H.-J. Wang et al. / Polymer 52 (2011) 326e338
327
region is ascribed to the absorption of the thiophene units with the
conjugated side-chains, and that in the visible region corresponds to
the absorption of the PT main chains [11e15]. Therefore, PT
absorption behavior is expected to be accurately modulated by
introducing conjugated moiety as pendants with various densities.
Generally, an open-circuit voltage (Voc) value of the PT-based PSCs is
about0.4e0.6 V [16,17]. Moreover, the Voc ofa BHJcell isproportional
to the difference between the highest occupied molecular orbital
(HOMO) level of the electron-donor polymer and the lowest unoc-
cupied molecular orbital (LUMO) level of the electron-acceptor
fullerene derivatives. The lower HOMO levels of the polymers will
provide a higher Voc value according to the theoretical prediction
[18]. With the introduction of bulky side-chains onto the polymer
backbone, the effective conjugation length can be curtailed via
toluene (40 mL) and 10 mL of degassed aqueous K2CO3 (2 M). The
reaction mixture was vigorously stirred at 90e95 ꢁC for 8 h under
a nitrogen atmosphere. After cooling, the reaction solution was
extracted with dichloromethane and water. The organic layer was
collected and dried over magnesium sulfate. Further purification
was performed using silica gel chromatography (EA/hexanes, 1:10)
to generate a yellow solid (2.13 g, yield ¼ 74%). 1H NMR (d-CDCl3,
300 MHz,
d
/ppm): 9.87 (s, 1H), 7.72 (d, 1H, J ¼ 3.5 Hz), 7.52 (d, 2H,
J ¼ 8 Hz), 7.22 (m, 7H), 6.99 (m, 6H). 13C NMR (d-CDCl3, 150 MHz,
d/
ppm):182.62, 154.59, 149.13, 146.95, 141.28, 137.75, 129.47, 127.23,
126.10, 125.16, 123.86, 122.84, 122.34. MS(m/z):355.2[Mþ]. Anal.
Calcd for C23H17NOS: C, 77.72; H, 4.82; N, 3.94. Found: C, 77.79; H,
4.59; N, 3.40.
twisting the conjugated
p
-system out of planarity and, thus, the
2.1.2. (E)-4-(5-(2-(2,5-dibromothiophen-3-yl)vinyl)thiophen-2-yl)-
N,N-diphenylaniline (6)
HOMO level can be decreased [15,19]. As a result, a high Voc value of
a PSC can be obtained by using a conjugated polymer with bulky
side-chains. On the other hand, the PV performance of polymer/
fullerene derivative-blend film-based PSCs is also heavily influenced
by the interpenetrating nanostructure formed by the two semi-
conductors. The literature has reported that polymer chains with
sufficient free volume are helpful for the intercalation of fullerene
molecules into polymer chains, which is favorable for the
improvement of PV performance [20,21]. Moreover, the packing
density and morphology in polymer thin film are strongly depen-
dent on the spaces between the polymer side-chains [22,23]. Hence,
the appropriate side-chain spacing of conjugated polymer is also an
important parameter to be considered for designing a new electron-
donor polymer for PSC applications.
In this study, we synthesized novel PT derivatives with bulky
conjugated side-chains, which comprised triphenylamine, thio-
phene, and vinylene groups (TPATh). The TPATh group was known
to exhibit a good electron-donating capacity and high hole-
mobility [24,25]. The copolymers regioregular-TPATh-PT (rr-TPATh-
PT) and random-TPATh-PT (r-TPATh-PT) with different copolymer
configurations and side-chain densities were synthesized by
Grignard metathesis (GRIM) and Stille coupling, respectively. A
lower side-chain density of r-TPATh-PT was designed and synthe-
sized as compared to that of rr-TPATh-PT. The influence of config-
uration and side-chain density of these copolymers on the
photophysical and electrochemical properties were investigated in
detail. Moreover, the morphological and PV characteristics of the
copolymer/fullerene derivative-blend films were also discussed.
A mixture of compound 5 (2 g, 5.10 mmol) and CH3ONa (0.75 g,
13.89 mmol) in 10 mL DMF was stirred under an ice water bath for
several minutes. Compound 4 (1.64 g, 4.62 mmol) was then added
to the solution. After 2 h, the reaction was quenched with ice water
and the yellow powder was precipitated. The precipitate was
filtered and washed with water several times. Further purification
was performed using silica gel chromatography (hexane as eluent)
to give a yellow solid (2.22 g, yield ¼ 81%). The product was filtered
and dried under vacuum. The yellow solid product obtained had an
81% yield (2.22 g). 1H NMR (d-CDCl3, 300 MHz,
d/ppm): 7.47 (d, 2H,
J ¼ 8.7 Hz), 7.27 (m, 4H), 7.14 (m, 6H), 7.05 (m, 5H), 6.99 (d, 1H,
J ¼ 15.6 Hz), 6.77 (d, 1H, J ¼ 15.9 Hz) 13C NMR (CDCl3, 150 MHz,
d/
ppm): 147.53, 147.35, 144.00, 140.37, 138.86, 129.33, 128.33, 127.89,
127.16, 126.45, 124.62, 124.48, 123.40, 123.23, 122.69, 118.89, 111.88,
109.57. MS (m/z):593.0[Mþ]. Anal. Calcd for C28H19Br2NS2: C, 56.67;
H, 3.23; N, 2.36. Found: C, 56.51; H, 3.86; N, 2.57.
2.1.3. rr-TPATh-PT
As shown in Scheme 1, rr-TPATh-PT was synthesized via
condensation polymerization using the GRIM method initially
reported by McCullough and coworkers [30,31]. Compound 6
(0.59 g, 1 mmol) and compound 7 (0.33 g, 1 mmol) were dissolved
in 10 mL of dry THF under an N2 atmosphere. Methylmagnesium
bromide (2 mL, 1.0 M solution in THF) was then added to the stirred
mixture. The mixture solution was refluxed for 2 h. Then, Ni(dppp)
Cl2 (10 mg, 1 mol%) was added to the reaction mixture and the
reactions were continued for a further 8 h. The whole mixture was
then poured into methanol (100 mL) and the precipitated material
was filtered into a Soxhlet thimble. Soxhlet extractions were per-
formed with methanol, hexane, acetone and chloroform. The
polymer was recovered from the chloroform fraction by rotary
evaporation. The solid was dried under vacuum for 24 h. After
drying, rr-TPATh-PT was obtained as a red solid with isolated yields
of 36%. Gel permeation chromatography (GPC) (THF): Mw ¼ 13.3 kg/
2. Experimental details
2.1. Chemical materials
The starting material triphenylamine, 5-Bromothiophene-2-
carbaldehyde (3), reagents and chemicals were purchased from
Aldrich, Alfa, TCI Chemical Co. and used as received without any
further purification. All the solvents, such as dichloromethane
(DCM), tetrahydrofuran (THF) and dimethylformamide (DMF), and
toluenewerefreshly distilled overappropriatedrying agents prior to
use and were purged with nitrogen. 4-Bromotriphenylamine (1)
[26], N,N-diphenyl-4-(4,4,5,5-tetramethyl-1,3,2- dioxaborolan-2-
yl)aniline (2) [27], diethyl (2,5-dibromothiophen-3-yl) methyl-
phosphonate (5) [15], 2,5-dibromo-3- hexylthiophene (7) [28] and
2,5-bis(trimethylstannyl)- thiophene (8) [29], as shown in Scheme 1,
were synthesized according to the literature.
mol and PDI ¼ 1.7. 1H NMR (d-DCM, 600 MHz,
d/ppm): 6.9e7.3 (br,
21H, vinylic and aromatic hydrogens), 2.75e2.85 (t, 4H,
chain), 1.25e1.7 (m, 16H, CH2), 0.92 (br, 6H, CH3).
aCH2 alkyl
2.1.4. r-TPATh-PT
The synthesis of r-TPATh-PT via the Stille coupling route is
shown in Scheme 1.Compound 6 (0.30 g, 0.5 mmol), compound 7
(0.17 g, 0.5 mmol), and compound 8 (0.41 g,1 mmol) were dissolved
in 10 mL of dry toluene. The reaction mixture was purged with N2
and subjected to three freeze-pump thaw cycles to remove O2. Pd
(PPh3)4 (11.5 mg, 1 mol %) was added to the mixture solution. Then,
the mixture was stirred and refluxed for 24 h. The whole mixture
was then poured into methanol (100 mL) and the precipitated
material was filtered into a Soxhlet thimble. Soxhlet extractions
were performed with methanol, hexane, acetone and chloroform.
2.1.1. 5-(4-(diphenylamino)phenyl)thiophene-2-carbaldehyde (4)
A mixture of compound 2 (3.0 g, 8.08 mmol), 5-Bromothio-
phene-2-carbaldehyde (3) (0.88 g, 8.08 mmol), and Pd(PPh3)4
(467 mg, 5 mol %) was added in a mixture solution of degassed
Wang, Hsing-Ju
