Deep-red electroluminescent polymers: Synthesis and characterization of new low-band-gap conjugated copolymers for light-emitting diodes and photovoltaic devices

Article abstract of DOI:10.1021/ma047969i

A novel series of semiconducting conjugated copolymers, derived from alkyl-substituted fluorene, 4,7-diselenophen-2′-yl-2,1,3-benzothiadiazole (SeBT), and 4,7-diselenophen-2′-yl-2,1,3-benzoselenadiazole (SeBSe), was synthesized by a palladium-catalyzed Su

Full text of DOI:10.1021/ma047969i

244
Macromolecules 2005, 38, 244-253
Deep-Red Electroluminescent Polymers: Synthesis and
Characterization of New Low-Band-Gap Conjugated Copolymers for
Light-Emitting Diodes and Photovoltaic Devices
Renqiang Yang, Renyu Tian, Jingai Yan, Yong Zhang, Jian Yang, Qiong Hou,
Wei Yang, Chi Zhang, and Yong Cao*
Institute of Polymer Optoelectronic Materials and Devices, Key Laboratory of Special Functional
Materials and Advanced Manufacturing Technology, South China University of Technology,
Guangzhou 510640, China
Received October 2, 2004; Revised Manuscript Received November 3, 2004
ABSTRACT: A novel series of semiconducting conjugated copolymers, derived from alkyl-substituted
fluorene, 4,7-diselenophen-2′-yl-2,1,3-benzothiadiazole (SeBT), and 4,7-diselenophen-2′-yl-2,1,3-benzose-
lenadiazole (SeBSe), was synthesized by a palladium-catalyzed Suzuki coupling reaction with various
feed ratios. The optical band gap of copolymers is very low, 1.87 eV for SeBT and 1.77 eV for SeBSe. The
efficient fast energy transfer from fluorene segments to narrow-band-gap sites was observed. The emission
of photoluminescence and electroluminescence is dominated by narrow-band-gap species and peaked at
670-790 nm, in the range from deep-red to near-infrared (NIR). The external electroluminescent (EL)
quantum efficiencies reached 1.1% and 0.3% for devices from these two types of copolymers, respectively.
Bulk-heterojunction polymer photovoltaic cells (PPVCs) made from composite thin film of the copolymer
9,9-dioctylfluorene and SeBT (PFO-SeBT) in blend with fullerene derivative [6,6]-phenyl C61 butyric
acid methyl ester (PCBM) as an active layer show promising performances. The energy conversion
efficiency (ECE) is up to 1% under AM1.5 solar simulator (78.2 mW/cm2). The spectral response is extended
up to 675 and 750 nm for PPVCs from PFO-SeBT and PFO-SeBSe, respectively.
Introduction
gap are needed for polymer photovoltaic cells (PPVCs)
being used for solar cells. One of the limiting factors of
polymer photovoltaic cells is their spectral response
mismatching with the solar-terrestrial radiation, which
comprises a substantial portion of emission in the red
and near-infrared. For a typical conjugated polymer
used for PPVCs, poly(2-methoxy-5-(2′-ethylhexyloxy)-
1,4-phenylenevinylene) (MEH-PPV), the onset and
maximum of its optical absorption are estimated at 590
and 510 nm, respectively. Recently, Brabec et al.
reported a low-band-gap alternating copolymer N-dodec-
yl-2,5-bis(2′-thienyl)pyrrole and 2,1,3-benzothiadiazole
(PTPTB) which has a band gap of around 1.77 eV.16
Color tuning to the deep-red and NIR region in
conjugated polymers can be achieved by incorporating
narrow-band-gap comonomer into polyfluorene back-
bone. Recently, we first reported a polyfluorene copoly-
mer with selen-containing heterocycle, benzoselenadi-
azole (BSeD), a selenium analogue of benzothiadiazole
(BT) in different compositions.11 We found out that
incorporating Se-containing heterocycle (BSeD) into the
polyfluorene main chain resulted in a significant red
shift in comparison with its sulfur analogue. We also
reported the synthesis and properties of the polyfluorene
copolymer with another selen-containing heterocycle,
2,1,3-naphthoselenadiazole (NSeD) as the narrow band-
gap component.12 The maximal EL external quantum
efficiency of the device fabricated with this type of
copolymer reaches 3.1%, and luminous efficiency is of
0.9 cd/A with an emission peak at 657 nm and Com-
mission International de l’Eclairage (CIE) coordinates
of (0.64, 0.33). These results indicate that synthesizing
selen-containing heterocycles with a narrower band gap
moves the absorption band edge to a further deep-red
or even NIR region without using the metallic complex
ions.
Electroluminescent polymers are a subject of consid-
erable academic and commercial interest when used as
the active materials in polymer light-emitting diodes
(PLEDs).1,2 π-π* transitions of conjugated polymers
with aromatic or heterocyclic units generally lead to the
optical band gap with an absorption peak of 300-600
nm.3 The color purity, quantum efficiency (QE) of light
emission, turn-on voltage, and stability of the devices
must be optimized for PLEDs to be applicable to
commercial light-emitting devices. A large number of
electroactive and photoactive conjugated polymers have
been introduced during the past few years,2a such as
poly(p-phenylenevinylene) (PPV),1a,4 poly(p-phenylene)
(PPP),5 polythiophene (PT),6 and polyfluorene (PF),7 etc.
Among those polymers, PLEDs based on polyfluorenes
have emerged as a promising candidate for the next
generation of flat panel displays2b,8 because of their
exceptional optoelectronic properties, such as good
thermal and chemical stability, high fluorescence quan-
tum yield, and good film-forming and hole-transporting
properties.7 Normally, polyfluorene homopolymers have
a large band gap and emit blue light. Significant efforts
and great success have been made to tune the light-
emitting color of polyfluorenes to a longer wave-
length.2b,9-12 However, these efforts were limited to the
visible region (500-650 nm). It will be of great interest
to extend the emission wavelength to the deep-red and
even to the near-infrared (NIR) region to apply these
light-emitting polymers to telecommunications.13 Near-
IR-emitting devices with around 800 nm emission have
been reported for ionic laser dyes.14 More efforts have
been devoted to complexes from rare-earth ions.15 On
the other hand, conjugated polymers with a small band
* To whom correspondence should be addressed. Phone: +86-
20-87114609. Fax: +86-20-87114535. E-mail: poycao@scut.edu.cn.
10.1021/ma047969i CCC: $30.25 © 2005 American Chemical Society
Published on Web 12/21/2004

Macromolecules, Vol. 38, No. 2, 2005
Deep-Red Electroluminescent Polymers 245
Experimental Section
Scheme 1. Synthetic Route of Monomers and
Copolymers
Materials and Characterization. All manipulations in-
volving air-sensitive reagents were performed in an atmo-
sphere of dry argon. All reagents, unless otherwise specified,
were obtained from Aldrich, Acros, and TCI Chemical Co. and
used as received. All the solvents were further purified before
use. 1H and 13C NMR spectra were recorded on a Bruker DRX
400 spectrometer operating, respectively, at 400 and 100 MHz
and were referred to tetramethylsilane. Analytical GPC was
obtained using a Waters GPC 2410 in tetrahydrofuran (THF)
via a calibration curve of polystyrene standards. Elemental
analyses were performed on a Vario EL Elemental Analysis
Instrument (Elementar Co.). The elemental selenium analysis
was recorded on a Polarized Zeeman Atomic Absorption
Spectrophotometer, Hitachi Co. UV-visible absorption spectra
were measured on a HP 8453 spectrophotometer. The PL
quantum yields were determined in an integrating sphere
under 405 nm excitation of a laser diode (CL-2000, Crystal
Laser). PL and EL spectra were recorded on an Instaspec 4
CCD spectrophotometer (Oriel Co.). Cyclic voltammetry was
measured on a Potentiostat/Galvanostat model 283 electro-
chemical workstation (Princeton Applied Research) at a scan
rate of 50 mV/s with a nitrogen-saturated solution of 0.1 M
tetrabutylammonium hexafluorophosphate (Bu4NPF6) in ac-
etonitrile (CH3CN) with platinum and saturated calomel
electrodes (SCE) as the working and reference electrodes,
respectively.
Synthesis. 2,7-Dibromofluorene (1) and 2,7-Dibromo-9,9-
dioctylfluorene (2). 1 and 2 were prepared following the
procedure described in ref 17. 1: white crystal, yield 93%, mp
161-163 °C. 2: white crystal, yield 85%, mp 49-50 °C.
2,7-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-di-
octylfluorene (3). 3 was prepared following the modified
procedure from 2,7-dibromo-9,9-dioctylfluorene (2).18 White
1
solid, yield 79%, mp128-130 °C. H NMR (400 MHz, CDCl3)
δ (ppm): 7.80 (d, 2H, fluorene ring), 7.74 (s, 2H, fluorene ring),
7.71 (d, 2H, fluorene ring), 1.99 (m, 4H, H-alkyl), 1.39 (s, 24H,
CH3), 1.22-1.00 (m, 20H, H-alkyl), 0.81 (t, 6H, H-alkyl), 0.56
(m, 4H, H-alkyl). 13C NMR (100 MHz, CDCl3) δ (ppm): 150.86,
144.30, 134.04, 129.29, 119.77 (fluorene ring), 84.11 (C-alkyl),
55.57 (C9, fluorene ring), 40.49, 32.18, 30.33, 29.58, 25.33,
23.98, 22.99, 14.48 (C-alkyl). Anal. Calcd for C41H64O4B2: C,
76.74; H, 10.04. Found: C, 76.68; H, 9.94).
4,7-Dibromo-2,1,3-benzothiadiazole (4).19 A mixture of 13.6
g (0.1 mol) of 2,1,3-benzothiadiazole in 30 mL of 45% hydro-
bromic acid was heated under reflux with stirring while 48 g
(0.3 mol, 15.3 mL) of bromine was added slowly. After
completion of the bromine addition, the reaction mixture
became a suspension of solid in hydrobromic acid and 15 mL
of hydrobromic acid was added, and the mixture was heated
under reflux for another 2.5 h. The mixture was filtered,
washed well with water, recrystallized from chloroform, and
dried to give white needle crystals (27.3 g, 93%). mp 187-188
°C. 1H NMR (400 MHz, CDCl3) δ (ppm): 7.73 (s, 2H). 13C NMR
(100 MHz, CDCl3) δ (ppm): 153.34, 132.67, 114.31.
This paper reports the syntheses of two new conju-
gated Se-containing heterocyclic monomers, 4,7-di(2′-
selenophenyl)-2,1,3-benzothiadiazole (SeBT) and 4,7-
di(2′-selenophenyl)-2,1,3-benzoselenadiazole (SeBSe)
(Scheme 1), which have a narrower band gap than
benzoselenadiazole and naphthoselenadiazole. A series
of novel copolymers derived from 9,9-dioctylfluorene
(FO) and the two Se-containing monomers were pre-
pared by the palladium-catalyzed Suzuki cross-coupling
reaction. The feed ratio of the narrow-band-gap compo-
nent is less than or equal to a molar ratio of 50%. The
light-emitting devices and photovoltaic devices were
fabricated from the obtained copolymers, the highest
external quantum efficiency of the device based on the
copolymer PFO-SeBT reaching 1.1% ph/el with an
emission peak of ca. 700 nm. The device with PFO-
SeBSe emits light with a maximal wavelength of 790
nm in the near-infrared region. The complete synthetic
details and device characteristics are presented. The
photovoltaic cells made from composite thin films of the
copolymer PFO-SeBT blending with the fullerene
derivative [6,6]-phenyl C61 butyric acid methyl ester
(PCBM) as an active layer show promising perfor-
mances. The energy conversion efficiency (ECE) is up
to 1% under AM1.5 solar simulator (78.2 mW/cm2). The
spectral response is extended to 750 nm for devices from
blend with PFO-SeBSe copolymers.
4,7-Dibromo-2,1,3-benzoselenadiazole (5).20 To a suspension
of 4,7-dibromo-2,1,3-benzothiadiazole (4) (5.88 g, 0.02 mol) in
ethanol (190 mL) was added portionwise sodium borohydride
(14 g, 0.37 mol) at 0 °C, and the mixture was stirred for 20 h
at room temperature. After evaporation of the solvent, 200 mL
of water was added, and the mixture was extracted with ether.
The extract was washed with brine and dried over anhydrous
sodium sulfate. Evaporation of the solvent gave 3,6-dibromo-
1, 2-phenylenediamine (6) (4.5 g) as a pale yellow solid in 85%
yield. To a solution of 6 (2.7 g, 10 mmol) in refluxing ethanol
(55 mL) was added a solution of selenium dioxide (1.17 g, 10.5
mmol) in hot water (22 mL). The mixture was heated under
reflux for 2 h. Filtration of the yellow precipitates and
recrystallization from ethyl acetate gave 4,7-dibromo-2,1,3-
benzoselenadiazole (5) (3.0 g) in 88% yield as golden yellow
needles. mp 285-287 °C. 1H NMR (400 MHz, CDCl3) δ (ppm):
7.63 (s, 2H, phenylene ring). 13C NMR (100 MHz, CDCl3) δ
(ppm): 157.2, 132.1, 116.5. Anal. Calcd for C6H2Br2N2Se: C,
21.1; H, 0.6; N, 8.2. Found: C, 21.4; H, 0.9; N, 7.8.

246 Yang et al.
Macromolecules, Vol. 38, No. 2, 2005
Tributyl(2-selenophenyl)stannane (7). 7 was prepared fol-
lowing the modified published procedures.21 n-Butyllithium (22
mL of a 2.0 M solution in hexane, 44 mmol) was added
dropwise to selenophene (10 g, 76.4 mmol) in anhydrous
tetrahydrofuran (70 mL) at -30 °C, and the mixture was
stirred at this temperature under argon for 3.5 h. Tributyltin
chloride (13.02 g, 40 mmol) was added, and the mixture was
stirred at -30 °C for a further 1 h. Saturated aqueous sodium
hydrogen carbonate (70 mL) was added, and the organic phase
was separated and washed with saturated aqueous sodium
hydrogen carbonate (70 mL) and brine (40 mL). The solvent
was evaporated from the dried sodium sulfate extract, and the
residue was purified by column chromatography on neutral
alumina (eluent petroleum ether) to give the title compound
7 (10.2 g, 61%) as colorless oil without further purification for
the next step synthesis. m/z, 419.6.
avoid a possible quenching effect or excimer formation by
boronic and bromine end groups in LEDs.24 The mixture was
then poured into methanol. The precipitated material was
recovered by filtration and washed for 3 h by stirring in
acetone to remove oligomers and catalyst residues. The
resulted polymers were air-dried overnight, followed by drying
under vacuum. Yields: ∼55-80%.
Poly[2,7-(9,9-dioctylfluorene)-co-5′,5′′-(4,7-diselenophen-2′-
yl)-2,1,3-benzothiadiazole] (PFO-SeBT). The ratio of 2,7-bis-
(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-dioctylfluo-
rene (3) (1.00 equiv) vs dibromides (2,7-dibromo-9,9-
dioctylfluorene (2) and 4,7-bis(5′-bromo-2′-selenophenyl)-2,1,3-
benzothiadiazole (10)) was kept always as (2+10):(3)) 1:1. The
comonomer feed ratios of (2+3) to 10 are 99:1, 95:5, 90:10, 85:
15, 70:30, and 50:50, and the corresponding copolymers were
named PFO-SeBT1, -5, -10, -15, -30, and -50, respectively.
Poly[2,7-(9,9-dioctylfluorene)-co-5′,5′′-(4,7-diselenophen-2′-
yl)-2,1,3-benzoselenadiazole] (PFO-SeBSe). In this polymeri-
zation 2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-
dioctylfluorene (3) (1.00 equiv) and dibromides 2,7-dibromo-
9,9-dioctylfluorene (2) and 4,7-bis(5′-bromo-2′-selenophenyl)-
2,1,3-benzoselenadiazole (11) (1.00 equiv of 2+11) were used.
The comonomer feed ratios of (2+3) to 11 are 99:1, 95:5, 90:
10, and 85:15, and the corresponding copolymers were named
PFO-SeBSe1, -5, -10, and -15, respectively.
4,7-Diselenophen-2′-yl-2,1,3-benzothiadiazole (8).22 To a solu-
tion of 4,7-dibromo-2,1,3-benzothiadiazole (4) (1.45 g, 4.9
mmol) and tributyl(2-selenophenyl)stannane (7) (5.0 g, 11.9
mmol) in anhydrous THF (30 mL), PdCl2(PPh3)2 (70 mg, 2 mol
%) was added. The mixture was refluxed for 4 h. After removal
of the solvent under reduced pressure, the residue was purified
by column chromatography on silica gel (eluent CH2Cl2/hexane,
1:1). Recrystallization from toluene-ethanol gave the title
compound 8 (1.45 g, 75%) as red needles. m/z, 394.
4,7-Diselenophen-2′-yl-2,1,3-benzoselenadiazole (9). 9 was
prepared according to the modified procedure for the synthesis
of compound 8. 4,7-Dibromo-2,1,3-benzoselenadiazole (5) (1.77
g, 5.2 mmol), tributyl(2-selenophenyl)stannane (7) (5.2 g, 12.4
mmol), and PdCl2(PPh3)2 (74 mg, 2 mol %) were dissolved in
35 mL of anhydrous THF. The mixture was refluxed for 4.5 h.
After removal of the solvent under reduced pressure, the
residue was purified by column chromatography on silica gel
(eluent CH2Cl2/hexane, 1:1). Recrystallization from toluene-
ethanol gave the title product 9 (1.74 g, 76%) as dark red
needles. m/z, 441.
Device Fabrication and Characterization. LED was
fabricated on prepatterned indium-tin oxide (ITO) with a
sheet resistance 10-20 Ω/0. The substrate was ultrasonically
cleaned with acetone, detergent, deionized water, and 2-pro-
panol subsequently. Oxygen plasma treatment was made for
10 min as the final step of substrate cleaning to improve the
contact angle just before film coating. Onto the ITO glass a
layer of polyethylenedioxythiophene-polystyrene sulfonic acid
(PEDT:PSS) film with a thickness of 50 nm was spin coated
from its aqueous dispersion (Baytron P 4083, Bayer AG),
aiming to improve the hole injection and avoid the possibility
of leakage. PEDT-PSS film was dried at 80 °C for 2 h in the
vacuum oven. The solution of the copolymers in toluene was
prepared in a nitrogen-filled drybox and spin coated on top of
the ITO/PEDT:PSS surface. The typical thickness of the
emitting layer was 70-80 nm. Then a thin layer of barium as
an electron injection cathode and the subsequent 200-nm thick
aluminum capping layers were thermally deposited by vacuum
evaporation through a mask at a base pressure below 2 × 10-4
Pa. The deposition speed and thickness of the barium and
aluminum layers were monitored by a thickness/rate meter,
model STM-100 (Sycon Instrument, Inc.). The cathode area
defines the active area of the device. The typical active area
of the devices in this study is 0.15 cm2. The spin coating of
the EL layer and device performance tests were carried out
within a glovebox (Vacuum Atmosphere Co.) with nitrogen
circulation. Current-voltage (I-V) characteristics were mea-
sured with a computerized Keithley 236 Source Measure Unit.
The luminance of the device was measured with a photodiode
calibrated by using a PR-705 SpectraScan Spectrophotometer
(Photo Research). External quantum efficiency was verified
by measurement in the integrating sphere (IS080, Labsphere)
after encapsulation of devices with UV-curing epoxy and thin
cover glass.
4,7-Bis(5′-bromo-2′-selenophenyl)-2,1,3-benzothiadiazole
(SeBT) (10).9 4,7-Diselenophen-2′-yl-2,1,3-benzothiadiazole (8)
(1.45 g, 3.7 mmol) was dissolved in a mixture of chloroform
(30 mL) and acetic acid (30 mL) under nitrogen, and N-
bromosuccinimide (1.4 g, 7.8 mmol) was added all at once.
After stirring the reaction mixture at room temperature all
night, the red precipitate was filtered off and recrystallized
from N,N-dimethylformamide (DMF) twice to provide the title
compound as shiny, red crystals (0.84 g, 41%). FAB, 552. mp
1
225-230 °C. H NMR (400 MHz, CDCl3) δ (ppm): 8.15, 7.98,
7.52. Anal. Calcd for C14H6Br2N2S1Se2: C, 30.46; H, 1.10; N,
5.07; S, 5.81; Se, 28.61. Found: C, 30.39; H, 1.16; N, 5.16; S,
5.90; Se, 28.86.
4,7-Bis(5′-bromo-2′-selenophenyl)-2,1,3-benzoselenadiazole
(SeBSe) (11). 11 was made following the modified procedure
for the preparation of compound 10. 4,7-Diselenophen-2′-yl-
2,1,3-benzoselenadiazole (9) (1.74 g, 4.0 mmol) was dissolved
in a mixture of chloroform (30 mL) and acetic acid (30 mL)
under nitrogen, and N-bromosuccinimide (1.5 g, 8.4 mmol) was
added all at once. After stirring the reaction mixture for 24 h
at room temperature, the dark red precipitate was filtered off
and recrystallized from DMF to give the title product as shiny,
dark red needle crystals (0.86 g, 36%). FAB, 599. mp 235-
239 °C. 1H NMR (400 MHz, CDCl3) δ (ppm): 8.17, 8.02, 7.54.
Anal. Calcd for C14H6Br2N2Se3: C, 28.08; H, 1.01; N, 4.68; Se,
39.54. Found: C, 27.96; H, 1.10; N, 4.59; Se, 39.25.
Results and Discussion
General Procedure for Polymer Synthesis.23 Carefully
purified 2,7-dibromo-9,9-dioctylfluorene (2), 2,7-bis(4,4,5,5-
tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-dioctylfluorene (3), 4,7-
bis(5′-bromo-2′-selenophenyl)-2,1,3-benzothiadiazole (SeBT)
(10), or 4,7-bis(5′-bromo-2′-selenophenyl)-2,1,3-benzoselena-
diazole (SeBSe) (11), (PPh3)4Pd(0) (0.5-2.0 mol %), and
several drops of Aliquat 336 were dissolved in a mixture of
toluene and aqueous 2 M Na2CO3 with THF as a cosolvent.
The solution was refluxed with vigorous stirring for 36 h under
an argon atmosphere. At the end of polymerization, polymers
were sequentially end-capped with 2,7-bis(4,4,5,5-tetramethyl-
1,3,2-dioxaborolan-2-yl)-9,9-dioctylfluorene and bromobenzene
to remove bromine and boronic ester end groups in order to
Synthesis and Chemical Characterization. The
general synthetic routes toward the monomers are
outlined in Scheme 1. 2,7-Dibromo-9,9-dioctylfluorene
(2) and 2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-
2-yl)-9,9-dioctylfluorene (3) were synthesized by pub-
lished procedures.17,18 4,7-Bis(5′-bromo-2′-selenophenyl)-
2,1,3-benzothiadiazole (10) and 4,7-bis(5′-bromo-2′-
selenophenyl)-2,1,3-benzoselenadiazole (11) were pre-
pared following the modified procedures in the litera-
ture.9,19-22 Following the polymer synthetic route in
Scheme 1, the conjugated copolymers derived from 2,7-
dibromo-9,9-dioctylfluorene (2) and 2,7-bis(4,4,5,5-tet-

Macromolecules, Vol. 38, No. 2, 2005
Deep-Red Electroluminescent Polymers 247
Table 1. Molecular Weights of the Copolymers and N and Se Content in the Copolymers
SeBT or SeBSe SeBT or SeBSe
content in the content in the
copolymers (%) copolymers (%)
N content
in the feed
N content
in the
Se content
in the feed
Se content
in the
according to
N content
according to
Se content
copolymers
Mn ( 103) Mw/Mn composition (%) copolymers (%) composition (%) copolymers (%)
PFO-SeBT1
PFO-SeBT5
PFO-SeBT10
PFO-SeBT15
PFO-SeBT30
PFO-SeBT50
PFO-SeBSe1
PFO-SeBSe5
PFO-SeBSe10
PFO-SeBSe15
21
28
25
24
12
8
23
20
28
13
1.60
1.43
2.01
1.59
1.78
1.51
1.86
1.83
1.95
1.72
0.07
0.36
0.72
1.08
2.16
3.59
0.07
0.36
0.71
1.06
<0.2
0.30
0.41
2.03
4.07
6.10
12.17
20.25
0.61
3.03
6.03
8.98
0.38
1.93
3.74
6.31
11.31
17.50
0.58
2.81
5.31
7.79
0.94
4.7
4.2
9.3
16.3
28.1
44.2
0.67
1.18
2.02
3.17
9.2
15.5
27.9
43.2
0.95
4.6
8.8
13.0
<0.2
0.31
0.64
0.97
4.3
9.0
13.7
Table 2. UV-Vis Absorption and Electrochemical Properties of the Copolymers in Thin Films
optical
Ered/Vb PFO,
SeBT/SeBSe
LUMO/eVc PFO,
SeBT/SeBSe
Eg(Echem)/eV PFO,
SeBT/SeBSe
copolymers
λmax, UV/nm
band gap/eVa
Eox/Vb
HOMO/eVc
PFO
382
2.92 (424)
2.92(424)
1.88 (658)
1.88 (661)
1.87 (664)
1.85 (670)
1.89 (656)
2.92 (425)
1.78 (696)
1.77 (700)
1.78 (695)
1.37
1.30
1.30
1.28
1.20
1.11
1.28
1.33
1.31
1.31
1.13
-2.24
-5.77
-5.70
-5.70
-5.68
-5.60
-5.51
-5.68
-5.73
-5.71
-5.71
-5.53
-2.16
-2.13
-2.09
-2.10
-2.17,
-2.76
-2.79
-2.11,
-2.17,
-2.28,
-2.93
3.61
PFO-SeBT1
PFO-SeBT5
PFO-SeBT10
PFO-SeB T15
PFO-SeBT30
PFO-SeBT50
PFO-SeBSe1
PFO-SeBSe5
PFO-SeBSe10
PFO-SeBSe15
383
-2.27
3.57
384, 561
385, 563
387, 563
389, 560
385, 537
382
380, 604
385, 609
393, 585
-2.31
3.61
-2.30
3.58
-2.23
3.43
-2.19, -1.64
-2.44, -1.61
-2.29
3.30, 2.75
3.72, 2.89
3.62
-2.23
-2.12
-2.17, -1.47
3.54
3.43
3.30, 2.60
b
a The optical band gap estimated from the onset wavelength (value in parentheses) of UV-vis spectra of the copolymer film. Eox and
Ered are the onset potential of oxidation and reduction, respectively. c Calculated from the empirical formula, E(HOMO) ) -(Eox + 4.40)
(eV), E(LUMO) ) -(Ered + 4.40) (eV).26
ramethyl-1,3,2-dioxaborolan-2-yl)-9,9-dioctylfluorene (3)
were synthesized separately with monomers 10 and 11
using the palladium-catalyzed Suzuki coupling method.
The obtained copolymers are completely soluble in
common organic solvents, such as toluene, THF, and
chloroform. The number-average molecular weights of
these polymers, as determined by GPC using a polysty-
rene standard, ranged from 8000 to 28 000 with a
polydispersity index (Mw/Mn) between 1.43 and 2.01
(Table 1).
The actual ratios of substituted fluorene to narrow-
band-gap monomer in the copolymers can be estimated
by elemental analysis. The selenium contents of these
polymers were measured by atomic absorption spec-
troscopy. The actual ratios calculated from elemental
analysis of Se or N and C are listed in Table 1, which is
in good agreement with the feeding ratios of the two
monomers within experimental error. Slightly larger
deviation for copolymers PFO-SeBT30, PFO-SeBT50,
and PFO-SeBSe15 calculated from the Se content
might be due to a larger error of the atomic absorption
spectroscopy for the samples with high Se content.
The chemical structure of the PFO-SeBT and PFO-
SeBSe copolymer was verified by 1H and 13C NMR. For
example, in the proton NMR spectra, the characteristic
signal of the benzene proton in benzothiadiazole can be
seen clearly at around δ 8.2-8.4 ppm (depending on
SeBT content) for all copolymers with SeBT content
equal to or greater than 5% molar ratio of SeBT. The
integration intensity of this signal increases with in-
creasing the SeBT content in comparison with the
fluorene proton at around δ 7.70 ppm. The low-field
carbon NMR signal at around δ 152.6 ppm, which can
be assigned to carbon in the benzothiadiazole ring, can
be clearly seen for all copolymers. Similar results were
observed for the NMR spectra of PFO-SeBSe copoly-
mers.
Optical Properties and Electrochemical Char-
acteristics. The UV-vis absorption properties of the
conjugated polymers based on 9,9-dioctylfluorene, SeBT,
and SeBSe are presented in Table 2. Figure 1 shows
the normalized UV-vis absorption of the polymers in
the thin solid films (Figure 1a and c) and in chloroform
solution (5 × 10-5 mol/L) (Figure 1b and d). Two distinct
absorption bands are observed. The absorption peak of
385 nm is due to the fluorene segments compared with
the absorption spectrum of pure poly(9,9-dioctylfluo-
rene) (PFO).11,25 A side peak of about 563 nm can be
assigned to the absorption of the SeBT unit (Figure 1a),
since its intensity increases with the increasing SeBT
content in the copolymers. For the PFO-SeBT50 co-
polymer, the shoulder is about 20 nm more blue shifted
than that of PFO-SeBT30, which is probably due to
band alternation. For the PFO-SeBSe copolymers
(Figure 1c), the intensities of the shoulder peak at about
605 nm increase linearly with the SeBSe content,
suggesting that the 605 nm absorption band originated
from SeBSe units. Two distinguished absorption fea-
tures demonstrate that the electronic states of the two
components in copolymers PFO-SeBT and PFO-SeBSe
are not mixed. The shoulder peaks are hard to see in
the absorption spectra for the copolymers with 1%
narrow-band-gap comonomer content. The optical band
gaps deducted from the onset of the absorption of
copolymers are about 1.87 and 1.77 eV for PFO-SeBT
and PFO-SeBSe copolymers, respectively. Maximal
wavelengths of the absorption peak, optical band gaps
of the copolymers, and their onset wavelengths are
summarized in Table 2.

248 Yang et al.
Macromolecules, Vol. 38, No. 2, 2005
Figure 1. UV-vis absorption spectra of the copolymers (a) PFO-SeBT in the thin films, (b) PFO-SeBT in chloroform solution,
(c) PFO-SeBSe in thin films, and (d) PFO-SeBSe in chloroform solution.
The electrochemical behavior of the copolymers was
investigated by cyclic voltammetry (CV). Table 2 sum-
marizes the oxidation and reduction potentials derived
from the onset of oxidation waves in the cyclic voltam-
mograms of the copolymers. We can record only one
p-doping process for these copolymers. The onset po-
tentials of the oxidation process (p-doping) occur at
about 1.3 V. The onset of distinct reduction waves (n-
doping) occur at about -2.24 V. When the content of
SeBT and SeBSe is more than 15%, a second n-doping
was recorded. For PFO-SeBT that occurs at -1.61 V
and for PFO-SeBSe at -1.47 V. The highest occupied
molecular orbital (HOMO) and lowest unoccupied mo-
lecular orbital (LUMO) levels of the copolymers can be
estimated by the empirical formulas E(HOMO) ) -(Eox
+ 4.40) (eV) and E(LUMO) ) -(Ered + 4.40) (eV).26 From
the differences between the onset potentials of the
oxidation and reduction processes, the electrochemical
band gaps, Eg(Echem) ) (Eox - Ered) (eV), of the copolymers
were calculated and are listed in Table 2. The oxidation
and reduction potentials of the copolymers at about 1.3
and -2.24 V are very close to those reported for the
previously by Janietz et al.,25 the electrochemical band
gap for polyfluorene homopolymer is much larger than
the optical band gap deducted from the absorption onset,
which is due to the existence of an additional interface
barrier for charge injection in the electrochemical
process.27
Photoluminescence Properties. The photolumi-
nescent (PL) spectra of the copolymers (PFO-SeBT and
PFO-SeBSe) were taken under excitation of the 405
nm line of the laser diode (CL-2000, Crystal Laser)
(Figures 2and 3). PL spectra of PFO homopolymer were
included for comparison. Once the narrow-band-gap unit
(SeBT or SeBSe) is incorporated into the polyfluorene
main chain, the PL emission corresponding to the
fluorene segment completely disappeared while the PL
emission consists exclusively of one peak at longer
wavelength responsible for SeBT and SeBSe units with
narrow-band-gap comonomer loading as low as 1% in
the copolymer (Figures 2a and 3a). The PL peaks are
significantly red shifted with increasing SeBT content
in the copolymers, from λmax ) 671 nm for copolymer
with 1% SeBT content to 713 nm for alternating
copolymer (Figure 2a and Table 4). For PFO-SeBSe
copolymers, the peak of the SeBSe unit emission was
shifted from 734 nm for PFO-SeBSe1 to 790 nm for
PFO-SeBSe15 (Figure 3a and Table 4). Since no large
red shift was observed for absorption spectra of the
corresponding copolymers (Figure 1), the significant red
shift in PL emission indicates that the Stokes shift
increases with the increase in SeBT or SeBSe content
polyfluorene homopolymer: Eox ) 1.4 V and Ered
)
-2.28 V.25 The two peaks are attributed to the p- and
n-doping of PFO segments. The reduction waves at
about -1.61 and -1.47 V correspond to the n-doping
processes on the SeBT and SeBSe units, respectively.
The electrochemical behavior of these copolymers is
consistent with the two separated absorption peaks
observed in the UV-vis spectra (Figure 1). As reported

Macromolecules, Vol. 38, No. 2, 2005
Deep-Red Electroluminescent Polymers 249
Figure 2. PL spectra of (a) PFO-SeBT copolymers in thin
films, (b) PFO-SeBT copolymers in chloroform solution at a
concentration of 1 × 10-4 mol/L, and (c) PFO-SeBT10 in
chloroform solution in different concentrations.
Figure 3. PL spectra of (a) PFO-SeBSe copolymers in thin
films, (b) PFO-SeBSe copolymers in chloroform solution at a
concentration of 5 × 10-4 mol/L, and (c) PFO-SeBSe10 in
chloroform solution in different concentrations.
in the copolymers. The reason for the increase of Stokes
shift on increasing narrow-band-gap units in the co-
polymers is probably due to the higher polarizability of
SeBT and SeBSe heterocycles, allowing greater adjust-
ment in the excited state. Similar phenomena were
observed for PFO-BSeD copolymers.11
The fact that the emission of the fluorene segment
completely disappeared at very low loading of narrow-
band-gap comonomers indicates that an efficient energy
transfer occurs from the fluorene segment to the nar-
row-band-gap unit, which serves as a powerful trap in
the copolymer chain. Excitons are confined and recom-
bined on narrow-band-gap units isolated from both sides
by fluorene segment and emit red light corresponding
to SeBT or SeBSe unit emission. Energy transfer from
fluorene segments to the narrow-band-gap unit may
occur via the intra- or intermolecular mechanism. To
elucidate the relative role of intra- and interchain
energy transfer, we investigated the concentration
dependence of PL spectra of the copolymers in chloro-
form solution. Figure 2b compares the PL spectra of the
copolymers with different SeBT contents at a concentra-
tion of 1 × 10-4 mol/L (a sufficiently dilute regime to
avoid strong intermolecular interaction between poly-

250 Yang et al.
Macromolecules, Vol. 38, No. 2, 2005
Table 3. Threshold Concentration of Copolymers at
Which Fluorescence of Fluorene Segments Is Just
Quenched
copolymers in the solution. However, the increase in the
threshold concentration with decreasing SeBT content
in the copolymer (Table 3) indicates that the interchain
energy transfer (interchain interaction) also plays a
certain role, especially in copolymers with a low content
of narrow-band-gap comonomers.
threshold
concentrationa
(mol/L)
threshold
concentrationa
(mol/L)
copolymers
copolymers
PFO-SeBT 1
PFO-SeBT 5
PFO-SeBT 10
PFO-SeBT 15
PFO-SeBT 30
PFO-SeBT 50
5 × 10-3
1 × 10-3
5 × 10-4
1 × 10-4
1 × 10-5
1 × 10-7
PFO-SeBSe 1
PFO-SeBSe 5
PFO-SeBSe 10
PFO-SeBSe 15
5 × 10-2
5 × 10-3
1 × 10-3
1 × 10-5
For PFO-SeBSe copolymers, similar characteristics
of the PL spectra in solution were observed (Figure 3b
and c). However, there was some difference between the
two types of copolymers. Figure 3b shows the PL spectra
of PFO-SeBSe copolymers in chloroform solution at a
concentration of 5 × 10-4 mol/L. As can be seen from
Figure 3b, for copolymers PFO-SeBSe1 and PFO-
SeBSe5, the PL spectra are dominated by PFO emission
while emission of the SeBSe unit at 760 nm was weak.
With increasing the content of SeBSe, the emission at
760 nm becomes gradually dominant. The threshold
concentration of PFO-SeBSe in solution for each co-
polymer was 5-10 times greater than that of the
corresponding counterpart of PFO-SeBT (Figure 3c,
Table 3). This fact indicates that the interchain interac-
tion between PFO-SeBSe chains is weaker than that
of PFO-SeBT copolymer, which is probably related to
more rigid chains of PFO-SeBSe because of more Se
atoms of larger atomic size in each monomer unit.
a Concentration at which fluorescence of PFO is completely
quenched.
Table 4. Comparison of Absolute PL Efficiencies for
PFO-SeBT and PFO-SeBSe Copolymers in Thin Films
Measured in the Integrating Sphere
photoluminescence
photoluminescence
λmax
/
QEPL
(%)
λmax
/
QEPL
(%)
copolymers
nm
copolymers
PFO
PFO-SeBSe1
PFO-SeBSe5
PFO-SeBSe10
PFO-SeBSe15
nm
PFO
426
671
692
694
697
713
713
47
59
39
16
15
5
426
734
754
770
790
47
22
9
9
1
PFO-SeBT1
PFO-SeBT5
PFO-SeBT10
PFO-SeBT15
PFO-SeBT30
PFO-SeBT50
3
In Table 4 we list the absolute PL quantum efficien-
cies of the copolymers in the thin film measured in an
integrating sphere under 405 nm excitation of an UV
laser diode (CL-2000, Crystal Laser). For PFO-SeBT
copolymers, the PL efficiencies increase with incorpora-
tion of a small amount of SeBT unit, reaching 59% and
39% for PFO-SeBT1 and PFO-SeBT5, respectively.
With further increases in the SeBT content, the PL
efficiencies decrease very quickly, falling to 3% for the
alternating copolymers. The initial increase in PL
efficiency with incorporation of SeBT content of 1% and
5% molar ratios in the copolymers is probably due to
disturbing the regularity of the PFO segments by a big-
size Se-containing heterocycle in the copolymer main
chains. The further decrease in PL efficiency with the
increase in SeBT content is related to the heavy atomic
quenching effect by Se atoms. This is consistent with
the fact that for PFO-SeBSe copolymers with three Se
atoms in each SeBSe comonomer unit there was no
initial increase in PL efficiency observed. For PFO-
SeBSe1, the PL efficiency decreases to 22%, in compari-
son with 47% for PFO homopolymers. The PL efficiency
of PFO-SeBSe copolymers decreases more rapidly with
increasing SeBSe content than for PFO-SeBT copoly-
mers. For PFO-SeBSe15, the PL efficiency decreases
to 1%. The heavy-atom effect from the selenium may
increase the intersystem crossing from the singlet to
triplet state, which quenches fluorescence. The maxi-
mum emission wavelengths of the copolymers in thin
films are also listed in Table 4. It is worthwhile to note
that the emission peaks of PFO-SeBSe copolymers
have been shifted to the near-infrared (from 734 nm for
PFO-SeBSe1 to 790 nm for PFO-SeBSe15). This is
among the longest emission wavelength realized with-
out using rare-earth metal ions or organic dye ions.16,30,31
mer chains in the solution). At this relatively dilute
concentration copolymers PFO-SeBT30 and PFO-
SeBT50 exhibit exclusively SeBT emission while fluo-
rene segment emission was completely quenched. This
seems to indicate that the transfer of the exciton energy
from fluorene segment to the narrow-band-gap site
(SeBT) occurs mainly along the polymer chain. In other
words, the efficient energy transfer from fluorene seg-
ment to SeBT unit occurs when an intramolecular
trapping mechanism takes place.28,29 However, when the
SeBT content was less than 30%, along with the main
emission peak in the red region, a small shoulder in the
blue region due to fluorene segment emission can be
observed in the PL spectra, and its intensity increased
with decreasing SeBT content (Figure 2b). Compared
with the PL spectra in thin films (Figure 2a), fluorene
emission completely disappears for 1% SeBT content.
The fact indicates that the energy transfer in such a
concentration is incomplete for copolymers with low
SeBT content in the solution. We have to note that
PFO-SeBT1 has an average molecular weight of 21 000,
corresponding to around 54 monomer units per polymer
chain (Table 1). Statistically, only one copolymer chain
out of two has one SeBT unit. In other words, about one-
half of the copolymer chains should be PFO homopoly-
mer in PFO-SeBT1. The interchain energy transfer
must take place in the solid-state film in order to quench
completely the PL emission from fluorene segment.
Figure 2c shows PL spectra of PFO-SeBT10 with
different concentrations in chloroform solution. Fluorene
segment emission in the copolymer was completely
depressed at a concentration of around 5 × 10-4 mol/L.
In Table 3 we list the threshold concentration of the
solution of each copolymer, where fluorene emission was
completely quenched. With increasing SeBT content in
the copolymers, the threshold concentration for complete
energy transfer decreases. A very low threshold con-
centration for completed depression of fluorene host
emission for PFO-SeBT copolymers seems to suggest
that the intramolecular energy transfer is an efficient
and quick process and plays a major role in such
Electroluminescent Properties. The EL properties
of the copolymers were investigated using a double-layer
device fabricated in the configuration ITO/hole transport
layer/polymer/Ba/Al, where the hole transport layer was
either PEDT or PEDT/poly(N-vinylcarbazole) (PVK).
Figure 4 shows the EL spectra from such a device,
compared with EL spectra of a device from polyfluorene

Macromolecules, Vol. 38, No. 2, 2005
Table 5. Device Performances of the Copolymers (ITO/hole transport layer/polymer/Ba/Al)
device performancesa
Deep-Red Electroluminescent Polymers 251
copolymers
PFO-SeBT1
hole transport layer
λELmax/nm
QEb (%)
Lc/cd‚m-2
current densityd/mA‚cm-2
Ve/V
PEDT
PEDT/PVK
PEDT
PEDT/PVK
PEDT
PEDT/PVK
PEDT
PEDT/PVK
PEDT
PEDT/PVK
PEDT
PEDT/PVK
PEDT
PEDT/PVK
PEDT
661
667
676
676
682
697
687
697
712
708
708
708
723
723
749
744
759
759
790
790
0.75
0.67
0.54
0.81
0.20
0.50
0.32
0.70
0.05
0.15
0.02
0.02
0.23
0.30
0.05
0.05
0.11
0.20
0.01
0.02
85
80
60
96
23
56
37
83
6
40.0
40.0
40.0
40.0
40.0
40.0
40.0
40.0
40.0
40.0
40.0
40.0
40.0
40.0
40.0
40.0
40.0
40.0
40.0
40.0
9.6
16.5
8.6
13.9
8.3
PFO-SeBT5
PFO-SeBT10
PFO-SeBT15
PFO-SeBT30
PFO-SeBT50
PFO-SeBSe1
PFO-SeBSe5
PFO-SeBS e10
PFO-SeBSe15
13.3
4.8
9.6
5.6
17
2
2
7.8
8.4
9.0
26
38
5
11.4
16.7
9.4
PEDT/PVK
PEDT
PEDT/PVK
PEDT
PEDT/PVK
6
21.2
11.5
17.7
8.6
13
24
1
2
9.7
a Device structure: ITO/hole transport layer/polymer/Ba/Al, active area 0.15 cm2. b External quantum efficiency. c Luminance. d Current
density. e Operating voltage at 40 mA/cm2.
quenched for copolymer with the narrow-band-gap
monomers loading as low as 1%. EL emission consists
exclusively of emission from SeBT or SeBSe units. With
increasing SeBT content, EL peaks are also red shifted
from 667 nm for PFO-SeBT1 to 708 nm for PFO-
SeBT50. For PFO-SeBSe copolymers, the emission
peak was moved from 723 nm for PFO-SeBSe1 to 790
nm for PFO-SeBSe15. This fact seems to indicate that
the energy-transfer process bears the same mechanism
and the recombination zone is the same for both PL and
EL processes. The fact that the incorporation of only
1% narrow-band-gap units into copolymer chain com-
pletely depresses both the fluorene host emission (λmax
) 439 nm) and the wide excimer emission (λmax ) 500
nm) (Figure 4) indicates that an intrachain energy
trapping process (from fluorene segment to SeBT or
SeBSe unit) must be very fast and intrachain energy
transfer should be more efficient than interchain inter-
action between fluorene segments in the polymer ag-
gregates. This indicates again that intramolecular
energy transfer plays the major role in devices from such
copolymers.
The device performance is summarized in Table 5. All
the devices were tested under a fixed current density
of 40 mA‚cm-2 for the convenience of comparison. The
external quantum efficiency of the device for PFO-
SeBT5 reaches 0.81% at an emission peak ca. 676 nm
with PVK as the hole transport layer. When PEDT was
used, the device efficiencies are much lower. This is very
consistent with the HOMO levels of these copolymers
(Table 2). As indicated by the CV measurement (Table
2), the HOMO level of PFO-SeBT and SeBSe copoly-
mers is estimated at around 5.6-5.8 eV toward the
vacuum level. Since PEDT has a work function around
4.8-5.0 eV,32 a significant barrier for hole injection for
the device with PEDT anode interlayer exists. Insertion
of PVK interlayer (work function 5.6-5.8 eV) signifi-
cantly improves hole injection in such devices. As can
be seen from Table 5, the external quantum efficiencies
of the devices for PFO-SeBSe copolymers were lower
than those from the corresponding PFO-SeBT copoly-
mers. This is probably related with the fact that the PL
efficiencies of PFO-SeBSe copolymers are much lower
Figure 4. EL spectra of (a) PFO-SeBT copolymers and (b)
PFO-SeBSe copolymers in device configuration: ITO/PEDT/
PVK/polymer/Ba/Al.
homopolymer. The shape and peak position of EL
emission are almost the same as the PL spectra in the
solid state (Figure 2a, 3a, and 4). As PL emission in thin
film, fluorene emission of EL device was completely

252 Yang et al.
Macromolecules, Vol. 38, No. 2, 2005
Figure 5. I-V curves of the ITO/PEDT/polymer:PCBM(1:4)/
Ba/Al cells under 78.2 mW/cm2 AM1.5 illumination and in the
dark.
Figure 6. Spectral responses of the ITO/PEDT/polymer:
PCBM (1:4)/Ba/Al cells at zero bias.
Table 6. The Optimized Performances of PVCs for
PFO-SeBT:PCBM and PFO-SeBSe:PCBM Blend
active layer
Jsc (mA/cm2) Voc (V) FF (%) ECE (%)
difference (Figure 1a and c). We also note that somehow
the fill factor (FF) for PFO-SeBSe devices is systemati-
cally lower than that for PFO-BSeT devices. Efforts to
optimize the device performance are in progress and will
be reported elsewhere.
PFO-SeBT30:PCBM ) 1:4
PFO-SeBSe15:PCBM ) 1:4
2.53
0.51
1.00
0.80
37.4
23.3
1.0
0.1
than that of PFO-SeBT. Efforts for further improve-
ment of the device efficiency and further extension of
the emission to the longer near-infrared region for
PFO-SeBSe are underway.
Conclusion
We synthesized a novel series of fluorene-based
copolymers with various molar ratios of narrow-band-
gap comonomers, SeBT or SeBSe, of low optical band
gaps (Eg ) 1.87 eV for SeBT and 1.77 eV for SeBSe,
respectively). The efficient and fast energy transfer due
to exciton confinement on narrow-band-gap sites has
been observed. The PL and EL emissions from the
fluorene segment were completely quenched at very low
narrow band-gap unit content (1%). The PL and EL
efficiencies decreased gradually with increasing the
narrow-band-gap unit content, probably due to the
heavy Se atom effect. The EL emission peak of the
device fabricated from PFO-SeBSe15 shifts to the near-
infrared region, around 790 nm. This is the first report
of an electroluminescent NIR polymer which contains
no rare-earth ions or dye ions. High-performance photo-
voltaic cells with extended spectral response and in-
creased open-circuit voltage have been achieved from a
composite thin film of PFO-SeBT copolymer and PCBM
blend.
Characteristics of Photovoltaic Devices. Bulk-
heterojunction photovoltaic cells (PVC) were made from
copolymers, PFO-SeBT or PFO-SeBSe as the donor
phase blending with PCBM as the acceptor phase. Since
the optical band gaps of the two narrow-band-gap units,
SeBT and SeBSe, were ca. 1.87 and 1.77 eV and
absorption peaks corresponding to SeBT and SeBSe
were 563 and 605 nm, respectively, we can expect that
the spectral response will shift to the deep-red region.
Solar cells were fabricated with a sandwich configura-
tion of ITO/PEDT:PSS/PFO-SeBT(or PFO-SeBSe):
PCBM/Ba/Al. Table 6 lists the PVC device performance
for blend composition, PFO-SeBT30(or PFO-SeBSe15):
PCBM ) 1:4. The best device performance was obtained
for the device from PFO-SeBT30:PCBM (1:4), reaching
Jsc (short-circuit current) ) 2.53 mA/cm2, Voc (open-
circuit voltage) ) 1.00 V, and ECE ) 1.0% under AM1.5
illumination (78.2 mW/cm2). We note that the Voc of
these devices is around 1.0 V, which is 0.20 V higher
than that of PFO-SeBSe/PCBM and also MEH-PPV/
PCBM (0.80 V)33 cells. Since the PCBM:PFO-SeBT
ratio, anode interlayer (PEDT), and cathode material
(Ba/Al) were the same for the devices listed in the Table
6 and in ref 33, the observed Voc (Figure 5) increase over
MEH-PPV:PCBM and PFO-SeBSe/PCBM devices must
be related with interfacial properties of PFO-SeBT in
the anode and/or acceptor interfaces.
Figure 6 compares the spectral response (normalized)
of three devices. Photosensitivity response curves of the
devices coincide with the absorption profiles of the
corresponding copolymers (Figure 1). Spectral responses
of the devices from PFO-SeBSe copolymers are around
750 nm, while that for PFO-SeBT devices is around
675 nm. However, the energy conversion efficiency of
PFO-SeBSe devices is lower than that of PFO-SeBT
devices (Table 6). Since the active layer of the device
are of the same thickness, a much smaller absorption
constant of the SeBSe unit in the copolymer than that
of PFO-SeBT is probably the main origin of this
Acknowledgment. This work was supported by
research grants from Ministry of Science and Technol-
ogy, China (MOST) (#2002CB613402), and National
Natural Science Foundation of China (NSFC#50433030).
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Deep-Red Electroluminescent Polymers 253
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