A record high electron mobility polymer semiconductor for organic thin-

film transistors

Bin Sun,1,3 Wei Hong,1,3 Zhuangqing Yan,1,3 Hany Aziz*,2,3 Yuning Li1,3,* 1 Department of Chemical Engineering, 2 Department of Electrical and Computer Engineering,

and 3 Waterloo Institute for Nanotechnology (WIN), University of Waterloo, 200 University Ave

West, ON N2L 3G1, Canada

Polymer based organic thin film transistors (OTFTs) can be used in radio-frequency

identification (RFID) tags, flexible displays, medical sensors, memory devices, etc.; but their

low charge carrier mobility has limited many applications.

Recently, several classes of π-conjugated polymers having electron donor (D) and acceptor

(A) units showed highly efficient charge transport performance with mobility higher than 1

cm2V-1s-1 in OTFTs. Particularly, Diketopyrrolopyrrole (DPP)-based polymers are the best

performing polymer semiconductors for OTFTs to date.1, 2 All the high mobility DPP

polymers reported are based on highly coplanar five-membered ring-flanked DPP building

blocks. The first reported phenyl-substituted DPP block (DBP) has been much less

investigated for the OTFT application. This is mainly due to the fact that DBP has a large

torsion angle of ~20-40° between the six-membered phenyl ring and the DPP core.3 Thus the

DBP-based polymers showed very low mobility values.4

In this study, we use the sterically less demanding 2-pyridinyl units to flank the DPP core to

achieve a high coplanarity (with a dihedral angle of 0°) of the resulting DBPy building block,

which was substantiated by computer simulations. Pyridine is an electron deficient moiety,

which would make DBPy a strong electron acceptor. Thus polymers based on DBPy are

expected to exhibit efficient electron transport characteristics in OTFTs. We report here the

first DBPy based polymer that showed record high electron mobility of up to 6.3 cm2V-1s-1.

Experimental

A copolymer of DBPy and bithiophene (BT), PDBPyBT, was prepared according to the route

shown in Scheme 1. The number (Mn) average molecular weights of PDBPyBT is 26,300,

with a polydispersity index (PDI) of 3.56, determined using a high-temperature gel-

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permeation chromatography (GPC) with 1,2,4-trichlorobenze as the eluent at a column

temperature of 140 °C.

Scheme 1. The synthetic route to PDBPyBT: i) tert-C4H9OK/2-methyl-2-butanol/100 °C; ii)

K2CO3/DMF/70 °C; iii) Pd2(dba)3/P(o-tolyl)3/chlorobenzene/130 °C. Copyright 2014 WILEY-VCH Verlag GmbH & Co. §

The polymer was evaluated in bottom-gate, bottom-contact (BGBC) OTFT devices and top-

gate, bottom-contact (TGBC) OTFT devices. Reflective XRD diagrams of polymer thin films

spin coated on SiO2/Si substrates were performed on a Bruker D8 Advance powder

diffractometer using standard Bragg-Brentano geometry with Cu Kα1 radiation (λ = 1.5406

Å). Transmission XRD measurements were carried out on a stack of polymer thin films 5

using Bruker Smart Apex2 CCD with Mo Kα radiation (λ = 0.71073 Å). Atomic force

microscopic (AFM) images were obtained on polymer thin films (~30-40 nm) spin coated on

SiO2/Si substrates with a Dimension 3100 Scanning Probe Microscope.

Results

We used the cyclic voltammetry (CV) to determine the frontier energy levels of PDBPyBT.

The HOMO and LUMO levels were calculated to be -5.69 eV and -4.33 eV, respectively. The

very low LUMO level is beneficial for electron transport in OTFTs.6

To investigate the polymer chain packing, X-ray diffractometry (XRD) measurements in a

reflective mode were performed on polymer thin films on SiO2/Si wafer substrates. For the

as-spun polymer thin film, an intense primary diffraction peak at 2θ = 4.36° was observed,

HN

NH

O

ON

N

Br

BrC2H5O

OC2H5

O

O

IC8H17

C10H21

SSnS SnN

N

O

ON

N

Br

Br

C10H21

C8H17

C8H17

C10H21

N

N

O

ON

N

C10H21

C8H17

C8H17

C10H21

SS n

PDBPyBT

DBPyBr-H

DBPyBr-20

NBr CN

i) ii)

iii)

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which corresponds to a d-spacing of 2.03 nm. When the thin film was thermally annealed, the

primary peak intensity increased dramatically, resulting from the improved crystallinity. To

determine the in-plane chain packing, we conducted transmission XRD measurements on

stacked polymer thin films. As shown in Fig. 1b, a prominent peak at 2θ = 11.34° was

observed, which corresponds to a π-π distance of 3.60 Å. This short π-π distance would be

beneficial for charge hoping along the in-plane π-stacking direction.

Fig. 1 (a) XRD spectrums of spin coated PBDPyBT thin films on bare SiO2/Si substrates

annealed at different temperatures measured in a reflection mode using Cu Kα1 radiation. (b)

XRD pattern of a stack of non-annealed PDBPyBT thin films in a transmission mode using Mo

Kα radiation. Copyright 2014 WILEY-VCH Verlag GmbH & Co. §

Fig. 2 AFM images (2µm × 2µm each) of PDBPyBT thin films on bare SiO2/Si substrates annealed at different temperatures: the root-mean square (RMS) roughness is 8.2 nm, 8.3 nm, and

11.7 nm for the as-spun (r.t.), 150 °C-annealed, and 200 °C-annealed thin films, respectively. Copyright 2014 WILEY-VCH Verlag GmbH & Co. §

PDBPyBT was evaluated in bottom-gate, bottom-contact (BGBC) OTFT devices using

heavily n-doped Si wafer substrates having a thermally grown SiO2 layer (~200 nm), where

the Si layer and the SiO2 layer were used as the gate electrode and the dielectric, respectively.

The polymer thin film was then annealed, followed by spin-coating with a PMMA

5 10 15 20 25 30

0.0

4.0k

8.0k

12.0k

16.0k 200 oC 100 oC r. t.

2θ (deg.)

Inte

nsity

5 10 15 20 250

10k

20k

30k

40k

2θ (deg.)

Inte

nsity

(a) (b)

( )

0 nm

70 nm r .t. 150 ° 200 °

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encapsulation layer (~500 nm). Devices annealed at 100 °C exhibited typical p-channel field

effect behavior with hole mobility of up to 0.088 cm2V-1s-1. The 200 °C-annealed devices

showed increased mobility of up to 0.46 cm2V-1s-1. It can be seen in the AFM image in Fig 2

that the polymer films are very rough surface. We found that the output and transfer curves of

the devices are very noisy, which is probably due to the large grain boundaries in the polymer

thin films and the poor contact of the polymer films to the DTS-modified SiO2 dielectric

layer.

To improve the interfacial contact between the dielectric and the PDBPyBT layer, we used a

commercial fluorinated polymer, Cytop, as the dielectric in a top-gate, bottom-contact

(TGBC) device configuration. Surprisingly, ambipolar charge transport performance was

observed for all the TGBC devices. High average mobility values of 4.54 cm2V-1s-1 for

electrons and 2.20 cm2V-1s-1 for holes were obtained for the 100 °C-annealed PDBPyBT

devices. The best device showed electron and hole mobility values of 6.30 cm2V-1s-1 and 2.78

cm2V-1s-1, respectively. These are the highest mobility values reported for ambipolar polymer

OTFTs so far. Especially, the electron mobility of 6.30 cm2V-1s-1 is the record high value

achieved for polymer semiconductor OTFTs to date. The high performance observed for

PDBPyBT was believed due to the high backbone coplanarity, very high crystallinity, and a

close π-π stacking distance of this polymer. The improved dielectric and polymer interface by

adopting the TGBC device structure is also greatly contributable to the achievement of the

record high electron mobility.

Fig. 3 Output (left) and transfer (right) curves of a typical top-gate, bottom-contact (TGBC)

OTFT device with a PDBPyBT thin film annealed at 100 °C. Device dimensions: channel length L = 30 µm; channel width W = 1 mm. Copyright 2014 WILEY-VCH Verlag GmbH & Co. §

-80 -60 -40 -20 00

-100

-200

-300

-400 V

GS

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

I DS/µ

A

0 20 40 60 800

100

200

300

400 V

GS

0V 20V 40V 60V 80V

VDS

/V-80 -40 0

1E-7

1E-6

1E-5

1E-4

I DS/A

0 40 801E-7

1E-6

1E-5

1E-4

VDS

=40V,60V,80VVDS

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

VGS

/V

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In conclusion, pyridin-flanked DPP (DBPy) was used to construct a polymer semiconductor

PDBPyBT with high backbone coplanaritystructure and a very short π-π stacking distance of

0.36 nm. The strong electron withdrawing effect of DBPy facilitated the electron injection

and transport of this polymer. Typical ambipolar charge transport performance was observed

for PDBPyBT in top-gate bottom-contact OTFT devices. A record high electron mobility

value of 6.30 cm2V-1s-1 was achieved. Our results demonstrate that DBPy is a promising

electron acceptor building block for high performance polymer semiconductors for printed

electronics.

Acknowledgements

The authors thank the Natural Sciences and Engineering Research Council (NSERC) of

Canada for financial support (Discovery Grants) and Jon Hollinger and Prof. Dwight Seferos

of University of Toronto for the measurements of molecular weight.

§ This work is published in Adv. Mater. 2014, DOI: 10.1002/adma.201305981

References

1. Holliday, S., J. E. Donaghey, et al. Chem. Mater. 2014, 26, 647

2. Li, Y.; Sonar, P.; Murphy, L.; Hong, W. Energy Environ. Sci. 2013, 6, 1684

3. Kim, C.; Liu, J.; Lin, J.; Tamayo, A. B.; Walker, B.; Wu, G.; Nguyen, T. Chem. Mater.

2012, 24, 1699

4. a) Wu, P.-T.; Kim, F. S.; Jenekhe, S. A. Chem. Mater. 2011, 23, 4618. b) Kanimozhi, C.;

Balraju, P.; Sharma, G. D.; Patil, S. J. Phys. Chem. B 2010, 114, 3095. c) Chen, L.; Deng,

D.; Nan, Y.; Shi, M.; Chan, P.; Chen, H. Z. J. Phys. Chem. C 2011, 115, 11282

5. Pan H., Li Y., Wu Y., Liu P., Ong B. S., Zhu S., Xu G., J. Am. Chem. Soc. 2007,129, 4112.

6. Jones, B. A.; Facchetti, A.; Wasielewski, M. R.; Marks, T. J. J. Am. Chem. Soc. 2007,

129, 15259. (b) Yan, H.; Chen, Z.; Zheng, Y.; Newman, C.; Quinn, J. R.; Dötz, F.;

Kastler, M.; Facchetti, A. Nature 2009, 457, 679 IPR 20

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A record high electron mobility polymer semiconductor for organic thin‐film 

transistors

Bin SunDepartment of Chemical Engineering, Institute of Polymer Research (IPR),  and Waterloo Institute for Nanotechnology (WIN), University of Waterloo

Supervisors: Profs Yuning Li & Hany Aziz

May 21/2014

IPR 36th ANNUAL SYMPOSIUM

Outline

• Background

• PDBPyBT Polymer semiconductor design and synthesis

• OTFTs evaluation

• Conclusion

2

Flexible displays

Electronic Paper Sensors

Plastic RFID

Electronics Based On Organic Thin‐film Transistors(OTFTs)

Light weightLarge areaFlexibilityLow cost

3

www.technologyreview.comHP, Arizona State's Paper‐Like, Flexible Displaywww.flexible‐display.net/cheap‐flexible‐displays‐using‐improved‐semiconducting‐polymers/Mol. Cryst. Liq. Cryst., 2006, 444, 13–22

Organic Thin‐Film Transistors

High mobility0 -20 -40 -60 -80

0.0

-2.0x10-6

-4.0x10-6

-6.0x10-6

-8.0x10-6

V

DS/V

I DS/A

VGS=0V

VGS=-10V

VGS=-20V

VGS=-30V

VGS=-40V

VGS=-50V

VGS=-60V

VGS=-70V

4

Dielectric layer

SemiconductorDrain

Source

Gate

Mobility5~10 cm2/Vsfor OLED

Conjugated polymer

Phys. Rev. Lett. 1977, 39, 1098-1101.Applied Physics Letters 1986, 49, 1210‐1212.

2000 Nobel Prize

Polyacetylene

Prof. Alan J. HeegerProf. Alan G. MacDiarmid Prof. Hideki Shirakawa

1980S first Polymer Field effect transistor

Hole Mobility 10‐5 cm2/(Vs)

UnmeltedInsoluble

1990s solution processwas introduced into OTFTs fabrication

5

Early 1970s

Charge moves all over the chain  

hop between different chains

Donor‐Acceptor Structure

Donor Accepter

Ong, B. S., et al, J. Am. Chem. Soc.  2004, 126, 3378.

Li, Y., et al, Adv. Mater. 2006, 18, 3029.

= ~0.10 cm2/(V.s)

= ~0.25 cm2/(V.s)

6

= ~12 cm2/(V.s)

Kang I ., et al  JACS 2013, 135, 14896

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Pigment red 254 or 'Ferrari Red'

Diketopyrrolopyrrole (DPP)

7

•An electron‐accepting moiety

•1974 first by Donald G. Farnum

.

Coplanarity of building block

Torsion angle

Monomer Angle

~26‐40°

~0°

<12°

~0°

N

N

O

O

R

R

S

S

N

N

O

O

R

R

N

N

O

O

R

R

O

O

N

N

O

O

R

R

Se

Se

Adv. Mater. 2013, 25,1859−1880

Chem. Sci. 2012, 3, 181−185

Chem. Mater. 2012, 24, 1699‐1709.

8

ELUMO, eV

EHOMO, eV

‐4

2‐Pyridine‐DPP

N

N O

O

R

R

N

N Dihedral angle~0 o

Five‐membered ring (electron rich)Six‐membered ring (electron deficient, lower LUMO, electron transport)

9

ELUMO, eV

EHOMO, eV

<‐4

Synthesis of PDBPyBT

10

Mn = 26,300; PDI = 3.56

Proton NMR of monomer

11

1H NMR (300 MHz, CDCl3) δ 8.93 (d, J = 8.5 Hz, 2H), 8.74 (d, J = 2.1 Hz, 2H), 8.01 (dd, J = 8.6, 2.3 Hz, 2H), 4.28 (d, J = 7.3 Hz, 4H), 1.61 (m, 2H), 1.24 (m, 64H), 0.93‐0.81 (m, 12H).

Energy Level of Polymer

-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5-60

-40

-20

0

20

40

60

Cur

rent

(A

)

Voltage (V)

Fc PDBPyBT

400 500 600 700 800 900 1000 11000.0

0.2

0.4

0.6

0.8

1.0 Solution Film

Wavelength

N

orm

aliz

ed A

bso

rban

ce

UV‐vis absorption spectra of PDBPyBT solution (chloroform) and as‐cast thin film on quartz.

Cyclic voltammetry diagrams of PDBPyBT and the reference ferrocene (Fc)  12

Maximum absorption wave length683nm in solution695nm in film

ELUMO, eV

EHOMO, eV

-4.33

-5.69

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Device Fabrication Process

13

Spin‐coatingDoped Si wafer with SiO2

Thermal evaporation

Photo lithography making subtrate

BGBC OTFTs 

Tannl.

C

SiO2 as dielectric layer

h,

cm2V‐1s‐1

Ion/Ioff

h max h ave

100 0.088 0.072 (±0.014) ~103

150 0.25 0.23 (±0.022) ~104

200 0.46 0.39 (±0.046) ~104SCSiO2

Au

PMMA

N++Si

14

SiO2 modified with dodecyltrichlorosilane (DDTS)

Poly(methyl methacrylate) (PMMA) 

Output and Transfer characteristics of BGBC OTFTs

0 -20 -40 -60 -80

20

0

-20

-40

VGS

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

I DS/u

A

VDS

/V

0 -20 -40 -60 -80

0

-10

-20

-30

-40

-50

-60

-70 V

GS

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

I G/u

A

VDS

/V

20 0 -20 -40 -60 -80

1E-8

1E-7

1E-6

1E-5

1E-4

VDS

=-80V

VGS

/V

I DS/A

0.000

0.005

0.010

0.015

0.020

I DS

1/2/A

1/2

(a) (b) (c)

(a) Output, (b) leakage current through gate, and (c) transfer curves of a bottom‐gate, bottom‐contact (BGBC)  OTFT device using a PDBPyBT thin film annealed at 200 C. Hole mobility: 0.46 cm2V‐1s‐1.

15

I μC V V

L

WVS

VG

VD

L=30um, W=1000um

Ci=17 nF/cm2

AFM image

16

AFM image of polymer film on Si/SiO2 substrate annealed on hotplate in nitrogenFilled glove box for 10min

RMS = 8.2 8.3 nm 11.7 nm

High performance in TGBC design

Tannl.

C

Cytop as dielectric

e,

cm2V‐1s‐1

h,

cm2V‐1s‐1

e max e ave (STD) h max h ave

100 6.30 4.54 (±1.27) 2.78 2.20 (±0.37)

150 3.91 3.50 (±0.40) 2.25 1.90 (±0.29)

200 3.32 2.30 (±0.62) 1.80 1.25 (±0.39)SCSiO2

Au

Dielctric

Al

Cytop

17

L=30um, W=1000um

I μC V V

Ci=3.2 nF/cm2

Output and Transfer

• Annealed at 100 °C

• e=6.30 cm2V‐1s‐1

• h=2.26 cm2V‐1s‐1

18

-80 -60 -40 -20 00

-50

-100

-150

-200

-250

-300

-350

-400 V

GS

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

I DS/

A

0 20 40 60 800

50

100

150

200

250

300 VGS

0V 20V 40V 60V 80V

VDS

/V-80 -40 0

1E-8

1E-7

1E-6

1E-5

1E-4

I DS/A

0 40 801E-8

1E-7

1E-6

1E-5

1E-4

VDS

=40V,60V,80VVDS

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

VGS

/VIP

R 2014

PDQT in TGBC OTFTs structure

Tannl.

C

Cytop as dielectric

h,

cm2V‐1s‐1

h max h ave (STD)

100 1.01 0.68 (0.19)

150 0.63 0.56 (0.048)

200 0.42 0.35 (0.039)

Hole mobility as high as 5.5 cm2V‐1s‐1

in BGBC structureB. Sun et.al. J. Mater. Chem. C 2013, 1, 4423

19

L=30um, W=1000um

PDQT in TGBC OTFTs configuration

0 -20 -40 -60 -8050

0

-50

-100

-150

VGS

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

I DS/u

A

VDS

/V

0 -20 -40 -60 -801E-7

1E-6

1E-5

1E-4

VDS

= -80V

VGS

/V

I DS/A

0.000

0.005

0.010

0.015

I DS

1/2/A

1/2

Output (left) and transfer (right) curves of a top‐gate, bottom‐contact (TGBC) OTFT device  using a PDQT thin film annealed at 100 C. Hole mobility: 1.01 cm2V‐1s‐1.

20L=30um, W=1000um

X Ray‐diffraction

2θ=2.12, d‐spacing distance 19.2Å

2θ=11.34, pi‐pi distance 3.6Å

PDQT3.70‐3.75Å

(a) XRD spectrums of spin coated PBDPyBT thin films on bare SiO2/Si  substrates annealed at different temperatures measured in a reflection mode using Cu K1  radiation. (b)  XRD pattern of a stack of non‐annealed PDBPyBT thin films in a transmission mode using Mo Kradiation.

21

Summary

• A donor‐acceptor copolymer of PDBPyBT was synthesized, which showed high crystallinity and a very short π‐π stacking distance of 0.36 nm. 

• High ambipolar charge transport performance with a record high electron mobility value of 6.30 cm 2 V −1 s −1 was achieved in OTFT devices at a mild annealing temperature of 100 °C. 

• Our results demonstrate that DBPy is a highly promising new building block for polymer semiconductors for OTFTs and other applications.

22

Acknowledgements• Zhuangqing Yan for computer simulation

• Dr. Wei Hong for UV‐Vis and CV measurements

• Jon Hollinger and Prof. Dwight Seferos of University of Toronto for the measurements of molecular weight

23

The authors thank the Natural Sciences and Engineering ResearchCouncil (NSERC) of Canada for financial support (Discovery Grants)

B. Sun, W. Hong, Z. Yan, H. Aziz, Y. Li, Adv. Mater.2014, 26, 2636.

Thank you !

24

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