Significantly improved photovoltaic performances of the dithiophene-benzothiadiazole-alt-fluorene copolymers by incorporating carbazole units in fluorene moiety

Article abstract of DOI:10.1002/pola.24825

To exploit an effective way to improve polymeric photovoltaic performance, a series of dithiophene-benzothiadiazole-alt-fluorene copolymers containing carbazole groups at C-9 positions of the alternating fluorene units (PFO-FCz-DBT) were synthesized and characterized. The effect of the carbazole groups on the optophysical, electrochemical, and photovoltaic properties of these copolymers was investigated. By comparison, this type of copolymers with carbazole units exhibited significantly improved photovoltaic properties than poly(2,7-(9,9-dioctyl-fluorene)-alt-5,5-(4′,7′-di-2-thienyl- 2′,1′,3′-benzothiadiazole) (PFO-DBT) in the bulk heterojunction solar cells. A maximum power-conversion efficiency (PCE) of 2.41% and a highest short-circuit current density (J_sc) of 9.68 mA cm ^-2 were obtained for the PFO-FCz-DBT30, which are about two times higher than the corresponding levels for the PFO-DBT30. This work demonstrated that introducing a hole-transporting carbazole unit into copolymer is a simple and effective method to improve the J_sc and PCE. Copyright

Full text of DOI:10.1002/pola.24825

Significantly Improved Photovoltaic Performances of the
Dithiophene-Benzothiadiazole-alt-Fluorene Copolymers by
Incorporating Carbazole Units in Fluorene Moiety
MING LIU, YAFEI WANG, ZHIYONG ZHANG, JIANMING LI, YU LIU, HUA TAN, MEIJUN NI, GANGTIE LEI, MEIXIANG ZHU,
WEIGUO ZHU
Key Lab of Environment-Friendly Chemistry and Application of the Ministry of Education, College of Chemistry,
Xiangtan University, Xiangtan 411105, China
Received 31 March 2011; accepted 8 June 2011
DOI: 10.1002/pola.24825
Published online 7 July 2011 in Wiley Online Library (wileyonlinelibrary.com).
ꢀ
2
ABSTRACT: To exploit an effective way to improve polymeric
photovoltaic performance, a series of dithiophene-benzothia-
diazole-alt-fluorene copolymers containing carbazole groups at
C-9 positions of the alternating fluorene units (PFO-FCz-DBT)
were synthesized and characterized. The effect of the carbazole
groups on the optophysical, electrochemical, and photovoltaic
properties of these copolymers was investigated. By compari-
son, this type of copolymers with carbazole units exhibited sig-
nificantly improved photovoltaic properties than poly(2,7-(9,9-
highest short-circuit current density (Jsc) of 9.68 mA cm were
obtained for the PFO-FCz-DBT30, which are about two times
higher than the corresponding levels for the PFO-DBT30. This
work demonstrated that introducing a hole-transporting carba-
zole unit into copolymer is a simple and effective method to
improve the
J
sc and PCE. VC 2011 Wiley Periodicals, Inc.
J Polym Sci Part A: Polym Chem 49: 3874–3881, 2011
KEYWORDS: bulk heterojunction; carbazole; conjugated poly-
mers; copolymers; dithiophene-benzothiadiazole-alt-fluorene;
functionalization of polymers; photovoltaic performances; solar
cell; structure-property relations
0
0
0
0
0
dioctyl-fluorene)-alt-5,5-(4 ,7 -di-2-thienyl-2 ,1 ,3 -benzothiadia-
zole) (PFO-DBT) in the bulk heterojunction solar cells. A maxi-
mum power-conversion efficiency (PCE) of 2.41% and
a
INTRODUCTION Organic/polymeric solar cells have been
intensely explored in the past decade on account of the
increasing demand of sustainable and renewable energy sup-
ply arising from rapid depletion of fossil fuels and disastrous
to the second factor. In general, the charge mobility for the
ꢀ
3
ꢀ11
2
ꢀ1
thin polymer films is in the order of 10 to 10
cm V
ꢀ
1 12–14
s
,
it is very hard for polymer solar cells to get the
high efficiency in this situation. To improve charge carrier
mobility, the carbazole functional group with hole-transport-
ing property instead of fluorene unit was incorporated into
1
2
environmental problems. Since the seminal report by Tang,
organic/polymeric photovoltaic cells have made noteworthy
progress. Up to now, devices with solar power conversion
1
5–17
the main chain of copolymers.
ing copolymers have not exhibited improved photovoltaic
properties.
Nevertheless, the result-
3
efficiencies over 8.1% have been demonstrated.
1
7
By reviewing the materials, the power-conversion efficiency
(
PCE) of organic/polymeric solar cells was limited by at least
In this article, we reported a novel way to improve poly-
meric photovoltaic performance by incorporating carbazole
moiety into the copolymeric side chain instead of main
chain. To adequately understand the effect of the incorpo-
rated carbazole group in the side chain on polymeric photo-
voltaic performance, a series of new dithiophene-benzothia-
diazole-alt-fluorene copolymers containing carbazole units at
the fluorene’s C-9 position (PFO-FCz-DBT) and poly(2,7-
two main factors. One is the spectral mismatch between the
absorption spectra of the organic/polymeric photoactive
materials and the terrestrial solar radiation, which leads to
only a small portion of solar energy that can be utilized. The
other is poor charge-transporting property of organic/poly-
meric donor (D) and accept or (A) materials. A feasible solu-
tion to enhance the absorbed solar radiation has been made
by constructing low-bandgap donor polymers with an
0 0 0 0 0
(
9,9-dioctyl-fluorene)-alt-5,5-(4 ,7 -di-2-thienyl-2 ,1 ,3 -benzo-
4
–7
absorption edge extended into the near-infrared region.
As a result, various low-bandgap polymers have been devel-
oped and investigated to enhance the PCEs of their organic/
thiadiazole) (PFO-DBT) were synthesized. Their photophysi-
cal, electrochemical, and photovoltaic performances were
studied. As expected, incorporation of carbazole groups at C-
6
–10
polymeric cells.
9
positions of the alternating fluorene units is available to
markedly improve the photovoltaic performance of their
resulting copolymers. The significantly increased short-
However, only few of these low-bandgap polymers give a
PCE higher than 5%. The low efficiency is probably related
1
1
Correspondence to: W. Zhu (E-mail: zhuwg18@126.com) or Y. Liu (E-mail: yuliu03b@126.com)
Journal of Polymer Science Part A: Polymer Chemistry, Vol. 49, 3874–3881 (2011) V
2011 Wiley Periodicals, Inc.
C
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ARTICLE
SCHEME 1 Synthesis of the monomers and their copolymers.
1
9
20
circuit current density (J ) and PCEs were observed for
PFO-FCz-DBT instead of PFO-DBT in the bulk heterojunc-
tyl-fluorene (2), 9-octyl-9H-carbazole (3), and 4,7-bis(5-
sc
2
1
bromo-2-thienyl)-2,1,3-benzothiadiazole (7) were synthe-
tion (BHJ) solar cells with the same configuration.
sized according to the described procedures before.
Measurement and Characterization
H NMR spectra were recorded on a Bruker AV-400 using
EXPERIMENTAL
1
Materials
deuterated chloroform as solvent. Elemental analyses were
performed on a Perkin–Elmer 240 instrument. Average mo-
lecular weight and polydispersity index (PDI) of copolymers
were determined by a Waters 1515 gel permeation chroma-
tography using THF as an eluent and polystyrene as a stand-
ard. Thermogravimetric analyses (TGAs) were performed on
All chemicals were purchased from commercial sources and
used without further purification. Solvents were dried and
distilled by a standard procedure under nitrogen atmosphere
1
8
when necessary. 2,7-Dibromo-9,9-dioctyl-9H-fluorene (1),
,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxa-borolan-2-yl)-9,9-dioc-
2
PHOTOVOLTAIC PERFORMANCES OF A COPOLYMER, M. LIU ET AL.
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
1
FIGURE 1 H NMR spectra of copoly-
mers in CDCl
3
.
a Netzsch TG 209 analyzer under nitrogen atmosphere at a
The devices were completed after thermal deposition of a
10 nm lithium fluoride and a 0.5 nm aluminum film as the
ꢁ
ꢀ1
heating rate of 20 C min . Differential scanning calorime-
ter (DSC) was performed on a TA DSCQ10 instrument under
ꢀ
4
cathode at a pressure of 6 ꢂ 10 Pa. The active area was
ꢁ
ꢀ1
2
nitrogen flow at a heating rate of 20 C min . Surface mor-
phologies were recorded by atomic force microscopy (AFM)
on a Veeco, DI Multimode NS-3D apparatus in a trapping
mode under normal air condition at room temperature. UV-
vis spectra were measured on a Perkin–Elmer Lamada 25
spectrometer. Cyclic voltammetry (CV) measurements were
conducted using a EG&G Princeton Applied Research Model
9 mm for each cell. The thicknesses of the spun-cast films
were recorded by a profilometer (Alpha-Step 200; Tencor
Instruments).
Device characterization was carried out under an AM 1.5G
ꢀ2
irradiation with an intensity of 100 mW cm (Oriel 91160,
300 W) calibrated by a NREL-certified standard silicon cell.
Current density–voltage (J–V) characteristics were measured
by a computer-controlled Keithley 2602 source measurement
unit in the dark. PCEs were detected under a monochromatic
illumination (Oriel Cornerstone 260 1/4 m monochromator
equipped with Oriel 70613NS QTH lamp) and the calibration
of the incident light was performed with a monocrystalline
silicon diode. All device characterizations were performed
under an ambient atmosphere at room temperature.
273 potentiostat/galvanostat equipped with an electrochemi-
cal analysis system software and a standard three-electrode
configuration under an argon atmosphere at room tempera-
ture. Typically, a platinum rod and a platinum wire were
used as working and counter electrodes, respectively. A satu-
rated calomel electrode (SCE) was used as reference elec-
trode. Polymeric thin films on platinum rod were prepared
by dipping the electrode into a 0.2–1.0 wt % chloroform so-
lution and dried under a house-nitrogen steam before mea-
surement. All CV curves were measured in anhydrous aceto-
nitrile using tetrabutylammonium hexafluorophosphate (0.1
9,9-Di(9-octyl-9H-carbazole-3-yl)-2,7-dibromofluorene (4)
Eaton’s reagent (7.7 wt % P2O5 in CH SO H, 4.0 mL) was
3
3
added to a solution of 3 (8.76 g, 31.4 mmol) and 2,7-
ꢀ
1
M) as supporting electrolyte at a scan rate of 100 mV s
.
dibromo-9H-fluorene-9-one (4.84 g, 14.3 mmol) in CH Cl2
2
(
50 mL). The mixture was stirred under nitrogen atmos-
Polymer Solar Cell Fabrication and Test
phere and heated to reflux for 2 h, then allowed to cool
down to ambient temperature. The cooled mixture was
diluted with CH Cl and washed with aqueous sodium bicar-
bonate. The combined organic phase was dried over MgSO4
and distilled to remove CH Cl . The residue was purified by
column chromatography on silica gel using CH Cl /hexane
(
cyclohexane to afford 4 (4.32 g, 34.6%) as white crystal.
The photovoltaic cells were constructed with a traditional
sandwich structure through the following steps. The iridium-
tin oxide (ITO)-coated glass substrates were cleaned by a se-
ries 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 subjected to the treat-
2
2
2
2
2
2
1:5, V/V) as the eluent, followed by recrystallization from
ment of Ar/O
2
plasma for 5 min. A filtered aqueous solution
of poly(3,4-ethylenedioxy-thiophene)-poly-(styrenesulfonate)
1
(
PEDOT:PSS; Bayer AG) was spun-cast onto the ITO surface
3
H NMR (400 MHz, CDCl ): 0.87 (t, 6H), 1.26–1.41 (m, 20H),
ꢁ
at 2000 rpm for 30 s and then baked at 150 C for 30 min
to form a PEDOT:PSS thin film with a thickness of 30 nm. A
1
8
.84–1.89 (m, 4H), 4.28 (t, 4H), 7.15 (td, 2H), 7.30 (d, J ¼
.4 Hz, 2H), 7.32 (dd, 2H), 7.36 (d, J ¼ 9.2 Hz, 2H), 7.41 (td,
ꢀ
1
blend of copolymer and PC BM (1:3 w/w, 20 mg mL for
6
0
2H), 7.53 (dd, 2H), 7.66–7.68 (m, 4H), 7.93–7.94 (m, 4H).
polymer) was dissolved in chlorobenzene, filtered through a
0.45 lm poly(tetrafluoroethylene) filter and spun-cast at
Synthesis of PFO-DBT5
3
000 rpm for 30 s onto the PEDOT:PSS layer. The substrates
2,7-Dibromofluorene (1) (120 mmol), 2,7-bis(4,4,5,5-tetra-
methyl-1,3,2-dioxaborolan-2-yl)-9,9-dioctylfluorene (2) (92.3
were dried under N at room temperature and then
annealed at 150 C for 15 min in a nitrogen-filled glovebox.
2
ꢁ
mmol),
4,7-bis-(5-bromo-2-thienyl)-2,1,3-benzothiadiazole
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ARTICLE
TABLE 1 Molecular Weight and Thermal and Optical Properties of the Copolymers
4
a
4 a
a
ꢁ
b
ꢁ
c
d
Polymers
M
n
(ꢂ10 )
M
w
(ꢂ10 )
PDI
T
d
( C)
T
g
( C)
UV-vis (nm)
PFO-DBT5
4.3
5.6
7.2
4.4
6.5
7.9
10.3
12.9
13.7
1.7
1.9
2.0
1.8
2.1
2.2
441
444
456
445
456
458
107
108
108
128
137
140
397, 538
PFO-DBT15
PFO-DBT30
PFO-FCz-DBT5
PFO-FCz-DBT15
PFO-FCz-DBT30
a
398, 541
397, 567
7.92
303, 398, 539
303, 399, 539
303, 399, 548
13.0
16.6
c
ꢁ
ꢀ1
Calculated from gel permeation chromatograpy (eluent: THF; polysty-
rene standard).
Determined by DSC at a scan rate of 20 C min under nitrogen flow.
Measured in spun-cast polymer films from chloroform solution.
d
b
Temperature at 5% weight loss measured by TGA at a heating rate of
ꢁ
ꢀ1
2
0 C min under nitrogen.
(
7) (8.6 mmol), Pd(OAc)2 (6 mmol), and tricyclohexylphos-
DBT5 (136 mg) was obtained as a red solid with a yield of
66%. ELEM. ANAL. for PFO-FCz-DBT5: Found: C, 88.31; H,
8.65; N, 2.37; S, 0.27. Calcd: C, 88.59; H, 8.40; N, 2.57; S,
0.44%.
phine (3 mmol) were dissolved in a mixture of 20% w/w
tetrabutylammonium hydroxide aqueous solution (2 mL) and
toluene (4 mL) under N
2
atmosphere. The mixture was
ꢁ
heated to 90 C and stirred vigorously for 72 h. Phenyl-bor-
onic acid and bromobenzene were then added in sequence
to endcap the polymer chain. After cooled to room tempera-
ture, the resulting mixture was poured into methanol. The
resulting solid was filtered off, dissolved in chloroform, and
then precipitated in methanol. After being extracted with ac-
etone in a Soxhlet apparatus for 24 h, the final polymer of
PFO-DBT5 (136 mg) was obtained as a red solid with a
yield of 73.7%. ELEM. ANAL. for PFO-DBT5: Found: C, 88.67;
H, 10.47; N, 0.09; S, 0.48. Calcd: C, 88.85; H, 10.35; N, 0.18;
S, 0.62%.
Synthesis of PFO-FCz-DBT15
PFO-FCz-DBT15 was prepared according to the synthetic
prcedure of PFO-FCz-DBT5 as a red solid with a yield of
68%. ELEM. ANAL. for PFO-FCz-DBT15: Found: C, 88.01; H,
8.45; N, 2.37; S, 0.67. Calcd: C, 88.14; H, 8.24; N, 2.67; S,
0.91%.
Synthesis of PFO-FCz-DBT30
PFO-FCz-DBT30 was prepared according to the synthetic
prcedure of PFO-FCz-DBT5 as a dark red solid with a yield
of 63%. ELEM. ANAL. for PFO-FCz-DBT30: Found, C, 86.02; H,
8
2
.33; N, 2.68; S, 2.67. Calcd: C, 86.16; H, 8.08; N, 2.84; S,
.92%.
Synthesis of PFO-DBT15
PFO-DBT15 was prepared according to the synthetic prce-
dure of PFO-DBT5 as a red solid with a yield of 58%. ELEM.
ANAL. for PFO-DBT15: Found: C, 87.23; H, 10.24; N, 0.37; S,
RESULTS AND DISCUSSION
Synthesis and Characterization
1.67. Calcd: C, 87.55; H, 10.03; N, 0.55; S, 1.87%.
The synthetic routes of the monomers and copolymers are
shown in Scheme 1. The copolymers were obtained by a pal-
ladium-catalyzed Suzuki polycondensation. The molar ratio
Synthesis of PFO-DBT30
PFO-DBT30 was prepared according to the synthetic prce-
dure of PFO-DBT5 as a dark red solid with a yield of 61%.
ELEM. ANAL. for PFO-DBT30: Found: C, 85.31; H, 9.78; N,
1.02; S, 3.57. Calcd: C, 85.54; H, 9.53; N, 1.11; S, 3.82%.
Synthesis of PFO-FCz-DBT5
,9-Di(9-octyl-9H-carbazole-3-yl)-2,7-dibromo-fluorene
90 mmol), 2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-
yl)-9,9-di-octylfluorene (2) (110.7 mmol), 4,7-bis-(5-bromo-
-thienyl)-2,1,3-benzothiadiazole (7) (6.4 mmol), Pd(OAc)2
6 mmol), and tricyclohexylphosphine (3 mmol) were dis-
9
(4)
(
2
(
solved in a mixture of 20% w/w tetrabutylammonium hy-
droxide aqueous solution (2 mL) and toluene (4 mL) (1/2,
V/V) under N atmosphere. The mixture was heated to 90
2
ꢁ
C and stirred vigorously for 72 h. Phenylboronic acid and
bromobenzene were then added in sequence to endcap the
polymer chain. After cooled to room temperature, the result-
ing mixture was poured into methanol. The resulting solid
was filtered off, dissolved in chloroform, and then precipi-
tated in methanol. After being extracted with acetone in a
Soxhlet apparatus for 24 h, the final polymer of PFO-FCz-
FIGURE 2 TGA curves of the PFO-FCz-DBT and PFO-DBT
ꢁ
ꢀ1
copolymers at a scan rate of 20 C min
under nitrogen
atmosphere.
PHOTOVOLTAIC PERFORMANCES OF A COPOLYMER, M. LIU ET AL.
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 3 AFM images of the
blend films at a weight radio of
polymer:PC60BM is 1:3 for PFO-
DBT30:PC60BM (a) and PFO-FCz-
DBT30: PC60BM (b).
of the monomers 2–1 and 7 (or 2–4 and 7) was always kept
as 1:1, in which the monomer feed ratios of 1–7 or 4–7
were 10:90, 30:70, and 60:40, respectively. The correspond-
ing copolymers consist of the PFO-DBT and PFO-FCz-DBT
species, which are named as PFO-DBT5, PFO-DBT15, PFO-
DBT30, PFO-FCz-DBT5, PFO-FCz-DBT15, and PFO-FCz-
DBT30, respectively. Their chemical structures were con-
weights (M ) and the PDIs of these copolymers appear to be
n
ꢀ
1
in the range of 43,000–79,000 g mol and 1.7–2.2, respec-
tively, which indicate the incorporated carbazole groups has
slight influence on the synthesis of its copolymers.
Thermal Stability
The thermal property of the copolymers was investigated by
the DSC and TGA. The corresponding data are also summar-
1
firmed by H NMR and are shown in Figure 1. The specific
peak at 2.2 ppm is observed for the PFO-DBT copolymers,
ized in Table 1. The glass transition temperatures (T ) of
g
ꢁ
which is attributed to the signal of ACH A near on the C-9
PFO-FCz-DBT and PFO-DBT are in the region of 128–140 C
2
ꢁ
position of fluorene. The typical peak at 4.2 ppm is pre-
and 107–108 C, respectively. Obviously, the PFO-FCz-DBT
sented for the PFO-FCz-DBT copolymers, which is assigned
copolymers exhibited higher T than the PFO-DBT copoly-
g
to the signal of ACH A near on the C-9 position of carba-
mers. Figure 2 shows the TGA curves of two classes of
2
zole. As shown in Table 1, the number-average molecular
copolymers. The degradation temperatures (T ) of 5%
d
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ARTICLE
carbazole group into the side chain has few effects on the
dispersibility of its copolymers.
Optical Property
The UV–vis absorption spectra of the PFO-FCz-DBT and
PFO-DBT copolymers in their neat films are shown in Figure
4. Their UV–vis absorption data are summarized in Table 1.
All copolymers show similar two absorption peaks at about
3
97 and 552 nm, which is a common feature of internal D–A
2
2–24
copolymers.
The high-lying absorption peak originates
from p–p* transition of the fluorene-based donor units and
the low-lying absorption peak is assigned to p–p* transition
2
5
of the DBT-based acceptor units. When compared with the
PFO-DBT copolymers, the PFO-FCz-DBT copolymers display
an additional absorption peak at high-lying region about
302 nm, which is assigned to p–p* transition of the attached
carbazole segment at C-9 position of the fluorene unit.
Simultaneously, the absorption intensity of low-lying absorp-
tion band enhance with an increasing content of DBT. The
FIGURE 4 UV–vis absorption spectra for the PFO-DBT and PFO-
FCz-DBT copolymers in their neat films.
opt
optical band-gaps (Eg ) of these polymers calculated from
weight loss for PFO-FCz-DBT5, PFO-FCz-DBT15, and PFO-
FCz-DBT30 are 445, 456, and 458 C, respectively, which
are almost identical to those of the PFO-DBT copolymers. It
means that an introduction of carbazole groups into the C-9
the absorption onset in their neat films are 1.89–1.95 eV for
the PFO-DBT copolymers and 1.87–1.94 eV for the PFO-FCz-
DBT copolymers, which are summarized in Table 2. Clearly,
the UV–vis absorption property and energy level of these
copolymers can be tuned by simply changing the ratio of the
DBT acceptor group in the main chain and the attached caba-
zole group with hole-transporting property in the side chain.
ꢁ
position of fluorene has minor effect on T but is available to
d
g
increase T for their conjugated polymers. The relative high
Tg and Td of these copolymers are available for improving
the operating lifetime of solar cells.
Electrochemical Study
The electrochemical behaviors of these copolymer-based
films were investigated by CV, and the CV data are listed in
Table 2 to further understand the influence of energy levels
of the highest occupied molecular orbital (HOMO) and the
lowest unoccupied molecular orbital (LUMO) on the photo-
voltaic properties for these copolymers. All of these copoly-
mers presented a redox potential. The onset oxidation poten-
Dispersible Property
As photovoltaic properties of the polymer solar cells are
greatly affected by the surface morphology of the blend film,
it is necessary to investigate dispersible property of the
copolymers in the PC BM acceptor. Based on this considera-
6
0
tion, the typical blend films of PFO-FCz-DBT30 and PFO-
DBT30 in PC BM were made for comparison, in which the
tial (E ) and reduction potential (Ered) were observed in a
6
0
ox
weight ratio of copolymer and PC BM is 1:3. Their surface
range of 0.82–1.0 and ꢀ0.86 to ꢀ1.06 V versus SCE, respec-
tively. The HOMO and LUMO energy levels for these copoly-
mers, that is, EHOMO and ELUMO, were calculated according to
the empirical formulas EHOMO ¼ ꢀ(E þ 4.40) eV and E
6
0
morphologies were recorded by AFM in a tapping mod and
are shown in Figure 3. The roughness of the PFO-
DBT30:PC BM and PFO-FCz-DBT30:PC BM blend films
6
0
60
ox
LUMO
2
6
are 0.388 and 0.418 nm, respectively. These results show
that both PFO-FCz-DBT and PFO-DBT copolymers have a
good dispersibility in the PC BM matrix, and incorporating
¼ ꢀ(E
þ 4.40) eV, respectively. The electrochemical
red
bandgaps (E ) of these copolymers were calculated based on
g
the formula of E ¼ E
ꢀ EHOMO. It is worth noting that
6
0
g
LUMO
TABLE 2 Electrochemical and Photovoltaic Properties and Energy Gaps of the Copolymers
a
b
b
el
opt
2
Polymer Blend
HOMO (eV)
LUMO (eV)
E
g
(eV)
E
g
(eV)
J
sc (mA/cm )
V
oc (V)
PCE (%)
FF (%)
PFO-DBT5
ꢀ5.40
ꢀ5.38
ꢀ5.34
ꢀ5.35
ꢀ5.28
ꢀ5.22
ꢀ3.53
ꢀ3.54
ꢀ3.36
ꢀ3.53
ꢀ3.54
ꢀ3.34
1.87
1.84
1.78
1.82
1.74
1.68
1.95
1.91
1.89
1.94
1.89
1.87
0.28
1.10
4.84
2.96
3.81
9.68
0.49
0.54
0.79
0.61
0.86
0.74
0.04
0.21
1.54
0.52
1.11
2.41
0.29
0.35
0.40
0.29
0.32
0.34
PFO-DBT15
PFO-DBT30
PFO-FCz-DBT5
PFO-FCz-DBT15
PFO-FCz-DBT30
a
Polymer:PC60BM ¼ 1:3 for all blends.
b
The HOMO and LUMO levels were calculated from the onset of the
oxidation and reduction potentials measured by cyclovoltammetry for
the polymers in CH CN, respectively.
3
PHOTOVOLTAIC PERFORMANCES OF A COPOLYMER, M. LIU ET AL.
3879

JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
CONCLUSIONS
In summary, a series of the dithiophene-benzothiadiazole-alt-
fluorene copolymers containing carbazole groups at C-9 posi-
tions of the alternating fluorene units (PFO-FCz-DBT) were
synthesized and characterized. By comparision under the
same conditions, these PFO-FCz-DBT copolymers containing
carbazole units exhibited higher PCE and Jsc than those cor-
responding PFO-DBT copolymers in the BHJ solar cells. A
ꢀ
2
maximum PCE of 2.41% and a highest Jsc of 9.68 mA cm
were obtained for PFO-FCz-DBT30, which are about two
times higher than those corresponding levels for PFO-
DBT30. This work demonstrated that introduction of hole-
transporting carbazole units into the copolymeric side chain
is a simple and effective method to improve the Jsc and PCE
for their resulting copolymers.
This work was supported by the National Natural Science
Foundation of China (50973093, 20772101, and 20872124),
the Science Foundation of Hunan Province (2009FJ2002), the
Specialized Research Fund for the Doctoral Program of
Higher Education (2009 4301110004), the New Teachers
Fund for the Doctoral Program of Higher Education of China
FIGURE 5 J–V characteristics of the bulk heterojunction poly-
meric solar cells with a structure of ITO/PEDOT: PSS/copoly-
mer:PC60BM(1:3,w/w)/LiF/Al under the illumination of AM 1.5 G
ꢀ
2
from a solar simulator (1000 W m ).
(
200805301013) and the Postgraduate Science Foundation
the E level is lower than that of the E og pt, and the ELUMO of
g
for Innovation in Hunan Province (CX2009B124).
the PFO-FCz-DBT copolymers is close to that of the PFO-
DBT copolymers in the same DBT ratio. However, the EHOMO
of the PFO-FCz-DBT copolymers is 0.1 eV higher than that
of the corresponding PFO-DBT copolymers, respectively. This
result indicates that an introduction of carbazole moieties
into the C-9 position of fluorene can obviously increase the
HOMO energy level for the resulting copolymers. However, it
has little influence on the LUMO energy level for the result-
ing copolymers.
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