A deeper insight into the dithiocarbamate precursor route: Synthesis of soluble poly(thienylene vinylene) derivatives for photovoltaic applications

Article abstract of DOI:10.1021/ma102101e

The synthesis of, two new poly(thienylene vinylene) derivatives is described, i.e. poly(3-octyl-2,5-thienylene vinylene) (O-PTV) and poly(bis[octylphenyl-2,5-thienylene vinylene]) (BOP-PTV). Both polymers have been prepared via the dithiocarbamate (DTC) precursor route. The polymerization protocol of the monomer toward the precursor polymer has been optimized by the use of different bases, leading to improved reproducibility of the polymerization step. Processability has been guaranteed by the introduction of alkyl side chains. Finally the precursor polymers were converted toward conjugated polymers and they were fully characterized by UV/vis, IR, GPC, and cyclic voltammetry. Bulk heterojunction solar cells with PCBM as acceptor showed promising power conversion efficiencies of 0.80% for BOP-PTV and 0.92% for O-PTV.

Full text of DOI:10.1021/ma102101e

Macromolecules 2010, 43, 10231–10240 10231
DOI: 10.1021/ma102101e
A deeper Insight into the Dithiocarbamate Precursor Route:
Synthesis of Soluble Poly(thienylene vinylene) Derivatives
for Photovoltaic Applications
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Hanne Dilien, Arne Palmaerts, Martijn Lenes, Bert de Boer, Paul Blom, Thomas J. Cleij,
Laurence Lutsen,‡ and Dirk Vanderzande*,†,‡
†Hasselt University, Institute for Materials Research (IMO), Universitaire Campus, Building D,
B-3590 Diepenbeek, Belgium, ‡IMEC, Division IMOMEC, Universitaire Campus, Wetenschapspark 1,
B-3590 Diepenbeek, Belgium, and §Molecular Electronics, University of Groningen, Nijenborgh 4,
NL-9747 AG Groningen, The Netherlands
Received September 10, 2010; Revised Manuscript Received November 4, 2010
ABSTRACT: The synthesis of, two new poly(thienylene vinylene) derivatives is described, i.e. poly(3-octyl-
2,5-thienylene vinylene) (O-PTV) and poly(bis[octylphenyl-2,5-thienylene vinylene]) (BOP-PTV). Both poly-
mers have been prepared via the dithiocarbamate (DTC) precursor route. The polymerization protocol of the
monomer toward the precursor polymer has been optimized by the use of different bases, leading to improved
reproducibility of the polymerization step. Processability has been guaranteed by the introduction of alkyl
side chains. Finally the precursor polymers were converted toward conjugated polymers and they were fully
characterized by UV/vis, IR, GPC, and cyclic voltammetry. Bulk heterojunction solar cells with PCBM as
acceptor showed promising power conversion efficiencies of 0.80% for BOP-PTV and 0.92% for O-PTV.
Introduction
into their corresponding conjugated polymers. They contain
typically a functional group in the main chain that is a precursor
There is a growing need for inexpensive renewable energy
sources, polymer solar cells can come up to this demand. During
the last decades the development of such photovoltaic cells has
progressed rapidly.1 An important limitation of the currently
applied materials is the mismatch between the absorption spec-
trum of the polymers and the solar emission spectrum AM1.5.
The use of low band gap polymers (Eg < 1.8 eV) is a viable
method to expand the absorption window of solar cells and
therefore increase their efficiency.2-5 From models it can be
estimated that for a classical bulk heterojunction solar cell with a
conjugated polymer as a donor in combination with PCBM as an
acceptor, a band gap of 1.4 eV for the donor material would be
optimal.6,7 Since π-conjugated polymers allow endless manipula-
tion of their chemical structure, tuning of the band gap of these
materials is a research topic of ongoing interest. It is anticipated
that this band gap engineering can give the polymer its desired
electrical and optical properties. Earlier, it has already been
proven that PTV’s are interesting conjugated polymers because
of their high mobility for charge carriers and low band gap of
about 1.7 eV.8-10
The synthesis of PTV derivatives can be achieved by many
pathways, all with their specific advantages and disadvantages.
A first well studied method is the synthesis by condensation
reactions, such as Horner-Wadsworth-Emmons coupling.11
Using this route, only low molecular weights are obtained, which
have a negative influence on the morphology of thin films made
from these materials. A general method to synthesize poly-
(arylene vinylene)s and more specifically PTV derivatives, is
based on precursor routes. In a precursor route, soluble precursor
polymers are first synthesized, which in a next step are converted
for a double bond.12 Actually the Durham route represents the
very first example of such precursor route.13 Recently a quite new
type of precursor route was introduced in which rather the
solubilizingside chains on the conjugated backbone are thermally
cleaved. This converts the conjugated polymer into an insoluble
material and may lead to a substantial stabilization of the
nanomorphology of the active layer in bulk heterojunction solar
cells.14,15 A similar advantage can be achieved in principle for the
many precursor routes that have been developed over the last
decades toward for example PTV derivatives. These routes differ
by their functional groups, their polymerization conditions and,
more important, their specific benefits. However it is quite a
challenge to tune conversion conditions such that in this way a
controlled nanomorphology is produced. Consequently, in gen-
eral and also in this work, preference is given to derivatives that
yield a conjugated polymer which maintain solubilizing side
chains to guarantee easy processing also after conversion.
The first precursor route toward PTV was based on the
Wessling route and has been patented by Harper and West,16
also Elsenbaumer et al. explored this route.17,18 Unfortunately
unstable precursor polymers, which spontaneous eliminate to-
ward insoluble PTV, were obtained. Later on, the Wessling route
has been modified by both Saito19-21 and co-workers and Murase.22
Their merit consists of an adaptation of the leaving group in the
precursor polymer. The sulfonium group has been replaced by a
methoxy leaving group. Because of this modification a precursor
polymer which is soluble in organic solvents has been created.
A second studied pathway is the sulphinyl precursor route.23 This
pathway has been investigated in our research group, but the
monomer synthesis seemed to be very problematic as a result of
the instability of the intermediary products toward the monomer.
In the search toward stable monomers also the xanthate route has
been studied, as well in our research group24 as by Burn et al.25
*To whom correspondence should be addressed at the Hasselt
University. Telephone þ32 (0) 11 26 83 21. Fax: þ 32 (0)11 26 83 01.
E-mail: dirk.vanderzande@uhasselt.be.
r 2010 American Chemical Society
Published on Web 11/19/2010
pubs.acs.org/Macromolecules

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Dilien et al.
10232 Macromolecules, Vol. 43, No. 24, 2010
Notwithstanding the fact that stable monomers can be formed
and moreover that these monomers can be polymerized and
converted toward PTV-derivatives, this route had still some
drawbacks. First, the polymerizations have a rather low yield
and second the obtained polydispersities are high (PD > 15). At
last the, in this study discussed, dithiocarbamate route can be
used.26 This procedure has been chosen as the precursor route of
preference to make PTVs, since it is the only route which enables
to prepare stable monomers from the electron rich thiophene
structure. The polymerization of the monomers starts with a base
induced elimination leading to the formation of a p-quinodi-
methane system which is the actual monomer. For this elimina-
tion, a relative strong and sterically hindered base has to be used.
In the past lithium diisopropylamide (LDA) and lithium bis-
(trimethylsilyl)amide (LHMDS) have been used for this pur-
pose.27 The use of LDA suffers from a lot of side reactions
(cleavage of the alkyl side chain, transfer of the isopropyl group)
and the polymerization of 3-alkyl substituted PTVs afford low
molecular weight polymers (Mw <14.000) in low yields (5%).28
To avoid these side reactions, LHMDS has been used as a base, to
achieve greater selectivity. Previous results showed that poly-
merization using LHMDS, leads to high molecular weights but
with a limited reproducibility.29
To gain more insight in the polymerizations of these monomers
a number of different bases have now been evaluated. Besides
LHMDS also the sodium and potassium equivalent of LHMDS
(NaHMDS and KHMDS), have been analyzed. The, less steri-
cally hindered, NatBuO has been used as well. The change of base
leads to more reproducible results while the precursor polymer
still reaches high molecular weights.
After optimization, two new conjugated polymers have been
synthesized; poly(3-octyl-2,5-thienylene vinylene) (O-PTV) and
poly[3,4-bis(octylphenyl)-2,5-thienylene vinylene] (BOP-PTV).
Octyl side chains have been chosen because in the past, it was
observed that butyl and hexyl side chains were insufficient to
guarantee solubility. The polymers were characterized by means
of UV-vis, IR, and cyclic voltammetry, and they were tested on
their photovoltaic properties in bulk heterojunction solar cells
with PCBM as acceptor.
Autolab PGSTAT 20 potentiostat/galvanostat using a conven-
tional three-electrode cell under N2 atmosphere (electrolyte:
0.1 M (TBA)PF6 in anhydrous CH3CN). For the measurements,
an Ag/AgNO3 reference electrode (0.01 M AgNO3 and 0.1 M
(TBA)PF6 in CH3CN), a platinum counter electrode and an
indium-tin oxide (ITO) coated glass substrate working elec-
trode were used. For the measurements, the polymers were de-
posited by spin coating directly onto the ITO substrates. Cyclic
voltammograms were recorded at 50 mV/s. All potentials were
referenced using a known standard, ferrocene/ferrocenium,
which in CH3CN solution is estimated to have an oxidation
potential of -4.98 eV vs vacuum.
Synthesis of O-PTV. Monomer Synthesis. 3-Octyl-2,5-dibro-
mothiophene (1c). To a solution of 3-octylthiophene (1b) (65 g,
333 mmol) in DMF (200 mL) a solution of N-bromosuccinimide
(130 g, 733 mmol) in DMF (200 mL) was added slowly at 0 °C
via a dropping funnel. The mixture was stirred for 72 h in
darkness. The reaction mixture was poured onto a solution of
NaOH (300 mL, 2.5 M) and stirred at 0 °C. The mixture was
extracted with diethyl ether and the combined organic layers
were washed with NaOH (2.5 M) and brine. The organic layers
were dried over MgSO4, the solvent was evaporated and product
1c was obtained as yellow oil (yield 86%, 101 g).
1H NMR (CDCl3): 6.77 (s, 1H), 2.50 (t, J = 7.78, 2H),
1.56-1.46 (m, 2H), 1.28-1.23 (m, 10H), 0.86 (t, J=1.23, 3H).
13C NMR (CDCl3): 143.0, 130.9, 110.3, 107.9, 31.8, 29.6, 29.5,
29.3, 29.2, 29.1, 22.7, 14.1. MS (EI, m/e): 352, 354, 356 (Mþ).
3-Octyl-2,5-thiophenedicarboxaldehyde (1d). In a three-necked
round-bottom flask a solution of 2,5-dibromo-3-octylthiophene
(1c) (35 g, 99 mmol) in THF (100 mL) was stirred under nitrogen
atmosphere at -78 °C. A solution of n-butyl lithium (135 mL,
215 mmol of a 1.6 M solution in hexane) was slowly added with a
cannula, the mixture was stirred for 30 min and afterward 1-for-
mylpiperidine (24.6 g, 217 mmol), freshly distilled, was slowly
added. The resulting mixture was stirred for 12 h at room
temperature. HCl (2M) was added to quench the excess of n-BuLi
followed by extraction with chloroform. The organic layers were
dried over MgSO4, filtered and the solvent was evaporated. The
obtained compound was purified by column chromatography on
silica gel with CHCl3/hexane (1/1) as a solvent. The dialdehyde 1d
was obtained as an orange oil (yield 71%, 18 g).
1H NMR (CDCl3): 10.12 (s, 1H), 9.95 (s, 1H), 7.62 (s, 1H),
2.97 (t, J=7.88, 2H), 1.73-1.63 (m, 2H), 1.31-1.23 (m, 10H),
0.86 (t, J=7.08, 3H). 13C NMR (CDCl3): 183.4, 183.0, 152.0,
147.8, 143.3, 137.1, 31.8, 31.2, 31.2, 29.2, 29.2, 28.5, 22.6, 14.1.
MS (EI, m/e): 252 (Mþ).
Experimental Section
General. All the commercially available chemicals were pur-
chased from Acros or Aldrich and were used without further
purification unless stated otherwise.
2,5-Bis(hydroxymethyl)-3-octylthiophene (1e). In a three-
necked round-bottom flask a mixture of LiAlH4 (3.8 g, 15 mmol)
in dry THF (50 mL) was made under argon atmosphere. This
slurry was cooled to 0 °C and a solution of aldehyde 1d (1.1 g,
0.03 mol) in THF (50 mL) was slowly added with a dropping
funnel. When the addition was complete the slurry was heated at
reflux temperature for 5 h. Sequently, the mixture was placed in
an ice bath and quenched very careful with water and an
aqueous 15% NaOH solution. The solution was extracted with
diethyl ether, dried over MgSO4 and the solvent was evaporated
under reduced pressure. The diol was obtained as a yellow oil in
a yield of 73% (5.6 g).
Tetrahydrofuran (THF) and diethyl ether used in the synth-
esis were dried by distillation from sodium/benzophenone.
Techniques. 1H NMR spectra were taken on a Varian Inova
300 spectrometer. For all synthesized substances spectra were
recorded in deuterated chloroform; the chemical shift at 7.24 ppm
(relatively to TMS) was used as reference. Molecular weights
and molecular weight distributions were determined relative to
polystyrene standards (Polymer Laboratories) by size exclusion
chromatography (SEC). Chromatograms were recorded on a
Spectra series P100 (Spectra Physics) equipped with two
MIXED-B columns (10 μm, 30 cm, Polymer Laboratories)
and a relative index (RI) detector (Shodex). A THF solution
of oxalic acid (1.1ꢀ10-3 M) was used as the eluent at a flow rate
of 1.0 mL/min. Toluene was used as flow rate marker. GC/MS
analyses werecarried out on TSQ-70 and Voyager massspectrom-
eters (Thermoquest); capillary column, Chrompack Cpsil5CB
or Cpsil8CB. Melting points (uncorrected) were measured with
a digital melting point apparatus, Electrothermal IA 9000 series.
Fourier transform-infrared spectroscopy (FT-IR) was perfor-
med on a Perkin-Elmer Spectrum One FT-IR spectrometer
(nominal resolution 4 cm-1, summation of 16 scans). Ultraviolet
visible spectroscopy (UV-vis) was performed on a VARIAN
CARY 500 UV-vis-NIR spectrophotometer. Thin film elec-
trochemical measurements were performed with an Eco Chemie
1H NMR (CDCl3): 6.76 (s, 1H), 4.73 (s, 2H), 4.70 (s, 2H), 2.52
(t, J=8.08, 2H), 1.57-1.47 (m, 2H), 1.28-1.22 (m, 10H), 0.85
(t, J=6.88, 3H). 13C NMR (CDCl3): 142.3, 140.3, 136.8, 127.7,
60.2, 57.8, 31.9, 31.0, 30.9, 29.4, 29.3, 28.3, 22.6, 14.1. MS (EI, m/e):
256 (Mþ).
2,5-Bis(chloromethyl)-3-octylthiophene (1f). To a cooled (0 °C),
stirred solution of diol 1e (4.67 g, 18.2 mmol) in THF (30 mL) was
added a solution of SOCl2 (5.41 g, 45.5 mmol) in THF (40 mL).
The temperature of the reaction mixture was allowed to increase
to room temperature under continuous stirring for 1 h. Then, the
mixture was cooled down again at 0 °C and a saturated sodium
bicarbonate solution was added dropwise until neutral. The mixture

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Macromolecules, Vol. 43, No. 24, 2010 10233
was extracted with diethyl ether, dried over MgSO4 and filtered.
The solvent was evaporated and the highly reactive dichloride 1f
was obtained as an orange oil. Because of the reactivity of 1f, the
dichloride (4.43 g) was used in the next reaction step without
purification.
(t, J=6.93, 3H). 13C NMR (CDCl3): 141.7, 141.5, 133.9, 128.8,
128.1, 123.5, 35.6, 31.9, 31.3, 29.5, 29.35, 29.3, 22.7, 14.1. MS
(EI, m/e): 460 (Mþ).
3,4-Bis(4-octylphenyl)-2,5-bis(chloromethyl)thiophene (2e). In a
three-necked round-bottom flask, a mixture of 2d (3.3 g, 7.1 mmol)
and paraformaldehyde (0.58 g, 19 mmol) was made. The mixture
was cooled to 0 °C under a nitrogen atmosphere, concentrated HCl
(4.0 g, 41 mmol) and acetic anhydride (7.3 g, 71 mmol) were added
dropwise. The resulting mixture was heated at 75 °C for 4.5 h. After
cooling down (0 °C) a cold saturated solution of sodium acetate and
a solution of sodium hydroxide (2M) were added. The solution was
extracted with diethyl ether, dried over MgSO4 and the solvents
were evaporated under vacuo. Because of the instability of the
product, the crude product 2e was used in the next reaction step
without further purification.
1H NMR (CDCl3): 6,81 (s, 1H) 4.69 (s, 2H), 4.68 (s, 2H), 2.52
(t, J=7.78, 2H), 1.61-1.51 (m, 2H), 1.32-1.22 (m, 10H), 0.86 (t,
J=7.14, 3H).
Synthesis of 3-Octythiophene-2,5-diylbismethylene N,N-Di-
ethyldithiocarbamate (1g). To a solution of bischloromethyl 1f
(4.43 g, 15 mmol) in ethanol (50 mL), sodium diethyldithiocar-
bamic acid salt trihydrate (13.61 g, 6 mmol) was added as a solid.
The mixture was stirred at ambient temperature overnight.
Then, water was added and the desired monomer was extracted
with diethyl ether and dried over MgSO4. The monomer was ob-
tained after column chromatography (eluent: CHCl3/hexane 1/1)
as a yellow oil (75%, 5.8 g).
1H NMR (CDCl3): 7.04-6.92 (m, 8H), 4.68 (s, 4H), 2.57
(t, J=7.94, 4H), 1.63-1.49 (m, 4H), 1.31-1.22 (m, 20H), 0.86
(t, J=7.08, 3H).
1H NMR (CDCl3): 6.75 (s, 1H), 4.65 (s, 2H), 4.58 (s, 2H), 4.01
(q, J=7.06, 4H), 3.69 (q, J=7.25, 4H), 2.48 (t, J=7.70, 2H),
1.59-1.47 (m, 2H), 1.29-1.23 (m, 10H), 0.85 (t, J=7.10, 3H).
13C NMR (CDCl3): 194.7, 194.5, 141.4, 136.9, 130.5, 129.0, 49.5,
49.3, 46.7, 36.9, 35.2, 31.9, 30.7, 29.5, 29.2, 28.4, 22.7, 14.1, 12.5,
11.6.
Synthesis of 3,4-bis(4-octylphenyl)thiophene-2,5-diylbismethy-
lene-N,N-diethyl dithiocarbamate (2f). Compound 2e (3.96 g,
7.1 mmol) was dissolved in methanol (50 mL), sodium diethyl-
dithiocarbamate trihydrate (4.8 g, 21 mmol) was added as a solid
and the mixture was stirred overnight. Subsequently the mixture
was extracted with diethyl ether, dried over MgSO4 and the
solvent was evaporated under vacuo. The crude product was
purified by column chromatography with a mixture of chloro-
form/pentane (1/1) as an eluent. The dithiocarbamate monomer
2f was obtained as orange oil (yield: 54%).
Precursor Polymer Synthesis
Polymerization. The monomer 1g (2.8 g, 5.4 mmol) was
freeze-dried. A solution, with a monomer concentration of
0.4 M, in dry THF (13.5 mL) was degassed by passing through
a continuous nitrogen flow. The solution was cooled to 0 °C.
Sodium bis(trimethylsilyl)amide (NaHMDS) (11 mL of a 1 M
solution in THF) was added in one go to the stirred monomer
solution. The resulting mixture was stirred for 90 min under
continuous nitrogen flow at 0 °C. The polymer was precipi-
tated in ice water and the water layer was neutralized with
diluted HCl before extraction with chloroform. The solvents
of the combined organic layers were evaporated under re-
duced pressure and a second precipitation was performed in
cold methanol. The polymer 1h was collected and dried in vacuo
(yield 56%, 1.1 g).
1H NMR (CDCl3): 7.01-6.87 (m, 8H), 4.62 (s, 4H), 4.06-
3.95 (m, 4H), 3.77-3.66 (m, 4H), 2.50 (t, J=7.58, 4H), 1.60-
1.51 (m, 4H), 1.32-1.20 (m, 32H), 0.86 (t, J=7.16, 3H). 13C NMR
(CDCl3): 194.6, 141.5, 141.4, 133.0, 132.7, 130.0, 128.0, 49.4,
46.8, 36.2, 35.6, 31.9, 31.2, 29.5, 22.7, 14.1, 12.5.
Precursor Polymer Synthesis
Polymerization. The monomer 2f (0.37 g, 0.48 mmol) was
freeze-dried. A solution, with a monomer concentration of
0.2 M, in dry THF (2.4 mL) was degassed by passing through
a continuous nitrogen flow. The solution was cooled to 0 °C.
Sodium bis(trimethylsilyl)amide (NaHMDS) (0.6 mL of a 1 M
solution in THF) was added in one go to the stirred monomer
solution. The resulting mixture was stirred for 90 min under
continuous nitrogen flow at 0 °C. The polymer was precipitated
in ice water and the water layer was neutralized with diluted HCl
before extraction with chloroform. The solvent of the combined
organic layers was evaporated under reduced pressure and a
second precipitation was performed in pure cold methanol. The
polymer 2g was collected and dried in vacuo (yield: 54%, 0.16 g).
UV-vis: λmax=242 nm (in film). IR: 2921; 2862; 1485; 1415;
1266; 1207. SEC: Mw=186ꢀ103. PD=2.7. 1H NMR (CDCl3):
7.00-6.68 (br, 8H), 4.07-3.93 (br, 4H), 3.91-3.38 (br, 4H), 2.44
(br, 4H), 1.61-1.40 (br, 24H), 1.32-1.15 (br, 12H), 0.91-0.79
(br, 6H).
UV-vis: λmax = 261 nm (in film). FT-IR(NaCl disk): 2931,
2846, 1486, 1415, 1268, 1206 cm-1. SEC: Mw=86ꢀ103. PD=
1
3.1. H NMR (CDCl3): 6.57 (br, 1H), 5.46 (br, 1H), 3.96 (br,
2H), 3.66 (br, 4H), 2.27 (br, 2H), 1.21 (br, 18H), 0.85 (br, 3H).
13C NMR (CDCl3): 193.96, 139.5, 138.8, 133.8, 127.6, 52.7, 48.9,
46.6, 31.8, 30.7, 29.5, 29.3, 28.2, 22.6, 14.0, 12.5, 11.5.
Conjugated Polymer
Thermal Conversion toward O-PTV. The precursor polymer
1h (1 g, 2.7 mmol) was dissolved in o-dichlorobenzene (50 mL)
and refluxed for 4.5 h. Afterward the solution was cooled to
room temperature and the obtained slurry was precipitated in
methanol. The precipitate was filtered off, washed several times
with methanol and dried in vacuo. A purple/black solid has been
obtained (yield 90%, 0.54 g).
Conjugated Polymer
UV-vis: λmax=620 nm (in film). λmax=582 nm (in CHCl3).
FT-IR: 2931, 2846, 1460, 1250, 1016, 926 cm-1. SEC: Mw=65ꢀ
103; PD=2.6. 1H NMR (CDCl3): 6.97 (s, 1H), 5.00 (s, 1H), 2.26
(br, 2H), 1.24 (br, 12H), 0.85 (br, 3H).
Thermal Conversion toward BOP-PTV. The precursor poly-
mer 2g (0.16 g, 0.25 mmol) was dissolved in o-dichlorobenzene
(6.2 mL) and stirred for 4,5 h at 150 °C. After being cooled to
room temperature, the resulting dark solution was precipitated
dropwise in methanol. The conjugated 2f polymer was filtered
off and dried under reduced pressure. A dark blue solid was
obtained in a yield of 60% (87 mg).
UV-vis: λmax=596 nm (in CHCl3 solution); λmax=601 nm,
shoulder at 646 nm (in film). FT-IR: 2930; 2850; 1269; 1095;
1030; 934; 802. SEC: Mw = 53 ꢀ 103. PD = 2.1. 1H NMR
(CDCl3): 6.99-6.80 (br, 8H), 2.54 (br, 4H), 1.61-1.48 (br, 4H),
1.31-1.18 (br, 20 H), 0.89-0.82 (br, 6H).
Device Preparation. All devices were made on prepatterned
ITO/Glass samples supplied by Philips. After a standardized
cleaning procedure in a wet station, a layer of PEDOT:PSS
(poly(3,4-ethylenedioxythiophene) doped with poly(styrenesul-
fonate) was spin coated on top of the ITO. This layer has a
Synthesis of BOP-PTV. Monomer Synthesis. Synthesis of
3,4-Bis(4-octylphenyl)thiophene (2d). A solution of p-octylphe-
nyl boronic acid (2b) (4.45 g, 19 mmol), dibromothiophene (2c)
(1 g, 4.1 mmol), and potassium fluoride (0.96 g, 17 mmol) was
made in a mixture of water and toluene (1/1). Pd(PPh3)4 (0.33 g,
0.029 mmol) was added as a catalyst. The resulting mixture was
refluxed for 18 h. Then an extraction was performed with
dichloromethane, the organic layers were dried over MgSO4
and the solvent was evaporated in vacuo. The desired product 2d
was obtained as colorless oil after purification by column
chromatography over silica with pentane as an eluent (84%,
7.4 g).
1H NMR (CDCl3): 7.27 (s, 2H), 7.11-7.02 (m, 8H), 2.58
(t, J=7.87, 4H), 1.64-1.54 (m, 4H), 1.34-1.20 (m, 20H), 0.89

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Dilien et al.
10234 Macromolecules, Vol. 43, No. 24, 2010
thickness of 50 to 60 nm. The aqueous PEDOT:PSS solution
(CLEVIOS P VP AI 4083 by H. C. Starck) was spin coated in
exactly the same way for each substrate (10 s at 500 rpm followed
by 50 s at 1500 rpm).
On top of the PEDOT:PSS, a layer of either BOP-PTV or
O-PTV was spin coated inside the glovebox. Table 1 shows the
spin procedures and concentrations for both polymers. Finally,
as a cathode, a metal top contact was evaporated, consisting of
5 nm samarium with 100 nm aluminum on top.
Device Measurements. All measurements were performed in a
glovebox, under nitrogen atmosphere. Current versus voltage
characteristics were determined using a Keithly 2400 Source
meter. The device current was registered using a Labview pro-
gram, while sweeping a voltage across the device, going up from
0 V to a positive voltage, then down to -2 V and up again to 0 V.
The thicknesses of all spin coated polymers, including the
PEDOT:PSS were determined using a Dektak 6 M Stylus
Profiler.
dithiocarbamate monomer 1g.6 Monomer purification was
performed by column chromatography (silica).
For the BOP-PTV monomer a slightly different pathway
has been used (Scheme 2). The monomer has been formed in
a 4 step synthesis. The first step is the synthesis of boronic
acid35 2b which is used for the formation of 3,4-di(octyl-
phenyl)thiophene (2e) via a Suzuki coupling with dibro-
mothiophene 2d. Bis(methylene chloride) 2f has been synthe-
sized by a direct chloromethylation reaction. The last reaction
step and purification toward monomer 2g are identical to the
first procedure.
Polymerization toward the Precursor Polymer. Both mono-
mers were freeze-dried to remove any traces of water. The
polymerizations have been initiated by different bases. All
the other conditions are the same for every reaction; 2 equiv
of base were used, the initial concentration of the monomer
was fixed at 0.4 M and all polymerizations were performed
at a definite reaction temperature (0 °C) and reaction time of
90 min.
Results and Discussion
To study the influence of the base on the formation of the
precursor polymer, 4 different bases have been used: LHMDS,
NaHMDS, KHMDS and NatBuO.
Monomer Synthesis. In view of the advantages compared
to other routes (e.g., Wessling, xanthate, ...), the dithiocar-
bamate (DTC) route has been chosen as the precursor route
of preference to obtain the PTV derivatives. The monomers
used were synthesized by two different synthetic pathways in
accordance with their substitution profile (mono- or disub-
stituted backbone).6,30 The mono substituted PTV (O-PTV)
has been synthesized by a six-step synthetic procedure
(Scheme 1). The synthetic scheme starts with substitution
of commercially available 3-bromothiophene (1a) by a
Kumada coupling.31 Second, bromination has been perfor-
med.32 Dialdehyde 1d was formed by a reaction with n-BuLi
and n-formylpiperidine.33 Reduction with lithium aluminum
hydride, followed by chlorination of the diol with thionyl
choride afforded intermediate 1f.34 Because of the instabil-
ity of thiophene 1f, it has to be converted in situ to the
The first base which has been tested is LHMDS. For this
base, it has already been proven that the formation of high
molecular weight materials is possible,29 but the reproduc-
ibility of the polymerization showed to be an important pro-
blem. To test the reproducibility, over 20 experiments have
been done. All the polymerizations were carried out under
exactly the same conditions as described above. All the poly-
mers reached a high molecular weight (Table 2), a moderated
yield of 38% (average of all the experiments), but the achieved
weight distribution differed significantly. Basically there
are 3 different situations (Figure 1): a monomodal distribu-
tion (1a) (Figure 1), an intermediate situation which can be
described as tailing (1b) and a bimodal distribution (1c). The
monomodal distribution only occurred 2 times. Seemingly
the polymerization process has the tendency to yield a
bimodal distribution or tailing. Both situations appear in
approximately the same frequency.
The different molecular weight distributions can be ex-
plained by the polymerization mechanism (Scheme 3). Poly-
merization starts with a base induced elimination from the
(pre)monomer; the base abstracts a proton and subsequently
one of the dithiocarbamate groups is expelled. This leads to
the actual monomer, a p-quinodimethane system. The actual
polymerization can proceed by either an anionic or radical
mechanism (Scheme 3). In former work it has been shown
Table 1. Applied Spin Coating Procedures and Concentrations for
Both Polymers
spin program
(lid, speed (rpm),
concn
(mg/mL)
acceleration (rpm/s),
time (s))
polymer
solvent
CHCl3
BOP-PTV
15
10
close, 300, 500, 4
close, 500, 500, 60
open, 500, 500, 30
open, 300, 100, 120
open, 800, 200, 30
O-PTV
o-dichlorobenzene
Scheme 1. Synthetic Route toward O-PTVa
a Key: (i) BrMgC8H17, NiCl2(dppp); (ii) NBS, DMF; (iii) (1) n-BuLi, (2) n-formylpiperidine; (iv) LiAlH4, THF; (v) SOCl2, THF; (vi) NaSC-
(S)NEt2 3H2O, EtOH; (vii) base, THF; (viii) ΔΤ.
3

Article
Macromolecules, Vol. 43, No. 24, 2010 10235
Scheme 2. Synthetic Pathway toward BOP-PTVa
a Key: (i) 1. n-BuLi, 2. triisopropylborate; (ii) Pd(PPh3)4, KF, toluene/H2O; (iii) CH2O, HCl, Ac2O; (iv) NaSC(S)NEt2 3H2O, MeOH; (v) NaHMDS,
3
THF; (vi) ΔΤ.
the molecular weight dropped extremely (Mw, 5.5 ꢀ 103;
Table 2. Polymerization Results for the Precursor Polymer after
Polymerization with LHMDS
PD, 3.1).
The second base which has been used is the sodium
equivalent of LHMDS, NaHMDS (Table 3 and Figure 3).
Its base strength is conditioned by the basicity or the pKa of
the parent amine, hexamethyldisilazide. Even so, the results
for these polymerizations differ significantly. Also high
molecular weights have been reached. But now, all polymer-
izations have more or less the same bimodal distribution.
Another advantage of the change of base is the fact that the
yield rises until 52% (average of the four experiments).
To complete the sequence lithium-sodium-potassium,
also the effect of KHMDS as a base has been tested. Here a
monomodal distribution with lower molecular weights (Mw,
14ꢀ103, PD, 3.2) has been achieved, but the yield rises until
64% (Figure 4).
entry
Mw (103)
PD
yield (%)
1
2
3
4
5
6
7
8
9
10
69
241
64
6.5
3.2
2.3
6.4
4.7
3
5.5
2.5
5.1
2
34
43
38
36
39
44
42
35
43
30
66
115
156
56
96
53
83
that the radical polymerization mechanism leads to the high
molecular weight fraction and the anionic mechanism results
in low molecular weight fractions.36
In case of the radical mechanism, the initiator is formed
when two p-quinodimethane systems combine and a diradi-
cal originates. For the anionic mechanism a nucleophile has
to be present as an initiator, it is possible that the base
interacts as such a nucleophile or that the anionic intermedi-
ate toward the p-quinodimethane system takes up that role.36
To prove that two different polymerization mechanisms
are present, two additional experiments have been done.37 In
a first experiment, 0.5 equiv of 2,2,6,6-tetramethylpiperi-
dine-1-oxyl (TEMPO) has been added to a typical reaction
mixture (Figure 2).38 Here, the molecular weight has been
reduced significantly (Mw, 28 ꢀ 103; PD, 6.4). This is an
indication that the high molecular weights originate from a
radical polymerization mechanism, which in this experiment
is suppressed by the radical inhibitor, TEMPO. To verify this
observation, a reversed addition, meaning that the monomer
is added dropwise to a solution of the base, has been done
(Figure 4). Such an addition should favor the anionic
polymerization.39,40 The experiment confirms this hypothesis,
When we compare the three sterically hindered hexam-
ethyldisilazane bases, the yield goes up through the sequence
Li-Na-K. A second phenomenon that can be recognized
concerns the molecular weight distribution. The larger the
counterion (Li < Na < K), the more low molecular weight
fractions are formed, so the anionic polymerization mecha-
nism is in favor. A possible explanation can relate the results
with differences in solvatation of the cation. The polymer-
izations are performed in THF, which is a solvent with a low
dielectric constant (ε = 7.58)41 and, therefore, a weakly
dissociating solvent. In such a solvent the nucleophilicity of
anions depends on the electrostatic attraction of the cat-
ion-anion pair and therefore the charge density on the
cation. The larger the cation, the lower the charge density
and the more the cation-anion pair will be separated. So the
probability of any nucleophile to act as an anionic initiator
increases.
Finally, sodium tert-butoxide has also been tested as a
base. Its base strength is lower than LHMDS. This also leads

€
Dilien et al.
10236 Macromolecules, Vol. 43, No. 24, 2010
Figure 1. Molecular weight distribution of O-PTV (SEC profiles).
Scheme 3. Mechanism of Dithiocarbamate Monomer Polymerization via p-Quinodimethane System
Figure 2. Influence of TEMPO and reversed addition on the molecular weight distribution.
to the formation of lower molecular weights (Mw=2ꢀ103)
(Figure 5) under normal reaction conditions (0 °C). The high
molecular weight peak is present in a very small amount.
From previous experience42 it is indeed to be expected that
due to the lower base strength, proton abstraction and for-
mation of the p-quinodimethane system is much slower. In
case of a competition between a radical and an anionic mech-
anism the competition occurs at the level of initiation
kinetics.39 As the rate of initiation for the radical mechanism
scale with the square and for the anionic mechanism in first
order in the concentration of the p-quinodimethane system,
initiation kinetics are for the former more sensitive to con-
centration of the true monomer than the latter.
Table 3. Polymerization Results with NaHMDS
high molecular weight low molecular weight
entry
M
w (103)
PD
Mw (103)
PD
yield
1
2
3
4
164
107
218
208
2
2.0
2.4
2.7
2.7
1.2
1.3
1.5
1.4
50
51
54
52
1.9
2.8
2.6
titive radical polymerization mechanism. Accordingly per-
forming the polymerization reaction at 35 and 60 °C results
in an increased contribution of the high molecular weight
fraction (Figure 5). The molecular weights observed at 202ꢀ
103 and 164ꢀ103 Da, respectively, stay virtually constant.
The precursor polymers which are used for further char-
acterization have been made in the same conditions. Both
monomers (1g and 2f) were polymerized using sodium
Consequently, this would imply that higher temperatures
leading to a more efficient formation of the true monomer
and keeping all other polymerization parameters (time,
equivalents base, ...) constant, would lead to a more compe-

Article
Macromolecules, Vol. 43, No. 24, 2010 10237
hexamethyldisilazide (2 M solution in THF) as a base in dry
THF at 0 °C for 90 min to afford the polymers (1g and 2f).
Workup and purification have been done as described above.
The isolated yields range from 56% for the octyl-precursor
polymer to 54% for the BOP-precursor polymer. Molecular
weights were determined by means of SEC in THF against
polystyrene standards (Table 4).
Conversion to the Conjugated Polymers. The precursor
polymers can be converted into their conjugated counter-
parts by a thermal treatment (Scheme 1 and 2). Upon heat-
ing, the dithiocarbamate group of the precursor polymer is
eliminated to form the conjugated backbone. The thermal
conversion has been performed in solution. The precursor
polymers are dissolved in o-dichlorobenzene and stirred for
4.5 h at 180 °C to form O-PTV and at 150 °C to form BOP-
PTV. Polymers are isolated by precipitation of the resulting
reaction mixture in MeOH. After isolation the conjugated
polymers were characterized by UV/vis (Figure 6) and SEC
(Table 4). Furthermore, the polymers were studied by FT-IR
spectroscopy, to be certain that all dithiocarbamate groups
disappeared and therefore full conjugation has been realized.
Transport Properties. Hole only devices have been made
resulting in a hole mobility of 3ꢀ10-9 m2/(V s) for BOP-PTV
and 5ꢀ10-9 m2/(V s) for O-PTV, which is slightly lower as
compared to the hole mobility of 3ꢀ10-8 m2/(V s) for P3HT,
but nevertheless acceptable when compared to other efficient
low band gap donors (Figure 7).8,43-45 Combined with the
good transport properties of PCBM no space charge effects
are expected, at least for moderately thick films, similar to
those used in MDMO-PPV:PCBM blends.
Figure 3. SEC profile (entry 4) for polymerization with NaHMDS.
Figure 4. SEC profile for polymerization with KHMDS.
Cyclic Voltammetry. The cyclic voltammograms of thin
films of BOP-PTV and O-PTV display distinct characteristic
irreversible oxidation and reduction processes during the
Figure 5. SEC profiles for polymerization at 0, 35, and 60 °C.
Table 4. SEC Data for the Precursor and the Conjugated Polymers
Mw
PD
O-PTV precursor polymer
O-PTV
BOP precursor polymer
BOP PTV
86 ꢀ 103
65 ꢀ 103
186 ꢀ 103
53 ꢀ 102
3.1
2.6
2.7
2.1
Figure 7. Current density versus voltage, corrected for built in voltage
and series resistance of a BOP-PTV and O-PTV hole only device. Data
(symbols) are fitted (line) using a space charge limited current with a
field dependent mobility.
Figure 6. UV-vis spectra of the conjugated polymers (film and solution).

€
Dilien et al.
10238 Macromolecules, Vol. 43, No. 24, 2010
first potential scan. Upon repeated scanning the currents
associated with these features significantly reduces, as ex-
pected for irreversible electrochemical processes. From the
onset potentials of the oxidation and reduction the position
of the energy levels can be estimated. For BOP-PTV the
onset of oxidation occurs at 0.46 V vs Ag/AgNO3 and the
onset of reduction occurs at -1.18 V vs Ag/AgNO3. The cor-
responding energy level of BOP-PTV for the highest occu-
pied molecular orbital (HOMO) is -5.39 eV and for the
lowest unoccupied molecular orbital (LUMO) is -3.75 eV.
For O-PTV the onset of oxidation occurs at 0.22 V vs Ag/
AgNO3 and the onset of reduction occurs at -1.30 V vs Ag/
AgNO3. The corresponding energy level of O-PTV for the
highest occupied molecular orbital (HOMO) is -5.15 eV and
for the lowest unoccupied molecular orbital (LUMO) is
-3.63 eV. The resulting electrochemical band gap of BOP-
PTV is 1.64 eV and of O-PTV is 1.52 eV. Although in the
same order of magnitude, these values are somewhat lower
than the observed optical band gaps (1.78 and 1.70 eV,
respectively). This is not surprising, since the determination
of the onsets of oxidation and reduction for irreversible elec-
trochemical processes may incur some level of error. Not-
withstanding, the cyclic voltammetry measurements clearly
confirm the occurrence of a lower band gap for O-PTV as
compared to BOP-PTV, likely a result of increased confor-
mational disorder due to steric effects of the substituents. In
addition, for BOP-PTV both the HOMO and the LUMO are
lowered as compared to O-PTV, which reflects the electronic
effects of the phenyl substituents (Table 5).
Solar Cells. Solar cells were made by sandwiching the
conjugated polymers in between an ITO/PEDOT:PSS anode
and a samarium/aluminum cathode. The best obtained per-
formance for bulk heterojunction solar cells with BOP-PTV
or O-PTV as donor polymer and PCBM as acceptor material
are depicted in Figure 8 and Table 6. For a 1:4 BOP-PTV:
PCBM solar cell spin coated from chloroform the highest
achieved efficiency is 0.80%. This cell has an open circuit
voltage (Voc) of 0.67 V and a fill factor of 60% and higher for
films with a thickness of more than 200 nm, indicating good
charge transport properties of the blend. For a 1:4 O-PTV:
PCBM solar cell spin coated from o-dichlorobenzene the
highest achieved efficiency is 0.92%. The open circuit voltage
(Voc) of O-PTV is 0.47 V, this is 0.2 V less compared to BOP-PTV.
This difference in Voc is most likely originated from the
differences in the HOMO levels of both polymers. Appar-
ently, the introduction of phenyl substituents results in a
lowering of the HOMO energy level, which translates itself in
a larger Voc. The difference in short circuit currents can be
explained by the differences in absorption of both films,
which will be discussed in the next section. Notwithstanding,
the short circuit currents (Jsc) are low, which results in the ob-
served efficiencies below 1.0%. Even so the performance of
the solar cells made by the optimized polymerization with
NaHMDS are significant better than the one made by
LHMDS. Girotto et all measured an efficiency of 0.15%
-0.17% for other 3-alkyl-substituted PTV’s.46 So by opti-
mizing the synthetic route toward the PTV-derivatives, the
efficiency of the solar cell has been increased by a factor
of 4.5.
Absorption Characteristics. To understand the moderate
efficiencies for these new low band gap polymers, the absorp-
tion characteristics, in solution as well as in thin films, of
both PTV derivatives have been studied and compared to
P3HT. To this end, thin films of P3HT, BOP-PTV, and
O-PTV were prepared from the same solvent, i.e., chloro-
form. After measuring the film thickness, the absolute
absorbance as a function of the wavelength can be readily
determined (Figure 9). Furthermore, the optical band gap
can be derived from the low energy side tangent to the π-π*
transition. The band gap and the absolute absorptivity in a
thin film are presented in Table 7.
The moderate short circuit currents (Jsc) of BOP-PTV and
O-PTV compared to P3HT can be understood in terms of the
absolute absorption maxima. It is noteworthy that the ab-
sorptivity of P3HT and O-PTV in a thin film follow the same
trend relative to the molar attenuation coefficient. On the
other hand where BOP-PTV has the highest molar attenua-
tion coefficient of the three compounds, this does not trans-
late to a high absorptivity. This can be explained in part by
the high structural order and even crystalline character of
P3HT47 and the expected nature or rotational disorder of the
larger side chains in BOP-PTV. This will be a subject for
further research.
Table 6. Solar Cell Performance for BOP-PTV:PCBM and O-PTV:
PCBM
Voc (V) Jsc (A/m2) FF % Efficiency %
Table 5. CV Data for O-PTV and BOP-PTV
1:4 O-PTV:PCBM
1:1 BOP-PTV:PCBM
1:2 BOP-PTV:PCBM
1:4 BOP-PTV:PCBM
0.47
0.66
0.67
0.67
52.8
9.1
15.8
18.3
35
59
57
60
0.92
0.34
0.66
0.80
HOMO LUMO Eg (electrochemical) Eg (optical)
O-PTV
BOP-PTV
-5.15
-5.39
-3.63
-3.75
1.52
1.64
1.70
1.78
Figure 8. Current density versus voltage of PTV:PCBM solar cells under illumination of a 1000 W/m2 simulated solar spectrum.

Article
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and P3HT
thin film absolute
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Acknowledgment. The authors gratefully acknowledge BEL-
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