Highly efficient deep-blue emitting organic light emitting diode based on the multifunctional fluorescent molecule comprising covalently bonded carbazole and anthracene moieties

Article abstract of DOI:10.1039/c1jm11111f

High performance deep-blue organic light-emitting diodes (OLEDs) have been investigated using new multifunctional blue emitting materials 3-(anthracen-9-yl)-9-ethyl-9H-carbazole (AC), 3,6-di(anthracen-9-yl)-9-ethyl-9H- carbazole (DAC), 3-(anthracen-9-yl-)-9-phenyl-9H-carbazole (P-AC), and 3,6-di(anthracen-9-yl)-9-phenyl-9H-carbazole (P-DAC) which comprise covalently bonded carbazole and anthracene moieties. We also have investigated the thermal, electrochemical, and morphological stability to find suitable molecular structure, consisting of carbazole and anthracene moieties. The non-doping deep-blue OLEDs using P-DAC, which showed the highest thermal, electrochemical, and morphological stability, proved the highest luminance efficiency and external quantum efficiency of 3.14 cd A^-1 and 2.75%, with the Commission Internationale de l'Eclairage (CIE) chromaticity coordinates (0.162, 0.136) at 100 mA cm^-2. Moreover, the doping devices using P-DAC as the host material showed blue emission, and the high luminance efficiencies and external quantum efficiencies of as high as 7.70 cd A^-1 and 4.86% with CIE chromaticity coordinates (0.156, 0.136) and (0.156, 0.217) at 100 mA cm^-2. Both the non-doping and doping devices using P-DAC uniquely exhibited high operational stability with virtually negligible efficiency roll-off over the broad current density range.

Full text of DOI:10.1039/c1jm11111f

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PAPER
Highly efficient deep-blue emitting organic light emitting diode based on the
multifunctional fluorescent molecule comprising covalently bonded carbazole
and anthracene moieties†
*
Se Hun Kim, Illhun Cho, Mun Ki Sim, Sanghyuk Park and Soo Young Park
Received 15th March 2011, Accepted 21st April 2011
DOI: 10.1039/c1jm11111f
High performance deep-blue organic light-emitting diodes (OLEDs) have been investigated using new
multifunctional blue emitting materials 3-(anthracen-9-yl)-9-ethyl-9H-carbazole (AC), 3,6-di
(anthracen-9-yl)-9-ethyl-9H-carbazole (DAC), 3-(anthracen-9-yl-)-9-phenyl-9H-carbazole (P-AC), and
3,6-di(anthracen-9-yl)-9-phenyl-9H-carbazole (P-DAC) which comprise covalently bonded carbazole
and anthracene moieties. We also have investigated the thermal, electrochemical, and morphological
stability to find suitable molecular structure, consisting of carbazole and anthracene moieties. The non-
doping deep-blue OLEDs using P-DAC, which showed the highest thermal, electrochemical, and
morphological stability, proved the highest luminance efficiency and external quantum efficiency of
3.14 cd Aꢀ1 and 2.75%, with the Commission Internationale de l’Eclairage (CIE) chromaticity
coordinates (0.162, 0.136) at 100 mA cmꢀ2. Moreover, the doping devices using P-DAC as the host
material showed blue emission, and the high luminance efficiencies and external quantum efficiencies of
as high as 7.70 cd Aꢀ1 and 4.86% with CIE chromaticity coordinates (0.156, 0.136) and (0.156, 0.217) at
100 mA cmꢀ2. Both the non-doping and doping devices using P-DAC uniquely exhibited high
operational stability with virtually negligible efficiency roll-off over the broad current density range.
amorphous film morphology are considered as important
Introduction
requirements in blue emitting materials.5 In addition, to obtain
excellent color purity, blue emitting materials should have
molecular structure not to form excimer which brings about
broad and red-shifted electroluminescent (EL) emission.6
It is well-known that a host–dopant emitter system can
significantly improve OLED performance in terms of the effi-
ciency, color purity, and device life-time.7 Various host materials
for blue emitting OLED have been reported to date, which
include anthracene,4,8 strylarylene,9 fluorene,10 quinoline,11 and
pyrene12 derivatives. In spite of their successful performance in
a given aspect, satisfactory blue host materials are still rare which
balance all the aspects of efficiency, color purity, and device life-
time.
Organic light-emitting diodes (OLEDs) have progressed into the
most desirable next generation flat-panel display and solid state
lighting devices over the past few decades after the pioneering
work of Tang and Van Slyke in 1987.1 Although OLEDs have
achieved remarkable development to date, there remain several
important problems to be solved currently. One of them is the
inferior performance of blue OLEDs to those of green and red
ones.2,3
Blue emitting materials intrinsically possess high band gap
energy, which results in the inferior OLED performance via
limited carrier injection demanding higher driving voltage for
appropriate operation.4 Due to the higher voltage applied for
effective carrier injection, the more Joule heat is also generated
during device operation. This Joule heat, in turn, brings about
the degradation and recrystallization of the blue emitting mate-
rials. As a result, color purity and device life-time are seriously
reduced. To address this problem, high thermal stability and
In this paper, we intended to report the multifunctional deep-
blue emitting host materials having carbazole and anthracene
moieties to achieve high efficiency, color purity, and long device
life-time all together in blue OLEDs. Carbazole-based
compounds have been well known for their excellent hole
transporting ability, luminescence efficiency, thermal stability,
versatile structural derivatization, and unique ability to form
amorphous film.13 Anthracene is more promising towards blue-
emitting OLED in terms of the excellent luminescence efficiency,
thermal and electrochemical stability, and ease of structural
modification.8a,8c,14 Unfortunately, however, it suffers from the
unfavorable crystallization and excimer formation in the film
Center for Supramolecular Optoelectronic Materials and WCU Hybrid
Materials Program, Department of Materials Science and Engineering,
Seoul National University, ENG 445, Seoul, 151-744, Korea. E-mail:
parksy@snu.ac.kr; Fax: +82 2 886 8331; Tel: +82 2 880 8327
† Electronic supplementary information (ESI) available: Synthesis and
characteristics of deep-blue emitting molecules and charge carrier
properties of P-DAC. See DOI: 10.1039/c1jm11111f
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state.4,6a It was rationally considered, therefore, that the covalent
bonding of carbazole and anthracene may give a multifunctional
molecule showing synergetic effect towards blue-emitting OLED
application.
N-alkylated or N-arylated to give 9-ethyl-9H-carbazole (1) and
9-phenyl-9H-carbazole (6), which were subsequently brominated
with N-bromosuccinimide (NBS) to generate the compounds 3-
bromo-9-ethyl-9H-carbazole
(2),
3,6-dibromo-9-ethyl-9H-
Herein, we report on the synthesis and OLED performances of
a novel series of new deep-blue emitting materials consisting of
covalently bonded carbazole and anthracene moieties; i.e. 3-
(anthracen-9-yl)-9-ethyl-9H-carbazole) (AC), 3,6-di(anthracen-
9-yl)-9-ethyl-9H-carbazole (DAC), 3-(anthracen-9-yl-)-9-phenyl-
9H-carbazole (P-AC), and 3,6-di(anthracen-9-yl)-9-phenyl-9H-
carbazole (P-DAC).
carbazole (4), 3-bromo-9-phenyl-9H-carbazole (7), and 3,6-
dibromo-9-phenyl-9H-carbazole (9). Then, these bromo-
compounds were transformed into boronic esters via lithiation
with n-butyllithium followed by reaction with 2-isopropoxy-
4,4,5,5-tetramethyl-1,3,2-dioxaborolane. Finally, AC, DAC, P-
AC, and P-DAC were obtained by the Suzuki–Miyaura coupling
reactions of boronic esters (3, 5, 8, 10) with 9-bromoanthracene.
All the compounds were purified either by the silica column
chromatography or recrystallization in solvent. The chemical
structure and purity of the intermediates and final compounds
were characterized by spectroscopic methods and elemental
analysis (see Experimental section for details).
Results and discussion
Scheme 1 illustrates the synthetic routes to a series of deep-blue
emitting materials AC, DAC, P-AC, and P-DAC. As shown in
the scheme, these compounds were all synthesized through four
step chemical reactions starting from carbazole. Carbazole was
Fig. 1 shows three-dimensional optimized geometry, the
calculated highest occupied molecular orbital (HOMO) and
Scheme 1 Synthetic routes to AC, DAC, P-AC, and P-DAC.
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lowest unoccupied molecular orbital (LUMO) electron density
maps, thermogravimetric analysis (TGA) trace, absorption and
photoluminescent (PL) spectrum, repeated cyclic voltammo-
grams, and scanning electron microscope (SEM) image of
P-DAC (see the ESI† for those of AC, DAC, and P-AC).
Theoretical molecular orbital calculation was carried out
using Gaussian03 at B3LYP/6-31g(dp)15 level, to characterize
optimized geometries, orbital energy, and electron densities of
the HOMO and LUMO states of each molecule. As shown in
Fig. 1(a), all molecules, including P-DAC (see Fig. S1 in the
ESI† for all molecules) have non-coplanar twisted conforma-
tion due to the large torsional angle between carbazole and
anthracene moieties. Such a unique structural feature is bene-
ficial in OLED application by preventing the excessive inter-
molecular interaction against fluorescence quenching and also
by suppressing the crystallization problem to establish the
uniform amorphous morphology in the film state of OLED
device.
Fig. 1 (a) Three-dimensional optimized geometry of P-DAC. (b) The calculated HOMO and LUMO electron density maps of P-DAC. (c) TGA trace of
P-DAC measured at a scan rate of 10 ꢁC minꢀ1 under N2. (d) Absorption and PL spectrum of P-DAC. (e) Repeated cyclic voltammograms of P-DAC. (f)
SEM image of P-DAC in vacuum deposited film on Si wafer. The scale bar represents 10 mm and image is magnified by a factor of 3000.
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Table 1 Physical properties of AC, DAC, P-AC, and P-DAC (ND: not detected)
AbPLQYe (%)
HOMO/LUMOf/eV
a
Compound
Tg/Tm/Td /ꢁC
labsb/nm
lemc/nm
lemd/nm
AC
ND/ND/300
ND/ND/395
89/196/326
351,368,387
351,368,388
348,368,387
351,368,388
443
441
435
431
456
464
451
460
45
44
51
40
ꢀ5.40/ꢀ2.36
ꢀ5.41/ꢀ2.37
ꢀ5.47/ꢀ2.42
ꢀ5.50/ꢀ2.45
DAC
P-AC
P-DAC
ND/ND/419
a
Tg determined by DSC with a heating rate of 10 ꢁC minꢀ1 under N2, Tm determined by DSC with a heating rate of 10 ꢁC minꢀ1 under N2, Td obtained
with 5% mass loss by TGA with a heating rate of 10 ꢁC minꢀ1 under N2. b Maximum absorption wavelength, measured in MC (10ꢀ5 M). c Maximum
emission wavelength, measured in MC (10ꢀ5 M). d Maximum emission wavelength, measured in film state (films prepared on quartz plate by the thermal
evaporation method). e AbPLQY values in film state were measured using the integrating sphere method. f HOMO energy level was measured by CV.
LUMO energy level was determined from the HOMO level and the optical band gap.
Fig. 1(b) shows the calculated HOMO and LUMO electron
density maps of P-DAC (see Fig. S2 in the ESI† for all the
molecules). It was peculiarly noted that HOMO and LUMO
electron densities of all the molecules are localized only at the
anthracene moiety. Thus, it was expected that the absorption and
emission properties of the multifunctional molecules are
controlled mostly by p–p* transition of anthracene moiety,
which means that the excellent luminescence efficiency of
anthracene moiety is maintained, while the crystallization and
aggregation problems are effectively suppressed.
to have the onset decomposition temperatures (Td), defined for
the 5% mass loss temperature by TGA, at 300, 395, 326, and 419
ꢁC, respectively. DSC thermograms of these molecules barely
showed glass transition temperatures (Tg) and melting temper-
atures (Tm) most likely due to their smaller heat capacity and
crystallinity. However, P-AC exhibited a somewhat noticeable Tg
ꢁ
ꢁ
at 89 C and Tm at 196 C.
Fig. 1(d) shows absorption and PL spectrum of P-DAC in
CH2Cl2 (MC) solution and also the PL spectra of thermally
evaporated film (see Fig. S4 in the ESI† for all other molecules).
The precise photophysical data are summarized in Table 1. In
MC solution state, all the multifunctional molecules showed the
characteristic vibrational patterns of isolated anthracene moiety
in the absorption spectra, while they were rather smooth and
The thermal properties of these multifunctional molecules
were characterized by differential scanning calorimetry (DSC)
and TGA in a N2 atmosphere (Table 1, Fig. 1(c), and see Fig. S3
in the ESI†). All the molecules showed good thermal stability as
Fig. 2 (a) Structures of non-doped devices and energy levels of materials. (b) Molecular structures of materials. (c) EL spectrum and emission
photograph of non-doped devices at 10 mA cmꢀ2
.
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carbazole activates the carbazole ring system by increasing the
electron density and makes the compound easier to oxidize than
corresponding phenyl-substituted compounds.18
Fig. 1(e) shows repeated scans of cyclic voltammograms which
were carried out to check the electrochemical stability of P-DAC
(see Fig. S5 in the ESI† for all the multifunctional molecules).
The oxidation process of AC and P-AC (compounds chemically
substituted at C3 and N9 positions, but with open C6 position of
carbazole moiety) was not reversible as to have the oxidation
potential gradually shifted to the lower value with concomitant
current increase in the repeated CV cycles (see Fig. S5(a) and (c)
in the ESI†). Such characteristics are attributed to the electro-
chemical dimerization of carbazoles through the active C6
position.19–22 In contrast, DAC and P-DAC with substituents at
all the C3, C6, and N9 positions, showed reversible oxidation
process as shown in Fig. S5(b) and (d)†.
SEM images were obtained to check morphological charac-
teristics of thermal evaporated films. Fig. 1(f) shows SEM image
of P-DAC (see Fig. S6 in the ESI† for all the molecules). Except
for AC, all molecules can form morphologically stable and
uniform amorphous films. For carbazole-based compound to
obtain sufficient morphological stability in thermally evaporated
film, it has been proposed that the molecule size should be
extended beyond the length of one carbazole molecule to obtain
bulky and sterically hindered molecular configuration.21–23 It is
considered that AC, with only C3 positioned anthracene and N9
positioned ethyl group, is rather small in its molecular size to
satisfy this condition.
In order to explore OLEDs characteristics of AC, DAC,
P-AC, and P-DAC, we have fabricated non-doping (ND) blue
devices, consisting of indium tin oxide (ITO) (150 nm)/4,40,400-tris
(N-(2-naphthyl)-N-phenyl-amino)triphenylamine (2-TNATA)
(60 nm)/N,N0-bis(naphthalen-1-yl)-N,N0-bis(phenyl)-benzidine
(NPB) (20 nm)/blue emitting layer (EML) (30 nm)/2,9-dimethyl-
4,7-diphenyl-1,10-phenanthroline (BCP) (10 nm)/tris(8-hydroxy-
quinolinato) aluminium (Alq3) (20 nm)/LiF (1 nm)/Al (100 nm).
2-TNATA was used as hole injection layer (HIL), NPB was used
as hole transporting layer (HTL), AC, DAC, P-AC, and P-DAC
were used as emitting layer (EML), BCP was used as hole
blocking layer (HBL), and Alq3 was used as electron trans-
porting layer (ETL), respectively.
Fig. 3 (a) J–V–L characteristics, (b) Luminance efficiency and power
efficiency of non-doped devices with AC, DAC, P-AC, and P-DAC.
featureless in the emission spectra. The PL spectrum in film state
was similarly shaped to that in solution but a little red-shifted
probably due to the rather trivial solid-state effect (see Fig. 1(d)
for the case of P-DAC).16 The absence of characteristic anthra-
cene excimer peaks in these PL spectrum evidences the successful
molecular design of our multifunctional blue emitting molecules.
Attributed to the suppressed intermolecular interaction, absolute
PL quantum yields (AbPLQY) of multifunctional molecules
measured by an integrating sphere method in film state,17 were
very high (>40%) as shown in Table 1. It should be noted that the
maximum emission wavelength of ethyl carbazole derivatives is
relatively larger than that of phenyl carbazole derivatives (AC vs.
P-AC, and DAC vs. P-DAC) in the PL spectra due to the
stronger electron donating properties of aliphatic group.
Fig. 2(a) shows the ND device structure and energy levels of
materials, (b) the molecular structures of each material used in
the device fabrication, and (c) the EL spectrum and emission
photograph at 10 mA cmꢀ2. Fig. 3 shows current density–
voltage–luminance (J–V–L) characteristics, and luminance
efficiency and power efficiency as a function of current density
for each blue OLED. The important EL properties were
analyzed from these graphs and summarized in Table 2. As
shown in Fig. 2(c), all the blue OLED devices showed deep-
blue emission, although we find a rather small exciplex emission
at around 580 nm range included in the EL spectra of DAC.
The Commission Internationale de l’Eclairage (CIE) chroma-
ticity coordinates of purely blue emitting AC, P-AC, and P-
DAC devices are (0.165, 0.151), (0.166, 0.152), and (0.167,
0.149) at 10 mA cmꢀ2 and (0.162, 0.134), (0.163, 0.132), and
(0.162, 0.136) at 100 mA cmꢀ2, which show the deep-blue
emission characteristics and very small change in the coordi-
nate values as a function of current density. All the AC, P-AC,
Table 1 also shows HOMO and LUMO energy levels of each
molecule. HOMO energy levels of AC, DAC, P-AC, and P-DAC
measured by cyclic voltammetry (CV) are ꢀ5.40, ꢀ5.41, ꢀ5.47,
and ꢀ5.50 eV, respectively. The optical band gap energies (Eg)
calculated from the threshold of the optical absorption are 3.04,
3.04, 3.05, and 3.05 eV, respectively, which are all appropriate for
blue emission. LUMO energy levels were calculated to be ꢀ2.36,
ꢀ2.37, ꢀ2.42, and ꢀ2.45 eV, respectively, from their HOMO
energy level and Eg. It is to be noted that the HOMO energy
levels of ethyl carbazole derivatives (AC and DAC) are slightly
higher than those of phenyl carbazole derivatives (P-AC and
P-DAC). It is speculated that ethyl group at N9 position of
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Table 2 Electroluminescent characteristics of blue non-doped OLEDs (V: driving voltage, hL: luminance efficiency, hP: power efficiency, hex: external
quantum efficiency)
EML
Va/V
Vb/V
)
hexb (%)
CIEb
Vc/V
)
CIEc
b
hL /cd Aꢀ1
b
hP /lm Wꢀ1
c
hL /cd Aꢀ1
c
hP /lm Wꢀ1
c
hex (%)
AC
4.40
4.60
4.40
3.80
7.91
7.54
8.17
7.57
3.45
2.09
3.11
3.09
1.39
0.87
1.19
1.28
2.84
1.38
2.53
2.50
0.165, 0.151
0.193, 0.196
0.166, 0.152
0.167, 0.149
10.62
11.08
11.18
10.66
2.32
1.82
2.56
3.14
0.69
0.52
0.72
0.92
2.07
1.33
2.35
2.75
0.162, 0.134
0.179, 0.171
0.163, 0.132
0.162, 0.136
DAC
P-AC
P-DAC
a
b
c
Turn-on voltage at 1 cd mꢀ2
.
At 10 mA cmꢀ2
.
At 100 mA cmꢀ2
.
Table 3 Electroluminescence data for the blue OLEDs (na: not available)
Materials code Moiety V/V
hL/cd Aꢀ1 hP/lm Wꢀ1 hex (%) CIE
Efficiency roll-offe (%) Reference
P-DAC
TPVAn
MAM
POAn
BFAn
DMIP-2-NA
9
MADN
DPVPA
Carbazole and anthracene
7.57a 3.09a
1.28a
na
2.50a
5.0b
3.90a
4.0a
4.8a
1.4b
1.05d
na
0.167, 0.149a +2
This paper
Anthracene and strylarylene 6.7b
5.0b
0.14, 0.12c
ꢀ16
8d
8e
8f
8g
8i
13c
24
24
Anthracene
Anthracene
Anthracene and fluorene
Anthracene and indene
Carbazole and arylamine
Anthracene
7.99a 2.94a
1.28a
na
0.163, 0.124a na
na
na
2.7a
5.4a
0.15, 0.07c
0.15, 0.12c
ꢀ33
ꢀ22
4.2a
0.64b
0.65d
0.4
5.77b 1.17b
0.151, 0.097b na
6.0d
6.2
1.26d
1.4
0.15, 0.14d
0.15, 0.10
0.14, 0.17
na
na
na
Anthracene and strylarylene 7.3
4.0
1.7
na
a
b
c
d
e
At 10 mA cmꢀ2
.
At 20 mA cmꢀ2 At 7 V. At 100 mA cmꢀ2 From 10 mA cmꢀ2 to 100 mA cmꢀ2
. . .
and P-DAC have shown high luminance efficiency larger than
3.0 cd Aꢀ1 at the current density level of 10 mA cmꢀ2. In the
case of AC and P-AC, however, the efficiencies were decreased
at the higher current density level of 100 mA cmꢀ2. The lumi-
nance efficiency was reduced by 33% (3.45 cd Aꢀ1 at 10 mA
cmꢀ2 and 2.32 cd Aꢀ1 at 100 mA cmꢀ2) in AC ND device, and it
was reduced by 18% (3.11 cd Aꢀ1 at 10 mA cmꢀ2 and 2.56 cd
Aꢀ1 at 100 mA cmꢀ2) in P-AC device. In P-DAC device, on the
contrary, the luminance efficiency was increased by 2% (3.09 cd
Aꢀ1 at 10 mA cmꢀ2 and 3.14 cd Aꢀ1 at 100 mA cmꢀ2). In fact,
in the P-DAC deep-blue ND device, showed virtually invariant
luminance efficiency as a function of current density. As shown
in Fig. 3(b), such an excellent OLED performance of P-DAC
consistent with its thermal, electrochemical, and morphological
stability. The efficiency, color purity, and device stability of P-
DAC non-doping device are the very good results, comparing
with blue OLED devices reported in the literature (Table
3).8d–g,8i,13c,24 Especially, device operational stability in terms of
efficiency roll-off tells that P-DAC is unique and excellent
deep-blue emission material, which suggests further potential
for the host material of deep-blue OLEDs. Fig. 4 shows the
device life-time of each non-doping device at the initial lumi-
nance of 400 cd mꢀ2 under constant current condition.
Although the overall device life-time is relatively short by using
BCP, which is lack of thermal and morphological stability,25
luminance half life-time of the P-DAC non-doping device is
largely superior to AC, DAC, and P-AC.
wt% dopant content. Fig. 5(a) shows the device structures of
P-DAC doping devices and energy levels of each material, (b)
the molecular structures of dopants, (c) absorption spectrum of
BCzVBi together with BDAVBi and PL spectra of P-DAC, and
(d) the EL spectrum of doping devices and emission photo-
graphs at 10 mA cmꢀ2. P-DAC doping devices showed the deep
blue emission whose shape is slightly different from that of
P-DAC ND device (Fig. 2(c)) and included the intrinsic
vibronic peak characteristics of dopants. Since the deep-blue
host material, P-DAC has the larger band gap energy than
dopants, and the spectral overlap efficiency between PL spectra
of P-DAC and absorption spectrum of dopants is very high,
€
Forster energy transfer was efficient from P-DAC to the
dopants. Fig. 6 shows J–V–L characteristics, and luminance
efficiency and power efficiency as a function of current density
for different doping devices. The important EL properties were
To further improve the device performance, doping devices
have been fabricated and evaluated by using P-DAC as the
host material which has shown the best performance among
non-doping device. In P-DAC doping devices, BCzVBi and
BDAVBi were investigated as blue-emitting dopants. The
device structures were the same as that of non-doping device,
when the each dopant was co-evaporated with P-DAC to give 3
Fig. 4 Device life-time of deep-blue non-doped OLEDs (initial lumi-
nance is 400 cd mꢀ2 and device is operated with constant current).
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Fig. 5 (a) Structures of P-DAC doping devices and energy levels of materials. (b) Molecular structures of blue dopants. (c) UV-visible absorption
spectrum of the BCzVBi, BDAVBi and PL spectra of the P-DAC. (d) EL spectrum and emission photographs of doping devices at 10 mA cmꢀ2
.
analyzed from these graphs and are summarized in Table 4.
Conclusions
The CIE chromaticity coordinates of BCzVBi and BDAVBi
High efficiency deep-blue OLEDs have been investigated using
new blue emitting materials AC, DAC, P-AC, and P-DAC
which comprise covalently bonded carbazole and anthracene
moieties. Theoretical molecular orbital calculations have shown
that AC, DAC, P-AC, and P-DAC have non-coplanar struc-
tures. We also have investigated the thermal, electrochemical,
and morphological stability to find suitable molecular struc-
ture, consisting of carbazole and anthracene moieties. The non-
doping deep-blue EL device using P-DAC, which showed the
highest thermal, electrochemical, and morphological stability,
showed the highest luminance efficiency of 3.14 cd Aꢀ1 and
2.75%, with CIE chromaticity coordinates (0.162, 0.136) at
100 mA cmꢀ2. Moreover, the doping devices using P-DAC as
the host material showed blue emission, and the high lumi-
nance efficiencies and external quantum efficiencies of as high
as 7.70 cd Aꢀ1 and 4.86%, with CIE chromaticity coordinates
(0.156, 0.136) and (0.156, 0.217). Both the non-doping and
doping devices using P-DAC uniquely exhibited high opera-
tional stability with virtually negligible efficiency roll-off over
the broad current density range.
devices are (0.158, 0.142), (0.158, 0.219) at 10 mA cmꢀ2 and
(0.156, 0.136), (0.156, 0.217) at 100 mA cmꢀ2, which show the
blue emission characteristics and very small change in the
coordinate values as a function of current density. Especially,
BCzVBi doping device shows the deep-blue OLED character-
istic. Luminance efficiency of BCzVBi doping device was 4.32
cd Aꢀ1 (E.Q.E: 3.65%) and that of BDAVBi was 7.36 cd Aꢀ1
(E.Q.E: 4.62%) at 10 mA cmꢀ2. The efficiency of BDAVBi
doping device was superior to that of BCzVBi, it is because of
the more spectral overlap between P-DAC and BDAVBi than
that between P-DAC and BCzVBi, resulting in the more
€
effective Forster energy transfer. Investigating the change in
luminance efficiency in doping devices from 10 mA cmꢀ2 to 100
mA cmꢀ2, the efficiency of BCzVBi doping device was increased
by 1% (4.32 cd Aꢀ1 at 10 mA cmꢀ2 and 4.37 cd Aꢀ1 at 100 mA
cmꢀ2) and the efficiency of BDAVBi doping device was also
increased by 5% (7.36 cd Aꢀ1 at 10 mA cmꢀ2 and 7.70 cd Aꢀ1 at
100 mA cmꢀ2). As shown in Fig. 6(b), it is noteworthy that
there is little roll-off of luminance efficiency as a function of
current density much like that of P-DAC non-doping device.
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a platinum wire counter-electrode, and a quasi Ag+/Ag electrode
as reference. Measurement was performed in 0.5 mM MC solu-
tion, with tetrabutylammonium tetrafluoroborate as supporting
electrolyte, at a scan rate of 100 mV sꢀ1. Each oxidation potential
was calibrated using ferrocene as a reference. LUMO levels were
calculated from HOMO level and energy band gap which was
obtained from the edge of the absorption spectra. SEM images
were obtained from HITACHI S-4300.
Device fabrication and evaluation
The blue OLEDs were fabricated by thermal evaporation onto
a cleaned glass substrate precoated with ITO without breaking
the vacuum. Prior to organic layer deposition, the ITO substrates
were exposed to UV–ozone flux for 20 min following degreasing
in isopropyl alcohol. All layers were grown by thermal evapo-
ration at the base pressure of <5 ꢂ 10ꢀ7 Torr. Layers were
deposited in following order: HIL/HTL/blue EML/HBL/ETL/
cathode. 60 nm-thick 2-TNATA (LT-E203 purchased from
Lumtec) was used as the HIL, 20 nm-thick NPB (LT-E101
purchased from Lumtec) was used as the HTL, 30 nm-thick AC,
DAC, P-AC, and P-DAC were used as the blue EML, 10 nm-
thick BCP (LT-E304 purchased from Lumtec) was used as the
HBL, 20 nm-thick Alq3 (LT-E401 purchased from Lumtec) was
used as the ETL, respectively. Finally, the cathode consisting of
a 1 nm-thick LiF and a 100 nm-thick layer of Al were deposited
onto the sample surface. In P-DAC doping devices, BCzVBi (LT-
E601 purchased from Lumtec) and BDAVBi (LT-E608
purchased from Lumtec) were investigated as blue-emitting
dopants. The doping device structures were the same as that of
non-doping device, when the each dopant was co-evaporated
with P-DAC to give 3 wt% dopant content. All the materials were
purified using train sublimation before use.
Fig. 6 (a) J–V–L characteristics, (b) Luminance efficiency and power
efficiency of doping devices.
Experimental
J–V–L characteristics, EL spectra, and CIE chromaticity
coordinates of the blue OLEDs were measured simultaneously
using a Keithley 237 programmable source meter and a Spec-
traScan PR650 (Photo Research). Assuming Lambertian emis-
sion, the external quantum efficiency (EQE) was calculated from
the luminance, current density, and electroluminescence
spectrum.
Characterization
1
Chemical structures were fully identified by H NMR (JEOL
JNM-LA300, 300 MHz), 13C NMR (Bruker, Avance DPX-300),
gas chromatography–mass spectrometry (GC–MS) (JEOL,
JMS-AX505WA), and elemental analysis (EA1110, CE Instru-
ment). The Tg and Tm of molecules were determined by DSC
under N2 atmosphere using a TA instruments Q1000 model. The
Td of molecules were obtained by TGA under N2 atmosphere
using a TA instruments Q50 model. UV-visible spectra were
recorded on a Varian Cary 100 Conc model, while PL spectra
were taken using a Varian Cary Eclipse fluorescence spectro-
photometer. AbPLQY was measured by the integrating method
with IESP-150B (Optel) quantum efficiency measurement
equipment. HOMO levels of molecules were obtained from the
CV measurement. CV measurement was performed using a 273A
(Princeton Applied Research) with a one-compartment elec-
Synthesis
P-DAC was synthesized according to the procedure in Scheme 1.
Unless stated otherwise, all reagents were purchased from Sigma
Aldrich, TCI, and Alfa Aesar (see Scheme S1 in the ESI† for all
molecules).
9-Phenyl-9H-carbazole (6). Carbazole (10 g, 59.8 mmol),
iodobenzene (12.445 g, 61 mmol), copper powder (11.628 g, 183
mmol), and potassium carbonate (25.293 g, 193 mmol) were
trolysis cell consisting of
a platinum working electrode,
Table 4 Electroluminescent characteristics of blue doping OLEDs
b
b
b
hex (%)
c
c
c
hex (%)
Dopant
Va/V
Vb/V
hL /cd Aꢀ1
hP /lm Wꢀ1
CIEb
Vc/V
hL /cd Aꢀ1
hP /lm Wꢀ1
CIEc
BCzVBi
BDAVBi
4.00
3.80
7.94
7.42
4.32
7.36
1.71
3.12
3.65
4.62
0.158, 0.142
0.158, 0.219
10.77
10.15
4.37
7.70
1.27
2.39
3.83
4.86
0.156, 0.136
0.156, 0.217
a
b
c
Turn-on voltage at 1 cd mꢀ2
.
At 10 mA cmꢀ2
.
At 100 mA cmꢀ2
.
9146 | J. Mater. Chem., 2011, 21, 9139–9148
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dissolved in dimethylformamide (DMF) (120 mL) and stirred
ꢁ
595.23. Anal. calcd for C46H29N: C 92.74, H 4.91, N 2.35; found:
C 92.58, H 4.93, N 2.32%.
overnight at 140 C under nitrogen atmosphere. After the reac-
tion was finished, the reaction mixture was poured into the brine,
washed, and the mixture was extracted by MC. The organic
extracts were dried over MgSO4 and concentrated by rotary
evaporation. Purification of solid residue by column chroma-
tography (ethyl acetate (EA) : n-hexane ¼ 1 : 10) gave product
(8.1 g, 33 mmol, 56%). 1H NMR (300 MHz, CDCl3, d): 8.12 (d,
J ¼ 7.8 Hz, 2H), 7.54–7.64 (m, 4H), 7.37–7.50 (m, 5H), 7.26–7.32
(m, 2H).
Acknowledgements
This work was supported in parts by the Basic Science Research
Program (CRI; RIAMIAM0209 (0417-20090011)) and WCU
(World Class University) Program (R31-2008-000-10075-0)
through the National Research Foundation of Korea (NRF)
funded by the Ministry of Education, Science and Technology.
3,6-Dibromo-9-phenyl-9H-carbazole (9). To a solution of 9-
phenyl-9H-carbazole (3.44 g, 14.13 mmol) in DMF (150 mL),
NBS (5.15 g, 28.97 mmol) was added slowly. The mixture was
stirred overnight at room temperature. After pouring into brine,
and washing, the mixture was extracted with MC. The organic
extracts were dried over MgSO4 and concentrated by rotary
evaporation. Purification of solid residue by reprecipitation with
methanol and tetrahydrofuran (THF) gave white powder (5.36 g,
Notes and references
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Phenyl-3,6-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9H-
carbazole (5.15 g, 10.4 mmol), 9-bromoanthracene (5.62 g, 21.84
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with a reflux condenser and dissolved in 80 mL of THF. After
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reaction mixture was heated at 85 ꢁC for 24 h. The cooled crude
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over MgSO4, filtered, and evaporated to yield a crude product.
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1
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