Novel red-emitting fluorene-based copolymers

Article abstract of DOI:10.1039/b203862e

A series of novel soluble conjugated copolymers derived from 9,9-dioctylfluorene (DOF) and 4,7-di-2-thienyl-2,1,3-benzothiadiazole (DBT) were synthesized by a palladium-catalyzed Suzuki coupling reaction with different feed ratios of DOF to DBT. Owing to exciton confinement on the DBT site, exciton emission is centred on the DBT chromophore and gives rise to saturated red emission. Polyfluorene fluorescence is quenched completely at DBT concentrations as low as 1% in the solid film. Devices based on these copolymers emit a saturated red light. Chromaticity coordinates are around x = 0.70, y = 0.30 for the copolymers. The emission peaks are shifted from 628 nm to 674 nm when DBT content increases from 1 to 35%. The highest quantum efficiency achieved with device configuration of ITO/PEDT/PFO-DBT/Ba/A1 was 1.4% for the copolymer with 15% DBT content.

Full text of DOI:10.1039/b203862e

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Novel red-emitting fluorene-based copolymers
Qiong Hou, Yishe Xu, Wei Yang, Min Yuan, Junbiao Peng and Yong Cao*
Institute of Polymer Optoelectrionic Materials and Devices, South China University of
Technology, Guangzhou, China 510640. E-mail: poycao@scut.edu.cn
Received 22nd April 2002, Accepted 3rd July 2002
First published as an Advance Article on the web 13th August 2002
A series of novel soluble conjugated copolymers derived from 9,9-dioctylfluorene (DOF) and 4,7-di-2-thienyl-
,1,3-benzothiadiazole (DBT) were synthesized by a palladium-catalyzed Suzuki coupling reaction with different
2
feed ratios of DOF to DBT. Owing to exciton confinement on the DBT site, exciton emission is centred on
the DBT chromophore and gives rise to saturated red emission. Polyfluorene fluorescence is quenched
completely at DBT concentrations as low as 1% in the solid film. Devices based on these copolymers emit a
saturated red light. Chromaticity coordinates are around x ~ 0.70, y ~ 0.30 for the copolymers. The emission
peaks are shifted from 628 nm to 674 nm when DBT content increases from 1 to 35%. The highest quantum
efficiency achieved with device configuration of ITO/PEDT/PFO-DBT/Ba/Al was 1.4% for the copolymer with
1
5% DBT content.
poly(3,4-ethylenedioxythiophene)–polystyrene sulfonate) was
.4% for a copolymer with 15% DBT content.
Introduction
1
In recent years, electroluminecent polymers have attracted
great attention due to their possible utilization in flat panel
displays. Polyfluorenes are promising new materials for light-
emitting diodes because of their thermal and chemical stability
and exceptionally high fluorenscence quantum yields (0.6–0.8)
Experimental
Instrumentation
1
in thin solid films. Normally, polyfluorene homopolymers
1 13
H and C NMR spectra were recorded on a Varian Inova 500
have large bandgaps and emit blue light. The emission color
of polyfluorenes can be tuned over the entire visible region
by incorporating narrow band-gap comonomers into the
or Bruker DRX 400 spectrometer in deuterated chloroform
¯
solution at 298 K. Number-average (M ) and weight-average
n
¯
(M
w
) molecular weights were determined by Waters GPC 2410
2
,3
polyfluorene backbone.
The most widely used narrow
band-gap comonomers are a variety of aromatic heterocycles
in THF using a calibration curve of polystyrene standards.
Elemental analysis was performed on a Vario EL Elemental
Analysis Instrument (ELEMENTAR Co.) or on an Antek
7000S/N (ANTEK Co.). UV-visible absorption spectra were
recorded on a HP 8453 spectrophotometer. Cyclic voltammetry
was performed using a CHI660A electrochemical workstation
4
–17
4–9
such as thiophene,
5–19
ethylenedioxythiophene
In most of these reports the copolymers
and ben-
1
zothiadiazole.
were alternating A–B-type copolymers which show low electro-
luminescence (EL) efficiencies. For example copolymerization
of fluorene with 4,7-di-2-thienyl-2,1,3-benzothiadiazole has
been reported in the patent literature by the Dow Chemical
2
1
with platinum electrodes at a scan rate of 50 mV s against a
calomel reference electrode with an anhydrous and nitrogen-
saturated solution of 0.1 M tetrabutylammonium hexafluor-
1
9
group. The three-component copolymer (fluorene–benzothia-
diazole–dithienylbenzothiadiazole) was prepared and white
light-emitting devices were fabricated from the polymer blend
of such a copolymer with a blue emitter. The power efficiences
4 6 3
ophosphate (Bu NPF )–acetonitrile (CH CN). The deposition
of polymers on the electrode was done by the evaporation of
a dilute chloroform solution. The PL quantum yields were
determined using an Integrating sphere IS080 (Labsphere)
with 325 nm excitation from a HeCd laser (Mells Griod). EL
efficiency and brightness measurements were carried out with
a calibrated silicon photodiode. Photoluminescence (PL) and
EL spectra were recorded on an Instaspec IV CCD spectro-
photometer (Oriel Co.).
2
1 19
.
for devices from such a blend increased to 0.2–0.3 Lm W
Recently, we reported the synthesis and properties of copoly-
mers with carbazole as a main chain building unit and DBT as
2
0
a narrow band-gap component.
In this paper we report the synthesis and characterization of
copolymers derived from fluorene and 4,7-di-2-thienyl-2,1,3-
benzothiadiazole (DBT, structure in Scheme 1) in less than 50%
in the polymer composition by the Suzuki coupling reaction.
A special feature of the Suzuki coupling is that it allows
an alternating type of copolymer to be obtained in the case of
A–B comonomers, while typical cross-coupling of dibromides
provides a multiblock copolymer. In the case of Suzuki coupl-
ing, each individual narrow band-gap unit is separated on both
sides by wide band-gap segments, once the narrow band-gap
component is less or equal to 50% in the copolymer. In this
case we could expect that the narrow band-gap unit functions
as an exciton trap which allows efficient intramolecular energy
transfer from the fluorene segment to the DBT unit. Devices
based on the copolymers emit a saturated red light. The highest
quantum efficiency achieved in the device of configuration
ITO/PEDT/PFO-DBT/Ba/Al (ITO: indium tin oxide; PEDT:
Scheme 1 Synthetic route of copolymers.
DOI: 10.1039/b203862e
J. Mater. Chem., 2002, 12, 2887–2892
This journal is # The Royal Society of Chemistry 2002
2887

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2
1
Materials
Polymer synthesis
n
Fluorene, Bu Li, n-octylbromide, iron powder and 2-isopro-
poxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane were purchased
from Aldrich Co. Tetrakis(triphenylphosphine)palladium was
obtained from TCI Co. Thiophene, tributylchlorostannane,
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-dioctylfluo-
rene (5), 4,7-bis(5-bromo-2-thienyl)-2,1,3-benzothiadiazole (7)
and (PPh
dissolved in a mixture of toluene and an aqueous solution of
2 M Na CO . The mixture was first put under an argon
3 4
) Pd(0) (0.5–1.5 mol%) ,and Aliquat 336 were
2
2 3 2
,1,3-benzothiadiazole, PdCl (PPh ) and Aliquat 336 were
purchased from Acros Co. N-Bromosuccinimide and bromine
were used as received. Anhydrous tetrahydrofuran was distilled
over sodium–benzophenone under nitrogen prior to use.
Chloroform, carbon tetrachloride and toluene were distilled
2
3
atmosphere and refluxed with vigorous stirring for 72 h. Then
2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-dioctyl-
fluorene and bromobenzene were added to end-cap the polymer
chain. The whole mixture was poured into methanol. The
precipitated material was recovered by filtration through a
funnel and washed with acetone to remove oligomers and
catalyst residues (yield: 70–83%). The resulting polymers were
soluble in conventional organic solvents (toluene, chloroform,
and tetrahydrofuran). The results of the elemental analyses
for sulfur (or nitrogen) and carbon for each copolymer were
used for calculating the actual copolymer composition.
Element Anal. Found: for PFO-DBT1: C, 86.36%; H, 9.78%;
N, 0.12%; S, 0.27% ; for PFO-DBT5: C, 86.99%; H, 9.93%; N,
2
1
over calcium chloride. 9,9-Dioctylfluorene (1), 2,7-dibromo-
2
,9-dioctylfluorene (2), tributyl(2-thienyl)stannane (3) and
1
22
9
4
2
,7-dibromo-2,1,3-benzothiadiazole (4) were prepared follow-
3
ing the already published procedures.
2,7-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-
2
dioctylfluorene (5)
1
To a solution of 2,7-dibromo-9,9-dioctylfluorene (2, 5.6 g,
0.22 mmol) in THF (130 mL) at 278 uC, Bu Li (19.7 mL of a
.6 M solution in hexane, 31.45 mmol) was added dropwise.
n
1
1
0
.51%; S, 1.18% (by Vario EL), 1.23% ( by Antek 7000S/N);
The mixture was stirred at 278 uC for 2 h. 2-Isopropoxy-4,4,5,5-
tetramethyl-1,3,2-dioxaborolane (25 ml, 123.24 mmol) was
added rapidly to the solution. The mixture was stirred at
for PFO-DBT10: C, 85.60%; H, 9.90%; N, 0.50%, S, 1.81%;
for PFO-DBT15: C, 83.90%; H, 9.22%; N, 1.04%; S, 3.90%;
for PFO-DBT25: C, 81.14%; H, 8.38%; N, 1.85%; S, 6.49%;
for PFO-DBT35: C, 75.57%; H, 7.76%; N, 2.72%; S, 9.07%.
2
78 uC for 2 h, then it was warmed to room temperature and
stirred for 36 h. The mixture was poured into water and it was
extracted with ether. The organic layer was washed with brine
and dried over anhydrous magnesium sulfate. The solvent
was removed under reduced pressure, and the residue was
recrystallized from tetrahydrofuran and methanol, then it was
purified by column chromatography (silical gel, 10% ethyl
Results and discussion
Synthesis and characterization of copolymers
¯
High molecular weight, readily soluble copolymers (Mn
~
acetate in hexane) to give the title product as a white solid in
1
11000–35000) of different compositions were prepared
from 2,7-dibromo-9,9-dioctylfluorene (DOF), 2,7-bis(4,4,5,5-
tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-dioctylfluorene and
narrow band gap comonomer 4,7-bis(5-bromo-2-thienyl)-2,1,3-
benzothiadiazole (DBT) using palladium catalyzed Suzuki
coupling methods (Scheme 1). The comonomer feed ratios
of DOF to DBT are 99 : 1, 95 : 5, 90 : 10, 85 : 15, 75 : 25 and
4
7
(
1
3
6% yield. Mp 128–131 uC. H NMR d(CDCl
.74 (s, 2H), 7.71 (d, 2H), 1.99 (m, 4H), 1.39 (s, 24H), 1.22–1.00
3
): 7.80 (d, 2H),
1
3
m, 20H), 0.81 (t, 6H), 0.56 (m, 4H). C NMR d(CDCl
3
):
50.86, 144.30, 134.04, 129.29, 119.77, 84.11, 55.57, 40.49,
2.18, 30.33, 29.58, 25.33, 23.98, 22.99, 14.48. Element Anal.
Calcd for C H O B : C, 76.74%; H, 10.04%. Found: C,
4
1
64
4 2
7
6.44%; H, 9.90%.
6
5 : 35 respectively, and the corresponding copolymers were
named PFO-DBT1, PFO-DBT5, PFO-DBT10, PFO-DBT15,
PFO-DBT25 and PFO-DBT35 respectively. For these copoly-
mers, n-octyl substituents in the 9-position of fluorene were
employed to improve the solubility of the resulting copolymers.
At the end of the polymerization, 2,7-bis(4,4,5,5-tetramethyl-
2
4
4
,7-Di-2-thienyl-2,1,3-benzothiadiazole (6)
To a solution of 4,7-dibromo-2,1,3-benzothiadiazole (2 g,
.8 mmol) and tributyl(2-thienyl)stannane (6.1 g, 16.4 mmol)
in THF (50 ml), PdCl (PPh ) (97 mg, 2 mol%) was added. The
mixture was refluxed under a nitrogen atmosphere for 3 h.
After removal of the solvent under reduced pressure, the
residue was purified by column chromatography on silica gel
( Cl –hexane, 1 : 1). Recrystallization from ethanol
2 2
gave the title compound (1.8 g, 88%) as red needles. Mp 124–
3
25 uC. H NMR d(CDCl ): 8.11 (dd, 2H), 7.87 (s, 2H), 7.46
dd, 2H), 7.21 (dd, 2H). C NMR d(CDCl
27.21, 127.90, 128.42, 139.75, 153.02. Element Anal. Calcd
for C14 : C, 55.97%; H, 2.68%; N, 9.32%; S, 32.02%.
Found: C, 56.34%; H, 2.82%; N, 9.50%; S, 32.38%.
6
2
3 2
1
,3,2-dioxaborolan-2-yl)-9,9-dioctylfluorene was added to
remove bromine end groups and bromobenzene was added
as a monofunctional end-capping reagent to remove boronic
ester end groups, because boron and bromine units could
quench emission and contribute to excimer formation in
eluent CH
2
5
1
LED applications.
The starting monomer ratios have
1
(
1
1
3
been adjusted in order to investigate the effect of copolymer
composition on the physical and optical properties. The actual
ratio of DOF : DBT in the copolymer, calculated from
elemental analysis, is very close to the feed ratio, as listed in
Table 1. The incorporation of DBT into the polyfluor-
ene main chain was also monitored by UV spectroscopy
3
): 126.15, 126.38,
8 2 3
H N S
1
9
4
,7-Bis(5-bromo-2-thienyl)-2,1,3-benzothiadiazole (7)
To a mixture of chloroform (52 ml) and acetic acid (52 ml),
4
Table 1 Molecular weights of the copolymers
,7-di-2-thienyl-2,1,3-benzothiadiazole (2 g, 6.67 mmol) was
DOF : DBT
in the feed
composition
added under a nitrogen flow. After the solid dissolved
completely, N-bromosuccinimide (NBS, 2.5 g, 14.01 mmol)
was added in one portion. The reaction mixture was stirred at
room temperature overnight, and the dark red precipitate
formed was filtered off and recrystallized from DMF to give
¯
M
(610 )
n
DOF : DBT
in the copolymers
3
a
¯
M
¯
/M
Copolymer
w
n
PFO-DBT1
PFO-DBT5
23
35
2.6
3.1
2.7
2.6
2.4
1.7
99 : 1
95 : 5
90 : 10
85 : 15
75 : 25
65 : 35
98.9 : 1.1
95.2 : 4.8
92.6 : 7.4
85.7 : 14.3
75.3 : 24.7
64.5 : 35.5
PFO-DBT10 33
PFO-DBT15 34
PFO-DBT25 29
PFO-DBT35 11
a
the title compound (1 g, 33%) as shiny red crystals. Mp 251–
1
2
52 uC. H NMR (500 MHz) d(CDCl ): 7.81 (dd, 2H), 7.78
3
(
6 2 3 2
s, 2H), 7.15 (dd, 2H). Element Anal. Calcd for C14H N S Br :
C, 36.70%; H, 1.32%; N, 6.11%; S, 20.99%; Br, 34.88%. Found:
C, 36.94%; H, 1.63%; N, 6.21%; S, 21.70%; Br, 33.52%.
Calculated from results of elemental analysis.
2
888
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the presence of the DBT unit is not apparent from the
absorption spectrum in the solid film (Fig. 1b), however, a
weak 540 nm peak is clearly seen in the solution spectrum of 1%
DBT copolymer (Fig. 1a). The two distinguishable absorption
features of the copolymers demonstrate that the electronic
configurations of the two components, DOF and DBT, in the
copolymers are not mixed.
The photoluminescent (PL) spectra of the polymers in thin
films and in CHCl solution, produced by excitation with the
3
3
25 nm line of the HeCd laser, are presented in Fig. 2a and 2b
respectively. In thin solid films, polyfluorene emission was
completely quenched and PL emission consists exclusively of
DBT emission at DBT concentrations as low as 1% (Fig. 2a).
In CHCl solution with a copolymer concentration of 1 6
3
2
5
21
1
0
mol L , PL emission consists of both fluorene and
DBT emission in the PFO-DBT1 and PFO-DBT5 copolymers
Fig. 2b). For copolymers with DBT content equal to or greater
than 10%, PL emission consists exclusively of DBT emission
(
2
5
21
at a concentration of 1 6 10 mol L (Fig. 2b). Since, for the
PFO-DBT1 copolymer with a number average molecular
4
weight of 2.3 6 10 (Table 1), statistically only one third of
the polymer chain contains the copolymer with the DBT unit,
the interchain interaction must play some role in PL quenching.
In Fig. 3, the PL spectra of PFO-DBT1 (Fig. 3a) and PFO-
DBT25 (Fig. 3b) are compared at different concentrations.
Polyfluorene emission was completely quenched when the
Fig. 1 (a) UV-Vis absorption spectra for the copolymers in CHCl
solution. (b) UV-Vis absorption spectra for the copolymers in solid
state films.
3
(lmax(oligofluorene segments) ~ 380 nm, lmax (incorporated
DBT) ~ 540 nm) (Fig. 1). Most of the polymers exhibit a
relatively high molecular weight with a polydispersity index
¯
M
¯
/M
(
w
n
) from 1.7 to 3.1 (Table 1). These polydispersity
indexes are consistent with a polycondensation reaction.
Optical and photoluminescent properties
The UV-Visible absorption properties are summarized in
Table 2 and Fig. 1. All the copolymers show similar absorption
spectra in solution (Fig. 1a) and in the solid films (Fig. 1b). The
absorption spectra of the copolymers measured in thin films
display two absorption peaks. The absorption peaks at ca.
3
80 nm are attributed to the fluorene segment, and the
additional long wavelength absorption bands at ca. 540 nm
are attributed to the DBT unit incorporated into the poly-
fluorene main chain. Moreover, the absorption intensity at
5
40 nm increased gradually with increasing concentration of
Fig. 2 (a) PL spectra of the copolymers in solid state films. (b) PL
spectra of the copolymers in CHCl solution at a concentration of
DBT in the polymer backbone. In the case of the copolymer
containing one percent of DBT in the copolymer main chain,
3
25
21
1 6 10 mol L
.
Table 2 UV-Vis and electrochemical properties of the copolymers
a
Optical band gap /eV
b
Copolymer
labsmax/nm
E
ox/V
EHOMO/eV
ELUMO /eV
PFO-DBT1
PFO-DBT5
PFO-DBT10
PFO-DBT15
PFO-DBT25
PFO-DBT35
a
382, —
2.93, —
1.32, —
1.28, —
1.32, 1.21
1.31, 1.15
1.32, 1.10
1.30, 1.07
b
25.72, —
25.65, —
25.72, 25.61
25.71, 25.55
25.72, 25.50
25.70, 25.47
22.79, —
22.72, —
22.79, 23.54
22.80, 23.52
22.84, 23.48
22.82, 23.46
382, 535
384, 536
382, 538
384, 542
388, 551
2.93, 2.08
2.93, 2.07
2.91, 2.03
2.88, 2.02
2.88, 2.01
Estimated from the onset wavelength of optical absorption in the solid state film. Calculated from the HOMO level and optical band gap.
J. Mater. Chem., 2002, 12, 2887–2892
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coupling). The DBT unit serves as a powerful trap. Excitons
are confined in the vicinity of the DBT unit isolated on both
sides by the fluorene segment and give rise to red light emission.
Significant enhancement of the external quantum efficiencies
of electroluminescence by exciton confinement in the random
copolymers that have different energy gap region within a
2
7
single chain was first reported by Burn et al.
Table 4 lists the PL efficiencies obtained with 325 nm
excitation using the HeCd laser. The PL efficiencies decrease
quickly with increasing DBT content in the copolymers from
1
1% for PFO-DBT1 to 4% for PFO-DBT35.
Electrochemical properties
The electrochemical behavior of the copolymers was investi-
gated by cyclic voltammetry (CV). Except for the copolymers
with 1 and 5% DBT, all of the copolymers show two p-doping
processes. Table 2 summarizes the oxidation potentials derived
from the onset in the cyclic voltammograms of the copolymers.
For copolymers with DBT content equal to or greater than
1
0%, the first onset of the p-doping processes decreases with
increasing DBT content from 1.21 V for PFO-DBT10 to 1.07 V
for PFO-DBT35. The second onset of the oxidation process
is almost constant for all of the copolymers at about 1.30–
1
.32 V. For PFO-DBT1 only one peak can be observed in the
voltammogram. The position of the onset of this peak is the
same (1.31 V) as the second onset with high DBT content. This
peak is attributed to p-doping of the DOF segments. It is
slightly unusual for PFO-DBT5, for which only one oxidation
peak is observed, the onset of which is moved to 1.28 V. This
peak is a result of the mixing of both oxidation processes in
the voltammogram. We were unable to record n-doping pro-
cesses after many attempts. The energies of the HOMO levels,
Fig. 3 (a) PL spectra of polymer PFO-DBT1 in CHCl
different concentrations. (b) PL spectra of polymer PFO-DBT25 in
CHCl solution at different concentrations.
3
solution of
3
calculated according to an empirical formula (E
~
HOMO
Table 3 Concentration of copolymers at which PFO fluorescence is
completely quenched
2
8
2e(Eox 1 4.4) (eV)), are listed in Table 2. In order to get some
idea about LUMO energies in these copolymers, they were
estimated from the optical band gap and HOMO energies. The
results are also listed in Table 2. The HOMO energies, 5.7 eV,
calculated from the second onset in the voltammograms
Concentration of copolymer
at which PFO fluorescence is
completely quenched/mol L
2
1
Copolymer
2
4
PFO-DBT1
PFO-DBT5
PFO-DBT10
PFO-DBT15
PFO-DBT25
4 6 102
are very similar to the data reported for polyfluorene homo-
29
polymers, Eox ~ 1.4 V, I ~ 5.8 eV. HOMO levels
p
5
5 6 102
6
5 6 10
1 6 102
1 6 10
corresponding to the DBT unit are slightly decreasing with
increasing DBT content e.g. EHOMO ~ 5.47 eV for PFO-
DBT35. The energies of the LUMO levels corresponding to
the DOF segments remain almost unchanged for the copoly-
mers with different DBT contents, while the LUMO levels
due to the DBT unit decrease with increasing DBT content.
Bearing in mind that a single DBT unit is isolated on both
sides by fluorene segments, the change in HOMO and LUMO
levels for the DBT units must originate from interchain
interactions. Two well-defined oxidation processes in the cyclic
voltammograms and the similarity of the EHOM2 values for
the DOF segment in the copolymers and for DOF in the
homopolymer demonstrate again that the electronic config-
uration of the two components, DOF and DBT, in the
copolymers are not mixed. This is very consistent with the two
distinct absorption peaks in the UV-Vis spectra (Fig. 1).
2
7
10
concentration of PFO-DBT1 is equal to or greater than
2
4
21
4
fluorescence was practically absent at concentrations as low
6 10 mol L (Fig. 3a). In case of PFO-DBT25 the PFO
2
10
21
mol L
as 1 6 10
(Fig. 3b). Table 3 lists the critical
concentration at which polyfluorene fluorescence was com-
pletely quenched. The observed concentrations are much lower
than the ‘‘overlap threshold concentration’’ of typical non-
2
6
conjugated polymers. Detailed analysis of solution properties
and energy transfer in solution are in progress and will
be presented elsewhere. These facts seem to suggest that
intramolecular energy transfer plays a major role in such
copolymers prepared by the cross-coupling method (Suzuki
Table 4 Optical Properties of the copolymers
Photoluminescence
Electroluminescence
Chromaticity coordinate
Copolymer
lPlsmax/nm
QPLsmax(%)
lElsmax/nm
Qextmax(%)
x
y
PFO-DBT1
PFO-DBT5
PFO-DBT10
PFO-DBT15
PFO-DBT25
PFO-DBT35
635
651
655
678
678
685
11.4
12.5
8.6
7.9
5.2
4
628
643
652
663
669
674
0.5
0.6
0.9
1.4
0.6
0.5
0.67
0.70
0.70
0.70
0.70
0.70
0.32
0.31
0.30
0.29
0.29
0.29
2
890
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Table 5 Device performance of PFO-DBT copolymers
a
Device performance
2
2
22
Copolymer
Bias/V I/mA cm
Luminance/cd m
QEext (%)
PFO-DBT1
PFO-DBT5
6.1
7.2
33.3
33.3
33.3
33.3
33.3
33.3
91
130
167
259
109
103
0.5
0.6
0.9
1.4
0.6
0.5
2
PFO-DBT10 7.7
PFO-DBT15 5.1
PFO-DBT25 3.1
PFO-DBT35 3.3
a
Device structure: ITO/PEDT/PFO-DBT/Ba/Al, active area 0.15 cm .
Fig. 4 EL spectra of devices from the copolymers.
Electroluminescent properties
QE vs. DBT content indicates that there are two opposite
processes controlling the device efficiency. Since the HOMO
level decreases with increasing DBT contents, the barrier height
for hole injection decreases accordingly. As the LUMO level
does not change very much with increasing DBT content
A single layer device was fabricated in the configuration ITO/
PEDT/PFO-DBT/Ba/Al. The EL spectra from such device
are presented in Fig. 4. A saturated red emission was obtained
for all of the copolymers. EL peaks are 10 nm blue shifted in
comparison with PL peaks with slightly larger bandwidth
(
Table 2), the electron injection should not change for copoly-
mers with different DBT contents. However, we note that PL
efficiencies decrease remarkably with increasing DBT content.
Thus, PL quenching leads to a decrease in EL efficiency with
increasing DBT content. The increase in the operating voltage
with increasing DBT content seems to support such an
interpretation.
(
cf. Fig 2 and 4). Emission peaks shift from 628 nm for
PFO-DBT1 to 674 nm for PFO-DBT35 (Fig. 4 and Table 4).
The chromaticity coordinates changed from x ~ 0.67, y ~ 0.32
for PFO-DBT1 to x ~ 0.70, y ~ 0.29 for PFO-DBT35. Table 4
lists the emission maximum and chromaticity coordinates
for each copolymer. It is very important to note the fact
that EL emission is exclusively from the DBT unit and
PFO emission is completely quenched. The PFO emission is
completely quenched even at 1% DBT loading. This is very
Conclusions
The synthesis of high molecular weight, readily soluble
copolymers of 9,9-dioctylfluorene and the low band gap
comonomer DBT can be accomplished by Pd-catalyzed
Suzuki coupling reactions. The efficient energy transfer due
to exciton trapping on narrow band-gap DBT sites has been
observed. Polyfluorene fluorescence is quenched completely at
DBT concentrations as low as 1% in the solid state film. All of
the copolymers emit saturated red color. Preliminary device
characteristics are promising for utilization. Devices fabricated
with copolymer with 15% DBT content shows the highest
external quantum efficiency at 1.4%. Further improvement by
manipulating the polymer/electrode interface is in progress.
3
0
similar to what was observed for the PL spectra. McGehee
3
1
and O’Brien and their co-workers reported large differences
between the PL and EL spectra at low doping concentrations
for Eu-complex/CNPPP and PtOEP/PFO devices. For these
systems host PL emissions were quenched completely only at
much higher doping concentrations than those of EL spectra.
3
0,31
This is quite common for dye-doped polymer systems.
The differences in the PL and EL spectra at low doping
concentrations were attributed to the differences in the
recombination zone for photo- and electric excitations. For
copolymers in this study, as mentioned above, energy transfer
probably occurs mainly within the polymer chain via intra-
molecular energy transfer. Again this fact demonstrates that
intramolecular energy transfer in the PFO-DBT copolymer is a
very quick and efficient process. As a result the generated
excitons are efficiently confined to DBT sites.
3
0
Acknowledgements
This research was supported by the Natural Science Founda-
tion of China (Project No. 29992530-6) and Guangdong
Province (Project No. 990623).
We note also that both PL (in solution and in solid films) and
EL emission are red-shifted with increasing DBT content in
polymer chains (Fig. 2 and 4). This fact is consistent with the
small change in HOMO (decreasing) and LUMO (increasing)
levels with increasing DBT content (Table 2). We note also a
slight red shift of the PL peaks with increasing copolymer
concentration (Fig. 3a and Fig. 3b). This fact indicates that the
red shift of the PL spectra must be related to the interchain
interactions too.
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