Synthesis of a new wide-bandgap conjugated copolymer with 3-trifluoromethylthiophene monomer and Its application to non-fullerene polymer solar cells

Article abstract of DOI:10.1080/15421406.2019.1648033

In this study, a novel wide-bandgap conjugate donor-acceptor type copolymer, PBDT-TFMTh, containing benzodithiophene (BDT) as a donor unit and trifluoromethyl (TFM)-substituted thiophene (Th) as an acceptor unit was designed and synthesized. The physical, optical, and electrochemical properties of PBDT-TFMTh were studied in detail and it was found to have lower-lying highest occupied and lowest unoccupied molecular orbitals. The polymer solar cell with an active layer consisting of PBDT-TFMTh and acceptor IDT(TCV)₂ showed a relatively high power conversion efficiency of 2.25% after thermal annealing at 120 °C.

Full text of DOI:10.1080/15421406.2019.1648033

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: https://www.tandfonline.com/loi/gmcl20
Synthesis of a new wide-bandgap conjugated
copolymer with 3-trifluoromethylthiophene
monomer and Its application to non-fullerene
polymer solar cells
Amaladass Pitchamuthu, Chang Geun Park, Su Hong Park, Min Ju Cho &
Dong Hoon Choi
To cite this article: Amaladass Pitchamuthu, Chang Geun Park, Su Hong Park, Min Ju Cho
& Dong Hoon Choi (2019) Synthesis of a new wide-bandgap conjugated copolymer with 3-
trifluoromethylthiophene monomer and Its application to non-fullerene polymer solar cells,
Molecular Crystals and Liquid Crystals, 686:1, 30-37, DOI: 10.1080/15421406.2019.1648033
To link to this article: https://doi.org/10.1080/15421406.2019.1648033
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MOLECULAR CRYSTALS AND LIQUID CRYSTALS
2019, VOL. 686, NO. 1, 30–37
https://doi.org/10.1080/15421406.2019.1648033
Synthesis of a new wide-bandgap conjugated copolymer
with 3-trifluoromethylthiophene monomer and Its
application to non-fullerene polymer solar cells
Amaladass Pitchamuthu, Chang Geun Park, Su Hong Park, Min Ju Cho, and
Dong Hoon Choi
Department of Chemistry, Research Institute for Natural Sciences, Korea University, 145 Anam-ro,
Sungbuk-gu, Seoul 02841, Republic of Korea.
KEYWORDS
ABSTRACT
3-trifluoromethylthiophene;
wide-bandgap copolymer;
polymer solar cell; power
conversion efficiency
In this study, a novel wide-bandgap conjugate donor-acceptor type
copolymer, PBDT-TFMTh, containing benzodithiophene (BDT) as a
donor unit and trifluoromethyl (TFM)-substituted thiophene (Th) as
an acceptor unit was designed and synthesized. The physical, optical,
and electrochemical properties of PBDT-TFMTh were studied in
detail and it was found to have lower-lying highest occupied and
lowest unoccupied molecular orbitals. The polymer solar cell with an
active layer consisting of PBDT-TFMTh and acceptor IDT(TCV)2
showed a relatively high power conversion efficiency of 2.25% after
thermal annealing at 120ꢀC.
Introduction
Over the last decade, the synthesis and application of thiophene-based oligomers or pol-
ymers to materials science have been of great interest because of their desirable func-
tional attributes in organic solar cells [1,2] and organic field effect transistors [3,4]. In
recent years, thiophene-based polymers have attracted significant attention and have
been extensively incorporated into polymer solar cells (PSCs) because of their low-cost
device fabrication, light weight, and flexible chemical structure [5].
It has been previously demonstrated that the preparation of the active layer in bulk
heterojunction (BHJ) type PSCs is based primarily on a blend of polymer electron
donors and small organic acceptor molecules or polymer electron acceptors. The
power conversion efficiency (PCE) of a BHJ-PSC device is directly proportional to
the short-circuit current density (Jsc), open-circuit voltage (Voc), and fill factor (FF).
Since Voc is mainly determined by the difference between the highest occupied
molecular orbital (HOMO) of a donor and the lowest unoccupied molecular orbital
(LUMO) of an acceptor, the exploration of a conjugated polymer with a low-lying
HOMO energy level is a feasible approach to increase Voc. In general, the HOMO
energy level of the donor polymer can be significantly lowered by reducing the
effective conjugation length to inhibit the delocalization of p-electrons in the
CONTACT Dong Hoon Choi
dhchoi8803@korea.ac.kr
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/gmcl.
ß 2019 Taylor & Francis Group, LLC

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
31
polymer backbone. For example, the HOMO level of a copolymer prepared by using
a fluorene monomer tends to be low [6].
An alternative method for lowering the HOMO energy level of a polythiophene
derivative is by copolymerization of the thiophene monomer with an electron-withdraw-
ing substituent. Some research groups have reported polythiophene-containing thio-
phene-3-carboxylate electron-withdrawing monomers [7,8]. In addition, polythiophenes
with cyano [9] and fluorine [10,11] electron-withdrawing substituents in the main chain
reportedly have a relatively low-lying HOMO level.
Thus, it may be quite advantageous to synthesize a conjugated copolymer by replacing
an electron donor monomer with an electron-withdrawing monomer and utilize the cor-
responding copolymer to fabricate a BHJ-type PSC. For example, Liang et al. introduced
a fluorine atom into the TT unit of the PBDTTs, and the HOMO level of the polymer
was altered such that the corresponding PSC demonstrated a higher Voc [12]. Apart from
the fluoro substituent, CF3 units have often been used to adjust the electronic structure of
small organic molecules. However, to the best of our knowledge, there is no report on
the synthesis of donor-acceptor (D–A) type copolymers containing a CF3-substituted thio-
phene monomer and its use as a polymer donor in the fabrication of BHJ-type PSCs.
In this work, for the first time, we report a new D–A type conjugated copolymer
PBDT-TFMTh, which contains a trifluoromethyl (TFM)-substituted thiophene (Th)
monomer and a benzodithiophene (BDT) unit. Compared to PBDT-Th, the polymer
bearing neat thiophene as the monomer, it was evident that replacing the thiophene
unit with the TFM-substituted thiophene unit in the polymer backbone structure led to
lowering of the HOMO and LUMO energy levels of PBDT-TFMTh (Scheme 1). The
PSC made of PBDT-TFMTh:IDT(TCV)2 showed a relatively high PCE of 2.25% after
thermal annealing at 120 ꢀC.
Experimental details
Materials
2,5-Dibromo-3-(trifluoromethyl)thiophene and (4,8-bis(5-(2-ethylhexyl)thiophen-2-
yl)benzo[1,2-b:4,5-b’]dithiophene-2,6-diyl)bis(trimethylstannane) were prepared accord-
ing to literature methods [8,13–15].
Instrumentation
The absorption spectra of the polymer were recorded using a UV ꢁ Vis absorption spec-
trometer (Agilent 8453, PDA type, k ¼ 190 ꢁ 1100 nm). The electrochemical properties
were characterized by cyclic voltammetry (CV, eDAQ EA161) with an electrolyte solu-
tion containing tetrabutylammonium hexafluorophosphate (Bu4NPF6) in acetonitrile. A
platinum wire and Ag/AgCl (3M NaCl) were purchased from BASi Corporation and
used as the counter and reference electrodes, respectively. Elemental analyses were per-
formed using an EA1112 (Thermo Electron Corp.) elemental analyzer. The molecular
weights of the synthesized polymers were measured relative to polystyrene (PS)
standards by gel permeation chromatography (GPC) with chlorobenzene at 85 ꢀC as an
eluent using an Agilent GPC 1200 series instrument. The surface morphologies of the

32
A. PITCHAMUTHU ET AL.
Scheme 1. Synthetic pathway of 3-trifluoromethylthiophene and copolymers PBDT-Th and PBDT-
TFMTh. (i) -78 ꢀC; (ii) K2S2O8, Cu(OAc)2ꢂH2O, 90 ꢀC, 24 h; (iii) Pd2(dba)3, P(o-tolyl)3, toluene, 120 ꢀC,
24 h; (iv) Pd2(dba)3, P(o-tolyl)3, toluene, 120 ꢀC, 48 h.
polymer films were investigated using an atomic force microscope (AFM, XE-100,
Advanced Scanning Probe Microscope, PSIA) with a silicon cantilever.
Synthesis of PBDT-TFMTh
(4,8-Bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b’]dithiophene-2,6-diyl)bis(trimethyl
stannane) (0.045 g, 0.05 mmol), 2,5-dibromo-3-(trifluoromethyl)thiophene (0.015 g,
0.05mmol), tris(dibenzylideneacetone)dipalladium(0) (2.0 mg, 5.0 mol%), tri(o-tolyl)phos-
phine (1.5 mg, 10mol%), and anhydrous toluene (5.0 mL) were added to a Schlenk tube.
The reaction mixture was then stirred at 120 ꢀC for 48h. After precipitation in MeOH,
the crude polymer was filtered and dried under vacuum. The precipitate was purified by
Soxhlet extraction using methanol, acetone, hexane, and chloroform sequentially. The
resultant polymer was collected, dried under vacuum, and obtained as a red solid in 69%
yield (25mg). GPC: Mn ¼ 15.4kg/mol, PDI ¼ 1.76. Anal. calcd for (C39H41F3S5)n: C,
64.43; H, 5.68; S, 22.05. Found: C, 64.37; H, 5.65; S, 21.94.
Synthesis of PBDT-Th
PBDT-Th was synthesized using (4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-
b’]dithiophene-2,6-diyl)bis(trimethyl stannane) (0.181 g, 0.2 mmol) and 2,5-dibromothio-
phene (0.048 g, 0.2 mmol) following the synthetic procedure outlined above for
PBDT-TFMTh. Yield: 81.5% (106 mg). GPC: Mn ¼ 8.8 kg/mol, PDI ¼ 2.40. Anal. calcd
for (C38H42S5)n: C, 69.25; H, 6.42; S, 24.33. Found: C, 69.21; H, 6.44; S, 24.27.
Results and discussion
Synthesis and characterization
In this work, we designed a new wide-bandgap conjugated copolymer (PBDT-TFMTh)
containing BDT as a donor unit and 3-trifluoromethylthienyl (TFMTh) unit as an

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
33
Table 1. Physical, optical, and electrochemical parameters of PBDT-TFMTh and PBDT-Th.
Absorption (nm)
Energy level (eV)
HOMOd
LUMOe
a
optb
c
Polymer
Mn (kg/mol) PDI
solution
film
kcut-off (nm) Eg
(eV) Eox (V)
PBDT-TFMTh
PBDT-Th
aFilm
15.3
8.8
1.75
2.40
470
518
518
525
605
600
2.05
2.07
0.87
1.35
ꢁ5.77
ꢁ5.29
ꢁ3.72
ꢁ3.22
bCalculated from the onset of optical absorption
cDetermined by CVs of thin films on the Pt electrode
dHOMO ¼ –(4.42 þ Eox)
eLUMO ¼ HOMO þ Eg
.
opt
acceptor unit, and synthesized it by Stille coupling polymerization. A heteroacene-type
donor, BDT was employed as a building block with a long alkyl-chain substituent that
imparted a high degree of planarity and better solubility. In addition, we introduced the
electron-withdrawing TFM-substituted thiophene monomer into the polymer backbone
to modulate the HOMO and LUMO levels.
The monomer, trifluoromethylated dibromothiophene, was synthesized by trifluoro-
methylation of dibromothiophene using freshly prepared dimethyl sulfoxide
(DMSO)ꢂCF3I adduct, potassium persulfate, and Cu medium in DMSO as solvent
according to the literature procedure [13,14]. Additionally, PBDT-Th as a control
copolymer was prepared as shown in Scheme 1. The number average molecular weights
(Mn) and polydispersity indices (PDI) of the obtained polymers, PBDT-TFMTh and
PBDT-Th, were 15.4 and 8.8 kg/mol, and 1.76 and 2.4, respectively (Table 1).
Optical and electrochemical properties
The UV-Vis absorption spectra of the conjugated copolymer PBDT-TFMTh in solution
and thin film states is shown in Figure 1. The corresponding parameters are summar-
ized in Table 1. As shown in Figures 1a and 1b, the maximum absorption wavelength
in the solution and film states were observed respectively at 470 and 518 nm for PBDT-
TFMTh, and 418 and 525 nm for PBDT-Th as the control polymer. Compared to
PBDT-Th, PBDT-TFMTh showed blue-shifted absorption spectrum which is due to
highly twisted structure with a large dihedral angle between BDT and TFMTh.
Interestingly, the absorption spectrum of the PBDT-TFMTh film was red-shifted com-
pared to that of PBDT-Th, which may be attributed to the strong interactions between
the polymer chains in the solid state. The optical bandgap of the copolymer PBDT-
TFMTh was estimated to be 2.05 eV from the onset wavelength (kcut-off ¼ 605 nm) of
the absorption spectrum of the thin film (Table 1). Thus, the copolymer having TFMTh
could be classified as a wide-bandgap polymer applicable to a non-fullerene PSC that
effectively induced complementary absorption when mixed with a low bandgap acceptor
to produce an active layer, as shown in Figure 1c.
We performed CV to investigate the electrochemical properties of the copolymers
PBDT-Th and PBDT-TFMTh in their thin film states, and the corresponding parame-
ters are shown in Table 1. The HOMO levels of the copolymers were estimated using
the onset oxidation potentials (Eoxs). As shown in Figure 2a, the values of Eox were 0.87
and 1.35 V for PBDT-Th and PBDT-TFMTh, respectively. The estimated HOMO
energy levels of PBDT-Th and PBDT-TFMTh were then calculated as –5.29 and

34
A. PITCHAMUTHU ET AL.
Figure 1. UV-Vis absorption spectra of PBDT-TFMTh and PBDT-Th in solution (a) and thin film states
(b). UV-Vis absorption spectra of PBDT-TFMTh as the donor polymer and IDT(TCV)2 in film states. (c)
Molecular structures of PBDT-TFMTh donor and IDT(TCV)2 acceptor.
ꢁ5.77 eV, respectively. The electron-withdrawing TFM unit in PBDT-TFMTh resulted
in a lower-lying HOMO energy level compared to that of PBDT-Th. The LUMO energy
levels were –3.22 and –3.72 eV for PBDT-Th and PBDT-TFMTh, respectively.
Non-fullerene polymer solar cells
The photovoltaic properties of the blend films bearing copolymer PBDT-TFMTh as the
donor and IDT(TCV)2 as the non-fullerene acceptor were investigated in an inverted
device configuration. The polymer solar cells used in this study were fabricated accord-
ing to a previously reported method [8]. The active layers were obtained by spin coating
the chlorobenzene solution (2 wt %) of the polymers (PBDT-TFMTh or PBDT-Th) and
IDT(TCV)2 with different weight ratio. The LUMO and HOMO energy levels of the
selected acceptor matched well with those of the copolymer PBDT-TFMTh. To obtain
the best PSC performance, the active layer thickness and the blending ratio of the donor
and acceptor were precisely optimized. The effect of varying blend compositions on
PSC performance parameters is presented in Table 2.
The current-voltage (J-V) and EQE curves of the optimized devices with the blend
film as the active layer are shown in Figures 3a and 3b. Among the as-cast active layers
with the same composition, the highest efficiency was obtained when the weight ratio
of PBDT-TFMTh to IDT(TCV)2 was 2:1. The PSC prepared using the PBDT-
TFMTh:IDT(TCV)2 (weight ratio ¼ 2: 1) blend film showed 1.96% PCE (Voc of 0.71 V,
Jsc of 6.54 mA cmꢁ2, and FF of 42.34%). We also conducted the thermal annealing of

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
35
Figure 2. (a) Cyclic voltammograms of the thin films of PBDT-Th and PBDT-TFMTh. (b) Energy dia-
gram of PBDT-Th and PBDT-TFMTh as donors and IDT(TCV)2 as an acceptor.
Table 2. Parameters of PSCs fabricated using PBDT-TFMTh:IDT(TCV)2 and PBDT-Th:IDT(TCV)2
Donor
Acceptor
IDT(TCV)2
D/A ratio (w/w%)
Annealing temp. (ꢀC)
Voc (V)
Jsc (mA/cm2)
FF (%)
PCE (%)
PBDT-TFMTh
1:1
1:1
–
100
–
–
120
–
0.70
0.69
0.71
0.71
0.69
0.71
0.31
6.25
6.09
6.02
6.54
6.44
4.64
2.53
44.39
49.99
48.73
42.34
50.14
42.23
46.20
1.95
2.08
2.07
1.98
2.25
1.39
0.37
1:1/CNa
2:1
2:1
1:2
2:1
PBDT-Th
IDT(TCV)2
120
a1-Chloronaphthalene (CN) was used as a solvent additive.
Figure 3. J-V curve of the PSC device based on PBDT-Th:IDT(TCV)2 and PBDT-TFMTh:IDT(TCV)2 blend
films (2:1) annealed at 120 ꢀC (a) and corresponding EQE curves (b).
the active layer at different temperatures. This resulted in slightly increasing the FF val-
ues of the different PSCs and their PCE values also increased from 1.96 to 2.25%
(Table 2). The EQE spectra of the PSCs were recorded in the range of 320 to 700 nm.
The maximum EQE value of the PSC based on PBDT-TFMTh:IDT(TCV)2 (2:1 wt%)
was observed at 610 nm. Thus, according to the preliminary experimental results, the
copolymer PBDT-TFMTh is a promising donor material for PSC devices.
It is well-known that the internal morphology of donor-acceptor blend films has a
significant effect on the performances of BHJ-type PSC devices [16]. The surface
morphology of the active layer in the PSC device was measured by atomic force micros-
copy (AFM). The AFM height images are displayed in Figure 4. It was found that the

36
A. PITCHAMUTHU ET AL.
Figure 4. AFM height (a) and phase (b) images (3 ꢃ 3 lm) of the PBDT-TFMTh:IDT(TCV)2 (2:1 wt.
ratio) blend.
high PSC device performance (g ¼ 2.25%) was observed when PBDT-TFMTh was
blended with IDT(TCV)2. This resulted in a fine surface morphology with a relatively
small surface roughness of 2.67 nm. The small surface roughness was likely due to the
fine internal morphology of the active layer so that efficient exciton diffusion and
exciton dissociation/charge transport were expected.
Conclusion
We have successfully synthesized a new wide-bandgap conjugated copolymer, PBDT-
TFMTh, having thiophene monomers substituted with a TFM group by Stille coupling
polymerization. The introduction of the TFM group into the thiophene ring in PBDT-
TFMTh effectively changed the HOMO and LUMO levels. The PSC fabricated using
PBDT-TFMTh as the donor and IDT(TCV)2 as the non-fullerene acceptor showed a
maximum PCE of 2.25%, which was attributable to its fine blend morphology. The PCE
of the PBDT-TFMTh-based PSC indicated that the introduction of the TFM substituent
is a promising strategy to prepare wide-bandgap donor polymers for PSC applications.
Acknowledgements
This research was supported by the National Research Foundation of Korea (NRF-
2019R1A2C2002647 and 2019R1A6A1A11044070).
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