Steady enhancement in photovoltaic properties of fluorine functionalized quinoxaline-based narrow bandgap polymer

Article abstract of DOI:10.3390/molecules24010054

To investigate the influence of fluoride phenyl side-chains onto a quinoxaline (Qx) unit on the photovoltaic performance of the narrow bandgap (NBG) photovoltaic polymers, herein, two novel NBG copolymers, PBDTT-DTQx and PBDTT-DTmFQx, were synthesized and

Full text of DOI:10.3390/molecules24010054

molecules
Article
Steady Enhancement in Photovoltaic Properties of
Fluorine Functionalized Quinoxaline-Based Narrow
Bandgap Polymer
1
2
3,4,
3
3
5
Zhonglian Wu , Huanxiang Jiang , Xingzhu Wang
*, Lei Yan , Wei Zeng , Xiu-Gang Wu ,
1
1
2,
Haiyu Zhuang , Wen Zhu and Renqiang Yang *
1
School of Materials Engineering, Jiangsu University of Technology, Changzhou 213001, China;
wuzhonglian@jsut.edu.cn (Z.W.); zhuanghaiyu@jstu.edu.cn (H.Z.); czzwen@163.com (W.Z.)
Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao 266101,
China; jianghx@qibebt.ac.cn
College of Chemistry, Xiangtan University, Xiangtan 411105, China; yanlei@xtu.edu.cn (L.Y.);
jonewel@gmail.com (W.Z.)
2
3
4
5
Southern University of Science and Technology, Shenzhen 518055, China
Jiangsu Key Laboratory of Environmentally Friendly Polymeric Materials, School of Materials Science &
Engineering, Changzhou University, Changzhou 213164, China; xgwu16@126.com
Correspondence: xz_wang@hotmail.com (X.W.); yangrq@qibebt.ac.cn (R.Y.); Tel.: +86-731-5829-2020 (X.W.);
*
+
86-532-8066-2700 (R.Y.)
ꢀ
ꢁꢂꢀꢃꢄꢅꢆꢇ
Received: 13 December 2018; Accepted: 21 December 2018; Published: 24 December 2018
ꢀ
ꢁꢂꢃꢄꢅꢆ
Abstract: To investigate the influence of fluoride phenyl side-chains onto a quinoxaline (Qx) unit
on the photovoltaic performance of the narrow bandgap (NBG) photovoltaic polymers, herein, two
novel NBG copolymers, PBDTT-DTQx and PBDTT-DTmFQx, were synthesized and characterized.
2
[
-ethylhexylthiothiophene-substituted benzodithiophene (BDTT), 2,3-diphenylquinoxaline (DQx)
or 2,3-bis(3-fluorophenyl)quinoxaline (DmFQx)] and 2-ethylhexylthiophene (T) were used as the
electron donor (D) unit, electron-withdrawing acceptor (A) unit and -bridge, respectively. Compared
π
to non-fluorine substituted PBDTT-DTQx, fluoride PBDTT-DTmFQx exhibited a wide UV-Vis
absorption spectrum and high hole mobility. An enhanced short-circuit current (Jsc) and fill factor (FF)
simultaneously gave rise to favorable efficiencies in the polymer/PC BM-based polymer solar cells
7
1
−
2
(
(
PSCs). Under the illumination of AM 1.5G (100 mW cm ), a maximum power conversion efficiency
PCE) of 6.40% was achieved with an open-circuit voltage (Voc) of 0.87 V, a Jsc of 12.0 mA cm
−
2
and a FF of 61.45% in PBDTT-DTmFQx/PC BM-based PSCs, while PBDTT-DTQx-based devices
71
also exhibited a PCE of 5.43%. The excellent results obtained demonstrate that PBDTT-DTmFQx by
fluorine atom engineering could be a promising candidate for organic photovoltaics.
Keywords: polymer solar cells; bulk heterojunction; quinoxaline; narrow bandgap conjugated
polymer; synthesis
1
. Introduction
Polymer solar cells (PSCs) have been attracting tremendous interest as next-generation solar cells
due to their unique advantages of low cost fabrication, light weight and potential for a large area
through solution-processing [ ]. At present, the state-of-the-art structure of PSCs is based on the
1–7
bulk heterojunction (BHJ) type, where the photoactive layer is composed of a blend of a conjugated
polymer as the donor material and a fullerene derivative or non-fullerene fuse-ring molecule as
the acceptor material [
8–10]. Through synergistic work of the development of efficient conjugated
polymers and device optimization, a series of PSCs whose power conversion efficiency (PCE) is over
Molecules 2019, 24, 54; doi:10.3390/molecules24010054
www.mdpi.com/journal/molecules

Molecules 2019, 24, 54
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1
0% have been obtained [
4,
11
–14]. However, the relationship between molecular structures and good
photovoltaic performance is still unclear. Hence, to obtain high-efficient PSCs, the donor materials
are required to have narrow bandgap energy and suitable molecular energy levels to maximize the
short-circuit current (Jsc), open-circuit voltage (Voc) and high carrier mobility to facilitate carrier
transport. These goals can be achieved by tuning the molecular structures of donor–acceptor (D–A)
conjugated polymers. Of these, quite a few donor and acceptor units have been developed in the past
decade, such as quinoxaline (Qx) [15
the acceptor unit and 2,7-fluorene [18], 2,7-carbazole [19], benzo[1,2-b:4,5-b ]dithiophene (BDT) as
,
16] or 2,5-dihydropyrrolo[3,4-c]-pyrrole-1,4-dione (DPP) [17] as
0
the donor unit [20,21]. Among these various donor units, the BDT unit has become one of the most
successful frameworks in the design and synthesis of efficient donor materials since it was first used
for PSCs in 2009, owing to its rigid and planar molecular structure [20]. Two-dimensional conjugated
BDT units were developed for the synthesis of donor materials, which not only broaden the absorption
spectra, but also enhance the hole mobilities [22,23]. For example, the Hou group [11] reported a high
power conversion efficiency (PCE) of 14.2% with an outstanding FF of 0.76 based on a DTBDT unit.
As a strong electron acceptor unit, quinoxaline (Qx) is widely used as the skeleton of a D–A
type conjugated polymer for PSC applications [24–31]. The molecular structure of the Qx unit can be
easily modified at the 2,3-position and the 6,7-position to tune the optoelectronic properties. As is well
known, the fluorine atom has a very small atomic radius (only slightly larger than that of the H atom),
strong electronegativity and intermolecular weak non-covalent interactions (such as F···H, F···S, F···π
,
etc.). Recently, introducing fluorine atoms in conjugated polymers as the donor materials in PSCs was
generally expected to modulate the optical and electrochemical properties, and molecular packing to
facilitate carrier transport to enhance photovoltaic properties [32]. The introduction of a fluorine atom
into the Qx unit of low bandgap polymers has been regarded as one effective strategy for improving
the photovoltaic properties of PSCs [25,29–31]. For example, the NBG polymer with a fluoride Qx unit
at the 5,6-position as the donor, and PC BM as the acceptor, achieved a high PCE value of 8.0% [29].
71
The Liu group synthesized the polymer based on a Qx unit with a 4-fluorophenyl side chain at the
,3-position. Its optimized PSCs based on the polymer/PC BM demonstrated an outstanding PCE
2
71
−
2
value of 7.2% with a Voc of 0.87 V, Jsc of 11.4 mA cm , and a fill factor (FF) of up to 73% [25].
Encouraged by these results, in this study we report the synthesis and properties
of non-fluorinated and fluorinated Qx units based on narrow-bandgap (NBG) polymers,
namely PBDTT-DTQx and PBDTT-DTmFQx (Scheme 1), in which a strong donor unit of
2-ethylhexylthiothiophene-substituted benzodithiophene (BDTT) has been introduced in the main
chains. We expected that the grafting of fluorine atoms can further pare away the highest occupied
molecular orbital (HOMO) energy level and improve the charge carrier mobility of the corresponding
polymer, and likewise, also enhance the photovoltaic properties for its polymer in PSCs. As a result,
PBDTT-DTmFQx/PC BM-based devices presented a maximum PCE of 6.40%, and were obtained
71
−
2
with a Voc of 0.87 V, Jsc of 12.0 mA cm and FF of 61.45%, while non-fluorinated PBDTT-DTQx-based
devices demonstrated a PCE of 5.43%. This result demonstrates that the fluorine-substituted side-chain
on the acceptor unit is a useful strategy to fine tune energy levels of NBG polymers.
2
. Results and Discussion
2
.1. Synthesis and Thermal Stability
The synthetic routes of the monomers and the copolymers PBDTT-DTQx and PBDTT-DTmFQx
are depicted in Scheme 1. The compounds 2a–b were prepared by a reduction reaction using NaBH4
with compound 1 and then a condensation reaction using sodium acetate as a catalyst with benzil or
0
3
,3 -difluorobenzil in good yield. The compounds 3a
–b
were obtained between compound 2a
–
b
and
tributyl(3-(2-ethylhexyl)-thiophen-2-yl)stannane in excellent yield, by a Stille coupling reaction using
Pd(PPh ) as the catalyst,. The compounds 3a were brominated with N-bromosuccinimide (NBS)
. Through a typical Stille coupling polymerization, PBDTT-DTQx and
–b
3
4
to afford the monomers 4a–b

Molecules 2019, 24, 54
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PBDTT-DTmFQx were obtained between 4a
–
b
and
5
with tris(dibenzylideneacetone) dipalladium
◦
[
Pd (dba) ] and tri(o-tolyl)phosphine [P(o-tol) ] as the catalysts in toluene at 110 C. The copolymers
2
3
3
were carefully purified by continuous Soxhlet extraction with n-hexane, acetone, and chloroform
successively. The chloroform fraction was concentrated under reduced pressure, and then precipitated
in methanol and collected by filtration. The chemical structure of all compounds was confirmed by
1
13
H-NMR and C-NMR. Both of the resulting copolymers were characterized by elemental analysis.
1
High-quality H-NMR spectra for the copolymers could not be obtained due to their limited solubility
in CDCl at room temperature (RT). The molecular weight of the copolymers was determined by
3
gel permeation chromatography (GPC) using tetrahydrofuran as the eluent and polystyrene as the
standard (Table 1). The average molecular weights (M ) of PBDTT-DTQx and PBDTT-DTmFQx
n
were 18.5 kDa (PDI = 2.4) and 21.2 kDa (PDI = 2.5), respectively. As four 2-ethylhexyl side chains
on repeating units of the copolymers were appended on the backbone, both the copolymers showed
good solubility, and could be easily dissolved in the commonly used solvents for device fabrication
such as THF, toluene, xylene, chlorobenzene (CB) and o-dichlorobenzene (ODCB). Thermal stability
of the copolymers was surveyed by thermogravimetric analysis (TGA). As shown in Figure 1 and
Table 1, PBDTT-DTQx and PBDTT-DTmFQx present outstanding thermal stability, with 5% weight
◦
loss temperatures (T ) of 316 and 338 C under an inert atmosphere, respectively.
d
Scheme 1. Synthetic routes of the copolymers.
Figure 1. Thermogravimetric analysis (TGA) curves of the copolymers.

Molecules 2019, 24, 54
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Table 1. Molecular weight and thermal properties of the copolymers.
◦
Polymer
Mn (kDa)
Mw (kDa)
PDI
Td ( C)
PBDTT-DTQx
PBDTT-DmFQx
18.5
21.2
44.5
53.2
2.4
2.5
316
338
2
.2. Optical Properties
The UV-Vis absorption spectra of PBDTT-DTQx and PBDTT-DTmFQx in diluted chloroform
solutions and thin neat films are presented in Figure 2, and the detailed optical data are listed in
Table 2. Both of the copolymers showed two main absorption peaks in CHCl solution, which is
3
a common feature of D–A type copolymers. The high-lying absorption band from 300 to 500 nm
was attributed to the
π-π* transition of the conjugated backbone, and the other band from 500 to
800 nm was assigned to the strong intramolecular charge transfer (ICT) state from the donor to the
acceptor unit [30]. Meanwhile, both of the copolymers showed similar absorption spectra in CHCl3.
In comparison with PBDTT-DTmFQx, the slight difference is that a shoulder peak centered at ca.
6
30 nm could be found for PBDTT-DTQx in CHCl solution. It is well known that the fluorine atom
3
has strong electronegativity, but a fluorine substituent shows a weak electron-withdrawing effect due
to meta-fluorinated phenyl side groups. Compared to the absorption profiles in solution, the ones in
thin films showed a certain degree of red-shift, which is ascribed to the intermolecular π-π stacking
interaction. Meanwhile, it is noted that the absorption spectra of PBDTT-DTmFQx vs PBDTT-DTQx
as neat films bathochromically shifted about 15 nm. It is inferred that the interchain interactions
among PBDTT-DTmFQx chains are stronger than that of PBDTT-DTQx chains. The absorption
onsets of PBDTT-DTQx and PBDTT-DmFQX as neat films are located at 730 nm and 800 nm, which
opt
are matched with their optical band gaps (Eg ) of 1.73 and 1.68 eV, respectively.
Figure 2. UV-Vis absorption spectra of the copolymers in solution and in thin films.
2
.3. Electrochemical Properties
Highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO)
levels of the donor materials strongly affect the Voc and the charge separation efficiency of PSCs.
Cyclic voltammetry (CV) was adopted to determine the HOMO and LUMO energy level of the two
copolymers, which was determined under nitrogen using n-Bu NPF (0.1 M in anhydrous acetonitrile)
4
6
+
as the supporting electrolyte [33]. The ferrocene/ferrocenium (Fc/Fc ) redox couple was used as the
standard. The CV curves and data are displayed in Figure 3 and Table 2. The HOMO and LUMO energy
ox
levels are calculated using the following Equations (1) and (2), where
E
is the onset oxidation
onset
red
potential and E
is the onset reduction potential [30].
onset
ox
EHOMO = −(4.80 − E1/2,Fc/Fc + E
) (eV)
(1)
onset

Molecules 2019, 24, 54
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red
ELUMO = −(4.80 − E1/2,Fc/Fc + E
) (eV)
(2)
onset
The HOMO energy level of PBDTT-DTmFQx
(
−
5.33 eV) is slightly lower than that of
PBDTT-DTQx 5.23 eV). Obviously, PBDTT-DTmFQx, by grafting two fluorine atoms at phenyl
(
−
side-chains into the Qx unit, can not only improve the electron-withdrawing ability of Qx, but also
cause a lower HOMO level. The comparatively deeper HOMO levels could be expected to achieve
higher Voc in PSC applications, which is one of main contributors of high efficiency PSCs. The
ec
bandgap values ( g
E
) in terms of the electrochemical study were 1.74 and 1.70 eV for PBDTT-DTQx
and PBDTT-DTmFQx, respectively, which are consistent with the optical bandgaps.
Figure 3. Cyclic voltammogram curves for the copolymers.
Table 2. Optical and electrochemical properties of the copolymers.
λmax,
λmax, Film
λonset,
opt
g
HOMO
(eV)
LUMO
(eV)
a
ec b
(eV)
g
polymer
Solution
E
(eV)
E
(
nm)
Film (nm)
(
nm)
PBDTT-DTQx
PBDTT-DTmFQx 578
581
586
652
717
737
1.73
1.68
−5.23
−5.33
−3.49
−3.63
1.74
1.70
opt
a
b
Optical bandgap, calculated from the absorption onset of the films, Eg = 1240/λonset. Electrochemical bandgap,
the offset between HOMO and LUMO.
2
.4. Photovoltaic Properties of the Copolymers
To investigate the photovoltaic properties of PBDTT-DTQx and PBDTT-DTmFQx, PSCs were
fabricated by blending PBDTT-DTQx or PBDTT-DTmFQx as the donor and PC BM as the
71
acceptor with the configuration of ITO/PEDOT:PSS/polymer:PC BM/PDINO (perylene diimide
71
functionalized with amino N-oxide)/Al. These PSCs were then tested under AM 1.5G irradiation
−
2
(
100 mW cm ). Photovoltaic performances of the devices were optimized by using different
donor/acceptor weight ratios, active layer thicknesses, and solvents with 1,8-diiodooctane (DIO)
as the additive. Table 3 lists the Voc, Jsc, FF, and PCE of the optimized devices. The optimized
device fabrication conditions include the donor/acceptor weight ratio of 1:1.5 for PBDTT-DTQx
and 1:2 for PBDTT-DTmFQx, and an active layer thickness of ca. 150 nm. The devices based on
PBDTT-DTQx
of 0.85 V and 0.91 V; Jsc values of 8.67 mA cm and 10.57 mA cm ; and FF values of 33.27% and
9.44%; with PCE values of 2.46% and 3.79%, respectively. The higher Voc value of PBDTT-DTmFQx
:PC BM (1:1.5, w/w) and PBDTT-DTmFQx:PC BM (1:2, w/w) showed Voc values
71 71
−
2
−2
3
compared with PBDTT-DTQx was derived from the corresponding lower HOMO value. Compared
to PBDTT-DTQx, the higher Jsc and FF values of PBDTT-DTmFQx results in superior PCE values.
To further optimize the photovoltaic properties of PSCs, DIO was frequently selected as the solvent
additive due to its ability to solvate fullerene derivatives as the acceptor and high boiling point. The
volume ratio of DIO to DCB varied from 1% to 3% in order to optimize the device performance.

Molecules 2019, 24, 54
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We found that the PSCs using 2% DIO as the additive showed the best photovoltaic performances
for the polymers. Figure 4 shows current density–voltage (J–V) curves of the optimal PSCs with
or without DIO as the additive. These devices for PBDTT-DTQx and PBDTT-TmFQx showed Voc
values of 0.87 and 0.91 V; Jsc values of 12.00 and 11.05 mA cm 2; FF values of 61.45% and 53.95%;
and PCE values of 6.40% and 5.43%, respectively. Noticeably, it was found that the addition of DIO
as the additive resulted into a slight variation of Voc (slight increase for PBDTT-DTQx and slight
decrease for PBDTT-DTmFQx). The variation can be attributed to the change of charge-separated
and charge-transfer-state energies derived from morphology evolution of the active layers upon
additive addition [34]. Although the device for PBDTT-TmFQx with 2% DIO showed a slightly lower
−
V
oc value, its Jsc and FF values were higher; this resulted in a higher efficiency owing to improved
intermolecular packing.
Figure 4. Current density–voltage (J–V) curves of the copolymer/PC BM-based polymer solar cells
7
1
(
PSCs) at optimized conditions under illumination of AM 1.5G, 100 mW/cm2.
To further explain the different photovoltaic properties of the copolymers in PSC devices, the hole
mobility of the blend films was measured by the space charge limited current (SCLC) model with a
typical device structure of ITO/PEDOT:PSS/polymer:PC BM/Au. The SCLC was calculated using
7
1
2
3
the Mott-Gurney law: J = (9/8)ε0εrµ (V /L ), where J stands for current density,
ε is the permittivity
0
h
of free space, εr is the relative dielectric constant of the transport medium,
µ is the hole mobility, V
h
is the applied voltage (Vapp) corrected from the built-in voltage (V ) arising from the difference in
bi
the work function of the contacts, and L is the thickness of the active layer [35]. The J–V curves of
these hole devices containing copolymers/PC BM active layers are exhibited in Figure 5. As listed in
7
1
−
5
Table 3, the hole mobility of PBDTT-DTQx and PBDTT-DTmFQx were calculated to be 8.38
×
10
−1 −1
s in the hole copolymer/PC BM-based devices, respectively. Obviously,
−
4
2
and 2.34 × 10 cm V
71
the higher hole mobility of PBDTT-DTmFQx:PC BM contributes in part to the higher Jsc and FF values
7
1
observed in the PSCs, which could be attributed to enhanced PCE in the devices.

Molecules 2019, 24, 54
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Figure 5. Current density–voltage (J–V) curves of the optimized hole-only copolymer/PC71BM devices.
Table 3. Photovoltaic properties of the copolymer/PC BM-based polymer solar cells (PSCs) at
7
1
optimized conditions under illumination of AM 1.5G, 100 mW/cm2.
b
Voc
Jsc
FF
[%]
PCE
[%]
µ
h
[cm V
Ratio a
Polymer
[V]
[mA cm 2]
−
2
−1 −1
s
]
PBDTT-DTQx
PBDTT-DTmFQx
1:2
0.85
0.91
0.91
0.87
8.67
11.05
10.52
12.00
33.27
53.95
39.44
61.45
2.46
5.43
3.79
6.40
8.38 × 10−
2.34 × 10−
5
1
:2 (2% DIO)
1:1.5
:1.5 (2% DIO)
4
1
a
Ratio = Polymer:PC71BM, w/w.; b
µ = the hole mobility.
h
The morphology of the blend films under optimized conditions (2% DIO) could also make
the improved photovoltaic performance of the PBDTT-DTQx and PBDTT-DTmFQx-based device
clear. The surface morphology of the copolymers/PC BM blend films was recorded by atomic force
71
microscopy (AFM), and the relevant topographical and phase images in a surface area of 2 2 µm are in
×
presented Figure 6. The root mean square (RMS) roughness from the height images were determined
as 2.15 and 1.74 nm for the PBDTT-DTQx and PBDTT-DTmFQx-based blend films, respectively.
Clearly, a smoother surface was observed for the PBDTT-DTmFQx-based blend film. Therefore, the
higher PCE value for the PBDTT-DTmFQx-based device can be ascribed to smoother morphologies,
more ordered bicontinuous interpenetrating networks and more efficient charge transport properties
−
2
(
Figure 5), which provided a Jsc of 12.00 mA cm , a FF of 61.45% and better device performance with
a PCE of up to 6.40%.

Molecules 2019, 24, 54
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Figure 6. Atomic force microscopy (AFM) height images (upper row) and phase images (lower row)
of PBDTT-DTQx:PC71BM blending film ( ) and PBDTT-DTmFQx:PC71BM blending film ( ). The
a,c
b,d
imaging size is 2 × 2 µm.
3
. Experimental Section
3
.1. Materials
4
,7-Dibromo-2,1,3-benzothidiazole (
1
) was purchased from Suna Tech Inc (Suzhou, Jiangsu,
0
China). The monomer (4,8-bis(5-((2-ethylhexyl)thio)thiophen-2-yl)benzo[1,2-b:4,5-b ]-dithio-phene-
,6-diyl)bis(trimethylstannane)(
0
2
5) [36], 3,3 -difluorobenzil [37] and tributyl-(3-(2-ethylhexyl)-
thiophen-2-yl)stannane [38] were synthesized according to the reported references. The other chemicals
and reagents were received from commercial sources and used without further purification.
3
.2. Characterization
1H-NMR and 13C-NMR measurements were carried on a Bruker 400 MHz DRX spectrometer
(
Bruker Analytische Messtechnik GmbH, Rheinstetten, Germany) with tetramethylsilane (TMS) as the
internal reference. UV-Vis absorption spectra were recorded on a Cary 60 UV-Vis spectrophotometer
Agilent technologies, Australia). Thermogravimetric analysis (TGA) (TA Instruments Corporate,
59 Lukens Drive New Castle, DE, USA) measurements were performed under nitrogen flow at a
(
1
◦
−1
heating rate of 20 C min . Cyclic voltammetry was carried out on a CHI600A electrochemical
workstation (Shanghai, China) using a polymer film on a platinum electrode as the working electrode,
a platinum wire as the counter electrode and Ag/AgCl as a reference electrode at a scan rate of
−
1
5
0 mV s . The surface morphology of the blend film was investigated by AFM on a Veeco, DI
multimode NS-3D apparatus (Plainview, NY, USA) in tapping mode under normal air conditions at
RT with a 5 mm scanner.

Molecules 2019, 24, 54
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3
.3. Fabrication and Characterization of PSCs
PSCs were fabricated through the conventional process. The basic device structure was
ITO/PEDOT:PSS/polymer:PC BM/PDINO/Al. Patterned ITO glass substrates were sequentially
71
cleaned with detergent, de-ionized (DI) water, acetone, and isopropanol. They were then treated
in an oxygen plasma (Plasma Preen II-862 Cleaner, North Brunswick, NJ, USA). A buffer layer of
PEDOT:PSS (ca. 40 nm) was spin-coated onto the precleaned ITO substrate and annealed in an oven
◦
at 150 C for 20 min. The photoactive layer was subsequently prepared by spin-coating a solution
of the polymer/PC BM in chlorobenzene (CB) with/without DIO additive on the PEDOT:PSS layer
71
with a typical concentration of 10 mg/mL. The thickness of active layers was varied by changing
the spin-coating speed, and the optimized thickness was about 120 nm. PDINO was dissolved into
methanol at a concentration of 1.5 mg/mL and spin-coated on the top of the photoactive layer. The
thickness of PDINO film was about 10 nm. Finally, the Al layer (ca.100 nm) was successively deposited
on the PIDNO layer in vacuum and used as the top electrode. The current density–voltage (J–V)
characterization of the devices was carried out on a computer-controlled Keithley source measurement
system. A solar simulator was used as the light source and the light intensity was monitored by a
2
standard Si solar cell. The active area was 0.1 cm for each cell. The thicknesses of the spuncast films
were recorded by a profilometer (Alpha-Step 200, Tencor Instruments, Milpitas, CA, USA).
3
.4. Synthesis of the Monomers and Copolymers
3
.4.1. Synthesis of 5,8-dibromo-2,3-diphenylquinoxaline (2a)
In a dry 100 mL flask, compound 1 (0.59 g, 2 mmol) and NaBH (0.76 g, 20 mmol) were mixed in
4
ethanol (40 mL) for 1 h at RT, and then refluxed for 4 h under a nitrogen atmosphere. After cooling
to RT, water was added and filtered. The residue was dried in vacuum. Then the crude product and
sodium acetate (0.33 g, 4 mmol) were added to a solution of benzil (0.42 g, 2 mmol) in anhydrous
ethanol (30 mL) under a nitrogen atmosphere. The mixture was refluxed for 24 h. After cooling to RT,
the solvent was removed under reduced pressure and the residue was purified by silica gel column
chromatography (eluent: n-hexane:CH Cl = 4:1) to give a pale yellow solid (yield 72.0%, 0.75 g).
2
2
1
13
H-NMR (400 MHz, CDCl3,
δ): 7.91(s, 2H), 7.65–7.66(m, 4H), 7.34–7.41(m, 6H). C-NMR (100 MHz,
CDCl , δ): 154.15, 139.36, 137.95, 133.10, 130.26, 129.59, 128.38, 123.74.
3
3
.4.2. Synthesis of 5,8-dibromo-2,3-bis(3-fluorophenyl)quinoxaline (2b)
The compound 2b was prepared according to the synthetic process of the above compound 2a
1
and gave a red powder. Yield 78%. H-NMR (400 MHz, CDCl ,
2
δ
): 7.95(s, 2H), 7.46(d, J = 12 Hz,
3
1
3
H), 7.26–7.36(m, 4H), 7.11–7.16(m, 2H). C-NMR (100 MHz, CDCl3,
δ): 163.71, 161.74, 152.52, 139.68,
1
39.41, 133.66, 130.04, 126.00. 123.76, 117.03.
3
.4.3. Synthesis of 5,8-bis(4-(2-ethylhexyl)thiophen-2-yl)-2,3-diphenylquinoxaline (3a)
Under a nitrogen atmosphere, Pd(PPh3)4 (56 mg) was added to a solution of
tributyl(4-(2-ethylhexyl)-thiophen-2-yl)stannane (1.94 g, 4 mmol) and compound 2a (0.44 g,
mmol) in dry toluene (20 mL). The mixture was refluxed for 12 h. After cooling to RT, the solvent was
1
removed under reduced pressure and the residue was purified by silica gel column chromatography
1
(
eluent: n-hexane: CH Cl = 5:1) to give a red solid (yield 91%, 0.61 g). H-NMR (400 MHz, CDCl ,
δ
):
2
2
3
8
1
1
.10(s, 2H), 7.73–7.77(m, 6H), 7.35–7.41(m, 6H), 7.08(s, 2H), 2.58–2.88(m, 4H), 1.65(m, 2H), 1.25–1.42(m,
): 151.48, 141.60, 138.77, 138.27, 137.29, 131.20,
30.49, 128.97, 128.65, 128.20, 126.90, 124.66, 40.49, 34.64, 32.59, 29.00, 25.66, 23.11, 14.21, 10.93.
6H), 0.89–0.93(m, 12H). 13C-NMR (100 MHz, CDCl3,
δ

Molecules 2019, 24, 54
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3
.4.4. Synthesis of 5,8-bis(4-(2-ethylhexyl)thiophen-2-yl)-2,3-bis(3-fluorophenyl)quinoxaline (3b)
The compound 3b was synthesized according to the synthetic process of the above compound
and gave a red powder. Yield 93%. 1H-NMR (400 MHz, CDCl3, ): 8.15(s, 2H), 7.73(s, 2H), 7.58(d,
3
a
δ
J = 12 Hz, 2H), 7.45(d, J = 8 Hz, 2H), 7.28–7.38(m, 2H), 7.13–7.16(m, 4H), 2.64(d, J = 8 Hz, 4H), 1.67(m,
2
H), 1.27–1.43(m, 16H), 0.90–0.95(m, 6H).
3
.4.5. Synthesis of 5,8-bis(5-bromo-4-(2-ethylhexyl)thiophen-2-yl)-2,3-diphenylquinoxaline (4a)
To a solution of compound 3a (0.34 g, 0.5 mmol) in CHCl and acetic acid (10 mL, CHCl : acetic
3
3
acid =5:1, v/v) was added NBS (0.20 g, 1.1 mmol) in portions over 20 min. The mixture was stirred
at RT in darkness for 12 h and then poured into water, and extracted with CH Cl for three times.
2
2
The combined organic layers were dried over anhydrous MgSO . After removing the solvent under
4
reduced pressure, the residue was purified by silica gel column chromatography to obtain a red
1
solid (yield 82%, 0.34 g). H-NMR (400 MHz, CDCl ,
7
δ
): 8.06(s, 2H), 7.70–7.72(m, 4H), 7.53(s, 2H),
3
13
.38–7.43(m, 6H), 2.57 (d, J = 8 Hz, 4H), 1.70(m, 2H), 1.31–1.40(m, 14H), 0.88–096(m, 12H). C-NMR
100 MHz, CDCl3, ): 151.95, 140.48, 138.27, 137.45, 136.82, 130.47, 130.26, 129.13, 128.25, 127.13, 125.68,
14.78, 40.07, 33.79, 32.53, 28.84, 25.68, 23.08, 14.16, 10.87.
(
δ
1
3
.4.6. Synthesis of 5,8-bis(5-bromo-4-(2-ethylhexyl)thiophen-2-yl)-2,3-bis-(3-fluoro-phenyl)
quinoxaline (4b)
The compound 4b was synthesized by the same procedure as the compound 4a. Yield 85%.
H-NMR (400 MHz, CDCl3, ): 8.05(s, 2H), 7.50(s, 2H), 7.36–7.46(m, 6H), 7.12–7.16(m, 2H), 2.56(d,
J = 8Hz, 4H), 1.68–1.71(m, 2H), 1.31–1.41(m, 16H), 0.88–0.94(m, 12H). 13C-NMR (100 MHz, CDCl , δ):
1
δ
3
1
1
64.32, 161.05, 150.35, 140.70, 140.11, 140.01, 137.16, 136.92, 130.57, 130.04, 129.93, 127.43, 126.24, 117.43,
17.12, 116.58, 116.30, 114.97, 40.10, 33.81, 32.55, 28.85, 25.70, 23.11, 14.19, 10.89.
3
.4.7. Synthesis of the Polymer PBDTT-DTQx
In a dry 25 mL flask, Pd (dba) , (2.0 mg) and P(o-Tol) , (4.0 mg) were added to a solution of
2
3
3
the monomer 4a (0.2 mmol) and
5 (194 mg, 0.2 mmol) in 8 mL degassed toluene under a nitrogen
atmosphere, and then stirred vigorously and refluxed for 19 h until the reaction system became a
viscous state. After cooling to RT, the mixture was poured into methanol and precipitation occurred. It
was collected by filtration and successively extracted in a Soxhlet apparatus with n-hexane, acetone
and chloroform (CHCl ), respectively. The collected CHCl solution was concentrated and precipitated
3
3
with methanol to get a dark solid (135 mg, 81.3%). Anal. Calcd for C H N S : C, 71.51; H, 6.77; N,
78
88
2 8
2
.14; S, 19.58. Found: C, 71.45; H, 6.90; N, 2.23; S, 19.34.
3
.4.8. Synthesis of the Polymer PBDTT-DTmFQx
PBDTT-DTmFQx was synthesized by the same procedure as PBDTT-DTQx. Yield 89.2%. Anal.
Calcd for C H F N S : C, 69.60; H, 6.44; N, 2.08; S, 19.05. Found: C, 70.01; H, 6.32; N, 2.13; S, 19.30.
78
86
2
2 8
4
. Conclusions
In summary, two novel D–A type NBG copolymers of PBDTT-DTQx and PBDTT-DTmFQx
were designed and synthesized. After grafting two F atoms on the Qx unit, the molecular energy
level, packing and charge-carrier transport capability of the PBDTT-DTmFQx have been finely tuned,
which resulted in a higher Voc, Jsc, FF of the PBDTT-DTmFQx-based device compared to that of
the nonfluorinated PBDTT-DTQx. The
µ of PBDTT-DTmFQx was 2.8 times more than that of
h
PBDTT-DTQx. The fluorinated PBDTT-DTmFQx/PC BM-based PSCs presented a maximum PCE
71
−
2
of 6.4% with a Voc of 0.87 V, Jsc of 12.0 mA cm , and FF of 61.45% under AM 1.5G illumination. The
results demonstrate that appending the fluorine atom onto the Qx unit through side-chain engineering
can effectively improve the photovoltaic properties of the corresponding copolymers.

Molecules 2019, 24, 54
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Author Contributions: Z.W., X.W., H.Z. and W.Z. (Wen Zhu) carried out the design, synthesis and characterization
of the polymers. H.J. and R.Y. designed, fabricated and characterized PSCs, X.-G.W., L.Y. and W.Z. (Wei Zeng)
performed the chemical structure characterization of all the compounds. Z.W. and X.W. wrote the manuscript.
Funding: The authors are deeply grateful to the Natural Science Foundation of Jiangsu Province (No.
BK20141151), and the National Natural Science Foundation of China (Nos. 51473139, 51773220, 51573205,
2
2
0974091), the Innovation Platform Open Foundation of University of Hunan Province (14K092) and Hunan
011 Collaborative Innovation Center of Chemical Engineering & Technology with Environmental Benignity
and Effective Resource Utilization, Science and Technology Program of Huizhou City (2017X0202012) for the
financial support.
Conflicts of Interest: The authors declare no conflict of interest.
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©
2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
CC BY) license (http://creativecommons.org/licenses/by/4.0/).
(