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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)
