Syntheses and properties of copolymers with N-alkyl-2,2′-bithiophene-3,3′-dicarboximide unit for polymer solar cells

Article abstract of DOI:10.1002/bkcs.10435

We report new random copolymers using the electron-deficient unit N-alkyl-2,2′-bithiophene-3,3′-dicarboximide (BTI) for organic solar cells. For absorption over a broader range of the solar spectrum, push-pull types of conjugated polymers PBTIBDT-3, PBTIBDT-5, and PBTIBDT-7, containing 4,8-bis(2-octyldodecyloxy) benzo[1,2-b;3,4-b']dithiophene (BDT) as electron-pushing unit and BTI as electron-pulling unit, were synthesized. The polymers were synthesized by coupling electron-pushing and electron-pulling units by Stille polymerization with Pd(0) catalyst. The incorporation of more BTI units induced more red shift of the absorption spectra of the polymer thin films. The device comprising PBTIBDT-5 and PC₇₁ BM (1:1) showed V_OC = 0.76 V, J_SC = 3.28 mA/cm², and fill factor (FF) = 0.51, giving a power conversion efficiency of 1.26%.

Full text of DOI:10.1002/bkcs.10435

Article
DOI: 10.1002/bkcs.10435
BULLETIN OF THE
J. Kim et al.
KOREAN CHEMICAL SOCIETY
Syntheses and Properties of Copolymers with N-Alkyl-2,20-bithiophene-
3,30-dicarboximide Unit for Polymer Solar Cells
Juae Kim,† Shin Hyun Kim,†,‡ Taehyo Kim,§ Joo Young Shim,† Dongkyung Park,†,‡ Jinwoo Kim,†
Il Kim,¶ Jin Young Kim,§ and Hongsuk Suh†,
*
†Department of Chemistry and Chemistry Institute for Functional Materials, Pusan National University,
Busan 609-735, Korea. *E-mail: hssuh@pusan.ac.kr
‡Songwon Industrial Co., Ltd., Ulsan 737-2, Korea
§Interdisciplinary School of Green Energy, Ulsan National Institute of Science and Technology,
Ulsan 689-798, Korea
¶The WCU Center for Synthetic Polymer Bioconjugate Hybrid Materials, Department of polymer Science and
Engineering, Pusan National University, Busan 609-735, Korea
Received March 11, 2015, Accepted May 14, 2015, Published online August 27, 2015
We report new random copolymers using the electron-deficient unit N-alkyl-2,20-bithiophene-3,30-dicarbox-
imide(BTI) fororganicsolarcells. Forabsorption overabroaderrangeofthesolarspectrum, push–pulltypesof
conjugated polymers PBTIBDT-3, PBTIBDT-5, and PBTIBDT-7, containing 4,8-bis(2-octyldodecyloxy)
benzo[1,2-b;3,4-b’]dithiophene (BDT) as electron-pushing unit and BTI as electron-pulling unit, were synthe-
sized. The polymers were synthesized by coupling electron-pushing andelectron-pulling units by Stille polym-
erization with Pd(0) catalyst. The incorporation of more BTI units induced more red shift of the absorption
spectra of the polymer thin films. The device comprising PBTIBDT-5 and PC71BM (1:1) showed VOC
=
0.76 V, JSC = 3.28 mA/cm2, and fill factor (FF) = 0.51, giving a power conversion efficiency of 1.26%.
Keywords: Polymer, Photovoltaic cells, N-Alkyl-2,20-bithiophene-3,30-dicarboximide unit (BTI),
Benzodithiophene
Introduction
benzothiadiazole (BT) and amide in diketopyrrolopyrrole
(DPP). In the case of bithiophene imide (BTI), it contains
bithiophene linked with imide, which has two carbonyl groups
and one nitrogen atom between them. Two carbonyl groups
attached to the thiophene units work as the electron-withdraw-
ing moieties to reduce the bandgap, and the five-membered
thiophene units reduce the torsional angles with the adjacent
aromatic groups.3 The fact that the alkyl group on nitrogen
atom of the imide is far away from the alkyl groups of the elec-
tron-pushing unit is another advantage of the BTI unit to pro-
vide better conjugation.10–12
To improve the morphology of the bulk heterojunction
(BHJ) using polymer and [6,6]-phenyl-C71-butyric acid
methyl ester (PC71BM) of PSCs and expand the absorption
range of the solar spectrum, copolymerization of the elec-
tron-rich and -deficient units with controlled ratios has been
reported.13,14
In this paper, we report push–pull type conjugated copoly-
mers PBTIBDTs for PSC devices utilizing BTI as the elec-
tron-pulling unit and BDT as the electron-pushing
unit (Scheme 1).15–17 These push–pull type conjugated
polymers were synthesized by the Stille coupling reaction of
2,6-dibromo-4,8-dioctyloxybenzo[1,2-b;3,4-b’]dithiophene,
N-(2-octyldodecyl)-5-50dibromo-2,20-bithiophene-3,30-dicar-
boximide and 2,5-bis(trimethylstannanyl)thiophene with var-
ious feed ratios. Novel conjugated polymers showed good
solubilities at room temperature in common organic solvents.
In order to solve the problems caused by the exhaustion of fos-
sil fuels, polymer solar cells (PSCs) have been investigated
intensively over the past decades as an alternative source of
green energy.1–4
The push–pull type of polymer synthetic strategy, which is
based on the copolymerization of electron-rich and electron-
deficient units, is a powerful way to induce intramolecular
charge transfer (ICT) for the generation of low-bandgap con-
jugated polymers and reduce the highest occupied molecular
orbital (HOMO) energy levelfortheenhancementof theopen-
circuit voltage (VOC).5,6
Benzo[1,2-b:4,5-b0]dithiophene (BDT) including a rigid
and coplanar fused ring system, which has a highly extended
π-conjugated structure for the improvement of π-stacking, is
one of the best materials as the electron-pushing unit. An addi-
tional advantage of the BDT unit is its two 5-membered thio-
phene rings, which are electron-rich and will reduce the
torsional angles with the adjacent aromatic moiety. In addition
to this, the phenylene ring of the BDT unit can be incorporated
with dialkyloxy groups to enhance the richness of electrons
and the solubility. Up to now, BDT-based copolymers have
been reported to make PSCs with a power conversion effi-
ciency of up to 9%.7–9
The electron-pulling units applied inPSCs include electron-
withdrawing groups such as the 1,2-quinoid form of
Correction added on 01 October 2015, after first online publication: ISSN (Print) has been corrected.
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KOREAN CHEMICAL SOCIETY
C8H17
constant, 21–78 N/m; scan rate, 2.0 Hz) at ambient conditions
(in air, 20 ꢀC).
C12H25
C10H21
O
O
N
Device Fabrication and Measurements. Solar cells were
fabricated on an indium tin oxide (ITO)-coated glass
substrate with the following structure; ITO-coated glass
substrate/poly(3,4-ethylenedioxythiophene)(PEDOT:PSS)/
polymer: PC71BM/Al. The ITO-coated glass substrate was
first cleaned with a detergent, ultrasonicated in acetone and
isopropyl alcohol, and subsequently dried overnight in an
oven. PEDOT:PSS (Baytron PH) was spin-cast from an aque-
ous solution to form a film of 40 nm thickness. The substrate
was dried for 10 min at 140 ꢀC in air and then transferred into a
glove box to spin-cast the active layer. A solution containing a
mixture of polymer:PC71BM in dichlorobenzene/chloroform
solvent with concentration of 10 mg/mL was then spin-cast
on top of the PEDOT/PSS layer. The film was dried for 60
min at 70 ꢀC in the glove box. Then, an aluminum (100 nm)
electrode was deposited by thermal evaporation in a vacuum
of about 5 × 10−7 Torr. Current density vs. voltage (J–V) char-
acteristics of the devices were measured using a Keithley
236 source measuring unit. Solar cell performance was meas-
ured using an Air Mass 1.5 Global (AM 1.5 G) solar simulator
with an irradiation intensity of 1000 W/m2. An aperture (12.7
mm2) was used on top of the cell to eliminate extrinsic effects
such as crosstalk, waveguiding, and shadow effects. The spec-
tral mismatch factor was calculated by comparing the solar
simulator spectrum with the AM 1.5 spectrum at room temper-
ature. In order to check the photocurrent of the devices, the
incident photon-to-current collection efficiency (IPCE) was
measured by using a solar cell spectral response measurement
system (PV Measurement Inc. (5757 Central Ave, Boulder,
USA)). A silicon calibration photodiode I772 was used for
spectral calibration before measuring IPCE of the devices.
IPCE is a measure of the photon-to-electron conversion effi-
ciency at a particular irradiation wavelength (Eq. (1)).
O
S
S
S
S
n
O
C10H21
P0
C8H17
Scheme 1. Previously reported alternating polymer.
The photovoltaic properties of the polymers were investigated
by fabrication of the polymer solar cells with the configuration
of ITO/PEDOT:PSS/polymer:PC71BM/Al.
Experimental Details
Instrumental Characterization. All reagents were pur-
chased from Aldrich (Gyeonggi-do, Republic of Korea) or
TCI (Seoul, Republic of Korea) and used without further puri-
fication. Solvents were purified by normal procedure and han-
dled under a moisture-free atmosphere. H and 13C NMR
1
spectra were recorded with a Varian Gemini-300 (300MHz)
spectrometer (Palo Alto, CA, USA), and chemical shifts were
recorded in ppm units with TMS as the internal standard. Flash
column chromatography was performed with Merck silica gel
60 (particle size 230–400 mesh ASTM) with ethyl acetate/
hexane or methanol/methylene chloride gradients, unless oth-
erwise indicated. Analytical thin layer chromatography (TLC)
was conducted using Merck 0.25 mm silica gel 60 F precoated
aluminum plates with the fluorescent indicator UV254.
High-resolution mass spectra (HRMS) were recorded on a
JEOL JMS-700 mass spectrometer (Akishima, Tokyo, Japan)
under electron impact (EI) conditions at the Korea Basic Sci-
ence Institute (Daegu). Molecular weight and polydispersity
ofthepolymerweredetermined bygelpermeationchromatog-
raphy (GPC) analysis with a polystyrene standard calibration.
Thermal analyses were carried out on an SDT Q600 instru-
ment (New castle, DE, USA) under N2 atmosphere with heat-
ing and cooling rates of 10 ꢀC/min at the Korea Basic Science
Institute (Busan). The UV–vis absorption spectra were
recorded by a Varian 5E UV–vis–NIR spectrophotometer
(Palo Alto, CA, USA). Cyclic voltammograms of the poly-
mers were obtained using an Wona-WPG100 instrument at
room temperature in a three-electrode cell under Ar atmos-
phere at a scan rate of 100 mV/s, with a Pt wire as the counter
electrode, a Ag/AgNO3 as reference electrode, and 0.1 M tet-
raethylammonium tetrafluoroborate (TBABF4) in acetonitrile
astheelectrolyte. Theenergy leveloftheAg/AgNO3 reference
electrode (calibrated by the FC/FC+ redox system) was 4.8 eV
below the vacuum level. The atomic force microscopy (AFM)
instrumentation consisted of a Veeco NanoScope AFM
(Dynamostr. 1368165 Mannheim, Germany) and a standard
silicon cantilever (Veeco; tip radius, 8 nm; normal spring
IPCE = ð1240 × JSCÞ=ðλi × PinÞ
ð1Þ
In Eq. (1), λi is the wavelength and Pin is the power of the
incident radiation per unit area.
Synthesis of Monomers and Polymers
Synthesis of 4,8-bis(2-octyldodecyloxy)benzo[1,2-b:3,4-b’]
dithiophene (2). To a mixture of benzo[1,2-b:4,5-b’]dithio-
phene-4,8-dion (1) (4.0 g, 18.0 mmol) and zinc powder (2.6
g, 40.0 mmol) in EtOH (16 mL) and DMF (16 mL) was added
5 NNaOHaqueoussolution(25 mL), andthemixture wasstir-
red under reflux for 3 h. Afterward, 1-iode-2-octyldodecane
(22.3 g, 54.0 mmol) and tetrabutylammoniumbromide (0.9
g, 3.6 mmol) were added to the mixture, and the reaction mix-
ture was further refluxed for 12 h. After extraction of the reac-
tion mixture with ethyl acetate, the organic layer was washed
with water and dried over anhydrous MgSO4. After removing
thesolventunder reducedpressure, theresidue was purifiedby
flash chromatography to give 4.9 g (35%) of compound 2 as a
1
colorless oil; H NMR (300 MHz, CDCl3) δ (ppm) 7.48 (d,
2H, J = 5.5 Hz), 7.36 (d, 2H, J = 5.5 Hz), 4.17 (d, 4H, J =
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4.7 Hz), 1.92–1.80 (m, 2H), 1.72–1.60 (m, 4H), 1.53–1.20 (m,
60H), 0.90–0.87 (t, 12H, J = 6.9 Hz); 13C NMR (75 MHz,
CDCl3) δ (ppm) 144.9, 131.7, 130.2, 126.1, 120.5, 39.4,
32.2, 31.6, 30.3, 30.1, 30.0, 29.9, 29.6, 27.2, 22.9, 14.4;
HRMS (FAB+, m/z) calcd for C50H86O2S2 783.6148, meas-
ured 783.6158.
6.3 mmol), 4-(dimethylamino) pyridine (0.3 g, 2.1 mmol),
and 10 mL of toluene. 2-Octyldodecylamine (7)13 (2.8 g,
6.6 mmol) was added dropwise to the suspension over 15
min, and the reaction mixture was allowed to stir for additional
15 min until no solid remained. Then, the reaction tube
was irradiated with microwaves (PMAX 300 W) for 2 h at a
constant temperature of 220 ꢀC. After running four
more batches of the reaction, all the resulting mixtures were
combined and extracted with diethyl ether. The organic phase
was washed six times with water, dried over MgSO4, and con-
centrated under reduced pressure to provide a yellow
oil, which was purified by column chromatography on silica
gel to give 1.8 g (68%) of compound 8 as a light yellow
Synthesis of 2,6-dibromo-4,8-di(2-octyldodecylloxy)benzo
[1,2-b;3,4-b]dithiophene (3). After adding bromine (0.1
mL, 2.8 mmol) in 5 mL of methylene chloride to a solution
of compound 2 (1.0 g, 1.4 mmol) in 10 mL of methylene chlo-
rideat0 ꢀC, thereactionmixture wasstirred for12 hatambient
temperature. After removing the solvent under reduced pres-
sure, the residue was purified by flash chromatography to give
1
1
1.2 g (50%) of compound 3 as a light yellow oil. H NMR
oil. H NMR (300 MHz, CDCl3) δ 7.70 (d, 2H, J = 5.30
(300 MHz, CDCl3) δ 7.41 (s, 2H), 4.08 (d, 4H, J = 5.21),
1.76–1.68 (m, 2H), 1.64–1.54 (m, 8H), 1.52–1.26 (m, 56H),
0.94–0.86 (m, 12H); 13C NMR (75 MHz, CDCl3) δ 142.92,
131.09, 130.98, 123.33, 115.10, 39.375, 32.19, 31.45,
30.30, 29.9, 29.93, 29.89, 29.62, 27.19, 22.96, 14.38;
HRMS(FAB+, m/z) [M]+ calcd for C26H36Br2O2S2
968.4279, measured 938.4283.
Hz), 7.22 (d, 2H, J = 5.3 Hz), 4.20 (d, 2H, J = 7.6 Hz),
1.92–1.88 (m, 1H), 1.35–1.19 (m, 32H), 0.9 (t, 6H, J = 6.5
Hz); 13C NMR (75 MHz, CDCl3) δ 162.29, 137.37, 133.26,
133.03, 124.21, 49.40, 36.39, 31.92, 31.63, 30.06, 29.64,
29.59, 29.54, 29.34, 29.31, 26.42, 22.67, 14.13; HRMS
(EI+, m/z) [M]+ calcd for C30H45O2NS2 515.2892, measured
515.2894.
Synthesis of2,20-bithiophene-3,30-dicarboxylic acid (5). This
material was synthesized following the published procedure11
from 3,30-dibromo-2,20-bithiophene. A solution of compound
4 (4.9 g, 15.1 mmol) in 50 mL of diethyl ether was added
dropwise over 1 h to a stirred solution of n-BuLi (24.2 mL,
2.5 M in hexane) in 50 mL of ethyl ether at −78 ꢀC. The reac-
tion mixture was then allowed to stir for 1 h at −78 ꢀC
before dry CO2 gas was bubbled into the reaction mixture
for 30 min. The reaction mixture was then allowed to stir
for an additional 30 min before 1 mL of methanol was added,
and the reaction mixture was filtered cold to afford a colorless
solid. After keeping under vacuum at 100 ꢀC overnight, the
solid was dissolved in 200 mL of water, and the resulting solu-
tion was acidified with 6 M HCl (aq.) to provide a colorless
precipitate. After filtration, the filtrate was dried overnight
under vacuum at 100 ꢀC to give 3.4 g (88%) of a colorless
Synthesis of N-(2-dodecyl)-5-50dibromo-2,20-bithiophene-
3,30-dicarboximide (9). Bromine (1.3 mL, 15.0 mmol) was
added to a solution of compound 8 (2.0 g, 3.8 mmol) in 30
mL of dichloromethane followed by addition of ferric chloride
(38 mg, 0.1 mmol). The reaction mixture was allowed to stir in
the dark for 6 h before 5 mL of sat. NaHSO3 (aq.) was added.
After stirring for 0.5 h, the reaction mixture was then poured
into dichloromethane, washed three times with water, and
dried over MgSO4. After removing the solvent under reduced
pressure, the residue was purified by flash chromatography to
give 3.8 g (78%) of compound 3 as a light yellow solid. Mp
1
50 ꢀC; H NMR (300 MHz, CDCl3) δ 7.67 (s, 2H), 4.18 (d,
2H, J = 7.7 Hz), 1.8–1.9 (m, 1H), 1.33–1.21 (m, 32H), 0.88
(t, 6H, J = 7.1 Hz); 13C NMR (75 MHz, CDCl3) δ 160.82,
137.37, 135.50, 133.36, 112.51, 49.67, 36.27, 31.92, 31.57,
30.02, 29.65, 29.59, 29.54, 29.36, 29.31, 26.37, 22.70,
14.13; HRMS (EI+, m/z) [M]+ calcd for C30H43O2NS2Br2
672.1180, measured 672.1177.
1
powder. Mp 196 ꢀC; H NMR (300 MHz, Acetone-d6) δ
7.60 (d, 2H, J = 5.4 Hz), 7.52 (d, 2H, J = 5.4 Hz); 13C NMR
was not measured due to very low solubility; HRMS (EI+,
m/z) [M]+ calcd for C10H6O4S2 253.9708, measured
253.9705.
Synthesis of poly(2-octyldodecyl-3-(2-thienyl)-4H-dithieno
[3,2-c:2,3e]azepine-4,6(5H)-dione-co-4,8-di(2-octyldodecy-
loxy)benzo[1,2-b;3,4-b]dithiophene) (PBTIBDT-3). BTI-
co-BDT with 7:3 feed ratio of BTI and BDT contents was
synthesized. Carefully purified 2,6-dibromo-4,8-dioctyloxy-
benzo[1,2-b;3,4-b’]dithiophene (3) (0.1676 g, 0.1780 mmol),
2,5-bis(trimethylstannanyl)thiophene (10) (0.2917 g, 0.5938
mmol) and N-(2-octyldodecyl)-5-50dibromo-2,20-bithio-
phene-3,30-dicarboximide (9) (0.2800 g, 0.4157 mmol), P(o-
tolyl)3 (40 mol%), and Pd2(0)(dba)3 (5 mol%) were mixed
with chlorobezene. The mixture was refluxed with vigorous
stirring for 8 h under argon atmosphere. After 36 h, phenyltri-
butylstannanewasaddedtothereaction mixture; 3 hlater, bro-
mobenzene was added and the reaction mixture was refluxed
overnight to complete the end-capping reaction. The whole
mixture was poured into methanol. The resulting solid was fil-
tered and washed with acetone to remove oligomers and
Synthesis of 2,20-bithiophene-3,30-dicarboxylic anhydride
(6). Compound 5 (3.2 g, 12.5 mmol) was stirred in 8 mL of
acetic anhydride at reflux for 6 h. Upon cooling to 0 ꢀC, the
reaction mixture provided a precipitate, which was washed
with cold acetic anhydride. This solid was dried overnight
under vacuum at 120 ꢀC to give 2.0 g (66%) of a light yellow
powder. Mp 261 ꢀC; 1H NMR (300 MHz, CDCl3) δ 7.66 (d,
2H, J = 5.4 Hz), 7.34 (d, 2H, J = 5.4 Hz); 13C NMR (75
MHz, CDCl3) δ 155.47, 139.99, 132.90, 128.18, 125.98;
HRMS (EI+, m/z) [M]+ calcd for C10H2O3S2 235.9602, meas-
ured 235.9605.
Synthesis of N-(2-octyldodecyl)-2,20-bithiophene-3,30-dicar-
boximide (8). A dry 10 mL microwave reaction tube was
charged with a micro-stirbar, anhydride compound 6 (1.5 g,
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Syntheses and Properties of Copolymers
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catalyst residues. Resulting PBTIBDT-3 (copolymer with 7:3
ratio of BTI (9) and BDT (3) contents in the feed) was soluble
in conventional organic solvents (toluene, chloroform, and
THF). Based on the elemental analyses of PBTIBDT-3, the
incorporation ratio was 7:3.9. Using the incorporated 7:3.9
ratio, Anal. Cald, for C44.9H65.8N0.7O2.18S3.2: C, 70.75; H,
8.55; N, 1.51; O, 4.77; S, 14.34. Found: C, 68.68; H,
8.38; N, 1.68; O, 5.24; S, 14.27.
C46.3H72.4N0.3O1.9S2.9: C, 72.95; H, 9.43; N, 0.72; O,
4.21; S, 12.66. Found: C, 72.18; H, 9.20; N, 0.66; O,
4.27; S, 13.36.
Results and Discussion
Synthesis and Characterization. The general synthetic
routes toward the intermediates and polymers are outlined
in Scheme 2. 2,6-Dibromo-4,8-di(2-octyldodecylloxy)benzo
[1,2-b;3,4-b’]dithiophene (3) was prepared from benzo[1,2-
b:4,5-b’]dithiophene-4,8-dione (1) over two steps. 3,30-
Dibromo-2,20-bithiophene (4)11 was converted, over two
steps, into 2,20-bithiophene-3,30-dicarboxylic anhydride (6),
which was coupled with 2-octyldodecylamine (7), prepared
using the known method,13 to provide N-(2-octyldodecyl)-
2,20-bithiophene-3,30-dicarboximide (8). N-(2-Octyldode-
cyl)-2,20-bithiophene-3,30-dicarboximide (8) was brominated
to generate N-(2-dodecyl)-5-50dibromo-2,20-bithiophene-
3,30-dicarboximide (9). 2,6-Dibromo-4,8-di(2-octyldodecyl-
loxy)benzo[1,2-b;3,4-b’]dithiophene (3), 2,5-bis(trimethyl-
stannanyl)thiophene (10), and N-(2-dodecyl)-5-50dibromo-
2,20-bithiophene-3,30-dicarboximide (9) were coupled to pro-
vide the copolymers through Stille reaction with
Pd(0) catalyst. The co-monomer feeding ratios of compounds
3–9 are 3:7, 5:5, and 7:3, respectively, and the corresponding
copolymers were named PBTIBDT-3, PBTIBDT-5, and
PBTIBDT-7, respectively. All polymers showed good solu-
bility at room temperature in organic solvents such as chloro-
form, THF, chlorobenzene, and o-dichlorobenzene (ODCB).
Table 1 summarizes the polymerization results including
the molecular weights, PDI, and thermal stability of the copo-
lymers. The number-average molecular weights (Mn) of
45 000, 46 000, and 37 000 and weight-average molecular
weights (Mw) of 64 000, 60 000, and 45 000 with PDI (poly-
dispersity index, Mw/Mn) values of 1.41, 1.31, and 1.18 were
determined by GPC for the PBTIBDT-3, PBTIBDT-5, and
PBTIBDT-7, respectively.
The thermal properties of the polymers were
characterized by both differential scanning calorimetry
(DSC) and thermogravimetric analysis (TGA), as shown
in Figure 1 and summarized in Table 1. The polymers show
good thermal stability, with onset decomposition tempera-
tures (Td, 5% weight loss) of 317, 321, and 318 ꢀC, respec-
tively. All polymers have glass transition temperature (Tg)
values over 100 ꢀC, confirming their thermal stability.
Regardless of the ratio, all polymers were shown to have
similar thermal properties. The high thermal stability of
the resulting polymers prevents the deformation of the
morphology and is important for organic photovoltaic
(OPV) device applications.
Synthesis of poly(2-octyldodecyl-3-(2-thienyl)-4H-dithieno
[3,2-c:2,3e]azepine-4,6(5H)-dione-co-4,8-di(2-octyldodecy-
loxy)benzo[1,2-b;3,4-b]dithiophene) (PBTIBDT-5). BTI-
co-BDT with 5:5 feed ratio of BTI and BDT contents was
synthesized. Carefully purified 2,6-dibromo-4,8-dioctyloxy-
benzo[1,2-b;3,4-b’]dithiophene (3) (0.2794 g, 0.2969 mmol),
2,5-bis(trimethylstannanyl)thiophene (10) (0.2917 g, 0.5938
mmol) and N-(2-octyldodecyl)-5-50dibromo-2,20-bithio-
phene-3,30-dicarboximide (9) (0.200 g, 0.2969 mmol), P(o-
tolyl)3 (40 mol%), and Pd2(0)(dba)3 (5 mol%) were mixed
with chlorobezene. The mixture was refluxed with vigorous
stirring for 8 h under argon atmosphere. After 36 h, phenyltri-
butylstannanewasaddedtothereaction mixture; 3 hlater, bro-
mobenzene was added, and the reaction mixture was refluxed
overnight to complete the end-capping reaction. The whole
mixture was poured into methanol. The resulting solid was fil-
tered and washed with acetone to remove oligomers and cat-
alyst residues. The resulting PBTIBDT-5 (copolymer with
5:5 ratio of BTI (9) and BDT (3) contents in the feed) was sol-
uble in conventional organic solvents (toluene, chloroform,
and THF). Based on elemental analysis of PBTIBDT-5, the
incorporation ratio was 5:6. Using the incorporated 5:6 ratio,
Anal. Cald for C49.4H75.3N0.5O2.2S3.3: C, 72.02; H, 9.05; N,
1.07; O, 4.46; S, 13.39. Found: C, 71.27; H, 8.71; N,
1.03; O, 4.53; S, 12.50.
Synthesis of poly(2-octyldodecyl-3-(2-thienyl)-4H-dithieno
[3,2-c:2,3-e]azepine-4,6(5H)-dione-co-4,8-di(2-octyldode-
cyloxy)benzo[1,2-b;3,4-b]dithiophene) (PBTIBDT-7). BTI-
co-BDT with 3:7 feed ratio of BTI and BDT contents was
synthesized. Carefully purified 2,6-dibromo-4,8-dioctyloxy-
benzo[1,2-b;3,4-b’]dithiophene (3) (0.3912 g, 0.4157 mmol),
2,5-bis(trimethylstannanyl)thiophene (10) (0.2917 g, 0.5938
mmol) and N-(2-octyldodecyl)-5-50dibromo-2,20-bithio-
phene-3,30-dicarboximide (9) (0.1200 g, 0.1781 mmol), P(o-
tolyl)3 (40 mol%), and Pd2(0)(dba)3 (5 mol%) were mixed
with chlorobezene. The mixture was refluxed with
vigorous stirring for 8 h under argon atmosphere. After 36
h, phenyltributylstannane was added to the reaction mixture;
3 h later, bromobenzene was added and the reaction
mixture was refluxed overnight to complete the end-capping
reaction. The whole mixture was poured into methanol. The
resulting solid was filtered and washed with acetone to remove
oligomers and catalyst residues. The resulting PBTIBDT-7
(copolymer with 3:7 ratio of BTI (9) and BDT (3)
contents in the feed) was soluble in conventional organic sol-
vents (toluene, chloroform, and THF). Based on the elemental
analysis of PBTIBDT-7, the incorporation ratio was 3:6.7.
Using the incorporated 3:6.7 ratio, Anal. Cald for
Optical Properties of the Polymers. The optical properties
of solutions and films of polymers were investigated by UV–
vis absorption spectroscopy. A uniform film was prepared on
quartz plates by spin-casting from their chloroform solution at
room temperature. The UV–vis absorption spectra of the poly-
mers in solution and thin films are shown in Figure 2 and
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C8H17
C8H17
C10H21
C10H21
O
O
O
O
O
1
Zn , NaOH , DMF, EtOH, H2O
2-octyl-1-dodecyliodide, TBAB
S
S
S
Br2
MC
Br
Br
S
S
S
O
C10H21
C10H21
C8H17
C8H17
2
3
O
O
CO2H
S
Br
O
S
S
AC2O
n-BuLi, CO2
S
ether
S
S
CO2H
Br
4
5
6
C8H17
C10H21
C8H17
C10H21
O
O
O
O
N
O
O
O
μ-wave 220 O
C
N
Br2, FeBr3
MC
C10H21
NH2
C8H17
4-(dimethylamino)pyridine
S
S
S
S
S
Br
S
Br
6
7
8
9
C8H17
C8H17
C10H21
O
C10H21
O
O
N
S
S
Sn
Sn
Br
Br
S
O
S
S
Br
Br
C10H21
C8H17
3
10
9
C8H17
C8H17
C10H21
C10H21
O
O
N
O
Pd2(dba)3 , P(o-tolyl)3
S
S
S
co
m
S
S
chlorobenzene
S
n
O
m : n = 7 : 3
m : n = 5 : 5
m : n = 3 : 7
PBTIBDT-3
PBTIBDT-5
PBTIBDT-7
C10H21
C8H17
Scheme 2. Synthetic route for the synthesis of the monomers and polymers.
summarized in Table 2. The solution was prepared using
ODCB as a solvent, and the thin film was prepared by spin-
coating on quartz plates from the solution in ODCB. The
UV–vis absorption spectra of the polymers in solution and
as thin films are shown in Figure 2 and summarized in Table 2.
As shown in the spectra of Figure 2, the solution of all the
polymers presents red-shifted maximum peaks and increased
full width at half-maximum (FWHM) with increasing
amounts of BTI content.
The short-wavelength absorption peaks have been ascribed
to a delocalized excitonic π–π* transition in the polymer
chains, and the long-wavelength absorption peaks are attribu-
ted to the ICT between the substituted BDT as the electron-
pushing unit and BTI as the electron-pulling unit.16 As
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Table 1. Polymerization results and thermal properties of polymers.
PDIa
DSC (Tg)
TGA (Td)b
a
a
Polymer
Mn
Mw
PBTIBDT-3
PBTIBDT-5
PBTIBDT-7
45 000
46 000
37 000
64 000
60 000
45 000
1.41
1.31
1.18
113
113
102
317
321
318
a Molecular weight (Mw) and polydispersity (PDI) of the polymers were determined by gel permeation chromatography (GPC) in THF using polystyrene
standards.
b Onset decomposition temperature (5% weight loss) was measured by TGA under N2.
1.2
100
80
60
40
20
PBTIBDT-3
PBTIBDT-5
PBTIBDT-7
PBTIBDT-3_sol
PBTIBDT-5_sol
PBTIBDT-7_sol
P0_sol
1.0
0.8
0.6
0.4
0.2
0.0
100
200
300
400
500
600
300
400
500
600
700
800
Temperature (°C)
Wavelength (nm)
-0.02
-0.04
-0.06
-0.08
PBTIBDT-3
PBTIBDT-5
PBTIBDT-7
1.2
1.0
0.8
0.6
0.4
0.2
0.0
PBTIBDT-3_film
PBTIBDT-5_film
PBTIBDT-7_film
P0_film
50
100
150
200
Temperature (°C)
300
400
500
600
700
800
Figure 1. Thermogravimetric analysis of polyemrs under N2. (Inset)
Differential scanning calorimetry of polymers under N2.
Wavelength (nm)
Figure 2. UV–visible absorption spectra of polymers in ODCB solu-
tion and in the solid state.
compared to the spectra of the solution, those of the films show
broader peaks slightly red-shifted due to the formation of
π-stacked structures in the solid state. The absorption maxi-
mum peak near 514 nm of PBTIBDT-7 underwent a relative
red shift to 561 and 604 nm of PBTIBDT-3 upon increasing
the contents of the BTI unit, implying that by tuning their com-
position the relative absorption area of the copolymers
PBTIBDT-3, PBTIBDT-5, and PBTIBDT-7 could be
adjusted effectively. The energy bandgaps calculated from
the absorption edges were ~1.79, 1.90, and 1.91 eV for
PBTIBDT-3, PBTIBDT-5, and PBTIBDT-7, respectively,
indicating an effective narrowing of the bandgaps by increas-
ing the BTI contents.
As reported with other random copolymers,17,18 which
improved absorptions and bandgaps as compared to the corre-
sponding alternating polymers, the random copolymer
PBTIBDT-3 with increased amounts of BTI has a wider range
of absorption with a lower bandgap than the corresponding
alternating polymer (P0).
Electrochemical Properties of the Polymers. The electro-
chemical data of the materials are summarized in Table 3 and
shown in Figure 3. The HOMO and LUMO) levels were calcu-
ox
lated according to the empirical formula (EHOMO = −([Eonset
+ 4.8) eV) and (ELUMO = −([Eonset
]
]
red + 4.8) eV), respectively.
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Table 2. Characteristics of the UV–vis absorption spectra.
Polymer
In solution
In thin film
Optical bandgapa (eV)
PBTIBDT-3
PBTIBDT-5
PBTIBDT-7
555
512
504
561, 604
536
514
1.79
1.90
1.91
a Optical energy bandgap was estimated from the onset wavelength of the optical absorption.
Table 3. Electrochemical potentials and energy levels of the polymers.
Polymers
HOMOa (eV)
LUMOb (eV)
Eoxc (V)
Eredc (V)
Electrochemical bandgapd (eV)
PBTIBDT-3
PBTIBDT-5
PBTIBDT-7
−5.68
−5.59
−5.57
−3.90
−3.97
−3.94
0.88
0.79
0.77
−0.90
−0.83
−0.87
1.78
1.62
1.64
a Calculated from the oxidation potentials.
b Calculated from the reduction potentials.
c Onset oxidation and reduction potential were measured by cyclic voltammetry.
d Calculated from the Eox and Ered
.
−0.87 eV, respectively, which correspond to LUMO energy
levels of −3.90, −3.97, and −3.94 eV, respectively. The elec-
trochemical bandgaps calculated from cyclic voltammetry
data are ~1.78, 1.62, and 1.64 eV, which are somewhat lower
than the optical bandgaps estimated from the onset wave-
lengths of the absorption spectra.20–21
1.0
PBTIBDT-3
PBTIBDT-5
PBTIBDT-7
0.5
Photovoltaic Properties of the Polymers. The photovoltaic
properties of PBTIBDT-3, PBTIBDT-5, and PBTIBDT-7
were investigated by fabricating OPV cells with ITO as the
positive electrode, the blend with copolymers (PBTIBDT-
3, PBTIBDT-5, and PBTIBDT-7) and PC71BM as the active
layer, and Al as the negative electrode. Figure 4 shows the cur-
rent vs. voltage (I–V) curves of the OPVs with the configura-
tion of ITO/PEDOT:PSS/polymer:PC71BM/Al under
AM1.5G irradiation (100 mW/cm2), and Table 4 summarizes
the results. The BHJ devices were fabricated by spin-coating
1% (w/v) ODCB solutions comprising a blend of copolymers
and PC71BM in different blend ratios (1:1 and 1:3). The
devices with PBTIBDT-3 and PC71BM with ratio of 1:1
showed VOC = 0.83 V, JSC = 2.14 mA/cm2, and FF = 0.56, giv-
ing a power conversion efficiency of 0.99%. The devices com-
prising PBTIBDT-5 with PC71BM (1:1) showed VOC = 0.76
V, JSC = 3.28 mA/cm2, and FF = 0.51, giving a power conver-
sion efficiency of 1.26%. The devices comprising PBTIBDT-
7 with PC71BM (1:1) showed VOC = 0.79 V, JSC = 2.48
mA/cm2, and FF = 0.47, giving a power conversion efficiency
of 0.92%. The PBTIBDT-5 device had better performance
because of good morphology, as shown in Figure 6(b), and
suitable LUMO energy level for efficient charge separation.
The IPCE spectra of the photovoltaic devices from polymer:
PC71BM blends are presented in Figure 5. The IPCE spectra
of the copolymers show a maximum of 20.1% at 350 nm for
PBTIBDT-3, 31.6% at 402 nm for PBTIBDT-5, and
30.2% at 383 nm for PBTIBDT-7. The enhanced efficiency
of PBTIBDT-5 results from the higher IPCE values between
330 and 630 nm. The higher IPCE values of PBTIBDT-5 led
0.0
-0.5
-1.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
Voltage (V)
Figure 3. Cyclic voltammetry curves of the polymers in 0.1 M TBAF
in acetonitrile solution at a scan rate of 100 mV/s at room temperature
(vs. a Ag quasi-reference electrode).
The polymers PBTIBDT-3, PBTIBDT-5, and PBTIBDT-
7, exhibited the absorption onset wavelengths of 689, 652, and
649 nm in solid thin films, which correspond to bandgaps of
1.79, 1.90, and 1.91 eV, respectively. The polymers exhibit
irreversible processes in an oxidation scan. The oxidation
onsets of the polymers PBTIBDT-3, PBTIBDT-5, and
PBTIBDT-7 were estimated to be 0.88, 0.79, and 0.77 V,
which correspond to HOMO energy levels of −5.68, −5.59,
and −5.57 eV, respectively. Obviously, the increase in oxida-
tion potentials of these copolymers can be attributed to the lar-
ger number of BTI units incorporated into the polymer main
chains, implying that they varied with respect to the ICT
strengths resulting from the content of the electron-pulling
unit.17–19
The reduction potential onsets of the polymers PBTIBDT-
3, PBTIBDT-5, and PBTIBDT-7 are −0.90, −0.83, and
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Table 4. Photovoltaic properties of polymers.
(a) PBTIBDT-3
Donor:
PC71BM
ratio
VOC
(V)
JSC
PCE
(%)
Donor
(mA/cm2) FF
0
-2
-4
PBTIBDT-3
1:1
1:3
1:1
1:3
1:1
1:3
0.83 2.14
0.79 1.03
0.76 3.28
0.73 1.31
0.79 2.48
0.68 1.40
0.56 0.99
0.48 0.39
0.51 1.26
0.51 0.52
0.47 0.92
0.45 0.42
PBTIBDT-5
PBTIBDT-7
Polymer:acceptor 10 mg/mL in DCB/CF.
PBTIBDT-3
1:1
1:3
40
30
20
10
0
0.0
0.2
0.4
0.6
0.8
1.0
PBTIBDT-3
PBTIBDT-5
PBTIBDT-7
Voltage (V)
(b) PBTIBDT-5
0
-2
-4
300
400
500
600
700
800
PBTIBDT-5
Wavelength (nm)
1:1
1:3
Figure 5. IPCE curves of the polymer:PC71BM = 1:1 under the illu-
mination of AM 1.5, 100 mW/cm2.
0.0
0.2
0.4
0.6
0.8
1.0
to a larger current density than that of PBTIBDT-3 and
PBTIBDT-7, which is caused by the optimum morphology
for higher performance.
Voltage (V)
(c) PBTIBDT-7
Morphology of the Polymers. AFM studies of the material
blends (polymer:PC71BM = 1:1.5) show the morphology of
the polymer/PC71BM blend films. The images were obtained
over a surface area of 1.0 × 1.0 μm2 in the tapping mode. The
AFM images of PBTIBDT-3, PBTIBDT-5, and PBTIBDT-
7 exhibit root-mean-square (rms) roughness of 1.31, 0.94, and
1.61 nm, as shown in Figure 6. The smoother surface without
largedomainsincreasesinterconnectionswithintheinterpene-
trating network for optimal charge transport, thereby resulting
in high Jsc values. 9
0
-2
-4
PBTIBDT-7
1:1
1:3
Conclusion
PBTIBDT-3, PBTIBDT-5, and PBTIBDT-7 with substi-
tuted-BDT as the electron-pushing unit and BTI as the elec-
tron-pulling unit were synthesized by the Stille coupling
reaction with Pd(0) catalyst to provide good solubility in com-
mon organic solvents. By increasing the content of the BTI
unit, the absorption spectra of PBTIBDT-3 thin film showed
a shift to longer wavelengths. Solution-processed OPVs with
0.0
0.2
0.4
0.6
0.8
1.0
Voltage (V)
Figure 4. Current density vs. potential characteristics of the polymer:
PC71BM = 1:1 and polymer:PC71BM = 1:3 under illumination of
AM 1.5, 100 mW/cm2.
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(b) PBTIBDT-5
KOREAN CHEMICAL SOCIETY
(a)
PBTIBDT-3
(c)
PBTIBDT-7
Figure 6. Atomic force microscopy images (2.5 × 2.5 μm2) of polymer:PC71BM = 1:1.
9. H.-C. Chen, Y.-H. Chen, C.-C. Liu, Y.-C. Chien, S.-W. Chou,
P.-T. Chou, Chem. Mater. 2012, 24, 4766.
10. X. Guo, R. P. Ortiz, Y. Zheng, Y. Hu, Y. Y. Noh,
K. J. Baeg, A. Facchetti, T. J. Marks, J. Am. Chem. Soc. 2011,
133, 1405.
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the polymers as donor and PC71BM as acceptor were
fabricated, and the experimental conditions with different
blending ratios were optimized. The device comprising
PBTIBDT-5 and PC71BM (1:1) showed VOC = 0.76 V, JSC
= 3.28 mA/cm2, and FF = 0.51, giving a power conversion
efficiency of 1.26%.
Acknowledgment. This work was supported by the National
Research Foundation of Korea (NRF) grant funded by the
Korea government (MEST) (NRF-2014R1A1A2055318).
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