Copolymers of diketopyrrolopyrrole and thienothiophene for photovoltaic cells

Article abstract of DOI:10.1039/c1jm10961h

Three copolymers of thieno[3,2-b]thiophene (TT) and diketopyrrolo[3,4-c] pyrrole (DPP) with a varying number of interconnecting thiophene units (nT) were synthesized by Suzuki polymerization. The PDPPnT-TT polymers display optical band gaps in the range of 1.33 to 1.52 eV and show ambipolar charge transport when applied in a field-effect transistor (FET) with hole and electron mobilities up to 3 × 10^-2 and 2 × 10^-3 cm ² V^-1 s^-1, respectively. Organic solar cells based on the PDPPnT-TT polymers as donors and the fullerene derivative [60]PCBM as acceptor achieve power conversion efficiencies up to 3.4% when the layer is deposited with the use of a co-solvent. The co-solvent strongly affects the morphology of the active layer and improves the performance. For these polymers the LUMO-LUMO offset with [60]PCBM correlates with the photocurrent and the maximum external quantum efficiency.

Full text of DOI:10.1039/c1jm10961h

View Article Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links <
C
Journal of
Materials Chemistry
Cite this: J. Mater. Chem., 2011, 21, 9224
www.rsc.org/materials
PAPER
Copolymers of diketopyrrolopyrrole and thienothiophene for photovoltaic
cells
*
ꢀ
Johan C. Bijleveld, R. A. Melanie Verstrijden, Martijn M. Wienk and Rene A. J. Janssen
Received 5th March 2011, Accepted 27th April 2011
DOI: 10.1039/c1jm10961h
Three copolymers of thieno[3,2-b]thiophene (TT) and diketopyrrolo[3,4-c]pyrrole (DPP) with a varying
number of interconnecting thiophene units (nT) were synthesized by Suzuki polymerization. The
PDPPnT-TT polymers display optical band gaps in the range of 1.33 to 1.52 eV and show ambipolar
charge transport when applied in a field-effect transistor (FET) with hole and electron mobilities up to
3 ꢀ 10ꢁ2 and 2 ꢀ 10ꢁ3 cm2 Vꢁ1 sꢁ1, respectively. Organic solar cells based on the PDPPnT-TT polymers
as donors and the fullerene derivative [60]PCBM as acceptor achieve power conversion efficiencies up
to 3.4% when the layer is deposited with the use of a co-solvent. The co-solvent strongly affects the
morphology of the active layer and improves the performance. For these polymers the LUMO–LUMO
offset with [60]PCBM correlates with the photocurrent and the maximum external quantum efficiency.
system thieno[3,2-b]thiophene (TT) have displayed charge carrier
mobility in FET devices up to 0.6 cm2 Vꢁ1 sꢁ1 7,8
These charac-
Introduction
.
Solar cells are rapidly developing as a prime application of
organic semiconductors.1 Published power conversion efficien-
cies (PCEs) are as high as 8.3% at present.2–4 Apart from having
a large overlap of the absorption spectrum with the solar emis-
sion and suitable HOMO and LUMO levels, many additional
prerequisites have to be met for a polymer to act as a successful
donor in organic photovoltaic cells such as molecular weight,
solubility and ability to form a nanoscale phase separated blend
with acceptor materials. In addition, a high charge carrier
mobility is required to optimize transport of photogenerated
charges to the electrodes.
teristics make the TT unit an interesting building block to be
further explored for incorporating in conjugated polymers for
solar cell applications. Moreover, we recently showed how a high
open circuit voltage of 1.15 V could be obtained with a TT-based
polymer.9
To control the band gap in conjugated polymers the electron
rich TT unit can be combined with an electron deficient unit
along the chain. The 1,4-diketopyrrolo[3,4-c]pyrrole (DPP) unit
has emerged as a promising unit in semiconducting polymers
for photovoltaic10,11–18 and transistor applications.19–21
Polymers based on 3,6-di-2-thienyl-1,4-diketopyrrolo[3,4-c]
pyrrole (T-DPP-T) alternating with intermediate 2,5-thienylene
(PDPP3T) and 1,4-phenylene units (PDPPTPT) (Fig. 1) can
provide power conversion efficiencies (PCEs) under simulated
AM1.5G light up to 5.5% and exhibit ambipolar charge trans-
port in FETs.17,18 A performance of PCE ¼ 4% was also obtained
when T-DDP-T was alternating with 3,30-didodecyl-2,20-bithio-
phene-5,50-diyl as in PBBTDPP2 (Fig. 1).10 On the other hand,
for the T-DPP-T homopolymer (PDPP2T) photoinduced elec-
tron transfer to PCBM was inhibited by a small LUMO–LUMO
offset.16 From these examples it is clear that by controlling the
aromatic segment alternating with the T-DPP-T segment, poly-
mers with high efficiencies can be obtained.
By alternating electron rich and electron poor moieties in the
main chain of the polymer it is possible to accurately tune the
optical band gap and redox potentials that control the coverage
of optical absorption with the solar spectrum and the open
circuit voltage of the cell. The design of materials with high
charge carrier mobility, however, is more difficult to control. The
charge carrier mobility is important for separating and collecting
charges efficiently. Increasing the tendency of semiconducting
polymers to aggregate and crystallize, as well as increasing the
size of p-conjugated segments, generally leads to more p-stack-
ing and to improved interchain charge transfer, which is benefi-
cial for charge transport. Consequently, fusing of aromatic rings
and optimizing thin film processing has resulted in field-effect
transistors (FETs) in which conjugated polymers display charge
carrier mobilities in the order of 0.1 cm2 Vꢁ1 sꢁ1 for electrons5 and
>1 cm2 Vꢁ1 sꢁ1 for holes.6 Polymers based on the fused ring
This justifies further exploring these structures to increase
performance and to combine the 1,4-diketopyrrolo[3,4-c]pyrrole
(DPP) units with thieno[3,2-b]thiophenes (TT). In this paper we
present three such copolymers with a varying number of inter-
connecting thiophene units (PDPPnT-TT, Fig. 1) to control the
optical band gap, the HOMO and LUMO levels and the charge
transport characteristics. A close derivative of PDPP2T-TTb
presented here, but with 2-octyldodecyl side chains instead of
Molecular Materials and Nanosystems, Eindhoven University of
Technology, PO Box 513, 5600 MB, The Netherlands. E-mail: r.a.j.
janssen@tue.nl; Fax: +31 40 451036; Tel: +31 40 2473597
9224 | J. Mater. Chem., 2011, 21, 9224–9231
This journal is ª The Royal Society of Chemistry 2011

View Article Online
Fig. 1 Structures of the new PDPPnT-TT polymers and previously reported PDPP2T, PDPP3T, PDPPTPT and PBBTDPP2.
2-hexyldecyl, was shown to have
a
hole mobility of
monomer 7 was a Stille coupling of 1 with 2-(tributylstannyl)
thiophene to yield 5. Bromination with NBS led to 6, which was
borylated in the same way as 1 and 3 to give monomer 7 in high
yield. The T-DDP-T monomers, 3,6-bis(5-bromothien-2-yl)-2,5-
dihydro-2,5-di(2-alkyl)-pyrrolo[3,4-c]pyrrolo-1,4-dione 8 and 9
(Scheme 2) were synthesized according to literature
procedures.19,23
0.94 cm2 Vꢁ1 sꢁ1 in an FET21 and recently DPP polymers
combined with TT were found to exhibit a maximum hole
mobility of 1.95 cm2 Vꢁ1 sꢁ1 and a PCE of 5.4% in solar cell when
combined with [70]PCBM.22 We show that the PDPPnT-TT
polymers (Fig. 1) described in this study exhibit ambipolar
charge transport and that they provide a PCE up to 3.4% with
[60]PCBM as acceptor in solar cells and discuss the structure
property relations that rationalize these results.
Results and discussion
Synthesis
The synthesis of the TT and T-TT-T monomers 2, 4 and 7 is
shown in Scheme 1. The synthesis of 2,5-dibromo-3,6-di(40-eth-
yloctyl)thieno[3,2-b]thiophene (1) has been described previ-
ously.9 2,5-Dibromothieno[3,2-b]thiophene (3) was obtained
from TCI. The corresponding boronic esters 2 and 4 were
obtained via debromination with n-butyllithium and subsequent
borylation of the formed anions. The conversion occurred with
reasonable yield for 2, but the yield for 4 was poor due to
a difficult purification and the high purity required for the
polymerization reaction. The first step in the synthesis of
Scheme 2 Synthesis of PDPPnT-TTs. i: Pd2(dba)3, PPh3, aliquat 336,
K3PO4, water, toluene, 105 ꢂC.
Scheme 1 Synthesis of the TT and T-TT-T monomers 2, 4, and 7.
This journal is ª The Royal Society of Chemistry 2011
J. Mater. Chem., 2011, 21, 9224–9231 | 9225

View Article Online
The PDPPnT-TT polymers were synthesized as described in
Scheme 2 using a Suzuki reaction with Pd2(dba)3 as the catalyst.
The polymers were obtained with number average molecular
weights (Mn) in the range of 9000–15 000 g molꢁ1 and poly-
dispersity indices varying from 2 to 4 (Table 1). The number
average degree of polymerization (DPn) of 10–14 is similar for
the three polymers and implies that they have up to ꢃ70 aromatic
units along the chain.
Optical, electrochemical and electrical properties
The optical absorption spectra of the polymers are shown in
Fig. 2 for solutions in chloroform and for thin films on glass.
Going from solution to the solid state there is a small red shift of
the absorption, signifying aggregation of chains. The optical
band gaps in the solid state, determined from the onsets of
absorption of PDPP2T-TTa and PDPP2T-TTb (1.31–1.33 eV)
are very similar to that of PDPP3T (1.36 eV) (Table 1),17 showing
that the effect of the thieno[3,2-b]thiophene-2,5-diyl unit (TT) on
the optical gap is similar to that of thiophene-2,5-diyl (T). The
similar band gap is somewhat unexpected because the higher
aromatic stabilization energy of thieno[3,2-b]thiophene
compared to a single thiophene, would result in an increased gap
between the aromatic and quinoid states. The optical band gap of
PDPP4T-TT is much larger (1.52 eV), also compared to
PBBTDPP2 (1.40 eV),10 and resembles that of PDPPTPT
(1.53 eV).18 This shows that extending the number of thiophene
units between the DPP units can be used to widen the optical
band gap.
Fig. 2 Normalized optical absorption spectra of PDPPnT-TTs in chlo-
roform solution (a) and as thin films on glass (b).
The HOMO and LUMO levels of the polymers were deter-
mined by cyclic voltammetry (CV) in o-DCB solution (Table 1)
using ꢁ5.23 eV as the offset of Fc/Fc+ vs. vacuum.24 PDPP2T-
TTa and PDPP2T-TTb have very similar HOMO levels, but their
LUMO levels differ by 80 mV. This difference is also reflected in
the optical band gap in solution, which is a little larger for
PDPP2T-TTa. The reason for the higher band gap could be that
the alkyl chains on the thieno[3,2-b]thiophene in PDPP2T-TTa
induce twisting between adjacent units, decreasing the orbital
overlap and conjugation. For PDPP4T-TT the low solubility in
pure o-DCB required a CV measurement in a mixture of o-DCB
and chloroform. Due to reduction of chloroform at negative
potentials, only the HOMO level of PDPP4T-TT could be
obtained from the CV experiments. The LUMO of PDPP4T-TT
was estimated at ꢁ3.69 ꢄ 0.03 V from the optical band gap and
the HOMO level, correcting for the average difference between
Eg,sol and eEg,CV for the other two polymers (DEg ¼ 0.06 ꢄ
0.03 eV) (Table 1), to give eELUMO z eEHOMO ꢁ Eg,sol ꢁ DEg.
Table 1 also shows the LUMO–LUMO offset a of the polymers
and PCBM, with ELUMO(PCBM) ¼ ꢁ4.16 eV.25
The pristine polymers were applied in an FET structure to
measure their charge carrier mobility. All polymers show ambi-
polar behaviour (Fig. 3). The transfer curves of PDPP2T-TTa
and PDPP2T-TTb are similar and the hole and electron mobil-
ities (mh and me, Table 2) determined from the slope of the square
root of source–drain current (|IDS|
1/2) vs. the gate voltage (VG) in
the saturation regime are within one order of magnitude.
Apparently, the presence of the alkyl side chains does not
significantly affect the charge transport properties. The transfer
curves of PDPP4T-TT are notably different from the other two
and especially me is lower. At present we do not have an expla-
nation for this fact. The low me results in a larger on/off ratio at
negative gate voltages. For PDPP2T-TTa and PDPP2T-TTb mh
and me are fairly balanced, but the hole mobility of PDPP2T-TTb
is less than the value reported for a closely related derivative
Table 1 Physical properties of the polymers
Polymer
Mn/kg molꢁ1
DPn
PDI
Eg,sol/eV
Eg,film/eV
EHOMO/eV
ELUMO/eV
Eg,CV/V
DEg/eV
aa/eV
PDPP2T-TTa
PDPP2T-TTb
PDP4T-TT
PBBTDPP2c
PDPP3Td
13
9
15
20
54
—
14
10
11
20
65
—
2.0
3.9
4.0
3.4
3.2
—
1.39
1.34
1.58
1.70
1.36
1.53
1.33
1.31
1.52
1.40
1.30
1.53
ꢁ5.31
ꢁ5.33
ꢁ5.33
ꢁ5.53
ꢁ5.30
ꢁ5.48
ꢁ3.84
ꢁ3.92
ꢁ3.69b
ꢁ3.83
ꢁ3.74
ꢁ3.66
1.47
1.41
0.08
0.03
0.32
0.24
ꢃ0.47
1.70
1.56
1.82
0.00
0.20
0.29
0.33
0.42
PDPPTPTe
0.50
a ¼ ELUMO – ELUMO(PCBM), with ELUMO(PCBM) ¼ ꢁ4.16 eV.25 Estimated, see text. c Ref. 10. Ref. 17. Ref. 18.
a
b
d
e
9226 | J. Mater. Chem., 2011, 21, 9224–9231
This journal is ª The Royal Society of Chemistry 2011

View Article Online
performance of the cells. Tapping mode AFM showed that layers
processed without DIO contain large (100–200 nm) globular
domains of semicrystalline [60]PCBM (Fig. 4a–c), but when
a small amount of DIO was added to the mixture of polymer and
[60]PCBM in chloroform before the spin coating, the large [60]
PCBM clusters disappeared and the length scale of phase sepa-
ration decreased (Fig. 4d–f). Simultaneously, the power
conversion efficiency of the cells increased significantly (Fig. 5).
For cells made with PDPP2T-TTb and PDPP4T-TT an optimal
performance was found when 25 mg mlꢁ1 DIO was added to the
chloroform, while cells with PDPP2T-TTa needed only 10 mg
mlꢁ1. The marked change in performance when using DIO is
Fig. 3 p-type (a) and n-type (b) transfer curves of FETs (channel width
and length are 10 mm and 2500 mm, respectively) made from PDDPnT-
TTs 2.
Table 2 Hole and electron mobilities of PDDPnT-TTs in FETs
mh/cm2
Vꢁ1 sꢁ1
me/cm2
Vꢁ1 sꢁ1
Polymer
PDPP2T-TTa
PDPP2T-TTb
PDPP4T-TT
1 ꢀ 10ꢁ2
3 ꢀ 10ꢁ2
5 ꢀ 10ꢁ3
2 ꢀ 10ꢁ3
9 ꢀ 10ꢁ3
2 ꢀ 10ꢁ5
recently.21 Differences in molecular weight, annealing tempera-
ture, morphology and device layout may account for these
differences.
Photovoltaic devices
Solar cells were built by spin coating the active layer from
a solution of [60]PCBM and the polymer on a sheet of glass,
covered with ITO and a layer of PEDOT:PSS. LiF and Al were
evaporated in vacuum and served as the reflecting second elec-
trode. Considering the small offset of the work function of
PEDOT:PSS (ꢁ5.1 eV26) with the HOMO energy levels
(ꢃꢁ5.3 eV) of the new polymers an ohmic contact can be
expected at the hole collecting contact, while LiF/Al is known to
form a good contact with [60]PCBM for collecting the elec-
trons.27 The composition of the active layer was optimized by
varying the polymer:[60]PCBM ratio and the highest efficiencies
were obtained using an excess of [60]PCBM (by weight)
(Table 3).
Fig. 4 Tapping mode AFM images (lateral dimensions 5 ꢀ 5 mm2) of
layers consisting of [60]PCBM and PDPP2T-TTa (a and d), PDPP2T-
TTb (b and e), and PDPP4T-TT (c and f) in the ratios mentioned in Table
3, without DIO (a–c) and with the optimal amount of DIO (d–f). Height
scales are 10 nm (a, c, d, and f), 15 nm (e) and 20 nm (b).
Similar to previous reports,28,29 we found that the use of
1,8-diiodooctane (DIO) as a co-solvent is beneficial in creating
a morphology of the active layer that leads to improved
Table 3 Characteristics of the photovoltaic cells based on [60]PCBM and DPP polymers
Ratioa
Voc/V
FF
PCE (%)
Max EQE
b
Jsc /mA cmꢁ2
Polymer
PDPP2T-TTa
PDPP2T-TTb
PDPP4T-TT
PBBTDPP2
PDPP3T
1 : 2
6.0
2.8
8.2
9.0
8.3
9.3
0.68
0.63
0.65
0.63
0.68
0.79
0.59
0.61
0.63
0.54
0.67
0.63
2.3
1.2
3.4
3.2
3.8
4.6
0.20
0.05
0.35
0.42
0.33
0.45
1 : 1.5
1 : 1.5
1 : 2
1 : 2
1 : 2
PDPPTPT
a
Polymer:[60]PCBM (w/w). b Jsc was derived from spectral response measurements under the bias of approximately one sun.
This journal is ª The Royal Society of Chemistry 2011
J. Mater. Chem., 2011, 21, 9224–9231 | 9227

View Article Online
Fig. 5 Estimated efficiency (a) and current density (b) as a function of
Fig. 6 J–V curves (a) and external quantum efficiency (b) of the
photovoltaic cells with [60]PCBM and PDPPnT-TT.
the amount of DIO used in the spin coating mixture.
molecular weight (Mn) is less than for the three previously
reported DPP polymers (Table 1). Especially for PDPP3T, the
influence of Mn on the PCE has been found to be significant and
this possibly hampers a straightforward correlation between
EQE and a.
a consequence of the change in morphology, which results in an
increase in short circuit current density (Fig. 5b) while open
circuit voltage (Voc) and fill factor (FF) remain fairly constant.
The variation in Voc was less than 5% and FFs were within 20%
of the best value. Addition of extra DIO to the spin coating
mixture resulted in a reduced performance of the cells as a result
of a lower photocurrent.
Conclusions
The J–V curves and the monochromatic external quantum
efficiencies (EQEs) of the photovoltaic cells are shown in Fig. 6.
The relevant parameters are listed in Table 3. For all cells, the
FFs are in the same range, despite the fact that the hole mobility
of PDPP4T-TT is less than for PDPP2T-TTa and PDPP2T-TTb
in an FET. The onsets of the EQEs correspond to the optical
band gaps of the polymers, extending up to 900 nm for the cells
made with PDPP2T-TTa and PDPP2T-TTb. Fig. 6b shows,
however, that for PDPP2T-TTb the EQE reaches only 5% in the
region where the polymer absorbs, while for PDPP2T-TTa and
PDPP4T-TT the EQE is reaching much higher values of 20% and
35%, respectively. The increase in EQE going from PDPP2T-
TTb, via PDPP2T-TTa, to PDPP4T-TT correlates with the
LUMO–LUMO offset energy a (Table 1). The very low EQE for
PDPP2T-TTb is consistent with the notion that its LUMO–
LUMO offset a ¼ 0.24 eV with [60]PCBM is less than the
minimal value of 0.3–0.4 eV that is generally considered to be
required for efficient exciton dissociation.30,31 This conclusion is
supported for three other high efficiency DPP derivatives, for
which relevant parameters are collected in Tables 2 and 3. We
note that for the PDPPnT-TT polymers listed in Table 3 the
Three new polymers based on thieno[3,2-b]thiophene and dike-
topyrrolo[3,4-c]pyrrole with a varying number of connecting
thiophene units have been synthesized and characterized. The
polymers exhibit suitable absorption spectra and energy levels
for use as a donor in OPCs. All polymers show ambipolar charge
transport with hole mobilities in the 10ꢁ2 cm2 Vꢁ1 sꢁ1 range. The
morphology of the active layers and the photovoltaic perfor-
mance of the resulting OPCs with [60]PCBM as acceptor strongly
depend on the use of a co-solvent. The best solar cells exhibited
a PCE of 3.4%.
In terms of structure property relations it is interesting to
consider that a high PCE in a solar cell can be reached when it
effectively converts photons into electrons and does not lose too
much energy in this process, i.e. keeping the open-circuit voltage
close to the optical band gap. The EQE is a measure for the
photon to electron conversion efficiency, while eVoc ꢁ Eg
measures the energy loss. Previously we have experimentally
established that eVoc ¼ Eg ꢁ 0.6 eV defines the maximum
attainable Voc in organic donor–acceptor solar cells at room
temperature under 1 sun illumination because at that point the
driving force for electron transfer from the optical band gap to
9228 | J. Mater. Chem., 2011, 21, 9224–9231
This journal is ª The Royal Society of Chemistry 2011

View Article Online
the charge transfer state is becoming very small (ꢃ0.1 eV).24
When Eg ꢁ eVoc becomes less than 0.6 eV photoinduced electron
transfer is impeded and energy transfer between the donor and
acceptor will be favoured but no charges are formed. For
PDPP2T-TTa and PDPP2T-TTb in combination with [60]
PCBM we find that Eg ꢁ eVoc ¼ 0.65–0.68 eV, i.e. very close to
the threshold value. Energetically this is favourable for solar
energy conversion, but we conjecture that approaching the limit
might be a reason for the reduced photon to electron conversion
that can be inferred from the lower EQE (Table 3). For PDPP4T-
TT:[60]PCBM, Eg ꢁ eVoc amounts to 0.87 eV and—reasoning
along the same line—this is consistent with its much higher EQE
(Table 3). For the three other polymers listed in Table 3, similar
arguments explain some of the differences. We note, however,
that for these materials the molecular weight is much higher
(Table 1), which is known to be favourable for achieving high
EQEs, most likely via affecting the blend morphology, size of the
domains and charge carrier mobility.17
thermally oxidized SiO2 layer as the gate dielectric. Using
conventional photolithography, gold source and drain electrodes
were defined in a bottom contact device configuration with
channel width and length of 10 mm and 2500 mm, respectively. A
10 nm layer of titanium was used acting as an adhesion layer for
the gold on SiO2. The SiO2 layer was exposed to the vapour of
the primer hexamethyldisilazane for 60 min. prior to semi-
conductor deposition in order to passivate the surface of the
dielectric. Polymer films were spun from a chloroform solution at
1500 rpm for 30 s. Freshly prepared devices were annealed in
a dynamic vacuum of 10ꢁ5 mbar at 100 C for 72 h to remove
ꢂ
traces of solvent. All electrical measurements were performed in
a vacuum using an HP 4155C semiconductor parameter
analyzer.
Photovoltaic devices were processed on cleaned patterned ITO
glass substrates. A mixed solution of polymer and [60]PCBM
(Solenne BV) in a weight ratio as indicated in Table 3 was spin
coated from chloroform with the appropriate amount of DIO on
50 nm of PEDOT:PSS (Clevios P VP AI 4083, H.C. Starck). The
devices were transported into a glovebox with nitrogen atmo-
sphere. LiF (1 nm) and Al (100 nm) were thermally evaporated
on top of the active layer at a pressure of 10ꢁ7 mbar. The devices
had an active area of 0.091 cm2 which is determined by the
overlap of the ITO and the evaporated aluminium. Current–
voltage curves were measured with a Keithley 2400 source meter
under nitrogen in the dark and under simulated solar illumina-
tion of 100 mW cmꢁ2 created by white light from a tungsten-
halogen lamp, filtered by a Schott GG385 UV filter and a Hoya
LB120 daylight filter. The EQE was measured with a Stanford
Research Systems Model SR830 DSP lock-in-amplifier by using
modulated monochromatic light from a tungsten halogen lamp
in combination with a monochromator (Oriel, Cornerstone 130).
During the measurement the device was operating at 1 equivalent
sun intensity by use of a bias laser (532 nm). A calibrated Si cell
was used as a reference. The device was kept behind a quartz
window in a nitrogen filled container. The thickness of the active
layers in the photovoltaic devices was measured on a Veeco
Dektak150 profilometer. Tapping mode AFM was measured on
In conclusion, the new PDPPnT-TT polymers offer promising
efficiencies in combination with [60]PCBM. For all polymers the
obtained molecular weights are not very high and this is most
likely one of the parameters that confines reaching higher PCEs.
In addition, for PDPP2T-TTa and PDPP2T-TTb, the photoin-
duced electron transfer is limited by a low offset between the
optical band gap and the energy of the charge transfer state. The
design of new materials should therefore concentrate on
achieving high molecular weights and creating an energy offset
Eg ꢁ eVoc that is at least 0.6 eV, but preferably somewhat larger
to ensure a high photon to electron quantum yield.
Experimental section
General procedures
All synthetic procedures were performed under an argon atmo-
sphere. Commercial chemicals were used as received. Dry
ꢁ
solvents were distilled over 4 A molecular sieves. 2,5-Dibromo-
3,6-di(40-ethyloctyl)thieno[3,2-b]thiophene (1) and 3,6-bis(5-
bromothien-2-yl)-2,5-dihydro-2,5-di(2-alkyl)-pyrrolo[3,4-c]pyr-
rolo-1,4-diones (8 and 9) were synthesized according to literature
procedures.9,19,23
a
MFP-3D (Asylum research) using PPP-NCHR probes
(Nanosensors).
1H NMR and 13C NMR spectra were recorded at 400 MHz
and 100 MHz respectively on a VARIAN mercury spectrometer
with CDCl3 as the solvent and tetramethylsilane (TMS) as the
internal standard. The peaks are given in ppm, relative to TMS
(0 ppm). Molecular weights were determined with GPC at 80 ꢂC
on a PL-GPC 120 system using a PL-GEL 5 m MIXED-C
column and o-DCB as the eluent.
2,5-Di(4,4,5,5-tetramethyl-1,3,2-dioxaboralan-2-yl)-3,6-di(40-
ethyloctyl)thieno[3,2-b]thiophene (2)
To a solution 2,5-dibromo-3,6-di(40-ethyloctyl)thieno[3,2-b]thiophene
(1) (141 mg, 0.24 mmol) in dry THF (6 ml), a 1.6 M solution of
n-buytllithium (0.8 ml, 1.28 mmol) in hexanes was added at ꢁ70 ꢂC.
ꢂ
This was stirred for 30 min at ꢁ70 C, after which 2-isopropoxy-
Optical absorption spectra were recorded on a Perkin Elmer
Lambda 900 UV/vis/near-IR spectrophotometer. Cyclic vol-
tammograms were recorded with a scan rate of 0.1 V sꢁ1 under an
(1,3,2)-dioxaboralane (0.3 ml, 1.47 mmol) was added at ꢁ70 ꢂC. This
mixture was allowed to reach room temperature overnight. The
reaction mixture was poured out in water and heptane and the organic
layer was washed with brine. The solvents were evaporated from the
organic layer and the resulting oil was subjected to column chroma-
tography (eluent heptane/ethyl acetate). The result of this was 89 mg
(51%) of a white solid. 1H NMR: 2.96 (t, 2H, J 6.0), 1.67 (m, 2H), 1.34
(s, 24H), 1.2 (m, 22H), 0.88 (m, 6H), 0.82 (m, 6H). 13C NMR: 146.22
(q), 146. 06 (q), 83.77 (q), 83.68 (q), 38.52 (s), 32.94 (s), 32.82 (s), 30.11
(t), 28.93 (s), 27.01 (s), 25, 80 (s), 24.78 (p), 23.14 (s), 14.17 (p), 10.85 (p).
MS (MALDI): calculated: 672.46, found: 672.46 (M+)
inert atmosphere with
1 M tetrabutylammonium hexa-
fluorophosphate (TBAPF6) in o-DCB as the electrolyte. The
working electrode was a platinum disk and the counter electrode
was silver rod. The concentration of the samples in the electrolyte
was approximately 2 mM, based on monomers. Fc/Fc+ was used
as internal standard.
Field-effect transistors were fabricated using heavily doped
silicon wafers as the common gate electrode with a 200 nm
This journal is ª The Royal Society of Chemistry 2011
J. Mater. Chem., 2011, 21, 9224–9231 | 9229

View Article Online
2,5-Di(4,4,5,5-tetramethyl-1,3,2-dioxaboralan-2-yl)-thieno[3,2-
2-isopropoxy-(1,3,2)-dioxaboralane (0.5 ml, 2.9 mmol) was added.
The resulting mixture was allowed to reach room temperature after
which it was poured out in heptane, which was washed with water
and brine. The resulting solid was subjected to column chroma-
tography (silica, eluent heptanes/ethyl acetate). This yielded 231 mg
(0.28 mmol, 97%) of a yellow-brown solid. 1H NMR: 7.58 (d, 2H, J
3.2), 7.23 (d, 2H, J 3.2), 2.88 (t, 4H, J 7.6), 1.72 (m, 4H), 1.35 (s,
24H), 1.30–1.2 (m, 22H), 0.88 (m, 6H), 0.82 (m, 6H). 13C NMR:
143.51 (q), 138.93 (q), 137.55 (t), 132.36 (q), 131.69 (q), 128.27 (t),
113.87 (t), 84.17 (q), 38.55 (s), 33.12 (s), 32.70 (s), 29.26 (s), 28.89 (s),
26.45 (s), 25.82 (s), 24.79 (s), 24.75 (p), 23.13 (s), 14.18 (p), 10.86 (p).
MS (MALDI): calculated: 836.43, found: 836.29 (M+).
b]thiophene (4)
To a solution of 2,5-dibromothieno[3,2-b]thiophene (3) (0.51 g,
1.71 mmol) in 12 ml of dry THF, a 1.6 M solution of n-butyl-
lithium (5.4 ml, 8.6 mmol) in hexanes was added at ꢁ70 ꢂC. This
was stirred for 60 min at ꢁ70 ꢂC, after which 2-isopropoxy-
(1,3,2)-dioxaboralane (2.1 ml, 10.3 mmol) was added at ꢁ70 ꢂC.
This mixture was allowed to reach room temperature overnight.
The reaction mixture was poured out in water and heptane and
the organic layer was washed with brine. The solvents were
evaporated from the organic layer and the resulting solid was
recrystallized multiple times from heptanes to yield 43 mg (5%) of
1
a white solid. H NMR: 7.75 (s, 2H), 1.24 (s, 24H). 13C NMR:
PDPP2T-TTa
146.57 (q), 128.80 (t), 84.46 (q), 77.2 (q), 24.83 (p). MS
(MALDI): calculated: 392.15, found: 392.06 (M+)
To a degassed solution of 2,5-di(4,4,5,5-tetramethyl-1,3,2-dioxa-
boralan-2-yl)-3,6-di(40-ethyloctyl)thieno[3,2-b]thiophene (2) (27
mg, 40 mmol), 3,6-bis(5-bromo-2-thienyl)-2,5-dihydro-2,5-di(20-
hexyldecyl)-pyrrolo[3,4-c]pyrrolo-1,4-dione (8) (28 mg, 40 mmol),
50% aliquat solution (v/v) in toluene (0.3 ml), K3PO4 (18 mg, 84
mmol), of triphenlyphosphine (2.0 mg, 2.1 mmol) and water (0.1
ml) in toluene (2 ml), tris(dibenzylideneacetone)dipalladium(0)
(2 mg, 1.7 mmol) was added. This mixture was stirred at 105 ꢂC for
3 days, after which it was precipitated in methanol. The solids were
filtered off, redissolved in CHCl3 (75 ml) and refluxed with 28%
NH3 solution in water (75 ml) for 3 h. The layers were separated
and the organic layer was stirred with EDTA (250 mg) overnight,
after which water (100 ml) was added and the liquids were stirred
for 1 h. The layers were separated and the organic layer was
reduced in volume by evaporating most of the chloroform. The
polymer was precipitated in methanol and filtered through
a Soxhlet thimble. The polymer was extracted with methanol,
after which the thimble was allowed to dry in ambient conditions.
Then the polymer was extracted with hexane and chloroform. The
solvent from the chloroform fraction was evaporated and the solid
was precipitated in methanol. The polymer was collected by
filtering over a 0.45 mm PTFE membrane filter and dried in
a vacuum oven to yield PDPP2T-TTa (21 mg, 56%) as a dark
green powder. 1H NMR: 9.15 (b, 2H), 7.26 (b, 2h), 4.08 (b, 4H),
2.95 (b, 4H), 2.70 (b, 2H), 1.97 (b, 4H), 1.75 (b, 4H), 1.5–1.1 (b,
34H), 0.95–0.8 (b, 24H). GPC: Mn ¼ 13 000 g molꢁ1, PDI ¼ 2.5.
Soxhlet extraction using o-dichlorobenzene to selectively extract
higher molecular weight polymer fractions resulted in degrada-
tion of the polymer at the increased temperature (180 ꢂC).
2,5-Di(thien-2-yl)-3,6-di(40-ethyloctyl)thieno[3,2-b]thiophene (5)
To a solution of 2,5-dibromo-3,6-di(40-ethyloctyl)thieno[3,2-b]
thiophene (1) (0.45 g, 0.78 mmol) and 2-(tributylstannyl)thio-
phene (0.55 ml, 1.8 mmol) in toluene (5 ml), tetrakis(triphenyl-
phosphine)palladium(0) (25 mg, 21 mmol) was added. The
resulting mixture was stirred at 130 ꢂC overnight and diluted with
10 ml heptane. This suspension was filtered over a Buchner
funnel and the solvents of the filtrate were evaporated in vacuum.
€
The resulting solid was subjected to kugelrohr distillation to
remove traces of bithiophene. This yielded 250 mg (0.43 mmol,
55%) of a yellow solid. 1H NMR: 7.36 (d, 1H), 7.18 (d, 1H), 7.1 (t,
1H), 2.86 (t, 2H), 1.75 (q, 2H), 1.28 (m, 22H), 0.89 (m, 12H). 13
C
NMR: 138.47 (q), 136.57 (q), 132.0 (q), 131.29 (q), 127.46 (t),
126.15 (t), 125.72 (t), 38.56 (s), 33.18 (s), 32.71 (s), 29.18 (s), 28.92
(s), 26.22 (s), 25.87 (s), 23.15 (s), 14.19 (p), 10.88 (p). MS
(MALDI): calculated: 584.26, found: 584.16 (M+)
2,5-Di(5-bromothien-2-yl)-3,6-di(40-ethyloctyl)thieno[3,2-b]
thiophene (6)
To a solution of 2,5-di(thien-2-yl)-3,6-di(40-ethyloctyl) thieno-
thiophene (5) (250 mg, 0.43 mmol) in DMF (30 ml), N-bromo-
succinimide (0.17 g, 0.95 mmol) was added and the reaction
mixture was stirred overnight. After this the mixture was poured
out in water/heptane and the heptane layer was washed with
water and brine. The solvents were evaporated and the resulting
yellow solid was subjected to column chromatography (silica,
eluent heptane) to yield 270 mg (0.36 mmol, 85%) of a yellow
PDPP2T-TTb
1
solid. H NMR: 7.04 (d, 2H, J 4.0), 6.90 (d, 2H, J 4.0), 2.80 (t,
4H, J 4.0), 1.71 (m, 4H), 1.35–1.2 (m, 22H), 0.88 (m, 6H), 0.82
(m, 6H). 13C NMR: 138.60 (q), 137.91 (q), 132.54 (t), 130.64 (q),
130.33 (q), 126.51 (t), 112, 43 (t), 38.54 (s), 33.12 (s), 32.69 (s),
29.13 (t), 28.92 (s), 26.21 (s), 25.87 (s), 23.16 (s), 14.21 (p), 10.89
(p). MS (MALDI): calculated: 742.08, found: 741.98 (M+)
Same procedure as for PDPP2T-TTa was used, but now 2,5-di
(4,4,5,5-tetramethyl-1,3,2-dioxaboralan-2-yl)-thieno[3,2-b]thio-
phene (4) (43 mg, 0.11 mmol) and 3,6-bis(5-bromo-2-thienyl)-
2,5-dihydro-2,5-di(2-hexyldecyl)-pyrrolo[3,4-c]pyrrolo-1,4-dione
(9) (100 mg, 0.11 mmol) were used as the monomers. Reaction
time was 16 h. Yield: 71 mg (67%). 1H NMR: 9.12 (b, 2H), 7.13
(b, 4H), 4.03 (b, 4H), 1.93 (b, 2H), 1.5–1.1 (b, 48H), 1.0–0.6 (b,
12H). GPC: Mn ¼ 9000 g molꢁ1, PDI ¼ 3.9.
2,5-Di(5-(4,4,5,5-tetramethyl-1,3,2-dioxaboralan-2-yl)thien-2-
yl)-3,6-di(40-ethyloctyl)thieno [3,2-b]thiophene (7)
To a solution of 2,5-di(5-bromothien-2-yl)-3,6-di(40-ethyloctyl)
PDPP4T-TT
thieno[3,2-b]thiophene (6) (205 mg, 0.28 mmol) in dry THF (20 ml)
ꢂ
at ꢁ78 C, 1.6 M butyllithium (0.8 ml, 1.28 mmol) solution was
Same procedure as for PDPP2T-TTa was used, but now 2,5-di(5-
(4,4,5,5-tetramethyl-1,3,2-dioxaboralan-2-yl)thien-2-yl)-3,6-di-
added. The reaction mixture was stirred for 30 min after which
9230 | J. Mater. Chem., 2011, 21, 9224–9231
This journal is ª The Royal Society of Chemistry 2011

View Article Online
(40-ethyloctyl)thienothiophene (7) (102.25 mg, 0.12 mmol) and
3,6-bis(5-bromo-2-thienyl)-2,5-dihydro-2,5-di(2-hexyldecyl)-pyr-
rolo[3,4-c]pyrrolo-1,4-dione (9) (111 mg, 0.12 mmol) were used
as the monomers. Reaction time was 40 min. Yield: 70 mg (44%).
1H NMR: 8.95 (b, 2H), 7.31 (b, 2H), 7.16 (b, 2H), 7.01 (b, 2H),
4.06 (b, 4H), 2.91 (b, 4H), 2.71 (b, 2H), 1.92 (b, 4H), 1.5–1.1 (b,
66H), 1.0–0.6 (b, 24H). GPC: Mn ¼ 15 000 g molꢁ1, PDI ¼ 4.0.
10 M. M. Wienk, M. Turbiez, J. Gilot and R. A. J. Janssen, Adv. Mater.,
2008, 20, 2556.
€
11 Y. Zou, D. Gendron, R. Badrou-Aıch, A. Najari, Y. Tao and
M. Leclerc, Macromolecules, 2009, 42, 2891.
12 E. Zhou, S. Yamakawa, K. Tajima, C. Yang and K. Hashimoto,
Chem. Mater., 2009, 21, 4055.
13 Y. Zou, D. Gendron, R. Neagu-Plesu and M. Leclerc,
Macromolecules, 2009, 42, 6361.
14 L. Huo, J. Hou, H.-Y. Chen, S. Zhang, Y. Jiang, T. L. Chen and
Y. Yang, Macromolecules, 2009, 42, 6564.
15 C.-Y. Yu, C.-P. Chen, D.-H. Chan, G.-W. Hwang and C. Ting, Chem.
Mater., 2009, 21, 3262.
Acknowledgements
16 A. P. Zoombelt, S. G. J. Mathijssen, M. G. R. Turbiez, M. M. Wienk
and R. A. J. Janssen, J. Mater. Chem., 2010, 20, 2240.
17 J. C. Bijleveld, A. P. Zoombelt, S. G. J. Mathijssen, M. M. Wienk,
M. Turbiez, D. M. de Leeuw and R. A. J. Janssen, J. Am. Chem.
Soc., 2009, 131, 16616.
18 J. C. Bijleveld, V. S. Gevaerts, D. Di Nuzzo, M. Turbiez,
S. G. J. Mathijssen, D. M. de Leeuw, M. M. Wienk and
R. A. J. Janssen, Adv. Mater., 2010, 22, E242.
We thank Dr Simon Mathijssen and Prof. Dago de Leeuw for
their help with FETs and mobility measurements. The research
was supported by a TOP grant of the Chemical Sciences (CW)
division of the Netherlands Organization for Scientific Research
(NWO) and is part of the Joint Solar Programme (JSP). The JSP
is co-financed by the Foundation for Fundamental Research on
Matter (FOM), Chemical Sciences of NWO and the Foundation
Shell Research. The research was further supported by the
Europees Fonds voor Regionale Ontwikkeling (EFRO) in the
Interreg IV-A project Organext.
€
19 L. Burgi, M. Turbiez, R. Pfeiffer, F. Bienewald, H. J. Kirner and
C. Winnewisser, Adv. Mater., 2008, 20, 2217.
20 T. L. Nelson, T. M. Young, J. Liu, S. P. Mishra, J. A. Belot,
C. L. Balliet, A. E. Javier, T. Kowalewski and R. D. McCullough,
Adv. Mater., 2010, 22, 4617.
21 Y. Li, S. P. Singh and P. Sonar, Adv. Mater., 2010, 22, 4862.
22 H. Bronstein, Z. Chen, R. S. Ashraf, W. Zhang, J. Du, J. R. Durrant,
P. S. Tuladhar, K. Song, S. E. Watkins, Y. Geerts, M. M. Wienk,
R. A. J. Janssen, T. Anthopoulos, H. Sirringhaus, M. Heeney and
I. McCulloch, J. Am. Chem. Soc., 2011, 133, 3272.
23 A. C. Rochat, L. Cassar and A. Iqbal, EP Pat., EP0094911A2, 1983.
24 D. Veldman, S. C. J. Meskers and R. A. J. Janssen, Adv. Funct.
Mater., 2009, 19, 1939.
Notes and references
1 Y. J. Cheng, S. H. Yang and C. S. Hsu, Chem. Rev., 2009, 109, 5868.
2 Y. Liang, Z. Xu, J. Xia, S. T. Tsai, Y. Wu, G. Li, C. R. Ray and
L. P. Yu, Adv. Mater., 2010, 22, E135.
3 H. Y. Chen, J. H. Hou, S. Q. Zhang, Y. Y. Liang, G. W. Yang,
Y. Yang, L. P. Yu, Y. Wu and G. Li, Nat. Photonics, 2009, 3, 649.
4 M. A. Green, K. Emery, Y. Hishikawa and W. Warta, Prog.
Photovolt: Res. Appl., 2011, 19, 8.
25 M. M. Wienk, J. M. Kroon, W. J. H. Verhees, J. Knol,
J. C. Hummelen, P. A. Van Hal and R. A. J. Janssen, Angew.
Chem., Int. Ed., 2003, 42, 3371.
26 A. M. Nardes, M. Kemerink, M. M. de Kok, E. Vinken, K. Maturova
and R. A. J. Janssen, Org. Electron., 2008, 9, 727.
27 S. E. Shaheen, C. J. Brabec, N. S. Sariciftci, F. Padinger, T. Fromherz
and J. C. Hummelen, Appl. Phys. Lett., 2001, 78, 841.
28 J. Peet, J. Y. Kim, N. E. Coates, W. L. Ma, D. Moses, A. J. Heeger
and G. C. Bazan, Nat. Mater., 2007, 6, 497.
5 H. Yan, Z. Chen, Y. Zheng, C. Newman, J. R. Quinn, F. Dotz,
M. Kastler and A. Facchetti, Nature, 2009, 457, 679.
6 H. N. Tsao, D. Cho, J. W. Andreasen, A. Rouhanipour,
€
D. W. Breiby, W. Pisula and K. Mullen, Adv. Mater., 2009, 21, 209.
7 I. McCulloch, M. Heeney, C. Bailey, K. Genevicius, I. Macdonald,
M. Shkunov, D. Sparrowe, S. Tierney, R. Wagner, W. M. Zhang,
M. L. Chabinyc, R. J. Kline, M. D. McGehee and M. F. Toney,
Nat. Mater., 2006, 5, 328.
29 C. V. Hoven, X. D. Dang, R. C. Coffin, J. Peet, T. Q. Nguyen and
G. C. Bazan, Adv. Mater., 2010, 22, E63.
30 D. Veldman, J. J. A. M. Bastiaansen, B. M. W. Langeveld-Voss,
J. Sweelssen, M. M. Koetse, S. C. J. Meskers and R. A. J. Janssen,
Thin Solid Films, 2006, 511–512, 581.
8 I. McCulloch, M. Heeney, M. L. Chabinyc, D. DeLongchamp,
€
B. Hamadani, R. Hamilton, L. Richter, A. Salleo, M. Shkunov,
D. Sparrowe, S. Tierney and W. Zhang, Adv. Mater., 2009, 21, 1091.
9 J. C. Bijleveld, R. A. M. Verstrijden, M. M. Wienk and
R. A. J. Janssen, Appl. Phys. Lett., 2010, 97, 073304.
R. J. Kline, M. Colle, W. Duffy, D. Fischer, D. Gundlach,
31 J. J. M. Halls, J. Cornil, D. A. dos Santos, R. Silbey, D. H. Hwang,
ꢀ
A. B. Holmes, J. L. Bredas and R. H. Friend, Phys. Rev. B:
Condens. Matter, 1999, 60, 5721.
This journal is ª The Royal Society of Chemistry 2011
J. Mater. Chem., 2011, 21, 9224–9231 | 9231