Synthesis and photovoltaic properties of conjugated copolymers bearing bis(9,9-di(2-ethylhexyl)-9H-fluoren-2-yl)quinoxaline subunit with deep HOMO level

Article abstract of DOI:10.14233/ajchem.2014.16346

A series of alternating copolymers (PT-BDFQx, PC-BDFQx and PBDT-BDFQx) have been synthesized bearing novel planar bis(9,9-di(2-ethylhexyl)-9H-fluoren-2-yl)quinoxaline (BDFQx) as acceptor unit, using benzo[1,2-b:4,5-b′]dithiophene (BDT), thiophene (T) and carbazole (C) as donor units via Stille or Suzuki coupling reactions. XRD characterization indicated that the presence of planar BDFQx unit (monomer 8) is favorable for the promotion of crystallization in the solid state and GPC results illustrated that the import of multiple chains in planar BDFQx unit raises the polymers molecular weight. Electrochemical measurement results suggested that three copolymers possess deep HOMO energy level of -5.50-5.77 eV. The polymer solar cell with structure of ITO/PEDOT:PSS (30 nm)/polymer: PCBM (60 nm)/Bphen (10 nm)/Ag (100 nm) exhibited the highest Voc of 0.80 V with PBDT-BDFQx as p-type polymer, while the best power conversion efficiency (PCE) of 0.9% was obtained using a blend of PBDT-BDFQx and PCBM(1:4) as active layer.

Full text of DOI:10.14233/ajchem.2014.16346

Asian Journal of Chemistry; Vol. 26, No. 18 (2014), 5959-5966
ASIAN JOURNAL OF CHEMISTRY
http://dx.doi.org/10.14233/ajchem.2014.16346
Synthesis and Photovoltaic Properties of Conjugated Copolymers Bearing
bis(9,9-di(2-ethylhexyl)-9H-fluoren-2-yl)quinoxaline Subunit with Deep HOMO Level
1
2
1
1
1
1,*
1,*
JING CHEN , XIANPING DENG , DAOBIN YANG , ERFU HUO , YUNQING CHEN , YAN HUANG and ZHIYUN LU
1
2
College of Chemistry, Sichuan University, Chengdu 610064, P.R. China
Key Laboratory of Environmentally Friendly Chemistry andApplications of Ministry of Education, College of Chemistry, Xiangtan University,
Hunan 411105, P.R. China
Corresponding authors: Tel/Fax: +86 28 85410059; E-mail: huangyan@scu.edu.cn; luzhiyun@scu.edu.cn; hef330@aliyun.com
Received: 12 September 2013; Accepted: 8 November 2013; Published online: 1 September 2014; AJC-15832
*
A series of alternating copolymers (PT-BDFQx, PC-BDFQx and PBDT-BDFQx) have been synthesized bearing novel planar bis(9,9-
di(2-ethylhexyl)-9H-fluoren-2-yl)quinoxaline (BDFQx) as acceptor unit, using benzo[1,2-b:4,5-b']dithiophene (BDT), thiophene (T)
and carbazole (C) as donor units via Stille or Suzuki coupling reactions. XRD characterization indicated that the presence of planar
BDFQx unit (monomer 8) is favorable for the promotion of crystallization in the solid state and GPC results illustrated that the import of
multiple chains in planar BDFQx unit raises the polymers molecular weight. Electrochemical measurement results suggested that three
copolymers possess deep HOMO energy level of -5.50-5.77 eV. The polymer solar cell with structure of ITO/PEDOT:PSS (30 nm)/
polymer: PCBM (60 nm)/Bphen (10 nm)/Ag (100 nm) exhibited the highest Voc of 0.80 V with PBDT-BDFQx as p-type polymer, while
the best power conversion efficiency (PCE) of 0.9 % was obtained using a blend of PBDT-BDFQx and PCBM(1:4) as active layer.
Keywords: Conjugated polymers, Polymer solar cells, Donor-acceptor, Bulk heterojunction, Deep HOMO energy level.
quinoxaline molecules have large flat-panel molecular
structure, which improve the material carrier mobility. The
most important thing is based on the receptor of copolymers,
INTRODUCTION
Organic/polymeric solar cells (PSCs) have been intensely
explored in the past decade on account of the increasing
demand of sustainable and renewable energy supply arising
from rapid depletion of fossil fuels and disastrous environ-
mental problems . Since the seminal report by Tang , organic/
poly-meric photovoltaic cells have made noteworthy progress.
After several generations of development, the concept of bulk
heterojunction (BHJ) structure solar cells is widely studied
due to good performances. Up to now, the power conversion
HOMO energy level of the polymer materials are relatively
10-11
low and located in -5.0 -5.4 eV
.
In order to use of the advantages of quinoxaline unit, get
lower HOMO energy level of conjugated polymer materials,
we developed a novel planar electron-deficient core of bis(9,9-
di(2-ethylhexyl)-9H-fluoren-2-yl)quinoxaline (BDFQx) to
1
2
12
expecting render deep HOMO polymers . Moreover, to gain
high molecular weight, fluorenes with long chains are imported
to the acceptor, which expected to improve the performance
of copolymers. By incorporation of different electron-donating
moieties such as carbazole, thiophene and benzo[1,2-b:4,5-
b']-dithiophene (BDT), three objective D-A copolymers with
DFAQx as acceptor motif have been synthesized. All of them
are found to show deep HOMO energy levels (-5.50-5.77 eV)
and high number-average molecular weight.
3
-5
efficiencies of BHJ-type polymeric solar cells have been
enhanced over 8 %.
There have been some research efforts employing electron-
deficient planar fused aromatic system asA subunit to construct
D-A copolymer backbones which both have low HOMO
energy level structure and the right energy gap, so as to enhance
6
-8
the open-circuit voltage of the device . It is known that high
oc is very important to improve power conversion efficiency,
V
which is decided by the energy difference between the HOMO
of the donor and the lowest unoccupied molecular orbital
EXPERIMENTAL
(
LUMO) of the acceptor (such as PCBM) in BHJ polymeric
The configuration of organic photovoltaic cells is ITO/
PEDOT:PSS (25 nm)/Polymer: PC60BM (m:n, w/w, 75 nm)/LiF
(0.7 nm)/Al (100 nm). The photovoltaic cells were constructed
9
solar cells . Quinoxaline (Qx) with pyrazine ring which has
power shortage shows to electron deficiency, combined with

5
960 Chen et al.
Asian J. Chem.
+
with a traditional sandwich structure through the following
steps. The iridium tin oxide (ITO)-coated glass substrates
were cleaned by a series of ultrasonic treatments for 10 min
in acetone, following by deionized water, then 2-propanol.
The substrates were dried under a stream of nitrogen and
a ferrocene-/ferrocenium (Fc/Fc ) redox couple as internal
standard. X-ray diffraction (XRD) data were generated using
the Philips DX-100 sealed-tube X-ray generator (Cu target; I
= 0.2 nm) with power of 40 kV and 35 mA and the scanning
angle was every 0.2° step over the range of 2-30°. The morpho-
logy of blend film was observed by an atomicforce microscopy
(AFM) (MFP 3D Asylum Research instrument) in contact
mode.
subjected to the treatment ofAr/O
aqueous solution of poly(3,4-ethylenedioxy-thiophene)-poly-
styrenesulfonate) (PEDOT:PSS; Bayer AG) was spun-cast
2
plasma for 5 min.A filtered
(
onto the iridium tin oxide surface at 2000 rpm for 30 s and
then baked at 150 °C for 0.5 h to form a PEDOT:PSS thin film
with a thickness of 25 nm. A blend of copolymer and PC60BM
Synthesis of monomers and polymers:All the materials
and reagents were commercially available and used without
further purification. Toluene was distilled from sodium freshly
(
1:3 w/w, 20 mg/mL for polymer) was dissolved in o-dichloro-
prior to use. Catalyst Pd(PPh
11 were synthesized according to the literature procedures
The detailed synthetic routes to the intermediates as well as
3
) and monomer 1, 2, 3, 9, 10,
4
13-17
benzene, filtered through a 0.45 µm poly (tetrafluoroethylene)
filter and spun-cast at 2100 rpm for 30 s onto the PEDOT:PSS
.
layer. The substrates were dried under N
2
at room temperature
objective polymers are outlined in Scheme-I and II.
18
and then annealed at 150 °C for 15 min in a nitrogen-filled
glove box. The devices were completed after thermal depo-
sition of a 10 nm lithium fluoride and a 0.5 nm aluminum film
2-Bromo-9H- fluorene (4) : In 250 mL round bottom
flask, fluorene (9.96 g, 60 mmol) and FeCl (1.46 g, 9 mmol)
dissolved in 80 mL DMF, then a solution of N-bromosuccimide
(NBS) (11.74 g, 66 mmol ) in 50 mL of DMF was added
dropwise to the flask in dark. The reaction mixture was stirred
for 24 h at room temperature. Then the mixture was washed
3
-5
as the cathode at a pressure of 6 × 10 Pa. The active area was
2
9
mm for each cell. The thicknesses of the spun-cast films
were recorded by a profilometer (Alpha-Step 200; Tencor
Instruments).
with 5 % of dilute hydrochloric acid, extracted with CHCl
3
Device characterization was carried out under an AM 1.5
G irradiation with an intensity of 100 mW/cm (Oriel 91160,
00 W) calibrated by a NREL-certified standard silicon cell.
(30 mL × 3) and dried over anhydrous magnesium sulfate.
After removal of solvent in the rotary evaporator, the residue
was collected and purified via recrystallization with ethanol/
water (v:v = 95:5) and methanol successively as solvent to
afford white crystal (13.2 g, 89.8 %). m.p.: 98-101 °C.
2-Bromo-9,9-di(2-ethylhexyl)-9H-fluorene (5) :2-Bromine
fluorene (13.2 g, 54 mmol), KI (1 g, 6 mmol), KOH (13.61 g,
243 mmol), 200 mL DMSO were added to a 500 mL round
bottom flask and stirred for 0.5 h, then a solution of brominated
isooctane (26.9 g, 139 mmol) was added dropwise to the flask.
The reaction mixture was stirred for 24 h at room temperature.
Then the mixture was washed with 200 mL ice water, extracted
2
3
Current density-voltage (J-V) characteristics were measured
by a computer-controlled Keithley 2602 source measurement
unit in the dark. Power conversion efficiencies (PCEs) were
detected under a monochromatic illumination (Oriel Corner-
stone 260 1/4 m monochromator equipped with Oriel 70613NS
QTH lamp) and the calibration of the incident light was perfor-
med with a monocrystalline silicon diode. All device charac-
terizations were performed under an ambient atmosphere at
room temperature.
18
1
13
H NMR spectra and C NMR spectra were recorded on
a Bruker AV-400 MHz spectrometer operating at 400 MHz
using deuterated chloroform as solvent. Number-average (M
and weight-average (M ) molecular weights and polydispersity
with CHCl (30 mL × 3) and dried over anhydrous magnesium
3
sulfate. After removal of solvent in the rotary evaporator, the
residue was collected and purified via column chromatography
on silica gel with petroleum ether as eluent to afford yellow
viscous liquid (16.7 g, 66 %).
n
)
w
indices (PDI) of the polymers were determined on a PL-GPC
model 210 chromatograph at 25 °C at a flow rate of 1 mL/min
and the calibration were based on THF as an eluent and poly-
styrene as a standard. UV-visible absorption spectra were
measured on a Shimadzu UV 2100 UV-visible spectrophoto-
2,2'-(1,2-Ethynediyl)-bis(9,9-di(2-ethylhexyl)-9H-
18
fluorene) (6) : In 250 mL three-necked flask, 9, 9-di(2-ethyl
hexyl)-2- bromide fluorene (9.8 g, 21 mmol) dissolved in 100
mL of toluene under the protection of argon atmosphere and
-6
meter in 10 mol/L o-dichlorobenzene (ODCB) solutions and
thin films casted from o-dichlorobenzene solution (30 mg/mL)
at 1,000 rpm on quartz substrates. Photoluminescence spectra
were measured using a Hitachi F-4500 spectrophotometer.
Thermogravimetric analysis (TGA) and differential scanning
calorimeter (DSC) were performed separately on Perkin-Elmer
TGA Q500 and Perkin-Elmer DSC Q100 instruments under
nitrogen atmosphere at a heating rate of 10 °C/min. Electro-
chemical measurements were conducted on a LK 2006
telectrochemical workstation at a scan rate of 100 mV/s using
tetrabutylammonium perchlorate (0.1 mol/ L) as supporting
stirred for 0.5 h. Then catalytic amount of PdCl
2
(PPh
3
) (0.71 g,
2
1.1 mmol), CuI (0.21 g, 1.1 mmol), 1,8-di-azabicyclo [5.4.0]
eleven carbon-7-ene (DBU) (15.98 g, 105 mmol), ethynyltri-
methylsilane (3.08 g, 31.5 mmol) and diisopropylamine (12.74
g, 105 mmol) were added to the flask under room temperature
and stirred for 1 h. Then 10 mL of benzene was injected into
the flask with a syringe at 80 °C for 10 h in dark. The mixture
was poured into the saturated ammonium chloride solution,
stirred for 0.5 h, extracted with CH
2
Cl (30 mL × 3) and dried
2
over anhydrous magnesium sulfate. After removal of solvent
in the rotary evaporator, the residue was collected and purified
+
electrolyte in acetonitrile with an Ag/Ag reference electrode
via column chromatography on silica gel with petroleum ether
1
and a platinum wire counter electrode. Polymer film was
formed by drop-casting 1 mL of polymer solutions in THF
analytical reagent, 1 mg/mL) onto the working electrode and
as eluent to afford white solid (10.9 g, 53 %). H NMR (CDCl
3
,
400 MHz, δ-ppm): 7.64-7.61 (m, 4H), 7.56-7.47 (m, 4H), 7.33-
7.26 (m, 6H), 1.99 (t, J = 8 Hz, 8H), 1.21-1.18 (m, 4H), 1.10-
1.00 (m, 36H), 0.83-0.60 (m, 20 H).
(
then dried in the air. Each measurement was calibrated using

Vol. 26, No. 18 (2014)
Synthesis and Photovoltaic Properties of Conjugated Copolymers 5961
Bis(9,9-di(2-ethylhexyl)-9H-fluoren-2-yl)-1,2-ethane-
General procedure for Stille cross coupling polymeri-
18
dione (7) : Intermediate 6(3.35 g, 4.5 mmol), I
2
(0.57 g, 2.3
zation: 0.606 mmol monomer 8, 0.606 mmol monomer 12,
mmol) and 25 mL DMSO was mixed in a flask in argon
atmosphere at 155 °C for 12 h. Then the reaction mixture was
cooled to room temperature, added to sodium sulfite solution
25 mL of degassed toluene and catalytic amount of Pd
2
(dba)
3
and P(o-Tol) were placed in a flask in argon atmosphere, then
3
stirred vigorously at 100 °C for 72 h. Then 1g bromobenzene
was added to the flask in argon atmosphere and then stirred
vigorously at 100 °C for 12 h. After the reactant was cooled
(25 mL, w = 4 %) and stirred for 0.5 h. The mixture was poured
into some water, extracted with CH Cl (30 mL × 3) and dried
2
2
over anhydrous magnesium sulfate. After removal of solvent
in the rotary evaporator, the residue was collected and
down to room temperature, 100 mL CHCl
organic layer was separated, washed with water (3 × 50 mL)
and dried over anhydrous MgSO . After removal of solvent
under vacuum, the resulted sticky residue was poured into
methanol (300 mL). The precipitate was filtered, washed with
3
was added. The
purified via recrystallization with ethanol as solvent to afford
4
1
yellow solid (2.44 g, 65 %). H NMR (CDCl
3
, 400 MHz, δ-
ppm): 8.09 (s, 2H), 7.91 (d, J = 8.8 Hz, 2H), 7.76 (m, 4H),
7
5
.42-7.36 (m, 6H), 2.02 (m, 8H), 1.26 (m, 4H), 0. 89-0.49 (m,
6H).
methanol and redissolved in CHCl (50 mL), followed by
3
precipitation from methanol for three times. The resulted
precipitate was collected and dried under vacuum.
5
,8-Dibromo-2, 3-di(9, 9-di (2-ethylhexyl)-9H-fluoren-
18
1
2-yl) quinoxaline (8) : In 100 mL two-necked flask, 30 mL
PT-BDFQx: Amaranth solid. Yield: 86.6 %. H NMR
glacial acetic acid was poured into it and deoxidized in argon
atmosphere for 0.5 h, then intermediates 7 (1.25 g, 1.5 mmol)
and 3 (0.3 g, 1.8 mmol) was added to the flask. The mixture
reacted at 110 °C for 4 h in argon atmosphere.After the reaction,
acetic acid was steamed to. The residue was collected and puri-
fied via column chromatography on silica gel with petroleum
(CDCl
3
, 400 MHz, δ-ppm): 8.38-8.01 (m, 6H), 7.66-7.52 (m,
6H), 7.38-7.29 (m, 6H), 2.06 (br, 8H), 0.90-0.70 (m, 40 H),
0.52 (br, 20H). C NMR (CDCl , 100 MHz, δ-ppm): 151.07,
150.23, 149.84, 141.21, 139.67, 136.81, 136.47, 130.82,
128.86, 127.35, 127.19, 126.66, 126.43, 125.78, 124.75,
123.19, 119.04, 117.92, 54.10, 44.00, 33.64, 32.33, 27.11,
25.50, 21.71, 13.02, 9.46.
PBDT-BDFQx: Atropurpuerus solid. Yield: 87.5 %. H
NMR (CDCl , 400 MHz, δ-ppm): 8.39-8.17 (m, 6H), 7.92-
7.54 (m, 6H), 7.42-7.31 (m, 6H), 4.44-4.31 (m, 4H), 2.09-
1.66 (m, 18H), 1.55-1.26 (m, 18H), 1.10-0.50 (m, 62H). C
13
3
ether/CH
2
1
Cl
2
(V:V = 5:1) as eluent to afford yellow solid (1.17
g, 73 %). H NMR (CDCl , 400 MHz, δ-ppm): 8.20 (br, 2H),
.89 (s, 2H), 7.65 (m, 2H), 7.45 (br, 4H), 7.31-7.25 (m, 6H),
.17-2.03 (m, 8H), 1.03-0.78 (m, 24H), 0.83-0.78 (m, 8H),
1
3
7
2
0
1
1
1
2
3
1
3
13
.70-0.54 (m, 28H). C NMR (CDCl
3
, 100 MHz, ppm):
54.58, 151.23, 151.14, 142.77, 140.51, 139.31, 136.48,
32.71, 130.79, 129.46, 127.09, 126.87, 125.98, 124.21,
23.79, 120.13, 118.75, 55.24, 44.83, 34.01, 28.31, 27.09,
6.57, 22.87, 22.72, 14.05.
NMR (CDCl , 100 MHz, δ-ppm): 151.27, 150.21, 149.98,
3
143.73, 141.41, 139.69, 138.30, 136.69, 136.30, 132.07,
131.21, 130.78, 128.85, 128.75, 128.68, 128.64, 128.61,
127.54, 127.45, 125.82, 124.84, 123.26, 119.76, 119.00,
119.00, 118.33, 54.17, 43.76, 39.76, 33.50, 32.32, 29.43,
28.68, 28.22, 27.02, 25.89, 25.42, 22.86, 22.28, 21.74, 13.18,
13.01, 10.28.
General procedure for Suzuki cross coupling polymeri-
zation: 0.675 mmol monomer 7, 0.675 mmol monomer 11,
0 mL of degassed toluene, 20 mL of 2 M aqueous potassium
carbonate and catalytic amount of Pd(PPh were placed in a
2
)
3 4
RESULTS AND DISCUSSION
flask in argon atmosphere, then stirred vigorously at 100 °C
for 72 h. Then 1 g bromobenzene and 1 g phenylboric acid
were added to the flask successively in argon atmosphere and
then stirred vigorously at 100 °C for 12 h, respectively. After
the reactant was cooled down to room temperature, 100 mL
The general synthetic routes to the monomers and
polymers are illustrated in Scheme-I and II. The conjugated
copolymers PBDT-BDFQx and PT-BDFQx bearing BDFQx
as subunit were synthesized via stille polycondensation
reaction with BDT and thiophene as electron-donating subunit,
while the conjugated copolymer PC-BDFQx was synthesized
via Suzuki polycondensation reaction with carbazole as
electron-donating subunit. The three copolymers possess
excellent solubility in common organic solvents such as THF,
chloroform, toluene and chlorobenzene. The molecular
structures of the monomers and polymers have been confirmed
CHCl
3
was added. The organic layer was separated, washed
with water (3 × 50 mL) and dried over anhydrous MgSO
4
.
After removal of solvent under vacuum, the resulted sticky
residue was poured into methanol (300 mL). The precipitate
was filtered, washed with methanol and redissolved in CHCl
50 mL), followed by precipitation from methanol for three
3
(
times. The resulted precipitate was collected and dried under
1
vacuum.
by H NMR. Molecular weight of the polymers was determined
1
PC-BDFQx:Yellow solid.Yield: 88 %. H NMR (CDCl
3
,
by gel permeation chromatography (GPC) using THF as the
eluent and polystyrenes as the standards at room temperature.
The results (vide Table-1.) indicate that number-average
4
6
0
1
1
1
1
3
2
00 MHz, δ-ppm): 8.31 (br, 4H), 8.02 (br, 2H), 7.56-7.42 (m,
H), 7.16 (br, 10H), 4.28 (br, 2H), 2.22-1.26 (m, 29H), 0.99-
13
.37 (m, 54H). C NMR (CDCl
3
, 100 MHz, δ-ppm): 150.21,
molecular weight (M ) of PC-BDFQx, PT-BDFQx and PBDT-
n
49.79, 149.83, 140.62, 140.27, 139.85, 138.80, 138.70,
37.71, 136.74, 136.56, 128.47, 128.24, 128.17, 126.90,
25.56, 125.35, 124.56, 124.44, 123.98, 123.08, 122.97,
22.20, 118.69, 117.20, 116.99, 107.60, 107.40, 53.95, 32.91,
0.90, 32.92, 30.91, 29.96, 28.67, 27.85, 27.01, 25.73, 25.5,
3.37, 22.19, 21.67, 21.10, 13.15, 9.88, 9.30.
BDFQx were found to be 10.6, 7.8 and 8.1kDa, with the
corresponding polydisperisty indices of 1.91, 1.63 and 1.93,
respectively (Table-1).
Thermal stability: Thermal properties of the copolymers
were determined via thermogravimetric analysis in nitrogen
atmosphere. As shown in Table-1 and Fig. 1, the 5 % weight-

5
962 Chen et al.
Asian J. Chem.
S
H2N
NH2
N
N
H N
2
NH2
Br
a
N
N
b
c
S
Br
Br
Br
2
3
Br
d
Br
e
f
R
R
R
R
R
R
4
5
6
7
Br
Br
N
N
O
C
O
C
g
h
;
3
+
7
R
R
R
R
R
R
R
R
7
R= -CH CH(C H )C H
9
8
2
2
5
4
Conditions: (a) SOCl , Et N; (b) Br , HBr(40%), reflux; (c) NaBH , EtOH, 0 °C; (d) DMF, NBS;
2
3
2
4
(
e) n-C H Br, KI, KOH, DMSO; (f) TMSA, Pd(PPh) Cl , CuI, DBU, i-Pr NH, Benzene; (g) I ,
8 17 3 2 2 2
3
DMSO; (h) CH COOH, 120 °C.
Scheme-I: Synthetic route to the intermediates
Scheme-II: Synthetic route to three polymers

Vol. 26, No. 18 (2014)
Synthesis and Photovoltaic Properties of Conjugated Copolymers 5963
TABLE-1
in solid films, all three polymers absorption spectra aren't
observed to red-shift, indicating the presence of unconspicuous
MOLECULAR WEIGHTS AND THERMAL
PROPERTIES OF COPOLYMERS
*
19
π-π stacking between the polymeric chains in solid state ,
which might be attributed to the long chain structure of difluo-
rene-substituted BDFQx unit. Additionally, from the onset
absorption wavelength of the spectra in films, the corres-
Polymer
PC-BDFQx
PT-BDFQx
PBDT-BDFQx
M (kDa)
M (kDa)
PDI
1.91
1.63
1.93
Td(°C )
373
371
w
n
20.3
12.7
15.6
10.6
7.8
8.1
326
20
ponding optical band gap of 2.58 eV for PC-BDFQx, 1.99
eV for PT-BDFQx and 1.97 eV for PBDT-BDFQx could be
estimated and data are summarized in Table-2.
loss temperatures for PC-BDFQx, PT-BDFQx and PBDT-
BDFQx are in the range of 326-373 °C and most of the polymers
show good thermostability which is adequate for application
of the copolymers as active materials in the optoelectronic
devices.
1
1
0
0
0
0
.2
.0
.8
.6
.4
.2
PBDT-BDFQx
PC-BDFQx
PT-BDFQx
1
10
100
BDFQx
9
8
7
6
5
4
3
0
0
0
0
0
0
0
PBDT-BDFQx
PC-BDFQx
PT-BDFQx
0.0
200
300
400
500
600
700
Wavelength (nm)
Fig. 2. Normalized absorption spectra of monomer BDFQx and three
polymers in chlorobenzene solution
100
200
300
Temperature (°C)
400
500
-1
Fig. 1. TGA plots of three polymers with a heating rate of 10 °C min in
nitrogen atmosphere
1.0
In film
PT-BDFQx
PBDT-BDFQx
PC-BDFQx
Optical properties: The absorption spectra of the polymers
0.8
-6
in dilute CHCl solution (10 mol/L) and in the solid films are
3
shown in Figs. 2 and 3 and the corresponding data are summa-
rized in Table-1. In the polymer solutions, to the polymers of
PT-BDFQx and PBDT-BDFQx, two intense absorption bands*
could be discernable, which could be assigned to the π-π
transition of BDFQx moiety (300-420 nm) and the intra-
molecular charge transfer (ICT) interactions between the
polymer backbone and the pendant acceptor groups transition
of the polymers (430-650 nm), respectively. As for polymer
PC-BDFQx, its absorption redshift only around 50 nm than
the monomer BDFQx, which indicated that forming polymer
did not increase the conjugate degree more. The reason may
be due to carbazole unit containing weaker electron-donating
ability, makes the polymer conjugate degree decreases. While
0
.6
0.4
0.2
0.0
2
00
300
400
500
600
700
800
Wavelength (nm)
Fig. 3. Normalized absorption spectra of three polymers in film state
TABLE-2
OPTICAL AND ELECTROCHEMICAL PROPERTIES OF THE SYNTHESIZED CONJUGATED POLYMERS
+
Absorption spectra
Cyclic voltammetry (vs. Ag/Ag ) (eV)
Egopt c(eV)
Polymer
a
b
d
d
d
Solution λ (nm) (log ε)
Film λ (nm)
HOMO
LUMO
E
g
max
max
PBDT-BDFQx
PT-BDFQx
529(1.12), 352(2.64), 289(2.48)
545(1.04), 346(3.02), 284(2.30)
398(1.69), 335(4.61), 307(5.12)
537, 352, 290
550, 346, 284
399, 339, 304
1.97
1.99
2.58
/
-3.53
-3.60
-3.40
/
-5.56
1.96
2.37
PC-BDFQx
-5.77
a
b
-1
UV–visible absorbance is determined in chlorobenzene solution, Thin films are spin-coated from 10 mg mL chlorobenzene solution,
Estimated from the onset of the absorption spectra of the thin films, Determined by CV of thin film samples
c
d

5
964 Chen et al.
Asian J. Chem.
onset
Structural properties: In order to gain the π-π accumu-
is because the polymer Ered is decided by the power shortage
units in the polymer. By formula, the HOMO energy level of
PC-BDFQx and PT-BDFQx are calculated to be -5.77 and
-5.56 eV, while the LUMO energy level of PC-BDFQx, PT-
BDFQx and PBDT-BDFQx are -3.40, -3.60 and -3.53 eV,
respectively. The energy gap of polymers can be calculated to
be 2.37 eV (PC-BDFQx) and 1.96 eV (PT-BDFQx), the results
are closed to the optical methods. According to the optical
energy gap, available the HOMO energy level of PBDT-
BDFQx is -5.50 eV. Compared with literature reports of similar
structure with quinoxaline unit for donor material (LUMO =
about -3.40 eV), the three polymers have lower LUMO energy
level (-3.40-3.60 eV) and HOMO energy level is reduced to
-5.50-5.77 eV, consistent with our design intent.
lation effect between polymer molecules and the formation of
the crystallite, the molecular organization of the copolymers
is examined by grazing incidence X-ray diffractometry
(
GIXRD). As shown in Fig. 4, two main diffraction peaks
around 12° and 22° could be observed in both the two polymeric
samples. The broad peaks at 22° (d ≈ 3.9 Å) may originate
from the face-to-face (π-π stacking) packing signal of the
21
aromatic groups in the main chain , especially that arises from
the packing of fused planar BDFQx block. The weak diffrac-
tion peak around at 12° (d ≈ 8.6 Å) probably comes from the
accumulation between fluorene units in the polymer monomer.
Hence, the presence of relative intense signals at 22° in all the
three polymer films implies the existence of closely packed
polymer chains in condensed state, which is good for the
22
enhancement of carrier mobility . Moreover, PB exhibits the
strongest signal at 22° in all three polymers, which should be
attributed to its larger planar BDT donar subunits.
600
PC-BDFQx
PT-BDFQx
400
2500
2000
1500
1000
PBDT-BDFQx
PC-BDFQx
PT-BDFQx
2
00
22°
0
–
200
–
0.5
0.0
0.5
1.0
1.5
2.0
Potential (V)
100
500
PC-BDFQx
1
0
20
(°)
Fig. 4. XRD diffractograms of the film samples of the polymers
30
PBDT-BDFQx
PT-BDFQx
5
0
2
θ
0
Electrochemical properties and electronic energy
levels: The highest occupied molecular orbital (HOMO) and
lowest unoccupied molecular orbital (LUMO) energy levels
of the conjugated polymers are important parameters in the
design of optoelectronic devices and they can be estimated
–50
100
150
–
23
from the onset oxidation and reduction potentials . As the
–
+
redox potential of Fc/Fc is considered to have an absolute
energy level of -4.8 eV relative to vacuum, the HOMO/LUMO
energy levels of the polymers are calculated using the
–
2.0
–1.5
–1.0
Potential (V)
Fig. 5. Cyclic voltammograms of polymer films on a platinum electrode
–0.5
0.0
0.5
2
4,25
onset
equation : HOMO/LUMO = -[E + 4.8] eV and electro-
chemical band gaps of these polymers are estimated according
-1
in 0.1 mol L Bu
4
NPF
6
acetonitrile solutions at a scan rate of 100
ec
onset
onset
to the equation: E
CV) of these macromolecules are shown in Fig. 5 and Table-
. In the scanning of the positive potential, except polymer
g
= Eox - Ered . Cyclic voltammograms
-1
mV s
(
2
Photovoltaic properties: To investigate the photovoltaic
properties of these copolymers, the BHJ polymer solar cells
with a sandwich structure of ITO/ PEDOT: PSS (25 nm)/
polymer: PC60BM (m: n, w/w, 75 nm)/ LiF (0.7 nm)/Al (100
nm) were fabricated. Each group of devices are made annealing
treatment at 80 °C. The J-V characteristics of the cells using
the three macromolecules as donor measured under the illumi-
PBDT-BDFQx, the other two polymers emerged oxidation
onset
initial potential (E ox ) at 0.97 V (PC-BDFQx) and 0.76 V
onset
(
PT-BDFQx), respectively. The Eox is decided by the electric
units of polymer and carbazole, thiophene and BDT for
electronic ability each are not identical. For the negative
onset
potential, three polymers started reduction potential (Ered
)
2
at -1.40 V (PC-BDFQx), -1.27 V(PT-BDFQx) and -1.20 V
PBDT-BDFQx), respectively. Three datas were similar. This
nation of AM 1.5 G (100 mW/cm ) from a solar simulator are
shown in Fig. 6 and the photovoltaic performances of these
(

Vol. 26, No. 18 (2014)
Synthesis and Photovoltaic Properties of Conjugated Copolymers 5965
copolymers-based cells are summarized in Table-3. We use
polymer PBDT-BDFQx which has the best absorption and
PCBM in different mass ratios to prepare polymer solar
cell devices, aimed to the ratio's influence to the device
performance and the test results shown in Tables 1-3. Results
can be seen from the data: when the quality ratio of 1:3, the
device has the highest efficiency of 0.90 %; when the ratio of
In addition, we prepare solar cell devices by making use
of three polymer materials and PCBM in mass ratio of 1:3,
observed the performance of different material cells and the
test results shown in Tables 1-3. Theoretical calculation of the
open circuit voltage through the HOMO and LUMO energy
level of the three kinds of polymers were above 1 V, but the
autual open circuit voltage of three polymers are lower than
theoretical data, that is because the open circuit voltage is also
affected by the film morphology. So we made blend membrane
with the mass ratio of 1:3 of polymer PT-BDFQx with PCBM
and morphology analysis of AFM (Fig. 7). Result shows that
the morphology of this kind of thin film is not ideal and the
RMS is up to 5.02 nm, which is not conductive to the contact
interface. This also leads to short circuit current and fill factor
are not too high, which is an important reason for the device
low efficiency. But the process of making film can be adjusted
to achieve a better state for the interface to improve efficiency,
so that, through the subsequent efforts to obtain high efficiency
promising device is expected.
1:2, the device has the highest open circuit voltage of 0.80 V;
the cause of the high efficiency for ratio of 1:3 due to the
improvement of short circuit current; the open circuit voltage
of three devices is above 0.60 V, but the fill factor and short
circuit current are low, which is a key factor of device low
efficiency.
0.0
–0.5
–1.0
–1.5
–2.0
–2.5
60.0 nm
5
4
3
0.0
0.0
0.0
PBDT-BDFQx:PCBM=1:1
PBDT-BDFQx:PCBM=1:2
PBDT-BDFQx:PCBM=1:3
PBDT-BDFQx:PCBM=1:4
6
0.0 nm
30.0
0
0.0
.4
.0
6
µ
m
20.0
10.0
0
8
.4 µm
4
.8
4
.8
0.0
0.2
0.4
0.6
Voltage (V)
0.8
1.0
1.2
3
.2
3
.2
1
.6
1
.6
0
.8
.4
.0
0
.0
0.0
Fig. 7. AFM image of PT-BDFQx/PCBM = 1:3 blend film (RMS = 5.02 n)
0
0
Conclusion
–
–
–
–
–
–
0.4
0.8
1.2
1.6
2.0
2.4
In summary, a series of alternating copolymers (PT-
BDFQx, PC-BDFQx and PBDT-BDFQx) have been synthe-
sized bearing novel planar bis(9,9-di(2-ethylhexyl)-9H-
fluoren-2-yl)quinoxaline (BDFQx) as acceptor unit, using
benzo[1,2-b:4,5-b']-dithiophene (BDT), thiophene (T) and
carbazole (C) as donor units. All polymers have deep HOMO
energy levels below -5.5 eV, but the PCE data are not ideal.
The reason may be due to the poor morphology of polymers
and require more study. By comparision under the same
conditions, these copolymers containing BDT units exhibited
higher PCE and Jsc than the other copolymers. A maximum
PC-BDFQx:PCBM=1:3
PT-BDFQx:PCBM=1:3
PBDT-BDFQx:PCBM=1:3
0
.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Voltage (V)
2
PCE of 0.9 % and a highest Jsc of 4.38 mA/cm were obtained
for PBDT-BDFQx. All these results demonstrate that BDFQx
Fig. 6. J-V curves of the polymer solar cells based on polymer:PC61BM
-2
under the illumination of AM 1.5, 100 mW cm
TABLE-3
PHOTOVOLTAIC PERFORMANCE OF THE POLYMER SOLAR CELLS BASED ON THE POLYMERS/PCBM BLENDS
2
Active layer
PBDT-BDFQx:PCBM=1:3
PBDT-BDFQx:PCBM=1:4
PBDT-BDFQx:PCBM=1:2
PBDT-BDFQx:PCBM=1:1
PT-BDFQx:PCBM=1:3
PC-BDFQx:PCBM=1:3
Voc (V)
0.70
0.60
0.76
0.68
0.61
0.43
J (mA/cm )
FF
PCE (%)
0.90
0.43
0.36
0.29
sc
4.38
2.21
1.75
1.59
0.77
0.29
0.29
0.33
0.27
0.27
0.35
0.30
0.17
0.04

5
966 Chen et al.
Asian J. Chem.
8
9
1
1
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Yu, Y. Liu and X. Pu, J. Mater. Sci., 47, 5535 (2012).
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is a promising building block as acceptor moiety for cons-
truction of high performance polymer solar cell.
ACKNOWLEDGEMENTS
0. E. Wang, L. Hou, Z. Wang, S. Hellström, F. Zhang, O. Inganäs and
M.R. Andersson, Adv. Mater., 22, 5240 (2010).
1. W.-H. Lee, S.K. Son, K. Kim, S.K. Lee, W.S. Shin, S.-J. Moon and
I.-N. Kang, Macromolecules, 45, 1303 (2012).
The authors gratefully thank the financial support from
the National Natural Science Foundation of China (Project
grant No. 50803040 and 20872103) and the Open Project
Program of Key Laboratory of Environmentally Friendly
Chemistry and Applications of Ministry of Education, China
12. M.H. Lai, J.H. Tsai, C.C. Chueh, C.F. Wang and W.C. Chen, Macromol.
Chem. Phys., 211, 2017 (2010).
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1
Center of Sichuan University for providing measurements of
NMR and UV-visible for the intermediates and objective
compounds.
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