Synthesis of novel dithienothiophene- and 2,7-carbazole-based conjugated polymers and H-bonded effects on electrochromic and photovoltaic properties

Article abstract of DOI:10.1002/pola.26336

Three kinds of dithienothiophene/carbazole-based conjugated polymers (P1-P3), which bear acid-protected and benzoic acid pendants in P2 and P3, respectively, were synthesized via Suzuki coupling reaction. Interestingly, P1-P3 exhibited reversible electrochromism during the oxidation processes of cyclic voltammogram studies, and P3 (with H-bonds) revealed the best electrochromic property with the most noticeable color change. According to powder X-ray diffraction (XRD) analysis, these polymers exhibited obvious diffraction features indicating bilayered packings between polymer backbones and π-π stacking between layers in the solid state. Compared with the XRD data of P2 (without H-bands), H-bonds of P3 induced a higher crystallinity in the small-angle region (corresponding to a higher ordered bilayered packings between polymer backbones), but with a similar crystallinity in the wide angle region indicating a comparable π-π stacking distance between layers. Moreover, based on the preliminary photovoltaic properties of PSC devices (P1-P3 blended individually with PCBM acceptor in the weight ratio of 1:1), P3 (with H-bonds) possessed the highest power conversion efficiency of 0.61% (with J _sc = 2.26 mA/cm², FF = 29.8%, and V_oc = 0.9 V). In contrast to P2 (without H-bands), the thermal stability, crystallinity, and electrochromic along with photovoltaic properties of P3 were generally enhanced due to its H-bonded effects.

Full text of DOI:10.1002/pola.26336

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Synthesis of Novel Dithienothiophene- and 2,7-Carbazole-Based
Conjugated Polymers and H-Bonded Effects on Electrochromic and
Photovoltaic Properties
1
1
1
2,3
Hsiao-Ping Fang, Jia-Wei Lin, I-Hung Chiang, Chih-Wei Chu,
1
1
Kung-Hwa Wei, Hong-Cheu Lin
1
Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu, Taiwan, Republic of China
Department of Photonics, National Chiao Tung University, Hsinchu, Taiwan, Republic of China
Research Center for Applied Sciences, Academia Sinica, Taipei, Taiwan, Republic of China
2
3
Correspondence to: C.-W. Chu (E-mail: gchu@gate.sinica.edu.tw) or H.-C. Lin (E-mail: linhc@mail.nctu.edu.tw)
Received 14 June 2012; accepted 5 August 2012; published online
DOI: 10.1002/pola.26336
ABSTRACT: Three kinds of dithienothiophene/carbazole-based
conjugated polymers (P1–P3), which bear acid-protected and
benzoic acid pendants in P2 and P3, respectively, were synthe-
sized via Suzuki coupling reaction. Interestingly, P1–P3 exhib-
ited reversible electrochromism during the oxidation processes
of cyclic voltammogram studies, and P3 (with H-bonds)
revealed the best electrochromic property with the most no-
ticeable color change. According to powder X-ray diffraction
angle region indicating a comparable p-p stacking distance
between layers. Moreover, based on the preliminary photovol-
taic properties of PSC devices (P1–P3 blended individually with
PCBM acceptor in the weight ratio of 1:1), P3 (with H-bonds)
possessed the highest power conversion efficiency of 0.61%
2
(with Jsc ¼ 2.26 mA/cm , FF ¼ 29.8%, and Voc ¼ 0.9 V). In con-
trast to P2 (without H-bands), the thermal stability, crystallinity,
and electrochromic along with photovoltaic properties of P3
were generally enhanced due to its H-bonded effects. VC 2012
(
XRD) analysis, these polymers exhibited obvious diffraction
features indicating bilayered packings between polymer back-
bones and p-p stacking between layers in the solid state. Com-
pared with the XRD data of P2 (without H-bands), H-bonds of
P3 induced a higher crystallinity in the small-angle region (cor-
responding to a higher ordered bilayered packings between
polymer backbones), but with a similar crystallinity in the wide
Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 000:
000–000, 2012
KEYWORDS: electrochromism; heterocyclic conjugated polymer;
solar cell; functionalization of polymers; supramolecular struc-
tures; WAXS
INTRODUCTION Novel materials are developed for organic
optoelectronic devices, such as polymeric solar cells (PSCs),
which is a popular research topic in recent decades, because
they are low cost and green materials for sustainable resour-
ces to reduce consumptions of fossil energy and nuclear
stabilities of polymers by longer conjugation lengths and
more rigid structures, novel heteroaromatic fused-ring deriv-
atives, including fused dithienothiophene and carbazole
units, are integrated into the polymer backbones. Carbazole
unit, which is well known to be a good electron-donating
1
7
power. In particularly, bulk heterojunction (BHJ) solar cells
moiety due to its fully aromatic structure, can improve
consisting of electron-donating conjugated polymers blended
with electron-accepting fullerenes are fabricated in solid thin
chemical stability in contrast to fluorene unit, and thus
poly(2,7-carbazole)s are attractive candidates for solar cells.
8
2
films. Up to now, regio-regular poly[2-methoxy-5-(3’,7’-
New carbazole-based copolymers utilized in BHJ solar cells
3
9
dimethyloctyloxy)-p-phenylenevinylene] (MDMO-PPV) and
were also reported to achieve a PCE value of 4.3%. Accord-
4
poly(3-hexylthiophene) (P3HT) as electron donors blended
ing to all these results, the development of new polymers
based on carbazole units should have interesting features for
the photovoltaic applications. Dithieno[3,2-b:2’,3’-d]thiophene
(DTT) unit is a sulfur-rich (three S atoms) and electron-rich
building block to make the polymer backbones more rigid
and coplanar, and thus to have longer p-conjugation and
with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as
an electron acceptor approached high power conversion effi-
ciency (PCE) values of 5.0% in PSCs. More recently, the PCE
values of BHJ solar cells using new low-band gap conjugated
5
,6
polymers have reached 6 to 8%. The PCE values of BHJ so-
lar cells were affected by, for example, the energy band gaps
of polymers, which is related to the chemical structure of
the conjugated polymers. In order to improve the thermal
1
0,11(a)
absorption lengths along with narrower band gaps.
Due to the limiting intramolecular rotation in the fused-ring
structures, such as DTT, the p-orbital overlaps in conjugated
V
C
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SCHEME 1 Synthetic routes of monomers M1–M3.
molecules could be maximized to enhance intermolecular
reported H-bonded assemblies of perylene bisimide and
melamine derivatives. In addition, El-ghayoury et al.
reported a PCE value of 0.39% for PSCs by utilizing a
1
1(b,c)
charge transports.
In order to improve the solubility
of poly(DTT), alkyl-substituted pendants were incorporated
into the polymer backbones, which can be applied to or-
H-bonded polymer containing oligo(p-phenylene vinyene)
1
2-15
15(b)
16(c)
ganic solar cells
and field effect transistors (FETs).
and ureido-pyrimidinone units.
Because of several
Hydrogen bonds (H-bonds) are ideal noncovalent interac-
tions to form self-assembled architectures due to their se-
lectivity and directionality. A numerous advantages of H-
bonded polymers, such as stronger light absorptions, lower
HOMO levels, higher Voc values, higher hole mobilities, and
advantages in polymers, including low cost, easy processing,
and tunable chemical properties, the conjugated polymers
consisting of different heteroaromatic rings, such as thio-
phene and carbazole, exhibit an electrochromic behavior as
well as photovoltaic properties. Based on this concept, two
different moieties, i.e., carbazole (M1 and M2) and fused
dithienothiophene (M3), were applied into electron-donor
monomers to synthesize fused-dithienothiophene-based
polymers P1–P3, where P1 is a model polymer and the
acid pendants of P3 are protected in P2 to compare the
1
6
higher crystallinities, were utilized for organic solar cells.
Therefore, great efforts have been taken toward the prepa-
ration and characterization of photo- and electroactive non-
covalent assemblies based on hydrogen bonds (H-bonds).
1
6(a,b)
16(c,d)
W u¨ rthner,
El-ghayoury et al., and Jonkheijm et al.
2
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SCHEME 2 Synthetic routes of polymers P1–P3.
7
H-bonded effects on their electrochromic behavior as well
as photovoltaic properties.
carbazole (4), and 3,5-didecanyldithieno[3,2-b:2’3’-d] thio-
1
7
phene) (5) were synthesized according to the literature
procedures. The synthetic routes of monomers 1–3 and poly-
mers P1–P3 are shown in Schemes 1 and 2, and the syn-
thetic procedures of their intermediates were described.
Chemicals and solvents were reagent grades and purchased
from Aldrich, ACROS, TCI, and Lancaster Chemical Co.
EXPERIMENTAL
Materials
All chemicals and solvents were used as received; 2,7-
7
dibromo-9-(heptadecan-9-yl)-9H-carbazole (1), 2,7-dibromo-
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Toluene, tetrahydrofuran, and diethyl ether were distilled to
keep anhydrous before use.
was prepared by spin coating from composite solutions of
P1–P3: PCBM (w/w ¼ 1:1) in dichlorobenzene (10 mg/mL)
by a spin rate of 500 to 800 rpm on the top of the PEDOT:
PSS layer. The PVC devices were completed by deposition
with a back electrode consisting of Ca (ca. 50 nm) and alu-
minum (100 nm). The film thicknesses were measured by a
profilometer (Dektak3, Veeco/Sloan Instruments Inc.). For
PVC measurements, I–V curves were recorded under a solar
Measurements
1
H NMR spectra were recorded on a Varian Unity 300 MHz
spectrometer using CDCl solvents. Elemental analyses were
3
performed on a HERAEUS CHN-OS RAPID elemental analyzer.
Transition temperatures were determined by differential
scanning calorimetry (DSC, Perkin-Elmer Pyris 7) with a
2
simulator with AM 1.5 irradiation (at 100 mW/cm ). A 300
ꢀ
heating and cooling rate of 10 C/min. Thermogravimetric
W xenon lamp (Oriel, #6258) with AM 1.5 filter (Oriel,
#81080 kit) was used as the white light source, and the op-
analyses (TGA) were conducted with a TA instrument Q500
ꢀ
2
at a heating rate of 10 C/min under nitrogen. Gel permea-
tical power at the sample was 100 mW/cm detected by
tion chromatography (GPC) analyses were conducted on a
Waters 1515 separation module using polystyrene as a
standard and tetrahydrofuran (THF) as an eluent. UV-Vis
absorption and photoluminescence (PL) spectra were
recorded in dilute THF solutions (10 M) on a HP G1103A
and Hitachi F-4500 spectrophotometer, respectively. Solid
films of UV-Vis and PL measurements were spin-coated on a
quartz substrate from THF solutions with a concentration of
Oriel thermopile 71964. The I–V characteristics were meas-
ured using a CHI 650B potentiostat/galvanostat. The external
quantum efficiency (EQE) was measured using a CHI 650B
coupled with Oriel Cornerstone 260 monochromator. All
PVC devices were prepared and measured under ambient
conditions.
ꢁ
6
Synthesis and Characterization
Materials
10 mg/mL. Cyclic voltammetry (CV) measurements were
9
-(Heptadecan-9-yl)-2,7-bis(4,4,5,5-tetramethyl-1,3,2-
performed using a BAS 100 electrochemical analyzer with a
standard three-electrode electrochemical cell in a 0.1 M tet-
dioxaborolan-2-yl)-9H-carbazole (M1). To a solution of
7
compound 1 (2,7-dibromo-9-(heptadecan-9-yl)-9H-carbazole)
rabutylammonium hexafluorophosphate (TBAPF ) solution
6
(3.62 g, 6.42 mmol) in 150 mL of dry THF, n-butyllithium (2.5
(
in dimethylformamide, DMF) at room temperature with a
M solution in hexane, 5.65 mL, 2.2 eq) was added dropwise,
scanning rate of 100 mV/s. In each case, a carbon working
electrode coated with a thin layer of these polymers, a plati-
num wire as the counter electrode, and a silver wire as the
quasi-reference electrode were used. Ag/AgCl (3 M KCl) elec-
trode was served as a reference electrode for all potentials
quoted herein. During the CV measurements, the solutions
were purged with nitrogen for 30 s, and the redox couple
ꢀ
and then stirred to react at ꢁ78 C under nitrogen. After reac-
ꢀ
tion for 2 h at ꢁ78 C, 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-
dioxaborolane (3.15 mL, 2.4 eq) was added carefully to the
ꢀ
mixture solution at ꢁ78 C and then the mixture was allowed
to warm up to react at room temperature overnight. The final
solution was acidified with 100 mL of 10% HCl solution and
stirred for 45 min at room temperature and the final solution
was extracted with diethyl ether. The organic layer was dried
over magnesium sulfate, and the solvent was evaporated. The
solvent was removed under reduced pressure, and the residue
was purified by recrystallization in methanol/acetone (ca.
þ
ferrocene/ferrocenium ion (Fc/Fc ) was used as an external
standard. The corresponding HOMO levels in polymer solu-
tions of P1–P3 could be calculated from Eox/onset values of
the electrochemical experiments (but no reduction curves,
i.e., no Ered/onset values and LUMO levels, were obtained in
the CV measurements). Each onset potential in the CV meas-
urements was defined by the intersection of two tangents
drawn at the rising current and background current. The
1
0:1) to obtain the final product as a white crystal (2.69 g,
1
yield: 64 %). H NMR(CDCl , 300 MHz): d 8.12 (d, J ¼ 8.0 Hz,
3
2
H), 8.02 (s, 1H), 7.89 (s, 1H), 7.66 (d, J ¼ 8.1 Hz, 2H), 4.73–
1
8
4.66 (m, 1H), 2.35–2.30 (m, 2H), 1.98–1.90 (m, 2H), 1.39–1.12
LUMO value of PCBM was in accordance with the literature
data. Film thickness and morphology were determined using
a Veeco Nanoscope DI 3100 AFM microscope operating in
tapping mode. Synchrotron powder X-ray diffraction (XRD)
measurements were performed at beamline BL13A of the
National Synchrotron Radiation Research Center (NSRRC),
Taiwan, where the wavelength of X-ray was 1.026503 Å. The
photovoltaic cell (PVC) device structure used in this study
was a sandwich configuration of ITO/PEDOT:PSS/active
layer/Ca/Al, where the active layer was made of electron
donor polymers P1–P3 mixed with electron acceptor [6,6]-
phenyl C61 butyric acid methyl ester (PCBM) in a weight ra-
tio1:1. The PVC devices were fabricated according to the pro-
cedures similar to those of EL devices. An ITO-coated glass
substrate was precleaned and treated with oxygen plasma
before use. A thin layer (ca. 50 nm) of PEDOT:PSS was spin-
þ
(
m, 48H), 0.82 (t, J ¼ 7.0 Hz, 6H). MS (FAB): m/z [M ] 657;
þ
calcd. m/z [M ] 657. Anal. Calcd.: C, 74.89; H, 9.96; N, 2.13.
Found: C, 74.42; H, 9.68; N, 2.26.
tert-Butyl 4-hydroxybenzoate (2). Seven grams of 4-
hydroxybenzoic acid (50.7 mmol) and 4-dimethylaminopyri-
dine (0.62 g, 0.1 eq) were dissolved in 60 mL of dry THF
and stirred in a two-necked flask. Then, 2-methylpropan-2-ol
(60 mL, 2-methylpropan-2-ol) and N,N’-dicyclohexylcarbodii-
mide (12.6 g, 1.2 eq) were added sequentially. The mixture
was purged with nitrogen and vigorously stirred overnight
at room temperature. Water was added after reaction, and
the reaction mixture was extracted with dichloromethane.
Consequently, the organic layer was separated and dried
with magnesium sulfate. Solvent was removed under vac-
uum, and the crude product was purified by chromatography
using hexane: ethyl acetate (4:1) as eluent. Subsequently, the
pure compound was obtained as a white powder. Yield: 3.95
ꢀ
coated on an ITO substrate and heated at 130 C for 1 h.
Subsequently, the preliminary active layer (ca. 100–160 nm)
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1
g (40 %). H NMR (CDCl , 300 MHz): d 7.87 (d, J ¼ 8.7 Hz,
as eluent. Subsequently, the pure compound was obtained as
3
1
2
H), 6.84 (d, J ¼ 8.5 Hz, 2H), 6.44 (s, 1H), 1.59 (s, 9H).
tert-Butyl 4-(6-bromohexyloxy) benzoate (3). tert-Butyl
-hydroxybenzoate (3.95 g, 20.3 mmol), K CO (8.43 g, 3
eq), and KI (0.2 g, 0.06 eq) were dissolved in acetone (150
mL) and stirred in a two-necked flask. The mixture was
purged with nitrogen and refluxed overnight. Water was
added after reaction, and the reaction mixture was extracted
with dichloromethane. Consequently, the organic layer was
separated and dried with magnesium sulfate. Solvent was
removed under vacuum, and the crude product was purified
by chromatography using hexane: ethyl acetate (30:1) as elu-
ent. Subsequently, the pure compound was obtained as a col-
orless oil. Yield: 6.48 g (89%). H NMR(CDCl , 300 MHz): d
.89 (d, J ¼ 8.7 Hz, 2H), 6.86 (d, J ¼ 8.5 Hz, 2H), 3.98 (t, J ¼
.4 Hz, 2H), 3.40 (t, J ¼ 6.8 Hz, 2H), 1.90–1.77 (m, 4H), 1.56
s, 9H), 1.51–1.46 (m, 4H).
a yellow powder. Yield: 2.12 g (86 %). H NMR (CDCl
3
, 300
MHz): d 7.35 (dd, J ¼ 5.1, 1.0 Hz, 2H), 7.18 (t, J ¼ 1.0 Hz,
2
H), 7.09 (dd, J ¼ 5.1, 3.6 Hz, 2H), 2.91 (t, J ¼ 8.1 Hz, 4H),
4
2
3
1.67–1.61 (m, 4H), 1.42–1.27 (m, 12H), 0.94–0.89 (m, 6H).
2,6-Bis(2’-bromothien-5’-yl) 3,5-dihexyldithieno[3,2-b:2’3’-d]
thiophene (M3). About 2.12 g of 2,6-dithienyl-3,5-dihexyldi-
thieno[3,2-b:2’3’-d]thiophene (4.01 mmol) and NBS (2.1 g, 2.1
eq) were dissolved in DMF (20 mL) and stirred in a flask.
The mixture was vigorously stirred overnight at room temper-
ature. Water was added after reaction, and the reaction mix-
ture was extracted with ethyl acetate. Consequently, the or-
ganic layer was separated and dried with magnesium sulfate.
Solvent was removed under vacuum, and the crude product
was purified by chromatography using hexane as eluent. Sub-
1
3
7
6
(
sequently, the pure compound was obtained as a pale yellow
1
powder. Yield: 2.0 g (73%). H NMR(CDCl , 300 MHz):d 7.04
3
(
4
6
d, J ¼ 3.5 Hz, 2H), 6.9 (d, J ¼ 3.5 Hz, 2H), 2.85 (t, J ¼ 7.3 Hz,
tert-Butyl 4-(6-(2,7-dibromo-9H-carbazol-9-yl)hexyloxy)-
H), 1.75-1.73 (m, 4H), 1.41–1.26 (m, 12H), 0.92–0.89 (m,
benzoate (M2). NaH (0.36 g, 1.8 eq) and 2,7-dibromo-car-
þ
þ
H). MS (EI): m/z [M ] 685; calcd. m/z [M ] 685. Anal.
8
bazole (2.71 g, 8.34 mmol) were dissolved in THF (10 mL)
Calcd.: C, 48.98; H, 4.40. Found: C, 48.89; H, 4.63.
and stirred in a two-necked flask and refluxed for 1 h. The
solution of tert-butyl-4-(6-bromohexyloxy)benzoate (3.7 g,
1
9
General Synthetic Procedures of Polymers P1–P3
1
.3 eq) in THF (10 mL) was added dropwise and refluxed
The synthetic routes of polymers P1–P3 are shown in Scheme
2. All of the polymerization procedures were carried out
through the palladium (0)-catalyzed Suzuki coupling reactions.
In a 25 mL two-necked flask, 0.5 eq of M1 and M2 with a
molar ratio of M2:M3 ¼ 0:0.5(P1) and 0.05:0.45 (P2, P3)
were added into 5 mL of anhydrous toluene. The Pd (0) com-
plex, tetrakis(triphenylphosphine)palladium (1 mol %), was
transferred into the mixture under dry environment. Then, 2
M aqueous potassium carbonate and a phase transfer catalyst,
that is, Aliquat 336 (several drops), were subsequently trans-
ferred to the previous mixture via dropping funnel. The reac-
overnight. Water was added after reaction, and the reaction
mixture was extracted with dichloromethane. Consequently,
the organic layer was separated and dried with magnesium
sulfate. Solvent was removed under vacuum, and the crude
product was purified by chromatography using hexane: ethyl
acetate (1:7) as eluent. Subsequently, the pure compound
was obtained as a colorless oil. Yield: 2.90 g (60 %).
1
H
NMR(CDCl , 300 MHz): d 7.90 (m, 4H), 7.53 (d, J ¼ 1.2 Hz,
3
2
4
H), 7.34 (dd, J ¼ 9.0, 1.6 Hz, 2H), 6.84 (d, J ¼ 8.9 Hz, 2H),
.22 (t, J ¼ 7.2 Hz, 2H), 3.96 (t, J ¼ 6.29 Hz, 2H), 1.91–1.75
þ
ꢀ
(m, 4H), 1.59–1.44 (m, 13H). MS (EI): m/z [M ] 601; calcd
tion mixture was stirred at 90 C for 2 days, and then both
þ
m/z [M ] 601. Anal. Calcd.: C, 57.92; H, 5.28; N, 2.33. Found:
C, 57.78; H, 5.04; N, 2.52.
excess amounts of end-cappers (i.e., iodobenzene and phenyl-
boronic acid) were correspondingly dissolved in 1 mL of anhy-
drous toluene and reacted for 4 h. The reaction mixture was
2
,6-Dibromo-3,5-dihexyldithieno[3,2-b:2’3’-d]thiophene (6).
ꢀ
cooled to 40 C and added slowly into a vigorously stirred mix-
1
7
Compound
5 (3,5-dihexyldithieno[3,2-b:2’3’-d]thiophene)
ture of methanol/water (10:1). The polymers were collected
by filtration and reprecipitation from methanol. The crude
polymers were further purified by washing with acetone for 3
days in a Soxhlet apparatus to remove oligomers and catalyst
residues. The chloroform fractions (350–400 mL) were
reduced to 40–50 mL under reduced pressure, and precipi-
tated in acetone along with air-dried overnight finally.
(
1.96 g, 5.38 mmol) and NBS (2.39 g, 2.5 eq) were dissolved
in 50 mL of DMF. The resulting solution was stirred to react
overnight at room temperature under nitrogen. Water (50
mL) was then added, and the organic phase was extracted
with ethyl acetate (100 mL) twice, washed with water, and
dried with magnesium sulfate. After that, the solvent was
removed under reduced pressure to obtain the product. The
crude product was purified by column chromatography with
P1
1
hexane to obtain a pale yellow oil (2.51 g). Yield: 89 %. H
Following the general polymerization procedure, M1 (0.5
NMR(CDCl , 300 MHz): d 2.72 (t, J ¼ 7.5 Hz, 4H), 1.75–1.65
equiv) and M3 (0.5 equiv) were used in this polymerization
3
1
(
m, 4H), 1.42–1.32 (m, 12H), 0.92–0.88 (m, 6H).
to obtain a red powder. Yield: 67%. H NMR (ppm, CDCl
3
): d
8
.06–7.23 (br, 10H), 4.62 (br, 1H), 2.99 (br, 4H), 2.5-0.5 (br).
Anal. Calcd.: C, 73.57; H, 7.69; N, 1.51. Found: C, 73.39; H,
.64; N, 1.35.
2
(
(
,6-Dithienyl-3,5-dihexyldithieno[3,2-b:2’3’-d]thiophene
7). Compound 5, 3,5-dihexyldithieno[3,2-b:2’3’-d]thiophene
2.46 g, 4.71 mmol), Pd(PPh (0.32 g, 0.06 eq), and tribu-
7
3 4
)
tyl(thiophen-2-yl)stannane (3.29 mL, 2.2 eq) were dissolved
in toluene (25 mL) and stirred in a two-necked flask to
reflux for 12 h. Solvent was removed under vacuum, and the
crude product was purified by chromatography using hexane
P2
Following the general polymerization procedure, M1 (0.5
equiv), M2 (0.05 equiv), and M3 (0.45 equiv) were used in
this polymerization to obtain a red powder. Yield: 67%.
1
H
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TABLE 1 Molecular Weights, Yields, and Thermal Data of
Polymers P1–P3
The thermal stabilities and phase transition temperatures of
polymers P1–P3 were characterized by thermogravimetric
analyses (TGA) and differential scanning calorimetry (DSC)
measurements under nitrogen atmosphere, and the thermal
d
e
T
g
ꢀ
T
d
ꢀ
Yield
(%)
a
b
w
c
Polymer
M
n
M
PDI
( C)
( C)
decomposition temperatures (T ) and glass transition tem-
d
peratures (T ) are summarized in Table 1. It is apparent that
all polymers exhibited good thermal stabilities, which
P1
P2
P3
a
16,600
19,200
18,200
19,700
53,100
50,100
1.2
2.7
2.7
130
138
146
421
380
428
71.4
79.5
70.5
g
showed less than 5% weight loss upon heating to 380 to
ꢀ
4
28 C. Regarding DSC experiments, samples (weighted 1–5
Number average molecular weight.
Weight average molecular weight.
mg) sealed in an aluminum pan were operated at 30 to 250
b
c
ꢀ
ꢀ
C under N2 atmosphere with a scan rate of 10 C/min.
Polydispersity indices (PDI ¼ M
w n
/M ).
d
e
These polymers showed glass transition (T ) temperatures at
g
Glass transition temperature.
ꢀ
Decomposition temperature at 5% weight loss.
130, 138, and 146 C for P1, P2, and P3, respectively. The
T and T values of P3 are higher than those of P2, which
d
g
were attributed to the rigid polymer networks of P3 formed
NMR (ppm, CDCl ): d 8.23-7.22 (br, 10H), 4.644 (br, 1H),
3
by H-bonds (due to its carboxyl group).
3.947 (br, 1H), 3.020 (br, 4H), 2.50–0.66 (br). Anal. Calcd.: C,
75.28; H, 7.49; N, 2.04. Found: C, 75.39; H, 7.44; N, 2.15.
Optical Properties
The optical absorption spectra of polymers P1–P3 in THF
P3
ꢁ
6
solutions (10 M) and solid films are shown in Figure 1,
and their photophysical properties are demonstrated in Ta-
ble 2. As can be seen, the absorption spectra of polymers
P2 (200 mg) was dissolved in 20 mL toluene, and excess
HCl solution was added slowly. The mixture was vigorously
stirred for 2 days at 80 C. The polymers were collected by
ꢀ
following the general polymerization procedure to gain a red
1
powder. Yield: 81%. H NMR (ppm, CDCl ): d 8.23–7.22 (br,
3
1
0
0H), 4.654 (br, 1H), 3.950 (br, 1H), 3.025 (br, 4H), 2.50–
.66 (br). Anal. Calcd.: C, 74.84; H, 7.20; N, 2.13. Found: C,
74.79; H, 7.44; N, 2.15.
RESULTS AND DISCUSSION
Syntheses and Chemical Characterization
As outlined in Scheme 1, two monomers M1 and M2 based
on carbazole moieties were prepared from 2,7-dibromo-9-
(
heptadecan-9-yl)-9H-carbazole (1) and 2,7-dibromo-carba-
7
zole (4) moieties. In addition, M3 based on dithienothio-
phene was prepared from 3,5-didecanyldithieno[3,2-b:2’3’-
1
7
d]thiophene (5) using a reduction procedure and followed
2
0
by dibromination, which were described by Coppo et al.
The electron-donating unit of compound 5 was prepared
according to the literature procedures. Monomers M1–M3
1
13
were satisfactorily characterized by H NMR, C NMR, MS
spectroscopies, and elemental analyses. Polymers P1–P3
were prepared successfully via Suzuki coupling, where P1
was produced by M1 and M3; P2 was synthesized by the
copolymerization of monomer M1 with M2 and M3; and P3
was prepared by the deprotection of acid in P2. The syn-
thetic procedures towards polymers P1–P3 are outlined in
Scheme 2. Most polymers are partly soluble in organic sol-
vents, such as chloroform, THF, and chlorobenzene at room
temperature, and completely soluble in high boiling point
solvents (e.g., chlorobenzene) at high temperatures. The
yields and molecular weights of polymers P1–P3 determined
by gel permeation chromatography (GPC) against polysty-
rene standards in THF are summarized in Table 1. These
results show that considerable molecular weights with high
yields (50–81% after Soxhlet extractions) were obtained in
these copolymers, where the weight-average molecular
FIGURE 1 Normalized optical absorption spectra of polymers
ꢁ6
weights (M ) of 19,700 to 53,100 with polydispersity indices
P1–P3 in (a) solutions (THF) (10 M) and (b) solid films (spin-
w
(
PDI ¼ M /M ) of 1.2 to 2.7 were obtained.
coating from THF solutions).
w
n
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solutions. As shown in Table 2, the optical band gaps (Eg,opt)
of 2.24 to 2.25 eV in polymers P1–P3 could be determined
by the cut-offs of the absorption spectra in solid films.
The PL emission spectra of the polymers in solutions and
solid films were illustrated in Figure 2. The photolumines-
cence (PL) spectra of polymers P1–P3 in THF solutions and
solid films were excited at incident wavelengths of 445 and
465 nm, respectively. Interestingly, in comparison with poly-
mer P2 in Figure 2, the PL spectra of P3 containing carbox-
ylic acid moieties were almost quenched in solution and com-
pletely quenched in solid film [see Fig. 2(a,b), respectively].
The PL emission of P3 containing carboxylic acid moieties
(
COOH) is quenched because its dimeric H-bonded structure
is formed to shorten the distance between chromophores and
to result in the stacking phenomena to quench the PL emis-
sion. In other words, the PL quenching phenomena of H-
bonds in P3 induced by the carboxylic groups might stem
from the intersystem crossing from the photo-excited singlet
state to the triplet one, where both intramolecular (in solu-
tions) and intermolecular (in solid films) energy transfers
along the conjugated backbones might occur. The correspond-
ing optical properties of these polymers in solid films, includ-
ing the broad and strong optical absorptions, proposed their
potential applications in photovoltaic cells described below.
Electrochemical Properties
The electronic states, that is, highest occupied molecular or-
bital (HOMO) and lowest unoccupied molecular orbital
(
LUMO) levels, of the polymers were investigated by cyclic
voltammetry (CV) measurements in order to understand the
charge injection processes in these polymers and their PSC
devices. The oxidation cyclic voltammograms of P1–P3 in
solid films and the corresponding electrochromic photos
FIGURE 2 Normalized photoluminescence (PL) spectra of poly-
ꢁ6
mers P1-P3 in (a) solutions (THF) (10 M), and (b) solid films
spin-coating from THF solutions).
(
with a distinct change from orange to black) of P3 are dis-
(
played in Figure 3(a), where the electrochromic activities in
P1 and P2 were not noticeable to be included due to the
lack of H-bonds. The formal potentials and HOMO energy
levels (estimated the average oxidation potentials from the
electrochemical measurements) are summarized in Table 2.
As shown in Figure 3(a), polymers P1–P3 showed one
quasi-reversible oxidation process but no detectable reduc-
tion behavior. Therefore, the HOMO energy levels of P1–P3
can be decided by their quasi-reversible oxidation peaks
P1–P3 covered broad wavelength ranges (300–600 nm) for
both solutions and solid films. In addition, P1–P3 possessed
similar maximum absorption wavelengths of 445 and 465 nm
in THF solutions and solid films, respectively. Due to the p-p
stacking of these polymer chains in solids, the maximum
absorption wavelength (465 nm) of P1–P3 in solid films was
red-shifted about 20 nm in contrast to that (445 nm) of their
TABLE 2 Photophysical Data in THF Solutions and Solid Films, Optical Band Gaps, Electrochemical Potentials, Energy Levels, and
Band Gap Energies of Polymers P1–P3
a
b
b
d
k
abs, Sol
k
abs, Film
k
PL, Film
E
1/2
E
HOMO
e
E
LUMO
f
E
g,opt
g
Polymer
(nm)
(nm)
(nm)
(ox)
(eV)
(eV)
(eV)
P1
P2
P3
a
445
445
445
465
465
465
546
543
0.89
0.91
0.91
ꢁ5.58
ꢁ5.60
ꢁ5.60
ꢁ3.34
ꢁ3.35
ꢁ3.35
2.24
2.25
2.25
c
–
e
The absorption spectra were recorded in dilute THF solution at room
temperature.
E
HOMO ¼ [ꢁ(E1/2 ꢁ 0.11) ꢁ 4.8] eV where 0.11 V is the value for ferro-
cene versus Ag/Agþ and 4.8 eV is the energy level of ferrocene below
b
The absorption and PL films were spin-coated from 10 mg/1 mL THF
the vacuum.
f
solution.
LUMO ¼ HOMO ꢁ Eg,opt.
c
g
PL peaks were not detectable due to the PL quenching behavior.
Optical band gaps were estimated from the absorption spectra in solid
d
E
1/2 was the average value of oxidation.
films by using the equation of E
g
¼ 1240/kedge
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correspondingly. The moderate onset oxidation potentials of
P1–P3 occurred between 0.6 and 1.2 V from which the esti-
mated HOMO levels of ꢁ5.58 to ꢁ5.60 eV were acquired
2
1,22
according to the following equation:
E
¼
HOMO/LUMO
[ꢁ(E
ꢁ E
ꢁ 4.8] eV,
onset (vs Ag/AgCl)
onset (Fc/Fc
þ
vs Ag/AgCl)
where 4.8 eV is the energy level of ferrocene below the vac-
uum level and Eonset (Fc/Fc
¼ 0.11 eV. Due to
þ
vs Ag/AgCl)
the lack of their reduction peaks, the LUMO energy levels of
P1–P3 can not be determined, but the LUMO levels can be
elucidated by subtracting the optical band gaps (Eg,opt)
from the HOMO energy levels of P1–P3. The electrochemical
reductions of polymers P1–P3 showed LUMO energy levels
at about ꢁ3.34 to ꢁ3.35 eV, which represent to possess high
electron affinities and also make these polymers suitable
donors for electron injection and transporting to PCBM
acceptor (with 0.4 eV offsets in LUMO levels regarding PCBM
2
3
with a LUMO level of ꢁ3.75 eV, as shown in Fig. 4) for the
2
4
bulk heterojunction polymer solar cell devices.
As the
potentials of P1–P3 were gradually increased to þ1.3 to 1.5
V in Figure 3(a), due to the higher stability and crystallinity
induced by H-bonds, only the electrochromic color of P3
changed noticeably from orange (in the neutral state) to
black (in the oxidation state) and the color change could be
easily detected by naked eye. The absorption spectra (at var-
ious applied potentials), and optical feature images along
with CIE chromaticity diagram of P3 film under neutraliza-
tion (0.0 V) and oxidation (1.3 V) states are demonstrated in
Figure 3(b,c), where P3 illustrated a hypsochromic absorp-
tion and an enhanced absorption in the range of 500 to 800
nm during the oxidation process (from 0.0 to 1.3 V). There-
fore, P3 is a good candidate for the electrochromic applica-
tion due to its distinct color change with easy processibility
in different solvents (in DCM, THF, and CHCl3).
X-Ray Diffraction (XRD) Analyses
As shown in Figure 5(a,b), powder X-ray diffraction (XRD)
patterns of polymers P1–P3 were acquired to investigate the
molecular organization and morphological change. The
FIGURE 3 (a) Cyclic voltammograms of polymers P1–P3 (in
solid films) at a scan rate of 100 mV/s, (b) absorption spectra
and optical images of P3 on ITO at various applied potentials
(
0 V–1.3 V), (c) CIE chromaticity diagram of P3 at ‘‘off’’ (0 V)
and ‘‘on’’ (þ1.3 V) states.
FIGURE 4 Energy band diagrams with HOMO/LUMO levels of do-
nor polymers P1–P3 in relation to the work functions of ITO and Al.
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FIGURE 5 Powder X-ray diffraction (XRD) spectra of (a) P1 and (b) P2–P3; and schematic representations of proposed three-dimen-
sional layered and p-p stacked arrangements of (c) P1 and (d) P2 in their XRD measurements (P3 is similar to P2).
measurements were proceeded on drop-cast films prepared
from 0.5 wt % solutions in THF after the thermal treatment
of about 150 C for 10 min and were then cooled to room
temperature. P1–P3 after thermal annealing exhibited sub-
stantially a primary diffraction feature with a angle at 2h
tic layers can be induced by the formation of supramolecular
26(b,c)
structures with highly ordered H-bonds.
The possible
ꢀ
packing motifs of polymers P1–P3 in the XRD measurements
with three-dimensional layered and p-p stacked arrangements
are represented in Figure 5(c,d), which show a model that the
alkyl side chains stack as bilayered packings within the same
layer. On the other hand, the reflections in the wide angle
region (corresponding to 3.9–4.5Å) are related to the p-p
ꢀ
¼
2.9 (corresponding to a large d-spacing value of 20 Å for
ꢀ
P1) and 2h ¼2.8 (corresponding to a large d-spacing value
of 21 Å for P2–P3), which were assigned to a distance
between the conjugated backbones separated by the long
side chains as reported for other similar p-conjugated poly-
2
7,28
stacked distance between the polymer layers,
which have
the similar wide-angle d-spacing values and XRD intensities in
all polymers P1–P3 to show their almost fixed p-p stacking
distances (ca. 3.9–4.5Å) with comparable crystallinities. The
diffraction features of polymers P1–P3 were often observed in
2
5
mers with long pendants. Moreover, a much stronger (100)
XRD characteristic peak of P3 (with H-bonds) was observed
in contrast to P2 (without H-bonds). The XRD data demon-
strate that P3 possessed a larger crystallinity than P2 which
was enhanced by the H-bonded interactions. In our previous
studies, the crystallinities of the supramolecular polymers
were much improved due to the presence of hydrogen
bonds, which enhanced the self-assembled behavior and thus
2
9
the XRD patterns of the p-conjugated polymers. Overall, the
proposed model can explain the possible structural arrange-
ments of the polymer chains in P1–P3, and the highest crystal-
linity of P3 in the small and large angle regions of XRD pattern
(corresponding to the bilayered packings between polymer
backbones and the p-p stacked distance between the polymer
layers, respectively) was induced by H-bonds.
2
6(a)
to induce higher PCE values of H-bonded polymers.
Moreover, the higher liquid crystalline arrangements of smec-
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FIGURE 6 AFM images obtained from solid films of P1–P3/PCBM (1:1 w/w).
2
Morphology
of 100 mW/cm . The preliminarily obtained properties are
summarized in Table 3, and the typical I–V characteristics of
all PSC devices are shown in Figure 7. Under monochromatic
illumination, the power conversion efficiency (PCE) values of
0.44 to 0.61% were obtained for the PSC devices composed
The AFM topographies of polymer blends (P1–P3: PCBM ¼
1:1 w/w) were investigated by the casting films of dichloro-
benzene solutions as shown in Figure 6, where the images
were obtained by the tapping mode. Furthermore, the solid
films of blended copolymers P1 and P3 showed a similar
surface roughness with moderate root mean square (RMS)
values about 18 nm. In comparison with blended polymers
P1 and P3, blended polymer P2 revealed a rather uneven
surface with a RMS roughness of 58.6 nm, which was attrib-
uted to the aggregation of polymer chains due to their poor
solubilities and lack of H-bonds, and thus to reduce the
interface between donor (P2) and acceptor (PCBM) signifi-
cantly. As shown in Table 3, owing to the unfavorable mor-
phology for charge transport offered by poor solubility of
P2, the PSC device based on P2 gave relatively low current
densities (I ) and thus the lowest PCE value. Therefore,
sc
the PCE values of blended polymers (P1–P3: PCBM ¼ 1:1
w/w) are inversely proportional to their RMS roughnesses
in AFM.
Photovoltaic Properties
The motivation for the design and syntheses of the conju-
gated polymers is to look for new polymers for the applica-
tion of PSCs. To investigate the potential use of polymers
P1–P3 in PSCs, bulk heterojunction devices were fabricated
from an active layer in which polymers P1–P3 were blended
with PCBM. PSC devices with a configuration of ITO/PEDOT:
PSS/P1–P3: PCBM (w/w ¼ 1:1)/Ca/Al were fabricated. The
PSC devices were measured under simulated AM 1.5 solar
illumination for a calibrated solar simulator with an intensity
TABLE 3 Photovoltaic Properties of PSC Devices Containing an
Active Layer of Polymer: PCBM (1:1 w/w) with a Device
a
Configuration of ITO/PEDOT: PSS/Polymer: PCBM/Ca/Al
Active Layer (Polymer:
V
oc
J
sc
FF
PCE
(%)
2
PCBM ¼ 1:1 w/w)
(V)
(mA/cm )
(%)
P1
P2
P3
a
0.73
0.71
0.90
2.27
2.02
2.26
2
0.33
0.31
0.31
0.54
0.44
0.61
FIGURE 7 I–V curves (under simulated AM 1.5 solar irradiation)
of solar cells with an active layer of P1–P3: PCBM measured by
(a) illuminated current and (b) dark current.
Measured under AM 1.5 irradiation, 100 mW/cm .
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of polymers P1–P3 with current density (J ), open circuit
006-MY2, NSC 99-2221-E-009-008-MY2, and National Chiao
Tung University through 97W807 are acknowledged. The pow-
der XRD measurements are supported by beamline BL13A
(charged by Ming-Tao Lee) of the National Synchrotron Radia-
tion Research Center (NSRRC), in Taiwan.
sc
voltage (Voc), and fill factor (FF) in the range of 2.02 to 2.27
2
mA/cm , 0.71 to 0.90 V, and 31 to 33%, respectively. The
photovoltaic properties of the PSC devices containing fused
dithienothiophene-based polymers P1–P3 were dependent
on the solubility and film-forming quality of the polymers.
As mentioned in our introductory content, some donor poly-
mers containing dithienothiophene units (designed and pre-
REFERENCES AND NOTES
1
2(b)
12(a)
pared by Millefiorini et al.,
et al.
Gong et al.,
) illustrated lower open circuit voltages (ꢂ 0.8 V)
and PCE values (ꢂ 0.4%) than those of P3, even certain do-
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Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Sci-
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0
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highest Voc value and highest crystallinity induced by H-
bonds.
5
(a) Liang, Y.; Feng, D.; Wu, Y.; Tsai, S. T.; Li, G.; Ray, C.; Yu,
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CONCLUSIONS
Ray, C.; Yu, L. Adv. Mater. 2010, 22, 135–138; (e) Chen, H. Y.;
Hou, J.; Zhang, S.; Liang, Y.; Yang, G.; Yang, Y.; Yu, L.; Wu, Y.;
Li, G. Nat. Photonics 2009, 3, 649–653.
We have successfully synthesized three dithienothiophene/
carbazole-based conjugated polymers (P1–P3) by Suzuki
coupling reaction. Interestingly, P1–P3 exhibited reversible
electrochromism during the oxidation processes of cyclic vol-
tammogram studies. Among P1–P3, polymer P3 (with
H-bonds) revealed the best electrochromic property with the
most noticeable color change. In powder X-ray diffraction
6
(a) Scharber, M. C.; M u¨ hlbacher, D.; Koppe, M.; Denk, P.;
Waldauf, C.; Heeger, A. J.; Brabec, C. J. Adv. Mater. 2006, 18,
89–794; (b) G u¨ nes, S.; Neugebauer, H.; Sariciftci, N. S. Chem.
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(
XRD) measurements, these polymers exhibited obvious dif-
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fraction features indicating distinct bilayered packings
between polymer backbones and similar p-p stacking
between layers in the solid state. Compared with the XRD
data of P2 (without H-bands), H-bonds of P3 induced a
higher crystallinity in the small angle region (corresponding
to a higher ordered bilayered packings between polymer
backbones), but with a similar crystallinity in the wide angle
8 (a) Leclerc, N.; Michaud, A.; Sirois, K.; Morin, J. F.; Leclerc,
M. Adv. Funct. Mater. 2006, 16, 1694–1704; (b) Fu, Y.; Kim, J.;
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9
(a) Blouin, N.; Michaud, A.; Gendron, D.; Wakim, S.; Blair, E.;
Neaguplesu, R.; Belletete, M.; Durocher, G.; Tao, Y.; Leclerc, M.
J. Am. Chem. Soc. 2008, 130, 732–742; (b) Zou, Y.; Gendron,
D.; A ¨ı ch, R. B.; Najari, A.; Tao, Y.; Leclerc, M. Macromolecules
2009, 42, 2891–2894; (c) Qin, R. P.; Li, W. W.; Li, C. H.; Du, C.;
Veit, C.; Schleiermacher, H. F.; Andersson, M.; Bo, Z. S.; Liu, Z.
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region indicating
a comparable p-p stacking distance
between layers. The potential applications of P1–P3 in bulk
heterojunction photovoltaic solar cells (PSCs) were further
investigated, where the PSC device containing P3 blended
with PCBM (by a weight ratio of 1:1) had the optimum
power conversion efficiency (PCE) up to 0.61% (with J
2
bonded effects, polymer P3 possessed higher thermal
decomposition temperature (T ), glass transition tempera-
ture (T ), RMS smoothness, open circuit voltage (V ), and
2
009, 131, 14612–14613; (d) Zhang, M.; Fan, H.; Guo, X.; He, Y.;
¼
sc
2
Zhang, Z.; Min, J.; Zhang, J.; Zhao, G.; Zhan, X.; Li, Y. Macro-
molecules 2010, 43, 5706–5712.
.26 mA/cm , FF ¼ 29.8%, and V ¼ 0.90 V). Due to the H-
oc
1
0 (a) Ku, S. Y.; Liman, C. D.; Burke, D. J.; Treat, N. D.; Coch-
d
ran, J. E.; Amir, E.; Perez, L. A.; Chabinyc, M. L.; Hawker, C. J.
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g
oc
PCE value than P2. These polymers demonstrate a novel
family of conjugated polymers along the path toward achiev-
ing the electrochromic and PSC applications.
11 (a) Zhang, S. M.; Guo,Y. L.; Fan, H. J.; Liu, Y.; Chen, H.
Y.;Yang, G. W.; Zhan, X. W.; Liu, Y. Q.; Li, Y. F.; Yang, Y. J.
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ACKNOWLEDGMENTS
The financial supports of this project provided by the National
Science Council of Taiwan (ROC) through NSC 99-2113-M-009-
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