Conjugated fluorene and silole copolymers: Synthesis, characterization, electronic transition, light emission, photovoltaic cell, and field effect hole mobility

Article abstract of DOI:10.1021/ma047561l

A novel series of soluble conjugated random and alternating copolymers (PFO-TST) derived from 9,9-dioctylfluorene (FO) and 1,1-dimethyl-3,4-diphenyl-2, 5-bis(2′-thienyl)silole (TST) were synthesized by palladium(0)-catalyzed Suzuki coupling reactions. The

Full text of DOI:10.1021/ma047561l

Macromolecules 2005, 38, 2253-2260
2253
Conjugated Fluorene and Silole Copolymers: Synthesis,
Characterization, Electronic Transition, Light Emission, Photovoltaic
Cell, and Field Effect Hole Mobility
Feng Wang, Jie Luo, Kaixia Yang, Junwu Chen,* Fei Huang, and Yong Cao
Institute of Polymer Optoelectronic Materials & Devices, Key Laboratory of Specially Functional
Materials and Advanced Manufacturing Technology of Ministry of Education, South China University
of Technology, Guangzhou 510640, China
Received November 25, 2004; Revised Manuscript Received December 21, 2004
ABSTRACT: A novel series of soluble conjugated random and alternating copolymers (PFO-TST) derived
from 9,9-dioctylfluorene (FO) and 1,1-dimethyl-3,4-diphenyl-2,5-bis(2′-thienyl)silole (TST) were synthesized
by palladium(0)-catalyzed Suzuki coupling reactions. The feed ratios of FO to TST were 99:1, 95:5, 90:
10, 80:20, and 50:50. Chemical structures and optoelectronic properties of the copolymers were
characterized by elemental analysis, NMR, UV absorption, cyclic voltammetry, photoluminescence (PL),
electroluminescence (EL), a photovoltaic cell, and a field effect transistor. The elemental analyses of the
copolymers indicated that FO and TST contents in the copolymers were very close to the feed compositions.
The random copolymers exhibited a PFO-segment-dominated UV absorption peak at ∼385 nm and a
narrow band gap TST absorption at ∼490 nm. For the alternating copolymer, only a broad absorption
band was found, demonstrating a mixed and TST-dominated electronic configuration. Compared with
the solution PL, complete PL excitation energy transfer from the PFO segment to the TST unit could be
achieved by film PL at lower TST content. It was found that the EL spectra of the copolymers with a
device configuration of indium-tin oxide/poly(3,4-ethylenedioxythiophene)/poly(vinylcarbazole) (PVK)/
copolymer/Ba/Al were red shifted and had better red light CIE coordinates with improved external
quantum efficiency, compared with corresponding device performances without the PVK layer. With the
alternating copolymer as the electron donor and methanofullerene [6,6]-phenyl C61-butyric acid methyl
ester as the electron acceptor, an energy conversion efficiency of 2.01% was achieved under an AM1.5
simulated solar light at 100 mW/cm2, which is among the highest values so far reported for bulk-
heterojunction photovoltaic cells. The field effect hole mobility of the alternating copolymer is 4.5 × 10-5
cm2/(V s) using polyacrylonitrile as an organic insulator on a gate electrode.
Introduction
incorporating narrow band gap comonomers into the PF
backbone.10-12 Besides the color tuning for PLED ap-
plications, recently excellent PVC performances have
also been achieved by some PF copolymers containing
2,1,3-benzothiadiazole derivatives.13
Conjugated polymers have drawn broad attention due
to their important applications in polymeric light-
emitting diodes (PLEDs),1,2 photovoltaic cells (PVCs),3
and field effect transistors (FETs).4 Ink-jet printing of
PLEDs is the key advantages in the commercialization
of full-color and large-area flat panel displays.5 Spin
coating and the low cost of polymer-based PVCs are the
attracting advantages for the fabrications of large-area
solar cells.3d,6 Conjugated polymers are the best candi-
dates to be used to fabricate flexible PLEDs, PVCs, and
FETs. A notable feature of conjugated polymers also lies
in the enormous versatility of the synthetic methodol-
ogy, which affords wide space to construct new polymers
with improved properties.
In the past decade, polyfluorenes (PFs) have emerged
as emitting materials suitable for use in PLEDs because
of their highly efficient photoluminescence (PL) and
electroluminescence (EL), their thermal and oxidative
stability, and their good solubility.7 Recently, amino-
alkyl-substituted PFs or their quarternized salts were
reported as novel electron injection layers for a high
work function metal such as the Al cathode.8 Though
PFs were typical blue-light-emitting polymers in the
beginning due to the large band gap,9 thereafter green
and red lights of PF copolymers were achieved by
Siloles or silacyclopentadienes are a group of five-
membered silacyclics that possess a unique low-lying
LUMO level associated with the σ*-π* conjugation
arising from the interaction between the σ* orbital of
two exocyclic σ bonds on the silicon atom and the π*
orbital of the butadiene moiety.14 Siloles exhibit high
electron acceptability14a and fast electron mobility.15
2,3,4,5-Tetraphenylsiloles show novel fluorescent char-
acteristics (aggregation-induced emission).16 Excellent
EL performances have been realized with 2,3,4,5-
tetraphenylsiloles derivatives as the emissive layer.17,18
Recently, poly(di-n-hexylfluorene-co-4,4-diphenyldithieno-
silole) with low silole content (e10%) was reported by
Jen and co-workers.19 Novel linear and hyperbranched
2,3,4,5-tetraphenylsilole-containing polymers were also
demonstrated.20 Siloles are narrow band gap building
blocks for conjugated polymers.21 Thus, the incorpora-
tion of silole into the main chain of PF can probably
afford copolymers with a new electron configuration and
new optoelectronic properties. So far, there are few
reports on silole-containing polymers as the emitting
materials in EL devices, and they only exhibited blue
and green emissions.19,20a To the best of our knowledge,
PVC and FET devices of silole-containing polymers were
not reported before. In this work, a novel series of
soluble conjugated random and alternating copolymers
* To whom correspondence should be addressed. E-mail:
psjwchen@scut.edu.cn. Phone: +8620-8711-4346. Fax: +8620-
8711-4535.
10.1021/ma047561l CCC: $30.25 © 2005 American Chemical Society
Published on Web 02/10/2005

2254 Wang et al.
Macromolecules, Vol. 38, No. 6, 2005
Table 1. Molecular Weights and Elemental Analyses of
the Copolymers
(PFO-TST) derived from 9,9-dioctylfluorene (FO) and
1,1-dimethyl-3,4-diphenyl-2,5-bis(2′-thienyl)silole (TST)
were successfully synthesized by palladium(0)-catalyzed
Suzuki coupling reactions. The optical and electronic
properties such as UV absorption, electrochemical prop-
erties, photoluminescence, electroluminescence, photo-
voltaic cell properties, and field effect transistor prop-
erties of the copolymers were evaluated.
elemental analysisb
a
a
copolymer Mw
Mw/Mn
C
H
S
PFO-TST1 29800
PFO-TST5 35500
PFO-TST10 34700
PFO-TST20 32900
PFO-TST50 44400
2.0
1.7
1.9
1.8
1.4
88.06 (89.62) 10.04 (10.36)
87.58 (88.75) 9.74 (10.07) 0.42 (0.82)
86.74 (87.89) 9.50 (9.77) 1.56 (1.63)
82.63 (86.17) 8.87 (9.17) 3.10 (3.24)
80.28 (81.22) 7.58 (7.44) 7.49 (7.89)
Experimental Section
a Estimated by GPC in THF on the basis of a polystyrene
calibration. b Data given in the parentheses are contents in the
feed compositions.
Materials. All manipulations involving air-sensitive re-
agents were performed under an atmosphere of dry argon. All
reagents, unless otherwise specified, were obtained from
Aldrich, Acros, and TCI Chemical Co. and were used as
received. All solvents were carefully dried and purified under
nitrogen flow. Compounds 1,1-dimethyl-3,4-diphenyl-2,5-bis-
(2′-thienyl)silole (1),22 2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxa-
borolan-2-yl)-9,9-dioctylfluorene (3),9b and 2,7-dibromo-9,9-
dioctylfluorene (4)9b were prepared according to the literature,
and all of them were recrystallized to reach the desirable
purity for the next reactions.
polymerization, a small amount of 3 was added to remove the
bromine end groups, and bromobenzene was added as a
monofunctional end-capping reagent to remove the boronic
ester end group because boron and bromine units could quench
emission and contribute to excimer formation in LED applica-
tion. The mixture was then poured into vigorously stirred
methanol. The precipitated solid was filtered and washed for
24 h with acetone to remove oligomers and catalyst residues.
A red powder of PFO-TST50 was isolated in 75% yield (610
mg). Mw ) 44400, Mw/Mn ) 1.4 (GPC, Table 1). 1H NMR (400
MHz, CDCl3): δ (TMS, ppm) 7.66-6.84 (m, br, 20H, Ar H),
1.91 (s, br, 4H, Ar CH2), 1.27-1.03 (m, br, 20H, CH2), 0.82
(m, br, 10H, CH2 and CCH3), 0.61 (s, br, 6H, SiCH3). 13C NMR
(100 MHz, CDCl3): δ (TMS, ppm) 152.9, 151.5, 144.4, 142.5,
140.0, 139.1, 133.3, 131.9, 129.6, 128.6, 128.1, 127.2, 124.7,
122.4, 119.9, 119.6, 55.0, 40.3, 31.8, 30.0, 29.7, 29.2, 23.7, 22.6,
14.1, -1.7. Anal. Found: C, 80.28; H, 7.58; S, 7.49. UV
(CHCl3): λmax/ꢀmax (mol-1 L cm-1) 510 nm/9.3 × 103.
Instrumentation. 1H and 13C NMR spectra were recorded
on a Bruker DRX 400 or AV 400 spectrometer with tetra-
methylsilane (TMS) as the internal reference. Molecular
weights of the polymers were obtained on a Waters GPC 2410
using a calibration curve of polystyrene standards, with
tetrahydrofuran as the eluent. Elemental analyses were
performed on a Vario EL elemental analysis instrument
(Elementar Co.). UV-vis absorption spectra were recorded on
an HP 8453 spectrophotometer. The PL quantum yields were
determined in an Integrating Sphere IS080 (LabSphere) with
405 nm excitation of a HeCd laser (Melles Griot). PL an EL
spectra were recorded on an Instaspec IV CCD spectropho-
tometer (Oriel Co.). Cyclic voltammetry was carried out on a
CHI660A electrochemical workstation with platinum elec-
trodes at a scan rate of 50 mV/s against a saturated calomel
reference electrode with a nitrogen-saturated solution of 0.1
M tetrabutylammonium hexafluorophosphate (Bu4NPF6) in
acetonitrile (CH3CN). The deposition of the copolymer on the
electrode was done by the evaporation of a dilute chloroform
solution.
1,1-Dimethyl-3,4-diphenyl-2,5-bis(5′-bromo-2′-thienyl)-
silole (2). N-Bromosuccinimide (NBS) (783 mg, 4.4 mmol) was
dissolved in 20 mL of anhydrous DMF. This mixture was
added dropwise to a mixture of 1 (853 mg, 2 mmol) in
anhydrous DMF (25 mL) in the dark. The mixture was stirred
for 15 min and poured into 100 mL of water. The organic
material was extracted with dichloromethane (2 × 150 mL).
The crude product was purified on a silica gel column using a
petroleum ether/chloroform mixture (5:1 by volume) as eluent.
A yellow solid of 2 was isolated in 45% yield (525 mg).
Recrystallization from a toluene/heptane mixture gave 409 mg
of pure compound. 1H NMR (400 MHz, CDCl3): δ (TMS, ppm)
7.16 (m, 6H, phenyl H), 6.93 (m, 4H, phenyl H), 6.80 (d, 2H,
thienyl H), 6.64 (d, 2H, thienyl H), 0.63 (s, 6H, CH3). 13C NMR
(100 MHz, CDCl3): δ (TMS, ppm) 152.9, 144.5, 138.3, 131.6,
129.3, 128.9, 128.7, 127.7, 127.0, 113.2, -1.9. GC-MS: m/e
584 (M+). Anal. Caled for C26H20Br2S2Si: C, 53.42; H, 3.42; S,
10.96. Found: C, 53.37; H, 3.52; S, 10.78.
LED Fabrication and Characterization. Polymers were
dissolved in toluene or p-xylene and filtered through a 0.45
µm filter. Patterned indium-tin oxide (ITO; ∼15 Ω/square)-
coated glass substrates were cleaned with acetone, detergent,
distilled water, and 2-propanol, subsequently in an ultrasonic
bath. After treatment with oxygen plasma, 150 nm of poly-
(3,4-ethylenedioxythiophene) doped with poly(styrenesulfonic
acid) (PEDOT:PSS) (Baytron P 4083, Bayer AG) was spin-
coated onto the ITO substrate followed by drying in a vacuum
oven at 80 °C for 8 h. For some devices, poly(vinylcarbazole)
(PVK; Aldrich) from 1,1,2,2-tetrachloroethane solution was
coated on top of a dried PEDOT:PSS layer subsequently. A
thin film of electroluminescent copolymer was coated onto the
anode by spin-casting inside a nitrogen-filled drybox (Vacuum
Atmospheres). The film thickness of the active layers was
around 80 nm, as measured with an Alfa Step 500 surface
profiler (Tencor). We note that since PVK is not soluble in
p-xylene, PVK remains untouched during spinning of the top
copolymer layer. A thin layer of Ba (4-5 nm) and subsequently
200 nm layers of Al were evaporated subsequently on the top
of an EL polymer layer under a vacuum of 1 × 10-4 Pa. Device
performances were measured inside a drybox (Vacuum Atmo-
spheres). Current-voltage (I-V) characteristics were recorded
with a Keithley 236 source meter. The luminance of the device
was measured with a calibrated photodiode. The external
quantum efficiency was verified by measurement in the
integrating sphere (IS-080, Labsphere), and luminance was
calibrated by using a PR-705 SpectraScan spectrophotometer
(Photo Research) after encapsulation of the devices with UV-
curing epoxy and a thin cover glass.
PVC Fabrication and Characterization. The PVC device
structure used in this study was a sandwich structure, ITO/
PEDOT:PSS/active layer/Ba/Al. ITO/PEDOT:PSS is the hole-
collecting electrode, and Ba/Al is the electron-collecting elec-
trode. We fabricated the PVC devices according to procedures
similar to that of EL devices. The I-V characteristics in the
dark and under illumination were measured with a Keithley
236 source meter. The photocurrent was measured under a
solar simulator with AM1.5 illumination (100 mW/cm2). The
spectral response was measured with a commercial photo-
modulation spectroscopic setup including a xenon lamp, an
optical chopper, a monochromator, and a lock-in amplifier
Polymerization. All of the polymerizations were carried
out by palladium(0)-catalyzed Suzuki coupling reactions with
an equivalent molar ratio of the diboronic ester monomer to
the dibromo monomers under dry argon protection. The
purifications of the polymers were conducted in air with yields
of 68-80%. A typical procedure for the copolymerization is
given below.
Carefully purified 2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxa-
borolan-2-yl)-9,9-dioctylfluorene (3) (642 mg, 1 mmol), 1,1-
dimethyl-3,4-diphenyl-2,5-bis(5′-bromo-2′-thienyl)silole (2) (584
mg, 1 mmol), (PPh3)4Pd(0) (6 mg, 0.005 mmol), and several
drops of Aliquat 336 were dissolved in a mixture of toluene
(10 mL) and aqueous 2 M Na2CO3 (2 mL). The solution was
refluxed with vigorous stirring for 48 h. At the end of the

Macromolecules, Vol. 38, No. 6, 2005
Conjugated Fluorene and Silole Copolymers 2255
Scheme 1
operated by a PC computer (Merlin, Oriel). A calibrated Si
photodiode was used as a standard in the determination of
photosensitivity.
10, 80:20, and 50:50, and the corresponding copolymers
are named PFO-TST1-50.23 PFO-TST50 is an alternat-
ing copolymer, and PFO-TST1-20 are random copoly-
mers. All the copolymers are soluble in common solvents
such as chloroform, toluene, tetrahydrofuran, etc. The
molecular weights of the copolymers are listed in Table
1. The Mw values of the copolymers are around (3-4) ×
104 with a polydispersity index (Mw/Mn) from 1.4 to 2.0.
The alternating copolymer PFO-TST50 has the highest
Mw and the narrowest molecular weight distribution.
The elemental analyses of the copolymers are also listed
in Table 1. The contents of C, H, and S of the copolymers
are very close to the feed compositions, and the rela-
tively larger deviation of the S content of PFO-TST5
may be attributed to the inherently low S content of the
copolymer, which causes larger experimental error.
Since S is derived from TST, the result demonstrated
that the narrow band gap TST was incorporated in the
copolymers nearly according to the feed ratios.
FET Fabrication and Characterization. The configura-
tion of the FET device is ITO/polyacrylonitrile (PAN)/PFO-
TST50/Au. ITO on glass, used as the gate electrode, was
patterned by photolithography. The thickness of the ITO
measured by the surface profiler (Tencor, ALFA-Step 500) was
110 nm. PAN, the insulator for the gate electrode, was
dissolved in N,N-dimethylformamide (DMF) and was deposited
on top of the ITO by spin-coating. Then the PAN layer was
dried at 90 °C for 20 h under vacuum, with a final thickness
of 450 nm. To analyze the dielectric properties, we made a
parallel capacitor with an ITO/insulator/Al structure. The
capacitance was measured by a Hewlett-Packard 4192 imped-
ance analyzer from 10 kHz to 1 MHz. On top of the dielectric
layer, the active layer (PFO-TST50) was obtained by spinning
of its toluene solution in a nitrogen atmosphere followed by
an annealing step at a temperature of 70 °C for 1 h under a
nitrogen atmosphere. The thickness of the active layer was
80 nm. On the top of the active layer, a gold layer with a
thickness of 50 nm was thermally deposited in a vacuum
through a shadow mask to obtain the source and drain
electrodes. Thus, the channel was formed with an interdigital
structure. The channel width-to-length ratio (W/L) was 100.
The output characteristics were measured under a nitrogen
atmosphere by a Keithley 2400 and a computer-controlled
source meter (Keithley 236 source-measure units) that con-
trolled the gate voltage and drain voltage, respectively. The
drain current was also recorded by the Keithley 236 source-
measure units.
1
Figure 1 shows evolution of H NMR spectra of the
copolymers from low TST content to high TST content.
The 13C NMR spectrum of the alternating copolymer is
shown in Figure 2. The spectrum indicates that the
chemical structure of each monomer unit in the alter-
nating copolymer is symmetric. We can assign some
characteristic carbon resonance, including one carbon
resonating at δ ) -1.7 ppm for SiCH3 of TST, one
carbon resonating at δ ) 152.8 ppm for the “ene” carbon
at the 3- or 4-position of the silole ring,24 one carbon at
δ ) 151.5 ppm for the 10- or 13-position carbon of
fluorene,9b one carbon at δ ) 55.0 ppm for the 9-position
carbon of fluorene, and the eight carbons resonating
between 40.3 and 14.1 ppm for the n-octyl group of the
fluorene. The NMR results confirm the chemical struc-
ture of the alternating copolymer (see the Experimental
Section for details).
Results and Discussion
Synthesis and Characterization. The general syn-
thetic routes toward the monomers and polymers are
shown in Scheme 1. We synthesized compound 1 ac-
cording to identical procedures reported by Yamaguchi
et al.22 Then monomer 2 was prepared in good yield by
the bromination of 1 with NBS in the dark. Monomers
3 and 4 were prepared according to published proce-
dures.9b Monomers 2-4 were recrystallized to reach the
desirable purity for copolymerization.
Conjugated copolymers derived from monomers 2-4
were prepared by palladium(0)-catalyzed Suzuki cou-
pling reactions with an equivalent molar ratio of the
diboronic ester monomer to the dibromo monomers. The
monomer ratios of fluorene to silole are 99:1, 95:5, 90:
UV Absorption and Electrochemical Properties.
The UV-vis absorption spectra of the copolymers in
chloroform solutions are shown in Figure 3A. The
random copolymers PFO-TST1-20 show absorption
maxima of around 385 nm, which are from the fluorene
segment. The absorption spectrum of PFO-TST1 is
practically identical to that of the PFO homopolymer
due to the very low content of TST. The absorption

2256 Wang et al.
Macromolecules, Vol. 38, No. 6, 2005
Figure 1. Evolution of 1H NMR spectra of the copolymers
from low TST content to high TST content (solvent CDCl3).
The solvent peaks are marked with an asterisk.
Figure 3. UV absorption spectra of the copolymers: (A)
chloroform solution (1 × 10-4 M), (B) thin solid film.
Table 2. Optical Band Gaps and Electrochemical
Properties of the Films of Copolymers
optical band
gapa (eV)
Eox
(V)
Ered
(V)
HOMOc LUMOd
copolymer
(eV)
(eV)
PFO-TST1
PFO-TST5
PFO-TST10
PFO-TST20
PFO-TST50
2.95
2.21b
2.15
2.14
2.08
1.35 -2.28
1.34 -2.29
1.35 -2.31
1.35 -0.80
1.31 -0.80
-5.75
-5.74
-5.75
-5.75
-5.71
-2.12
-2.11
-2.09
-3.60
-3.60
a Estimated from the onset wavelength of optical absorption of
the thin solid film. b Using the absorption spectrum of its chloro-
form solution. c Calculated according to HOMO ) -e(Eox + 4.4).
d Calculated according to LUMO ) -e(Ered + 4.4).
indicates that the electronic configurations of both
components, fluorene and silole, in the alternating
copolymer are mixed and silole-dominant.
The UV-vis absorption spectra of thin solid films of
the copolymers are shown in Figure 3B. The absorption
features of the films of the copolymers are similar to
those of their chloroform solutions, but the TST absorp-
tion peaks of PFO-TST5 and PFO-TST10 are not so
well-resolved, compared with those of the corresponding
chloroform solutions. The TST absorption peak of the
PFO-TST20 film is at 496 nm, which is practically
identical to that of its chloroform solution. The TST
absorption peak of the PFO-TST50 film is at 520 nm,
with a 10 nm red shift to that of its chloroform solution.
The optical band gaps of the films of the copolymers are
estimated from the onset wavelengths of optical absorp-
tions of thin solid films and are listed in Table 2. The
optical band gap decreases with increasing TST content.
The electrochemical behavior of the copolymers was
investigated by cyclic voltammetry (CV). We could
record only one p-doping and one n-doping process of
the copolymers. The onsets of oxidation processes of the
copolymers are between 1.31 and 1.35 V (see Table 2),
which are very close to the data reported for polyfluo-
Figure 2. 13C NMR spectrum of the alternating copolymer
(PFO-TST50) in CDCl3. The solvent peaks are marked with
an asterisk.
spectra of copolymers PFO-TST5-20 also show obvious
absorptions at longer wavelengths, which are from the
TST moiety. The higher the TST content in the random
copolymer, the larger the absorbance ratio of TST to
fluorene. The maxima of the TST absorption of PFO-
TST5-20 are at 488, 488, and 495 nm, respectively. The
alternating copolymer PFO-TST50 has a completely
different absorption spectrum, compared with the ran-
dom copolymers. The absorption peak at around 385 nm
corresponding to the fluorene segment disappeared
completely, and only a broad and strong absorption
peaked at 510 nm, quite different from two separate
absorptions of other alternating copolymers derived
from fluorene and narrow band gap nitrogen-containing
aromatic heterocyclics.12d,13a The absorption spectrum

Macromolecules, Vol. 38, No. 6, 2005
Conjugated Fluorene and Silole Copolymers 2257
Table 3. PL Properties of the Copolymersa
solution
film
λmax
(nm)
λmax
(nm)
QEb
(%)
copolymer
PFO-TST1
PFO-TST5
PFO-TST10
PFO-TST20
PFO-TST50
418, 441
458, 510
458, 549
557
15.2
24.8
20.0
12.1
6.4
418, 441, 571
418, 441, 572
418, 439, 577
585
562
591, 616
a Excitation wavelength 390 nm. b Absolute PL quantum yield
measured in the integrating sphere.
Figure 4. Photoluminescence spectra of the copolymers
(excitation wavelength 390 nm): (A) chloroform solution (1 ×
10-4 M), (B) thin solid film.
rene homopolymer, Eox ) 1.4 V and Ip ) 5.8 eV.25 So
the oxidations are attributed to p-doping of PFO seg-
ments. The onsets of the n-doping processes of PFO-
TST1-10 are between -2.28 and -2.31 V, which are
the reductions of the PFO segments. The onsets of the
n-doping processes of PFO-TST20 and PFO-TST50 are
both at -0.80 V, which are the reductions of the TST
moieties. The TST reduction was not found for PFO-
TST1-10, probably due to the low TST contents in the
copolymers. HOMO and LUMO levels calculated ac-
cording to an empirical formula (HOMO ) -e(Eox + 4.4)
eV and LUMO ) -e(Ered + 4.4) eV)26 are also listed in
Table 2. The LUMOs of PFO-TST20 and PFO-TST50
demonstrate the low-lying LUMO structural character-
istics of siloles. The electrochemical band gaps of PFO-
TST1, PFO-TST20, and PFO-TST50 are very close to
the corresponding optical band gap. The large deviations
of the electrochemical band gap from the optical band
gap of PFO-TST5 and PFO-TST10 indicate that the
LUMOs of the two copolymers may not be reliable.
Photoluminescence Properties. Photolumines-
cence spectra of the copolymers in chloroform solution
at a concentration of 1 × 10-4 M are shown in Figure
4A. The PL peaks are listed in Table 3. The feature of
the PL spectrum of PFO-TST1 is completely PFO-
dominant with two peaks at 418 and 441 nm. There is
no signature of silole emission. Increasing the TST
contents to 5%, the silole emission emerges with a
maximum at 571 nm. The silole emission of PFO-TST10
is further enhanced but is still weaker than the PFO
emission. The intensity of the silole emission of PFO-
TST20 is stronger than that of the PFO emission,
indicating the intramolecular energy transfer from the
PFO segment to the silole moiety. For the alternating
copolymer, there is only one peak at 585 nm, and the
PFO emission disappears, demonstrating the important
Figure 5. EL spectra of the copolymers with device configu-
rations of (A) ITO/PEDOT/copolymer/Ba/Al and (B) ITO/
PEDOT/PVK/copolymer/Ba/Al.
role of the intramolecular energy transfer in the PL
emission.
Photoluminescence spectra of the thin solid films of
the copolymers are shown in Figure 4B. The PL peaks
and the absolute PL quantum yields (QEs) are also
listed in Table 3. Once the TST unit is incorporated into
the PFO main chain, the PL emission is changed
dramatically. Even for PFO-TST1, the blue emission of
the PFO segment becomes very weak and the copolymer
shows a stronger green emission peak at 510 nm. This
can be attributed to strong intermolecular energy
transfer or partial excimer formation of PFO segment27
in the aggregation state. The QE of the copolymer is
15.2%. For the other four copolymers, the emission
signature of PFO at 458 nm is gradually weakened and
disappears finally. The silole emission also continuously
red shifts to the red light region. The QE of PFO-TST5
is the highest among the five copolymers.
Electroluminescence Properties. A double-layer
device with a configuration of ITO/PEDOT (50 nm)/
copolymer (80 nm)/Ba/Al was fabricated. The EL spectra
of the copolymers are shown in Figure 5A. For PFO-
TST1, the EL spectrum is very broad and peaks at 556
nm, close to white light on the basis of CIE coordinates

2258 Wang et al.
Macromolecules, Vol. 38, No. 6, 2005
Table 4. EL Properties of the Copolymers with a Device
Configuration of ITO/PEDOT/Copolymer/Ba/Ala
bias current brightness ηb
λmax
(nm)
CIE
x, y
copolymer (V) (mA)
(cd/m2)
(%)
PFO-TST1
7.8
4.4
5.3
5.2
4.8
4.9
73
140
101
36
0.21 556
0.39 585
0.28 586
0.11 590
0.30, 0.39
0.58, 0.42
0.59, 0.40
0.61, 0.39
PFO-TST5 11.8
PFO-TST10 8.6
PFO-TST20 7.2
PFO-TST50 3.2
2
0.01 590, 633 0.62, 0.38
a The active area is 0.15 cm2. b External quantum efficiency.
Table 5. EL Properties of the Copolymers with a Device
Configuration of ITO/PEDOT (50 nm)/PVK (30 nm)/
Copolymer (80 nm)/Ba/Al
bias current brightness ηb
λmax
(%) (nm)
CIE
x, y
copolymer
(V)
(mA)
(cd/m2)
PFO-TST1 14.6
PFO-TST5 17.2
PFO-TST10 16.8
PFO-TST20 13.4
PFO-TST50 12.2
5.5
5.3
5.1
5.6
4.7
186
97
0.89 610 0.49, 0.42
0.48 618 0.59, 0.40
0.40 622 0.60, 0.40
0.44 624 0.61, 0.39
0.17 638 0.63, 0.36
Figure 6. Photocurrent and dark current of the PVC device
with PFO-TST50:PCBM ) 1:4 as the active layer. The photo-
current was recorded under an AM1.5 solar simulator at 100
mW/cm2.
77
94
30
a The active area is 0.15 cm2. b External quantum efficiency.
Table 6. Photovoltaic Properties of the Copolymersa
b
c
Voc
Isc
FFd
(%)
ECEe
(%)
(mA/m2)
of the EL spectrum (Table 4). This is also a signature
of incomplete intra- and intermolecular energy transfer
from the PFO segment to the silole moiety due to the
low content of TST or partial excimer formation of the
PFO segment. The EL spectrum of PFO-TST5 demon-
strates that a complete energy transfer is fulfilled at a
TST content of 5%. The silole moiety even at the low
content can serve as an efficient trap for all of the
generated excitons. As a result, the generated excitons
are confined and recombined on the silole sites, and the
copolymer emits exclusively from the narrow band gap
component. Increasing the TST contents to 10% and
20%, their EL spectra are quite similar to that of PFO-
TST5. Different from the two comparable peaks at 591
and 616 nm of the PL spectrum of PFO-TST50, its EL
spectrum shows a main peak at 590 nm and a shoulder
peak at 633 nm. All the EL spectra of the copolymers
are red shifted to the corresponding PL spectra of their
films. Similar results were reported before.12 The device
performances are also listed in Table 4. At a current of
∼5 mA, the EL device with PFO-TST5 as the emissive
layer shows the highest external quantum efficiency (η
) 0.39%). Among the devices, the highest brightness is
1226 cd/m2 at 11.0 V, obtained by the white light device
with PFO-TST1 as the emissive layer. The turn-on
voltage of PFO-TST50 is as low as 2.67 V; however, its
η is only 0.01%.
active layer
(V)
PFO-TST50:PCBM ) 1:2
PFO-TST50:PCBM ) 1:4
a With a device configuration of ITO/PEDOT/active layer/Ba/
Al. b Open-circuit voltage. c Short-circuit current. d Fill factor.
e Energy conversion efficiency.
0.70
0.65
5.40
8.67
31.5
35.8
1.20
2.01
injected electron and hole was improved with the
insertion of the PVK layer.
Photovoltaic Properties. In the fabrication of bulk-
heterojunction photovoltaic cells, methanofullerene [6,6]-
phenyl C61-butyric acid methyl ester (PCBM) is the
typical electron acceptor.3 On the basis of the UV
absorptions of the films of the PFO-TST copolymers,
PFO-TST50, the alternating copolymer, possesses the
best spectral coverage range of the visible light. The
HOMO and LUMO levels of PFO-TST50 also match that
of an electron donor for bulk-heterojunction photovoltaic
cells with a PCBM acceptor. Thus, PFO-TST50 was
chosen to fabricate photovoltaic cells with a configura-
tion of ITO/PEDOT/PFO-TST50:PCBM/Ba/Al. The en-
ergy conversion efficiency (ECE) and fill factor (FF) were
calculated according to the following equations:3
EQE ) (FF)JscVoc/Pin
FF ) JmVm/JscVoc
Since the HOMO for the copolymer is around 5.71-
5.75 eV (Table 2), while the work function of PEDOT is
around 5.0-5.2 eV, it would be possible to expect a
better hole injection once a PVK (work function 5.5-
5.6 eV) is used as the hole injection anode. So we further
fabricated EL devices with a configuration of ITO/
PEDOT (50 nm)/PVK (30 nm)/copolymer (80 nm)/Ba/
Al. The EL spectra of the devices are shown in Figure
5B. All the peaks of the EL spectra are red shifted
compared to those of the EL devices without a PVK
layer. For PFO-TST5-50, exchange of the emission
peaks and shoulders was found when the PVK layer was
inserted. This might be attributed to different contribu-
tions of phonon vibration, and also possibly due to a
microcavity effect.28 Better red light CIE coordinates
were achieved (Table 5). Despite the increasing voltage
to reach a current of ∼5 mA, the η for every copolymer
was increased, indicating that the recombination of an
where Pin is the incident radiation flux, Jsc and Voc are
the short-circuit current and open-circuit voltage, re-
spectively, and Jm and Vm are the current and voltage
at the maximum power output, respectively.
The J-V characteristic of a PVC device with PFO-
TST50:PCBM ) 1:4 as the active layer is shown in
Figure 6 as an example. The dark J-V curve shows a
small leakage current, suggesting a continuous, pinhole-
free active layer in the device. Energy conversion
efficiencies were measured under an AM1.5 solar simu-
lator at 100 mW/cm2. The photovoltaic performances of
the devices are summarized in Table 6. At a 1:2 ratio of
PFO-TST50 to BCBM, the Jsc, Voc, and FF are 5.4 mA/
m2, 0.7 V, and 31.5%, respectively. The ECE was further
improved to 2.01% by using PFO-TST50:PCBM ) 1:4
as the active layer, which is among the highest values
so far reported for bulk-heterojunction photovoltaic

Macromolecules, Vol. 38, No. 6, 2005
Conjugated Fluorene and Silole Copolymers 2259
Figure 7. Spectral response of the photovoltaic cell with PFO-
TST50:PCBM ) 1:4 as the active layer.
Figure 8. Schematic structure of the FET device.
Figure 9. Output and transfer characteristics of the FET
device: (A) Ids vs Vds for a series of gate voltages and (B) (Ids)1/2
vs Vds with gate voltage equivalent to Vds.
cells.3,13 Donor:PCBM ) 1:4 is also the optimal ratio for
MEH-PPV and another fluorene-containing alternating
copolymer (PFDTBT).13 The Voc of the device slightly
decreases but with higher FF, compared with those of
PFO-TST50:PCBM ) 1:2 as the active layer. The Voc
values of our devices are relatively lower than those of
MEH-PPV and PFDTBT devices, but Jsc of the device
with PFO-TST50:PCBM ) 1:4 is remarkably higher
than those of MEH-PPV and PFDTBT devices. The
spectral response of the device with PFO-TST50:PCBM
) 1:4 was measured (Figure 7). The spectral response
peaks at 528 nm with a photosensitivity of 0.12 A/W,
all comparable to the corresponding values of MEH-
PPV.13b The spectral response of the device is quite
similar to the absorption spectrum of the film of PFO-
TST50.
(Vg), the drain current (Ids) of the device could reach
saturation along with the drain voltage (Vds) (Figure
9A). The on:off current ratio is ∼100. To calculate the
hole mobility (µ) from the curve of (Ids)1/2 vs Vds, the gate
voltage was set equivalent to the drain voltage to ensure
saturated Ids.31 The field effect hole mobility was
estimated in the saturation regime using the following
equation:32
Ids ) (W/2L)µCi(Vg - Vth)2
where Vth is the threshold voltage.
From the slope of the line as shown in Figure 9B,31
the hole mobility of PFO-TST50 is estimated to be 4.5
× 10-5 cm2/(V s), which is higher than that of MDMO-
PPV and Dow Red PF.33 The Vth of the device is -32 V.
An electron-only FET device with aluminum as the
source and drain electrodes was also fabricated, but the
FET performance was not successful. So PFO-TST50 is
probably a p-type semiconducting polymer.
Field Effect Hole Mobility. Carrier mobilities play
an important role in bulk-heterojunction PVCs. PFO-
TST50, the alternating copolymer, should possess enough
high hole mobility on the basis of its good electron donor
performance in the PVC. To measure the hole mobility
of PFO-TST50, we fabricated an FET device with a top
contact configuration of ITO (110 nm)/polyacrylonitrile
(PAN) (450 nm)/PFO-TST50 (80 nm)/Au (50 nm), as
shown in Figure 8. The width to length ratio (W/L) of
the FET device is 100. Introduction of a polymeric
insulator for the gate electrode is an important step
toward fully organic FET. Here PAN was utilized as a
novel organic insulator for the ITO gate electrode.29 We
made a parallel capacitor, indicating the capacitance (Ci)
of the PAN film (450 nm) was 12.4 nF/cm2. The dielectric
constant (ꢀ) was calculated to be 6.27, which is higher
than that of cross-linked poly(vinylphenol) (PVP) (ꢀ )
5) but lower than that of poly(vinyl alcohol) (PVA) (ꢀ )
10).30 The breakdown electric field of the capacitor was
∼1 MV/cm.
Conclusions
In summary, a novel series of soluble conjugated
random and alternating copolymers (PFO-TST) derived
from 9,9-dioctylfluorene (FO) and 1,1-dimethyl-3,4-
diphenyl-2,5-bis(2′-thienyl)silole (TST) were synthe-
sized by palladium(0)-catalyzed Suzuki coupling reac-
tions. The random copolymers exhibited PFO-segment-
dominated UV absorption and narrow band gap TST
absorption. For the alternating copolymer, only a broad
absorption band was found, demonstrating a mixed and
TST-dominated electronic configuration. Compared with
the solution PL, complete PL excitation energy transfer
from the PFO segment to TST unit could be achieved
by film PL at lower TST content. By using the copoly-
The output and transfer characteristics of the FET
device are shown in Figure 9. At different gate voltages

2260 Wang et al.
Macromolecules, Vol. 38, No. 6, 2005
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conductor devices, PLEDs, PVCs, and FET, were dem-
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the electron donor, an energy conversion efficiency of
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light at 100 mW/cm2. The field effect hole mobility of
the alternating copolymer is 4.5 × 10-5 cm2/(V s) using
polyacrylonitrile as an organic dielectric layer.
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Acknowledgment. Financial support from the Natu-
ral Science Foundation of China (Grants 50303006 and
50373014) and Ministry of Science and Technology of
China (Grant 2002CB613404) is gratefully acknowl-
edged.
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