300
V. Cimrov aꢁ et al. / Polymer 59 (2015) 298e304
(
1
2
(
2H, br s, 2 ꢁ CH), 1.44 (8H, br s, 4 ꢁ CH
.10 (24H, br s, 12 ꢁ CH ), 0.93 (12H, m, 4 ꢁ CH
ꢁ CH ). (75.45 MHz, CDCl , 330 K, Me Si O): 157.23 (2C),152.22
2C), 145.21 (2C), 140.94 (2C), 140.84 (2C), 133.28 (2C), 129.26 (2C),
26.97 (2C), 124.97 (2C), 120.31 (2C), 120.22 (2C), 113.19 (2C), all
aromatic, 55.76 (1C, fluorene central ring), 40.56 (2C), 40.22 (2C),
2
), 1.30 (8H, br s, 4 ꢁ CH
2
),
comonomers), phase transfer agent (Aliquat 336), the ratio be-
tween reactants, solvent, and base solution was kept constant. The
2
3
), 0.87 (6H, m,
3
d
C
3
6
2
1 2 3
copolymers CEHTF8-t , CEHTF8-t and CEHTF8-t were prepared by
ꢂ
Suzuki coupling in: THF at 66 C with reaction times of 24, 48 and
72 h, respectively; 1 mol-% of Pd catalyst per monomers; and with
the same ratio between reactants, solvent and base solution as for
the experiments using the high temperature boiling solvents
mentioned above. All CEHTF8 copolymers were soluble in common
organic solvents such as THF, toluene, xylene, chloroform, CB and
DCB.
1
3
2
1
5.24 (2C), 33.05 (2C), 31.92 (2C), 30.17 (2C), 29.30 (4C), 29.06 (2C),
6.35 (2C), 24.09 (2C), 23.23 (2C), 22.69 (2C), 14.14 (2C), 14.05 (2C),
1.00 (2C), all aliphatic. FTIR (ATR):
n
¼ 2954, 2922, 2852, 1606,
1
540, 1486, 1464, 1420, 1376, 1264, 1146, 1134, 876, 834, 814, 722,
ꢀ
1
5
92 cm
Elemental analysis. Found: C, 73.70; H, 8.30; N, 2.9 (CEHTF8-d
C, 74.43; H, 8.64; N, 3.00 (CEHTF8-d ). C, 74.47; H, 8.61; N, 3.03
CEHTF8-x). Calc. for C57 structural unit: C, 74.62; H, 8.35;
N, 3.05%.
.
Rapid polycondensation with formation of copolymer pro-
ceeded in all high-boiling solvents (CB, DCB and xylene). It could be
easily followed because of colour change from blue (colour of ter-
thiophene comonomer) to green (colour of longer conjugated
structures). After 30 min, the reaction was stopped by pouring
resulting mixture into MeOH and the dark green precipitated ma-
terial was subjected to usual workup. The copolymers were char-
acterized by SEC, NMR, IR and elemental analysis data.
1
).
2
(
76 2 4
H N S
2
.4. Device preparation and measurements
Thin films of copolymers and their blends with [60]PCBM were
Using high-boiling solvents CB, DCB and xylene, copolymers
prepared by spin coating from DCB solutions. Thin films for optical
studies were spin-coated onto fused silica substrates or coated on Pt
wire electrode by dipping for electrochemical measurements. For
polymer photovoltaic (PV) devices, polymer layers were prepared
on indiumetin oxide (ITO) substrates covered with a thin layer of
poly[3,4-(ethylenedioxy)thiophene]/poly(styrenesulfonate) (PED-
with high molar weights M
w
¼ 60 000e222 000, monomodal and
narrow distributions of molar masses PDI ¼ 1.4e1.5 were prepared
in very short time with yields in the range 75e96 %. SEC curves are
displayed in Fig. 1 and the molar masses are given in Table 1. The
molar weights M
w
¼ 222 000 of CEHTF8-c and M ¼ 143 000 of
w
CEHTF8-d are the highest values we succeeded using CB and DCB,
respectively. The longest copolymers, CEHTF8-c and CEHTF8-
d consist of 242 and 155 repeating units, respectively. It must be
noted that molar weights of polymers prepared by Suzuki poly-
condensation are strongly dependent on a molar ratio between
comonomers. An exact mole ratio of 1:1 is needed in order to
achieve high molar masses. It was reported that a mere 0.5 mol-%
excess of one monomer leads to more than ten-fold decrease of an
average degree of polymerization [25]. At the same time, a perfect
equimolar ratio of comonomers is difficult to ensure. Therefore,
Exps. 3 and 4 were performed with stock comonomer solutions to
check the reproducibility of the short time synthesis. Exps. 3 and 4
showed good reproducibility of the copolymer synthesis in DCB as
an example. The same molar weights were obtained as can be
documented by the comparison of SEC curves of the copolymers
ꢀ
3
OT:PSS). All polymer films were dried in vacuum (10 Pa) at 353 K
for 2 h. The ITO glass substrates were purchased from Merck (Ger-
many) and PEDOT:PSS (Baytron PH500) from HC Starck AG (Ger-
many). The 50-nm thick PEDOT:PSS layers were prepared by spin
coating and dried in air at 396 K for 15 min. The calcium (20 nm),
and subsequently aluminium electrodes (60e80 nm) were vacuum-
evaporated on the top of polymer films to form PV devices. Typical
2
active areas of the PV devices were about 12 mm . Layer thickness
(d) was measured using a KLA-Tencor P-10 profilometer.
Ultravioletevisible (UVeVIS) spectra were measured on a
Perkin-Elmer Lambda 35 UVeVIS spectrometer. Cyclic voltamme-
try (CV) was performed with a PA4 polarographic analyser (Labo-
ratory Instruments, Prague, CZ) with a three-electrode cell.
Platinum (Pt) wire electrodes were used as both working and
þ
counter electrodes, and a non-aqueous Ag/Ag electrode (Ag in
1 2
CEHTF8-d and CEHTF8-d (identical curves in Fig. 1).
0
3
.1 M AgNO solution) was used as the reference electrode. CV
measurements were made in a solution of 0.1 M tetrabutylammo-
nium hexafluorophosphate (TBAPF6) in anhydrous acetonitrile in
nitrogen atmosphere. Typical scan rates were 20, 50 and
ꢀ1
1
00 mV s . Currentevoltage characteristics of PV devices were
measured using the Keithley 237 source measure unit. A 300 W
xenon lamp (Oriel) with a 1.5 AM filter Daylight-Q X-7460 (Labimex
CZ) was used for illumination. The light intensity was set using a
calibrated solar reference cell SRC-1000-TC-QZ (VLSI Standards,
Inc.) and checked by radiometer Rs 3960 (Laser Precision Corp.). All
thin film preparations and the device fabrication were conducted in
a glove box under a nitrogen atmosphere.
3
. Results and discussion
3.1. Synthesis
The synthesis of comonomer EHTBr was performed according to
the procedure described in our previous paper [19], and both the
EHTBr and copolymer CEHTF8 syntheses are shown in Scheme 1.
The results of the copolymerization by Suzuki coupling reaction in
chlorinated and non-chlorinated solvents are shown in Table 1.
Copolymers CEHTF8-c, CEHTF8-d and CEHTF8-x were prepared by
Fig. 1. SEC curves of the copolymer CEHTF8 samples prepared in various solvents: in
CB CEHTF8-c (red), in DCB CEHTF8-d (blue) CEHTF8-d (orange), CEHTF8-d (yellow-
1
2
identical with orange), CEHTF8-x (green), in THF CEHTF8-t (black), for various reaction
times: 24 h (dashed), 48 h (dotted) and 72 h (solid), measured with DCB as the mobile
ꢂ
phase at 60 C. Polystyrene standards were used for calibration. (For interpretation of
ꢂ
Suzuki copolymerization (at 131 C for 30 min) in CB, DCB and
the references to colour in this figure legend, the reader is referred to the web version
xylene, respectively. The amount of Pd catalyst (1 mol-% per both
of this article.)