Novel distyrylcarbazole derivatives as hole-transporting blue emitters for electroluminescent devices

Article abstract of DOI:10.1039/b510035f

We have synthesized three novel distyrylcarbazole derivatives for use as simultaneously hole-transporting and light-emitting layers in blue light-emitting diodes. Each compound, which contains a rigid carbazole core and two 2,2-diphenylvinyl end groups su

Full text of DOI:10.1039/b510035f

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Novel distyrylcarbazole derivatives as hole-transporting blue emitters for
electroluminescent devices{
Fang-Iy Wu, Ping-I Shih, Mao-Chuan Yuan, Ajay Kumar Dixit, Ching-Fong Shu,* Zhu-Ming Chung and
Eric Wei-Guang Diau*
Received 14th July 2005, Accepted 13th September 2005
First published as an Advance Article on the web 30th September 2005
DOI: 10.1039/b510035f
We have synthesized three novel distyrylcarbazole derivatives for use as simultaneously
hole-transporting and light-emitting layers in blue light-emitting diodes. Each compound, which
contains a rigid carbazole core and two 2,2-diphenylvinyl end groups substituted through either
the 3,6- or the 2,7- position, forms films satisfactorily and exhibits a blue emission with its PL
maximum in the range 459–470 nm. Photophysical measurements indicate that twisting of the
adjacent C–C bonds in the 3,6-position of the carbazole core in dilute solutions causes an efficient
nonradiative relaxation to occur, yielding a much smaller quantum yield for fluorescence in
3,6-linked carbazoles. As intense emissions of 2,7-linked carbazoles are observed, such
deactivation from an excited-state is inefficient therein. Electrochemical studies revealed that
incorporation of the carbazole core increased the HOMO energy effectively; this feature facilitates
hole injection. These distyrylcarbazole derivatives are promising bifunctional, blue-emitting, hole-
transporting molecules for use in simple double-layer devices of a general structure ITO/emitting
layer/TPBI/Mg : Ag, in which TPBI—1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene—serves as a
hole-blocking and electron-transporting material. The devices prepared using 2,7-
distyrylcarbazole as the emitter produced bright blue emissions having activating voltages below
3.0 V. A DPVTCz-based device attained a luminance efficiency of 3.11 cd A21 at 5 V, a brightness
of 3062 cd m22, and CIE color coordinates of (0.14, 0.22).
most salient feature of DPVBi is that two nonplanar phenyl
Introduction
rings are located at each end of the molecule; this arrangement
Conjugated organic materials that function as both efficient
DPVBi layer and its neighboring charge-transporting layer.4c
light emitters and/or charge transporters have attracted much
prevents exciplex formation at the interface between the
interest because of their prospective applications in organic
Furthermore, distyrylarylene derivatives that incorporate
light-emitting diodes (OLED).1 Organic materials that emit
arylamine moieties can exhibit both intense fluorescence and
light and have large-energy band gaps—so that they emit
great mobility for the transport of holes. For instance, the hole
blue light efficiently—have, in particular, stimulated a wide
mobility of BCzVBi, a DSA derivative possessing carbazoryl
range of interest because they might be applicable as sources of
groups at both ends of the molecule, was determined to be as
large as 1023 cm2 V21 s21 (1–3 MV cm21) through measure-
blue light in full-color display applications or as hosts for
exothermic energy transfer to lower-energy fluorophores.2
(120 nm)/Mg : Ag.4a In a double-layer device with the
Such materials’ large band gaps lead to an inherent difficulty
ment of the transient behavior of the EL of ITO/BCzVBi
when attempting to inject charges into the blue emitters; this
phenomenon hampers the performance of devices fabricated
from these materials, especially for single- or double-layer
devices.3
configuration ITO/BCzVBi/PBD/Mg : Ag, a large power
efficiency (2.1 lm W21) was obtained at 5 V, with a brightness
of 135 cd m22 4b
As the emission of this BCzVBi-based device
.
occurred in the blue-green region, its use as a blue emitter is
restricted in OLED applications.
Among the various known blue emitters, the most promising
are distyrylarylene derivatives (DSA) because their neat solids
fluoresce intensely and because they form satisfactory films.4
Efficient organic diodes that emit blue and white light
have been prepared using blue-emitting 4,49-distyrylbiphenyl
Our objective in this study was to prepare distyryl
derivatives that display both hole-transporting ability and
highly efficient fluorescence in the blue region. Polymers and
small molecules containing carbazole units are often effective
hole-transporting materials because the nitrogen atom in
the carbazole ring bestows an electron-donating ability.6 We
designed three analogues of DPVBi in which we retained the
2,2-diphenylvinyl end groups and replaced the biphenyl core
with a carbazole ring, substituted through either the 3,6- or the
2,7- position, in an attempt to realize blue distyrylcarbazole
derivatives that might serve as both the hole-transporting layer
(HTL) and the emitting layer (EML).3b,4b,7 We examined the
derivatives—e.g.,
4,49-bis(2,2-diphenylvinyl)-1,19-biphenyl
(DPVBi)—that function as the electroluminescent layer.5 The
Department of Applied Chemistry, Institute of Molecular Science and
Center for Interdisciplinary Molecular Science, National Chiao Tung
University, Hsinchu, Taiwan, 30010
{ Electronic supplementary information (ESI) available: details of
experimental procedures and fabrication of light-emitting devices. See
DOI: 10.1039/b510035f
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thermal, optical, and electrochemical properties of these
distyrylcarbazole derivatives, and fabricated double-layer
devices to demonstrate the potential of these distyrylcarbazole
compounds as blue light-emitting and hole-transporting
materials for use in EL device applications.
Results and discussion
Synthesis
Scheme 1 illustrates the synthetic procedures used to prepare
the 2,7-linked carbazole derivatives. The key intermediate,
2,7-dibromo-9H-carbazole (1), was synthesized according to a
procedure described in the literature: nitration of 4,49-dibro-
mobiphenyl with concentrated nitric acid followed by a
reductive Cadogan ring-closure.8 N-Alkylation of 1 with
isopropyl bromide in NMP–K2CO3 gave 2. The copper-
catalyzed Ullman coupling of 1 with 4-iodotoluene yielded 3.
We prepared the borolane 4 through lithiation of 2,2-
diphenylvinyl bromide9 with excess n-BuLi and subsequent
treatment with butyl borate and hydrolysis in aqueous HCl to
give 2,2-diphenylvinyl boronic acid, which we then converted
to the boronic ester through its reaction with pinacol. The
Scheme 2
Photophysical properties
We measured the absorption and fluorescence spectra of the
three carbazole derivatives in dilute toluene solutions and as
neat films on a quartz substrate (Fig. 1); Table 1 summarizes
the corresponding spectral data. We observed no significant
difference between the absorption spectra of DPVICz
and DPVTCz in toluene; their absorption maxima occur at
376–378 nm. This finding indicates that the p-tolyl group in
DPVTCz does not contribute significantly to extending the
length of
p
conjugation.11 In contrast, the absorption
Pd-catalyzed Suzuki coupling of borolane
4 with the
maximum of 3,6-DPVTCz is located at 325 nm, i.e., it is
blue-shifted by ca. 50 nm with respect to that of DPVTCz. This
shift presumably originates from the disparate lengths of
conjugation that result from the varied substitution patterns of
2,2-diphenylvinyl groups on the carbazole cores. In 2,7-linked
carbazoles, conjugation can extend over the entire molecule,
whereas in the 3,6-linked carbazole there is little conjugation
over large distances.12
2,7-dibromocarbazole derivatives 2 and 3 afforded target
compounds DPVICz and DPVTCz, respectively. Similarly, we
prepared 3,6-DPVTCz through Suzuki coupling of 4 with the
3,6-dibromocarbazole compound10 5 (Scheme 2). DPVICz,
1
DPVTCz, and 3,6-DPVTCz, which we characterized using H
and 13C NMR spectroscopy, were sublimable in a vacuum
chamber without their thermal decomposition. High-resolu-
tion mass spectrometric and elemental analyses provided
additional verification of the proposed structures.
Fig. 1 UV-Vis absorption and PL spectra of (a) DPVICz, (b)
DPVTCz, and (c) 3,6-DPVTCz in dilute toluene solutions (solid lines)
and in the solid state (dashed lines).
Scheme 1
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angle (d) of ca. 37–38u for both molecules in their S0 states.
In Fig. 2, we display a comparison of the corresponding
HOMO-1, HOMO, and LUMO of the carbazole derivatives
optimized in the S0 state. Because electronic excitation from
the HOMO to the LUMO produces the first singlet excited
state (S1), the orbital features presented in Fig. 2 might provide
important clues toward understanding the distinct photo-
physical properties of the two types of carbazole substituents.
For the LUMO, the orbitals in the carbazole core are
almost identical for both molecules, but an extension of the
p-conjugation over the two dimethylvinyl groups occurs
only for the substituents in the 2,7-positions, i.e., not in the
3,6-positions, which makes the bridged C–C bonds in the 2,7-
linked carbazole display double-bond character. For the
HOMO, p-conjugation of the orbitals is extended effectively
to the dimethylvinyl groups in both the 2,7- and 3,6-linked
carbazoles, but the bridged C–C bonds retain their single-bond
character in both cases. Thus, we expected that a feasible
twisting motion would occur in the ground-state surface for
both molecules. According to the LUMO presented in Fig. 2,
we expect that rotation about the bridged C–C bonds is more
difficult to perform in the 2,7-linked carbazole than it is in the
3,6-linked carbazole in the S1 state.
Table 1 Optical properties of distyrylcarbazole derivatives
Absorption,
Photoluminescence,c
lmax/nm
lmax/nm
Quantum
Solution
yields (Wf)a,b (log e)a
Filmd Solutiona Filmd
DPVICz
DPVTCz
3,6-DPVTCz 0.01
a
Measured in toluene. The relative quantum yield was measured in
toluene with reference to that of 9,10-diphenylanthracene in
cyclohexane. The photoluminescence maximum was recorded upon
irradiation at the absorption maximum. Prepared by spin-coating
from their CHCl3 solutions.
0.37
0.51
378 (4.71) 378
376 (4.71) 378
325 (4.70) 327
457
456
456
470
469
459
b
c
d
Both 2,7-disubstituted carbazoles display intense photo-
luminescence in the blue region, with their respective maxima
occurring at ca. 460 nm. The fluorescence quantum yields (Wf)
of DPVICz and DPVTCz in dilute toluene solution were 0.37
and 0.51, respectively; they are significantly greater than that
of DPVBi (Wf = 0.25) when measured under the same
conditions with reference to 9,10-diphenylanthracene in
cyclohexane (Wf = 0.90).13 In contrast, 3,6-DPVTCz exhibits
a weak emission in toluene solution, with the maximum
intensity occurring at ca. 460 nm; its fluorescence quantum
yield is only ca. 0.01. The observed disparate magnitudes of
Wf —a factor of ca. 50—between 3,6-DPVTCz and DPVTCz
indicates the existence of an efficient channel for nonradiative
relaxation that is activated in the 3,6-linked carbazole but
inhibited in the 2,7-linked carbazole. Our control experiments
indicate that the carbazole core itself displays an intense
emission in dilute solution. Moreover, we observed an
enhanced emission of the 3,6-linked carbazole in a 3,6-
DPVTCz/PMMA solid film when the nuclear motions of the
molecule were substantially restricted in a PMMA matrix.
Thus, we obtained strong evidence supporting the notion that
deactivation of a carbazole derivative in its excited state is
efficient when a key nuclear motion is activated for the system
possessing p-conjugated substituents in the 3,6-positions, but
not in the 2,7-positions.
Rotation about the bridged C–C bonds toward a twisted
geometry would involve an energy barrier for both model
systems because of the lack of extended p-conjugation in the
LUMO for the 2,7-linked carbazole and in the HOMOs of
both molecules. To confirm this hypothesis, we performed
geometry optimization calculations at the CIS/6-31G(d) level
of theory in the S1 state. We found that the S1 minimum (min)
of the 2,7-linked carbazole is located at d = ca. 28u, whereas the
S1 (min) of the 3,6-linked carbazole has an asymmetric
geometry with the two bridged C–C bonds being twisted by
35 and 49u, respectively. Along the single C–C bond twist
reaction coordinate (RC), the corresponding transition struc-
tures are located at the perpendicular geometry (the single C–C
bond twist angle is 90u); note that d = ca. 37–38u at the
Franck–Condon (FC) geometry in both cases. To obtain
the energy levels in an effort to improve our understanding of
the interactions between excited-state surfaces, we performed
single-point energy calculations at the TD B3LYP/6-31+G(d)
level of theory for three key geometries: the FC, the S1 (min),
We used G03 software to perform quantum chemical
calculations of the excited states of model systems14 in an
attempt to rationalize the observed large discrepancy in the
values of Wf between the 3,6- and 2,7-linked carbazoles. Our
preliminary calculations indicate that the phenyl groups are
twisted substantially with respect to the carbazole ring.
Therefore, we believe that the interactions between the phenyl
groups and the carbazole center are weak. To diminish the
computational cost and to allow a systematic comparison, we
replaced all of the phenyl groups with saturated hydrocarbon
units to serve as model systems in these calculations. As a
result, the model systems contain one ethyl group connected to
the central N atom and two dimethylvinyl groups connected
to the carbazole core in either the 2,7- or 3,6-position. The
geometries of both the 2,7-linked and 3,6-linked carbazoles in
the ground state (S0) were optimized based on density
functional theory (DFT) calculations at the B3LYP/6-31G(d)
level. The optimized geometries of the 2,7- and 3,6-linked
species are basically nonplanar with the two bridged C–C
bonds being twisted substantially through a common torsion
Fig. 2 Calculated HOMO-1, HOMOs, and LUMOs of dimethyl-
vinylcarbazole derivatives; the geometries were optimized at the
B3LYP/6-31G(d) level of theory.
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and the perpendicular species (P1 or P2). According to our
calculations, efficient nonradiative electronic relaxation might
occur through twisting of the bridged C–C bond(s) along
either the single C–C twist RC (via P1 species) or the concerted
C–C twist RC (via P2 species) leading to a much smaller value
of Wf being observed for the 3,6-linked carbazole than for the
2,7-linked carbazole.15 The larger Stokes shift observed for
3,6-DPVTCz than for DPVTCz (Fig. 1) is consistent with our
calculations that the relative energy between the FC geometry
(S3 for the 3,6-linked carbazole; S2 for the 2,7-linked
carbazole) and the S1 minimum (given the maximum emission
intensity) of the 3,6-linked carbazole (ca. 0.5 eV) is larger than
that of the 2,7-linked carbazole (ca. 0.35 eV).
Table 2 Thermal properties of distyrylcarbazole derivatives
DSCa TGAb
5% weight 10% weight
Tg (uC) Tc (uC) Tm (uC) loss
loss
DPVICz
DPVTCz
80
92
121
142
n.a.
193
244
n.a.
351
380
376
368
395
394
3,6-DPVTCz 100
a
Measured under nitrogen at heating rates of 20 uC min21 and
cooling rates of 40 uC min21. n.a.: not available. Measured under
b
nitrogen at a heating rate of 20 uC min21
.
temperature range 30–320 uC. DPVICz and DPVTCz exhibit
distinct glass transition temperatures (Tg) at 80 and 92 uC,
respectively, followed by broad crystallization features at 121
and 142 uC, respectively, and well-defined melting points at
193 and 244 uC, respectively. For 3,6-DPVTCz, we observed a
glass transition at 100 uC, but no crystallization or melting of
the sample at temperatures up to 320 uC. This result might
reflect the different symmetry in the molecular structure:6c the
2,7-linked carbazoles have a rod-like architecture, whereas the
3,6-linked carbazole has a L-shaped structure, which might
hinder close packing and diminish its tendency to crystallize.
For comparison with these carbazole-based compounds, the
related analogue DPVBi has a glass transition at 64 uC prior to
crystallizing at 106 uC.17 Consequently, these distyrylcarbazole
derivatives form more-stable amorphous glasses than does
DPVBi and are, therefore, more promising—in terms of their
thermal stability—for their application into OLEDs. We
attribute the enhanced morphological stability of the carba-
zole-based materials to the presence of their rigid carbazole
linkages.6c These distyrylcarbazole derivatives also exhibit
satisfactory thermochemical stability; as evident from the
According to these quantum chemical calculations, we
provide a schematic illustration (Scheme 3) to rationalize the
observed discrepancy in Wf between the 2,7- and 3,6-linked
carbazoles. Because the singlet p,p* excited state is zwitterionic
in nature,16 its positive and negative charges are completely
separable through electronic resonance in the 2,7-linked
molecule, whereas charge separation is localized in only the
carbazole core of the 3,6-linked molecule (Scheme 3). As a
result, electron redistribution in the 2,7-linked carbazole causes
the original flexible C–C single bond between the diphenyl-
vinyl moiety and the carbazole core to become a rigid bridge,
having double-bond character, because rotation about the
C–C double-bond axis becomes unfavorable in the 1(p,p*)
excited state. Localization of the p-conjugation in the
carbazole core of the 3,6-linked molecule in the 1(p,p*) state
does not affect its feasible C–C single bond twisting motion,
which, thus, enables efficient nonradiative relaxation. Hence,
we conclude that localization of the p conjugation and the
bridged C–C bond twisting motion in the excited-state surface
are responsible for the much smaller values of Wf observed for
the 3,6-linked carbazole.
thermogravimetric analyses performed under
a nitrogen
The absorption spectra of thin films prepared through spin-
coating of the carbazole derivatives from chloroform solutions
onto quartz plates are almost identical to those obtained in
dilute solutions (see the dashed curves in Fig. 1). The emission
spectra of the 2,7-linked carbazoles in thin films were red-
shifted by ca. 10 nm, but the spectrum of the 3,6-linked
carbazole is similar to that obtained in solution, reflecting the
fact that molecular packing of the 2,7- and 3,6-linked
carbazoles is different in their respective solid states.
atmosphere, their 5%-weight-loss temperatures reach as high
as 350 uC.
Electrochemistry
We performed cyclic voltammetry (CV) measurements of the
carbazole-containing compounds near 295 K using ferrocene
as an internal standard. Cyclic voltammetry is a simple method
Thermal properties
We investigated the thermal properties of these distyryl-
carbazole derivatives using differential scanning calorimetry
(DSC) and thermogravimetric analysis (TGA); Table 2 pre-
sents the results. Fig. 3 displays DSC curves recorded over a
Fig. 3 DSC traces of distyrylcarbazole derivatives, recorded at a
heating rate of 20 uC min21
.
Scheme 3
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Fig. 5 Energy levels of materials used to prepare the double-layer
devices.
Electroluminescence properties
To evaluate the utility of distyrylcarbazole derivatives as
simultaneously emitting and hole-transporting layers, we
fabricated double-layer EL devices with the configuration
ITO/EML/TPBI/Mg : Ag. The emitting layer was deposited on
ITO glass and a layer of TPBI was then deposited to function
as the electron-transporting and hole-blocking layer. We co-
evaporated a Mg–Ag alloy as the cathode, which we then
protected with an additional layer of Ag. For comparison, we
also constructed a control device using DPVBi as the emitting
layer. Fig. 5 provides the energy levels of the materials used for
the double-layer devices. According to this diagram, DPVICz,
DPVTCz, and 3,6-DPVTCz have ionization energies of 5.38,
5.42, and 5.28 eV, respectively; these values are considerably
smaller than that of DPVBi (5.59 eV, obtained through CV
measurement), which indicates that holes can be injected from
the ITO electrode (WITO = 4.8 eV) into the HOMO levels of the
distyrylcarbazole derivatives more readily than into the
HOMO of DPVBi. In contrast, the LUMO level of the TPBI
layer is 2.7 eV;19 thus, electrons can readily cross the internal
heterojunction into the LUMO levels of the emitters by
conquering a small energy barrier of 0.3–0.5 eV (Fig. 5). Fig. 6
displays plots of current density vs. applied voltage for the
devices based on these distyryl derivatives. The I–V curve of
the DPVBi-based device clearly differs from those of the other
Fig. 4 Cyclic voltammograms of (a) DPVICz, (b) DPVTCz, and (c)
3,6-DPVTCz.
that allows the HOMO/LUMO energy levels of these materials
to be determined; in this case, we calculated them with regard
to the energy level of the ferrocene reference (4.8 eV below the
vacuum level).18 Fig. 4 displays cyclic voltammograms of these
carbazole derivatives. Both of the 2,7-distyrylcarbazoles
exhibit irreversible redox behavior. The onset potentials for
the oxidations of DPVICz and DPVTCz are 0.58 and 0.62 V,
respectively, and those for the reductions are 22.44 and
22.41 V, respectively, vs. Fc/Fc+. For 3,6-DPVTCz, we
observed a reversible oxidation at 0.55 V, with the onset
potential at 0.48 V, and an irreversible reduction with its onset
potential at 22.64 V. Similar phenomena have been reported
previously for the 2,7- and 3,6-linked carbazole trimers in their
cyclic voltammograms, with the reversible oxidation process
occurring only for the latter.12 Hence, protection of the 3 and 6
positions through substitution prevents C–C coupling at these
positions; such chemical reactions are the cause of irreversible
oxidation.11,12 Table 3 summarizes electrochemical properties
of the three carbazole derivatives. From the electrochemistry
data we estimate that the band gaps of DPVICz, DPVTCz,
and 3,6-DPVTCz are 3.02, 3.03, and 3.12 eV, respectively;
these values agree satisfactorily with the optical band gaps
calculated from the absorption edges of the corresponding
UV-Vis spectra. Again, the larger band gap of 3,6-DPVTCz is
related to its smaller conjugation length.
Table 3 Electrochemical properties of distyrylcarbazole derivatives
Eegl/ Eogpt
eVd eVe
/
ox
onset
red
onset
E
/
E
/
HOMO/ LUMO/
eVb
Va
Va
eVc
DPVICz
DPVTCz
0.58
0.62
22.44 25.38
22.41 25.42
22.64 25.28
22.36
22.39
22.16
3.02 2.90
3.03 2.92
3.12 3.12
3,6-DPVTCz 0.48
a
Potential values are listed vs. Fc/Fc+. Determined from the onset
oxidation potential. Determined from the onset reduction potential.
b
c
d
Electrochemical band gap, estimated with a relation Eegl = E
2
ox
onset
e
red
onset
E
the UV-Vis spectrum.
.
Optical band gap, calculated from the absorption edge of
Fig. 6 Current density-voltage (I–V) curves of devices with the
configuration ITO/EML/TPBI/Mg : Ag.
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Fig. 7 EL spectra of devices having the configuration ITO/EML/
Fig. 8 Luminescence–voltage (L–V) curves of devices having the
TPBI/Mg : Ag, recorded at an applied voltage of 7 V.
configuration ITO/EML/TPBI/Mg : Ag.
three, and it is shifted to a greater voltage. We ascribe this
increased operating voltage for the DPVBi-based device to its
larger injection barrier for holes; the similar I–V characteristics
that we observe among the three distyrylcarbazole compounds
reflect their similar HOMO levels. Fig. 7 displays EL spectra of
devices based on the distyrylcarbazole derivatives and DPVBi.
These devices exhibit blue emissions that nearly coincide with
their corresponding fluorescence spectra in the solid state;
this observation implies that only the emitting layer con-
tributes to the light emission and that no exciplexes form at the
EML/TPBI interface. Moreover, the appearance of the EL
spectra of each device that we investigated is independent of
the applied voltage.
In this bilayer device geometry, blocked holes accumulate at
the internal heterojunction, which results in a positive space–
charge interfacial zone. This accumulated positive charge in
turn enhances electron injection to neutralize the accumulated
holes, which improves the charge recombination.20 With the
facilitation of both hole and electron injections, the distyryl-
carbazole-based devices become activated at potentials
below 4.0 V and emit brighter light and have more efficient
performance than does the DPVBi-based device (cf. Fig. 8 and
9). We obtained the maximum external quantum efficiency
(1.94%) at 5.0 V with a brightness of 3062 cd m22 for the
device based on DPVTCz; for the device based on DPVICz,
these values were 0.92%, 6.0 V, and 3059 cd m22, respectively.
Because of its small quantum yield for fluorescence, the
maximum external quantum efficiency of the 3,6-DPVTCz-
based device was only 0.11%; this performance occurred at an
applied voltage 6.0 V and was accompanied by a brightness
of 80 cd m22. For the DPVBi-based device, the maximum
external quantum efficiency was only 0.03% at 9.5 V and it
decreased rapidly upon increasing the current (Fig. 9). From
our investigations of these EL devices, we find that the
incorporation of the p-type carbazole core, rather than a
biphenyl one, can significantly decrease the operating voltage
and improve the device performance in a low-voltage region.
Furthermore, taking the DPVTCz-based device as an example,
the maximum power efficiency was 2.18 lm W21, which
occurred at 4.0 V, with a brightness of 708 cd m22. This power
Fig. 9 Plots of luminance efficiency vs. current density for the distyryl
derivative-based devices.
efficiency is comparable to that of other multi-layer devices
that exhibit efficient blue fluorescence.21 Furthermore, the
DPVTCz-based device can attain a brightness greater than
104 cd m22 at a potential below 7.5 V. Table 4 summarizes
the characteristics of the devices based on the four DPVBi
analogues.
Conclusions
We have synthesized novel distyrylcarbazole derivatives, each
of which contains part of the well-known DPVBi functionality,
but with the biphenyl core replaced by a carbazole ring; we
characterized their photophysical and electrochemical pro-
perties with respect to their functions in OLEDs. In dilute
solutions the 2,7-linked carbazole exhibits an intense emission
(Wf = 0.4–0.5), whereas the 3,6-linked carbazole displays a
weak emission (Wf = ca. 0.01). According to our theoretical
calculations, ISC deactivation in the 3,6-linked carbazole is
activated efficiently through the twisting motion of the bridged
C–C single bond(s), whereas this electronic relaxation is
inefficient in the 2,7-linked carbazole because the extensive
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Table 4 Performance of ITO/EML/TPBI/Mg : Ag devices
EML
DPVBi
DPVICz
DPVTCz
3,6-DPVTCz
Activating voltage/Va
6.5
7.5 (9.0)
5 (31)
0.02 (0.03)
0.02 (0.03)
80 (@ 11.5 V)
0.03
0.03
457
67
0.15 and 0.13
3.0
2.5
3.6
Voltage/Vb
4.2 (5.1)
171 (1486)
0.79 (1.48)
0.48 (0.90)
9012 (@ 10.5 V)
1.52
0.92
470
70
0.15 and 0.22
3.8 (5.0)
475 (3109)
2.16 (3.11)
1.35 (1.93)
11134 (@ 9.0 V)
3.11
1.94
470
71
0.14 and 0.22
c
5.0 (6.2)
19 (102)
0.09 (0.10)
0.10 (0.11)
511 (@ 10.5 V)
0.10
0.11
449
67
0.15 and 0.11
Brightness/cd m22 b
Luminance efficiency/cd A21 b
External quantum efficiency (%)
Maximum brightness/cd m22
b
Maximum luminance efficiency/cd A21
Maximum external quantum efficiency (%)
EL maximum/nmc
FWHM/nmc
CIE coordinates,c x and y
a
b
Recorded at 1 cd m22
.
Recorded at 20 mA cm22; the data in parentheses were recorded at 100 mA cm22
. Recorded at 7 V.
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degree of p-conjugation of the orbitals provided the bridged
C–C bond with double-bond character. These compounds
form satisfactory films that possess the dual functions of
emitting blue light and transporting holes. Double-layer
devices prepared using 2,7-distyrylcarbazole derivatives—as
the simultaneously emitting and hole-transporting layer—in
combination with TPBI—as the hole-blocking and electron-
transporting material—produced bright blue emissions and
had activating voltages below 3.0 V. The luminance efficiency
of the DPVTCz-based device reached 3.11 cd A21 at 5 V, a
brightness of 3062 cd m22, and CIE color coordinates of (0.14,
0.22). In contrast, the device based on pristine DPVBi displayed
poor performance and required a larger operating voltage.
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Acknowledgements
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We thank the National Science Council for financial support.
Our special thanks go to Professor C.-H. Cheng, Dr J.-P.
Duan, and Dr H.-T. Shih for their support and cooperation
during the preparation and characterization of the light-
emitting devices.
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