Synthesis and characteristics of bis(2,4-dimethyl-8-quinolinolato)(triphenylsilanolato)aluminum (III): A potential hole-blocking material for the organic light-emitting diodes

Article abstract of DOI:10.1016/j.jorganchem.2006.02.002

A new five-coordinated bis(2,4-dimethyl-8-quinolinolato)(triphenylsilanolato)aluminum (III) (24MeSAlq) material, having bulky substituents, was prepared in one-step reaction and was characterized. The photoluminescent (PL) spectrum of 24MeSAlq shows the largest hypsochromic shift exhibiting the maximum wavelength at the peak of 461 nm among the blue-emitting q₂AlOR-type complexes (q = 8-quinolinolato ligand and OR = aryloxy or alkoxy ligand) reported. The deep blue device composed of ITO/2-TNATA (60 nm)/NPB (15 nm)/24MeSAlq (20 nm)/Alq₃ (45 nm)/LiF (1 nm)/Al (100 nm), which uses 24MeSAlq as a hole-blocking layer and applies a principle efficiently confining an exciton recombination zone into a hole transporting layer, shows the maximum electroluminescent (EL) at the peak of 446 nm originating from the NPB emissive layer. This is attributed to an excellent hole-blocking property due to the high HOMO (highest occupied molecular orbital) energy level (6.14 eV).

Full text of DOI:10.1016/j.jorganchem.2006.02.002

Journal of Organometallic Chemistry 691 (2006) 2701–2707
www.elsevier.com/locate/jorganchem
Synthesis and characteristics of bis(2,4-dimethyl-8-quinolinolato)-
(triphenylsilanolato)aluminum (III): A potential hole-blocking
material for the organic light-emitting diodes
a
a
a
a,
b
J.T. Lim , C.H. Jeong , J.H. Lee , G.Y. Yeom *, H.K. Jeong ,
b
b
b
S.Y. Chai , I.M. Lee , W.I. Lee
a
Department of Materials Science and Engineering, SungKyunKwan University, 300 Chun-Chun-Dong, Suwon, Kyung-Gi-Do 440-746, Republic of Korea
b
Department of Chemistry, Inha University Inchon, Yonghyun-dong, Nam-ku 402-751, Republic of Korea
Received 16 August 2005; received in revised form 31 January 2006; accepted 2 February 2006
Available online 20 March 2006
Abstract
A new five-coordinated bis(2,4-dimethyl-8-quinolinolato)(triphenylsilanolato)aluminum (III) (24MeSAlq) material, having bulky
substituents, was prepared in one-step reaction and was characterized. The photoluminescent (PL) spectrum of 24MeSAlq shows the
largest hypsochromic shift exhibiting the maximum wavelength at the peak of 461 nm among the blue-emitting q2AlOR-type complexes
(q = 8-quinolinolato ligand and OR = aryloxy or alkoxy ligand) reported. The deep blue device composed of ITO/2-TNATA (60 nm)/
NPB (15 nm)/24MeSAlq (20 nm)/Alq3 (45 nm)/LiF (1 nm)/Al (100 nm), which uses 24MeSAlq as a hole-blocking layer and applies a
principle efficiently confining an exciton recombination zone into a hole transporting layer, shows the maximum electroluminescent
(EL) at the peak of 446 nm originating from the NPB emissive layer. This is attributed to an excellent hole-blocking property due to
the high HOMO (highest occupied molecular orbital) energy level (6.14 eV).
ꢀ 2006 Elsevier B.V. All rights reserved.
Keywords: 24MeSAlq; HBL; Hole-blocking material; 8-Hydroxy-quinoline; OLED
1. Introduction
have been developed and have been demonstrated to be
useful emissive materials or/and hole-blocking/electron-
transporting materials [11]. Kwong et al. [11] reported on
the operating stability and the luminous efficiency of an
orange-red electrophosphorescent device with a variety of
hole and exciton blocking materials of the q2AlOR-type
complexes. Among these complexes, the device using
bis(2-methyl-8-quinolinolato)(4-phenylphenolato)alumi-
num (III) (BAlq) showed the excellent hole blocking prop-
erty indicating a maximum efficiency of 17.6 cd Aꢀ1 with a
projected operational lifetime of 15,000 h normalized to
The organic light-emitting diode (OLED) has become
one of the most promising next-generation large-area flat
panel displays since Tang and VanSlyke reported an effi-
cient double layered OLED utilizing tris(8-quinolinolato-
N1,O3) aluminum (III) (Alq3) [1,2] and has been widely
studied since the display panel fabricated by a Japanese
OLED manufacturer, Pioneer, was put to the market in
1998 [3].
Since then, many aluminum complexes based on the
quinoline ligands such as q3Al (q = 8-quinolinolato ligand
and Al = aluminum) [4–6], q2AlOR (OR = aryloxy or alk-
oxy ligand) [5,7–9], and q2AlOAlq2-type complexes [10]
100 cd mꢀ2
.
Concerning the aluminum complexes based on the quin-
oline ligands, several luminescent characteristics have been
demonstrated [4,12–15]. The light-emission of q3Al-type
and q2AlOR-type complexes is mostly attributed to the
p ! p* transition of a quinolinolato ligand rather than a
*
Corresponding author. Tel.: +82 31 299 6560; fax: +82 31 299 6565.
E-mail address: gyyeom@skku.edu (G.Y. Yeom).
0022-328X/$ - see front matter ꢀ 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2006.02.002

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J.T. Lim et al. / Journal of Organometallic Chemistry 691 (2006) 2701–2707
phenolato ligand. The filled p orbital (location of the high-
est occupied molecular orbital or HOMO) is located on the
phenoxide ring and unfilled p* orbital (location of the low-
est unoccupied molecular orbital or LUMO) on the pyridyl
ring. Generally, in the case of q3Al-type complexes, the
introduction of an electron-donating substituent into the
pyridyl ring raises the energy level of LUMO relatively
more as compared with the energy raising of HOMO,
resulting in a hypsochoromic shift (blue shift) of the emis-
sive light. On the other hand, the introduction of the elec-
tron-donating substituent into the phenoxide ring raises the
energy level of the HOMO relatively more as compared
with the energy raising of LUMO, leading to a bathochro-
mic shift of the emissive light. Also, as it decreases the
covalent nature between the central atom (boron, alumi-
num, etc.) and nitrogen atom on the quinoline ligand, the
wavelength of complexes shows a hypsochromic shift.
For example, the lithium terta-(2-methyl-8-quinolino-
lato)boron complex, in which boron is coordinated to the
ligand only through a B–O bond due to the small size of
boron atom relative to aluminum atom, shows more hyp-
sochromic-shifted emission than the electroluminescent
(EL) spectrum of Alq3 (about 530 nm), exhibiting that at
470 nm [16]. Therefore, the q2AlOR-type complexes with
the five-coordination number is expected to show an inten-
sive hypsochromic shifts rather than the q3Al-type com-
plexes with the six-coordination number due to
decreasing of the number of the M–N bonding between
the central aluminum metal (M) and a nitrogen atom (N)
on the quinolinolato ligand [5,7–9].
folders or inert-atmosphere dry boxes. All solvents were
freshly distilled using the appropriate drying agents under
a nitrogen atmosphere and all chemicals were a reagent
grade without further purification.
1H and 13C nuclear magnetic resonance (NMR) spectra
were measured using a NMR spectrometer (400.133 MHz)
with tetramethylsilane (TMS) as an internal standard (Var-
ian Unity-400, Varian Inc.). The infrared (IR) spectrum was
measured as KBr pellets on an IR spectrophotometer (IFS
66/S, BRUKER Co., Ltd.). The identification of fragment
patterns and molecular ions of the synthesized material
was obtained by a gas chromatography/mass spectrometry
data (GC/MSD) system (Agilent 6890GC/5973N MSD,
Agilent Technologies, Ltd.). The quantitative analysis of
carbon, proton, and nitrogen was performed by an elemental
analyser (EA) (EA-1110, CE Inst. Co., Ltd.). The photolu-
minescence (PL) spectrum of the films deposited on the
quartz substrate and the absorption spectrum in the solu-
tion were recorded with a spectrofluorophotometer (RF-
5301PC, Shimazu Co., Ltd.) and a ultraviolet–visible
(UV–Vis) spectrophotometer (Lambda 40, Perkin–Elmer
Inc.), respectively.
The differential scanning calorimeter (DSC) measure-
ment and the thermogravimetric analysis (TGA) were per-
formed using a DSC analyser (DSC6100, SEICO INST.)
and a Seiko Exster6000 system with a thermogravimetic
analyser (TG/DTA6100, SEICO INST.), respectively. A
heating rate of 10 ꢁC minꢀ1 with a nitrogen flow of
75 ml minꢀ1 was used with the temperature range from
25 to 490 ꢁC for the DSC or to 940 ꢁC for the TGA.
In this paper, in order to develop a large band-gap emis-
sive material among the q2AlOR-type complexes for the
OLEDs, a five-coordinated 24MeSAlq molecule was
designed, where two methyl groups were substituted to
2,4-position proton on the pyridyl ring of the 8-qunolino-
lato ligand to obtain a hypsochromic-shifted emission
and where the triphenylsiloxy group was used as a bulky
substituent to reduce an intermolecular interaction. The
24MeSAlq material was characterized through spectro-
scopic analyses, thermal behaviour, the electrochemical
band gap, and so on. Also, for the application of the
24MeSAlq material as a hole-blocking layer (HBL) to
OLEDs, the hole-blocking properties of 24MeSAlq was
discussed through the electroluminescent spectrum and
the current–voltage–luminance characteristic of the blue
device utilizing 24MeSAlq as a hole-blocking material
and compared with the device of the same structure using
other hole-blocking material, which is fabricated by a prin-
ciple confining an exciton recombination zone only into the
hole-transporting layer (HTL).
2.2. Cyclic voltammetric measurements
Experiments on the cyclic voltammetry were carried out
in a three-electrode compartment cell with a total volume
of electrolyte solution (0.1 M tetra(n-butyl)ammonium per-
chlorate [(n-Bu)4NClO4] in acetonitrile) of 20 ml. All
potential were measured against a Ag/AgCl (sat. KCl) elec-
trode (0.197 V versus NHE) as the reference electrode, a
platinum wire as a counter electrode, and an indium tin
oxide (ITO) film having the effective area of 12 mm2 as a
working electrode, on which the 100 nm 24MeSAlq thin
film was deposited by thermal evaporation. The measure-
ments were performed using a potentiostat (CHI660B,
CH Instruments, Ltd.) under an argon atmosphere.
2.3. Preparation of bis(2,4-dimethyl-8-
quinolinolato)(triphenylsilanolato)aluminum (III)
(24MeSAlq)
2,4-Dimethyl-8-hydroxyquinoline was prepared by
modifying the procedure reported by Phillips et al. [17],
which included the synthesis of a ketoenamine intermediate
by the reaction between 2-amino-phenol and 2,4-pentandi-
one followed by the condensation reaction to prepare 2,4-
dimethyl-8-hydroxyquinoline under an acid catalyst such
as sulfuric acid.
2. Experimental
2.1. General
The reaction was carried out under vacuum or under a
nitrogen atmosphere using either standard vacuum-mani-

J.T. Lim et al. / Journal of Organometallic Chemistry 691 (2006) 2701–2707
2703
24MeSAlq was synthesized through the homogeneous-
phase reaction between aluminium iso-propoxide and two
ligands (2,4-dimethyl-8-quinolinol and triphenylsilanol) at
the reflux condition. A three-necked flask (250 ml) contain-
ing a magnetic stir bar and fitted with a reflux condenser, a
rubber septum, and a gas inlet tube was charged with the
tetramer of aluminum iso-propoxide (1.00 g, 1.22 mmol)
and isopropanol (100 ml) under a nitrogen atmosphere.
The slurry solution was heated to about 100 ꢁC for several
minutes until the solid was dissolved. The solution of iso-
propanol containing 2,4-dimethyl-8-quinolinol (0.84 g,
4.88 mmol) was added to the colorless solution formed
over a period of 0.5 h under the reflux condition using a
cannula to have a light yellow solution followed by rapid
stirring for 1 h. Another solution composed of 2,4-
dimethyl-8-quinolinol (0.84 g, 4.88 mmol) and triphenylsil-
anol (2.70 g, 9.76 mmol) dissolved in 50 ml of isopropanol
was added drop by drop to the above light yellow solution
over a period of 1 h under the reflux condition using the
cannula. A white solid was precipitated immediately, but
the solution was stirred for an additional 1 h under the
reflux condition after all reactants had been added. The
white precipitates were collected by filtration, washed with
50 ml of hot isopropanol and then twice with 50 ml of hot
diethylether, and finally allowed to dry under a reduced
pressure to yield the white powder of 24MeSAlq (2.46 g,
78%, based on aluminum iso-propoxide).
NMR (400.267 MHz, CDCl3). d (ppm): 156.9, 156.7,
148.2, 139.1, 138.7, 134.9, 134.8, 128.4, 127.0, 126.7,
124.6, 112.3, 110.4, 22.8, 18.7; Thermal Anal. (ꢁC): no
Tg, Tm (245.8), Td (461.9); GC/MSD (EI) (m/z): 645
(M+); FT-IR (KBr pellet) (cmꢀ1): 3066, 3042, 1605, 1576,
1508, 1462, 1427, 1410, 1393, 1354, 1323, 1285, 1221,
1153, 1111, 1055, 1026, 991, 937, 862, 812, 752, 702, 554,
513, 448; Anal. Calc. for C40H35O3N2AlSi: C, 74.28; H,
5.45; N, 4.17. Found: C, 74.52; H, 5.90; N, 4.09%.
2.4. Fabrication of the blue OLEDs
Fig. 1 exhibits the schematic device configuration and
molecular structures of OLEDs. The blue OLED devices
with the structure of glass/ITO (120 nm, about 10 X/h)/
2-TNATA (60 nm)/NPB (15 nm)/24MeSAlq (20 nm) or
BAlq (20 nm)/Alq3 (35 nm)/LiF (1 nm)/Al(100 nm) were
fabricated by vacuum deposition, in which 24MeSAlq or
BAlq was used as a HBL/an electron-transporting layer
(ETL), 4,40,400-tris[2-naphthylphenyl-1-phenylamino]triph-
enylamine (2-TNATA) as a hole-injecting layer (HIL),
4,40-bis[N-(1-napthyl)-N-phenyl-amino]-biphenyl (NPB) as
a
HTL, tris[8-hydroxyquinolinonato]aluminum (III)
(Alq3) as an ETL, and lithium fluoride (LiF) as an elec-
tron-injecting layer (EIL), respectively. Before loading into
a deposition chamber, the ITO substrate was cleaned with
detergents and deionized water, dried in an oven at the
temperature of 120 ꢁC for 2 h. The devices were fabricated
by evaporating the organic layers at the rate of 0.05–
0.2 nm sꢀ1 onto the ITO substrate sequentially at a pres-
sure below 1 · 10ꢀ6 Torr. Finally, onto the EIL layer, a
100-nm-thick aluminum metal was deposited at the rate
The characterization results of the 24MeSAlq material
are summarized as follows: 1H NMR (400.267 MHz,
CDCl3)
d (ppm): 7.47 (t, 2H, 6-H in quinoline,
3
3J6,5 = 8.1 Hz, J6,7 = 7.8 Hz), 7.28–7.19 (m, 11H), 7.09–
7.02 (m, 10H), 2.68 (s, 3H, Me), 2.62 (s, 3H, Me); 13C
Fig. 1. Schematic configuration of the blue devices and molecular structures.

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J.T. Lim et al. / Journal of Organometallic Chemistry 691 (2006) 2701–2707
of 0.1–2 nm sꢀ1 as a cathode. The emissive active area of all
devices was 2 mm · 2 mm.
and no glass transition was observed prior to Tm, while it
shows a sharp melting point at 246 ꢁC. In addition, when
1
the remelting stability was tested using H NMR and FT-
2.5. Photometric properties of the blue OLEDs
IR with the material left in the melting tube just past the
major endothermic transition (Tm), no decomposition
product could be observed by the analyses. Also, the
24MeSAlq material exhibited an onset of the endothermic
decomposition at about 340 ꢁC and showed the largest one
at 462 ꢁC as shown in the DSC curve. Meanwhile, as shown
in Fig. 2 for the TGA curve of 24MeSAlq, the maximum
rate of weight loss of 24MeSAlq took place at 420 ꢁC
and the weight loss of 5% and 10% were occurred at 329
and 349 ꢁC, respectively. In results, the temperature range
to deposit the thermally stable 24MeSAlq thin film by ther-
mal evaporation is expected to be from 246 ꢁC (melting
point) to about 340 ꢁC (the temperature just below the start
of decomposition).
Current–voltage characteristics were measured with a
source-measure unit (236, Keithley Instrument Inc.). The
intensities from the blue emission of the devices were mea-
sured by the photocurrent induced on the silicon photodi-
odes using a picoammeter (485, Keithley Instruments Inc.).
The EL spectra of the as-fabricated blue devices were mea-
sured by optical emission spectroscopy (PCM-420, SC
Tech. Inc.).
3. Results and discussion
3.1. Thermal stability of 24MeSAlq
Thermal properties of 24MeSAlq were evaluated by
simultaneous thermal analysis, where TGA and DSC
experiments were run simultaneously; therefore, thermal
events observed in the DSC could be directly correlated
with weight loss events.
3.2. Optical properties of 24MeSAlq
Fig. 3 shows the PL and photoluminescence excitation
(PLE) spectra of the 24MeSAlq film deposited on the
quartz substrate by thermal evaporation.
The morphological stability of Alq3, desirable for the
practical OLED applications, has been attributed to its
intrinsic polymorphic, a racemic nature, and strong dipolar
interactions. Several phase transitions were observed for
Alq3 and these phase transitions affecting the optical prop-
erties of the OLED device are attributed to polymorphism
of the crystalline material as discussed by Brinkmann et al.
[18], assigning an exothermic phase transition from a-Alq3
to c-Alq3 phase at about 395 ꢁC, as it was confirmed by X-
ray diffraction. In the case of 24MeSAlq, as shown in the
inset of Fig. 2 for the DSC curve of 24MeSAlq measured
at the heating rate of 10 ꢁC minꢀ1, no phase transition
The effect of the ligand in q2AlOR-type complexes was
discussed by Wang et al. [13]. As bulkiness of OR in
q2AlOR is increased, the maximum intensity in PL spectra
were hypsochromic shifted because an intermolecular inter-
action among molecules is decreased [12]. The triphenylsi-
lanolato ligand as a bulky ligand was introduced to reduce
an intermolecular interaction. In result, as bulkiness of OR
is increased, we are expected to be hypsochromic shifted in
PL spectrum, due to a reduction of an electrostatic binding
force among molecules.
Meanwhile, the optical transition responsible for the PL
of 24MeSAlq is due to a p ! p* charge transfer from the
electron rich phenoxide ring (HOMO) to the electron
deficient pyridyl ring (LUMO) [13]. The energy band gap
is increased with increasing the number of the methyl
Fig. 2. TGA trace of the as-synthesized 24MeSAlq material. Inset: DSC
trace of the same material. Data were measured with the heating rate of
Fig. 3. PL spectrum and PLE spectrum of the 24MeSAlq thin film with
the thickness of 100 nm on the quartz substrate.
10 ꢁC minꢀ1 in flowing nitrogen at 75 ml minꢀ1
.

J.T. Lim et al. / Journal of Organometallic Chemistry 691 (2006) 2701–2707
2705
Table 1
substituent on the pyridyl ring in the 8-quinolinolato
ligand. The PL spectrum of 24MeSAlq (kmax: 461 nm)
which contains the 2,4-dimethyl-8-quinolinolato ligand
substituting two methyl groups on the pyridyl ring showed
Electrochemical properties and energy levels of 24MeSAlq and BAlq as
the hole blocking materials
Hole-blocking
materials
Eonset(ox) vs. Eonset(red) vs. EHOMO ELUMO EgCHEM
Ag/AgCl (V) Ag/AgCl (V) (eV)
(eV)
ðeVÞ
the hypsochromic shift of 14 nm, relative to SAlq (kmax
:
24MeSAlq
BAlq
1.80
1.54
ꢀ1.40
ꢀ1.43
6.14
5.88
2.94
2.91
3.20
2.97
475 nm) [19] which has the 2-methyl-8-quinolinolato ligand
substituting one methyl group on the pyridyl ring. Also, the
PL spectrum of 24MeSAlq exhibited the narrow wave-
length with a full-width at half-maximum (FWHM) of
81 nm when the spectrum was obtained by the excitation
with the pumping wavelength of 355 nm. In the case of
the PLE spectrum of 24MeSAlq, as shown in Fig. 3, two
peaks at 266 and 355 nm were shown together with a shoul-
der peak around 331 nm.
0.1 M n-BuN4ClO4 solution dissolved in anhydrous aceto-
nitrile at the scanning rate of 50 mV sꢀ1. During the
sweeping of the materials from the positive to negative
bias, the color of the films was not changed and was
remained colorless. The anodic scans of two complexes,
corresponding to p-doping, show well-defined irreversible
peaks. The oxidation onset potentials (versus Ag/AgCl-
saturated) of 24MeSAlq and BAlq (not shown) were 1.8
and 1.54 V, respectively, and the reduction onset poten-
tials (versus Ag/AgCl-saturated) were found to be ꢀ1.4
and ꢀ1.43 V, respectively, as shown in Fig. 4 for
24MeSAlq.
3.3. Electrochemical properties of 24MeSAlq
In order to investigate the optical band gap, the UV–Vis
absorption spectrum of the as-synthesized 24MeSAlq
material in a dilute solution of chloroform (10ꢀ4 M) was
measured and the result is shown in the inset of Fig. 4.
The band gap can be estimated from a plot of (ahm)2 versus
the photon energy (hm) [20]. The intercept of the tangent to
the plot will give a good approximation of the band gap
energy as shown in the inset of Fig. 4. The optical band
gaps (p ! p*) of 24MeSAlq and BAlq (not shown) com-
plexes estimated by the intercept (or UV absorption onset)
were about 3.2 and 2.97 eV, respectively.
The cyclic voltammogram of 24MeSAlq is shown in
Fig. 4 and the data obtained from the cyclic voltammo-
grams of 24MeSAlq and BAlq are summarized in Table
1. All these hole-blocking materials were deposited by
thermal evaporation onto the ITO glass substrate and sep-
arately scanned both positively and negatively with a
From these onset potentials of the oxidation and the
reduction of cyclic votammograms, the energy levels
(EHOMO and ELUMO) and an electrochemical band gap
ðECg HEMÞ could be calculated. With an empirical relation
obtained through the conversion of the experimental data
using the valence effective Hamiltonian technique,
E
HOMO; ELUMO and EgCHEM of each complexes were calcu-
lated [21,22]. The EgCHEM values of 24MeSAlq and BAlq
were calculated to be 3.20 and 2.97 eV, respectively. These
ECg HEM values agree with the aforementioned optical band
gap of UV–Vis spectrum. However, when the energy levels
of the 24MeSAlq material with two methyl groups were
compared with those of the BAlq one with one methyl
group, the HOMO energy level (6.14 eV) of 24MeSAlq
was higher than that (5.88 eV) of BAlq material but the
LUMO energy values of 24MeSAlq (2.94 eV) and BAlq
(2.91 eV) materials were similar each other. The obtained
results are different from the results obtained by Sugimoto
and Sakaki [14], possibly due to the different material char-
acteristics between the q3Al-type complexes and q2AlOR-
type complexes, where the LUMO energy level was raised
relatively higher than the HOMO energy level in the
q3Al-type complexes as the number of the methyl group
on the pyridyl ring were increased. The higher HOMO level
of 24MeSAlq than that of BAlq obtained in the experiment
is expected to confine a hole/exciton effectively inside the
emissive layer of HTL due to the better hole-blocking
property. The electron-transporting ability of two materi-
als is also expected to have a similar trend.
3.4. Optical properties of the blue OLEDs
The inset of Fig. 1 shows a schematic configuration of
the blue OLED which is composed of glass/ITO
(120 nm)/2-TNATA (60 nm)/NPB (15 nm)/HBL (20 nm)/
Alq3 (35 nm)/LiF (1 nm)/Al (100 nm) (HBL: 24MeSAlq
(device A) or BAlq (device B)). Meanwhile, the green
Fig. 4. Cyclic voltammogram of the 24MeSAlq film deposited onto the
ITO glass in a 0.10 M (n-Bu)4NClO4 solution dissolved in anhydrous
acetonitrile at a scanning rate of 50 mV sꢀ1. The counter electrode was a
platinum wire. Inset: plot of (ahm)2 vs. photon energy (hm) for the
24MeSAlq thin film.

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J.T. Lim et al. / Journal of Organometallic Chemistry 691 (2006) 2701–2707
OLED of device C is composed of glass/ITO (120 nm)/2-
TNATA (60 nm)/NPB (15 nm)/Alq3 (55 nm)/LiF (1 nm)/
Al (100 nm). The BAlq material of device B is commonly
used as a hole-blocking layer in an organic light-emitting
diode [11].
The luminescence of OLEDs arises from radiative
recombination of an electron and a hole. In the case of
device A and device B, the fabricated blue-emitting devices
yield a singlet exciton in the interfacial region of a hole-
transporting layer (NPB) between NPB and 24MeSAlq.
In general, by lowering the charge carrier injection barrier
and in turn increasing the hole and electron injection effi-
ciency, a better balance of opposite carrier concentration
in an emissive layer can be achieved resulting in increased
electroluminescent efficiency. Insertion of carrier injection
assisted layers (e.g., HIL, HTL, EIL, and ETL) at both
anode and cathode sides are one of such interfacial modifi-
cation techniques. Consider a carrier injection in device A
of Fig. 1, it is expected to process by the following the pro-
posed hopping mechanism. Just starting from ITO (work
function: about 4.7 eV) [23] as anode, a hole is injected
by a hopping through 2-TNATA (EHOMO: 5.1 eV) [24] as
a hole-injecting layer into NPB (EHOMO: 5.4 eV) [23] as a
hole-transporting layer. An electron, just starting from Al
(work function: 4.3 eV) [23] as cathode, is injected by a
hopping through LiF (work function: 2.6 eV) [25] as an
electron-injecting layer, Alq3 (ELUMO: 3.1 eV) [23] as the
electron-transporting layer and 24MeSAlq (ELUMO in
Fig. 4: 2.9 eV) as both an electron-transporting layer and
a hole-blocking layer into a NPB layer. In the case of the
green-emitting device C, an exciton recombination zone
can be formed broadly in the Alq3 layer having functions
as EML and ETL.
Fig. 5. EL spectra of glass/ITO/2-TNATA (60 nm)/NPB (15 nm)/HBL
(20-x nm)/Alq3 (35 + x nm)/LiF (1 nm)/Al (100 nm) (h: x = 0, 24MeSAlq
(20 nm)/Alq3 (35 nm); s: x = 0, or BAlq (20 nm)/Alq3 (35 nm); n: x = 20,
Alq3 (55 nm)).
DH, completely showing a real blue emission with the 65-
nm-FWHM wavelength.
Meanwhile, an exciton recombination cannot be formed
in the HIL of 2-TNATA (PLmax: 480 nm) [24] and the HBL
of 24MeSAlq (PLmax: 461 nm). In general, the EL spectrum
cannot be moved to the hypsochromic-shifted wavelength
than the PL spectrum in OLEDs for itself. Meanwhile,
the device B with BAlq as the HBL containing one methyl
In addition, in blue devices fabricated by using a hole-
blocking layer, the exciton recombination efficiency by
hopping of a hole and an electron is expected to depend
on a difference of HOMO energy level (DH) between
NPB and HBL, which would confine most of a hole carrier
concentration only into a NPB layer. The device A is
expected to be more hypsochromic shifted in EL spectra
than device B because 24MeSAlq (DH: 0.74 eV) have lager
hole-blocking property than BAlq (DH: 0.48 eV).
Fig. 5 shows the EL spectra of the device A, B, and C,
respectively. Comparing with device C containing the
Alq3 emitter having the maximum EL peak (ELmax) at
543 nm, the device A with 24MeSAlq and device B with
BAlq as a hole-blocking layer exhibit ELmax peaks at 446
and 503 nm, respectively. Also, the EL spectra of device
A, B, and C exhibit the FWHM of 65, 122, and 117 nm,
respectively. As the number of methyl substituent on the
pyridyl ring of the quinoline ligand is increased, the hole-
blocking properties of the q2AlOR-type complexes are
improved in the blue device leading to the emissive zone
in the NPB layer. As expected in device A with 24MeSAlq,
bearing two methyl substituents on the pyridyl ring of the
quinoline ligand, the exciton recombination zone was
nearly restricted into the neighbouring NPB layer by a high
Fig. 6. Current density–voltage–luminance characteristics of the blue
OLED devices composed of ITO/2-TNATA (60 nm)/NPB (15 nm)/
24MeSAlq (20 nm)/Alq3 (35 nm)/LiF (1 nm)/Al (100 nm). The inset
shows the corresponding CIE chromaticity diagram with the coordinates
of the spectra for the same device as a function of the voltage.

J.T. Lim et al. / Journal of Organometallic Chemistry 691 (2006) 2701–2707
2707
substituent on the quinolinolato ligand is mostly emitted a
Acknowledgements
broad bluish-green color with 122-nm-FWHM as a result
of a poor hole-blocking property.
This work was supported by Electronics and Telecom-
munications Research Institute (ETRI), by the National
Research Laboratory Program (NRL) of Ministry of Sci-
ence and Technology, and by the Ministry of Commerce,
Industry and Energy (MOCIE).
Fig. 6 shows the current density–voltage–luminance
characteristic of the deep blue OLED device (device A)
with 20-nm-thick 24MeSAlq tuned by confining the recom-
bination zone into a neighbouring HTL. The onset voltage
of 20-nm-thick 24MeSAlq is 3.2 V and the external quan-
tum efficiency and the power efficiency are about 0.82%
and 0.58 lm Wꢀ1 at the luminance of 100 cd mꢀ2 (bias volt-
age of 7.4 V and current density of 3.32 mA cmꢀ2). The
maximum luminance is about 18,850 cd mꢀ2 at 14.6 V
and 1.37 A cmꢀ2. The inset of Fig. 6 shows the change of
the corresponding the CIE (Commission Internationale
de L’Eclairege) chromaticity coordinates for the device A
when the forward bias is changed from 9 V (about
30 cd mꢀ2) to 11 V (about 1300 cd mꢀ2). As shown in the
figure, the change of the forward bias from 9 to 11 V did
not change the CIE chromaticity coordinates significantly,
therefore, remaining in the deep blue range of 0.164–0.168,
0.127–0.135.
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age of 14.6 V and the current density of 1.37 A cmꢀ2
.