Synthesis and optoelectronic properties of a luminescent fluorene derivative containing hole-transporting triphenylamine terminals

Article abstract of DOI:10.1039/c0nj00886a

Solution-processable 4,4′,4″-{[9,9-bis(hexyl)-9H-fluorene-2,4, 7-triyl]tri-2,1-ethenediyl}tris(N,N-diphenyl)benzeneamine (TF), consisting of a fluorene core and terminal triphenylamine groups, was synthesized by the Wittig reaction. The characteristics of

Full text of DOI:10.1039/c0nj00886a

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PAPER
Synthesis and optoelectronic properties of a luminescent fluorene
derivative containing hole-transporting triphenylamine terminalsw
Hsiang-Fu Yang, Wen-Fen Su and Yun Chen*
Received (in Victoria, Australia) 10th November 2010, Accepted 6th January 2011
DOI: 10.1039/c0nj00886a
Solution-processable 4,40,400-{[9,9-bis(hexyl)-9H-fluorene-2,4,7-triyl]tri-2,1-ethenediyl}tris-
(N,N-diphenyl)benzeneamine (TF), consisting of a fluorene core and terminal triphenylamine
1
groups, was synthesized by the Wittig reaction. The characteristics of TF were analyzed by H-NMR,
elemental analysis, differential scanning calorimetry, optical spectra, cyclic voltammetry, and
atomic force microscopy. The triphenylamine terminals were intentionally incorporated to
increase its hole-transporting ability that reduces the barrier height of hole-injection. A multilayer
light-emitting diode using TF as a hole-transporting layer [ITO/PEDOT:PSS/TF/Alq3(70 nm)/
LiF(0.5 nm)/Al(100 nm)] was readily fabricated by the spin-coating process. Its maximum
brightness (9390 cd mꢀ2) and current efficiency (2.90 cd Aꢀ1) were superior to those using
conventional NPB as a hole-transporting layer. In addition, the TF could also be blended with
polyfluorene (PF) to be applied as an emitting layer; the devices [ITO/PEDOT:PSS/blend
(110 nm)/LiF(0.5 nm)/Ca(50 nm)/Al(100 nm)] exhibited high maximum brightness (4040 cd mꢀ2
)
and current efficiency (1.62 cd Aꢀ1). Our results indicate that the TF is a potential optoelectronic
material which is readily processed by wet processes.
For example, Promarak et al.12 fabricated a double-layer
Introduction
OLED device using an amorphous fluorene derivative 2,7-
bis(4-diphenylaminophenyl)-9,9-bis-n-hexylfluorene as a hole-
transporting layer (HTL) [highest occupied molecular orbital
(HOMO) level: ꢀ5.37 eV]. The HTL was spin-coated onto an
ITO followed by vacuum deposition of the Alq3 layer; the
device exhibited a bright green emission of 6500 cd mꢀ2
and a low turn-on voltage of 3.8 V. Yang et al. synthesized
triphenylamine-based dendrimers with truxene as the core and
used as HTL in OLEDs.13 The HTM was solution-processable
and possessed a HOMO level of ꢀ4.95 eV. In addition, Kong
and coworkers developed a macrocyclic oligomer consisting of
triphenylamine and oligofluorene to be applied as a hole-
transporting emitting layer,14 with a HOMO level being
ꢀ5.06 eV. The device emitted blue light; and the maximum
brightness and maximum current efficiency were 1716 cd mꢀ2
and 0.93 cd Aꢀ1, respectively. These results indicate that an
optimal HTL should have a proper HOMO level between the
ITO and emitting layer to facilitate hole-injection. In addition,
to improve the performance and extend the life-time of
optoelectronic devices the materials should meet significant
requirements, such as thermal stability, uniform film morphology
without pinholes, and electrochemical stability.2
Organic light-emitting devices (OLEDs) have attracted consider-
able attention due to their potential applications in large area flat
panel displays owing to outstanding advantages such as low cost,
self-emission, low voltage driving and high brightness.1–5 The high
efficiency multilayer OLEDs have been reported by Tang and
VanSlyke, which used small molecular triphenylamine as a hole
transporting layer (HTL) and Alq3 as an emitting and electron
transporting layer (ETL) to balance the transport and injection
of charge carriers.6 Triarylamine derivatives such as N,N0-di(1-
naphthyl)-N,N0-diphenyl-[1,10-biphenyl]-4,40-diamine (NPB) and
N,N0-bis(tolyl)-N,N0-diphenyl-1,10-biphenyl-4,40-diamine (TPD),
due to their high hole mobility (10ꢀ5–10ꢀ7 m Vꢀ1 sꢀ1) and good
optoelectronic properties, have been widely used as hole-
transporting materials (HTMs).7–11 However, low molecular
weight hole-transporting materials, such as NPB, must be
deposited by vacuum evaporation, which usually requires strict
control of deposition conditions. Therefore, homogeneous films
easily obtained by solution processes such as spin-coating are
highly desirable in fabricating OLEDs. Some low molecular
weight HTMs whose film can be obtained by wet processes have
been synthesized and applied in OLEDs.
This paper presents the synthesis and device characteristics
of solution-processable trifunctional 4,40400-{[9,9-bis(hexyl)-
9H-fluorene-2,4,7-triyl]tri-2,1-ethenediyl}tris(N,N-diphenyl)-
benzeneamine (TF), which is employed as a hole-transporting
layer or emitting component. It is expected that the combination
Department of Chemical Engineering, National Cheng Kung
University, Tainan, Taiwan. E-mail: yunchen@mail.ncku.edu.tw;
Fax: +886 6 2344496; Tel: +886 6 2085843
w This article is part of a themed issue on Molecular Materials: from
Molecules to Materials, commissioned from the MolMat2010 conference.
c
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of the rigid fluorene core and triphenylamine terminals not
only retains thermal and morphological stabilities but also
improves device performance.15 Although a similar structure
has been synthesized previously by the Horner–Emmons
coupling reaction;16 only its two-photon absorption charac-
teristics had been investigated. In addition, we synthesized
the TF by the Wittig reaction. The TF was used as the
hole-transporting layer (HTL) or emitting component in
fabricating electroluminescent devices consisting of Alq3 or
polyfluorene (PF), respectively, as the emitting layer. The
optoelectronic performance of TF-based devices surpasses that
based on conventional NPB. The TF is a promising hole-
transporting material for optoelectronic devices; and more
importantly, it can be deposited by spin-coating processes.
standard. The FT-IR spectra were measured as KBr disk using
a Fourier transform infrared spectrometer, model 7850 from
Jasco. The elemental analysis was carried out on a Heraus
CHN-Rapid elemental analyzer. Differential scanning calori-
metry (DSC) was performed on a Mettler Toledo DSC 1
Star System under a nitrogen atmosphere at a heating rate
of 4.72 K sꢀ1. Absorption spectra were measured with a Jasco
V-550 spectrophotometer and PL spectra were obtained using
a Hitachi F-4500 fluorescence spectrophotometer. The cyclic
voltammograms were recorded using a voltammetric appara-
tus (model CV-50W from BAS) at room temperature under a
nitrogen atmosphere. The measuring cell was composed of a
glassy carbon as the working electrode, an Ag/AgCl electrode
as the reference electrode and a platinum wire as the auxiliary
electrode. The electrodes were immersed in an acetonitrile
solution containing 0.1 M n-Bu4NClO4 under a nitrogen
atmosphere. The energy levels were calculated using the
ferrocene (FOC) value of ꢀ4.8 eV with respect to the vacuum
level, which is defined as zero.20 Morphology images of films
were captured using an atomic force microscope (AFM),
a Veeco/Digital Instrument Scanning Probe Microscope
(tapping mode) with a Nanoscope IIIa controller. The film
thicknesses of hole-transporting and emitting layers were
measured by a surface profiler, a-step 500.
Experimental
Materials and characterization
9,9-Dihexylfluorene (2), 2,4,7-tris(bromomethyl)-9,9-dihexyl-
fluorene (3), 2,4,7-tris[methylene(triphenylphosphonium bromide)]-
9,9-dihexylfluorene (4), 4-(diphenylamino)benzaldehyde (5)
and poly(9,9-dihexylfluorene) (PF, molecular weight = 3 ꢁ
104 g molꢀ1) were prepared according to previously reported
procedures.17–19 All the chemicals were purchased from
Aldrich, Acros, TCI, and Lancaster Chemicals Co. and used
without further purification. Poly(ethylene dioxythiophene):-
polystyrenesulfonate (PEDOT:PSS, Baytron P 4083) was
obtained from H.C. Starck. All newly synthesized compounds
were identified by 1H NMR, FT-IR, and elemental analysis
(EA). The 1H NMR spectra were recorded with a Bruker
AVANCE-400 NMR spectrometer, with the chemical shifts
reported in ppm using tetramethylsilane (TMS) as an internal
Synthesis of 4,40,400-{[9,9-bis(hexyl)-9H-fluorene-2,4,7-triyl]tri-
2,1-ethenediyl}tris(N,N-diphenyl)benzeneamine (TF)
A
mixture of 2,4,7-tris[methylene(triphenylphosphonium
bromide)]-9,9-dihexylfluorene (4: 0.69 g, 0.49 mmol) and
4-(diphenylamino)benzaldehyde (5: 0.514 g, 1.83 mmol) in
15 mL N,N-dimethylformamide (DMF) was stirred at room
temperature under a nitrogen atmosphere. After complete
dissolution of the solids, it was added dropwise to an ethanol
Scheme 1 The synthetic procedures of TF.
c
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1220 New J. Chem., 2011, 35, 1219–1225

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solution of sodium ethoxide (21%) and stirred for 24 h. The
mixture was poured into a large amount of distilled water; the
appearing precipitates were collected by filtration and dried in
a vacuum oven. Then the crude products were further purified
by column chromatography on silica gel using n-hexane/ethyl
acetate (V/V = 3/1) as the eluent and crystallized from
cyclohexane/n-hexane to afford yellow solids of TF (yield:
58%). FT-IR (KBr pellet, cmꢀ1): n 696, 742, 958, 1172,
1280, 1328, 1490, 1585, 2856, 2925, 3027, 3058. 1H NMR
(CDCl3, 400 MHz): d 7.85–7.83 (d, J = 8 Hz, 1H), 7.78–7.74
(m, 1H), 7.59 (s, 1H), 7.52–7.30 (m, 8H), 7.30–7.26 (m, 14H),
7.17–7.00 (m, 28H), 2.02 (t, J = 8.0 Hz, 4H, –CH2–),
1.20–1.00 (m, 12H, –CH2–), 0.78–0.66 (m, 10H, –CH2– and
–CH3). Anal. calcd for C85H79N3: C, 89.35%; H, 6.97%; N,
3.68%. Found: C, 89.19%; H, 7.11%; N, 3.54%. FAB-MS: m/
z 1142 [M+] (calcd: 1141.63).
Fig. 1 1H-NMR spectrum of TF in CDCl3.
Fabrication of electroluminescent devices
Multilayer light-emitting diodes, with a structure of ITO/
PEDOT:PSS/TF/Alq3/LiF/Al or ITO/PEDOT:PSS/TF
+
its branched structure and rather high molecular weight
(1140). The thermal transition property of TF was evaluated
by differential scanning calorimetry (DSC); TF exhibited a
glass transition temperature (Tg) of 89 1C which is higher than
that of widely used hole-transporting material such as
4,40-bis(m-tolylphenylamino)biphenyl (TPD) (Tg = 60 1C).8
Organic materials with high Tg are necessary to suppress
morphology deformation and degradation under high driving
voltage in LEDs.
PF/LiF/Al, were fabricated for the investigation of optoelectronic
characteristics. The ITO-coated glasses were washed successively
in ultrasonic baths of neutral cleaner/de-ionized water mixture,
de-ionized water, acetone and 2-propanol, followed by treatment
in a UV-ozone chamber. A thick hole-injection layer of
PEDOT:PSS was spin-coated on top of the cleaned ITO glass
and annealed at 423 K for 900 s in a dust-free atmosphere. The
hole-transporting layer (HTL) was formed by spin-coating a
TF solution (15 mg mLꢀ1 in chlorobenzene) on top of the
PEDOT:PSS layer. Then the emitting Alq3 layer (70 nm) was
deposited onto the surface of the HTL layer by thermal
evaporation. Finally, the thin layer of LiF (0.5 nm) and
aluminium (100 nm) was deposited as the cathode by thermal
evaporation at about 1 ꢁ 10ꢀ6 Torr. The luminance versus
bias, current density versus bias, and emission spectral char-
acteristics of the devices were recorded using a combination of
a Keithley power source (model 2400) and an Ocean Optics
usb2000 fluorescence spectrophotometer. The fabrication of
the devices was done in ambient conditions, with the following
performance tests conducted in a glove-box filled with
nitrogen.
Optical and electrochemical properties
The absorption and photoluminescence (PL) spectra of TF in
solution and film states are shown in Fig. 2. In the solution
state, the absorption peak locates at 413 nm which can
be assigned to the p–p* transitions along the conjugated
backbone. Furthermore, the absorption peak of the TF thin
film is slightly shifted to 425 nm, suggesting the existence of
certain intermolecular interactions. The maximum absorption
peak of TF is red-shifted when compared with its linear
counterpart,21 which can be attributed to the presence of an
additional conjugated branch in TF. The PL spectral peaks of
Results and discussion
Synthesis and characterization
Scheme 1 illustrates the synthetic routes of a target material
TF by the well-known Wittig reaction. The chemical shifts of
vinylene and aromatic protons overlap around 7–7.8 ppm,
making the estimation of the ratio of protons in triphenyl
terminals over the fluorene core unsuccessful (Fig. 1). However,
the peak area ratio of aromatic and vinylene protons over
aliphatic protons is about 2.07, roughly coincident with the
real ratio (2.04) due to the presence of CHCl3. In combination
with the data of elemental analysis, the successful synthesis of
TF can be confirmed. The TF is soluble in common organic
solvents such as chloroform, THF, and chlorobenzene.
qMoreover, uniform thin films could be readily obtained by
using the solution spin-coating technique, probably owing to
Fig. 2 Absorption and photoluminescence spectra of TF in the film
state (lex: 425 nm) and the CHCl3 solution state (lex: 413 nm).
c
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(ꢀ5.0 eV) and Alq3 (ꢀ5.44 eV), forming a cascade of HOMO
levels to facilitate hole injection. Moreover, the barrier height
(DEh) for hole injection between PEDOT:PSS and TF is
0.12 eV, smaller than that between PEDOT:PSS and NPB
(DEh = 0.40 eV). This means that the hole-injection from
PEDOT:PSS to TF is much easy than from PEDOT:PSS to
NPB. Accordingly, the enhanced device efficiency can be expected
from the improvement in charges injection and transport.
Morphological properties
To achieve high performance in multilayer optoelectronic
devices, it is imperative for each deposited layer to be highly
homogeneous. Small organic molecules such as NPB are
widely used as hole-transporting materials in the fabrication
of OLEDs. However, the NPB layer must be deposited by
thermal evaporation under vacuum due to its poor film-
forming properties obtainable in solution processes such as
spin-coating. However, the TF demonstrated a uniform sur-
face morphology by spin-coating processes (7.5 mg TF in
0.5 mL chlorobenzene; 1000 rpm), probably attributed to its
branched structure and higher molecular weight. Moreover,
the root-mean-square (rms) roughness of the TF film surface
(1.02 nm) was much smaller than 1.51 nm of the NPB film
obtained by thermal deposition (at 2 ꢁ 10ꢀ6 Torr). The
morphological results indicate that TF can be deposited by
facile wet-processes to obtain a highly homogeneous film
required by OLEDs. This advantage in deposition processes
makes TF a promising hole-transporting material in opto-
electronic devices.
Fig. 3 Cyclic voltammograms of TF films coated on a glassy carbon
electrode; scan rate: 100 mV sꢀ1
.
TF in CHCl3 and in the film state locate at 474 and 506 nm,
respectively. The great red-shift in the film state (32 nm) is
attributable to the aggregate formed by the strong inter-
molecular interactions.
Cyclic voltammetry (CV) is an effective method in
investigating reduction–oxidation behaviors of thin films
insoluble in electrolyte solution. Because the TF readily
formed as a homogeneous film which was insoluble in aceto-
nitrile, it was deposited on a glassy carbon to be applied as the
working electrode for CV measurements. The cyclic
voltammograms obtained were employed to evaluate the
electrochemical properties of TF, such as HOMO and LUMO
energy levels (Fig. 3). The HOMO and LUMO levels can be
readily estimated from onset oxidation (Eox) and onset
Electroluminescent properties of multilayer OLEDs
Multilayer OLEDs having a structure of ITO/PEDOT:PSS/
HTL/Alq3/LiF(0.5 nm)/Al (100 nm) were fabricated to inves-
tigate their optoelectronic properties. The TF and Alq3 were
used as a hole-transporting layer (HTL) and an emitting layer
respectively, and a reference EL device using NPB as the HTL
was also fabricated to compare with a TF-based device. The
current density versus bias and brightness versus bias curves
(J–B–V) of the EL devices are shown in Fig. 5 with the
corresponding characteristic data summarized in Table 1.
The turn-on voltages (at 10 cd mꢀ2) of TF- and NPB-based
devices are 6.1 and 8.1 V, respectively. The lower turn-on
reduction potentials (Ered), using the equations EHOMO
=
ꢀ(Eox + 4.8) eV and ELUMO = ꢀ(Ered + 4.8) eV,
respectively, where Eox and Ered are the onset oxidation and
reduction potentials relative to the ferrocene/ferrocenium
couple (EFOC = 0.47 V versus Ag/AgCl electrode). The
HOMO and LUMO levels are estimated to be ꢀ5.12 eV and
ꢀ2.55 eV, respectively. Fig. 4a illustrates the energy levels of
TF, NPB and Alq3. Clearly the HOMO level of TF (ꢀ5.12 eV)
is higher than ꢀ5.40 eV of conventional NPB. Furthermore,
the HOMO level of TF lies between those of PEDOT:PSS
Fig. 4 Energy level diagrams: (a) TF, NPB and Alq3; (b) TF and PF.
c
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Fig. 5 Current density–luminance–voltage characteristic of the EL devices using TF or NPB as a hole-transporting layer. Device structure:
ITO/PEDOT:PSS/HTL(100 nm)/Alq3(70 nm)/LiF(0.5 nm)/Al(100 nm).
Table 1 Electroluminescent properties of the multilayer devicesa
HTL
Vonb/V
Lmaxc/cd mꢀ2
LEmaxd/cd Aꢀ1
Emissione (lmax, nm)
CIE coordinatesf (x, y)
TF
NPB
6.1
8.1
9390
7100
2.90
1.24
517
518
(0.29, 0.55)
(0.29, 0.53)
a
b
c
Device structure: ITO/PEDOT:PSS/HTL(100 nm)/Alq3(70 nm)/LiF(0.5 nm)/Al(100 nm). Turn-on voltage at 10 cd mꢀ2
.
Maximum
luminance. Maximum luminance efficiency. Electroluminescent spectra at ca. 400 cd mꢀ2
. .
The 1931 CIE coordinate at ca. 400 cd mꢀ2
d
e
f
voltage of the TF-based device is reasonably explained by the
lower hole barrier height (DEh = 0.12 eV) from PEDOT:PSS
to TF layers, compared to that from PEDOT:PSS to NPB
layers (Fig. 4a). The results suggest that hole injection from
PEDOT:PSS to TF is much easier than from PEDOT:PSS to
NPB. The maximum brightness and maximum current
efficiency of the TF-based device are 9390 cd mꢀ2 and
2.90 cd Aꢀ1, respectively, which are substantially higher than those
of the NPB-based device (7100 cd mꢀ2, 1.24 cd Aꢀ1). The EL
spectra of TF-based and NPB-based devices are similar and
show bright green emission with an emission peak at 518 nm,
which are coincident with the PL spectrum of Alq3.12 To
conclude, the performance of the TF-based device surpasses
that of the NPB-based one in turn-on voltage, brightness,
and current efficiency. This enhanced performance can be
explained by its uniform surface morphology (rms = 1.02 nm)
and reduced hole-injection barrier. Current results indicate
that TF is a promising hole-transporting material which can
be employed in fabricating optoelectronic devices by wet-
processes.
At first, the surface morphologies of the deposited films
were investigated by AFM. The root-mean-square (rms)
roughness of the blends spin-coated on the ITO glass was in
the range of 0.92–1.15 nm (for TF1–TF8). The small rms
roughness indicates that there occurred no obvious phase
separation. Therefore, we fabricated a series of EL devices
[ITO/PEDOT:PSS/EML(110 nm)/LiF(0.5 nm)/Ca(50 nm)/
Al(100 nm)] using blends of TF and PF as emitting layers.
The weight percents of TF in PF were 1, 2, 4, and 8 wt% for
TF1, TF2, TF4, and TF8 respectively. Fig. 6 shows the current
density and brightness versus bias curves (J–B–V) of the blend
devices (TF1–TF8), with the corresponding characteristic data
summarized in Table 2. The maximum brightness and maximum
current efficiency of the blend devices are 2020–4040 cd mꢀ2
and 0.67–1.62 cd Aꢀ1, respectively. The brightness–voltage
(B–V) curves shift to higher voltages with the increase of TF
weight percents, suggesting that the TF traps the charges in the
emitting layer. As shown in Fig. 4b, the HOMO level of TF
(ꢀ5.12 eV) is much higher than that of PF (ꢀ5.51 eV), while
their LUMO levels are very close (ꢀ2.55 eV, ꢀ2.61 eV).
Therefore, the TF acts mainly as a hole trapping site in the
emitting layer. The TF4-based device exhibits the best perfor-
mance, with the maximum brightness and maximum current
efficiency being 4040 cd mꢀ2 and 1.62 cd Aꢀ1, respectively.
This is attributable to enhanced recombination of electrons
and holes at TF which act as hole trapping sites. Fig. 7 shows
emission spectra of the electroluminescent (EL) devices, which
Electroluminescent properties of OLEDs using blends of TF and
PF as emitting layers
The TF is an efficient hole-transporting material applicable in
OLEDs as mentioned above. The hole-transporting role of TF
can also be extended to the emitting layer by simple blending.
c
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Fig. 7 Emission spectra of EL devices using TF + PF blends as
emitting layers. Device structure: ITO/PEDOT:PSS/TF + PF/LiF
(0.5 nm)/Ca(50 nm)/Al(100 nm).
Fig. 6 Current density–luminance–voltage characteristic of the blend
devices. Device structure: ITO/PEDOT:PSS/TF + PF(110 nm)/
LiF(0.5 nm)/Ca(50 nm)/Al(100 nm).
Table 2 Electroluminescent properties of the devices using blends of
TF and PF as emitting layersa
Fig. 8 Normalized UV/Vis absorption and photoluminescence spectra
of PF and TF in the film state.
Vonb/ Lmaxc/cd LEmaxd/cd Emissione
CIE co-
ordinatesf (x, y)
Devicea
V
mꢀ2
Aꢀ1
(lmax, nm)
TF1g
TF2
TF4
TF8
5.7 2940
6.2 3270
7.8 4040
8.8 2020
1.24
1.57
1.62
0.67
494
494
504
502
(0.15, 0.29)
(0.15, 0.29)
(0.17, 0.39)
(0.17, 0.37)
expected. Therefore, the TF is not only a suitable hole-
transporting material but also applicable as emitting material.
a
Device structure: ITO/PEDOT:PSS/TF + PF(110 nm)/LiF(0.5 nm)/
b
Conclusions
Ca(50 nm)/Al(100 nm). Turn-on voltage at 10 cd mꢀ2
c
. Maximum
d
luminance. Maximum luminance efficiency. Electroluminescent
e
We have successfully prepared a hole-transporting and emitting
4,40400-{[9,9-bis(hexyl)-9H-fluorene-2,4,7-triyl]tri-2,1-ethenediyl}-
tris(N,N-diphenyl)benzeneamine (TF) by the Wittig reaction.
The TF displayed a moderate glass transition temperature
(Tg = 89 1C) and uniform surface morphology (rms roughness =
1.02 nm) in the film obtained by spin-coating. The HOMO
level of TF (ꢀ5.12 eV), estimated from the onset oxidation
potential, is higher than the conventional NPB (ꢀ5.40 eV),
facilitating hole injection from PEDOT:PSS to TF layers.
The device using TF as a hole-transporting layer [ITO/
PEDOT:PSS/HTL/Alq3/LiF(0.5 nm)/Al(100 nm)] exhibited
maximum current efficiency and brightness (2.90 cd Aꢀ1 and
9390 cd mꢀ2), which are superior to those of the NBP-based
device (1.24 cd Aꢀ1 and 7100 cd mꢀ2). Moreover, the TF could
be blended with PF to be used as an emitting layer. The
emission was blue-green which originated mainly from TF,
with the best maximum brightness and maximum current
spectra ca. 400 cd mꢀ2
400 cd mꢀ2
f
. The 1931 CIE coordinate at ca.
g
.
The number represents weight percent of TF in the film.
are obviously different from that of PF (emission peak at
429 nm). The EL spectra of TF1–TF8 illustrate blue-green
emission with major emission peaks at ca. 492–504 nm which
resemble the PL spectrum of TF. Clearly TF is the main
emitting species in the blend devices. This is attributable to
hole trapping in TF and efficient energy transfer from poly-
fluorene segments to TF molecules. These two processes may
occur simultaneously under an external electric field. As
shown in Fig. 8, the absorption and emission peaks of the
TF film locate at 425 and 506 nm, whereas those of poly-
fluorene (PF) situate at 384 and 427 nm, respectively. The
absorption band of TF overlaps extensively with the emission
band of PF, so efficient energy transfer from PF to TF can be
c
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efficiency being 4040 cd mꢀ2 and 1.62 cd Aꢀ1 respectively. This
is attributed to charge recombination at TF and efficient
energy transfer from polyfluorene segments to TF. Current
results demonstrate that the TF is a promising optoelectronic
material which can be used in hole-transporting and emitting
layers. More importantly, it can be readily deposited by wet
processes.
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Acknowledgements
The authors thank the National Science Council of Taiwan
for the financial aid through project NSC 98-2221-E-006-
002-MY3
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