Enhanced open-circuit voltages of trifluoromethylated quinoxaline-based polymer solar cells

Article abstract of DOI:10.1016/j.orgel.2018.11.022

Three quinoxaline-based conjugated polymers with donor-π-acceptor configurations have been synthesized by Stille coupling reaction. The electron-donating 2,3-dioctylthienyl- substituted benzodithiophene (BDT) unit was linked to the electron-accepting 2,3-

Full text of DOI:10.1016/j.orgel.2018.11.022

Accepted Manuscript
Enhanced open-circuit voltages of trifluoromethylated quinoxaline-based polymer
solar cells
Sella Kurnia Putri, Ho Cheol Jin, Dong Ryeol Whang, Joo Hyun Kim, Dong Wook
Chang
PII:
S1566-1199(18)30600-1
DOI:
https://doi.org/10.1016/j.orgel.2018.11.022
ORGELE 4985
Reference:
To appear in: Organic Electronics
Received Date: 17 July 2018
Revised Date: 2 October 2018
Accepted Date: 13 November 2018
Please cite this article as: S.K. Putri, H.C. Jin, D.R. Whang, J.H. Kim, D.W. Chang, Enhanced open-
circuit voltages of trifluoromethylated quinoxaline-based polymer solar cells, Organic Electronics (2018),
doi: https://doi.org/10.1016/j.orgel.2018.11.022.
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ACCEPTED MANUSCRIPT
Graphical Abstract
A series of trifluoromethylated quinoxaline-based polymers were systematically synthesized
by Stille coupling reaction. The results obtained in this study clearly reveal the significant
contribution of the electron-withdrawing trifluoromethyl groups in enhancing the open-circuit
voltages, as well as producing a sharp improvement in the power conversion efficiencies of
polymer solar cells with bulk-heterojunction structure.

ACCEPTED MANUSCRIPT
Enhanced Open-Circuit Voltages of Trifluoromethylated Quinoxaline-Based Polymer Solar
Cells
Sella Kurnia Putri,a† Ho Cheol Jin,b† Dong Ryeol Whang,c Joo Hyun Kim,*, b and Dong Wook
Chang*, a
aDepartment of Industrial Chemistry, Pukyong National University, 48547 Busan, Republic of
Korea.
E-mail: dwchang@pknu.ac.kr
bDepartment of Polymer Engineering, Pukyong National University, 48547 Busan, Republic of
Korea.
E-mail: jkim@pknu.ac.kr
cLinz Institute for Organic Solar Cells (LIOS)/Institute of Physical Chemistry, Johannes Kepler
University Linz, 4040 Linz, Austria.
† S. K. Putri and H. C. Jin equally contributed to this research.
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Abstract
Three quinoxaline-based conjugated polymers with donor-π-acceptor configurations have
been synthesized by Stille coupling reaction. The electron-donating 2,3-dioctylthienyl-
substituted benzodithiophene (BDT) unit was linked to the electron-accepting 2,3-
diphenylquinoxaline (DPQ) group through a thiophene bridge, to produce a reference
polymer PTBDT-Qx. Furthermore, the strong electron-withdrawing trifluoromethyl
moieties were introduced at the para-position of the phenyl groups in the 2,3-positions of
DPQ and 6,7-difluorinated DPQ, to afford PTBDT-QxCF3 and PTBDT-FQxCF3
,
respectively. Owing to the continuous reduction in their HOMO energy levels with
increasing number of electron-withdrawing groups, the open-circuit voltage (Voc) in
polymer solar cells (PSCs) shows a gradual improvement in the order of PTBDT-Qx
<
PTBDT-QxCF3 PTBDT-FQxCF3. The inverted-type PSC based on PTBDT-FQxCF3
<
with a configuration of ITO/ZnO/polymer:PC71BM/MoO3/Al provides the best power
conversion efficiency of 6.47%, together with a Voc of 0.99 V, a short-circuit current of
10.03 mA/cm2, and a fill factor of 65.1%.
Keywords. Quinoxaline; electron-withdrawing; 2,3-diphenylquinoxaline; trifluoromethyl; polymer
solar cells
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1. Introduction
Polymer solar cells (PSCs) based on bulk heterojunction (BHJ) structures between polymeric
semiconductors and fullerene derivatives have attracted significant attention in the last decades, due
to their unique advantages such as low cost, light weight, easy preparation, and outstanding
flexibility [1, 2]. As a result of continuous efforts to optimize the material design as well as the
device fabrication process, the power conversion efficiencies (PCEs) of state-of-the-art PSCs can
exceed 10% [3-6]. In order to prepare conjugated polymers appropriate for high-performance PSCs,
various studies have attempted to incorporate alternating electron-donating (D) and electron-
accepting (A) units along the polymer backbones [7]. In this D-A configuration, the band gap of the
polymers can be significantly reduced by the facile formation of an intramolecular charge transfer
(ICT) state. Furthermore, important parameters of the D-A polymers including energy levels, band
gap and carrier mobility can be easily tuned by the selection and combination of the suitable D and
A components [8]. Among various building blocks for constructing the D-A polymers, the
quinoxaline (Qx) unit has attracted attention as a promising moiety for the A component, due not
only to its strong electron-withdrawing capability but also to other beneficial features such as simple
synthesis and a rigid/flat conjugated structure [9-11].
Increasing the open-circuit voltage (Voc) by controlling the energy levels of the conjugated
polymers in the active layer of PSCs is currently considered one of the critical tasks for improving
the photovoltaic properties of these systems [12]. The Voc of BHJ PSCs is usually determined as the
difference between the lowest unoccupied molecular orbital (LUMO) of the electron acceptor and
the highest occupied molecular orbital (HOMO) of the electron donor [13]. Therefore, the
development of polymeric donors with a low-lying HOMO energy level has become a task of great
importance. In this regard, the simple strategy involving the incorporation of electron-withdrawing
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substituents such as fluorine into the polymer backbone has been intensively investigated [14-18].
Compared to their non-fluorinated counterparts, fluorinated polymers exhibit a substantial reduction
in both LUMO and HOMO without significant alteration of their band gaps, which implies an
enhanced Voc value when applied in PSCs. Furthermore, the energy levels and photovoltaic
properties of D-A type polymers are strongly influenced by the location and concentration of F
atoms in their structures [19-21].
Besides fluorine atoms, the introduction of strong electron-withdrawing trifluoromethyl (CF3)
groups in the polymer chains can also lead to a sharp improvement in the Voc of PSCs, through the
efficient lowering of the HOMO energy level of the polymers [22, 23]. In particular, the CF3 groups
not only possess a higher electron-withdrawing capability than fluorine atoms but also impart
unique advantages to the resultant polymers such as high thermal and chemical stability, along with
good solubility [24]. The Hammett constants (σ, which are important parameters to determine the
electronic characteristics of the substituents) of CF3 and F at the para-position (σp) of benzene rings
were determined to be 0.54 and 0.06, respectively [25]. For example, the presence of the CF3 unit
either on the electron-withdrawing or electron-donating moiety in D-A polymers can induce the
sharp increase in the Voc value of the related PSCs [22, 23, 26, 27].
Herein, we synthesized a series of D-A polymers, in which the electron-donating 2,3-
dioctylthienyl substituted-benzodithiophene (BDT) unit was connected to the electron-withdrawing
2,3-diphenylquinoxaline (DPQ) derivatives through a thiophene bridge. Owing to advantageous
features including high electron affinity, simple preparation, and facile modification, the DPQ unit
was selected as one of basic building blocks for the preparation of reference PTBDT-Qx polymer
(Figure 1). It has also been reported that the incorporation of the electron-withdrawing F atoms at
the para-position of the phenyl rings located in the 2,3-positions of DPQ can efficiently enhance the
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Voc of the corresponding BHJ PSCs [19, 28]. Inspired by these results, strong electron-withdrawing
CF3 moieties (in alternative to fluorine atom) were incorporated at the same positions of DPQ and
6,7-difluorinated DPQ, to afford the target polymers PTBDT-QxCF3 and PTBDT-FQxCF3,
respectively (Figure 1). Owing to the presence of the strong electron-withdrawing moieties,
PTBDT-QxCF3 and PTBDT-FQxCF3 provide markedly different optical and electrochemical
properties compared to the reference PTBDT-Qx polymer. In particular, the Voc of inverted-type
PSCs based on the three polymers shows a gradual increase in the order of PTBDT-Qx < PTBDT-
QxCF3 < PTBDT-FQxCF3, with corresponding values of 0.71, 0.91, and 0.99 V, respectively. The
increasing trend of Voc is also in good agreement with the progressive increase in the PCEs of PSCs
prepared using PTBDT-Qx, PTBDT-QxCF3, and PTBDT-FQxCF3 (2.61, 4.61, and 6.47%,
respectively).
Figure 1. Chemical structures of PTBDT-Qx, PTBDT-QxCF3, and PTBDT-FQxCF3.
2. Results and Discussion
2.1. Synthesis and characterization
The synthetic approaches used to prepare monomers and polymers are illustrated in Scheme 1.
First,
the
dithiophene-attached
benzothiadiazole
derivatives
of
4,7-di(thiophen-2-
yl)benzo[c][1,2,5]thiadiazole (1) and 5,6-difluoro-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole (2)
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were converted to the associated DPQ units through zinc-mediated reduction followed by a
condensation process between the intermediate
of 1 with benzyl and 1,2-bis(4-(trifluoromethyl)phenyl)ethane-1,2-dione afforded 2,3-diphenyl-5,8-
di(thiophen-2-yl)quinoxaline (3) and 5,8-di(thiophen-2-yl)-2,3-bis(4-
ο-diamine and α-diketone (Scheme 1). The reaction
(trifluoromethyl)phenyl)quinoxaline (4), respectively. Moreover, the DPQ possessing two CF3
groups and two F atoms on its 2,3- and 6,7-positions, respectively, (5) was successfully produced by
the reaction between 2 and 1,2-bis(4-(trifluoromethyl)phenyl)ethane-1,2-dione. Finally, bromination
of 3, 4, and 5 using N-bromosuccinimide (NBS) yielded the dibrominated DPQ-based monomers 6,
7, and 8, respectively.
Scheme 1. Synthesis of monomers and polymers: (i) Zn/acetic acid, at 100 °C for 12 h; (ii) α-
diketone/acetic acid, reflux for 12 h; (iii) NBS, room temperature (RT) overnight; (iv)
tetrakis(triphenylphosphine)palladium(0) (Pd(PPh3)4), at 90 °C for 48 h.
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In order to prepare conjugated polymers with a typical D-A configuration, the electron-donating
2,3-dioctylthienyl-substituted BDT monomer (9) was polymerized with the electron-withdrawing
DPQ-based monomers 6, 7, and 8 under Stille coupling conditions affording the PTBDT-Qx,
PTBDT-QxCF3, and PTBDT-FQxCF3 target polymers, respectively. The number average
molecular weights (Mn) of PTBDT-Qx, PTBDT-QxCF3, and PTBDT-FQxCF3, recorded by gel
permeation chromatography (GPC), were 23.05, 20.21, and 14.61 KDa, respectively, with
polydispersity indexes of 2.74, 2.86, and 2.83, respectively. In addition, the three polymers
exhibited not only good solubility in common organic solvents such as chloroform, tetrahydrofuran
(THF), and toluene, but also high thermal stability. The onset decomposition temperature at 5%
weight loss (Td5%) for all polymers could reach up to 415°C (Figure 2).
Figure 2. Thermogravimetric analysis (TGA) thermograms of PTBDT-Qx, PTBDT-QxCF3, and
PTBDT-FQxCF3 at a heating rate of 10 °C/min under N2.
2.2.Optical and electrochemical properties
The UV-Vis absorption spectra of PTBDT-Qx, PTBDT-QxCF3, and PTBDT-FQxCF3
measured for films grown on a glass substrate are shown in Figure 2a, and the corresponding data
are summarized in Table 1. Two distinct UV-Vis absorption bands at 400–490 and 500–750 nm
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were observed for all polymers. The peak at shorter wavelengths (400–490 nm) corresponds to the
localized π-π* transition of polymer chains, while the peak in the longer wavelength region (500–
750 nm) is related to the ICT between the D and A components in the polymer backbone. Interestin
gly, the absorption maxima of PTBDT-QxCF3 and PTBDT-FQxCF3 at longer wavelengths were s
hifted to the higher-energies compared to the corresponding peak of PTBDT-Qx. These results wer
e attributed to the increased band gap caused by the significant reduction in the HOMO levels of PT
BDT-QxCF3 and PTBDT-FQxCF3 (vide infra). The similar blue-shift in absorption spectra have b
een also reported for other conjugated polymers possessing electron-withdrawing groups [22, 29]. In
addition, weak shoulders at ca. 650 nm were observed in the absorption spectra of PTBDT-QxCF3
and PTBDT-FQxCF3, respectively, due to the forced intermolecular aggregation through existing
F-F and F-H interactions [30, 31]. The optical band gaps of PTBDT-Qx, PTBDT-QxCF3, and
PTBDT-FQxCF3 determined from the absorption edges were 1.68, 1.69, and 1.71 eV, respectively
(Table 1).
Cyclic voltammetry (CV) measurements were carried out to investigate the energy levels of the
polymers, and the results are shown in Figure 2b. The HOMO energy level of the polymers was
calculated from the onset potential of oxidation by assuming the absolute energy level of ferrocene
is 4.8 eV below the vacuum level. The HOMO energy levels of PTBDT-Qx, PTBDT-QxCF3, and
PTBDT-FQxCF3, calculated from the oxidation onset potential of the cyclic voltammograms with
a ferrocene(Fc)/ferrocenium(Fc+) external standard were -5.19, -5.47, and -5.57 eV, respectively.
Upon incorporation of the strong electron-withdrawing CF3 moieties into the polymer structure, the
HOMO energy of PTBDT-QxCF3 (-5.47 eV) was significantly reduced compared to that of
PTBDT-Qx (-5.19 eV). A further slight decrease of the HOMO energy (from -5.47 to -5.57 eV) was
observed for PTBDT-FQxCF3, due to the additional introduction of two F atoms. In addition, the
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LUMO energy levels of PTBDT-Qx, PTBDT-QxCF3, and PTBDT-FQxCF3, calculated from the
difference between HOMO level and the optical band gap, were -3.55, -3.78, and -3.86 eV,
respectively. The electrochemical parameters of all polymers are also listed in Table 1. Again, the
overall results of the UV-Vis and CV measurements highlight the marked effect of the electron-
withdrawing CF3 unit on the optical and electrochemical properties of DPQ-based polymers.
Figure 3. (a) UV-visible spectra of polymer films on a glass substrate (spectra are offset for clarity)
and (b) cyclic voltammograms of polymers.
Table 1. Summary of optical and electrochemical properties.
ꢋꢌꢍꢉ
(nm)c
ꢀ
ꢁꢂꢃꢁ(nm)a, ꢄꢃꢇꢅꢆꢆꢈ(eV)b
HOMO (eV)c
-5.19
LUMO (eV)d
-3.55
ꢀ
ꢉꢅꢊ
738, 1.68
441, 622
443, 597
PTBDT-Qx
733, 1.69
-5.47
-3.78
PTBDT-QxCF3
PTBDT-
FQxCF3
725, 1.71
441, 601
-5.57
-3.86
a
b
c
absorption edge of polymer film; estimated from the absorption edge; maximum absorption
wavelength of the film; c estimated from the oxidation onset potential; d estimated from the HOMO
and the optical band gap.
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2.3.Theoretical calculations
To investigate the electronic structures and frontier molecular orbitals of PTBDT-Qx, PTBDT-
QxCF3, and PTBDT-FQxCF3, density functional theory (DFT) calculations were performed at the
B3LYP/6-31G** level using the Gaussian 09 program [32]. For simplicity, the octyl side chains of
the polymers were replaced by short methyl groups, and a two-repeating unit model was adopted in
calculations. As shown in Figure 4, the electron density of the HOMO is almost completely
delocalized along the polymer backbone, while that of the LUMO is localized on the electron-
withdrawing DPQ units. The calculations also show that the introduction of the strong electron-
withdrawing CF3 substituents significantly lowers both the HOMO and LUMO energy levels of the
polymers (from -4.73 and -2.43 eV for PTBDT-Qx, respectively, to -4.93 and -2.71 eV for
PTBDT-QxCF3, respectively). Moreover, the incorporation of two additional fluorine atoms in the
DPQ unit to afford PTBDT-QxCF3 causes further reduction in the HOMO and LUMO energy
levels (-4.95 and -2.80 eV). The trends in the theoretical HOMO and LUMO levels of the polymers
agree well with determined from the experimental results (Table 1).
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Figure 4. Frontier molecular orbitals of two-repeating unit models with HOMO and LUMO energy
levels calculated at the B3LYP/6-31G** level for (a) PTBDT-Qx, (b) PTBDT-QxCF3, and (c)
PTBDT-FQxCF3.
2.4. Photovoltaic properties
The photovoltaic properties of the polymers were investigated by fabricating inverted-type PSCs
with indium tin oxide(ITO)/ZnO(25 nm)/active layer(polymer: [6,6]-Phenyl C71 butyric acid methyl
ester (PC71BM))(80 nm)/MoO3(10 nm)/Ag(100 nm) structure. The energy levels of all materials
used for the device fabrication, shown in Figure 5, demonstrate the efficient charge separation and
charge transport process in the PSCs. We tested different polymer:PC71BM weight rations (from 3:2
to 3:7), in the active layer of BHJ PSC, and the optimum blend ratios of PTBDT-Qx, PTBDT-
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QxCF3, and PTBDT-FQxCF3 to PC71BM were determined to be 3:3, 3:5, and 3:5, respectively
(Table S1).
Figure 5. Energy level diagram of all materials used for the fabrication of inverted-type PSCs.
Figure 6a shows the optimized current density-voltage (J-V) curves of the polymers with the
highest PCEs under 1.0 sun condition (the inset shows the curves obtained under dark conditions),
and the corresponding device parameters are summarized in Table 2. Interestingly, the PCEs of the
polymers gradually increased in the order of PTBDT-Qx (2.61%) < PTBDT-QxCF3 (4.61%) <
PTBDT-FQxCF3 (6.47%). Moreover, the trend in the PCE values is strongly correlated with the
similar gradual increase in the Voc of the PSCs. The Voc values measured for the devices fabricated
with PTBDT-Qx, PTBDT-QxCF3, and PTBDT-FQxCF3 were 0.71, 0.91, and 0.99 V,
respectively. Therefore, the introduction of CF3 groups at the para-position of the phenyl rings
located in the 2,3-positions of DPQ moieties in the polymer structure can induce a sharp increase in
Voc, of about 28% relative to the original value (from 0.71 V for PTBDT-Qx to 0.91 V for PTBDT-
QxCF3). A similar improvement in the Voc of the corresponding PSCs was obtained upon the
incorporation of fluorine atoms at the same positions of DPQ moieties of D-A type polymers [19].
The attachment of fluorine atoms (in addition to the CF3 groups) on the 6,7-positions of DPQ,
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affording PTBDT-FQxCF3 resulted in a further enhancement in Voc up to 0.99 V. The observed
changes in the Voc values of PSCs in good agreement with the HOMO levels of the applied
polymers (vide supra). In addition to the Voc values, the incorporation of the electron-withdrawing
groups in the polymer skeleton also improve fill factors (FFs) of the devices, and the highest value
of 65.1% was achieved for the PSC fabricated with PTBDT-FQxCF3. The device based on
PTBDT-FQxCF3 also displayed the highest short-circuit current (Jsc) value of 10.03 mA/cm2,
relative to those of the devices based on PTBDT-Qx (9.13 mA/cm2) and PTBDT-QxCF3 (8.38
mA/cm2). Overall, these results show that the incorporation of the strong electron-withdrawing CF3
groups in the polymer backbone is highly beneficial for the photovoltaic properties of PSCs, mainly,
through a significant enhancement in the Voc and FF values. As shown in Figure 6b, the incident
photon-to-current efficiency (IPCE) curves of all PSCs are well consistent with the absorption
behavior of the polymers in the range of 300–800 nm, and provide an adequate explanation for the
trend of the Jsc values. The calculated Jsc values based on incident photon-to-current efficiency
(IPCE) spectra showed very good agreement with the Jsc data of the devices under 1.0 sun.
Furthermore, the series resistance (Rs) of the devices was calculated from the J-V curves under dark
conditions (inset of Figure 6a). was calculated. The Rs of the device based on PTBDT-Qx (9.61
cm2) was much larger than the values measured for the devices based on PTBDT-QxCF3 (3.36
Ω
Ω
cm2) and PTBDT-FQxCF3 (2.54
Ω
cm2) (Table 2). Therefore, the Rs values are gradually
decreased with increasing number of electron-withdrawing groups on the DPQ unit, in good
agreement with the photovoltaic properties of the devices.
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Figure 6. (a) Current density vs. voltage curves of PSCs with optimum blend ratio of polymeric
donors to PC71BM, showing the best performances under 1.0 sun condition (inset: under dark
conditions); (b) IPCE spectra of PSCs based on PTBDT-Qx (squares), PTBDT-QxCF3 (circles),
and PTBDT-FQxCF3 (triangles)
.
Table 2. Best photovoltaic parameters of the PSCs. The average photovoltaic parameters of each
device are shown in parentheses.
Calculated Jsc
Jsc (mA/cm2)
Voc (V)
FF (%)
PCE (%) Rs (Ω cm2)b
(mA/cm2)a
9.13
0.71
40.3
2.61
9.61
9.07
PTBDT-Qx
(9.05)
(0.71)
(39.7)
(2.55)
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8.38
(8.07)
10.03
0.91
(0.91)
0.99
60.5
4.61
(4.47)
6.47
8.43
3.36
2.54
PTBDT-QxCF3
PTBDT-FQxCF3
(60.9)
65.1
10.11
(10.04)
(0.99)
(64.0)
(6.46)
acalculated from the IPCE curves, bSeries resistance estimated for the corresponding best device.
To investigate the charge carrier transport properties of the polymers, we fabricated and tested
hole- and electron-only devices with ITO/PEDOT:PSS (35 nm)/ polymer:PC71BM (~ 90 nm)/Au
(50 nm) and ITO/Al (50 nm)/polymer:PC71BM (~ 90 nm)/Al (100 nm) structures, respectively. As
shown in figure 7a and 7b, the J-V curves of all polymers exhibited space charge-limited current
(SCLC) behavior, and could be fitted by the well-known Mott-Gurney law [33]. We calculated the
charge mobility using the permittivity of 3.9 for the active layer. The hole mobilities of the devices
based on PTBDT-Qx, PTBDT-QxCF3, and PBDT-FQxCF3 were 6.03 × 10-4, 5.61 × 10-4, and
6.29 × 10-4 cm2V-1s-1, respectively. In addition, the electron mobilities of the devices fabricated with
PTBDT-Qx, PTBDT-QxCF3, and PBDT-FQxCF3 were 6.00 × 10-4, 4.98 × 10-4, and 7.25 × 10-4
cm2V-1s-1, respectively. The hole and electron mobilities of PBDT-FQxCF3 were higher than those
of PTBDT-Qx and PTBDT-QxCF3. This result is also consistent with the better performances of
the PTBDT-FQxCF3-based device compared with those of based on PTBDT-Qx and PBDT-FQx.
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Figure 7. Current density vs. voltage curves of (a) hole-only (ITO/PEDOT:PSS (35 nm)/
polymer:PC71BM (~ 90 nm)/Au (50 nm)) and (b) electron-only (ITO/Al (20 nm)/polymer:PC71BM
(~ 90 nm)/Al (100 nm)) devices based on PTBDT-Qx (squares), PTBDT-QxCF3 (circles), and
PTBDT-FQxCF3 (triangles) The insets show square root of current density vs. voltage (V)-built-in
voltage (Vbi), together with fitted lines.
The morphologies of the active layers in PSCs were investigated by the transmission electron
microscopy (TEM) and the results were shown in figure 8. In comparison with the active layer
based on PTBDT-QxCF3 and PTBDT-FQxCF3, the active layer based on PTBDT-Qx showed
relatively larger phase separation and aggregation. In addition, the active layer based on PTBDT-
QxCF3 and PTBDT-FQxCF3 showed the better bicontinuous interpenetrating networks than that
of the active layer based on PTBDT-Qx. The best FF of the device based on PTBDT-QxCF3 and
PTBDT-FQxCF3 were 60.5 and 65.1%, respectively, which are a 50 and 61% increase compared to
that of the device based on PTBDT-Qx. The morphologies of the active layer support that the FFs
of the devices based on PTBDT-QxCF3 and PTBDT-FQxCF3 are superior to that of the device
based on PTBDT-Qx. The surface morphologies of the active layers based on PTBDT-Qx,
PTBDT-QxCF3, and PBDT-FQxCF3, inspected by tapping-mode atomic force microscopy
(AFM) measurements as well, are shown in Figure S1. The root-mean-square (RMS) of the
surfaces of the active layers based on PTBDT-Qx, PTBDT-QxCF3, and PBDT-FQxCF3 were
1.56, 1.70, and 1.60 nm, respectively, indicating that the performances of PSCs are almost
independent of the surface morphology and roughness.
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Figure 8. TEM images of the active layer based on (a) PTBDT-Qx, (b) PTBDT-QxCF3, and (c)
PTBDT-FQxCF3.
3. Conclusions
Three conjugated polymers, in which the electron-donating 2,3-dioctylthienyl-substituted BDT
unit was connected to the electron-accepting DPQ derivatives through a thiophene bridge, have been
prepared by Stille coupling reaction. To investigate the effects of the strong electron-withdrawing
CF3 substituent on the intrinsic properties of the polymers, CF3 was systematically incorporated at
the para-position of the phenyl groups located in the 2,3-positions of both DPQ, 6,7-difluorinated
DPQ, to afford PTBDT-QxCF3 and PTBDT-FQxCF3, respectively. Owing to the strong
influences of the CF3 unit, PTBDT-QxCF3 and PTBDT-FQxCF3 exhibit the significant lowering
of the HOMO energy levels, which can induce the great enhancement in Voc values of photovoltaic
cells. In addition, the incorporation of additional F atom in the presence of CF3 can trigger other
positive effects such as enhancement of charge carrier mobility, and formation of better
bicontinuous interpenetrating networks in an active layer of device. So, Jsc and FF of the
photovoltaic cells with CF3/F groups can increase sharply. As a result, the PCEs of inverted-type
PSCs based on these polymers show a gradual increase in the order of PTBDT-Qx < PTBDT-
QxCF3 < PTBDT-FQxCF3, with corresponding values of 2.61, 4.61, and 6.47%, respectively. The
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PCE enhancement can be attributed to the beneficial effect of the CF3 and CF3/F substituents
incorporated in the polymer backbone, such as the increase in Voc through the lowering HOMO
energy levels and the concomitant enhancement in FF. Therefore, this study provides meaningful
insight into the synthesis and structure-property relationships of conjugated polymers with strong
electron-withdrawing CF3 substituents, which will support their use in various applications such as
photovoltaic cells and field-effect transistors.
Acknowledgements
This research was supported by Korea Institute of Energy Technology Evaluation and Planning
(KETEP-20174010201460 and 2018201010636A).
References
Supplementary information (experimental details including materials, methods, and the synthesis of
monomer and polymers. In addition, summary of the photovoltaic parameters of the PSCs based on
PTBDT-Qx, PTBDT-QxCF3, and PTBDT-FQxCF3 at different weight ratios between donor and
PC71BM, AFM images of the active layers based on PTBDT-Qx, PTBDT-QxCF3, and PTBDT-
FQxCF are available in the electronic supporting information) associated with this article can be
found in online
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Electronic Supporting Information (ESI) for:
20

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Enhanced Open-Circuit Voltages of Trifluoromethylated Quinoxaline-Based Polymer Solar
Cells
Sella Kurnia Putri,a† Ho Cheol Jin,b† Dong Ryeol Whang,c Joo Hyun Kim,*, b and Dong Wook
Chang*, a
aDepartment of Industrial Chemistry, Pukyong National University, 48547 Busan, Republic of
Korea.
E-mail: dwchang@pknu.ac.kr
bDepartment of Polymer Engineering, Pukyong National University, 48547 Busan, Republic of
Korea.
E-mail: jkim@pknu.ac.kr
cLinz Institute for Organic Solar Cells (LIOS)/Institute of Physical Chemistry, Johannes Kepler
University Linz, 4040 Linz, Austria.
† S. K. Putri and H. C. Jin equally contributed to this research.
Table S1. The photovoltaic parameters of the PSCs based on PTBDT-Qx, PTBDT-QxCF3, and
PTBDT-FQxCF3. The average and deviation (10 devices are averaged) for the photovoltaic
parameters of each device are given in parentheses.
Blend
ratioa
Jsc
Voc
FF
PCE
Rs
Donor
(mA/cm2)
(ꢎ)
(%)
(%)
(Ω cm2)
21

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9.13
(9.05±0.08)
8.92
0.71
(0.71±0.00)
0.68
40.3
(39.7±0.58)
40.9
2.61
(2.55±0.04)
2.48
3:3
3:4
3:5
3:3
3:4
3:5
3:6
3:2
3:3
3:4
3:5
3:6
3:7
9.61
-
PTBDT-Qx
(8.84±0.06)
8.42
(0.68±0.00)
0.68
(40.4±0.36)
40.6
(2.43±0.03)
2.32
-
(8.37±0.09)
8.06
(0.68±0.00)
0.93
(40.5±0.23)
52.6
(2.31±0.02)
3.94
-
(7.98±0.06)
8.03
(0.93±0.00)
0.92
(52.7±0.15)
60.4
(3.91±0.04)
4.46
-
(8.03±0.13)
8.38
(0.92±0.00)
0.91
(59.6±0.92)
60.5
(4.40±0.05)
4.61
PTBDT-QxCF3
3.36
(8.07±0.21)
7.63
(0.91±0.00)
0.89
(60.9±0.52)
58.5
(4.47±0.09)
3.97
-
(7.70±0.14)
8.44
(0.87±0.02)
1.00
(55.3±2.16)
55.5
(3.70±0.17)
4.69
-
(8.46±0.15)
9.56
(1.00±0.00)
1.00
(54.1±1.23)
61.0
(4.57±0.08)
5.83
-
(9.34±0.22)
10.00
(1.00±0.00)
0.99
(60.7±0.08)
63.6
(5.67±0.16)
6.30
-
(9.90±0.06)
10.03
(0.99±0.00)
0.99
(63.8±0.54)
65.1
(6.25±0.04)
6.47
PTBDT-FQxCF3
2.54
(10.04±0.01)
8.78
(0.99±0.00)
0.98
(64.9±0.20)
66.4
(6.46±0.01)
5.71
-
-
(8.67±0.07)
8.35
(0.98±0.00)
0.98
(66.7±0.62)
67.0
(5.68±0.03)
5.49
(8.21±0.13)
(0.98±0.00)
(67.2±0.24)
(5.41±0.08)
a mass ratio of polymeric donor to PC71BM, b series resistance estimated from the corresponding best device.
Experimental section
22

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Materials and methods. 1 [1], 2 [2],
diphenylquinoxaline (6) [3], and 2,6-bis(trimethyltin)-4,8-di(2,3-dioctylthiophen-5-yl)-benzo[1,2-
b:4,5-b ]dithiophene (9) [4], and 1,2-bis(4-(trifluoromethyl)phenyl)ethane-1,2-dione [5] were
3 [3], 5,8-di(5-bromothiophen-2-yl)-2,3-
′
prepared following previously reported methods. All other chemicals and solvents were purchased
1
from Aldrich. H, 13C, and 19F NMR spectra were obtained with a JEOL JNM ECP-400
spectrometer. Matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) analyses
were carried out using an Ultraflex spectrometer (Bruker). UV-Vis spectra were recorded using a
JASCO V-530 spectrometer. GPC measurements were conducted on an Agilent 1200 series
instrument in the presence of THF eluent. CV analyses were carried out using a VersaSTAT3
potentiostat (Princeton Applied Research), with tetrabutylammonium hexafluorophosphate
(Bu4NPF6, 0.1 M) electrolyte in acetonitrile. A glassy carbon electrode coated with polymers and a
platinum wire were used as a working and counter electrode, respectively. A silver wire was used as
pseudo-reference electrode with a Fc/Fc+ external standard. AFM imaging measurement were
carried out using a NanoScope V (Bruker) microscope operated in tapping mode.
Fabrication and analysis of PSCs. PC71BM,(catalog no. nano-cPCBM-SF) was purchased from
Nano-C, Inc. Inverted-type of PSCs with ITO/ZnO/active layer (polymer:PC71BM)/MoO3/Ag
structure were fabricated to investigate the photovoltaic properties of the polymers. First, a ZnO
film was deposited on a cleaned ITO surface by a sol-gel process followed by thermal curing at 200
°C for 10 min. Then, the active layer composed of polymer and PC71BM was spin-coated using
chlorobenzene solution at 600 rpm for 60 s. 1.8-diiodooctane (DIO, 3% v/v) was added to as a
processing additive during the formation of the active layer. Finally, MoO3 and Ag layers were
sequentially deposited by thermal evaporation at 2 × 10-6 Torr through a shadow mask with an
active area of 0.13 cm2. The J-V characteristics of the PSCs were examined using a Keithley 2400
23

ACCEPTED MANUSCRIPT
source- measure unit under AM 1.5G illumination at 100 mW/cm2 from a 150 W Xe lamp. The
calibration of the solar simulation was conducted using a Si reference cell with a KG5 filter certified
by the National Institute of Advanced Industrial Science and Technology.
Synthesis of 5,6-di(thiophen-2-yl)-2,3-bis(4-(trifluoromethyl)phenyl)quinoxaline (4). A mixture
of 4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole (1) (0.70 g, 2.33 mmol) and zinc powder (3.05 g,
46.66 mmol) in acetic acid (60 mL) was stirred at 80 °C for 12 h. Once the reaction was completed,
zinc powder was removed by filtration and the filtrate was carefully collected. After adding 1,2-
bis(4-(trifluoromethyl)phenyl)ethane-1,2-dione (0.81 g, 2.34 mmol) to the filtrate, the mixture was
refluxed overnight. After cooling to room temperature, the mixture was poured into water and
extracted with ethyl acetate. The ethyl acetate fractions were the separated and dried over
magnesium sulfate. Solvents were removed using a rotary evaporator, and the crude residue was
purified by recrystallization (ethanol/chloroform 1:4, v/v) to give 3 as a light brown solid (0.61 g,
yield = 45%). 1H NMR (400 MHz, CDCl3):
J = 8.32 Hz), 7.53 (d, 2H, J = 5.12 Hz), 7.19 (dd, 2H, J = 5.12, 3.76 Hz). 13C NMR (100 MHz,
δ (ppm) = 8.20 (s, 2H), 7.85-7.83 (m, 6H), 7.66 (d, 4H,
CDCl3):
δ (ppm) = 149.8, 141.5, 138.1, 137.3, 131.3, 130.7, 129.2, 127.7, 126.7, 126.6, 125.4,
124.9, 123.1. 19F NMR (376 MHz, CDCl3):
found, 582.202 [M+].
δ (ppm) = -62.5. MALDI-TOF, m/z: Calcd, 582.066;
Synthesis of 6,7-difluoro-5,8-di(thiophen-2-yl)-2,3-bis(4-(trifluoromethyl)phenyl)quinoxaline
(5). A mixture of 5,6-difluoro-4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole (2) (0.3 g, 0.9 mmol),
zinc powder (1.17 g, 18 mmol), and acetic acid (24 mL) was stirred at 80 °C for 12 h. After
filtering, 1,2-bis(4-(trifluoromethyl)phenyl)ethane-1,2-dione (0.31 g, 0.9 mmol) was added to the
filtrate, followed by heating under reflux overnight. After cooling to room temperature, the mixture
was poured into water and extracted with ethyl acetate. The ethyl acetate fractions were separated
24

ACCEPTED MANUSCRIPT
and dried over magnesium sulfate. Solvents were removed using a rotary evaporator, and the crude
residue was purified by recrystallization (ethanol/chloroform 1:3, v/v) to give 5 as a yellow solid
(0.14 g, yield = 32%). 1H NMR (400 MHz, CDCl3, ppm):
= 8.32 Hz), 7.68 (m, 6H), 7.28 (m, 2H). 13C NMR (100 MHz, CDCl3):
135.2, 131.5, 131.3, 131.1, 130.7, 130.5, 130.3, 126.8, 125.6, 124.8, 123.0. 19F NMR (376 MHz,
(ppm) = -62.6, -126.8. MALDI-TOF, m/z: Calcd, 618.563; found, 619.046 [M+].
δ 8.04 (d, 2H, J = 3.76 Hz), 7.83 (s, 4H, J
δ
(ppm) = 149.8, 141.0,
CDCl3):
δ
Synthesis of 5,8-bis(5-bromothiophen-2-yl)-2,3-bis(4-(trifluoromethyl)phenyl)quinoxaline (7).
5,6-di(thiophen-2-yl)-2,3-bis(4-(trifluoromethyl)phenyl)quinoxaline (4) (0.35 g, 0.60 mmol) and
NBS (0.24 g, 1.32 mmol) were dissolved in dry THF (20 mL). The mixture was stirred at room
temperature for 12 h in the dark. Once the reaction was completed, the mixture was poured into
water and extracted with ethyl acetate. The ethyl acetate fractions were separated and dried over
magnesium sulfate. Solvents were removed using a rotary evaporator, and the crude residue was
purified by recrystallization (methanol/ethyl acetate 1:4, v/v) to give 7 as a red solid (0.32 g, yield =
72%). 1H NMR (400 MHz, CDCl3):
δ
(ppm) = 8.17 (s, 2H), 7.81 (d, 4H, J = 5.36 Hz), 7.72 (d, 4H, J
13
= 5.36 Hz), 7.59 (d, 2H, J = 2.68 Hz), 7.16 (d, 2H, J = 2.68 Hz). C NMR (100 MHz, CDCl3):
δ
(ppm) = 150.8, 141.2, 139.0, 136.9, 130.8, 130.7, 129.4, 126.6, 126.0, 125.6, 124.8, 123.0, 117.5.
19F NMR (376 MHz, CDCl3):
[M+].
δ
(ppm) = -62.5. MALDI-TOF, m/z: Calcd, 739.885; found, 739.888
Synthesis
of
5,8-bis(5-bromothiophen-2-yl)-6,7-difluoro-2,3-bis(4-
(trifluoromethyl)phenyl)quinoxaline
(8).
6,7-difluoro-5,8-di(thiophene-2-yl)-2,3-bis(4-
(trifluoromethyl)phenyl)quinoxaline (5) (0.14 g, 0.23 mmol) and NBS (0.09 g, 0.51 mmol) were
dissolved in THF (15 mL). The reaction mixture was stirred at room temperature for 12 h in a dark
room. After that, the mixture was poured into water and extracted with chloroform. The chloroform
25

ACCEPTED MANUSCRIPT
fractions were separated and dried over magnesium sulfate. Solvents were removed using a rotary
evaporator, and the crude residue was purified by recrystallization (methanol/chloroform 1:4, v/v) to
give 8 as an orange solid (0.14 g, yield = 81%). 1H NMR (400 MHz, CDCl3, ppm):
δ
7.83-7.78 (m,
(ppm) =
13
6H), 7.72 (d, 4H, J = 8.32 Hz), 7.22 d, 2H, J = 4.32 Hz). C NMR (100 MHz, CDCl3):
δ
19
150.2, 140.5, 134.7, 131.9, 131.7, 131.6, 131.5, 131.4, 129.7, 127.1, 125.7, 119.7, 117.6. F NMR
(376 MHz, CDCl3):
[M+].
δ (ppm) = -62.6, -126.5. MALDI-TOF, m/z: Calcd, 776.355; found, 775.893
Synthesis of polyquinoxalines via Pd-catalyzed Stille reaction
BDT monomer (0.20 mmol), dibrominated quinoxaline monomer (0.20 mmol), and Pd(PPh3)4 (3
mol%) were dissolved in dry toluene (10 mL) in a Schlenk flask. After flushing with N2 for 15
min, the reaction mixture was stirred at 90 °C for 48 h under N2 atmosphere. At the end of the
polymerization, 2-trimethylstannylthiophene and 2-bromothiophene were successively added as
end-capping agents with an interval of 3 h. After cooling to room temperature, the mixture was
poured into methanol, and the precipitated dark solids were collected by filtration. The solids were
further purified by sequential Soxhlet extractions with methanol, acetone, hexane, and chloroform.
The polymers in the chloroform fraction were recovered by precipitation into methanol, and finally
dried in a vacuum oven at 50 °C.
PTBDT-Qx: 9 and 6 were used as monomers. Yield: 87% (deep purple solid). GPC (THF): Mn =
23.04 KDa, Mw = 63.24 KDa, PDI = 2.74. 1H NMR (400 MHz, CDCl3, ppm):
δ
7.93-7.72 (br, 4H),
0.97-0.81
δ
7.65-7.10 (br, 14H), 3.15-2.62 (br, 8H), 1.88-1.66 (br, 8H), 1.52-1.21 (br, 40H), δ
δ
δ
δ
(br, 12H). Elemental analysis calcd (%) for C78H90N2S6: C 75.07, H 7.27, N 2.24; found: C 74.65, H
7.01, N 2.22.
26

ACCEPTED MANUSCRIPT
PTBDT-QxCF3: 9 and 7 were used as monomers. Yield: 85% (deep purple solid). GPC (THF):
Mn = 20.31 KDa, Mw = 58.16 KDa, PDI = 2.86. 1H NMR (400 MHz, CDCl3, ppm):
4H), 7.78-7.59 (br, 4H), 7.37-7.08 (br, 8H), 3.12-2.58 (br, 8H), 1.90-1.64 (br, 8H),
1.19 (br, 40H), 0.96-0.79 (br, 12H). Elemental analysis calcd (%) for C80H88F6N2S6: C 69.43, H
δ
7.99-7.81 (br,
1.53-
δ
δ
δ
δ
δ
δ
6.41, N 2.02; found: C 69.05, H 6.19, N 1.96.
PTBDT-FQxCF3: 9 and 8 were used as monomers. Yield: 83% (deep purple solid). Mn = 14.61
KDa, Mw = 41.40 KDa, PDI = 2.83. 1H NMR (400 MHz, CDCl3, ppm):
7.52 (br, 4H), 7.41-7.11 (br, 6H), 3.09-2.54 (br, 8H), 1.91-1.63 (br, 8H),
0.98-0.77 (br, 12H). Elemental analysis calcd (%) for C80H86F8N2S6: C 67.67, H 6.10, N 1.97;
found: C 67.34, H 6.09, N 1.89.
δ
8.05-7.82 (br, 4H),
δ 7.77-
δ
δ
δ
δ
1.54-1.18 (br, 40H),
δ
Figure S1. AFM topography of the active layer based on (a) PTBDT-Qx, (b) PTBDT-QxCF3,
and (c) PTBDT-FQxCF3.
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