amphiphilic block copolymers for morphology control in organic...

Amphiphilic Block Copolymers for Morphology Control in Organic Solar Cells

Valerie Mitchell ACAP Fellow, University of Melbourne

2 4 Dec 2018 Bio21 Institute Valerie D. Mitchell

Jones Group, University of Melbourne

Can we improve the industrial viability of organic photovoltaics through material design?


Johnson, K.; Huang, Y.-S.; Huettner, S.; Sommer, M.; Brinkmann, M.; Mulherin, R.; Niedzialek, D.; Beljonne, D.; Clark, J.; Huck, W. T. S.; Friend, R. H., J. Am. Chem. Soc. 2013, 5074-5083

Importance of Morphology

Bulk Heterojunction Solar Cell

Efficiency decreases with •  Large domains (>20 nm) •  Small domains (<7 nm) •  Lack of percolation pathways

⇢ Exciton decay ⇢ Recombination ⇢ Recombination


Issues to address for industrial application

In order to be industrially viable, these morphologies must be:

•  Stable

•  Reproducible

•  Obtainable with minimal and environmentally friendly processing


Block Copolymer Self-Assembly

Block Copolymers

•  Covalent linkage of donor and acceptor molecule

•  Self-assembly leads to well defined domains

•  Allows direct control over size and interface of the donor and acceptor domains


Challenges in Block Copolymer Applications

•  Morphological control: achieving phase separation

•  Synthetic difficulties: reliance on polycondensations

•  Purification: Isolation of block copolymer

NS

NS

H13C6

SS

C6H13

H13C6

n m

SSPh

H13C6 C6H13

H17C8 C8H17

NS

NS

H13C6

SS

C6H13

H13C6

n m

SSPh

H17C8 C8H17

NS

NS

H13C6

SS

C6H13

H13C6

n m

SSPh

R2 R2

R1 R1

R1 = CH3, C6H13, C8H17

R2 = C6H13, 2-ethylhexyl

P3HT-b-PFTBT

P3HT-b-PFT6BT

3.1% efficiency

No microphase separation

1.  Sommer, M. et al. Macromolecules 2012, 45(10): 4142-4151 2.  Smith, K. A. et al. Macromolecules 2015, 48(22): 8346-8353.


Challenges in Block Copolymer Applications

•  Morphological control: achieving phase separation

•  Synthetic difficulties: reliance on polycondensations

•  Purification: Isolation of block copolymer

BB AA+A +

m

m

Am

NS

NS

H13C6

SS

C6H13

H13C6

n m

SSPh

H13C6 C6H13

H17C8 C8H17

NS

NS

H13C6

SS

C6H13

H13C6

n m

SSPh

H17C8 C8H17

NS

NS

H13C6

SS

C6H13

H13C6

n m

SSPh

R2 R2

R1 R1

R1 = CH3, C6H13, C8H17

R2 = C6H13, 2-ethylhexyl

P3HT-b-PFTBT

P3HT-b-PFT6BT

P3HT-b-PFTBT, 3.1% efficiency •  52% block copolymer, •  48% additional polymer

contaminants

1. Guo C. et al. Nano Lett 2013, 13(6): 2957-2963.


Amphiphilic Block Copolymers

Amphiphilic Analog

NSN

S

H13C6

SS

C6H13

H13C6

nSS

OO4 4

b.

H13C6 C6H13

NSN

m

SS

OO4 4

Ph

H13C6 C6H13

S

H13C6

SS

C6H13

n

OO4 4

C6H13

PhS

H13C6

SS

C6H13

H13C6

nBr

NSN

m

SS

OO4 4

Ph

H13C6 C6H13

S

H13C6

SS

C6H13

H13C6

n

P3HT-b-PFTEGT6BT

•  Enhanced phase separation

•  Increased solubility

•  Facilitate purification


Synthesis of P3HT-b-PFTEGT6BT

S

H13C6

SS

C6H13

H13C6

nBr +

NS

NS

H13C6

SS

C6H13

H13C6

n m

SS

OO4 4

NS

NSS

OO4 4

BB Br BrO

O

O

O

Ph

+

H13C6 C6H13

H13C6 C6H13

NS

NSS

OO4 4

BB Br BrO

O

O

O+

H13C6 C6H13

NS

N

m

SS

OO4 4

Ph

H13C6 C6H13

Ph

1 eq 0.9 eq

0.07 eq 1 eq 1 eq

Pd(PPh3)4TEA-OH (20%)Aliquat 336Toluene90˚C O/N

Pd(PPh3)4TEA-OH (20%)Aliquat 336

Toluene90˚C O/N

P3HT-b-PFTEGT6BT Synthesis

V. D. Mitchell; W. W. H. Wong; M. Thelakkat; D. J. Jones, Polym. J., 2017, 49, 155-161.


Purification of Reaction Mixture

CHCl310% MeOH

CH2Cl210% MeOHCHCl3

P3HT PFTEGT(R)BT P3HT-b-PFTEGT(R)BTPolymer mixture

loaded on silica gel

Purification Technique

-Amphiphilic design allows easy separation of polymeric impurities -Improvement over other reported systems with no selectivity between blocks

Polymeric impurities BCP Product

NSN

S

H13C6

SS

C6H13

H13C6

nSS

OO4 4

b.

H13C6 C6H13

NSN

m

SS

OO4 4

Ph

H13C6 C6H13

S

H13C6

SS

C6H13

n

OO4 4

C6H13

PhS

H13C6

SS

C6H13

H13C6

nBr

NSN

m

SS

OO4 4

Ph

H13C6 C6H13

S

H13C6

SS

C6H13

H13C6

n


Properties of P3HT-b-PFTEGT6BT

V. D. Mitchell; W. W. H. Wong; M. Thelakkat; D. J. Jones, Polym. J., 2017, 49, 155-161.

Mn(kg/mol) PDI P3HT(%) Yield(%)

P3HT 17.6 1.1 41P3HT-b-PFTEGT6BT 37.3 1.9 47 26


AFM of Homopolymer Blend and BCP

As-Spun Annealed

P3HT/PFTEGT6BT Blend •  As-spun: some fibrillar domains •  Nodules after annealing

BCP/P3HT/PFTEGT6BT Blend (50:25:25) •  As-spun: some linear domains •  Mostly featureless

P3HT-b-PFTEGT6BT •  Nice linear domains in as-spun films,

retained with annealing


Resonant Soft X-Ray Scattering Domain Analysis

Showed domains of 17 nm in as-spun films, 20 nm after annealing

2

46

10-7

2

46

10-6

2

46

I2 q (a

u)

90.01

2 3 4 5 6 7 8 90.1

2 3 4 5 6

q (nm-1)

2

3

4

5

6

7

8

9

2

3

4

5

6

10

100

d spacing (2π/q) (nm)

P3HT-b-PFTEGTBT As-Spun P3HT-b-PFTEGTBT Annealed As-spun roughness Annealed roughness

Thanks to A/Prof Chris McNeill (Monash University, Australian Synchrotron), Dr Eliot Gann (NIST), and Dr. Lars Thomsen (Australian Synchrotron) for RSoXS and NEXAFS measurements


P3HT-b-PFTEGT6BT

•  Great morphology

•  Poor performance due to lack of electron mobility

NSN

S

H13C6

SS

C6H13

H13C6

nSS

OO4 4

b.

H13C6 C6H13

NSN

m

SS

OO4 4

Ph

H13C6 C6H13

S

H13C6

SS

C6H13

n

OO4 4

C6H13

PhS

H13C6

SS

C6H13

H13C6

nBr

NSN

m

SS

OO4 4

Ph

H13C6 C6H13

S

H13C6

SS

C6H13

H13C6

n


P3HT-b-PNDIegT2 Materials

N

N

OO

O O

O

O O

O2 2

O

O O

O2 2

S

C6H13

SS

C6H13

C6H13

nS

Sm

N

N

OO

O O

O

O O

O2 2

O

O O

O2 2

SS

m

P3HT-b-PNDISDEGT2 PNDISDEGT2CHCl3 wash

DCM/MeOHwash

CHCl3/MeOHwash

P3HT PNDIEGT2 BCP

Mn(kg/mol) PDI P3HT(%) Yield(%)

P3HT 19.3 1.1 51P3HT-b-PNDISDEGT2 71 1.6 26 12


Solution UV/Vis

300 400 500 600 700 8000.0

0.5

1.0

0.0

0.5

1.0

300 400 500 600 700 800

Wavelength (nm)

Nor

mal

ized

Abs

orba

nce

(a.u

.)

P3HT-b-PNDIDEG

T2

CBTolTol/Ani

PNDIDEG

T2

•  Preaggregation in solution has been shown to induce microphase separation in film

•  P3HT block unchanged

•  PNDI block shows some aggregation

Solution UV/Vis in chlorobenzene, toluene, or toluene/anisole (1:1)


AFM of BCP films

200nm200nm200nm200nm200nm200nm 200nm200nm200nm200nm200nm200nm200nm200nm

6.276.276.276.276.276.276.276.276.27

0.000.000.000.000.000.000.000.000.00

-6.27-6.27-6.27-6.27-6.27-6.27-6.27-6.27-6.27

°2.652.652.652.652.652.652.652.652.65

0.000.000.000.000.000.000.000.000.00

-2.65-2.65-2.65-2.65-2.65-2.65-2.65-2.65-2.65

°

200nm200nm200nm200nm200nm200nm200nm200nm200nm200 nm 200 nm 200 nm200 nm 200 nm

3.403.403.403.403.403.403.403.40

0.000.000.000.000.000.000.000.000.00

-3.40-3.40-3.40-3.40-3.40-3.40-3.40-3.40-3.40

3.4˚

-3.4˚

0.0˚

3.403.403.403.403.403.403.403.40

0.000.000.000.000.000.000.000.000.00

-3.40-3.40-3.40-3.40-3.40-3.40-3.40-3.40-3.40

6.3˚

-6.3˚

0.0˚

3.403.403.403.403.403.403.403.40

0.000.000.000.000.000.000.000.000.00

-3.40-3.40-3.40-3.40-3.40-3.40-3.40-3.40-3.40

2.7˚

-2.7˚

0.0˚

Chlorobenzene Toluene Toluene/Anisole(50/50)

Distinct variations in microstructure depending on solvent choice visible in AFM. Globular domains in CB, linear in toluene and toluene/anisole.


GIWAXS of BCP films

As-spun Annealed

CB

Tol/Ani

•  Greater degree of P3HT crystallization in films spun from CB

•  Higher order reflections attributable to both blocks with annealing


RSoXS of BCP films

Anisole/TolueneChlorobenzene

Thanks to A/Prof Chris McNeill (Monash University, Australian Synchrotron), Dr Eliot Gann (NIST), and Dr. Lars Thomsen (Australian Synchrotron) for RSoXS and NEXAFS measurements

-Clear peak in film deposited from chlorobenzene indicates 22 nm domains. -Weak separation in toluene/anisole, annealing film at 220˚C for 15 minutes did not increase phase separation


Device performance

CB 2.3 0.54 41 0.5Tol/Ani 3.1 0.54 48 0.8

Solvent J SC

(mA/cm2)VOC (V) FF (%) PCE (%)

P3HT-b-PNDIDEGT2

AgMoO3

ZnOITO

•  Despite weaker phase separation, the film deposited from Tol/Ani performed better than the CB film

•  The morphology obtained with preaggregation of PNDI is more favorable

Thanks to Dr. Jegadesan Subbiah (University of Melbourne) for device measurements


Conclusions

•  Our amphiphilic block copolymer system improved purification and phase separation

•  We demonstrated self-assembly in as-spun films and stability with annealing

•  We showed the morphological benefit of BCP purification

•  Future work: develop synthetic strategies to allow efficient incorporation of high-perfomance polymers

200nm

Self-AssemblyDonor-AcceptorBlock Copolymer


Acknowledgements

Monash University and the Australian Synchrotron Dr. Eliot Gann, Prof. Chris McNeill and Dr. Lars Thomsen for RSoXS and NEXAFS measurements, analysis, and beamtime Nigel Kirby, Stephen Mudie, and Adrian Hawley at the SAXS/WAXS beamline

University of Melbourne Dr. David Jones, Prof Ken Ghiggino, and Dr. Wallace Wong Jianing Lu for advice on AFM measurements Thanks to members of the Jones and Wong labs past and present

Sincere thanks to


Analysis of Purification

GPC Analysis of Fractions

P3HT PFTEGT6BT P3HT-b-PFTEGT6BT

Insets show traces from isolated homopolymers for comparison

10 15 20

DCM/MeOH

Abso

rban

ce (a

.u.)

10 15 2

CHCl3

0.00.20.40.60.81.01.2

12 14 16 18 20

CHCl3/MeOH

360 nm 450 nm

Retention time (min)

0

0 12 14 16 181 12 14 16 1801 01

NSN

S

H13C6

SS

C6H13

H13C6

nSS

OO4 4

b.

H13C6 C6H13

NSN

m

SS

OO4 4

Ph

H13C6 C6H13

S

H13C6

SS

C6H13

n

OO4 4

C6H13

PhS

H13C6

SS

C6H13

H13C6

nBr

NSN

m

SS

OO4 4

Ph

H13C6 C6H13

S

H13C6

SS

C6H13

H13C6

nNSN

S

H13C6

SS

C6H13

H13C6

nSS

OO4 4

b.

H13C6 C6H13

NSN

m

SS

OO4 4

Ph

H13C6 C6H13

S

H13C6

SS

C6H13

n

OO4 4

C6H13

PhS

H13C6

SS

C6H13

H13C6

nBr

NSN

m

SS

OO4 4

Ph

H13C6 C6H13

S

H13C6

SS

C6H13

H13C6

nNSN

S

H13C6

SS

C6H13

H13C6

nSS

OO4 4

b.

H13C6 C6H13

NSN

m

SS

OO4 4

Ph

H13C6 C6H13

S

H13C6

SS

C6H13

n

OO4 4

C6H13

PhS

H13C6

SS

C6H13

H13C6

nBr

NSN

m

SS

OO4 4

Ph

H13C6 C6H13

S

H13C6

SS

C6H13

H13C6

n


Analysis of Charge Mobility

μh#(cm2/V!s)#100˚C% 150˚C% 220˚C%

P3HT# 1.25x10*2%P3HT/b/PFTEGTBT# 1.52x10*4% 7.31x10*4% *%P3HT/b/PFTEGT6BT# 4.22x10*5% 9.89x10*4% 1.13x10*3%

Hole mobilities (µh) calculated from the slope of (Id)0.5 vs V(g) in the saturated regime

-Materials showed no electron transport in OFET configuration in as-spun films or after annealing at 100˚C, 150˚C, and 220˚C for 10 min -Measurement of hole mobility as a function of annealing showed development of charge percolation pathways -Charge mobility determined by SCLC confirmed low electron mobility

μe##(cm2/V!s)#

Avg#μe#(cm2/V!s)#

μh##(cm2/V!s)#

Avg#μh##(cm2/V!s)#

PFTEGTBT#4.5x10'10( 1.7x10'9( 1.4x10'4( 8.4x10'5(3.0x10'9( 2.7x10'5(

PFTEGT6BT#1.3x10'8(

1.3x10'8(2.3x10'8(2.0x10'9(

SCLC Analysis

OFET Analysis

SCLC analysis performed with the help of Dr. Chetan Singh, Bayreuth University


Analysis of Device Performance

Active'Layer'(~100'nm)'Al'(100'nm)'

PEDOT:PSS'(40'nm)'ITO'Glass'Substrate'

Active'layer'(~100'nm)'

Ag'(100'nm)'

ZnO'(40'nm)'ITO'Glass'Substrate'

MoO3'(10'nm)'

JSC$$(mA/cm2)$

VOC$(V)$

FF$(%)$

PCE$(%)$

P3HT5b5PFTEGTBT$Inverted$

PA$ 0.22$ 0.89$ 27.6$ 0.05$A$ 0.25$ 0.32$ 37.8$ 0.03$

Normal$PA$ 0.18$ 1.03$ 27.2$ 0.05$A$ 0.63$ 0.95$ 21.2$ 0.13$

P3HT5b5PFTEGT6BT$Inverted$$

PA$ 0.18$ 0.80$ 35.9$ 0.05$A$ 0.10$ 0.47$ 39.9$ 0.02$

Normal$PA$ 0.03$ 0.50$ 23.1$ 0.003$A$ 0.03$ 0.53$ 24.5$ 0.003$

Normal Architecture Inverted Architecture

PA- before annealing, A- annealed at 150˚C for 10 min

Device analysis performed with the help of Dr. Chetan Singh, Bayreuth University


UV/Vis

4.1 3.1 2.5 2.1 1.8 1.6

300 400 500 600 700 8000.0

0.2

0.4

0.6

0.8

1.0 Photon Energy (eV)

Abso

rban

ce (

a.u.)

Wavelength (nm)

PFTEG

T6BT

P3HT-b-PFTEG

T6BT

P3HT

.

amphiphilic block copolymers for morphology control in organic...

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