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  • TEMPLATE DESIGN © 2008

    www.PosterPresentations.com

    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

    W e

    ig h

    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

    N o

    rm a

    liz e

    d a

    b s o

    rb a

    n c e

    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

    N o

    rm a

    liz e

    d a

    b s o

    rb a

    n c e

    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.

    Polymer λmax 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

    I n

    te n

    s it y

    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 0 1E-8

    1E-7

    1E-6

    1E-5

    1E-4

    1E-3

    I D S /A

    0 40 80 1E-8

    1E-7

    1E-6

    1E-5

    1E-4

    1E-3

    V DS

    =60V,80V,100VV DS

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

    V GS

    /V -100 -80 -60 -40 -20 0 0

    -10

    -20

    -30

    -40

    -50

    -60

    -70

    -80 V

    GS

    0V

    -20V

    -40V

    -60V

    -80V

    -100V

    I D S /

    A

    0 20 40 60 80 100 0

    10

    20

    30

    40

    50

    60

    70

    80 V

    GS

    0V

    20V

    40V

    60V

    80V

    100V

    V DS

    /V

    -80 -40 0 1E-9

    1E-8

    1E-7

    1E-6

    1E-5

    I D S /A

    0 40 80 1E-10

    1E-9

    1E-8

    1E-7

    1E-6

    1E-5

    V DS

    =40V,60V,80VV DS

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

    V GS

    /V -80 -60 -40 -20 0 0

    -1

    -2

    -3

    -4

    -5 V

    GS

    0V

    -20V

    -40V

    -60V

    -80V

    I D S /

    A

    0 20 40 60 80 0

    1

    2

    3

    4

    5 V

    GS

    0V

    20V

    40V

    60V

    80V

    VDS/V

    Polymer Temperature Average 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)