Synthesis and characterization of phenanthrocarbazole-diketopyrrolopyrrole copolymer for high-performance field-effect transistors

Article abstract of DOI:10.1002/pola.26602

In this study, we successfully designed and synthesized a novel phenanthro[1,10,9,8-c,d,e,f,g]carbazole (PCZ)-based copolymer poly[N-(2-octyldodecyl)-4,8-phenanthro[1,10,9,8-c,d,e,f,g]carbazole-alt-2, 5-dihexadecyl-3,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione] (PPDPP) with an extended π-conjugation along the vertical orientation of polymer main chain. This polymer exhibited excellent solubility in common solvent and high thermal stability, owning good properties for solution-processed field-effect transistors (FETs). Besides, absorption spectra demonstrated that annealing PPDPP thin films led to obviously red-shifted maxima, indicating the formations of aggregation or orderly π-π stacking in their solid-state films. X-ray diffraction measurements indicated the crystallinity of PPDPP thin films was enhanced after high temperature annealing, which was favorable for charge transport. The solution-processed PPDPP-based FET devices were fabricated with a bottom-gate/bottom-contact geometry. A high hole mobility of up to 0.30 cm²/Vs and a current on/off ratio above 10⁵ had been demonstrated. These results indicated that the copolymers constructed by this kind of ladder-type cores could be promising organic semiconductors for high-performance FET applications.

Full text of DOI:10.1002/pola.26602

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Synthesis and Characterization of Phenanthrocarbazole–
Diketopyrrolopyrrole Copolymer for High-Performance
Field-Effect Transistors
Huajie Chen, Yunlong Guo, Xiangnan Sun, Dong Gao, Yunqi Liu, Gui Yu
Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190,
People’s Republic of China
Correspondence to: G. Yu (E-mail: yugui@iccas.ac.cn)
Received 19 December 2012; accepted 22 January 2013; published online 14 February 2013
DOI: 10.1002/pola.26602
ABSTRACT: In this study, we successfully designed and
synthesized novel phenanthro[1,10,9,8-c,d,e,f,g]carbazole
PCZ)-based copolymer poly[N-(2-octyldodecyl)-4,8-phenanthro
1,10,9,8-c,d,e,f,g]carbazole-alt-2,5-dihexadecyl-3,6-di(thiophen-
-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione] (PPDPP) with an
the crystallinity of PPDPP thin films was enhanced after high
temperature annealing, which was favorable for charge trans-
port. The solution-processed PPDPP-based FET devices were
fabricated with a bottom-gate/bottom-contact geometry. A high
a
(
[
2
2
hole mobility of up to 0.30 cm /Vs and a current on/off ratio
5
extended p-conjugation along the vertical orientation of poly-
mer main chain. This polymer exhibited excellent solubility in
common solvent and high thermal stability, owning good prop-
erties for solution-processed field-effect transistors (FETs).
Besides, absorption spectra demonstrated that annealing
PPDPP thin films led to obviously red-shifted maxima, indicat-
ing the formations of aggregation or orderly p–p stacking in
their solid-state films. X-ray diffraction measurements indicated
above 10 had been demonstrated. These results indicated that
the copolymers constructed by this kind of ladder-type cores
could be promising organic semiconductors for high-perform-
ance FET applications. VC 2013 Wiley Periodicals, Inc. J. Polym.
Sci., Part A: Polym. Chem. 2013, 51, 2208–2215
KEYWORDS: diketopyrrolopyrrole; field-effect transistors; high
performance; ladder-type; phenanthro[1,10,9,8-c,d,e,f,g]carbazole
INTRODUCTION Polymer field-effect transistors (FETs) have
attracted significant attention because of their potential
applications for achieving flexible and low-cost photoelec-
tronic devices through large-scale solution deposition techni-
extending p-conjugated structures along the parallel orienta-
13–20
tion of polymer main chains.
For example, Hsu and his
coworkers had successfully developed a series of ladder-type
dithienosilolo-carbazole (DTSC), dithienopyrrolo-carbazole
(DTPC), and dithienocyclopenta-carbazole (DTCC) units, in
which two adjacent thiophene units were covalently fused to
the central 2,7-carbazole cores along the parallel orientation.
The polymers (PDTCCBT, PDTSCBT, and PDTPCBT), con-
structed from these ladder-type units, exhibited a good
charge transporting characteristic, with the hole mobility
1
–4
ques.
Recently, Apart from the optimizations of device
engineerings, the development of novel polymer semiconduc-
tors still play a key role to obtain high charge mobility for
1
–4
FETs.
An important design principle for developing high-
performance polymers is to achieve strong p–p stacking
structures, which determine the charge-transporting charac-
teristic in their solid-state films. Usually, to ensure strong
intermolecular interactions between the neighboring poly-
mer main chains, the incorporation of p-extended fused-rings
ꢁ4
2
18
range from 6.1 ꢀ 10 to 0.11 cm /V s. Despite a large
number of reports on the copolymers constructed by some
13–20
ladder-type cores,
there are very few studies on the
5–12
should be one of the most effective design strategies.
ladder-type cores extending p-conjugation along the vertical
orientation of polymer main chains. Therefore, strategies to
further develop novel ladder-type p-conjugation cores and
their polymers, and thoroughly study structure-performance
relationships are well worth exploring for researchers.
Consequently, an increasing number of ladder-type fused-
rings with forced coplanarity structures have been reported
to develop some excellent copolymers for the applications in
1
3–20
high-performance FETs.
However, the realization of this
kind of chemical structures generally suffers from synthetic
challenges, and thus requires complex molecular design.
Furthermore, most ladder-type cores are designed and syn-
thesized according to a conventional approach, namely,
Herein, we designed and synthesized a novel ladder-type phe-
nanthro[1,10,9,8-c,d,e,f,g]carbazole (PCZ) building-block by
extending p-conjugation along the vertical orientation of 2,7-
V
C
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2
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SCHEME 1 Chemical structures of carbazole, PCZ, PCBTDPP, and PPDPP.
carbazole core (see Scheme 1). The electron-rich PCZ
derivative unit was copolymerized with an electron-deficient
diketopyrrolopyrrole (DPP) unit by Suzuki Coupling to
afford an alternating donor–acceptor (D–A) copolymer,
poly[N-(2-octyldodecyl)-4,8-phenanthro[1,10,9,8-c,d,e,f,g]
carbazole-alt-2,5-dihexadecyl-3,6-di(thiophen-2-yl)pyrrolo[3,4-c]
pyrrole-1,4(2H,5H)-dione] (PPDPP) (see Sheme 2). Herein, the
DPP building-block was used as an electron acceptor, because
its backbone’s coplanarity would be favorable for strengthen-
trophotometer. Cyclic voltammetry (CV) measurements were
recorded on an electrochemistry workstation (CHI660A,
Chenhua Shanghai) to determine electrochemical redox
potentials. The working electrode was a Platinum stick,
coated with a layer of polymer thin film. Platinum wire was
used as the counter electrode. Ag/AgCl (Ag in a 0.01 mol/L
KCl) electrode was used as the reference electrode. An anhy-
drous solution 0.1 M tetrabutylammonium hexylfluorophos-
phate (Bu NPF ) in acetonitrile was used as the electrolyte.
4
6
2
1,22
ing intermolecular p–p stackings.
Compared with
Thermal gravity analyses (TGA) were carried out on a TA
Instrument Q600 analyzer with a heating and cooling rate of
carbazole core, PCZ-based polymers would have much larger
p-conjugation structure and stronger electron-donating ability,
but weaker solubility. To solve low solubility issue and
enhance self-organization, long branched 2-octyldodecyl side
chains and hexadecyl side chains are substituted on the nitro-
gen atoms of PCZ and DPP units, respectively. Previous works
had demonstrated a D–A low bandgap polymer (PCBTDPP,
see Scheme 1), containing carbazole and DPP units, exhibited
ꢂ
10 C/min under an inert atmosphere. Differential scanning
calorimetry analyses (DSC) were recorded on a METTLER
TOLEDO Instrument DSC822 calorimeter with a heating/
ꢂ
cooling rate of 10 C/min under an inert atmosphere. Atomic
force microscopy (AFM) measurements were carried out on
a Digital Instruments Nanoscope V instrument, and operated
in a tapping mode. X-ray diffraction (XRD) measurements
were carried out in a reflection mode, using a D/MAX-TTR
III Rigaku X-ray diffraction system.
2
23
hole mobility of 0.02 cm /Vs. Herein, the highly p-extended
polymer, PPDPP with large p-conjugated DPP and ladder-type
2
PCZ units, exhibited higher hole mobility of up to 0.30 cm /
5
Vs, along with a current on/off ratio above 10 . The enhanced
Fabrication and Measurement of FET Devices
mobility demonstrated that the extension of p-conjugation
along the vertical orientation should be a versatile approach
to develop novel ladder-type fused heteroarenes.
To investigate the charge transporting proprieties of PPDPP
thin-film, bottom-gate/bottom-contact (BGBC) FET devices
þ
were fabricated using n -Si/SiO
substrates (300 nm). The
2
gold (Au) source-drain electrodes were prepared by a photo-
lithography technique to control the channel length (L) and
channel width (W ¼ 1400 lm). Here, different channel
lengths of FET devices (L ¼ 5 and 50 lm) were investigated
EXPERIMENTAL
General Measurement and Characterization
Nuclear magnetic resonance (NMR) spectra were conducted
on a Bruker ARX-400 spectrometer. Elemental analyses were
recorded on a Carlo Erba model 1160 elemental analyzer.
MALDI-TOF mass spectrometric measurements were per-
formed on Bruker Bifiex III MALDI-TOF. Electron-impact
mass spectra (EIMS) were performed on a Bruker BIFLEX III
mass spectrometer. Gel permeation chromatography (GPC)
2
to optimize device performance. First, the SiO substrates
were washed with ultrasonication in acetone, a cleaning
agent, deionized water, and iso-propanol, respectively. The
ꢂ
cleaned substrates were then dried at 80 C under vacuum.
To form a layer of octadecyltrichlorosilane (OTS) monolayer,
OTS-modification was performed on the SiO gate dielectrics
2
in a vacuum. After that, a layer of PPDPP thin film (40 nm)
ꢂ
was conducted on a waters 515–2410 at 30 C, using THF as
was deposited on the OTS-modified SiO substrates by spin-
2
eluent and polystyrenes as reference standard. UV–vis-NIR
absorption spectra were measured on a Hitachi U-3010 spec-
coating a PPDPP solution in o-dichlorobenzene (10 mg/mL)
with a speed of 2000 rpm for 60 s. For annealing FETs, the
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ꢂ
samples were placed on a hotplate for 5 min in air directly.
The FETs characteristics of the devices were also determined
in air by using a Keithley 4200 SCS semiconductor parame-
ter analyzer. The mobility of the devices was calculated in
the saturation regime. The equation is listed as follows:
50 mL of dry THF was stirred at 65 C for 12 h under
nitrogen. After cooling to room temperature, the mixture
ꢂ
was added 5 mL of water and stirred for 10 min at 0 C.
The resulting mixture was then extracted with diethyl ether.
The crude product was purified by chromatography using
petroleum ether as an eluent to afford 14 g of yellow oil
(3) containing some 1-iodo-2-octyldodecane. MALDIꢁTOF:
found 545.8. The proportion of compound 3 was about
2
IDS ¼ ðW=2LÞCilðVGS ꢁ VthÞ
1
where W/L is the channel width/length, C is the insulator
78% in this yellow oil, which was determined by H NMR
i
capacitance per unit area, and VGS and Vth are the gate volt-
age and threshold voltage, respectively.
spectra. So the crude product 3 was used directly without
further purification. To the solution of compound 3 (14 g,
2
0 mmol) in 300 mL of CH Cl was added NBS (7.48 g, 42
2 2
Materials
mmol) in four portions. And then the mixture was stirred
at room temperature for 3 h. The resulting mixture was
then extracted with diethyl ether. The crude product was
purified by column chromatography on silica gel (petroleum
ether) to yield the yellow solid. The yellow solid was added
All starting materials were purchased from Aldrich, Alfar,
and used directly without further purification. Tetrahydrofu-
ran (THF) and toluene were dried from benzophenone ketyl
and sodium. 3,6-Bis-(5-bromo-thiophen-2-yl)-2,5-dihexade-
cylpyrrolo[3,4-c]pyrrole-1,4(2H,5H)dione (M2) was synthe-
1
00 mL of methanol and sonicated for 10 min, and then
5
sized according to the reported literature.
filtered to afford the pure product as a light yellow solid
Nitroperylene (1)
At 60 C, to a solution of perylene (30.0 g, 119 mmol) in 1 L
of 1,4-dioxane was added a mixture of 24 mL of water and
(13.4 g, 95%).
ꢂ
1
H NMR (CDCl , 400 MHz): d (ppm) 8.18ꢁ8.20 (d, 2 H),
3
8
3
0
1
1
2
1
C
.04ꢁ8.06 (d, 2 H), 7.62ꢁ7.65 (t, 2 H), 7.50 (s, 2 H),
2
4
1
5.0 mL of nitric acid (d ¼ 1.5). The resulting solution was
.76ꢁ3.78 (d, 2 H), 1.85 (m, 1 H), 1.16ꢁ1.26 (m, 32 H),
ꢂ
stirred at 60 C for 40 min, and then poured into 1.5 L of
water. The resulting mixture was filtered and washed with
13
.83ꢁ0.90 (m, 6 H). C NMR (CDCl , 100 MHz): d (ppm)
3
30.83, 129.01, 127.08, 124.93, 124.48, 123.01, 121.16,
17.57, 116.36, 115.19, 49.57, 39.51, 31.93, 31.87, 31.57,
9.85, 29.62, 29.56, 29.51, 29.34, 29.28, 26.34, 22.72, 22.66,
4.18, 14.14. MALDIꢁTOF: found, 703. Anal. Calcd. for
2
00 mL of water and 200 mL of ethanol. The red solid was
further purified by column chromatography on silica gel (pe-
troleum ether/CH Cl
2
2
¼ 2:1) to afford a red solid (10.6 g,
30%).
40 2
H49Br N: C, 68.28; H, 7.02; N, 1.99; found: C, 68.39; H,
1
H NMR (400 MHz, DMSO): d (ppm) 8.52 (dd, 2 H), 8.01–
.93 (m, 4 H), 7.81 (d, 1 H), 7.76 (dd, 2 H), 7.66 (t, 1 H),
.56 (t, 1 H). C NMR (DMSO, 100 MHz): d (ppm) 146.0,
7.11; N, 1.96.
7
7
1
3
N-(2-octyldodecyl)-4,8-bis(4,4,5,5-tetramethyl-1,3,2-dioxa-
borolan-2-yl)phenanthro[1,10,9,8-c,d,e,f,g]carbazole (M1)
1
1
1
C
3
34.3, 133.8, 131.3, 130.2, 129.0, 128.9, 128.8, 128.0,
27.8, 127.6, 127.2, 126.8, 125.6, 124.8, 122.8, 122.7,
22.2. EIMS ((M/z (%)): 297 [M ]. Anal. Calcd. for
ꢂ
At –78 C, to a solution of compound 4 (3.5 g, 5 mmol) in
þ
100-mL dry THF was added dropwise 5.2 mL of 2.5 M solu-
tion of n-butyllithium (n-BuLi) in n-hexane. The mixture was
stirred at –78 C for 30 min, then warmed to 0 C for 1 h,
and then cooled down to –78 C again, 2-isopropoxy-4,4,5,5-
tetramethyl-1,3,2-dioxaborolane (2.8 g, 15 mmol) was added
rapidly into the solution. The resultant mixture was allowed
to stir at room temperature overnight, then quenched with
20
H
11NO
2
: C, 80.80; H, 3.73; N, 4.71; found C, 80.58; H,
ꢂ
ꢂ
.68; N, 4.74.
ꢂ
6
H-phenanthro[1,10,9,8-c,d,e,f,g]carbazole (2)
A mixture of 5.0 g (16.8 mmol) of 1-nitroperylene (1) and
ꢂ
3
0 mL of triethyl phosphite was stirred at 160 C under
2
4
nitrogen for 5 h. After cooling down to room temperature,
the reactant was precipitated by addition of 200 mL of pe-
troleum ether, and then filtered to afford the yellow solid.
The crude product was further purified by column chroma-
tography on silica gel using CH Cl as an eluent to afford a
10 mL of water and extracted with dichloromethane. The or-
ganic extracts were washed with brine and dried over mag-
nesium sulfate and concentrated in vacuum. The crude prod-
uct was purified by chromatography using petroleum ether/
dichloromethane (3:1) as an eluent to give a yellow solid.
Yield: 1.28 g, 32%.
2
2
light yellow solid (3.7 g, 83%).
1
H NMR (400 MHz, DMSO): d (ppm) 12.19 (s, 1 H), 8.76 (d,
1
1
3
H NMR (CDCl
3
, 400 MHz): d (ppm) 8.97ꢁ8.99 (d, 2 H),
2
H), 8.19 (d, 2 H), 7.98 (dd, 4 H), 7.82 (t, 2 H). C NMR
8
2
.68ꢁ8.70 (d, 2 H), 8.45 (s, 2 H), 7.86 (t, 2 H), 4.64ꢁ4.65 (d,
H), 2.35 (m, 1 H), 1.49 (s, 24 H), 1.36ꢁ1.40 (m, 6 H),
(
DMSO, 100 MHz): d (ppm) 130.6, 129.7, 128.3, 125.0, 124.5,
1
24.2, 123.5, 120.8, 116.9, 115.5. EIMS ((M/z (%)): 265
13
þ
1.19ꢁ1.27 (m, 26 H), 0.81ꢁ0.87 (m, 6 H). C NMR (CDCl
,
3
[
M ]. Anal. Calcd. for C20
H
11N: C, 90.54; H, 4.18; N, 5.28;
1
1
00 MHz): d (ppm) 132.97, 132.41, 130.74, 126.76, 124.99,
24.40, 122.91, 120.66, 119.90, 83.76, 49.91, 39.89, 32.08,
found: C, 90.23; H, 4.15; N, 5.21.
N-(2-octyldodecyl)-4,8-dibromo-phenanthro[1,10,9,8-
c,d,e,f,g]carbazole (4)
A mixture of compound 2 (5.3 g, 20 mmol), NaH (0.72 g,
32.05, 31.79, 30.08, 29.80, 29.71, 29.50, 29.45, 26.54, 25.24,
22.84, 22.82, 14.29. MALDIꢁTOF: found, 797.7. Anal. Calcd.
for C H B NO : C, 78.29; H, 9.22; N, 1.76; found: C, 78.01;
52
73
2
4
3
0 mmol), 1-iodo-2-octyldodecane (12.5 g, 30.6 mmol) in
H, 9.23; N, 1.79.
2
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SCHEME 2 Synthetic routes of the novel ladder-type polymer PPDPP.
2
4
Poly[N-(2-octyldodecyl)-4,8-phenanthro[1,10,9,8-c,d,e,f,g]-
carbazole-alt-2,5-dihexadecyl-3,6-di(thiophen-2-yl)pyr-
rolo[3,4-c]pyrrole-1,4(2H,5H)-dione] (PPDPP)
synthesized following a reported procedure, which was
then converted to compound 3 via alkylation. Then com-
pound 3 was directly brominated to obtain an intermediate
4. The structure of compound 4 was carefully confirmed by
NMR spectra, MALDI-TOF, and elemental analysis. Here, a
long branched alkyl-chain substituted on the nitrogen atom
of the PCZ core was rationally designed for better solubility
and self-assembly purpose. Then, compound 4 was con-
verted to monomer M1 by the present of n-BuLi and 2-iso-
propoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane. Another im-
portant monomer M2 was prepared according to a reported
Monomer M1 (0.16 g, 0.2 mmol), monomer M2 (0.18 g, 0.2
mmol), 2 M aqueous K CO3 solution (5 mL), toluene (10
2
mL), and three drops of N-methyl-N, N-dioctyloctan-1-ammo-
nium chloride (Aliquat 336) were added into a 50-mL flask.
The mixture was charged with nitrogen through a freeze-
pump-thaw cycle for three times, and then tetrakis(triphenyl-
phosphine)palladium (20 mg) was added. The mixture was
ꢂ
stirred for 48 h at 110 C under nitrogen atmosphere. Then
5
phenylboronic acid (0.3 g, 2.5 mmol) was added and stirred
at reflux for 3 h. Next, bromobenzene (5 mL) was then
added under nitrogen, and then the mixture was stirred for
another 3 h. After the reactant was cooled down to room
temperature, the polymer was precipitated by addition of
literature. Palladium-catalyzed Suzuki-coupling reaction of
M1 and M2 was carried out to obtain the target polymer
PPDPP in 68% yield. After removing the residual catalytic
metal and low-molecular-weight oligomers of the resultant
polymer by Sohxlet extraction with methanol (12 h), acetone
(12 h), and hexane (12 h), the residue was extracted with
chloroform to afford copolymer PPDPP with weight-average
200 mL of methanol and 10 mL of hydrochloric acid. The
precipitate was filtered and washed by methanol. Finally, the
polymer was subjected to soxhlet extraction with methanol
for 12 h, acetone for 12 h, hexane for 12 h, and finally chlo-
roform for 12 h. The evaporation of chloroform fractions
afforded a dark solid. Yield: 175 mg, 68%.
molecular weights (M ) of 16.6 KDa and polydispersity
w
indexes (PDI) of 1.63. Because of the introduction of long 2-
octyldodecanyl and hexadecyl side-chains into polymer back-
bone, PPDPP exhibited excellent solubility (>30 mg/mL, at
ꢂ
1
25 C) in common organic solvents, including THF, chloro-
H NMR (CDCl , 400 MHz): d (ppm) 8.94ꢁ9.35 (br, 2 H),
3
form, toluene, and chlorobenzene. Moreover, PPDPP showed
good film-forming ability and could obtain high-quality thin
films by simple spin-coating techniques.
8
.40ꢁ8.68 (br, 4 H), 7.51ꢁ7.86 (br, 6 H), 3.99ꢁ4.55 (br, 6
H), 2.41 (br, 1 H), 1.97ꢁ2.03 (br, 4 H), 1.25ꢁ1.58 (br, 84 H),
0
.86 (br, 12 H). GPC: M ¼ 16.6 KDa, M ¼ 10.2 KDa, M /
w
n
w
Mn ¼ 1.63. Anal. Calcd. for (C H N O S ) : C, 79.88; H,
86
121 3 2 2 n
Thermal Analysis
9
.43; N, 3.25; found: C, 79.42; H, 9.20; N, 3.30.
To determine the thermal properties of PPDPP, TGA, and
DSC had been performed under nitrogen. As shown in Figure
, PPDPP exhibits excellent thermal stability with the
decomposition temperature (5% weight loss) above 400 C.
This result shows that PPDPP matches well with the fabri-
cating needs of organic optoelectronic devices. Besides, DSC
RESULTS AND DISCUSSION
1
ꢂ
Synthesis and Characterization
The synthetic routes of monomers and copolymer are
presented in Scheme 2. Compounds 1 and 2 were readily
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FIGURE 3 The UV–vis-NIR absorption spectra of PPDPP in
chloroform solution and thin films annealing at different
temperature.
ꢂ
FIGURE 1 TGA curve of PPDPP with a heating rate of 10 C/
min under an inert atmosphere.
measurement of PPDPP reveals no clear liquid-crystalline
tively. These red-shifted features indicate that the aggrega-
ꢂ
phase transition from 25 to 300 C (Fig. 2), however, has a
tion or orderly p–p stacking occurs in the solid-state
ꢂ
melting transition at 188 C, which is attributable to the
25,26
film.
These results indicate that the annealing treatment
melting of polymer backbone.
would be helpful for enhancing the charge transport in poly-
mer films. Besides, the optical bandgap was estimated to be
1.39 eV, which was calculated from the onset absorption
(ꢃ890 nm) of polymer thin film without annealing.
Optical Properties
The UV–vis-IR absorption spectra of the dilute PPDPP solu-
tion in chloroform and the spin-coated thin film on the
quartz glass substrate are presented in Figure 3, and the cor-
responding data are summarized in Table 1. In the case of
chloroform solution, PPDPP displayed a broad absorption
band extended from 500 to 850 nm due to the strong intra-
molecular charge transfer arising from the strong D–A inter-
actions. Compared with the absorption maximum (653 nm)
in solution, the absorption peak in the solid-state film was
significantly red-shifted to 702 nm. When the thin films
Electrochemical Properties
CV measurement revealed that polymer PPDPP exhibited a
reversible and strong oxidation process but a weak reduction
one, indicating PPDPP might be operated as a p-type trans-
26
porting semiconductor. As shown in Figure 4, the onset
oxidation potential (E ) of PPDPP is 0.81 V. The corre-
ox
sponding energy level of the highest occupied molecular
orbital (HOMO) is –5.21 eV, which is calculated according to
onset
ox
27
ꢂ
the equation of EHOMO ¼ – (E
þ 4.4) eV. Combining
were annealed at 120 and 200 C, the absorption maximum
with the optical bandgap of 1.39 eV, the energy level of
lowest unoccupied molecular orbital (LUMO) is determined
(kmax) were further red-shifted to 714 and 734 nm, respec-
to be –3.82 eV, calculated according to the equation of E
LUMO
optical
27
¼
(EHOMO þ Eg
) eV. One can note that this deep-lying
HOMO value matches well with the work function of bare
28
gold electrodes (5.13 eV), which might facilitate the hole
injection from gold electrodes into polymer thin films and
27
thus realize high-performance charge transport.
Field-Effect Transistor Characteristics
To investigate the carrier transport properties of PPDPP thin
film, the FET devices with a conventional BGBC configuration
were fabricated. The gold (Au) source-drain electrodes were
TABLE 1 Optical Property and field-Effect Characteristics for
PPDPP Thin Film Based on Different Annealing Temperatures
ꢂ
2
T ( C)
kmax (nm)
M (cm /Vs)
I
on/Ioff
Vth (V)
4
As-spun/25
702
714
734
0.035 6 0.002
0.063 6 0.005
0.13–0.30
>10
–8 6 2
4
1
20
00
>10
–11 6 2
–10 6 5
ꢂ
FIGURE 2 DSC curves of PPDPP with a heating rate of 10 C/min
5
7
2
10 –10
under an inert atmosphere.
2
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in Table 1. The field-effect results showed the FET perform-
ance of PPDPP thin film was dependent on annealing tem-
perature, which was consistent with the results of absorption
spectra. For the unannealed FET devices with L ¼ 50 mm,
the average saturation mobilities extracted from transfer
2
curves were equal to 0.035 cm /Vs, with a current on/off
4
ꢂ
ratio of 10 . Annealing FET devices at 120 and 200 C, the
2
average saturation mobilities reached 0.063 and 0.13 cm /
Vs, respectively. To further optimize device performance, FET
devices with L ¼ 5 lm were investigated under the anneal-
ꢂ
ing temperature of 200 C. The average saturation mobilities
2
extracted from transfer curves reached 0.25 cm /Vs, together
with a current on/off ratio above 10 and a threshold
5
voltage of –15 V. Some hero transistors exhibited the highest
2
hole mobility of up to 0.30 cm /Vs. Previous work had
FIGURE 4 Cyclic voltammetry of PPDPP film in 0.1 mol/L
demonstrated that the molecular weight played a key role in
improving charge mobilities, because the thin film crystallin-
Bu4NPF6, acetonitrile solution, with a scan rate of 0.1 V/s.
12(b)
ity could be enhanced with growing molecular weight.
Herein, despite relatively low molecular weight for PPDPP
(M
¼ 16.6 KDa, PDI ¼ 1.63), the highly p-extended copoly-
mer PPDPP exhibited much higher mobility (0.30 cm /Vs)
compared with PCBTDPP (M
¼ 30 KDa, PDI ¼ 2.1, and l
prepared by photolithography method. A layer of PPDPP
thin film (40 nm) was then deposited onto OTS-modified Si/
w
2
SiO (300 nm) substrate by a simple spin-coating technique.
2
Then PPDPP-based FETs were annealed and measured in air
directly. Figure 5 shows the typical p-type output and
transfer curves of PPDPP-based FETs. Data are summarized
w
2
¼ 0.02 cm /Vs), which was higher by one order of magni-
15
tude than that of PCBTDPP. The results indicated that the
ꢂ
FIGURE 5 Output (a) and transfer (b) curves of PPDPP-based FETs annealing at 200 C with W ¼ 1400 lm and L ¼ 50 lm, exhibit-
2
ꢂ
ing a hole mobility of 0.13 cm /Vs; Output (c) and transfer (d) curves of PPDPP-based FETs annealing at 200 C with W ¼ 1400 lm
2
and L ¼ 5 lm, exhibiting a hole mobility of 0.30 cm /Vs.
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2
FIGURE 6 AFM topography (top) and phase (bottom) images (5 ꢀ 5 lm ) of PPDPP thin films on the OTS-modified SiO
2
/Si sub-
ꢂ
ꢂ
ꢂ
strates. (a and b) annealed at 25 C and (c and d) annealed at 120 C, and (e and f) annealed at 200 C.
largely conjugated PCZ units and DPP units would provide a
strong force to facilitate p–p stacking between polymer back-
bones, therefore successfully enhancing hole transport.
(100). This interlayer distance (26.26 Å) is shorter than that
26
of theoretically calculated alkyl side-chain (ꢃ36 Å). Thus,
these side-chains should be closely interdigitated with each
other between adjacent layers, which is useful for high-per-
formance charge transport.
Microstructure of PPDPP Thin Films
To understand the reasons why annealing treatments
improved charge-transporting ability, AFM and XRD meas-
urements were used to investigate the change in the micro-
structure of PPDPP thin films upon annealing treatments.
AFM images revealed that a uniform and intertwined fiber
structure was formed for the as-spun thin films (Fig. 6). This
kind of fiber structure is a typical one observed for most
conjugated polymer films, like regioregular poly(3-hexythio-
CONCLUSIONS
In summary, a novel ladder-type copolymer, PPDPP, with
alkyl substituted PCZ unit as an electron donor and DPP
unit as an electron acceptor, was designed and synthesized
by conventional Suzuki-coupling reaction. The long branched
alkyl-chains ensured PPDPP excellent solubility in most or-
ganic solvent, along with good film-forming ability and self-
organization ability. UV–vis-IR spectra measurements showed
PPDPP thin film covered the whole visible light band (from
2
9
10,30
phene) and DPP-based polymers.
However, the poly-
crystalline grains of the annealed thin films were clearly
unlike the ones of as-spun films, and were a fine nodule-like
structure. This obvious change in film morphology might be
caused by polymer backbone’s melting. Compared with thin
400 to 900 nm) due to the strong intramolecular charge
ꢂ
films annealed at 120 C, a larger polycrystalline grains was
ꢂ
obtained for the ones annealed at 200 C, which was helpful
for charge transport.
Consistent with the annealing-induced changes in their film
morphologies, different crystallinity of PPDPP was observed
by comparing the XRD data of annealed PPDPP thin films.
Figure 7 shows XRD patterns of PPDPP thin films deposited
on OTS-modified Si/SiO2 substrates. For as-spun samples
ꢂ
and the ones annealed at 120 C, two diffraction peaks were
clearly observed in XRD patterns, suggesting a strong tend-
ency for PPDPP thin films to self-assemble into an orderly
lamellar structure in solid-state films. Once thin films were
ꢂ
thermal annealed at 200 C, thin films displayed clear dif-
fraction peaks up to the third orders, indicating the increases
in crystallinity of polymer thin films. Such an increase of
crystallinity could be an important reason for the improved
charge transporting ability. Besides, a new sharp peak
FIGURE 7 X-ray scattering curves of PPDPP thin films on the
assigned to a-axis (h00) direction was observed at 2y ¼
OTS-modified SiO /Si substrates under annealing treatments at
2
ꢂ
3
.36 corresponding to a interlayer distance of 26.26 Å
different temperature.
2
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transfer. Besides, a suitable HOMO energy level of –5.21 eV
was favorable for hole injection from source electrode into
polymer thin film. The solution-processed FETs based on
PPDPP film exhibited good field-effect performance in air.
22, 3696–3698; (b) H. N. Tsao, D. M. Cho, I. Park, M. R. Hansen,
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Polym. Sci. Part A: Polym. Chem. 2012, 50, 4266–4271.
2
The highest hole mobility reached 0.30 cm /Vs with a cur-
1
3 X. C. Wang, H. Luo, Y. P. Sun, M. J. Zhang, X. Y. Li, G. Yu,
5
rent on/off ratio above 10 , which fully matched with the
Y. Q. Liu, Y. F. Li, H. Q. Wang, J. Polym. Sci. Part A: Polym.
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requirements for fabricating low-cost and flexible display
and smart card. This study demonstrated that the extension
of p-conjugation along the vertical orientation should be an
effective method to develop ladder-type polymer semicon-
ductors for high-performance organic optoelectronic
applications.
1
4 B. C. Schroeder, Z. G. Huang, R. S. Ashraf, J. Smith, P.
D’Angelo, S. E. Watkins, T. D. Anthopoulos, J. R. Durrant, I.
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1
ACKNOWLEDGMENTS
hee, A. Salleo, J. Kirkpatrick, S. Ashraf, T. Anthopoulos, M.
Heeney, I. McCulloch, J. Am. Chem. Soc. 2010, 132,
This work was supported by the National Natural Science Foun-
dation of China (20825208, 61101051, 51233006, and
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