Thiophene-benzothiadiazole-thiophene (D-A-D) based polymers: Effect of donor/acceptor moieties adjacent to D-A-D segment on photophysical and photovoltaic properties

Article abstract of DOI:10.1039/c1jm10649j

New push-pull copolymers based on thiophene (donor) and benzothiadiazole (acceptor) units, poly[4,7-bis(3-dodecylthiophene-2-yl) benzothiadiazole-co- thiophene] (PT3B1) and poly[4,7-bis(3-dodecylthiophene-2-yl) benzothiadiazole-co-benzothiadiazole] (PT2B2), are designed and synthesized via Stille and Suzuki coupling routes respectively. Gel permeation chromatography shows the number average molecular weights are 31100 and 8400 g mol^-1 for the two polymers, respectively. Both polymers have shown absorption throughout a wide range of the UV-vis region, from 300 to 650 nm. A significant red shift of the absorption edge is observed in thin films compared to solution of the copolymers; the optical band gap is in the range of 1.7 to 1.8 eV. Cyclic voltammetry indicates reversible oxidation and reduction processes with HOMO energy levels calculated to be in the range of 5.2 to 5.4 eV. Upon testing both materials for organic field-effect transistors (OFETs), PT3B1 showed a hole mobility of 6.1 × 10^-4 cm² V^-1 s ^-1, while PT2B2 did not show any field effect transport. Both copolymers displayed a photovoltaic response when combined with a methanofullerene as an electron acceptor. The best performance was achieved when the copolymer PT3B1 was blended with [70]PCBM in a 1:4 ratio, exhibiting a short-circuit current of 7.27 mA cm^-2, an open circuit voltage of 0.85 V, and a fill factor of 41% yielding a power conversion efficiency of 2.54% under simulated air mass (AM) 1.5 global (1.5 G) illumination conditions (100 mW cm^-2). Similar devices utilizing PT2B2 in place of PT3B1 demonstrated reduced performance with a short-circuit current of 4.8 mA cm ^-2, an open circuit voltage of 0.73 V, and a fill factor of 30% resulting in a power conversion efficiency of roughly 1.06%.

Full text of DOI:10.1039/c1jm10649j

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PAPER
Thiophene–benzothiadiazole–thiophene (D–A–D) based polymers: effect of
donor/acceptor moieties adjacent to D–A–D segment on photophysical and
photovoltaic properties†
Prashant Sonar,* Evan L. Williams, Samarendra P. Singh and Ananth Dodabalapur‡*
Received 14th February 2011, Accepted 20th April 2011
DOI: 10.1039/c1jm10649j
New push–pull copolymers based on thiophene (donor) and benzothiadiazole (acceptor) units,
poly[4,7-bis(3-dodecylthiophene-2-yl) benzothiadiazole-co-thiophene] (PT3B1) and poly[4,7-bis(3-
dodecylthiophene-2-yl) benzothiadiazole-co-benzothiadiazole] (PT2B2), are designed and synthesized
via Stille and Suzuki coupling routes respectively. Gel permeation chromatography shows the number
ꢀ1
average molecular weights are 31 100 and 8400 g mol for the two polymers, respectively. Both
polymers have shown absorption throughout a wide range of the UV-vis region, from 300 to 650 nm. A
significant red shift of the absorption edge is observed in thin films compared to solution of the
copolymers; the optical band gap is in the range of 1.7 to 1.8 eV. Cyclic voltammetry indicates reversible
oxidation and reduction processes with HOMO energy levels calculated to be in the range of 5.2 to 5.4
eV. Upon testing both materials for organic field-effect transistors (OFETs), PT3B1 showed a hole
ꢀ4
2
ꢀ1 ꢀ1
mobility of 6.1 ꢁ 10 cm V s , while PT2B2 did not show any field effect transport. Both
copolymers displayed a photovoltaic response when combined with a methanofullerene as an electron
acceptor. The best performance was achieved when the copolymer PT3B1 was blended with [70]PCBM
ꢀ2
in a 1 : 4 ratio, exhibiting a short-circuit current of 7.27 mA cm , an open circuit voltage of 0.85 V, and
a fill factor of 41% yielding a power conversion efficiency of 2.54% under simulated air mass (AM) 1.5
ꢀ2
global (1.5 G) illumination conditions (100 mW cm ). Similar devices utilizing PT2B2 in place of
ꢀ2
PT3B1 demonstrated reduced performance with a short-circuit current of 4.8 mA cm , an open circuit
voltage of 0.73 V, and a fill factor of 30% resulting in a power conversion efficiency of roughly 1.06%.
1
. Introduction
primarily controlled by the modulation in chemical structures
through changes in the conjugation backbone which allows for
band gap tuning. Of these materials, low band-gap conjugated
materials are of particular interest because of their utility in
harvesting visible and near infrared wavelength photons and
possible ambipolar charge transport properties in electronic
Conjugated semiconducting polymers, oligomers, and small
molecules are an important class of materials for the develop-
1
ment of printable and flexible electronics. These materials have
drawn significant attention over the last two decades for
a variety of organic electronic device applications [organic light
emitting diodes (OLEDs), organic field effect transistors
5
devices. One of the successful and elegant approaches to
making low band gap materials is the incorporation of strong
electron donating and electron accepting moieties [push–pull
6
system] along the conjugated backbone. The alternating donor
(
OFETs) and organic photovoltaics (OPVs)] due to their tune-
2–4
able optical and electrical characteristics.
In such devices,
active layers of the semiconducting materials on the order of
0–100 nm can be deposited between two electrodes and the
and acceptor arrangement causes the hybridization of the
highest occupied molecular orbital (HOMO) of the donor with
lowest unoccupied molecular orbital (LUMO) of the acceptor
and results in a reduction of the band gap. The incorporation of
a conjugated heterocyclic unit can greatly influence the optical,
electrochemical, morphological and electrical properties of
2
optoelectronic properties can be investigated. Photophysical
and electrochemical properties of these conjugated materials are
Institute of Materials Research and Engineering, Agency for Science,
Technology, and Research (A*STAR), 3 Research Link, Singapore,
7
conjugated materials. Additionally, this incorporation may
117602, Republic of Singapore. E-mail: sonarp@imre.a-star.edu.sg;
ananth.dodabalapur@engr.utexas.edu
enhance the intramolecular charge transfer, which in turn can
improve the charge carrier mobility. The utilization of thio-
phene as a donating moiety and 2,1,3-benzothiadiazole as an
accepting moiety is a well-studied and successful strategy for
†
1
‡
Electronic supplementary information (ESI) available. See DOI:
0.1039/c1jm10649j
Permanent address: Microelectronics Research Center, University of
8–12
Texas at Austin, Austin, TX, 78758, USA.
making low band gap materials.
Furthermore, nitrogen
1
0532 | J. Mater. Chem., 2011, 21, 10532–10541
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based heterocycles such as thiazole, thiadiazole, pyrazine and
their derivatives incorporated with thiophene in the conjugated
backbone have shown non-covalent sulfur–nitrogen (S/N)
interactions within the molecule. These interactions influence
The highest achieved power conversion efficiencies were 2.5%
and 1%, using a polymer to fullerene ratio of 1 : 4 for PT3B1 and
PT2B2, respectively. The detailed synthesis, characterization,
and correlation of structure to device performance are discussed
herein.
13
molecular assembly in the solid state. In an earlier report, we
successfully synthesized thiophene–benzothiadiazole based alkyl
end capped solution processable donor–acceptor (D–A) conju-
gated small molecules with varying ratios of thiophene and
2. Results and discussion
14
benzothiadiazole. In these materials, we observed the energy
levels, liquid crystallinity, and thin film morphological proper-
ties to all be influenced by the ratio of the donor–acceptor
moiety. Following successful utilization of these D–A small
molecules in solution-processed OFETs, we decided to synthe-
size analogous copolymers of these model compounds in order
to study structure–property relationships and performance in
both OFET and OPV devices. Polymers often display better
thin film-forming characteristics through various solution-based
processing techniques such as spin coating, dip coating and ink-
jet printing, than small molecules, which offer greater promise
2
.1 Syntheses and characterization
The synthetic strategy for making the soluble version of thio-
phene–benzothiadiazole–thiophene segment is outlined in
Scheme 1. In order to provide good solubility of the polymer,
a dodecyl group was introduced on thiophene. First, commer-
cially available 3-dodecyl thiophene (1) was brominated at the
2-position by using N-bromosuccinamide (NBS) in a mixture of
chloroform and acetic acid (1 : 1) solvent providing the 2-bromo-
3-dodecyl thiophene (2) (93% yield). Selective stannylation at the
2-position of 3-dodecyl thiophene was carried out by synthe-
sizing intermediate Grignard reagent (GR) using magnesium
turnings in dry tetrahydrofuran (THF). After the addition of
15
in terms of printing organic electronic devices. Conjugated
polymers comprising donor–acceptor blocks are an interesting
system to use as a tool to study the effect of materials design,
energy levels and optical shifts on the performance in bulk
heterojunction (BHJ) OPV devices. Whereas in OFET devices,
it is particularly interesting to study the charge transporting
properties of these D–A materials through variations in
designing such as strength of donor and acceptor units used in
the conjugated backbone, their location relative to each other
and the overall planarity.
ꢂ
tributyltin chloride at ꢀ78 C in the above synthesized GR gave
2-tributyltin-3-dodecylthiophene (3) (61% yield). Compound 3
was then reacted (3 equivalents) with commercially available 4,
7-dibromobenzo[c][1,2,5]thiadiazole in dry N,N-dimethylforma-
ꢂ
mide (DMF) in the presence of Pd(PPh ) catalyst at 80 C for
3
4
24 h by Stille coupling, which produced 4,7-bis(3-dodecylth-
iophene-2-yl)benzo[c][1,2,5]thiadiazole (5) (65% yield) after
column chromatography. Bromination of compound 5 was
carried out using NBS in DMF at room temperature for 24 h
There are reports of using a thiophene–benzothiadiazole–
thiophene segment with a variety of other comonomers such as
producing
4,7-bis(5-bromo-3-dodecylthiophen-2-yl)benzo[c]-
1
1a
11b,c
9a
indenofluorene, thienothiophene,
9b,c
cyclopentadithiophene,
[1,2,5]thiadiazole (6) (62% yield). This dibromo derivative
(compound 6) was used as a common block for polymerization
with other two comonomers such as thiophene (donor) and
benzothiadiazole (acceptor) respectively.
10
8b,c
fluorene, carbazole,
dibenzosilole
and longer oligothio-
12
phenes to yield low band gap copolymers. The thiophene–
benzothiadiazole–thiophene has thus been established as
a promising segment which can be attached with other novel and
interesting donor/acceptor conjugated units to make a library of
copolymers with tailored photophysical properties. It is well
known that a polymer’s molecular weight, solubility, thin film
morphology, crystallinity, band gap, and charge carrier mobility
are important parameters which may have great influence on the
device performance. In this article, we report the design and
synthesis of two donor–acceptor based copolymers poly[4,7-bis-
The synthesis of two new alternating copolymers comprised
of three thiophene with one benzothiadiazole units (PT3B1) and
two thiophene with two benzothiadiazole units in the conju-
gated backbone (PT2B2) is shown in Scheme 2. 2,5-Bi(trime-
thylstannyl)thiophene (7) and 4,7-bis(4,4,5,5-tetramethyl-1,3,2-
dioxabrolan-2-yl)benzothiadiazole
(8)
were
synthesized
(
(
3-dodecylthiophene-2-yl)
benzothiadiazole-co-thiophene]
PT3B1) and poly[4,7-bis(3-dodecylthiophene-2-yl) benzothia-
diazole-co-benzothiadiazole] (PT2B2). A thiophene–benzothia-
diazole–thiophene segment with either thiophene or
a
benzothiadiazole comonomer is correspondingly used for
making these copolymers, with the objective to investigate the
effect of adjacent donor and acceptor comonomer units on
optoelectronic properties. These materials were characterized by
NMR spectroscopy, gel permeation chromatography, UV-Vis
absorption and photoluminescence spectroscopy, thermogravi-
metric analysis and cyclic voltammetry. OFETs made from
PT3B1 on octyltrichlorosilane (OTS) treated Si/SiO substrate
2
Scheme 1 Reagents and conditions: (i) N-bromosuccinamide, CHCl
3
:
ꢀ
4
2
ꢀ1 ꢀ1
showed hole mobilities of 6.1 ꢁ 10 cm V
s
using gold for
CH COOH (1 : 1), room temp., overnight, 93%; (ii) magnesium turnings,
3
source and drain electrodes. Bulk heterojunction OPV devices
were fabricated by blending the donor copolymer with [6,6]
phenyl-C71-butyric acid methyl ester (PC70BM) as the acceptor.
ꢂ
anhydrous THF, reflux for 2 h, tributyltin chloride, ꢀ78 C, overnight,
83%; (iii) Pd(PPh ) , anhydrous DMF, 81%; (iv) N-bromosuccinamide,
3
4
DMF, 24 h, 88%.
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thermogravimetric analysis (TGA) under a nitrogen flow.
ꢂ
PT3B1 and PT2B2 polymers show 5% weight loss at 449 C and
ꢂ
4
33 C, respectively, which indicates an excellent thermal
stability of the materials. GPC and thermal data have been
summarized in Table1. DSC characterization was carried out up
ꢂ
to 300 C but no thermal transition in the bulk sample was
observed.
2
.2 Optical properties
The absorption spectra of both the copolymers were measured in
chloroform solutions (1 ꢁ 10ꢀ M based on repeat units) and are
shown in Fig. 1a (Table 2). Both copolymers show absorption
peaks in short and long wavelength regions. These peaks are
related with the p–p* transition band and charge transfer tran-
sition between donor and acceptor blocks. The copolymer
PT3B1 shows short wavelength absorption peaks at 320 and 384
nm, respectively, which could be attributed to the oligothiophene
segment. A longer wavelength peak is seen at 500 nm and is
Scheme 2 Reagents and conditions: (i) Pd
ꢂ
2
(dba)
3
, P(o-tolyl)
3
P, chloro-
, 2 M
5
benzene, reflux, 120 C, 72 h, 90%; (ii) Aliquat 336, Pd(PPh
3
)
4
2 3
K CO , toluene, reflux, 48 h, 87%.
16
according to a reported procedure. Compounds 6 and 7 were
reacted in a Schlenk flask using tris(dibenzylideneacetone)-
2 3 3
dipalladium(0) (Pd dba ), tri(o-tolyl) phosphine [P(o-tol) ] as
the catalytic system in chlorobenzene solvent and then refluxed
ꢂ
at 110 C under argon atmosphere for 72 h via Stille coupling.
attributed to charge transfer associated with oligothiophene–
11b,c,17
After completion of the copolymerization, the polymer PT3B1
benzothiadiazole (D–A) interaction.
In the case of PT2B2
was precipitated out in methanol. Another copolymerization
ꢂ
peaks are located at 320 nm in short wavelength and 521 nm in
longer wavelength regions. The peak at 320 nm is common in the
absorption spectra of both PT3B1 and PT2B2 polymers, though
weaker in the former. The peaks, in long and short wavelength
regions, observed for both polymers are red shifted with respect
to our earlier reported small molecule (T3B1 and T2B2) model
was carried out at 80 C under argon atmosphere for 72 h using
compounds 6 and 8 in the presence of Pd(PPh ) as a catalyst,
3
4
and 2 M K CO as a base in toluene via Suzuki coupling. The
3
2
reaction was terminated by adding the bromobenzene and
phenyl boronic acid, respectively, and stirred additionally for
another 5 h. The resulting mixture was poured into a mixture of
methanol and 2 M HCl and stirred for few hours to precipitate
the polymer. Such a non-solvent precipitation is a common
method to recover the polymer sample e.g. polythiophene dilute
solution in organic solvent can be precipitated either in meth-
anol or acetone antisolvent. Both crude polymers were then
subjected to purification using Soxhlet extraction for 2 days
using methanol, acetone, and hexane solvents, respectively, for
the removal of oligomer and catalytic impurities. The residue
was finally extracted with chloroform and precipitated again
from methanol, filtered, washed with methanol and dried.
Purified polymers PT3B1 and PT2B2 showed the
14b
compounds. Such kinds of multiple absorption bands have
been noticed in various donor–acceptor based conjugated
ꢀ
1
number average molecular weights (M
n
) 31 100 g mol and
ꢀ1
8
400 g mol with polydispersity index (PDI) of 1.80 and 1.36,
respectively, measured by Gel Permeation Chromatography
ꢂ
(
GPC) at a column temperature of 40 C with THF as an eluent
and polystyrene (PS) as standards. Both polymers are nicely
soluble in many organic solvents such as THF, chloroform,
toluene, chlorobenzene and dichlorobenzene. These polymers
also exhibit nice film forming properties and large flakes of
polymer films were obtained after precipitation in a non-solvent.
The thermal stability of the polymers was analyzed by
Table 1 Polymerization results and thermal stability of the copolymers
a
ꢀ1
b
w
ꢀ1
c
T
d
Polymers
M
n
/g mol
M
/g mol
PDI
PT3B1
PT2B2
31 100
8400
56 100
11 500
1.80
1.36
449
433
a
b
Number-average molecular weight. Weight-average molecular weight
c
equivalent to PS. Decomposition temperature (with 5% weight loss)
Fig. 1 (a) Normalized UV-vis and (b) emission spectrum of PT3B1 and
ꢀ
2
determined by TGA under N .
5
PT2B2 in chloroform solution (10 M).
1
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Table 2 Optical (UV-vis and PL) and electrochemical properties of the copolymers
a
UV-Vis solution lmax / UV-Vis thin film lmax / Optical band gap / PL solution in film lmax
b
c
a
/
Electrochemical band
e f
d
HOMO LUMO gap /eV
Polymers nm
nm
eV
nm
PT3B1 384, 500
PT2B2 320, 521
395, 539
338, 589
1.80
1.71
618
662
5.20
5.40
3.24
3.60
1.96
1.80
a
b
Diluted solution in chloroform. Polymer film on a glass plate by spin coating from chloroform solution. Optical band gap calculated from the
c
d
e
f
absorption onset of the thin films. Measured from the oxidation onset of the CV. Obtained from the reduction onset of the CV. Obtained from
the HOMO–LUMO gap.
systems both in oligomers and polymers due to donor–acceptor
solution as shown in Fig. 2. In films, the peak bandwidth of the
copolymers is broader and red shifted as compared to in solu-
tion. Solid state absorption maxima were recorded at 395 and
539 nm, for PT3B1 films which is 11 nm and 39 nm red shifted,
as compared to solution, in shorter and longer wavelength
regions, respectively. Whereas in the case of PT2B2, the
measured thin film absorption maxima at 338 and 589 nm are
red shifted by18 nm and 68 nm, as compared to solution
spectra. This red shift is associated with p–p stacking and
interchain interactions of the polymeric chain in solid state.
Optical band gaps, calculated from absorption cut off values,
are determined to be 1.78 and 1.71 eV for PT3B1 and PT2B2
respectively (these values are lowered by 0.25 and 0.50 eV than
T3B1 and T2B2 small molecule model compounds reported
8–13
orbital hybridization.
From the comparison of the optical
data from both copolymers, it is clear that PT2B2 shows a red
shift of 21 nm (longer wavelength region) relative to PT3B1. This
shift is attributed to the more extended delocalization of
p-electrons due to the increased benzothiadiazole content in the
D–A system. The increased benzothiadiazole content could lead
18
to a stronger internal charge transfer interaction. PT3B1 shows
a peak at 384 nm that is not observed in PT2B2. The origin/
absence of this peak is not well understood, but it is clear that
PT2B2 lacks the adjacent thiophene units that are present in
PT3B1.
The fluorescence spectra for both copolymers in chloroform
are shown in Fig. 1b (Table 2). PT3B1 and PT2B2 copolymers
exhibited weak red to dark red emission with maxima at 618 and
14b
earlier ). Materials with such a low band gap and wide
absorption range are expected to have potential for OPV
devices.
6
34 nm, respectively. It is highly expected that the weak fluo-
rescence is due to the fast intramolecular charge transfer between
donor and acceptor units after photoexcitation. The 16 nm red
shift in emission for polymer PT2B2, compared to PT3B1, is
attributed to the more extended conjugation of alternative
thiophene–benzothiadiazole segments and the strong donor–
acceptor interaction in the former. It is noteworthy that the
difference in absorption and emission peaks is greater for PT2B2
than for PT3B1 (141 nm and 118 nm, respectively). From the
optical data of both copolymers, it is evident that the donor–
acceptor interaction in the backbone strongly depends on several
factors i.e. strength of donors and acceptors, their attachment in
backbone, steric hindrance between the blocks and planarity of
the polymeric chains.
2
.3 Electrochemical properties
The electrochemical properties of the thiophene–benzothiadia-
zole based copolymers were examined by cyclic voltammetry
(CV). Using this technique the energy levels of the copolymer
such as HOMO and LUMO can be determined. These energy
levels are crucial for the selection of appropriate acceptor
materials in OPV blends. CV measurements were carried out in
a 0.1 M tetrabutylammonium tetrafluoroborate (Bu NBF )
4
4
solution in acetonitrile at room temperature using a scan rate of
ꢀ1
50 mV s under nitrogen. A Pt electrode coated with the
polymer film by drop casting was used as working electrode and
the Ag/AgCl electrode was used as reference electrode. The
HOMO and LUMO levels of the copolymer were calculated
from the oxidation and reduction onset potential relative to
ferrocene as an internal standard, by treating the ionisation
Solid state absorption measurements of these newly synthe-
sized copolymers were compared with standard P3HT, widely
used donor in the OPV system. P3HT, PT3B1 and PT2B2 thin
films were processed by spin coating on glass from chloroform
+
potential as ꢀ4.8 eV for the ferrocene/ferrocenium (Fe/Fe )
redox system. The onset potentials were determined from the
intersection of two tangents drawn as the rising current and
baseline charging current of the CV traces. The electrochemical
data for the two copolymers are shown in Fig. 3 and summa-
rized in Table 2. Both copolymers exhibited a predominant
oxidation peak (anodic peak) due to the electron donating oli-
gothiophene units and a slight reduction peak related with the
electron withdrawing benzothiadiazole moiety. The oxidation
onsets for PT3B1 and PT2B2 copolymers were observed at
1
.08 V and 1.28 V relative to ferrocene, respectively. Using these
values, HOMO energy levels (EHOMO) for PT3B1 and PT2B2
copolymers were calculated as 5.20 eV and 5.40 eV, respectively.
LUMO values for PT3B1 and PT2B2 were 3.24 eV and 3.60 eV,
Fig. 2 Normalized UV-vis spectrum of P3HT, PT3B1 and PT2B2 thin
film layer deposited on glass from chloroform solution.
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2
.4 Organic field-effect transistors (OFETs)
Polymer OFETs were fabricated using bottom gate top contact
device geometry with 40–50 nm solution processed thin films of
PT3B1 and PT2B2 channel semiconductors. Out of the two
polymers, only PT3B1 exhibited p-channel OFET behaviour
whereas polymer PT2B2 did not show any channel conductance.
In our earlier study for small molecule donor–acceptor systems,
we also did not observe OFET performance for the T2B2
small molecule whereas for T3B1 we observed a mobility of
2
2
ꢀ1 ꢀ1 14b
2
.0 ꢁ 10ꢀ cm V s . One of the possibilities for no charge
accumulation of PT2B2 thin film in OFET devices could be
related with the structural disorder at the semiconductor/dielec-
tric interface leading to charge trapping. The output and transfer
characteristics of OFETs using PT3B1 channel semiconductor
are shown in Fig. 5. The devices exhibit p-channel performance
with the mobility, calculated from the saturation regime, of
4
2
ꢀ1 ꢀ1
6
.1 ꢁ 10ꢀ cm V
s
and threshold voltage of ꢀ15.5 Volts. The
substrates have
OFET devices fabricated on OTS treated Si/SiO
2
shown more than one order of magnitude higher hole mobility as
compared to the transistors fabricated on bare Si/SiO substrates
2
ꢀ
5
2
ꢀ1 ꢀ1
(
m ¼ 3.0 ꢁ 10 cm V s ). Higher hole mobility exhibited in
h
OFET devices fabricated on OTS treated substrates can be
attributed to better packing of PT3B1 polymer thin films on
20
SAM treated substrates. This mobility values are within the
range of previously reported thiophene–benzothiadiazole based
21
copolymers.
Fig. 3 Cyclic voltammograms showing the first scan of cathodic and
anodic cycles of PT3B1 and PT2B2 spin coated polymers on working
ꢀ
1
electrode at a scan rate 50 mV s in acetonitrile. The electrolyte was
.1 M TBAPF
0
6
.
2
.5 Organic photovoltaic (OPV) devices
Organic photovoltaic devices were made utilizing either PT3B1
or PT2B2 as the donor material, paired with [70]PCBM as the
acceptor (note, devices utilizing [60]PCBM instead showed
a much reduced short circuit current, suggesting significant
photocurrent can be attributed to absorption by [70]PCBM and
subsequent hole transfer to the polymer). The donor–acceptor
ratio and thickness were varied for the optimization of devices in
an attempt to maximum device performance. Devices made with
a 1 : 4 ratio demonstrated better performance than devices with
1 : 1 or 1 : 2 ratios, for both donor polymers. An active layer
thickness around 60 nm proved optimal and the highest achieved
power conversion efficiencies were 2.54% and 1.06% for PT3B1
and PT2B2-based devices, respectively. Current density–voltage
curves for 1 : 4 ratio devices of varied thickness, under
calculated from the difference between optical band gap and
HOMO values. The polymer PT2B2 shows a lower lying
HOMO level than PT3B1, this is due to shifting of the LUMO
further away from vacuum caused by the stabilization effect due
to the two electron-withdrawing benzothiadiazole moieties in
19
the polymer backbone. Due to the higher number of thio-
phenes in PT3B1 (increased donor nature), the HOMO level is
higher than PT2B2. As expected, PT2B2 shows a lower LUMO
level than PT3B1 which can be correlated to the higher content
19
of benzothiadiazole units. These studies confirm that the
composition of donor and acceptor moieties in the polymeric
backbone can alter the energy levels. An energy level diagram of
the OPV device structure using these new donor polymers
(
PT3B1 and PT2B2) and the PCBM acceptor is shown in Fig. 4
with references to the vacuum level.
Fig. 4 Energy level diagram of OPV device structure using new donor
polymers (PT3B1 and PT2B2) and PCBM acceptor references to the
vacuum level.
Fig. 5 (a) Output (VGS ¼ 0 V to ꢀ70 V) and (b) transfer (VDS ¼ ꢀ70 V)
characteristics of OTFTs with PT3B1 thin films.
1
0536 | J. Mater. Chem., 2011, 21, 10532–10541
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known to adversely affect the V , and the very low fill factor for
oc
PT2B2 devices may also be indicative of significant recombina-
tion. Although slightly higher, close to 40%, the fill factor for
PT3B1 devices suggests that charge transport of electrons and
holes may not be balanced for OPV devices employing either of
these donor polymers.
PT2B2-based devices show a pronounced kink in the J–V
curve, as can be seen for all thicknesses of 1 : 4 blend ratio
devices in Fig. 6. Although not shown, it should be noted that
this kink was evident at 1 : 1 and 1 : 2 blend ratios, as well. A
kink in the J–V curve has been observed in a range of solar cell
22–24
systems,
including CIGS, DSSC, and a number of organic
photovoltaics, and with a variety of characteristics and shapes. It
is commonly and generally attributed to an interfacial effect and/
or poor or unbalanced charge carrier collection. It is interesting
to note that the kink displayed here is predominantly a
th
4
quadrant characteristic; detailed evaluation of this kink
is beyond the scope of this paper and will be discussed
elsewhere.
A strong dependence of performance on active layer thickness
is clearly evident from examination of the cells’ Jsc values. It is of
interest to look at the IPCE spectra with thickness for these
devices in Fig. 7. A clear change in shape occurs over a fairly
small change in thickness, as revealed by PT3B1 : [70]PCBM
(
1 : 4) devices of 45, 55, and 85 nm. A similar trend with thickness
is observed for PT2B2 devices. The significant amount of overlap
between the absorption spectra of the donor polymers and [70]
PCBM makes it difficult to unequivocally attribute changes to
primarily the donor or acceptor. Qualitatively, the thinnest
devices have a more pronounced blue response, while thicker
devices show a more significant red response. Optical absorption
measurements revealed spectra that closely resembled that of the
fullerene, which might be expected given the 1 : 4 ratio in
composition. Optical interference effects have been known to
influence device performance, but it would seem unlikely that
such relatively small changes in thickness could result in these
significant changes over such a broad spectral range. Morpho-
logical effects, changes in donor–acceptor segregation and
intermixing, may be playing an important role in determining the
IPCE spectra. Likewise, possible differences in blend
morphology, between PT3B1 devices and PT2B2 devices, make
it difficult to ascertain the extent of the role of the varied energy
levels on device performance.
Fig. 6 Current density–voltage curves for various thicknesses of PT3B1
and PT2B2 devices, as blended with [70]PCBM in a 1 : 4 ratio.
illumination, are shown in Fig. 6, and solar cell characteristics
are summarized in Table 3.
Devices employing PT3B1 showed significantly higher Jsc
values than devices with PT2B2. The lower lying LUMO level of
PT2B2 suggests that exciton dissociation by the fullerene may
not be as efficient as in the PT3B1 case, yielding less photocur-
rent. The Voc of organic photovoltaic devices is often correlated
with the difference between the donor HOMO level and the
acceptor LUMO level from which holes and electrons, respec-
tively, are extracted. Despite the lower lying HOMO level of
PT2B2, devices incorporating this polymer displayed a lower Voc
than devices using PT3B1. Recombination of charge carriers is
Table 3 Summary of photovoltaic device performance
ꢀ
2
Donor
Acceptor
Ratio
Thickness/nm
Jsc/mA cm
V
oc/V
FF (%)
PCE (%)
PT2B2
PT2B2
PT2B2
PT2B2
PT2B2
PT2B2
PT2B2
PT3B1
PT3B1
PT3B1
PT3B1
PT3B1
PT3B1
PT3B1
[70]PCBM
[70]PCBM
[70]PCBM
[70]PCBM
[70]PCBM
[70]PCBM
[60]PCBM
[70]PCBM
[70]PCBM
[70]PCBM
[70]PCBM
[70]PCBM
[70]PCBM
[60]PCBM
1 : 4
1 : 4
1 : 4
1 : 4
1 : 1
1 : 2
1 : 4
1 : 4
1 : 4
1 : 4
1 : 4
1 : 1
1 : 2
1 : 4
30
40
60
100
60
65
60
35
45
55
85
60
60
60
2.4
3.9
4.8
3.7
1.7
3.9
0.93
3.4
5.33
7.27
7.13
1.5
5.6
3.3
0.64
0.63
0.73
0.70
0.73
0.74
0.71
0.81
0.85
0.86
0.85
0.88
0.93
0.78
31
28
30
28
21
25
27
42
39
41
35
24
33
34
0.48
0.7
1.06
0.73
0.26
0.73
0.18
1.15
1.79
2.54
2.12
0.32
1.7
0.88
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J. Mater. Chem., 2011, 21, 10532–10541 | 10537

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spectra were obtained on a Bruker Autoflex TOF/TOF instru-
ment using dithranol as a matrix. GPC analysis against PS
standards was performed in THF on a Waters high pressure
GPC assembly with separation model 2690 and Evaporative
Light Scattering (ELS 2420) detector. The pore sizes of 3 Phe-
6
4
ꢀ
nogel columns in use are 10 , 10 , and 500 A. The flow rate of
ꢀ
1
ꢂ
THF is 1 mL min and column oven temperature is 40 C. A
typical concentration 1.5 mg weight of polymer dissolved in 1 mL
of THF was used for running GPC samples. UV-Vis spectra were
recorded on a Shimadzu model 2501-PC. Photoluminescence
(
PL) spectra were measured on a Perkin-Elmer (LS50B) spec-
trofluorimeter. Cyclic voltammetry experiments were performed
using an Autolab potentiostat (model PGSTAT30) by Echochi-
mie. All CV measurements were recorded in solid state with
0
.1 M tetrabutylammonium hexafluorophosphate as the sup-
ꢀ1
porting electrolyte (scan rate of 50 mV s ). The experiments
were performed at room temperature with a conventional three-
electrode configuration consisting of a platinum disc working
electrode, a gold counter electrode, and an Ag/AgCl reference
electrode. Differential scanning calorimetry (DSC) was carried out
under nitrogen on a TA Instrument DSC Q100 (scanning rate of
ꢂ
ꢀ1
1
0 C min ). Thermal gravimetric analysis (TGA) was carried out
ꢀ1
ꢂ
using a TA Instrument TGA Q500 (heating rate of 10 C min ).
Synthesis of 2-bromo-3-dodecylthiophene (2). To a solution of
Fig. 7 IPCE spectra of PT3B1 and PT2B2 devices, as blended with [70]
3
-dodecylthiophene (5.00 g, 19.8 mmol) in the mixture of CHCl
20 mL) and CH COOH (20 mL) N-bromosuccinamide (NBS)
3.52 g 19.8 mmol) was added portion wise over a period of 1 h.
3
PCBM in a 1 : 4 ratio.
(
(
3
3
. Conclusion
The reaction mixture was stirred overnight at room temperature.
After completion of the reaction it was then poured into water
and extracted with chloroform. The combined organic phase was
washed with chloroform, brine solution and dried over magne-
sium sulfate. After filtration, the chloroform was removed on
a rotavap and the crude product was purified by column chro-
matography using hexane as the solvent (6.00 g, 93%).
Novel
push–pull
thiophene–benzothiadiazole–thiophene
(
D–A–D) based low band gap organic semiconducting copoly-
mers have been designed, synthesized and characterized. The
effect of a donating or accepting moiety adjacent to the thio-
phene–benzothiadiazole–thiophene (D–A–D) segment on
optical properties, energy levels, and performances in OFET and
OPV devices has been investigated. Solution processed thin film
photovoltaic devices utilizing the corresponding donor polymers
with a PCBM acceptor exhibited promising performance. A
highest power conversion efficiency of 2.54% has been achieved
for devices using PT3B1 donor polymer with [70]PCBM as
the acceptor. These findings suggest that the careful design of
D–A–D blocks in the polymer backbone can alter the photo-
physical properties.
1
H NMR (400 MHz, CDCl , ppm): d ¼ 0.88 (t, 3H, CH ,
3
3
J ¼ 6.7 Hz), 1.29 (m, 18H, 9CH
2
), 1.62 (q, 2H, CH
2
, J ¼ 7.2 Hz,
J¼ 7.2 Hz), 2.55 (t, 2H, CH , J ¼ 7.5 Hz), 6.77 (d, 2H, Th,
2
J ¼ 5.8 Hz), 7.18 (d, 2H, Th, J ¼ 5.8 Hz).
Synthesis of 2-tributylstannyl-3-dodecylthiophene (3). Magne-
sium turnings (0.400 g 16.6 mmol) in anhydrous THF (15.0 mL)
were heated to mild reflux until totally disappearing. This solu-
tion was added drop wise to the solution of 2-bromo-3-dode-
cylthiophene (5.0 g, 15 mmol) in 15 mL of THF. The reaction
mixture was refluxed for 2 h before being transferred to a solu-
tion of tributyltin chloride (4.50 mL, 16.02 mmol) in 25 mL of
4
. Experimental
General
ꢂ
anhydrous THF at ꢀ78 C. The mixture was allowed to warm to
All the chemicals were purchased from Strem, Acros and Sigma-
Aldrich and used without further purification. All reactions were
carried out using Schlenk techniques in an argon or nitrogen
atmosphere with anhydrous solvents.
room temperature and stirred overnight before pouring into
water. The aqueous layer was extracted with hexanes, and the
combined organic phase was washed with brine and dried over
MgSO
4
. The crude compound was used as such for the next step
(
5.00 g, 61%).
1
H NMR (400 MHz, CDCl
3
): d ¼ 0.89–1.60 (m, 50H, 4CH
3
,
Characterization
1
9CH ), 2.63 (t, 2H, CH , J ¼ 7.8 Hz), 7.11 (d, 1H, Th, J ¼ 4.8
2
2
1
13
H and C NMR data were performed on a Bruker DPX
00 MHz spectrometer with chemical shifts referenced to
residual CHCl and H O in CDCl . Matrix assisted laser
desorption/ionization time-of-flight (MALDI-TOF) mass
Hz), 7.54 (d, 2H, Th, J ¼ 4.8 Hz).
4
Synthesis of 4,7-bis(3-dodecylthiophene-2-yl)benzo[c][1,2,5]-
thiadiazole (5). 2-(Trimethylstannyl)-3-dodecylthiophene (4.50 g,
3
2
3
1
0538 | J. Mater. Chem., 2011, 21, 10532–10541
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1
0. 60 mmol) and 4,7-dibromobenzothiadiazole (1.06 g, 3.62
Synthesis of 4,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-
yl)benzo[c][1,2,5]thiadiazole (8). 4,7-Dibromo-2,1,3-benzothiadia-
zole (5) (4.0 g, 13.6 mmol), bis(pinacolato)diboron (8.0 g,
mmol) were dissolved in dry DMF (50 mL). Then Pd (PPh3)4
(
0.125 g, 3 mol%) was added to the above reaction mixture
ꢂ
under argon and heated to reflux at 110 C for 48 hours. The
mixture was allowed to cool to room temperature and poured
into water. The aqueous layer was extracted with dichloro-
methane, and the combined organic phase was washed with
2
31.2 mmol), PdCl (dppf) (2.0 g, 2.4 mmol), and KOAc (8.0 g,
80 mmol) were kept under vacuum for 10 min and then degassed
before 1,4-dioxane (50 mL) was added under argon. The reaction
ꢂ
mixture was stirred at 80 C overnight and quenched by adding
brine and dried over MgSO
4
. The crude product was purified
water. The resulting mixture was extracted with ethyl acetate
(100 mL). The combined organic layers were washed with brine,
by column chromatography using hexane : DCM as eluent
(
1.5 g, 65%).
1
2 4
dried over Na SO , and filtered. After removing the solvent, a dark
H NMR (400 MHz, CDCl ): d ¼ 0.88 (t, 3H, CH ,
red solid was obtained, which was purified by silica gel chroma-
tography by using 3% ethyl acetate in hexane as eluent to give the
3
3
J ¼ 6.8 Hz), 1.23 (m, 18H, CH ), 1.63 (q, 2H, CH , J ¼ 7.8 Hz,
2
2
J ¼ 7.4 Hz), 2.67 (t, 2H, CH , J ¼ 7.5 Hz), 7.11(d, 2H, Th,
title compound as a pink solid (2.4 g, 46%).
1
2
J ¼ 5.4 Hz), 7.45 (d, 2H, Th, J ¼ 5.3 Hz), 7.63(s, 1H, btz).
H NMR (400 MHz, CDCl ): d ¼ 8.12 (s, 2H), 1.43 (s, 24H).
3
+
MALDI-TOF-MS (dithranol) m/z: 635.68 (M ); calcd for
13
C NMR (100 MHz, CDCl
3
, ppm): d ¼ 157.55, 138.11, 84.91,
C
38
H
56
N
2
S
3
¼ 636.36.
25.3.
MALDI-TOF-MS (dithranol) m/z:
¼
388.0; calcd for
S: C,
Synthesis of 4,7-bis(5-bromo-3-dodecylthiophen-2-yl)benzo[c]-
1,2,5]thiadiazole (6). 4,7-Bis(3-dodecylthiophene-2-yl)benzo[c]
C
18
H
26
2
B N
2
O
4
S ¼ 388.18. Anal. Calcd for C18
2 2 4
H26Br N O
[
55.71; H, 6.75; N, 7.22; S, 8.26%. Found: C, 55.65; H, 6.82; N,
7.16; S, 8.15%.
[
1,2,5]thiadiazole (1.27 g 2.00 mmol) was dissolved in 25 mL
dimethylformamide (DMF) under argon in the absence of light.
N-Bromosuccinamide (NBS) (0.90 g 5.040 mmol) was dissolved
in another 25 mL dimethylformamide (DMF) and added drop
wise through a dropping funnel to the above reaction mixture.
The resulting reaction mixture was stirred overnight at room
temperature. The reaction was worked up by pouring the reac-
tion mixture into water and extracting the aqueous layer with
dichloromethane, and the combined organic phase was washed
Synthesis of poly[4,7-bis(3-dodecylthiophene-2-yl) benzothia-
diazole-co-thiophene] (PT3B1). To a 50 mL Schlenk flask, 4,7-bis
(5-bromo-3-dodecylthiophen-2-yl)benzo[c][1,2,5]thiadiazole
6
(0.400 g, 0.50 mmol) and 2,5-bis(trimethylstannyl)thiophene
(0.207 g, 0.50 mmol), tris(dibenzylideneacetone)dipalladium(0)
(9.14 mg, 0.001mmol), tri(o-tolyl)phosphine (12.16 mg,
0.04 mmol), and anhydrous chlorobenzene (20 mL) were charged.
The solution was purged with argon for 30 min, and then the
4
with brine and dried over MgSO . The crude compound was
ꢂ
purified by column chromatography using hexane : DCM as
reaction mixture was stirred at 80 C for 2 d. On the third day,
eluent (1.0 g, 62%).
1
H NMR (400 MHz, CDCl
0.5 mL of bromobenzene was added to the reaction mixture to
react with the unreacted trimethylstannyl end groups. The
3
): d ¼ 0.89 (t, 3H, CH
3
,
ꢂ
J ¼ 6.7 Hz), 1.23 (m, 18H, CH ), 1.61 (q, 2H, CH , J ¼ 6.8 Hz,
mixture was further stirred at 80 C for 6 h before cooling down
2
2
J ¼ 8.3 Hz), 2.64 (t, 2H, CH , J ¼ 7.5 Hz), 7.06 (s, 1H, Th), 7.60
to room temperature, and poured into 200 mL of stirring meth-
anol. The solid was filtered off, washed with methanol, and dried.
The solid was further purified by Soxhlet extraction using
acetone, methanol and hexane, respectively, and then finally
dissolved with chloroform. A dark red, shiny, metallic solid was
obtained after removing the solvent and precipitating in methanol
2
1
3
(
s, 1H, btz). C NMR (100 MHz, CDCl , ppm): d ¼ 154.33,
3
1
2
42.89, 134.00, 132.380, 130.05, 127.09, 113.54, 32.27, 30.85,
9.97, 29.81, 29.69, 23.02, 14.42.
MALDI-TOF-MS (dithranol) m/z: 793.59(M + 1); calcd for
C
38
H
54Br
2
N
2
S
3
¼ 792.18. Anal. Calcd for C38
2 2 3
H54Br N S : C,
5
3
7.42; H, 6.85; N, 3.52; S, 12.10%. Found: C, 57.38; H, 6.79; N,
.46; S, 12.25%.
(0.365 g, 97%). M /M
w
n
(GPC) ¼ 56 180/31,170. UV-vis: 384 and
500 nm (in chloroform); 395 and 539 nm (thin film).
1
H NMR (400 MHz, CDCl , d): 0.88 (m, 3H, CH ), 1.26 (m, 18H),
3
3
Synthesis of 2,5-bis(trimethylstannyl)thiophene (7). A hexane
1.70 (m, 2H), 2.72 (m, 2H), 7.10–7.30 (br, 2H), 7.71 (s, 1H).
solution of butyllithium (1.6 M, 30 mmol, 19 mL) was added to a
ꢂ
THF (50 mL) solution of thiophene (1.26 g, 15 mmol) at ꢀ78 C.
Synthesis of poly[4,7-bis(3-dodecylthiophene-2-yl) benzothia-
Upon formation of white precipitate, the mixture was warmed to
rt and stirred for 1 h followed by addition of trimethyltin chlo-
ride. The clear solution was heated to gentle reflux for 1 h, and it
turned cloudy. The reaction mixture was poured into water. The
diazole-co-benzothiadiazole] (PT2B2). To a Schlenk flask, 4,7-bis
(5-bromo-3-dodecylthiophen-2-yl)benzo[c][1,2,5]thiadiazole
6
(0.300 g, 0.37 mmol) and 4,7-bis(4,4,5,5-tetramethyl-1,3,2-diox-
abrolan-2-yl)benzothiadiazole (0.150 g, 0.37 mmol), 2 M
organic layer was separated and dried over MgSO
4
. After
2 3
aqueous K CO solution (5 mL) and 2 drops of Aliquat 336 were
removal of the solvent by evaporation, the residue was recrys-
tallized 3 times from isopropanol to obtain the title compound
dissolved in toluene (10 mL). The solution was purged with
argon for 30 min, and then tetrakis(triphenylphosphine)palla-
dium (13 mg, 0.011 mmol) was added. The reaction was stirred at
(
3.9 g, 60%).
1
H NMR (400 MHz, CDCl
ꢂ
3
): d ¼ 0.37 (s, 18H), 7.38 (s, 2H).
80 C for 3 d. Then a toluene solution of phenyl boronic acid was
13
C NMR (100 MHz, CDCl , ppm): d ¼ ꢀ7.86, 136.21, 144.42.
added and the mixture was stirred for additional 4 h, followed by
addition of a few drops of bromobenzene and stirred overnight.
The resulting mixture was poured into a mixture of methanol and
water and stirred overnight. The dark precipitate was re-dis-
solved in chloroform and added drop wise to methanol (250 mL).
3
+
MALDI-TOF-MS (dithranol) m/z: ¼ 408, 410, 412 (M , three
most intense peaks of the isotope pattern); calcd for C H SSn ¼
10
20
2
4
11.93. Anal. Calcd for C H SSn : C, 29.31; H, 4.92; S, 7.83%.
10 20 2
Found: C, 29.25; H, 4.98; S, 7.75%.
This journal is ª The Royal Society of Chemistry 2011
J. Mater. Chem., 2011, 21, 10532–10541 | 10539

View Article Online
The resulting solid was filtered off and subjected to Soxhlet
extraction for 2 d in methanol, acetone, and hexane for the
removal of oligomers and catalytic impurities. The remaining
polymer was extracted with chloroform and precipitated again
from methanol, filtered, washed with methanol, and dried under
was set using a reference cell (Hamamatsu S1787-04) and the
calculated spectral mismatch factor.
Acknowledgements
The authors acknowledge the Visiting Investigatorship Pro-
gramme (VIP) of the Agency for Science, Technology and
Research (A*STAR), Republic of Singapore, for financial
support. We are also thankful to Chua Pei Lin and Pang Lusha
from Republic Polytechnic, Singapore, for scaling up starting
precursors during their attachment program with IMRE.
w n
vacuum at room temperature (0.300 g, 69% yield). M /M
(
GPC) ¼ 8490/11 560. UV-vis: 320, 521 nm (in chloroform); 338,
5
89 nm (thin film).
1
3 3
H NMR (400 MHz, CDCl , d): 0.87 (m, 3H, CH ), 1.20–1.35
(
m, 18H), 1.78 (m, 2H), 2.84 (m, 2H), 7.80 (s, 1H), 7.93 (s, 1H),
8
.20 (s, 1H).
OFET fabrication and characterization. Top contact/bottom
+
gate OTFT devices were fabricated using p -Si/SiO substrates
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film was spin cast using 8 mg mL solution in chloroform on the
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polymer active layer, a roughly 100 nm thick gold (Au) thin film
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(
(
15 min), de-ionized water (15 min), acetone (15 min), methanol
15 min) and isopropanol (15 min). The substrates were then
ꢂ
dried at 80 C for several hours in an oven to remove residual
solvents. The substrates were the subjected to oxygen plasma
cleaning for 10 minutes prior to the spin coating of a 40 nm thick
PEDOT:PSS hole transporting layer (Clevios P VP AI 4083). The
active layer of PT3B1 : [70]PCBM or PT2B2 : [70]PCBM in
varied ratios and concentrations was spin coated to obtain
a range of thicknesses. An aluminium cathode was deposited by
thermal evaporation through a shadow mask under a pressure of
8
9
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roughly 10 mbar to complete the devices, having device square
2
area of 9 mm . The IPCE (Incident Photon-to-current Conver-
sion Efficiency) was measured with a testing system consisting of
an Oriel 300 W Xe lamp in combination with a monochromator
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(
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Research Systems, SRS 810); the incident light intensity was
determined by a calibrated Si photodiode. The current–voltage
characteristics were measured using a source meter (Keithley
2
400), while irradiance was provided by a solar simulator
(
Steuernagel, Germany model 535). The simulator lamp intensity
1
0540 | J. Mater. Chem., 2011, 21, 10532–10541
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