Influence of side chain length and bifurcation point on the crystalline structure and charge transport of diketopyrrolopyrrole-quaterthiophene copolymers (PDQTs)

Article abstract of DOI:10.1039/c3tc32219j

The influence of the side chain on the molecular organization and charge transport performance of diketopyrrolopyrrole-quaterthiophene copolymers (PDQTs) was studied. It was found that, by increasing the side chain length from 2-octyldodecyl (PDQT-20) to 2-decyltetradecyl (PDQT-24), the mobility increased from 2.10 cm² V^-1 s^-1 up to 3.37 cm² V^-1 s^-1 in organic thin film transistors (OTFTs). The increase was found to be due to the improved surface morphology, rather than the changes in crystallinity and π-π stacking distance. A new side chain substituent, 4-decylhexadecyl, was developed for studying the effects of the bifurcation point of the branched side chains in comparison with 2-octyldodecyl and 2-decyltetradecyl. The 4-decylhexadecyl substituted PDQT (PDQT-26) showed a surge in mobility up to 6.90 cm² V^-1 s^-1. The remarkably enhanced charge transport performance observed for PDQT-26 was believed to originate from its much shorter π-π distance (3.68 A) than those of PDQT-20 (3.79 A) and PDQT-24 (3.86 A). The improvement was the result of a farther distance of the bifurcation point of 4-decylhexadecyl from the polymer backbone, which could effectively minimize the steric interference of the bulky side chain branches with the backbone to facilitate the co-facial π-π stacking.

Full text of DOI:10.1039/c3tc32219j

Journal of
Materials Chemistry C
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PAPER
Influence of side chain length and bifurcation point
on the crystalline structure and charge transport of
diketopyrrolopyrrole-quaterthiophene copolymers
(PDQTs)†
Cite this: J. Mater. Chem. C, 2014, 2,
2183
a
a
c
b
Shaoyun Chen,‡ab Bin Sun,‡ Wei Hong, Hany Aziz, Yuezhong Meng
*
a
*
and Yuning Li
The influence of the side chain on the molecular organization and charge transport performance of
diketopyrrolopyrrole-quaterthiophene copolymers (PDQTs) was studied. It was found that, by increasing
the side chain length from 2-octyldodecyl (PDQT-20) to 2-decyltetradecyl (PDQT-24), the mobility
increased from 2.10 cm2 Vꢀ1 sꢀ1 up to 3.37 cm2 Vꢀ1 sꢀ1 in organic thin film transistors (OTFTs). The
increase was found to be due to the improved surface morphology, rather than the changes in crystallinity
and p–p stacking distance. A new side chain substituent, 4-decylhexadecyl, was developed for studying
the effects of the bifurcation point of the branched side chains in comparison with 2-octyldodecyl and 2-
decyltetradecyl. The 4-decylhexadecyl substituted PDQT (PDQT-26) showed a surge in mobility up to 6.90
cm2 Vꢀ1 ꢀ1. The remarkably enhanced charge transport performance observed for PDQT-26 was believed
s
Received 12th November 2013
Accepted 27th December 2013
to originate from its much shorter p–p distance (3.68 A˚) than those of PDQT-20 (3.79 A˚) and PDQT-24
(3.86 A˚). The improvement was the result of a farther distance of the bifurcation point of 4-decylhexadecyl
from the polymer backbone, which could effectively minimize the steric interference of the bulky side
chain branches with the backbone to facilitate the co-facial p–p stacking.
DOI: 10.1039/c3tc32219j
www.rsc.org/MaterialsC
based D–A polymers represent a class of high performing polymer
semiconductors for OTFTs.3 Many DPP based polymers exhibit
mobility greater than 1 cm2 Vꢀ1 sꢀ1, outperforming amorphous
Introduction
Printed organic thin lm transistors (OTFTs) are of extensive
research interest mainly because they can be fabricated using
printing technologies on plastic substrates, which have great
potential to be used in exible, low cost, and large area elec-
tronics.1 In the past few years, there have been remarkable
improvements in the charge carrier mobility of polymer semi-
conductors by using push–pull or donor–acceptor (D–A) struc-
tures.2,3 The D–A moiety shortens the intermolecular distance
along the co-facial p–p stacking direction, greatly facilitating
interchain charge carrier hopping. Diketopyrrolopyrrole (DPP)
silicon that has a typical mobility range of 0.1–1 cm2 Vꢀ1 sꢀ1. We
reported a simple DPP polymer, PDQT, comprising DPP and
b-unsubstituted quaterthiophene (QT), which showed a high hole
mobility of ꢁ1 cm2 Vꢀ1 sꢀ1
. Recently, we demonstrated that a
3b
higher mobility value of up to 5.5 cm2 Vꢀ1 sꢀ1 could be achieved
for PDQT through optimization of the purication procedure.3g
This polymer was also found to be a good donor material for
organic photovoltaics (OPVs), reaching a power conversion effi-
ciency as high as 5.62%.4 Interestingly, PDQT exhibited a mobility
value of 2.0 ꢂ 10ꢀ2 cm2 Vꢀ1 sꢀ1 along the direction perpendicular
to the substrate, determined by the space charge limited current
(SCLC) method, which is the highest value reported for donor
polymers for OPVs.5 The high hole mobility of PDQT enabled OPV
devices with a very high active layer thickness of up to ꢁ800 nm,
which will be very useful for more feasible manufacturing of large
area OPV devices via high speed roll-to-roll printing processes.
Therefore, PDQT is a very promising polymer semiconductor for
printed electronics. Recently, it was found that side chain engi-
neering has signicant impacts on a number of conjugated poly-
mers6–10 and small molecules.11 Side chains with appropriate
length were demonstrated to increase the charge carrier mobility
signicantly, which are accounted for by the improved
aDepartment of Chemical Engineering and Waterloo Institute for Nanotechnology
(WIN), 200 University Ave W, Waterloo, Ontario, N2L 3G1, Canada. E-mail: yuning.
li@uwaterloo.ca; Fax: +1-519-888-4347; Tel: +1-519-888-4567 ext. 31105
bThe Key Laboratory of Low-carbon Chemistry & Energy Conservation of Guangdong
Province/State Key Laboratory of Optoelectronic Materials and Technologies, Sun
Yat-Sen University, Guangzhou 510275, P. R. China. E-mail: mengyzh@mail.sysu.
edu.cn
cDepartment of Electrical and Computer Engineering/Waterloo Institute for
Nanotechnology (WIN), University of Waterloo, 200 University Avenue West,
Waterloo, Ontario, N2L 3G1, Canada
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c3tc32219j
‡ These authors contributed equally.
This journal is © The Royal Society of Chemistry 2014
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crystallinity/molecular ordering6 or shortened p–p stacking malonate (28.82 g, 0.058 mol) in ethanol (62 mL) at room
distance.7 Another recent nding is that positioning the bifurca- temperature. The reaction mixture was reuxed for 4 h and
tion point of the branched side chains away from the polymer ethanol was distilled off. Water (300 mL) was added and the
backbone could have noted inuences on the charge transport of mixture was acidied with conc. HCl (ꢁ12 N). Aer phase
polymer semiconductors.8–10 The steric interference of the bulky separation, the combined organic phase was washed with water,
side chain branches with the co-facial p–p stacking of the conju- dried over anhydrous Na2SO4 and ltered. Removal of solvent
gated polymer backbone could be minimized by moving away the from the ltrate gave the diacid as a yellow solid, which was
bifurcation point, leading to dramatically increased mobility.
heated under reduced pressure at 180 ꢃC until there were no
To further explore the potential of PDQT as an OTFT channel bubbles coming out to afford 4-decylhexadecanoic acid (22.0 g,
semiconductor, in this study, we investigated the effects of the 95.5%). This product was used in the next step without further
1
chain length and the bifurcation point position of branched purication. H-NMR (CDCl3, 300 MHz, ppm) d 2.33 (t, J ¼ 8.1
alkyl side chains on the molecular organization and the charge Hz, 2H), 1.62 (m, 3H), 1.26 (m, 40H); 0.88 (t, 6H). 13C-NMR
transport characteristics of this polymer. We found that under (CDCl3, 75 MHz, ppm) d 179.52, 37.09, 33.39, 32.08, 31.48,
the same device fabrication conditions, by increasing the size of 30.20, 29.84, 29.82, 29.51, 28.60, 26.67, 22.58, 14.27.
the side chain from 2-octyldodecyl to 2-decyltetradecyl, the
mobility of PDQT increased from 2.10 cm2 Vꢀ1 sꢀ1 to 3.37 cm2 Synthesis of 4-decylhexadecan-1-ol (3)
Vꢀ1 sꢀ1. Increasing the distance of the bifurcation point from
To
a suspension of 4-decylhexadecanoic acid (21.67 g,
two carbon atoms (for 2-octyldodecyl) away from the polymer
backbone to four carbon atoms (for 4-decylhexadecyl) further
54.6 mmol) in dry diethyl ether (170 mL) was added a LiAlH4
solution (109.2 mmol, 109.2 mL, 1 M in diethyl ether) dropwise
at room temperature. The reaction mixture was then reuxed
for 12 h before cooling down to room temperature. The mixture
was added slowly to water and extracted with diethyl ether three
times. The combined organic phase was dried over anhydrous
Na2SO4 and ltered. Solvent was removed under reduced pres-
signicantly improved the mobility up to 6.90 cm2 Vꢀ1 sꢀ1
.
Experimental
Materials
1
All chemicals were obtained from commercial sources and used
as received without further purication. 3,6-Di(thiophen-2-yl)-
pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (DBT-H),3g 3,6-bis(5-bro-
mothiophen-2-yl)-2,5-bis(2-octyldodecyl)pyrrolo[3,4-c]pyrrole-
1,4(2H,5H)-dione (M-20),3g and 3,6-bis(5-bromothiophen-2-yl)-
2,5-bis(2-decyltetradecyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione
(M-24)7 were prepared according to the literature methods.
sure to give a colourless liquid. Yield: 20.7 g (99.0%). H-NMR
(CDCl3, 300 MHz, ppm) d 3.63 (q, J ¼ 6.3 Hz, 2H), 1.65–
1.45 (m, 2H), 1.40–1.15 (m, 43H), 0.88 (t, 6H). 13C-NMR (CDCl3,
75 MHz, ppm) d 63.72, 37.39, 33.75, 32.09, 30.29, 30.13, 29.91,
29.90, 29.88, 29.85, 29.83, 29.72, 29.53, 26.82, 22.85, 14.27.
Synthesis of 4-decyl-1-hexadecyl bromide (4)
To a solution of triphenylphosphine (PPh3) (13.12 g, 50 mmol)
in dichloromethane (CH2Cl2) (90 mL) was added dropwise a
solution of bromine (7.99 g, 50 mmol) in CH2Cl2 (10 mL) at
room temperature. Aer stirring for 1 h, compound 3 (19.14 g,
50 mmol) and pyridine (4 mL, 50 mmol) were added. The
mixture was stirred at room temperature overnight before a
saturated Na2SO3 aqueous solution (6 mL) was added with
stirring. The organic phase was separated, dried over anhydrous
Na2SO4, and ltered. Aer removal of solvent, the residue was
puried by column chromatography on silica gel with hexane to
give the title compound as a colourless liquid. Yield: 20.3 g
Synthesis of diethyl 2-(2-decyltetradecyl)malonate (1)
Under argon protection, sodium (1.84 g, 0.08 mol) was reacted
with dry ethanol (65 mL) and diethyl malonate (10.57 g, 0.08
mol). Aer stirring for 1 h at room temperature, 2-decylte-
tradecyl bromide (33.40 g, 0.08 mol) was added. Aer reuxing
overnight, excess ethanol was evaporated under reduced pres-
sure. The residue was washed with water and extracted with
diethyl ether three times. The combined organic phase was
dried over anhydrous Na2SO4, ltered, and evaporated under
reduced pressure. The residue was further puried by column
chromatography on silica gel (hexane : ethyl acetate ¼ 20 : 1–
10 : 1) to give a colourless liquid, as a mixture of ethyl (ꢁ90 mol
%) and methyl (ꢁ10 mol%) esters. Yield: 29.0 g (73.3%). 1H-
NMR (CDCl3, 300 MHz, ppm) d 4.20 (q, ꢁ3.6H, OCH2, J ¼
7.2 Hz), 3.73 (s, ꢁ0.6H, OCH3), 3.52–3.34 (t, 1H, J ¼ 7.5 Hz). 1.84
(m, 2H), 1.50–1.15 (m, 40H + 1H + ꢁ4.8H, CH2, CH and
OCH2CH3), 0.88 (t, 6H). 13C-NMR (CDCl3, 75 MHz, ppm) d
170.48, 169.99, 169.89, 61.49, 61.40, 52.52, 50.29, 50.13, 35.61,
33.32, 33.35, 33.27, 33.20, 32.08, 30.14, 29.84, 29.80, 29.51,
26.46, 22.85, 14.27, 14.24.
1
(91.1%). H-NMR (CDCl3, 300 MHz, ppm) d 3.39 (t, J ¼ 6.9 Hz,
2H), 1.89–1.76 (m, 2H), 1.45–1.10 (m, 43H), 0.86 (t, 6H). 13C-
NMR (CDCl3, 75 MHz, ppm) d 37.03, 34.51, 33.70, 32.35, 32.12,
30.35, 30.26, 29.89, 29.86, 29.85, 29.56, 26.80, 22.88, 14.29.
Synthesis of 2,5-bis(4-decylhexadecyl)-3,6-di(thiophen-2-yl)-
pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (DBT-26)
To a suspension of DBT-H (0.451 g, 1.5 mmol) in anhydrous
N,N-dimethylformamide (DMF) (18 mL) were added potassium
carbonate (K2CO3) (0.622 g, 4.5 mmol) and 4-decyl-1-hexadecyl
ꢃ
bromide (2.01 g, 4.5 mmol). The mixture was stirred at 130 C
Synthesis of 4-decylhexadecanoic acid (2)
for 12 h under argon protection. DMF was then removed under
A solution of KOH (16.27 g, 0.29 mol) in water (30 mL) was reduced pressure. The residue was washed with water and
slowly added to a solution of diethyl 2-(2-decyltetradecyl)- extracted with CH2Cl2. The organic phase was separated, dried
2184 | J. Mater. Chem. C, 2014, 2, 2183–2190
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over Na2SO4, and ltered. Aer removal of solvent, the residue
was puried by column chromatography on silica gel
(CH2Cl2 : hexane ¼ 1 : 1) to afford the product. Yield: 0.95 g
(61.7%). 1H-NMR (CDCl3, 300 MHz, ppm) d 8.92 (d, J ¼ 3.6 Hz,
2H), 7.62 (d, J ¼ 4.8 Hz, 2H), 7.29 (d, J ¼ 4.5 Hz, 2H), 1.80–
1.60 (m, 4H), 1.45–1.05 (m, 86H), 0.88 (t, 6H). 13C-NMR (CDCl3,
75 MHz, ppm) d 161.42, 140.10, 135.42, 130.70, 129.91, 128.73,
107.79, 42.68, 37.26, 33.66, 32.07, 30.60, 30.23, 29.86, 29.81,
29.51, 27.18, 26.82, 22.84, 14.27.
Instrumentation and methods
1H and 13C NMR spectra were acquired on a Bruker DPX 300
MHz spectrometer in CDCl3 and the chemical shis were
referenced to internal tetramethylsilane (TMS, 0 ppm). Ther-
mogravimetric analysis (TGA) and differential scanning calo-
rimetry (DSC) measurements were carried out on a TA
Instruments SDT 2960 under nitrogen at a heating rate of 10 ꢃC
minꢀ1 and TA Instruments DSC Q2000 at a scanning rate of 10
ꢃC minꢀ1, respectively. High-temperature gel-permeation chro-
matography (HT-GPC) was carried out on a Malvern 350 HT-
GPC system at 140 ꢃC using 1,2,4-trichlorobenzene as an eluent
and polystyrene as standards. UV-Vis spectra were recorded
with polymer solutions in chloroform and polymer lms spin-
coated onto quartz substrates using a Thermo Scientic Gen-
esys 10 UV spectrometer. Cyclic voltammetry (CV) measure-
ments were conducted on a Digi-Ivy DY2111 Potentiostat under
argon with 0.1 M tetrabutylammonium hexauorophosphate in
anhydrous acetonitrile as the electrolyte. A platinum disk elec-
trode coated with a thin layer of polymer was used as the
working electrode, Ag/AgCl was used as the reference electrode,
and a platinum wire was used as the counter electrode. The
HOMO (highest occupied molecular orbital) energy level was
calculated using the equation:
Synthesis of 3,6-bis(5-bromothiophen-2-yl)-2,5-bis(4-
decylhexadecyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (M-26)
N-Bromosuccinimide (NBS) (0.153 g, 0.86 mmol) was added in
small portions into a solution of DBT-26 (0.422 g, 0.41 mmol) in
chloroform (13 mL) at 0 ꢃC. The reaction mixture was protected
from light and stirred at room temperature for 20 h. Solvent was
evaporated and the residue was puried by column chroma-
tography on silica gel (hexane : CH2Cl2 ¼ 1 : 1) to give M-26.
Yield: 0.37 g (77.1%). 1H-NMR (CDCl3, 300 MHz, ppm) d 8.69 (d,
J ¼ 4.2 Hz, 2H), 7.24 (d, J ¼ 4.2 Hz, 2H), 1.76–1.60 (m, 4H), 1.45–
1.10 (m, 86H), 0.88 (t, 6H). 13C-NMR (CDCl3, 75 MHz, ppm) d
161.10, 139.11, 135.55, 131.78, 131.26, 119.28, 107.91, 42.75,
37.21, 33.65, 32.09, 30.52, 30.25, 39.87, 29.52, 27.17, 26.85,
22.85, 14.28.
ꢀ
ꢁ
onset
ox
onset
EHOMO ðeVÞ ¼ ꢀ E
ꢀ E
ꢀ 4:80 eV;
Fc=Fcþ
onset
onset
where E
and E
are the onset oxidation potentials for
Fc=Fcþ
ox
Synthesis of PDQT-26
the polymer sample and ferrocene relative to the Ag/AgCl elec-
trode, while the value of ꢀ4.80 eV is the HOMO energy level of
ferrocene.12 Reection X-ray diffraction (XRD) patterns were
obtained using polymer thin lms (ꢁ35 nm) spin-coated on
dodecyltrichlorosilane (DTS)-modied Si/SiO2 substrates on a
Bruker D8 Advance diffractometer with Cu Ka radiation (l ¼
To a 50 mL dry ask was added M-26 (0.2494 g, 0.21 mmol), 5,50-
bis(trimethylstannyl)bithiophene (0.1033 g, 0.21 mmol) and
tri(o-tolyl)phosphine (P(o-tolyl)3) (5.1 mg,
8
mol%,
0.0168 mmol). Under argon protection, anhydrous chloroben-
zene (18 mL) and tris(dibenzylideneacetone)dipalladium(0)
(Pd2(dba)3) (3.9 mg, 2 mol%, 0.0042 mmol) were added. The
˚
1.5418 A). Transmission XRD measurements were carried out
ꢃ
reaction mixture was stirred at 130 C for 72 h. Aer cooling,
on a stack of polymer thin lms using a Bruker Smart Apex2
2-bromothiophene (0.5 mL) was added and the mixture was
stirred at 130 ꢃC for an additional 2 h. Aer cooling to room
temperature, the reaction mixture was poured into methanol
(100 mL). Aer ltration, the obtained solid was subjected to
Soxhlet extraction successively with acetone, hexane, and chlo-
roform. Removing solvent from the chloroform fraction gave
the product PDQT-26. Yield: 250 mg (99.8%). HT-GPC (in
˚
CCD with Mo Ka radiation (l ¼ 0.71073 A). Atomic force
microscopy (AFM) measurements were carried out on a
Dimension 3100 Scanning Probe Microscope.
OTFT device fabrication
Bottom-gate, bottom-contact OTFT devices were fabricated on
heavily n++-doped silicon wafer with a ꢁ300 nm thermally
grown silicon oxide (SiO2) top layer having a capacitance of ꢁ11
nF cmꢀ2. Gold source and drain patterns were deposited by
thermal evaporation on the SiO2 layer using a conventional
lithography technique. Prior to device fabrication, the
substrates were subjected to cleaning with air plasma, acetone,
and isopropanol, and then modied with dodecyltri-
chlorosilane (DTS) in toluene (10 mg mLꢀ1) at 60 ꢃC for 20 min.
Subsequently the substrates were washed with toluene, and
dried under a nitrogen ow. A layer of PDQT-20, PDQT-24 or
PDQT-26 semiconductor lm (ꢁ35 nm) was deposited on the
1,2,4-trichlorobenzene at 140 ꢃC): Mn ¼ 62.6 kDa; Mw
¼
265.2 kDa; Mw/Mn ¼ 4.24.
Synthesis of PDQT-20 (ref. 3g)
This polymer was synthesized using M-20 according to the
procedure described for PDQT-26. HT-GPC (in 1,2,4-tri-
chlorobenzene at 140 ꢃC): Mn ¼ 40.0 kDa; Mw ¼ 128.8 kDa; Mw/
Mn ¼ 3.22.3g
Synthesis of PDQT-24
This polymer was synthesized using M-24 according to the substrate by spin-coating a 5 mg mLꢀ1 polymer solution in
procedure described for PDQT-26. HT-GPC (in 1,2,4-tri- 1,1,2,2-tetrachloroethane (TCE) at a speed of 2000 rpm for 90 s.
chlorobenzene at 140 ꢃC): Mn ¼ 54.9 kDa; Mw ¼ 171.5 kDa; Mw/ The substrate was then placed on a hotplate at 150 ꢃC or 200 ꢃC
Mn ¼ 3.12.
for 15 min in a glove box. Then a poly(methyl methacrylate)
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(PMMA) solution in butyl acetate (8 wt%) was spin-coated on voltage and threshold voltage, respectively. VT was determined
1/2
top of the polymer semiconductor layer at 3000 rpm for 60 s, from the VGS axis intercept of the linear extrapolation of (IDS
followed by drying on a hotplate at 80 ꢃC for 30 min in the same ꢀ VGS in the saturation regime at IDS ¼ 0.
glove box. The resulting PMMA encapsulation layer is ꢁ500 nm
)
thick. The devices were characterized in air by using an Agilent
4155C Semiconductor Parameter Analyzer. The carrier mobility Results and discussion
in the saturated regime, m, was calculated from the slope of the
1/2
In this study, we choose three different alkyl side chains,
(IDS
)
versus VGS plot according to the following equation
namely, 2-octyldodecyl, 2-decyltetradecyl, and 4-decylhexadecyl,
which can be incorporated into the DPP unit by using respective
alkyl bromides in the presence of K2CO3 in dry DMF (Scheme 1).
2-Octyldodecyl bromide and 2-decyltetradecyl bromide can be
W
2
IDS ¼ mCi ðVGS ꢀ VTÞ ;
2L
where IDS is the drain current, L and W are the channel length
(30 mm) and channel width (1000 mm), Ci is the gate dielectric readily prepared from the commercial alcohols using bromine
layer capacitance per unit area, and VGS and VT are the gate and triphenylphosphine (PPh3). 4-Decyl-1-hexadecyl bromide
Scheme 1 Synthetic route towards PDQT polymers, PDQT-20, PDQT-24, and PDQT-26: (a) K2CO3/DMF/130 ꢃC; (b) NBS/DMF/50 ꢃC; (c)
Pd2(dba)3/P(o-tolyl)3/chlorobenzene/130 ꢃC; (d) NaOEt/EtOH/50 ꢃC; (e) (i) KOH/EtOH–H2O/50 ꢃC, (ii) HCl (conc.), (iii) 180 ꢃC, vacuum; (f) LiAlH4/
Et2O; (g) Br2/PPh3/pyridine/CH2Cl2/r.t.
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Table 1 Optical and electrochemical properties of PDQTs
lambasxa (nm)
abs
onset
a
Energy levelse
l
(nm)
Compounds
Mn (kDa)/PDI
Solutiona
Filmb
Filmb
Eogptc (eV)
Eoonx setd (eV)
EHOMO (eV)
ELUMO (eV)
PDQT-20
PDQT-24
PDQT-26
40.0/3.22
54.9/3.12
62.6/4.24
783
781
791
786
784
798
932
939
954
1.33
1.32
1.30
0.54
0.58
0.58
ꢀ5.29
ꢀ5.33
ꢀ5.33
ꢀ3.96
ꢀ4.02
ꢀ4.03
a
ꢁ10ꢀ5 M in chloroform. b Spin-coated lms from chloroform solutions on glass substrates. c Calculated from the onset absorption, Eogpt ¼ 1240/
abs
onset
d
l
.
Calculated using the equation Eooxnset (vs. Fc/Fc+) ¼ Eoonx set (vs. Ag/AgCl) ꢀ Eoonx set (Fc/Fc+ vs. Ag/AgCl). e Calculated using the equation EHOMO
(eV) ¼ ꢀEoonx set (vs. Fc/Fc+) ꢀ 4.8 eV, ELUMO ¼ EHOMO + Eogpt
.
(4) was synthesized starting from 2-decyltetradecyl bromide blue shied for 2 nm to 781 nm in solution and to 784 nm in
according to the procedure outlined in the inset of Scheme 1. thin lm (Fig. 1), suggesting that the bulkier side chains in
The key intermediate carboxylic acid 2 was synthesized using PDQT-24 only slightly inuenced the conjugation. On the other
the literature method.13 2 was then quantitatively reduced to hand, notable redshis in absorption were observed for PDQT-
alcohol 3, which was subsequently converted to the target 26 that exhibited the lmax at 791 nm in solution and 798 nm in
bromide 4 using bromine and PPh3 in the presence of pyridine. the solid thin lm. The UV-Vis data indicate that PDQT-26 has a
The overall yield of four steps from 2-decyltetradecyl bromide to more coplanar backbone structure than that of PDQT-20 and
4 is 67%. The synthetic route towards PDQTs with different alkyl PDQT-24 in both solution and solid states, which most likely
side chains, PDQT-20, PDQT-24, and PDQT-26, is depicted in originates from the larger distance of the bifurcation point of
Scheme 1. The dibromo monomers M-20, M-24, and M-26 were the side chain from the backbone in PDQT-26.
conveniently prepared by incorporation of alkyl chains at the
The electrochemical properties of the polymer thin lms were
nitrogen atoms of the DPP unit using respective alkyl bromides, characterized by cyclic voltammetry (CV). Reversible oxidation
followed by bromination using N-bromosuccinimide (NBS). M- processes were observed for all three polymers (ESI†). The high-
20, M-24, and M-26 were then copolymerized with 5,50-bis- est occupied molecular orbital (HOMO) energy levels were
(trimethylstannyl)-2,20-bithiophene with a catalytic amount of calculated from the onset oxidation potentials to be ꢀ5.29 eV for
Pd2(dba)3/P(o-tolyl)3 in chlorobenzene to produce PDQT-20, PDQT-20, ꢀ5.33 eV for PDQT-24, and ꢀ5.33 eV for PDQT-26,
PDQT-24, and PDQT-26, respectively. All polymers were puried
extensively using Soxhlet extraction with acetone and hexane to
remove oligomers. The remaining polymers were dissolved with
chloroform. Molecular weights of all polymers were evaluated
by gel permeation chromatography (GPC) with 1,2,4-tricholor-
ꢃ
obenzene as the eluent at 140 C (Table 1). All three polymers
have high molecular weights with a Mn (the number average
molecular weight) of 40.0 kDa, 54.9 kDa and 62.6 kDa for PDQT-
20, PDQT-24 and PDQT-26, respectively.
These polymers show good thermal stability with the 5%
weight loss temperatures (Tꢀ5%) of ꢁ395 ꢃC (ESI). PDQT-20
showed no thermal transition on its differentꢃial scanning
ꢃ
calorimetric (DSC) curves in the range from ꢀ20 C to 320 C.
On the other hand, PDQT-24 showed an obvious endothermic
peak at 293 ꢃC during the heating sweep and an exothermic
peak at 276 ꢃC upon cooling. These results indicate that
increasing the size of the side chain from 2-octyldodecylin
PDQT-20 to 2-dodecyltetradecyl in PDQT-24 could dissociate the
polymer chain packing at a much lower temperature due to the
easier side chain motion in the latter. Interestingly, PDQT-26
with an even larger side chain substituent, 4-decylhexadecyl,
showed no thermal transition on its DSC prole. This might be
due to the lower degree of crystallinity (as conrmed by XRD) or
a higher thermal transition temperature of PDQT-26.
The UV-Vis absorption spectra of polymers were recorded
both in solutions and with thin lms. PDQT-20 showed the
Fig. 1 Normalized UV-Vis absorption spectra of PDQT-20, PDQT-24
and PDQT-26 (a) in ꢁ10ꢀ5 M solutions in chloroform (CHCl3) and (b) as
wavelength of absorption maximum (lmax) at 783 nm in solu-
tion and 786 nm in the solid state, while the lmax of PDQT-24 thin films spin-coated from CHCl3 solutions on glass substrates.
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which indicated that the position of the bifurcation point of the crystalline lamellar domains. The calculated p–p distances are
ꢃ
ꢃ
˚
˚
side chains had little inuence on the HOMO energy levels of 3.79 A (2q ¼ 10.78 ) for PDQT-20, 3.86 A (2q ¼ 10.58 ) for PDQT-
ꢃ
˚
these polymers. The reduction processes showed signicantly 24, and 3.68 A (2q ¼ 11.08 ) for PDQT-26. The results indicate
lower currents and were less reversible, indicating that these that increasing the side chain length weakens intermolecular
polymers favour hole conduction over electron conduction.
p–p interaction of polymers (PDQT-24 vs. PDQT-20), while
We used reection and transmission X-ray diffraction (XRD), positioning the bifurcation point of the side chain farther from
and atomic force microscopy (AFM) techniques to gain insights the backbone can dramatically facilitate the p–p stacking due
into the polymer chain packing and thin lm morphologies. to the reduced steric effect (PDQT-26 vs. PDQT-24). The
Fig. 2a–c show the reection-mode XRD results of polymer thin morphology of polymer thin lms with increasing annealing
lms spin-coated on dodecyltrichlorosilane (DTS)-modied temperature was visualized under an atomic force microscope
SiO2/Si substrates. For the 150 ꢃC annealed thin lms, PDQT-20 (AFM) (Fig. 3). A rough surface comprising clearly dened
ꢃ
exhibited an intense primary diffraction peak along with higher grains can be seen in the 150 C annealed PDQT-20 thin lm.
order peaks up to the 4th order. PDQT-24 showed a slightly On other hand, the PDQT-24 and PDQT-26 lms are much
stronger primary peak than that of PDQT-20, but the 3rd and 4th smoother and contain very small grains. Increasing the
order peaks are not observed. PDQT-26 showed the weakest annealing temperature to 200 ꢃC had insignicant effects on the
diffraction peaks, indicating its less ordered layer-by-layer surface morphology of all three polymers.
polymer chain packing.
These polymers were tested as channel semiconductors in
The interlayer d-spacing distances of PDQT-20, PDQT-24, bottom-gate, bottom-contact OTFT devices constructed on SiO2/
and PDQT-26 lms were calculated using Bragg's law to be 1.94, Si wafer substrates. A polymer solution was spin coated on the
2.12 and 2.45 nm, respectively, in good agreement with their substrate with pre-patterned gold source–drain electrode pairs
increasing side chain length. Since no peaks representing the and annealed at 150 ꢃC or 200 ꢃC in a glove box under nitrogen.
˚
p–p distance (ꢁ3–4 A) were observed, the polymer chains were The devices were encapsulated with ꢁ500 nm of PMMA by spin-
likely packed in a lamellar crystalline structure with an edge-on coating a PMMA solution in butyl acetate and dried at 80 ꢃC on a
orientation in these thin lms.3b In order to further substantiate hotplate. The devices were characterized in air using an Agilent
the edge-on molecular orientation in the polymer thin lms and 4155C I–V source measurement unit. All devices showed p-
elucidate the p–p stacking distance, transmission XRD channel charge transport characteristics (Fig. 4 and Table 2).
measurements were carried out on a stack of polymer thin lms The average (and maximum) mobꢀil1ities of the polymer lms
2
with a Mo Ka radiation source (l ¼ 0.71073 A). It can be clearly annealed at 150 ꢃC were 1.58 cm V sꢀ1 (2.10 cm2 Vꢀ1 sꢀ1) for
˚
seen in Fig. 2d that the transmission XRD proles are markedly PDQT-20, 2.65 cm2 Vꢀ1 sꢀ1 (3.37 cm2 Vꢀ1 sꢀ1) for PDQT-24, and
different from those obtained in the reection mode. For all 5.18 cm2 Vꢀ1 sꢀ1 (6.90 cm2 Vꢀ1 sꢀ1) for PDQT-26. The increase
polymer samples, the primary peaks corresponding to the in mobility of PDQT-24 compared with PDQT-20 is considered a
interlayer distance are very weak. On the other hand, very strong result of the more interconnected grains of the former that
peaks at 2q ¼ ꢁ10–12ꢃ were clearly observed. These peaks facilitates the charge hopping across the polymer thin lms.
represent the p–p distances between the polymer chains in the Since PDQT-26 has the lowest crystallinity among three poly-
mers and the morphologies of PDQT-26 and PDQT-24 are very
similar, the signicantly improved mobility observed for PDQT-
26 is most likely attributable to its higher backbone coplanarity
Fig. 2 Reflection XRD diagrams (a–c) of spin coated PDQT-20,
PDQT-24 and PDQT-26 thin films (ꢁ35 nm) on
a dodecyltri-
chlorosilane (DTS)-modified SiO2/Si substrate annealed at different
temperatures and transmission XRD diagrams obtained with Cu Ka
radiation (l ¼ 1.5406 A˚) (d) of PDQT-20, PDQT-24 and PDQT-26 thin Fig. 3 Height AFM images (2 mm ꢂ 2 mm) of spin coated PDQT-20,
film stacks (non-annealed) obtained with Mo Ka radiation (l ¼ 0.71073 PDQT-24 and PDQT-26 thin films (ꢁ35 nm) on the DTS-modified
A˚).
SiO2/Si substrate annealed at different temperatures.
2188 | J. Mater. Chem. C, 2014, 2, 2183–2190
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Journal of Materials Chemistry C
3.57 cm2 Vꢀ1 sꢀ1 with the highest value of 5.50 cm2 Vꢀ1 sꢀ1 due
to the increased crystallinity and improved molecular ordering
of the polymer thin lms at this annealing temperature.
However, decreased performance was observed for both PDQT-
24 and PDQT-26 at this annealing temperature although both
polymers showed improved crystallinity. This might be due to
the deteriorated polymer/dielectric interfacial contact caused by
their more mobile side chains.14
Conclusions
The effects of the side chain length and bifurcation point posi-
tion on the molecular organization and charge transport prop-
erties of
a series of diketopyrrolopyrrole-quaterthiophene
polymers (PDQTs) in OTFTs were investigated. A comparative
study of PDQTs substituted with 2-octyldodecyl (PDQT-20),
2-decyltetradecyl (PDQT-24), and 4-decylhexadecyl (PDQT-26) was
conducted. It was found that PDQT-24 that has longer side chains
showed a much improved mobility of up to 3.37 cm2 Vꢀ1 sꢀ1
compared to that of PDQT-20 (2.10 cm2 Vꢀ1 sꢀ1) in OTFT devices
annealed at 150 ꢃC. The crystallinity of PDQT-24 was found to be
similar to PDQT-20, which did not contribute to the improved
˚
performance of the former. The p–p distance of PDQT-24 (3.86 A)
˚
increased with respect to that of PDQT-20 (3.79 A), which also
had no benet to the increase in mobility. Therefore, the better
performance of PDQT-24 was considered to be due to its
smoother surface morphology and better interconnected
networks for more efficient charge transport between grains.
PDQT-26 showed a remarkable increase in mobility up to
6.90 cm2 Vꢀ1 sꢀ1 in comparison with PDQT-24 and PDQT-20,
even though PDQT-26 thin lms showed lower crystallinity than
that of both PDQT-20 and PDQT-24. The main reason for this
surge in the mobility of PDQT-26 most likely originates from its
Fig. 4 Output (left) and transfer (right) characteristics of OTFT devices
using 150 ꢃC annealed thin films of (a) PDQT-20 (mh ¼ 2.10 cm2 Vꢀ1
sꢀ1),3g (b) PDQT-24 (mh ¼ 3.37 cm2 Vꢀ1
s
ꢀ1), and (c) PDQT-26 (mh
¼
6.90 cm2 Vꢀ1
s
ꢀ1). Device dimensions: L ¼ 30 mm; W ¼ 1 mm.
˚
much shorter p–p distance (3.68 A). Our results demonstrated
that 4-decylhexadecyl is a very effective new substituent group for
polymer semiconductors to enhance the charge transport
performance by increasing the main chain coplanarity and
shortening the p–p distance.
and shorter p–p distance brought about by the more distant
bifurcation point of the side chains from the polymer backbone.
The highest mobility achieved for the 150 ꢃC annealed
PDQT-26 lms is 6.90 cm2 Vꢀ1 sꢀ1, which is among the best
mobility values reported so far for polymer semiconductors.
Acknowledgements
ꢃ
OTFT devices with polymer thin lms annealed at 200 C were
also evaluated. The average mobility of PDQT-20 improved to
The authors thank the Natural Sciences and Engineering
Research Council (NSERC) of Canada for nancial support
(Discovery Grants and NSERC-DALSA Industry Research Chair
Program) of this work. The authors also thank Angstrom Engi-
neering Inc. for providing the thermal deposition system for the
fabrication of OTFT devices and Jon Hollinger and Prof. Dwight
Seferos of University of Toronto for measuring molecular
weights using HT-GPC. SC thanks the Oversea Study Program of
Guangzhou Elite Project provided by Guangzhou City, China.
Table
2 OTFT device performance of the polymers at different
annealing temperatures
Annealing
mha (cm2 Vꢀ1 sꢀ1
)
c
Polymer
temperature (ꢃC) average (max.)
VT (V)b
Ion/Ioff
PDQT-20 150
(ref. 3g) 200
PDQT-24 150
200
PDQT-26 150
200
1.58 (2.10)
3.57 (5.50)
2.65 (3.37)
1.46 (1.51)
5.18 (6.90)
1.52 (1.55)
ꢀ10.0
ꢀ6.8
18.2
17.6
27.1
13.5
7.4 ꢂ 103
1.4 ꢂ 106
2.6 ꢂ 104
1.6 ꢂ 104
1.6 ꢂ 104
2.9 ꢂ 103
Notes and references
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