Synthesis and characterization of graft polystyrenes with para-substituted π-conjugated oligo(carbazole) and oligo(carbazole-thiophene) moieties for organic field-effect transistors

Article abstract of DOI:10.1016/j.polymer.2013.04.002

In this research, two styrene-derivative monomers based on bicarbazole and carbazole-bithiophene-carbazole (StCz2 and StCzT2Cz) were prepared and further free-radical polymerized (PStCz2 and PStCzT2Cz) to form graft π-conjugated polymers. The thermal, optical, and electrochemical properties of these studied monomers and polymers were fully characterized for studying the relationships of the structures to the properties. The field-effect characteristics of the polymers were also studied, a highest hole mobility on the order of 1.1 × 10^-3 cm²V^-1s^-1 was obtained with PStCzT2Cz. The morphology of the polymeric active layers was also investigated with X-ray diffraction and atomic force microscopy to realize the relationships of the structures to the electric characteristics.

Full text of DOI:10.1016/j.polymer.2013.04.002

Polymer 54 (2013) 3548e3555
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Polymer
journal homepage: www.elsevier.com/locate/polymer
Synthesis and characterization of graft polystyrenes with para-
substituted
p-conjugated oligo(carbazole) and oligo(carbazole-
thiophene) moieties for organic field-effect transistors
Gung-Pei Chang a, Ching-Nan Chuang b, Jong-Yih Lee a, Yung-Shen Chang a,
,
,b,
*
Man-kit Leung b c, Kuo-Huang Hsieh a
a Department of Chemical Engineering, National Taiwan University, Taipei 106, Taiwan
b Institute of Polymer Science and Engineering, National Taiwan University, Taipei 106, Taiwan
c Department of Chemistry, National Taiwan University, Taipei 106, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history:
In this research, two styrene-derivative monomers based on bicarbazole and carbazole-bithiophene-
Received 26 December 2012
Received in revised form
25 March 2013
Accepted 1 April 2013
Available online 9 April 2013
carbazole (StCz2 and StCzT2Cz) were prepared and further free-radical polymerized (PStCz2 and
PStCzT2Cz) to form graft p-conjugated polymers. The thermal, optical, and electrochemical properties of
these studied monomers and polymers were fully characterized for studying the relationships of the
structures to the properties. The field-effect characteristics of the polymers were also studied, a highest
hole mobility on the order of 1.1 ꢀ 10ꢁ3 cm2Vꢁ1sꢁ1 was obtained with PStCzT2Cz. The morphology of the
polymeric active layers was also investigated with X-ray diffraction and atomic force microscopy to
realize the relationships of the structures to the electric characteristics.
Keywords:
Organic field-effect transistor (OFET)
Graft polymers
Ó 2013 Elsevier Ltd. All rights reserved.
Free radical polymerization
1. Introduction
5.6 ꢀ 10ꢁ7 cm2Vꢁ1sꢁ1 [13]. In the same year, Marder et al. prepared
a polynorbornene with pendant perylene diimides, the highest
Main chain nonconjugated polymers containing pendant
p
-
electron mobility was on the order of 4.9 ꢀ 10ꢁ5 cm2Vꢁ1sꢁ1 [14].
Before obtaining such graft polymers as mentioned above, the
semiconducting monomers were ranged diversely in vinyl-,
methacrylate-, and norbornene-derivatives by specific molecular
designs and synthetic strategies. Among the various techniques
used to polymerize the semiconducting monomers, the most
convenient polymerization remains in free-radical polymerization
[11,15e17]. By comparing with main chain conjugated polymers,
using graft polymers as OFET active layers could be considered as a
pioneering development of semiconducting materials for OFET
[18e22]. However, the FET performances based on graft polymers
as mentioned above are quite low and could be further improved,
especially for the charge carrier mobility.
Among these graft polymers with specific semiconducting
groups, the [3,30]-dicarbazole-yl-[2,20]-bithiophene co-oligomer
(CzT2Cz) showed the best field-effect hole mobility on the order
of 1.2 ꢀ 10ꢁ1 cm2Vꢁ1sꢁ1 in our previous report [23]. However, this
oligomeric semiconductor has to be deposited via solution casting
to obtain such high order of hole mobility. Thus, for advancing the
processability of CzT2Cz to spin coating process and maintaining
the good field-effect mobility, the graft polymer with pendant
CzT2Cz might give the advantages of short deposition time via spin
conjugated moieties have been widely studied in electrochromic
materials [1e3], polymer light-emitting diodes (PLEDs) [4e7], and
memory devices [8e10] in the past decades. These graft polymers
with isolated semiconducting groups in their side chains show the
advantages of controlled
p-conjugating lengths and improved
solubility in organic solvents [11]. Thus, this molecular architecture
is easy to be tailor-made and processed for the applications of
organic electronics.
Recently, some graft polymers with pendant semiconducting
groups were reported in organic field-effect transistors (OFETs)
rarely. In 2010, Thelakkat et al. prepared a series of polystyrenes
with triarylamine as pendant groups, the best hole mobility was on
the order of 1 ꢀ 10ꢁ4 cm2Vꢁ1sꢁ1 [12]. In 2012, Reichmanis et al.
reported a series of graft polymethacrylates containing diphe-
nyldithiophene moieties, the best hole mobility was on the order of
* Corresponding author. Department of Chemical Engineering, National Taiwan
University, Taipei 106, 1, Sec. 4, Roosevelt Road, Taipei, Taiwan. Tel.: þ886
2
33663044; fax: þ886 2 33665237.
E-mail address: khhsieh@ntu.edu.tw (K.-H. Hsieh).
0032-3861/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.polymer.2013.04.002

G.-P. Chang et al. / Polymer 54 (2013) 3548e3555
3549
coating process, uniform film formation, mechanical flexibility, and
good FET performances.
Absorption spectra were recorded with a Jasco model UV/vis/
NIR V-570 spectromete. For the thin film spectra, the studied
materials were first dissolved in toluene (4 mg-mlꢁ1) and filtered
According to the inspirations as mentioned above, two semi-
conducting styrene-derivatives monomers were designed and
synthesized (StCz2 and StCzT2Cz) initially in this research, as
shown in Scheme 1. The styrene moiety was attached to the sem-
iconducting oligomers as the polymerizable groups for free-radical
polymerization. Then these two styrene-derivatives monomers
were further free-radical polymerized to yield graft polymers
(PStCz2 and PStCzT2Cz), as shown in Scheme 2. Before studying
the FET characteristics based on these graft polymers, the thermal,
optical, and electrochemical properties of monomers and their
corresponding polymers were also investigated.
through 0.45
mm pore size polytetrafluoroethene (PTFE) mem-
brane syringe filters, spin-coated at a speed rate of 1000 rpm for
60 s onto quartz substrates. Cyclic voltammetry (CV) was per-
formed with the use of a three-electrode cell in which indium tin
oxide was used as a working electrode. A platinum wire was used
as an auxiliary electrode. All cell potentials were taken with the
use of a homemade Ag/AgCl, KCl (sat.) reference electrode. The
highest occupied molecular orbital energy levels (HOMO) were
oxidation
determined from the onset oxidation ðE
Þ and estimated on
onset
the basis of the reference energy level of ferrocence (4.8 V below
the vacuum level) according to the following equation: HOMO ¼
1=2
oxidation
ꢁeðE
ꢁ Eferrocene þ 4:8Þ (eV) [24,25]. The lowest unoccupied
onset
2. Experimental section
molecular orbital (LUMO) levels were estimated from the HOMO
values and the values of optical band gaps (Egopt.) according to
the equation: LUMO ¼ HOMO þ Egopt (eV) [25].
2.1. Characterization
1H NMR spectra were recorded by a Bruker Avance DRX
500 MHz. GPC analysis was performed on a Laboratory Alliance
RI2000 instrument connected with one refractive index detector
from Schambeck SFD Gmbh. All GPC analyses were performed on
polymer/THF solution at a flow rate of 1 mL-minꢁ1 at 40 ꢂC and
calibrated with polystyrene standards.
Thermogravimetric analysis (TGA) and differential scanning
calorimetry (DSC) measurements were performed under a nitrogen
atmosphere at a heating rate of 10 ꢂC-minꢁ1 using TA instruments
(TGA-951 and DSC-910S). Atomic force microscopy (AFM) mea-
surements were obtained with a NanoScope IIIa AFM (Digital In-
struments, Santa Barbara, CA) at room temperature. Commercial
silicon cantilevers (Nanosensors, Germany) with typical spring
constants of 21e78 N-mꢁ1 was used to operate the AFM in tapping
mode. Images were taken continuously with the scan rate of 1.0 Hz.
2.2. Fabrication and characterization of thin film transistors
Organic thin film transistors were prepared from polymer thin
films with bottom-gate-top-contact geometries on the heavily n-
doped silicon wafers. A thermally grown 300 nm SiO2 was used as
the gate dielectric with a capacitance of 11 nF-cmꢁ2. Afterward, the
surface of SiO2 was modified with phenyltrichloro silane (PTS) to
form a phenyl self-assemble monolayer (SAM) [26]. Polymer solu-
tions (0.5 wt % in chlorobenzene) were filtered through 0.20 mm
pore size PTFE membrane syringe filters, and further spin-coated
onto the PTS-treated Si/SiO2 substrates at 1000 rpm for 60 s and
annealed at the corresponding Tg for 30 min. The source-drain
electrodes were defined by a 100 nm thick gold contact electrode
through a regular shadow mask, and the channel length (L) and
Scheme 1. The synthetic routes of monomers StCz2 and StCzT2Cz. (i) Pd(PPh3)4, K2CO3(aq), THF, 60 ꢂC, 24 h, under nitrogen; (ii) Pd(PPh3)4, K2CO3(aq), THF, 60 ꢂC, 24 h, under
nitrogen; (iii) Pd(pddf)Cl2, KOAc, THF, 60 ꢂC, 24 h, under nitrogen; (iv) Pd(PPh3)4, K2CO3(aq), THF, 60 ꢂC, 24 h, under nitrogen; (v) Pd(PPh3)4, K2CO3(aq), THF, 60 ꢂC, 24 h, under
nitrogen.

3550
G.-P. Chang et al. / Polymer 54 (2013) 3548e3555
Schlenk flask and subjected to three vacuum/nitrogen refill cycles.
THF (15 mL) was added and stirred for 10 min under nitrogen. The
mixture was heated at 60 ꢂC for 24 h and monitored via thin layer
chromatography for reaction completion. THF was removed using a
rotovap and the product was extracted with dichloromethane,
successively washed with water, and dried over MgSO4. Removal of
the solvent afforded the crude product. The crude product was
subjected to a silica gel column using a hexane/dichloromethane
mixture (3:1 by volume) as eluent. A white solid of compound 4
was isolated in 72% yield (0.8 g). 1H NMR (DMOS-d6)
d (ppm): 0.85e
0.88 (t, 3H, eCH3), 1.27e1.29 (t, 2H, eCH2e), 1.33 (s, 12H, eCH3),
1.74e0.177 (t, 2H, eCH2e), 4.40e4.43 (t, 2H, eNeCH2e), 5.26e5.28
(d, 1H, eCH¼CH2e), 5.86e5.89 (d, 1H, eCH¼CH2e), 6.76e6.81 (q,
1H, eCH¼CH2e), 7.55e8.63 (10H, PheH).
Scheme 2. The synthetic routes of polymers PStCz2 and PStCzT2Cz. (i) AIBN, NMP,
75 ꢂC, 24 h, under nitrogen; (ii) AIBN, NMP, 75 ꢂC, 24 h, under nitrogen.
3.3. 5-(N-butylcarbazole-3-yl)-50-bromo-2,20-bithiophene,
compound 5
width (W) were 25
mm and 1500 mm, respectively. Output and
Compound 3 (2.00 g, 6.17 mmol), 5,50-dibromo-2,20-bithiophene
(4.85 g, 14.97 mmol), and (PPh3)4Pd(0) (0.87 g, 0.75 mmol) were
added to a 100 mL Schlenk flask and subjected to three vacuum/ni-
trogen refill cycles. THF (60 mL) and a nitrogen degassed aqueous
solution of 2 M K2CO3 (25 mL) were added and stirred for 10 min
under nitrogen. The mixture was heated at 60 ꢂC for 24 h and
monitored via thin layer chromatography for reaction completion.
THF wasremoved using a rotovapand the product wasextracted with
dichloromethane, successively washed with water, and dried over
MgSO4. Removal of the solvent afforded the crude product. The crude
product was subjected to a silica gel column using a hexane/
dichloromethane mixture (8:1 by volume) as eluent. Ayellow solid of
compound 5 was isolated in 61% yield (2.13 g). 1H NMR (CDCl3)
transfer characteristics of the FET devices were measured using a
Keithley 4200 semiconductor parametric analyser under ambient
conditions.
3. Synthesis of the monomers and polymers
All the starting materials for preparing monomers and poly-
mers were purchased from Aldrich (Missouri, USA) and used as
received, such as 4-vinylphenylbronic acid, bis(pinacolato)
diboron,
5,50-dibromo-2,20-bithiophene,
tetrakis(triphenyl-
phosphine)-pd(0) [(PPh3)4Pd(0)], potassium carbonate, potas-
sium acetate, [1,10-bis(diphenylphosphino)-ferrocene]dichlor-
opalladium(II), complex with dichloromethane [Pd(dppf)Cl2
DCM], tetrahydrofuran (THF), azobisisobutyronitrile (AIBN), and
N-metheyl-2-pyrrolidone (NMP). The 3,6-dibromo-N-butylcar-
bazole (compound 1) and N-butylcarbazole-3-ylboronic acid
(compound 3) were prepared according to the previous litera-
ture [27,28].
d
(ppm): 0.86e0.89 (t, 3H, eCH3),1.27e1.32 (q, 2H, eCH2e),1.74e1.77
(t, 2H, eCH2e), 4.39e4.41 (t, 2H, eNeCH2e), 7.17e8.47 (11H, PheH
and TheH).
3.4. 4-[6-(N-butylcarbazole-3-yl)-N-butylcarbazole-3-yl]-styrene,
StCz2
3.1. 4-(6-Bromo-N-butylcarbazole-3-yl)-styrene, compound 2
Compound
2 (1 g, 2.47 mmol), compound 3 (0.97 g,
2.97 mmol), and (PPh3)4Pd(0) (0.14 g, 0.12 mmol) were added to a
50 mL Schlenk flask and subjected to three vacuum/nitrogen refill
cycles. THF (20 mL) and a nitrogen degassed aqueous solution of
2 M K2CO3 (7 mL) were added and stirred for 10 min under ni-
trogen. The mixture was heated at 60 ꢂC for 24 h and monitored
via thin layer chromatography for reaction completion. THF was
removed using a rotovap and the product was extracted with
dichloromethane, successively washed with water, and dried over
MgSO4. Removal of the solvent afforded the crude product. The
crude product was subjected to a silica gel column using a hex-
ane/dichloromethane mixture (4:1 by volume) as eluent. A white
solid of StCz2 was isolated in 68% yield (0.92 g). 1H NMR (DMSO-
4-vinylphenylbronic acid (1.00 g, 6.76 mmol), Compound 1
(5.15 g, 13.52 mmol), and (PPh3)4Pd(0) (0.78 g, 0.66 mmol) were
added to a 100 mL Schlenk flask and subjected to three vacuum/
nitrogen refill cycles. THF (60 mL) and a nitrogen degassed
aqueous solution of 2 M K2CO3 (25 mL) were added and stirred
for 10 min under nitrogen. The mixture was heated at 60 ꢂC for
24 h and monitored via thin layer chromatography for reaction
completion. THF was removed using a rotovap and the product
was extracted with dichloromethane, successively washed with
water, and dried over MgSO4. Removal of the solvent afforded the
crude product. The crude product was subjected to a silica gel
column using a hexane/dichloromethane mixture (5:1 by volume)
as eluent. A white solid of compound 2 was isolated in 63% yield
d6),
d (ppm): 0.88e0.92 (t, 6H, eCH3), 1.28e1.38 (s, 4H, eCH2e),
1.76e1.84 (s, 4H, eCH2e), 4.41e4.46 (q, 4H, eNeCH2e), 5.27e5.29
(d, 1H, eCH¼CH2e), 5.86e5.90 (d, 1H, eCH¼CH2e), 6.76e6.83 (q,
1H, eCH¼CH2e), 7.21e8.72 (17H, PheH). Elementary analysis of
(C40H38N2): C, 87.87; H, 7.01; N, 5.12. Found: C, 87.69; H, 7.15; N,
5.06. ESIMS: calculated for 546.74. Found: 546.6 [M], 547.5 [Mꢁ],
548.3 [M2ꢁ], 549.5 [M3ꢁ].
(1.72 g). 1H NMR (DMOS-d6)
d (ppm): 0.85e0.88 (t, 3H, eCH3),
1.26e1.31 (q, 2H, eCH2-), 1.73e1.76 (t, 2H, eCH2-), 4.40e4.43
(t, 2H, eNeCH2e), 5.26e5.29 (t, 1H, eCH¼CH2), 5.86e5.90 (d,
1H, eCH¼CH2), 6.76e6.82 (q, 1H, eCH¼CH2), 7.57e8.60 (10H,
PheH).
3.2. 4-[6-(4,4,5,5-tetramethyl-1,3,2-dioxaboroyl)-N-
butylcarbazole-3-yl]-styrene, compound 4
3.5. 4-{6-[50-(N-butylcarbazole-3-yl)-2,20-bithiophene-5-yl]-N-
butylcarbazole-3-yl}-styrene, StCzT2Cz
Compound
2
(1.00 g, 2.47 mmol), bis(pinacolato)diboron
Compound 4 (1 g, 2.22 mmol), compound 5 (1.24 g, 2.66 mmol),
and (PPh3)4Pd(0) (0.15 g, 0.13 mmol) were added to a 50 mL Schlenk
flask and subjected to three vacuum/nitrogen refill cycles. THF
(0.75 g, 2.97 mmol), potassium acetate (1.21 g, 12.36 mmol), and
Pd(dppf)Cl2 DCM (0.1 g, 0.12 mmol) were added to a 100 mL

G.-P. Chang et al. / Polymer 54 (2013) 3548e3555
3551
(20 mL) and a nitrogen degassed aqueous solution of 2 M K2CO3
(7 mL) were added and stirred for 10 min under nitrogen. The
mixture was heated at 60 ꢂC for 24 h and monitored via thin layer
chromatography for reaction completion. THF was removed using a
rotovap and the product was extracted with dichloromethane,
successively washed with water, and dried over MgSO4. Removal of
the solvent afforded the crude product. The crude product was
recrystallized using an acetone/methanol mixture (2:1 by volume)
for three times. A dark green solid of StCzT2Cz was isolated in 73%
mixture was heated at 75 ꢂC for 24 h, and poured into methanol. The
precipitated material was dissolved into a small amount of chloro-
benzene and then reprecipitated into methanol to afford a crude
polymer. The crude polymer was washed for 24 h with hot acetone
to remove oligomers and afforded PStCz2 in 61% yield. Mw and PDI
estimated from GPC are 17,708 and 1.46, respectively.
4. Results and discussion
yield (1.15 g).1H NMR (DMSO-d6),
d
(ppm): 0.87e0.90 (q, 6H, eCH3),
4.1. Synthesis of styrene-derivative monomers and polymers
1.30e1.33 (s, 4H, eCH2e), 1.76e1.79 (q, 4H, eCH2e), 4.41e4.44 (t,
4H, eNeCH2e), 5.27e8.68 (24H, eCH¼CH2, PheH, TheH).
Elementary analysis of (C48H42N2S2), calcd.: C, 81.09; H, 5.95; N,
3.94. Found: C, 81.83; H, 5.50; N, 3.61. ESIMS: calculated for 710.99.
Found: 710.3 [M], 711.3 [Mꢁ], 712.3 [M2ꢁ], 713.3 [M3ꢁ].
The styrene-derivative monomers StCz2 and StCzT2Cz were
synthesized via the Suzuki coupling reaction sequentially, as shown
in Scheme 1. The butyl substituent of carbazole was attached
initially to maintain the overall solubility of the products during the
reaction period. These monomers show good solubility in THF,
chloroform, toluene, chlorobenzene, and NMP. The structure char-
acterizations of the studied monomers were well-defined by 1H
NMR, elementary analysis, and mass spectrometer, respectively.
Polymers PStCz2 and PStCzT2Cz were prepared in dry and
degassed NMP, under a nitrogen atmosphere, using AIBN as the
radical initiator, as shown in Scheme 2. The polymers were char-
acterized by GPC using polystyrene as the standard and THF as the
eluent. The weight-average molecular weights (Mw) and poly-
dispersity index (PDI) of PStCz2 was 4.5 ꢀ 104 and 2.35, PStCzT2Cz
was 1.8 ꢀ 104 and 2.35, respectively. Since the repeat unit mass of
PStCz2 is 532 Da, the Mw value of this polymer corresponded to
more than 84 repeat units. For PStCzT2Cz (repeat unit mass 711 Da)
the Mw value corresponded to 25 repeat units [11]. Which indicates
the styrene moiety of StCz2 and StCzT2Cz can be free-radical
polymerized efficiently.
3.6. Poly-(4-[6-(N-butylcarbazole-3-yl)-N-butylcarbazole-3-yl-
]-styrene), PStCz2
StCz2 (0.7 g, 1.28 mmol) and AIBN (12.6 mg, 6 mol%) were added
to a 5 mL Schlenk flask and subjected to three vacuum/nitrogen
refill cycles. Anhydrous NMP (1.5 mL) was added sequentially then
the mixture was heated at 75 ꢂC for 24 h, and poured into methanol.
The precipitated material was dissolved into a small amount of
chlorobenzene and then reprecipitated into methanol to afford a
crude polymer. The crude polymer was washed for 24 h with hot
acetone to remove oligomers and afforded PStCz2 in 82% yield.
Weight average molecular weight (Mw) and polydispersity index
(PDI) estimated from gel permeation chromatographic (GPC) are
45,284 and 2.35, respectively.
3.7. Poly-(4-{6-[50-(N-butylcarbazole-3-yl)-2,20-bithiophene-5-yl]-
N-butylcarbazole}-styrene), PStCzT2Cz
4.2. Thermal properties
StCzT2Cz (0.7 g,1.0 mmol) and AIBN (9.8 mg, 6 mol%) were added
to a 5 mL Schlenk flask and subjected to three vacuum/nitrogen refill
cycles. Anhydrous NMP (1.2 mL) was added sequentially then the
TGA and DSC were studied to realize the thermal degradations
and thermal phase transition behaviours of the studied monomers
and polymers, as shown in Fig. 1. According to the degradation
Fig. 1. The thermograms of the studied materials. (a) TGA, (b) DSC thermograms of StCz2, (c) DSC thermograms of StCzT2Cz, (d) DSC thermograms of PStCz2 and PStCzT2Cz.

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G.-P. Chang et al. / Polymer 54 (2013) 3548e3555
Table 1
Table 2
Molecular weight, polydispersity index (PDI), thermal phase transitions and sta-
bilities of the styrene-derivative monomers and their corresponding polymers.
Optical and electrochemical properties of styrene-derivative monomers and their
corresponding polymers.
GPCa
DSCb
TGAb
UV-vis absorption
CV
Egopt:
(eV)a (V)
E
HOMO(eV) LUMO(eV)b
Oxidation
Mw
PDI 1st heating scan Texo
2nd heating Td5wt% (ꢂC)/
scan Tg (ꢂC) Char yield
(wt%)c
lmax (nm)
Onset
(g-molꢁ1
)
or Tendo (ꢂC)/
DH (J/g)
StCz2
Solution 214 242 304 3.25 0.86
Film 242 304
Solution 254 272 306 3.13 0.76
Film 252 278 310
StCzT2Cz Solution 214 244 400 2.49 0.67
Film 242 278 406
PStCzT2Cz Solution 248 276 402 2.58 0.50
ꢁ5.22
ꢁ5.11
ꢁ5.03
ꢁ4.85
ꢁ1.97
ꢁ1.98
ꢁ2.54
ꢁ2.27
StCz2
Texo, onset: 134
Texo, max: 160
130
468/22.4
PStCz2
D
Hexo: 71.5
Tendo Hendo Texo
172/54.5 179/81.2
StCzT2Cz
PStCz2
/D
/DHexo 133
497/54.3
4.5 ꢀ 104 2.35
194
180
445/13.5
466/7.2
PStCzT2Cz 1.8 ꢀ 104 1.46
Film
250 282 408
a
a
eluent: THF, standard: polystyrene.
1240/lonset [30].
heating rate: 10 ꢂC-minꢁ1, under nitrogen atmosphere.
char yield at 800 ꢂC.
LUMO ¼ HOMO þ Egopt.
.
b
c
b
indicates that StCzT2Cz melted at this temperature range and fol-
lowed a self-thermal polymerization instantaneously. As the ob-
servations of the exothermic peaks recorded from the first heating
scan for both monomers, the self-thermal polymerizations were
caused in the same cell of StCz2 and StCzT2Cz, and given a glass
transition (Tg) state at 130 ꢂC and 133 ꢂC during second heating
scan, respectively. The glass transition temperature of the free-
radical polymerized polymers PStCz2 and PStCzT2Cz were at
194 ꢂC and 180 ꢂC, as shown in Fig.1d. By comparing with the main-
patterns in Fig. 1a, the studied monomers and polymers exhibit
high thermal stability over than 440 ꢂC (5wt% loss temperature).
The high thermal stability of these studied materials could ensure
them to be operated in crucial conditions. The specific thermal
stabilities such as 5wt% loss temperature and char yield at 800 ꢂC of
each studied monomers and polymers are also summarized in
Table 1.
The thermal phase transitions of the studied materials were
traced by DSC, as shown in Fig. 1b,c and d. For StCz2, a specific
chain p-conjugated polymers based on carbazole-thiophene as the
repeating units, the higher Tg of PStCz2 and PStCzT2Cz were
exothermic peak was observed (Tonset: 134 ꢂC, Tmax: 160 ꢂC,
DHexo:
71.5 J/g) at the first heating scan. Which indicates the self-thermal
polymerization was observed to the vinyl groups at elevated tem-
perature. Interestingly, StCzT2Cz showed an endothermic peak and
attributed to the bulky grafted groups [29].
4.3. Optical and electrochemical properties
followed an exothermic peak immediately at 172 ꢂC (
DHendo: 54.5 J/
g) and 179 ꢂC (
DHexo: 81.2 J/g) at the first heating scan, which
The optical properties of the studied materials were character-
ized with UV-vis spectrometer, as shown in Fig. 2 and summarized
in Table 2. In dilute chloroform solution (10ꢁ6 M), the absorption of
StCzT2Cz showed a red-shift (in web version) in 96 nm to StCz2 due
to the extended p-conjugation by bithiophene moiety. This similar
result was also observed to their corresponding polymers. The
Fig. 2. The UV-vis absorption spectra of the studied monomers and polymers. (a) In
chloroform solution (10ꢁ6 M), (b) thin films.
Fig. 3. The cyclic voltammograms of the studied materials.

G.-P. Chang et al. / Polymer 54 (2013) 3548e3555
3553
4.4. Morphologies of the thin films
The molecular packing characteristics of the spin-coated and
thermal-annealed polymer thin films were investigated by XRD, as
show in Fig. 4. It can be seen that PStCz2 thin film showed no
specific molecular packing. As the
p-conjugated unit was extended
by bithiophene moiety, PStCzT2Cz showed a clear reflection peak at
2q
13.1ꢂ (dspacing: 6.7 A), which indicates a specific regular molec-
ꢀ
ular packing was observed. The feature of regular molecular pack-
ing is a favour key requirement for charge carrier to travel easily
and efficiently in active layer during the operation of an organic
electronic device.
The surface morphologies of both thin films were also charac-
terized with AFM, as shown in Fig. 5. `Both spin-coated and
thermal-annealed PStCz2 and PStCzT2Cz showed smooth and
dense morphologies with low surface roughness. The dense and flat
morphologies of OFET active layer is also a favour factor for mini-
mizing the contact resistance between active layer and source-
drain electrodes.
Fig. 4. The XRD of thermal-annealed PStCz2 and PStCzT2Cz thin films.
longer absorption band around 400 nm of StCzT2Cz is indicated to
the intramolecular charge transfer of the conjugated unit, while the
shorter absorption band (200e250 nm) is attributed to the
pe
p*
transition [25].
As the similar absorption patterns were observed with the
studied monomers to their corresponding polymers, which in-
dicates the vinyl-to-aliphatic transformations after polymerization
showed no significant effects to the UV-vis absorption. Thus, the
optical energy gaps (Egopt.) of the studied monomers are also
similar to their corresponding polymers, as listed in Table 2.
However, the onset oxidative potentials of the studied mono-
mers and their corresponding polymers are interrelated to the
vinyl-to-aliphatic transformations, as shown in Fig. 3. The CV
indicated the onset oxidative potentials of StCz2, StCzT2Cz, PStCz2,
and PStCzT2Cz were at 0.86, 0.67, 0.76, and 0.50 V (vs Ag/AgCl),
respectively. The calculated HOMO energy levels of StCz2,
StCzT2Cz, PStCz2, and PStCzT2Cz were at ꢁ5.22, ꢁ5.03, ꢁ5.11,
and ꢁ4.85 eV, respectively. Before the polymerizations, the
electron-withdrawing property of the vinyl groups would reduce
4.5. Field-effect transistor characteristics
To prepare OFET devices based on the studied polymers, the
studied polymers were spin-coated onto the PTS-treated Si/SiO2
substrates, than Au was deposited onto the active layers as
source-drain electrodes. These two bottom-gate-top-contact
OFET devices were all measured under constant ambient condi-
tions. The average field effect mobility was measured by the
transfer curves in the saturation regime at Vds ¼ ꢁ100 V, using
the equation:
À
Á
WCi
2L
2
Ids;sat
¼
msat Vg ꢁ Vth
where W and L are channel width and length, respectively, Ci is the
capacitance of gate insulator (SiO2), Vth is the threshold voltage, Ids
is the drain current, Vg is the gate voltage, and Vds is the drain
the electron density in
p-conjugated units and further lower the
HOMO energy levels of the monomers. However, the aliphatic
substituents would transfer to the electron-donating groups after
voltage. msat is the carrier mobility in the saturation regime and can
1/2
the polymerizations. Thus, the electron density in
p-conjugated
be determined from slope of (ꢁIds
)
verses Vg.
units would be increased to gain high-lying HOMO energy levels.
The LUMO energy levels of the studied materials were also esti-
mated from the HOMO energy levels and their corresponding Egopt
[17,25]. The calculated LUMO energy levels of StCz2, StCzT2Cz,
PStCz2, and PStCzT2Cz were at ꢁ1.97, ꢁ2.54, ꢁ1.98, and ꢁ2.27 eV,
respectively, as summarized in Table 2.
Figs. 6 and 7 show the output and transfer characteristics of
PStCz2 and PStCzT2Cz. The FET performances without- and with-
thermal-annealing were also summarized in Table 3. The as-spun
PStCz2 showed no FET performance. After annealing the PStCz2
at its Tg, weak FET performances were observed with a hole
mobility on the order of 1.7 ꢀ 10ꢁ6 cm2Vꢁ1sꢁ1
.
Fig. 5. The AFM images of thermal-annealed PStCz2 and PStCzT2Cz thin films.

3554
G.-P. Chang et al. / Polymer 54 (2013) 3548e3555
Table 3
OFET performances of PStCz2 and PStCzT2Cz.
PStCz2
PStCzT2Cz
As-spun
msat (cm2Vꢁ1sꢁ1
Vt (V)
)
NA
NA
NA
1.4 ꢀ 10ꢁ5
ꢁ20
Ion/Ioff ratio
102.0
Annealeda
msat (cm2Vꢁ1sꢁ1
Vt (V)
)
1.7 ꢀ 10ꢁ6
ꢁ21.1
101.9
1.1 ꢀ 10ꢁ3
ꢁ35.7
Ion/Ioff ratio
103.9
a
The devices were annealed at the corresponding Tg of the studied polymers.
The elevation of threshold voltage might be attributed to the
conformational changes in the polymer and altered spatial ordering
to the n-butyl substituents toward the gate dielectric layer [12]. By
comparing with the thermal-annealed PStCz2 and other graft
polymers for OFET as mentioned in introduction, the remarkable
enhancements in FET performances of PStCzT2Cz are due to the
regular molecular packing in thin film as discussed in XRD, and the
extended side chain conjugation would also facilitate the charge
carrier travelling.
5. Conclusions
To conclude, two styrene-derivative monomers based on
bicarbazole and carbazole-bithiophene-carbazole (StCz2 and
StCzT2Cz) were designed and free-radical polymerized (PStCz2 and
PStCzT2Cz). The thermal, optical, and electrochemical properties of
the studied monomers and their corresponding polymers were
highly affected by the insertion of bithiophene moiety and the
vinyl-to-aliphatic transformations after polymerizations. For FET, a
highest hole mobility was obtained with thermal-annealed
PStCzT2Cz on the order of 1.1 ꢀ 10ꢁ3 cm2Vꢁ1sꢁ1 due to the
Fig. 6. IeV characteristics of thermal-annealed PStCz2. (a) Output curves, (b) transfer
curve.
For PStCzT2Cz, the hole mobilities of as-spun and thermal-
annealed device were 1.4 ꢀ 10ꢁ5 cm2Vꢁ1sꢁ1 and 1.1 ꢀ 10ꢁ3
cm2Vꢁ1sꢁ1, respectively. However, the threshold voltage of ther-
mal-annealed-PStCzt2Cz was elevated from ꢁ20 V to ꢁ35.7 V.
extended
p-conjugation and regular molecular packing. By
comparing with other graft polymers for OFET, this study improve
the field-effect mobility in facility and substantially. This research
also provides a new path to design semiconducting graft polymers
for other applications in organic electronics.
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.polymer.2013.04.002.
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