.Sariciftci, A.J. Heeger, USPatents 1992/1993- G. Yu, J. Gao, J.C. Hummelen, F. Wudl, A.J. Heeger, Science 270, 1789-91 (1995)

- S. E. Shaheen, C. J. Brabec, F. Padinger, T. Fromherz, J. C. Hummelen, N. S. Sariciftci, Appl. Phys.Lett. 78, 841 (2001). GCEP2004

Opening the box of Pandora:from bilayer to mixture

p/n

-metal top electrode

transparent bottom electrodeglass

Light

DA

p:n

GCEP2004G. Yu, J. Gao, J.C. Hummelen, F. Wudl, A.J. Heeger, Science 270, 1789 (1995)

Basic processes in a PV cell

Light

e-

h+

excitonElectrode 1 Electrode 2

GCEP2004

Commonly used PPVs

O

O

O

O

O

O

O

O

O

O

O

O

O

O

O

O

O

O

O

O

MEH-PPV

MDMO-PPV(OC1C10PPV)

M

EH

DMO

M GCEP2004

The standard fullerene acceptor

O

O

PCBMMicrosoft

owerPoint Presentatio

A highly processable methanofullerene(50-80 weight % of the D:A blend!)

Available from Nano-C Inc. (Westwood, Mass)nano-c.com

GCEP2004

PCBM (PhCl) crystal structureO

O

Fullerene moieties at < 10 Å in THREE dimensionsSpheres: orientation always OK!

M.T. Rispens, A. Meetsma, R. Rittberger, C.J. Brabec, N.S. Sariciftci, J.C. Hummelen, Chem. Commun. 2116-2118 (2003) GCEP2004

O

O

Charge carrier mobility in PCBM

• (MDMO-PPV: holes µh = 5×10 -7 cm 2 /Vs)• PCBM: electrons µe = 2×10 -3 cm 2 /Vs

(C60 films: 8.10-2 cm2/Vs ; C60 single crystals: 0.5 cm2/Vs)Mobility measurements on PCBM films

Experimental and calculated (solid lines) J-V characteristics of ITO/PEDOT:PSS/PCBM/LiF/Al devices with thicknesses L=90 nm and 170 nm (symbols),using Vbi=1.4 eV and VRs=30 Ω

The device band diagram is indicated in the inset. The electron transport is described by SCLC,with an electron mobility µe=2.0 ⋅10-7 m2/Vs and a dielectric constant εr =3.9.

SCLC:

i.e. PCBM does 4000 times better than MDMO-PPV!

Average hopping distance in PCBM ~ 3 nm!

Mihailetchi, van Duren, Blom, Hummelen, Janssen, Kroon, Rispens, Verhees, Wienk, Adv. Funct. Mat. 13, 43-46 (2003) GCEP2004

20 30 40 50 60 70 80 90 10010-11

10-10

10-9

10-8

10-7

10-6

µe

µh

weight percentage PCBM [wt.-%]

µ [m

2 /Vs]

µh – pure MDMO-PPV

T=295K

Charge carrier mobility in blendO

O

MDMO-PPV

• enhanced intermolecular interaction by adding PCBM.

C. Melzer et al., Adv. Funct. Mat., (accepted)M. Kemerink et al., Nano Lett., 2003, 3, 1191

MDMO-PPV:PCBM

• electron mobility increases due to the increase number of percolated pathways.

• more balanced transport: µe ≈ 10 × µh. GCEP2004

Morphology: influence of spin cast solvent:MDMO-PPV:PCBM

from toluene from chlorobenzenea

0.5 µm

b

0.5 µm

0.0 0.5 1.0 1.5 2.0 2.5-4

0

4

8

Sur

face

Hei

ght (

nm)

Distance (µm)0.0 0.5 1.0 1.5 2.0 2.5

-4

0

4

8

Sur

face

Hei

ght (

nm)

Distance (µm) GCEP2004GCEP2004

Morphology: influence of spin cast solvent:

Glass

PEDOT

Active layer

LiF

AluminumSMU

+-

ITO

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9-6

-5

-4

-3

-2

-1

0

from toluene from chlorobenzene

Cur

rent

Den

sity

(mA

/cm

2 )

Voltage (V)

S. E. Shaheen, C. J. Brabec, F. Padinger, T. Fromherz, J. C. Hummelen, N. S. Sariciftci, Appl. Phys.Lett. 78, 841-3 (2001). GCEP2004

Morphology control: compatibilizingwith the PCBX series

O

O

O

O

O

O

O

O

O

O

O

OH O

O

O

O

PCBM PCBE PCBPr

PCBEH PCBDMO

GCEP2004

Dropcasting using different acceptorsGilch-PPV/PCBM

Toluene 1:4Gilch-PPV/PCBDMO

Toluene 1:42001 01096 2002 01505

TEMT. Martens, Z. Beelen, J. D’Haen, J. Manca, IMEC

Unfiltered

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Dropcasting using different acceptors

2001 01078

Gilch-PPV/PCBMChlorobenzene 1:4

Gilch-PPV/PCBDMOChlorobenzene 1:4

2002 01521

T. Martens, Z. Beelen, J. D’Haen, J. Manca, IMEC

TEM Unfiltered GCEP2004

The photoinduced LUMO-LUMO electron transfer process(from excited donor)

Eref

LUMO

HOMO

Light

Voc(max)

D A

Anode CathodeGCEP2004

Improving Voc

Eref

Anode Cathode

AD

Voc(max)

is there some truth in this picture? GCEP2004

Soluble fullerene derivativeswith varying acceptor strength

O

OO

N OO

O

N

EHO-Azafulleroid-[5,6] EHO-Ketolactam PCBM-[6,6]

GCEP2004

PCBM Ered tuning

O

O

O

O

OCH3

CH3O

O

O

OCH3

CH3O OCH3

O

O

OCH3

O

O

OCH3

CH3O

O

O

CH3O

GCEP2004Joop Knol, Floris Kooistra

Voc versus Ered acceptorin MDMO-PPV/acceptor cells (CB)

-1.15 -1.10 -1.05 -1.00 -0.95

600

700

800

900

V oc

Ered

vs Fc/Fc+

KL

C60

AF

PCBM3,4-(MeO)2PCBM, TCBM

2,3,4-(MeO)3PCBM

.PCBM

GCEP2004

Higher fullerenes: [70]PCBM

M.M. Wienk, J. M. Kroon, W. J. H. Verhees, J. Knol, J. C. Hummelen,

P. A. van Hal, and R. A. J. Janssen, Angew. Chemie 42, 3371-5 (2003)

GCEP2004

The photoinduced HOMO-HOMO electron transfer process(to excited acceptor)

Evac

LUMO

HOMO

LightVoc(max)

D A

Anode CathodeGCEP2004~ ‘hole transfer’

IPR Fullerenes C60-C80 (calculated)

GCEP2004

UV-Vis spectrum of [60]PCBM

300 400 500 600 700 8000

10

20

30

40

50ex

tinct

ion

coef

ficie

nt /

L g-1cm

-1

wavelength / nm

O

O

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Synthesis of [70]PCBM

(chiral)85 % 15 %

OMe

O

OMeO

OMe

O

+ +

NNHTs

OMeO

base, C70 ODCB

GCEP2004

UV-Vis spectra of [60]PCBM and [70]PCBM

300 400 500 600 700 8000

10

20

30

40

50ex

tinct

ion

coef

ficie

nt /

L g-1cm

-1

wavelength / nm[60]PCBM:MDMO-PPV (4:1, w/w) and [70]PCBM:MDMO-PPV (4:1, w/w) (normalized)

All in toluene GCEP2004

PL decay of the fullerene emission at 720 nm of [70]PCBM:MDMO-PPV (4:1 w/w) films

0 1 2 3 410

100

1000

10000

Cou

nts

Time / ns

Spun from chlorobenzene, o-xylene, and ODCB. Yellow: pristine [70]PCBM film

PL quenching:0, 30, 60, 95 %

GCEP2004

AFM tapping mode height images[70]PCBM:MDMO-PPV (4:1 w/w) films on glass

z-range = 37 nm rms roughness = 7 nm

o-xylenez-range = 86 nm

rms roughness = 12 nm

CB ODCBz-range = 8.2 nm

rms roughness = 1.0 nm

0 1 2 3 410

100

1000

10000

Cou

nts

Time / nsGCEP2004

Spectral response (EQE) of ITO/PEDOT-PSS/fullerene:MDMO-PPV/LiF/Al cells

400 500 600 700 800 9000.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7EQ

E

wavelength / nm

[70]PCBM:MDMO-PPV cells, spun from CB and ODCB[60]PCBM: MDMO-PPV cell spun from CB; active areas = 0.1 cm2 GCEP2004

I / V characteristics of [70]PCBM:MDMO-PPV devices

-1.0 -0.5 0.0 0.5 1.0-10

-5

0

5

10

15

20

Cur

rent

Den

sity

/ m

Acm

-1

Voltage / V

-2 -1 0 1 21E-5

1E-3

0.1

10

1000

VOC = 0.77 VISC = 7.6 mA/cm2

FF = 0.51η = 3.0 %

M.M. Wienk, J. M. Kroon, W. J. H. Verhees, J. Knol, J. C. Hummelen, P. A. van Hal, and R. A. J. Janssen, Angew. Chemie 42, 3371-5 (2003)

GCEP2004

[70]PCBM Available from Nano-C Inc. (Westwood, Mass)nano-c.com

Morphology of the bulk heterojunction

• Formationphases, domain sizes, percolation, wetting, processing, annealing, thickness

• Efficiencyexciton diffusion, charge separation, charge recombination, (balanced) mobilities

• Stabilityphase separation (crystallization),(photo-)chemistry

GCEP2004

Some provoking remarks

• No theoretical grounds for inferior efficieny of molecular devices

• Higher efficiency => improved stability• Molecular materials offer infinite architectural

opportunities (even for 3rd generation tricks)• General: 225M$ is very attractive! Beware of

groups that not have a real focus on sustainable energy? (PV in this case)

GCEP2004

• Joop Knol• Minze T. Rispens• Floris Kooistra• Luis Sanchez• Jan Alma• Patrick van ‘t Hof• Valentin Mihailetchi• Paul Blom (RuG)

• Paul van Hal • René Janssen (TU/e)

• Jan M. Kroon• Wiljan Verhees• Martijn Wienk (ECN)

• Christoph Brabec (now Siemens/Konarka)

• Serdar Sariciftci (Uni Linz)

• David Kronholm• Henning Richter (Nano-C)

Novem (The Netherlands’ Organization for Energy and the Environment)

E.E.T. (“Economy, Ecology, Technology” by Dutch EZ, OC&W, VROM depts)

Acknowledgments

GCEP2004

Acknowledgments