In situ monitoring the thermal degradation of PCPDTBT low band gap polymers with varying alkyl side-chain patterns

Article abstract of DOI:10.1002/pola.26920

The degradation pattern of a series of low band gap PCPDTBT polymers under thermal stress is investigated by in situ UV-vis and FT-IR techniques combined with thermal degradation analysis. Thermogravimetric analysis is used to predetermine the decomposition intervals, revealing that thermolysis occurs in two stages. TG-TD-GC/MS shows that loss of the alkyl side chains predominantly happens within the first temperature regime and degradation of the polymer backbone occurs thereafter. UV-vis spectroscopy is used to monitor the evolution of the optical properties upon heating, reflecting the thermal stability of the conjugated backbone, whereas FT-IR spectroscopy is applied to evaluate the chemical changes under thermal stress, with an emphasis on the polymer periphery. The influence of the side chains and possible defects, dependent on the synthesis protocol applied, on the thermal stability of the final polymers is discussed and is related to the application of said materials in organic photovoltaics.

Full text of DOI:10.1002/pola.26920

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In Situ Monitoring the Thermal Degradation of PCPDTBT Low Band Gap
Polymers with Varying Alkyl Side-Chain Patterns
Lidia Marin,1,2 Huguette Penxten,1 Sarah Van Mierloo,1 Robert Carleer,3 Laurence Lutsen,2
Dirk Vanderzande,1,2 Wouter Maes1,2
1Design and Synthesis of Organic Semiconductors (DSOS), Institute for Materials Research (IMO-IMOMEC), Hasselt University,
Agoralaan 1 – Building D, 3590 Diepenbeek, Belgium
2IMEC, IMOMEC Ass. Lab., Universitaire Campus, Wetenschapspark 1, 3590 Diepenbeek, Belgium
3Applied and Analytical Chemistry, Institute for Materials Research (IMO-IMOMEC), Hasselt University, Agoralaan 1 – Building D,
3590 Diepenbeek, Belgium
Correspondence to: W. Maes (E-mail: wouter.maes@uhasselt.be)
Received 13 June 2013; accepted 15 August 2013; published online
DOI: 10.1002/pola.26920
ABSTRACT: The degradation pattern of a series of low band gap
PCPDTBT polymers under thermal stress is investigated by in situ
UV–vis and FT-IR techniques combined with thermal degradation
analysis. Thermogravimetric analysis is used to predetermine the
decomposition intervals, revealing that thermolysis occurs in two
stages. TG-TD-GC/MS shows that loss of the alkyl side chains pre-
dominantly happens within the first temperature regime and deg-
radation of the polymer backbone occurs thereafter. UV–vis
spectroscopy is used to monitor the evolution of the optical proper-
ties upon heating, reflecting the thermal stability of the conjugated
backbone, whereas FT-IR spectroscopy is applied to evaluate the
chemical changes under thermal stress, with an emphasis on the
polymer periphery. The influence of the side chains and possible
defects, dependent on the synthesis protocol applied, on the ther-
mal stability of the final polymers is discussed and is related to the
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application of said materials in organic photovoltaics. 2013 Wiley
Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 00, 000–
000
KEYWORDS: alkyl side chains; degradation pattern; FT-IR and
UV-vis spectroscopy; low band gap conjugated polymers;
PCPDTBT; thermal stability
factors such as light, heat, and humidity, require light harvest-
ing materials that are stable under UV–vis irradiation and
thermal stress, as well as efficient encapsulation to prevent
oxygen and humidity to penetrate in the device. For the state
of the art bulk heterojunction (BHJ) organic solar cells the sta-
bility of the required phase-separated blend nanomorphology
is also of particular concern, as the conjugated low band gap
donor polymer5 and fullerene acceptor materials tend to reor-
ganize in separate domains over time, a process strongly
accelerated at higher (use) temperature.4,6
INTRODUCTION Organic photovoltaics (OPV) show appealing
properties such as simple preparation, novel aesthetical possi-
bilities, reduced weight and mechanical flexibility, semi-
transparency, and better performance in diffuse light, which
makes organic solar cells particularly attractive for portable
or wearable electronics and building-integrated photovoltaics.
OPV has the potential of becoming a competitive renewable
energy technology if the requirements of the “critical triangle
for photovoltaics,” that is, low cost, high efficiency, and pro-
longed lifetime, are simultaneously fulfilled.1 Solution process-
ability and the small amounts of semiconducting materials
required in the fabrication of organic solar cells by roll to roll
printing make these devices a cheap and viable alternative.2
With certified efficiencies having surpassed the 10% threshold
for single modules as well as for tandem configurations, and
considering their almost exponential evolution over the last
decade, the efficiency requirement for OPV can probably also
be met.3 The last and most challenging hurdle is then to
improve the stability of OPV devices and modules.4 Photovol-
taic modules, being continuously exposed to external stress
OPV performance and stability being strongly dependent on
the integrity of the photoactive materials, which depends on
a series of external and internal factors, it is important to
determine the evolution of the compounds under certain
stress conditions, for example, increased temperature, irradi-
ation by solar light, the presence of oxygen and/or humidity,
or a combination of them. It has already been shown on sev-
eral occasions that under accelerated artificial aging condi-
tions semiconducting polymers start to degrade.4,7 In terms
Additional Supporting Information may be found in the online version of this article.
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which the monomer was obtained via a cyclization step
rather than by alkylation under basic conditions.10 Neverthe-
less, the formation of the “keto defect” was not fully pre-
vented, its apparition also being noticed in ladder-type
structures. Moreover, no correlation was found between the
concentrations of the monoalkylfluorene and those of 9-
fluorenone, the formation of the latter being enhanced in PF
films, which underwent additional thermal treatment, sug-
gesting a more complex degradation process.11
Poly[(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b0]dithiophene-
2,6-diyl)-alt-(2,1,3-benzothiadiazole-4,7-diyl)] (PCPDTBT; 2-
ethylhexyl-substituted, as in P1 in Fig. 1) was among the
first low band gap donor-acceptor copolymers to be explored
for OPV applications, affording power conversion efficiencies
(PCEs) above 2.8% in combination with
a fullerene
acceptor.12 In the presence of a suitable processing additive,
the efficiency was almost doubled, surpassing the 5% thresh-
old.13 As a result, PCPDTBT has become one of the work-
horse low band gap materials in the field and seminal
studies have been conducted using this donor polymer (and
derivatives thereof).14 Although other systems have been
shown to afford higher PCEs in the last couple of years, the
interest in PCPDTBT and related polymers has very recently
been given a novel boost by the high efficiencies obtained
with some fluorinated analogues.3(h),15 The dialkylated CPDT
building block has been shown to have a rather poor photo-
chemical stability,4(c) attributed to the quaternary carbon
bridge, but in general information on the stability of CPDT-
based polymers, either as crude materials or in BHJ OPV
blends, is very limited.16
FIGURE 1 Structures of PCPDTBT-type donor-acceptor copoly-
mers P1–P3.
of stability, the alkyl side chains—mandatory to obtain con-
jugated polymers that can be processed from solution—are
regarded as the Achilles’ heel of the polymer materials. In
general, the side chains are (oxidatively) cleaved off in a first
degradation step, starting a chain reaction that eventually
leads to the degradation of the whole conjugated backbone.
Additionally, defects that are built up during the polymer
synthesis, and which are consequently present along the
polymer backbone, and residual metal impurities (e.g., from
the Ni or Pd catalysts employed during the polymer synthe-
sis) can considerably speed up the degradation process, lead-
For this reason, we have conducted a detailed study on the
behavior of three PCPDTBT polymers P1–P3 with different
alkyl side-chain patterns (Fig. 1) under induced thermal
stress (thermolysis and thermo oxidation). PCPDTBT was
chosen for its importance in the field and the ample experi-
ence on the synthesis and OPV applications of such materials
within the group.17 The aim of the study was two-fold: (i)
analyze the purely thermal stability of the PCPDTBT model
low band gap polymer, with an emphasis on the influence of
the alkyl side chains and the synthetic history and (ii) iden-
tify if a similar keto defect appears as was identified during
PF degradation. Although light (in combination with oxygen)
is a more aggressive constraint for most OPV polymers than
temperature, the latter cannot be neglected either and
knowledge on the purely thermal degradation pathway of
these materials is certainly relevant and complementary to
photooxidation studies. For this purpose, UV–vis and FT-IR in
situ spectroscopic techniques were combined with thermal
degradation analysis.18 Such an approach is quite unique as
usually just thermogravimetric analysis (TGA) data are pro-
vided for novel photoactive push-pull polymers. The mass
loss observed by TGA is, however, not sufficient for OPV sta-
bility screening as degradation of the chromophore system
without cleavage of any subunit also results in a strong
decrease in OPV performance. A number of studies tackling
the stability issues of semiconducting OPV polymers have
ing to
a deterioration of the optical and conducting
properties.4,7,8
Elaborate degradation studies have already been conducted
on poly[2-methoxy-5-(30,70-dimethyloctyloxy)21,4-phenylene
vinylene] (MDMO-PPV), poly(3-hexylthiophene) (P3HT), and
poly(9,9-dialkyl-9H-fluorene) (PF) derivatives.7,8 The solubi-
lizing side chains (alkyl or alkoxy) and the exocyclic vinylene
bonds (in MDMO-PPV) were proven to be the weakest points
of these photoactive materials. The complex mechanisms
involved in the degradation process are strongly influenced
by the nature of the side chains, the position of the substitu-
ents and the interconnection (CAC vs. C@C) of the aromatic
repeating units. Even though, in all cases, initial degradation
of the side chains triggers the scission of the backbone, the
decomposition products are nevertheless strongly dependent
on the nature of the repeating units. Detailed investigations
on the degradation mechanisms of modern low band gap
donor-acceptor copolymers have only been performed to a
minor extent so far.4
In PFs, the monoalkylfluorene defect is believed to be
responsible for the alteration of the optical characteristics of
the pristine polymer due to oxidative formation of a ketone
functionality on the carbon bridge of the fluorene unit.9 This
process was drastically suppressed in fully alkylated PFs for
2
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already been conducted, generally addressing either one sin-
gle material or comparing different polymer classes. It was
revealed that small changes in the chemical structure of the
repeating units and the fashion by which they are intercon-
nected can have a huge impact on the photochemical stabil-
ity of a given system.4,16 Although the impact of the side
chains on the purely thermal stability of low band gap poly-
mers is expected to be smaller, this aspect has not been
investigated in detail before for a specific material class.
solution (10 mg mL21). All experiments were performed at a
heating rate of 2 ꢀC min21 under a continuous flow of air.
TGA measurements were performed on a TA instrument 951
thermogravimetric analyzer under a continuous nitrogen
flow of 80 mL min21 and a heating rate of 20 ꢀC min21
.
Polymer samples (5 mg) were inserted in the solid state.
Collection of the released volatiles occurred on self-packed
Tenax tubes (Tenax TA 60/80 Grace) in two fractions, 20–
550 and 550–1000 ꢀC, as predetermined by TGA experi-
ments. Analysis of the adsorbed VOCs was done using a
Unity-1 thermal desorber (Markes) setup equipped with an
Ultra-TD autosampler (Markes), operating under the follow-
ing experimental conditions: tube desorption 20 min at 270
ꢀC, cold trap temperature 21ꢀ0 ꢀC, heating of general pur-
pose cold trap 15 min at 320 C, helium desorption flow 20
mL min21, inlet and outlet split flow 75 mL min21. Analytes
were transferred to a Trace Ultra GC Thermo gas chromato-
graph equipped with a ZB5-MS [30 m 3 0.25 mm 3 0.25
mm (df)] fused silica column. The initial temperature of 30
EXPERIMENTAL
Materials and Methods
Unless stated otherwise, all reagents and chemicals were
obtained from commercial sources and used without further
purification. 2,1,3-Benzothiadiazole-4,7-bis(boronic acid pina-
col ester), 95%, (2) was purchased from Sigma Aldrich and
used without further purification.
ꢀ
ꢀC was maintained for 5 min, ramped to 100 C at a rate of
NMR chemical shifts (d, in ppm) were determined relative to
the residual CHCl3 absorption (7.26 ppm) or the 13C reso-
nance shift of CDCl3 (77.16 ppm). Gas chromatography-mass
spectrometry (GC-MS) analyses were carried out applying
Chrompack CPsil5CB or CPsil8CB capillary columns. HPLC
experiments were carried out using an Agilent 1100 series
system. A Phenomenex (OOF-4248-EO) Luna 3 m C8(2) 100
A 2 150 3 4.60 mm column was used at 25 ꢀC with a
MeOH/THF/H2O gradient at a flow rate of 0.5 mL/min. Chro-
matograms were obtained by scanning the samples from 190
to 600 nm with a resolution of 2 nm and the chromato-
graphic peaks were monitored at 250 and 330 nm. Molecu-
lar weights and molecular weight distributions were
determined relative to polystyrene standards (Polymer Labo-
ratories) by size exclusion chromatography (SEC). Chromato-
grams were recorded on a Spectra Series P100 (Spectra
Physics) equipped with two mixed-B columns (10 mm, 0.75
3 30 cm2, Polymer Laboratories) and a refractive index
8
ꢀC min21, and then ramped to 310 ꢀC at a rate of 12 ꢀC
min21 with a subsequent hold time of 5 min. Mass spectro-
metric analyses were carried out on a DSQ-I mass spectrom-
eter (Thermo) in EI1 mode (electron energy 70 eV, scan
from m/z 5 33 to 480 in 0.4 s).
SYNTHESIS
2,6-Dibromo-4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b’]-
dithiophene (1a/1a0) was synthesized according to literature
procedures.17(a,c),19 Material identity and purity were con-
1
firmed by HPLC (>99%), GC-MS and H NMR.
Poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b0]-
dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT)
(P1/P10) was synthesized using a general polymerization pro-
cedure from 1a/1a0 and 2, according to a modified literature
procedure.14(c) The phase transfer catalyst Aliquat 336 was
replaced by a high temperature resistant variant, that is, Ali-
quatTM HTA-1. The crude polymer was precipitated in metha-
nol and purified by repetitive Soxhlet extraction with
methanol, acetone, and n-hexane. The final polymer P1 was
extracted with chloroform, isolated upon precipitation in
methanol and dried under vacuum to afford a dark green
powder (yield 5 84%).20 GPC (THF, PS standards) Mn 5 1.8 3
104 g mol21, Mw 5 3.6 3 104 g mol21, PDI 5 2.0. P10 was
extracted with chloroform, isolated upon precipitation in
methanol, and dried under vacuum to afford a dark green
powder (yield 5 72%). GPC (THF, PS standards) Mn 5 1.1 3
104 g mol21, Mw 5 2.3 3 104 g mol21, PDI 5 2.1. Polymer P1
ꢀ
detector (Shodex) at 40 C. Tetrahydrofuran (THF) was most
often used as the eluent at a flow rate of 1.0 mL min21. If
chlorobenzene (CB) was used as the eluent, the temperature
ꢀ
was raised to 80 C. In situ UV–vis measurements were per-
formed on
a Cary 500 UV–vis-NIR spectrophotometer,
adapted by Agilent to contain a Harrick high-temperature
cell, with a scan rate of 600 nm min21 in a continuous run
from 1000 to 200 nm. Polymer films were drop-cast from a
CB solution (10 mg mL21) on a quartz glass substrate, which
was subsequently placed and heated in the Harrick high
temperature cell, positioned in the beam of the spectropho-
tometer. The temperature of the film was controlled by a
dual channel temperature controller from Watlow (series
999). Spectra were taken every 10 min. The heating rate was
showed
a reasonable PCE of 2.7% in a BHJ device
21
ꢀ
ꢀ
2 C min up to 250 C. All measurements were performed
under a continuous flow of nitrogen. Step-heating experi-
ments with in situ collection of IR were also performed in a
Harrick high temperature cell, this time positioned in the
beam of a Perkin-Elmer Spectrum One FT-IR spectrometer.
Infrared spectra were collected with a resolution of 4 cm21
(16 scans) using films drop-cast on a NaCl disk from a CB
PCPDTBT:PC71BM (1:3; spin-coated from CB; no additives
used).
4,4-Dioctyl-4H-cyclopenta[2,1-b:3,4-b0]dithiophene was syn-
thesized according to our previously reported procedure.17(a)
Material identity and purity were confirmed by HPLC
1
(>99%), GC-MS, and H NMR.
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2,6-Dibromo-4,4-dioctyl-4H-cyclopenta[2,1-b:3,4-b’]dithio-
phene (1b0) was synthesized according to a literature proce-
dure.17(c) NBS (1.41 g, 7.96 mmol) was added to a solution
of 4,4-dioctyl-4H-cyclopenta[2,1-b:3,4-b0]dithiophene (1.00 g,
2.49 mmol) in DMF (100 mL). The reaction mixture was
stirred for 3 h at r.t. Subsequently, the mixture was poured
onto ice and extracted with diethyl ether (3 3 100 mL). The
combined organic layers were washed with water and brine,
dried over MgSO4, filtered, and concentrated by evaporation
in vacuo. The crude product was purified by column chroma-
tography (silica, eluent n-hexane). The compound was
obtained as a yellow oil (0.77 g, 55%); 1H NMR (300 MHz,
CDCl3) d 6.93 (s, 2H), 1.78–1.72 (m, 4H), 1.27–1.13 (m,
24H), 0.86 (t, J 5 6.9 Hz, 6H); 13C NMR (75 MHz, CDCl3) d
156.0, 136.4, 124.6, 111.2, 55.1, 37.7, 31.9, 31.7, 30.1, 29.4,
24.6, 22.8, 14.3; GC-MS (EI) m/z 558/562 [M1]; IR (NaCl,
cm21) mmax 3083 (w, unsaturated CAH), 2954/2926/2854
(s, saturated CAH).
matography (silica, eluent hexanes) to afford a colorless
oil (0.146 g, 56%). GC-MS (EI) m/z 290 [M1]; 1H NMR
(300 MHz, CDCl3) d 7.15 (d, J 5 4.8 Hz, 2H), 7.05 (d,
J 5 4.8 Hz, 1H), 7.04 (d, J 5 5.1 Hz, 1H), 3.70 (t, J 5 8.1 Hz,
1H), 1.66–1.58 (m, 2H), 1.49–1.39 (m, 1H), 1.32–1.24 (m,
8H), 0.92–0.86 (m, 6H); 13C NMR (75 MHz, CDCl3) d
154.90, 154.84, 137.5, 124.4, 122.8, 42.4, 37.4, 36.1, 32.8,
28.7, 25.9, 23.2, 14.3, 10.6.
2,6-Dibromo-4-(2-ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b0]
dithiophene (5a) and 2,6-dibromo-4-(2-ethylhexylidene)-
4H-cyclopenta[2,1-b:3,4-b0]dithiophene (5b): 4-(2-Ethylhexyl)-
4H-cyclopenta[2,1-b:3,4-b0]dithiophene (4a; 0.400 g, 1.38
mmol) was ꢀdissolved in DMF (25 mL) and the mixture was
cooled to 0 C by means of an ice bath (the reaction was run
in the dark and under nitrogen atmosphere). Subsequently, a
solution of NBS (0.491 g, 2.76 mmol) in DMF (25 mL) was
added drop-wise and the mixture was allowed to react at r.t.
for 1 h. Water (10 mL) was added and the aqueous layer was
extracted with CH2Cl2. The combined organic layers were
dried over MgSO4, filtered, and concentrated by evaporation
in vacuo. The crude product was purified by column chroma-
tography (silica, eluent hexanes).
Poly[2,6-(4,4-dioctyl-4H-cyclopenta[2,1-b:3,4-b0]dithiophene)-
alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT) (P20) was syn-
thesized according to the general polymerization procedure
from 1b0 and 2. P20 was extracted with CB, isolated upon
precipitation in methanol, and dried under vacuum to afford
a dark green powder (yield 5 72%).14(g) GPC (CB, PS stand-
The crude reaction mixture, as analyzed by GC-MS and 1H
NMR (Supporting Information Fig. S1), consisted of a mixture
of 5a and 5b, which were present in a ꢁ1:0.1 ratio. GC-MS
(EI) m/z 446/448/450 and 444/446/448 [M1]. Upon col-
umn chromatographic purification, a yellow oil was recov-
ered (0.492 g, 80%), which was identified as pure 5b. 1H
NMR (300 MHz, CDCl3) d 7.17 (s, 1H), 7.15 (s, 1H), 6.13 (d,
J 5 10.8 Hz, 1H), 2.80–2.71 (m, 1H), 1.68–1.57 (m, 2H),
1.47–1.37 (m, 2H), 1.29–1.25 (m, 4H), 0.91–0.83 (m, 6H);
13C NMR (75 MHz, CDCl3) d 144.0, 140.44, 140.36, 139.1,
135.4, 130.5, 125.6, 122.8, 111.0, 110.4, 42.5, 35.3, 29.9,
28.7, 23.0, 14.1, 12.1; IR (NaCl, cm21) mmax 3102/3083 (w,
unsaturated CAH), 2957/2922/2869/2854 (s, saturated
CAH), and 1650 (w, unsaturated CAH).
ards) Mn 5 8.3 3 103 g mol21, Mw 5 1.4 3 104 g mol21
PDI 5 1.6.
,
2,6-Dibromo-4,4-didodecyl-4H-cyclopenta[2,1-b:3,4-b’]dithio-
phene (1c) was synthesized according to a literature proce-
dure.17(c) Material identity and purity were confirmed by
1
HPLC (>98.5%), GC-MS and H NMR.
Poly[2,6-(4,4-didodecyl)-4H-cyclopenta[2,1-b:3,4-b0]dithio-
phene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT) (P3)
was synthesized according to the general polymerization
procedure from 1c and 2. P3 was extracted with CB, isolated
upon precipitation in methanol, and dried under vacuum to
afford a dark green powder (yield 5 95%).17(d) GPC (CB, PS
standards) Mn 5 1.6 3 104 g mol21, Mw 5 7.2 3 104
mol21, PDI 5 4.5.
g
RESULTS AND DISCUSSION
4H-Cyclopenta[2,1-b:3,4-b0]dithiophene (3) was synthesized
Synthesis and Characterization
Polymer synthesis
according to a literature procedure.21(a) Material identity was
1
The synthesis of the dialkylated CPDT derivatives (1a–c;
Scheme 1) was achieved using two different strategies. Com-
pounds 1a and 1c were synthesized by the traditional route,
involving direct alkylation of parent 4H-cyclopenta[2,1-b:3,4-
b’]dithiophene under basic conditions in the presence of 2-
ethylhexyl bromide and 1-bromododecane, respectively, fol-
lowed by bromination with N-bromosuccinimide (NBS).21
However, monomers 1a0 and 1b0 were obtained upon bromi-
nation of the CPDT scaffold prepared via the three-step syn-
thetic protocol as recently developed in our group.17(a),22 In
the first step, 3-bromo-2,20-bithiophene was synthesized by
Kumada coupling of 2-thienylmagnesium bromide with 2,3-
dibromothiophene. Lithiation in position 3 followed by addi-
tion of the corresponding dialkyl ketone afforded dialkylated
tertiary alcohol derivatives. The CPDT scaffold was ultimately
confirmed by GC-MS and H NMR.
4-(2-Ethylhexyl)-4H-cyclopenta[2,1-b:3,4-b0]dithiophene
(4a): n-BuLi (2.5 M in hexane, 0.39 mL) was added drop-
wise to a solution of 3 (0.160 g, 0.90 mmol) in THF (8
mL), cooled to 0 ꢀC by means of an ice bath. The mixture
was allowed to warm-up to r.t. and was left at this tem-
perature for 1.5 h. A solution of 2-ethylhexyl bromide
(0.191 g, 0.99 mmol) in THF (5 mL) was then added and
the mixture was allowed to react at r.t. overnight. The
reaction was quenched by addition of water. The organic
layer was separated and the aqueous layer was extracted
with diethyl ether. The combined organic extracts were
dried over MgSO4, filtered, and concentrated by evapora-
tion in vacuo. The residue was purified by column chro-
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TABLE 1 Analytical SEC Data of PCPDTBT Polymers P1, P10,
P20, and P3 (Profiles in Supporting Information Fig. S5)
Polymer
Mn (g mol21
)
PDI
P1a
1.8 3 104
1.1 3 104
8.3 3 103
1.6 3 104
2.0
2.1
1.6
4.5
a
P10
b
P20
P3b
a
SEC in THF.
SEC in CB.
b
SCHEME 1 PCPDTBT polymer synthesis via Suzuki polycondensation.
during the monomer synthesis and hence built-in in the final
polymers. Polymers P10 and P20, for which the CPDT scaffold
was obtained by a final ring closure reaction, should be free
of any monoalkyl defects.
obtained by means of Friedel-Crafts dehydration cyclization
in sulfuric acid medium. Regardless of the applied synthetic
route, all monomers were highly pure, as confirmed by HPLC
analysis (Supporting Information Figs. S2–S4). It has to be
mentioned though that the three-step synthetic protocol
requires more extensive chromatographic purification.
The monoalkyl defect - Synthesis of 4-(2-ethylhexyl)-4H-
cyclopenta[2,1-b:3,4-b0]dithiophene. Based on the structural
resemblance between fluorene and CPDT, both consisting of
two aryl rings connected by an sp3-hybridized carbon bridge,
and the similar conditions used to synthesize the dialkylated
building blocks (via direct alkylation and/or ring closure),
one might presume a common degradation pathway for
these materials, that is, apparition of a keto defect. To verify
this assumption, we have synthesized a monoalkylated CPDT,
with the goal to incorporate this defect in the final donor-
acceptor PCPDTBT polymers (for comparison) deliberately.
Polymers P1–P3 were then obtained by Suzuki polyconden-
sation of the CPDT derivatives 1a–c and the commercially
available 2,1,3-benzothiadiazole-4,7-bis(boronic acid pinacol
ester) (2) using a modified literature procedure (Scheme
1).14(c) End-capping was performed in two stages by addition
of phenylboronic acid and bromobenzene, respectively.
Molecular weights of the polymers were determined by SEC
ꢀ
ꢀ
in THF at 40 C for P1 and P10 and in CB at 80 C (to mini-
mize aggregation) for P20 and P3 (Table 1, Supporting Infor-
mation Fig. S5). Regardless of the method by which the
CPDT scaffold was prepared, and taking into account the
high purity of the monomers (as determined by HPLC) and
the clean SEC profiles, all polymers were considered “defect-
free” materials (vide infra). Polymers P1 and P3, for which
the alkyl chains were introduced on preformed CPDT sys-
tems, may possibly contain very low percentages of monoal-
kylated CPDTs, which were left undetected (by 1H NMR)
Alkylation of CPDT 3 using 1.1 equiv of n-BuLi and 2-
ethylhexyl bromide afforded monoalkyl-CPDT derivative 4a
as the major reaction product, together with small amounts
of dialkylated derivative 4b (Scheme 2).23 Due to the reason-
able difference in polarity, the pure compounds (4a and 4b,
respectively) could smoothly be separated and purified by
means of column chromatography (on silica), and they
showed normal stability under ambient conditions. As a
result, the presence of a monoalkyl defect in the dialkyl-
CPDT monomer is rather unlikely. Subsequent bromination
SCHEME 2 Synthesis of monoalkylated CPDT derivatives 5a and 5b.
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of 4a in the presence of NBS was required before proceeding
to the Suzuki polycondensation reaction.17(c) The formation
of the desired 2,6-dibromo-4-(2-ethylhexyl)-4H-cyclopenta-
[2,1-b:3,4-b0]dithiophene (5a) was confirmed by 1H NMR of
the crude reaction mixture (Supporting Information Fig. S1).
However, upon purification by means of column chromatog-
raphy (on silica, hexanes eluent) exclusive isolation of the
corresponding 2,6-dibromo-4-(2-ethylhexylidene)-4H-cyclo-
penta [2,1-b:3,4-b’]dithiophene (5b) was observed. The
formation of the dibromoalkylidene derivative is believed
to occur during the purification step and/or during the
handling and analysis of the fractions recovered after
chromatography. These findings suggest that, should a
rough mixture of 1a still contain traces of compound 5a,
the latter will undoubtedly degrade to afford compound
5b. In consequence, this is a strong confirmation that the
corresponding monomers and polymers are free of any
monoalkyl defects.
The relatively unstable nature of products 4a and 5a may
arise from the presence of the labile proton situated on the
bridging carbon atom (C-4). Even under mild basic condi-
tions as applied in the Suzuki polycondensation reaction,
that is, cesium fluoride in 1,2-dimethoxyethane at 80 ꢀC,24
abstraction of the proton in position 4 of compound 4a
occurred. Addition of deuterium oxide at the end of the reac-
tion resulted in an exchange of D and H atoms, as observed
by disappearance of the proton signal at 3.70 ppm. In conse-
quence, the instability of 4a and 5a (the latter never being
isolated in pure form) hampered the synthesis of the corre-
sponding PCPDTBT-like polymers incorporating different per-
centages (e.g., 10–100%) of the monoalkyl defect in the
backbone. Nevertheless, these findings point out a possible
degradation pathway for the dibrominated monoalkyl-CPDT
(5a), leading to the formation of an alkene (5b) rather than
a ketone.
Thermolysis
UV–vis Analysis
For semicrystalline polymers such as P3HT, short exposure
(in the order of minutes) of polymer films to temperatures
ꢀ
between 100 and 150 C has proven to be beneficial for the
organization of the polymer chains. Annealed films show a
higher degree of crystallinity and therefore improved mobil-
ity of charge carriers compared with the “as-cast” ones.25
Higher temperatures and/or prolonged exposure times lead
to degradation of the material, this process being consider-
ably accelerated in the presence of oxygen and/or humidity.
Cleavage of the alkyl side chains accompanied by scission of
the polymer backbone results in a loss of conjugation. Such
structural changes cause an alteration of the chromophore,
modifying the absorption spectrum of the photoactive
species.20
FIGURE 2 UV–vis spectra (not normalized) of polymer thin
films as-cast (solid lines) and after thermolysis for 1 day at 80
ꢀC (top), 150 ꢀC (middle), or 200 ꢀC (bottom) (dotted lines).
[Color figure can be viewed in the online issue, which is avail-
able at wileyonlinelibrary.com.]
the maximum solar cell operation temperature—under an
inert atmosphere, the absorption features remained virtually
unchanged after 24 h of exposure. For polymers P20 and P3,
carrying linear alkyl side chains, an annealing effect was
observed at a more elevated temperature of 150 ꢀC (Fig. 2).
The evolution of the optical properties of thin films of poly-
mers P1–P3, drop-cast from a CB solution, upon thermal
stress was followed by UV–vis spectroscopy (Fig. 2). Upon
ꢀ
isothermal annealing of the polymer films at 80 C—close to
6
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TABLE 2 Overview of the Theoretical (Estimated) Weight Losses and the Values Issued from TGA
Theoretical
TGA Weight Loss (%)/Temperature (ꢀC)
Side Chains (%)
Polymer
Backbone (%)
First Step
Second Step
Third Step
Total
P1 and P10
P20
42.3
42.3
52.4
57.7
57.7
47.6
20/447
27/473
22/472
75/910
52/672
47/577
–
83/963
55/962
73/963
–
P3
58/640
The broadening/shoulder at the low energy side can be attrib-
uted to a better organization and interaction between the poly-
mer chains. For PCPDTBT polymers P20 and P3, the
chromophore appeared to be stable under inert atmosphere
for up to 24 h at 150 ꢀC. In contrast, P10 showed a small
TD-GC/MS (thermogravimetry-thermal desorption-gas chro-
matography/mass spectrometry) experiments were per-
formed. This hyphenated technique allows, during the
heating steps, trapping of the volatile organic compounds
(VOCs) evolving from the polymers on Tenax adsorbent
tubes and later on, in a second step, thermal desorption of
the VOCs and separation of the analytes in function of their
interaction with the stationary phase (based on the intrinsic
properties of the analytes such as molecular weight, volatil-
ity, polarity, etc.). Identification of the thermal decomposition
products provided a clear overview of the degradation mech-
anism taking place in these polymers. In the first tempera-
ture interval (20–550 ꢀC), the expected major degradation
process is the loss and/or fragmentation of the side chains
present on the CPDT bridge. Surprisingly, side-chain degrada-
tion was not only restricted to cleavage of the 2-ethyhexyl,
octyl and dodecyl substituents in P1/P10, P20, and P3,
respectively, but apparently also degradation of the cyclopen-
tadiene ring occurred (Table 3). The expected a- and b-
cleavage of the alkyl chains—b and a bonds, respectively—
was in this case accompanied by cleavage of both c bonds
(see Figure in Table 3). This explains the detection of alka-
nes and alkenes for which the number of carbon atoms is
superior to the length of the alkyl substituents on the CPDT
building block. However, their formation could also take
place during a termination step in which two free alkyl radi-
cals combine. Side-chain degradation was not complete in
the first temperature range and continued at temperatures
ꢀ
decrease in the low energy absorption band. Heating at 200 C
resulted in significant changes in the absorption pattern of P1,
for which a clear decrease in the intensity of the low energy
absorption band was noticed. However, P20 and P3 showed no
or little degradation, respectively, which suggests superior
thermal stability of the polymers bearing linear side chains.
Step-heating experiments, in which the studied polymer
films are subjected to repetitive heating-cooling cycles (Sup-
porting Information Fig. S6), follow a more relevant tempera-
ture stress pattern compared with isothermal heating and
can give an indication of the stability of the polymers when
exposed to important temperature variations, as occurring in
working solar cells. Unlike P10 and P3, which showed minor
changes, the heating-cooling cycles led to a small enhance-
ment of the optical properties for P20 (Supporting Informa-
tion Fig. S7).
In conclusion, the chromophore systems of polymers P1–P3
seem to be stable up to 150 ꢀC under inert atmosphere. At
higher temperatures (200 ꢀC) there is a clear difference
depending on the alkyl side chains, the linear ones affording
higher thermal stability.
TG-TD-GC/MS Analysis
TGA screening experiments were conducted to predetermine
the temperature interval(s) wherein weight loss occurs. For
this purpose, the samples were heated up to ꢁ1000 ꢀC in a
N2 atmosphere (80 mL min21), at a rate of 20 ꢀC min21
,
resulting in pyrolysis of the polymer species. The prelimi-
nary TGA results showed that P1 and P10 lose 20% weight
ꢀ
at ꢁ450 C, while P20 and P3 showed an initial 27 and 22%
weight loss, respectively, at ꢁ475 ꢀC (Table 2, Fig. 3). The
ꢀ
total weight loss, recorded at ꢁ960 C, was 83, 55, and 73%
for P1/P10, P20, and P3, respectively (Fig. 3). This result is
in agreement with the observed stability of the polymer
chromophores according to UV–vis experiments, which
increases from P1/P10 to P3 and P20, albeit looking at a
complete different temperature regime.
FIGURE 3 TGA profiles of the PCPDTBT polymers. [Color figure
can be viewed in the online issue, which is available at
wileyonlinelibrary.com.]
After this screening and according to the observed tempera-
ture intervals (20–550 ꢀC and 550–1000 ꢀC), combined TG-
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TABLE 3 Most Abundant Degradation Compounds as Revealed
by TG-TD-GC/MS Analysis [Color table can be viewed in the
online issue, which is available at wileyonlinelibrary.com.]
phine oxide, presumably from the palladium catalyst
(Pd(PPh3)4) used in the polymerization reaction, was
detected for polymers P10 and P3. Moreover, siloxane was
observed upon degradation of P3. This contamination is
believed to have occurred during the synthesis and/or Soxh-
let extraction of the polymer, as polydimethylsiloxane
(PDMS) was used for lubricating the glass stopcocks and
joints. If present, PDMS alters the film forming properties of
the polymers, leading to “slippery” hydrophobic polymer sol-
utions, which afford inhomogeneous films. The knowledge
on the presence of residual amounts of these materials in
the purified polymer batches is highly relevant as they might
have a direct influence on active layer processing and solar
cell performance.3,4,14(h)
First Step (20–550 ꢀC)
P1 and P10
Second Step (550–1000 ꢀC)
Heptane
Butane 1 butene
Benzenea
3-Methylheptane 1 alkene
Tridecane
Thiophene
Pentadecane
2,1,3-Benzothiadiazole
Undecene
Thermo Oxidation - FTIR Analysis
In situ FT-IR spectroscopy was then used as a tool to moni-
tor the loss of alkyl functionalities present along the
1-Hexene
Butane 1 butene
Heptane
Triphenylphosphine oxidea
P20
Hexane 1 hexene
Heptane 1 heptene
Octane 1 octene
Undecene
Benzene
Thiophene
Heptane 1 heptene
Octane 1 octene
2,1,3-Benzothiadiazole
2,1,3-Benzothiadiazole
P3
Undecane 1 alkene(s)
Dodecane 1 dodecane
Pentadecane
Undecane 1 alkene(s)
Dodecane 1 dodecane
Octane
Decane 1 decene
Pentane 1 pentene
Benzene
Nonene
Cyclopentadiene
Triphenylphosphine oxide
Color codes refer to the bond scissions as indicated in the Figure
above.
a
Detected in P10.
superior to 550 ꢀC. Above this temperature, degradation of
the polymer backbone also took place. Thiophene, 2,1,3-ben-
zothiadiazole, benzene and cyclopentadiene appeared as the
most abundant volatiles in this temperature regime. The
presence of thiophene and 2,1,3-benzothiadiazole provides
a proof of polymer chain scission. Benzene units could be
generated following further degradation of the 2,1,3-benzo-
thiadiazole unit and/or loss of the benzene groups intro-
duced during the end-capping step. Finally, the detection of
cyclopentadiene could be explained by further degradation
of the CPDT unit. TG-TD-GC/MS revealed identical degrada-
tion pathways for both P1 and P10, being an indication that
the synthetic route used for the generation of the CDPT unit
has no (or very little) influence on the thermal stability of
PCPDTBT low band gap copolymers.
FIGURE 4 IR spectra of polymer P10 for the as-cast film and
after sequential heating-cooling cycles under air (top) and plot
of the calculated area (between 3010 and 2750 cm21) corre-
sponding to the alkyl CAH vibrations, recorded at room tem-
perature (25 ꢀC), in function of temperature (bottom). [Color
figure can be viewed in the online issue, which is available at
annalsofneurology.org.]
An important issue concerning conjugated polymers for
organic electronics is purity. The presence of traces of metal
catalysts has for instance been proven to accelerate the deg-
radation of photoactive species.8 Residual triphenylphos-
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PCPDTBT backbone and the apparition of certain degrada-
tion products during thermo oxidation. Particular attention
was devoted to observation of a possible keto defect, as
observed for PFs. For this purpose, thin films fabricated
from PCPDTBT polymers P1–P3 were again subjected to
repetitive heating-cooling cycles (in the absence of UV–vis
light). The experiments were conducted by continuously
flushing the measurement cell with compressed air and in
situ monitoring of the evolution of the chemical composition
within these films.
mer materials was analyzꢀed as well. Under normal solar cell
operating conditions (80 C, encapsulated), the optical prop-
erties and chemical composition of the thin films remained
unchanged after 24 hours of exposure. Polymer degradation
set in at higher temperatures (150/200 ꢀC) and was most
pronounced for P1/P10, suggesting superior stability of the
derivatives bearing linear alkyl side chains. Repetitive
heating-cooling cycles up to 200 ꢀC induced degradation of
the chromophore for P3 and (more pronounced) for P10,
whereas further annealing was seen for P20. According to
TGA, thermolysis occurred in two stages, cleavage of the
alkyl side chains and additional degradation of the polymer
backbone. A combined TG-TD-GC/MS analysis allowed identi-
fication of the volatile derivatives released upon cleavage of
the side chains, as well as aromatic units arising from degra-
dation of the polymer backbone. Moreover, it could be seen
that the synthetic route by which the CDPT scaffold was
obtained had no or very little influence on the thermostabil-
ity of the final polymers. In situ FT-IR studies revealed that
the intensities of the CAH vibrations decreased considerably
at temperatures above 150 ꢀC in the presence of air. The
thermo oxidation was accompanied by the apparition of a
broad band between 1800 and 1600 cm21, suggesting the
formation of carbonyl derivatives. Deliberate incorporation of
monoalkyl defects along the polymer chain, presumed to be
responsible for accelerated degradation (leading to keto
defects) in the case of PFs, was not possible. In ambient con-
ditions and during the purification step of the dibrominated
monoalkyl-CPDT derivative 5a oxidation occurred, resulting
in the formation of the corresponding alkylidene derivative.
This finding is in agreement with the results of the thermo
oxidation experiments, which demonstrates that the degrada-
tion of PCPDTBT derivatives probably does not involve
keto intermediates, as such signatures were not clearly
identified.
The only functionalities present along the PCPDTBT polymer
backbone are the solubilizing alkyl side chains. During the
thermo oxidation experiments, their signature (located
between 3010 and 2750 cm21) was used for evaluating the
cumulative effect of temperature and air on the polymer film
characteristics. In all cases, a slight reduction of the area of
the aliphatic CAH stretching bands (calculated between
3010 and 2750 cm21) was noticed already after heating the
films to 150 ꢀC (Fig. 4 and Supporting Information Fig. S8).
For P10/P20 and P3 this became very important at 220 and
200 ꢀC, respectively. At the end of the experiments, after
having heated the polymer films to 250 ꢀC, complete disap-
pearance of the CAH stretches was observed. This process
was accompanied by the apparition of a broad band situated
between 1800 and 1600 cm21, most probably due to the for-
mation of degradation products possessing carbonyl func-
tionalities, for example, aromatic and/or aliphatic ketones,
carboxylic acids, etc. A possible way for identifying these car-
bonyl functionalities could be to apply a chemical derivatiza-
tion method.26 For none of the PCPDTBT polymers, a keto
defect could clearly be observed, in contrast to the analogous
polyfluorenes. Monitoring the thermal degradation of poly-
mer P1 under inert atmosphere (step-heating to 250 ꢀC) by
in situ FT-IR revealed no identifiable changes in the spectra
(Supporting Information Fig. S9), pointing to a significantly
slower loss of the alkyl functionalities in the absence of air.
In general, this study showed that P20, bearing linear octyl
side chains, has superior thermal stability compared to P3
and P1/P10, bearing linear dodecyl and branched 2-
ethylhexyl side chains, respectively. It became clear that the
pendant alkyl groups, introduced merely for granting solubil-
ity to the final polymers, are not just dormant entities, but
dictate both the performance of the device (by altering the
active layer blend nanomorphology) and the (thermal) stabil-
ity of the photoactive material blend.27
In the case of P3, traces of PDMS were clearly noticed by
FT-IR as well, as unambiguously identified by the fingerprint
located at 1261 cm21 (tCASi, see Supporting Information Fig.
S10). Subsequent washing of the polymer with dichlorome-
thane,
a solvent, which preferentially dissolves PDMS,
allowed removal of this impurity. The as-purified polymer
indeed showed a considerable reduction of the characteristic
PDMS bands (albeit not completely absent) and improved
film forming properties. Avoiding the use of silicon grease
during the synthesis of OPV materials seems therefore
advisable.
ACKNOWLEDGMENTS
The authors acknowledge Hasselt University for continuing
financial support and BELSPO in the frame of the IAP P6/27
and IAP 7/05 networks. The reported activity is partly sup-
ported by the projects ORGANEXT (EMR. INT4-1.2.-2009-04/
054), selected in the frame of the operational program INTER-
REG IV-A Euregio Maas-Rijn, and ORION (FP7-NMP, grant no:
229036). The authors also thank the IWT (Institute for the Pro-
motion of Innovation through Science and Technology in Flan-
ders) for financial support by means of the SBO project
PolySpec (060843).
CONCLUSIONS
We have monitored the direct effect of temperature as an
external stress factor on the stability of a workhorse low
band gap OPV copolymer, PCPDTBT, with an emphasis on the
differences related to variation of the alkyl side-chain pat-
tern. To this extent, in situ UV–vis and FT-IR techniques were
combined with TG-TD-GC/MS, and the influence of the syn-
thetic sequence applied to prepare the light harvesting poly-
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11 L. Liu, S. Qiu, B. Wang, W. Zhang, P. Lu, Z. Xie, M. Hanif, Y.
Ma, J. Shen, J. Phys. Chem. B 2005, 109, 23366–23370.
REFERENCES AND NOTES
1 C. J. Brabec, Sol. Energy Mater. Sol. Cells 2004, 83, 273–292.
€
12 D. Muhlbacher, M. Scharber, M. Morana, Z. Zhu, D. Waller,
2 N. Espinosa, M. Hosel, D. Angmo, F. C. Krebs, Energy Envi-
ron. Sci. 2012, 5, 5117–5132.
R. Gaudiana, C. Brabec, Adv. Mater. 2006, 18, 2884–2889.
13 J. Peet, J. Y. Kim, N. E. Coates, W. L. Ma, D. Moses, A. J.
Heeger, G. C. Bazan, Nat. Mater. 2007, 6, 497–500.
3 (a) T. S. van der Poll, J. A. Love, T.-Q. Nguyen, G. C. Bazan,
Adv. Mater. 2012, 24, 3646–3649; (b) Y.-W. Su, S.-C. Lan, K.-H.
Wei, Mater. Today 2012, 15, 554–562; (c) M. A. Green, K.
Emery, Y. Hishikawa, W. Warta, E. D. Dunlop, Progr. Photovolt:
Res. Appl. 2012, 20, 12–20; (d) G. Li, R. Zhu, Y. Yang, Nat. Pho-
ton. 2012, 6, 153–161; (e) L. J. A. Koster, S. E. Shaheen, J. C.
Hummelen, Adv. Energy Mater. 2012, 2, 1246–1253; (f) A.
14 (a) J. Y. Kim, K. Lee, N. E. Coates, D. Moses, T.-Q. Nguyen,
M. Dante, A. J. Heeger, Science 2007, 317, 222–225; (b) C. Soci,
I. W. Hwang, D. Moses, Z. Zhu, D. Waller, R. Gaudiana, C. J.
Brabec, A. J. Heeger, Adv. Funct. Mater. 2007, 17, 632–636; (c)
M. Zhang, H. N. Tsao, W. Pisula, C. Yang, A. K. Mishra, K.
€
Mullen, J. Am. Chem. Soc. 2007, 129, 3472–3473; (d) R. C.
€
Mishra, P. Bauerle, Angew. Chem. Int. Ed. 2012, 51, 2020–2067;
Coffin, J. Peet, J. Rogers, G. C. Bazan, Nat. Chem. 2009, 1, 657–
661; (e) H. N. Tsao, D. Cho, J. W. Andreasen, A. Rouhanipour,
(g) V. S. Gevaerts, A. Furlan, M. M. Wienk, M. Turbiez, R. A. J.
Janssen, Adv. Mater. 2012, 24, 2130–2134; (h) J. You, L. Dou,
K. Yoshimura, T. Kato, K. Ohya, T. Moriarty, K. Emery, C.-C.
Chen, J. Gao, G. Li, Y. Yang, Nat. Commun. 2013, 4, 1446; (i)
www.heliatek.com. Heliatek world record cells with 12.0%
efficiency.
€
D. W. Breiby, W. Pisula, K. Mullen, Adv. Mater. 2009, 21, 209–
212; (f) M. Lenes, M. Morana, C. J. Brabec, P. W. M. Blom,
Adv. Funct. Mater. 2009, 19, 1106–1111; (g) A. P. Zoombelt, S.
G. J. Mathijssen, M. G. R. Turbiez, M. M. Wienk, R. A. J.
Janssen, J. Mater. Chem. 2010, 20, 2240–2246; (h) J. Kettle, M.
Horie, L. A. Majewski, B. R. Saunders, S. Tuladhar, J. Nelson,
M. L. Turner, Sol. Energy Mater. Sol. Cells 2011, 95, 2186–2193;
(i) A. Teichler, R. Eckardt, S. Hoeppener, C. Friebe, J. Perelaer,
A. Senes, M. Morana, C. J. Brabec, U. S. Schubert, Adv. Energy
ꢀ
4 (a) S. Cho, J. H. Seo, S. H. Park, S. Beaupre, M. Leclerc, A. J.
Heeger, Adv. Mater. 2010, 22, 1253–1257; (b) H. Cha, H. Kong,
D. S. Chung, W. M. Yun, T. K. An, J. Hwang, Y.-H. Kim, H.-K.
Shim, C. E. Park, Org. Electron. 2010, 11, 1534–1542; (c) M.
Manceau, E. Bundgaard, J. E. Carle, O. Hagemann, M.
Helgesen, R. Sondergaard, M. Jorgensen, F. C. Krebs, J. Mater.
Chem. 2011, 21, 4132–4141; (d) M. Jïrgensen, K. Norrman, S.
A. Gevorgyan, T. Tromholt, B. Andreasen, F. C. Krebs, Adv.
Mater. 2012, 24, 580–612; (e) D. M. Tanenbaum, M. Hermenau,
E. Voroshazi, M. T. Lloyd, Y. Galagan, B. Zimmermann, M.
Hosel, H. F. Dam, M. Jorgensen, S. A. Gevorgyan, S. Kudret,
W. Maes, L. Lutsen, D. Vanderzande, U. Wurfel, R. Andriessen,
R. Rosch, H. Hoppe, G. Teran-Escobar, M. Lira-Cantu, A.
Rivaton, G. Y. Uzunoglu, D. Germack, B. Andreasen, M. V.
Madsen, K. Norrman, F. C. Krebs, RSC Adv. 2012, 2, 882–893;
(f) S. Alem, S. Wakim, J. Lu, G. Robertson, J. Ding, Y. Tao,
ACS Appl. Mater. Interfaces 2012, 4, 2993–2998.
€
Mater. 2011, 1, 105–114; (j) K. F. Jeltsch, M. Schadel, J.-B.
Bonekamp, P. Niyamakom, F. Rauscher, H. W. A. Lademann, I.
Dumsch, S. Allard, U. Scherf, K. Meerholz, Adv. Funct. Mater.
2012, 22, 397–404; (k) M. Horie, J. Kettle, C.-Y. Yu, L. A.
Majewski, S.-W. Chang, J. Kirkpatrick, S. M. Tuladhar, J.
Nelson, B. R. Saunders, M. L. Turner, J. Mater. Chem. 2012, 22,
381–389; (l) A. V. Tunc, A. De Sio, D. Riedel, F. Deschler, E. Da
Como, J. Parisi, E. von Hauff, Org. Electron. 2012, 13, 290–296.
15 (a) S. Albrecht, S. Janietz, W. Schindler, J. Frisch, J.
Kurpiers, J. Kniepert, S. Inal, P. Pingel, K. Fostiropoulos, N.
Koch, D. Neher, J. Am. Chem. Soc. 2012, 134, 14932–14944; (b)
Y. Zhang, J. Zou, C.-C. Cheuh, H.-L. Yip, A. K. Y. Jen, Macro-
molecules 2012, 45, 5427–5435.
5 In this article, we only address conjugated polymers and do
16 A. Distler, P. Kutka, T. Sauermann, H.-J. Egelhaaf, D. M.
Guldi, D. Di Nuzzo, S. C. J. Meskers, R. A. J. Janssen, Chem.
Mater. 2012, 24, 4397–4405.
not consider small molecule OPV.
ꢀ
6 (a) B. J. Kim, Y. Miyamoto, B. Ma, J. M. J. Frechet, Adv. Funct.
Mater. 2009, 19, 2273–2281; (b) J. Vandenbergh, B. Conings, S.
Bertho, J. Kesters, D. Spoltore, S. Esiner, J. Zhao, G. Van
Assche, M. M. Wienk, W. Maes, L. Lutsen, B. Van Mele, R. A. J.
Janssen, J. Manca, D. J. M. Vanderzande, Macromolecules
2011, 44, 8470–8478; (c) S. Bertho, B. Campo, F. Piersimoni, D.
Spoltore, J. D’Haen, L. Lutsen, W. Maes, D. Vanderzande, J.
Manca, Sol. Energy Mater. Sol. Cells 2013, 110, 69–76.
17 (a) S. Van Mierloo, P. J. Adriaensens, W. Maes, L. Lutsen, T.
J. Cleij, E. Botek, B. Champagne, D. J. Vanderzande, J. Org.
Chem. 2010, 75, 7202–7209; (b) S. Van Mierloo, A. Hadipour,
M.-J. Spijkman, N. Van den Brande, B. Ruttens, J. Kesters, J.
D’Haen, G. Van Assche, D. M. de Leeuw, T. Aernouts, J.
Manca, L. Lutsen, D. J. Vanderzande, W. Maes, Chem. Mater.
2012, 24, 587–593; (c) L. Marin, S. Van Mierloo, Y. Zhang, K.
Robeyns, B. Champagne, P. Adriaensens, L. Lutsen, D.
Vanderzande, W. Maes, Tetrahedron 2013, 69, 2260–2267; (d) L.
Marin, Y. Zhang, K. Robeyns, B. Champagne, P. Adriaensens,
L. Lutsen, D. Vanderzande, D. Bevk, W. Maes, Tetrahedron Lett.
2013, 54, 526–529.
7 (a) S. Chambon, A. Rivaton, J.-L. Gardette, M. Firon, L.
Lutsen, J. Polym. Sci. Part A: Polym. Chem. 2007, 45, 317–331;
(b) M. Manceau, A. Rivaton, J.-L. Gardette, S. Guillerez, N.
^
Lemaıtre, Polym. Degrad. Stab. 2009, 94, 898–907; (c) M.
Manceau, S. Chambon, A. Rivaton, J.-L. Gardette, S. Guillerez,
^
N. Lemaıtre, Sol. Energy Mater. Sol. Cells 2010, 94, 1572–1577;
18 E. Kesters, D. Vanderzande, L. Lutsen, H. Penxten, R.
Carleer, Macromolecules 2005, 38, 1141–1147.
(d) S. Chambon, A. Rivaton, J.-L. Gardette, M. Firon, Polym.
Degrad. Stab. 2011, 96, 1149–1158.
19 B. P. Karsten, J. C. Bijleveld, L. Viani, J. Cornil, J.
Gierschner, R. A. J. Janssen, J. Mater. Chem. 2009, 19, 5343–
5350.
8 (a) R. Grisorio, G. P. Suranna, P. Mastrorilli, C. F. Nobile,
Adv. Funct. Mater. 2007, 17, 538–548; (b) R. Abbel, M. Wolffs,
R. A. A. Bovee, J. L. J. van Dongen, X. Lou, O. Henze, W. J.
Feast, E. W. Meijer, A. P. H. J. Schenning, Adv. Mater. 2009,
21, 597–602.
20 (a) A. Rivaton, S. Chambon, M. Manceau, J.-L. Gardette,
^
N. Lemaıtre, S. Guillerez, Polym. Degrad. Stab. 2010, 95,
278–284; (b) M. Manceau, J. Gaume, A. Rivaton, J.-L.
Gardette, G. Monier, L. Bideux, Thin Solid Films 2010, 518,
7113–7118.
9 (a) E. J. W. List, R. Guentner, P. Scanducci de Freitas, U.
Scherf, Adv. Mater. 2002, 14, 374–378; (b) M. R. Craig, M. M. de
Kok, J. W. Hofstraat, A. P. H. J. Schenning, E. W. Meijer, J.
Mater. Chem. 2003, 13, 2861–2862.
21 (a) P. Coppo, D. C. Cupertino, S. G. Yeates, M. L. Turner,
Macromolecules 2003, 36, 2705–2711; (b) P. Coppo, M. L.
Turner, J. Mater. Chem. 2005, 15, 1123–1133.
10 S. Y. Cho, A. C. Grimsdale, D. J. Jones, S. E. Watkins, A. B.
Holmes, J. Am. Chem. Soc. 2007, 129, 11910–11911.
10
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22 The prime (’) denotes that the monomer is prepared via the
26 C. Wilhelm, J.-L. Gardette, J. Appl. Polym. Sci. 1994, 51,
more recent three-step protocol (as reported in ref. 17a).
1411–1420.
23 G. Zotti, G. Schiavon, A. Berlin, G. Fontana, G. Pagani, Mac-
romolecules 1994, 27, 1938–1942.
27 Although one can argue that the results are not directly
transferable to device use conditions, as much higher tempera-
tures were used for the accelerated degradation tests, they are
still of relevance to the community as they certainly contribute
to enhanced fundamental understanding of OPV (material)
stability.
24 A. N. Cammidge, K. V. L. Crepy, Chem. Commun. 2000,
1723–1724.
25 N. D. Treat, C. G. Shuttle, M. F. Toney, C. J. Hawker, M. L.
Chabinyc, J. Mater. Chem. 2011, 21, 15224–15231.
WWW.MATERIALSVIEWS.COM
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