Alternating copolymers of carbazole and triphenylamine with conjugated side chain attaching acceptor groups: Synthesis and photovoltaic application

Article abstract of DOI:10.1021/ma101491c

Four alternating copolymers of carbazole (Cz) and triphenylamine (TPA) with thienylenevinylene (TV) conjugated side chain containing different acceptor end groups of aldehyde (PCzTPATVCHO), monocyano (PCzTPA-TVCN) dicyano (PCzTPA-TVDCN), and 1,3-diethyl-2-thiobarbituric acid (PCzTPA-TVDT), have been designed and synthesized. The structures and properties of the main chain donor-side chain acceptor D-A copolymers were fully characterized. Through changing the acceptor group attached to the TV conjugated side chain on TPA, the electronic properties and energy levels of the copolymers were effectively tuned. The effect of substituent on the electronic structures of the copolymers was also studied by molecular simulation. These results indicate that it is a simple and effective approach to tune the bandgap in a conjugated polymer by attaching an acceptor end group on the conjugated side chains. PCzTPA-TVCN, PCzTPA-TVDCN, and PCzTPA-TVDT were used as donor in polymer solar cells; the device based on PCzTPA-TVDT/PC70BMdemonstrates a power conversion efficiency of 2.76% withVoc of 0.87 V under the illumination of AM1.5G, 100 mW/cm².

Full text of DOI:10.1021/ma101491c

9376 Macromolecules 2010, 43, 9376–9383
DOI: 10.1021/ma101491c
Alternating Copolymers of Carbazole and Triphenylamine
with Conjugated Side Chain Attaching Acceptor Groups:
Synthesis and Photovoltaic Application
Zhi-Guo Zhang,† Yi-Liang Liu,‡ Yang Yang,† Keyue Hou,† Bo Peng,† Guangjin Zhao,†
Maojie Zhang,† Xia Guo,† En-Tang Kang,‡ and Yongfang Li*,†
†Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences,
Beijing 100190, China, and ‡Department of Chemical & Biomolecular Engineering, National University of
Singapore, Kent Ridge, Singapore 119260
Received July 4, 2010; Revised Manuscript Received October 16, 2010
ABSTRACT: Four alternating copolymers of carbazole (Cz) and triphenylamine (TPA) with thienylene-
vinylene (TV) conjugated side chain containing different acceptor end groups of aldehyde (PCzTPA-
TVCHO), monocyano (PCzTPA-TVCN) dicyano (PCzTPA-TVDCN), and 1,3-diethyl-2-thiobarbituric
acid (PCzTPA-TVDT), have been designed and synthesized. The structures and properties of the main chain
donor-side chain acceptor D-A copolymers were fully characterized. Through changing the acceptor group
attached to the TV conjugated side chain on TPA, the electronic properties and energy levels of the copoly-
mers were effectively tuned. The effect of substituent on the electronic structures of the copolymers was also
studied by molecular simulation. These results indicate that it is a simple and effective approach to tune the
bandgap in a conjugated polymer by attaching an acceptor end group on the conjugated side chains.
PCzTPA-TVCN, PCzTPA-TVDCN, and PCzTPA-TVDT were used as donor in polymer solar cells; the
device based on PCzTPA-TVDT/PC70BM demonstrates a power conversion efficiency of 2.76% with Voc of
0.87 V under the illumination of AM1.5G, 100 mW/cm2.
Introduction
acceptor moiety provides a means for narrowing the bandgap
and tuning the HOMO and LUMO energy levels of conjugated
Polymer solar cells (PSCs) are considered as a promising
alternative to inorganic semiconductor-based solar cells for the
generation of affordable, clean, and renewable energy.1 PSCs are
commonly composed of a blend film of conjugated polymer
donor and fullerene derivative (such as PCBM) acceptor sand-
wiched between an ITO positive electrode and a low workfunc-
tion metal negative electrode. Advantages of the PSCs include
low-cost fabrication of large-area devices, lightweight, mechan-
ical flexibility, and easy tunability of chemical properties of the
organic photovoltaic materials. Therefore, there have been great
interests from academia to application-oriented companies to
increase the efficiency of PSCs for future commercial application,
primarily through development of new conjugated polymers2-4
and device engineering.5-7 It has been realized that an ideal poly-
mer donor in PSCs should exhibit broad absorption with high
absorption coefficient in the visible region, high hole mobility,
suitable energy level matching with the fullerene acceptor, and
appropriate compatibility with fullerene acceptor to form nano-
scale bicontinuous interpenetrating network.8 Optimization of
the properties will offer high values of short-circuit current density
(Jsc), open-circuit voltage (Voc), and fill-factor (FF) of the PSCs.
Power conversion efficiency (PCE) of the PSCs depends on the
parameters by the following equation: PCE = (JscꢀVocꢀFF)/
Pin, where Pin is the input light power.9
polymers.10,11 On the basis of this approach, main chain D-A
copolymers have been witnessed great success for achieving high
performance PSCs.12-22 However, main chain D-A copolymers
may suffer from lower hole mobility due to the influence of the
acceptor units on the polymer main chain.
Previous studies in our group have developed a new family of
conjugated polymers with conjugated side chains.2 This type of
polymers features high hole mobility thanks to the overlapping of
the side chain interactions with the conjugated main chains, and
broad absorptions deriving from both the main chains and the
conjugated side chains, thus demonstrated prominent device
performances in PSCs23 and OFETs (organic field-effect
transistors).24 Here, we are trying to attach appropriate acceptor
end groups to the conjugated side chains to construct new D-A
copolymers with main chains as donor unit and side chains as
acceptor unit. Compared with well developed main chain D-A
copolymers, these main chain donor and side chain acceptor
D-A copolymers are expected to have some interesting features
for optoelectronic applications, such as higher hole mobility of
the polymer main chains and having the advantage of allowing
charge separation through sequential transfer of electrons from
the main chains to the acceptor side chains and then to fullerene
acceptor in the active layer of PSCs.25 As successful examples,
Huang et al recently reported copolymers of fluorene (or
silafluorene) and triphenylamine (TPA) with the side chains
containing acceptor unit, which exhibited promising photo-
voltaic properties.26,27
One feasible approach toward broadening the visible absorp-
tion and tuning the energy levels is to design alternating do-
nor-acceptor (D-A) copolymers, in which the hybridization of
HOMO located on the donor moiety with LUMO located on the
Poly(2,7-carbazole) derivatives are among the most promising
conjugated polymers used in fabricating the optoelectronic
devices.28,29 They possess the advantages of high hole mobility,
*Corresponding author. E-mail: liyf@iccas.ac.cn.
pubs.acs.org/Macromolecules
Published on Web 10/29/2010
r 2010 American Chemical Society

Article
Macromolecules, Vol. 43, No. 22, 2010 9377
Scheme 1. Synthesis Route of PCzTPA-TVCHO
good solution processability, deeper HOMO energy level and air
stable features, deriving from the nature of the carbazole unit,
such as rigid biphenyl unit, delocalization of the lone electron pair
of nitrogen atom over aromatic structure and the bulky alkyl
chains attached. The alternating copolymers of 2,7-carbazole and
various acceptors have been synthesized for applications as the
donor materials in PSCs.17,29-31 The combination of the unique
features of the 2,7-carbazole and the advantages of the main
chain donor-side chain acceptor architecture may offers some
promising donor materials for high-performance PSCs.
On the basis of the consideration mentioned above, alternating
copolymers of 2,7-carbazole and TPA with thienylenevinylene
conjugated side chain containing various acceptor end groups
including aldehyde (PCzTPA-TVCHO), monocyano (PCzTPA-
TVCN) dicyano (PCzTPA-TVDCN), and 1,3-diethyl-2-thiobar-
bituric acid (PCzTPA-TVDT), were designed and synthesized in
this work, for the application as donor materials in PSCs. Among
the four polymers, PCzTPA-TVDT shows broad absorption band
in the visible region with absorption edge at 710 nm and promising
photovoltaic properties.
100 mV/s with an Ag/Agþ reference electrode and a platinum
wire counter electrode. Polymer film was formed by drop-
casting 1.0 μL of polymer solutions in THF (analytical reagent,
1 mg/mL) onto the working electrode, and then dried in the air.
Device Fabrication and Characterization of PSCs. The PSCs
were fabricated with a configuration of ITO/PEDOT:PSS (40 nm)/
active layer (40-90 nm)/Al (40 nm). A thin layer of PEDOT:PSS
(poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate)) was
spin-cast on precleaned ITO-coated glass from a PEDOT:PSS
aqueous solution (Baytron AI 4083 from H. C. Starck) at 2000
rpm and dried subsequently at 150 °C for 30 min in air, then the
device was transferred to a glovebox, where the active layer of
the blend of the copolymers and PC70BM was spin-coated onto
the PEDOT:PSS layer. Finally, an Al top electrode was depos-
ited in vacuum onto the active layer at a pressure of ca. 5ꢀ10-5
Pa. The active area of the device was defined by the mask when
depositing Al and it was ca. 4 mm2.
The thickness of the active layer was determined by an
Ambios Tech. XP-2 profilometer. The current-voltage (I-V)
characteristics were measured on a computer-controlled Keith-
ley 236 Source-Measure Unit. A xenon lamp coupled with AM
1.5 solar spectrum filter was used as the light source, and the
optical power at the sample was around 100 mW/cm2.
Experimental Section
Synthesis of the Monomers and Copolymers. The synthesis
routes of the monomers and the copolymers are shown in
Schemes 1 and 2. The detailed synthetic processes are as follows.
N-90-Heptadecanyl-2,7-dibromocarbazole (2). 2,7-Dibromo-
carbazole (15 g, 46.2 mmol), 9-heptadecane p-toluenesulfonate
(20.8 g, 50.8 mmol) and the phase-transfer catalyst tetrabuty-
lammonium hydrogensulfate(TBAHS, 0.3 g) were dissolved in
300 mL of acetone, followed by addition of 10.3 g (184 mmol) of
powdered potassium hydroxide. The reaction mixture was
stirred and refluxed for 2 h. After cooling to room temperature,
the inorganic solids were filtered off and washed with acetone.
After the combined filtrates were evaporated to dryness, water
and hexane were added to the residue, and the organic layer was
separated, washed with water twice, and dried over MgSO4.
After the organic solution was concentrated, the product was
recrystallized from petroleum ether (60-90 °C) as a white solid
at -20 °C, to get compound 2 in a yield of 85%. MS: m/z = 563
(Mþ). Melting point: 67-68 °C. 1H NMR (400 MHz, CDCl3, δ,
ppm): 7.90 (br, 2H); 7.70 (br, 1H); 7.54 (br, 1H); 7.33 (br, 2H)
4.42 (br, 1H); 2.19 (br, 2H); 1.90(br, 2H); 1.14 (br, 22H); 0.97
(br, 2H); 0.83 (t, J = 6.3 Hz, 6H). 13C NMR (100 MHz, CDCl3,
δ, ppm): 143.11; 139.65; 122.54; 121.66; 121.41; 121.06; 119.99;
119.40; 114.73; 112.37; 57.16; 33.69; 31.94; 29.48; 29.33; 26.94;
22.81; 14.28. Anal. Calcd for C29H41Br2N: C, 61.82; H, 7.33; N,
2.49; Br, 28.36. Found: C, 62.73; H, 7.50; N, 2.60; Br, 28.11.
2,7-Bis(40,40,50,50-tetramethyl-10,30,20-dioxaborolan-20-yl)-N-
900-heptadecanylcarbazole (3). A mixture of compound 2 (13.5 g,
24 mmol), bis(pinacolato)diboron (14.6 g, 57.6 mmol), (diphenyl-
phosphino)ferrocene palladium dichloride (PdCl2(dppf), 1.2 g,
Materials. 9-Heptadecane p-toluenesulfonate and 2-[2-[4-[N,
N- di(4-bromophenyl) amino]phenyl]ethenyl]thien-5-al (compound
4 in Scheme 1) were prepared according to the method reported
in the literatures.26,32 Tetrakis(triphenylphosphine)palladium,
trioctylmethylammonium chloride (Aliquat 336), octyl cyanoa-
cetate, malononitrile and 1,3-diethyl-2-thiobarbituric acid were
purchased from Sigma-Aldrich Chemical Co, while 2,7-dibro-
mocarbazole was obtained from Synwit Technology. The sol-
vents (pyridine, DMF and toluene) were dried by standard
procedure and distilled before use.
Measurements and Instrumentation. 1H NMR and 13C NMR
spectra were measured on a Bruker DMX-400 or Bruker DMX-
600 spectrometer with d-chloroform as the solvent and tetra-
methylsilane as the internal reference. UV-visible absorption
spectra were measured on a Hitachi U-3010 UV-vis spectro-
photometer. Photoluminescence spectra were measured using a
Hitachi F-4500 spectrophotometer. Melting points were deter-
mined on a WRS-1B Digital Melting Point Apparatus and were
uncorrected. Mass spectra were recorded on a Shimadzu spec-
trometer. Elemental analyses were carried out on a flash EA
1112 elemental analyzer. Thermogravimetric analysis (TGA)
was conducted on a Perkin-Elmer TGA-7 thermogravimetric
analyzer at a heating rate of 20 °C/min and under a nitrogen flow
rate of 100 mL/min. Gel permeation chromatography (GPC)
measurements were conducted, using polystyrene as standard
and THF as the eluent. The electrochemical cyclic voltammetry
was conducted on a Zahner IM6e Electrochemical Workstation,
in a 0.1 mol/L acetonitrile solution of tetrabutylammonium
hexafluorophosphate (n-Bu4NPF6) at a potential scan rate of

9378 Macromolecules, Vol. 43, No. 22, 2010
Scheme 2. Synthesis Routes of PCzTPA-TVCN, PCzTPA-TVDCN, and PCzTPA-TVDT
Zhang et al.
3.36 mmol), KOAc (16.5 g, 168 mmol), and degassed 1,4-dioxane
(260 mL) was stirred under an argon atmosphere at 90 °C for 8 h.
After cooling to room temperature, the crude mixture was filtered
and washed with dichloromethane, and the filtrates were com-
bined. Upon the removal of solvent, the dark-black residue was
subjected to column chromatography on silica gel with ethyl
acetate/hexane (v/v, 1:3) as the eluent, followed by recrystalliza-
tion from ethanol to offer a white solid of 3 in a yield of 87%. MS:
m/z=657 (Mþ). Melting point: 141-143 °C. 1H NMR (400 MHz,
CDCl3, δ, ppm): 8.13 (t, J = 8.4 Hz, 2H,); 8.02 (S, 1H); 7.89
(S, 1H);7.66(d, J=5.4 Hz, 2H); 4.70(t, J=5 Hz, 2H); 2.33 (m, 2H);
1.90 (m, 2H); 1.61 (m, 2H); 1.39 (S, 24H); 1.19 (br, 24 H); 0.98(br,
2H); 0.87 (t, J=6.4 Hz, 6H). 13C NMR (100 MHz, CDCl3, δ,
ppm): 142.04; 138.78; 126.44, 126.41; 126.31; 126.28; 126.16;
124.78; 124.69; 120.13; 119.82; 118.19; 115.53; 83.76; 56.44;
33.92; 31.86; 29.41; 29.30; 26.85; 25.03; 22.70; 22.59; 14.17. Anal.
Calcd for C41H65B2NO4: C, 74.89; H, 9.96; N, 2.13. Found: C,
74.70; H, 10.13; N, 2.20.
Synthesis of PCzTPA-TVCHO. 2,7-Bis(4,4,5,5-tetramethyl-
1,3,2-dioxaborolane-2-yl)-N-90-heptadecanylcarbazole (0.657 g,
1 mmol), compound 4 (4.311 g, 1 mmol), Aliquat 336 (20 mg)
and (PPh3)4Pd (34 mg, 0.03 mmol) (1.5 mol % based on total
monomers) were dissolved in degassed toluene (6 mL) in a 25 mL
two-necked round-bottom flask. A 2 M aqueous solution of
K2CO3 (4 mL) was added. The solution was stirred vigorously
under argon at 90 °C for 48 h. The resulting mixture was then
poured into 250 mL of methanol. The precipitate was collected
by filtration and washed with 2 M HCl and methanol. The
yellowish-red solid product of PCzTPA-TVCHO was ex-
tracted with acetone for 24 h in a Soxhlet apparatus to remove
oligomers and catalyst residues to yield PCzTPA-TVCHO as a
yellowish-red solid (620 mg, yield: 79%). 1H NMR (400 MHz,
CDCl3, δ, ppm): 9.85 (s, 1H), 8.20 (br, 2H),7.80 (s, 1H), 7.67(s,
5H), 7.60 (s, 1H), 7.48 (br, 5H), 7.32 (d, 8 Hz, 4H), 7.22 (d, 8 Hz,
4H), 7.14 (s, 2H), 4.68 (br, 1H), 2.34 (m, 2H), 1.98 (br, 2H), 1.26
(br, 8H), 1.12 (br,16H), 0.80 (t, J = 6.4 Hz, 6H). 13C NMR
(150 MHz, CDCl3, δ, ppm): 182.50; 153.13; 148.30; 146.20;
143.16; 141.13; 139.84; 137.60; 137.41; 132.60; 132.18; 131.93;
129.93; 128.82; 128.58; 128.51; 128.08; 126.05; 125.12; 123.24;
120.69; 120.43; 119.03; 118.30; 109.68; 106.98; 56.484; 33.87;
31.80; 32.51; 29.51; 29.44; 29.39; 29.23; 26.94; 22.67; 22.63;
21.99; 14.10. Anal. Calcd for C54H58N2S: C, 82.82; H, 7.47; N,
3.58; S, 4.09. Found: C, 82.70; H, 7.50; N, 3.49; S, 4.09.
Synthesis of PCzTPA-TVCN. A 20 equiv sample of octyl
cyanoacetate (847 mg) was added to a solution of PCzTPA-
TVCHO (168 mg, 0.25 mmol of basic units) in 20 mL of
pyridine. The solution turned red immediately and was heated
to 55 °C for 48 h. The solution was concentrated under reduced
pressure, and the residue was added dropwise to excess metha-
nol to precipitate the polymer of PCzTPA-TVCN as a red solid
(120 mg). Further purification was performed by dissolution
of the polymer in CHCl3 (100 mL) and subsequently passing
through a column packed with cotton and silica gel. The com-
bined polymer solution was concentrated and was poured into
1
methanol. H NMR (400 MHz, CDCl3, δ, ppm): 8.26 (s, 1H),
8.18 (br, 2H),7.76 (s, 1H), 7.68 (s, 4H), 7.63 (br, 1H), 7.48 (br,
5H), 7.22 (br, 4H), 7.17 (d, J=8 Hz, 2H), 7.12 (s, 1H), 4.68 (br,
1H), 4.29 (dd, J=6.4 Hz, 2H), 2.39 (br, 2H), 1.98 (br, 2H), 1.76
(br, 2H), 1.40 (br, 10 H), 1.12 (br, 24 H), 0.89 (t, J=7.0 Hz, 3H),
0.80 (t, J=6.4 Hz, 6H). 13C NMR (150 MHz, CDCl3, δ, ppm):
163.22; 153.85; 148.47; 146.33; 146.132; 143.16; 139.84; 139.41;
137.68; 133.97; 133.27; 132.18; 132.11; 128.82; 128.54; 128.19;
127.66; 126.43; 125.25; 123.02; 120.69; 120.42; 118.66; 118.28;
116.33; 109.69; 106.99; 97.17; 66.57; 33.87; 31.80; 29.75; 29.52;
29.39; 29.23; 29.20; 28.626; 26.95; 25.86; 22.69; 22.63; 14.15;
14.11. Anal. Calcd for C65H75N3O2S: C, 81.12; H, 7.86; N, 4.37,
S, 3.33. Found: C, 81.05; H, 7.90; N, 4.40; 3.35.
Synthesis of PCzTPA-TVDCN. A 20 equiv sample of mal-
ononitrile (282 mg) was added to a solution of PCzTPA-TV-
CHO (168 mg, 0.25 mmol of basic units) in a mixture of 20 mL of

Article
Macromolecules, Vol. 43, No. 22, 2010 9379
chloroform and 1 mL pyridine. The solution turned purple
immediately and was heated to 40 °C for 24 h. The solution
was concentrated under reduced pressure, and the residue was
added dropwise, to excess methanol to precipitate the polymer
of PCzTPA-TVDCN as a purple solid (116 mg). Further puri-
fication of the polymer was carried out as that of PCzTPA-
TVCN. 1H NMR (400 MHz, CDCl3, δ, ppm): 8.18 (m, 2H),7.80
(s, 1H), 7.74 (s, 1H), 7.68 (br, 4H), 7.60 (br, 1H), 7.47 (m, 4H),
7.32 (m, 4H), 7.22 (m, 4H), 7.14 (m, 2H), 4.68 (br, 1H), 2.34 (m,
2H), 1.98 (br, 2H), 1.26 (br, 8H), 1.12 (br, 16H), 0.80 (t, J=6.4
Hz, 6H). 13C NMR (150 MHz, CDCl3, δ, ppm): 155.82; 150.07;
148.89; 145.98; 143.14; 140.33; 139.83; 137.86; 134.65; 133.21;
132.10; 129.24; 128.82; 128.58; 128.51; 128.43; 126.50; 125.41;
122.67; 120.71; 120.44; 118.26; 118.04; 113.79; 109.70; 106.99;
56.48; 33.86; 31.79; 29.74; 29.50; 29.38; 29.22; 26.94; 22.62;
14.10. Anal. Calcd for C57H58N4S: C, 82.37; H, 7.03; N, 6.74,
S, 3.86. Found: C, 82.35; H, 7.13; N, 6.59; 3.92.
Figure 1. FT-IR spectra of the copolymers.
desirable.34 To overcome the reactivity difference among the
monomers, a postmodification approach was adopted to
modify the pendant groups on the thienylenevinylene side
chains of PCzTPA-TVCHO, forming PCzTPA-TVCN,
PCzTPA-TVDCN, and PCzTPA-TVDT. Thus, all the
three polymers have the same number of conjugation units
as PCzTPA-TVCHO with fine-tuned electronic structure.
For the synthesis of PCzTPA-TVCN, the optimum reac-
tion condition to achieve complete conversion of the alde-
hyde groups was found to be at 55 °C for 48 h in pyridine as
both solvent and base, while for PCzTPA-TVDCN and
PCzTPA-TVDT, the optimum reaction condition is in
chloroform at 40 °C for 24 h and using 1 mL pyridine as
base. After purification and drying, PCzTPA-TVCHO,
PCzTPA-TVCN, PCzTPA-TVDCN, and PCzTPA-TVDT
were obtained as yellowish-red, red, purple, and dark blue
solids, respectively, indicating significant change in their optical
properties. The bulky alkyl side chains attaching on the nitro-
gen atom of the carbazole unit combined with the amorphous
nature of the triphenylamine group, account for the high
solubility of the copolymers in chlorinated solvents, such as
dichloromathane, chloroform and dichlorobenzene, which is
important for their application in PSCs.
Synthesis of PCzTPA-TVDT. A 20 equiv sample of 1,3-
diethyl-2-thiobarbituric acid (855 mg) was added to a solution of
PCzTPA-TVCHO (168 mg, 0.25 mmol of basic units) in a
mixture of 20 mL of chloroform and 1 mL pyridine. The solu-
tion turned blue immediately and was heated to 40 °C for 24 h.
The solution was concentrated under reduced pressure, and the
residue was added dropwise to excess methanol to precipitate
the polymer of PCzTPA-TVDT as a dark blue solid (127 mg).
Further purification of the polymer was carried out as that of
1
PCzTPA-TVCN. H NMR (400 MHz, CDCl3, δ, ppm): 8.63
(s, 1H), 8.16 (br, 2H), 7.70 (m, 5H), 7.64 (s, 1H), 7.50 (r, 6H), 7.35
(m, 3H), 7.20(m, 3H), 4.68 (br, 5H), 2.39 (br, 2H), 1.98 (br, 2H),
1.26 (br, 30H), 0.80 (t, J=6.4 Hz, 6H). 13C NMR (150 MHz,
CDCl3, δ, ppm): 178.68; 161.17; 160.95; 160.01; 149.12; 148.82;
147.54; 145.99; 143.15; 139.84; 137.86; 135.99; 134.73; 129.62;
128.82; 128.58; 128.48; 126.84; 125.42; 122.92; 122.73; 120.70;
120.45; 118.93; 118.27; 109.69; 109.41; 106.99; 56.49; 43.99;
43.18; 33.87; 31.80; 29.74; 29.51; 29.39; 29.23; 26.94; 22.63;
14.11; 12.60; 12.44. Anal. Calcd for C62H68N4O2S2: C, 77.14;
H, 7.10; N, 5.58, S, 6.64. Found: C, 77.20; H, 7.09; N, 5.49; 6.73.
Results and Discussion
Polymer Synthesis and Characterization. Chemical struc-
tures and synthetic routes of the main chain donor-side chain
acceptor D-A copolymers are depicted in Schemes 1 and 2.
The synthesis of alkylated compound 2 was well described by
Leclerc et al. in a DMSO-KOH system.29 To overcome the
inconvenience brought by the high boiling point solvent
(DMSO) along with low solubility of sulfonate in DMSO,
we developed an easier synthesis procedure for Compound 2
in this work. The N-alkylation of 2,7-dibromocarbazole (1)
was attained by the reaction with 9-heptadecane p-toluene-
sulfonate for 2 h using KOH as the base and acetone as the
solvent, in the presence of TBAHS as a phase transfer
reagent. The aryl bromide obtained was converted to the bo-
ronic ester using bis(pinacolato)diboron via Pd-mediated
borylation. This procedure does not require generation of
the corresponding lithiated species (which are very sensitive
to air and moisture) typically used to form arylboronates.33
The dibromide, 2-[2-[4-[N,N-di(4-bromophenyl) amino]-
phenyl]ethenyl]thien-5-al (4), was synthesized from commer-
cially available triphenylamine in four steps.26,32 The “basic”
copolymer, viz. PCzTPA-TVCHO, was synthesized by Suzuki
polycondensation of an equimolecular mixture of the dibor-
onate (3) and dibromide (4), as illustrated in Scheme 1. The
copolymerization was carried out for 48 h using Pd(PPh3)4 as
the catalyst in a two-phase mixture of degassed toluene and
aqueous K2CO3 (2.0 M) and in the presence of Aliquat 336 as
the phase transfer reagent. To study the structure-property
relationship, polymers with similar molecular architecture
and bearing comparable numbers of conjugation units are
The conversion of aromatic aldehyde of PCzTPA-TV-
CHO to other acceptor groups was confirmed by Fourier
transform infrared (FT-IR) absorption spectroscopy. As
shown in Figure 1, the characteristic absorption peaks of
the -CHO group appeared at 1666 and 2796 cm-1, belong-
ing to the C-H and CdO stretching vibrations, respec-
tively.35 After transformation into PCzTPA-TVCN and
PCzTPA-TVDCN, these characteristic absorptions of al-
dehyde disappeared, and new absorption peaks at 2217 cm-1
(corresponding to CtN stretching for the two polymers) and
1710 cm-1 (corresponding to CdO stretching for PCzTPA-
TVCN) emerged, indicative of complete conversion of the
terminal aldehyde group. It should be noted that for
PCzTPA-TVDT, due to overlapping of vibration in the
aldehyde group and that of 1,3-diethyl-2-thiobarbituric acid
moiety, the conversion of aldehyde group into the 1,3-
diethyl-2-thiobarbituric acid group in PCzTPA-TVDT can-
not be identified in the FT-IR spectrum. However, it can be
clearly confirmed in the 1H NMR spectra. In the 1H NMR
spectra (see Figure S1 in the Supporting Information), the
complete disappearance of the aldehyde proton signal at
9.8 ppm10 and the appearance of olefinic proton signals at
8.26 ppm for PCzTPA-TVCN, 7.74 ppm for PCzTPA-
TVDCN, and 8.62 ppm27 for PCzTPA-TVDT, confirmed
the complete structural conversion. In addition, the ratios of
integrated peak areas of olefinic to aliphatic protons and that
of olefinic to methine group adjacent to the carbazole N
atom protons are consistent with the proposed structures of

9380 Macromolecules, Vol. 43, No. 22, 2010
Table 1. Molecular Weights, Thermal Properties, and Physical Properties of the Copolymers
Zhang et al.
polymers
Mn (g/mol)
PDI (Mw/Mn)
Tda (°C)
λ absb (nm)
Egc (eV)
HOMOd (eV)
LUMOe (eV)
PCzTPA-TVCHO
PCzTPA-TVCN
PCzTPA-TVDCN
PCzTPA-TVDT
14.2K
18.7K
15.9K
19.8K
3.93
3.58
3.62
3.49
360
289
399
264
269, 388, 445
270, 385, 519
271, 383, 548
269, 380, 588
2.38
2.00
1.89
1.74
-5.27
-5.29
-5.30
-5.29
-2.89
-3.29
-3.41
-3.55
a 5% weight loss temperature measuredby TGA under N2. b For the polymer films. c Band gap, estimated from the optical absorption band edge of the
films. d Calculated from the onset oxidation potentials of the polymers. e Estimated using empirical equations ELUMO = EHOMO þ Eg.
Figure 2. UV-visible absorption spectra of the copolymers (a) in toluene solutions and (b) in films.
PCzTPA-TVCN, PCzTPA-TVDCN, and PCzTPA-
TVDT, as depicted in Scheme 2.
groups produces new absorption bands at 445 nm for
PCzTPA-TVCHO, 519 nm for PCzTPA-TVCN, 548 nm
for PCzTPA-TVDCN, and 588 nm for PCzTPA-TVDT,
attributable to the intramolecular charge transfer (ICT) inter-
action10,38 between the polymer backbone and the pendant
acceptor groups. Thus, both the main chain and side chain
contribute to the broad nature of the absorption spectra of
the copolymers, which is much like that of the conjugated
side chain polymers developed in our group.18 Evidently, the
ICT interactions are progressively enhanced with the in-
crease of the electron affinity of the pendant acceptor group
in the D-A copolymers, due to the enhanced electron
accepting ability of the acceptor moiety.10 For the as-spun
film on quartz substrate, the UV-visible absorption spectra
of the copolymers are red-shifted and broadened (Figure 2b),
as compared with that of their corresponding dilute solu-
tions. The red-shift and broadening of the absorption spectra
of the polymer films is a common phenomenon, which results
from the enhanced interchain interaction in the solid films
and is probably related to increased polarizability of the
film.10 It seems that the interchain interaction in the main
chain donor-side chain acceptor D-A copolymers is not as
strong as those of the main chain D-A copolymers, prob-
ably due to the fact that the bulky side chains and propeller-
twisting conformation of the TPA moiety retard the con-
formation change of the polymer chains in the solid state.38,39
The absorption edges of the copolymer films are located at
521 nm, 620 nm, 656 and 713 nm for PCzTPA-TVCHO,
PCzTPA-TVCN, PCzTPA-TVDCN, and PCzTPA-TVDT
respectively, from which the optical bandgaps were calcu-
lated to be 2.38 eV for PCzTPA-TVCHO, 2.00 eV for
PCzTPA-TVCN, 1.89 eV for PCzTPA-TVDCN, and 1.74
eV for PCzTPA-TVDT. The absorption peak wavelength and
optical bandgaps of the copolymer films are also listed in
Table 1. The variation in the absorption and bandgap suggests
the possibility of tuning the photophysical properties by the
simple manipulation of acceptor strength in the pendant
groups.
Molecular weight of the polymers was determined by gel
permeation chromatography (GPC). Number-average mo-
lecular weight (Mn) of PCzTPA-TVCHO is 1.42 ꢀ 104 g/
mol, corresponding to a polymerization degree of ca. 18,
with a polydispersity index of 3.93. The structural transfor-
mation of PCzTPA-TVCHO into other three copolymers
has led to a slight increase in molecular weights of the resulting
copolymers, as shown in Table 1. The Mn’s of PCzTPA-
TVCN, PCzTPA-TVDCN, and PCzTPA-TVDCN were
found to be 1.87 ꢀ 104, 1.59 ꢀ 104, and 1.98 ꢀ 104 g/mol
respectively, with the corresponding polydispersity indices of
3.58, 3.62, and 3.49.
Thermal Stability. Thermal property of the copolymers
was determined via thermogravimetric analysis (TGA) under a
nitrogen atmosphere. As shown in Table 1 and Figure S2 (see
Supporting Information), the 5% weight-loss temperatures
for PCzTPA-TVCN, PCzTPA-TVDCN, and PCzTPA-
TVDT are in the range of 264-399 °C, which is adequate for
application of the copolymers as active materials in the
optoelectronic devices.36
Optical Properties. Figure 2 shows the absorption spectra
of the copolymers in solutions and in solid films. All the
polymers are featured by three distinct absorption peaks. In
the polymer solutions, the sharp absorption peak around 270
nm originates from n-π* transition of the backbone,37 the
maximum absorptions of the copolymers at 388 nm for
PCzTPA-TVCHO, 385 nm for PCzTPA-TVCN, 383 nm
for PCzTPA-TVDCN, and 380 nm for PCzTPA-TVDT
(Figure 2a), correspond to the π-π* transition of the poly-
mer backbone. The π-π* transition absorption peaks of the
polymer backbones, are slightly blue-shifted when the pen-
dant acceptor groups are varied from the aldehyde group to
the monocyano group, to the dicyano group and further to
1,3-diethyl-2-thiobarbituric acid group. This phenomenon is
probably related to the increasing extent of distortion of the
backbones, thereby reducing the effective conjugation length
of the TPA-containing copolymers.10
Electrochemical Properties and Electronic Energy Levels.
The highest occupied molecular orbital (HOMO) and lowest
unoccupied molecular orbital (LUMO) energy levels of the
In addition to the absorptions of carbazole-alt-TPA back-
bones, the introduction of electron-withdrawing pendant

Article
Macromolecules, Vol. 43, No. 22, 2010 9381
conjugated polymers are important parameters in the design
of optoelectronic devices, and they can be estimated from the
onset oxidation and reduction potentials.40 Figure S3 (see
Supporting Information) shows the cyclic voltammograms
of the four copolymers. The onset oxidation potentials
(Eoxonset) are 0.56 V for PCzTPA-TVCHO, 0.58 V for
PCzTPA-TVCN, 0.59 V for PCzTPA-TVDCN, and 0.58 V
vs. Ag/Agþ for PCzTPA-TVDT. The HOMO energy levels
of the polymers were estimated according to the following
onset
equations:41 EHOMO=-e(Eox
þ4.71) (eV), where the unit
of Eoxonset is V vs. Ag/Agþ. The HOMO levels of PCzTPA-TV-
CHO, PCzTPA-TVCN, and PCzTPA-TVDCN, calculated in
this way, are -5.27, -5.29, and -5.30, and -5.29 eV, re-
spectively, as listed in Table 1. Since we could not obtain
reliable onset reduction potentials for the four copolymers,
the optical band gaps (Eg) of the polymer films (which were
calculated from their absorption band edge) were utilized to
estimate the LUMO energy levels of the copolymers based
on the relation of Eg = ELUMO - EHOMO. The estimated
LUMO energy levels of PCzTPA-TVCHO, PCzTPA-
TVCN and PCzTPA-TVDCN, and PCzTPA-TVDT are
-2.89, -3.29, -3.41, and -3.55 eV, respectively, which are
also shown in Table 1. Obviously, the HOMO energy levels
of the four copolymers remain almost unchanged, in spite of
the presence of different pendant acceptor groups, while
their LUMO energy levels decreased after attaching the
acceptor cyano and 1,3-diethyl-2-thiobarbituric acid (DT)
acceptor groups in the conjugated side chains on the TPA
units. Regarding that the threshold HOMO level for air
stable conjugated polymers being estimated to be -5.2 eV,42,43
lower HOMO levels of the three copolymers with cyano and
DT acceptor groups should be beneficial to their chemical
stability in ambient conditions. In addition, the deeper
HOMO level of the copolymers is desirable for higher open
circuit voltage (Voc) of the PSCs with polymers as donor
materials because the Voc is usually proportional to the
difference between the LUMO level of the acceptor and the
HOMO level of the donor.9
Theoretical Calculations. To provide further insight into
the fundamentals of molecular architecture, molecular simu-
lation was carried out for PCzTPA-TVCHO, PCzTPA-
TVCN, PCzTPA-TVDCN, and PCzTPA-TVDT with a
chain length of n=1 at the DFT B3LYP/6-31G(d) level with
the Gaussian 03 program package.44 The bulky N-alkyl
chain was not included in the calculation to avoid excessive
computation demand, since they do not significantly affect
the equilibrium geometry and thus the electronic properties.
All CdC bonds outside the aromatic rings were set to be in
trans configuration. As depicted in Figure 3, the LUMOs for
all the four polymers are located on the peripheral tripheny-
lamine groups and the adjacent carbazole rings, and the
computed HOMO values are close to one another for the
four polymers. The electron density distributions at LUMO
become highly localized near the electron-withdrawing
group, and the well-separated distribution of HOMO and
LUMO levels indicate that a transition between them can be
considered as a charge-transfer transition. The unique elec-
tron distribution patterns of PCzTPA-TVCN, PCzTPA-
TVDCN, and PCzTPA-TVDT have the advantage of al-
lowing charge separation through sequential transfer of
electrons from the main chains to the side chain acceptor
groups and then to fullerene acceptor in a bulk-heterojunc-
tion PSC. Also, it can be envisioned from the DFT results
that LUMO energy levels can be further decreased by
appending a stronger pedant acceptor within this molecular
architecture, indicative of flexibility of this special main
chain donor-side chain acceptor D-A copolymer architec-
Figure 3. HOMO and LUMO energy levels and the frontier molecular
orbital obtained from DFT calculations on PCzTPA-TVCHO,
PCzTPA-TVCN, PCzTPA-TVDCN, and PCzTPA-TVDT with a
chain length n = 1 at the B3LYP/6-31G* level of theory.
Table 2. Photovoltaic Parameters of the PSCs Based on Copolymer/
PC70BM (1:4, w/w)
Voc
(V)
J
sc (mA
)
FF
(%)
PCE
(%)
thickness
(nm)
copolymers
cm-2
PCzTPA-TVCN 0.79
PCzTPA-TVDCN 0.84
PCzTPA-TVDT 0.87
6.41
7.00
7.50
0.383 1.94
0.415 2.44
0.424 2.76
60
67
60
ture for tuning the optoelectronic properties of the conju-
gated polymers.
In addition, it can be seen from Figure 3 and Table 1 that
although discrepancies exist between the calculation and
experimental results, the trends of variation in the HOMO
and LUMO energy levels, as well as the energy gaps, are
similar. The presence of pendant cyano and 1,3-diethyl-2-
thiobarbituric acid groups with strong electron affinity leads
to the decrease of the LUMO energy levels of PCzTPA-
TVCN, PCzTPA-TVDCN, and PCzTPA-TVDT but af-
fects their HOMO levels to a much lesser extent. This
phenomenon is significantly different from most main chain
D-A type copolymers, whose HOMO levels were elevated a
lot when using different acceptors to reduce the band gaps.45
Photovoltaic Properties. Bulk heterojunction PSCs using the
three copolymers PCzTPA-TVCN, PCzTPA-TVDCN, and
PCzTPA-TVDT as donor and PC70BM as acceptor were
investigated with the device configuration of ITO/PEDOT:
PSS/copolymer:PC70BM(1:4, w/w)/Al. Photovoltaic perfor-
mance characteristics of the PSCs are summarized in Table 2.
PC70BM was chosen as the acceptor because it has stronger
absorption in the visible region from 400 to 530 nm,46 which
can complement the absorption valley between the π-π transi-
tion of the backbone and ICT interactions, as demonstrated by
the absorption spectra of the polymer film (Figure 2) and the
polymer/PC70BM blend film (see Figure S4 in Supporting
Information). Figure 4 shows the current density-voltage
(J-V) curves of the PSCs based on the copolymers/PC70BM.
PCzTPA-TVDT demonstrated better photovoltaic perfor-
mance over the other two copolymers with Jsc of 7.5 mA/
cm2, FF of 0.42, Voc of 0.87 V, corresponding to a PCE of
2.76%. For PSCs based on the three copolymers as donor, the
Voc is around ∼0.8 V, which is similar to that of the PSCs based
on other carbazole copolymer donors. The high Voc is the result
of the lower HOMO level of the copolymers. From
PCzTPA-TVCN to PCzTPA-TVDCN and further to
PCzTPA-TVDT, the progressively improved absorption
spectra brought by the enhanced ICT interaction, should
account for the increase in Jsc. The incident-photon-to-con-
verted current efficiency (IPCE) spectra of the PSCs, as shown

9382 Macromolecules, Vol. 43, No. 22, 2010
Zhang et al.
Figure 4. Current density-voltage characteristics of PSCs based on the 1:4 weight ratio composite films of PCzZTPA-TVCN:PC70BM (a),
PCzTPA-TVDCN:PC70BM (b), and PCzTPA-TVDT:PC70BM (c). The incident-photon-to-converted-current efficiency (IPCE) spectra of the
corresponding devices: PCzTPA-TVCN:PC70BM (d), PCzTPA-TVDCN:PC70BM (e), and PCzTPA-TVDT:PC70BM (f).
Figure 5. Tapping mode AFM topography images (5 ꢀ 5 μm2) of the 1:4 (weight ratio) composite films of (a) PCzTPA-TVCN/PC70BM,
(b) PCzTPA-TVDCN/PC70BM, and (c) PCzTPA-TVDT/PC70BM.
in Figure 4, are consistent with the absorption spectra of the
blend films of the active layer and the short circuit current
values of the PSCs.
Therefore, it can be expected that even better device perfor-
mance could be achieved, by further device engineering, such
as thermal annealing, solvent annealing, and additives to
promote proper phase separation.
AFM Morphology. The morphologies of the blend films
of PC70BM and the three copolymers PCzTPA-TVCN,
PCzTPA-TVDCN, and PCzTPA-TVDT were investigated
by atomic force microscopy (AFM). As shown in Figure 5,
all the blend films show smooth surface with root-mean-
square (rms) roughness of 0.50 nm for PCzTPA-TVCN,
0.45 nm for PCzTPA-TVDCN, and 0.47 nm for PCzTPA-
TVDT, indicating excellent miscibility between the copoly-
mers and PC70BM. As suggested by the optimized geometry
of the copolymers, the two-dimensional architecture com-
bined with twisted propeller conformation of the TPA
moiety would create large free volume, where fullerene mole-
cules can be dispersed well over the polymer network, thus no
evident phase separation was observed. Although the well-
distribution of fullerene molecules over polymer network
can offer large heterojunction interface area for exciton
dissociation, proper phase separation is desirable in the
composite films to enable a continuous percolating path
for hole and electron transport to the corresponding electrodes.
Conclusions
A conjugated alternating copolymer of carbazole and triphe-
nylamine bearing the thienylene-vinylene conjugated side chain
with aldehyde terminal group, PCzTPA-TVCHO, was synthe-
sized via the Suzuki coupling method. Then three copolymers,
with the side chains containing acceptor groups of monocyano
(PCzTPA-TVCN), dicyano (PCzTPA-TVDCN), and 1,3-di-
ethyl-2-thiobarbituric acid (PCzTPA-TVDT), were synthesized
via chemical transformation of the aldehyde group in PCzTPA-
TVCHO, into respective acceptor groups. The synthesis strategy
allows direct comparison of polymers with the same number of
conjugation units for elucidating their structure-property rela-
tionships. The copolymers exhibit good solubility in common
organic solvents and good thermal stability. Through manipulat-
ing the strength of the acceptor groups attached at the conjugated
side chain of the copolymer backbone, the energy levels, and thus

Article
Macromolecules, Vol. 43, No. 22, 2010 9383
the photophysical and electronic properties, as well as the
absorption spectra of the copolymers were effectively tuned. The
power conversion efficiency of the PSCs based on the copolymers/
PC70BM (1:4, w/w) reached 1.94% for PCzTPA-TVCN, 2.44%
for PCzTPA-TVDCN and 2.76% for PCzTPA-TVDT, under
the illumination of AM1.5, 100 mW/cm2. The chemical transfor-
mation approach thus provides a simple and effective way for
tuning the optoelectronic properties of the conjugated polymers.
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50633050, 20721061 and 50933003), and the Chinese Academy
of Sciences. The authors thank Dr. Shanghui Ye for the AFM
measurements.
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and UV-vis absorption spectra of the blend films of copolymer/
PC70BM. This material is available free of charge via the
Internet at http://pubs.acs.org.
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