Synthesis and photovoltaic properties of an alternating polymer based fluorene and fluorine substituted quinoxaline derivatives

Article abstract of DOI:10.1016/j.reactfunctpolym.2013.07.015

An alternating polymer (PFOFTQx) with 9,9-dioctylfluorene (FO) as electron-rich unit and fluorine substituted quinoxaline (FTQx) as electron-withdrawing unit was synthesized and characterized. PFOFTQx showed similar absorption property with that of the counterpart polymer without fluorine atom (synthesized APFO-15). However, the low-lying highest occupied molecular orbit (HOMO) energy level of PFOFTQx was -5.37 eV, about 0.07 eV smaller than that of synthesized APFO-15. In order to study the photovoltaic properties of the materials, polymer solar cells (PSCs) were fabricated with PFOFTQx as donor blended with [6,6]-phenyl-C61-butyric acid methyl ester (PC₆₁BM) as acceptor. The power conversion efficiency (PCE) of PSC was 1.77% with a high open-circuit voltage (V_oc) of 0.90 V for an optimized PFOFTQx:PC₆₁BM weight ratio of 1:5, in comparison with that of synthesized APFO-15-based device (PCE of 1.60% with V_oc of 0.77 V). This study indicated that fluorine substituted quinoxaline-based polymers would be promising material with a higher V_oc for the application in polymer solar cells.

Full text of DOI:10.1016/j.reactfunctpolym.2013.07.015

Reactive & Functional Polymers 73 (2013) 1432–1438
Contents lists available at ScienceDirect
Reactive & Functional Polymers
journal homepage: www.elsevier.com/locate/react
Synthesis and photovoltaic properties of an alternating polymer based
fluorene and fluorine substituted quinoxaline derivatives
a
a
d
a
a
Haimei Wu a,b, Bo Qu c, , Di Tian , Zhiyuan Cong , Bowen Gao , Jianqun Liu , Zhongwei An ,
⇑
Chao Gao a, , Lixin Xiao c, Zhijian Chen c, Qihuang Gong c, Wei Wei b,
a Xi’an Modern Chemistry Research Institute, Xi’an Shaanxi 710065, People’s Republic of China
⇑
⇑
b Institute of Advanced Materials, Nanjing University of Posts and Telecommunications, Nanjing 210003, People’s Republic of China
c State Key Laboratory for Artificial Microstructures and Mesoscopic Physics, Department of Physics, Peking University, Beijing 100871, People’s Republic of China
d State Key Laboratory of Transient Optics and Photonics, Xi’an Institute of Optics and Precision Mechanics, Chinese Academy of Science (CAS), Xi’an Shaanxi 710119, People’s
Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
An alternating polymer (PFOFTQx) with 9,9-dioctylfluorene (FO) as electron-rich unit and fluorine substi-
tuted quinoxaline (FTQx) as electron-withdrawing unit was synthesized and characterized. PFOFTQx
showed similar absorption property with that of the counterpart polymer without fluorine atom (synthe-
sized APFO-15). However, the low-lying highest occupied molecular orbit (HOMO) energy level of
PFOFTQx was ꢀ5.37 eV, about 0.07 eV smaller than that of synthesized APFO-15. In order to study the
photovoltaic properties of the materials, polymer solar cells (PSCs) were fabricated with PFOFTQx as
donor blended with [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) as acceptor. The power conver-
sion efficiency (PCE) of PSC was 1.77% with a high open-circuit voltage (Voc) of 0.90 V for an optimized
PFOFTQx:PC61BM weight ratio of 1:5, in comparison with that of synthesized APFO-15-based device
(PCE of 1.60% with Voc of 0.77 V). This study indicated that fluorine substituted quinoxaline-based poly-
mers would be promising material with a higher Voc for the application in polymer solar cells.
Ó 2013 Elsevier Ltd. All rights reserved.
Received 26 December 2012
Received in revised form 15 June 2013
Accepted 29 July 2013
Available online 3 August 2013
Keywords:
Polymer solar cells
D–A polymers
Fluorine substituted quinoxaline
Fluorene
Energy level
1. Introduction
molecular orbit (HOMO) energy level to provide a big open-circuit
voltage (Voc) and a suitable lowest unoccupied molecular orbit
As a solar energy converter, polymer bulk heterojunction (BHJ)
solar cells based on conjugated polymers have attracted much
attention for their advantages of low cost, roll to roll printing pro-
cess, and light weight [1–4]. Rapid developments in this field have
led to power conversion efficiency (PCE) over 8% and reaching 10%
reported recently by Yang [5,6]. However, the efficiency of polymer
solar cells is still significantly lower than their inorganic counter-
parts, such as silicon, CdTe and CIGS, which prevents practical
applications in large scale.
There are many factors limiting the performance of BHJ solar
cells [7,8]. Among them, the materials of the active layer play an
important role in the whole performances of the polymer solar
cells (PSCs). Ideally, the polymers should have a broad absorption
to ensure effective harvesting of the solar photons and a high hole
mobility for charge transport. Furthermore, suitable energy levels
of the polymer are required that match those of the fullerene deriv-
atives, the polymer should have a low-lying highest occupied
(LUMO) energy level to provide enough offset for charge separa-
tion. In addition, it is imperative that a bicontinuous network of
the morphology with a domain size approximately twice that of
the exciton diffusion length and a large donor/acceptor interfaces
are formed, which favors the exciton dissociation and transport
of the separated charges to the respective electrode [9,10].
To satisfy above-mentioned requirements, many kinds of do-
nor–acceptor (D–A) copolymers have been designed for their intra-
molecular charge transfer from electron-rich unit to the electron-
withdrawing unit with tunable energy levels and bandgap. It is
an effective way to achieve large Voc through a deeper HOMO en-
ergy level material by the introduction of strong electron with-
drawing group into the polymer structure. For instance, through
replacing the ester with ketone and even more electronegative sul-
fonyl on thieno[3,4-b]thiophene (TT), rational deeper HOMO en-
ergy level and higher Voc of the corresponding devices were
achieved [11,12]. As an effective electron withdrawing group, fluo-
rine have attracted broad attention for its high electron affinity,
small size and without any deleterious steric effects when intro-
duced into the polymer [13,14]. For example, the two famous high
efficiency polymers based on benzodithiophene (BDT) and fluorine
⇑
Corresponding authors. Tel.: +86 29 88291190 (C. Gao).
E-mail addresses: bqu@pku.edu.cn (B. Qu), chaogao74@gmail.com (C. Gao),
weiwei@njupt.edu.cn (W. Wei).
1381-5148/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.reactfunctpolym.2013.07.015

H. Wu et al. / Reactive & Functional Polymers 73 (2013) 1432–1438
1433
substituted thieno[3,4-b]thiophene (TT) (PBDTTT-CF and PTB7)
were obtained with relatively low-lying HOMO energy level
[15,16]. Schroeder also confirmed that the HOMO energy level of
SilDT-2FBT was lowered than that of SilDT-BT in their study [17].
As a type of electron-withdrawing unit, quinoxaline has an elec-
tron-deficient N-heterocycle, which facilitates tuning the band gap
and energy levels. Quinoxaline-based copolymer TQ1 exhibited a
high Voc of 0.9 V and a very low-lying HOMO energy level of
ꢀ5.7 eV [18]. Another polymer with quinoxaline unit (P(T-Qx))
with HOMO energy level of ꢀ5.57 eV was reported recently [19].
Among these quinoxaline-based polymers, alternating polymer
poly[2,7-(9,9-dioctylflrorene)-alt-5,5-(50,80-di-2-thienyl-(20,30-
bis-(30-octyloxyphenyl)-quinoxaline))] (APFO-15) showed the lowest
HOMO energy level of ꢀ6.30 eV and highest Voc of 1.0 V [20].
We are interested in the quinoxaline-based polymers due to
their good charge-transfer characteristics and stability for device
application [21,22]. Through the introduction of fluorine atom into
the quinoxaline unit, a promising copolymer poly[6-fluoro-2,3-bis-
(3-octyloxyphenyl)quinoxaline-5,8-diyl-alt-thiophene-2,5-diyl]
(FTQ) with very deep HOMO energy level (ꢀ5.51 eV) and high PCE
of 5.3% were achieved [23]. However, the copolymers based on 9,9-
dioctylfluorene and fluorine substituted quinoxaline is less re-
ported to the best of our knowledge. In order to decrease the
HOMO energy level of APFO-15 and subsequently obtain enhanced
photovoltaic performance, in this work, an alternating fluorine
substituted quinoxaline copolymer poly[2,7-(9,9-dioctylflrorene)-
alt-5,5-(50,80-di-2-thienyl-(60-fluoro-20,30-bis-(300-octyloxyphenyl)-
quinoxaline))] (PFOFTQx) was synthesized. It is demonstrated that
the HOMO energy level of this polymer was lowered to ꢀ5.37 eV,
about 0.07 eV smaller than that of synthesized APFO-15. As a con-
sequence, Voc and PCE of polymer solar cells based on
PFOFTQx:[6,6]-phenyl-C61-butyric acid methyl ester (PC61BM)
were enhanced simultaneously in comparison with that of synthe-
sized APFO-15:PC61BM-based devices under the same conditions.
2.1.2. 1,2-Bis(3-(octyloxy)phenyl)ethane-1,2-dione (4)
A THF solution of anhydrous LiBr (3.19 g, 36.8 mmol, 15 ml) was
added to a suspension of CuBr (2.64 g, 18.4 mmol) in THF (15 ml)
under nitrogen. The mixture was stirred at room temperature until
it became homogeneous and was then cooled to 0 °C in an ice-
water bath. A freshly prepared solution of 3-octyloxybenzene mag-
nesium bromide in THF (5.23 g, 18.4 mmol, 20 ml) was added drop
wise to the mixture. After 10 min, oxalyl chloride (1.04 g,
8.22 mmol) was added and the mixture was stirred at 0 °C for
20 min. Then the reaction was quenched with saturated aqueous
NH4Cl and extracted with ethyl acetate, the combined organic
layer was washed with brine and dried over anhydrous magnesium
sulfate. After removal of the solvent under reduced pressure, the
residue was purified by column chromatography on silica gel using
hexane/ethyl acetate (100:1) as eluent to afford compound 4
(2.03 g) as a white solid in yield 53%. 1H NMR (CDCl3, 500 MHz)
d(ppm):
7.19(ddd,2H,J = 5.5 Hz),
7.51(s,2H),
7.45(m,2H),
4.00(t,4H,J = 6.5 Hz),
7.38(t,2H,J = 8 Hz),
1.79(m,4H),
1.46(m,4H), 1.33(m,16H), 0.89(t,6H,J = 7 Hz). 13C NMR(500 MHz,
CDCl3): d(ppm), 194.56, 158.36, 133.91, 129.88, 122.80, 122.15,
113.72, 68.39, 31.81, 29.33, 29.23, 29.13, 26.01, 22.67, 14.11. GC–
MS: m/z = 466.
2.1.3. 5,8-Dibromo-6-fluoro-2,3-bis(3-(octyloxy)phenyl)quinoxaline
(5)
A mixture of compound 1 (2.44 g, 8.58 mmol), compound 4 (4 g,
8.58 mmol), and acetic acid (70 ml) was briefly warmed to 60 °C, and
the solution was then stirred at room temperature for 2 h. The
precipitate was collected by filtration, washed with ethanol, and
dried to afford 5,8-dibromo-6-fluoro-2,3-bis (3-(octyloxy)
phenyl)quinoxaline
5
(5.83 g)
as
awhite
solide
in
yield 95.23%. 1H NMR(CDCl3, 500 MHz) d(ppm): 7.94(d,1H,J =
10 Hz), 7.21–7.24(m,4H),7.17(t,2H,J = 10 Hz),6.94(m,2H), 3.86
(t,4H,J = 6.5 Hz),
1.73(m,4H),
1.29–1.42(m,20H),
0.89(t,6H,
J = 6.5 Hz). 13C NMR (CDCl3, 500 MHz) d(ppm): 160.36, 159.09,
158.27, 154.52, 153.29, 139.60, 139.56, 139.00, 138.91, 136.43,
129.36, 122.56, 122.46, 116.72, 115.79, 115.73, 108.21, 108.05,
68.13, 31.56, 29.35, 29.28, 29.12, 26.04, 22.69, 14.12. mp: 89 °C.
Anal. Calcd for (C36H43Br2FN2O2) (%):C 60.51, H 6.07, N 3.92. Found
(%):C 59.96, H 6.14, N 3.97.
2. Experimental section
2.1. Materials and methods
All chemicals and reagents are obtained from Aldrich and Alfa
Aesar. THF is dried over Na/benzophenone ketyl and freshly dis-
tilled prior to use. 4,7-dibromo-5-fluoro-2,1,3-benzothiadiazole
(1) was purchased from Beijing Allmers Chemical S&T Co., Ltd.,
5,8-bis(5-bromothiophen-2-yl)-2,3-bis(3-(octyloxy)phenyl)quin-
oxaline [20] and 2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-
yl)-9,9-dioctylfluorene [24] were prepared according to the re-
ported literatures. The synthetic routes of monomers and copoly-
mers are shown in Scheme 1.
2.1.4. 5,8-Bis(5-bromothiophen-2-yl)-6-fluoro-2,3-bis(3-
(octyloxy)phenyl)quinoxaline (7)
A mixture of 5,8-dibromo-6-fluoro-2,3-bis-(3-octyloxyphenyl)-
quinoxaline (1.26 g, 1.76 mmol), 2-(tributylstannyl)thiophene
(1.44 g,
3.88 mmol),
dichlorobis-(triphenylphosphine)palla-
dium(II) (49 mg), and toluene (100 ml) were heated under reflux
overnight. It was then cooled and the toluene was removed under
reduced pressure and the residue was recrystallized by hexane and
obtained compound 6 (1.01 g) in 80% yield. To a suspension of
compound 6 (0.5 g, 0.7 mmol) in THF (20 ml) was added NBS
(0.261 g, 0.47 mmol). The mixture was heated at 40 °C for 4 h
and poured into methanol, the precipitate was collected by
filtration and recrystallized by hexane and afford 5,8-bis(5-bromo-
thiophen-2-yl)-6-fluoro-2,3-bis(3-(octyloxy)phenyl)quinoxaline
(0.43 g) in yield 70%. 1H NMR(CDCl3, 500 MHz) d(ppm): 7.90
2.1.1. 3,6-Dibromo-4-fluoro-1,2-phenylenediamine (1)
4,7-Dibromo-5-fluoro-2,1,3-benzothiadiazole (5 g, 0.016 mol)
was dissolved in ethanol (150 ml), then sodium borohydride
(12.1 g, 0.32 mol) was added portion wise at 0 °C, and the reactants
were stirred for 20 h at room temperature. After evaporation of the
solvent, 160 ml water was added, and the mixture was extracted
with ethyl acetate. The extract was washed with brine and dried
over anhydrous magnesium sulfate. The residue was purified by
column chromatography on silica gel using hexane/ethyl acetate
(25:1) as eluent to afford 3,6-dibromo-4-fluoro-1,2-phenylenedi-
amine (3.5 g) as a pale solid in 78% yield. 1H NMR (CDCl3,
(d,1H,J = 15 Hz),
7.22(ddd,2H,J = 5 Hz), 7.14(dd,2H,J = 5 Hz), 7.09(d,2H,J = 5 Hz),
6.98(d,2H,J = 5 Hz), 4.04(q,4H), 1.80(m,4H), 1.49(m,4H),
7.77(d,1H,J = 3.5 Hz),
7.53(dd,3H,J = 10 Hz),
1.33(m,16H), 0.89(t,6H,J = 5 Hz). 13C NMR (CDCl3, 500 MHz)
d(ppm): 159.94, 159.40, 157.91, 152.32, 150.98, 139.09, 139.00,
137.98, 137.75, 137.68, 133.60, 133.48, 133.43, 131.25, 131.16,
130.36, 130.21, 129.17, 129.09, 129.06, 126.07, 123.01, 122.90,
118.48, 117.82, 117.79, 117.40, 117.18, 116.23, 116.14, 115.77,
115.53, 115.18, 115.13, 68.36, 31.84, 29.45, 29.35, 29.33, 26.21,
500 MHz)
d(ppm):
6.81(d,1H,J = 8 Hz),
3.63(s,4H).
13C
NMR(500 MHz, CDCl3): d(ppm), 154.16, 152.25, 135.75, 135.73,
128.73, 128.71, 109.44, 109.35, 108.83, 108.62, 96.78, 96.58. mp:
94 °C. Anal. Calcd for (C6H5Br2FN2) (%):C25.38, H 1.78, N 9.87.
Found (%):C 24.29, H 1.83, N 10.14.

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H. Wu et al. / Reactive & Functional Polymers 73 (2013) 1432–1438
Scheme 1. Synthetic routes of monomers and copolymers.
22.67, 14.15. mp: 134 °C. Anal. Calcd for (C44H47Br2FN2O2S2) (%):C
(tetrahydrofuran,
polystyrene
standard):
Mn = 22.23 kDa,
60.14, H 5.39, N 3.19. Found (%):C 59.75, H 5.30, N 3.14.
Mw = 38.42 kDa, PDI = 1.73. Anal. Calcd for (C73H88N2O2S2)n (%):C
80.4, H 8.14, N 2.57. Found (%):C 79.68, H 8.28, N 2.12.
For PFOFTQx (190 mg, yield 85.7%). 1H NMR (CDCl3, 500 MHz)
d(ppm): 8.11(br,1H), 8.00(br,2H), 7.74(br,6H), 7.50(br,4H),
7.38(br,4H), 7.00(d,2H,J = 8 Hz), 3.91(t,4H,J = 6 Hz). GPC (tetrahy-
drofuran, polystyrene standard): Mn = 37.13 kDa, Mw = 102.69 kDa,
PDI = 2.76. Anal. Calcd for (C73H87N2O2FS2)n (%):C 79.16, H 7.92, N
2.53. Found (%):C 78.31, H 7.94, N 1.84.
2.1.5. For 5,8-bis(5-bromothiophen-2-yl)-2,3-bis(3-
(octyloxy)phenyl)quinoxaline (8)
(Yield 77%) 1H NMR(CDCl3, 500 MHz) d(ppm): 8.07(s,2H),
7.55(dd,4H,J = 5 Hz). 7.22(t,2H,J = 10 Hz), 7.11(dd,4H,J = 10 Hz),
6.98(ddd,2H,5 Hz), 4.04(t,4H,6.5 Hz), 1.81(m,4H), 1.42(m,20H),
0.85(t,6H,J = 5 Hz).
2.1.6. Synthesis of APFO-15 and PFOFTQx
2.2. Measurement and characterization
In a 50 ml dry flask, monomer 7 (175.7 mg, 0.2 mmol) or 8
(172.2 mg, 0.2 mmol) and 2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxa-
borolan-2-yl)-9,9-dioctylfluorene (128.5 mg, 0.2 mmol) were dis-
All compounds were characterized by nuclear magnetic reso-
nance spectra (NMR) recorded on a Bruker AV 500 spectrometer
in CDCl3 at room temperature. Molecular weights and distributions
of the copolymers were estimated by gel permeation chromatogra-
phy (GPC) method, THF as eluent and polystyrene as standard. The
absorption spectra were determined by a Unico UV-2102 scanning
spectrophotometer. Thermogravimetric analysis (TGA) of the poly-
mers was investigated on a Universal V2.6D TA instruments. Dif-
ferential scanning calorimetry (DSC) measurements were
performed on DSC Q20 V24.9 Build 121 with a heating rate of
20 °C/min. The electrochemical cyclic voltammetry was conducted
on a CHI 660D Electrochemical Workstation with Pt disk, Pt plate,
and Ag/Ag+ electrode as working electrode, counter electrode, and
reference electrode respectively in a 0.1 mol/L tetrabutylammo-
nium hexafluorophosphate (Bu4NPF6) acetonitrile solution. Poly-
mer thin films were formed by drop-casting chloroform solution
(analytical reagent, 1 mg/mL) onto the working electrode, and then
dried in the air. Atomic force microscopy (AFM) images were col-
lected in air under ambient conditions using the MultiMode scan-
ning probe microscope (Agilent Technologies 5500).
solved in
a mixture of 20% aqueous tetraethylammonium
hydroxide(1.5 ml) and degassed toluene (10 ml), the mixture was
flushed with nitrogen for 30 min, tris(dibenzylideneacetone)dipal-
ladium(0) (Pd2(dba)3) (3.7 mg) and tri(o-tolyl)phosphine (P(o-
Tol)3) (6.5 mg) were added, flushed with nitrogen again. Then the
mixture was vigorously stirred at 100 °C for 72 h under a nitrogen
atmosphere. The polymer was end-capped by adding phenyl boro-
nic acid (2.4 mg) and bromobenzene (2.2 lL) at the end of poly-
merization. After cooling to room temperature, the solution was
poured into methanol. The obtained polymer was then subjected
to Soxhlet-extracted with methanol, hexane and chloroform suc-
cessively. The chloroform solution was concentrated to a small vol-
ume, and precipitated by pouring this solution into methanol.
Finally, the polymer was collected by filtration, dried under vac-
uum at 50 °C overnight.
For APFO-15 (184 mg, yield 84.7%). 1H NMR (CDCl3, 500 MHz)
d(ppm): 8.22(br,2H), 7.98(br,2H), 7.72(br,6H), 7.48(br,4H),
7.36(br,4H),
7.00(d,2H,J = 8 Hz),
3.90(t,4H,J = 6 Hz).
GPC

H. Wu et al. / Reactive & Functional Polymers 73 (2013) 1432–1438
1435
2.3. Solar cell device fabrication and characterization
The organic photovoltaic cells were fabricated with ITO glass as
the anode, Al as the cathode and the blend films of polymer:
PC61BM as the active layer. The ITO glass was pre-cleaned and
modified by a thin layer of PEDOT:PSS, which was spin-coated
from a PEDOT:PSS aqueous solution (Bayer), and the thickness of
the PEDOT:PSS layer was about 40 nm. The photosensitive layer
was prepared by spin-coating a blend solution of the polymer
and PC61BM in chlorobenzene (CB) with the concentration of
15 mg/ml on top of ITO/PEDOT:PSS substrate and the thickness
of the active layer was about 80 nm. Then the blend system was
put in glove box overnight. Finally, 0.5 nm-thick LiF layer and
85 nm-thick Al layer were evaporated in sequence under the vac-
uum of 3 ꢁ 10ꢀ6 Torr. The effective area of the device was
0.0314 cm2. The current density–voltage (J–V) curves were ob-
tained using a Keithley 2611 source-measure unit. The photocur-
rent was measured with a solar simulator (Newport Thermal
Oriel 69911 300W, 4 in. ꢁ 4in. beam size) with AM 1.5G illumina-
tion at 100 mW/cm2. A calibrated mono silicon diode is used as a
reference.
PFOFTQx
APFO-15
50
100
150
200
250
Temperature (oC)
Fig. 2. DSC scans of polymers at a heating rate of 20 °C/min in nitrogen.
APFO-15 solution
APFO-15 film
PFOFTQx solution
PFOFTQx film
1.0
0.8
0.6
0.4
0.2
0.0
3. Results and discussion
3.1. Thermal properties
The thermal properties of polymers were determined by using
TGA (Fig. 1) and DSC (Fig. 2), the thermal decomposition tempera-
ture (5% weight loss) were 370 °C for synthesized APFO-15 and
340 °C for PFOFTQx, respectively. Consequently, the thermal prop-
erties of the two polymers are adequate for application in photo-
voltaic solar cells and other optoelectronic devices. From the DSC
measurement, synthesized APFO-15 showed a glass transition
temperature about 142 °C. However, no transition was observed
for PFOFTQx.
400
500
600
700
800
Wavelength/nm
3.2. Absorption properties
Fig. 3. Absorption spectra of polymers solution and film.
The absorption properties of polymers were investigated both
in CHCl3 solution and film state as shown in Fig. 3. The two copoly-
mers showed very similar absorption curves with two main
absorption peaks, the band in short-wavelength near 390 nm can
0.1mA
be assigned to the
p–
p⁄ transition of their conjugated backbone,
while the other peak of 525 nm for synthesized APFO-15 and
PFOFTQx
521 nm for PFOFTQx could be ascribed to the strong intramolecular
100
90
APFO-15
80
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
70
Potential / V vs. Ag/Ag+
60
Fig. 4. Cyclic voltammograms of polymer films on a platinum electrode in 0.1 M
Bu4NPF6/CH3CN solution at room temperature.
APFO-15
PFOFTQx
50
charge transfer between donor and acceptor. In the solid state, the
main absorption peak became broader and shifted toward longer
wavelength at 542 nm for synthesized APFO-15 and 540 nm for
PFOFTQx. The red shift about 20 nm from solution to film, which
0
100
200
300
400
500
Temperature (oC)
Fig. 1. Thermalgravimetric analysis curves of polymers with a heating rate of
10 °C/min in nitrogen.
indicated that
p stacked structure in the solid state might be

1436
H. Wu et al. / Reactive & Functional Polymers 73 (2013) 1432–1438
Table 1
Electrochemical and optical properties of the polymers.
Eegc (eV)
Eogpt (eV)a
Polymer
Eox (V)
Ered (V)
EHOMO (eV)
ELUMO (eV)
Synthesized APFO-15
PFOFTQx
0.58
0.65
ꢀ1.61
ꢀ1.50
ꢀ5.30
ꢀ5.37
ꢀ3.11
ꢀ3.22
2.19
2.15
1.90
1.91
ꢀ
ꢁ
Optical band gaps were estimated from the onset of electronic absorption of the polymer films Eogpt ¼ 1240=kðnmÞ
.
a
1
PFOFTQx:PC BM=1:1
61
PFOFTQx:PC BM=1:2
61
0
-1
-2
-3
-4
-5
PFOFTQx:PC BM=1:3
61
PFOFTQx:PC BM=1:4
61
PFOFTQx:PC BM=1:5
61
0.0
0.2
0.4
0.6
0.8
1.0
Fig. 5. Energy level diagrams of synthesized APFO-15, PFOFTQx and PC61BM.
Voltage (V)
Fig. 7. J–V curves of the PSCs based on PFOFTQx:PC61BM with different donor/
acceptor weight ratios (1:1, 1:2, 1:3, 1:4 and 1:5), under illumination of AM1.5G,
100 mW/cm2.
APFO-15:PC BM=1:1
61
0
-1
-2
-3
-4
APFO-15:PC BM=1:2
61
Eegc ¼ eðEox ꢀ Ered
Þ
ð3Þ
APFO-15:PC BM=1:3
61
APFO-15:PC BM=1:4
61
where the units of Ered and Eox are V vs. Ag/Ag+. The electrochemical
potentials and energy levels of polymers as shown in Table 1, the
onset potentials (Eox/Ered) of synthesized APFO-15 and PFOFTQx
are 0.58/ꢀ1.61 and 0.65/ꢀ1.50 V, respectively. From the Eox and Ered
values of the polymers, the corresponding HOMO, LUMO energy
APFO-15:PC BM=1:5
61
ꢀ
ꢁ
levels and the electrochemical bandgap Eegc of the polymers were
calculated and listed in Table 1. As expected, compared to synthe-
sized APFO-15, a lower HOMO energy level of ꢀ5.37 eV was ob-
tained for PFOFTQx, which was advantageous to the Voc for the
corresponding polymer solar cells. In addition, the LUMO energy le-
vel of PFOFTQx was significantly higher than that of PC61BM
(ꢀ3.91 eV, as shown in Fig. 5), the relative large offset facilitate
electron transferring from the polymer to the PC61BM. It should
be noted that the HOMO energy level of APFO-15 was ꢀ6.30 eV
[20], which was too deep to understand. However, a rational HOMO
of ꢀ5.30 eV for synthesized APFO-15 was obtained in this work.
0.0
0.2
0.4
0.6
0.8
Voltage (V)
Fig. 6. J–V curves of the PSCs based on synthesized APFO-15:PC61BM with different
donor/acceptor weight ratios (1:1, 1:2, 1:3, 1:4 and 1:5), under illumination of
AM1.5G, 100 mW/cm2.
formed. The optical bandgap of two copolymers can be calculated
about 1.90 eV for synthesized APFO-15 and 1.91 eV for PFOFTQx
from the absorption edges of the films.
3.4. Photovoltaic properties
3.3. Electrochemical properties
To investigate the photovoltaic properties of the copolymers,
polymer solar cells were fabricated with a general device structure
of ITO/PEDOT:PSS/polymer:PC61BM/LiF/Al. Synthesized APFO-15/
PFOFTQx was used as electron donor and PC61BM as electron
acceptor, chlorobenzene as solvent. The weight ratio of donor
and acceptor in the bulk-heterojunction (BHJ) layer was tuned
from 1:1 to 1:5, and the corresponding J–V curves of the devices
were shown in Figs. 6, 7 and summarized in Table 2. For synthe-
sized APFO-15, a reasonable Voc (0.77 V), Jsc (3.98 mA/cm2), FF
(0.52) and PCE (1.60%) were obtained by the optimized weight ra-
tio 1:4. However, this is inferior to the reported 3.7% [20]. It is well
known that the morphology of the active layer plays an important
role in the properties of the photovoltaic devices, so the morphol-
ogy of blend films (from 1:1 to 1:5) spin-coated from
Fig. 4 shows the cyclic volatmmograms of polymer films on Pt
electrode. The HOMO and LUMO energy levels of the polymers
were determined from the onset oxidation potentials (Eox) and
the onset reduction potentials (Ered) and by assuming the energy
level of ferrocene/ferrocenium (Fc/Fc+) to be ꢀ4.72 eV below the
vacuum level. The formal potential of Fc/Fc+ was measured as
ꢀ
ꢁ
0.09 eV against Ag/Ag+. The energy gap Eegc of the polymer was
calculated from the HOMO and LUMO energy levels. The calculat-
ing equations are as follows:
EHOMO ¼ ꢀeðEox þ 4:72Þ
ELUMO ¼ ꢀeðEred þ 4:72Þ
ð1Þ
ð2Þ

H. Wu et al. / Reactive & Functional Polymers 73 (2013) 1432–1438
1437
Table 2
Photovoltaic results of the PSCs based on synthesized APFO-15, PFOFTQx and PC61BM with different weight ratio under the illumination of AM1.5G (100 mW/cm2).
Polymer
D/A ratio
Voc (V)
Jsc (mA/cm2)
FF
PCE (%)
Synthesized APFO-15
1:1
1:2
1:3
1:4
1:5
0.73
0.75
0.75
0.77
0.78
2.73
4.13
3.82
3.98
3.41
0.38
0.46
0.52
0.52
0.51
0.76
1.42
1.49
1.60
1.33
PFOFTQx
1:1
1:2
1:3
1:4
1:5
0.96
0.88
0.84
0.84
0.90
1.91
3.32
4.21
4.25
4.40
0.26
0.30
0.42
0.47
0.45
0.47
0.90
1.48
1.69
1.77
(a) APFO-15:PC61BM=1:5
(b) APFO-15:PC61BM=1:5
(d) PFOFTQx:PC61BM=1:5
(c) PFOFTQx:PC61BM=1:5
Fig. 8. AFM images for polymer:PC61BM films (1:5) (0.5
lm ꢁ 0.5 lm). (a and c) The topography of polymer film. (b and d) The corresponding phase images.
chlorobenzene were investigated by atomic force microscopy
(AFM) (Fig. S14 and Fig. 8). Films 1:2 and 1:4 showed a much more
uniform than others but still had an average large domain size of
20–60 nm, which is disadvantageous to exciton dissociation and
charge transfer, and this might be the main reason for the differ-
ence of PCE with reported material [20].
As expected, it is notable that every device based on PFOFTQx
has a larger Voc than that of synthesized APFO-15-based device un-
der the same conditions as shown in Table 2. As illumination
above, the HOMO energy level of PFOFTQx was 0.07 eV smaller
than that of synthesized APFO-15 through introduction the fluo-
rine atom into the polymer, which could contribute to enhance-
ment of Voc and the PCE. In addition, both the Jsc, FF and PCE
vary monotonically with increasing PC61BM content (from 1:1 to
1:5), this increase as a function of PC61BM content might be
attributed to improved carrier transport, the formation of appro-
priate hole and electron percolation paths that is advantageous
to efficient charge collection to electrodes and decreasing in bulk
resistance, in addition to efficient dissociation of excited states
[25,26]. When the weight ratio of PFOFTQx:PC61BM reached 1:5,
reasonable Voc (0.90 V), FF (0.45), Jsc (4.40 mA/cm2) and PCE
(1.77%) were obtained.
In order to further study the difference in the photovoltaic per-
formance between PFOFTQx and synthesized APFO-15, the
morphology of PFOFTQx:PC61BM (1:5) and synthesized APFO-
15:PC61BM (1:5) were selected, carried out and shown in Fig. 8.
The PFOFTQx:PC61BM film showed a much more uniform and finer
domain structure with an average domain size of 20–40 nm (Fig. 8c
and d), which was not ideal but suitable for the exciton diffusion to
some extent. While the average domain size of synthesized APFO-

1438
H. Wu et al. / Reactive & Functional Polymers 73 (2013) 1432–1438
15 blended film was 30–70 nm (Fig. 8a and b), which indicated the
increase of the exciton diffusion length and recombination rate of
charges. So, if further improvement of the morphology can be
adopt (such as thermal annealing, solvent annealing and additives)
[27,28] to obtain idea morphology and domain length, enhanced
photovoltaic performance might be anticipated for the devices.
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Acknowledgements
This work was supported by NSFC (Grand Nos. 60907012,
60907015, 61177031, 61274054, 10934001 and 10821062), the
National Basic Research Program under Grant No. 2009CB930504.
Dr. Bo Qu was also supported by the Research Fund for the Doctoral
Program of Higher Education (RFDP) under Grant No.
20110001120124.
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Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.reactfunctp-
olym.2013. 07.015.