Synthesis of fluorinated benzotriazole (BTZ)- and benzodithiophene (BDT)-based low-bandgap conjugated polymers for solar cell applications

Article abstract of DOI:10.1016/j.dyepig.2016.12.007

A series of donor–acceptor (D–A) polymers (P1–P3) based on benzodithiophene (BDT) and electron-accepting benzotriazole (BTZ) units containing thiophene linkers with/without alkyl side-chains were designed and synthesized via Stille coupling polymerization

Full text of DOI:10.1016/j.dyepig.2016.12.007

Accepted Manuscript
Synthesis of fluorinated benzotriazole (BTZ)- and benzodithiophene (BDT)-based low-
bandgap conjugated polymers for solar cell applications
Murali Krishna Pola, Karunakara Moorthy Boopathi, Harihara Padhy, Raghunath
Putikam, Ashutosh Singh, Ming-Chang Lin, Chih-Wei Chu, Hong-Cheu Lin
PII:
S0143-7208(16)30789-6
10.1016/j.dyepig.2016.12.007
DYPI 5632
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 21 September 2016
Revised Date: 23 November 2016
Accepted Date: 5 December 2016
Please cite this article as: Pola MK, Boopathi KM, Padhy H, Putikam R, Singh A, Lin M-C, Chu C-
W, Lin H-C, Synthesis of fluorinated benzotriazole (BTZ)- and benzodithiophene (BDT)-based low-
bandgap conjugated polymers for solar cell applications, Dyes and Pigments (2017), doi: 10.1016/
j.dyepig.2016.12.007.
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ACCEPTED MANUSCRIPT
Graphical Abstract

Synthesis of fluorinated benzotriazole (BTZ)- and benzodithiophene
ACCEPTED MANUSCRIPT
(BDT)-based low-bandgap conjugated polymers for solar cell applications
Murali Krishna Pola,a Karunakara Moorthy Boopathi,b Harihara Padhy,a Raghunath Putikam,c
Ashutosh Singh,a Ming-Chang Lin,c Chih-Wei Chu,b Hong-Cheu Lina,*
a Department of Materials Science and Engineering, National Chiao Tung University, Hsinchu,
Taiwan
b Research Center for Applied Sciences, Academia Sinica, Taipei, Taiwan
c
Center for Interdisciplinary Molecular Science, Department of Applied Chemistry, National
Chiao Tung University, Hsinchu 300, Taiwan
Supporting Information Placeholder
Keywords: fluorine-substituted benzotriazole, benzodithiophene, donor–acceptor polymer, bulk
heterojunction solar cell, low-bandgap polymer.
1

Abstract
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A series of donor–acceptor (D–A) polymers (P1-P3) based on benzodithiophene (BDT) and
electron-accepting benzotriazole (BTZ) units containing thiophene linkers with/without alkyl
side-chains were designed and synthesized via Stille coupling polymerization method. The
effects of polymers with multiple fluorinated BTZ groups on their thermal, optical,
electrochemical, and photovoltaic properties were investigated. These polymers possessed the
highest occupied molecular orbital (HOMO) levels ranged -5.38 to -5.6 eV and the lowest
unoccupied molecular orbital (LUMO) levels ranged -3.55 to -3.57 eV, which covered broad
absorption ranges with low optical bandgaps. The bulk heterojunction (BHJ) polymer solar cell
(PSC) devices containing an active layer of D-A polymers blended with different weight ratios
of electron-acceptor [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) were explored under
100 mW cm-2 of AM 1.5 white-light illumination, where the maximum power conversion
efficiency (PCE) value of 3% (with Jsc = 7.70 mA/cm2, FF = 54.04, and Voc = 0.72 V) was
obtained in the PSC device consisting of polymer P3
.
2

1. Introduction
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To generate unlimited energy directly from sunlight using photovoltaic (PV) technology has
become an essential factor in order to meet the global energy demand. Organic solar cells
consisting of conjugated polymers with promising state-of-the-art photovoltaic technology
are due to several benefits, such as low-cost, light-weight, and large-area devices by using
solution-processed roll-to-roll printing methods.[1-7] In the past decade, significant
progress[11-14] in power conversion efficiencies (PCEs) over 11 % has been achieved in bulk
heterojunction (BHJ) solar cell devices containing conjugated polymers as electron-donating
materials along with [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) (or
[6,6]-phenyl-C61-butyric acid methyl ester, PC61BM) as electron-accepting materials.[8-10]
Nevertheless, intensive attentions to solve the problems of polymer solar cells (PSCs) are
required, including poor fill factor (FF), short-circuit current (Jsc), open-circuit voltage (Voc),
morphology, and balanced charge mobilities.[15] To meet the above challenges, it is necessary
to develop novel alternating donor-acceptor (D-A) building blocks.[16-18]
Recently, several low bandgap (LBG) polymer materials are designed to harvest higher
amounts of light from the solar spectra and larger bandgaps in D-A polymers with a
lower-lying highest occupied molecular orbitals (HOMO) and the higher-lying lowest
unoccupied molecular orbital (LUMO), which provides an opportunity to increase the open
circuit voltage.[19-28] A high Voc is readily achieved by tuning of HOMO energy level of
donor, because the Voc is directly proportional to HOMO level of donor and LUMO levels of
3

acceptor i.e PC71BM or PC61BM.[29],[30] To increase the values of Voc in conjugated polymers
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while maintaining the bandgap constant, both HOMO and LUMO energy levels must be
decreased simultaneously. Thus, the electron-withdrawing groups need to be placed
alternatively on polymer backbones.[31] The fluorine atom has been recently gained
considerable attention as an electron-withdrawing group for high-performance polymer solar
cells, which with a very small size can be easily introduced onto polymer backbones without
any steric effects compared with the other electron-withdrawing groups.[32],[33]
Fluorine-substituted conjugated polymers exhibited higher thermal and oxidative stabilities,
resistance to degradation, and inverted charge distribution in polymers. These unique features
of the fluorine atom are due to the highest electronegative element and smallest
electron-withdrawing group among all electron-withdrawing groups (e.g., cyano, nitro, and
trifluoromethyl groups). Moreover, these fluorine atoms showed great influences on inter-
and intra-molecular interactions.[34-39] Furthermore, M. P. Felix and F. G. Michael et al.
reported that the introduction of fluorine atoms onto polymer backbones decreased HOMO
energy levels in the copolymers based on 2-alkyl-2H-benzotriazole derivatives as acceptor
units and fluorine and carbzole as donor units with PCE values of 1.8 and 4.6%,
respectively.[40],[41]
The arrangements of alternating donor and acceptor units were highly effective to
design photovoltaic polymers with desired intrinsic optoelectronic properties (such as
light-harvesting windows and energy levels) by optimal selections of electron-donating
4

(donor) and electron-withdrawing (acceptor) building blocks. The benzodithiophene (BDT)
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unit has emerged as an attractive donor building block for PSCs as well as small molecule
organic solar cells (SMOSCs) owing to its high-coplanarity, π-π stacking, high-mobility and
enhanced solubility. Moreover, BDT-based polymers exhibiting low HOMO levels
potentially provide high open circuit voltages and different LUMO levels, which can be
manipulated effectively through the use of various acceptors.[42-51] Very recently,
Zhong-Sheng Xu and coworkers reported a series of BDT-based polymers featuring
mono-fluorinated benzothiadiazole unit as the electron-acceptor that displayed a highest PCE
value of 6.2%.[52]
In the spirit of further exploring structure-property and manipulation acceptor groups,
herein we report the design, synthesis, and investigation of BDT and benzotriazole (BTZ) units
as donor and acceptor, respectively. Three polymers (P1–P3) are developed to utilize
multiple-fluorine-substituted benzene ring units in BTZ to fine tune the light absorption
capabilities, Voc values, and morphlogy due to favorable intramolecular charge transfer (ICT)
between BDT and BTZ units.
2. Experimental Section
2.1. Materials
All of the chemicals and solvents were reagent grade and were purchased from Aldrich,
ACROS, TCI, TEDIA, and Alfa-Aesar Chemical Co. Tetrahydrofuran (THF), toluene, and
5

diethyl ether were distilled over sodium/benzophenone. Other reagents were used as received
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without further purification.
2.2 Measurements and characterization
1H and 13C NMR spectra were recorded using a Varian VNMRS-500 and 700 MHz
spectrometer using CDCl3 as the solvent and the chemical shifts are reported as δ values
relative to a tetramethylsilane (TMS) internal standard. Elemental analyses were performed
on a HERAEUS CHN-O RAPID elemental analyzer. Thermogravimetric analyses (TGA)
were conducted on a TA Instruments Q500 instrument at a heating rate of 10o°C min-1 under
nitrogen. Gel permeation chromatography (GPC) was conducted using a Waters 1515
separation module, using polystyrene as a standard and THF as the eluent. UV–vis absorption
spectra were recorded in dilute chloroform solutions (10-6 M) using an HP G1103A
spectrophotometer. The solid films for UV–vis measurements were spin-coated on glass
substrates from chlorobenzene solution with a concentration of 5 mg/mL. Cyclic
voltammetry (CV) was measured using a BAS 100 electrochemical analyzer with a standard
three-electrode electrochemical cell in anhydrous acetonitrile containing 0.1 M
tetrabutylammonium hexafluorophosphate (TBAPF6) at a scanning rate of 100 mV s-1 at
room temperature. The solutions were purged with nitrogen for 30 s during the CV
measurements. In each case, a carbon rod coated with a thin layer of polymers as the working
electrode, a platinum wire as the counter electrode, and a silver wire as the quasi-reference
electrode were used. An Ag/AgCl (3M of KCl in acetonitrile) electrode served as the
6

reference electrode for all of the potentials quoted herein. The ferrocene/ferrocenium (Fc/Fc+)
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redox couple was used as an external standard. The highest occupied molecular orbital
(HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels were estimated by
the onset oxidation and reduction potentials from the reference energy level of ferrocene (4.8
eV below the vacuum level), according to the following equations.[53]
+
EHOMO/LUMO = [- (Eonset
-
Eonset(Fc/Fc vs.Ag/Ag+))-4.8] eV
(1)
_
Bandgap = Eox,onset Ered,onset
(2)
+
+
4.8 eV is the energy level of ferrocene below the vacuum level and Eonset(Fc/Fc vs.Ag/Ag ) = 0.45
eV. For the CV experiments with solid films of the polymers, which were performed by
drop-casting films with similar thicknesses from THF solutions (5 mg mL-1), the LUMO
level of PC71BM employed was in accordance with the literature data. The onset potentials
were determined from the intersections of two tangents drawn at the rising currents and
background currents of the CV measurements. The topologies of the films were investigated
using atomic force microscopy (AFM, Veeco diInnova; scanning rate: 3–4 Hz), performed in
the tapping mode in a dry environment at room temperature over a sampling area of
5 × 5 ꢀm2. The images had a resolution of 512 × 512 pixels and were acquired at an
approximate scanning rate of 0.5 Hz. The images obtained were processed using diSPMLab
software (Veeco). Surface morphologies and particle distributions were investigated using a
thermal field emission scanning electron microscope (JEOL JSM-6500F).
7

2.3 Fabrication of polymer solar cells
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The polymer solar cells (PSCs) in this study contained an active layer of polymers (P1–P3)
blended with PC71BM in a solid film, which was sandwiched between a transparent
indium/tin oxide (ITO) anode and a calcium cathode (see Fig. S1). Prior to device
fabrication, the ITO-coated glass substrates (1.5 × 1.5 cm2) were subjected to cleaning with
ultrasonication in detergent, deionized water, acetone, and isopropyl alcohol successively for
10 min each. After routine solvent cleaning, the substrates were treated with UV ozone for 15
min. Then, a modified ITO surface was obtained by the spin-coating of a layer of
poly(ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) (40 nm). After baking
at 120 °C for 1 h, the substrates were transferred to a nitrogen-filled glove box. Subsequently,
on the top of PEDOT:PSS layer, the active layer was prepared by spin-coating from blended
solutions of polymers P1–P3:PC71BM (with 1:2 w/w) at a spin rate of 1500 rpm. The
thickness of the active layer was typically 200 nm. Initially, the blended solutions were
prepared by dissolving both polymers and PC71BM in trichlorobenzene (TCB) (20 mg mL-1),
followed by continuous stirring for 12 h at 50°C. In the slow-growth approach, the blended
polymers in solid films were kept in the liquid phase after spin-coating by using a solvent
with a high boiling point (e.g., in a glass Petri dish) and allowing the solvent to dry slowly.
Finally, a calcium layer (30 nm) and a subsequent aluminum layer (60 nm) were thermally
evaporated through a shadow mask at a pressure below 6 × 10-6 Torr, to obtain an active
device area of 0.12 cm2. The solar-cell testing was done inside a glove box under simulated
8

AM 1.5 G irradiation (100 mW/cm2) using a xenon-lamp-based solar simulator (Thermal
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Oriel 1000 W). The light intensity was calibrated using a monosilicon photodiode with a
KG-5 color filter (Hamamatsu). The external quantum efficiency (EQE) action spectra were
obtained at short-circuit conditions. The light source was a 450 W xenon arc lamp (Oriel
Instrument, model 6266) equipped with a water-based IR filter (Oriel Instruments, model
6123NS). The light output from the monochromator (Oriel Instruments, model 74100) was
focused on the photovoltaic cell under test.
2.4 Synthesis
2.4.1 General suzuki-coupling procedures for 3a–3c:
A solution of dibromo compound 1a or 1b (1.0 equiv) and thiophene-2-boralone compound
2a or 2b (3.0 equiv) in a mixture anhydrous toluene (40 mL), 2 M aqueous sodium carbonate
(NaCO3) (10 mL) solution and a few drops of Aliquat 336 was degassed and purged with
nitrogen. Afterward, tetrakis(triphenylphosphine)palladium(0) (Pd(PPh3)4) (0.05 equiv) was
added in one portion, and the reaction mixture was heated to 90°C with vigorous stirring until
reaction completion by TLC analyses. After cooling to room temperature, the reaction
mixture was poured into water and extracted with dichloromethane. The combined organic
layer was washed thrice with water, once with brine and dried over anhydrous magnesium
sulfate (MgSO4). The solvent was evaporated and the residue was purified by column
chromatography on silica gel using hexanes/dichloromethane (1/2, v/v) as eluent, and
9

recrystallized from dichloromethane/MeOH to obtain the desired product. Their chemical
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characterization analyses are shown as follows.
2.4.1.1 2-(3,5-Difluoro-4-(octyloxy)phenyl)-4,7-di(thiophen-2-yl)-2H-benzo[d][1,2,3]triazole
(
3a)
3a was prepared according to the general Suzuki-coupling procedure. 1a (2.0 g, 3.9 mmol),
2a (2.5 g, 11.6 mmol), Pd(PPh3)4 (225 mg, 0.17 mmol) were used and stirred at 90°C for 24
h. The resulting product 3a was obtained as a green solid with a yield of 80%. 1H NMR (500
MHz, CDCl3) δ (ppm): 8.12 (d,
J = 4.0 Hz, 2H), 8.06 (d, J = 9.0 Hz, 2H), 7.65 (s, 2H), 7.41
(d, = 5.0 Hz, 2H), 7.21 (t, = 4.0 Hz, 2H), 4.22 (t,
J
J
J
= 6.5 Hz, 2H), 1.83-1.77 (quin, J = 6.5
13
Hz, 2H), 1.52-1.46 (m, 2H), 1.34-1.29 (m, 8H), 0.88 (t,
J = 6.5 Hz, 3H). C NMR (500
MHz, CDCl3) δ (ppm): 157.01, 155.02, 142.98, 139.36, 128.18, 127.13, 126.00, 123.88,
123.73, 105.32, 105.25, 105.09, 75.14, 31.79, 29.97, 29.26, 29.21, 25.63, 22.64, 14.10.
2.4.1.2
2-(3,5-Difluoro-4-(octyloxy)phenyl)-4,7-bis(4-hexylthiophen-2-yl)-2H-benzo[d][1,2,3]triazol
e (3b
)
3b was prepared according to the general Suzuki-coupling procedure. 1a (2.0 g, 3.9 mmol),
2b (3.45 g, 11.6 mmol), Pd(PPh3)4 (225 mg, 0.17 mmol) were used and stirred at 90°C
overnight. The resulting product 3b was obtained as a light green solid with a yield of 84%.
1H NMR (500 MHz, CDCl3) δ (ppm): 8.04 (d,
J
= 9.0 Hz, 2H), 7.90 (d,
J
= 1.5 Hz, 2H), 7.60
(s, 2H), 6.99 (d,
J
= 1.5 Hz, 2H), 4.22 (t,
J
= 6.5 Hz, 2H), 2.70 (t, J = 8.0 Hz, 4H), 1.83-1.77
10

(quin,
J = 6.5 Hz, 2H), 1.73-1.67 (quin, J = 8.0 Hz, 4H), 1.52-1.46 (m, 2H), 1.44-1.38 (m,
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13
4H), 1.35-1.29 (m, 16H), 0.91-0.87 (m, 9H). C NMR (500 MHz, CDCl3) δ (ppm): 157.05,
157.00, 155.08, 155.02, 144.42, 142.96, 138.93, 134.44, 128.34, 123.76, 123.65, 120.81,
105.34, 105.27, 105.18, 105.11, 75.15, 31.79, 31.73, 30.64, 30.44, 29.96, 29.27, 29.21,
29.04, 25.63, 22.64, 14.11.
2.4.1.3 4,7-Bis(4-hexylthiophen-2-yl)-2-(3,4,5-trifluorophenyl)-2H-benzo[d][1,2,3]triazole
(3c)
3c was prepared according to the general Suzuki-coupling procedure. 1b (2.0 g, 4.9 mmol),
2b (4.4 g, 14.8 mmol), Pd(PPh3)4 (285 mg, 0.02 mmol) were used and stirred at 90°C for 24
1
h. The resulting product 3c was obtained as a yellow solid with a yield of 70%. H NMR
(500 MHz, CDCl3) δ (ppm): 8.14-8.11 (m, 2H), 7.87 (s, 2H), 7.60 (s, 2H), 7.00 (s, 2H), 2.70
(t,
J
= 8.0 Hz, 4H), 1.73-1.67 (quin, J = 6.5 Hz, 4H), 1.42-1.38 (m, 4H), 1.35-1.31 (m, 8H),
13
0.89 (t,
J = 7.5 Hz, 6H). C NMR (500 MHz, CDCl3) δ (ppm): 152.42, 150.47, 150.42,
144.43, 143.14, 139.18, 138.75, 135.29, 128.33, 123.93, 123.80, 120.96, 105.57, 105.52,
105.42, 105.36, 31.72, 30.62, 30.44, 29.04, 22.63, 14.11.
2.4.2 General bromination procedures for monomers M1-M3:
4,7-Di(2-thienyl)arene (3a
anhydrous chloroform (CHCl3) (30 mL) and acetic acid (3 mL) under nitrogen and cooled to
0°C. -bromosuccinimide (NBS) (2.0 equiv) was added portion wise over a period of 15 min
, 3b or 3c) (1.0 equiv) was dissolved in a mixture solution of
N
protection in the dark. Then reaction mixture was allowed to warm to room temperature and
11

stirred for until the reaction completion by TLC analyses. After, the reaction mixture was
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poured into water and extracted with dichloromethane. The combined organic layer was
washed with water, once with brine and dried over anhydrous MgSO4. The residue was
purified by column chromatography on silica gel with hexanes/dichloromethane (1/2, v/v) to
give the desired products. Their chemical characterization analyses are shown as follows.
2.4.2.1
4,7-Bis(5-bromothiophen-2-yl)-2-(3,5-difluoro-4-(octyloxy)phenyl)-2H-benzo[d][1,2,3]triazole (M1)
M1 was prepared according to the general bromination procedure, where 3a (1.5 g, 2.9
mmol) and NBS (1.02 g, 5.7 mmol) were used. The resulting product M1 was obtained as a
1
yellow solid with a yield of 76%. H NMR (500 MHz, CDCl3) δ (ppm): 7.92 (d,
J
= 8.5 Hz,
= 6.4 Hz, 2H),
= 6.5 Hz, 2H), 1.53-1.49 (m, 2H), 1.34-1.31 (m, 8H), 0.90 (t, = 7.0 Hz,
3H). 13C NMR (500 MHz, CDCl3) δ (ppm): 142.70, 140.94, 131.31, 127.39, 123.45, 123.25,
114.07, 105.66, 105.27, 75.55, 32.22, 30.42, 29.71, 29.65, 26.06, 23.08, 14.54. MS (EI, ):
2H), 7.71 (d,
J = 3.9 Hz, 2H), 7.38 (s, 2H), 7.11 (d, J = 3.9 Hz, 2H), 4.25 (t, J
1.85-1.78 (quin,
J
J
m/z
calcd for C28H25Br2F2N3OS2: 679.0; found: 678.9. Anal. calcd (%) for C28H25Br2F2N3OS2: C,
49.35; H, 3.70; N, 6.17; found: C, 50.08; H, 3.81; N, 6.28. The detailed NMR and Mass
spectra of M1 are shown in Figs. S4-S6.
2.4.2.2
4,7-Bis(5-bromo-4-hexylthiophen-2-yl)-2-(3,5-difluoro-4-(octyloxy)phenyl)-2H-benzo[d][1,2,3]triaz
ole (M2)
12

M2 was prepared according to the general bromination procedure, where 3b (1.5 g, 2.2
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mmol) and NBS (0.77 g, 4.4 mmol) were used. The resulting product M2 was obtained as an
1
orange solid with a yield of 78%. H NMR (500 MHz, CDCl3) δ (ppm): 7.88 (d,
J
= 7.7 Hz,
= 7.7 Hz, 4H), 1.84-1.80
J = 7.0 Hz, 4H), 1.52-1.50 (m, 2H), 1.42-1.3 (m,
2H), 7.57 (s, 2H), 7.31 (s, 2H), 4.24 (t,
J = 6.3 Hz, 2H), 2.60 (t, J
(quin, = 6.3 Hz, 2H), 1.67-1.63 (quin,
J
13
20H), 0.92-0.88 (m, 9H). C NMR (700 MHz, CDCl3) δ (ppm): 156.66, 156.62, 155.23,
155.20, 143.03, 143.02, 142.31, 142.29, 142.27, 138.39, 136.69, 136.61, 134.06, 133.98,
127.28, 127.27, 122.80, 122.77, 122.74, 110.62, 105.08, 104.92, 75.12, 31.81, 31.67, 30.02,
29.70, 29.66, 29.30, 29.24, 28.98, 25.66, 22.66, 22.63, 14.11. MS (EI,
m/z): calcd for
C40H49Br2F2N3OS2: 849.2; found: 849.6. Anal. calcd (%) for C40H49Br2F2N3OS2: C, 56.54;
H, 5.81; N, 4.94; found: C, 56.79; H, 5.91; N, 4.74. The detailed NMR and Mass spectra of
M2 are shown in Figs. S7-S9.
2.4.2.3 4,7-Bis(5-bromo-4-hexylthiophen-2-yl)-2-(3,4,5-trifluorophenyl)-2H-benzo[d][1,2,3]triazole
(M3)
M3 was prepared according to the general bromination procedure, where 3c (1.5 g, 2.6
mmol) and NBS (920 mg, 5.2 mmol) were used. The resulting product M3 was obtained as
1
an orange solid with a yield of 72%. H NMR (500 MHz, CDCl3) δ (ppm): 8.14-8.10 (m,
2H), 7.68 (s, 2H), 7.53 (s, 2H), 2.66 (t,
1.42-1.34 (m, 12H), 0.89 (t,
J
= 7.2 Hz, 4H), 1.71-1.64 (quin, J = 6.3 Hz, 4H),
13
J
= 6.9 Hz, 6H). C NMR (500 MHz, CDCl3) δ (ppm):
143.15, 142.66, 138.25, 127.38, 123.24, 123.01, 110.89, 105.58, 105.23, 31.67, 29.72, 29.67,
13

28.95, 22.62, 14.10. MS (EI,
m
/
z
): calcd for C32H32Br2F3N3S2: 737.0; found: 737.4. Anal.
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calcd (%) for C32H32Br2F3N3S2: C, 51.97; H, 4.36; N, 5.68; found: C, 52.11; H, 4.47; N, 5.85.
The detailed NMR and Mass spectra of M3 are shown in Figs. S10-S12.
2.4.3 4,8-Bis(octyloxy)benzo[1,2-b:4,5-b']dithiophene (5)
Compound
4 (5.0 g, 22.7 mmol) was mixed with zinc powder (3.27 g, 50.0 mmol) and
aqueous sodium hydroxide (NaOH) (60 mL, 12.0 wt%) solution under nitrogen. The
resulting mixture was refluxed for 2 h. Then, 1-bromoctane (13.2 g, 68.2 mmol) and a
catalytic amount of tetrabutylammonium bromide were sequentially added into the mixture
solution. After being refluxed for overnight, the reaction was poured into cold water and
extracted with diethyl ether. The organic layer was dried over anhydrous MgSO4 and
concentrated under vacuum. The crude product was purified by recrystallization from ethyl
1
alcohol twice. The compound
(500 MHz, CDCl3) δ (ppm): 7.48 (d,
6.6 Hz, 4H), 1.92-1.83 (quin, = 6.9 Hz, 4H), 1.42-1.34 (m, 20H), 0.89 (t,
5
was obtained as a white solid with a yield of 86%. H NMR
J
= 5.7 Hz, 2H), 7.36 (d,
J
= 5.4 Hz, 2H), 4.28 (t,
J =
J
J
= 6.9 Hz, 6H).
13C NMR (500 MHz, CDCl3) δ (ppm): 144.52, 131.6, 130.14, 125.95, 120.3, 73.94, 31.82,
30.53, 29.41, 29.27, 26.07, 22.65, 14.1.
2.4.4 2,6-Dibromo-4,8-bis(octyloxy)benzo[1,2-b:4,5-b']dithiophene (6)
Compound
5 (4.46 g, 10.0 mmol) was dissolved in anhydrous CHCl3 (100 mL) and cooled to
0°C under nitrogen. Bromine (3.2 g, 20.0 mmol) in methylene chloride (60 mL) was slowly
dropped to the reaction solution, and then the reaction was allowed to warm to room
14

temperature and stirred for 4 h. After completion, the solvent was removed by evaporation.
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The residue was recrystallized by hexane. The compound
5
was obtained as a white solid
= 6.5 Hz,
= 7.0 Hz, 4H), 1.55-1.50 (m, 4H), 1.39-1.31 (m, 16H), 0.91 (t, = 6.5
1
with a yield of 90%. H NMR (500 MHz, CDCl3) δ (ppm): 7.42 (s, 2H), 4.18 (t,
J
4H), 1.86-1.80 (quin,
J
J
13
Hz, 6H). C NMR (500 MHz, CDCl3) δ (ppm): 142.53, 131.15, 130.88, 123.12, 114.96,
74.16, 31.81, 30.42, 29.35, 29.24, 25.96, 22.66, 14.12.
2.4.5 2,6-Bis(trimethyltin)-4,8-dioctyloxybenzo[1,2-b;3,4-b]dithiophene (7)
Compound
cooled down to -78°C under nitrogen.
was added slowly over a period for 15 min, and then the reaction mixture was stirred for 30
min. Then, trimethyltin chloride (18 mL, 1 M in -hexane) was added in one portion, and the
6
(5.0 g, 8.3 mmol) was dissolved in tetrahydrofuran (THF) (100 mL) which was
n
-Butyllithium ( -BuLi) (7.3 mL, 2.5 M in -hexane)
n
n
n
reactant turned to clear rapidly. The resulting mixture was allowed to warm to room
temperature and stirred for 2 h. The reaction was quenched with cool water (200 mL) and
extracted with diethyl ether. The combined organic layer was washed with water, and dried
over anhydrous MgSO4. After removing solvent under vacuum, the residue was recrystallized
by ethyl alcohol twice. The compound
7
was obtained as a colorless solid with a yield of
= 6.4 Hz, 4H), 1.90-1.83
= 6.4 Hz, 4H), 1.60-1.53 (m, 4H), 1.36-1.29 (m, 16H), 0.88 (t, = 6.0 Hz, 6H), 0.44
1
72%. H NMR (500 MHz, CDCl3) δ (ppm): 7.50 (s, 2H), 4.28 (t,
J
(quin,
J
J
13
(s, 18H). C NMR (500 MHz, CDCl3) δ (ppm): 143.40, 140.78, 134.32, 133.28, 128.31,
15

73.89, 32.15, 30.83, 29.73, 29.64, 26.41, 22.99, 14.42. Anal. calcd (%) for C32H54O2S2Sn2: C,
ACCEPTED MANUSCRIPT
49.76; H, 7.05; found: C, 50.01; H, 7.12.
2.4.6 General Still-coupling polymerization procedures for polymers P1-P3:
Dibromo monomer (M1, M2 or M3) (1.0 equiv) and distannyl monomer 7 (1.0 equiv) in
anhydrous toluene (15 mL) were degassed with nitrogen. Then Pd(PPh3)4 (1.5 mol %) was
added in one portion. The reaction mixture was heated to reflux for 24 h under nitrogen.
After, an excess amount of 2-bromothiophene (0.34 g, 2.0 mmol) was added the trimethyl
stannyl groups as an end-capped and reacted for 2 h. The reaction mixture was cooled to
room temperature and poured into methanol (200 mL) for precipitation. The precipitated
solid was collected and was further purified by Soxhlet extraction with methanol, hexane,
acetone, and CHCl3. Finally, the remaining solid was dried under vacuum to afford the
desired polymer.
2.4.6.1 P1
:
P1 was prepared according to the general Still-coupling polymerization
(0.74 g, 1.0 mmol) were used. The P1
procedure. Monomers M1 (0.68 g, 1.0 mmol) and
7
1
was obtained as a black solid with a yield of 80%. H NMR (500 MHz, CDCl3) δ (ppm):
7.95-7.44 (br, 10H), 4.22 (br, 6H), 1.90-1.81 (br, 10H), 1.55-1.12 (br, 36H), 0.91 (br, 9H).
2.4.6.2 P2
:
P2 was prepared according to the general Still-coupling polymerization
(0.74 g, 1.0 mmol) were used. The P2
procedure. Monomers M2 (0.85 g, 1.0 mmol) and
7
1
was obtained as a black solid with a yield of 88%. H NMR (500 MHz, CDCl3) δ (ppm):
16

8.07-7.53 (br, 8H), 4.23 (br, 6H), 2.95 (br, 4H), 1.91-1.82 (br, 12H), 1.35-1.30 (br, 40H),
ACCEPTED MANUSCRIPT
0.88 (br, 15).
2.4.6.3 P3: P3 was prepared according to the general Still-coupling polymerization procedure.
Monomers M3 (0.74 g, 1.0 mmol) and 7 (0.74 g, 1.0 mmol) were used. The P3 was obtained as a
black solid with a yield of 82%. 1H NMR (500 MHz, CDCl3) δ (ppm): 8.11-7.33 (br, 8H), 4.26 (br,
4H), 2.96 (br, 4H), 1.90-1.81 (br, 18H), 1.39-1.33 (br, 22H), 0.89 (br, 12H).
3. Results and discussion
3.1 Synthesis and thermal stability
The synthetic routes of monomers M1
1. The syntheses of compounds 1a [54] 1b [54]
Three 4,7-dibromo-2-(phenyl)-2 -benzotriazoles (1a and 1b) with different substitution
–
M3 and polymers P1
–P3 are shown in Scheme
,
,
and 4[55] are followed by the previous literature.
H
patterns on the phenyl group are subjected to Suzuki reaction conditions with
3-hexylthiophene-2-boronicacidpinacol ester and thiophene-2-boronicacidpinacol ester to
give the substituted BTZ (i.e., 3a
–
3c). After bromination of the prepurified compounds
M3 could
M3 are proven by EA data
3a 3c in CHCl3 with -bromosuccinamide (NBS), pure dibromo monomers M1–
–
N
be obtained in reasonable yields, and the purities of M1
–
demonstrated in Fig. S13.
The stannylated BDT was polymerized with the corresponding monomers M1–M3 by
Stille-coupling, in which polymers P1–P3 were obtained in very good yields, respectively. All
17

polymers were end-capped with 2-bromothiophene, and precipitated from methanol. The solubilities
ACCEPTED MANUSCRIPT
and polydispersity indices (PDI = Mw/Mn) were subjected to Soxhlet extraction with various solvents
or simply filtered through a plug of celite. After dissolving the polymers again in a small amount of
an appropriate solvent (e.g., CHCl3) the polymers could be precipitated from methanol. The
polymers were collected through filtration and dried in vacuum. To determine the actual ratio
1
between M1–M3 and 7 in the polymers, their chemical compositions were characterized by H
NMR spectroscopy. The actual and feed ratios of these polymers are in good agreements. Detailed
synthetic procedures and characterization data are given in the Experiment Section. All polymers
exhibit good solubilities in common organic solvents such as chloroform, THF, DCB, and TCB, etc.
at room temperature. As summarized in Table 1, the number-average molecular weights (Mn),
weight-average molecular weights (Mw) and PDI values of P1–P3 were determined by GPC using
THF as eluent against polystyrene standards. These results show that considerable molecular weights
were obtained in these polymers, where the values of Mn were ranged among 163000–32400g/mol
with PDI values of 1.1–1.3. Among all polymers, P1 showed a remarkably higher molecular weight
of 32400g/mol. This could be attributed to the lack of alkyl chains on thiophene of poymer backbone
along with a more planar structure, which led to higher molecular weight. The thermal stabilities of
conjugated polymers play a key role for device fabrication. The thermal stabilities of P1–P3 were
investigated using thermogravimetric analyses (TGA), and their corresponding results are
summarized in Fig. S2 and Table 1. It is apparent that all polymers exhibited excellent thermal
stabilities as the temperatures for 5% weight loss (under nitrogen) were found to be 368–391°C. Due
18

to the highest molecular weight and the most planar backbone conformation by the density
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functional theory (DFT) calculations of P1 in these polymers, P1 showed the highest decomposition
temperature at 391°C. Whereas, polymer P2 exhibited the lowest decomposition temperature at
368°C lower than P3 (381°C), because of three fluorine-substituted phenyl pendant attached to the
BTZ unit for P3 in contrast to the alkoxy-substituted phenyl pendant for P2. Thus, these polymers
showed sufficient thermal stabilities for the applications of polymer solar cells and other
optoelectronic devices.
3.2 Optical and electrochemical properties
The photophysical properties of the polymers in chloroform solutions and solid films
were investigated by UV-vis absorption spectroscopy. The UV–vis absorption spectra are
shown in Figs. 1a and 1b, and the absorption maxima (for both solutions and solid films) and
the optical bandgaps (Eg,opt) are listed in Table 2. All polymers in solution state showed
similar absorption spectra and exhibited two main absorption peaks at ca. 400 and 500 nm.
The shorter wavelength absorption bands located between 385 and 411 nm were
correspondent to the π–π* transitions of polymer backbones. On other hand, the longer
absorption bands located between 505 and 524 nm were mainly due to the intramolecular
charge transfer (ICT) interactions from the donor BDT units to acceptor fluorine substituted
BTZ moieties, which is a typical characteristic for D-A polymers. These similar
characteristic bands were also obtained in solid films. All polymers have absorption maxima
accompanied by a distinctive shoulder, indicating a substantial degree of backbone
19

coplanarity. The blue-shift of 12 nm in polymer P2 was likely originated from the reduced
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backbone coplanarity, which was due to the high alkylation density and interchain repulsions
and thus to induce less aggregation in solution. Whereas polymer P3 showed a red shift of 19
nm, as a result of three fluorine-substituted phenyl pendant attached to the BTZ unit to
promote backbone coplanarity. As expected, all three polymers demonstrated a red shifted
absorption from solutions to solid films in Fig. 1b. The absorption maxima of P1–P3 were in
the range of 508–534 and 560–618 nm for solutions and solid films, respectively. Whereas
P3 showed a little blue-shifted absorption maximum at 576 nm and a shoulder peak at 619
nm in contrast to P2. The optical bandgaps (Eg,opt) of P1, P2, and P3 in solid films were
calculated to be 1.79, 1.77, and 1.81 eV, respectively, from the absorption edges of the
corresponding UV–vis absorption spectra (see Table 2). In order to investigate the
temperature effect on absorption spectra of polymers, we carried out UV-Vis experiments at
various temperatures between 20-50°C in chloroform solutions. As shown in Fig. S3, the
absorption spectra of polymers P1-P3 displayed a systematic trend of an absorption peak
broadening accompanied with a blue-shift upon increasing temperature. At elevated
temperatures, all polymers are well dissolved and disaggregated. The blue-shift and
broadening of the absorption band are also related to the thermally induced less-restricted and
less-planar conformation distributions at higher temperatures.[56],[57] These results imply that
it is an efficient way to design LBG photovoltaic polymers to have higher light-harvesting
capabilities by tuning various electron-acceptor moieties.
20

To investigate the redox behavior of polymers and their electronic states (i.e., the
ACCEPTED MANUSCRIPT
HOMO/LUMO levels), the electrochemical properties of polymers P1–P3 were investigated by CV
measurements. The electrochemical properties, such as the onset potentials of oxidation and
reduction processes, (i.e., the estimated positions of the upper edges of valence bands (HOMO) and
the lower edges of conduction bands (LUMO), respectively) and the electrochemical bandgaps of all
polymers are summarized in Table 2. As shown in Fig. 2, the oxidation onsets of P1, P2, and P3
were achieved to be 1.03, 1.09, and 1.20 V, which corresponded to HOMO energy levels of -5.38,
-5.44, and -5.60 eV, respectively. Comparing HOMO energy levels of P1–P3, it showed a lower
HOMO level in P3 due to one more fluorine atom in the polymer pendant. This result implies that
P3 is promising for achieving a higher Voc value in PSCs than its counterparts P1 and P2. Moreover,
the reduction onsets of P1, P2, and P3 were measured to be -0.80, -0.85, and -0.78 V, which
correspond to LUMO energy levels of –3.55, -3.50, and -3.57 eV, respectively. Hence, the
introduction of fluorine units seems to affect mostly the HOMO energy levels of the resulting
polymers while the LUMO energy levels remained almost constant. In summary, these polymers
P1–P3 showed roughly similar optical and electronic bandgaps.
3.3. Theoretical studies
To further understand the geometrical structures and absorption spectra of one repeating unit
for these polymers (P1-P3), we carried out the density functional theory (DFT) calculations using
the Gaussian 09 software package. We optimized the ground state geometries of all polymers at the
21

B3LYP/6-31G level. The obtained minima are further confirmed by vibrational frequency
ACCEPTED MANUSCRIPT
calculations at the same level; only the lowest energy conformation is reported here. The optimized
geometries are shown in Fig. 3. Table S1 summarizes the dihedral angles of their optimal
geometries. Furthermore, we also calculated the average dihedral angles between
BDT-thophene-BTZ units to estimate the coplanarity of molecular backbone; the values are 3.0°,
6.7°, and 4.2° between BDT-thophene units and 2.8°, 2.7°, and 0.1° between thophene-BTZ units for
P1, P2, and P3, respectively. These results imply that P2 features the most twisted conformation
among three polymers. Anchoring the alkyl side chain on thiophene implies more molecular twist
between BDT-thiophne, which reduces the coplanarity of polymer backbone. Whereas, the dihedral
angles between thiophene-BTZ units are very low and mostly similar, and the coplanarity with the
lowest dihedral angle (φ2 = 0.1°) of P3 is induced in the presence of three fluorine-substituted phenyl
pendant attached to the BTZ unit.
Using the optimized structures of all these polymers (P1-P3) with the B3LYP function, as
shown in Figs. 4 and 5 we have plotted the iso-surfaces (iso-value=0.02) of the frontier molecular
orbitals (FMOs) for all these polymers. The results show that the molecular orbitals of HOMO and
LUMO for all these polymers are predominantly localized in the backbone. The molecular orbitals
of the HOMO electronic densities are mainly located on the backbones of these polymers, whereas
the contribution to LUMO is mainly from the terminal group of BTZ unit. The detailed analysis of
these wavelength absorptions as well as their main contributions to the first excitation can be made
on the basis of time-dependent DFT (TDDFT) calculations as collected in Table S1. For the S0 →
22

S1 transition, the calculated wavelengths for P1, P2, and P3 polymers are 554, 572, and 606 nm with
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oscillator strengths of 0.853, 0.802, and 0.641, respectively, resulting mainly from the HOMO →
LUMO transition. All these theoretical calculations are highly supportive to the experimental results.
3.4 Photovoltaic properties
The motivation to design and synthesize these D-A polymers containing BDT and BTZ moieties
is to look for new LBG polymers for the application of PSCs. The BHJ photovoltaic devices were
fabricated with a configuration of ITO/PEDOT:PSS(40nm)/(P1–P3):PC71BM(200nm)(1:2,
w/w)/Ca(30nm)/Al(60nm). Fig. 6 showed the current density–voltage (J–V) curves for polymers
under the condition of AM 1.5G simulated solar illumination at 100 mW/cm2. The photovoltaic
properties, including the open-circuit voltage (Voc), short-circuit current density (Jsc), fill factor (FF),
and PCE values, of the polymers for PSC devices are summarized in Table 3. The PSC devices
based on polymers P1–P3 showed different Voc values in the range of 0.58–0.72 V, which are related
to the differences between the HOMO energy levels of the polymers and the LUMO energy levels of
the PC71BM. Comparing photovoltaic properties of P1–P3, the PSC device based on P3 showed the
best photovoltaic performance with the highest measured PCE value of 3.00%, with Jsc = 7.70
mA/cm2, FF = 54.04, and Voc = 0.72 V.
In comparison with P1, polymer P2 with alkyl side chains on thiophene linker has a stronger
electron-donating trend to induce a higher oxidation strength of 1.09 V (vs 1.03 V of P1) and a
deeper lying HOMO level of 5.44 eV (vs 5.38 eV of P1).[58],[59] The oxidation potential of polymer
P3 incorporated with the electron-withdrawing BTZ unit showed the highest value of 1.20 V, so P3
23

showed the lowest HOMO level of 5.60 eV among these three polymers. Since all polymers P1–P3
ACCEPTED MANUSCRIPT
had the same electron-donating BDT moiety, P3 with alkyl side chains on thiophene linker and
all-fluorine-substituted benzene ring units in BTZ unit has the strongest electron-withdrawing
tendency to enhance the oxidation strength (high oxidation state) of the polymer.
Due to the lowest HOMO level of P3, its PSC device achieved the largest Voc value of 0.72 V so
as to acquire the highest PCE value (3.00%). On the other hand, the PSC device containing P1
obtained the smallest Voc of 0.58 V and thus to have the lowest PCE value of 1.23% owing to the
highest HOMO level of 5.38 eV. The external quantum efficiency (EQE) spectra (matched with
absorption properties of all polymers) of PSC devices containing P1–P3 are demonstrated in Fig. 7,
where the PSC device containing P3 exhibited the highest EQE value about 43 % at 545 nm. This
may attributed to the highest efficiency of light absorption for P3 in the visible region of 400 to 700
nm.
24

3.5. Morphology
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The surface morphological studies of the active layer in PSC devices were investigated by atomic
force microscopy (AFM) (tapping mode) to record heights and phase images (5 × 5 ꢀm2) of the
polymer blend films (P1–P3:PC70BM, 1:2 w/w). The AFM phase (2-D) and height (3-D) graphs of
all blended polymer films with PC70BM showed in Figs. 8 and 9, respectively. The root-mean square
(RMS) values of roughnesses of P1, P2, and P3 measured by the topographic images were 10.22,
8.07, and 8.37 nm, respectively. Due to the highest molecular weight of P1, the largest RMS value
of 10.22 nm in P1 indicated the worst solubility with the most large-scaled phase separation, which
decreased the film quality and thus to result in the lowest PCE value of 1.23% in the PSC device.
The most efficient PSC device with the maximum PCE value of 3.00% was achieved by the active
layer of P3:PCBM (1:2 w/w) with the medium roughness value of 8.37 nm, which possessed higher
roughness and PCE values than its analogous polymer P2.
4. Conclusions
In summary, three new conjugated polymers P1–P3, which were based on electron-donating BDT
and electron-accepting BTZ units containing multiple fluorine-substituted benzene ring along with
thiophene linkers with/without alkyl side-chains, were designed and successfully synthesized for
BHJ PSCs. All polymers demonstrated good solubilities in common organic solvents and possessed
excellent thermal stabilities. The optical properties showed that these new polymers revealed strong
π-π stacking and an enhanced absorption-maximum band in solid films due to strong ICT in the
polymer backbone. Thus, the optical and electrochemical bandgaps and energy levels of the
25

polymers could be effectively tuned by incorporation of highly electronegative fluorine atoms on the
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polymer pendants. Furthermore, among PSC devices fabricated from the active layer of
P1–P3:PC71BM (1:2, w/w), P3 illustrated the best photovoltaic properties with the highest PCE
value of 3.00%, Jsc = 7.70 mA/cm2, FF = 54.04, and Voc = 0.72 V. This work demonstrates that D-A
polymer P3 containing electron-donating BDT and electron-accepting multiple fluorinated BTZ
units along with thiophene linkers with alkyl side-chains can enhance the photovoltaic performance
of PSC devices. Our study provides a new concept of polymer design towards low bandgap
conjugated polymers for the fabrication of efficient BHJ PSCs.
Acknowledgements
We thank the Ministry of Science and Technology (MOST) in Taiwan for the financial
supports of this project through MOST 103-2113-M-009-018-MY3 and MOST
103-2221-E-009-215-MY3.
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