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Novel Solubilizing Side Chains for Large π-conjugated

Polymers in OTFTs

Yinghui He, Chang Guo, Zhuangqing Yan, Bin Sun and Yuning Li

Department of Chemical Engineering, University of Waterloo

Introduction

Organic thin film transisotors (OTFTs) start to play an important

role in various electronic devices, such as displays, radio

frequency identification tags and sensors, as their mobility

improved by orders in the last decade. Polymer based OTFTs are

extremely desired because of their solution processability, low

cost and flexibility.

A typical Bottom gate bottom contact configuration of OTFTs.

The strategy to obtain to high performance polymers for OTFTs is

to construct donor-acceptor (D-A) polymers, which tend to have

strong π-π stacking induced by the D-A interaction. However, as

π-π stacking gets stronger, solubility of the polymer will decrease.

So far, the most widely used solubilizing side chains are alkyl

side chains. In this poster, a new type of side chains, which has

an ester group is introduced into D-A polymers to tackle the

solubility issue caused by the large π-conjugated backbone. The

polymer will be characterized by different techniques, such as

GPC, UV and XRD. Finally, the OTFTs based on these polymers

will be characterized.

Synthetic Work

GPC Characterization

TGA Characterization

UV-Vis Characterization

XRD Characterization

OTFT Characterization

Conclusions

Polymer semiconductor

Source Drain

Dielectric layer

Gate

Three polymers (P11, P2 and P3) were synthesized with different side

chains. Even though P1 incorporated the largest commercially

available side chain, 1-bromo-2decyltetradecane, it is still essentially

insoluble in common organic solvents. On the other hand, P2 and P3,

incorporating the ester side chains, show good solubility in toluene and

chloroform.

The synthesis of the ester side chains is simple. The Grignard reaction

with ethyl formate brought two long side chain into a secondary alcohol.

Then the alcohol was linked to 4-bromobutyric acid through

condensation, affording the corresponding ester side chains. The

length of the branches can be easily adjusted accordingly, as well as

the branching point, by choosing different starting materials.

Polymer Molecular weight Mn Polydispersity (PDI)

P1 N/A N/A

P2 54 kDa 2.3

P3 61 kDa 3.3

The mechanism of charge transport in polymer semiconductors

Synthetic Work

• The GPC measurement was carried out using chlorobenzene

as eluent and polystyrenes as standards at 50°C.

• P1’s molecular weight couldn’t be measured due to its poor

solubility. P2 and P3 both show high molecular weight and

small PDI.

0 100 200 300 400 500

40

50

60

70

80

90

100

We

igh

t%

Temperature (°C)

P2

P3

• These two polymers show similar thermal stability. A

5% weight loss was not observed until 300°C, which

means the ester group and the backbone are both

quite thermally stable.

300 400 500 600 700 800 900 1000 1100 1200

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

No

rma

lize

d a

bso

rba

nce

Wavelength (nm)

Solution in chloroform

Film

300 400 500 600 700 800 900 1000 1100 1200

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

No

rma

lize

d a

bso

rba

nce

Wavelength (nm)

Solution in chloroform

Film

• Both polymers show a narrow optical band gap, indicating the

large π-conjugation along the backbone.

• The blue shift of the maximum adsorption of P3 is suspected to

be caused by the twisting of backbone in solid state.

Polymerλmax in

solution (nm)

λmax in film

(nm)

Optical band

gap (eV)

P2 844 844 1.35

P3 840 837 1.34

• Both thin films of P2 and P3 show enhanced intensity

after annealing at 200°C, indicating the improved

crystallinity.

• The d-spacing is 2.61 nm for P2 and 2.81 nm for P3

0 5 10 15 20 25 30 35 40

-1000

0

1000

2000

3000

4000

5000

6000

7000

8000 100°C

150°C

200°C

2

P3

0 5 10 15 20 25 30 35 40

-1000

0

1000

2000

3000

4000

5000

6000

7000

8000 100°C

150°C

200°C

2

In

ten

sity

P2

AFM Characterization

• For films of P2, as the annealing temperature increases,

the grains start to fuse and the gaps get smaller. The

annealed (at 200°C) film shows a more uniform

morphology, which is considered to be in favor of charge

transport.

• For films of P3, annealing at 150°C gave an relatively

uniform morphology. After annealing at 200°C, the grains

start to aggregate, making the gaps larger.

• It is expected that P2 will show higher charge mobility

than P3.

Donor Accepter

4

• A novel type of side chains with ester group was introduced to

solubilize large π-conjugated polymers. The synthesis of the

ester side chains is simple, and the branching point and branch

length can be easily manipulated.

• Effect of the branch length was studied. P3 with longer branches

showed lower mobility than P2. It is probably because the π-π

stacking was compromised by the over-sized side chains.

• The effect of the branching point would be interesting to study.

References

1. Yan et al., Chem. Commun., 2013, 49, 3790--3792

Thin film of P2:

annealed at

00°C, 150°C

and 200°C

respectively

(from left to right)

Thin film of P3:

annealed at

00°C, 150°C

and 200°C

respectively

(from left to right)

AFM Characterization

Transfer and output

of the OTFT device

based P2 (200°C)

Transfer and output

of the OTFT device

based P2 (150°C)

-80 -40 01E-8

1E-7

1E-6

1E-5

1E-4

1E-3

I DS/A

0 40 801E-8

1E-7

1E-6

1E-5

1E-4

1E-3

VDS

=60V,80V,100VVDS

=-60V,-80V, -100V

VGS

/V -100 -80 -60 -40 -20 00

-10

-20

-30

-40

-50

-60

-70

-80 V

GS

0V

-20V

-40V

-60V

-80V

-100V

I DS/

A

0 20 40 60 80 1000

10

20

30

40

50

60

70

80 V

GS

0V

20V

40V

60V

80V

100V

VDS

/V

-80 -40 01E-9

1E-8

1E-7

1E-6

1E-5

I DS/A

0 40 801E-10

1E-9

1E-8

1E-7

1E-6

1E-5

VDS

=40V,60V,80VVDS

=-40V,-60V, -80V

VGS

/V-80 -60 -40 -20 00

-1

-2

-3

-4

-5 V

GS

0V

-20V

-40V

-60V

-80V

I DS/

A

0 20 40 60 800

1

2

3

4

5 V

GS

0V

20V

40V

60V

80V

VDS

/V

Polymer TemperatureAverage electron

mobility (cm2 V-1 S-1)

Average hole mobility

(cm2 V-1 S-1)

P2

100°C 7.7 × 10-2 2.7 × 10-2

150°C 0.10 2.9 × 10-2

200°C 0.12 4.3 × 10-2

P3

100°C 4.4 × 10-2 2.0 × 10-2

150°C 3.1 × 10-2 1.7 × 10-2

200°C 3.2 × 10-2 1.6 × 10-2

• All OTFT devices were fabricated under bottom gate bottom

contact configuration.

• Both polymers showed a typical ambipolar charge transport

property.

• P2 showed higher mobility due to its better morphology. (AFM)