Synthesis and characterization of new thieno[3,4-c]pyrrole-4,6-dione derivatives for photovoltaic applications

Article abstract of DOI:10.1002/adfm.201001771

A new class of low-bandgap copolymers based on benzodithiophene (BDT) and thieno[3,4-c]pyrrole-4,6-dione (TPD) units is reported. Chemical modifications of the conjugated backbone promote both high molecular weights and processability while allowing for tuning of the electronic properties. Copolymers with substituted thiophene spacers (alkyl chains facing the BDT unit) or unsubstituted thiophene spacers tend to have low power conversion efficiencies (PCE less than 1%) due to a bad morphology of the active layer, whereas copolymers with substituted thiophene spacers (alkyl chains facing TPD unit) show enhanced morphology and PCEs up to of 3.9%. Finally, BDT-TPD copolymers without any thiophene spacers still show the best performances with power conversion efficiencies up to 5.2%. The synthesis and characterization of new electroactive and photoactive copolymers based on benzodithiophene (BDT) and thieno[3,4-c]pyrrole-4,6-dione (TPD) units are described. The copolymers are synthesized with or without thiophene spacers in order to tune the electronic properties and to control the morphology of the active layer in bulk heterojunction solar cells. The potential of these new materials in photovoltaic devices is investigated.

Full text of DOI:10.1002/adfm.201001771

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Synthesis and Characterization of New Thieno[3,4-c]
pyrrole-4,6-dione Derivatives for Photovoltaic Applications
Ahmed Najari, Serge Beaupré, Philippe Berrouard, Yingping Zou, Jean-Rémi Pouliot,
Charlotte Lepage-Pérusse, and Mario Leclerc*
between 1.2 and 1.9 eV with broad absorp-
tion to match the spectral flux, a HOMO
energy level between −5.2 and −5.8 eV
and a lowest unoccupied molecular orbital
(LUMO) energy level between −3.7 and
−4.0 eV to ensure efficient charge separa-
tion while maximizing the open circuit
voltage (Voc). The past few years have wit-
nessed the development of several new
classes of conjugated polymers that have
been used as donors in BHJ solar cells.[10–12]
Lately, PCEs up to 8.1% have been reported,
which confirms that organic photovoltaic
technology can become a cost-effective and
competitive technology.[13]
A new class of low-bandgap copolymers based on benzodithiophene (BDT)
and thieno[3,4-c]pyrrole-4,6-dione (TPD) units is reported. Chemical modifica-
tions of the conjugated backbone promote both high molecular weights and
processability while allowing for tuning of the electronic properties. Copoly-
mers with substituted thiophene spacers (alkyl chains facing the BDT unit) or
unsubstituted thiophene spacers tend to have low power conversion efficien-
cies (PCE less than 1%) due to a bad morphology of the active layer, whereas
copolymers with substituted thiophene spacers (alkyl chains facing TPD
unit) show enhanced morphology and PCEs up to of 3.9%. Finally, BDT-TPD
copolymers without any thiophene spacers still show the best performances
with power conversion efficiencies up to 5.2%.
Recently, we have reported for the first
1. Introduction
time a new class of copolymers based on benzodithiophene
(BDT) and thieno[3,4-c]pyrrole-4,6-dione (TPD).[14] The TPD
unit is attractive due its compact planar structure that could
be beneficial to the electron delocalization when incorporated
into various conjugated polymers. Moreover, it seems that it
should promote intra- and interchain interactions along and
between coplanar polymer chains and its strong electron-
withdrawing effect should lead to lower HOMO and LUMO
energy levels, a desired property to increase the stability and
the Voc in BHJ solar cells. Furthermore, TPD can be easily pre-
pared in few steps from commercially available compounds. A
PCE of 5.5% was obtained using a poly((4,8-diethylhexyloxyl)
benzo[1,2-b:4,5-b’]dithiophene)-2,6-diyl)-alt-((5-octylthieno[3,4-
c]pyrrole-4,6-dione)-1,3-diyl) (PBDTTPD)/[70]PCBM blend
for an active area of 100 mm2.[14] Following this paper, several
research groups confirmed that copolymers based on BDT
and TPD units can reach high power conversion efficiencies.
Jen et al.[15] reported power conversion efficiency of 4.1% for
PBDTTPD/[70]PCBM (ratio 1:2) while Fréchet et al.[16] and Xie
et al.[17] reported PCEs ranging from 4.0% to 6.8% for a series
of alkylated TPD-based copolymers. Recently, Wei et al.[18]
reported a power conversion efficiency of 4.7% with a high Voc
of 0.95 V using a copolymer based on TDP and bithiophene
derivatives.
Harvesting the unlimited and renewable energy from sunlight
to produce electricity through photovoltaic devices is a prom-
ising way to address growing global energy needs while mini-
mizing detrimental effects on the environment. Among all
photovoltaic technologies, polymer bulk heterojunction (BHJ)
solar cells offer a compelling option for tomorrow’s photo-
voltaic devices since they can be easily prepared using low-cost
and energy efficient roll-to-roll manufacturing processes.[1–3]
During the past decade, BHJ solar cells based on regioregular
poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric
acid methyl ester ([60]PCBM) blend have been widely inves-
tigated and power conversion efficiencies (PCEs) up to 5–6%
have been reported.[4–6] However, a relatively large bandgap
of 1.9 eV and a highest occupied molecular orbital (HOMO)
energy level of 5.1 eV prevent P3HT/PCBM-based BHJ solar
cells from reaching higher PCE values.[7] Numerous studies on
BHJ photovoltaic devices led to a widely accepted guideline for
the design of new conjugated polymers used as donor in BHJ
solar cells.[8,9] Indeed, when combined with the mostly used
electron acceptor [60]PCBM, the donor should have a bandgap
Following these first studies on copolymers based on BDT
and TPD units (Figure 1), we report the syntheses and the char-
acterization of new copolymers based on BDT and alkylated-
TPD comonomers where thiophene spacers were added to tune
the electronic properties and to enhance photons harvesting.
Effects on the morphology and phase separation using these
new copolymers were also investigated using grazing incidence
X-ray scattering (GIXS) and atomic force microscopy (AFM).
Dr. A. Najari, Dr. S. Beaupré, P. Berrouard, Dr. Y. Zou,
J.-R. Pouliot, C. Lepage-Pérusse, Prof. M. Leclerc
Canada Research Chair on Electroactive and Photoactive Polymers
Department of Chemistry
Université Laval
Quebec City, Quebec, G1V 0A6, Canada
E-mail: Mario.Leclerc@chm.ulaval.ca
DOI: 10.1002/adfm.201001771
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investigated the influence of the alkyl chains
of the TPD unit (P1–P3) on the morphology of
the copolymer/[60]PCBM blend.
The synthesis of comonomers 4–6 was
straightforward and proceeded in good yields
according to reported procedures.[20] The
Stille coupling between comonomers 4–6
and BDT comonomer gave soluble copoly-
mers with moderate molecular weights
(P1–P3) (Scheme 3). In order to modulate
the electronic properties and bandgap of
BDT-TPD-based copolymers, an alkylated-
TPD unit bearing on each side a thiophene
unit (comonomers 19–26) was also prepared.
Unfortunately, copolymers with unsubstituted
thiophene spacers and straight alkyl chains
on the TPD unit (P4,P6) were found to be
insoluble in most common organic solvents
and could not be characterized. However, the
use of a branched chain (ethylhexyl group) on
the TPD unit led to a soluble and processable
copolymer with moderate molecular weights
(P5). Finally, P7–P11 were prepared to inves-
tigate the influence of the length and of the
positioning of alkyl chains on thiophene
spacers on the solubility; molecular weights;
and optical, electronic, and morphological
properties.
It is worth noting that during the syn-
thesis of precursors 11–18, some by-prod-
ucts that indicate a side reaction involving
a
dimerization of the TPD unit (com-
Figure 1. a) Molecular structures of the active materials: poly(thieno[3,4-c]pyrrole-4,6-dione)
derivatives (left) and [60]PCBM (right). b) The device structure (left) and scanning electron
microscopy (SEM) cross-sectional image (right) of the polymer solar cell.
pounds 4–6) were isolated. This side reac-
tion might occur during the preparation of
copolymers P1–P3 and might lead to an
imperfect alternating structure. The presence
Since the thickness of the active layer may be a critical issue for
a reliable roll-to-toll processing of BHJ solar cells, both thin and
and the amount of these possible structural defects are under
investigation.
thick active layers were investigated using a 25 mm2 active area.
2.2. Molecular Weights and Thermal Properties
2. Results and Discussion
All polymers were characterized by size exclusion chromato-
graphy (SEC) using monodisperse polystyrene standard in
2.1. Monomers and Copolymers
R1
N
R1
N
The synthetic routes for comonomers are
shown in Scheme 1 and Scheme 2. Our work
on BDT-TPD copolymers (not reported here)
and other studies on BDT-based copolymers
have shown that benzo[1,2-b:3,4-b]dithiophene
(BDT) bearing ethylhexyl side chain led to sol-
uble copolymers with high molecular weights,
enhanced morphology for films obtained from
copolymer/[60]PCBM blends, and high power
conversion efficiency in BHJ solar cells.[14–17,19]
Since alkyl side chains on the TPD unit can
also influence the solubility, the molecular
weights, the organization, and the charge
mobility of BDT-TPD copolymers, we have
HOOC
COOH
O
O
O
O
(i, ii, iii)
(iv)
S
Br
Br
S
S
R1= C8H17 (1)
R1= EthylHexyl (2)
R1= C12H25
R1= C8H17 (4)
R1= EthylHexyl (5)
(6)
R1= C12H25
(3)
Reagents and conditions: (i)Ac2O, 140°C, overnight; (ii) 1-octylamine or
2-ethyl-1-hexylamine or 1-dodecylamine, toluene, reflux 24h; (iii) SOCl2, reflux, 12h, (iv)
H2SO4, CF3COOH, NBS, r.t., overnight.
Scheme 1. Synthesis of TPD precursors.
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R1
N
2.3. Optical and Electrochemical Properties
R1
O
O
The optical properties of all copolymers are
summarized in Table 1. For all polymeric
materials, the absorption spectra are broad,
which means that a large part of the solar
spectral flux will be absorbed (Figure 2) and
(i)
N
O
R3
R2
SnMe3
O
S
S
+
S
S
Br
S
Br
R3
R2
R2
R3
should contribute to get high short circuit
current (Jsc) in BHJ solar cells. One can see
that the absorption spectra of P1–P5 remain
almost the same in solution and films (Figure
2b). Density functional theory (DFT) calcu-
lations performed on the P1–P3 repeating
R2= H; R3= H (7)
R1= C8H17 (4)
R1= EthylHexyl (5)
R1= C12H25 (6)
R1= C8H17; R2= H; R3= H (11)
R2= C8H17; R3= H (8)
R2= C2H5; R3= H (9)
R2= H; R3= C8H17 (10)
R1= EthylHexyl; R2= H; R3= H (12)
R1= C12H25; R2= H; R3= H (13)
R1= C8H17; R2= C8H17; R3= H (14)
R1= C8H17; R2= C2H5; R3= H (15)
R1= C8H17; R2= H; R3=C8H17 (16)
R1= C12H25; R2= C8H17; R3= H (17)
R1= C12H25; R2= H; R3= C8H17 (18)
units revealed coplanar structures (data not
R1
N
shown). Indeed, Wei et al.[18] reported that
O
O
the electron-deficient TPD moiety exhibits a
symmetric, rigidly fused, and coplanar struc-
ture. Moreover, neither the length nor the
type of the tail chain on the TPD seems to
affect the optical properties of these copoly-
(ii)
S
S
Br
Br
R3
S
R3
R2
R2
mers. Absorption spectrum of film of P5
R1= C8H17; R2= H; R3= H (19)
showed that the thiophene spacers (without
R1= EthylHexyl; R2= H; R3= H (20)
R1= C12H25; R2= H; R3= H (21)
R1= C8H17; R2= C8H17; R3= H (22)
R1= C8H17; R2= C2H5; R3= H (23)
R1= C8H17; R2= H; R3=C8H17 (24)
R1= C12H25; R2= C8H17; R3= H (25)
R1= C12H25; R2= H; R3= C8H17 (26)
alkyl chains) did not modulate the optical
properties and the optical bandgap remains
the same as P1–P3. In order to obtain soluble
and high molecular weights BDT-TPD deriv-
atives with thiophene spacers, copolymers
P7–P11 were prepared. For these copolymers,
significant red shifts (up to 40 nm for P8)
were observed when comparing solution to
films. This typical behavior observed in dense
Reagents and conditions: (i) PdCl2(PPh3)2, THF, reflux, 24h; (ii) NBS, acetic acid,
chloroform, 50°C, 24 h.
thin-films of conjugated polymers is related
Scheme 2. Synthesis of TPD derivatives with flanked thiophene.
to an increase in conjugation length due to
an ordering in the solid state. However, the
hot 1,2,4-trichlorobenzene (TCB) and data are summarized in
Table 1. The number average molecular weight (Mn) ranges
from 8.3 kDa to 131 kDa with a polydispersity index (PDI)
between 1.5 and 3.2. Recently, Fréchet et al.[16] have reported
higher number-average molecular weight for P1 (Mn = 35 kDa;
PDI = 2.7). Xie et al.[17] have also reported higher molecular
weights for similar BDT-TPD copolymers with high polydisper-
sity indexes. In both cases, the SEC measurements were carried
out in tetrahydrofuran (THF). In order to adequately com-
pare the molecular weights, SEC measurements in THF were
attempted for P1. However, the P1 sample was only slightly
soluble in THF and the number-average molecular weight
obtained for the soluble fraction was low (Mn < 5 kDa). Since
P1 was soluble in chloroform, SEC measurements in chloro-
form were also performed and a broad molecular weight dis-
tribution, which was attributed to aggregation, was found. This
aggregation led to an overestimation of the apparent molecular
weights and high polydispersity indexes.
absorption spectra of these copolymers were blue shifted when
compared to P1–P3 but again the optical bandgap remained
almost the same. One can see that only a small range of the
optical bandgap (onset of the absorption spectra) was obtained
for BDT-TPD derivatives meaning that the influence of thi-
ophene spacer on the optical properties is somehow limited.
Cyclic voltammetry was utilized to evaluate the HOMO and
LUMO energy levels as well as the bandgap of BDT-TPD deriva-
tives. These data are summarized in Table 1 and Figure 3. The
HOMO energy levels were estimated by using the onset of the oxi-
dation potential (where the current differs from the baseline) from
the first scan since all polymers showed non-reversible oxidation
processes. On the other hand, all polymers showed reversible
reduction processes and LUMO energy levels were also estimated
the same way as HOMO energy levels. As observed for optical
properties, the alkyl chain on the TPD unit did not dramatically
change the electronic properties. Here, the relatively deep HOMO
energy levels should contribute to obtain high Voc in BHJs while
the LUMO levels are within the desirable range for proper electron
transfer from the donor to the electron acceptor ([60]PCBM).[8,9]
Thermogravimetric analyses (TGA) showed that all poly-
mers were thermally stable with degradation temperatures
(Td) ranging from 330 to 380 °C. With the exception of P1
(Tg = 138 °C), differential scanning calorimetry (DSC) per-
formed on P2–P11 did not reveal any noticeable glass transi-
tion temperatures. Similar observations have been reported by
Xie et al.[17]
2.4. Photovoltaic Properties
The photovoltaic properties of copolymers P1–P11 were
investigated in BHJ solar cells using the following configura-
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R1
N
R1
N
O
O
O
O
O
O
O
S
Pd2(dba)3/P(o-Tolyl)3
S
Me3Sn
SnMe3
+
S
dry Toluene
110°C/72h
S
S
Br
Br
S
O
n
R1= C8H17 (4)
R1= EthylHexyl (5)
R1= C12H25 (6)
(27)
R1= C8H17 (P1)
R1= EthylHexyl (P2)
(P3)
R1= C12H25
R1
N
(19)
R1= C8H17; R2= H; R3= H
O
O
O
R1= EthylHexyl; R2= H; R3= H (20)
R1= C12H25; R2= H; R3= H (21)
R1= C8H17; R2= C8H17; R3= H (22)
R1= C8H17; R2= C2H5; R3= H (23)
S
S
S
Me3Sn
SnMe3
Br
R3
Br
+
S
S
(24)
R1= C8H17; R2= H; R3=C8H17
R1= C12H25; R2= C8H17; R3= H (25)
(26)
O
R3
R2
R2
R1= C12H25; R2= H; R3= C8H17
(27)
Pd2(dba)3/P(o-Tolyl)3
dry Toluene
110°C/72h
R1
(P4)
R1= C8H17; R2= H; R3= H
O
N
R1= EthylHexyl; R2= H; R3= H (P5)
R1= C12H25; R2= H; R3= H (P6)
R1= C8H17; R2= C8H17; R3= H (P7)
R1= C8H17; R2= C2H5; R3= H (P8)
O
S
O
n
R3
S
S
S
(P9)
R1= C8H17; R2= H; R3=C8H17
R1= C12H25; R2= C8H17; R3= H (P10)
R2
S
(P11)
R1= C12H25; R2= H; R3= C8H17
R2
R3
O
Scheme 3. Synthesis of poly(thieno[3,4-c]pyrrole-4,6-dione) derivatives.
T a b le 1 . Molecular weights and optical and electrochemical properties of copolymers P1 to P11.
cv
opt
Polymer
Mn
(kDa)
12.0
PDI
Td
EHOMO
ELUMO
(eV)
Eg
Eg
Solution λmax
Film λmax
(nm)
(nm)
(eV)
−5.56
−5.66
(eV)
1.81
1.79
(eV)
1.80
1.84
(°C)
380
340
P1
P2
2.37
2.56
308, 360, 448, 614
308, 360, 448, 614
−3.75
−3.87
20.6
447, 556,
609
444, 555,
609
P3
16.1
—
2.23
—
330
—
555, 610
—
556, 615
—
1.78
—
1.84
—
−5.60
—
−3.82
—
P4a)
P5
8.3
1.50
—
330
—
512
516
1.65
—
1.84
—
−5.49
—
−3.70
—
P6a)
P7
—
—
—
131.0
11.6
19.9
41.6
22.9
2.82
3.19
2.11
2.27
2.30
340
340
330
340
330
524
539
1.86
1.76
1.83
1.86
1.95
1.88
1.84
1.86
1.88
1.89
−5.56
−5.54
−5.66
−5.56
−5.73
−3.70
−3.78
−3.83
−3.70
−3.78
P8
513
553
P9
517
539
P10
P11
512
540
519
538
a)P4 and P6 were not soluble.
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found to be 1:2 (wt:wt) for all polymers, excepted for P2 (1:1).
Solar cells were tested under AM 1.5G (AM = air mass) illu-
mination of 100 mW cm−2 and the active area of the devices
was 25 mm2 (Table 2). Current density–voltage (J–V) curves
are shown in Figure 4 and data on BHJ solar cells are sum-
marized in Table 2.
Energy (eV)
4
3,5
3
2,5
2
1,2
(a)
1,0
0,8
0,6
0,4
0,2
P1
P2
P3
P5
P7
P8
P9
P10
P11
Based upon the equivalent circuit of a photovoltaic cell,[21,22]
the current vs voltage (I–V) characteristics can be described by
the following equation:[21,23]
ꢀ
ꢁ
ꢂ
ꢃ
V − I RS
nkT
V − I RS
I = I0 exp
e
− 1 +
− Iph
(1)
Rsh
0,0
where I0 is the dark current, e is the electron charge, n is the diode
ideality factor, V is the applied voltage, Rs is the series resist-
ance, Rsh is the shunt resistance, and Iph is the photocurrent.
Rs and Rsh can be described by the following equations.[24]
300
400
3
500
600
700
800
Wavelength (nm)
Energy (eV)
4
3,5
2,5
2
1,2
ꢀ
ꢀ
ꢃ
(b)
dI
≈ RS−1
1,0
0,8
0,6
0,4
0,2
(2)
(3)
P1
P2
P3
P5
P7
P8
P9
P10
P11
dV
I= 0
ꢃ
dI
−1
≈ R
sh
dV
V= 0
Thus, to obtain high short-circuit currents, Isc (V = 0), solar
cell devices must have small Rs and large Rsh. In a physical
point of view, the series resistance Rs can be associated with the
materials’ conductivity, thus the charge carrier mobility in the
donor/acceptor (D/A) blends (electron mobility in the acceptor
and hole mobility in the donor). Rsh interprets the charge
recombination close to dissociation charge sites (interface D/A
and electrodes).
0,0
300
400
3
500
600
700
800
Wavelength (nm)
Energy (eV)
2,5
According to reported data, P1 and P3 show the best
photovoltaic performances with high Voc (0.94 V and 0.96 V,
respectively) and Jsc of 10.8 mA cm−2 and 10.0 mA cm−2 ,
respectively. Moderate fill factors (FFs), approaching 51% for
both copolymers, led to PCEs of 5.2% for P1 and 4.8% for
P3 (thin films). Fréchet et al.[16] reached a PCE up to 6.3%
for P1 (with Mn = 35 kDa) in BHJ solar cells and Jen et al.[15]
have reported a PCE up to 4.1% for P1 for devices using [70]
PCBM. These high PCEs underline a significant potential
of this copolymer in organic solar cells applications. Since
high-speed roll-to-roll manufacturing process can be used
to print polymeric solar cells,[25] the use of relatively thick
films is important in order to obtain a uniform and defect
free active layer. P1 is a promising material that could ful-
fill those requirements. Indeed, we obtained BHJ solar cells
with a PCE up to 3.8% using an active layer with a thickness
of 170 nm. While the Voc and Jsc remain almost the same
when compared to thin films devices, the FF drops by more
than 20%. This can be attributed to the low mobility of holes
for P1 (μh = 1.6 × 10−4 cm2 V−1 s−1 ), which increase Rs and
decrease Rsh for thicker films. Despite this fact, the PCE
reported here for P1 is among the highest values reported
in literature for thick-layer BHJ solar cells. According to data
reported in Table 2, the length of the alkyl on the TPD unit
(from octyl to docecyl) led to a lower PCE (4.8%), caused by
a drop of Jsc even though Rsh was higher for P3 (1163 Ω cm2)
4
3,5
2
2,5
(c)
P1
P2
P3
P5
P7
P8
P9
P10
P11
2,0
1,5
1,0
0,5
0,0
300
400
500
600
700
800
Wavelength (nm)
Figure 2. a) Normalized absorption spectra of TPD derivatives for dilute
ortho-dichlorobenzene (oDCB) solutions for P1–P5, P8; for dilute chloro-
form solutions of P7, P10, and P11. b) In film. c) Films from copolymer/
[60]PCBM blends.
tion: glass/indium tin oxide (ITO)/poly(3,4-ethylenedioxythi
ophene):poly(styrenesulfonate) (PEDOT:PSS)/active layer/Al
(Figure 1). The active layer was a blend of P1–P11 with [60]
PCBM spin-coated from chloroform or oDCB solutions. For
P1 and P10, both thin and relatively thick active layers were
prepared. The ratio of donor:acceptor was optimized and
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with an Voc of 0.87 V and a FF of 58%.
Data reported in Table 2 clearly show that
devices using P7 are better than ones using
P8 and P10 due to a higher shunt resistance
value (Rsh = 1904 Ω cm2). For P10, a lower
shunt resistance causes power losses in
solar cells by providing an alternate current
path for the light-generated current. How-
ever, when used in a thick-film configura-
tion (165 nm), P10 shows a high Voc (0.88
V) and a PCE of 3.1%. As for P1 and P3,
the length of the alkyl chain causes a small
drop in the short circuit current Jsc, prob-
ably due to the morphology of the blend
with the [60]PCBM. P8 was prepared to
obtain soluble copolymers with the shortest
alkyl chain on the thiophene space since
P4 and P6 were insoluble in most common
organic solvents. Despite a high Jsc of 9.0
mA cm−2 , a drop in the Voc of 0.23 V led to
a PCE of 3.5%. Finally, the BDT-TPD deriv-
atives with thiophene spacer bearing alkyl
chains facing the BDT unit gave the worst
results with PCEs of 0.2% and 0.7% for P9
and P11, respectively. For both copolymers,
the values of the Jsc and FF were low and
Figure 3. Energy levels of copolymers P1 to P11.
are easily explained by the nanoscale mor-
phology observed using AFM. While the
than for P1 (994 Ω cm2). Fréchet et al.[16] and Xie et al.[17]
have reported similar PCEs with P2.
nanoscale morphology of blends of P1–P5,P7, P8, and P10
with [60]PCBM are quite similar with only small variations of
the roughness nanoscale surface, morphologies of blends for
P9 and P11 were completely different. As shown in Figure
5, “donut” shapes with a diameter of 500 nm were observed.
The percolation pathways were poorly formed, which limits
the transport of the charges to the respective electrodes and
leads to poor short circuit current. To confirm these assump-
The BDT-TPD derivatives with thiophene spacers with
alkyl chains facing the TPD unit also revealed promising photo-
voltaic results (P7, P8, and P10). Among these copolymers,
the high molecular weight copolymer P7 gave the best photo-
voltaic results with a PCE of 3.9% (thickness of 102 nm).
P10 (with C12H25 on the TPD unit) reached a PCE of 3.6%
T a b le 2 . Device Characteristics of Photovoltaic solar cells based on TPD containing Polymers with [60]PCBM.
Polymer
P1
Solvent
D:A Ratio
(wt:wt)
1:2
Jsc
(mA cm−2
−10.8
−10.2
−6.2
−10.0
−
Voc
(V)
FF
PCE
(%)
5.2
3.8
2.6
4.8
−
Rs
(Ω cm2)
18
Rsh
(Ω cm2)
994
Active Layer Thickness
(nm)
98
)
oDCB
oDCB
oDCB
oDCB
−
0.94
0.92
0.96
0.93
−
0.51
0.41
0.43
0.51
−
1:2
30
407
170
67
P2
1:1
52
480
P3
1:2
23
1163
−
95
P4a)
P5
-
−
−
oDCB
−
1:2
0.76
−
0.43
−
0.95
−
61
884
88
−2.9
−
P6a)
P7
−
−
−
−
CHCl3
oDCB
CHCl3
CHCl3
CHC3
CHCl3
1:2
0.89
0.76
0.66
0.87
0.88
0.92
0.57
0.51
0.26
0.58
0.53
0.34
3.9
3.5
0.2
3.6
3.1
0.7
19
1904
700
102
70
−7.6
−9.0
−1.2
−7.2
−6.5
−2.3
P8
1:2
20
P9
1:2
690
19
1222
533
115
90
P10
1:2
1:2
32
958
165
85
P11
1:2
97
772
a)P4 and P6 are not soluble.
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P7 gave a better power conversion efficiency
with 3.9%, the GIXS diffractogram did not
show any lamellar distance related peak as
for the less efficient BDT-TPD derivatives
(P5, P9, and P11).
In brief, copolymers with substituted
thiophene spacers (alkyl chains facing the
BDT unit) or the ones with unsubstituted
thiophene spacers have the lower perform-
ances due to a poor morphology of the
active layer. Indeed the “donut” shapes
observed in AFM measurements for poly-
mers P9 and P11 suggest poor charge
transport properties due to a poor perco-
lation in the polymer/[60]PCBM blends.
Moreover, unlike other copolymers in the
series, GIXS measurements did not reveal
the presence of a peak around 20 Å, which
is usually attributed to lamellar spacing. In
addition to its lack of solubility in oDCB,
P5 shows low PCEs (less than 1%) due
to a weak interpenetrated network of the
donor and the acceptor. It is important to
note that the carrier lifetime is largely con-
trolled by the phase morphology between
the donor and acceptor materials.[10]
Furthermore, the molecular weight for this
polymer is rather low at 12.5 kDa. As for
P1–P3, AFM and GIXS measurements of
copolymers (P7, P8, P10) show suitable
film morphology that could lead to high
PCE. It is noteworthy that the photovoltaic
properties of this new class of materials
Figure 4. J–V curves of the polymers solar cells based on poly(thieno[3,4-c]pyrrole-4,6-dione)
derivatives under the illumination of AM 1.5, 100 mW cm−2 . a) Polymers without thiophene
spacer, b) polymers with thiophene spacer, c) polymers with thiophene spacer having alkyl chains
facing TPD unit, and d) polymers with thiophene spacer having alkyl chains facing BDT unit.
have been obtained without any optimization processes.
According to the tool proposed by the Konarka Technologies
Inc. team,[9] it seems that a PCE up to 8.5% can be reached
using this new class of materials so optimization processes
are underway.
tions, we investigated the influence of alkyl chains on the
TPD unit (P1–P3) and also the influence of the thiophene
spacers (P5, P7–P11) on the molecular organization in pol-
ymer thin films using GIXS. P1–P3, P5, P7–P11/[60]PCBM
GIXS analyses were conducted on glass/ITO/PEDOT:PSS
solid support. As shown in Figure 6 , a peak at 4.2 Å, has
been observed for all copolymers/[60]PCBM blends. This
peak corresponds to the distance between the coplanar
π-conjugated backbones (d2). Indeed, Fréchet et al.[16] have
reported smaller π-stacking distances for blends of P1 (d2 =
3.6 Å) and P2 (d2 = 3.8 Å) with [60]PCBM. According to the
authors,[16] the fact that the π-stacking peak was observed for
the entire BDT-TPD series indicating that these copolymers
are able to retain the same face-on orientation when blended
with the electron acceptor ([60]PCBM). For P1–P3, one can
see an additional peak that corresponds to lamellar spacing
(d1). Since this distance is related to the length of the side
chain, it is longer for the dodecyl derivative P3 (d1 = 24.6 Å)
than the octyl derivative P1 (d1 = 21.2 Å) and ethylhexyl deriv-
ative P2 (d1 = 18.6 Å). These results are in good agreement
with those reported by Fréchet et al.[16] These peaks were also
present for P8 (d1 = 17.3 Å) and P10 (d1 = 20.6 Å). One can
think that the shorter distance observed for P8 (ethyl on the
thiophene spacer) led to a better order in the blend, which
led to higher Jsc (9.0 mA cm−2 ) in BHJ solar cells when com-
pared to P10 (Jsc = 7.2 mA cm−2 ). On the other hand, while
3. Conclusions
We have investigated a new class of BDT-TPD-based copoly-
mers. Preliminary results on the photovoltaic devices based
on these materials revealed that the length of the alkyl chain
on the TPD unit can modify the morphology of the copoly-
mers/[60]PCBM blends. A PCE of 5.2% has been obtained
for a thin layer of P1. Moreover, thick active layer BHJ solar
cells have been studied and a PCE up to 3.8% was reached
with P1. This result is among the highest PCE values
reported so far for relatively thick films and could contribute
to address issues related to roll-to-roll manufacturing proc-
esses of BHJ solar cells. Optimization of the fabrication of
the photovoltaic devices is therefore underway to push for-
ward the performances of these new copolymers. Finally,
compounds 19 to 26 could be homopolymerized or copo-
lymerized with new electron-rich comonomers to develop
totally new well-defined photoactive and electroactive
polymers.
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4. Experimental Section
Instrumentation: 1H and 13C NMR spectra were
recorded using a Varian AS400 or 300 in deuterated
chloroform, dimethyl sulfoxide, or THF solution
at 298 K. Number-average (Mn) and weight-
average (Mw) molecular weights were determined
by size exclusion chromatography (SEC) using
a high temperature Varian Polymer Laboratories
GPC220 equipped with
a refractive index (RI)
detector and a PL BV400 HT Bridge Viscometer.
The column set consists of 2 PLgel Mixed C
(300 mm × 7.5 mm) columns and a PLgel Mixed
C
guard column. The flow rate was fixed at
1.0 mL min−1 using 1,2,4-trichlorobenzene (TCB)
(with 0.0125% BHT w/v) as the eluent. The
temperature of the system was set to 140 °C. The
sample was prepared at concentration of nominally
1.0 mg mL−1 in hot TCB. Dissolution was performed
using a Varian Polymer Laboratories PL-SP 260VC
sample preparation system. The sample vial was
held at 160 °C with shaking for 1 h for complete
dissolution. The solution was filtered through
a 2 μm porous stainless steel filter used with
the SP260 pipettor into a 2 mL chromatography
vial. The calibration method used to generate
the reported data was the classical polystyrene
method using polystyrene narrow standards Easi-
Vials PS-M from Varian Polymer Laboratories,
which were dissolved in TCB. TGA measurements
were carried out using a Mettler Toledo TGA SDTA
851e apparatus (heating rate of 20 °C min−1 under
nitrogen flow) and the temperature of degradation
(Td) corresponds to a 5% weight loss. Differential
scanning calorimetry (DSC) analysis was performed
on a Perkin-Elmer DSC-7 instrument, calibrated
with ultrapure indium. Glass transition temperature
(Tg) was determined using a scanning rate of
20 °C min−1 under a nitrogen flow. UV-vis-NIR
absorption spectra were recorded using a Varian
Cary 500 UV-vis-NIR spectrophotometer with path
length quartz cells of 1 cm. Spin-coated films on
glass plates were used for solid-state UV-vis-NIR
measurements. Optical bandgaps were determined
from the onset of the absorption band. Cyclic
voltammograms (CV) were recorded on a Solartron
1287 potentiostat using platinum wires as working
electrode and counter-electrode at a scan rate of
50 mV s−1 . The reference electrode was Ag/Ag+
(0.1 M of AgNO3 in acetonitrile) and the electrolyte
was a solution of 0.1 M of tetrabutylammonium
tetrafluoborate in dry acetonitrile. In these
conditions, the oxidation potential of ferrocene
was 0.09 V versus Ag/Ag+, whereas the oxidation
potential of ferrocene was 0.41 V versus saturated
calomel electrode (SCE). The HOMO and LUMO
energy levels were determined from the oxidation
and reduction onsets (where the current differs
from the baseline) assuming that SCE electrode
is −4.7 eV from vacuum. The operation power was
40 kV, 30 mA and the collimator was 0.8 mm in
diameter. GIXS experiments were performed using
a Siemens D5000 X-ray diffractometer with a CuKα
radiation source (1.540598 Å). The operation power
was 40 kV, 30 mA. The surface topographies of BHJ
active layers composed of donor polymers and [60]
PCBM were obtained using AFM in tapping mode
(Dimension V SPM, Veeco). The Si AFM tip was
Figure 5. The AFM tapping mode height and simultaneously acquired phase insets images of
poly(thieno[3,4-c]pyrrole-4,6-dione) derivatives (P1–P11, excepted P4 and P6). The scan size for
height and phase images is 10 mm × 10 mm.
Å
140
120
100
80
140
120
100
80
24.6
21.2
18.6
Å
Å
Å
Å
4.2
4.2
P1
P2
P3
P5
60
60
40
40
20
20
(a)
25
(b)
25
0
0
0
5
10
15
20
30
0
5
10
15
20
30
Å
140
120
100
80
140
120
100
80
20.6
17.3
Å
Å
4.2
Å
4.2
P7
P8
P10
P9
P11
60
60
40
40
20
20
(c)
25
(d)
25
0
0
0
5
10
15
20
30
0
5
10
15
20
30
2 Theta (Degrees)
Figure 6. X-ray diffraction patterns of polymers P1 to P11-[60]PCBM blend films at room
temperature.
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used with a force constant of 42 N m−1 and AFM images were collected
in air under ambient conditions.
The organic phase was washed with water then brine. The solvent was
removed under reduced pressure and the crude product (brownish oil) was
purified by distillation under high vacuum to afford 4.57 g of the product as
a colorless oil (Y = 63%) (boiling point 73–75 °C at 0.35 mmHg).
Chemicals: Thiophene-3,4-dicarboxylic acid was purchased from Frontier
Scientific. 2-(Tributylstannyl)thiophene was purchased from Aldrich. THF was
distilled over a potassium/benzophenone system. Toluene and acetonitrile
were distilled over calcium hydride before use. Synthesis of 5-octylthieno[3,4-c]
pyrrole-4,6-dione[14] (1), 1,3-dibromo-5-octyl-thieno[3,4-c]pyrrole-4,6-dione[14]
(4), 2-trimethyltin thiophene (7), 2-(trimethyltin)-3-octylthiophene[26] (8),
2-(trimethyltin)-4-octylthiophene[27] (10), and 2,6-bis(trimethyltin)-4,8-di(2-
ethylhexyloxyl)benzo[1,2-b:3,4-b]dithiophene[14] (27) were prepared according
to procedures reported in literature. All the monomers were carefully purified
prior to use in the polymerization reaction.
Synthesis of 5-(2-ethylhexyl)thieno[3,4-c]pyrrole-4,6-dione (2): A solution
of thiophene-3,4-dicarboxylic acid (0.90 g, 5.23 mmol) in 21 mL of acetic
anhydride was stirred at 140 °C overnight. The solvent was removed and
the crude product was used for the next step without any purification. The
brown solid (assuming 5.23 mmol) was dissolved into 55 mL of toluene
and then 2-ethylhexylamine (1.02 g, 7.85 mmol) was added. The reaction
mixture was refluxed for 24 h. The reaction mixture was cooled down and
the solvent was removed under reduced pressure. The resulting solid
was dissolved into 68 mL of thionyl chloride. The mixture was refluxed
for 3 h then cooled. The solvent was removed under reduced pressure
and the crude product was purified by column chromatography using
dichloromethane:hexanes as the eluent (ratio 1:1) to afford 0.90 g of the
product as a white solid (Y = 65%).
1H NMR (300 MHz, CDCl3, δ): 7.55 (d, 1H, J = 4.6 Hz); 7.13 (d, 1H, J =
4.7 Hz); 2.68 (q, 2H, J = 7.7 Hz); 1.24 (t, 3H, J = 7.5 Hz); 0.39 (s, 9H).
Synthesis
of
1,3-di(thien-2’-yl)-5-octylthieno[3,4-c]pyrrole-4,6-dione
(11): Compound 4 (2.10 g, 4.96 mmol) was dissolved into dry THF
(200 mL). 2-(tributylstannyl)thiophene (15.00 mmol, 4.76 mL), and
bis(triphenylphosphine) palladium(II) dichloride (210 mg, 6%) were
added to the reaction mixture. The solution was refluxed for 24 h then
cooled and poured into water. The mixture was extracted twice with
dichloromethane. The organic phases were combined, washed with
brine, and dried over anhydrous magnesium sulphate. The solvent was
removed under reduce pressure and the crude product was purified by
column chromatography using dichloromethane:hexanes as the eluent
(ratio 1:1) to afford 1.60 g of the product as a green powder (Y = 75%).
1H NMR (400 MHz, d8-THF, δ): 8.33 (d, 2H); 7.78 (d, 2H); 7.34 (t,
2H, J = 4.2 Hz); 3.83 (t, 2H, J = 7.1 Hz); 1.89 (m, 2H); 1.53–1.47 (m,
10H); 1.05 (t, 3H, J = 7.2 Hz).
13C NMR (100 MHz, d8-THF, δ): 162.08; 135.71; 132.57; 130.09;
129.12; 128.94; 128.42; 38.14; 32.00; 29.36; 28.48; 26.99; 22.73; 13.62.
One peak is missing due to the deuterated solvent.
Synthesis of 1,3-di(thien-2’-yl)-5-(2-ethylhexyl)thieno[3,4-c]pyrrole-4,6-
dione (12): This compound was synthesized as described for 11 using
5 (2.10 g, 4.96 mmol), dry THF (100 mL), 2-(tributylstannyl)thiophene
(15.00 mmol, 4.76 mL), and bis(triphenylphosphine) palladium(II)
dichloride (210 mg, 6%) to afford 1.90 g of the product as a green
powder (Y = 89%).
1H NMR (400 MHz, CDCl3, δ): 7.78 (s, 2H); 3.48 (d, 2H, J = 7.3 Hz);
1.76 (t, 1H, J = 6.1 Hz); 1.32–1.24 (m, 8H); 0.87 (t, 6H, J = 7.4 Hz).
13C NMR (100 MHz, CDCl3, δ): 163.16; 136.81; 125.71; 42.54; 38.35;
30.67; 28.67; 24.00; 23.23; 14.30; 10.62.
Synthesis of 5-(dodecyl)thieno[3,4-c]pyrrole-4,6-dione (3): This compound
was synthesized as described for 2 using thiophene-3,4-dicarboxylic acid
(10.00 g, 58.08 mmol) and 1-dodecylamine (16.14 g, 87.12 mmol) to
afford 10.27 g of the product as a white solid (Y = 55%).
1H NMR(400 MHz, CDCl3, δ): 8.03 (d, 2H, J = 3.1 Hz); 7.44 (d, 2H,
J = 4.6 Hz); 7.13 (t, 2H, J = 4.0 Hz); 3.58 (d, 2H, J = 7.3 Hz); 1.87–1.84
(m, 1H); 1.46–1.26 (m, 8H); 0.94–0.88 (m, 6H).
13C NMR(100 MHz, CDCl3, δ): 163.17; 136.72; 132.68; 130.15; 128.87;
128.69; 128.65; 42.75; 38.49; 30.83; 28.84; 24.14; 23.28; 14.33; 10.70.
Synthesis of 1,3-di(thien-2’-yl)-5-dodecylthieno[3,4-c]pyrrole-4,6-dione
(13): This compound was synthesized as described for 11 using 6
(1.00 g, 2.09 mmol), dry THF (50 mL), 2-(tributylstannyl)thiophene
(4.18 mmol, 1.45 mL), and bis(triphenylphosphine) palladium(II)
dichloride (73 mg, 6%) to afford 0.82 g of the product as a yellow green
powder (Y = 82%).
1H NMR (400 MHz, CDCl3, δ): 7.80 (s, 2H); 3.60 (t, 2H, J = 7.3 Hz);
1.65–1.62 (m, 2H); 1.30–1.24 (m, 18H); 0.87 (t, 3H, J = 6.5 Hz).
13C NMR (100 MHz, CDCl3, δ): 162.92; 136.91; 125.71; 38.75; 32.15;
29.87; 29.85; 29.81; 29.75; 29.59; 29.45; 28.72; 27.12; 22.93; 14.39.
Synthesis of 1,3-dibromo-5-(2-ethylhexyl)thieno[3,4-c]pyrrole-4,6-dione (5):
5-(2-ethylhexyl)-thieno[3,4-c]pyrrole-4,6-dione (0.45 g, 1.70 mmol) was
dissolved in a mixture of sulfuric acid (2.6 mL) and trifluoroacetic acid (8.7
mL). The solution was kept in the dark. N-Bromosuccinimide (0.94 g, 5.28
mmol) was added in four portions and the reaction mixture was stirred
at room temperature overnight. The brown-red solution was poured into
water and extracted twice with dichloromethane. The organic phases were
combined and dried over anhydrous magnesium sulphate. The solvent
was removed under reduced pressure and the crude product was purified
by column chromatography using dichloromethane:hexanes as the eluent
(ratio 3:2) to afford 1.00 g of the product as white powder (Y = 78%).
1H NMR (400 MHz, CDCl3, δ): 3.49 (d, 2H); 1.8 (m, 1H); 1.34–1.27
(m, 8H); 0.89 (t, 6H, J = 7.1 Hz).
1H NMR (400 MHz, CDCl3, δ): 8.01 (d, 2H, J = 3.0 Hz); 7.45 (d, 2H,
J = 0.6 Hz); 7.13 (t, 2H, J = 1.0 Hz); 3.66 (t, 2H, J = 7.3 Hz); 1.70–1.65 (m,
2H); 1.37–1.25 (m, 18H); 0.95 (t, 3H, J = 7.3 Hz).
13C NMR (100 MHz, CDCl3, δ): 162.87; 136.73; 132.68; 130.11;
128.89; 128.67; 128.63; 38.83; 32.16; 29.91; 29.88; 29.83; 29.74; 29.60;
29.48; 28.73; 27.20; 22.94; 14.39.
Synthesis of 1,3-di(3’-octylthien-2’-yl)-5-octylthieno[3,4-c]pyrrole-4,6-
dione (14): This compound was synthesized as described for 11 using 4
(1.26 g, 2.98 mmol), dry THF (100 mL), 2-(trimethyltin-3-octylthiophene)
(8) (7.45 mmol, 3.58 g), and bis(triphenylphosphine) palladium(II)
dichloride (210 mg, 6%). The crude product was purified by column
chromatography using dichloromethane:hexanes as the eluent (ratio
3:2) to afford 1.52 g of the product as a sticky oil (Y = 79%).
1H NMR (400 MHz, CDCl3, δ): 7.42 (d, 2H, J = 5.2 Hz); 7.02 (d, 2H, J =
5.2 Hz); 3.63 (t, 2H, J = 7.2 Hz); 2.80 (t, 4H, J = 7.9 Hz); 1.66–1.63 (m,
6H); 1.30–1.25(m, 30H); 0.88 (t, 9H, J = 6.4 Hz).
13C NMR (100 MHz, CDCl3, δ):160.92; 134.95; 113.18; 42.86; 38.40;
30.74; 28.77; 24.05; 23.18; 14.32; 10.60.
Synthesis of 1,3-dibromo-5-(dodecyl)thieno[3,4-c]pyrrole-4,6-dione (6): This
compound was synthesized as described for 5 using 5-(dodecyl)-thieno[3,4-c]
pyrrole-4,6-dione (3.00 g, 9.32 mmol), a mixture of sulfuric acid (17.4 mL) and
trifluoroacetic acid (56.4 mL), and N-bromosuccinimide (4.44 g, 24.94 mmol)
to afford 3.03 g of the product as white powder (Y = 68%).
13C NMR (100 MHz, CDCl3, δ): 162.55; 144.55; 137.25; 130.83;
130.06; 127.83; 125.27; 38.69; 32.11; 32.04; 30.77; 29.89; 29.77; 29.64;
29.50; 29.43; 29.40; 28.69; 27.19; 22.90; 22.87; 14.34; 14.33.
1H NMR (400 MHz, CDCl3, δ): 3.59 (t, 2H, J = 7.2 Hz); 1.64–1.61 (m,
2H); 1.30–1.25 (m, 18H); 0.87 (t, 3H, J = 6.5 Hz).
13C NMR (100 MHz, CDCl3, δ): 160.63; 135.03; 113.17; 39.08; 32.16;
29.86; 29.84; 29.81; 29.69; 29.59; 29.40; 28.50; 27.04; 22.94; 14.38.
1,3-di(3’-ethylthien-2’-yl)-5-octylthieno[3,4-c]pyrrole-4,6-dione
(15):
This compound was synthesized as described for 11 using 4 (0.97
g; 22.90 mmol), dry THF (60 mL), 2-(trimethyltin)-3-ethylthiophene
(9) (1.89 g; 69.00 mmol), and bis(triphenylphosphine) palladium(II)
dichloride (96 mg, 6%) to afford 0.75 g of the product as a sticky orange
oil (Y = 68%).
Synthesis of 2-(trimethyltin)-3-ethylthiophene (9): n-Butyllithium (2.5
M
in hexane) (27.60 mmol, 11.0 mL) was added dropwise to a solution of
2-bromo-3-ethylthiophene (5.00 g, 26.17 mmol) in dry THF (50 mL)
at −78 °C. The solution was stirred at −78 °C for 2 h. Then, trimethyltin
chloride (39.40 mmol, 40.0 mL) was added at once to the reaction mixture.
The cooling bath was removed and the reaction was warmed to room
temperature overnight. The reaction mixture was then poured into hexanes.
1H NMR (300 MHz, CDCl3, δ ): 7.42 (d, 2H, J = 5.2 Hz); 7.05 (d, 2H, J =
5.2Hz); 3.06 (t, 2H, J = 7.2Hz); 2.82 (q, 4H, J = 7.5Hz); 1.67–1.59
(m, 2H); 1.28 (m, 16H); 0.86 (t, 3H, J = 6.3Hz).
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13C NMR (75 MHz, CDCl3, δ): 162.33; 145.55; 137.04; 130.71; 129.35;
127.76; 124.63; 38.47; 31.80; 29.18(2C); 28.48; 26.95; 22.91; 22.65; 14.79;
14.10.
using hexanes to dichloromethane:hexanes (ratio 1:1 to 2:3) to afford
1.72 g of the product as a bright yellow solid (Y = 98%).
1H NMR (400 MHz, CDCl3, δ): 7.65 (d, 2H, J = 4.0 Hz); 7.07 (d, 2H,
J = 4.0 Hz); 3.54 (d, 2H, J = 7.3 Hz); 1.82 (t, 1H, J = 6.0 Hz); 1.38–1.29
(m, 8H); 0.92 (t, 6H, J = 2.9 Hz).
Synthesis of 1,3-di(4’-octylthien-2’-yl)-5-octylthieno[3,4-c]pyrrole-4,6-
dione (16): This compound was synthesized as described for 11 using 4
(1.00 g, 2.36 mmol), dry THF (50 mL) 2-(trimethyltin)-4-octylthiophene
(10) (1.87 g; 5.20 mmol), and bis(triphenylphosphine) palladium(II)
dichloride (9.9 mg, 6%) to afford 0.55 g of the product as a sticky orange
oil (Y = 36%).
13C NMR (100 MHz, CDCl3, δ): 162.92; 135.37; 133.96; 131.38; 130.01;
128.79; 116.98; 42.87; 38.51; 30.83; 28.86; 28.83; 24.13; 23.28; 14.34.
Synthesis of 1,3-di(2’-bromothien-5’-yl)-5-dodecylthieno[3,4-c]pyrrole-4,6-
dione (21): This compound was synthesized as described for 19 using
13 (0.80 g, 1.65 mmol), a mixture of acetic acid and chloroform (40 mL)
(ratio 1:1) at 0 °C, and N-bromosuccinimide (0.65 g, 3.62 mmol).
The crude product was purified by column chromatography using
dichloromethane:hexanes as the eluent (ratio 1:1) to afford 0.80 g of the
product as a yellow solid (Y = 76%).
1H NMR (400 MHz, CDCl3, δ ): 7.87 (s, 2H); 7.02 (s, 2H); 3.65 (t, 2H,
J = 7.4 Hz); 2.26 (t, 4H, J = 7.6 Hz); 1.67–1.61 (m, 6H); 1.33–1.29 (m,
30H); 0.89 (t, 9H, J = 6.5 Hz).
13C NMR (100 MHz, CDCl3, δ):162.94; 145.18; 137.02; 132.38; 131.32;
128.28; 123.77; 38.83; 32.13; 30.70; 29.65; 29.52; 29.49; 29.43; 28.80;
27.24; 22.93; 14.37. Some peaks are missing due to overlapping.
Synthesis of 1,3-di(3’-octylthien-2’-yl)-5-dodecylthieno[3,4-c]pyrrole-
4,6-dione (17): This compound was synthesized as described for 11
using 6 (0.95 g, 1.98 mmol), dry THF (46 mL), 2-(trimethyltin)-3-
octylthiophene) (8) (4.35 mmol, 1.56 g), and bis(triphenylphosphine)
palladium(II) dichloride (8.3 mg, 6%). The crude product was purified
by column chromatography using dichloromethane:hexanes as the
eluent (ratio 2:3) to afford 0.96 g of the product as a yellow solid (Y
= 68%).
1H NMR (400 MHz, CDCl3, δ): 7.65 (d, 2H, J = 4.0 Hz); 7.08 (d, 2H,
J = 4.0 Hz); 3.64 (t, 2H, J = 7.3 Hz); 1.66 (m, 2H); 1.32–1.25 (m, 18H);
0.87 (t, 3H, J = 6.5 Hz).
13C NMR (100 MHz, CDCl3, δ): 162.63; 135.39; 133.96; 131.39;
129.99; 128.87; 116.97; 38.95; 32.16; 31.20; 29.87; 29.81; 29.72; 29.60;
29.45; 28.69; 27.18; 22.94; 14.28.
Synthesis of 1,3-di(5’-bromo-3-octylthien-2’-yl)-5-octylthieno[3,4-c]pyrrole-
4,6-dione (22): This compound was synthesized as described for 19 using
14 (1.10 g, 1.72 mmol), a mixture of acetic acid and chloroform (50 mL)
(ratio 1:1) at 0 °C, and N-bromosuccinimide (0.68 g, 3.76 mmol). After
the addition of NBS, the cooling bath was removed and the reaction was
stirred at ambient temperature for 24 h. The solution was then heated
up to 55 °C for 24 h. The reaction was carefully monitored by thin layer
chromatography (TLC). The reaction mixture was cooled and poured
into water. The mixture was extracted three times using chloroform. The
chloroform parts were combined, washed with brine, and dried over
anhydrous magnesium sulphate. The crude product was purified by
column chromatography using dichloromethane:hexanes as the eluent
(ratio 1:1) to afford 1.30 g of the product as a yellow solid (Y = 82%).
1H NMR (400 MHz, CDCl3, δ): 6.96 (s, 2H); 3.60 (t, 2H, J = 7.1 Hz);
2.75- 2.71 (t, 4H, J = 7.6 Hz); 1.64–1.60 (m, 6 H); 1.30–1.25 (m, 30 H);
0.88–0.84 (t, 9H, J = 5.7 Hz).
1H NMR (400 MHz, CDCl3, δ): 7.46 (d, 2H, J = 5.1 Hz); 7.04
(d, 2H, J = 5.2 Hz); 3.64 (t, 2H, J = 7.2 Hz); 2.86 (t, 4H, J = 7.7 Hz);
1.70–1.59 (m, 6H); 1.31–1.25 (m, 38H); 0.86 (t, 9H, J = 3.9 Hz).
13C NMR (100 MHz, CDCl3, δ): 167.24; 145.19; 131.58; 130.30;
130.04; 128.31; 112.99; 38.64; 31.90; 31.79; 30.56; 29.50; 29.40; 29.27;
29.17; 26.95; 22.68; 22.62; 14.10; 14.07. Some peaks are missing due to
overlapping.
Synthesis of 1,3-di(4’-octylthien-2’-yl)-5-dodecylthieno[3,4-c]pyrrole-4,6-
dione (18): This compound was synthesized as described for 11 using 6
(0.91 g, 1.90 mmol), dry THF (45 mL), 2-(trimethyltin)-4-octylthiophene
(10) (1.5 g, 4.17 mmol), and bis(triphenylphosphine) palladium(II)
dichloride (8.0 mg, 6% mol). The crude product was purified by column
chromatography using dichloromethane:hexanes as the eluent (ratio
2:3) to afford 0.73 g of the product as a yellow wax (Y = 75%).
1H NMR (400 MHz, CDCl3, δ): 7.87 (s, 2H); 7.01 (s, 2H); 3.65
(t, 2H, J = 7.3 Hz); 2.62 (t, 4H, J = 7.6 Hz); 1.65–1.61 (m, 6H); 1.33–1.25
(m, 38H); 0.88 (t, 9H, J = 4.9 Hz).
13C NMR (100 MHz, CDCl3, δ): 162.26; 145.13; 135.63; 132.81;
130.94; 126.63; 115.69; 38.78; 32.11; 32.04; 30.57; 30.06; 29.73; 29.61;
29.49; 29.42(2C); 28.68; 27.19; 22.92; 22.89; 14.35; 14.32.
Synthesis of 1,3-di(5’-bromo-3’-ethylthien-2’-yl)-5-octylthieno[3,4-c]
pyrrole-4,6-dione (23): This compound was synthesized as described
for 22 using 15 (0.36 g, 0.75 mmol), a mixture of acetic acid and
chloroform (24 mL) (ratio 1:1) at 0 °C, and N-bromosuccinimide (0.29 g,
1.65 mmol) to afford 0.32 g of the product as a highly viscous orange
oil (Y = 66%).
13C NMR (100 MHz, CDCl3, δ ): 162.91; 145.17; 136.99; 132.38;
131.31; 128.27; 123.74; 38.83; 32.17; 32.13; 30.70; 30.66; 29.89; 29.84;
29.77; 29.66; 29.60; 29.57; 29.53; 28.80; 27.25; 22.93; 14.37. Some peaks
are missing due to overlapping.
Synthesis
of
1,3-di(2’-bromothien-5’-yl)-5-octylthieno[3,4-c]pyrrole-
1H NMR (300 MHz, CDCl3, δ): 7.01 (s, 2H); 3.61 (t, 2H, J = 7.4 Hz);
2.7 (q, 4H, J = 7.5 Hz); 1.66–1.60 (m, 2H); 1.28–1.23 (m, 16H); 0.87 (t,
3H, J = 6.2 Hz).
4,6-dione (19): Compound 11 (0.86 g, 2.00 mmol) was dissolved in a
mixture of acetic acid and chloroform (60 mL) (ratio 1:1). The solution
was cooled to 0 °C and kept in the dark. N-Bromosuccinimide (0.79 g,
4.43 mmol) was added to the solution in several portions. The cooling
bath was removed and the reaction was stirred at ambient temperature
for 24 h. The reaction solution was poured into water and extracted
several times with chloroform. The organic phases were combined,
washed with brine, and dried over anhydrous magnesium sulphate. The
solvent was removed under reduced pressure and the crude product
was purified by column chromatography using dichloromethane:hexanes
as the eluent (ratio 1:1) to afford 1.16 g of the product as a bright yellow
solid (Y = 99%).
13C NMR (75 MHz, CDCl3, δ): 162.07; 146.13; 135.42; 132.19; 130.83;
126.08; 115.53; 38.58; 31.79; 29.17(2C); 28.45; 26.95; 23.09; 22.65; 14.61;
14.11.
Synthesis of 1,3-di(5’-bromo-4’-octylthien-2’-yl)-5-octylthieno[3,4-c]pyrrole-
4,6-dione (24): This compound was synthesized as described for 22 using
16 (0.55 g, 0.85 mmol), a mixture of acetic acid and chloroform (20 mL)
(ratio 1:1) at 0 °C, and N-bromosuccinimide (0.29 g, 1.65 mmol) to afford
0.52 g of the product as a yellow solid (Y = 72%).
1H NMR (400 MHz, CDCl3, δ): 7.60 (s, 2H); 3.61 (t, 2H, J = 7.1 Hz);
2.54 (t, 4H, J = 7.5 Hz); 1.66–1.60 (m, 6H); 1.33–1.28 (m, 30H); 0.87 (t,
9H, J = 6.6 Hz).
1H NMR (400 MHz, CDCl3, δ ): 7.65 (d, 2H, J = 4 Hz); 7.09 (d, 2H, J =
4 Hz); 3.64 (t, 2H, J = 6.9 Hz); 1.67 (m, 2H); 1.32–1.26 (m, 10H); 0.87
(t, 3H, J = 5.4 Hz).
13C NMR (100 MHz, CDCl3, δ): 162.61; 143.96; 135.62; 132.09; 130.47;
128.49; 113.76; 38.91; 32.13; 29.96; 29.89; 29.75; 29.61; 29.52; 29.46;
28.76; 27.25; 22.93; 14.38. Some peaks are missing due to overlapping.
Synthesis of 1,3-di(5’-bromo-3’-octylthien-2’-yl)-5-dodecylthieno[3,4-c]
pyrrole-4,6-dione (25): This compound was synthesized as described
for 22 using 17 (1.47 g, 2.07 mmol), a mixture of acetic acid and
chloroform (50 mL) (ratio 1:1) at 0 °C, and N-bromosuccinimide
(0.82 g, 4.55 mmol) to afford 1.26 g of the product as a yellow solid
(Y = 70%).
13C NMR (100 MHz, CDCl3, δ): 162.54; 135.32; 133.96; 131.37;
129.97; 128.83; 116.98; 38.93; 32.03; 29.41(2C); 28.69; 27.20; 22.87;
14.34.
Synthesis of 1,3-di(2’-bromothien-5’-yl)-5-(2-ethylhexyl)thieno[3,4-c]
pyrrole-4,6-dione (20): This compound was synthesized as described for
19 using 12 (1.29 g, 3.00 mmol), a mixture of acetic acid and chloroform
(60 mL) (ratio 1:1) at 0 °C, and N-bromosuccinimide (1.20 g, 6.67 mmol).
The crude product was purified by gradient column chromatography
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Adv. Funct. Mater. 2011, 21, 718–728
2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
727

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1H NMR (400 MHz, CDCl3, δ): 6.98 (s, 2H); 3.61 (t, 2H, J = 7.2 Hz);
2.74 (t, 4H, J = 7.8 Hz); 1.64–1.58 (m, 6H); 1.30–1.26 (m, 38H); 0.87
(t, 9H, J = 6.4 Hz).
Development Canada (DRDC), Valcartier, Qc, Canada. A special thanks
to Sheila Rodman from Konarka Technologies Inc. for the molecular
weight measurements.
13C NMR (100 MHz, CDCl3, δ): 162.34; 145.17; 135.69; 132.84;
130.97; 126.55; 115.68; 38.81; 32.10; 32.03; 30.59; 30.03; 29.71; 29.60;
29.47; 29.42; 29.40; 28.67; 27.18; 22.90; 22.87; 14.35. Some peaks are
missing due to overlapping.
Received: August 26, 2010
Revised: October 4, 2010
Published online: December 22, 2010
Synthesis of 1,3-di(5’-bromo-4’-octylthien-2’-yl)-5-dodecylthieno[3,4-c]
pyrrole-4,6-dione (26): This compound was synthesized as described for
22 using 18 (0.60 g, 0.85 mmol), a mixture of acetic acid and chloroform
(20 mL) (ratio 1:1) at 0 °C, and N-bromosuccinimide (0.29 g, 1.65 mmol)
to afford 0.45 g of the product as a yellow solid (Y = 65%).
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1H NMR (400 MHz, CDCl3, δ ): 6.98 (s, 2H); 3.61 (t, 2H, J = 7.2 Hz);
2.74 (t, 4H, J = 7.8 Hz); 1.64–1.58 (m, 6H); 1.30–1.26 (m, 38H); 0.87
(t, 9H, J = 6.4 Hz).
13C NMR (100 MHz, CDCl3, δ): 162.61;143.95; 135.63; 132.09; 130.46;
128.50; 113.76; 38.92; 32.17; 32.13; 29.97; 29.89; 29.84; 29.77; 29.61;
29.52; 28.76; 27.25; 22.95; 22.94; 14.38. Some peaks are missing due to
overlapping.
Representative procedure for polymerization: 2,6-Bis(trimethyltin)-4,8-
di(2-ethylhexyloxyl)-benzo[1,2-b:3,4-b]dithiophene (27): (0.5 mmol),
compounds (4–6; 19–26) (0.5 mmol), Pd2(dba)3 (2 mol%), and P(o-Tol)3
(8 mol%) were put in a round bottom flask (25 mL). The system was
then purged three times by vacuum/argon cycling. The solids were
dissolved in 16 mL of dry and oxygen free toluene. The temperature was
increased from room temperature to 110 °C using an oil bath equipped
with a temperature controller. After 24 to 36 h of polymerization time,
100 μL of bromobenzene was added to the viscous reaction mixture as
an end-capping agent. After an additional hour of reaction, 100 μL of
trimethyltin phenyl was added to complete the end capping procedure.
After an additional hour of reaction, the whole mixture was cooled
to room temperature and poured in 500 mL of cold methanol. The
precipitate was filtered. Soxhlet extraction with acetone followed by
hexane removed catalyst residues and low molecular weight material.
Polymers were then extracted with chloroform. The solvent was reduced
to about 30 mL and the mixture was poured into cold methanol.
Polymers were recovered by filtration and further purification was done
to remove all metals impurities. Typical yields from 80 to 95% were
obtained.
Photovoltaic devices: The bulk heterojunction photovoltaic solar cell
were fabricated using the following structure: glass/ITO/PEDOT:PSS/
polymer:[60]PCBM/Al as illustrated in the Figure 1. Commercial ITO
(ITO-coated glass substrate (25 × 25 mm2)) with a sheet resistance
of ≤10 Ohms per sq (Thin Film Devices Inc, USA) were cleaned using
following sequence in an ultrasonic bath: detergent, water, acetone,
and 2-propanol. After treatments in plasma-O2 for 5 min, each ITO
substrate was patterned using photolithography techniques. Then
PEDOT:PSS, (Baytron P, H. C. Starck) was spin-coated (2000 rpm, 60 s)
on the ITO surface and dried at 120 °C for 1 h. After cooling the substrate,
an oDCB solution of polymer and [6,6]-phenyl-C61 butyric acid methyl
ester ([60]PCBM) (Nano-C, USA) mixture (1:2, wt/wt) was spin-coated.
The substrates were then put in a thermal evaporation chamber in order
to evaporate 70 nm of the aluminum layer (0.2–0.3 nm s−1 ) under high
vacuum (2 × 10−5 torr) through a shadow mask (active area 25 mm2). The
current–voltage characteristics of the photovoltaic cells were measured
using a Kheithey 2400 (I–V) Digital SourceMeter under a collimated
beam. The measurements were conducted under the irradiation
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by using a 150 W Oriel Instruments Solar Simulator and xenon
lamp. Light intensity was adjusted using a Gentec-eo power detector
(PS-330).
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Acknowledgements
This work was supported by grants from NSERC, a research contract
from the Sustainable Development Technology Canada (SDTC)
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program, and
a
research contract from Defence Research and
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2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Funct. Mater. 2011, 21, 718–728