Low band-gap polymers based on quinoxaline derivatives and fused thiophene as donor materials for high efficiency bulk-heterojunction photovoltaic cells

Article abstract of DOI:10.1039/b823536h

In this study we synthesized three low band-gap polymers - poly[2,3-bis(4-hexyloxyphenyl)quinoxaline-alt-2,5-thieno[3,2-b]thiophene] (PQTT), poly[2,3-bis(4-hexyloxyphenyl)quinoxaline-alt-3,6-dipentadecylthieno[3, 2-b]thiophene] (PQPDTT) and poly[2,3-bis(4

Full text of DOI:10.1039/b823536h

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Low band-gap polymers based on quinoxaline derivatives and fused thiophene
as donor materials for high efficiency bulk-heterojunction photovoltaic cells
a
Jang-Yong Lee, Won-Suk Shin, Jung-Rim Haw and Doo-Kyung Moon
b
a
a
*
Received 6th January 2009, Accepted 21st April 2009
First published as an Advance Article on the web 3rd June 2009
DOI: 10.1039/b823536h
In this study we synthesized three low band-gap polymers—poly[2,3-bis(4-
hexyloxyphenyl)quinoxaline-alt-2,5-thieno[3,2-b]thiophene] (PQTT), poly[2,3-bis(4-
hexyloxyphenyl)quinoxaline-alt-3,6-dipentadecylthieno[3,2-b]thiophene] (PQPDTT) and poly[2,3-
bis(4-hexyloxyphenyl)quinoxaline-alt-2,5-bis(thieno-2-yl)-3,6-dipentadecylthieno[3,2-b]thiophene]
(PQTPDTT)—that are based on quinoxaline and thieno[3,2-b]thiophene, and examined their
photovoltaic properties. The band-gaps of PQTT and PQTPDTT were 1.65eV and 1.7eV, respectively.
For PQTPDTT, a HOMO level of ꢀ5.12eV, which was lower than that of P3HT, was observed.
According to the analysis of the characteristics of the photovoltaic device using PC61BM and PC71BM
as acceptors, an open circuit voltage (VOC) of 0.71V, a short circuit current (JSC) of 8.80mA/cm2, a fill
factor of 0.36 and a power conversion efficiency (PCE) of 2.27% were observed at 100mW/cm2 (AM
1.5 illumination) when a PQTPDTT/PC71BM blend film was used as the active layer. A very high
incident photon to current efficiency (IPCE) was detected (60% at 400nm, 57% at 500nm).
P3HT cannot absorb a wide range of photovoltaic power due to
Introduction
a relatively broad band-gap (1.9eV) and may lose much energy
because of high LUMO levels.27,28 As a result, it appears that the
band-gap of bulk-heterojunction organic solar cells is optimum
at 1.3–1.5eV with a LUMO level of 3.8eV, which is higher than
that of an acceptor by 0.3–0.5eV.29–33
p-Conjugated polymers have been drawing great attention
recently due to their application to related fields (e.g. OTFT,
OLED, etc.),1–4 their potential for fabricating low-cost integrated
circuit elements for large areas and the feasibility of low-cost
production using various methods such as spin coating, ink-jet
printing and roll-to-roll through solution processes.5–11 Thanks
to the ease of cell production and efficient charge separation,
bulk heterojunction-type organic solar cells, which contain
a conjugated polymer and a fullerene as donor and acceptor,
respectively, have been most widely studied out of the various
organic solar cells.12–17 In the beginning, the efficiency of organic
solar cells was less than 1%. Since the adoption of fullerene as
an acceptor, however, this figure has increased greatly. Due to
a sharp advance in the field, devices reaching up to 5% in power
conversion efficiency have recently been developed.18–21
However, to achieve a 7% or higher level of power conversion
efficiency, which is essential for the commercialization of organic
solar cells, a variety of polymer materials that can overcome
the existing limitations (stability, morphology, production tech-
niques, etc.) must be developed.22–24
For the synthesis of lower band-gap polymers, many polymers
have adopted an internal charge transfer between the electron-
rich unit and the electron-deficient moiety.34–36 It is regarded as
a promising method for producing low band-gap polymers via
which photons can be absorbed in a wide range of photovoltaic
power, since alternating donor–acceptor units lower the effective
band-gap by orbital mixing.
In this study, we synthesized new donor–acceptor-type low
band-gap polymers—poly[2,3-bis(4-hexyloxyphenyl)quinoxaline-
alt-2,5-thieno[3,2-b]thiophene] (PQTT), poly[2,3-bis(4-hexyloxy-
phenyl)quinoxaline-alt-3,6-dipentadecylthieno[3,2-b]thiophene]
(PQPDTT) and poly[2,3-bis(4-hexyloxyphenyl)quinoxaline-
alt-2,5-bis(thieno-2-yl)-3,6-dipentadecylthieno[3,2-b]thiophene]
(PQTPDTT)—which contain quinoxaline (excellent electron-
attraction unit) and thieno[3,2-b]thiophene, which has excellent
planarity and abundant electrons. To improve the solubility of
n-type materials and stabilize the excitons that are produced
in molecules by the resonance effect, in particular, hexylox-
ybenzene was utilized. Furthermore, pentadecyl (a long alkyl
chain) was used to increase the solubility of the fused thiophene.
Bulk heterojunction-type devices with PC61BM and PC71BM as
acceptors were fabricated in order to investigate the photovoltaic
properties of PQTT, PQPDTT and PQTPDTT. Devices using
the synthesized polymers and PCBM-blend films as active layers
displayed a 1–2% power conversion efficiency. In those devices
that used a PQTPDTT/PC71BM blend film, in particular, a very
A high level (ꢁ5%) of PCE has been reported in polymer-
based organic solar cells that use a regioregular poly(3-hexylth-
iophene) (P3HT) and [6,6]-phenyl C61 butyric acid methyl ester
(PCBM) blend as the active layers.25,26 Despite excellent charge
transfer and separation, however, the polymer-based donor
aDepartment of Materials Chemistry and Engineering, Konkuk University,
1 Hwayang-dong, Gwangjin-Gu, Seoul, 143-701, South Korea. E-mail:
dkmoon@konkuk.ac.kr; Fax: +82-2-444-0765; Tel: +82-2-450-3498
bEnergy Materials Research Division, Korea Research Institute of
Chemical Technology, P. O. BOX 107, Sinseongno 19, Yuseong,
Daejeon, 305-600, South Korea. E-mail: shinws@krict.re.kr; Fax: +82-
42-860-7868; Tel: +82-42-860-7106
high level of PCE (2.27%) was detected (VOC ¼ 0.71V, JSC
¼
8.80mA/cm2, FF ¼ 0.36). The process of the synthesis of
monomers and polymers is shown in Scheme 1.
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Scheme 1 The process of the synthesis of monomers and polymers. I) NaBH4, ethanol; II) 4,40-dimethoxybenzil, butanol; III) C5H5N$HCl, reflux; IV)
n-bromohexane, ethanol, reflux; V) THF, n-BuLi, Me3SnCl, ꢀ50 ꢂC; VI) CH2Cl2/AcOH, NBS; VII) THF, n-BuLi, Me3SnCl; VIII) THF, 2-tribu-
tyltinthiophene, Cl2(PPh3)2Pd; IX) THF, n-BuLi, Me3SnCl; X) DMF/THF, Cl2(PPh3)2Pd, reflux.
investigated using H-NMR, 13C-NMR and elemental analysis.
1
Results and discussion
The synthesized polymers displayed high solubility in common
To improve the solar absorption spectra and reduce the band-
organic solvents such as THF, chloroform, toluene, chloroben-
gap of polymers, thiophene and thieno[3,2-b]thiophene were
zene and o-dichlorobenzene. Molecular weights were measured
used as electron donors and quinoxaline as an electron acceptor.
through GPC using THF as a solvent. According to these
Pentadecyl and hexyloxybenzene were attached to thieno[3,2-
measurements, PQTT, PQPDTT and PQTPDTT had Mn values
b]thiophene and quinoxaline respectively to increase solubility.
of 9 800, 8 668 and 14 065, Mw values of 14 600, 12 041 and
Polymers were synthesized through a Stille coupling reaction,
26 700, and PDIs of 1.44, 1.39 and 1.9, respectively.
and low-molecular-weight materials were removed through
Fig. 1 shows the UV-vis absorption spectra of polymers
reprecipitation and Soxhlet extraction of the solid product with
measured in the chloroform solution and in the film. For PQTT,
methanol, acetone and hexane. The synthesis of polymers was
the maximum absorption spectra were detected at 373nm and
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means that PQTPDTT is more effective than PQTT in solar
absorption with potential use as a polymer for organic solar cells.
In terms of the optical band-gaps, which were measured through
UV-vis absorption spectra, PQTT and PQTPDTT were 1.65eV
and 1.7eV. For PQPDTT, however, the maximum absorption
peaks were detected at 326nm, 397nm and 540nm (shoulder) in
solution and there were no differences between the solution and
the film. Furthermore, the UV-vis absorption spectra of
PQPDTT were similar to those of quinoxaline derivatives. This
indicates that there are no effective interactions between
quinoxaline derivatives and fused thiophene derivatives. It is
supposed to be caused by steric effects between long alkyl chains
of quinoxaline derivatives and fused thiophenes.37 The optical
band-gap, which was calculated from the absorption onset, of
PQPDTT was 2.3eV.
Fig. 1 UV-vis spectra of polymers in solution and in films.
Fig. 2 shows the cyclic voltammograms of synthesized poly-
mers. According to the analysis, PQTT and PQTPDTT were
found to have oxidation potentials of 1.08V and 1.25V and
reduction potentials of ꢀ1.3V and ꢀ1.23V. However, the
oxidation and reduction potentials of PQPDTT were not clear.
In contrast, the oxidation and reduction onset potentials of
PQTT, PQPDTT and PQTPDTT were 0.52V, 1.21V and 0.68V,
and ꢀ0.83V, ꢀ0.91V and ꢀ0.76V, respectively. As shown in
Table 1, the HOMO and LUMO levels were ꢀ4.96eV and
ꢀ3.61eV for PQTT, ꢀ5.6eV and ꢀ3.53eV for PQPDTT and
ꢀ5.12eV and ꢀ3.68eV for PQTPDTT, respectively. Electro-
chemical band-gaps (which show the difference between the
HOMO and LUMO levels) of 1.35eV (PQTT), 2.07eV
(PQPDTT) and 1.44eV (PQTPDTT) were observed. Although
all the polymers showed a small band-gap for the absorption of
a wide range of photons, a high HOMO level was detected in
PQTT, which implies poor oxidative stability due to a high
HOMO level of thieno[3,2-b]thiophene (in polymers, an electron
donor). Although the overall HOMO and LUMO energy levels
decreased with the inclusion of an electron acceptor, the decrease
in HOMO levels was not as great as the decrease in LUMO
levels.38 Conversely, compared to P3HT, PQTPDTT displayed
excellent oxidative stability with a HOMO level of ꢀ5.12eV.
Furthermore, a decrease in the LUMO level by intra-molecular
interaction would be expected to improve the energy conversion
efficiency by reducing energy loss.
597nm in solution and at 379nm and 598nm in the film. In
contrast, the maximum absorption spectra were detected at
396nm and 540nm in solution and at 420nm and 600nm (about
60nm red-shifted) in the film for PQTPDTT. Similar results are
often observed in polymers with an active inter-chain interaction.
When compared to PQTT, PQTPDTT displayed greater 2D
stacking in the film. Therefore, it appears that PQTPDTT is
more effective than PQTT in shifting electrons and holes.
Furthermore, PQTPDTT displayed better optical absorption
within the range of 500–700nm, a range rich in photons, which
The morphologies of the PCBM/polymer blend films were
examined using AFM. The RMS thicknesses of PQTT/PC61BM
and PQTT/PC71BM blend films (Fig. 3a and d) were relatively
durable at 11.33nm and 25.45nm, respectively. It was also found
Fig. 2 Cyclic voltammograms of synthesized polymers.
Table 1 The optical and electrochemical properties of polymers
Absorption lmax (nm)a
Energy level (eV)d
c
e
In filmb
HOMO
LUMO
Eg ꢀ Egec (eV)
op
op
Eg (eV)
ec
Eg (eV)
Polymer
In solution
PQTT
PQPDTT
PQTPDTT
376, 598
326, 397, 540
398, 544
378, 604
325, 395, 540
420, 600
1.65
2.3
1.7
ꢀ4.96
ꢀ5.6
ꢀ3.61
ꢀ3.53
ꢀ3.68
1.35
2.07
1.44
0.3
0.23
0.26
ꢀ5.12
All potentials are given vs. Ag/AgCl electrode.a lmax was determined from UV-vis data. Spin-coated from chloroform. Estimated from the onset
absorption of the thin film. Calculated from the reduction and oxidation potentials under the assumption that the absolute energy level of Fc/Fc+
was 4.8eV below a vacuum. Band-gaps derived from the difference between onset potentials of oxidation and reduction.
b
c
d
e
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Fig. 3 AFM images of a) PQTT/PC61BM, b) PQPDTT/PC61BM, c)
PQTPDTT/PC61BM, d) PQTT/PC71BM, e) PQPDTT/PC71BM and f)
PQTPDTT/PC71BM blend films.
Fig. 4 Comparison of IPCE properties of the devices that have a)
PQTT/PC61BM, PQPDTT/PC61BM and PQTPDTT/PC61BM, and b)
PQTT /PC71BM, PQPDTT/PC71BM and PQTPDTT/PC71BM blend
films as active layers.
that polymer/PCBM films were poorly formed due to a local
aggregation of PCBM. Conversely, the RMS thicknesses of
PQPDTT/PC61BM and PQPDTT/PC71BM blend films (Fig. 3b
and e) were 3.5nm and 3.4nm, and those of PQTPDTT/PC61BM
and PQTPDTT/PC71BM blend films (Fig. 3c and f) were 3nm
and 2.8nm. When compared to the film using PQTT as a donor,
therefore, a far smoother morphology was observed. Although
the channels of polymers and PCBM were not completely
formed, charge transfer was smooth as the PCBM channels were,
for the most part, well formed. It therefore appears that relatively
good film properties were detected.39 The optical and electro-
chemical properties of the polymers are summarized in Table 1.
Fig. 4 visually demonstrates the IPCE of polymer/PCBM
bulk-heterojunction photovoltaic devices. In terms of IPCE,
a clear difference was observed between the polymer/PCBM films
depending on type. As shown in Fig. 4, PQTT/PC61BM and
PQTPDTT/PC61BM films exhibited IPCE values of 27% and
45% at 400nm and 7–17% and 3–28% at 500nm–700nm,
respectively. PQTT/PC71BM and PQTPDTT/PC71BM films also
displayed high IPCEs at 400nm (35% and 60%). In contrast to
the use of PC61BM, high IPCE was observed at 500nm as well
(33% and 57%). Although a decrease was detected at 500–700nm
as with the introduction of PC61BM, the IPCE almost doubled.
This kind of change at 500nm occurred because the number of
photons increased due to solar absorption by PC71BM.40 In
comparison with PQTT and PQTPDTT, PQPDTT/PCBM films
showed low IPCE values. This stems from poor absorption
propertyies in the visible range of the spectrum.
Fig. 5 illustrates the I–V properties of fabricated devices. The
thicknesses of active layers were adjusted with different spin
coating conditions, and the devices were thermally treated at
120 ꢂC before the LiF/Al electrodes were deposited thereon. The
I–V curves of the organic solar cells show pronounced kinks.
Fig. 5 Comparison of photovoltaic properties of the polymers.
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Table 2 The photovoltaic properties of the devicesa
Active layer
Layer thickness (nm)
VOC (V)
JSC (mA/cm2)
FF
IPCE/l (%/nm)
PCE (%)
PQTT/PC61BM ¼ 1:3
60
54
64
50
56
99
0.61
0.36
0.7
0.69
0.37
0.71
4.12
1.37
4.63
5.29
2.60
8.8
0.33
0.32
0.39
0.38
0.31
0.36
28/400, 20/500
17/400, 10/470
35/400, 33/500
45/420, 30/510
25/400, 23/500
60/420, 56/510
0.84
0.16
1.28
1.39
0.30
2.27
PQPDTT/PC61BM ¼ 1:3
PQTPDTT/PC61BM ¼ 1:3
PQTT/PC71BM ¼ 1:3
PQPDTT/PC71BM ¼ 1:3
PQTPDTT/PC71BM ¼ 1:3
a
The measurements were carried out at 100mW/cm2 (AM 1.5). The structure of the device was: ITO/PEDOT:PSS/polymer:PCBM/LiF/Al. Devices were
annealed at 120 ꢂC for 5 minutes before deposition of the LiF/Al electrode. Active area was 9mm2 in all of the devices.
The peculiarity in the production of this device was the extremely
fast evaporation rate of the top electrode. One might assume that
there is some problem with the electrical properties of the top
electrode. It would be one of factors which would substantially
reduce the fill factor and therefore the photovoltaic perfor-
mance.41,42 As shown in Fig. 5, PQTT, PQPDTT and PQTPDTT
exhibited VOC of 0.61V, 0.36V and 0.7V, and JSC values of
4.12mA/cm2, 1.37mA/cm2 and 4.63mA/cm2, respectively, when
PC61BM was used as an acceptor. When PC71BM was used, on
the other hand, 0.69V, 0.37V and 0.71V VOC, and 5.29mA/cm2,
2.6mA/cm2 and 8.8mA/cm2 JSC were observed. While no differ-
ence was detected in VOC, there was a substantial difference in
JSC in both PQTT and PQTPDTT. This difference can be
explained in terms of the results of IPCE in Fig. 4. In the case of
PQTPDTT, in particular, a larger difference was observed in JSC
depending on the type of acceptor because IPCE was high even at
500–700nm. In terms of power conversion efficiency (PCE), the
devices that used PQTT/PC61BM, PQPDTT/PC61BM and
PQTPDTT/PC61BM as active layers exhibited values of 0.84%,
0.16% and 1.28% while the devices that used PQTT/PC71BM,
PQPDTT/PC71BM and PQTPDTT/PC71BM as active layers
gave values of 1.39%, 0.3% and 2.27%, respectively. The
photovoltaic properties of the devices are summarized in Table 2.
Although VOC and JSC were high, complete efficiency was
controlled by low fill factors. Low fill factors were apparently
caused by poor morphology as in the results of AFM (Fig. 3). It
appears that charge dissociation by poor morphology, decline in
charge carrier transport and recombination of the electrons and
holes would reduce fill factors. The morphology of a polymer/
PCBM blend film could be improved by using various solvents at
spin coating, changing the annealing temperature and adding
various additives for better morphology. Due to the expansion of
FF after the improved morphology, the properties of polymers
could be greatly improved.
bis(2-thienyl)-3,6-dipentadecylthieno[3,2-b]thiophene 1147 were
prepared as described in the literature.
Synthesis
2,3-Bis(4-methoxyphenyl)-5,8-dibromoquinoxaline
3.
To
a mixture of 2 (6g, 12.4mmol) and 4,40-dimethoxybenzil (7.2g,
26.9mmol) in butanol (100ml), glacial acetic acid was added in
several droplets, and then the solution was stirred at about 120
ꢂC for 5 h. The solution was cooled to 0 ꢂC and filtered. The
separated solid was washed twice with hot ethanol and dried
under vacuum oven to give a yellow solid (4g, 65%). Found: C,
52.68; H, 3.17; N, 5.6. C22H16N2O2Br2 requires C, 52.76; H, 3.17;
N, 5.6%; dH(400MHz; CDCl3; Me4Si) 3.85 (6H, s), 6.88 (4H, d, J
¼ 8 Hz), 7.65 (4H, d, J ¼ 8Hz), 7.85 (2H, s); dC(400MHz; CDCl3;
Me4Si) 62.4, 114.6, 130.2, 132.9, 133.2, 142.2, 149.8, 157.3, 161.4.
2,3-Bis(4-hydroxyphenyl)-5,8-dibromoquinoxaline 4. In a two-
neck-ed flask, 3 (4g, 8mmol) and pyridine hydrochloride (30eq)
were added. After the mixture was stirred at 200 ꢂC for 8 h, it was
cooled to room temperature. Aqueous hydrochloric acid was
poured into the mixture, and ether was added to extract the
product from the aqueous layer. The organic layer was treated
with dilute aqueous hydrochloric acid, aqueous sodium
hydroxide and aqueous hydrochloric acid, in that order, and then
further extracted with ether. The organic layer was washed with
water and brine and dried over anhydrous Na2SO4. After the
solvent was removed by rotary evaporation, the product was
obtained as a yellow solid (3.58g, 95%). Found: C, 50.77; H, 2.55;
N, 5.93. C20H12N2O2Br2 requires C, 50.85; H, 2.54; N, 5.93%;
dH(400MHz; CDCl3; Me4Si) 5.17 (2H, s), 6.82 (4H, d, J ¼ 8 Hz,),
7.62 (4H, d, J ¼ 8Hz), 7.85 (2H, s); dC(400MHz; CDCl3; Me4Si)
118.7, 130.2, 132.9, 133.2, 142.2, 149.8, 158.1, 169.1.
2,3-Bis(4-hexyloxyphenyl)-5,8-dibromoquinoxaline
5.
To
a solution of 4 (3.58g, 7.6mmol) in ethanol (50ml) was added
aqueous potassium hydroxide (4eq). After the mixture was stir-
red for 1 h at room temperature, n-bromohexane was added. The
reaction mixture was allowed to react at 70 ꢂC for 24 h, then
cooled to ꢀ20 ꢂC. After filtration the crude product was purified
by recrystallization from methanol to give a yellow solid (2.35g,
48%). Found: C, 59.88; H, 5.64; N, 4.33. C32H36N2O2Br2
requires C, 59.97; H, 5.63; N, 4.38%; dH(400MHz; CDCl3;
Me4Si) 0.91 (6H, t, J ¼ 8Hz), 1.34 (8H, m, J ¼ 4Hz), 1.48 (4H, m,
J ¼ 6Hz), 1.79 (4H, m, J ¼ 6Hz), 3.99 (4H, t, J ¼ 6Hz), 6.88 (4H,
d, J ¼ 8 Hz), 7.65 (4H, d, J ¼ 8Hz), 7.85 (2H, s); dC(400MHz;
Experimental
Materials
All reagents and chemicals were purchased from Aldrich. Chlo-
roform was dried over CaCl2 and THF, toluene was dried over
sodium under a nitrogen atmosphere. Other reagents and
chemicals were used as received. 4,7-Dibromo-2,1,3-benzothia-
diazole 1,43 3,6-dibromo-1,2-phenylenediamine 2,44 thieno[3,2-
b]thiophene 6,45,46 3,6-dipentadecylthieno[3,2-b]thiophene 8,47
2,5-dibromo-3,6-dipentadecylthieno[3,2-b]thiophene 947 and 2,5-
4942 | J. Mater. Chem., 2009, 19, 4938–4945
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CDCl3; Me4Si) 14.5, 23.1, 29.6, 29.7, 30.1, 32.3, 68.8, 114.6,
130.2, 132.9, 133.2, 142.2, 149.8, 157.3, 160.4.
room temperature, it was poured into methanol. A powder was
obtained by filtration was reprecipitated with methanol several
times. The polymer was further purified by washing with meth-
anol, acetone and hexane, in a Soxhlet apparatus for 24h and
dried under reduced pressure at 50 ꢂC.
2,5-Bis(trimethylstannyl)thieno[3,2-b]thiophene 7. To a mixture
of 6 (1 g, 7.13mmol) in anhydrous THF (50ml) was added a 2.5M
solution of n-BuLi in hexane (6ml) at ꢀ50 ꢂC under N2. After 2h,
a 1M solution of Me3SnCl (15.7ml) in THF was added to the
solution at ꢀ50 ꢂC. After stirring at ꢀ50 ꢂC for 2h, the solution
was warmed to room temperature and stirred for 24h. The
resulting mixture was poured into deionized water. CHCl3 was
added to extract the product from the aqueous layer, and the
combined organic layers were washed with water and brine. The
organic layer was dried over anhydrous Na2SO4, and the solvent
was removed by rotary evaporation. The residue was purified by
recrystallization from Et2O to give colorless crystals (1.96g,
59%). Found: C, 57.12; H, 9.08; S, 7.23. C12H20S2Sn2 requires C,
56.93; H, 9.04; S, 7.23%; dH(400MHz; CDCl3; Me4Si) 0.38 (6H, s,
J ¼ 6.5 Hz), 7.25 (2H, s). dC(400MHz; CDCl3; Me4Si) ꢀ7.8,
126.5, 141.6, 147.9.
Poly[2,3-bis(4-hexyloxyphenyl)quinoxaline-alt-thieno[3,2-b]thio-
phene] (PQTT). Dark red solid (0.24g, 48.5%). Found: C, 73.42;
H, 6.18; N, 4.53; S, 10.33; O, 5.32. C38H38N2S2O2 requires C,
73.77; H, 6.19; N, 4.53; S, 10.34; O, 5.17%; Tg ¼ 75 ꢂC, Tm ¼ 168
ꢂC, Td ¼ 314 ꢂC; dH(400MHz; CDCl3; Me4Si) 0.92 (6H, br), 1.37
(6H, br), 1.80 (4H, br), 3.95 (4H, br), 6.45 (4H, br), 6.9 (4H, br),
7.70 (4H, d); dC(400MHz; CDCl3; Me4Si) 14.4, 14.5, 23.0, 23.1,
26.1, 29.4, 29.6, 29.7, 29.9, 30.1, 32.0, 32.3, 68.4, 114.5, 130.9,
131.7, 132.1, 133.9, 139.1, 141.2, 147.1, 151.7, 156.3, 160.3.
Poly[2,3-bis(4-hexyloxyphenyl)quinoxaline-alt-3,6-dipentadecyl-
thieno[3,2-b]thiophene] (PQPDTT). Red solid (0.11g, 22.2%).
Found: C, 77.92; H, 9.46; N, 2.60; S, 6.22; O, 3.27. C68H98N2S2O2
requires C, 78.57; H, 9.50; N, 2.69; S, 6.16; O, 3.08%; Tg ¼ 49 ꢂC,
Tm ¼ 141 ꢂC, Td ¼ 298 ꢂC; dH(400MHz; CDCl3; Me4Si) 0.78 (6H,
br), 0.92 (6H, br), 1.14 (40H, br), 1.25 (6H, br), 1.36 (4H, br), 1.69
(4H, br), 1.81 (8H, br), 2.83 (4H, br), 3.87 (4H, br), 6.75 (4H, br),
7.57 (4H, br), 7.80 (2H, br); dC(400MHz; CDCl3; Me4Si) 14.4,
14.5, 23.0, 23.1, 26.1, 29.4, 29.6, 29.7, 29.9, 30.1, 32.0, 32.3, 68.4,
114.5, 130.9, 131.7, 132.1, 133.9, 139.1, 141.2, 147.1, 151.7, 156.3,
160.3.
2,5-Bis(trimethylstannyl)-3,6-dipentadecylthieno[3,2-b]thiophene
10. To a mixture of 9 (3 g, 4.17mmol) in anhydrous THF (30ml)
was added a 2.5M solution of n-BuLi in hexane (3.5ml) at ꢀ50 ꢂC
under N2. After 2h, a 1M solution of Me3SnCl (8.5ml) in THF
was added to the solution at ꢀ50 ꢂC. After stirring at ꢀ50 ꢂC for
2h, the solution was warmed to room temperature and stirred for
24h. The resulting mixture was poured into a deionized water.
CHCl3 was added to extract the product from the aqueous layer,
and the combinded organic layers were washed with water and
brine. The organic layer was dried over anhydrous Na2SO4, and
the solvent was removed by rotary evaporation. It was used
directly in the next step without further purification. (2.88g,
78%). (Found: C, 57.43; H, 8.04; S, 12.18. C42H80S2Sn2 requires
C, 57.18; H, 8.01; S, 12.2%) dH(400MHz; CDCl3; Me4Si) 0.38
(6H, s, J ¼ 6.5 Hz), 7.25 (2H, s); dC(400MHz; CDCl3; Me4Si)
-7.8, 14.4, 23.1, 29.1, 29.6, 31.5, 32.0, 32.1, 130.1, 132.7, 141.8.
Poly[2,3-bis(4-hexyloxyphenyl)quinoxaline-alt-2,5-bis(thieno-2-yl)-
3,6-dipentadecylthieno[3,2-b]thiophene] (PQTDPT). Dark red solid
(0.3g, 63.7%). Found: C, 75.14; H, 8.54; N, 2.20; S, 10.66; O, 2.68.
C76H102N2S4O2 requires C, 75.84; H, 8.54; N, 2.33; S, 10.64; O,
2.66%; Tg ¼ 70 ꢂC, Tm ¼ 161 ꢂC, Td ¼ 303 ꢂC; dH(400MHz; CDCl3;
Me4Si) 0.85 (6H, br), 0.92 (6H, br), 1.24 (40H, br), 1.35 (6H, br),
1.48 (4H, br), 1.56 (4H, br), 1.81 (8H, br), 3.0 (4H, br), 3.99 (4H, br),
6.88 (4H, br), 7.09 (2H, br), 7.6 (4H, br), 7.69 (2H, br), 7.91(2H, br);
dC(400MHz; CDCl3; Me4Si) 14.4, 14.5, 23.0, 23.1, 26.1, 29.4, 29.6,
29.7, 29.9, 30.1, 32.0, 32.3, 68.4, 114.5, 130.9, 131.7, 132.1, 133.9,
139.1, 141.2, 147.1, 151.7, 156.3, 160.3.
2,5-Bis(5-trimethylstannyl-thienyl-2yl)-3,6-dipentadecylthieno[3,2-
b]thiophene 12. To a mixture of 11 (1 g, 1.38mmol) in anhydrous
THF (20mL) at ꢀ25 ꢂC was added n-butyllithiumꢂ (1.6M in
hexane) by syringe. The mixture was stirred at ꢀ25 C for 1h.
Me3SnCl was added dropwise to the solution, and the resulting
mixture was warmed to room temperature and stirred for 24h. The
mixture was poured into water and extracted with chloroform.
The organic extracts were washed with brine and dried over
anhydrous Na2SO4. The solvent was removed by rotary evapo-
ration and the crude product was obtained as a yellow solid (0.57g,
40%). It was used directly in the next step without further purifi-
cation. dH(400MHz; CDCl3; Me4Si) 7.34 (d, J ¼ 4.9 Hz, 2H) 0.88
(6H, t, J ¼ 6.5 Hz), 1.25–1.40 (48H, m), 1.76 (4H, m), 2.87 (4H, t,
J ¼ 7.9 Hz), 7.09 (2H, dd, J1 ¼ 4.9 Hz, J2 ¼ 2.6 Hz), 7.16 (2H, d, J
¼ 2.6 Hz); dC(400MHz; CDCl3; Me4Si) ꢀ8.1, 14.4, 23.1, 29.1,
29.6, 31.5, 32.0, 32.1, 128.2, 129.6, 131, 133.3, 137.2, 140.9, 142.3.
Instruments
1H-NMR and C-NMR spectra were performed in a Bruker
13
€
ARX 400 spectrometer using solutions in CDCl3 and chemical
shifts were recorded in ppm units with TMS as the internal
standard. Elemental analyses were measured with EA1112 in
a CE Instrument. The UV-vis spectra were measured with a HP
Agilent 8453 UV-Vis. All GPC analyses were made using THF as
eluent and polystyrene standard as reference. Cyclic voltam-
metric waves were produced by using a Wonatech WMPG 1000
with a 0.1 M acetonitrile (substituted a nitrogen in 20 min)
solution containing tetrabutylammonium tetrafluoroborate
(TBABF4, Fluka 99.9%) at constant scan rate of 50 mV/s. A
three-electrode cell was used and silver/silver chloride [Ag in
0.1M KCl] was used as a reference electrode. The luminance of
devices was measured with a Spectra Scan PR-670. Current
density-voltage (J-V) characteristics of all polymer photovoltaic
cells were measured under the illumination of simulated solar
light with 100 mW/cm2 (AM 1.5 G) from an Oriel 1000W solar
General procedure of polymerization
To a mixture of Pd(PPh3)Cl2 (1.0 mol%) in DMF/THF (1/1)
moꢂnomers were added. The mixture was vigorously stirred at 85–
90 C for 48h under nitrogen. After the mixture was cooled to
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J. Mater. Chem., 2009, 19, 4938–4945 | 4943

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simulator. Electric data were recorded using a Keithley 236
source-measure unit and all characterizations were carried out in
the ambient atmosphere. The illumination intensity used was
calibrated by a standard Si photodiode detector from Bunkou-
keiki Co., Ltd. The incident photon-to-current conversion effi-
ciency (IPCE) was measured as a function of wavelength from
360 to 800 nm (PV measurement Inc.) equipped with a halogen
lamp as a light source, and calibration was performed using
a silicon reference photodiode. Thicknesses of the thin films were
measured using a KLA Tencor Alpha-step 500 surface profil-
ometer with an accuracy of 1 nm.
on the improvement of morphology are continued to enhance the
fill factor (FF).
Acknowledgements
This work was supported by the Korea Research Foundation
Grant funded by the Korean Government (MOEHRD, Basic
Research Promotion Fund) (KRF-2007-313-D00213). This work
was supported by New & Renewable Energy R & D Program
(2008-A011-0073) under the Korea Ministry of Knowledge
Economy (MKE).
Photovoltaic device fabrication
Composite solutions with polymers and PCBM were prepared
using 1,2-dichlorobenzene (DCB). The concentration was
controlled adequately in the range 1.0–2.0 wt%. For the
measurement of optical absorption spectra of PCBM, the solu-
tions were spun on pre-cleaned UV-grade silica substrates.
The polymer photovoltaic devices were fabricated with
a typical sandwich structure of ITO/PEDOT:PSS/active layer/
LiF/Al. The ITO coated glass substrates were cleaned through
a routine cleaning procedure, including sonication in detergent
followed by distilled water, acetone, and 2-propanol. A 45 nm
thick layer of PEDOT:PSS (Baytron P) was spin coated on
a cleaned ITO substrate after exposing the ITO surface to ozone
for 10 min. The PEDOT:PSS layer was baked on a hot plate at
120 ꢂC for 10 min. The active layer was spin-coated from the pre-
dissolved composite solution after filtering through 0.45 mm PP
syringe filters. The device structure was completed by depositing
a 200 nm Al cathode as the top electrode onto the polymer active
layer under 3 ꢃ 10ꢀ6 torr vacuum in a thermal evaporator.
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J. Mater. Chem., 2009, 19, 4938–4945 | 4945