Syntheses and photovoltaic properties of 6-(2-thienyl)-4H-thieno[3,2-b]indole based conjugated polymers containing fluorinated benzothiadiazole

Article abstract of DOI:10.1016/j.polymer.2016.12.040

In this report, a series of copolymers based on 6-(2-thienyl)-4H-thieno[3,2-b]indole (TTI) as an electron-rich unit and fluorinated DTBT as an electron-deficient unit were synthesized, namely PTTIF1 and PTTIF2, and applied to photovoltaic devices. TTI unit was coupled with fluorinated DTBT to utilize the merit of introduction of fluorine atom leading to the lowering of the HOMO energy level while keeping high planarity of the conjugated backbone. The synthesized copolymers show a noticeable change in HOMO energy levels as compared with non-fluorinated polymer (PTTIDTBT-h). Optimized photovoltaic devices of PTTIF2 exhibited power conversion efficiency of 4.36% with decent J_SC and FF values, which can be explained by the higher charge transporting ability of PTTIF2 with preferable face-on crystallite population than PTTIF1 in grazing incident wide-angle X-ray scattering (GIWAXS).

Full text of DOI:10.1016/j.polymer.2016.12.040

Accepted Manuscript
Syntheses and photovoltaic properties of 6-(2-thienyl)-4H-thieno[3,2-b]indole based
conjugated polymers containing fluorinated benzothiadiazole
Ina Jeong, Sangmin Chae, Ahra Yi, Juae Kim, Jung Hyeong Cho, Hyo Jung Kim,
Hongsuk Suh
PII:
S0032-3861(16)31129-6
10.1016/j.polymer.2016.12.040
JPOL 19276
DOI:
Reference:
To appear in: Polymer
Received Date: 13 September 2016
Revised Date: 4 November 2016
Accepted Date: 13 December 2016
Please cite this article as: Jeong I, Chae S, Yi A, Kim J, Cho JH, Kim HJ, Suh H, Syntheses and
photovoltaic properties of 6-(2-thienyl)-4H-thieno[3,2-b]indole based conjugated polymers containing
fluorinated benzothiadiazole, Polymer (2017), doi: 10.1016/j.polymer.2016.12.040.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Syntheses and Photovoltaic Properties of 6-(2-Thienyl)-
4H-thieno[3,2-b]indole Based Conjugated Polymers
containing Fluorinated Benzothiadiazole
Ina Jeong†1, Sangmin Chae†2, Ahra Yi2, Juae Kim1, Jung Hyeong Cho2, Hyo Jung Kim*2,
and Hongsuk Suh*1
1Department of Chemistry and Chemistry Institute for Functional Materials, Pusan National
University, Busan 609-735, Korea
2Department of Organic Material Science and Engineering, Pusan National University, Busan
609-735, Korea
† Contributed equally to this work
AUTHOR EMAIL ADDRESS: hssuh@pusan.ac.kr
hyojkim@pusan.ac.kr

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ABSTRACT
In this report, a series of copolymers based on 6-(2-thienyl)-4H-thieno[3,2-b]indole (TTI) as
an electron-rich unit and fluorinated DTBT as an electron-deficient unit were synthesized,
namely PTTIF1 and PTTIF2, and applied to photovoltaic devices. TTI unit was coupled
with fluorinated DTBT to utilize the merit of introduction of fluorine atom leading to the
lowering of the HOMO energy level while keeping high planarity of the conjugated backbone.
The synthesized copolymers show a noticeable change in HOMO energy levels as compared
with non-fluorinated polymer (PTTIDTBT-h). Optimized photovoltaic devices of PTTIF2
exhibited power conversion efficiency of 4.36% with decent JSC and FF values, which can be
explained by the higher charge transporting ability of PTTIF2 with preferable face-on
crystallite population than PTTIF1 in grazing incident wide-angle X-ray scattering
(GIWAXS).
KEYWORDS: Photovoltaic cells, 6-(2-thienyl)-4H-thieno[3,2-b]indole, Fluorinated DTBT

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1. Introduction
Polymer solar cells (PSCs) have drawn much interest as an alternative inorganic
photovoltaics because of their advantages such as light weight, flexibility and cost-effective
manufacturing process.1-6 Due to effort toward the improvement of new p-type conjugated
polymers and n-type fullerene derivates for high power conversion efficiency (PCE),
conjugated PSCs have reached PCE up to10%.7-10
One of the main efforts in the research of PSCs is exploration of new structure, for the
improvement of efficiency, through modification of conjugated polymers such as
introductions of various kinds of functional groups and the fused structures of aromatic rings
along with the optimization of the side chains.11 Also, much effort has met with success to
develop numerous types of low bandgap conjugated polymers, for the better coverage of the
solar spectrum, which resulted in the improvement of the performance of PSCs.
Reduction of the bandgap by utilization of intramolecular charge transfer (ICT) has been
achieved by the introduction of electron deficient moiety composed of strong electron-
withdrawing groups such as fluorine atom, nitrile and carbonyl group in conjugated polymer
backbone.12 Especially, introduction of fluorine atom with small radius and highest
electronegativity in polymer backbone has provided strong ICT effect and high charge carrier
mobility. High electronegativity which is providing strong inductive effect and the small
radius of fluorine atom are suitable to provide conjugated structure having reduced torsional
angle and higher planarity.13-15 The highest electro-negativity of fluorine atom is inducing
low-lying highest occupied molecular orbital (HOMO) energy levels which should be helpful
to provide high values of open circuit voltage (VOC) as depicted in Figure 1.15
6-(2-Thienyl)-4H-thieno[3,2-b]indole (TTI) which has five-membered thiophene ring to
substitute the six-membered benzene ring of carbazole shows good hole-transporting property,
electron-donating ability and planarity for high JSC value and ICT effect.16-19

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Herein, we synthesized two conjugated polymers through Stille cross-coupling
polymerization, poly(4-(1-octylnonyl)-6-(2-thienyl)-4H-thieno[3,2-b]indole-alt-5,6-difluoro-4,7-
bis(4-hexyl-2-thienyl)-2,1,3-benzothiadiazole) (PTTIF1) and poly(4-(1-octylnonyl)-6-(2-
thienyl)-4H-thieno[3,2-b]indole-alt-5-fluoro-4,7-bis(4-hexyl-2-thienyl)-2,1,3-benzothiadiazole)
(PTTIF2), to provide ICT effect using TTI as an electron-rich unit and fluorinated DTBT as
an electron-deficient units. The thermal, optical, electrochemical, and photovoltaic properties
of the conjugated polymers were investigated. The synthetic routes and chemical structures
for the electron-rich/deficient units and polymers are outlined in Scheme 1 and 2,
respectively.20
2. Experimental
2.1. Instrumental characterization
All reagents were purchased from Aldrich or TCI, and used without further purification.
Solvents were purified by normal procedure and handled under moisture-free atmosphere. 1H
and 13C NMR spectra were recorded with a Varian Gemini-300 (300 MHz) spectrometer 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 otherwise
indicated. Analytical thin layer chromatography (TLC) was conducted using Merck 0.25 mm
silica gel 60F pre-coated aluminum plates with fluorescent indicator UV254. High resolution
mass spectra (HRMS) were recorded on a JEOL JMS-700 mass spectrometer under electron
impact (EI) conditions in the Korea Basic Science Institute (Daegu). Molecular weight and
poly dispersity (PDI) of the polymer were determined by gel permeation chromatography
(GPC) analysis with a polystyrene standard calibration and carried out at 35 °C. TGA was
performed with a SDT Q600 under nitrogen atmosphere at a heating rate of 10 °C/min to 800

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°C in the Korea Basic Science Institute (Busan). The DSC analysis was performed on a DSC
Q200 at heating rates of 10 °C/min. The UV-vis absorption spectra were recorded by a
Varian 5E UV/VIS/NIR spectrophotometer. Cyclic voltammogram of the polymers was
obtained using an Wona-WPG100 at room temperature in a three-electrode cell under a Ar
atmosphere at a scan rate of 100 mV/s, with Pt wire as the counter electrode, an Ag/AgNO3
reference electrode, and 0.1 M tetraethylammonium tetrafluoroborate (TBABF4) in
acetonitrile as the electrolyte. The AFM instrumentation consisted of a Veeco Nano Scope
AFM and standard silicon cantilever (Veeco; tip radius, 8 nm; normal spring constant, 21-78
N/m; scan rate, 2.0 Hz) at ambient conditions (in air, 20 °C). GIWAXS were measured at the
3C beamline in the Pohang Accelerator Laboratory (PLS II, South Korea). 2D GIWAXS data
were recorded using CCD detector and operation conditions were set to a X-ray wavelength
of 1.256 Å. A sample-to-detector distance (SDD) was 212 mm. The incidence angle was
carefully chosen to allow for complete X-ray penetration of the film. The scattering spectra
were collected as a 2D image map oriented along the plane of the substrate (qy) and the plane
perpendicular to the substrate (qz).
2.2. Organic field effect transistor (OFET) fabrication and characterization
Bottom-gate/top-contact OFETs were fabricated using heavily doped n-type Silicon wafer
(Si) as the bottom gate electrode with 300 nm of silicon dioxide (SiO2) layer as the dielectric.
The substrates were pre-cleaned by sonicating in deionized water, acetone and isopropyl
alcohol, and then dried in an oven at 110 ˚C. The substrates were treated with ultraviolet
(UV)-ozone for 30 min. The polymer films were spin-casted at 2000 revolution per minute
(rpm) to get 40 nm thickness and then annealed on a hot plate at 100 ˚C for 20 min in a
nitrogen (N2) glove box. The concentration of all solution was 10 mg mL-1 in chlorobenzene.
The source and drain Au electrodes were thermally deposited through a shadow mask under 3

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× 10-6 Torr, with 50 nm thickness, 50 µm channel length (L) and 1.0mm channel width (W).
OFETs were characterized under a vacuum using Keithley 4200 and vacuum chamber probe
station. The mobility (µ) was determined using I-V relationship in the saturation regime
described in the following equation where the capacitance per unit area Ci was 11.3 nF cm-2
and VT was the threshold voltage.
ꢃ
⁄
ꢉ ꢃ
ꢉꢎ
Iꢀꢁꢂ = ꢄꢅꢆꢇ 2ꢈ ∙ ꢊꢋꢌ − ꢊꢍ
2.3. OPV fabrication and characterization
Photovoltaic devices were fabricated with conventional type OPVs with structure of the
indium tin oxide (ITO) glass/poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate)
(PEDOT:PSS)/polymer:PC71BM/LiF/Al. The patterned ITO (160 nm) glass substrates were
first cleaned with detergent, ultrasonicated in acetone and isopropyl alcohol at room
temperature, and subsequently dried overnight in an oven. PEDOT:PSS (Clevios PV P
AI4083) was spin-casted from aqueous solution to form a 40 nm thick film. 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 in a N2 environment. A prepared solution was then spin-cast with various rpm on
top of the PEDOT:PSS layer. The total solution concentration was 30 mg mL-1 in
cholorobenzen (CB) with different blend ratio and additives. The film was dried for 30 min at
room temperature in the glove box. Then, an aluminum (Al, 100nm) electrode was deposited
by thermal evaporation in a vacuum of about 3 x 10-6 Torr. Current density-voltage (J-V)
characteristics of the devices were measured using a Keithley 4200 Source Measure Unit
under AM 1.5G (100mW/cm2) simulated by an oriel solar simulator (Newport, Class AAA
solar simulator). The light intensity was calibrated using a NREL-certificated Si solar cell.
The external quantum efficiency (EQE) was measured using an Oriel IQE-EQE 200B. The

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light intensity at each wavelength was calibrated with a standard single-crystal Si
photovoltaic cell.
2.4. Synthesis of monomers
2.4.1. Synthesis of 3-hexylthiophene (1)
After stirring a mixture of magnesium turnings (10.3 g, 421.2 mmol) and 1-bromohexane
(59.3 mL, 422.2 mmol) in 80 mL of anhydrous THF for 2 hr at 70 °C under argon atmosphere,
the mixture was treated with a solution of 3-bromothiophene (20 mL, 211.1 mmol) and
Ni(dppp)Cl2 (1.1 g, 2.1 mmol) in 30 mL of THF. The mixture was allowed to reflux for 24 hr,
treated with a solution of 2 N HCl, washed with a 10% solution of NaHCO3, and extracted
with ether. The organic layer was washed with brine, concentrated under reduce pressure and
1
purified by flash chromatography to give 19.1 g (54%) of compounds 1 as colorless oil. H
NMR (300 MHz, CDCl3): δ (ppm) 7.28 (m, 1 H), 6.97 (m, 2H), 2.68 (t, 2H, J = 7.42 Hz),
1.67 (qi, 2H, J = 4.2 Hz), 1.36 (m, 6H), 0.95 (t, 3H, J = 6.59 Hz).; 13C NMR (75 MHz,
CDCl3): δ (ppm) 143.14, 128.195, 124.958, 119.7, 31.688, 30.523, 30.265, 29.013, 22.617,
14.080. HRMS (m/z, FAB+) calcd for C10H16S, 168.0973, found 168.0974.
2.4.2. Synthesis of 5-fluoro-2,1,3-benzothiadiazole (4)
To a solution of 4-fluoro-1,2-phenylenediamine (3) (10.0 g, 79.3mmol) and triethylamine
(44.2 mL, 317.1 mmol) in 100 mL CHCl3, thionyl chloride (11.6 mL, 158.6 mmol) was
added dropwise at 0 oC. After stirring for overnight at 60 oC, the reaction mixture was treated
with water and extracted with methylene chloride. The solvent was removed under reduced
pressure and the residue was purified by flash chromatography to give 8.2 g (65%) of
1
compound 4. H NMR (300 MHz, CDCl3): δ (ppm) 7.8 (dd, 1H, J = 9.4and 5.3 Hz) 7.4 (dd,
1H, J = 8.8and 2.3 Hz) 7.3 (td, 1H, J = 9.4 and 2.3 Hz); 13C NMR (75 MHz, CDCl3): δ (ppm)

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163.5 (d, 1JC-F = 253.1 Hz), 154.9, 152.0, 122.5 (d, 3JC-F = 11.6 Hz),131.3 (d, 2JC-F = 30.0 Hz),
104.5 (d, 3JC-F = 23.1 Hz). HRMS (m/z, EI+) calcd for C6H3FN2S 154.0001 found 153.9999.
2.4.3. Synthesis of 4,7-dibromo-5-fluoro-2,1,3-benzothiadiazole (5)
To a solution of compound 4 (11.0 g, 71.4 mmol) in 120 mL HBr, bromine (17 ml, 356.8
mmol) was added dropwise slowly at room temperature. After the addition, the reaction
mixture was stirred for 48 hr at 120 oC. After cooling down the mixture to room temperature,
the residual bromine was quenched with saturated NaHSO3 solution. The precipitated
material was recovered by filtration and then recrystallizied from methylene chloride/ethanol
1
to give 9.8 g (44%) of compound 5. H NMR (300 MHz, CDCl3): δ (ppm) 7.81 (d, 1H, J =
1
3
8.2 Hz); 13C NMR (75 MHz, CDCl3): δ (ppm) 160.1 (d, JC-F = 255.4 Hz), 152.7 (d, JC-F
=
7.0 Hz), 150.3, 123.9 (d, 2JC-F = 32.4 Hz), 114.0 (d, 3JC-F = 11.5 Hz), 98.2 (d, 2JC-F = 24.3 Hz).
HRMS (m/z, EI+) calcd for C6HBr2FN2S 309.8211, found 309.8213.
2.4.4. Synthesis of 5-fluoro-4,7-bis(4-hexyl-2-thienyl)-2,1,3-benzothiadiazole (6)
To a solution of 3-hexylthiophene (1) (5 g, 29.0 mmol) in anhydrous 40 mL of THF was
added n-BuLi (1.6 M in hexane) (21.8 mL, 34.8 mmol) over 30 min at -40 °C under argon
atmosphere. After stirring for 1 hr at room temperature, the reaction mixture was treated with
o
dropwise addition of tributyltin chloride (9.9 mL, 36.3 mmol) at -40 C, followed by stirring
at RT overnight. The mixture was poured into saturated NaHCO3 solution and the organic
phase was extracted with diethyl ether and washed with water. The organic phase was dried
over MgSO4 and solvent was removed under reduced pressure. Tributyl-(4-hexyl-2-
thienyl)stannane (2) as yellow liquid was acquired and used in the next step without further
purification. The solution of compound 5 (3.0 g, 9.7 mmol) and compound 2 (9.7 g, 29.0
mmol) in anhydrous 10 mL of THF was treated with Pd(PPh3)2Cl2 (5mg, 0.05mmol). After

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stirring for overnight at 100 oC under argon atmosphere, the reaction mixture was
concentrated under reduced pressure and purified by flash column chromatography to give
compound 6, 0.9 g (19%). 1H NMR (300 MHz, CDCl3): δ (ppm) 8.1 (s, 1H), 8.0 (s, 1H), 7.75
(d, 1H, J = 12.9 Hz), 7.16 (s, 1H), 7.10 (s, 1H), 2.71 (t, 2H, J = 7.00 Hz), 2.68 (t, 2H, J = 7.00
Hz), 1.60-1.78 (m, 4H), 1.51-1.21 (m, 12H), 0.93 (t, 6H, J = 7.00 Hz), 0.91 (t, 3H, J = 7.1
3
Hz); 13C NMR (75 MHz, CDCl3): δ (ppm) 160.4, 157.0, 153.3 (d, JC-F = 10.4 Hz), 149.6,
144.4, 143.4, 137.5, 132.3 (d, 5JC-F = 5.7 Hz), 131.4 (d, 4JC-F = 8.1 Hz), 125.6 (d, 3JC-F = 10.4
Hz), 125.6 (d, 1JC-F = 255.4 Hz), 122.9 (d, 4JC-F = 7.0 Hz), 116.5 (d, 2JC-F = 33.6 Hz), 111.0 (d,
2JC-F = 17.3 Hz), 31.8, 30.6, 30.5, 30.4, 29.1, 28.3, 26.8, 22.7, 17.3, 14.2, 13.7.1. HRMS (m/z,
EI+) calcd for C26H31FN2S3 486.1633 found 486.1635.
2.4.5. Synthesis of 4,7-bis(5-bromo-4-hexyl-2-thienyl)-5-fluoro-2,1,3-benzothiadiazole (7)
A solution of compound 6 (2 g, 4.1 mmol) in 25 mL THF under argon atmosphere was
treated with N-bromosuccinimide (NBS) (1.5 g, 8.2 mmol) portionwise and stirred for 2 hr at
o
40 C. The mixture was concentrated under reduced pressure and recrystallized to give an
orange solid 2.3 g (80%) of compound 7. 1H NMR (300 MHz, CDCl3): δ (ppm) 7.94 (s, 1H),
7.74 (s, 1H), 7.62 (d, 1H, J = 12.9 Hz), 2.65 (t, 2H, J = 7.0 Hz), 2.63 (t, 2H, J = 7.6 Hz), 1.60-
1.78 (m, 4H), 1.51-1.20 (m, 12H), 1.2-0.3 (m, 6H); 13C NMR (75 MHz, CDCl3): δ (ppm)
160.3, 156.9, 152.6 (d, 3JC-F = 11.6 Hz), 149.0, 143.1, 142.3, 136.9, 132.1 (d, 5JC-F = 5.8 Hz),
4
3
1
130.8 (d, JC-F = 9.3 Hz), 124.6 (d, JC-F = 11.6 Hz), 120.8 (d, JC-F = 1162.9 Hz), 115.5 (d,
4
2
2JC-F = 31.3 Hz), 113.2 (d, JC-F = 9.3 Hz), 110.4 (d, JC-F = 15.0 Hz), 31.7, 29.8, 29.7, 29.6,
29.6, 29.0, 22.7, 14.3. HRMS (m/z, EI+) calcd for C26H29Br2FN2S3 641.9844 found 641.9541.
2.4.6. Synthesis of 2,3-difluoro-1,4-bis(trimethylsilyl)benzene (9)
To the solution of 1,2-difluorobenzene (8) (10 mL, 102.6 mmol) in 80 mL of anhydrous

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THF, lithium diisopropylamide (LDA) (118 mL, 236.1 mmol) was added at -78 oC under the
argon atmosphere. After stirring for 30 min, chlorotrimethylsilane (28.7 mL, 225.8 mmol)
was added dropwise to the reaction mixture over 1 hr at -78oC. After treating with 30 mL of 1
M H2SO4 solution, the reaction mixture was extracted with 100 mL of diethyl ether. After
washing with 100 mL of H2O, the organic layer was dried with MgSO4 and concentrated in
1
vacuo to give colorless needle-shaped solid 15.2 g (57%) of compound 9. H NMR (300
13
1
MHz, CDCl3): δ (ppm) 7.09 (s, 2H); C NMR (75 MHz, CDCl3): δ (ppm) 154.1 (d, JC-F
=
=
246.2 and 2JC-F = 17.3 Hz), 130.27 (dd, JC-F = 27.7 Hz and 3JC-F = 15 Hz), 129.2 (dd, 3JC-F
2
12.7 Hz and 4JC-F = 5.8 Hz), -1.1. HRMS (m/z, FAB+) calcd for C12H20F2Si2, 258.1072, found
258.1074.
2.4.7. Synthesis of 2,3-difluoro-1,4-dibromobenzene (10)
To a solution of compound 9 (26 g, 98.7 mmol) in 10 mL THF, bromine (14.8 mL, 296.0
mmol) in 10 mL THF was added dropwise at 0 oC. The reaction mixture was stirred for 12 hr
at 60 °C and slowly poured into saturated NaHSO3 solution at 0 oC. After extraction with 200
mL of CHCl3, the organic phase was washed with 3 × 50 mL of water and dried over MgSO4.
After removing the organic solvent underreduced pressure, the residue was purified by flash
chromatography to give 24.1 g (90%) of compound 10. 1H NMR (300 MHz, CDCl3): δ (ppm)
1
2
7.22 (s, 2H); 13C NMR (75 MHz, CDCl3): δ (ppm) 148.7 (dd, JC-F = 254.9 and JC-F = 17.3
2
3
Hz), 128.3. 109.5 (dd, JC-F = 10.4 Hz and JC-F = 8.1 Hz). HRMS (m/z, EI+) calcd for
C6H2Br2F2 269.8490, found 269.8491.
2.4.8. Synthesis of 1,4-dibromo-2,3-difluoro-5,6-dinitrobenzene (11)
To a fuming nitric acid (15 mL, 353.1 mmol) was added trifluoromethanesulfonic acid (40
o
mL, 441.4 mmol) dropwise at 0 C under argon atmosphere. A solution of compound 10 (24

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o
g, 88.3 mmol) was added dropwise to the reaction mixture. After 24 hr at 50 C, the mixture
was quenched with 1N NaOH aqueous solution. The precipitated material was recovered by
filtration and then recrystallizied from ethanol to give 18 g (56%) of compound 11 as a
1
2
yellow solid. 13C NMR (75 MHz, CDCl3): δ (ppm) 149.9 (dd, JC-F = 266.5 Hz and JC-F
=
17.3 Hz ), 141.4 (br), 105.7 (dd, 2JC-F = 13.9 Hz and 3JC-F = 11.6 ). HRMS (m/z, EI+) calcd for
C6Br2F2N2O4 359.8198, found 359.8195.
2.4.9. Synthesis of 1,4-dibromo-2,3-difluoro-5,6-diaminobenzene (12)
To a stirred solution of compound 11 (12.5 g, 34.5 mmol) in 120 mL of acetic acid under
o
argon was added iron powder (13.1 g, 234.9 mmol). After 1.5 hr at 80 C, to the reaction
mixture was treated with 60 mL of 1N NaOH aqueous solution and 50 mL of ethyl acetate.
After the hot filteration with ethyl acetate, the residue was diluted with 100 mL of ethyl
acetate. The organic phase was washed with 3 × 50 mL of water and the total organic extracts
were combined and dried over MgSO4. The solvent was removed under reduced pressure to
give 9.2 g (86%) of compound 12 as a brown solid. 13C NMR (75 MHz, CDCl3): δ (ppm)
141.7 (dd, 1JC-F = 241.6 Hz and 2JC-F = 18.5 Hz), 129.5, 98.2 (dd, 2JC-F = 11.5 Hz and 2JC-F
10.4 Hz). HRMS (m/z, EI+) calcd for C6H4Br2F2N2 299.8709, found 299.8707.
=
2.4.10. Synthesis of 4,7-dibromo-5,6-difluoro[2,1,3]benzothiadiazole (13)
Toa stirred solution of compound 12 (2.72 g, 9.0 mmol) and triethylamine (17 mL, 119.2
mmol) in 100 mL of methylene chloride, thionyl chloride (4.3 mL, 59.6 mmol) was added
o
o
dropwise over 1 hr at 0 C. After 24 hr at 70 C, the reaction mixture was diluted with 200
mL of CHCl3 and dried with MgSO4. The solvent was removed under reduced pressure, and
the solid residue was purified by column chromatography to give 5.8 g (59%) of the
1
compound 13, a light yellow solid. 13C NMR (75 MHz, CDCl3): δ (ppm) 151.7 (dd, JC-F
=

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2
2
3
261.3 Hz and JC-F = 20.8 Hz), 148.8, 99.3 (dd, JC-F = 13.9 Hz and JC-F = 10.4 Hz). HRMS
(m/z, EI+) calcd for C6Br2F2N2S 329.8096, found 329.8104.
2.4.11. Synthesis of 5,6-difluoro-4,7-bis(4-hexyl-2-thienyl)-2,1,3-Benzothiadiazole (14)
The solution of compound 13 (3.2 g, 9.7 mmol) and compound 2 (9.7 g, 29.0 mmol),
obtained using the same method as used in the synthesis of compound 6, in anhydrous 10 mL
of THF was treated with Pd(PPh3)2Cl2 (5 mg, 0.05 mmol). After stirring for overnight at 100
oC under nitrogen atmosphere, the reaction mixture was concentrated under reduced pressure
and purified by flash column chromatography to give 0.9 g (19%) of compound 14, a light
orange colored-compound. 1H NMR (300 MHz, CDCl3): δ (ppm) 8.1 (s, 2H), 7.3 (s, 2H), 2.7
(t, 4H, J = 7.6 Hz) 1.80-1.62 (m, 4H), 1.48-1.24 (m, 12H), 0.930 (t, 6H, J = 7.0 Hz); 13C
1
2
NMR (75 MHz, CDCl3): δ (ppm) 149.8 (dd, JC-F = 280.9 Hz and JC-F = 20.8 Hz), 148.8,
143.7, 132.3,131.2,124.0, 111.7,31.7, 30.5, 30.4, 29.0, 22.7, 14.1. HRMS (m/z, EI+) calcd for
C26H30F2N2S2 504.1539, found 504.1537.
2.4.12.
Synthesis
of
4,7-bis(5-bromo-4-hexyl-2-thiophenyl)-5,6-difluoro-2,1,3-
benzothiadiazole (15)
A solution of compound 14 (1 g, 1.9 mmol) in 15 mL THF under argon atmosphere was
treated with N-bromosuccinimide (NBS) (0.7 g, 4.0 mmol) portionwise and stirred at room
temperature for 2 hr. After removal of the solvent under reduced pressure, the solid residue
was purified by flash chromatography column to provide 1.8 g (82%) of compound 15 with
orange-color. 1H NMR (300 MHz, CDCl3): δ (ppm) 7.96 (s, 1H), 2.74-2.58 (t, 4H, J = 7.7 Hz)
1.76-1.62 (m, 4H), 1.46-1.22 (m, 12H), 0.91 (m, 6H); 13C NMR (75 MHz, CDCl3): δ (ppm)
1
2
177.0, 149.5 (dd, JC-F = 260.1 Hz and JC-F = 20.8 Hz), 148.4, 142.6, 131.7, 131.3, 114.5,
110.8, 31.7, 29.8, 29.6, 29.0, 22.7, 14.1. HRMS(m/z, EI+) calcd for

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C26H28Br2F2N2S3659.9749, found 659.9747.
2.5. Synthesis of the polymers
2.5.1. Synthesis of poly(4-(1-octylnonyl)-6-(2-thienyl)-4H-thieno[3,2-b]indole-alt-5-fluoro-
4,7-bis(4-hexyl-2-thienyl)-2,1,3-benzothiadiazole) (PTTIF1)
Carefully purified 4-(1-octylnonyl)-2-(trimethylstannyl)-6-[5-(trimethylstannyl)-2-thienyl]-
4H-thieno[3,2-b]indole (16) (200 mg, 0.304 mmol), 4,7-bis(5-bromo-4-hexyl-2-thienyl)-5-
fluoro-2,1,3-benzothiadiazole (7) (210 mg, 0.304 mmol), tri(o-tolyl)phosphine (0.035 g,
0.13 mmol) and Pd2(dba)3 (3 mol%) were dissolved in 4 mL of chlorobezene. After reflux
over 36 hr under argon atmosphere, the reaction mixture was treated with 0.3 mL of
trimethyl(phenyl)tin. After additional reflux for 6 hr, the reaction mixture was treated with
0.3 mL of bromobenzene and refluxed overnight to complete the end-capping reaction.
After cooling to room temperature, the reaction mixture was poured into methanol. The
precipitated material was recovered by filtration. The resulting solid was purified by
Soxhlet extractor for two days using acetone/methanol solution to remove additional
oligomers and catalyst residue. The final purple product, PTTIF1 with 37% yield, was
obtained after drying under vacuum at 60 °C.
2.5.2. Synthesis of poly(4-(1-octylnonyl)-6-(2-thienyl)-4H-thieno[3,2-b]indole-alt-5,6-
difluoro-4,7-bis(4-hexyl-2-thienyl)-2,1,3-benzothiadiazole) (PTTIF2)
Carefully purified 4-(1-octylnonyl)-2-(trimethylstannyl)-6-[5-(trimethylstannyl)-2-thienyl]-
4H-thieno[3,2-b]indole (16) (250 mg, 0.225 mmol), 4,7-bis(5-bromo-4-hexyl-2-
thiophenyl)-5,6-difluoro-2,1,3-benzothiadiazole (15) (155 mg, 0.225 mmol), tri(o-
tolyl)phosphine (0.027 g, 0.094 mmol) and Pd2(dba)3 (3 mol%) were dissolved in
chlorobenzene. The reaction mixture was treated under the same conditions used for

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PTTIF1 and the obtained solid was purified using the same method. The final purple
product, PTTIF2 with 49% yield, was obtained after drying under vacuum at 60 °C.
3. Results and Discussion
3.1. Synthesis and characterization
The synthetic routes toward the monomers and polymers are described in Scheme 1 and 2.
4,7-Bis(5-bromo-4-hexyl-2-thienyl)-5-fluoro-2,1,3-benzothiadiazole (7) or 4,7-bis(5-bromo-
4-hexyl-2-thiophenyl)-5,6-difluoro-2,1,3-benzothiadiazole (15) was coupled with 4-(1-
octylnonyl)-6-(2-thienyl)-4H-thieno[3,2-b]indole (16) by using Stille cross coupling reaction
condition and then final polymers, PTTIF1 and PTTIF2, were obtained. Table 1 summarized
the polymerization results including molecular weights, PDI, and thermal stability of the
copolymers. For PTTIF1 and PTTIF2, the number-average molecular weights (Mn) by gel
permission chromatography (GPC) of 9300 and 9800 g mol-1 and the weight-average
molecular weights (Mw) of 17000 and 15000 g mol-1 with PDI (Mw/Mn) values of 1.8 and 1.5
were determined. The thermal properties of the polymers were characterized by thermal
gravimetric analysis (TGA). Data of TGA of the polymers are shown in Figure 2 and
summarized in Table 1. TGA data showed that PTTIF1 and PTTIF2 are thermally stable
with only about 5% weight loss in air at temperatures of 400 and 421 °C, respectively. The
glass transition temperature (Tg) values of two polymers showed above 100 °C. The results
suggested that both polymers have high thermostablity and are suitable for organic
photovoltaic application.
3.2. Optical properties
The solution was prepared using 1,2-dichlororobenzene (ODCB) as a solvent and the thin film

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by spin-coating on quartz plates from the solution in ODCB at room temperature. Figure 3a
shows the absorption spectra of the polymers in solution and in thin films. Both the PTTIF1
and PTTIF2 solution exhibited similar absorption spectra, originating from similar polymer
backbone, with two distinct absorption bands in the range of 350-750 nm.21 A high-energy
absorption band around 350-470 nm can be assign to the π- π* transition of the polymer
backbone and a low-energy absorption band between 470 and 700 nm are due to the ICT
between the electron-rich and electron-deficient units in the main chain.22 The solid film
absorption spectra of PTTIF1 and PTTIF2 show red-shift in comparison with their solution
spectra, which indicates improved molecular packing ordering and π- π stacking in the solid
state.13 The film of PTTIF1 shows maximum peaks at 426 and 598 nm and the onset (λonset
)
of absorption at 742 nm, corresponding to a band gap of 1.68 eV. The maximum absorption
peaks of PTTIF2 film appear at 431 and 603 nm and the onset (λonset) of absorption at 748
nm, corresponding to a band gap of 1.66 eV, which are quite similar as compared to the case
of PTTIF1, as listed in Table 2. Interestingly, while the absorption spectrum of PTTIF2
solution was slightly blue-shifted as compared the case of PTTIF1 solution,23 absorption
maxima of PTTIF2 film are showing almost same λmax as compared to the case of PTTIF1
film, implying that ICT was more efficient by effective molecular packing between the
backbone of PTTIF2. Figure 3b exhibited molar extinction coefficients of absorption spectra
of two polymers in ODCB (concentration: (5± 0.1) × 10−5 M). The molar extinction
coefficients of PTTIF2 are around 2.8 × 10−5 M−1 cm−1 at 560 nm which are higher than the
case of PTTIF1, corresponding to good light-harvesting ability and short-circuit current
density.24
3.3. Electrochemical properties

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The electrochemical properties of the polymers were determined from the bandgaps
estimated from the absorption onset wavelength, and the HOMO energy levels which were
estimated from the cyclic voltammetry (CV). The CV was performed with a solution of
tetrabutylammonium tetrafluoroborate (Bu4NBF4) (0.10M) in acetonitrile at a scan rate of
100mV/s at room temperature under argon atmosphere. A platinum electrode (~0.05 cm2)
coated with a thin polymer film was used as the working electrode. Pt wire and Ag/AgNO3
electrode were used as the counter electrode and reference electrode, respectively. The energy
level of the Ag/AgNO3 reference electrode (calibrated by the Fc/Fc+ redox system) was 4.8
eV below the vacuum level. The CV spectra are shown in Figure 4, and the oxidation
potentials derived from the onsets of electrochemical p-doping are summarized in Table 3.
The HOMO and LUMO levels were calculated according to the empirical formula (EHOMO
=
ox
–([Eonset
]
+ 4.8) eV) and (ELUMO = EHOMO+Egopt) eV), respectively. Both of the HOMO
energy levels of the polymers, PTTIF1 and PTTIF2, were estimated to be -5.69 and -5.76
eV which is lower than the case of the non-fluorinated polymer, PTTHDTBT-h (-5.31
eV).18 The introduction of fluorine atom to the conjugated polymer backbone leads to the
lower HOMO energy level, caused by the inductive effect of fluorine atom which has the
highest electronagativity.12-14
The optimized geometry and the HOMO and LUMO energy levels were calculated at the
B3 LYP/6-31+G (2p, 2d) level of theory using the Gaussian 03 package, as shown in Figure
5. The HOMO and LUMO surfaces were plotted using the Gauss View version 4.1. To
simplify the calculations, the alkyl chains of the polymers were replaced by isopropyl groups.
The HOMOs of PTTIF1 and PTTIF2 are distributed evenly over the molecular backbone,
while the LUMOs are mainly located on the electron-deficient unit,25 as shown in Figure 5.
The orbital overlap was increased by the enlarged delocalization of HOMO.26

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3.4. OFET properties
The hole mobilities of the polymers were obtained from I-V transfer curves of OFET
devices based on PTTIF1 as well as PTTIF2 as shown in Figure 6 and the detail device
parameters are summarized in Table 4. The field-effect hole mobility of PTTIF2 (4.45 × 10-
3cm2 V-1 s-1) was higher than that of PTTIF1 (6.28 × 10-4 cm2 V-1 s-1), which revealed that
the more fluorination increased the charge mobility of the polymer.15
3.5. Photovoltaic properties
The photovoltaic properties of the polymers were investigated by fabricating the OPVs
with ITO as the positive electrode, the hole transporting PEDOT:PSS layer, the blend of
polymers (PTTIF1 and PTTIF2) and PC71BM as active layer, LiF and Al as cathode. We
optimized the properties as a function of polymer:PC71BM ratios, thickness of the active
layers and addition of various additives. The J-V curves of the OPV devices under different
processing conditions and their parameters are summarized in supporting information. The
optimum blend ratio of PTTIF1 to PC71BM was 1:2. The device based on PTTIF1:PC71BM
with 1:2 ratio exhibited a PCE of 2.44% with the highest VOC value of 0.83 V, a JSC value of
7.38 mA cm-2 and a FF of 40.0 %. When PTTIF1 was treated with various additives, 1,8-
diiodeoctane (DIO) and 1-chloronaphthalene (CN) provided negative effect to the device
properties. With 1,1-diphenylethylene (DPE) additive, the PCE was increased to 2.83% with
a FF 42.4% and increased JSC value of 8.33 mA cm-2 and slightly decreased VOC value of 0.80
V. In case of PTTIF2, the optimum blend ratio of polymer to PC71BM was 1:1. The device
based on PTTIF2:PC71BM with 1:1 ratio exhibited a PCE of 3.71% with a VOC value of 0.76
V, a JSC value of 8.87 mA cm-2 and a FF of 55.2 %. Among various additives, DIO and DPE
provided negative effect to the device properties. In case of CN additive, the highest PCE of
4.36% and a FF of 57.1% were obtained with a VOC value of 0.76 V and increased JSC of 10.0

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mA cm-2. The EQE spectra of the devices are shown in Figure 7b. The device of PTTIF2
exhibited a maximum EQE of 75% at 400 nm, while the case of PTTIF1 shows 53% at 450
nm, which is in accordance with the higher JSC of PTTIF2 in the PSC.
3.6. Thin film morphology
The surface morphology of the polymer thin films were investigated by using atomic force
microscopy (AFM), as shown in Figure 8, because the morphology of the active layer is the
important factor to OPV properties. While the PTTIF1 shows smooth morphology with 0.36
nm root mean square (RMS) roughness without additives, PTTIF1 with DPE showed a few
nano-size pinholes relating to the small decrease of VOC property of devices. In case of
PTTIF2, the RMS roughness (0.85nm) of the active layer is larger than the case of PTTIF1
due to large aggregations. The distinct aggregations revealed morphological coarsening of the
active layer (as shown in the inset of figure 8c) leading reduced VOC by increasing charge-
carrier recombination.27 Therefore, although HOMO energy levels of PTTIF1 and PTTIF2
were similar as described in the electrochemical properties section, the surface aggregation of
PTTIF2 induced the small decrease of VOC in PTTIF2 devices in comparison with PTTIF1
devices. The Figure 8d and 8h showed that CN additive induced smaller RMS value
indicating better morphology for the increase of JSC and FF of the PTTIF2 devices.
3.7. Crystallinity and the crystal orientation
Since the crystal formation of the active layer is important parameter to increase charge
carrier mobility, GIWAXS was measured to elucidate the crystal properties of the polymer
thin films. In 2D GIWAXS images of all the thin films, both (h00) and (010) diffraction peaks
were observed in the out-of-plain direction as well as in plane direction of 2D images of the
films respectively, as shown in the Figure 9. The (100) diffraction peaks of PTTIF1, PTTIF1

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with DPE, PTTIF2 and PTTIF2 with CN were observed at qz = 0.27, 0.27, 0.26 and 0.26 Å-1,
respectively, corresponding to the lamella distance of 23.2, 23.2, 23.8, and 23.9 Å. The (010)
diffraction of the polymer films are observed at qz= 1.81, 1.81, 1.75 and 1.73Å-1, respectively,
corresponding to the π-π distances of 3.47, 3.47, 3.60 and 3.63 Å. Additionally, a pole figure
analysis of the polymer films to gain the crystal orientation of the polymers was performed.
Figure 9e shows the pole figures extracted from the (100) lamella diffraction. Integrated areas
with polar angle χ ranges of 0°-45° (including 135°-180° (Ay)) and 55°-125°c (Az) were
defined as those corresponding to the volume fraction of face-on and edge-on crystallinity,
respectively. As shown in Figure 9e, Ay for PTTIF2 (55%) and PTTIF2 with CN (57%) are
higher than Ay for PTTIF1 (51%) and PTTIF1 with DPE (45%) indicated that the face-on
crystallite population of PTTIF2 based active layers was more dominant than the case of
PTTIF1 based active layers. Therefore, this strong tendency to take face-on orientation in
PTTIF2 contributed to more favorable vertical charge mobility increasing JSC, FF and PCE
of the solar cells.15,28

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4. Conclusion
In conclusion, we synthesized a series of copolymers, PTTIF1 and PTTIF2, incorporating
6-(2-thienyl)-4H-thieno[3,2-b]indole (TTI) as electron-rich unit and fluorinated DTBT as
electron-deficient units by Stille coupling reaction. Due to the strong electron-withdrawing
ability of introduced fluorine atom in PTTIF1 and PTTIF2, low HOMO energy levels were
achieved. In case of PTTIF2, OTFT exhibited higher charge carrier mobility (4.45 × 10-3cm2
V-1 s-1) than those of PTTIF1 (6.28 × 10-4 cm2 V-1 s-1), which leads to enhanced values of JSC
and FF. The device of PTTIF2 with CN additive exhibited the best photovoltaic performance
with a VOC value of 0.76 V, a JSC value of 10.00 mA/cm2, and a FF of 57.1% giving a PCE of
4.36%
Acknowledgements
This work was supported by the National Research Foundation of Korea (NRF) grant funded
by the Korea government (MEST) (NRF-2014R1A1A2055318), the Basic Science Research
Program through the National Research Foundation of Korea (NRF) funded by the Korean
Government (NRF-2015M2A2A6A03044907) and was also supported by the World Class
300 project (S2317456) of the SMBA (Korea).

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Table 1. Polymerization results and thermal properties of polymers.
Mn
Mw
Polymer
PDIa
TGA(Td)b
DSC (Tg)c
[g mol-1]a [g mol-1]a
9300
9800
17000
15000
1.8
1.5
400
421
100
100
PTTIF1
PTTIF2
aMolecular weight (Mw) and poly dispersity index (PDI) of the polymers were determined by gel permeation
b
chromatography (GPC) in CHCl3 using polystyrene standards. Onset decomposition temperature (5% weight
loss) measured by TGA under N2.cDifferential scanning calorimetry of the polymers under N2.

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Table 2. Characteristics of the UV-vis absorption spectra of the polymers.
opt,a
b
Eg
sol λmax
film λmax
film λonset
Polymers
(eV)
(nm)
(nm)
(nm)
1.68
410,567
405,559
432,602
429,602
745
748
PTTIF1
PTTIF2
1.66
aOptical energy bandgaps was estimated from the onset wavelength of the optical absorption, Eg=1240/λonset
.
bSolution absorption in ODCB.

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Table 3. Characteristics of the electrochemical potentials and energy levels of the polymers.
c
c
HOMOa
(eV)
LUMOb
(eV)
Eox
Ered
Polymers
(V)
(V)
-5.69
-5.76
-4.01
-4.10
0.89
-0.82
PTTIF1
PTTIF2
0.96
-0.77
opt
b
c
aCalculated from the oxidation potential. LUMO = (HOMO + Eg
), Onset oxidation potential measured by
cyclic voltammetry.

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Table 4. Hole mobility measured by OFET devices.
Polymer
Vth [V]
4.06
Mobility [cm2/V-s]
6.28 x 10-4
on / off ratio
7.9 x 102
PTTIF1
26.9
4.45 x 10-3
1.6 x 103
PTTIF2

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Table 5. Photovoltaic properties of the polymer:PC71BM blend based solar cells.
JSC
[mA/cm2]
VOC
FF
PCEmax (avg)
[%]
Polymer
Ratio
1:2
Additive
[V]
[%]
w/o
DPE
w/o
CN
7.38
8.33
0.83
0.80
0.76
0.76
40.1
42.4
55.2
57.1
2.44 (2.42)
2.83 (2.77)
3.71 (3.50)
4.36 (4.24)
PTTIF1
8.87
1:1
PTTIF2
10.00