Regioisomeric π-conjugated terpolymers bearing carboxylate substituted thienothiophenyl quarterthiophene and their application to fullerene-free polymer solar cells

Article abstract of DOI:10.1016/j.polymer.2018.05.027

Two regioisomeric π-conjugated terpolymers bearing carboxylated thiophene (CT) as electron accepting unit and thienothiophene (TT) and bithiophene (BT) as electron donating units were successfully synthesized for enhancing the performance of polymer solar cells (PSCs). Regio-regular and regio-random binary copolymers based on CT and BT were also prepared as control polymers. All polymers showed unique optical properties and crystalline behaviors. Among the PSC devices fabricated in this study, the PSC based on the regio-random terpolymer (Ran-TT) and 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis (4-hexylphenyl) dithieno [2,3-d:2′,3′-d′]-s-indaceno [1,2-b:5,6b′]-dithiophene (ITIC) exhibited the highest power conversion efficiency of 5.65% with a high short circuit current density (J_sc) of 12.63 mA cm^?2 and an open circuit voltage (V_oc) of 0.81 V. The promising PCE value is attributed to the low-lying highest occupied molecular orbital (HOMO) of Ran-TT, effective complementary absorption spectrum and favorable internal morphology of the blend film. The internal morphology of the Ran-TT was observed to be more fine phase separation than that of the regio-regular terpolymer, facilitating exciton diffusion and dissociation in the bulk heterojunction.

Full text of DOI:10.1016/j.polymer.2018.05.027

Accepted Manuscript
Regioisomeric π-conjugated terpolymers bearing carboxylate substituted
thienothiophenyl quarterthiophene and their application to fullerene-free polymer solar
cells
Chang Geun Park, Gi Eun Park, Ji Hyung Lee, Aesun Kim, Young Un Kim, Seo Yeon
Park, Su Hong Park, Min Ju Cho, Dong Hoon Choi
PII:
S0032-3861(18)30423-3
10.1016/j.polymer.2018.05.027
JPOL 20588
DOI:
Reference:
To appear in: Polymer
Received Date: 26 February 2018
Revised Date: 24 April 2018
Accepted Date: 8 May 2018
Please cite this article as: Park CG, Park GE, Lee JH, Kim A, Kim YU, Park SY, Park SH, Cho MJ,
Choi DH, Regioisomeric π-conjugated terpolymers bearing carboxylate substituted thienothiophenyl
quarterthiophene and their application to fullerene-free polymer solar cells, Polymer (2018), doi:
10.1016/j.polymer.2018.05.027.
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ACCEPTED MANUSCRIPT
TOC

ACCEPTED MANUSCRIPT
Regioisomeric π-Conjugated Terpolymers bearing Carboxylate
substituted Thienothiophenyl Quarterthiophene and Their
Application to Fullerene-free Polymer Solar Cells
Chang Geun Park a, Gi Eun Park a,b, Ji Hyung Lee a, Aesun Kim a, Young Un Kim a, Seo Yeon
Park a, Su Hong Park a, Min Ju Cho a,* and Dong Hoon Choi a.*
a Department of Chemistry, Research Institute for Natural Sciences, Korea University, 145
Anam-Ro, Sungbuk-gu, Seoul 02841, Republic of Korea
b
Photo-electronic Hybrids Research Center, Korea Institute of Science and Technology,
Seoul 02792, Republic of Korea
Abstract
Two regioisomeric π-conjugated terpolymers bearing carboxylated thiophene (CT) as
electron accepting unit and thienothiophene (TT) and bithiophene (BT) as electron donating
units were successfully synthesized for enhancing the performance of polymer solar cells
(PSCs). Regio-regular and regio-random binary copolymers based on CT and BT were also
prepared as control polymers. All polymers showed unique optical properties and crystalline
behaviors. Among the PSC devices fabricated in this study, the PSC based on the regio-
random terpolymer (Ran-TT) and 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-
indanone))-5,5,11,11-tetrakis(4-hexylphenyl) dithieno[2,3-d:2′,3′-d′]-s-indaceno[1,2-b:5,6b′]-
dithiophene (ITIC) exhibited the highest power conversion efficiency of 5.65% with a high
short circuit current density (Jsc) of 12.63 mA cm-2 and an open circuit voltage (Voc) of 0.81 V.
The promising PCE value is attributed to the low-lying highest occupied molecular orbital
(HOMO) of Ran-TT, effective complementary absorption spectrum and favorable internal
1

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morphology of the blend film. The internal morphology of the Ran-TT was observed to be
more fine phase separation than that of the regio-regular terpolymer, facilitating exciton
diffusion and dissociation in the bulk heterojunction.
Keywords: carboxylated thiophene, regioisomeric, terpolymer, organic solar cell,
semiconductor
Correspondence to: M. J. Cho (chominju@korea.ac.kr) and D. H. Choi (E-mail:
dhchoi8803@korea.ac.kr)
Introduction
Polymer solar cells (PSCs) have been studied intensively due to the excellent film-forming
ability, persistent internal morphology, desirable thin film structure, high light absorptivity,
and mechanical flexibility of the conjugated polymers composing the active layer.1-4
The most commonly known PSCs include bulk heterojunctions (BHJs) with an active layer
composed of a conjugated polymer as an electron donor and a low molecular weight
compound or polymer as an electron acceptor. In the early study of BHJ-type PSC, fullerene
derivatives such as [6,6]-phenyl-(C61 or C71)-butyric acid methyl ester (PC61BM or PC71BM),
which exhibit electron-withdrawing ability, were mainly used as acceptor materials. However,
the inflexibility of the molecular structure, low absorption in the visible light region, and self-
associating characteristics have been pointed out as disadvantages in enhacing the efficiency
of PSC.5,6 To overcome these drawbacks, a number of studies were recently conducted on
non-fullerene acceptors. For example, materials such as perylenediimide (PDI) derivatives,
naphthalene diimide (NDI) derivatives, and indaceno-dithiophene (IDT)-based small
molecules have been employed for fabricating high-performing PSCs.7-10 To achieve efficient
2

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complementary absorption behavior with blend films containing the new non-fullerene
acceptors described above, new donor polymers with a wide bandgap energy and strong
absorption in the short wavelength region need to be developed.11,12
Recently, high-performing wide-bandgap donor polymers bearing donor(D) and
acceptor(A) moieties in the repeating group have been utilized to fabricate high-efficiency
non-fullerene PSCs .13-18 On the other hand, polythiophene (PT) derivatives with wide
bandgap energies have previously been used as donor polymers due to their accessibility and
ease of synthesis. However, PT derivatives have a relatively high-lying HOMO level and a
resulting low open circuit voltage (Voc), which is directly related to the gap between the
HOMO of the donor and the LUMO of the acceptor, making it difficult to implement these
molecules in PSCs. To remedy this problem, extensive efforts have been devoted to
developing new PT derivatives by employing more effective conjugative units to tune their
energy levels and modify the morphology of the blend film with the acceptors. For instance,
Hou et al. demonstrated that the PT derivative P3HDTTT allowed for a relatively low-lying
HOMO level (EHOMO = -5.3 eV) by reducing the number of alkyl groups and reducing the
electron donating effect. Using this approach, the Voc was increased from 0.64 V to 0.82 V.13
Many attempts have been made to introduce carboxylates into the thiophene unit, as well as
to employ side-chain engineering by modifying the alkyl chain. This approach has been
shown to reduce the HOMO level without significantly changing the optical bandgap. Zang et
al. developed a series of PT derivatives (e.g. PT-C1, PT-C2 and PT-C3) containing
carboxylate substituents and the PSC based on PT-C3 having the largest branched alkyl side
chain, showed improved Voc of 0.78 V, which is higher than the PSC made with P3HT (Voc =
0.60 V), having no carboxylates.19
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As noted above, most previous studies have focused on tuning the energy level of the PT
polymer. To further improve the efficiency of the BHJ-type PSC, studies on improving the
internal morphology of the blend films with non-fullerene acceptors should be performed. An
effective design strategy to obtain a favorable blend film morphology is to control the regio-
regularity of the donor polymer backbone structure. The effect of regio-random and regio-
regular conjugated donor polymer structures on the performance of PSC devices using high
performance acceptors has been studied.20-28
Most recently, a number of studies have been conducted on conjugated terpolymer as one
useful method for precisely manipulating the physical properties of binary D-A copolymers.
By introducing a new third conjugated monomer, the resulting polymer properties such as the
spectral absorption, charge transport, solubility, and blend film morphology can be finely
controlled compared to conventional binary copolymers.
In this study, we proposed conjugated terpolymers by employing regio-random and regio-
regular polythiophene derivatives with a carboxylate side chain and further introducing
thienothiophene as a third monomer. Thus, new asymmetric carboxylate substituted PT-based
terpolymers, Ran-TT and Reg-TT, were successfully synthesized herein, along with PT-
C3(Ran-BT) and Reg-BT as control polymers. Eventually, they were employed to fabricate
inverted type PSCs for their comparison. The blend film of 3,9-bis(2-methylene-(3-(1,1-
dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl) dithieno[2,3-d:2′,3′-d′]-s-
indaceno[1,2-b:5,6b′]-dithiophene (ITIC) and Reg-TT exhibits poor photovoltaic
performance with a PCE of 2.43%. To modulate the performance of the PSC based on Reg-
TT, we also employ the random terpolymer Ran-TT in corresponding PSCs, where the
terpolymer displays an fine internal morphology with ITIC in the blend film state. The fully
4

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optimized Ran-TT:ITIC based device provides the highest PCE of over 5.65% with a high
Jsc of 12.57 mA cm-2. Compared with the PSC performance achieved using Ran-BT, Ran-
TT-based PSC provides a significantly enhanced PCE.
Experimental
Materials
The chemicals required to synthesize the polymers used in this study were purchased
from Sigma-Aldrich, Acros Organics, and Tokyo Chemical Industry and were used
without further purification. Compounds
2, 3, 4, M3, and M4 were synthesized
−
32
according to published methods.29
Synthesis of monomers and polymers
2,5-Dibromothiophene-3-carboxylic acid (2).
Compound (2.00 g, 15.6 mmol) was dissolved in glacial acetic acid (25.0 mL) and
1
Br2 (12.4 g, 78.0 mmol) was added to the above solution dropwise at room
temperature. The mixture was heated and stirred at 60 °C for 24 h. After the reaction
was completed, the mixture was poured into 200 mL of deionized water to quench the
reaction. The precipitate was filtered and dissolved in ethyl acetate (EA). The solution
in EA was dried with Na2SO4. After evaporation of the solvent, the crude product was
purified by recrystallization from EtOH−H2O to give the product as a white powder
1
(3.60 g, yield 80.7%; H NMR (500 MHz, CDCl3; δ (ppm)): 13.29 (s, 1H), 7.44 (s,
1H)).
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2-Hexyldecyl 2,5-dibromothiophene-3-carboxylate (3).
A mixture of compound
2 (2.50 g, 8.74 mmol) and K2CO3 (3.63 g, 9.62 mmol) was
dissolved in dimethyl formamide (DMF; 20.0 mL). The mixture was stirred for 1 h
under Ar atmosphere and 1-iodo-2-hexyldecane (2.60 g, 9.62 mmol) was added and
stirred overnight. Deionized water (50.0 mL) with a small amount of 1 M HCl (2−3
mL) was poured into the mixture to quench the reaction, followed by extraction with
CH2Cl2. The organic phase was dried with Na2SO4 and the solvent was removed in
vacuo. The product was purified by column chromatography on silica gel using
hexane/CH2Cl2 (4:1 v/v), yielding the pure compound as a colorless oil (1.70 g, yield
1
38.0%; H NMR (500 MHz, CDCl3; δ (ppm)): 7.33(s, 1H), 4.19–4.18 (d,
J = 5.8 Hz,
2H), 1.74–1.71 (m, 1H), 1.42–1.27 (m, 14H), 0.89–0.86 (m, 6H)).
2-Bromothiophene-3-carboxylic acid (4).
An oven-dried 100 mL round bottom flask was charged with compound
1
(2.00 g,
o
15.6 mmol) and tetrahydrofuran (THF; 30 mL). The solution was cooled to -78.0 C
and 2.50 M
n
-butyllithium in hexane (12.6 mL, 31.5 mmol) was slowly added
o
dropwise. The reaction mixture was stirred for 30 min at -78.0 C and bromine (0.86
o
mL, 16.4 mmol) was added dropwise. Stirring was continued for 1 h at -78.0 C and
the mixture was heated to room temperature. After stirring the mixture overnight, a
small amount of dilute 1 M HCl (50.0 mL) was added to quench the reaction and the
solution was then concentrated. After extracting the resultant mixture using ether and
water, the organic layer was dried with Na2SO4. The compound was recrystallized
6

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twice using a H2O/ether mixture to furnish the compound as faint yellow needles (2.13
1
g, yield 66.0%; H NMR (500 MHz, DMSO-d6, δ (ppm)): 7.63−7.62 (d,
J = 5.8 Hz,
1H), 7.32−7.31 (d, J = 5.8 Hz, 1H)).
2-Hexyldecyl 2-bromothiophene-3-carboxylate (5).
A mixture of compound (2.10 g, 10.1 mmol) and K2CO3 (4.21 g, 30.4 mmol) was
4
dissolved in DMF (15.0 mL). The mixture was stirred for 1 h under argon atmosphere;
1-iodo-2-hexyldecane (3.93 g, 11.2 mmol) was added and stirred overnight. After the
reaction was completed, the mixture was poured into 50 mL of deionized water and a
small amount of 1 M HCl (3−5 mL) was added to quench the reaction, followed by
extraction with CH2Cl2. The organic phase was dried with Na2SO4 and the solvent was
removed. The product was purified via column chromatography on silica gel using
hexane/CH2Cl2 (4:1), yielding the pure compound as a colorless oil (3.00 g, yield
1
68.5%; H NMR (500 MHz, CDCl3, δ(ppm)): 7.37−7.36 (d,
J
= 5.8 Hz, 1H),
7.23−7.21 (d,
1.45−1.27 (m, 24H), 0.89−0.86 (m, 6H)).
J = 5.8 Hz, 1H), 4.21−4.20 (d, J = 5.5 Hz, 2H), 1.76−1.73 (m, 1H),
Bis(2-hexyldecyl)-[2,2':5',2'':5'',2'''-quaterthiophene]-3,3'''di-carboxylate (6).
Compound (1.50 g, 3.50 mmol) and 5,5'-bis(trimethylstann-yl)-2,2'-bithiophene
5
(0.85 g, 1.70 mmol) were dissolved in toluene (18.0 mL) and dimethylformamide
(DMF) (2.00 mL). The reaction mixture was degassed with argon, after which
tris(dibenzylideneacetone)dipalladium (Pd2(dba)3) (80.0 mg, 5 mol%) and tri(o-
tolyl)phosphine (P(
o
-tolyl)3) (53.0 mg, 5 mol%) were added to the mother mixture.
7

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The mixture was heated to 110 °C and continuously stirred for 24 h. After the reaction
was completed, the mixture was poured into 50.0 mL of deionized water and then
extracted with CH2Cl2. The organic phase was dried with Na2SO4 and the solvent was
removed. The product was purified via column chromatography on silica gel using
hexane/CH2Cl2 (1.5:1), yielding the pure compound as a light yellow oil (1.0 g, yield
1
66.7%).; H NMR (500 MHz, CDCl3, δ(ppm)): 7.49−7.48 (d,
J
= 5.5 Hz, 2H),
= 3.7 Hz, 2H),
= 5.8 Hz, 4H), 1.69 (m, 2H), 1.29−1.24 (m, 48H), 0.88−0.85 (m, 12H).
7.39−7.38 (d,
4.17−4.16 (d,
J
J
= 4.0 Hz, 2H), 7.20−7.19 (d, J = 5.8 Hz, 2H), 7.16 (d, J
; MALDI-TOF: m/z = 866.13
Bis(2-hexyldecyl) 5,5'''-dibromo-[2,2':5',2'':5'',2'''-quaterthio-phene]-3,3'''dicar-
boxylate (7).
Compound
6 (1.00 g, 1.20 mmol) and N-bromosuccinimide (NBS) (0.41 g, 2.30
mmol) were dissolved in chloroform (5.00 mL) and acetic acid (5.00 mL). The
mixture was stirred for 24 h at room temperature. After the reaction was finished, the
mixture was poured into 50.0 mL of deionized water and then extracted with
hexane/ether to remove the acid completely. The product was purified by column
chromatography on silica gel using hexane/CH2Cl2 (2:1), yielding the pure compound
1
as a yellow oil (0.60 g, yield 50.9%).; H NMR (500 MHz, CDCl3, δ(ppm)): 7.42 (s,
2H), 7.33 (d,
J = 4.0 Hz, 2H), 7.15 (d, J = 4.0 Hz, 2H), 4.15−4.14 (d, J = 5.5 Hz, 4H),
1.68 (m, 2H), 1.28−1.25 (m, 48H), 0.88−0.87 (m, 12H). MALDI-TOF: m/z = 1025.27
8

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Polymerization.
All four polymers were synthesized by the Stille coupling reaction. Ran-BT and
Reg-BT were prepared by using the monomer (1 equiv). 5,5'-Bis(trimethylstannyl)-
2,2'-bithiophene (1 equiv), Pd2(dba)3, and P(o-tolyl)3 were added to a 2-neck round
bottom flask. Dry toluene (10 mL) was then added and the mixture was degassed.
o
Compound
3
was stirred and refluxed for 24 h at 110 C (Ran-BT) and
7
was reacted
in dry toluene (50 mL) under dilute conditions for 5 h at 110 oC (Reg-BT). Ran-TT
and Reg-TT were prepared by using the monomers (1 equiv); 5,5'-
bis(trimethylstannyl)-2,2'-bithiophene
bis(trimethylstannyl)thieno[3,2-b]thiophene (1 equiv) were added to a 2-neck round
bottom flask in the presence of Pd2(dba)3 and P( -tolyl)3. Dry toluene (50 mL) was
added and the mixture was degassed with argon. Ran-TT and Reg-TT were prepared
(1
equiv)
and
2,5-
o
o
o
under reflux for 48 h at 110 C and 5 h at 110 C, respectively. After cooling to room
temperature, the resulting solution was precipitated into methanol (300 mL). To
remove the by-products and oligomers, Soxhlet extraction was conducted with
acetone, n-hexane, methyl chloride, and chloroform successively. The chloroform
fraction was then precipitated into methanol (300 mL) and dried under vacuum at 40
oC for 24 h. (Ran-BT Mn = 12.3 kDa, PDI = 1.86; Reg-BT Mn = 32.4 kDa, PDI =
1.97; Ran-TT Mn = 33.8 kDa, PDI = 2.01; Reg-TT Mn = 23.5 kDa, PDI = 2.24).
Instrumentation
1
The H NMR spectra of all the compounds were analyzed by using a Bruker 500
MHz spectrometer (Ascend 500, Bruker). The absorption spectra of the small
9

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molecules and polymers in chloroform solution and thin films were recorded using a
UV−vis absorption spectrometer (Agilent 8453, photodiode array,
λ = 190−1100 nm).
The electrochemical properties were character-ized by cyclic voltammetry (CV, eDAQ
EA161) with an electrolyte solution comprising tetrabutylammonium
hexafluorophosphate (Bu4NPF6) in acetonitrile. A platinum wire and Ag/AgCl were
used as the counter and reference electrodes, respectively. The number average
molecular weight (Mn) and polydispersity index (PDI) of the polymers were measured
relative to polystyrene (PS) standards by gel permeation chromatography (GPC) with
chloroform as the eluent using an Agilent GPC 1200 series instrument. Grazing
incidence wide-angle X-ray diffraction (GIWAXD) measurements were conducted at
the 9A (U-SAXS) beamline (energy = 11.05 keV, pixel size = 79.59 µm, wavelength
(λ) = 1.12199 Å, 2θ = 0−20°) of the Pohang Accelerator Laboratory (PAL). Samples
were prepared by spin-coating the molecule solutions onto a SiO2 wafer. The surface
morphologies of the films were explored using an atomic force microscope (XE-100,
Advanced Scanning Probe Microscope, PSIA) with a silicon cantilever. The
morphology of each polymer film surface was investigated by atomic force
microscopy (AFM; Advanced Scanning Probe Microscope, XE-100, PSIA).
Transmission electron microscopy (TEM) imaging was performed using a Tecnai
G2F30 transmission electron microscope (FEI Inc.; accelerating voltage = 300 kV).
Fabrication of Polymer Solar Cell
Inverted solar cells were fabricated with the ITO (150 nm)/ZnO (30
nm)/polymer:ITIC /MoO3 (10 nm)/Ag (100 nm) configuration. The pre-patterned ITO
10

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glass (150 nm, R = 10.0 Ω cm-2) was washed with acetone, deionized water, and
isopropyl alcohol for 10 min each in the stated order. A ZnO layer was spin-coated on
the washed ITO glass at 3000 rpm for 40 s after UV-ozone processing for 20 min and
then annealed at 165 °C for 1 h. The substrates were then transferred into a N2
protected glove box. The polymer:ITIC blends were obtained by spin coating the
chloroform solution (1.0 wt%) with a 1:1 weight ratio of the polymer and ITIC. The
blend solutions were stirred overnight at 60 °C and spin coated onto an ITO/ZnO
substrate at 3000 rpm for 40 s. Then, the spin coated films was annealed at 100 oC for
10 min. Finally, MoO3 (10 nm) and Ag (100 nm) were deposited on the photoactive
layer by thermal evaporation. The current density−voltage (J−V) characteristics were
measured by using a Keithley 2400 source meter under AM 1.5 G illumination at 100
mW cm−2 (Oriel, 1000 W). The incident light intensity was measured by using a
calibrated broadband optical power meter (Spectra Physics, model 404). EQE spectra
were recorded with a K3100 EQX instrument with a K240 XE300 lamp source. The
optimized device is illustrated in Fig. S1
.
Space-charge-limited current (SCLC) method for measuring charge carrier mobility
The structures of the hole only devices (HODs) and electron only devices (EODs)
were ITO/PEDOT:PSS/active layer/Au and ITO/ZnO/active layer/LiF/Al,
respectively. Each active layer was fabricated using the optimum conditions described
for the PSCs. The thickness of the active layers was adjusted to be 110 ± 5 nm. After
spin-coating the solution, Au or LiF/Al was deposited on top of the blend film under
vacuum. The hole and electron carrier mobilities were determined using the modified Mott-
11

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Gurney equation, J = (9/8)ε0εrµ(V2/L3), where J is the current density, µ is the mobility, and
V is the applied voltage, and the device thickness, L, is defined. ε0 is the vacuum permittivity
and εr is the relative permittivity.33
Result and discussion
Synthesis of a regioisomeric PT-based conjugated polymer
To demonstrate the feasibility of conjugated terpolymers as p-type materials in BHJ-
type PSCs, we synthesized three new PT derivative polymers; Reg-BT, Ran-TT, and
Reg-TT. The Ran-BT was synthesized as a control polymer.34,35 Detailed procedures
for the synthesis of the monomers and polymers are depicted in Scheme 1. By
employing the terpolymer structure and tuning the structural regularity of the polymer
backbone, numerous attempts were made to find PT-based analogues with the optimal
structure for PSCs. First, the CT unit in the repeating group of Ran-TT was arranged
randomly. Reg-TT was also synthesized to regulate the arrangement of the CT
monomeric unit in a regio-regular manner. The TT moiety was then introduced in a
periodic fashion to increase the planarity and rigidity in the direction of the polymer
chain. For TT-containing polymers, intermolecular interactions between the polymer
chains should be enhanced with increasing crystallinity. As shown in Scheme 1, the
binary copolymers (Ran-BT and Reg-BT) and regular terpolymers (Ran-TT and
Reg-TT) were prepared through the Stille coupling reaction using a Pd-catalyst (e.g.
Pd2(dba)3) and P(
o-tolyl)3 as a ligand. The terpolymer Ran-TT was prepared by
polymerizing M3, and M4 in toluene at 110 °C. Reg-TT was also prepared from
3
,
7
3
and M4 in toluene at 110 °C. The control polymer Ran-BT was synthesized from
12

ACCEPTED MANUSCRIPT
and M3 through the same coupling reaction, while Reg-BT was prepared from
7 and
M3. The obtained crude polymers were purified by successive Soxhlet extraction with
acetone, hexane, dichloromethane, and chloroform. After concentrating the chloroform
solution, reprecipitation was performed in methanol. The molecular weights and
polydispersity indices (PDIs) of the polymers were measured by gel permeation
chromatography (GPC) using 1,2-dichlorobenzene (
o
-DCB) as the solvent. The
Ran-BT, and Reg-BT
number average molecular weights (Mn) of Ran-TT Reg-TT
,
,
were 33.8, 23.5, 12.3, and 32.4 kDa, and the PDIs were 2.01, 2.24, 1.86, and 1.97,
respectively. Even though Ran-TT and Reg-TT have the same backbone, they differ
significantly in terms of the regularity of the arrangement of the CT monomer, thus
their solubility will differ due to the difference in the polymer structure. Reg-TT has a
relatively high degree of crystallinity due to the rigid and planar repeating unit
(M3)
(M4)
i
ii
S
R
R
3 (M1)
v
O
O
2
O
O
S
O
S
Br
HO
S
S
S
iii
iv
vi
1
Br
S
S
S
S
R O
O
Br
O
O
6
4
5
7 (M2)
O
O
R
R
R
O
O
vii
vii
S
S
S
S
S
S
n
M3
M3
O
Ran-BT
Reg-BT
O
R
M1 M2
vii
vii
M3, M4
M4
Ran-TT
Reg-TT
Scheme 1. Synthetic procedure of monomers and polymers. (i) Br2, AcOH; (ii) 1-iodo-2-
hexyldecane, K2CO3, DMF; (iii) n-BuLi, Br2, THF; (iv) 1-iodo-2-hexyldecane, K2CO3, DMF; (v)
M3, Pd2(dba)3, P(o-tolyl)3, toluene/DMF (9:1 v/v); (vi) NBS, chloroform/AcOH (1:1 v/v); (vii)
13
Pd2(dba)3, P(o-tolyl)3, toluene. (R = 2-hexyldecyl).

ACCEPTED MANUSCRIPT
structure and the strong intermolecular interactions between the polymer chains.(Fig.
S8) Therefore, it is expected to show poor solubility in various organic solvents.
Moreover, during fabrication of the blend film with ITIC, large polymer domains may
be formed. A regular terpolymer system was introduced to resolve the disadvantages
of Ran-BT and Reg-BT, providing a regular monomer arrangement composed of two
monomeric units. This terpolymer system may provide an avenue for modifying
various physical properties of the polymer, such as the solubility, thermal properties,
and crystallinity, by changing the type of repeating unit and thus the conjugative
effect.36,37
Thermal properties
Differential scanning calorimetry (DSC) thermograms were recorded to observe the thermal
transition behavior of the synthesized polymers. The thermal properties of conjugated
polymers are important factors that can be used to predict the thermal stability of the organic
14
Fig. 1. DSC thermograms of Ran-BT, Reg-BT, Ran-TT, and Reg-TT

ACCEPTED MANUSCRIPT
semiconductor devices in which they are employed. For Reg-BT and Reg-TT polymers, the
glass transition temperatures were 206 °C and 204 °C, respectively, as reproducibly
confirmed in the cooling cycle. In addition, no melting of the polymers was observed
between 25 °C and 330 °C, and no thermal decomposition was observed. The cold
crystallization temperatures (Tc) of Reg-BT and Reg-TT were observed at 215 °C and
214 °C, respectively, as indicated by clear Tc peaks, attributed to the rigid, regular backbone,
in contrast to the Ran-BT and Ran-TT polymers.38,39
Optical and electrochemical properties
To investigate the optical properties of the synthesized PT derivatives and the ITIC acceptor,
UV-vis absorption spectra were acquired in chloroform (CF) solution and in the film state.
The data is summarized in Table 1. The absorption wavelength range for the PT derivatives,
Ran-BT, Reg-BT, Reg-TT, and Ran-TT, was complementary to the absorption range of
Table 1. Optical and electrochemical properties of Ran-BT, Reg-BT, Ran-TT, Reg-TT,
and ITIC.
a
b
c
opt,d
onset
onset
Energy Level (eV)
HOMOe LUMOe
Mn
(kDa)
λpeak
λpeak
λedge
Eg
Eox
(V)
Ered
(V)
Polymer
PDI
(nm) (nm) (nm)
(eV)
12.3 1.86
32.4 1.97
33.8 2.01
23.5 2.24
493
519
539
555
678
552
557
552
558
700
651
649
640
648
766
1.90
0.78
0.82
0.89
0.84
-
-5.23
-5.27
-5.34
-5.29
-5.63
-3.33
-3.34
-3.40
-3.38
-4.00
Ran-BT
Reg-BT
Ran-TT
Reg-TT
ITIC
1.91
1.94
1.91
1.62
-
-
-
-
-
-
-0.45
b
c
d
a chloroform solution, film, Absorption edge of the polymer films. Calculated from the absorption edge of
opt
e
the polymer films: Eg = 1240 / λedge, Calculated according to the equation EHOMO/LUMO = -e (Eox/red + 4.40)
(eV).
ITIC and spanned the range of 300−650 nm for the blend films. Generally, the absorption
spectrum of a regio-regular conjugated polymer in the solution state is red-shifted and much
15

(a)
(b)
1.2
1.0
0.8
0.6
0.4
0.2
0.0
1.2
ACCEPTED MANUSCRIPT
Ran-BT
Reg-BT
Ran-TT
Reg-TT
ITIC
Ran-BT
Reg-BT
1.0
Ran-TT
Reg-TT
ITIC
0.8
0.6
0.4
0.2
0.0
400
500
600
700
800
400
500
600
700
800
Wavelength (nm)
Wavelength (nm)
Fig. 2. Normalized optical absorption spectra of Ran-BT
,
Reg-BT
,
Ran-TT
,
Reg-TT and
ITIC (a) in chloroform solution and (b) in the as-cast film states
.
wider than that of a regio-random conjugated polymer. This suggests that regio-regular
conjugated polymers have a well-ordered, uniform monomer arrangement with strong
intermolecular interactions between the polymer chains.40
Interestingly, in the spectra of the regio-regular copolymers, Reg-BT and Reg-TT
,
shoulder peaks were observed at a wavelength of 608 nm. These were attributed to the
strong intermolecular interactions arising from the regularity of the polymer
backbones. However, as shown in Fig. 2b, all four polymers exhibited similar
absorption peaks in the film form. In the polymer film state, it is common to observe
absorption at longer wavelengths due to intermolecular interactions, as opposed to in
the solution state. The profiles of Ran-BT, Reg-BT, Ran-TT, and Reg-TT displayed
spectral red-shifts of 59, 38, 13, and 3 nm, respectively, and the maximum absorption
was observed at 555−560 nm for all polymers. For the solution-state absorption
spectrum, the shoulder peak at 600 nm was still observed in the film state.41
16

ACCEPTED MANUSCRIPT
Fig. 3. Temperature-dependent absorption (TD-Abs) of the four polymers in chlorobenzene
solution: (a) Ran-BT, (b) Reg-BT, (c) Ran-TT, (d) Reg-TT, and (e) photographic images
of the four polymer solutions taken at 20 oC and 100 oC.
The aggregation behavior of the four polymers in chlorobenzene (CB) solution was
investigated via UV-vis absorption spectroscopy while varying the solution
temperature. Generally, as the solution temperature increased, regio-regular polymers
(
Fig. 3b and 3
d
) showed a much larger hypochromic shift compared to regio-random
17

ACCEPTED MANUSCRIPT
polymers (Fig. 3a and 3c). This is because the regio-regular polymer undergoes
aggregation, even in solution. When the temperature was increased, the intermolecular
interactions were weakened and the spectrum was shifted to the short wavelength
region. Because the interaction between the main chains can be reduced by increasing
the temperature, the main absorption peak is due to an intramolecular π-π* transition.42
Compared with binary copolymers (e.g. Ran-BT and Reg-BT; Fig. 3a and 3b),
terpolymers with the TT moiety (e.g. Ran-TT and Reg-TT) (Fig. 3c and 3d) showed
stronger interchain interactions, allowing the difference between the regio-random and
regio-regular polymers to be investigated in terms of the interchain interactions of the
binary copolymer and terpolymer. Based on these results, it was confirmed that the
intermolecular interactions in the regio-regular polymer were more effective than in
the regio-random polymer, as predicted.43
Electrochemical cyclic voltammetry (CV) was performed to investigate the HOMO
and LUMO energy levels of the conjugated polymers. All CV measurements were
performed in a solution of 0.1 M Bu4NPF6 in acetonitrile. The oxidation and reduction
potentials were calibrated using the ferrocene/ferrocenium (Fc/Fc+) redox couple and
the electrochemical parameters of the Ran-BT
films on a Pt electrode are summarized in Table 1. Detailed CV data and energy levels
are summarized in Fig. S2. The HOMO energy levels of Ran-BT Reg-BT Ran-TT,
, Reg-BT, Ran-TT, Reg-TT, and ITIC
,
,
Reg-TT, and ITIC were -5.23, -5.27, -5.34, -5.29, and -5.63 eV, respectively, and the
corresponding LUMO energy levels were calculated to be -3.33, -3.34, -3.40, -3.38,
and -4.00 eV, respectively. The terpolymers, Ran-TT and Reg-TT, have lower-lying
HOMO energy levels than do the binary copolymer systems, with Ran-TT having the
18

ACCEPTED MANUSCRIPT
lowest HOMO energy value (-5.34 eV). The PSC prepared using Ran-TT as the donor
polymer and ITIC as the acceptor should have a relatively high Voc
.
(a)
(b)
2
0
Ran-BT:ITIC
Reg-BT:ITIC
Ran-TT:ITIC
Reg-TT:ITIC
Ran-BT:ITIC
Reg-BT:ITIC
Ran-TT:ITIC
Reg-TT:ITIC
14
12
10
8
60
50
40
30
20
10
0
-2
-4
-6
6
-8
4
-10
-12
-14
2
0
0.0
0.2
0.4
0.6
0.8
400
500
600
700
800
Voltage (V)
Wavelength (nm)
Fig. 4. (a) Representative J-V characteristics and (b) external quantum efficiency (EQE)
spectra and integrated Jsc of BHJ-PSC devices using the polymer:ITIC blend films (1:1,
w/w ; annealed at 100 oC for 10 min) as the active layer.
Performance of PSCs with ITIC
To investigate the influence of the regio-regularity and terpolymer structure on the
photovoltaic performance of the PT derivatives, PSC devices were fabricated with an
inverted device structure: ITO/ZnO (30 nm)/BHJ active layer/MoO3 (10 nm)/Ag (100
nm), where the polymers Ran-BT, Reg-BT, Ran-TT, and Reg-TT were used as
electron donors and ITIC was used as a fullerene-free acceptor. The blend composition
was optimized with a 1:1 donor/ITIC weight ratio. The blend film made using
chloroform exhibited the best performance in the PSCs. Fig. 4a shows the current-
density versus voltage (J−V) curves measured under AM 1.5 G illumination (100 mW
cm-2). Representative characteristics of the inverted PSCs, such as the Voc
, Jsc, fill
factor (FF), and PCE, are summarized in Table 2. The device fabricated with Ran-BT
19

ACCEPTED MANUSCRIPT
in the absence of any solvent additive showed a Voc of 0.70 V, a Jsc of 6.98 mA cm-2,
an FF of 56.5%, and a PCE of 2.75%. The device with Reg-BT exhibited a Voc of 0.72
V, a Jsc of 8.57 mA cm-2, an FF of 43.7%, and a PCE of 2.70%. Due to the high
crystallinity of Reg-BT, the FF was lowered while the Voc and Jsc increased slightly,
resulting in unprecedentedly similar PCEs. The devices made with Ran-TT and Reg-
TT displayed a Voc of 0.81 and 0.75 V, Jsc of 12.57 and 7.62 mA cm-2, FF of 55.3 and
42.6%, and PCE of 5.65 and 2.43%, respectively. Although three of the polymers
showed similar efficiencies, it is noteworthy that the Jsc of the device fabricated with
Ran-TT was almost twice as high as that of the other polymers, giving the best
efficiency.
In order to observe the characteristics of the PSC manufactured in this study more
closely, the device was fabricated under various conditions to obtain the J-V curves
(
Fig. S3-S6) and the photovoltaic parameters (Table S1-S4) were shown in the
support information. The data in Table S1 demonstrates that the PSC devices
fabricated using CB as a solvent had relatively low FF, Jsc, and PCE values compared
to the devices made by employing CF as a solvent. However, same tendency could be
observed that regio-random polymer based PSCs displayed higher performance than
the devices with regio-regular polymers. In brief, the BHJ photovoltaic performance of
Ran-TT based PSC was superior to that of the other polymers.
The Ran-TT:ITIC blend active layer showed the highest PCE of 5.32% without
thermal annealing. For the PSC fabricated by thermal annealing at 100 °C for
modification of the internal structure of the active layer, the FF increased and the
measured PCE was 5.65%, which is higher than that without thermal annealing. The
20

ACCEPTED MANUSCRIPT
high Jsc value of the PSC was confirmed by external quantum efficiency (EQE)
spectroscopy (Fig. 4b). As predicted, the EQE spectrum generally matched the light
absorption spectrum of the corresponding blend films for the active layers. The
integrated Jsc values of the devices are well consistent with those obtained from Fig.
4b. The EQE spectra of all blend films showed a broad, strong band in the 300−800
nm region due to the complementarity between the absorption spectra of the PT
derivatives and ITIC. The maximum EQE values of the PSCs based on Ran-BT
, Reg-
BT Ran-TT, and Reg-TT were 36, 42, 58, and 37%, respectively. The highest EQE
,
Table 2. Photovoltaic properties of PSCs made from chloroform solution with a
ITO/ZnO/polymer:ITIC (1:1, w/w)/MoO3/Ag device structure measured under AM 1.5G
illumination (100 mW cm-2); The active layers were annealed at 100 oC for 10 min.
Voc
(V)
Jsc
FF
(%)
PCE
(%)
Integrated Jsc
Donor
Ran-BT
Reg-BT
Ran-TT
Reg-TT
Acceptor
(mA cm-2)
(mA cm-2)
0.70
(0.71±0.01)
6.98
(6.93 ±0.05)
56.5
(51.2±5.30)
2.75
(2.65±0.10)
6.93
8.49
0.72
(0.73±0.01)
8.57
(8.12±0.45)
43.7
(40.9±2.10)
2.70
(2.44±0.26)
ITIC
0.81
12.63
55.3
(52.2±3.10)
5.65
(4.98±0.67)
12.46
7.55
(0.80±0.01) (11.72±0.91)
0.75
(0.74±0.01)
7.62
(7.33±0.29)
42.6
(39.9±2.65)
2.43
(2.15±0.28)
Average values and standard deviations were obtained from more than 10 devices. They are in parentheses.
values observed with the Ran-TT-based PSCs indicated highly efficient photon and
charge collection in the active layer.
We also investigated the charge mobilities of the single carrier devices such as hole-
only device (HOD) and electron-only device (EOD) to understand the different PSC
performance described above. The hole and electron mobilities of the blend films were
measured using the space-charge limited current (SCLC) method. HOD and EOD
21

ACCEPTED MANUSCRIPT
properties were summarized in Fig. S7 and Table S5. Compared to the blends film of
regio-regular polymers (e.g. Reg-BT and Reg-TT) and ITIC, the blend films of regio-
random polymers and ITIC exhibited better hole and electron charge balance, which
may contribute to the higher FF values of the corresponding PSCs.44,45 In particular,
Reg-TT:ITIC blend film displays the worst charge mobility balance, resulting in the
lowest PCE in the corresponding PSC.
Morphological studies of the blend film of the polymer and ITIC
22

ACCEPTED MANUSCRIPT
Fig. 5. (a−d) AFM height images, (e−h) phase images, and (i−l) TEM images of
polymer:ITIC (1:1) blend films. Ran-BT:ITIC (a, e, i), Reg-BT:ITIC (b, f, j), Ran-
TT:ITIC (c, g, k), Reg-TT:ITIC (d, h, l); all samples were annealed at 100 oC for 10 min.
Atomic force microscopy (AFM) and transmission electron microscopy (TEM) were
employed to investigate the internal and surface morphology of the polymer:ITIC
blend films prepared from chloroform after annealing at 100 °C for 10 min.46
Fig. 5a−h shows height and phase images of the optimized blend films obtained
using AFM. Overall, the regio-random polymers (e.g. Ran-BT and Ran-TT) showed
a finer morphology than the regio-regular polymers (Reg-BT and Reg-TT). The
images of Reg-BT in Fig. 5b and
square (RMS) roughness, which was attributed to severe aggregation due to the unique
structure of the polymer. In contrast, the images of Ran-TT in Fig. 5c and show
f show a very coarse surface with a high root-mean-
g
desirably fine phase separation and a highly homogeneous interpenetrating network,
23

ACCEPTED MANUSCRIPT
which is beneficial for exciton diffusion/dissociation and charge carrier transport, and
a low RMS roughness value.47 It was deduced that the suitable morphology of the
Ran-TT:ITIC film lead to PSCs with relatively high performance.
TEM measurements were performed to monitor the internal morphology of the four
polymers and ITIC mixtures prepared under optimal conditions. As shown in Fig.
5i−l, the internal morphology of Reg-BT comprised large aggregates, similar to those
observed in the AFM surface images. In contrast, the fine internal morphology of the
Ran-TT films was observed. The latter is much more advantageous for exciton
diffusion, charge separation, and charge transport within the active layer of the PSC
due to the effective formation of small charge migration channels. These results
explained the higher Jsc values of the Ran-TT:ITIC-based PSC relative to those of the
PSCs employing the other polymers.
Conclusion
In conclusion, we successfully synthesized two regioisomeric π-conjugated
terpolymers bearing thiophene carboxylate, bithiophene, and thienothiophene (e.g.
Ran-TT and Reg-TT). Conjugated polymers with thiophene carboxylate showed
significant differences in their absorption properties, HOMO / LUMO energy levels,
and bandgap compared to polythiophene only with neat thiophene. This study
confirmed that the characteristics of binary conjugated copolymers can be tuned by
synthesizing conjugated terpolymers with a third monomer (e.g. TT) in the repeating
group. Regio-regular and regio-random terpolymers also displayed different
absorption properties and HOMO / LUMO energy levels. Of the four polymers
24

ACCEPTED MANUSCRIPT
synthesized, the Ran-TT terpolymer exhibited a lower-lying HOMO level (-5.34 eV)
than conventional polythiophene derivatives, and was expected to show a higher Voc
value in the corresponding BHJ-type PSCs.
Among the PSC devices fabricated in this study, the Ran-TT:ITIC-based PSC
exhibited the highest PCE of 5.65% with a high short circuit current density of 12.63
mA cm-2 and an open circuit voltage of 0.81 V. The results of this study confirmed
that the internal morphology of the blend films of ITIC and regio-random terpolymer
facilitated exciton diffusion and charge migration, in contrast to that of regio-regular
polymer-based blend films. This gives a higher short circuit current density, which is
advantageous for increasing the efficiency of the PSC.
Acknowldgements
This research was supported by the National Research Foundation of Korea
(NRF2015R1A2A1A05001876) and by the Key Research Institute Program
(NRF20100020209). We are grateful to Pohang Accelerator Laboratory (Pohang,
Korea) for allowing us to conduct the grazing incidence wide angle X-ray diffraction
(GIWAXD) measurements.
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