Alternating copolymers of dithienyl-s-tetrazine and cyclopentadithiophene for organic photovoltaic applications

Article abstract of DOI:10.1021/cm200330c

As a new emerging electron deficient building block, s-tetrazine (Tz) shows high electron affinity, which is even higher than the commonly used benzothiadiazole units. This property makes Tz a very promising electron withdrawing unit for low band gap conjugated polymers, especially for organic solar cell materials. We report the synthesis and property of five alternating copolymers of s-tetrazine and cyclopenta[2,1-b:3,4-b′]dithiophene (CPDT), which are bridged with a thiophene unit. Methyl, hexyl, and/or 2-ethylhexyl groups are introduced onto thiophene and CPDT units to tune the solubility, UV absorption, frontier molecular orbital energy levels, and interchain stacking property of the resulting polymers. These polymers are stable up to 220 °C and decompose to dinitrile compounds with the breaking of Tz linkage at a higher temperature. Efficient bulk heterojunction solar cells were fabricated by blending these polymers with (6,6)-phenyl-C₇₁-butyric acid methyl ester (PC₇₁BM), and they reached a power conversion efficiency up to 5.53% under simulated AM 1.5 G irradiation of 100 mW/cm². The morphological structures of the active layers from different polymers or under different processing conditions were then analyzed by atomic force microscopy (AFM) and correlated with their device performance.

Full text of DOI:10.1021/cm200330c

ARTICLE
pubs.acs.org/cm
Alternating Copolymers of Dithienyl-s-Tetrazine and
Cyclopentadithiophene for Organic Photovoltaic Applications
,†
,†
†
†
‡
‡
‡
Zhao Li,* Jianfu Ding,* Naiheng Song, Xiaomei Du, Jiayun Zhou, Jianping Lu, and Ye Tao
†
‡
Institute for Chemical Process and Environmental Technology (ICPET) and Institute for Microstructural Sciences (IMS),
National Research Council of Canada (NRC), 1200 Montreal Road, Ottawa, ON, Canada K1A 0R6
ABSTRACT: As a new emerging electron deficient building block, s-tetra-
zine (Tz) shows high electron affinity, which is even higher than the
commonly used benzothiadiazole units. This property makes Tz a very
promising electron withdrawing unit for low band gap conjugated polymers,
especially for organic solar cell materials. We report the synthesis and
property of five alternating copolymers of s-tetrazine and cyclopenta[2,1-
0
b:3,4-b ]dithiophene (CPDT), which are bridged with a thiophene unit.
Methyl, hexyl, and/or 2-ethylhexyl groups are introduced onto thiophene
and CPDT units to tune the solubility, UV absorption, frontier molecular
orbital energy levels, and interchain stacking property of the resulting
polymers. These polymers are stable up to 220 °C and decompose to
dinitrile compounds with the breaking of Tz linkage at a higher temperature.
Efficient bulk heterojunction solar cells were fabricated by blending these
polymers with (6,6)-phenyl-C -butyric acid methyl ester (PC BM), and
71
71
they reached a power conversion efficiency up to 5.53% under simulated AM
2
1
.5 G irradiation of 100 mW/cm . The morphological structures of the active
layers from different polymers or under different processing conditions were
then analyzed by atomic force microscopy (AFM) and correlated with their device performance.
KEYWORDS: s-tetrazine, low band gap, conjugated polymer, bulk heterojunction, solar cell
9
’
INTRODUCTION
repeating unit. However, this polymer was prepared by electro-
chemical polymerization, so the resulting material is insoluble in
organic solvent. Thus, detailed investigation of its properties and
application is difficult.
As one of the most electron-deficient CꢀN heterocycles,
s-tetrazine (Tz)-containing compounds have drawn much re-
1
search attention recently because of their unique properties.
Recently, we reported the synthesis of the first solution-proces-
First, although the Tz ring has a small size, it renders unusually
high electron affinity in its aromatic compounds, which makes Tz
an ideal building block for electro-optically active materials. For
example, Tz compounds have been used as sensing or fluorescent
10
sable Tz backboned conjugated copolymer, PCPDTTTz. Its main
chain contains alternating Tz and CPDT units bridged by thipohene
(Figure 1). This kind of structure is unique for polymers and
2
especially for conjugated polymers because the conjugation through
Tz is introduced into the main chains, instead of the commonly
used ꢀCdCꢀ bond. The simple and straightforward synthetic
approach opens a door for the preparation of various Tz-containing
conjugated polymers and allows us to carry out detailed studies on
their structure and property relationship. More importantly, efficient
polymer solar cells have been obtained by blending these polymers
with (6,6)-phenyl-C -butyric acid methyl ester (PC BM), de-
materials and are even claimed to be the world’s smallest organic
3
fluorophores. Second, Tz derivatives have a very high nitrogen
content. These nitrogen atoms provide multiple binding sites
and can be employed in coordination chemistry for metal
1
c
complexation, and some other Tz compounds have been
intensively studied as explosive materials, which have remarkably
4
high energy content. Third, the Tz ring shows high reactivity as a
71
71
diene in cycloaddition reaction; thus, Tz has been used as
precursor for some new pyridazines or natural products using
monstrating an important application of these novel electro-opti-
11
cally active materials.
5
the CarboniꢀLindsey reaction. However, up to now, most of
Benzothiadiazole (BT) is one of the most commonly em-
ployed electron withdrawing building blocks because of its very
strong electron-deficient property. High efficiency polymer solar
cells were fabricated by several groups on the basis of its
these studies are focused on small molecular compounds because
of the absence of an efficient synthetic approach for the forma-
tion of Tz based polymers. Wiggins and Pasquinet et al. reported
the synthesis of Tz-based unconjugated linear or hyperbranched
12
polymers. To compare the effect of Tz and BT moieties, we
6
,7
polymers. Grote et al. attached Tz to ion-exchange resin as side
chains to modify its anion-exchange properties toward precious
Received: February 1, 2011
8
metal ions. Audebert et al. reported the first example of an
Revised:
March 1, 2011
electroactive conjugated polymer containing a Tz moiety in each
Published: March 17, 2011
r 2011 American Chemical Society
1977
dx.doi.org/10.1021/cm200330c
|Chem. Mater. 2011, 23, 1977–1984

Chemistry of Materials
ARTICLE
synthesized two polymer analogues with the structures shown in
(Scheme 1). However, this approach will inevitably produce
about 12% isomer (3-alkyl-2-thiophene carboxaldehyde). It is
difficult to remove from the product at this stage. We noticed that
to use lithium 2,2,6,6-tetramethylpiperidide (LiTMP) instead of
10
Figure 1. Interestingly, as compared with the energy levels of the BT
polymer (PCPDTTBTT), both the highest occupied molecular
orbital (HOMO) and the lowest unoccupied molecular orbital
1
4
(LUMO) energy levels of the Tz polymer (PCPDTTTz) became
n-butyl lithium will improve the selectivity of the reaction. We
found that this 3-substituted isomer will not interfere with the
next step reaction for the formation of the nitrile compound, and
it can be completely removed from the desired product at this
stage by a careful column chromatography. A modified sulfur-
assisted Pinner synthesis was employed to prepare the symme-
much lower, with the values reduced by 0.25 and 0.12 eV, respectively.
This led to a larger open circuit voltage (V ) of the photovoltaic
oc
devices when using PCPDTTTz-based devices (0.75 V) than when
13
using PCPDTTBTT-based devices (0.56 V). In addition, it also
resulted in a slightly broader band gap (1.86 vs 1.73 eV).
10
15
Following our previous study, in this work, we synthesized
several bisthienyl-s-tetrazine (TTz) monomers with a methyl,
hexyl, or 2-ethylhexyl side group on the 4-position of the
thiophene ring. These electron-deficient monomers were then
trical alkyl substituted dithienyl-s-tetrazine (TTz), where the
thiophene nitrile was treated with hydrazine in the presence of
sulfur to form 1,4-dihydro-s-tetrazine, which was than oxidized to
s-tetrazine by isoamyl nitrite. We noticed that the size of alkyl
groups has a steric effect for this reaction, as the alkyl increases
from hydrogen, methyl, hexyl, to 2-ethylhexyl; the yield of this
ring-closing reaction decreased from 60% to 45%, to 41%, and
0
copolymerized with electron-rich cyclopenta[2,1-b:3,4-b ]
dithiophene (CPDT) monomers to obtain low band gap poly-
mers. We investigated their thermal decomposition, optical,
electrochemical, and chain packing properties, and compared
their performance in solar cell devices.
15c
then to 36%. These TTz compounds can then be easily
brominated to corresponding dibromide monomers.
0
Cyclopenta[2,1-b:3,4-b ]dithiophene (CPDT) has been suc-
’
RESULTS AND DISCUSSION
cessfully used to prepare high performance polymer solar cell
materials and has been demonstrated as an efficient electron-rich
Monomer and Polymer Synthesis. The bisthienyl-s-tetra-
16
unit; thus, it was chosen as the comonomer unit in this work.
zine (TTz) monomer synthesis starts from 4-alkyl-2-thiophene
carboxaldehyde, which can be synthesized from 3-alkyl thiophene
by reacting with n-butyl lithium and then 1-formylpiperidine
16a
The trimethyltin CPDT monomer was prepared as reported,
which was then copolymerized with the above TTz monomers by
the Stille coupling reaction to yield five polymers with different
side chain combinations (Scheme 2). As shown in Table 1, after
careful purification by solvent extraction, the average molecular
weight (M ) of the polymers ranged from 11.9 to 29.4 kDa, with
n
the polydispersity index (PDI) ranged from 1.50 to 2.14.
As expected, the solubility of these polymers improves with
increasing content of the branched alkyl chain. For example,
polymers P1 and P3 can be dissolved in chloroform, chloroben-
zene, THF, and o-dichlorobenzene (o-DCB) at room tempera-
ture, while P2 and P5 are only soluble in these solvents at an
elevated temperature. P4 can be only dissolved in chlorobenzene
Scheme 2. Synthesis of the Conjugated Polymers
Figure 1. Structures and energy levels of PCPDTTBTT, PCPDTTTz,
and PCBM.
Scheme 1. Synthesis of the Tetrazine Monomers
1
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Chemistry of Materials
ARTICLE
Table 1. Polymer Characterization
a
a
b
opt
g
c
red
d
ox
d
e
e
polymers
M
n
(kDa)
PDI
soln λmax (nm)
film λmax (nm)
E
(eV)
E
1/2 (V)
E
1/2 (V)
LUMO (eV)
HOMO (eV)
P1
P2
P3
P4
P5
17.0
11.9
29.4
20.0
15.6
2.00
2.14
1.51
1.41
1.50
546
563
562
588
564
551
577
560
590
575
1.72
ꢀ0.87
ꢀ0.88
ꢀ0.88
ꢀ0.89
ꢀ0.94
0.94
0.95
0.95
0.97
ꢀ3.53
ꢀ3.52
ꢀ3.52
ꢀ3.51
ꢀ3.46
ꢀ5.34
ꢀ5.35
ꢀ5.35
ꢀ5.37
ꢀ5.26
1.61
1.66
1.60
1.60
0.86
a
c
b
Average molecular weight number and polydispersity index (GPC vs polystyrene standards in chlorobenzene). Solution absorption in chlorobenzene.
Optical energy gap estimated from the onset of the UV curve measured in thin film. Half wave potentials from CV measurements of thin films in a 0.1
d
e
red
ox
M Bu
4
NPF
6
/CH
3
CN solution vs Ag. Estimated from ELUMO = ꢀ(E1/2 þ 4.40) eV and EHOMO = ꢀ(E1/2 þ 4.40) eV.
agrees well with previously suggested decomposition pathways of
17
small molecular 3,6-substituted s-tetrazines. This decomposi-
tion reaction may be employed to prepare symmetric dinitirle
18
compounds.
Optical Study and X-ray Diffraction. The UVꢀvis spectra of
the polymers in chlorobenzene solution and in thin solid film are
shown in Figure 4, and the corresponding data are summarized in
Table 1. All polymer solutions show high molar absorptivity
4
4
ꢀ1
ꢀ1
(
εmax) from 4.4 ꢁ 10 to 5.6 ꢁ 10 (M cm ). Comparatively,
the solid films show slightly broader absorption peaks, and the
maximum absorption peaks only have a small red shift (less than
1
4 nm) as compared with those of the solution spectra. Inter-
estingly, P4 displays an absorption peak at the longest wave-
length in the five polymers, with a red shift of about 25 nm in
solution and 13ꢀ39 nm in solid film when compared with the
others. Its solution also has the highest molar absorptivity, and
the UV curve contains a small shoulder near 680 nm, indicating
some aggregation even in the dilute solution. Because P4
contains all linear hexyl side chains, we believe this red shift
was caused by stronger interchain interaction due to lower
steric hindrance from the side groups on thiophene and CPDT
units for chain stacking. In this regard, one might suggest that
P5 should have even smaller steric hindrance because there is
only a methyl group on the thiophene bridge as compared with
that of the hexyl or 2-ethylhexyl groups for the other polymers,
but in fact P5 has a UV absorption maximum located between
those of P4 and the other polymers. This result, once again,
proved that the branched 2-ethylhexyl groups on the CPDT
unit have more significant steric hindrance than that on the
thiophene bridge.
X-ray diffraction (XRD) curves of these polymer films are
shown in Figure 5. Polymers P1 to P4 all have a broad peak at 2θ
of 6.6°, corresponding to an interchain distance of 15.5 Å, while
only P4 shows a clear and broad πꢀπ stacking peak at 2θ of 27°,
corresponding to a short πꢀπ stacking distance of 3.8 Å. This
result agrees well with our previous discussion that P4 has
stronger interchain stacking than the other polymers.
Electrochemical Properties. The cyclic voltammetry (CV)
curves of these polymer thin films on the platinum electrode
coated from chlorobenzene solution are shown in Figure 6, and
half wave potentials are summarized in Table 1. All polymers
show reversible electroactive properties. Polymers P1 to P4 have
similar oxidation and reduction potentials at 0.95 and ꢀ0.88 V,
respectively, versus the Ag quasi-reference electrode. So the
HOMO and LUMO energy levels were calculated to be ꢀ5.35
and ꢀ3.52 eV with an electrochemical band gap of 1.83 eV,
which is slightly larger than their optical band gaps (1.60ꢀ1.72
eV). Comparatively, polymer P5 shows raised HOMO and
LUMO energy levels at ꢀ5.26 and ꢀ3.46 eV. It is interesting
Figure 2. DSC and TGA curve of polymer P4 with a heating rate of 10
C/min in nitrogen.
°
or o-DCB at higher temperatures. It is interesting that P3 shows
better solubility than P2, although both of them bear two hexyl
and two 2-ethylhexyl groups in each repeat unit but at different
positions. This indicates that whether the branched alkyl chains
are on the thiophene or CPDT units will significantly affect the
solubility and interchain stacking. It appears that the branched
alkyl chains on the CPDT units can create stronger steric
hindrance for chain stacking than these on the thiophene units
when these two branched chains are crowded on the same carbon
in the fused triple-ring coplanar structure of CPDT.
Thermal Stability and Decomposition. The thermal stability
of the polymers was investigated with differential scanning
calorimetry (DSC) and thermal gravimetric analysis (TGA).
Typical DSC and TGA curves of P4 are shown in Figure 2.
The DSC analysis showed that the polymers are all stable up to
2
2
20 °C, while a huge exothermal peak appeared with an onset at
50 °C. The TGA curve also showed about 4% weight loss
between 250 and 300 °C. We noticed that the dark blue polymer
solid changed to yellow oil after a scan from 0 to 300 °C,
indicating an occurrence of dramatic chain scission of the
polymer. H and C NMR spectra of this yellow oil showed
that a highly pure dinitrile (compound 6 in Scheme 3) was
obtained even without any purification (Figure 3), and its FT-IR
spectrum gave a strong peak at 2214 cm , which was attributed
to nitrile group. GPC analysis of the resulting yellow liquid
revealed a main decomposition product with a monomodel
1
13
ꢀ
1
distribution (PDI = 1.03) and M of 720 g/mol (inset of
n
Figure 3). This result agrees with the decomposition species
displayed in Scheme 3 and confirms the decomposition route of
the polymer, where the Tz unit decomposed to nitrile with
release of N gas and cleavage of the remaining NꢀN bond. This
2
1
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Chemistry of Materials
ARTICLE
Scheme 3. Thermal Decomposition of Polymer P4
1
Figure 3. H NMR spectrum (in CDCl
3
) of the crude product from the
thermal decomposition of P4, see Scheme 3 for peak assignment. (Inset:
GPC curve of the crude product; the molecular weights of correspond-
ing peaks are shown with an arrow).
that the replacement of hexyl or 2-ethylhexyl with the methyl
group on the thiophene bridge has such an effect on their
electrochemical properties.
Photovoltaic Performance. Bulk heterojunction solar cells
were fabricated with a general structure of ITO/PEDOT-PSS
Figure 4. UV absorption spectra of dilute solutions in chlorobenzene
(a) and thin films (b) of the polymers.
(40 nm)/polymer:PC BM(100 nm)/LiF(1 nm)/Al. As re-
71
10
ported in our previous paper, P4 produced an optimized device
performance with a short-circuit current density (J ) of 12.5
sc
2
mA/cm , a V of 0.75 V, a FF of 59%, and a PCE of 5.53%, under
oc
the following device processing condition: polymer/PC BM
71
ratio of 1/2, DIO content of 3.0%, and spin-coating temperature
of 80 °C (device D4). Similar processing conditions were applied
to the other polymers (D1 for P1, D2 for P2, D3 for P3, and D5
for P5). Figure 7a compares the current densityꢀvoltage (JꢀV)
curves of these five devices, with the device fabrication conditions
and performance data summarized in Table 2. Among these five
devices, device D4 (from P4) has the best performance with the
data listed above. Device D5 (from P5) shows the lowest Voc
(0.60 V) in the five devices, about 0.15 V lower than that of D4,
agreeing well with the CV results (Table 1). It should be noted
that although device D1 and D 3 have higher V , their J values
oc
sc
are extremely lower. This is probably due to the poor film
morphology formed in the active layers. As mentioned above,
polymers P1 and P3 have the best solubility and are soluble in
o-DCB even at room temperature. The combined effect of the
good solubility of the polymer and the enhance solubility of
PC BM due to the use of the processing additive, 1,8-diiodooc-
Figure 5. XRD spectra of the polymer films.
coating, resulting in a poorly phase-segregated active layer.
Therefore, P1 and P3 were also fabricated at room temperature
without using DIO to generate D6 and D7 in order to obtain a
7
1
tane (DIO), might hamper the phase separation during spin-
1
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Chemistry of Materials
ARTICLE
DIO (D1 and D3), indicating a better morphological structure in
D6 and D7. Intrigued by this result, we also prepared a device
from P4 without using DIO; the obtained device (D8) showed a
much poorer performance than that of D4. This contradicted
result of the DIO effect for the polymers with poor and good
solubility indicates that the effect of DIO on the device
performance depends on the solubility and chain-stacking
capability of the individual polymer. To further explore the
DIO effect, devices with different amount of DIO (0.0%, D8;
1
.3%, D12; 3.0%, D4, and 4.5%, D12) were fabricated from
P4. The results showed that the best performance was still
from D4, while devices D11 and D12 also performed well.
This result indicates that DIO is beneficial in creating a better
morphological structure for P4. Different weight ratios of P4
to PC BM (1/1 for D10; 1/2 for D4; 1/3 for D9) were also
7
1
tested, and their JꢀV curves were compared in panel (b) of
Figure 7. It is shown that these three devices have similar V
oc
and FF values, while the J value of D4 is much better than
sc
those of D10 and D9, indicating that a ratio of 1:2 of P4/
PC BM gives the best balance in light absorption and charge
7
1
transport, as is proved in the following discussion.
The external quantum efficiency (EQE) spectra of devices
D10, D4, and D9 from polymer P4 with weight ratios of 1:1, 1:2,
and 1:3, respectively, are compared in panel (a) of Figure 8, while
the corresponding UV absorption spectra of the active layers are
shown in panel (b) of Figure 8. As listed in Table 2, the integrated
Figure 6. Cyclic voltammograms of polymer films on the platinum
electrode in Bu NPF /CH CN solution.
4
6
3
J from the EQE spectra agrees well with the data from the JꢀV
sc
curves. The EQE of D4 reaches 60% from 450 to 600 nm, while
that of D10 covers a broader range from 400 to 650 nm at over
5
0%. D9 shows a lower EQE from 400 to 700 nm because of the
lower polymer weight ratio that resulted in a limited light
absorption. These results prove the importance of the weight
ratio between donor and acceptor, which will also influence the
morphological structure of the resulting bicontinuous network
for balancing the electron and hole transport.
Atomic Force Microscopy (AFM). The surface morphological
structure of the active layer of the polymer/PC BM blend was
71
analyzed by tapping mode AFM (Figure 9). The films on D6 and
D7 show obscure domains without clear phase separation,
corresponding to the modest FF of 30.2% and 38.4%, respec-
tively. The films on D2 and D9 have a clear interpenetrated two-
phase morphology, while the film on D9 shows a finer structure.
This may explain the better performance of D2 and D9 among
these devices. Large scale (5 μm ꢁ 5 μm) images of the film on
D8 show large round domains with sizes over 100 nm, which
apparently indicates the formation of large spherical crystals of the
polymer or PC BM during spin-coating (Figure 9g,h). Interest-
71
ingly, besides these big domains, other regions of the image still
show a phase-segregated structure (Figure 9i,j). We believe that
this is caused by the mismatched crystallization rates of the
polymer and PC BM during the spin coating. PC BM tends
71
71
to precipitate and crystallize faster than P4; thus, large PC BM
7
1
islands formed first during spin-coating at a high temperature.
These islands span across the film and might act as defects in the
device and limit interfacial area between the two phases, which
might explain why the device performance of D8 is so poor. DIO
has a higher boiling point than the host solvent, o-DCB, and shows
good solubility only for PC BM, and during spin coating, DIO
Figure 7. JꢀV curve of (a) polymer:PC71BM (weight ratio 1:2) bulk
heterojunction solar cells and (b) P4:PC71BM solar cells under a
71
ꢀ
2
simulated AM 1.5 G irradiation of 100 mW cm
.
willevaporate slower than o-DCB. So, its addition will decrease the
tendency of precipitation and crystallization of the PC BM phase
7
1
19
better phase separation. The resulting devices showed better
performance than those fabricated at elevated temperature using
to match with that of P4; thus, better phase segregation can be
expected as in D9 (Figure 9k,l).
1
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Chemistry of Materials
ARTICLE
Table 2. Summary of the Device Fabrication Conditions and Device Performance
a
b
c
2
c
devices
polymer/PC71BM w/w ratio
DIO (%)
temp
J
sc (mA/cm )
Voc (V)
FF (%)
PCE (%)
D1
D2
P1, 1/2
P2, 1/2
P3, 1/2
P4, 1/2
P5, 1/2
P1, 1/2
P3, 1/2
P4, 1/2
P4, 1/3
P4, 1/1
P4, 1/2
2.5
2.5
2.5
3.0
3.0
0.0
0.0
0.0
2.5
2.5
1.3
80
80
80
85
80
21
21
85
85
85
85
85
1.70
11.1
0.83
0.74
0.89
0.75
0.60
0.78
0.81
0.60
0.72
0.72
0.73
0.75
32.8
47.7
41.5
59.0
37.1
30.2
38.4
35.2
53.2
54.8
55.2
57.6
0.46
3.89
D3
3.57
1.32
D4
12.5 (12.2)
9.08
5.53 (5.40)
2.02
D5
D6
4.73
1.11
D7
9.44
2.93
D8
4.83
1.03
D9
9.69 (9.31)
9.93 (9.92)
3.70 (3.55)
3.92 (3.92)
4.42 (4.36)
4.96
D10
D11
D12
10.97 (10.81)
11.49
P4, 1/2
b
4.5
a
c
Volume ratio to o-DCB. Device fabrication temperature. EQE calibrated data are shown in parentheses.
fabricated on the basis of the blends of these polymers with
PC BM. Polymer P4, which has all linear hexyl side chains, gives
71
the best performance with a PCE up to 5.53%. The use of DIO as
a processing additive improves the device performance for the
polymers with poor solubility in the processing solvent, but its
influence to the polymers with good solubility is less significant.
This effect is closely correlated with the formation of the
bicontinuous interpenetrating domain structure, which is con-
firmed by AFM analysis.
’
EXPERIMENTAL SECTION
3
General Methods. NMR spectra were recorded in CDCl , acet-
one-d , or o-DCB-d using a Varian Unity Inova spectrometer at a
6
4
1
13
resonance frequency of 399.96 MHz for H and 100.58 MHz for C.
UVꢀvis spectra were measured using a Varian Cary 5000 spectrometer.
Gel permeation chromatography (GPC) (Waters Breeze HPLC system
with 1525 Binary HPLC Pump and 2414 differential refractometer) was
used to measure the molecular weight and polydispersity index. Chlor-
obenzene was used as eluent, and commercial polystyrenes were used as
standard. The differential scanning calorimetry (DSC) analysis was per-
formed under a nitrogen atmosphere (50 mL/min) using a TA Instrument
DSC 2920 at a heating rate of 10 °C/min, calibrated with the melting
transition of indium. The thermal gravimetric analysis (TGA) was per-
formed using a TA Instrument TGA 2950 at a heating rate of 10 °C/min
under a nitrogen atmosphere (60 mL/min). Cyclic voltammetry (CV)
measurements were carried out under argon in a three-electrode cell using
0
.1 M Bu NPF in anhydrous CH CN as the supporting electrolyte. The
4 6 3
polymer was coated on the platinum-working electrode. The CV curves
were recorded with reference to an Ag quasi-reference electrode, which was
þ
calibrated using a ferrocene/ferrocenium (Fc/Fc ) redox couple (4.8 eV
below the vacuum level) as an external standard. The E1/2 of the Fc/Fc
Figure 8. (a) EQE spectra and (b) UV absorption of the active layer of
devices D4, D10, and D9.
þ
redox couple was found to be 0.40 V vs the Ag quasi-reference electrode.
X-ray diffraction (XRD) spectra were recorded on a Bruker AXS D8
1
Advance instrument with Co KR radiation (λ = 1.789 Å). Tapping mode
’
CONCLUSION
AFM images were obtained with a Veeco AFM instrument.
In summary, we synthesized five alternating copolymers of
Chemicals. Monomer 4b, 4c, 5a, and 5b were synthesized accord-
TTz and CPDT with different alkyl side chains (methyl, hexyl, or
-ethylhexyl) connected to thiophene and CPDT units. Whether
10,20,16a
ing to the procedure reported in the literatures.
Synthesis of 4-Methylthiophene-2-carboxaldehyde (1a).
2
using linear or branched side alkyl chains on these two units,
especially on the CPDT unit, dramatically affects the solubility,
chain stacking, and energy levels of the polymers. These polymers are
stable up to 220 °C and will decompose to dinitrile compounds by
breaking Tz linkage at 250ꢀ300 °C. Polymer solar cells were
A vacuumed dried 500 mL round-bottomed flask was charged with
3-methylthiophene (9.8 g, 0.10 mol) and anhydrous tetrahydrofuran
(
200 mL) under argon. The solution was cooled to ꢀ78 °C with an
acetone/dry ice bath. To the solution was slowly added 2.5 M n-butyl
lithium in hexane (40 mL, 0.10 mol). The reaction solution was stirred
1
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Chemistry of Materials
ARTICLE
Figure 9. AFM topography (first and third column) and phase (second and forth column) images of the active layer in devices D6 (a,b), D2 (c,d), D7
e,f), D8 (g,h,i,j), D9 (k,l). The size of images is 5 μm ꢁ 5 μm (g,h) and 500 nm ꢁ500 nm (for the others).
(
at ꢀ78 °C for 30 min before 1-formylpiperidine (12.4 g, 0.11 mol) was
added in one shot. Then, the solution was allowed to slowly warm to
room temperature and stirred overnight. The solution was poured into
ice water (200 mL). The organic layer was separated, and the aqueous
layer was extracted with ethyl acetate. The organic phases were
combined and washed with distilled water, dried over anhydrous
magnesium sulfate, and rotary evaporated to remove the solvent.
The liquid residue was subjected to silica-gel column chromatography
of gas evolved. The solution was heated to reflux and stirred for 2 h.
After cooling in an ice bath, the resulting red solid was collected and
washed with cold ethanol and dried under vacuum. To a solution of the
above red solid in chloroform (50 mL) was added isopentyl nitrite
(24.6 g, 210 mmol). The resulting solution was stirred at room
temperature overnight. The solvent was removed by rotary evapora-
tion, and the red solid residue was washed with ethanol to give pure
1
product (8.7 g, 45% yield). H NMR (CDCl
3
, δ): 8.04 (s, 2H), 7.25 (s,
1
3
using ethyl acetate/hexanes (15/85) as eluent to yield a clear liquid
2H); 2.36 (s, 6H). C NMR (CDCl , δ): 161.31, 139.81, 135.52,
3
1
product (6.5 g, 52% yield). H NMR (CDCl
3
, δ): 9.86 (s, 1H); 7.56 (d,
132.74, 128.22, 15.70.
J = 1.2 Hz, 1H); 7.34 (d, J = 1.2 Hz, 1H): 2.31 (s, 3H).
Synthesis of 3,6-Bis[4-methyl-5-bromothien-2-yl]-s-tetra-
zine (1d). To a suspension solution of 3,6-bis[4-methylthien-2-yl]-s-
tetrazine (3.00 g, 10.9 mmol) in chloroform (80 mL) and acetic acid
(25 mL) was added N-bromosuccinimide (5.85 g, 32.8 mmol). The
mixture was stirred at 80 °C for 4 h, and the resulting solid product was
Synthesis of 4-Methyl-2-cyanothiophene (1b). A mixture
solution of 4-methylthiophene-2-carboxaldehyde (5.1 g, 41 mmol) and
hydroxylamine hydrochloride salt (4.0 g, 57.5 mmol) in pyridine/
ethanol (30 mL, 1/1, v/v) was stirred at 80 °C overnight. After cooling,
the solution was rotary evaporated to yield a viscous liquid, which was
dissolved in acetic anhydride (30 mL) containing 0.2 g of potassium
acetate. The resulting mixture was stirred at 150 °C for 4 h. After cooling,
the solution was poured into 5% aqueous sodium hydroxide solution and
extracted with ethyl acetate. The organic phases were combined and
washed with 5% aqueous sodium hydroxide solution until basic. Then
the organic phase was washed with distilled water to neutral, dried over
anhydrous magnesium sulfate, and rotary evaporated to remove the
solvent. The residue was subjected to silica-gel column chromatography
collected by filtration and recrystallized from toluene to give red needle
1
crystals (3.2 g, 68% yield). H NMR (CDCl
C NMR (CDCl
, δ): 7.92 (s, 2H); 2.29 (6H).
3
13
3
, δ): 160.79, 139.78, 134.92, 132.38, 118.42, 15.35.
General Procedure for Stille Reaction. To a 25 mL flask was
added 4 (0.420 mmol), 5 (0.400 mmol), N,N-dimethylformamide
(DMF, 0.5 mL), and toluene (8 mL). The system was purged with Ar
under vacuum, and then (PPh ) Pd(0) (0.06 g, 0.006 mmol) was added
3
4
in a glovebox. The solution was stirred and refluxed for 24 h under the
protection of Ar. After the solution was cooled to room temperature, the
solution was dropped into acetone to precipitate the copolymer. The
copolymer was Soxhlet extracted with methanol, acetone, hexanes, and
dichloromethane to remove the oligmer and then with chloroform and
toluene to obtain the polymer (yield 50ꢀ90%). The GPC analysis data
are shown in Table 1.
Thermal Decomposition. Polymer P4 (6.0 mg, 0.0079 mmol)
was weighted into a test tube, which was then purged with Ar. Under Ar
protection, the tube was placed in a 280ꢀ300 °C oil bath for 80 min.
1
to yield a clear light yellow liquid product (3.0 g, 61% yield). H NMR
13
(
CDCl , δ): 7.40 (s, 1H); 7.16 (s, 1H); 2.27 (s, 3H). C NMR (CDCl3,
3
δ): 138.98, 138.40, 128.08, 114.35, 109.21, 15.12.
Synthesis of 3,6-Bis[4-methylthien-2-yl]-s-tetrazine (1c).
To a mixture solution of 4-methyl-2-cyanothiophene (17.4 g, 141
mmol) and sulfur (2.24 g, 70.0 mmol) in anhydrous ethanol
(
20 mL) was added hydrazine monohydrate (99%, 10.5 g, 210 mmol)
at room temperature. The solution turned into red, and large amount
1
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dx.doi.org/10.1021/cm200330c |Chem. Mater. 2011, 23, 1977–1984

Chemistry of Materials
ARTICLE
After cooling to room temperature, the crude product (brown oil) was
(11) (a) Cheng, Y.-J.; Yang, S.-H.; Hsu, C.-S. Chem. Rev. 2009,
109, 5868. (b) Dennler, G.; Scharber, M. C.; Brabec, C. J. Adv. Mater.
2009, 21, 1323. (c) Chen, L.-M.; Hong, Z.; Li, G.; Yang, Y. Adv. Mater.
1
analyzed. H NMR (CHCl , δ): 7.43 (s, 2H), 7.05(s, 2H), 2.77 (t, J = 7.8
3
Hz, 4H), 1.87 (m, 4H), 1.64 (m, 4H), 1.37 (m, 4H), 1.31 (m,8H),
2
009, 21, 1334. (d) Thompson, B. C.; Fr ꢁe chet, J. M. J. Angew. Chem., Int.
1
6
1
2
,23ꢀ1,10 (br, 12H), 0.99 (m, 4H), 0.88 (t, J = 6.8 Hz, 6H), 0.81 (t, J =
13
Ed. 2008, 47, 58. (e) G €u nes, S.; Neugebauer, H.; Sariciftci, N. S. Chem.
Rev. 2007, 107, 1324. (f) Mayer, A. C.; Scully, S. R.; Hardin, B. E.;
Rowell, M. W.; McGehee, M. D. Mater. Today 2007, 28.
(12) (a) Peet, J.; Kim, J. Y.; Coates, N. E.; Ma, W. L.; Moses, D.;
Heeger, A. J.; Bazan, G. C. Nat. Mater. 2007, 6, 497. (b) Blouin, N.;
Michaud, A.; Gendron, D.; Wakim, S.; Blair, E.; Neagu-Plesu, R.;
Bellet ^e te, M.; Durocher, G.; Tao, Y.; Leclerc, M. J. Am. Chem. Soc.
.8 Hz, 6H). C NMR (CHCl , δ): 159.09, 140.24, 139.45, 139.34,
3
38.22, 134.47, 122.12, 114.62, 106.46, 54.60, 37.78, 31.79, 31.75, 30.43,
ꢀ
1
n
9.79, 29.46, 29.30, 24.80, 22.78, 14.24, 14.21. GPC: M = 720 g mol ,
ꢀ
1
PDI = 1.03 for the main peak. FT-IR: 2214 cm (ꢀCN).
Device Fabrication and Testing. Patterned ITO glass substrates
were cleaned with detergent before sonication in CMOS grade acetone
and isopropanol for 15 min. The organic residue was further removed by
treating with UVꢀozone for 10 min. Then a thin layer of PEDOT:PSS
2
008, 130, 732.
13) Moul ꢁe , A. J.; Tsami, A.; B €u nnagel, T. W.; Forster, M.; Kronen-
(
(Clevios P, H. C. Starck, 45 nm) was spin coated and dried for 1 h at
berg, N. M.; Scharber, M.; Koppe, M.; Morana, M.; Brabec, C. J.;
Meerholz, K.; Scherf, U. Chem. Mater. 2008, 20, 4045.
120 °C. The polymer and PC71BM (ADS) were dissolved in o-DCB
containing different amounts of diiodooctane. The solution was filtered
and spin coated on top of the PEDOT:PSS layer. Then, 1.0 nm of LiF
and 100 nm of Al layer were thermally evaporated through a shadow
(14) Smith, K.; Barratt, M. L. J. Org. Chem. 2007, 72, 1031.
(15) (a) Pinner, A. Chem. Ber. 1893, 26, 2126. (b) Abdel-Rahman,
M. O.; Kira, M. A.; Tolba, M. N. Tetrahedron Lett. 1968, 9, 3871. (c)
Audebert, P.; Sadki, S.; Miomandre, F.; Clavier, G.; Verni ꢀe res, M. C.;
Saoud, M.; Hapiot, P. New J. Chem. 2004, 28, 387. (d) Soloducho, J.;
Doskocz, J.; Cabaj, J.; Roszak, S. Tetrahedron 2003, 59, 4761.
ꢀ
7
mask at a pressure of 5 ꢁ 10 mbar in a Boc Edwards Auto 500 system.
2
The active area is 50 mm . The currentꢀvoltage (JꢀV) characteristics
were measured with a Keithley 2400 digital source meter under
ꢀ
2
(16) (a) Zhu, Z.; Waller, D.; Gaudiana, R.; Morana, M.; M €u hlbacher,
simulated air mass (AM) 1.5 solar irradiation of 100 mW cm
Sciencetech, Inc., SF150). The light intensity was calibrated with a
D.; Brabec, C. Macromolecules 2007, 40, 1981. (b) Coffin, R. C.; Peet, J.;
Rogers, J.; Bazan, G. C. Nat. Chem. 2009, 1, 657. (c) Hoven, C. V.; Dang,
X.-D.; Coffin, R. C.; Peet, J.; Nguyen, T.-Q.; Bazan, G. C. Adv. Mater.
(
power meter (Gentec Solo PE laser power and energy meter). The
external quantum efficiency (EQE) was performed using a JobinꢀYvon
Triax 180 spectrometer, a JobinꢀYvon xenon light source, a Merlin
lock-in amplifier, a calibrated Si UV detector, and a SR 570 low-noise
current amplifier.
2
010, 22, E63.
17) Oxley, J. C.; Smith, J. L.; Zhang, J. J. Phys. Chem. A 2000,
04, 6764.
18) Jones, B. A.; Facchetti, A.; Marks, T. J.; Wasielewski, M. R.
Chem. Mater. 2007, 19, 2703.
19) Lee, J. K.; Ma, W. L.; Brabec, C. J.; Yuen, J.; Moon, J. S.; Kim,
J. Y.; Lee, K.; Bazan, G. C.; Heeger, A. J. J. Am. Chem. Soc. 2008,
(
1
(
’
AUTHOR INFORMATION
(
Corresponding Author
130, 3619.
*E-mails: zhao.li@nrc-cnrc.gc.ca (Z.L.), jianfu.ding@nrc-cnrc.gc.
(20) Ding, J.; Song, N.; Li, Z. Chem. Commun. 2010, 46, 8668.
ca (J.D.).
’
ACKNOWLEDGMENT
We acknowledge the financial support from NRC-CSIC,
NRC-NSERC-BDC and NRC-NANO projects. This work is
NRC Publication Number NRCC52846.
’
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1
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