Synthesis of triarylamine-based alternating copolymers for polymeric solar cell

Article abstract of DOI:10.1016/j.polymer.2014.08.018

Two donor-acceptor alternating copolymers based on electron-rich triarylamine, di(1-(6-(2-ethylhexyl))naphthyl)phenylamine (DNPA), and electron-deficient benzothiadiazole and benzoselenadiazole derivatives were designed and synthesized via Suzuki coupling reaction. The resulting triarylamine-based alternating copolymers PDNPADTBT and PDNPADTBS showed good solubility in common organic solvents and good thermal stability. The optical band gaps determined from the onset absorption were 1.93 and 1.81 eV, respectively. By introducing the naphthalene ring into the triarylamine, copolymers had relatively deep HOMO energy levels of -5.48 and -5.45 eV, which led to a high open circuit voltage (V_oc) and good air stability for photovoltaic application. Bulk heterojunction solar cells were fabricated with a structure of ITO/PEDOT-PSS/copolymers-PC₇₀BM/LiF/Al by blending the copolymer with PC₇₀BM. Both blend systems showed remarkably high V_oc near 0.9 V, and the highest performance of 2.2% was obtained from PDNPADTBT, with V_oc = 0.88 V, J_sc = 7.4 mA/cm², and a fill factor of 34.4% under AM 1.5 G.

Full text of DOI:10.1016/j.polymer.2014.08.018

Polymer 55 (2014) 4837e4845
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Synthesis of triarylamine-based alternating copolymers for polymeric
solar cell
Jinhee Lee a, Hyojung Cha b, Hoyoul Kong c, Myungeun Seo d, Jaewon Heo a,
,
, *
In Hwan Jung a, Jisung Kim a, Hong-Ku Shim a, Chan Eon Park b **, Sang Youl Kim a
a Department of Chemistry, KAIST, Daejeon 305-701, Republic of Korea
b Polymer Research Institute, Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784,
Republic of Korea
c Korea Research Institute of Chemical Technology (KRICT), Division of Chemical R&BD at Ulsan, Ulsan 681-310, Republic of Korea
d Graduate School of Nanoscience and Technology KAIST, Daejeon 305-701, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Two donor-acceptor alternating copolymers based on electron-rich triarylamine, di(1-(6-(2-ethylhexyl))
naphthyl)phenylamine (DNPA), and electron-deficient benzothiadiazole and benzoselenadiazole de-
rivatives were designed and synthesized via Suzuki coupling reaction. The resulting triarylamine-based
alternating copolymers PDNPADTBT and PDNPADTBS showed good solubility in common organic sol-
vents and good thermal stability. The optical band gaps determined from the onset absorption were 1.93
and 1.81 eV, respectively. By introducing the naphthalene ring into the triarylamine, copolymers had
Received 23 May 2014
Received in revised form
4 August 2014
Accepted 7 August 2014
Available online 15 August 2014
relatively deep HOMO energy levels of ꢀ5.48 and ꢀ5.45 eV, which led to a high open circuit voltage (Voc
)
Keywords:
and good air stability for photovoltaic application. Bulk heterojunction solar cells were fabricated with a
structure of ITO/PEDOT-PSS/copolymers-PC70BM/LiF/Al by blending the copolymer with PC70BM. Both
blend systems showed remarkably high Voc near 0.9 V, and the highest performance of 2.2% was obtained
from PDNPADTBT, with Voc ¼ 0.88 V, Jsc ¼ 7.4 mA/cm2, and a fill factor of 34.4% under AM 1.5 G.
© 2014 Elsevier Ltd. All rights reserved.
Triarylamine-based alternating copolymer
Benzothiadiazole derivative
Polymeric solar cell
1. Introduction
commercialization [9]. Furthermore, concurrent increase in short
circuit current (Jsc) and open circuit voltage (Voc) have been realized
Efforts to find sustainable energy sources have led development
of new renewable energy technologies which use wind power,
geothermal heat, tidal power, and solar energy [1]. Among them,
solar energy has been studied intensively for decades to transform
sunlight into electrical energy and various types of solar cells were
designed on the basis of materials [2e6]. Polymer-based organic
solar cells (PSCs) have attracted scientific interest owing to its low
cost, easy processability, portability, and ability to make large-area
devices [7,8]. Particularly, fabrication of PSCs with a bulk hetero-
junction structure, typically a bicontinuous network of electron-
donating polymer and electron-acceptor such as phenyl-C61-
butyric acid methyl ester (PC61BM), has enhanced charge separa-
tion and transportation and therefore significantly increased power
by development of low band gap conjugated polymers which
consist of electron-rich donors and electron-deficient acceptors in
the polymer main chains in an alternating fashion, which resulted
low HOMO energy levels and low band gaps via pushepull effect
[10]. PCE values up to 8% have been reported by using low band gap
alternating copolymers [11e15].
Triarylamine is a well-known electron-rich moiety that readily
interacts with electron-deficient acceptor species via charge-
transfer interaction and also undergoes oxidation to form stable
radical cation species [16]. Many triarylamine derivatives have been
widely investigated and applied in various electro-optical materials
such as organic light-emitting diodes (OLEDs), organic field-effect
transistors (OFETs), organic solar cells (OSCs), and dye-sensitized
solar cells (DSCs) [17e20]. Especially, excellent electron-donating
ability of the triarylamine moiety was intensively exploited for
bulk heterojunction OSC and PSC applications [21e33]. Moreover,
recent reports have suggested that triarylamine-based alternating
copolymers could be promising candidates for donor material due
to its amorphous film formability and highly reproducible perfor-
mances [34e36]. However, there have been only few papers on
conversion efficiency (PCE) close to
a level required for
* Corresponding author. Tel.: þ82 42 350 2834.
** Corresponding author. Tel.: þ82 54 279 2269.
E-mail addresses: cep@postech.ac.kr (C.E. Park), kimsy@kaist.ac.kr, polysky@
gmail.com (S.Y. Kim).
http://dx.doi.org/10.1016/j.polymer.2014.08.018
0032-3861/© 2014 Elsevier Ltd. All rights reserved.

4838
J. Lee et al. / Polymer 55 (2014) 4837e4845
incorporating triarylamine moieties with an electron-deficient
acceptor along the conjugated backbone to achieve low band gap
polymers [34e43]. Among them, copolymers composed of triphe-
nylamine and 2,1,3-benzothiadiazole (BT) derivatives showed
remarkable result, but they showed relatively high HOMO level and
solubilizing groups on BT derivatives were required due to low
solubility [34,36e38,40,41].
following compounds were synthesized according to the pro-
cedures in the literature: 5-iodo-2-naphthol (2), 4,7-bis(5-bromo-
2-thienyl)-2,1,3-benzothiadiazole (6), and 4,7-bis(5-bromo-2-
thienyl)-2,1,3-benzoselenadiazole (7) [49,53e55].
2.2. Material characterization
To this end, we designed new low band gap conjugated poly-
mers consisting of a triarylamine moiety. The target polymer
structure is shown in Fig. 1. Replacement of the phenyl rings of the
triphenylamine with other aromatic rings such as naphthalene
could be a quite efficient method to control the energy level of the
copolymer, and Kwon group has reported incorporation of naph-
thyl groups lowers the HOMO energy level of the copolymers and
increase Voc of the solar cell devices [35,44,45]. From this point of
view, we introduced two (2-ethylhexyloxy)-naphthyl groups to the
triarylamine moiety to render solubility of the copolymer and
further control the HOMO energy level and Voc. Meanwhile, we
chose BT unit as electron-deficient moiety to form alternating
copolymer with the triarylamine moiety. It has been proved that BT
is excellent electron-deficient unit and successfully incorporated in
low band gap conjugated polymers for electroluminescent and
photovoltaic applications [46,47]. In addition, to enhance associa-
tion between the polymer chains, we designed to increase conju-
gated length of the electron-deficient moiety by introducing two
thiophene rings at both sides of BT unit [48]. Its analog 2,1,3-
benzoselenadiazole (BS) was also incorporated into the copol-
ymer as electron-deficient unit. BS containing polymers are known
to be more effectively moving the absorption of light toward the
longer wavelength region than BT while its HOMO energy level
remains similar value with BT [49e51]. Therefore, it is expected
that BS based polymers are more efficient to harvest solar energy,
but the photovoltaic performance of polymers-based on carbazole
and 4,7-di-2,1,3-benzoselenadiazole (DTBS) units lagged behind
result that of 4,7-di-2-thienyl-2,1,3-benzothiadiazole (DTBT) [52].
Thus, we developed new triarylamine-based alternating co-
polymers and compared the effect of BT and BS on the properties of
copolymers.
The Fourier-transform infrared (FTIR) spectra of the copolymers
were obtained with a Burker EQUINOX-55 spectrophotometer us-
ing a KBr pellet. 1H and 13C NMR spectra of synthesized materials
were recorded on a Bruker Fourier Transform AC 400 (400 MHz)
spectrometer. Chemical shifts were expressed in part per million
(ppm) with reference to the peak of residual DMSO (2.49 for 1H and
39.52 ppm for 13C) and CHCl3 (7.24 ppm for 1H and 77.16 ppm for
13C), respectively. Mass spectra were obtained with a Bruker Dal-
tonik microTOF-QII mass spectrometer using electrospray ioniza-
tion (ESI) method, and elemental analysis was conducted using a
Thermo Scientific FLASH 2000 series elemental analyzer. Molecular
weights and molecular weight distributions of polymers were
measured by the means of size exclusion chromatography (SEC).
The SEC diagrams were obtained with Viscotek TDA302 equipped
with a RI detector and three packing columns (PLgel 10 mm MIXED-
B) using tetrahydrofuran (THF) as an eluant at 35 ꢁC. The number-
and weight-average molecular weights of the polymers were
calculated relative to linear polystyrene standards. Thermogravi-
metric analysis (TGA) and differential scanning calorimetry (DSC)
were conducted on a TA Instruments TGA Q500 and a DSC Q2000,
respectively. The TGA and DSC measurements were performed
under nitrogen atmosphere at a heating rate of 10 ꢁC/min. UV/vis
spectra were measured on a JASCO V-530 and an Optizen Pop
spectrometer. Films for the UV-vis measurements were prepared
by spin coating of the polymer solution in chloroform (1 wt%).
Cyclic voltammetry (CV) was performed on an AUTOLAB/PG-
STAT12 model with a three-electrode cell in a solution of tetrabu-
tylammonium tetrafluoroborate (Bu4NBF4) (0.1 M) in acetonitrile at
a scan rate of 50 mV sꢀ1. A film of each polymer was coated onto a Pt
wire electrode by dipping the electrode into a solution of the
polymer. The Ag/Agþ reference electrode was calibrated using a
ferrocene/ferrocenium redox couple as an internal standard, whose
oxidation potential is set at ꢀ4.8 eV with respect to zero vacuum
level.
In this study, a new triarylamine-containing di(boronic ester)
monomer was synthesized and corresponding alternating co-
polymers with BT- or BS-containing dibromo monomers were
successfully produced via Suzuki coupling. Properties of the poly-
mers and photovoltaic characteristics of bulk heterojunction PSCs
fabricated by blending the polymers with PC70BM as an acceptor
were investigated.
2.3. Device fabrication and characterization of PSCs
PSCs were fabricated with the structure glass/indium tin oxide
(ITO)/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)
(PEDOT:PSS)/polymer:PC70BM/LiF/Al in the N2 globe box. The glass
2. Experimental
substrate with pre-patterned ITO (Freemteck, resistance: 10
U/
2.1. Materials
square) active layer with a thickness of 0.09 mm2 was ultra-
sonicated in detergent, D.I. water, CMOS-grade acetone, and iso-
propanol, and the surface of the glass substrate was modified with
UV-ozone treatment for 20 min. PEDOT:PSS (Bay P VPAI4083, Bayer
All chemicals were purchased from commercial sources
(Aldrich, TCI, or Junsei) and used without further purification. The
Fig. 1. Low band gap conjugated polymers consisting of triarylamine moiety.

J. Lee et al. / Polymer 55 (2014) 4837e4845
4839
AG) was spin-coated for hole injection at 4000 rpm for 60 s to a
thickness of 30e40 nm on the cleaned ITO-patterned glass sub-
Found: C, 83.78; H, 8.64; N, 2.31. ESI-MS: 624.38 for [M þ Na]þ
(Calcd for [M]: 601.39).
strate after filtration through a 0.45 mm filter, followed by baking in
an oven at 120 ꢁC for more than an hour. The blended solutions
were spin-coated onto the above substrate at 1000 rpm for 60 s to a
thickness of 100 nm. The LiF and Al cathodes were thermally
deposited to thicknesses of 1 and 100 nm, respectively, onto the
surface of the active layer. The current density-voltage (JeV) char-
acteristics of the device were measured using a Keithley 4200
source measurement unit, in the dark and under AM 1.5 G solar
illumination (Oriel 1 kW solar simulator) with respect to a refer-
ence cell PVM 132 calibrated at the National Renewable Energy
Laboratory, at an intensity of 100 mW/cm2. External quantum ef-
ficiency (EQE) was measured using a photomodulation spectro-
scopic setup (model Merlin, Oriel), a calibrated Si UV detector, and
an SR570 low noise current amplifier. The atomic force microscope
(AFM) (Multimode IIIa, Digital Instruments) was operated in tap-
ping mode to acquire images of the surfaces of polymer:PC70BM
blend layers.
2.3.3. Di(1-(4-bromo-6(2-ethylhexyloxy)naphthyl))phenylamine
(5)
To a solution of compound 4 (5.576 g, 9.26 mmol) in anhydrous
DMF (60 mL), N-bromosuccinimide (NBS) (3.38 g, 18.99 mmol) was
added. The mixture was stirred for 24 h, then poured into water and
extracted with ethyl acetate. The extract was successively washed
with water and dried over anhydrous MgSO4. The solvent was
evaporated and the residue was purified by column chromatog-
raphy on silica gel with 1:4 dichloromethane/hexane as an eluant to
afford the product as a yellowish viscous liquid (5.276 g, 75%).
1H NMR (CDCl3, 400 MHz, ppm): 7.89 (2H, d, J ¼ 9.2 Hz), 7.58
(2H, d, J ¼ 8 Hz), 7.49 (2H, d, J ¼ 2.8 Hz), 7.10 (2H, t, J ¼ 7.8 Hz), 6.99
(2H, dd, J ¼ 9.2, 2.4 Hz), 6.70 (2H, dd, J ¼ 8.6, 1.2 Hz), 3.98 (4H, d,
J ¼ 5.6 Hz), 1.85e1.70 (2H, m), 1.59e1.38 (8H, m), 1.37e1.26 (8H, m),
1.02e0.81 (12H, m). 13C NMR (CDCl3, 100 MHz, ppm): 158.64,
150.04, 144.64, 134.78, 130.53, 129.05, 126.33, 126.20, 122.70,
121.36, 120.90, 120.03, 118.11, 106.78, 70.48, 39.28, 30.60, 29.12,
23.94, 23.03, 14.08, 11.18. Anal. Calcd. for C42H49Br2NO2: C, 66.41; H,
6.50; Br, 21.04; N, 1.84; O, 4.21. Found: C, 66.64; H, 6.46; N, 1.98. ESI-
MS: 780.20 for [M þ Na]þ (Calcd for [M]: 757.21).
2.3.1. 6-(2-Ethylhexyloxy)-1-iodonaphthalene (3)
5-Iodo-2-naphthol (4.45 g, 16.48 mmol), anhydrous potassium
carbonate (6.910 g, 50 mmol), and 2-ethylhexyl bromide (9.657 g,
50 mmol) were dissolved in anhydrous N,N-dimethylformamide
(DMF, 50 mL). The solution was heated at 70 ꢁC and stirred for 16 h
under nitrogen atmosphere. The reaction mixture was then poured
into water and extracted with ethyl acetate. The organic layer was
then successively washed with water and dried over anhydrous
MgSO4. The solvent was evaporated and the residue was purified by
column chromatography on silica gel with 1:4 ethyl acetate/hexane
as an eluant to yield orange viscous liquid (5.859 g, 93%).
2.3.4. Di(1-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-6(2-
ethylhexyloxy)naphthyl))-phenylamine (1)
To a solution of compound 7 (6.675 g, 8.79 mmol) in 20 mL of
anhydrous THF at ꢀ78 ꢁC, 8.09 mL (20.22 mmol) of n-butyllithium
(2.5 M in hexane) was added by syringe. The mixture was stirred
at ꢀ78 ꢁC for 2 h, and then 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-
dioxaborolane (4.13 mL, 20.22 mmol) was added to the solution.
After 1 h, the reaction mixture was warmed to room temperature
and stirred for further 12 h. The reaction mixture was poured into
water, extracted with diethyl ether, and dried over anhydrous
MgSO4. The solvent was evaporated and the residue was purified by
column chromatography on silica gel with 1:5 hexane/toluene as an
eluant to afford the product as a yellowish viscous liquid (5.404 g,
72%).
1H NMR (CDCl3, 400 MHz, ppm): 7.95 (d,1H, J ¼ 9.2 Hz), 7.89 (dd,
1H, J ¼ 7.4,1.2 Hz), 7.68 (d,1H, J ¼ 8 Hz), 7.20 (1H, dd, J ¼ 9.2, 2.4 Hz),
7.08 (1H, t, J ¼ 8 Hz), 7.05 (1H, d, J ¼ 2.4 Hz), 3.95 (2H, dd, J ¼ 5.8,
1.2 Hz), 1.78 (1H, m), 1.59e1.40 (4H, m), 1.35e1.30 (4H, m),
1.00e0.85 (6H, m). 13C NMR (CDCl3, 100 MHz, ppm): 157.92, 135.20,
134.90, 133.57, 129.72, 127.72, 127.20, 120.65, 106.88, 99.16, 70.71,
39.34, 30.56, 29.09, 23.91, 23.04, 14.08, 11.12. Anal. Calcd. for
1H NMR (CDCl3, 400 MHz, ppm): 8.39 (2H, d, J ¼ 2.4 Hz), 8.04
(2H, d, J ¼ 9.2 Hz), 8.02 (2H, d, J ¼ 8.4 Hz), 7.19 (2H, t, J ¼ 8 Hz), 7.09
(2H, d, J ¼ 7.6 Hz), 7.05 (2H, dd, J ¼ 9.4, 2.4 Hz), 6.96 (1H, t,
J ¼ 7.2 Hz), 6.87 (2H, d, J ¼ 7.6 Hz), 4.11 (4H, d, J ¼ 6 Hz), 1.97e1.91
(2H, m), 1.70e1.42 (40H, m), 1.08e0.97 (12H, m). 13C NMR (CDCl3,
100 MHz, ppm): 157.61, 150.25, 148.21, 140.52, 136.55, 128.91,
128.79, 128.12, 125.88, 124.67, 121.67, 121.37, 118.55, 108.05, 83.39,
70.34, 38.96, 30.50, 28.91, 24.94, 23.83, 23.06, 14.07, 10.98. Anal.
Calcd. for C54H73B2NO6: C, 75.97; H, 8.62; B, 2.53; N, 1.64; O, 11.24.
Found: C, 76.23; H, 8.77; N, 1.74. ESI-MS: 876.55 for [M þ Na]þ
(Calcd for [M]: 853.56).
C
18H23IO: C, 56.55; H, 6.06; I, 33.20; O, 4.19. Found: C, 57.76; H, 6.06.
ESI-MS: N/A [56].
2.3.2. Di(1-(6-(2-ethylhexyloxy)naphthyl))phenylamine (4)
The synthesis of the compound 4 was carried out following a
modified literature procedure [57]. To a two-neck round-bottomed
flask (RBF) equipped with a reflux condenser, 3 (6.6 g, 17.26 mmol),
aniline (0.723 g, 7.76 mmol), tris(dibenzylideneacetone)dipalla-
dium (Pd2(dba)3) (0.158 g, 0.17 mmol), tri-tert-butylphosphine
(P(tBu)3) (0.69 mL, 1 M in toluene), sodium tert-butoxide (NaOtBu)
(2.322 g, 24.16 mmol), and anhydrous toluene (100 mL) were added
and stirred at 90 ꢁC for 24 h under nitrogen atmosphere. The
mixture was poured into water and extracted with ethyl acetate.
After drying over anhydrous MgSO4, the solvent was evaporated.
The residue was purified by column chromatography on silica gel
with 1:4 dichloromethane/hexane as an eluant to yield greenish
yellow viscous liquid (4.017 g, 86%).
2.3.5. Poly[4,4-(di(1-(6-(2-ethylhexyl)naphthyl))phenylamine)-alt-
5,5-(40,70-di-2-thienyl-20,10,30-benzothiadiazole)] (PDNPADTBT)
In a two-necked RBF equipped with a condenser, monomer 1
(0.338 g, 0.395 mmol), monomer 6 (0.181 g, 0.395 mmol), palla-
dium(II) acetate (Pd(OAc)2) (5.3 mg, 0.008 mmol), and tricyclohexyl
phosphine (3.3 mg, 0.012 mmol) were dissolved in toluene (3.1 mL).
To this solution, tetramethyl ammonium hydroxide (Me4NOH)
(0.256 g, 1.74 mmol, 20 wt% in water) was added, and then the
reaction mixture was vigorously stirred at 90 ꢁC for 24 h under
nitrogen. 2-Bromothiophene and phenylboronic acid were added at
the end of the reaction to endcap the polymer chain end. After
cooling to room temperature, the reaction mixture was poured into
methanol, and filtered. The precipitate was washed in Soxhlet
apparatus with methanol, acetone, and chloroform for 24 h,
respectively. The chloroform fraction was condensed, and
1H NMR (DMSO-d6, 400 MHz, ppm): 7.79 (2H, d, J ¼ 9.6 Hz), 7.65
(2H, d, J ¼ 8.4 Hz), 7.37 (2H, d, J ¼ 2.4 Hz), 7.35 (2H, d, J ¼ 7.6 Hz), 7.11
(2H, t, J ¼ 8.2 Hz), 7.02 (2H, dd, J ¼ 9.4, 2.4 Hz), 6.95 (2H, dd, J ¼ 7.4,
1.2 Hz), 6.83 (1H, t, J ¼ 7.2 Hz), 6.54 (2H, dd, J ¼ 8.6, 1.2 Hz), 3.94 (4H,
d, J ¼ 6 Hz),1.79e1.62 (2H, m),1.55e0.121 (16H, m), 0.91e0.84 (12H,
m). 13C NMR (DMSO-d6, 100 MHz, ppm): 156.84, 149.96, 144.12,
136.36, 129.07, 126.80, 125.12, 124.66, 124.60, 122.13, 120.52, 119.62,
118.96, 107.50, 69.96, 38.60, 29.92, 28.39, 23.31, 22.49, 13.92, 10.86.
Anal. Calcd. for C42H51NO2: C, 83.82; H, 8.54; N, 2.33; O, 5.32.

4840
J. Lee et al. / Polymer 55 (2014) 4837e4845
precipitated in methanol. The final product was obtained after
drying in vacuo at 60 ꢁC, yielding 0.231 g (65%).
120.68, 118.98, 105.84, 70.32, 39.31, 30.57, 29.11, 23.88, 23.04, 14.05,
11.19. Anal. Calcd. for C56H57N3O2S2Se: C, 71.01; H, 6.07; N, 4.44; O,
3.38; S, 6.77; Se, 8.34. Found: C, 71.21; H, 6.17; N, 4.16; O, 3.38; S,
6.28.
IR (KBr, cmꢀ1): 2957, 2925, 2860, 1619, 1579, 1490, 1437, 1361,
1322, 1266, 1221, 1028, 876, 746, 695. 1H NMR (CDCl3, 400 MHz,
ppm): 8.24 (2H, br), 8.05 (2H, d, J ¼ 9.2 Hz), 7.93 (2H, br), 7.74 (2H,
s), 7.50 (2H, br), 7.38 (2H, br), 7.20e7.07 (4H, m), 7.05 (2H, d,
J ¼ 8.8 Hz), 6.93e6.77 (3H, m), 3.90 (4H, br), 1.78e1.65 (2H, m),
1.61e1.09 (16H, m), 1.01e0.64 (12H, m). 13C NMR (CDCl3, 100 MHz,
ppm): 158.06, 157.22, 152.66, 150.29, 145.31, 143.99, 139.23, 134.72,
129.19, 129.05, 128.25, 128.18, 128.06, 126.38, 125.77, 125.47, 121.93,
121.13, 120.71, 119.04, 105.78, 70.34, 39.31, 30.58, 29.12, 23.89,
23.01, 14.06, 11.19. Anal. Calcd. for C56H57N3O2S3: C, 74.71; H, 6.38;
N, 4.67; O, 3.55; S, 10.69. Found: C, 74.08; H, 6.52; N, 4.28; O, 3.49; S,
10.06.
3. Results and discussion
3.1. Synthesis and characterization
The synthesis of triarylamine-containing monomer 1 was car-
ried out as shown in Scheme 1. Compound 2 was prepared by
azotation and halogenations of 5-amino-2-naphthol [53]. Before
palladium-catalyzed arylation reaction, the hydroxyl group of
compound 2 was reacted with 2-ethylhexyl bromide to endow
solubility of the polymer to form compound 3. Then the compound
3 was reacted with 0.45 equivalents of aniline to result in the tri-
arylamine compound 4 [57]. The dibromo compound 5 was syn-
thesized by bromination of compound 4 with N-bromosuccinimide
(NBS), then lithiation of compound 5 with n-butyllithium (n-BuLi)
and quenching with 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-
dioxaborolane produced a desired monomer 1. The electron-
deficient dibromo monomers consisting of BT and BS units were
also synthesized following the literature procedures [49,54,55].
The alternating copolymers were synthesized via palladium-
catalyzed Suzuki coupling reaction of the electron-rich monomer
1 and the electron-deficient monomers 6 and 7. The synthetic route
and structures of alternating copolymer are shown in Scheme 2.
The obtained copolymers were purified by reprecipitation in
methanol and Soxhlet extraction with methanol and acetone to
remove catalyst and low molecular weight polymers. The co-
polymers showed good solubility in common organic solvents, such
as chloroform, toluene, and chlorobenzene at room temperature.
2.3.6. Poly[4,4-(di(1-(6-(2-ethylhexyl)naphthyl))phenylamine)-alt-
5,5-(40,70-di-2-thienyl-20,10,30-benzoselenadiazole)] (PDNPADTBS)
In a two-necked RBF equipped with a condenser, monomer 1
(0.3 g, 0.352 mmol), monomer 6 (0.178 g, 0.352 mmol), Pd(OAc)2
(4.7 mg, 0.007 mmol), and tricyclohexyl phosphine (3.0 mg,
0.011 mmol) were dissolved in toluene (2.7 mL). To this solution,
Me4NOH (0.228 g,1.55 mmol, 20 wt% in water) was added, and then
the reaction mixture was vigorously stirred at 90 ꢁC for 24 h under
nitrogen. 2-Bromothiophene and phenylboronic acid were added at
the end of the reaction to endcap the polymer chain end. After
cooling to room temperature, the reaction mixture was poured into
methanol, and filtered. The precipitate was washed in Soxhlet
apparatus with methanol, acetone, and chloroform for 24 h,
respectively. The chloroform fraction was condensed, and precipi-
tated in methanol. The final product was obtained after drying in
vacuo at 60 ꢁC, yielding 0.22 g (66%).
IR (KBr, cmꢀ1): 2955, 2924, 2861, 1618, 1579, 1490, 1435, 1362,
1322, 1266, 1219, 1026, 821, 747, 693. 1H NMR (CDCl3, 400 MHz,
ppm): 8.15 (2H, br), 8.05 (2H, d, J ¼ 9.2 Hz), 7.86 (2H, br), 7.74 (2H, s),
7.51 (2H, br), 7.37 (2H, br), 7.18e7.09 (4H, m), 7.04 (2H, d, J ¼ 9.2 Hz),
6.94e6.71 (3H, m), 3.90 (4H, br), 1.78e1.65 (2H, m), 1.60e1.09 (16H,
m), 1.01e0.64 (12H, m). 13C NMR (CDCl3, 100 MHz, ppm): 158.22,
158.01, 157.19, 150.29, 145.23, 144.26, 139.55, 134.71, 129.15, 129.02,
128.35, 127.94, 127.74, 127.23, 126.34, 125.65, 125.48, 121.92, 121.11,
The chemical structure of the copolymers was confirmed by 1H, 13
C
nuclear magnetic resonance (NMR) spectroscopy (Fig. S1, S2), FTIR
(Fig. S3), and elemental analysis. All the data is provided in Sup-
porting Information. The number-average molecular weights (Mn)
of PDNPADTBT and PDNPADTBS were determined by size exclu-
sion chromatography (SEC) using linear polystyrene standards and
Scheme 1. Synthetic routes of monomer 1.

J. Lee et al. / Polymer 55 (2014) 4837e4845
4841
Scheme 2. Synthetic route of copolymers.
were estimated to be 10,900 (Mw/Mn ¼ 1.85), 10,300 (Mw/
Mn ¼ 1.89), respectively (Fig. S4).
incorporation of two thiophene rings at both ends of BT and BS
units generated a more planar backbone structure and facilitated
association between chains. The maximum absorption spectra of
PDNPADTBS exhibited greater bathochromic shift compared with
the PDNPADTBT, due to the larger size and more electron-rich
character of the selenium atom than that of sulfur [50,58,59].
However, PDNPADTBT showed better absorption coefficient than
PDNPADTBS at the long-wavelength region as shown in Fig. S5 and
it was well matched with previous report [50]. The optical band
gaps of the copolymer, which were calculated from the onset ab-
sorption of the film, were 1.93 (PDNPADTBT) and 1.81 eV
(PDNPADTBS).
Thermal properties of the copolymers were investigated by
differential scanning calorimetry (DSC) and thermogravimetric
analysis (TGA) under nitrogen atmosphere. As shown in Fig. 2 and
Table 1, these copolymers lost less than 5% of their weight up to 406
(PDNPADTBT) and 369 ꢁC (PDNPADTBS), the glass transition
temperatures (Tg) of the copolymers appeared at 151 (PDNPADTBT)
and 137 ꢁC (PDNPADTBS). No crystallization or melting transition
was observed upon further heating beyond the Tg. The molecular
weights and thermal properties are summarized in Table 1.
3.2. Optical properties
3.3. Electrochemical properties
Normalized UV-vis absorption spectra of the copolymers in
chloroform solutions and films are shown in Fig. 3, and optical
properties of the copolymers are summarized in Table 2. The co-
polymers exhibited short-wavelength absorption at ~400 nm cor-
The electrochemical properties of the copolymers also were
investigated by cyclic voltammetry (CV) with Bu4NBF4 (0.1 M in
responding to the
pep* transition of their conjugated backbone
Table 1
and long-wavelength absorption which is attributed to intra-
molecular charge transfer (ICT) between the electron-rich mono-
mer 1 and the electron-deficient BT derivatives. The maximum
absorption spectra of PDNPADTBT and PDNPADTBS in solution
appeared at 504 and 540 nm, while those in films at 521 and
558 nm, respectively. The absorption spectra in the solid state of
copolymers were more red-shifted than that of the corresponding
spectra in solution indicating strong interchain interaction in the
solid state. Although the kinked structure of the triarylamine
moiety tends to reduce the interchain interaction, we posit that
Molecular weights and thermal properties of the copolymers.
Mn (10ꢀ3
a
)
Mw (10ꢀ3
a
)
PDI
Td ꢁC
Tg ꢁC
b
c
Copolymer
Yield (%)
PDNPADTBT
PDNPADTBS
65
66
10.9
10.3
20.2
19.5
1.85
1.89
406
369
151
137
a
Molecular weights and polydispersity index (PDI) values were measured by SEC
using THF as an eluant, polystyrene as a standard.
b
Td exhibited 5% weight loss temperature of the polymers measured by TGA at a
heating rate of 10 ꢁC/min under nitrogen.
c
Glass transition temperature determined by a second heating DSC curve under
nitrogen, at a heating rate of 10 ꢁC/min.
Fig. 2. TGA plot (a) and DSC plot (b) of the copolymers.

4842
J. Lee et al. / Polymer 55 (2014) 4837e4845
Fig. 3. Normalized UV-vis absorption spectra of the copolymers (a) in chloroform and (b) in film.
acetonitrile) at a scan rate of 50 mV sꢀ1 under nitrogen. The Pt
electrode coated with the polymer film was used as a working
electrode and the Ag/AgNO3 electrode was used as a reference
electrode. The cyclic voltammograms of the copolymers are shown
in Fig. 4 and the results are summarized in Table 2. The HOMO
levels of the copolymers are calculated from the onset oxidation
potentials according to the equation, EHOMO ¼ [ꢀ(Eonset ꢀ Eonset(Fc/
Fcþ vs. Ag/Agþ)) ꢀ 4.8] eV, relative to the ferrocene/ferrocenium
(Fc/Fcþ) redox system, whose potential is assumed to be ꢀ4.8 eV
relative to vacuum [60]. The LUMO energy levels of the copolymers
were calculated from the HOMO energy levels and the optical band
gap using the equation, ELUMO ¼ EHOMO þ Eogpt. The calculated
HOMO energy values of PDNPADTBT and PDNPADTBS were ꢀ5.48
and ꢀ5.45 eV, and the LUMO energy values were ꢀ3.55
and ꢀ3.64 eV, respectively. The HOMO levels of copolymers were
not raised apparently despite of the presence of different acceptors.
However, PDNPADTBS showed lower LUMO energy levels than
those of PDNPADTBT, and this result indicated that BS was more
effective on lowering the LUMO level of copolymer as previously
reported [50,61,62]. Intriguingly, the copolymers exhibited low
HOMO levels, which allow PSCs to have high Voc [63]. Compared
with previous polymer systems composed of electron-rich triphe-
nylamine and electron-deficient BT derivatives, such low HOMO
levels of copolymers may be originated from the naphthalene unit
introduced into the triarylamine [36,41,44,45].
PSS (40 nm)/polymers-PC70BM (100 nm)/LiF (1 nm)/Al (100 nm)
(ITO indium tin oxide, PEDOT poly(3,4-
ethylenedioxythiophene), PSS poly(styrene sulfonate)). The
¼
¼
¼
copolymer-PC70BM blend in o-dichlorobenzene (40 mg/ml) was
spin-coated as an active layer onto the PEDOT-PSS layer. All devices
were tested without thermal annealing. In the polymer-fullerene
blend system with intercalation, high level of fullerene loading is
needed to create the optimum phase separation [64]. Therefore,
initial photovoltaic systems were fabricated with the copolymer-
PC70BM blended solutions prepared in a weight ratio of 1:4 (w/w).
AFM image of the copolymer-PC70BM blend films was shown in SI
(Fig. S6) and the surfaces of both blend films were similarly ho-
mogeneous. The JeV characteristics of the devices under the illu-
mination of AM 1.5G (100 mW/cm2) are shown in Fig. 5 and
summarized in Table 3. The highest PCE of 2.0% was obtained from
the PDNPADTBT-PC70BM solar cell with a Voc of 0.92 V, a Jsc of
6.8 mA/cm2, and a fill factor (FF) of 31.7%. Under the same condition,
a slightly lower PCE of 1.5% was observed from the PDNPADTBS-
PC70BM solar cell with a Voc of 0.88 V, a Jsc of 5.6 mA/cm2, and a FF of
31.2%. The slightly lower Voc of PDNPADTBS-PC70BM is probably
because of the higher HOMO energy level of PDNPADTBS. It is
noteworthy that the Voc of PDNPADTBT and PDNPADTBS exhibited
a value close to 0.9 V, which is comparable to the best result of
poly(2,7-carbazole) derivatives reported in the literature,
3.4. Photovoltaic properties
The photovoltaic properties of triarylamine-based alternating
copolymers were investigated using the bulk heterojunction
photovoltaic devices which had a layered structure of ITO/PEDOT-
Table 2
Electrochemical and optical properties of the copolymers.
a
a
c
d
Copolymer
lmax (nm) lmax (nm) Eogptb Eox/onset EHOMO
ELUMO
Solution
Film
(eV)
(V)
(eV)
(eV)
PDNPADTBT 370, 504
PDNPADTBS 350, 540
376, 521
358, 558
1.93
1.81
0.72
0.69
ꢀ5.48
ꢀ5.45
ꢀ3.55
ꢀ3.64
a
lmax: the absorption maxima from the UV-vis spectra in chloroform solution or
in film.
b
c
Eogpt: optical band gap determined from the absorption onset in film.
EHOMO: HOMO levels of the polymers was determined with the first oxidation
onset potential relative to ferrocene (Fc).
d
ELUMO: LUMO energy levels of the polymers were estimated from the HOMO
energy levels and Egopt
.
Fig. 4. Cyclic voltammograms of the copolymers.

J. Lee et al. / Polymer 55 (2014) 4837e4845
4843
Fig. 5. J-V characteristics of PSCs with an active layer of copolymer-PC70BM (1:4, w/w)
Fig. 7. Characteristics of PSCs at various PDNPADTBT/PC70BM weight ratios (AM 1.5G
mixture under AM 1.5G illumination (100 mW/cm2).
illumination at 100 mW/cm2).
Table 3
thermal annealing on the photovoltaic response of the PSCs were
further investigated. The photovoltaic properties of the polymer
blends PDNPADTBT-PC70BM in various weight compositions (1:1,
1:2, 1:3, and 1:4 w/w) were estimated under same conditions and
currentevoltage characteristic of these devices are shown in Fig. 7
and summarized in Table 4. Power conversion efficiency (PCE)
values of the devices ranged from 0.94% (1:1 w/w) to 2.0% (1:4 w/
w) according to variation in the composition. The lower ratios of
PC70BM in the polymer blends led to reduction in Jsc values because
of the inefficient charge separation and electron transporting
properties, and considerable reduction of Voc and FF values was
observed when the weight composition became 1:1. The effect of
thermal annealing on blends PDNPADTBT-PC70BM (1:4 w/w) solar
cell was also investigated. It was found that the best PCE of 2.2% was
obtained when the device was annealed under 140 ꢁC for 20 min.
The morphologies of the PDNPADTBT-PC70BM blend films before
and after heat treatment were estimated by atomic force micro-
scopy (AFM) as shown in Fig. 8. Both films possessed smooth sur-
faces, with a low root mean square (RMS) roughness of 0.367 and
0.333 nm, respectively. However, smoother surface was obtained
after annealing and showed more uniform morphology without
significant phase separation. Since interpenetrating polymer-PCBM
networks with a smaller domain size is known to be better for
efficient charge transfer and separation, we ascribe that formation
Photovoltaic properties of triarylamine-based PSCs processed with copolymer-
PC70BM (1:4, w/w) blend in dichlorobenzene.
Copolymer-PC70BM (1:4)
Voc (V)
Jsc (mA/cm2)
FF (%)
PCEa (%)
PDNPADTBT
PDNPADTBS
0.92
0.88
6.8
5.6
31.7
31.2
2.0
1.5
a
Maximum values.
indicating that the low HOMO energy level is one of the important
factor to obtain the high Voc values of PSCs [10,47,63,65]. Fig. 6(a)
shows the UV-vis absorption spectra of copolymer-PC70BM blend
films at 1:4 weight ratio. Although PDNPADTBS-PC70BM blend film
had a bathochromic absorption shift compared to PDNPADTBT-
PC70BM blend film as expected from the absorption spectra of the
copolymers, much larger absorption coefficients of PDNPADTBT
than PDNPADTBS in the range of 450e600 nm (Fig. S5) appeared to
dominate efficiency of the PSC up to 600 nm with negligible dif-
ference from 600 to 700 nm. Thus higher performance of the PSC
consisting of PDNPADTBT was attributed to lower HOMO energy
level and higher absorption coefficient of PDNPADTBT than
PDNPADTBS.
By selecting the PDNPADTBT-PC70BM solar cell which showed a
better performance, effects of donor-acceptor blend ratios and
Fig. 6. (a) Absorption spectra of copolymer-PC70BM (1:4, w/w) blend films spin-coated from dichlorobenzene and (b) EQE curves of copolymer-PC70BM based PSCs.

4844
J. Lee et al. / Polymer 55 (2014) 4837e4845
Table 4
sufficiently high due to limited solar absorption and a low FF.
Currently we are exploring other electron-deficient acceptor which
could reduce the band gap of the copolymers to further improve
solar cell performances.
Photovoltaic properties of PSCs at various PDNPADTBT/PC70BM weight ratios (AM
1.5G illumination at 100 mW/cm2).
Annealing temp. (ꢁC)
Solar cell performance
Ratio
Voc (V)
Jsc (mA/cm2)
FF (%)
PCEa (%)
Acknowledgments
As-casted
As-casted
As-casted
As-casted
Annealed (140 ꢁC)
1:1
1:2
1:3
1:4
1:4
0.68
0.89
0.88
0.92
0.88
5.0
5.5
6.2
6.8
7.4
27.4
30.5
31.6
31.7
34.4
0.94
1.5
1.7
2.0
2.2
This work was supported by a NRF grant funded by MSIP
through the NRL (R0A-2008-000-20121-0) and the ERC (R11-2007-
050-00000-0) programs.
a
Maximum values.
Appendix A. Supplementary data
of the more uniform phase separated blend by thermal annealing
leads to improvement of Jsc, FF, and PCE [66,67]. PDNPADTBT-
PC70BM solar cell showed improved Voc and Jsc values compared to
previously reported copolymers composed of triphenylamine and
DTBT derivatives supporting our approach to obtain high perfor-
mance PSCs [36,40,41].
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.polymer.2014.08.018.
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