Thermal behaviour of dicarboxylic ester bithiophene polymers exhibiting a high open-circuit voltage

Article abstract of DOI:10.1039/c7tc04322h

Nine different polythiophene derivatives based on dialkyl-(2,2′-bithiophene-5,5′-diyl)-4,4′-dicarboxylate (DCB) alternating with thiophene (T), bithiophene (2T) or thienothiophene (TT) as co-monomer have been synthesized to study the effect of the polymer backbone and side chain length on the thermal properties, the tendency to aggregate, and the photovoltaic performance. Polymers incorporating DCB and 2T show increased crystallinity and a large effect of the side chain length on the morphology of the photoactive layer blends. Thermal annealing increases the crystallinity of the polymers and enhances the long-wavelength light absorption. The concomitant increase in polymer fibre width, however, deteriorates the photovoltaic performance. The best devices were made using the PDCB-2T polymer with 2-butyloctyl side chains providing a power conversion efficiency of 5.18%. The PDCB-T polymer with 2-ethylhexyl substituents shows a comparable efficiency (5.08%), but with a significantly higher open-circuit voltage due to deeper frontier orbitals levels.

Full text of DOI:10.1039/c7tc04322h

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Nguyên T. K. Thanh, Xiaodi Su et al.
Fine-tuning of gold nanorod dimensions and plasmonic properties using
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DOI: 10.1039/C7TC04322H
Thermal behaviour of dicarboxylic ester bithiophene polymers
exhibiting a high open‐circuit voltage†
Ruurd Heuvel,a Fallon J. M. Colberts,a Martijn M. Wienk ab and René A. J. Janssen*ab
Nine different polythiophene derivatives based on dialkyl‐(2,2′‐bithiophene‐5,5′‐diyl)‐4,4′‐
dicarboxylate (DCB) alternating with thiophene (T), bithiophene (2T) or thienothiophene (TT) as co‐
monomer have been synthesized to study the effect of the polymer backbone and side chain length
on the thermal properties, the tendency to aggregate, and the photovoltaic performance. Polymers
incorporating DCB and 2T show increased crystallinity and a large effect of the side chain length on
the morphology of photoactive layer blends. Thermal annealing increases the crystallinity of the
polymers and enhances the long‐wavelength light absorption. The concomitant increase in polymer
fibre width however deteriorates the photovoltaic performance. Best devices were made using the
PDCB‐2T polymer with 2‐butyloctyl side chains providing a power conversion efficiency of 5.18 %.
The PDCB‐T polymer with 2‐ethylhexyl substituents shows a comparable efficiency (5.08 %), but with
significantly higher open‐circuit voltage due to deeper frontier orbitals levels.
a
Molecular Materials and Nanosystems & Institute for Complex Molecular Systems, Eindhoven
University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands. E‐mail:
r.a.j.janssen@tue.nl
b
Dutch Institute for Fundamental Energy Research, De Zaale 20, 5612 AJ Eindhoven, The
Netherlands
† Electronic Supplementary Information (ESI) available: See DOI: 10.1039/x0xx00000x
1

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DOI: 10.1039/C7TC04322H
Introduction
Since the early 1990’s organic solar cells based on active layers consisting of conjugated polymer
donors and fullerenes acceptors have been considered for renewable energy applications.1‐4 Strong
light absorption by the polymer5,6 permitted the fabrication of thin film devices, leading to flexible,
lightweight and low‐cost solar cells.7‐9 The photovoltaic performance has been increasing with leaps
and bounds over the past two decades and recently power conversion efficiencies (PCEs) have been
reported in excess of 10% using fullerene acceptors10‐13 and over 11% using non‐fullerene
acceptors.14−16 In most cases, large jumps in efficiency coincided with the development of new
donor‐acceptor copolymers, and lately with the advance of efficient non‐fullerene acceptors.14−16
Conjugated polymers based on the donor‐acceptor paradigm offer a high degree of freedom in
tuning the energy levels of the highest occupied (HOMO) and lowest unoccupied (LUMO) molecular
orbitals and consequently also the optical bandgap (Eg).17−20
With increasing efficiencies, performances are moving towards thresholds for application and
theoretical limits.21 The development of multi‐junction solar cells is becoming important as a viable
way to surpass these limits.22 For high performance multi‐junction solar cells, there is not only a
need for well‐performing low (Eg < 1.4 eV23‐29) and medium (Eg = 1.4‐1.7 eV10‐12) bandgap polymers,
which both have been the subject of many studies. Also wide bandgap polymers (Eg > 1.7 eV) need
to be developed to ensure complete spectral overlap with a minimum of photon energy loss and
parasitic absorption. One of the important benefits of wide bandgap polymers, is their potential to
generate high voltages as direct consequence of their large optical bandgap. However attaining
these high voltages has been the main challenge for wide bandgap polymers, because presently
many of these are hampered by comparatively high photon energy losses. By carefully tuning the
HOMO and LUMO energy levels, the energy offset with a fullerene or alternative electron acceptor
can be minimized while retaining the offset required for exciton separation.30 In this way the photon
energy loss might be reduced,31‐35 thereby increasing the open‐circuit voltage (Voc) with a minimum
of penalty in short‐circuit current density (Jsc).
Polythiophene and especially poly(3‐hexylthiophene) (P3HT) as advanced by Wudl et al. in the
1980’s,36,37 has become the workhorse in polymer field‐effect transistors and in polymer solar cells
as wide bandgap (Eg = 1.9 eV) absorber.38‐40 P3HT is semi‐crystalline and has a correspondingly high
charge carrier mobility. In combination with Wudl’s [6,6]‐phenyl‐C61‐butylric acid methyl ester
(PC61BM) electron acceptor,41 P3HT has become the most studied donor material for polymer solar
cells. The PCE of P3HT‐PC61BM solar cells is mainly limited by a comparatively low Voc (0.6 V) as
consequence of the low oxidation potential of P3HT. The cell has a photon energy loss (Eg − qVoc)
well over 1 eV, which exceeds the minimal loss of ~0.6 eV thought to be sufficient for efficient
current generation. Although the Voc of P3HT based solar cells can be increased by employing
fullerene bis‐adducts as acceptors with more negative reduction potentials, the tunablity of such
systems is limited.42,43
As homopolymer P3HT lacks the donor‐acceptor motif which would enable controlling the
oxidation potential and thereby Voc.44,45 However, the oxidation potential of polythiophenes can be
increased by using electron‐withdrawing ester side chains.46 As a result, the Voc can be increased.47
Significant progress was made by the introduction of dicarboxylic bithiophene polymers (PDCBT)
which exhibit a high Voc of 0.91 V and a PCE up to 7.2 %,48 in combination with [6,6]‐phenyl‐C71‐
butylric acid methyl ester (PC71BM). We note that for the same material also a lower Voc of 0.80 V
has been reported more recently.49 Such variation maybe related to the fact that the energy levels,
2

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bandgap and performance of polythiophenes can differ depending on the degree ofDaOgI:g1r0e.1g0a39t/iCo7nT,C04322H
which is affected by the molecular weight, regioregularity and the processing conditions.50,51
Numerous methods have been used to ensure proper organization, aggregation and crystallization
of conjugated polymers in bulk heterojunction blend layers, ranging from the use of co‐solvents to
pre‐aggregation by solution aging or the addition of additives,50 including peroxides.52 Established
ways of ensuring aggregation are slow drying, hot solution processing53 and solvent or thermal
annealing (TA).54
Here we investigate the effect of backbone and side chain length on the energetics, thermal
and aggregation behaviour and crystallinity of polythiophenes based on dialkyl‐(2,2′‐bithiophene‐
5,5′‐diyl)‐4,4′‐dicarboxylate (DCB) with thiophene (T), bithiophene (2T) or thienothiophene (TT) as
co‐monomers to control energy levels with 2‐ethylhexyl (EH), 2‐butyloctyl (BO) or 2‐hexyldecyl (HD)
side chains to influence the tendency to aggregate (Scheme 1). A total of nine different polymers
have been synthesized for which we correlate aggregation, thermal behaviour, film formation and
morphology with photovoltaic performance.
Results and discussion
The synthesis of the monomers M1 – M3 (Scheme 1) started from the procedure reported by Hou et
al.48 However, poor regioselectivity during the final bromination step and challenging purification,
especially for the longer side chain derivatives, called for a change in the synthesis route (Scheme 1).
In the new approach 5‐bromo‐3‐carboxylic acid (2) was protected as a methyl ester, rather than
functionalizing with a long, branched side chain. Dimerization was then carried out in a Suzuki type
fashion using bis(pinacolato)diboron. The resulting bithiophene (4) was brominated resulting in the
highly versatile monomer precursor 5 which was easily purified by column chromatography and
recrystallization. Monomers M1 – M3 could then be obtained in very high purity and with near
quantitative yield via a transesterification reaction using oxo[hexa(trifluoroacetato)]tetrazinc
(ZnTAC24)55,56 and 4‐dimethylaminopyridine (DMAP).57 Co‐polymerization of monomers M1 – M3
with the distannyl derivatives of thiophene, bithiophene and thienothiophene was done in Stille‐
type polycondensation reactions and afforded a total of nine polymers. Purification of the polymers
was carried out by precipitation in methanol, followed by treatment with
ethylenediaminetetraacetic acid (EDTA) to remove any residual palladium, and Soxhlet extraction.
3

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DOI: 10.1039/C7TC04322H
Scheme 1 Synthesis routes towards 2‐ethylhexyl (EH), 2‐butyloctyl (BO) and 2‐hexydecyl (HD)
substituted dicarboxylic ester bithiophene monomer (DCB) (top) and polymerization conditions for
all nine combinations of monomers M1 – M3 and thiophene (T), bithiophene (2T) and
thienothiophene bistannyl monomers (TT) (bottom).
The newly synthesized polymers show reasonable to good molecular weights as determined
using high‐temperature (140 °C) gel permeation chromatography (GPC) in o‐dichlorobenzene (oDCB)
(Table 1). Poor solubility of the PDCB‐TT‐EH polymer even at high temperature prohibited
determining the molecular weight. All other polymers were sufficiently soluble under these
conditions and showed no signs of aggregation using an in‐line UV‐vis detector. The polymers
containing thiophene as co‐monomer show somewhat reduced molecular weights with respect to
others. Increased steric effects due to the smaller co‐monomer can limit the reaction rate in the
polymerization and increase the likelihood of introducing (smaller) terminating groups. Furthermore,
a lower number of fractions during Soxhlet purification, caused by a higher solubility, increases the
yield of the highest molecular weight fraction. An increased polydispersity index (PDI) of PDCB‐T‐EH
is caused by a low molecular weight tail in the GPC. The high PDI of PDCB‐2T‐EH is probably caused
by low solubility of the polymer, resulting in a less well defined material.
Table 1 Physical, optical and electrical properties of the PDCB polymers
PDIa
Egb
HOMOc
[eV]
LUMOc
[eV]
a
a
Mn
[kDa]
Mw
[kDa]
102
53.1
81.3
226
69.2
94.7
Ins.
Yield
[eV]
1.96
1.95
2.05
1.87
1.90
1.91
1.85
2.16
2.02
PDCB‐T‐EH
PDCB‐T‐BO
PDCB‐T‐HD
PDCB‐2T‐EH
PDCB‐2T‐BO
PDCB‐2T‐HD
PDCB‐TT‐EH
PDCB‐TT‐BO
PDCB‐TT‐HD
86 % 19.0
81 % 17.9
88 % 25.8
78 % 43.8
78 % 34.8
75 % 35.5
53 % Ins.
58 % 54.9
67% 52.3
5.36
2.96
3.16
6.07
1.99
2.67
Ins.
−5.75
−5.81
−6.00
−5.63
−5.55
−5.76
−5.95
−5.84
−5.85
−3.40
−3.40
−3.42
−3.40
−3.30
−3.30
−3.38
−3.35
−3.30
158
127
2.88
2.42
a GPC versus polystyrene standards in oDCB at 140 °C; b Low energy absorption onset from pristine
polymer thin film spectrum, spin coated from a 6 mg/mL solution in chloroform at 2000 rpm; c Cyclic
voltammetry of thin polymer films on ITO, using 0.1 M TBA+PF6 solution in acetonitrile as electrolyte
−
and versus Fc/Fc+ as external standard; EHOMO/LUMO = − q(Eox/red + 5.23).
Optical absorption spectra recorded in chloroform solution (Fig. 1) show a single absorption
band at room temperature originating from ‐* transitions58,59 for most polymers, except for of
the PDCB‐2T‐EH and PDCB‐TT‐EH. The latter two are sparsely soluble under these conditions and
show extensive aggregation, evidenced from a long‐wavelength vibronically‐structured absorption
4

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band.60 With polymer chains being more ordered in their aggregated state, the additiDoOnIa: 1l0s.1p0e39c/tCr7aTlC04322H
features arise from an increased effective conjugation length due to a higher degree of order and
stronger  stacking interactions. The three T‐based polymers show overlapping absorption spectra
that are blue shifted compared to TT‐based polymers. Extended conjugation and a more pronounced
donating character of the co‐monomer increases delocalization and intramolecular charge transfer
character of the ‐* transition. Furthermore, 2T‐based polymers have a slightly blue shifted
absorption maximum with respect to TT‐based polymers due to the higher degree of freedom in the
thiophene‐thiophene bond.
View Article Online
Spin coating from hot solution can be an effective way of controlling the moment of polymer
aggregation and thereby the morphology of the resulting layer.53 To better understand the
behaviour of the polymers in solution at elevated temperatures, temperature dependent UV‐vis
absorption spectra were measured in oDCB solution. A typical example (PDCB‐2T‐EH) of the
temperature dependent studies in oDCB solution is shown in Fig. 1b. Two other examples (PDCB‐2T‐
BO and PDCB‐TT‐EH) are depicted in the supplementary information (Fig. S1, ESI†). A summary of
the temperature dependent UV‐vis data is collected in Table 2. The majority of the PDCB polymers is
molecularly dissolved in oDCB at room temperature. At elevated temperature these polymers show
a slight blue shift and a decrease of the absorption maximum due to backbone twisting and solvent
expansion respectively. PDCB‐2T‐BO shows a transition to the aggregated state after cooling the
solution to 10 °C. PDCB‐2T‐EH shows a similar transition, but already at an elevated temperature
between 60 and 70 °C. For the least soluble material, PDCB‐TT‐EH, a significant portion of the
polymer remains aggregated even at 100 °C. These results suggest increased ‐stacking interactions
between bithiophene and thienothiophene polymers with respect to thiophene containing
polymers.
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
(b)
(a)
1.0
0.8
0.6
0.4
0.2
0.0
PDCB-2T-EH
CHCl3
T-EH
oDCB
T (°C)
20
T-BO
T-HD
40
60
70
80
2T-EH
2T-BO
2T-HD
TT-EH
TT-BO
TT-HD
100
400
500
600
700
800
400
500
600
700
800
Wavelength (nm)
Wavelength (nm)
Fig. 1 (a) UV‐vis spectra of PDCB polymers in chloroform solution. (b) Temperature dependent UV‐vis
spectra of PDCB‐2T‐EH in oDCB.
A similar trend can be seen in the thin film absorption spectra (Fig. 2), for layers spin coated
from chloroform solution. In addition to the co‐monomer trend, also a side chain trend can be seen.
Polymers with longer side chains show more blue‐shifted absorption maxima and onsets than those
with shorter side chains. Apparently, the higher fraction of branched alkyl chain content in the film
disrupts interchain interactions and three‐dimensional ordering and thereby effective conjugation
length. Consequently the PDCB‐T‐HD polymer shows highest optical bandgap suggesting that it is the
most amorphous polymer, whereas PDCB‐TT‐EH is extremely aggregated and shows the lowest
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bandgap. PDCB‐TT‐BO does not follow the observed trend; both in solution and in tDhOinI: 1f0i.l1m03s9/tCh7TeC04322H
optical absorption is blue‐shifted compared to PDCB‐TT‐EH, for reasons that we do not understand
at present. X‐ray diffraction of PDCB‐TT‐EH and PDCB‐TT‐BO films did not reveal significant
differences between these two, except for the lamellar spacing which is larger (20.0 vs. 17.2 Å) for
the BO derivative compared to the EH polymer (Fig. S2, ESI†). In general, these results shows that by
varying co‐monomer and side chain length, control can be exerted over the optical properties of this
type of polymers. Furthermore, the 2T‐based polymers show an on average smaller bandgap and a
less pronounced side chain effect. This confirms the previous observation related to the bithiophene
co‐monomer providing more aggregation to the polymer.
Annealing at elevated temperatures is a known way for polythiophenes to increase
crystallinity and thereby increase the long‐wavelength absorption of light.54 Thermal annealing at
different temperatures has been performed on both pristine (Fig. 2 and Fig. S3, ESI†) and blend films
with PC71BM (Fig. S4, ESI†) for 10 min, after which UV‐vis absorption was recorded at room
temperature. Results of the pristine film are summarized in Table 2. Several polymers (PDCB‐2T‐EH,
PDCB‐TT‐EH, and PDCB‐2T‐BO) were already aggregated after spin coating and little changes were
observed in the UV‐vis spectra upon thermal treatment. The PDCB‐T‐HD polymer remained in a non‐
aggregated state after thermal annealing at high temperatures. However, three interesting cases
present themselves as significant changes could be seen with PDCB‐T‐BO, PDCB‐TT‐BO, and PDCB‐
TT‐HD. The spectra of pristine PDCB‐TT‐BO shown in Fig. 2b reveal that thermal annealing to
temperatures above 150 °C are necessary to bring the polymer from a non‐ordered to an ordered
state. This is confirmed by more intense reflections in the X‐ray diffractogram (Fig. S2, ESI†). For
PDCB‐T‐BO and PDCB‐TT‐HD, results are very similar but the transition occurs at significantly lower
temperature (~100 °C). For PDCB‐T‐BO increased aggregation was be observed when annealing up to
120 °C. Higher temperatures decreased the degree of aggregation (Fig. S3a, ESI). In blends with
PC71BM (polymer:fullerene weight ratio 1:1.5) the polymers show the same trends and no significant
influence of the PC71BM can be seen on the aggregation behaviour of the polymers. In a few cases
additional scattering is observed when annealing at temperatures over 150 °C, probably related to
the formation of large PC71BM crystallites.
(a)
(b)
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
PDCB-TT-BO
Annealing T (°C)
Film
T-EH
T-BO
T-HD
2T-EH
2T-BO
2T-HD
TT-EH
TT-BO
TT-HD
20
80
100
120
150
200
400
500
600
700
800
400
500
600
700
800
Wavelength (nm)
Wavelength (nm)
Fig. 2 (a) Optical absorption of PDCBT polymer thin films cast from chloroform. (b) Optical
absorption spectra after thermal annealing of a PDCB‐TT‐BO film. The annealing temperature is
shown in the legend, all spectra were recorded at room temperature after thermal annealing.
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The thermal behaviour was further studied by differential scanning calorimetrDyO(I:D10S.C10)3.9D/CS7CTC04322H
traces are shown in Fig. 3 and Fig. S5 (ESI†). Transition peak temperatures for both melting and
crystallization are summarized in Table 2. From these results a general trend can be observed that,
for a single co‐monomer, an increased side chain length results in a lower transition temperature.
Furthermore, an additional trend can be observed when changing the co‐monomer from T, via 2T to
TT. A shift of transition peak temperature towards higher temperatures is seen, putting them
outside of our measuring range (up to 300 °C) for PDCB‐TT‐EH and PDCB‐TT‐BO. Although glass
transition temperatures were not identified in DSC, at least some kind of extra mobility of the
polymer is already introduced from 80 °C onwards for PDCB‐T‐BO and PDCB‐TT‐HD. This could be
seen from the thermal annealing induced aggregation, evidenced by the optical absorption
experiments (Fig. S3a, ESI†). Interestingly PDCB‐T‐BO shows an additional transition, suggesting
some kind of intermediate phase between 98 and 136 °C. Further studies will have to be performed
to determine the exact nature of these transitions and the state between them. Whether the drop in
the aggregation shoulder intensity of PDCB‐T‐BO at temperatures over 120 °C (Fig. S3a, ESI†) has any
relation to the extra transition around 130 °C is not certain.
Comparing the melting (ΔHm) and fusion (ΔHf) enthalpies shows a clear trend of decreased
latent heat of crystallization when extending side chain length, especially for T‐based polymers
(Table 2). Longer side chains combined with a smaller co‐monomer introduce steric hindrance and
produce more amorphous polymers. Furthermore, the thiophene polymers introduce an additional
loss of crystallinity due to the configuration of the thiophene carboxylate moieties on either side of
the thiophene co‐monomer. Retaining the alternating directivity of thiophene units will place both
carboxylate moieties on the same side introducing additional steric hindrance for longer side chains.
Placing the side chains on different sides of the polymer however breaks the alternating directivity
and will introduce a bending of the polymer chain. Bithiophene and thienothiophene both retain the
alternating directivity of the thiophene units as well as the side chains. These results confirm the
added crystallinity of the bithiophene containing polymers. Polymers containing the bithiophene co‐
monomer show an on average higher transition enthalpy indicating a more crystalline material.
Table 2 Overview of thermal behaviour of PDCB polymers by optical absorption or differential
scanning calorimetry.
1st λmax [nm]( oDCB)
Eg [eV] (film)
pristine TAa
Temperature
Enthalpy
ΔHm [J/g] ΔHf [J/g]
10 °C
100 °C
462
462
461
477
478
478
615
476
476
Tm [°C]
Tf [°C]
PDCB‐T‐EH
477
1.94
1.95
2.08
1.86
1.91
1.93
1.82
2.07
1.94
1.92
1.92
2.07
1.88
1.91
1.92
1.82
1.90
1.89
232
198
8.0
9.3
PDCB‐T‐BO 478
PDCBT‐HD 476
(98) 136 (76) 127
(3.9) 7.4 (3.6) 8.0
75
48
2.9
‐
17
10
‐
0.8
‐
20
10
‐
‐
11
PDCB‐2T‐EH 607
PDCB‐2T‐BO 599
PDCB‐2T‐HD 497
PDCB‐TT‐EH 619
PDCB‐TT‐BO 486
PDCB‐TT‐HD 509
>300
282
227
>300
>300
275
270
260
197
>300
>300
247
‐
4.5
a Thermally annealed at 120 °C for 10 min.
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1.0
0.8
0.6
0.4
0.2
0.0
-0.2
-0.4
-3.0
(b)
(a)
-2.0
T-EH
T-BO
T-HD
-3.5
-4.0
-4.5
-1.5
-1.0
0.5
1.0
1.5
-6.0
-6.5
LUMO
+ TA
HOMO
+ TA
Exo up
0
50
100
150
200
250
300
Temperature (°C)
Fig. 3 (a) DSC traces of PDCB‐T polymers. (b) Redox potentials and electrochemical energy levels for
the PDCB polymers.
The electrochemical properties of the PDCB polymers were measured using pristine and
thermally annealed (10 min at 150 °C) thin films that were spin coated from pure chloroform on
glass/ITO substrates (Fig. 3b). The HOMO and LUMO levels (Table 1) were calculated using the
onsets of the oxidation and reduction waves vs. ferrocene/ferrocenium using EHOMO/LUMO = −(Eox/red
+
5.23). Figure 3b shows that the LUMO levels are only slightly affected by backbone, side chain, or
thermal history. On the other hand, the HOMO levels show some variation and weak trends can be
identified. First, both T‐ and TT‐based polymers show somewhat deeper HOMO levels than the 2T‐
based polymers, owing to the increased electron rich character of the latter. Second, the HOMO
level becomes more negative for longer side chains. This effect is most likely caused by the
decreased tendency of the polymers to aggregate for longer and bulkier side chains. Third, thermal
annealing either raises the HOMO level or leaves it unaltered. Surprisingly, there is no correlation
between the polymers that change the optical band gap (PDCB‐T‐BO, ‐TT‐BO, and ‐TT‐HD) and those
that change the oxidation potential (PDCB‐T‐HD, ‐2T‐EH, and ‐2T‐HD) after thermal annealing. Due to
the additional interaction between the measured film and the solvent and ions in the electrolyte, it
is difficult to unambiguously determine the origin of the small differences in energy levels observed
by cyclic voltammetry. However, an increased tendency to aggregate, either by using shorter side
chains or by thermal annealing, often raises the HOMO level and reduces the optical bandgap. All
polymers, in either pristine or annealed state, show sufficiently high offset (> 0.3 eV) with the energy
levels of PC71BM for photoinduced charge transfer to occur. Based on the results shown in Fig. 3b, an
increased Voc can be expected for solar cells incorporating T‐ and TT‐based polymers, compared to
2T‐based polymers.
The photovoltaic performance of the PDCB polymers was investigated using a regular device
configuration. A bulk heterojunction of the PDCB polymer as donor and PC71BM as acceptor in a
1:1.5 weight ratio was sandwiched between an ITO/PEDOT:PSS hole collecting electrode and a LiF/Al
electron collecting electrode. Photovoltaic devices were characterized by measuring the J‐V
dependence in the dark and under simulated AM1.5G (100 mW/cm2) illumination (Table 3 and Fig.
4a‐c). The external quantum efficiency (EQE) (Fig. 4d‐f) was determined by measuring the spectral
response using a calibrated silicon reference cell and was used to correct short‐circuit current
densities (Table 3) for spectral mismatch. The photoactive layers were processed from chloroform,
followed by a performance optimization using different amounts of diphenyl ether (DPE) as co‐
solvent to vary aggregation behaviour (Table S1, ESI†). Furthermore, optimized devices were
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subjected to thermal annealing for 10 min at 120 °C to study the effect on the DpOhI:o10to.10v3o9l/tCa7iTcC04322H
parameters. PDCB‐2T‐EH and PDCB‐TT‐EH were too poorly soluble to be processed in working
photovoltaic devices. Whereas PDCB‐TT‐EH was only soluble in superheated chloroform at
temperatures exceeding 160 °C, PDCB‐2T‐EH was soluble at around 70 °C, but showed rapid
aggregation upon spin coating resulting in rough layers and shorted devices. For the sparsely soluble
polymer (PDCB‐TT‐BO), the reproducibility was limited due to the need for hot spin coating (at ~60
°C). When hot spin coating is employed, the morphology is essential frozen as the solvent cools
rapidly. The resulting morphology is therefore very much depending on parameters that are difficult
to control, like substrate and solution temperature as well as cooling rate.
Table 3 Photovoltaic characteristics of optimized bulk heterojunction solar cells of PDCB polymers
combined with PC71BM in regular configuration.
Polymer
TAa
Jsc [mA/cm2]b
Voc [V]
FF
PCE [%]
PDCB‐T‐EH
No
Yes
No
Yes
No
Yes
8.01 ± 0.04 (8.06) 0.94 ± 0.00 (0.94) 0.67 ± 0.01 (0.67) 5.05 ± 0.06 (5.08)
6.62 ± 0.07 (6.62) 0.96 ± 0.00 (0.94) 0.67 ± 0.00 (0.67) 4.26 ± 0.05 (4.26)
2.43 ± 0.14 (2.60) 0.99 ± 0.01 (0.99) 0.59 ± 0.01 (0.57) 1.42 ± 0.06 (1.47)
1.26 ± 0.04 (1.30) 1.03 ± 0.01 (1.04) 0.43 ± 0.01 (0.43) 0.56 ± 0.02 (0.58)
0.48 ± 0.01 (0.49) 0.98 ± 0.09 (0.97) 0.41 ± 0.06 (0.39) 0.19 ± 0.04 (0.19)
0.37 ± 0.01 (0.36) 0.61 ± 0.21 (0.67) 0.32 ± 0.03 (0.32) 0.07 ± 0.03 (0.08)
PDCB‐T‐BO
PDCB‐T‐HD
PDCB‐2T‐EH
‐
‐
‐
‐
PDCB‐2T‐BO No
9.66 ± 0.16 (9.86) 0.74 ± 0.00 (0.74) 0.71 ± 0.01 (0.71) 5.04 ± 0.12 (5.18)
8.46 ± 0.15 (8.71) 0.82 ± 0.00 (0.82) 0.69 ± 0.06 (0.70) 4.78 ± 0.44 (5.00)
8.76 ± 0.11 (8.80) 0.75 ± 0.00 (0.75) 0.65 ± 0.00 (0.65) 4.24 ± 0.10 (4.29)
6.95 ± 0.16 (7.09) 0.86 ± 0.01 (0.86) 0.66 ± 0.01 (0.67) 3.94 ± 0.18 (4.08)
Yes
PDCB‐2T‐HD No
Yes
PDCB‐TT‐EH
PDCB‐TT‐BO No
Yes
‐
‐
‐
‐
7.60 ± 0.14 (7.56) 0.92 ± 0.01 (0.92) 0.67 ± 0.01 (0.68) 4.75 ± 0.09 (4.73)
6.49 ± 0.13 (6.65) 0.94 ± 0.00 (0.94) 0.56 ± 0.01 (0.56) 3.42 ± 0.09 (3.50)
6.43 ± 0.12 (6.61) 0.88 ± 0.01 (0.88) 0.58 ± 0.01 (0.57) 3.25 ± 0.04 (3.32)
PDCB‐TT‐HD No
Yes
6.68 ± 0.09 (6.73) 0.84 ± 0.02 (0.84) 0.59 ± 0.05 (0.59) 3.31 ± 0.32 (3.34)
a
b
Thermal annealing for 10 min at 120 °C. Obtained by integrating the EQE with the AM1.5G
spectrum.
The most efficient devices were made using PDCB‐T‐EH, PDCB‐2T‐BO and PDCB‐TT‐BO (Table
3), which correspond to the polymers with the shortest side chain for each co‐monomer (T, 2T, TT)
where the PDCB polymer is soluble enough to be processed. Longer side chains generally reduced
the performance. The Voc‘s follow the same general trend as was seen in the cyclic voltammetry on
pristine polymer films, showing higher Voc’s for T‐ and TT‐based polymers. Fill factors (FF) increase as
the crystallinity of the polymer increases (i.e. for shorter side chains). Especially PDCB‐2T‐BO shows a
high FF. Thermal treatment of the active layer does not increase the performance. In general, Voc
and FF are little affected, while Jsc decreases. The EQE spectra show a broader response for the more
crystalline 2T‐based polymers. The more disordered polymers yield generally lower EQEs, which is
especially clear in the T‐based polymers with BO and HD side chains. For most polymers the effect of
thermal annealing on the shape of the EQE is surprisingly small, but it causes a distinct broadening
for PDCB‐TT‐HD, consistent with the optical absorption experiment (Figure S3, ESI). For PDCB‐TT‐BO
annealing up to 120 °C shows a small effect only, but significant broadening of the EQE occurs at 200
°C (Fig. S6, ESI†). Aggregation in this polymer is only induced at higher temperatures as is known
9

Journal of Materials Chemistry C
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from temperature dependent optical absorption experiments. The most clear and rather general
DOI: 10.1039/C7TC04322H
effect of thermal annealing, however, is a drop of the maximum EQE explaining the reduced Jsc.
3
2
3
2
3
2
(b)
(c)
(a)
T-EH
T-EH+TA
T-BO
T-BO+TA
T-HD
T-HD+TA
2T-BO
2T-BO+TA
2T-HD
TT-BO
TT-BO+TA
TT-HD
1
1
1
0
0
0
2T-HD+TA
TT-HD+TA
-1
-2
-3
-4
-5
-6
-7
-8
-9
-1
-2
-3
-4
-5
-6
-7
-8
-9
-1
-2
-3
-4
-5
-6
-7
-8
-9
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
1.2
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
1.2
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Bias (V)
Bias (V)
Bias (V)
(d)
(f)
(e)
0.6
0.5
0.4
0.3
0.2
0.1
0.0
0.6
0.5
0.4
0.3
0.2
0.1
0.0
0.6
0.5
0.4
0.3
0.2
0.1
0.0
T-EH
T-EH+TA
T-BO
T-BO+TA
T-HD
T-HD+TA
2T-BO
2T-BO+TA
2T-HD
TT-BO
TT-BO+TA
TT-HD
2T-HD+TA
TT-HD+TA
400
500
600
700
800
900
400
500
600
700
800
900
400
500
600
700
800
900
Wavelength (nm)
Wavelength (nm)
Wavelength (nm)
Fig. 4 (a‐c) J‐V‐characteristics of optimized bulk heterojunction solar cells of PDCB polymers
containing (a) thiophene, (b), bithiophene, and (c) thienothiophene with PC71BM. (d‐f)
Corresponding EQE spectra of the same devices. TA is thermal annealing for 10 min at 120 °C.
To clarify the effect of the morphology on the photovoltaic performance, the active layers of
optimized devices were studied using transmission electron microscopy (TEM). Typical micrographs
are shown in Fig. 5. A complete overview of TEM images of pristine photoactive blends is shown in
Fig. S7 and for thermally annealed blends in Fig. S8 in the ESI†. In general, TEM micrographs
correlate very well with the results and conclusions inferred from optical absorption, DSC and
photovoltaic performance. For the most soluble material (PDCB‐T‐HD) the blend film shows droplet‐
like features that are associated with PC71BM‐rich domains that form via spinodal liquid‐liquid phase
separation. Moving towards shorter BO and EH side chains for the T‐based polymers results in a
crossover from coarse morphologies caused by liquid‐liquid phase separation as the dominant
mechanism for phase separation to more intimately mixed phases that are formed because polymer
chains aggregate before liquid‐liquid phase separation occurs. Hence, whereas the PDCB‐T‐BO
derivative still shows sizeable domains (Fig. S7b, ESI†), moving to even shorter side chains eventually
results in an intimately‐mixed fibrillary morphology for PDCB‐T‐EH (Fig. S7a, ESI†) that provides the
highest PCE for the T‐based PDCB polymers. For PDCB‐2T‐BO and PDCB‐2T‐HD also fibrillary
morphologies are found (Fig. 5b,c). The fibres are semi‐crystalline as evidenced by the crystal lattice
fringes that are visible at higher magnification (Fig. S9, ESI†). Such semi‐crystalline, fibre‐like
morphology is known to be beneficial because of the small domain sizes and percolating pathways in
both phases. It is interesting to see that for the more soluble PDCB‐2T‐HD, the crystalline fibres
appear somewhat broader than for PDCB‐2T‐BO. For PCDB‐TT‐BO and PDCB‐TT‐HD the trends are
similar (Fig. S7, ESI†). The broadening of polymer fibres for more soluble semiconducting polymers
has also been observed for diketopyrrolopyrole polymers,61 and was rationalized by considering the
kinetics of nucleation and growth of polymer fibres in which higher solubility leads to wider polymer
fibrils.62 Because for wider fibres the fibril width may exceed the exciton diffusion length, the EQE
10

Page 11 of 19
Journal of Materials Chemistry C
and Jsc are reduced when the side chains become longer.61 The present results are consistent with
View Article Online
DOI: 10.1039/C7TC04322H
this explanation.
Thermal annealing of the active layer increases the width of the fibres for the PDCB‐2T‐
HD:PC71BM blend (Fig. 5c,d), thereby decreasing the effective area for charge separation and
reducing the EQE and Jsc even further. Thermal annealing generally increases the fraction of
crystalline regions, except for blends of PC71BM with PDCB‐T‐HD, PDCB‐2T‐BO and PDCB‐2T‐HD that
show already extensive crystallization in the as‐cast films. For the morphologies that form via liquid‐
liquid the contrast between the phases in the TEM images is decreased, which suggests a flattening
of the films, but crystallization does not take place.
(a)
(b)
(d)
(c)
Fig. 5 Bright‐field TEM micrographs of optimized, pristine active layer blends with PC71BM for (a)
PDCB‐T‐HD; (b) PDCB‐2T‐BO; (c) PDCB‐2T‐HD. (d) A thermally annealed blend of PDCB‐2T‐
HD:PC71BM. In each case the mixing ratio was 1:1.5 (polymer/fullerene) by weight.
Conclusion
In conclusion, a total of nine different polymer were synthesized and studied for their optical
properties, thermal behaviour, and performance in polymer‐fullerene solar cells. PDCB polymers,
functionalized with short side chains, or containing the bithiophene co‐monomer show higher
crystallinity and better performance in photovoltaic devices. Care had to be taken to ensure
solubility and processability of the polymers in order to be able to make smooth, closed layers.
Furthermore, thermal treatment of the polymers was most effective for the more amorphous
polymers as these crystallize and show a steepening of the absorption onset. The JSC of these
polymers, however, does not increase upon thermal annealing. The EQE drops as a result of a
widening of the fibre‐like polymer domains, leading to smaller D‐A surface for charge carrier
11

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generation. We found that the PDCB‐2T‐BO polymer, using a moderate length side chaDinOIa: 1lo0.n10g39w/Ci7tThC04322H
the increased crystallinity of the bithiophene moiety, performed best with PCE = 5.18% (Jsc = 9.86
mA/cm2, Voc = 0.74 V, FF = 0.71). However, for multi‐junction devices PDCB‐T‐EH can be an
interesting polymer due to a comparable PCE of 5.08 % but significantly increased open‐circuit
voltage (0.94 V). Ultimately, this class of PDCBT type polymers presents an interesting group of wide
band‐gap polymers, where especially the high voltage might prove useful for the application in
multi‐junction solar cells.
Experimental section
Materials and Methods
Tris(dibenzylideneacetone)dipalladium (Pd2dba3) (Strem Chemicals Inc), thiophene‐3‐carboxylic acid,
and ZnTAC24 (TCI) were used as received. Triphenylphosphine (PPh3) was recrystallized from
absolute ethanol. All solid monomers were freshly recrystallized from absolute ethanol prior to use,
liquid monomers were subjected to column chromatography and used within two days.
1H and 13C NMR spectra were recorded at respectively 400 and 100 MHz on a Bruker Avance III
spectrometer at 25 °C. Molecular weights of small molecules were determined using matrix assisted
laser desorption ionization time of flight (MALDI‐TOF) mass spectroscopy (Bruker Autoflex Speed
spectrometer) or gas chromatography/mass spectroscopy (GC‐MS) (Shimadzu GC‐2010
chromatograph, equipped with a Zebron ZB‐5MS column and a GCMS‐QP2010 plus mass
spectrometer). Molecular weights of polymers were determined by gel permeation chromatography
(GPC) on a PL‐GPC 220 using a PL‐GEL 10 m MIXED‐B column. The system was operated at 140 °C
with o‐dichlorobenzene (oDCB) as the eluent. Samples, dissolved at 0.1 mg/mL, were measured
against polystyrene standards. UV/visible/NIR spectroscopy was conducted on a PerkinElmer
Lambda 1050 spectrophotometer equipped with a 3D WB PMT/InGaAs/PbS detector module.
Temperature control was realized with a PerkinElmer PTP1 Peltier temperature programmer. Cyclic
voltammetry was carried out using an AutoLab PGSTAT 30 in an inert atmosphere. The electrolyte
consisted of 0.1 M tetrabutylammonium hexafluorophosphate in acetonitrile. The sample was
applied as a thin film, spin coated on an ITO coated glass substrate as working electrode. A silver rod
was used as counter electrode and a silver chloride coated silver rod (Ag/AgCl) as quasi‐reference
electrode. The measurements were performed at a scan speed of 0.2 V/s and potentials are quoted
vs. Fc/Fc+ as external standard. For conversion to energy levels vs. vacuum we used EFc/Fc+ = −5.23
eV. Differential scanning calorimetry was done using a TA Instruments DSC Q2000.
Photovoltaic devices were fabricated with active areas of 0.09 and 0.16 cm2.
Poly(ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) (Clevios P, VP Al4083) was spin
coated at 3000 rpm on pre‐cleaned, patterned indium tin oxide (ITO)/glass substrates (Naranjo
Substrates). The active layer was spin coated at 2000 rpm from either a chloroform(/co‐solvent)
solution (6 mg/mL polymer, 9 mg/mL [6,6]‐phenyl‐C71‐butyric acid methyl ester (PC71BM)). The back
electrode was evaporated at ~10−7 mbar and consisted of LiF (1 nm) and Al (100 nm) layers. Current
density – voltage (J−V) characteristics were recorded with a Keithley 2400 source meter using a
tungsten‐halogen lamp as light source. The light was filtered by a Schott GG385 UV filter and a Hoya
LB120 daylight filter to provide 100 mW/cm2 AM1.5G light. Short‐circuit currents and PCEs were
calculated by integrating the solar spectrum and the spectral response of the cells. External quantum
efficiencies (EQE) were determined using modulated monochromatic light from a 50 W tungsten‐
12

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halogen lamp (Philips Focusline) passing through a monochromator (Oriel, CornerstonDeO1I:3100.1)0a39n/Cd7TaC04322H
mechanical chopper. The response was recorded as the voltage produced by a preamplifier
(Stanford Research Systems SR570) with a lock‐in amplifier (SR830). All measurements were done
against a Si‐reference cell with known spectral response.
X‐ray diffraction was measured on a Bruker 2D Phaser using Cu Kα radiation with a
wavelength of 0.15406 nm between 2θ angles of 2° to 30°, using increments of 0.02°. Transmission
electron microscopy (TEM) was performed on Tecnai G2 Sphera transmission electron microscope
(FEI) operating at 200 kV. Layer thicknesses were determined using a Vecoo Dektak 150
profilometer, subtracting the thickness of any underlying layers.
Synthesis
5‐bromothiophene‐3‐carboxylic acid (2). Thiophene‐3‐carboxylic acid (5.00 g, 39.0 mmol) was
dissolved in glacial acetic acid (60 mL). A solution of bromine (1.9 mL, 36.9 mmol) in glacial acetic
acid (30 mL) was added dropwise at room temperature. The solution was then stirred for 15 min at
room temperature, after which it was poured in cold water (300 mL). The precipitate was washed
with cold water and recrystallized hot water. The solid was filtered off, washed with cold water and
dried in vacuo at 40 °C. The product was obtained as a white solid with a yield of 4.42 g (21.3 mmol
or 54.7 %). 1H‐NMR (400 MHz, CDCl3, δ) 8.119 (d, J = 1.4 Hz, 1H), 7.514 (d, J = 1.5 Hz, 1H). 13C‐NMR
(100 MHz, CDCl3, δ) 166.553, 135.783, 133.026, 130.360, 113.336. GC‐MS: [M]+ calc: 205.90, found:
205.95.
Methyl 5‐bromothiophene‐3‐carboxylate (3). 5‐bromothiophene‐3‐carboxylic acid (2) (5.55 g,
26.8 mmol) was dissolved in thionyl chloride (50 mL) and refluxed for 30 min. The solution was
allowed to cool to room temperature and thionyl chloride was evaporated in vacuo. Methanol (100
mL) was added slowly and the solution refluxed for 60 min. All solvent was evaporated and the
1
product obtained as a transparent liquid with a yield of 4.387 g (19.8 mmol, 73.9 %). H‐NMR (400
MHz, CDCl3, δ) 7.984 (d, J = 1.5, 1H), 7.470 (d, J = 1.5 Hz, 1H), 3.858 (s, 3H). 13C‐NMR (100 MHz,
CDCl3, δ) 162.024, 133.837, 130.238, 112.913, 52.023. GC‐MS: [M]+ calc: 219.92, found: 219.95.
Dimethyl [2,2'‐bithiophene]‐4,4'‐dicarboxylate (4). Methyl 5‐bromothiophene‐3‐
carboxylate (3) (1.97 g, 8.91 mmol) and bis(pinacolato)diboron (1.130 g, 4.45 mmol) were dissolved
in THF in a dry, oxygen‐free Schlenk vial and degassed for 15 min. with argon. A solution of
potassium carbonate (5.02 g, 36.3 mmol) in deionized water (18 mL) was added and the solution
degassed for another 15 min. First tri‐tert‐butylphosphonium tetrafluoroborate (155 mg, 0.534
mmol), then tris(dibenzylideneacetone)dipalladium (123.2 mg, 0.135 mmol) was added and the
solution degassed for another 15 min. The vial was sealed under argon atmosphere and the mixture
stirred over night at room temperature. Diethyl ether was added and the mixture washed with
water. The water phase was then extracted with chloroform and the combined organic phases dried
over magnesium sulfate. After evaporation of the solvents, the crude material was subjected to a
short silica plug using chloroform as the solvent and recrystallized from a mixture of ethanol. The
1
product was obtained as colourless crystals with a yield of 0.978 g (3.46 mmol or 77.8 %). H‐NMR
(400 MHz, CDCl3, δ) 8.005 (d, J = 1.3, 2H), 7.590 (d, J = 1.2 Hz, 2H), 3.887 (s, 6H). 13C‐NMR (100 MHz,
CDCl3, δ) 162.762, 136.953, 134.110, 131.942, 124.586, 52.000. GC‐MS: [M]+ calc: 282.00, found:
282.10.
Dimethyl 5,5'‐dibromo‐[2,2'‐bithiophene]‐4,4'‐dicarboxylate (5). In a dry, oxygen‐free
Schlenk vial, a solution of dimethyl [2,2'‐bithiophene]‐4,4'‐dicarboxylate (4) (1.55 g, 5.48 mmol) in
chloroform (30 mL) was bubbled for 15 min with argon. Trifluoroacetic acid (30 mL) was added and
13

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the solution cooled to 0 °C. N‐bromosuccinimide (2.15 g, 12.1 mmol) was added andDtOhI:e10s.1o0l3u9t/Cio7TnC04322H
stirred in the dark for 6 hours under argon at 0 °C. Overnight, the solution was allowed to warm
slowly to room temperature while stirring in the dark. Chloroform was added and the solution
washed with a saturated solution of sodium bicarbonate and water. Aqueous phases were extracted
with chloroform and the combined organic phases dried over magnesium sulfate. Solvents were
evaporated and the crude material purified by column chromatography (silica, chloroform) and
recrystallization from a mixture of ethanol and chloroform (3:1). The product was obtained as a
white solid with a yield of 1.821 g (4.138 mmol or 75.5 %). 1H‐NMR (400 MHz, CDCl3, δ) 7.395 (s, 2H),
3.925 (s, 6H). 13C‐NMR (100 MHz, CDCl3, δ) 161.851, 135.323, 131.720, 125.844, 119.508, 52.134.
GC‐MS: [M]+ calc: 437.82, found: 437.95.
Bis(2‐ethylhexyl) 5,5'‐dibromo‐[2,2'‐bithiophene]‐4,4'‐dicarboxylate (M1). A dry, oxygen‐
free Schlenk vial was charged with a solution of 2‐ethylhexanol (1.10 g, 8.44 mmol) in toluene (12
mL) and bubbled with argon for 15 min. Sequentially, dimethyl 5,5'‐dibromo‐[2,2'‐bithiophene]‐4,4'‐
dicarboxylate (5) (417 mg, 0.947 mmol), 4‐dimethylaminopyridine (220 mg, 1.96 mmol), ZnTAC24
(204 mg, 0.191 mmol) and 4 Å molecular sieves were added. The flask was then sealed under argon
atmosphere and the mixture stirred overnight at 110 °C. After allowing to cool to room temperature,
the solution was transferred using chloroform and the solvent evaporated in vacuo. The crude
material was purified using column chromatography (silica, chloroform/heptane, 3:2) to obtain the
product as a colourless oil with a yield of 592 mg (0.931 mmol or 98.3 %). 1H‐NMR (400 MHz, CDCl3,
δ) 7.380 (s, 2H), 4.263 (dd, J = 2.2 Hz, 5.6 Hz, 4H), 1.734 (m, 2H), 1.527‐1.329 (m, 16H), 0.948 (m,
12H). 13C‐NMR (101 MHz, CDCl3, δ) 161.698, 135.274, 132.311, 126.051, 118.964, 67.680, 38.821,
30.519, 28.972, 23.936, 22.978, 14.079, 11.073. MALDI‐TOF‐MS: [M]+ calc: 634.04., found: 634.06.
Bis(2‐butyloctyl) 5,5'‐dibromo‐[2,2'‐bithiophene]‐4,4'‐dicarboxylate (M2). Same procedure
as M1, using 2‐butyloctanol (1.40 g, 7.49 mmol), toluene (12 mL), dimethyl 5,5'‐dibromo‐[2,2'‐
bithiophene]‐4,4'‐dicarboxylate (5) (405 mg, 0.920 mmol), 4‐dimethylaminopyridine (214 mg, 1.91
mmol) and ZnTAC24 (206 mg, 0.193 mmol). Purification by column chromatography (silica,
chloroform/heptane, 2:3) to obtain the product as a colourless oil with a yield of 685 mg (0.918
mmol or 99.8 %). 1H‐NMR (400 MHz, CDCl3, δ) 7.353 (s, 2H), 4.218 (d, J = 5.5 Hz, 4H), 1.762 (m, 2H),
1.417‐1.285 (m, 32H), 0.895 (m, 12H). 13C‐NMR (101 MHz, CDCl3, δ) 161.692, 135.275, 132.324,
126.044, 118.931, 68.103, 37.322, 31.836, 31.384, 31.054, 29.623, 28.961, 26.728, 22.989, 22.668,
14.113, 14.088. MALDI‐TOF‐MS: [M]+ calc: 746.17., found: 746.15.
Bis(2‐hexyldecyl) 5,5'‐dibromo‐[2,2'‐bithiophene]‐4,4'‐dicarboxylate (M3). Same procedure
as M1, using 2‐hexyldecanol (736 mg, 3.04 mmol), toluene (5 mL), dimethyl 5,5'‐dibromo‐[2,2'‐
bithiophene]‐4,4'‐dicarboxylate (5) (152 mg, 0.362 mmol), 4‐dimethylaminopyridine (82.0 mg, 0.731
mmol) and ZnTAC24 (83 mg, 77.6 mol). Purification by column chromatography (silica,
chloroform/heptane, 1:1) to obtain the product as a colourless oil with a yield of 270 mg (0.314
mmol or 86.7 %). 1H‐NMR (400 MHz, CDCl3, δ) 7.352 (s, 2H), 4.216 (d, J = 5.6 Hz, 4H), 1.762 (m, 2H),
1.413‐1.281 (m, 48H), 0.873 (m, 12H). 13C‐NMR (101 MHz, CDCl3, δ) 161.677, 135.271, 132.328,
126.052, 118.929, 68.120, 37.341, 31.913, 31.837, 31.393, 31.380, 29.951, 29.623, 29.578, 29.325,
26.748, 26.732, 22.688, 22.670, 14.118. MALDI‐TOF‐MS: [M]+ calc: 858.29., found: 858.28.
Polymerization reactions
PDCB‐T‐EH. To a dry Schlenk vial equipped with screw cap was added, bis(2‐ethylhexyl) 5,5'‐
dibromo‐[2,2'‐bithiophene]‐4,4'‐dicarboxylate
(M1)
(150.3
mg,
0.236
mmol),
2,5‐
bis(trimethylstannyl)thiophene (97.2 mg, 0.237 mmol), PPh3 (7.380 mg, 28.14 μmol), Pd2dba3 (6.426
14

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mg, 7.017 μmol), anhydrous toluene (3.3 mL), and anhydrous dimethylformamide (DMF) (0.3 mL).
DOI: 10.1039/C7TC04322H
The solution was degassed with argon for 15 min and the flask sealed. Five pump purge cycles with
argon were performed to remove any remaining oxygen, after which the mixture was heated to 115
°C overnight. The polymer was end capped with tributyl(thiophen‐2‐yl)stannane (0.1 mL) and 2‐
bromothiophene (0.2 mL) heating to 115 °C for 20 min after each addition. The reaction mixture was
diluted with chloroform and precipitated in a 0.3 M solution of HCl in methanol. The resulting solids
were treated with a palladium scavenger by redissolving in 1,1,2,2‐tetrachloroethane, adding
ethylenediaminetetraacetic acid and heating to 115 °C under argon atmosphere for 1 hour. After
cooling, water was added and the polymer extracted with chloroform. The organic phase was dried
over magnesium sulfate and concentrated in vacuo. The concentrated solution was precipitated in
methanol. The solids where further purified using Soxhlet extraction with acetone, hexane, and
dichloromethane. The dichloromethane fraction was concentrated and precipitated in methanol to
obtain PDCB‐T‐EH as a dark solid (103 mg, yield 78%). The 1H‐NMR spectrum is shown in Fig. S10.
PDCB‐T‐BO. Same procedure as PDCB‐T‐EH, using M2 (102.6 mg, 0.137 mmol), 2,5‐
bis(trimethylstannyl)thiophene (56.3 mg, 0.137 mmol), PPh3 (4.313 mg, 16.44 μmol), Pd2dba3 (3.762
mg, 4.108 μmol), anhydrous toluene (3.3 mL), and anhydrous DMF (0.3 mL). Palladium scavenging in
1
chloroform at 62 °C. Soxhlet extraction with acetone and hexane. Yield 78 mg, 81 %. The H‐NMR
spectrum is shown in Fig. S11.
PDCB‐T‐HD. Same procedure as PDCB‐T‐EH, using M3 (102.4 mg, 0.119 mmol), 2,5‐
bis(trimethylstannyl)thiophene (48.5 mg, 0.118 mmol), PPh3 (3.737 mg, 14.25 μmol), Pd2dba3 (3.254
mg, 3.553 μmol), anhydrous toluene (2.2 mL), and anhydrous DMF (0.2 mL). Palladium scavenging in
1
chloroform at 62 °C. Soxhlet extraction with acetone and hexane. Yield 82 mg, 88 %. The H‐NMR
spectrum is shown in Fig. S12.
PDCB‐2T‐EH. Same procedure as PDCB‐T‐EH, using M1 (103.3 mg, 0.162 mmol), 5,5'‐
bis(trimethylstannyl)‐2,2'‐bithiophene (80.0 mg, 0.163 mmol), PPh3 (5.093 mg, 19.42 μmol), Pd2dba3
(4.330 mg, 4.729 μmol), anhydrous toluene (3.3 mL), and anhydrous DMF (0.3 mL). Palladium
scavenging in 1,1,2,2‐tetrachloroethane at 115 °C. Soxhlet extraction with acetone, hexane,
dichloromethane and chloroform. The residue was dissolved in hot TCE, filtered, concentrated and
1
precipitated in methanol. Yield 81 mg, 78 %. Solubility was insufficient to record the H‐NMR
spectrum.
PDCB‐2T‐BO. Same procedure as PDCB‐T‐EH, using M2 (151.2 mg, 0.204 mmol), 5,5'‐
bis(trimethylstannyl)‐2,2'‐bithiophene (101.5 mg, 0.206 mmol), PPh3 (6.391 mg, 24.37 μmol),
Pd2dba3 (5.578 mg, 6.091 μmol), anhydrous toluene (4.4 mL), and anhydrous DMF (0.4 mL).
Palladium scavenging in 1,1,2,2‐tetrachloroethane at 115 °C. Soxhlet extraction with acetone,
hexane, dichloromethane and chloroform. Yield 81 mg, 78 %. The 1H‐NMR spectrum is shown in Fig.
S13.
PDCB‐2T‐HD. Same procedure as PDCB‐T‐EH, using M3 (103.9 mg, 0.121 mmol), 5,5'‐
bis(trimethylstannyl)‐2,2'‐bithiophene (59.8 mg, 0.122 mmol), PPh3 (3.834 mg, 14.62 μmol), Pd2dba3
(3.233 mg, 3.531 μmol), anhydrous toluene (2.2 mL), and anhydrous DMF (0.2 mL). Palladium
scavenging in chloroform at 62 °C. Soxhlet extraction with acetone, hexane and dichloromethane.
Yield 78 mg, 75 %. The 1H‐NMR spectrum is shown in Fig. S14.
PDCB‐TT‐EH. Same procedure as PDCB‐T‐EH, using M1 (111.8 mg, 0.176 mmol), 2,5‐
bis(trimethylstannyl)thieno[3,2‐b]thiophene (82.2 mg, 0.176 mmol), PPh3 (5.561 mg, 21.20 μmol),
Pd2dba3 (4.727 mg, 5.162 μmol), anhydrous toluene (3.3 mL), and anhydrous DMF (0.3 mL).
Palladium scavenging in 1,1,2,2‐tetrachloroethane at 115 °C. Soxhlet extraction with acetone,
15

Journal of Materials Chemistry C
Page 16 of 19
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hexane, dichloromethane and chloroform. The residue was dissolved in hot TCE, filtered,
DOI: 10.1039/C7TC04322H
concentrated and precipitated in methanol. Yield 57 mg, 53 %. Solubility was insufficient to record
the 1H‐NMR spectrum.
PDCB‐TT‐BO. Same procedure as PDCB‐T‐EH, using M2 (97.9 mg, 0.131 mmol), 2,5‐
bis(trimethylstannyl)thieno[3,2‐b]thiophene (61.2 mg, 0.131 mmol), PPh3 (4.113 mg, 15.68 μmol),
Pd2dba3 (3.612 mg, 3.944 μmol), anhydrous toluene (3.3 mL), and anhydrous DMF (0.3 mL).
Palladium scavenging in 1,1,2,2‐tetrachloroethane at 115 °C. Soxhlet extraction with acetone,
hexane, dichloromethane and chloroform. The residue was dissolved in hot TCE, filtered,
concentrated and precipitated in methanol. Yield 58 mg, 58 %. Solubility was insufficient to record
the 1H‐NMR spectrum.
PDCB‐TT‐HD. Same procedure as PDCB‐T‐EH, using M3 (151.5 mg, 0.176 mmol), 2,5‐
bis(trimethylstannyl)thieno[3,2‐b]thiophene (82.3 mg, 0.177 mmol), PPh3 (5.575 mg, 21.26 μmol),
Pd2dba3 (4.815 mg, 5.258 μmol), anhydrous toluene (3.3 mL), and anhydrous DMF (0.3 mL).
Palladium scavenging in 1,1,2,2‐tetrachloroethane at 115 °C. Soxhlet extraction with acetone,
hexane, dichloromethane and chloroform. Yield 99 mg, 67 %. The 1H‐NMR spectrum is shown in Fig.
S15.
Acknowledgements
We thank B.J. Bruijnaers for assistance with the X‐ray measurements. The research has received
funding from the European Research Council under the European Union's Seventh Framework
Programme (FP/2007‐2013) / ERC Grant Agreement No. 339031 and from the Ministry of Education,
Culture and Science (Gravity program 024.001.035).
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Graphic and text for table of contents
6
5
4
3
2
1
0
T
2T TT
EH
BO
HD
Pristine
Annealed
50
100
150
200
250
300
DSC transition (oC)
Polythiophene derivatives with ester side chains enable high open-circuit voltages in polymer-fullerene
solar cells. The side chains affect solubility, thermal properties, tendency to aggregate, and photovoltaic
performance by modulating the morphology of the blends.