Synthesis and thin-film transistor performance of benzodipyrrolinone and bithiophene donor-acceptor copolymers

Article abstract of DOI:10.1039/c2jm34867e

Three novel benzodipyrrolinone-bithiophene based donor-acceptor polymers are synthesized and used as p-type semiconductors in organic thin-film transistors (OTFTs). Hole mobility of up to 0.03 cm² V^-1 s^-1 is achieved. A ne

Full text of DOI:10.1039/c2jm34867e

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PAPER
Synthesis and thin-film transistor performance of benzodipyrrolinone and
bithiophene donor-acceptor copolymers†
a
a
a
Wei Hong, Chang Guo, Yuning Li, Yan Zheng,b Chun Huang,b Shaofeng Lub and Antonio Facchettib
*
Received 24th July 2012, Accepted 5th September 2012
DOI: 10.1039/c2jm34867e
Three novel benzodipyrrolinone–bithiophene based donor-acceptor polymers are synthesized and used
as p-type semiconductors in organic thin-film transistors (OTFTs). Hole mobility of up to 0.03 cm2 Vꢀ1
sꢀ1 is achieved. A new acetal-type branched substituent is introduced to significantly improve the
solubility of the resulting polymer.
hole mobility up to 1.6 cm2 Vꢀ1 sꢀ1 and electron mobility up to
Introduction
2.0 cm2 Vꢀ1 sꢀ1 for polymer films annealed at high tempera-
In recent years, polymer semiconductors have attracted consid-
tures.7 Within less than two years, a number of other DPP-based
erable research interests because they enable solution-processing
(or printing) organic thin film transistors (OTFTs),1 organic
photovoltaics (OPVs),2 organic light-emitting diodes (OLEDs),3
D-A polymers with mobility of ꢁ1 to 2 cm2 Vꢀ1 sꢀ1 were
reported,6,8 demonstrating that DPP is indeed a very useful
building block for high-performance polymer semiconductors
etc., which are essential components for inexpensive, flexible, and
for OTFTs.
large-area organic opto-electronics. Charge carrier mobility is
DPP can be viewed as a fused aromatic ring of two pyrroli-
one of the most important parameters for OTFTs, which deter-
nones. A natural extension of DPP by fusing two pyrrolinones to
mines the performance of the electronic devices using OTFTs.
cyclohexa-1,4-diene would form a larger conjugated moiety,
Mobility for polymer semiconductors, which is typically less than
0.1 cm2 Vꢀ1 ꢀ1, has long been inadequate for several applica-
s
pyrrolo[3,4-f]isoindole-1,5(2H,6H)-dione or benzodipyrrolinone
(BDP) (Scheme 1). When used as a building block in polymer
semiconductors, the more extended p-conjugation and the larger
ring size of BDP could provide stronger intermolecular interac-
tions and larger p–p overlapping, which are desirable charac-
teristics for efficient charge transport. While the preparation of
BDP was proven to be a synthetic challenge, its isomeric structure,
pyrrolo[2,3-f]indole-2,6(1H,5H)-dione (BDP0) is readily acces-
sible.9 Based on its analogy with BDP in terms of its molecular
geometry and electronic structure, BDP0 is also highly likely a
useful building block for high mobility D-A polymers for OTFTs.
tions such as backplane drivers for displays and radio-frequency
identification (RFID) tags. In the last couple of years, however, a
number of polymers were reported to show ultra-high mobility
values close to or exceeding 1 cm2 Vꢀ1 ꢀ1, even outperforming
s
amorphous silicon (a-Si).4 These high mobility polymer semi-
conductors are almost exclusively donor-acceptor (D-A)-type
polymers,5 which consist of an alternating arrangement of an
electron-donating (D) unit and an electron-accepting (A) unit.
Recently we found that combination of the electron-accepting
pyrrolo[3,4-b]pyrrole-1,4(2H,5H)-dione or diketopyrrolopyrrole
(DPP) and the electron-donating thieno[3,2-b]thiophene could
form a p-type D-A polymer, PDBT-co-TT, which showed high
hole mobility of ꢁ0.94 cm2 Vꢀ1 sꢀ1 in OTFTs.6 The strong
intermolecular donor-acceptor interactions led to the reduced
ꢀ
p–p stacking distance (ꢁ3.71 A) and the well-interconnected film
morphology, which are responsible respectively for the improved
interchain and interdomain charge hopping. Later, this polymer
was reinvestigated in ambipolar OTFTs resulting in even higher
aDepartment of Chemical Engineering and Waterloo Institute for
Nanotechnology (WIN), University of Waterloo, 200 University Ave
West, Waterloo, ON, Canada, N2L 3G1. E-mail: yuning.li@uwaterloo.
ca; Fax: +1 519-888-4347; Tel: +1 519-888-4567 ext. 31105
bPolyera Corporation, 8045 Lamon Avenue, Skokie, IL 60077, USA. Fax:
+1 847-6777510; Tel: +1 847-6777517
Scheme 1 A structural comparison of various fused pyrrolinones,
wherein R is hydrogen or a suitable substituent such as an alkyl group.
† Electronic supplementary information (ESI) available: NMR spectra of
compounds. See DOI: 10.1039/c2jm34867e
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Small molecules that comprise BDP0 have been known for
many years as pigments and dyes.9 Very recently, BDP0-based
polymers comprising a thiophene unit (PBDP0DP-T) was
reported.10a The resulting polymers showed ambipolar behaviour
in OTFT devices. Electron mobility of up to 3.5 ꢂ 10ꢀ3 cm2 Vꢀ1
sꢀ1 and hole mobility of up to 6.4 ꢂ 10ꢀ3 cm2 Vꢀ1 sꢀ1 were
achieved. Another lately reported BDP0-thiophene copolymer
in concentrated sulphuric acid afforded 2 in 96% isolated yields.
The1H NMR spectrum of 2 confirmed that it contains three
stereoisomers. Oxidation of 2 with H2O2 produced 3 in 45%
yields. The subsequent alkylation of 3 with 2-octyldodecyl
bromide, 2-decyltetradecyl bromide or 2,2-bis((3,7-dimethyloc-
tyl)oxy)ethyl bromide was conveniently accomplished in anhy-
drous DMF in the presence of potassium carbonate, a procedure
similar to the one for the preparation of alkylated DPP
compounds.6,11 The products M1–M3 were obtained in 57–68%
yields. Substitution of the BDP0 unit with long branched alkyl
and acetal type side chains aims to provide sufficient solubility to
resulting polymers. The precursor to novel acetal type side chain,
2,2-bis((3,7-dimethyloctyl)oxy)ethyl bromide, could be easily
synthesized in high yields (ꢁ90%) by heating the commercially
available bromoacetaldehyde diethyl acetal and 3,7-dimethyl-1-
octanol in the presence of a catalytic amount of methanesulfonic
acid.
containing N-acylated side chains showed improved electron
mobility of up to 0.012 cm2 Vꢀ1 sꢀ1
In this work, we report
10b
.
three new BDP0-based D-A polymers, PBDP0DP-BT’s, synthe-
sized via Stille coupling polymerization and their charge trans-
port performance as channel semiconductors in OTFTs. The
incorporation of the longer and more electron donating bithio-
phene (BT) unit aims to suppress the electron transport behav-
iour and promote hole transport of the resulting BDP0 polymers.
All polymers showed unipolar p-channel charge transport
characteristics with the best hole mobility value of up to 0.03 cm2
Vꢀ1
s
ꢀ1. The use of a new acetal-type side-chain significantly
Polymers P1 (PBDP0DP20-BT), P2 (PBDP0DP24-BT) and P3
(PBDP0DP24a-BT) were synthesized by Stille coupling reactions
of 5,50-bis(trimethylstannyl)-bithiophene with M1, M2, and M3,
improved the solubility of the resulting BDP0-based polymer.
Our results demonstrate that BDP0 is a promising acceptor
building block for the construction of high performance D-A
type polymer semiconductors and that an extension of the donor
unit could be able to convert the BDP0 based polymers into
unipolar p-type semiconductors with improved charge transport
mobility.
respectively, in the presence of
a catalytic amount of
ꢃ
Pd(PPh3)2Cl2 in toluene at 90–110 C. After being precipitated
from methanol, the polymers were extensively purified by
Soxhlet extraction successively with acetone and hexane to
remove low molecular mass fractions and other impurities. The
higher molecular weight polymer fractions were dissolved with
refluxing chloroform. However, P1 substituted with 2-octyldo-
decyl groups was found only sparingly soluble in chloroform and
thus 1,1,2,2-tetrachloroethane (TCE) that has a higher boiling
point (146 ꢃC) than that (61 ꢃC) of chloroform was used to
extract P1. The majority of P1 (73%) was dissolved in TCE, while
Results and discussion
The synthetic route to PBDP0DP-BT is outlined in Scheme 2.
Compound 1 was prepared in good yields (ꢁ87%) according to
the established literature methods.9 Ring-closure of compound 1
Scheme 2 Synthesis of (a) PBDP0DP-BT polymers (P1, P2, and P3) and (b) 2,2-bis((3,7-dimethyloctyl)oxy)ethyl bromide.
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there still remained some insoluble mass in the extraction
thimble. Polymer P2, which has longer side chains (2-decylte-
tradecyl groups), showed better solubility with 35% of the
theoretical mass (as P2a) dissolved with refluxing chloroform.
The remaining higher molecular-weight fraction was completely
dissolved with refluxing TCE, affording another 59% yield of
polymer as P2b. The lower solubility of these two polymers than
that of DPP-based copolymers6,11 could be attributed to the
stronger intermolecular interactions in P1 and P2 due to the
presence of larger fused BDP0 rings. On the other hand, polymer
P3 substituted with the highly branched acetal-type side chain,
2,2-bis((3,7-dimethyloctyl)oxy)ethyl,
showed
significantly
improved solubility with all the high molecular weight fractions
(P3, ꢁ90% yield) being dissolved in refluxing chloroform.
Besides chloroform, P3 is also soluble in tetrahydrofuran (THF),
chlorobenzene, 1,2-dichlorobenzene, 1,1,2-trichloroethane, and
TCE. Since P1 and P2 showed very poor solubility in THF, the
molecular weights of all polymers were determined by high-
temperature gel permeation chromatography (HT-GPC) with
polystyrene as standards and 1,2,4-trichlorobenzene as an eluent
at a high temperature of 140 ꢃC. For P1, the number average
molecular weight (Mn) was found to be 6.0 kDa, while its weight
average molecular weight (Mw) is 27.4 kDa (with a polydispersity
index, PDI, of 4.57). The average number of repeat unit for P1 is
calculated to be 5.7, indicating that P1 consists of a significant
portion of oligomers. P2a, which was extracted with chloroform,
is also mainly composed of oligomers with a low Mn of 4.3 kDa
(PDI ¼ 3.51). The molecular weights of the TCE extracted
fraction, P2b, are much higher with a Mn of 19.8 kDa (PDI ¼
1.78). The chloroform extracted P3 has an even higher Mn of 22.8
kDa (PDI ¼ 2.89), which is close to that (Mn ¼ 25.4 kDa, PDI ¼
2.39) of a DPP-based high mobility polymer PDQT11 measured
using a HT-GPC system.
Fig. 1 Normalized UV-Vis absorption spectra of M1–M3 and polymers
P1–P3: (a) ꢁ10ꢀ5 M solutions in 1,1,2,2-tetrachloroethane; and (b): thin
films spin-coated from 1,1,2,2-tetrachloroethane solutions on glass
substrates.
The UV-Vis absorption properties of P1–P3, as well as their
monomers M1–M3 in TCE solutions and in thin films are shown
in Fig. 1 and Table 1. In solutions, all three monomers showed
almost an identical maximum absorption wavelength (lmax) at
ꢁ471 nm. In thin films, their lmax red-shifted to 519–522 nm,
suggesting that these molecules are more coplanar and have
strong intermolecular interactions in the solid state. As expected,
polymers P1–P3 displayed large redshifts in their absorption
spectra in comparison to their corresponding monomers M1–
M3. Redshifts of ꢁ22 to 47 nm in the lmax from solution to the
solid thin film for all polymers are observed, which are indicative
of the more coplanar conformation of the polymer chains and
the stronger intermolecular interactions (such as the D–A
interactions) in the solid state. P1 showed a lmax at 590 nm in
solution, while the lmax of P2b in solution is 605 nm, 15 nm
longer than that of P1. This difference is apparently a result of
their different molecular weights; P2b with a higher molecular
weight has a more extended p-conjugation length and thus a
smaller band gap. Interestingly, in the solid state, P1 and P2b
exhibited almost identical absorption maxima (637 nm for P1
and 639 nm for P2b). This result might be interpreted by the
enhanced intermolecular interactions and polymer chain aggre-
gation of P1 in the thin films. On the other hand, P3 showed a
longer lmax at 653 nm in the thin film. The red-shifted absorption
profile of the P3 thin film compared with those of P1 and P2
might be due to the stronger intermolecular interactions
enhanced by the polar acetal side chains in the solid state. The
optical band gaps (Eogpt) of P1, P2b and P3 thin-films are
calculated to be 1.61, 1.62 and 1.59 eV from their absorption
edges of 769, 766 and 779 nm, respectively, which are narrower
than that (1.68 eV) of BDP0-thiophene copolymer (PBDP0DP-T)
reported previously.10a
The electrochemical properties of P1, P2b, and P3 thin films
were examined with cyclic voltammetry (CV) in 0.1 M Bu4NPF4
solution in anhydrous acetonitrile, using ferrocene (with a
highest occupied energy molecular orbital (HOMO) energy level,
EHOMO ¼ ꢁꢀ4.8 eV)12 as an external standard. For all the
polymers, a reversible oxidation process was observed (Fig. 2).
The HOMO energy levels of P1, P2b, and P3 were calculated
from their onset oxidation potentials to be ꢀ5.64, ꢀ5.66 eV, and
ꢀ5.62 eV, respectively (Table 1). On the other hand, the reduc-
tion processes of these polymers showed weak and irreversible
I–V loops, indicating that these polymers are unstable during the
reduction processes under our experimental conditions. There-
fore the lowest unoccupied molecular orbital (LUMO) energy
levels (ELUMO) of P1, P2b, and P3 were calculated using their
HOMO levels and optical band gaps to be ꢀ4.03, ꢀ4.04, and
ꢀ4.03 eV, respectively, which are comparable to that of their
analogous DPP-bithiophene copolymer PDQT (ꢀ4.0 eV).11 The
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Table 1 Properties of BDP0-containing monomers M1–M2 and polymers P1–P3
abs a
max
abs a
onset
l
(nm)
l
(nm)
Energy levelse
OTFT performance
mh
(eV) EHOMO (eV) ELUMO (eV) (cm2 Vꢀ1
onsetd
ox
Compounds Solutiona Filmb Solutiona Filmb Eogptc (eV)
E
s
ꢀ1) range (average) Ion/Ioff
M1
M2
M3
P1
P2b
P3
471
470
471
590
605
631
519
519
522
637
639
653
535
537
536
707
738
750
560
556
558
769
766
779
—
—
—
1.61
1.62
1.59
—
—
—
0.84
0.86
0.82
—
—
—
ꢀ5.64
ꢀ5.66
ꢀ5.62
—
—
—
ꢀ4.03
ꢀ4.04
ꢀ4.03
—
—
—
—
—
—
0.003–0.030(0.015)
0.007–0.027(0.018)
0.024–0.030(0.026)
ꢁ103ꢀ6
ꢁ103ꢀ5
ꢁ105ꢀ6
a
b
c
ꢁ10ꢀ5 M in 1,1,2,2-tetrachloroethane. Spin-coated films from 1,1,2,2-tetrachloroethane solutions on glass substrates. Calculated from the onset
absorption, Eogpt ¼ 1240/l
.
(vs. Fc/Fc+) ¼ E
(vs. Ag/AgCl) ꢀ E
(Fc/Fc+ vs. Ag/AgCl). e Calculated
abs
d
onset
Calculated using the equation E
ox
onset
ox
onset
ox
onset
using the equation EHOMO (eV) ¼ ꢀ Eoonxset (vs. Fc/Fc+) ꢀ 4.8 eV, ELUMO ¼ EHOMO + Eogpt
.
Fig. 2 Cyclic voltammograms of films of P1, P2b, and P3 in 0.1 M
tetrabutylammonium hexafluorophosphate in dry acetonitrile at
a
sweeping rate of 50 mV sꢀ1 under nitrogen, using ferrocene/ferrocenium
(Fc/Fc+) couple as a standard.
low LUMO levels of P1, P2b, and P3 indicate that BDP0 is a
strong electron accepting building block. The very similar
HOMO and LUMO levels measured for these three polymers
indicate that the energy levels are mainly determined by their
conjugated backbone, while different side chain substitutions
only exerted slight effects.
To investigate the molecular organization of these polymers in
the thin film state, X-ray diffraction (XRD) analysis of the
polymer thin films spin-coated on glass substrates was per-
formed. As shown in Fig. 3, the as-spun P1 thin film showed a
primary reflection peak at 2q ¼ ꢁ4.9ꢃ, which remained similar
ꢃ
after the film was annealed at 150 and 200 C, respectively. The
as-spun P2b film showed a noticeable peak at 2q ¼ ꢁ4.8ꢃ. The
intensity of this peak increased as the film was annealed at
150 ꢃC. Further raising the annealing temperature to 200 ꢃC had
little effect on increasing the intensity of this peak. Very weak
reflections were observed for the as-spun P3 film. After the film
was annealed at 150 ꢃC, a primary peak at 2q ¼ ꢁ4.1ꢃ appeared.
Interestingly, the reflection intensꢃity of this peak decreased when
the sample was annealed at 200 C, which might be due to the
side chain motion at this temperature.13 The diffraction patterns
of these polymers resemble those of many other conjugated
polymers with a layer-by-layer lamellar packing structure.6,8,11
Fig. 3 XRD diagrams obtained from spin-coating polymer thin films on
glass substrates annealed at different temperatures.
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The low angle diffraction peak represents the interlayer
ꢀ
but large gaps between grains remained. The as-spun P2b thin
film showed smaller grains than those in the as-spun P1 thin film.
Some grains have joined to form clusters when the film was
annealed at 150 ꢃC. For the film annealed at 200 ꢃC, all grains are
well connected. On the other hand, the AFM image of the as-
spun P3 thin film showed a much smoother surface, which
consists of tightly packed grains of ꢁ50 nm in diameter. The
d-spacing distance, which was calculated to be ꢁ18.0 A for P1,
ꢀ
ꢀ
ꢁ18.4 A for P2b, and 21.5 A for P3, respectively. The broad peak
in the region of 2q ¼ ꢁ20 to 30ꢃ is mainly due to the glass
substrate since no such intense peaks were observed for samples
prepared on the Si/SiO2 wafer substrates (ESI†). The polymer
thin film samples on the Si/SiO2 substrates was also found to be
less crystalline.
ꢃ
grains become well defined for the film annealed at 150 C, but
The surface morphology of P1, P2b, and P3 thin films (ꢁ30 to
40 nm) spin-coated using their solutions in TCE on glass
substrates was characterized with the atomic force microscopy
(AFM) technique (Fig. 4). For the as-spun P1 thin film, spherical
grains with an average diameter of ꢁ50 nm formed. Thermal
annealing at 150 and 200 ꢃC improved the granular connectivity,
turned less discernible again once the annealing temperature was
increased to 200 ꢃC. The changes in the morphology with
annealing temperature observed for the thin films of all three
polymers are in good agreement with the changes in crystallinity
shown in their XRD diagrams (Fig. 3). It seems that the film
consisting of more defined grains is more crystalline. The
Fig. 4 AFM height images (2 ꢂ 2 mm) of polymer thin films (ꢁ30 to 40 nm) on glass substrates annealed at different temperatures: (a) P1/r. t.,
(b) P1/150 ꢃC; (c) P1/200 ꢃC, (d) P2b/r. t., (e) P2b/150 ꢃC, (f) P2b/200 ꢃC, (g) P3/r. t., (h) P3/150 ꢃC, and (i) P3/200 ꢃC.
22286 | J. Mater. Chem., 2012, 22, 22282–22289
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device based on P3 are shown in Fig. 5. The hole mobility in the
saturation regime at VDS ¼ ꢀ60 V is 0.03 cm2 Vꢀ1
s
ꢀ1, with a
high current on-to-off ratio of ꢁ106.
Conclusions
Three new pyrrolo[2,3-f]indole-2,6(1H,5H)-dione (BDP0)-
bithiophene copolymers P1, P2, and P3 were synthesized by
Stille coupling polymerization. The BDP0 moiety showed similar
electron-accepting effects as the DPP unit in conjugated poly-
mers, but it induced stronger intermolecular interactions due to
its large ring size. All three polymers showed good hole mobility
up to ꢁ0.03 cm2 Vꢀ1 sꢀ1, demonstrating that BDP0 is a poten-
tially useful acceptor building block for polymer semiconductors
for OTFTs. The incorporation of an electron-donating bithio-
phene unit in these polymers could suppress the electron trans-
port characteristics, making these polymers explicit in hole
transport. The phenylene units adjacent to BDP0 are found to
cause unfavoured twisting of the polymer backbone.10a It is
expected that replacing the phenylene units with sterically less
encumbered thiophene or furan rings would planarize the poly-
mer backbone and thus greatly enhance the charge transport
properties, as similarly observed in the analogous DPP-based
polymers. It was found that the long branched alkyl side chain, 2-
octyldecyl and even 2-decyltetradecyl, which are sufficient for
solubilising the analogous DPP-based polymers, are unable to
offset the strong aggregation tendency enhanced by the BDP0
units to render the resulting polymers soluble. A novel acetal-
type side chain substituent, 2,2-bis((3,7-dimethyloctyl)oxy)ethyl,
was found to be an excellent solubilising group for the BDP0-
based polymers. Besides solubility, the acetal substitution has
significant impacts on the molecular ordering, thin film
morphology, and OTFT performance of the resulting polymers.
Fig. 5 Output (left) and transfer (right) curves of a typical OTFT device
with a P3 thin film annealed at 120 ꢃC. Device dimensions: channel length
L ¼ 50 mm; channel width W ¼ 500 mm.
formation of smoother and well interconnected morphology
observed for P3 is probably due to the better solubility and the
amorphous nature of the as-spun thin films of this polymer as
compared to P1 and P2b. AFM images of polymer thin films on
Si/SiO2 substrates were also measured (ESI†). P1 films on Si/SiO2
are similar as on the glass substrates, but P2b and P3 films
showed dramatically different surface morphology. This again
affirms that the substrate surface plays an important role in the
molecular organization.
Polymers P1, P2b, and P3 were evaluated as channel semi-
conductors in the top-gate, bottom-contact OTFT device archi-
tecture. A glass substrate was first patterned with gold source-
drain pairs with a channel length (L) of 50 mm and a channel
width (W) of 500 mm. A polymer thin film (ꢁ30 nm) was then
deposited by spin-coating a polymer solution in 1,2-dichloro-
benzene on the substrate, followed by thermal annealing at 120
ꢃC for 5 min on a hotplate. Next, a polyolefin-polyacrylate
dielectric layer (capacitance ¼ 4.2 nF cmꢀ2) was deposited by
spin-coating a Polyera ActivInkTM D2200 solution and baked
at 120 ꢃC for 5 min. Finally a ꢁ30 nm gold thin film was
deposited as a top gate electrode to complete the device fabri-
cation. OTFT devices were characterized under ambient condi-
tions. All polymers tested showed typical p-type field effect
behaviour. The hole mobility (mh) and current on-to-off ratio
(Ion/Ioff) are summarized in Table 1. The average mobility values
Experimental
Instrumentation and materials
Reagents and anhydrous solvents were purchased from Sigma-
Aldrich or Alfa Aesar and used as received. N,N0-(1,4-Phenylene)
bis(2-(4-bromophenyl)-2-hydroxyacetamide) (1) and 3,7-bis(4-
bromophenyl)-5,7-dihydropyrrolo[2,3-f]indole-2,6(1H,3H)-dione
(2) were prepared using the literature method9 in comparable
yields as reported previously.10 2-Octyldodecyl bromide and 2-
decyltetradecyl bromide14 and 5,50-bis(trimethylstannyl)bithio-
phene15 were synthesized according to the literature. NMR data
were collected on a Bruker DPX 300 MHz spectrometer with
chemical shifts relative to tetramethylsilane (TMS, 0 ppm). UV-
Vis spectra were recorded on a Shimadzu Mandel 2501-PC
instrument. Cyclic voltammetry (CV) measurements were per-
formed on a potentiostat/galvanostat model EPP-4000 (Prince-
ton Applied Research) using an Ag/AgCl reference electrode, a
platinum wire counter electrode, and a platinum foil working
electrode. The working electrode was coated with a polymer film
by drop-casting a polymer solution in 1,1,2,2-tetrachloroethane
(TCE). CV measurements were recorded in 0.1 M tetrabuty-
for P1, P2b, and P3 are 0.015, 0.018, and 0.026 cm2 Vꢀ1 sꢀ1
,
respectively. The mobility values for these polymers are greater
than the ones reported for the BDP0 copolymer having one
thiophene unit (PBDP0DP-T).10 In addition, the electron trans-
port characteristics of P1–P3 are minimal, indicating that the
incorporation of the stronger electron-donating bithiophene unit
effectively suppressed the electron transport and enhanced the
hole transport performance. OTFTs with P1 and P2b showed
large variations in mobility and current on-to-off ratio. This is
due to the poor quality of the P1 and P2b thin films as shown in
their AFM images, where there are large gaps between grains. On
the other hand, the mobility and the current on-to-off ratio of the
P3 based OTFTs are very consistent, which is due to the
smoother morphology of the P3 thin films consisting of tightly
joined grains (Fig. 4). The output and transfer curves of a typical
lammonium hexafluorophosphate in dry acetonitrile at
a
sweeping rate of 50 mV sꢀ1 under nitrogen, using ferrocene as a
ref. 12 The HOMO energy levels were calculated using the
onset
equations of EHOMO (eV) ¼ ꢀ{E
ꢀ EFc} ꢀ 4.8 eV,
ox
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onset
ox
respectively, where E
and EFc are the onset oxidation
130 ꢃC. The reaction mixture was further heated at 130 ꢃC under
stirring overnight. Then the reaction mixture was allowed to cool
down to room temperature, poured into water (200 mL), and
stirred for 5 min. The product was extracted with ethyl acetate
(100 mL for 3 times), then washed with water. Upon removal of
solvent, the crude product was further purified using column
chromatography on silica gel (eluted with CH2Cl2) to give
potentials of the polymer and ferrocene, respectively, relative to
the Ag/AgCl reference electrode. The LUMO energy levels are
estimated by using HOMO levels and optical band gaps. XRD
diagrams of polymer thin films (ꢁ100 nm) on glass substrates or
DTS-modified Si/SiO2 substrates deposited by drop-casting
polymer solutions in TCE were obtained with a Bruker D8
Advance powder diffractometer using standard Bragg-Brentano
1
compound M1 as a deep-green solid (1.33 g, 62.9%). H NMR
ꢀ
geometry with Cu Ka radiation (l ¼ 1.5406 A). Data were
(CDCl3, 300 MHz) d ppm 7.61 (d, J ¼ 8.7 Hz, 4H), 7.57 (d, J ¼
8.7 Hz, 4H), 6.29 (s, 2H), 3.50 (d, J ¼ 6.9 Hz, 4H), 1.65–1.85 (m,
2H), 1.15–1.45(m, 66H), 0.80–0.95 (m, 12H). 13C NMR (CDCl3,
75 MHz) d ppm 169.71, 144.47, 134.64, 132.20, 131.08, 130.19,
126.47, 123.74, 97.29, 44.18, 37.26, 32.06, 32.04, 31.91, 30.15,
29.78, 29.75, 29.50, 26.83, 22.82, 14.26. UV-Vis: lmax ¼ 471 (in
TCE), 519 (film).
collected from 2 to 35ꢃ in 2q, using a step size of 0.02ꢃ and a count
time of 0.7 s. High-temperature gel-permeation chromatography
(HT-GPC) measurements were performed on a Malvern 350 HT-
GPC system using 1,2,4-trichlorobenzene (stabilized with
butylated hydroxytoluene) as an eluent with polystyrene as
standards at column temperature of 140 ꢃC. AFM images of
polymer thin films were obtained on a Dimension 3100 Scanning
Probe Microscope. The polymer thin films (ꢁ30 to 40 nm) for
AFM measurements were deposited by spin-coating polymer
solutions in TCE (0.4 wt%) on glass substrates or DTS-modified
Si/SiO2 substrates.
Synthesis of 3,7-bis(4-bromophenyl)-1,5-bis(2-decyltetradecyl)
pyrrolo[2,3-f]indole-2,6(1H,5H)-dione (M2)
Compound M2 was prepared from compound 3 and 2-decylte-
tradecyl bromide following a procedure as described for
compound M1. Data for M2 follow. Yield: 68.4% yield. 1H
NMR (CDCl3, 300 MHz) d ppm 7.61 (d, J ¼ 8.7 Hz, 4H), 7.56 (d,
J ¼ 8.7 Hz, 4H), 6.29 (s, 2H), 3.50 (d, J ¼ 7.2 Hz, 4H), 1.65–1.85
(m, 2H), 1.15–1.45(m, 80H), 0.80–0.95 (m, 12H). 13C NMR
(CDCl3, 75 MHz) d ppm 169.68, 144.43, 134.61, 132.18, 131.06,
130.16, 126.43, 123.72, 97.28, 44.14, 37.23, 32.06, 31.88, 30.14,
29.80, 29.50, 26.82, 22.83, 14.26. UV-Vis: lmax ¼ 470 nm (in
TCE); 519 nm (film).
Synthesis of 2,2-bis((3,7-dimethyloctyl)oxy)ethyl bromide
2-Bromoacetaldehyde diethyl acetal (3.3 mL, 20 mmol) was
dissolved in 42 mL of 3,7-dimethyl-1-octanol, and 3 drops of
methanesulfonic acid was added. The mixture was heated at
120 ꢃC overnight with a slow stream of argon to remove the
ethanol formed. The mixture was cooled down and sodium
bicarbonate (0.84 g, 10 mmol) was added to neutralize the acid.
Upon filtration and removal of solvent, the excess 3,7-dimethyl-
1-octanol was removed under a reduced pressure (95 ꢃC, 2 mbar)
to give a brown oil, which was passed through a flash silica gel
column with hexane as an eluent to give the title compound as a
light yellow liquid (7.59 g, 90.0%). 1H NMR (CDCl3, 300 MHz) d
ppm 4.65 (t, J ¼ 5.4 Hz, 1H), 3.72–3.57 (m, 2H), 3.57–3.45 (m,
2H), 3.36 (d, J ¼ 5.4 Hz, 2H), 1.70–1.05 (m, 20H), 0.88 (t, 18H).
13C NMR (CDCl3, 75 MHz) d ppm 101.91, 65.29, 39.42, 37.44,
36.90, 31.86, 29.84, 28.10, 24.80, 22.84, 22.75, 19.74.
Synthesis of 1,5-bis(2,2-bis((3,7-dimethyloctyl)oxy)ethyl)-3,7-
bis(4-bromophenyl)pyrrolo[2,3-f]indole-2,6(1H,5H)-dione (M3)
Compound M3 was prepared from compound 3 and 2,2-bis((3,7-
dimethyloctyl)oxy)ethyl bromide following a procedure as
described for compound M1. Data for M3 follow. Yield: 56.7%.
1H NMR (CDCl3, 300 MHz) d ppm 7.59 (s, 8H), 6.58 (s, 2H),
4.58 (t, J ¼ 5.4 Hz, 2H), 3.80–3.60 (m, 8H), 3.60–3.42 (m, 4H),
1.70–0.95 (m, 40H), 0.90–0.75 (m, 36H). 13C NMR (CDCl3, 75
MHz) d ppm 169.72, 144.25, 135.12, 132.22, 131.09, 130.03,
126.48, 123.76, 98.97, 67.02, 66.93, 43.32, 39.41, 37.40, 37.07,
29.89, 28.09, 24.81, 24.77, 22.84, 22.74, 19.68, 19.63. UV-Vis:
lmax ¼ 471 nm (in TCE), 522 nm (film).
Synthesis of 3,7-bis(4-bromophenyl)pyrrolo[2,3-f]indole-
2,6(1H,5H)-dione (3)
A 2 M aqueous NaOH solution (24 mmol, 12 mL) was added
dropwise to a suspension of 3,7-bis(4-bromophenyl)-5,7-dihy-
dropyrrolo[2,3-f]indole-2,6(1H,3H)-dione (2) (4.4 g, 8.83 mmol)
Synthesis of PBDP0DP20-BT (P1)
ꢃ
in ethanol (75 mL) at 20 C. After 15 min, hydrogen peroxide
M1 (0.3172 g, 0.3 mmol) and 5,50-bis(trimethylstannyl)-bithio-
phene (0.1476 g, 0.3 mmol) were charged into a 50 mL dry flask.
After degassing and refilling argon for 3 times, anhydrous
toluene (20 mL) and bis(triphenylphosphine)palladium(II)
dichloride (7 mg) were added and the reaction mixture was
heated to 90 ꢃC and stirred for 48 h. The temperature of reaction
was raised to 100 ꢃC and reacted for another 10 h. Then 0.5 mL
of bromobenzene was added and the mixture was further stirred
at 100 ꢃC for 8 h to eliminate unreacted trimethylstannyl end
groups. The mixture was then poured into 200 mL of stirring
methanol. The solid was filtered off, washed with methanol, and
dried. The solid was further purified by Soxhlet extraction using
acetone, hexane, and then dissolved with chloroform. A low
molecular weight polymer fraction was obtained from the
(15 mL, 30 wt%) was added and the mixture was stirred at room
temperature overnight. The reaction mixture was filtered,
washed with a saturated NH4Cl aq. solution, water, i-PrOH and
MeOH, then dried in vacuo to give compound 3 as a brown
powder (1.98 g, yield 45%). 1H NMR (DMSO, 300 MHz) d ppm
10.35 (s, 2H), 6.36 (s, 2H), 7.71 (d, J ¼ 8.4 Hz, 4H), 7.61 (d, J ¼
8.4 Hz, 4H).
Synthesis of 3,7-bis(4-bromophenyl)-1,5-bis(2-octyldodecyl)
pyrrolo[2,3-f]indole-2,6(1H,5H)-dione (M1)
To a mixture of 3 (1.0 g, 2 mmol) and K2CO3 (0.830 g, 6 mmol) in
anhydrous N,N-dimethylformamide (DMF) (30 mL) was added
portion-wise 2-octyl-1-dodecylbromide (2.241 g, 6.2 mmol) at
22288 | J. Mater. Chem., 2012, 22, 22282–22289
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chloroform extract as a blue-purple solid (40 mg, 12.5%). The
remaining material was extracted with TCE, to give polymer P1
as a deep-blue solid film (234 mg, 73.4%). 1H NMR (CDCl3, 300
MHz) d ppm 7.90–7.50, 7.30–7.10, 6.70–6.10 (aromatic), 3.65–
3.35 (NCH2), 1.90–1.65, 1.65–1.40, 1.40–1.00 (CH2 and CH in
the chains), 0.95–0.65 (CH3). HT-GPC: Mn ¼ 6032; Mw ¼ 27,
426; Mw/Mn ¼ 4.54. UV-Vis: lmax ¼ 590 nm (in TCE), 637 nm
(film).
Acknowledgements
The authors thank Polyera Corporation for financial support
and Bin Sun for collecting the XRD and AFM data.
Notes and references
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Synthesis of PBDP0DP24-BT (P2)
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P2 was prepared as described above for P1 from M2 (0.3508 g,
0.3 mmol) and 5,50-bis(trimethylstannyl)-bithiophene (0.1476 g,
0.3 mmol). After Soxhlet extraction with acetone and hexane, the
first polymer fraction P2a was obtained by extraction with
chloroform as a blue-purple solid (124 mg, 34.9%). 1H NMR
(CDCl3, 300 MHz) d ppm 7.90–7.50, 7.50–7.25, 7.00–6.15
(aromatic), 3.65–3.35 (NCH2), 1.90–1.60, 1.60–1.40, 1.40–0.95
(CH2 and CH in the chains), 0.95–0.70 (CH3). HT-GPC results
for P2a: Mn ¼ 4293; Mw ¼ 15, 061; Mw/Mn ¼ 3.50. The higher
molecular weight polymer P2b was obtained as a deep-blue solid
film by extraction with TCE (208 mg, 59.0%). HT-GPC: Mn ¼
19, 841; Mw ¼ 35, 301; Mw/Mn ¼ 1.78. UV-Vis: lmax ¼ 605 nm
(in TCE), 639 nm (film).
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Synthesis of PBDP0DP24a-BT (P3)
P3 was prepared as described above for P1 from M3 (0.2943 g,
0.25 mmol) and 5,50-bis(trimethylstannyl)-bithiophene (0.1230 g,
0.25 mmol). After Soxhlet extraction with acetone and hexane,
P3 was obtained as a deep-blue solid by extraction with chlo-
1
roform (265 mg, 89.7%). H NMR (CDCl3, 300 MHz) d ppm
8.00–7.45, 7.45–7.25, 7.10–6.35 (aromatic), 4.75–4.50 (O–CH–
O), 3.90–3.65 (OCH2), 3.65–3.40 (NCH2), 1.75–1.30, 1.30–0.95
(CH2 and CH in the chains), 0.95–0.70 (CH3). HT-GPC: Mn ¼
22 826; Mw ¼ 65 975; Mw/Mn ¼ 2.89. lmax: 631 nm (in TCE), 653
nm (film).
Fabrication and characterization of OTFT devices
The top-gate, bottom-contact OTFT devices were fabricated on
glass substrates (Precision Glass & Optics, Eagle 2000). The
gold source and drain electrodes (ꢁ30 nm) were deposited by
thermal evaporation using a shadow mask (L ¼ 50 mm, W ¼
500 mm). The polymer semiconductor films were deposited onto
the substrates by spin-coating a polymer solution in 1,2-
dichlorobenzene (DCB) (4 mg mLꢀ1) at 1000 rpm for 60 s and
were annealed on a 120 ꢃC on a hotplate for 5 min. Then a
polyolefin-polyacrylate dielectric layer (Polyera ActivInkTM
D2200, capacitance ¼ 4.2 nF cmꢀ2) was spin-coated on top of
the polymer semiconductor film. The dielectric film was then
baked at 120 ꢃC on a hotplate for 5 min before the deposition of
a ꢁ30 nm gold thin film as the gate electrode on top of the
dielectric layer. The finished devices were tested under ambient
conditions.
15 Y. Wei, Y. Yang and J.-M. Yeh, Chem. Mater., 1996, 8, 2659.
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