A low band gap donor-acceptor copolymer containing fluorene and benzothiadiazole units: Synthesis and photovoltaic properties

Article abstract of DOI:10.1039/c0nj00378f

A new low band gap copolymer containing dialkylfluorene and 4,7-dithienyl-2,1,3-benzothiadiazole (TBT), poly(fluorenevinylene-alt-4,7- dithienyl-2,1,3-benzothiadiazole) (PF-TBT) was synthesized by Heck cross-coupling polymerization. The copolymer is soluble in common organic solvents such as chloroform, tetrahydrofuran and chlorobenzene. The TGA result indicated that the copolymer possesses good thermal stability. The absorption, electrochemical and photovoltaic properties of PF-TBT were investigated and compared with those of poly(fluorenevinylene-alt-4,7-diphenyl-2,1,3- benzothiadiazole) (PF-DBT) whose structure is similar to PF-TBT. The copolymer exhibited a broad absorption band with an absorption edge close to 700 nm and an optical band gap of 1.82 eV. Cyclic voltammetry studies indicated that the relatively low HOMO energy level assured a higher open circuit voltage (V _oc) when PF-TBT is used as the donor material in a photovoltaic cell. The bulk heterojunction (BHJ) solar cell using PF-TBT as the donor and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) as the acceptor with the structure of ITO/PEDOT:PSS/copolymer:PCBM/LiF/Al, exhibited a V_oc of 0.86 V, short-circuit current density (J_sc) of 3.97 mA cm ^-2, fill factor (FF) of 0.35, and a power conversion efficiency (PCE) of 1.18% under one sun of AM 1.5 solar simulator illumination (100 mW cm ^-2).

Full text of DOI:10.1039/c0nj00378f

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PAPER
A low band gap donor–acceptor copolymer containing fluorene and
benzothiadiazole units: synthesis and photovoltaic properties
Jianing Pei, Shanpeng Wen, Yinhua Zhou, Qingfeng Dong, Zhaoyang Liu,
Jibo Zhang and Wenjing Tian*
Received (in Montpellier, France) 19th May 2010, Accepted 5th July 2010
DOI: 10.1039/c0nj00378f
A new low band gap copolymer containing dialkylfluorene and 4,7-dithienyl-2,1,3-
benzothiadiazole (TBT), poly(fluorenevinylene-alt-4,7-dithienyl-2,1,3-benzothiadiazole) (PF-TBT)
was synthesized by Heck cross-coupling polymerization. The copolymer is soluble in common
organic solvents such as chloroform, tetrahydrofuran and chlorobenzene. The TGA result
indicated that the copolymer possesses good thermal stability. The absorption, electrochemical
and photovoltaic properties of PF-TBT were investigated and compared with those of
poly(fluorenevinylene-alt-4,7-diphenyl-2,1,3-benzothiadiazole) (PF-DBT) whose structure is similar
to PF-TBT. The copolymer exhibited a broad absorption band with an absorption edge close to
7
00 nm and an optical band gap of 1.82 eV. Cyclic voltammetry studies indicated that the
relatively low HOMO energy level assured a higher open circuit voltage (V ) when PF-TBT is
oc
used as the donor material in a photovoltaic cell. The bulk heterojunction (BHJ) solar cell using
PF-TBT as the donor and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) as the acceptor
with the structure of ITO/PEDOT : PSS/copolymer : PCBM/LiF/Al, exhibited a Voc of 0.86 V,
À2
short-circuit current density (Jsc) of 3.97 mA cm , fill factor (FF) of 0.35, and a power
conversion efficiency (PCE) of 1.18% under one sun of AM 1.5 solar simulator illumination
À2
(
100 mW cm ).
Introduction
The state of internal charge transfer (ICT) transition from
donor to acceptor inside the copolymer absorbs photons with
long wavelengths which could extend the absorption and
decrease the band gap of the copolymer. Furthermore, the
physical properties of the copolymers could be tuned flexibly
by changing the donor or acceptor units and the linking bridge
between the donor and the acceptor. As usual, thiophene,
Polymer solar cells have great potential as candidates for future
1
energy due to low cost, flexibility and large-area fabrication.
–3
During the past few decades, bulk heterojunction (BHJ) solar
cells consisting of a polymer : fullerene blend film as the active
layer have been one of the most successful types of polymer
4
solar cells. Up to now, significant effort has been made
5
20
21,22
pyrrole, fluorene and carbazole
23
electron-donating units, whereas quinoline, quinoxaline,
are widely used as
24
–7
to synthesise new materials
8
and to optimize device
–10
structures
to enhance the efficiency of the polymer solar
25
and pyridazine are employed as electron-withdrawing units,
and the electron-donating unit and electron-withdrawing unit
were connected through a single bond inside the copolymer.
Polyfluorene (PF) and its derivatives are a new class of
organic semiconductors due to their unique electro-optical
cells. In order to further improve the efficiency of BHJ solar
cells, researchers have paid more attention to the synthesis of
novel polymers with a suitable band gap to match solar
1
1–13
irradiation,
a suitable energy level for a high open-circuit
15
1
voltage (Voc), a high charge carrier mobility and good film
4
properties, such as high charge carrier mobility and good
26,27
1
6
forming ability.
processability.
However, the optical band gap of
To expand the spectral absorption of polymers, one promising
method is to introduce an electron-donating unit and an
electron-withdrawing unit into the polymer main chain to
28
poly(9,9-dialkylfluorene) is ca. 3.0 eV, which is too high
for efficient sunlight harvesting because the maximum photon
flux of sunlight is around 1.7 eV. In order to narrow the band
gap of the polymers containing PF for optimal sunlight
collection, researchers incorporated an electron-withdrawing
unit into the PF main chain to form a D–A alternating
1
7–19
achieve the alternative donor–acceptor (D–A) copolymers.
State Key Laboratory of Supramolecular Structure and Materials,
Jilin University, 2699 Qianjin Avenue, Changchun 130012,
P. R. China. E-mail: wjtian@jlu.edu.cn; Fax: + 86431 85193421;
Tel: + 86431 85155359
29
arrangement. Previously, we reported a new narrower optical
3
0
band gap PF-based copolymer,
poly(fluorenevinylene-
alt-4,7-diphenyl-2,1,3-benzothiadiazole) (PF-DBT; the band
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2
8
gap of PF-DBT is 2.40 eV, and that of PF is 3.0 eV), which is
synthesized by using a phenyl unit as a conjugated bridge
between the electron-donating fluorene unit and electron-
accepting 2,1,3-benzothiadiazole unit.
and the dark red precipitate formed was filtered off and
recrystallized from DMF to give the title compound (1.1 g,
36.3%) as shiny red solid.
1
H NMR (500 MHz, CDCl ): d 7.81 (d, J = 3.5 Hz, 2H),
3
To further lower the band gap of PF-based copolymer for
efficient sunlight harvesting, here we used a thiophene unit
to replace the phenyl unit between the electron-donating
fluorene unit and electron-withdrawing 2,1,3-benzothiadiazole
unit to synthesize a low band gap D–A copolymer poly-
7.79 (s, 2H), 7.15 (d, J = 4.0 Hz, 2H). Anal. calcd for
C H N S Br : C, 36.70; H, 1.32; N, 6.11; S, 20.99. Found:
2
1
4
6
2
3
C, 36.98; H, 1.61; N, 6.32; S, 21.52.
2,7-Diformyl-9,9-di-n-octylfluorene (3). Into a solution of
2,7-dibromo-9,9-di-n-octylfluorene (2) (6.00 g, 11.0 mmol) in
anhydrous THF (100 mL) was added n-BuLi (2.5 M in hexane,
9.60 mL, 24.0 mmol) dropwise at À78 1C. After the solution
had been stirred for 1 h, anhydrous DMF (2.2 mL, 28.4 mmol)
was added dropwise. The resulting mixture was stirred at
À78 1C for 30 min and 16 h at room temperature, and
then poured into water (400 mL). The mixture was extracted
with dichloromethane (3 Â 100 mL). The organic extracts
(
(
fluorenevinylene-alt-4,7-dithienyl-2,1,3-benzothiadiazole)
PF-TBT). Thiophene was used because it has been shown to
have excellent optical and hole-transporting properties as
well as an exceptional electron-donating ability and chemical
2
8
stability in BHJ solar cells. Compared with the previous
copolymer PF-DBT, PF-TBT exhibited a broader absorption
band with an absorption edge close to 700 nm and the lower
optical band gap (1.82 eV). The BHJ solar cell was fabricated
using PF-TBT as the donor and PCBM as the acceptor. The
solar cell with a structure of ITO/PEDOT : PSS/copolymer :
4
were washed with brine, dried with anhydrous MgSO ,
and concentrated. The product was purified on a silica gel
column with petroleum ether/dichloromethane as the eluent to
PCBM/LiF/Al exhibited an open circuit voltage (Voc) of
À2
0
.86 V, short-circuit current density (Jsc) of 3.97 mA cm
,
give pale yellowish green solid in a yield of 69% (3.40 g).
1
fill factor (FF) of 0.35, and a PCE of 1.18% under one sun of
À2
3
H NMR (500 MHz, CDCl ): d 10.10 (s, 2 H), 7.90–7.94
AM 1.5 solar simulator illumination (100 mW cm ).
(m, 6 H), 2.06–2.10 (m, 4 H), 0.91–1.29 (m, 20 H), 0.80
t, J = 6.9 Hz, 6 H), 0.45–0.61 (m, 4 H). Anal. calcd for
C H O : C, 83.36; H, 9.48. Found: C, 83.10; H, 9.16.
(
3
1
42
2
Experimental section
Materials
2,7-Diethenyl-9,9-di-n-octylfluorene (M-2). To a solution of
,7-diformyl-9,9-di-n-octylfluorene (3) (1.5 g, 3.36 mmol) and
2
All starting materials were purchased from either Acros or
Aldrich Chemical Co. and used without further purification,
unless otherwise noted. In synthetic preparations, diethyl ether
and THF were dried by distillation from sodium/benzophenone
under nitrogen. Similarly, DMF and dichloromethane were
triphenylmethylphosphonium bromide (2.93 g, 8.07 mmol) in
degassed anhydrous THF (40 mL) was added potassium
tert-butoxide (1.0 M solution in THF, 9.5 mL, 9.50 mmol)
dropwise at 0 1C. The resulting mixture was stirred for 8 h at
room temperature and then poured into 150 mL water. The
mixture was extracted with petroleum ether (3 Â 100 mL). The
organic extracts were washed with brine, dried with anhydrous
3
1
distilled from CaH
2
under nitrogen. 9,9-Di-n-octylfluorene,
2
,7-dibromo-9,9-di-n-octylfluorene and 4,7-dibromo-2,1,3-
32
benzothiadiazole were prepared according to known literature
procedures.
MgSO
4
, and concentrated. The product was purified on silica
gel column with petroleum ether as eluent to afford 1.16 g
(
78%) product as a colorless oil.
1
Synthesis
H NMR (500 MHz, CDCl ): d 7.62 (d, J = 8.0 Hz, 2 H),
3
Monomers synthesis
7
5
1
6
1
.34–7.40 (m, 4 H), 6.80 (dd, J = 11.0 Hz, J = 6.5 Hz, 2 H),
1 2
4
,7-Di-2-thienyl-2,1,3-benzothiadiazole (5). To a solution of
,7-dibromo-2,1,3-benzothiadiazole (4) (2.0 g, 6.8 mmol) and
tributyl(2-thienyl) stannane (6.1 g, 16.4 mmol) in THF (50 ml),
PdCl (PPh (97 mg, 2 mol%) was added. The mixture was
.80 (d, J = 17.5 Hz, 2 H), 5.25 (d, J = 11.0 Hz, 2 H),
.93–1.97 (m, 4 H), 1.01–1.30 (m, 20 H), 0.81 (t, J = 7.0 Hz,
4
H), 0.58–0.67 (m, 4 H). Anal. calcd for C33
0.47. Found: C, 89.27; H, 10.65.
H46: C, 89.53; H,
2
3 2
)
refluxed in a nitrogen atmosphere for 3 h. After the removal of
the solvent at a reduced pressure, the residue was purified by
Polymer synthesis
A flask was charged with a mixture of M-1 (0.174 g,
.393 mmol), M-2 (0.18 g, 0.393 mmol), Pd(OAc) (0.0045 g,
column chromatography on silica gel (eluent CH
2 2
Cl –hexane,
1
: 1). Recrystallization from ethanol gave the title compound
0
2
(
1.8 g, 80%) as red needles.
1
H NMR (500 MHz, CDCl ): d 8.11 (d, J = 8.5 Hz, 2H),
0.035 mmol), P(o-tolyl)
(0.037 g, 0.195 mmol), DMF
3
3
(
8 mL), and triethylamine (2 mL). The flask was degassed
7
.87 (s, 2H), 7.46 (d, J = 5.0 Hz, 2H), 7.21 (m, 2H). Anal.
2
and purged with N . The mixture was heated at 90 1C for
2
calcd for C H N S : C, 55.97, H, 2.68, N, 9.32, S, 32.02.
1
4
8
2 3
4 h under nitrogen atmosphere. The polymer was precipitated
Found: C, 56.24; H, 2.92; N, 9.58; S, 32.35.
into 200 mL of methanol, and filtered through a Soxhlet
thimble, which was then subjected to Soxhlet extraction
with methanol, hexane, acetone, and chloroform. The fraction
from chloroform was concentrated under reduced pressure
and precipitated into methanol (200 mL), and collected by
filtration. The final product was dried under vacuum overnight
to afford PF-DBT as a black solid (0.17 g, 64%).
4
,7-Bis(5-bromo-2-thienyl)-2,1,3-benzothiadiazole (M-1).
To a mixture of chloroform (50 mL) and acetic acid
50 mL), 5 (2.0 g, 6.67 mmol) was added in a nitrogen flow.
After the solid dissolved completely, N-bromosuccinimide
NBS, 2.5 g, 14.01 mmol) was added in one portion. The
reaction mixture was stirred at room temperature overnight,
(
(
3
86 New J. Chem., 2011, 35, 385–393
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1
H NMR (500 MHz, CDCl
H), 7.71–7.60 (m, 2H), 7.53–7.13 (m, 10H, aromatic and
vinylic), 1.92–2.08 (m, 4 H), 1.00–1.29 (m, 20 H), 0.81 (t, J =
.5 Hz, 6 H), 0.60–0.76 (m, 4 H). Anal. calcd for C H N S :
3
): d 8.10 (br, 2 H), 7.90 (br,
2
6
4
7
50
2 3
C, 76.38; H, 6.82; N, 3.79; S, 13.02. Found: C, 75.78; H, 7.25;
N, 3.47; S, 12.84.
Measurements and characterization
1
H NMR spectra were recorded on a Bruker AVANCE
5
00-MHz spectrometer with d-chloroform as solvent and
tetramethylsilane (TMS) as the internal standard. The elemental
analysis was carried out with a Thermoquest CHNS-
Ovelemental analyzer. The gel permeation chromatographic
Scheme 1 Chemical structures of PF-TBT and PF-DBT.
(
GPC) analysis was carried out with a Waters 410 instrument
À1
with tetrahydrofuran as the eluent (flow rate: 1 mL min , at
3
5 1C) and polystyrene as the standard. The thermo-
gravimetric analysis (TGA) was performed on a Perkin Elmer
À1
Pyris 1 analyzer under nitrogen atmosphere (100 mL min ) at
À1
a heating rate of 10 1C min . UV-visible absorption spectra
were measured using a Shimadzu UV-3100 spectrophoto-
meter. Electrochemical measurements of these derivatives
were performed with a Bioanalytical Systems BAS 100 B/W
electrochemical workstation. Atomic force microscopy (AFM)
images of blend films were carried out using a Nanoscope IIIa
Dimension 3100. Transmission electron microscopy (TEM)
2
images of blend films were performed on a FEI Technai-G
transmission electron microscope operated at an acceleration
voltage of 200 kV.
Photovoltaic device fabrication and characterization
The BHJ solar cells were fabricated with the active layer
consisting of the copolymers (PF-TBT, and PF-DBT) : PCBM
with varied blend ratios (1 : 1, 1 : 2, 1 : 3, 1 : 4 w/w), respectively.
ITO glass was cleaned by detergent, acetone and boiled in
Scheme 2 Synthetic routes of the monomers and PF-TBT.
Results and discussion
H O . A 50 nm layer of poly(3,4-ethylene dioxythiophene) :
2
2
Synthesis and characterization
poly(styrenesulfonate) (PEDOT : PSS)(Bayer PVP Al 4083) as
a modified layer was spin-coated onto the pre-cleaned ITO
glass substrate and then dried at 120 1C for 15 min on a hot
plate. The copolymers were dissolved in chlorobenzene to
The chemical structures of PF-TBT and PF-DBT are shown in
Scheme 1. The PF group was bridged to the p-conjugated
systems through the vinylene linkage. An octyl group was
À1
make 4 mg ml solutions, followed by blending with PCBM
(
purchased from Lumtec. Corp) in varied blend ratios (1 : 1,
: 2, 1 : 3, 1 : 4 w/w). The active layers were obtained by spin-
1
coating the blend solutions and the thickness of films were
B70 nm, as measured with the Ambios Technology XP-2.
Finally, the cathode of LiF (0.6 nm)/Al (100 nm) was
thermally deposited to finish the device fabrication. The
hole-only devices with a general structure of ITO/PEDOT :
PSS/copolymer : PCBM(50 nm)/Au (60 nm) were fabricated
for hole mobility measurements by space charge limited
2
current (SCLC) method. The active area was about 5 mm .
Current–voltage (J–V) characteristics were recorded using a
À2
Keithley 2400 Source Meter in the dark and under 100 mW cm
simulated AM 1.5 G irradiation (Sciencetech SS-0.5 K Solar
Simulator). The spectral response was recorded by a SR830
lock-in amplifier under short circuit conditions when devices
were illuminated with a monochromatic light from a Xeon
lamp. All fabrication and characterizations were performed
under ambient atmosphere at room temperature.
1
Fig. 1 The H NMR spectrum and chemical structure of monomer
3
(M-2) in CDCl solution.
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introduced onto the C
9
atom for the sake of solubility. PF-TBT
Table 1 Polymerization results and thermal properties of copolymers
was synthesized by Heck cross-coupling polymerization with
the synthesis route shown in Scheme 2. 4,7-Bis(5-bromo-
4
(10 )
4
(10 )
Polymers
M
w
M
n
PDI
d
TGA (T )
2
-thienyl)-2,1,3-benzothiadiazole (M-1) was synthesized
3
according to the procedures described in the literature.
PF-TBT
PF-DBT
0.76
12.2
0.66
6.25
1.15
1.95
402
389
3
2
,7-Diethenyl-9,9-di-n-octylfluorene (M-2) was synthesized in
1
a multistep synthesis. The H NMR spectrum of monomer
M-2 in CDCl is shown in Fig. 1. Starting from commercially
3
available fluorene, in three steps 2,7-diformyl-9,9-di-n-
octylfluorene was obtained. Similarly, 2,7-diethenyl-9,9-di-n-
octylfluorene was synthesized by a modified procedure in high
yield. In the conversion from aldehyde to vinylene, we
added potassium tert-butoxide as strong base under room
temperature conditions instead of n-BuLi at À40 1C, which
can reach higher yields. Because fluorenevinylene is less stable
and radical polymerization happens at room temperature
under irradiation, M-2 should be copolymerized quickly with
2 3
M-1 by Heck coupling reaction with Pd(OAc) and P(o-tol) as
catalysts and triethylamine as a base in DMF to obtain
PF-TBT. The synthesized copolymer was highly soluble in
common organic solvents such as chloroform, tetrahydrofuran,
and chlorobenzene at room temperature.
Thermal properties
The thermal property of PF-TBT was evaluated by
thermogravimetric analysis (TGA). The thermal property of
3
PF-DBT was reported previously. Fig. 2 shows the TGA
0
curves of PF-TBT and PF-DBT at the heating rate of
À1
1
0 1C min
Fig. 2, the decomposition temperatures (T
under a nitrogen atmosphere. As shown in
) of PF-TBT and
d
PF-DBT were 402 1C and 389 1C, respectively, which suggests
relatively high thermal stability. The high thermal stability of
the resulting polymers prevents the deformation and degrada-
tion of the polymeric active layer under applied electric fields.
Data on the polymerization results and thermal properties of
PF-TBT and PF-DBT are listed in Table 1.
Fig. 3 Normalized UV-vis absorption: PF-TBT and PF-DBT in
chloroform solution and film (a); PF-TBT in PMMA film, chloroform
solution and film (b).
Optical properties
The UV-vis absorption spectra of the PF-TBT in dilute
À5
solution and film are also shown in Fig. 3a and the maximum
absorptions are listed in Table 2. The absorption spectrum of
PF-TBT in solution shows three peaks with the maximum
absorption peaks located at 318, 400 and 536 nm. The shorter
wavelength peak of PF-TBT in the region of 300–500 nm
which is consistent with the absorption of PF-DBT, can
be assigned to the absorption of a fluorine unit and p–p*
transition of the conjugated polymer backbone, whereas the
longer wavelength peak at 536 nm originates from the ICT
interaction between the electron-donating fluorene unit and
the electron-accepting benzothiadiazole unit.
chloroform solution (concentration 1 Â 10 M) and thin film
are shown in Fig. 3a. For comparison, those of PF-DBT in
The optical absorption spectra of the copolymers in a thin
film are generally similar in shape to that in dilute solution.
Compared to its counterparts in dilute solution, the optical
absorption of PF-TBT in a film exhibited red shift by ca.
2
4 nm, presumably indicating the formation of a p-stacked
structure in the solid state that could facilitate harvesting of
3
0,34
sunlight for photovoltaic applications.
In order to see
Fig. 2 TGA curves of the PF-TBT and PF-DBT at the heating rate of
À1
whether p-stacking was the cause of the 24 nm red-shift in
the absorption spectra, we prepared the film of PF-TBT
1
0 1C min under nitrogen atmosphere.
3
88 New J. Chem., 2011, 35, 385–393
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Table 2 Optical and electrochemical properties of PF-TBT and PF-DBT
abs, sol
max
abs, film
max
opt
onset
onset
ec
E
g
Polymers
l
l
E
g
E
ox
HOMO
E
red
LUMO
PF-DBT
PF-TBT
400/436
400/536
400/449
409/560
2.32
1.82
0.71
0.64
À5.45
À5.37
À1.69
À1.32
À3.05
À3.41
2.40
1.96
dispersed in PMMA (with a weight ratio of 1 : 100) and
measured the absorption spectrum, shown in Fig. 3b, together
with the absorption spectra of PF-TBT in solution and
film. It can be seen that the absorption behavior of PF-TBT
dispersed in PMMA is similar to that in solution. There is
a 24 nm red shift of the absorption peak of pure PF-TBT
film compared to that of the PF-TBT : PMMA film, which
confirms that p-stacking causes the 24 nm red shift in
the absorbance spectra of PF-TBT. From the low energetic
edge of the absorption spectrum of the individual copolymer,
the band gap of PF-DBT was estimated to be 2.32 eV
Electrochemical properties
Cyclic voltammetry of the copolymer films was performed in
acetonitrile with 0.1 M tetrabutylammonium hexafluoro-
phosphate (TBAPF
6
) as the supporting electrolyte at scan
À1
rates of 30–50 mV s . Platinum wire electrodes were used
as both counter and working electrodes, and silver/silver ion
(
Ag in 0.1M AgNO
3
solution, from Bioanalytical Systems, Inc.)
was used as the reference electrode. Ferrocene/ferrocenium
+
(
Fc/Fc ) was used as the internal standard.
The redox curves of PF-TBT and that of PF-DBT are
shown in Fig. 4a as a comparison. A complete picture of the
band structure of the copolymers is presented in Fig. 4b. On
the anodic sweep, two copolymers showed an irreversible
(
absorption edge: B534 nm), while smaller band gaps of
.82 eV were calculated for PF-TBT (absorption edge:
1
B680 nm), respectively, indicating the strong electron-
donating conjugated bridge between the electron-donating
unit and electron-accepting unit could lead to an enhanced
ICT transition band, resulting in the extension of the
absorption spectral range.
+
oxidation with onset potentials of 0.71 V (versus Ag/Ag )
for PF-DBT, and 0.64 V for PF-TBT, respectively. In contrast,
the cathodic sweep showed onset reduction potentials of À1.69 V
+
(
versus Ag/Ag ) for PF-DBT, and À1.32 V for PF-TBT.
onset
From the onset oxidation potentials (Eox
) and the onset
) of the polymers, HOMO and
LUMO energy levels as well as the energy gap were calculated
onset
reduction potentials (Ered
7
,30
according to the following equations:
onset
HOMO (eV) = Àe(Eox
+ 4.73)
onset
LUMO (ev) = Àe(Ered
+ 4.73)
ec
Eg (eV) = e(Eox
onset
onset
À Ered
)
onset
onset
where Eox
and E
are the measured onset potentials
red
+
relative to Ag/Ag .
The results of the electrochemical measurements and
calculated energy levels of the copolymers are listed in
ec
Table 2. From Table 2 we can see that the band gaps (E
g
)
of PF-DBT and PF-TBT were estimated to be about 2.40 and
.96, respectively, which are in good agreement with the
opt
1
optical band gap (E
g
). For the copolymer PF-TBT, the
substitution of the stronger electron-donating conjugate
bridge (thiophene unit) resulted in a lower LUMO level of
À3.41 eV, a stable HOMO energy level of À5.37 eV and a
ec
g
lower E
compared with that of PF-DBT. The HOMO
energy level of the donor polymer in a BHJ solar cell is very
important for high device efficiency, as the Voc of BHJ solar
cell is determined by the difference between the HOMO level
of the donor polymer and the LUMO of the acceptor. The
relatively low HOMO level (À5.37 eV) of PF-TBT compared
with polythiophene derivatives, such as P3HT (HOMO =
3
5
À4.75 eV), may be favored for the improvement of the
oc when fabricating the BHJ solar cell with PF-TBT as the
donor and PCBM as the acceptor. Besides the V , J is
V
Fig. 4 (a) Cyclic voltammograms of the copolymer thin films on Pt
wires in 0.1M TBAPF
À1
6
in acetonitrile. The scan rates used were
oc
sc
30–50 mV s ; (b) energy level diagrams for PF-TBT, PF-DBT, and
another key parameter to determine the efficiency of the solar
PCBM.
cells. We know the Jsc is directly related with the absorption of
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New J. Chem., 2011, 35, 385–393 389

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À2
the active layer. The broad absorption of the photon flux in
the solar spectrum is the premise for high Jsc. The broad
absorption means a narrow bandgap of the materials. We
3.23 mA cm , FF of 0.48 and PCE of 1.64% under one sun of
À2
AM 1.5 solar simulator illumination (100 mW cm ).
The higher Voc of the device based on PF-DBT : PCBM is
due to the lower lying HOMO energy level (À5.45 eV) than
want a low-lying HOMO level for high V . At the same time,
oc
we also want a narrow bandgap for high J . Therefore
sc
that of PF-TBT (À5.37 eV), because V is related to the
oc
the LUMO level of the material should also go towards a
difference between the LUMO energy level of the acceptor and
the HOMO energy level of the donor within the active layer.
The higher Jsc of the device based on PF-TBT : PCBM than
that of the device based on PF-DBT:PCBM could be
explained by the broader absorption and lower band gap
of PF-TBT, which is consistent with the broader external
quantum efficiency (EQE) spectra (shown in Fig. 6) of the
devices based on PF-TBT : PCBM.
3
6
lower-lying position accordingly. However, to guarantee the
efficient electron transfer from the donor material to PCBM,
the energy level cannot be too low. At least 0.3 eV over the
4
LUMO level of PCBM (À4.2 eV) is required. Here, PF-TBT
with a LUMO level of À3.41 eV. The bandgap is 1.96 eV.
Comparing with PF-DBT, PF-TBT has a lower-lying LUMO
level and narrower bandgap (Fig. 4b) and accordingly a high
Jsc for the solar cells based on PF-TBT is expected.
As for FF, the devices based on PF-DBT : PCBM exhibited
higher FF than the devices based on PF-TBT : PCBM. The FF
of the solar cells is strongly influenced by the mobility and the
Photovoltaic properties
37
surface morphology of the active layer. We analysed the hole
We fabricated BHJ solar cells with the structure of ITO/
PEDOT : PSS/copolymer : PCBM/LiF/Al using the copolymer
PF-TBT as the electron donor and PCBM as the electron
acceptor. For comparison, BHJ solar cells based on PF-DBT
with the same structure were also fabricated. We tried different
compositions of the copolymer : PCBM (1 : 1, 1 : 2, 1 : 3, 1 : 4, w/w)
to optimize the device performance. Fig. 5 shows the
current–voltage (J–V) characteristics of the solar cells based
on the two copolymers blended with PCBM and the photo-
voltaic data are summarized in Table 3. The solar cell based
mobility of the polymer : PCBM composite films through
3
8,39
the space charge limited current (SCLC) method.
The
hole-only devices with a structure of ITO/PEDOT : PSS/
copolymer : PCBM/Au were fabricated. Fig. 7 shows the J–V
characteristics of the hole-only devices based on PF-TBT :
PCBM and PF-DBT : PCBM with varied composition and the
corresponding hole motilities are listed in Table 2. As the
concentration of PCBM increased, the hole mobility of
polymer : PCBM was enhanced. The hole mobility of the
À6
2
À1 À1
PF-DBT : PCBM (1 : 4, w/w) blend is 8.0 Â 10 cm V
S ,
on PF-TBT : PCBM exhibited a V of 0.86 V, Jsc of
oc
which is about 2.5 times of that of the PF-TBT : PCBM
À6
À2
2
À1 À1
S ) blend. The higher hole
3
.97 mA cm , FF of 0.35 and a PCE of 1.18%, while the cell
(1 : 4, w/w, 3.05 Â 10 cm V
based on PF-DBT : PCBM exhibited a Voc of 1.04 V, Jsc of
mobility of PF-DBT:PCBM blend might contribute to a more
Fig. 5 J–V curves of the devices based on PF-TBT : PCBM (a) and PF-DBT : PCBM (b) with different blend ratios (1 :1, 1 :2, 1 : 3, 1 : 4, w/w)
À2
under the illumination of AM 1.5, 100 mW cm
.
Table 3 Photovoltaic performance of the solar cells based on PF-TBT : PCBM and PF-DBT : PCBM
À2
2
Hole mobility/cm V
À1 À1
Donor (polymer)
Blend ratio (D : A)
Voc/V
Jsc/mA cm
FF
PCE (%)
S
À7
PF-TBT
PF-TBT
PF-TBT
PF-TBT
PF-DBT
PF-DBT
PF-DBT
PF-DBT
1 : 1
1 : 2
1 : 3
1 : 4
1 : 1
1 : 2
1 : 3
1 : 4
0.88
0.84
0.84
0.86
1.06
1.06
1.06
1.04
1.57
2.66
2.90
3.97
0.82
1.88
2.58
3.23
0.24
0.26
0.27
0.35
0.21
0.33
0.40
0.48
0.33
0.57
0.67
1.18
0.19
0.66
1.09
1.64
3.82 Â 10
À7
5.38 Â 10
À6
1.07 Â 10
À6
3.05 Â 10
À7
4.02 Â 10
À6
1.06 Â 10
À6
2.89 Â 10
À6
8.0 Â 10
3
90 New J. Chem., 2011, 35, 385–393
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Fig. 8 TEM images and AFM topography images (5 mm  5 mm) of
PF-DBT : PCBM (a, c) and PF-TBT : PCBM (b, d) in 1 : 4 w/w blend
ratio cast from chlorobenzene solutions.
Fig. 6 The external quantum efficiency (EQE) based on PF-TBT :
AFM and TEM images (Fig. 8) reveal a smooth surface with
a root mean square (RMS) of 0.462 nm and the formation of
more homogeneous morphology without obvious PCBM-
rich domains in the PF-TBT : PCBM blend film. However
the image of the PF-DBT : PCBM film becomes relatively
rough with a RMS of 0.63 nm and significant phase separation
with the formation of large PCBM-rich domains appearing.
The PCBM-rich domains could improve the charge trans-
portation and carrier collection effectively, which results
in a reduction of recombination loses and an increase of
PCBM (a) and PF-DBT : PCBM (b) with different blend ratios
(1 : 1, 1 : 2, 1 : 3, 1 : 4, w/w).
balanced charge transport in the PF-DBT : PCBM composite
layer and a more efficient carrier collection at the interface
3
7,40
between the active layer and the respective electrode.
In order to investigate the reason for the different FF of the
cells based on PF-TBT : PCBM and on PF-DBT:PCBM, we
also analyzed the surface morphology of PF-TBT : PCBM and
PF-DBT : PCBM composite films (1 : 4, w/w) using atom force
microscopy (AFM) and transmission electron microscopy
4
0,42
FF.
So, the higher hole mobility and the rough morphology with
obvious phase separation of PF-DBT : PCBM blends could be
the reason why the FF of the cell based on PF-TBT : PCBM
was lower than that of the cells based on PF-DBT : PCBM.
(
TEM). In the TEM image, the dark areas are attributed to
PCBM domains because the electron scattering density of
4
PCBM is higher than that of conjugated polymers. The
1
Fig. 7 J–V curve in the dark of an ITO/PEDOT/PF-TBT (PF-DBT) : PCBM/Au device in log axis for estimating the hole mobility of
PF-DBT : PCBM (a) and PF-TBT : PCBM (b). The thickness of PF-TBT : PCBM and PF-DBT : PCBM are around 50 nm. The open rectangle
symbols are the experimental data. The solid line from 0.01 to 0.25 V means logJ is fitted linearly dependent on logV with a slope of 1. The solid
line from 0.25 to 0.8 V logJ is fitted linearly dependent on logV with a slope of 2 (SCLC area).
This journal is c The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2011
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The PCE of a solar cell is determined by the open circuit
voltage, the short circuit current density and the fill factor. We
found that the solar cells based on PF-TBT : PCBM exhibited
5 H. Y. Chen, J. H. Hou, S. Q. Zhang, Y. Y. Liang, G. W. Yang,
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a Voc of 0.86 V, Jsc of 3.97 mA cm , FF of 0.35 and PCE of
À2
1
.18%, while a Voc of 1.04 V, Jsc of 3.23 mA cm
,
FF of 0.48 and PCE of 1.64% were found for the cell
based on PF-DBT : PCBM under one sun of AM 1.5 solar
1
8 J. Y. Kim, K. Lee, N. E. Coates, D. Moses, T. Q. Nguyen,
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À2
simulator illumination (100 mW cm ). As compared to
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