Molecular structure-property engineering for photovoltaic applications: Fluorene-acceptor alternating conjugated copolymers with varied bridged moieties

Article abstract of DOI:10.1016/j.polymer.2010.01.039

A series of novel soluble conjugated copolymers consisting of electron-accepting 2-pyran-4-ylidenemalononitrile (PM) and electron-donating fluorene connected by different electron-donating ability conjugated moieties were synthesized by Suzuki coupling polymerization. The structures of the copolymers were characterized and their physical properties were investigated. High molecular weight (M_n up to 43.8 kg/mol) and thermostable copolymers were obtained. The conjugated bridge between PM and fluorene building block with gradually increased electron-donating ability moieties results in enhanced intramolecular charge transfer (ICT) transition bands, which lead to an extension of their absorption spectral range. Cyclic voltammetry measurement displayed that the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of the copolymers can be fine-tuned. The resulting copolymers possessed relatively low HOMO energy levels, promising good air stability and high open circuit voltage (V_oc) for photovoltaic application. Bulk heterojunction photovoltaic devices were fabricated by using the copolymers as donors and (6,6)-phenyl C₆₁-butyric acid methyl ester (PCBM) as acceptor. The power conversion efficiencies (PCE) of the devices were in the range of 0.02-0.52% under simulated AM 1.5 solar irradiation of 100 mW/cm², and the highest V_oc reached 0.82 V. The significant improvement of PCE indicates a novel concept for developing donor-acceptor (D-A) conjugated copolymers with high photovoltaic performance by adjusting electron-donating ability of conjugated bridge.

Full text of DOI:10.1016/j.polymer.2010.01.039

Polymer 51 (2010) 1786–1795
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Molecular structure–property engineering for photovoltaic applications:
Fluorene-acceptor alternating conjugated copolymers with varied
bridged moieties
*
Yaowen Li, Hui Li, Bin Xu, Zaifang Li, Feipeng Chen, Dongqing Feng, Jibo Zhang, Wenjing Tian
State Key Laboratory of Supramolecular Structure and Materials, Jilin University, Changchun 130012, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of novel soluble conjugated copolymers consisting of electron-accepting 2-pyran-4-ylidene-
malononitrile (PM) and electron-donating fluorene connected by different electron-donating ability
conjugated moieties were synthesized by Suzuki coupling polymerization. The structures of the copol-
ymers were characterized and their physical properties were investigated. High molecular weight (Mn up
to 43.8 kg/mol) and thermostable copolymers were obtained. The conjugated bridge between PM and
fluorene building block with gradually increased electron-donating ability moieties results in enhanced
intramolecular charge transfer (ICT) transition bands, which lead to an extension of their absorption
spectral range. Cyclic voltammetry measurement displayed that the highest occupied molecular orbital
(HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of the copolymers can be fine-
tuned. The resulting copolymers possessed relatively low HOMO energy levels, promising good air
stability and high open circuit voltage (Voc) for photovoltaic application. Bulk heterojunction photovoltaic
devices were fabricated by using the copolymers as donors and (6,6)-phenyl C61-butyric acid methyl
ester (PCBM) as acceptor. The power conversion efficiencies (PCE) of the devices were in the range of
0.02–0.52% under simulated AM 1.5 solar irradiation of 100 mW/cm2, and the highest Voc reached 0.82 V.
The significant improvement of PCE indicates a novel concept for developing donor–acceptor (D–A)
conjugated copolymers with high photovoltaic performance by adjusting electron-donating ability of
conjugated bridge.
Received 22 August 2009
Received in revised form
4 January 2010
Accepted 16 January 2010
Available online 28 January 2010
Keywords:
Photovoltaic cells
Conjugated polymers
Synthesis
Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction
(around 700 nm) [7]. In order to further improve the properties of
PVCs, people paid more attention to the donor–acceptor (D–A)
In recent years, considerable efforts have been directed towards
the development of new polymer photovoltaic cells (PVCs). PVCs
are becoming more and more attractive because they represent
a low cost and flexible devices, tunable electronic properties, and
ease of processing [1–4]. One of the promising strategies for
devising efficient PVCs involves the use of interpenetrating
networks bulk heterojunctions (BHJ) based on a blend of electron-
donating conjugated polymers and soluble fullerene acceptors as
the active layer [5]. For instance, bulk heterojunction PVCs made
from a blend of regioregular poly(3-hexylthiophene) (P3HT) as the
donor and PCBM as the acceptor have recently been shown the
power conversion efficiencies (PCE) up to 4–5% [6,34]. However,
P3HT only harvests photons with wavelengths below 650 nm,
while the energy of the majority of the solar photons is much lower
conjugated polymers whose optical and electronic properties could
be tunable through the intramolecular charge transfer (ICT) from
the donor to acceptor. So, design and synthesis D–A conjugated
copolymers which can efficiently harvest the majority energy of the
solar spectrum, are effective ways to obtain low band gap polymers
[8]. However, several reported D–A copolymers showed PCE much
lower than the wide band gap counterparts [9], because of the
mismatch of the energy level between electron-donating polymer
and electron acceptor (e.g., PCBM), and large-scale blend phase
separation between the donor and acceptor [10]. Most recently,
several D–A copolymer systems have achieved better efficiency by
tuning the energy level of the polymers through modifying the
monomer structures based on the known thienopyrazine or ben-
zothiadiazole systems [11,15]. However, the relatively low Voc
(around 0.6 V) still limits the PCE of the devices.
On all accounts, the D–A copolymers with low band gaps are
needed for harvesting solar photons in a broader spectrum. To
fulfill this requirement, well-chosen donor and acceptor groups are
* Corresponding author. Tel.: þ86 431 85155359; fax: þ86 431 85193421.
E-mail address: wjtian@jlu.edu.cn (W. Tian).
0032-3861/$ – see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2010.01.039

Y. Li et al. / Polymer 51 (2010) 1786–1795
1787
particularly desirable for low band gap polymers due to a signifi-
cant enhancement of the ICT intensity and conjugated length,
which lead to a better extended absorption and higher absorption
coefficient. In addition, the D–A copolymers should possess the
following two features in order to achieve high efficiency of PVCs.
One is sufficient driving force for electron transfer from the electron
donor to the acceptor, and suitable HOMO energy level of electron
donor. It means that the LUMO energy level of the electron donor
must be higher than that of the electron acceptor (at least 0.3 eV) in
order to have sufficient driving force for electron transfer from the
donor to acceptor, and the HOMO energy level of electron donor
must be low enough in order to get relative high Voc, which is
determined by the difference between the HOMO energy level of
the donor and LUMO energy level of the acceptor [13]. The other is
to have good miscibility with the electron acceptor (e.g., PCBM) to
form an interpenetrating network. In order to efficiently dissociate
the excitons occurred at the interface between the electron-
donating component and the electron-accepting one, the control
of the BHJ morphology is of crucial importance. Ideally, the phase
separation length scale should match the exciton diffusion length
of conjugated polymer, which is approximately 10 nm [12].
Consequently, to further increase the photovoltaic performances
for practical application, it is important to design and synthesize
polymers with the possibility to tune their energy levels and
miscibility with the electron acceptor (e.g., PCBM).
It is widely accepted that the alternative copolymer structures
with donor and acceptor functionalities are very effective in
decreasing the band gap and modulating electronic and physical
properties of the polymers. Most of the D–A copolymer system was
symbolized as [D*–D–A–D]n, where the D* is the donor, A is
acceptor unit and D is an aromatic donor conjugated bridge
between the D* and A which can increase the conjugated length of
copolymers. Numerous attempts to develop new donor/acceptor
[D*–D–A–D]n combinations have been made by changing electron-
donating D*. For example, the 4,7-dithien-2-yl-2,1,3-benzothia-
diazole (DTBT) unit is very effective as an acceptor, and it can
copolymerize with many kinds of donor segments (D*), such as
fluorene [13], silafluorene [14], carbezole [15], dithienosilole [16],
and cyclopenta[2,1-b:3,4-b]dithiophene [17]. Meanwhile, many
electron-accepting moieties (A), including quinoline [18], qui-
noxaline [19], 2,1,3-benzothiadiazole [13], and pyridazine[20] have
been also introduced into [D*–D–A–D]n copolymer systems. All the
above research efforts have led to major progress in the synthesis of
new D–A copolymers. However, the conjugated bridged-D which
connects the D* and A also plays an important role in tuning the
optical and electronic properties by changing varied electron-
donating ability and coplanarity. To our knowledge, the research
of the conjugated bridged-D copolymers for photovoltaic devices
has been scarcely considered.
conjugated length, which could further reduce the band gap of the
conjugated system. We reasoned that incorporation of varied
electron-donating ability functional bridged-D moieties will bring
different degrees of ICT to the conjugated system and thus provide
a means to tune the energy levels. Indeed, we found that the energy
level and absorption spectra of copolymers can be fine-tuned by
changing the moieties of different electron-donating ability. Both
the HOMO energy level (ꢀ5.13 to ꢀ5.64 eV) and LUMO energy level
(around ꢀ3.35 eV) of the copolymers were close to the ideal energy
level [39]. The photovoltaic performances of copolymers demon-
strated that appropriate energy level and molecular structure
engineering resulted in the improvement of the PCE to 0.52% (in the
case of PFPMP), which was more than 13 times higher than that of
PFTMT.
2. Experimental section
2.1. Materials
2,7-Bis(4,4,5,5-tetramathyl-1,3,2-dioxaborolan-2-yl)-9,9-dihex-
ylfluorene [22], 1,4-dihexyl- 2,5-dibromobenzene (1) [23], 2,5-
dibromo-3,4-dihexylthiophene (3) [22], 3,7-dibromo-10-hex-
ylphenothiazine (5) [24], 2-(2,6-dimethyl pyran-4-ylidene)
malononitrile [25], was synthesized according to known literature
procedures. All reagents and chemicals were purchased from
commercial sources(Aldrich, Across, Fluka) andused without further
purification unless stated otherwise. All solvents were distilled over
appropriate drying agent(s) prior to use and were purged with
nitrogen.
2.2. Characterization
The infrared spectroscopy spectra were recorded via the KBr
pellet method by using a Nicolet Impact 410 FT-IR spectrophotom-
eter. The elemental analysis was carried out with a Thermoquest
CHNS-Ovelemental analyzer. The gel permeation chromatographic
(GPC) analysis was carried out with a Waters 410 instrument with
tetrahydrofuran as the eluent (flow rate: 1 mL/min, at 35 ꢁC) and
polystyrene as the standard. Differential scanning calorimetry (DSC)
were performed under nitrogen flushing at a heating rate of 20 ꢁC/
min with a NETZSCH (DSC-204) instrument. Thermogravimetric
analysis were performed on a Perkine Elmer Pyris 1 analyzer under
nitrogen atmosphere (100 mL/min) at a heating rate of 10 ꢁC/min.
1H NMR and 13C NMR spectra were measured using a Bruker
AVANCE-500 NMR spectrometer and a Varian Mercury-300 NMR,
respectively. UV–visible absorption spectra were measured using
a Shimadzu UV-3100 spectrophotometer. The photoluminescenece
spectra of spin-cast films and solution were measured with an RF-
5301PC spectrofluorophotometer. 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.
Herein, we synthesized three novel
p-conjugated copolymers
consisting of PM-based donor unit coupled to different electron-
donating moieties: poly{(9,9-dihexyl-9H-fluorene-2,7-ylene)-alt-
2-(2,6-bis((E)-2,5-bis(hexyloxy)styryl)-4H-pyran-4-ylidene)malon
onitrile}(PFBMB), poly{(9,9-dihexyl-9H-fluorene-2,7-ylene)-alt-2-
(2,6-bis((E)-2-(5-bromo-3,4-dihexylthiophen-2-yl)vinyl)-4H-pyran-
4-ylidene)-malononitrile} (PFTMT), poly{(2,20-bithiophene-5,50-
ylene)-alt-2-(2,6-bis((E)-2-(10-hexyl-10H-phenothiazin-3-yl)vinyl)-
4H-pyran-4-ylidene)malononitrile}(PFPMP). The functional electron-
donating moieties fluorene (D*) can increase the solubility and
coplanarity of the copolymers. 2-Pyran-4-ylidenemalononitrile (A)
is a strong electron-accepting group, which can increase electron
affinity and reduce the band gap of the conjugated system [21]. To
allow for the systematic modulation of the electronic and optical
properties, derivatives with a chain structure [D*–D–A–D]n were
designed, where the varied alkylated bridged-D assured the better
2.3. Photovoltaic device fabrication and characterization
For device fabrication, the ITO glass was precleaned and modi-
fied by a thin layer of PEDOT:PSS, which was spin-cast from
a PEDOT:PSS aqueous solution (H. C. Starck) on the ITO substrate,
and the thickness of the PEDOT:PSS layer is about 50 nm. The active
layer contained a blend of copolymers as electron donor and PCBM
as electron acceptor, which was prepared by weight ratio (1:1 w/w,
1:2 w/w, 1:3 w/w, 1:4 w/w) in chlorobenzene (8 mg/mL) for
copolymers. After spin-coating the blend from solution at
3000 rpm, The devices were completed by evaporating a 0.6 nm LiF

1788
Y. Li et al. / Polymer 51 (2010) 1786–1795
layer protected by 100 nm of Al at a base pressure of 5 ꢂ10ꢀ4 Pa.
The effective photovoltaic area as defined by the geometrical
overlap between the bottom ITO electrode and the top cathode
was 4 or 5 mm2. The thickness of the photoactive layer was
50–60 nm, measured by the Ambios Technology XP-2. The
current–voltage (I–V) characterization of PV devices in the dark
and under white-light illumination from an SCIENCETECH 500-W
solar simulator (AM 1.5100 mW/cm2) were measured on
computer-controlled Keithley 2400 Source Meter measurement
system. All the measurements were performed under ambient
atmosphere at room temperature.
with chloroform. The combined organic extractions were washed
three times with water, dried over anhydrous MgSO4, evaporated
under vacuum and purified with column chromatography on silica
gel with dichloromethane:petroleumether (3:1) as the eluant to
a dark red solid (2.64 g, 2.91 mmol, yield 63.0%). 1H NMR (500 MHz,
CDCl3, TMS):
d
(ppm) 7.76 (d, J ¼ 16.5 Hz, 2H, –vinylic), 7.15 (s, 2H,
-Ph), 7.02 (s, 2H, –Ph), 6.87 (d, J ¼ 16.0 Hz, 2H, –vinylic), 6.68 (s, 2H,
–Ph), 4.01 (m, 8H, –OCH2), 1.84 (m, 8H, –CH2), 1.53–1.44 (m, 8H,
–CH2), 1.37 (m, 8H, –CH2) 1.23 (m, 8H, –CH2), 0.92 (m, 6H, –CH3),
0.79 (m, 6H, –CH3). 13C NMR (75 MHz, DMSO, TMS):
d(ppm) 158.46,
155.61, 151.44, 149.19, 131.03, 123.77, 119.72, 117.93, 114.95, 114.34,
111.94, 106.77, 69.39, 30.71, 30.56, 28.37, 25.06, 24.81, 21.68, 21.56,
13.44, 13.29. Elem. Anal. Calcd. for C48H62Br2N2O5 : C, 63.58; H,
6.89; N, 3.09. Found: C, 63.60; H, 6.85; N, 3.05.
2.4. Synthesis of monomers
2.4.1. Synthesis of 4-bromo-2,5-di(hexyloxy)benzaldehyde (2)
n-BuLi (1.07 mL of 2.5 M solution in hexane, 2.68 mmol) was
added dropwise to a solution of compound 1 (1.064 g, 2.44 mmol)
in THF (30 mL) at ꢀ78 ꢁC under argon. The solution was stirred at
ꢀ78 ꢁC for 2 h, then dried DMF (0.24 mL) was added quickly and it
was kept at room temperature and stirred for 24 h before being
poured into water. The product was extracted with ether. The
organic layer was subsequently washed with water and brine and
dried over MgSO4, and the solvent was removed by rotary evapo-
ration. The crude product was purified by column chromatography
with CH2Cl2:petroleum ether (1:2) to give a red bright yellow solid
(0.71 g, 1.85 mmol, yield 76%). 1H NMR (500 MHz, CDCl3, TMS):
2.4.5. Synthesis of 2-(2,6-bis((E)-2-(5-bromo-3,4-dihexylthiophen-
2-yl)vinyl)-4H-pyran-4-ylidene)malononitrile (M-2)[22]
The synthetic procedure for M-2 was similar to that for M-1 gave
a dark green solid (2.1 g, 2.46 mmol, yield 53.2%). 1H NMR
(500 MHz, CDCl3, TMS):
6.61 (s, 2H, –PM), 6.37 (d, J ¼ 15.5 Hz, 2H, –vinylic), 2.68 (t,
CH2), 1.50 (m, 8H,
d(ppm) 7.51 (d, J ¼ 15.5 Hz, 2H, –vinylic),
J ¼ 7.8 Hz, 4H, –
a
CH2), 2,53 (t, J ¼ 7.8 Hz, 4H, –
a
–CH2),1.33 (m,16H, –CH2),1.29 (m, 8H, –CH3), 0.91 (t, J ¼ 6.8 Hz, 6H,
–CH3), 0.84 (t, J ¼ 6.8 Hz, 6H, –CH3). 13C NMR (75 MHz, CDCl3, TMS):
d(ppm) 157.79, 157.69, 155.16, 144.95, 143.32, 134.63, 127.84, 116.50,
115.13, 113.72, 106.64, 59.60, 31.45, 31.42, 29.34, 29.19, 28.31, 27.96,
22.49, 13.95, 13.87. Elem. Anal. Calcd. for C44H58Br2N2OS2 : C, 61.82;
H, 6.84; N, 3.28. Found: C, 61.88; H, 6.87; N, 3.22.
d
(ppm) 10.41 (s, 1H, –CHO), 7.31 (s, 1H, –Ph), 7.23 (s, 1H, –Ph), 4.01
(m, 4H, –OCH2), 1.82 (m, 4H, –CH2), 1.47 (m, 4H, –CH2), 1.35 (m, 8H,
–CH2), 0.91 (m, 6H, –CH3). 13C NMR (125 MHz, CDCl3, TMS):
(ppm)
d
189.29, 156.17, 150.29, 124.71, 121.36, 118.88, 111.05, 70.25, 69.89,
31.87, 29.44, 29.40, 26.07, 26.02, 22.96, 14.40. Elem. Anal. Calcd. for
C19H29BrO3 : C, 59.22; H, 7.59. Found: C, 59.15; H, 7.60.
2.4.6. Synthesis of 2-(2,6-bis((E)-2-(7-bromo-10-hexyl-10H-
phenothiazin-3-yl)vinyl)-4H-pyran-4- ylidene)malononitrile (M-3)
The synthetic procedure for M-3 was similar to that for M-1 give
a dark solid (6.84 g, 7.47 mmol, yield 73.5%). 1H NMR (500 MHz,
2.4.2. Synthesis of 5-bromo-3,4-dihexylthiophene-2-carbaldehyde (4)
The synthetic procedure for 3 was similar to that for 2 give a red
brown oil (0.46 g, 1.29 mmol, yield 53%), 1H NMR (500 MHz, CDCl3,
CDCl3, TMS): d(ppm) 7.37–7.34 (m, 4H, –Ph and –vinylic), 7.30 (m,
2H, –Ph), 7.27–7.24 (m, 4H, –Ph), 6.86 (d, J ¼ 8.5 Hz, 4H, –Ph), 6.72
(d, J ¼ 8.5 Hz, 2H, –Ph), 6.62 (s, 2H, -PM), 6.59 (d, J ¼ 16.0 Hz, 2H,
–vinylic), 3.84 (t, J ¼ 7.0 Hz, 4H, –NCH2), 1.80 (m, 4H, –CH2), 1.44 (m,
4H, –CH2), 1.32 (m, 8H, –CH2), 0.89 (t, J ¼ 7.0 Hz, 6H, –CH3). 13C NMR
TMS):
(t, J ¼ 8.0 Hz, 2H, –
(m, 4H, –CH2), 1.32 (m, 8H, –CH2), 0.90 (m, 6H, –CH3). 13C NMR
(75 MHz, CDCl3, TMS): (ppm) 181.23, 151.350, 143.70, 138.25,
d
(ppm) 9.91 (s, 1H, -CHO), 2.87 (t, J ¼ 8.0 Hz, 2H, –
aCH2), 2.54
a
CH2),1.57 (m, 2H, –CH2),1.50 (m, 2H, –CH2),1.50
(75 MHz, CDCl3, TMS):
d(ppm) 158.22, 155.59, 146.73, 143.13,
d
136.35, 130.20, 129.68, 129.10, 127.72, 126.40, 125.96, 124.57, 116.84,
116.50, 115.50, 115.42, 115.20, 106.70, 59.03, 47.90, 31.38, 26.66,
26.50, 22.57, 13.98. Elem. Anal. Calcd. for C48H44Br2N4OS2:C, 62.88;
H, 4.84; N, 6.11. Found: C, 62.95; H, 4.82; N, 6.13.
122.48, 32.23, 31.459, 31.43, 29.36, 29.21, 27.91, 27.71, 22.53, 22.48,
13.99. Elem. Anal. Calcd. for C17H27BrOS : C, 56.82; H, 7.57. Found: C,
56.80; H, 7.60.
2.4.3. Synthesis of 7-bromo-10-hexyl-10H-phenothiazine-3-
carbaldehyde (6)
2.5. General procedures of polymerization
The synthetic procedure for 6 was similar to that for 2 give
Suzuki cross-coupling reaction was used to synthesize the
copolymers shown in Scheme 2. 2,7-Bis(4,4,5,5-tetramathyl-1,3,2-
dioxaborolan-2-yl)-9,9-dihexyl-fluorene, dibromo monomer, and
(PPh3)4Pd(0) (2 mol% with respect to the monomer) were dis-
solved in a mixture of toluene (15 mL) and aqueous 2 M K2CO3 (3/2
volume ratio). The solution was stirred under an Ar atmosphere and
refluxed with vigorous stirring for 48 h. The resulting solution was
then poured into methanol and followed by washing with water.
The precipitated solid was extracted with methanol and acetone for
24 h in a Soxhlet apparatus to remove the oligomers and catalyst
residues, respectively. The soluble fraction was then collected via
extraction with CHCl3 for 24 h. The chloroform solution was then
concentrated to afford the copolymers.
a yellow solid (0.62 g, 1.58 mmol, yield 65%). 1H NMR (500 MHz,
CDCl3, TMS): d(ppm) 9.79 (s, 1H, –CHO), 7.64–7.63 (m,1H, –Ph), 7.55
(m, 1H, –Ph), 7.25–7.20 (m, 2H, –Ph), 6.88 (d, J ¼ 8.5 Hz, 1H, –Ph),
6.70 (d, J ¼ 9.0 Hz, 1H, –Ph), 3.83 (t, J ¼ 7.3 Hz, 2H, –NCH2), 1.77 (m,
2H, –CH2), 1.42 (m, 2H, –CH2), 1.30 (m, 4H, –CH2), 0.87 (m,
J ¼ 6.3 Hz, 3H, –CH2). 13C NMR (75 MHz, DMSO, TMS):
d(ppm)
189.79, 150.22, 142.57, 131.22, 130.18, 130.17, 129.70, 128.34, 126.06,
124.30,117.01, 115.72, 114.92, 48.03, 31.28, 26.55, 26.38, 22.49, 13.91,
Elem. Anal. Calcd. for C19H20BrNOS : C, 58.46; H, 5.16; N, 3.59.
Found: C, 58.50; H, 5.15; N, 3.55.
2.4.4. Synthesis of 2-(2,6-bis((E)-4-bromo-2,5-bis(hexyloxy)styryl)-
4H-pyran-4-ylidene)malononitrile (M-1)
A
mixture of 4-bromo-2,5-di(hexyloxy)benzaldehyde (2)
2.5.1. Poly{(9,9-dihexyl-9H-fluorene-2,7-ylene)-alt-2-(2,6-bis((E)-
2,5-bis(hexyloxy)styryl)-4H-pyran-4-ylidene)malononitrile}
(PFBMB)
(4.44 g, 10.17 mmol), 2-(2,6-dimethylpyran-4-ylidene)-malononi-
trile (7) (0.80 g, 4.62 mmol), piperidine (20 drops), and n-propyl
alcohol were refluxed under N2 for 24 h. The reaction mixture was
cooled to room temperature and poured into water and extracted
The resulting copolymer PFBMB was obtained as a dark green
shining powder with a yield of 74%. 1H NMR (500 MHz, CDCl3, TMS):

Y. Li et al. / Polymer 51 (2010) 1786–1795
1789
d
(ppm) 7.94 (d, J ¼ 16.5 Hz, 2H, –vinylic), 7.78 (2 m, H, -Ph),, 7.68 (br,
23.02, 22.98, 14.46. Elem. Anal. Calcd. for C73H76N4OS2: C, 80.40; H,
6.98; N, 4.41. Found: C, 79.83; H, 7.68; N, 4.56.
2H, –Ph), 7.53 (m, 2H, –Ph), 7.17 (br, 2H, –Ph), 7.05 (br, 2H, –Ph), 6.97
(d, J ¼ 15.5 Hz, 2H, –vinylic), 6.73 (s, 2H, –PM), 4.12 (br, 4H, –OCH2),
3.99 (br, 4H, –OCH2), 2.04 (br, 4H, -aCH2), 1.88 (br, 4H, –CH2), 1.75
3. Results and discussion
(br, 4H, -CH2), 1.56–1.05 (m, 40H, –CH2), 0.88 (br, 6H, –CH3) 0.81 (br,
6H, –CH3), 0.76 (br, 6H, –CH3). 13C NMR (125 MHz, CDCl3, TMS):
3.1. Material synthesis and structural characterization
d(ppm) 159.45, 156.53, 152.99, 151.05, 150.80, 144.19, 140.67, 137.10,
135.46, 128.47, 124.75, 123.72, 119.89, 119.30, 116.02, 113.31, 107.71,
70.20, 55.57, 40.93, 32.56, 31.98, 31.40, 30.36, 29.77, 29.06, 28.82,
26.36, 26.18, 24.41, 23.06,14.43. Elem. Anal. Calcd. for C73H94N2O5: C,
81.14; H, 8.71; N, 2.59. Found: C, 80.34; H, 8.63; N, 2.13.
The general synthetic routes toward the monomers are outlined
in Scheme 1. In order to increase the solubility of the monomer, the
active proton of the hydroquinone, thiophene and phenothiazine
was alkylated with 1-bromohexane before dibromination. Critical
to the synthetic strategy was the selective halogen-metal exchange
followed by conversion to the aldehyde. The monoaldehyde of
dibromo compounds (1, 3, 5) was prepared by a modified procedure
with 1.1 equative n-Buli followed by DMF. The monomer (M-1, M-2,
M-3) was prepared through Knoevenagel condensation of 2-(2,6-
dimethypyran-4-ylidene)malomonitrile (7) with monoaldehyde
compound (2, 4, 6). The structures of monomer (M-1, M-2, M-3)
were confirmed by 1H NMR, 13C NMR, and elemental analysis. In 1H
NMR spectroscopy of monomer (M-2), the coupling constant
(J w 15.5 Hz) of olefinic protons indicates that the Knoevenagel
reaction afforded the pure all-trans isomers, which is further
confirmed by the characterization of vibration band of trans double
bond at 960 cmꢀ1 in the FT-IR spectra. The 2,7-Bis(4,4,5,5-
tetramathyl-1,3,2-dioxaborolan-2-yl)-9,9-dihexylfluorene (8) was
prepared according to previously reported methods. The poly-
merization reaction was proceeded by the well-known palladium-
catalyzed Suzuki coupling reaction between bis(4,4,5,5-tetra
methyl-1,3,2-dioxaborolane) fluorene (8) and varied functional-
ized dibromo aromatic monomer (M-1, M-2, M-3) [26]. The
synthetic routes of copolymers are shown in Scheme 2. The 1H NMR
spectra of all the copolymers and their assignments shown in Fig. 1
are consistent with the proposed structure. NMR spectra clearly
indicate that well defined the copolymers has been obtained.
All the copolymers exhibited excellent solubility in common
organic solvents such as chloroform, tetrahydrofuran, dichloro-
methane, and chlorobenzene. Molecular weights and poly-
dispersities of the resulting copolymers were determined by GPC
analysis with the number average molecular weight (Mn) of 5240–
43,800 g/mol and PDI (polydispersity index, Mw/Mn) of 1.49–4.2.
Table 1 summarized the polymerization results including molec-
ular weights, PDI and thermal stability of the copolymers. The steric
hindrance of alkoxy and low solubility of the monomer M-1 and M-
2.5.2. Poly{(9,9-dihexyl-9H-fluorene-2,7-ylene)-alt-2-(2,6-bis((E)-
2-(3,4-dihexylthiophen-2-yl) vinyl)-4H-pyran-4-ylidene)-
malononitrile} (PFTMT)[22]
The resulting copolymer PFTMT was obtained as a dark green
shining powder with a yield of 80%. 1H NMR (500 MHz, CDCl3,
TMS):
7.47 (br, 4H, –Ph), 6.64 (s, 2H, –PM), 6.55 (d, 2H, J ¼ 16.0 Hz,
–vinylic), 2.79 (br, 4H, – CH2), 2.67 (br, 4H, – CH2), 2.04 (br, 4H,
d(ppm) 7.80 (br, 2H, –Ph), 7.71 (d, J ¼ 16.0 Hz, 2H, –vinylic),
a
a
–aCH2), 1.10–1.65 (m, 48H, –CH2), 0.88 (br, 12H, –CH3), 0.81 (t,
J ¼ 7.3 Hz, 12H, –CH3). 13C NMR (125 MHz, CDCl3, TMS):
d(ppm)
158.68, 155.90, 151.83, 147.35, 142.75, 141.03, 140.26, 133.85, 129.18,
129.01,128.93,128.69,128.44,123.83,120.60,120.40,120.33,116.48,
115.99, 107.08,106.91, 59.13, 55.79, 40.90, 32.34, 32.23, 32.09, 32.02,
31.34, 30.50, 30.02, 29.96, 29.47, 28.32, 27.99, 25.37, 24.39, 23.10,
23.00, 14.45. Elem. Anal. Calcd. for C69H90N2OS2: C, 80.65; H, 8.77;
N, 2.72. Found: C, 79.66; H, 9.11; N, 2.44.
2.5.3. Poly{(2,20-bithiophene-5,50-ylene)-alt-2-(2,6-bis((E)-2-(10-
hexyl-10H-phenothiazin-3-yl) vinyl)-4H-pyran-4-
ylidene)malononitrile} (PFPMP)
The resulting copolymer PFPMP was obtained as a dark powder
with a yield of 52%. 1H NMR (500 MHz, CDCl3, TMS):
d(ppm) 7.75 (br,
2H, –vinylic), 7.50 (br, 6H, –Ph),, 7.43 (br, 2H, –Ph), 7.34 (br, 6H, –Ph),
6.95 (br, 2H, –Ph), 6.88 (br, 2H, –Ph), 6.58 (m, 4H, –vinylic and –PM),
3.91 (br, 4H, –NCH2), 2.03 (br, 4H, –aCH2), 1.86 (br, 4H, –CH2), 1.48
(br, 4H, –CH2), 1.35 (br, 8H, –CH2), 1.15 (br, 4H, –CH2), 1.06 (br, 12H,
–CH2), 0.90 (br, 6H, –CH3), 0.77 (br, 6H, –CH3). 13C NMR (125 MHz,
CDCl3, TMS): d(ppm) 158.78, 156.12, 152.17, 147.38, 143.30, 140.39,
138.92, 137.13, 129.25, 128.07, 127.59, 126.60, 126.24, 125.80, 125.22,
124.40, 121.21, 120.48, 116.63, 116.24, 115.93, 115.69, 107.04, 55.72,
48.33, 40.86, 32.19, 31.87, 30.41, 29.34, 27.21, 27.02, 25.36, 24.26,
Scheme 1. Synthetic routes of the monomers.

1790
Y. Li et al. / Polymer 51 (2010) 1786–1795
Scheme 2. Synthetic routes of the copolymers.
3, which prevented the polymerization due to its low solubility,
should be the main reason for the low polymerization of PFBMB
and PFPMP. The large polydispersity in the molecular weight of
PFTMT may be a result of the precipitation of copolymer from the
reaction solution.
the copolymer morphology and the degradation of active layer
applied in PVCs.
3.3. Optical properties
The normalized UV–vis absorption spectra of PFBMB, PFTMT and
PFPMP in dilute chloroform solution (concentration 10ꢀ5 M) are
shown in Fig. 3, and the main optical properties are listed in Table 2.
PFBMB with the weak electron-donating dialkylfluorene moiety
showed two absorption bands at 348 nm and 460 nm in dilute
3.2. Thermal properties
All the copolymers exhibited good thermal stability with 5%
weight-loss temperatures (Td) higher than 301 ꢁC under N2 and
high glass transition temperatures (Tg) of 115–175 ꢁC, as revealed
by thermogravimetric analysis (TGA) and the differential scanning
calorimetry (DSC), respectively (see Fig. 2). The high thermal
stability of the resulting copolymers prevents the deformation of
solution (Fig. 3a), which can be assigned to
p-p* transition of the
conjugated copolymer backbone and ICT interaction between the
fluorene donor and BVM-based acceptor. Similarly, the absorption
spectra of other copolymers (PFTMT and PFPMP) in dilute solutions
Fig. 1. 1H NMR spectra and chemical structures of PFBMB, PFTMT and PFPMP in CDCl3 solution.

Y. Li et al. / Polymer 51 (2010) 1786–1795
1791
Table 1
Polymerization Results for Copolymers PFBMB, PFTMT and PFPMP.
a
Copolymer
Mn (Kg/mol)a
Mw (Kg/mol)
PDI
Tgb (ꢁC)
Tdc (ꢁC)
PFBMB
PFTMT
PFPMP
5.24
43.8
6.16
8.28
183.8
9.20
1.58
4.20
1.49
116
115
175
353
380
301
a
Calculated from GPC (eluent : THF; polystyrene standards).
b
c
Determined by DSC at a heating rate of 20 ꢁC/min under nitrogen.
Temperature at 5% weight loss by a heating rate of 10 ꢁC/min under nitrogen.
also showed two bands near 334, 335 nm and 482, 500 nm due to
the * transition and the ICT interaction, respectively. The solu-
tion absorption spectrum of PFPMP, with an absorption maximum
p–p
abs
(
l
) at 500 nm, is red-shifted compared to those of PFBMB
max
abs
max
abs
(
l
¼ 460 nm), and PFTMT (
l
¼ 482 nm), which can be
max
explained by much stronger ICT effect in PFPMP than that in PFBMB
and PFTMT. Among these three copolymers, there is an alternating
‘‘D*–(D–A–D)’’ structure, where the D* is the donor of fluorene, and
A is the PM-based acceptor unit which is bridged by varied electron-
donating ability donor
D (1,4-linked styryl; 2,5-linked thio-
phenevinyl; 1,4-linked phenothiazinevinyl). If the bridged-D
possesses the strong electron-donating ability, the electronic delo-
calization degree and the ICT intensity of the copolymer will be
enhanced. Since the order of the electron-donating abilities of the
three bridged-D is benzene < thiophene < phenothiazine, the
strongest electron-donating ability of phenothiazine compared to
benzene and thiophene improves the effective conjugation length
along copolymer backbone, resulting in an increase in the ICT
strength and thus electronic delocalization [27]. Moreover, the
relatively high absorption coefficients (3max) could be calculated
from Beer’s law equation with the same dilute concentration of the
copolymers in chloroform, which assures the copolymers can
absorb enough photons.
Fig. 3b shows the optical absorption spectra of thin films of the
copolymers. The thin film absorption spectra are generally similar
in shape to those in dilute solution. The maximum absorption peak
of PFBMB at visible region shows a 7 nm red shift between in
solutions and thin films. In the case of PFTMT and PFPMP, the
absorption spectra in thin films also exhibit 9 nm and 12 nm red
Fig. 3. Normalized absorption spectra of the copolymers (a) in chloroform solutions
with the concentration of 10ꢀ5 mol/L; (b) films spin-coated from a 10 mg/mL chloro-
form solution.
shift, respectively, which can be explained by the formation of p-p
stacking structure in the solid state that could facilitate charge
transportation for photovoltaic applications. The small red shift
between in solutions and thin films of the three copolymers maybe
caused by the nonplanar conformations due to the fluorene and
phenothiazine moieties and the long alkyl side chains can induce
the twist of the copolymer backbone, which is not beneficial to the
p–
p
stacking in the solid state [29].
The ICT absorption bands of the three copolymers are tuned
from 467 to 512 nm, and the optical band gap Eg,opt of the copoly-
mers derived from the absorption edge of the thin film spectra is in
the range of 2.23–1.92 eV (Table 2). As expected, among the three
copolymers, PFPMP with the strongest intramolecular charge
transfer interaction thus has the lowest optical band gap of 1.92 eV,
which is lower than that of poly(3-alkylthiophene) homopolymer
(w2.0 eV) [28,30]. It is evident that the ICT interaction between
bridged-D and acceptor moieties in D*–(D–A–D) copolymers is
a practical approach to lower the band gap. We also investigated
the photoluminescence properties of the synthesized copolymers
in solutions and films. It has to be noted that all the copolymers
discussed here are poorly emissive except PFTMT in solution. The
observed strong quenching of the luminescence is in good agree-
ment with the occurrence of an ICT process [31].
To further understand the optical properties of PFBMB, PFTMT
and PFPMP, we have carried out the molecular orbital distribution
calculations of the basic structure units (D*–D–A–D model) of the
three copolymers using quantum mechanical package Gaussian 03.
The optimum geometry and electron-state-density distribution of
the highest occupied molecular orbital (HOMO) and the lowest
unoccupied molecular orbital (LUMO) were calculated by using the
Fig. 2. TGA thermograms of the copolymers.

1792
Y. Li et al. / Polymer 51 (2010) 1786–1795
Table 2
Optical and electrochemical data of the PFBMB, PFTMT and PFPMP.
c
In solutiona
In filmb
E
E
Electrochem.
Optical
onset
Ox
onset
Red
Copolymer
abs
max
abs
max
l
[nm] 3max([Mꢀ1 cmꢀ1])
ledge [nm]
l
[nm]
ledge [nm]
(V)/HOMO(eV)
(V)/LUMO(eV)
Eg,ec (eV)
Eg,opt (eV)c
PFBMB
PFTMT
PFPMP
460 (60230)
483 (49270)
500 (70369)
532
568
604
467
492
512
556
581
646
0.94/ꢀ5.64
0.91/ꢀ5.61
0.47/ꢀ5.17
ꢀ1.39/ꢀ3.31
ꢀ1.32/ꢀ3.38
ꢀ1.35/ꢀ3.35
2.38
2.23
1.78
2.23
2.13
1.92
a
1 ꢂ 10ꢀ5 M in anhydrous chloroform.
b
c
Spin-coated from a 10 mg/mL chloroform solution.
The optical band gap (Eg,opt) was obtained from absorption edge.
density functional theory (DFT) as approximated by the B3LYP
functional and employing the 6-31G* basis set (see in Fig. 4). Abi-
nitio calculations on the model compound for the three copolymers
show that the different degree of torsion between D*and D lead to
the noplanar molecule, which weaken the electrons to be delo-
calized within the molecule by conjugation, and also decreased the
onset oxidation potentials (EOonxset) of the three copolymers are
observed in the range of 0.47–0.94 V. The onset reduction poten-
tials (ERonedset) are almost the same for the three copolymers
(ꢀ1.35 V). The redox potential of Fc/Fcþ which has an absolute
energy level of ꢀ4.8 eV relative to the vacuum level for calibration
is located at 0.1 V in 0.1 M TBAPF6/acetonitrile solution [32]. So the
p–
p
stacking structure in the solid state. This result is in agreement
evaluation of HOMO and LUMO levels as well as the band gap (Eg, ec)
with the slight red shift of copolymers between in solutions and
thin films. The HOMO state density was distributed some extent
could be done according to the following equations:
onset
HOMOðeVÞ ¼ ꢀeðE
þ 4:7ÞðeVÞ
þ 4:7ÞðeVÞ
over
p–conjugated molecule, the electron density of LUMO was
Ox
mainly localized on the PM moiety with some extending to
bridged-D, which suggests that the transition is accompanied by
ICT from D* and D units to the PM moiety, and as a result, the low
band gap of the copolymers are due to the introduction of the PM
segment.
onset
Red
LUMOðeVÞ ¼ ꢀeðE
onset
Ox
onset
Eg;ec ¼ E
ꢀ E
ðeVÞ
Red
onset
Ox
onset
are the measured potentials relative to Ag/
Red
where E
and E
Agþ. The electrochemical properties as well as the energy level
parameters of copolymers are list in Table 2.
3.4. Electrochemical properties
The estimated HOMO and LUMO energy levels of PFBMB are
ꢀ5.64 and ꢀ3.31 eV, respectively. The LUMO energy levels of
PFTMT and PFPMP are ꢀ3.38 and ꢀ3.35 eV, which are very similar
to that of PFBMB. Therefore, the substitution of bridged-D with
varied electron-donating ability moieties has slight effect on the
Fig. 5 shows the cyclic voltammetry (CV) diagrams of the
copolymers using TBAPF6 as supporting electrolyte in acetonitrile
solution with platinum button working electrodes, a platinum wire
counter electrode and an Ag/AgNO3 reference electrode under the
N2 atmosphere. Ferrocene was used as the internal standard. The
Fig. 4. Molecular orbital surfaces of the HOMO and LUMO of D*–D–A–D model obtained at B3 LYP/6-31G* level.

Y. Li et al. / Polymer 51 (2010) 1786–1795
1793
Fig. 6. Band diagram for accepting PCBM and donor PFBMB, PFTMT, and PFPMP
copolymers. Dashed lines indicate the thresholds for air stability (5.2 eV) and effective
charge transfer PCBM (4.3 eV).
Fig. 5. Cyclic voltammetry curves of PFBMB, PFTMT, PFPMP films on platinum elec-
trode in 0.1 mol/L n-Bu4NPF6 in CH3CN solution, at a scan rate of 100 mV/s.
investigated from 1:1 to 1:4. It is known that solvents used for the
preparation of the active layer have a strong impact on the
performance of the cell [35]. Here, we chose chlorobenzene for the
three copolymers in order to obtain the films with the relative good
quality. The devices were characterized in the dark and under solar
simulator AM1.5 (100 mW/cm2) with simultaneous recording of
their current–voltage characteristics. The current–voltage charac-
teristics of the photovoltaic cell based on PFBMB:PCBM,
PFTMT:PCBM and PFPMP:PCBM with weight ratio (1:3 w/w)
showed the best performance, which are presented in Fig. 7. The
photovoltaic parameters of the photovoltaic cells are summarized
in Table 3.
The cells based on PFBMB:PCBM (1:3 w/w) and PFTMT:PCBM
(1:3 w/w) showed an open circuit voltage (Voc) of 0.78 V and 0.80 V,
a short circuit current (Jsc) of 0.11 mA/cm2 and 0.20 mA/cm2, a fill
factor (FF) of 0.27 and 0.28, giving a power conversion efficiency
(PCE) of 0.02% and 0.04%, respectively. The low Jsc of the cells may
be caused by the weak ICT interaction inside PFBMB and PFTMT and
the bad aggregated configuration in solid state of PFTMT:PCBM
blend film which leads to the low absorption of solar spectrum and
hindered charge transport. However, greatly enhanced device
performance was obtained for the other two cells based on
reduction potential of the copolymers, which is consistent with
molecular orbital distribution calculations that the electron density
of LUMO was mainly localized on the PM moiety with some
extending to bridged-D. Besides, the relatively low LUMO energy
levels of the three copolymers result from the stronger reduction of
PM-based acceptor unit. On the other hand, the HOMO energy
levels of copolymers behave quite differently. The HOMO energy
levels of the copolymers PFBMB, PFTMT and PFPMP are in the range
of ꢀ5.64 to ꢀ5.17 eV, which are clearly affected by the varied
electron-donating ability of the three bridged-D due to the
modulation of the ICT strength. The HOMO levels of PFBMB
(ꢀ5.64 eV), PFTMT (ꢀ5.61 eV) and PFPMP (ꢀ5.17 eV) are gradually
increased with the enhanced electron-donating ability of the three
bridged-D. Generally, the stronger electron-donating ability of
bridged-D resulted in a higher HOMO energy level.
The HOMO energy level of the donor copolymers is very
important for high performance photovoltaic cell. Firstly, the
copolymers should have good air stability with HOMO energy level
being below the air oxidation threshold (ca. ꢀ5.2 eV) [33,39].
Secondly, the relatively low HOMO level of the copolymers can
allow a high open circuit potential (Voc) value for the photovoltaic
cell [22]. A complete picture of the band structure of the copoly-
mers is presented in Fig. 6. The first dashed line indicates the
threshold for air stability, and the second dashed line represents
the threshold value for an effective charge transfer from the
copolymers to PCBM (ꢀ4.3 eV) [39]. Both the HOMO energy levels
and LUMO energy levels of the copolymers are close to the ideal
range. The destabilization of HOMO and stabilization of LUMO level
result in reduced band gaps, which also demonstrated the signifi-
cance of intramolecular charge transfer through the D–A structures
inside the copolymers. Although there is deviation between the
optical band gap (Eg,opt) and electrochemical band gap (Eg,ec), the
trend between the band gap and copolymer strcture is similar. So,
the HOMO energy level and band gap of the copolymers can be
controlled strictly by introducing different electron-donating
ability electron donors.
3.5. Photovoltaic properties
In order to investigate the photovoltaic properties of the
copolymers, the BHJ photovoltaic cells with a structure of ITO/
PEDOT-PSS/copolymers:PCBM/LiF/Al were fabricated, where the
copolymers were used as donors and PCBM as the acceptor [34].
The weight ratios of blend films (copolymers:PCBM) were
Fig. 7. Current–voltage characteristics of copolymer photovoltaic cells based on
PFBMB, PFTMT and PFPMP in the dark and under illumination of AM 1.5, 100 mW/cm2
white light.

1794
Y. Li et al. / Polymer 51 (2010) 1786–1795
Table 3
a high degree of homogeneity is observed for the film. Since
PFPMP and PCBM molecules have good miscibility, increased
interfacial area is expected, and the Jsc of PFPMP:PCBM (1:3 w/w)
based device increases to 2.1 mA/cm2, which is more than 13
times higher than that of PFTMT:PCBM based device. It has been
recently reported that chemical similarity between the func-
tional groups attached to the donor copolymers and the fullerene
can effectively affect the formation of films with uniform and
stable nanophase morphologies [37]. In the present case, the
significant morphological difference between the three blend
copolymer films suggests that similar chemical polarity of the
D–A copolymers with PCBM by adjusting the ICT intensity
may improve the miscibility between them in the blend film,
thereby efficiently suppressing the tendency of the PCBM
molecules to phase segregate into microscale clusters or uniform
morphologies.
Characteristic current–voltage parameters from device testing at standard AM 1.5G
conditions and blend films roughness of AFM measurement.
Copolymer/PCBM
(w/w ratio)
Voc(v)a
Jsc
FFa
RMS
PCE
(%)a
(mA/cm2)a
(nm)b
PFBMB
PFTMT
PFPMP
1:3
1:3
1:3
0.78
0.80
0.82
0.11
0.20
2.1
0.27
0.28
0.30
60.89
5.73
1.79
0.02
0.04
0.52
a
Photovoltaic properties of copolymer/PCBM-based devices spin-coated from
a chlorobenzene solution for PFBMB (1:3 w/w), PFTMT (1:3 w/w) and PFPMP
(1:3 w/w).
b
Root mean-square (RMS) roughness from AFM measurement.
PFPMP:PCBM (1:3 w/w) with Voc of 0.82, Jsc of 2.1 mA/cm2, FF of
0.30, and PCE of 0.52%, respectively. The reason for the improve-
ment PCE could be explained by the strong ICT interaction and
better p–p stacking of PFPMP in the solid state.
Fig. 9 depicts photocurrent action spectra for the PFBMB:PCBM
(1:3 w/w), PFTMT:PCBM (1:3 w/w), PFPMP:PCBM (1:3 w/w)
photovoltaic devices under monochromatic illumination. The
external quantum efficiency or incident photon-to-current effi-
To gain better insight into what might be controlling the low
short circuit current and open circuit voltage, atomic force
microscopy (AFM) was used to examine the surface topography
of the films. Film morphology of the active layer, i.e. the blend
film of the donor copolymer and the acceptor (e.g. PCBM) has
been found to be one of the key elements in determining the PCE
of copolymer photovoltaic cell [36]. Fig. 8a–c shows the AFM
height images of PFBMB:PCBM, PFTMT:PCBM, PFPMP:PCBM
blend films with the same weight ratio (1:3 w/w) for effective
comparison and further elucidation of the difference in PVCs
performance. As is clearly evidenced by AFM, the PFBMB:PCBM
blend film shows a high coarse surface with the root mean
square (RMS) of 60.89 nm (Fig. 8a and Table 3), and substantial
PCBM grain-aggregation with the size distribution around
ciency, IPCE (%), is displayed as
a function of wavelength.
Comparing the IPCE curves of PFBMB:PCBM, PFTMT:PCBM and
PFPMP:PCBM, it can be seen that the IPCE values of PFBMB:PCBM
and PFTMT:PCBM in the wavelength ranging from 400 to 600 nm
are similar and showed extremely low values (no more than 2%).
However, the IPCE curve of PFPMP:PCBM shows a significantly
increased spectral response in the visible region. The maximum
IPCE values of the device based on PFPMP:PCBM is 20% at 489 nm in
the wavelength ranging from 400 to 600 nm. It is further confirmed
that the PFPMP:PCBM based device with higher IPCE values
obtained higher Jsc.
1.50 mm are almost homogeneously dispersed in the PFBMB
Furthermore, as discussed above, the Voc is directly governed by
the difference between the HOMO energy levels of the donor and
the LUMO of the acceptor. However, several other parameters must
be taken into account such as carrier recombination, resistance
related to thickness of the active layer and degree of phase sepa-
ration between the components in the blend, which can modify the
energetically expected Voc value. Therefore, although the HOMO
energy levels of PFBMB and PFTMT are higher than that of PFPMP,
the Voc value of PFBMB and PFTMT-based device was a bit lower
than that of PFPMP-based devices. The relative low Voc for PFBMB
and PFTMT-based device could be explained by the large numbers
of carrier recombination due to the obvious grain-aggregation and
large-scale blend phase separation. But all the three copolymers
have a satisfying Voc (around 0.80 V), which is higher than the
P3HT:PCBM based device (0.6 V) [38].
matrix, which results in a most large-scale phase separation,
decreased diffusional escape probability for mobile charge
carriers and hence increased recombination. This is fully
consistent with the low short circuit obtained for the
PFBMB:PCBM cell (0.11 mA/cm2). Compared with the
PFBMB:PCBM blend film, the blend film of PCBM and PFTMT with
a relatively strong electron-donating abilities moiety, thiophene
instead of benzene, shows a lower degree of grain-aggregation
with the size distribution around 150 nm, the relative flat
surface with the RMS of 5.73 nm (Fig. 8b) and smaller phase
separation, which lead to an improvement of the short circuit
current for the PFTMT:PCBM cell (0.20 mA/cm2). In the case of
PFPMP:PCBM blend film (Fig. 8c), an even stronger electron-
donating abilities moiety phenothiazine was brought in, and
Fig. 8. Topography image obtained by tapping-mode AFM showing the morphology of the blend films spin-coated from chlorobenzene for (a) PFBMB/PCBM (w/w, 1:3) (size
20 m); (b) PFTMT/PCBM (w/w, 1:3) (size 5 m ꢂ 5 m); (c) PFPMP/PCBM (w/w, 1:3) (size 5 m ꢂ 5 m).
m ꢂ 20
m
m
m
m
m
m

Y. Li et al. / Polymer 51 (2010) 1786–1795
1795
would like to thank the financial support of the Project 20091008
from Graduated Innovation Fund of Jilin University.
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We have designed and synthesized three novel D–A conju-
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Optical property and molecular orbital distribution calculations
investigations unequivocally indicate that these new copolymers
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This work was supported by the State Key Development
Program for Basic Research of China (Grant No.2009CB623605), the
National Natural Science Foundation of China (Grant No.50673035,
20874035), Program for New Century Excellent Talents in Univer-
sities of China Ministry of Education, the 111 Project (Grant No.
B06009), and the Project of Jilin Province (20080305), Yaowen Li
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