New carbazole-based conjugated polymers containing pyridylvinyl thiophene units for polymer solar cell applications: Morphological stabilization through hydrogen bonding

Article abstract of DOI:10.1002/pola.24465

We have synthesized two conjugated polymers (P1, P2) containing alternating electron-donating and -accepting units, based on N-alkyl-2,7-carbazole, 4,7-di(thiophen-5-yl)-2,1,3-benzothiadiazole and 3-[2-(4-pyridyl)vinyl]thiophene units. These conjugated polymers contained different contents of pyridine units, which were incorporated to form hydrogen bonds with [6,6]-phenyl-C ₆₁-butyric acid (PCBA). When these hydrogen bonding interactions were present in the polymer thin films, their thermal stability improved; deterioration, which occurred through aggregation of PCBA methyl ester after lengthy annealing times, was also suppressed. The power conversion efficiency of a device incorporating the film featuring hydrogen bonding interactions remained at 75% of the original value after thermal annealing for 5 h at 140 °C.

Full text of DOI:10.1002/pola.24465

New Carbazole-Based Conjugated Polymers Containing Pyridylvinyl
Thiophene Units for Polymer Solar Cell Applications: Morphological
Stabilization Through Hydrogen Bonding
SO-LIN HSU, CHIA-MIN CHEN, YU-HSIN CHENG, KUNG-HWA WEI
Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu 30010, Taiwan
Received 27 October 2010; accepted 28 October 2010
DOI: 10.1002/pola.24465
Published online 2 December 2010 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: We have synthesized two conjugated polymers (P1,
P2) containing alternating electron-donating and -accepting
units, based on N-alkyl-2,7-carbazole, 4,7-di(thiophen-5-yl)-2,1,3-
benzothiadiazole and 3-[2-(4-pyridyl)vinyl]thiophene units.
These conjugated polymers contained different contents of pyri-
dine units, which were incorporated to form hydrogen bonds
with [6,6]-phenyl-C61-butyric acid (PCBA). When these hydrogen
bonding interactions were present in the polymer thin films,
their thermal stability improved; deterioration, which occurred
through aggregation of PCBA methyl ester after lengthy anneal-
ing times, was also suppressed. The power conversion effi-
ciency of a device incorporating the film featuring hydrogen
bonding interactions remained at 75% of the original value after
ꢀ
C
V
thermal annealing for 5 h at 140 C.
2010 Wiley Periodicals,
Inc. J Polym Sci Part A: Polym Chem 49: 603–611, 2011
KEYWORDS: conjugated polymers; phase separation; photovol-
taic cells; polycarbazole; polycondensation
INTRODUCTION Polymer solar cells (PSCs) have attracted
much attention because they offer great potential to produce
light-weight, flexible, inexpensive, large-area devices. Among
the many, conjugated polymers have been developed for so-
lar cell applications;1–5 regioregular poly(3-hexylthiophene)
(P3HT) remains one of the most suitable for PSCs.6–14 The
power conversion efficiency (PCE) of a bulk heterojunction
accelerated under the constant flux of thermal energy gener-
ated by solar irradiation, decreases the donor–acceptor inter-
facial area and thereby decreases the PCE.29 To achieve dura-
ble PSCs, the nanometer-scale morphological features of the
BHJ device must be preserved. Two strategies have been
developed to obtain stable BHJ morphologies. In the first, a
thermal or photocrosslinkable P3HT material is used in the
BHJ solar cell,30,31 stabilizing the BHJ morphology and par-
tially suppressing the deterioration of the PSC’s performance.
The hole mobility of the crosslinked P3HT domain, however,
also decreased, leading to lower values for the short-circuit
current density (Jsc) and PCE. In the second approach, the
fullerene derivatives are grafted onto the side chains of the
P3HT backbone32–34 to reduce the rate of degradation of the
PSC performance, as evidenced through accelerated thermal
testing. In those reported polymers, however, the fullerene
grafting percentage was relatively low (1%); as a result, the
morphological stabilization was limited.
(BHJ) PSC made from blends of P3HT and [6,6]-phenyl-C71
-
butyric acid methyl ester (PC71BM) is approximately 5%.12–14
The efficiency of PSCs rises to over 6% when using recently
developed low-band gap conjugated polymers;15–18 the
highest PCE reported to date (7.73%) was that for a PSC
fabricated from a poly[4,8-bis-substituted-benzo[1,2-b:4,5-b0]
dithiophene-2,6-diyl-alt-4-substituted-thieno[3,4-b]thiophene-
2,6-diyl]/PC71BM blend.19
For a BHJ PSC, controlling the blend morphology at the mi-
croscopic scale is critical for optimizing its PCE.20 Several
methods have been developed to control the blend morphol-
ogy, including thermal annealing and solvent annealing of
blends,7,20–22 slow drying of spin-casted films,12,23–25 and
addition of high-boiling point additives.26,27 Although the
best BHJ morphology in a solar cell is believed to be a bicon-
tinuous composite of donor and acceptor components with
phase segregation on a suitable length scale (20–30 nm),3,28
this morphology is not stable toward annealing for long peri-
Because our ability to control the morphology directly through
covalent bonding is somewhat limited, an alternative approach
toward achieving ideal morphologies is to take advantage of
supramolecular noncovalent interactions, namely hydrogen
bonding, p stacking, and Coulombic interactions. Huang
et al.35 reported photovoltaic devices built through hierarchi-
cal self-assembly using a self-assembled monolayer (capable
of forming hydrogen bonds) on gold and a combination of
a barbituric acid-appended fullerene and a complementary
melamine-terminated p-conjugated thiophene-based oligomer.
ꢀ
ods (>30 min) at high temperatures (>130 C). During ther-
mal annealing, the molecules of PCBM diffuse through the
film to form microsized aggregates. This process, which is
Correspondence to: K.-H. Wei (E-mail: khwei@mail.nctu.edu.tw)
C
V
Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 49, 603–611 (2011) 2010 Wiley Periodicals, Inc.
NEW CARBAZOLE-BASED CONJUGATED POLYMERS, HSU ET AL.
603

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
The incorporation of these electron donor (oligomer) and
acceptor (fullerene derivative) assemblies into simple photo-
voltaic devices as thin film led to a 2.5-fold enhancement in
photocurrent relative to that of the analogous system lacking
hydrogen bonding between the donor and acceptor units. The
development of such self-assembly techniques remains a sci-
entific and technological challenge; if overcome, however,
it could lead to a breakthrough in the creation of new molecu-
lar devices. Following such an approach, in this study, we syn-
thesized two conjugated polymers (P1, P2) containing alter-
nating electron-donating and -accepting units based on N-
alkyl-2,7-carbazole, 4,7-di(thiophen-5-yl)-2,1,3-benzothiadia-
zole and 3-[2-(4-pyridyl)vinyl]thiophene units. Both polymers
were synthesized through Suzuki coupling of 2,7-bis(4,4,5,
5-tetramethyl-1,3,2-dioxaborolane-2-yl)-N-90-heptadecanylcar-
bazole with 4,7-(20-dibromothien-5-yl)-2,1,3-benzothiadiazole
and 2,5-dibromo-3-[2-(4-pyridyl)vinyl]thiophene. The conju-
gated polymers contained various amounts of 3-[2-(4-pyri-
dyl)vinyl]thiophene units, which were capable of forming
hydrogen bonds with [6,6]-phenyl-C61-butyric acid (PCBA)
units. We also investigated the effect of thermal annealing
(140 ꢀC) for long periods of time on the morphological stabili-
zation of the corresponding BHJ films and on the device
performance.
(HOMO) of which is ꢁ4.8 eV with respect to zero vacuum
level] as the standard. The topographies of the polymer/
PCBM films were measured using atomic force microscopy
(AFM); a Digital Instruments Nanoscope IIIa apparatus was
operated in tapping mode under ambient conditions.
Synthetic Procedures
2,7-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane-2-yl)-N-90-hep-
tadecanylcarbazole and 4,7-di(20-bromothien-50-yl)-2,1,3-ben-
zothiadiazole were prepared according to the reported proce-
dures.36 The monomer M1, PCBA, and the copolymers P1
and P2 were prepared as outlined in Scheme 1.
2,5-Dibromo-3-methylthiophene (1)
N-Bromosuccinimide (NBS, 38.1 g, 0.210 mol) was added to
a solution of 3-methylthiophene (10.0 g, 0.110 mol) in THF
(80 mL) and AcOH (80 mL) at room temperature. After stir-
ring for 3 h, CHCl3 was added and the resulting mixture
washed three times with water. The organic layer was dried
(MgSO4) and concentrated under reduced pressure. The
crude product was distilled at 90 ꢀC under vacuum to yield
1 (22.6 g, 87%).
1H NMR (300 MHz, CDCl3, ppm): d 6.74 (s, 1H), 2.12 (m,
3H). 13C NMR (75 MHz, CDCl3, ppm): d 138.3, 132.1, 110.3,
108.6, 15.4. Anal. Calcd: C, 23.46; H, 1.58. Found: C, 23.42; H,
1.60.
EXPERIMENTAL
Materials
PCBM was purchased from Nano-C. All solvents were pur-
chased from Aldrich, J. T. Baker, or Tedia and used as
received, except for tetrahydrofuran (THF), N,N-dimethylfor-
mamide, and toluene, which were purified prior to use. All
other chemicals were purchased in reagent grade from
Aldrich, Acros, TCI, or Lancaster Chemical and used as
received.
2,5-Dibromo-3-(bromomethyl)thiophene (2)
NBS (8.90 g, 50.0 mmol) and benzoyl peroxide (10 mg, 0.04
mmol) were added to a solution of 1 (12.8 g, 50.0 mmol) in
CCl4 (100 mL), which was then heated under reflux for 6 h.
After cooling to room temperature, the mixture was washed
sequentially with water (2ꢂ 300 mL), saturated NaHCO3
(1ꢂ 300 mL), and water again (1ꢂ 300 mL). The organic
phase was dried (MgSO4) and concentrated under reduced
pressure. The crude product was distilled at 105 ꢀC under
vacuum to yield 2 (14.1 g, 84%).
Characterization
1H and 13C NMR spectra were recorded using a Varian
Unity-300 spectrometer. The molecular weights of the poly-
mers were measured using gel permeation chromatography,
with a Waters 2414 differential refractometer and a Waters
Styragel column (polystyrene standards, THF as the eluent).
UV–Vis absorption spectra were recorded using an HP 8453
1H NMR (300 MHz, CDCl3, ppm): d 6.97 (s, 1H), 4.34 (m,
2H). Anal. Calcd: C, 17.93; H, 0.90. Found: C, 16.53; H, 1.00.
4-[2-(2,5-Dibromothiophen-3-yl)vinyl]pyridine (M1)
spectrophotometer.
Photoluminescence
spectra
were
A mixture of 2 (10.0 g, 30.0 mmol) and triethyl phosphite
(4.98 g, 30.0 mmol) was heated at 160 C for 3 h. After cool-
ꢀ
recorded using a Hitachi F-4500 luminescence spectrometer.
Thermogravimetric analysis (TGA) was performed under a
N2 atmosphere using a Du Pont TGA 2950 apparatus oper-
ated at a heating rate of 10 ꢀC minꢁ1. Differential scanning
calorimetry (DSC) was performed under a N2 atmosphere
using a PerkinElmer Pyris DSC1 instrument operated at a
heating rate of 10 ꢀC minꢁ1. Cyclic voltammetry (CV) was
performed using a BAS 100 electrochemical analyzer and a
conventional three-electrode cell: a carbon glass electrode
coated with the polymer thin film as the working electrode,
a Pt wire as the counter electrode, and Ag/Agþ (0.01 M
AgNO3) as the reference electrode. Tetrabutylammonium hex-
afluorophosphate (0.1 M) in acetonitrile was used as the elec-
trolyte for the CV measurements; the curves were calibrated
using ferrocene [the highest occupied molecular orbital
ing to room temperature, the triethyl phosphite was distilled
ꢀ
off at 120 C. The residue was dissolved in dry THF (20 mL)
ꢀ
and cooled in an ice-water bath (0 C). A solution of NaOMe
(1.80 g, 33.3 mmol) in dry THF (10 mL) was added portion-
wise to the mixture, which was then stirred for 10 min. A
solution of 4-pyridinecarboxaldehyde (3.21 g, 30.0 mmol) in
dry THF was added via syringe to the mixture, which was
then stirred at room temperature for 12 h before being
poured into water and extracted with diethyl ether. The com-
bined organic phases were washed three times with water
and then dried (MgSO4). After evaporating the solvent under
reduced pressure, the crude product was purified through
chromatography (SiO2; EtOAc/hexane, 1:5). Recrystallization
(MeOH/hexane) afforded M1 as a white solid (6.91 g, 67%).
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SCHEME 1 Synthesis of monomers,
the copolymers P1 and P2, and PCBA.
1H NMR (300 MHz, CDCl3, ppm): d 8.58 (d, J ¼ 1.5 Hz, 1H),
8.56 (d, J ¼ 1.5 Hz, 1H), 7.34 (d, J ¼ 1.8 Hz, 1H), 7.32 (d, J
¼ 1.8 Hz, 1H), 7.21 (s, 1H), 7.14 (d, J ¼ 16.2 Hz, 1H), 6.82
(d, J ¼ 16.2 Hz, 1H). 13C NMR (75 MHz, CDCl3, ppm): d
150.6, 144.1, 138.4, 128.7, 127.6, 124.4, 123.0, 121.1, 112.7.
Anal. Calcd: C, 38.29; H, 2.04; N, 4.06. Found: C, 38.73; H,
2.49; N, 3.99.
(s, 1H), 4.66 (br, 5H), 2.39 (br, 10H), 1.98 (br, 10H), 1.23–
1.13 (br, 120H), 0.76 (s, 30H). Anal. Calcd: C, 74.94; H, 7.02;
N, 5.77. Found: C, 72.65; H, 6.90; N, 5.68.
P2 (m:n ¼ 1:1)
Using the same procedure described above for P1, the reac-
tion of M1 (52.5 mg, 0.150 mmol), 2,7-bis(4,4,5,5-tetra-
methyl-1,3,2-dioxaborolane-2-yl)-N-90-heptadecanylcarbazole
(200 mg, 0.300 mmol), 4,7-di(20-bromothien-50-yl)-2,1,3-ben-
zothiadiazole (69.6 mg, 0.150 mmol), K2CO3 (276.4 mg, 2.00
mmol), H2O (1 mL), Aliquat 336 (36 mg), toluene (5 mL),
and Pd(PPh3)4 (7 mg, 0.006 mmol) afforded P2 as a black
solid (151 mg, 77%).
P1 (m:n ¼ 4:1)
Pd(PPh3)4 (7 mg, 0.006 mmol) was added to a degassed
mixture of M1 (21.0 mg, 0.0600 mmol), 2,7-bis(4,4,5,5-tetra-
methyl-1,3,2-dioxaborolane-2-yl)-N-90-heptadecanylcarbazole
(200 mg, 0.30 mmol), 4,7-di(20-bromothien-50-yl)-2,1,3-ben-
zothiadiazole (111.4 mg, 0.240 mmol), K2CO3 (276.4 mg,
2.00 mmol), H2O (1 mL), Aliquat 336 (36 mg), and toluene
1H NMR (300 MHz, CDCl3, ppm): d 8.53 (s, 2H), 8.20–8.11
(m, 6H), 7.97–7.43 (m, 13H), 7.19 (s, 1H), 7.11 (s, 2H), 6.90
(s, 1H), 4.70 (br, 2H), 2.39 (br, 4H), 2.02 (br, 4H), 1.25–1.16
(br, 48H), 0.79 (s, 12H). Anal. Calcd: C, 77.21; H, 7.43; N,
5.43. Found: C, 75.37; H, 7.46; N, 5.14.
ꢀ
(5 mL) at 60 C and then the mixture was stirred and heated
under reflux for 4 h under a N2 atmosphere. At this point,
phenylboronic acid (37 mg, 0.03 mmol) was added and the
mixture was heated under reflux for a further 1 h; bromo-
benzene (0.03 mL, 0.03 mmol) was then added and the mix-
ture was heated under reflux for another 1 h. The reaction
mixture was poured into MeOH (100 mL) and the resulting
black precipitate was collected through filtration. The pre-
cipitated polymer was washed with MeOH, acetone, and hex-
ane in a Soxhlet apparatus for 24 h to remove any oligomers
and catalyst residues, thereby providing P1 (172 mg, 83%).
PCBA
AcOH (3.8 mL) and conc. HCl (1.5 mL) were added at room
temperature to a solution of PCBM (50.0 mg, 0.05 mmol) in
chlorobenzene (6 mL) and then the mixture was stirred for
ꢀ
10 min before being heated at 110 C for 12 h. After cooling,
the reaction mixture was poured into MeOH (100 mL) and
the precipitate was purified through repeated (three times)
precipitation from CS2 solution into MeOH (172 mg, 83%).
1H NMR (300 MHz, CDCl3, ppm): d 8.50 (s, 2H), 8.17–8.11
(m, 15H), 7.97–7.41 (m, 34H), 7.19 (s, 1H), 7.17 (s, 8H), 6.89
NEW CARBAZOLE-BASED CONJUGATED POLYMERS, HSU ET AL.
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JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
TABLE 1 Molecular Weights and Thermal Properties of the
2,1,3-benzothiadiazole, and M1 through Suzuki cross-cou-
pling polycondensation to afford the copolymers P1 and P2
containing different mole percentages (10% and 25%,
respectively) of pyridine side chains. According to size exclu-
sion chromatography experiments (using monodisperse poly-
styrene standards and THF as the solvent), the number-
average molecular weights of P1 and P2 were 88,031 and
26,357 g molꢁ1, respectively, with corresponding polydisper-
sity indices (PDIs) of 1.69 and 4.91, respectively (Table 1).
We suspect that the molecular weight of P2 was less than
that of P1 because the greater number of pyridylvinyl thio-
phene units disturbed the reactivity of the polycondensation.
The copolymers P1 and P2 exhibited good solubility in 1,2-
dichlorobenzene, CHCl3, and THF; therefore, uniform thin
films of the polymers were readily produced through spin
coating for further study of PSC devices.
Copolymers
Polymer Mn (g molꢁ1
)
Mw (g molꢁ1
)
PDI Td (ꢀC) Tg (ꢀC)
P1
P2
88,031
26,357
148,954
129,517
1.69 344
4.91 393
129
148
1H NMR (300 MHz, CDCl3/CS2, ppm): d 9.58 (s, 1H), 7.91 (s,
2H), 7.54–7.45 (m, 3H), 2.94 (s, 2H), 2.54 (s, 2H), 2.18 (s,
2H). Anal. Calcd: C, 95.08; H, 1.35. Found: C, 95.02; H, 1.37.
Photovoltaic Device Fabrication
The current density–voltage (J–V) properties of the polymers
were determined for devices having the sandwich structure
[indium tin oxide (ITO)/poly(3,4-ethylenedioythiophene):
poly(styrene sulfonic acid) (PEDOT:PSS)/polymer:PCBM/Ca/
Al]. First, an ITO-coated glass substrate was etched with acid
and cleaned sequentially using detergent, deionized water,
acetone, and isopropyl alcohol. The cleaned substrate was
treated with ozone plasma for 5 min and then a 30 nm layer
of PEDOT (Batron P AL 4083, HC Stark) was spin coated
onto it. The PEDOT-coated substrate was heated at 150 ꢀC
for 30 min to ensure that all of the solvents had been
removed. The active layer of the device was spin coated at
1200 rpm from a 1,2-dichlorobenzene solution containing
the polymer and PCBM (1:2, w/w). The thickness of the
polymer/PCBM layer was about 100 nm. After thermal
annealing of the active layer (140 ꢀC, 10 min), a layer of Ca
(35 nm) was vapor deposited as the cathode and a layer of Al
(100 nm) as the protecting layer under a base pressure of
less than 1 ꢂ 10ꢁ6 Torr. The effective area of an accomplished
device was 0.04 cm2. The sample devices were tested under
simulated AM 1.5G illumination (100 mW cmꢁ2) using a Xe
lamp-based Newport 66902 150W solar simulator equipped
with an AM1.5 filter as the white light source. An OPHIR ther-
mopile 71964 was used to determine that the optical power
at the sample was 100 mW cmꢁ2. The J–V characteristics of
the samples were measured under a N2 atmosphere using a
Keithley 236 source-measurement meter. The external quan-
tum efficiencies (EQEs) were measured using a Keithley 236
source-measure unit coupled with an Oriel Cornerstone 130
monochromator; the light intensity at each wavelength was
calibrated using an OPHIR 71580 diode.
We synthesized PCBA through hydrolysis of its corresponding
ester (PCBM) with HCl/AcOH (Scheme 1). The solubility of
PCBA is good in CS2, but it could also be dissolved sparingly
in 1,2-dichlorobenzene, CHCl3, and THF for further study. Fig-
ure 1 presents FTIR spectra of P1, PCBA, and a P1/PCBA
blend. For polymer P1, characteristic C¼¼C and C¼¼N ring
stretching of pyridine rings appeared in the range 1430–1600
cmꢁ1. When P1 was mixed with PCBA, a broad peak appeared
at 3100–3500 cmꢁ1, consistent with the formation of hydro-
gen bonds between the pyridine units on the polymer side
chains and the carboxylic acid groups of PCBA units.
Thermal Properties
Table 1 summarizes the thermal properties of P1 and P2,
determined through DSC and TGA analyses performed under
a N2 atmosphere. TGA indicated that both polymers exhib-
ited good thermal stability, with 5% decomposition tempera-
tures (Td) of 344 and 393 ꢀC; the glass transition tempera-
tures were 129 and 148 ꢀC, respectively. This good thermal
stability, which we ascribe to the presence of stiff carbazole
RESULTS AND DISCUSSION
Synthesis and Characterization
Scheme 1 displays the chemical structures and the synthetic
route for preparing the copolymers P1 and P2 and PCBA.
We synthesized the monomer M1 through
a Horner–
Emmons reaction to form the conjugated pyridine–vinylene-
substituted thiophene. The 1H NMR spectrum of the mono-
mer M1 featured two sets of doublets (7.19–6.77 ppm) rep-
resenting the two olefinic protons in the vinyl unit. The cou-
pling constant of these two doublets (16.2 Hz) indicates that
the main product of the reaction was the trans isomer. We
polymerized 2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolane-
2-yl)-N-90-heptadecanylcarbazole, 4,7-di(20-bromothien-50-yl)-
FIGURE 1 FTIR spectra of the copolymer P1, PCBA, and a P1/
PCBA blend. [Color figure can be viewed in the online issue,
which is available at wileyonlinelibrary.com.]
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ARTICLE
we calculated HOMO energy levels of P1 and P2 as ꢁ5.50
and ꢁ5.66 eV, respectively; these similar values are also close
to the HOMO energy level of PCDTBT (ꢁ5.5 eV), presumably
because of the similar main chain structures of these poly-
mers.36 The LUMO energy levels (ca. ꢁ3.6 eV for P1; ꢁ3.58
eV for P2) were located 0.2–0.3 eV above the LUMO energy
level of the PCBM acceptor (ꢁ3.8 eV), thereby suggesting the
possibility of energetically favorable electron transfer.
Photovoltaic Properties
To study the photovoltaic properties of these two polymers,
we fabricated BHJ PSC devices having the structure ITO/
PEDOT:PSS/polymer:PCBM/Ca/Al. The polymer (donor)/
PCBM (acceptor) blend active layers were spin coated from
1,2-dichlorobenzene solutions at a concentration of 7 mg
mLꢁ1. The photovoltaic properties of P1 and P2 were inves-
tigated initially using a 1:2 blending ratio, which was found
to be optimal for the corresponding poly[N-90-heptadecanyl-
2,7-carbazole-alt-5,5-(40,70-di-2-thienyl-20,10,30-benzothiadia-
zole)] (PCDTBT) system.17 For the system based on the P1/
PCBM blend, we calculated a PCE of 1.05%, with a value of
Voc of 0.58 V, a value of Jsc of 4.89 mA cmꢁ2, and a fill factor
(FF) of 0.37. For the device based on the P2/PCBM blend, the
PCE was 0.36%, with values of Voc, Jsc, and FF of 0.38 V, 2.56
mA cmꢁ2, and 0.34, respectively. The performance of the de-
vice based on P2/PCBM was considerably lower than that
based on P1/PCBM, presumably because the increasing quan-
tity of pyridylvinyl side-chain units accompanied with decreas-
ing content of benzothiadiazole might significantly reduce the
charge transfer ability in the polymer chain and alter the
intermolecular interactions. It might also be attributed to the
fact that P2 had a lower molecular weight and poorer PDI
value than that of P1. Therefore, we used P1 in our subse-
quent studies of the photovoltaic properties of this system.
Next, we introduced PCBA to observe the effects of hydrogen
bond formation between the tethered pyridine units of the
polymer and the carboxylic acid groups of PCBA units. The
weight ratio of P1 to PCBM to PCBA in the blend is 3:5:1. In
this case, the molar ratio of pyridylvinyl side chain in P1 to
carboxylic acid in PCBA is equal to 1:1 because each pyridyl-
vinyl group has only one active position to form a hydrogen
bond with a carboxylic acid. The active layer was annealed at
140 ꢀC for 10 min prior to deposition of Ca/Al cathode. The
device based on this blend exhibited a PCE of 1.12%, with a
value of Voc of 0.58 V, a value of Jsc of 4.97 mA cmꢁ2, and a
FF of 0.39 (the J–V characteristics are presented in Fig. 3).
This device performed slightly better (Jsc ¼ 4.97 mA cmꢁ2; FF
¼ 0.39) than that based simply on a corresponding P1/PCBM
FIGURE 2 UV–Vis absorption spectra of the copolymers P1 and
P2. [Color figure can be viewed in the online issue, which is
available at wileyonlinelibrary.com.]
units on the main chains, is also an important property for
PSC devices to ensure resistance against deformation or deg-
radation of the active layer.
Optical Properties
Figure 2 displays the absorption spectra of the two copoly-
mers, measured both from dilute THF solutions and from thin
films; Table 2 summarizes the correlated optical parameters.
Both polymers exhibited two characteristic bands in their
absorption spectra; the first, at about 395 nm, represents p–
p* transitions of the conjugated side chains, whereas the sec-
ond, at longer wavelength, represents intramolecular charge
transfer from the carbazole groups to the benzothiadiazole
moieties in the conjugated main chains. The signal for the p–
p* transitions of P2 (ca. 395 nm) has greater intensity and is
slightly red shifted relative to that of P1 (ca. 519 nm), due to
the higher content of incorporated pyridylvinyl conjugated
side chain on the polymer backbone of P2. Notably, the two
characteristic bands in the absorption spectra (ca. 395, 550
nm) changed only slightly (by ca. 5 nm) after transfer from
solution to the solid state, suggesting that only weak packing
forces exist between the polymer chains, possibly due to
steric hindrance resulting from the tethered pyridylvinyl
units. The optical band gaps (Eogpt), calculated from absorption
onsets in the films, were 1.87 eV for P1 and 1.91 eV for P2.
Electrochemical Properties
We used CV to estimate the HOMO and lowest unoccupied
molecular orbital (LUMO) energy levels of the conjugated
polymers (Table 2). Using ferrocene as the internal standard,
TABLE 2 Optical and Electrochemical Properties of the Copolymers
k
max, solution
k
max, film
Eogpt
(eV)a
EEgC
(eV)
EHOMO
ELUMO
Polymer
(nm)
(nm)
(eV)
(eV)
P1
P2
a
395, 545
401, 542
395, 553
403, 549
1.87
1.91
1.90
2.08
ꢁ5.50
ꢁ5.66
ꢁ3.60
ꢁ3.58
Calculated from the absorption onset in the absorption spectrum of the film.
NEW CARBAZOLE-BASED CONJUGATED POLYMERS, HSU ET AL.
607

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 3 J–V characteristics and EQE spectra of devices based on P1/PCBM and P1/PCBM/PCBA blends. [Color figure can be
viewed in the online issue, which is available at wileyonlinelibrary.com.]
blend (4.89 mA cmꢁ2 and 0.37, respectively). EQE spectra of
devices based on P1/PCBM and P1/PCBM/PCBA blends were
also illustrated (Fig. 3). The PSC device based on the P1/
PCBM/PCBA blend exhibited higher efficiency in its absorp-
tion in the UV region relative to that of the device based on
P1/PCBM, presumably because p–p* transitions were induced
by the formation of hydrogen bonds in the polymer side
chain, thereby increasing the photocurrent of the device.
Moreover, the curves display great coherence with their cor-
relative absorptions of the active layers. These results suggest
that incorporation of PCBA into the blending system dose not
affect the charge transporting properties and film morphology.
Finally, to verify the additional stability provided by hydrogen
bonding interactions, we measured the device performances
after various annealing times. Figure 4 reveals decreases in
the PCEs of both devices over time; the rate of degradation of
FIGURE 4 Devices performance plotted with respect to the annealing time at 140 ꢀC for the P1/PCBM and P1/PCBM/PCBA blends.
[Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]
608
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ARTICLE
FIGURE 5 Optical micrographs of (a–c) P1/PCBM and (d–f) P1/PCBM/PCBA blend films that had been subjected to annealing at 140
ꢀC for (a,d) 10 min, (b,e) 5 h, and (c,f) 10 h.
the P1/PCBM/PCBA film was, however, slower (decreasing by
only 25% after thermal annealing for 5 h) than that of the
P1/PCBM film (ca. 50% decrease). We ascribe this behavior
to partial stabilization of the morphology of the active layer
as a result of hydrogen bonding between the pyridine func-
tional groups of the polymer and the carboxylic acid groups
of the PCBA units; thus, while assembling with the polymer,
PCBA simultaneously attracts its analog, PCBM, to form partial
aggregates, thereby improving the degree of phase separation
and increasing the interfacial area between the donor and
acceptor units in the BHJ thin film.
Because the morphology of the active layer blends in the
devices plays an important role in determining the device
performance, we used optical microscopy to observe the
morphological alteration on the micrometer scale. Figure 5
displays the morphological evolution within the thin films
spin coated from P1/PCBM and P1/PCBM/PCBA blends after
various annealing times. It is clear to find that thermal
annealing induced the formation of needle-like crystals of
PCBM molecules (length: hundreds of micrometers) in the
P1/PCBM film; this phenomenon was greatly suppressed in
the case of the P1/PCBM/PCBA blend. Thus, the assembled
FIGURE 6 Topographic AFM images of the
films based on (a,c) P1/PCBM and (b,d) P1/
PCBM/PCBA blends; that had been sub-
jected to annealing at 140 ꢀC for (a,b) 10
min, (c,d) 5 h. Scale: 3 ꢂ 3 lm2. [Color fig-
ure can be viewed in the online issue,
which is available at wileyonlinelibrary.
com.]
NEW CARBAZOLE-BASED CONJUGATED POLYMERS, HSU ET AL.
609

JOURNAL OF POLYMER SCIENCE: PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
5 Cheng, Y. J.; Yang, S. H.; Hsu, C. S. Chem Rev 2009, 109,
PCBA units successfully inhibited the large-scale aggregation
of diffused PCBM molecules over time, thereby enhancing
the morphological stabilization relative to that of the system
lacking hydrogen bonding interactions.
5868–5923.
6 Gunes, S.; Neugebauer, H.; Sariciftci, N. S. Chem Rev 2007,
¨
107, 1324–1338.
To study of the morphological issues on the nanoscale, we
monitored the topographies of the blend films using tapping-
mode AFM. Figure 6 reveals that the P1/PCBM/PCBA and
P1/PCBM films gave root-mean-square roughnesses of 0.65
and 1.07 nm after annealing for 10 min, respectively. How-
ever, after annealing for 5 h, the P1/PCBM/PCBA blend had
a more uniform surface than that of the P1/PCBM blend,
with root-mean-square roughnesses of 1.06 and 4.80 nm,
respectively. Thus, it appears that the presence of an appro-
priate quantity of PCBA in the polymer blend prevented the
PCBM units from undergoing severe aggregation during
long-term annealing, possibly because the PCBM units were
well dispersed through attraction to the PCBA units in each
active site for hydrogen bonding. In summary, decreased sur-
face roughness when hydrogen bonding was present, on
both the micrometer and nanoscale scales, improved the
photovoltaic performance of the PSC devices.
7 Hoppe, H.; Sariciftci, N. S. J Mater Chem 2006, 16, 45–61.
8 Thompson, B. C.; Fre´chet, J. M. J. Angew Chem Int Ed 2008,
47, 58–77.
9 Chang, Y.-T.; Hsu, S.-L.; Su, M.-H.; Wei, K.-H. Adv Funct
Mater 2007, 17, 3326–3331.
10 Chang, Y.-T.; Hsu, S.-L.; Chen, G.-Y.; Su, M.-H.; Singh, T. A.;
Diau, E. W.-G.; Wei, K.-H. Adv Funct Mater 2008, 18, 2356–
2365.
11 Chang, Y.-T.; Hsu, S.-L.; Su, M.-H.; Wei, K.-H. Adv Mater
2009, 21, 2093–2097.
12 Li, G.; Shrotriya, V.; Huang, J. S.; Yao, Y.; Moriarty, T.; Em-
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13 Ma, W. L.; Yang, C. Y.; Gong, X.; Lee, K. H.; Heeger, A. J.
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CONCLUSIONS
15 Hau, S. K.; Yip, H.-L.; Ma, H.; Jen, A. K.-Y. Appl Phys Lett
We have synthesized two polymers, P1 and P2, through
Suzuki couplings of 2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxa-
borolane-2-yl)-N-90-heptadecanylcarbazole with 4,7-(20-dibro-
mothien-5-yl)-2,1,3-benzothiadiazole and 2,5-dibromo-3-[2-
(4-pyridyl)vinyl]thiophene. Both polymers exhibited rela-
tively high glass transition temperatures (129 and 148 ꢀC,
respectively). Although the photovoltaic properties decreased
when we increased the content of side chain-tethered pyri-
dine units from 10% to 25% (possibly because P2 had a
lower molecular weight and poorer PDI than did P1), the
thermal stability improved after incorporating PCBA into P1,
thereby forming hydrogen bonds in the polymer blend; the
PCE of the PSC device based on P1/PCBM/PCBA remained
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16 Blouin, N.; Michaud, A.; Gendron, D.; Wakim, S.; Blair, E.;
Neagu-Plesu, R.; Belleteˆte, M.; Durocher, G.; Tao, Y.; Leclerc,
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18 Chu, T. Y.; Alem, S.; Verly, P. G.; Wakim, S.; Lu, J.; Tao, Y.;
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ꢀ
at 75% of its original value after annealing for 5 h at 140 C,
but that based on P1/PCBM plunged to less than 55% of the
original value under the same conditions. The morphological
deterioration of the BHJ film of the P1/PCBM/PCBA blend
was also suppressed, as observed through optical and AFM
imaging.
20 Campoy-Quiles, M.; Ferenczi, T.; Agostinelli, T.; Etchegoin,
P. G.; Kim, Y. K.; Anthopoulos, T. D.; Stavrinou, P. N.; Bradley,
D. D. C.; Nelson J. Nat Mater 2008, 7, 158–164.
21 Al-Ibrahim, M.; Ambacher, O.; Sensfuss, S.; Gobsch, G.
The authors thank the National Science Council for financial
support.
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22 Chirvase, D.; Parisi, J.; Hummelen, J. C.; Dyakonov, V.
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