Solution-processed red iridium complexes based on carbazole-phenylquinoline main ligand: Synthesis, properties and their applications in phosphorescent organic light-emitting diodes

Article abstract of DOI:10.1016/j.jorganchem.2010.11.016

New carbazole-phenylquinoline (CVz-PhQ) based iridium complexes were designed and synthesized for their application in red phosphorescence organic light-emitting diodes (PhOLEDs) and their photophysical, electrochemical and electroluminescence (EL) properties were investigated. The PhOLEDs were fabricated using bis[9-(2-(2-methoxyethoxy)ethyl)-3-(4-phenylquinolin-2-yl)-9H- carbazolato-N,C2′]iridium 2-pyrazinecarboxylic acid (EO-CVz-PhQ) ₂Ir(prz) and bis[9-(2-(2-methoxyethoxy)ethyl)-3-(4-phenylquinolin-2- yl)-9H-carbazolato-N,C2′]iridium 5-methyl-2-pyrazinecarboxylic acid (EO-CVz-PhQ)₂Ir(mprz) as the emitter and PVK, co-doped with OXD-7 as the electron transport material and TPD as the hole transport material, as the polymer host. The red emissive PhOLEDs, based on the ITO/poly(3,4- ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS)/4,4′,4″- tris(carbazole-9-yl)triphenylamine (TCTA)/poly(N-vinylcarbazole) (PVK):N,N′-diphenyl-N,N′-(bis(3-methylphenyl)-[1,1-biphenyl]-4, 4′-diamine (TPD):1,3-bis[5-(4-tert-butylphenyl)-1,3,4-oxadiazole-2-yl] benzene (OXD-7):Ir complex/cathode configuration, exhibited a maximum external quantum efficiency of 3.68% and a maximum luminance efficiency of 6.69 cd/A. Furthermore, by introducing a TCTA interlayer, the PhOLEDs showed only a slight efficiency roll off of 5.4% from a low current density (1.81 mA/cm²) to a high current density (44.59 mA/cm²).

Full text of DOI:10.1016/j.jorganchem.2010.11.016

Journal of Organometallic Chemistry 696 (2011) 2122e2128
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Solution-processed red iridium complexes based on carbazole-phenylquinoline
main ligand: Synthesis, properties and their applications in phosphorescent
organic light-emitting diodes
Myungkwan Song a, Jin Su Park a, Mikyoung Yoon a, Ae Jin Kim a, Young Inn Kim a, Yeong-Soon Gal b,
**
,
a,
*
Jae Wook Lee c , Sung-Ho Jin
a Department of Chemistry Education, Interdisciplinary Program of Advanced Information and Display Materials and Center for Plastic Information System, Pusan National University,
Busan 609-735, Republic of Korea
b Polymer Chemistry Lab., Kyungil University, Hayang 712-701, Republic of Korea
c Department of Chemistry, Dong-A University, Busan 604-714, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
New carbazole-phenylquinoline (CVz-PhQ) based iridium complexes were designed and synthesized
for their application in red phosphorescence organic light-emitting diodes (PhOLEDs) and their
photophysical, electrochemical and electroluminescence (EL) properties were investigated. The
PhOLEDs were fabricated using bis[9-(2-(2-methoxyethoxy)ethyl)-3-(4-phenylquinolin-2-yl)-9H-carba-
zolato-N,C20]iridium 2-pyrazinecarboxylic acid (EO-CVz-PhQ)2Ir(prz) and bis[9-(2-(2-methoxyethoxy)
ethyl)-3-(4-phenylquinolin-2-yl)-9H-carbazolato-N,C20]iridium 5-methyl-2-pyrazinecarboxylic acid (EO-
CVz-PhQ)2Ir(mprz) as the emitter and PVK, co-doped with OXD-7 as the electron transport material and
TPD as the hole transport material, as the polymer host. The red emissive PhOLEDs, based on the ITO/poly
(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS)/4,40,400-tris(carbazole-9-yl)triphenyl-
amine (TCTA)/poly(N-vinylcarbazole) (PVK):N,N0-diphenyl-N,N0-(bis(3-methylphenyl)-[1,1-biphenyl]-
4,40-diamine (TPD):1,3-bis[5-(4-tert-butylphenyl)-1,3,4-oxadiazole-2-yl]benzene (OXD-7):Ir complex/
cathode configuration, exhibited a maximum external quantum efficiency of 3.68% and a maximum
luminance efficiency of 6.69 cd/A. Furthermore, by introducing a TCTA interlayer, the PhOLEDs showed only
a slight efficiency roll off of 5.4% from a low current density (1.81 mA/cm2) to a high current density
(44.59 mA/cm2).
Received 15 September 2010
Received in revised form
11 November 2010
Accepted 12 November 2010
Keywords:
Solution-process
PhOLEDs
Phosphorescent
Red
Hole transporting material
Crown Copyright Ó 2010 Published by Elsevier B.V. All rights reserved.
1. Introduction
efficiency of up to 100% is theoretically possible. Many PhOLEDs
have been fabricated by simultaneous or sequential vapor deposi-
Organic light-emitting diodes (OLEDs) have been the subject of
intense research in the past decade, because of their potential
applications for large area flat panel displays and solid state lighting
sources. Recently, OLEDs using phosphorescent materials such as
iridium or platinum complexes as the emitting materials were
reported, which showed a much higher efficiency than those using
fluorescent emitting materials [1e3]. In particular, phosphorescent
OLED (PhOLED) materials offer the possibility of developing highly
efficient OLEDs, since they are able to achieve emission from both
the singlet and triplet excited states. Therefore, an internal quantum
tion of several organic layers [4,5], making the fabrication process
relatively complicated and expensive, especially if large substrates
are used. However, solution based manufacturing processes, such as
spin coating and inkjet printing, provide an attractive alternative to
vacuum deposition for the preparation of OLEDs, mainly due to their
significantly reduced production cost. Therefore, it is desirable to
investigate approaches to solution processable phosphorescent
materials. Some of the research groups have been tried, including
doping polymer host systems with phosphorescent materials and
the synthesis of soluble phosphorescent complexes [6e8]. The
reported quantum efficiencies were high, but were measured in an
integrating sphere, and include both light emission from forward
direction and waveguided light, thus are not directly compared to
other device performances. The majority of OLEDs containing
solution processed layers produced so far have contained oligomers
* Corresponding author. Tel.: þ82 51 510 2727; fax: þ82 51 581 2348.
** Corresponding author.
E-mail addresses: jlee@donga.ac.kr (J.W. Lee), shjin@pusan.ac.kr (S.-H. Jin).
0022-328X/$ e see front matter Crown Copyright Ó 2010 Published by Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2010.11.016

M. Song et al. / Journal of Organometallic Chemistry 696 (2011) 2122e2128
2123
or conjugated polymers [9e14] and the more recently fabricated
2. Results and discussion
ones contained dendrimers [15e20]. Phosphorescent polymers
have been employed as solution processed emitting materials by
chemically bonding iridium complexes in the side chain, main chain,
or chain terminals [21e23]. Phosphorescent dendrimers are also
employed, because of their inherent topological features [24], in
which an iridium complex is surrounded by a branched shell to
prevent the self-aggregation of the concentration quenching of the
emissive core in the solid state. However, the performance of poly-
mer- and dendrimer-based phosphorescent materials is signifi-
cantly lower than that of their solution processable small molecule
counterparts. Many attempts have been made to tailor the ligand
structure in order to establish the relationship between the struc-
tures of the ligands and properties of the iridium complexes, and
then develop good materials for red [25], green [26] and blue [27]
OLEDs. It has been found that the emission wavelength of iridium
complexes can usually be tuned by choosing appropriate substitu-
ents and changing their positions. For instance, the emission
wavelengths of iridium complexes can be made to cover the entire
visible region by modifying or varying the cyclometalated 2-aryl-
pyridine main ligands and ancillary ligands [28]. Using this
approach, we recently reported a novel red emitting iridium
complexes based on carbazole (CVz) containing phenylquinoline
(PhQ) derivatives as an emitting layer for the fabrication of PhOLEDs
by solution processing [29]. Therefore, in this paper, we report the
synthesis, redox, photophysical, and electroluminescent (EL) prop-
erties of new red emitting heteroleptic iridium complexes contain-
ing pyrazine-based ancillary ligands for the solution processing of
the emitting layer in the fabrication of PhOLEDs.
2.1. Synthesis and characterization
Scheme 1 illustrates the synthetic procedures used for the prep-
aration of (EO-CVz-PhQ)2Ir(prz) and (EO-CVz-PhQ)2Ir(mprz). The
main ligand, 9-(2-(2-methoxyethoxy)ethyl)-3-(4-phenylquinolin-
2-yl)-9H-carbazole (EO-PhQ-CVz) was prepared byan acid-catalyzed
Friedlander condensation reaction. The reaction of 3-acetyl-N-
2-(2-methoxyethoxy)ethylcarbazole with 2-aminobenzophenone
resulted in the formation of the desired donoreacceptor type of CVz-
PhQ with 80% yield. The cyclometalated iridium m-chloride bridged
dimer was synthesized from iridium trichloride hydrate with an
excess of the main ligand using Nonoyama’s method [30]. Two
separate sodium carbonate-mediated ligand exchange reactions
with 5-methyl-2-pyrazinecarboxylic (mprz) acid and 2-pyrazine-
carboxylic acid (prz) ancillary ligands, respectively, were carried out
to give heteroleptic iridium complexes in quantitative yield. These
two iridium complexes were purified using silica column chroma-
tography and recrystallization. The final products prior to device
fabrication were characterized by 1H and 13C NMR spectroscopy and
elemental analysis.
2.2. Thermal properties
The thermal properties of the iridium complexes were examined
by thermogravimetric analysis (TGA) and differential scanning
calorimetry (DSC) under an N2 atmosphere at a heating rate of 10 ꢀC/
min and their decomposition temperatures (Td, corresponding to
O
NH2
O
N
NaH, THF
AlCl3 , (CH3CO)2O
CH2Cl2
N
O
N
O
O
Br
N
H
O
N
O
DPP, m-cresol
O
O
O
O
N
OH
O
O
IrCl3 H2O
N
N
N
N
Cl
Ir
N
Ir
Ir
2-ethoxyethanol
THF, H2O
Cl
N
N
Na2CO3
N
N
N
2-ethoxyethanol
24hr
N
O
2
2
2
O
O
O
O
O
(EO-CVz-PhQ)2Ir(prz)
O
O
O
N
OH
O
N
O
N
N
Ir
Na2CO3
N
2-ethoxyethanol
N
24hr
2
O
O
(EO-CVz-PhQ)2Ir(mprz)
Scheme 1. Synthetic routes for (EO-CVz-PhQ)2Ir(prz) and (EO-CVz-PhQ)2Ir(mprz).

2124
M. Song et al. / Journal of Organometallic Chemistry 696 (2011) 2122e2128
110
100
90
a 100
(EO-CVz-PhQ) Ir(mprz)
2
(EO-CVz-PhQ) Ir(mprz)
2
(EO-CVz-PhQ) Ir(prz)
2
80
60
40
20
0
(EO-CVz-PhQ) Ir(prz)
2
1.0
0.8
0.6
0.4
0.2
0.0
1.0
Host, PL
0.8
0.6
0.4
0.2
0.0
80
70
60
300
400
500
600
700
50
Wavelength (nm)
40
30
300
400
500
Wavelength (nm)
600
700
0
200
400
600
800
o
Temperature ( C)
Fig. 1. TGA trace of iridium complexes measured at a scan rate of 10 ꢀC/min under N2
atmosphere.
b
Ir(mprz): Solution
(EO-Cvz-PhQ)
1.0
0.8
0.6
0.4
0.2
0.0
Ir(mprz): Film
(EO-Cvz-PhQ)
(EO-Cvz-PhQ)
Ir(prz): Solution
5% weight loss) are given in Fig. 1 and summarized in Table 1. The
two iridium complexes are thermally stable up to 340 ꢀC, as deter-
mined by TGA, which is beneficial to the long-term stability of the
PhOLEDs devices fabricated from these materials. From the DSC
measurements, we observed distinct glass transition temperatures
(Tg) of (EO-CVz-PhQ)2Ir(prz) and (EO-CVz-PhQ)2Ir(mprz) at 160 and
164 ꢀC, respectively. To prevent their degradation through the
morphological changes of the amorphous organic layer, materials
having relatively high Tg values are needed for the fabrication of
PhOLEDs [31].
Ir(prz): Film
Ir(mprz): 77K
Ir(prz): 77K
(EO-Cvz-PhQ)
(EO-Cvz-PhQ)
(EO-Cvz-PhQ)
2.3. Photophysical properties
550
600
650
700
750
800
Fig. 2 displays the UVevisible absorption (a) and photo-
luminescence (PL) (b) spectra of the two iridium complexes dis-
solved in chloroform solution. The photophysical parameters of the
iridium complexes are summarized in Table 1. The main absorption
peak at 315 nm, with shoulder peaks at 356 and 410 nm and
additional broad tail peaks in the range of 450e550 nm, are
observed. In all cases, the higher energy absorption peaks below
Wavelength (nm)
Fig. 2. Absorption (a) and PL (b) spectra of the (EO-CVz-PhQ)2Ir(prz) and (EO-CVz-
PhQ)2Ir(mprz) in CHCl3 at 293 K. Inset : Absorption spectra of the (EO-CVz-PhQ)2Ir
(prz) and (EO-CVz-PhQ)2Ir(mprz) and PL spectrum of host.
spectra of (EO-CVz-PhQ)2Ir(prz) and (EO-CVz-PhQ)2Ir(mprz), their
spectra were measured, as shown in the inset of Fig. 2(a). This
overlap should enable the efficient Förster energy transfer from the
singlet-excited state in the host to the MLCT band of the guest,
followed by fast intersystem crossing to the triplet state of (EO-CVz-
PhQ)2 Ir(prz) and (EO-CVz-PhQ)2 Ir(mprz) and, consequently,
emission from their triplet state. The solution state PL spectra of
(EO-CVz-PhQ)2Ir(prz) and (EO-CVz-PhQ)2Ir(mprz) show the emis-
sion of red light with main peaks at 613 and 600 nm, respectively,
as shown in Fig. 2(b). The spectra are broad and structureless,
implying that the lowest triplet states of these iridium complexes
400 nm are assigned to the spin-allowed 1p p* transitions of the
-
cyclometalated ligands. The absorption peaks extending into the
visible region around 480 nm can be assigned to the spin-allowed
metal-ligand charge-transfer band (1MLCT) and the weak bands at
the longer wavelength can be assigned to both spin-orbit coupling
enhanced 3p
-
p* and 3MLCT transition [32]. As a result, the two
iridium complexes exhibit very similar 1p
-
p* 1MLCT and 3MLCT
,
absorptions, suggesting that the ancillary ligand makes only a small
contribution to the absorption process. To investigate the effective
overlap between the PL spectrum of the host and the absorption
Table 1
Photophysical, electrochemical and thermal data for synthesized red emitting iridium complexes.
Compound
T
d (ꢀC)a
Tg (ꢀC)b
labs (log
3
(nm))c
lem (nm)d
lem (nm)e
lem (nm)f
Fpl (%)g
HOMO/LUMO (ev)h
(EO-CVz-PhQ)2Ir(mprz)
(EO-CVz-PhQ)2Ir(prz)
369
346
164
160
315(1.96), 352(1.74), 408(1.57)
315(1.95), 356(1.74), 410(1.56)
600
613
594
593
584
582
0.015
0.006
ꢁ5.23/ꢁ2.70
ꢁ5.23/ꢁ2.85
a
Temperature with 5% mass loss measure by TGA with a heating rate of 10 ꢀC/min under N2.
Glass transition temperature, determined by DSC with a heating rate of 10 ꢀC/min under N2.
Measured in CHCl3 solution.
b
c
d
e
f
Maximum emission wavelength, measured in CHCl3 solution (298 K).
Maximum emission wavelength, measured in PMMA film (298 K).
Maximum emission wavelength, measured in CHCl3 solution (77 K).
g
h
Measured in 1 x 10ꢁ5 M degassed CHCl3 solution relative to Ir(piq)2(acac) (Fpl ¼ 0.20) with 450 nm excitation.
Determined from the half-wave potentials for oxidation and reduction.

M. Song et al. / Journal of Organometallic Chemistry 696 (2011) 2122e2128
2125
are likely dominated by the 3MLCT excited state. On the other hand,
40
20
the PL spectra of the iridium complexes doped (8 wt%) in poly
(methyl methacrylate) (PMMA) films exhibit smaller hypsochromic
shifts compared to those in the solution state. The characteristic
hypsochromic shift of the PL spectra of the iridium complexes in
the film state are thought to be related to the motional relaxation of
the excited-state geometry, which is prone to be affected by the
rigidity of the matrix. Furthermore, a similar hypsochromic shift
was also observed when the CHCl3 solution of the two iridium
complexes was frozen (77 K). The hypsochromic shift observed at
77 K is mainly due to the solvent reorganization in a fluid solution
at low temperature that can stabilize the charge-transfer states
prior to emission [33]. This process is significantly impeded in
a rigid matrix at 77 K and, hence, phosphorescence occurs at
a higher energy. We further investigated the photophysical prop-
erties of the iridium complexes by measuring their PL quantum
yields (Fpl) in dilute degassed CHCl3 solutions at 298 K (Table 1).
(Cvz-PhQ) Ir(mprz)
2
(Cvz-PhQ) Ir(prz)
2
0
-20
-40
-60
-80
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
Potential (V)
Fig. 3. Cyclic voltammograms of iridium complexes in tetra-n-butylammonium hex-
afluorophosphate (TBAPF6) at a scan rate of 100 mV/s.
2.4. Electrochemical properties
To investigate the electrochemical properties of (EO-CVz-PhQ)2Ir
(prz) and (EO-CVz-PhQ)2Ir(mprz), we estimated their highest
occupied molecular orbital (HOMO) and lowest unoccupied
molecular orbital (LUMO) energy levels by cyclic voltammetry
(CV). The CV measurements were carried out in a three electrode
cell setup with a 0.1M solution of tetra-n-butylammonium hexa-
fluorophosphate (TBAPF6) in anhydrous acetonitrile/benzene (1:1.5
v/v). The CV curves were referenced to an Ag/AgNO3 reference
electrode, which was calibrated using a ferrocene/ferrocenium
(Fc/Fcþ) redox couple (4.8 eV below the vacuum level) as an external
standard [34]. The E1/2 of the Fc/Fcþ redox couple was found to be
0.50 V vs. the Ag/AgNO3 reference electrode. Therefore, the HOMO
and LUMO energy levels of (EO-CVz-PhQ)2Ir(prz) and (EO-CVz-
PhQ)2Ir(mprz) can be calculated using the empirical equation
EHOMO ¼ E1/2 (ox) þ 4.30 eV and ELUMO ¼ E1/2 (red) þ 4.30 eV,
respectively, where E1/2(ox) and E1/2(red) stand for the half-wave
potentials for oxidation and reduction relative to the Ag/AgNO3
reference electrode, respectively. The CV curves of (EO-CVz-PhQ)2Ir
(prz) and (EO-CVz-PhQ)2Ir(mprz) are shown in Fig. 3. In all of the
cases, the electrochemical switching was reversible and the CV
curves remained unchanged under multiple successive potential
scans, suggesting their excellent stability against electrochemical
oxidation and reduction. On the basis of the oxidation and reduction
potentials, (EO-CVz-PhQ)2Ir(prz) and (EO-CVz-PhQ)2Ir(mprz)
exhibited in the ranges of 0.93 V and ꢁ1.45 to ꢁ1.60 V, respectively.
From the results, the HOMO and LUMO energy levels of (EO-CVz-
PhQ)2Ir(prz) and (EO-CVz-PhQ)2Ir(mprz) were calculated to be
ꢁ5.23 eV and ꢁ2.85 to ꢁ2.70 eV, respectively. The HOMO and LUMO
energy levels are summarized in Table 1.
complexes would be able to trap both electrons and holes in the
emitting layer. Moreover, a hole blocking or electron transport
(OXD-7) layer was introduced for effective electron injection/
facilitate and charge carrier balance within the emitting layer. The
LUMO energy level of OXD-7 is closely aligned to the LUMO energy
level of the iridium complexes in the emitting layer.
Fig. 4 displays the relative HOMO and LUMO energy levels of the
materials used to fabricate the PhOLEDs. The energy level diagrams
show that the energy levels of (EO-CVz-PhQ)2Ir(prz) and (EO-CVz-
PhQ)2Ir(mprz) (both HOMO and LUMO) lie above and below those
of the host, respectively. Consequently, it is possible that iridium
complexes would be able to trap both electrons and holes in the
emitting layer. Moreover, a hole blocking or electron transport
(OXD-7) layer was introduced for effective electron injection/
facilitate and charge carrier balance within the emitting layer. The
LUMO energy level of OXD-7 is closely aligned to the LUMO energy
level of the iridium complexes in the emitting layer.
Fig. 5 displays the electroluminescent (EL) spectra of (EO-CVz-
PhQ)2Ir(prz) and (EO-CVz-PhQ)2Ir(mprz) at a current density of
10 mA/cm2 and their PL film spectra. The EL spectra exhibited a red
emission with a peak maximum at 600 nm and excellent color
purity at a CIE coordinate of (0.60, 0.39). The PL film spectra of (EO-
CVz-PhQ)2Ir(prz) and (EO-CVz-PhQ)2Ir(mprz) showed a slight
difference from their EL spectra, corresponding to a red-shift of
2.5. Electroluminescence properties
We evaluated the potential of (EO-CVz-PhQ)2Ir(prz) and (EO-
CVz-PhQ)2Ir(mprz) as red emissive materials in PhOLEDs applica-
tions with the configuration of ITO/PEDOT:PSS/TCTA (30 nm)/EML
(50 nm)/OXD-7 (20 nm)/Ba (3 nm)/Al (100 nm). As a host, we
selected a blend of poly(vinylcarbazole) (PVK), N,N0-diphenyl-N,N0-
(bis(3-methylphenyl)-[1,1-biphenyl]-4,40-diamine (TPD) as the hole
transport material and 1,3-bis[5-(4-tert-butylphenyl)-1,3,4-oxa-
diazole-2-yl]benzene (OXD-7) as the electron transport material.
Fig. 4 displays the relative HOMO and LUMO energy levels of the
materials used to fabricate the PhOLEDs. The energy level diagrams
show that the energy levels of (EO-CVz-PhQ)2Ir(prz) and (EO-CVz-
PhQ)2Ir(mprz) (both HOMO and LUMO) lie above and below those
of the host, respectively. Consequently, it is possible that iridium
Fig. 4. The relative HOMO/LUMO energy levels of the materials used for the PhOLEDs.

2126
M. Song et al. / Journal of Organometallic Chemistry 696 (2011) 2122e2128
Table 2
EL performance of red PhOLED based on a solution processed emitting layer.
1.0
0.8
0.6
0.4
0.2
0.0
(EO-CVz-PhQ) Ir(mprz)-PL
2
Emitter
Turn-on Lmax
(V)a
(EO-CVz-PhQ)2Ir(mprz) 5.6
EQEmax LEmax PEmax CIE (x, y)b
(cd/A) (lm/W)
(EO-CVz-PhQ) Ir(prz)-PL
2
(cd/m2) (%)
(EO-CVz-PhQ) Ir(mprz)-EL
2
7219
4024
3.68
2.33
6.69
3.96
1.80
1.03
0.60, 0.39
0.60, 0.39
(EO-CVz-PhQ) Ir(prz)-EL
2
(EO-CVz-PhQ)2Ir(prz)
5.6
a
Luminance is 1 cd/m2.
b
Recorded at current density 10 mA/cm2.
the PhOLEDs using (EO-CVz-PhQ)2Ir(prz) and (EO-CVz-PhQ)2Ir
(mprz) were found to be 3.96 and 6.69 cd/A and their maximum
power efficiencies were 1.03 and 1.80 lm/W, respectively. When
using (EO-CVz-PhQ)2Ir(mprz) as an emitter, the PhOLEDs exhibited
an efficiency of 6.69 cd/A at a low current density of 1.81 mA/cm2.
On the other hand, at a high current density of 44.59 mA/cm2, the
PhOLEDs exhibited a luminance efficiency of 6.33 cd/A, with only
a 5.4% luminance efficiency roll-off being observed. From this
result, it can be concluded that the problematic efficiency roll-off in
PhOLEDs can also be suppressed by introducing a TCTA interlayer
between the emitting layer and PEDOT:PSS. This phenomenon may
be explained by the fact that the TCTA interlayer renders the carrier
injection to the PVK host more efficient and blocks the electrons at
the TCTA/EML interface, thus allowing them to be better confined at
the interface. The roll-off of the PhOLEDs using (EO-CVz-PhQ)2Ir
(prz) as an emitter showed a similar trend to that of the PhOLEDs
based on (EO-CVz-PhQ)2Ir(mprz).
550
600
650
700
750
800
Wavelength (nm)
Fig. 5. EL and PL spectra (film state) of (EO-CVz-PhQ)2Ir(prz) and (EO-CVz-PhQ)2Ir
(mprz).
about 5 nm. These differences between the EL and PL spectra are
attributed to the optical effect caused by the recombination zone
shift [35].
Fig. 6(a) displays the current density-voltage-luminance (J-V-L)
characteristics of these red PhOLEDs. The maximum luminances of
the PhOLEDs using (EO-CVz-PhQ)2Ir(prz) and (EO-CVz-PhQ)2Ir
(mprz) were found to be 4024 and 7219 cd/m2 and their turn-on
voltages were 5.6 and 5.6 V, respectively. The PhOLEDs with the
(EO-CVz-PhQ)2Ir(mprz) dopant exhibited a higher current density
than that of those with the (EO-CVz-PhQ)2Ir(prz) dopant at the
same voltage. This result can be explained by the efficient electron
injection or transport from the Al cathode to the emitting layer.
(EO-CVz-PhQ)2Ir(prz) and (EO-CVz-PhQ)2Ir(mprz) have LUMO
energy levels of ꢁ2.85 and ꢁ2.70 eV, respectively. As can be seen in
Fig. 4, electrons are somewhat trapped by (EO-CVz-PhQ)2Ir(prz),
due to the LUMO gap of 0.15 eV between OXD-7 and (EO-CVz-
PhQ)2Ir(prz). On the other hand, the LUMO gap between OXD-7 and
(EO-CVz-PhQ)2Ir(mprz) is 0 eV, which is very well aligned with the
LUMO energy level of (EO-CVz-PhQ)2Ir(mprz). Fig. 6(b) displays the
luminance efficiency and power efficiency versus the current
density characteristics and the performances of the red PhOLEDs
are summarized in Table 2. The maximum luminance efficiencies of
3. Conclusion
We successfully synthesized a new iridium complexes with
a carbazole containing phenylquinoline (CVz-PhQ) derivative, in
order to fabricate solution processed, red PhOLEDs. The fabricated
red PhOLEDs with the configuration of ITO/PEDOT:PSS/TCTA
interlayer/PVK:TPD:OXD-7:iridium complex/cathode exhibited
maximum external quantum and luminance efficiency values of
3.68% and 6.69 cd/A, respectively. Furthermore, by introducing
a TCTA interlayer between the emitting layer and PEDOT:PSS, the
efficiency roll off can be suppressed through effective carrier
injection and electron blocking within the emitting layer.
4. Experimental
4.1. Characterization
1H NMR and 13C NMR spectra were recorded on a Varian
Mercury Plus 300 MHz spectrometer and the chemical shifts were
recorded in ppm units with CDCl3 as the internal standard. The
thermal analyses were carried out on a Mettler Toledo TGA/SDTA
851e, DSC 822e analyzer under an N2 atmosphere at a heating rate
of 10 ꢀC/min. CV was carried out with a CHI 600C potentiostat (CH
Instruments) at a scan rate of 100 mV/s in a 0.1 M solution of tetra-
n-butylammonium hexafluorophosphate (TBAPF6) in anhydrous
acetonitrile/benzene (1:1.5 v/v). A platinum wire was used as the
counter electrode and an Ag/AgNO3 electrode was used as the
reference electrode. UVevisible spectra were measured using
a JASCO V-570 spectrophotometer. PL spectra and low-temperature
phosphorescence spectra were obtained using Hitachi F-4500
fluorescence spectrophotometer.
10
10
10
10
10
10
10
10
,
(EO-CVz-PhQ) Ir(mprz)
(EO-CVz-PhQ) Ir(prz)
,
(b)
0
20 40
60 80 100 120 140 160
Current density [mA/cm ]
4.2. Synthesis of (EO-CVz-PhQ)2Ir(prz) or (EO-CVz-PhQ)2Ir(mprz)
9-(2-(2-Methoxy-ethoxy)ethyl)-3-(4-phenylquinolin-2-yl)-9H-
carbazole (EO-CVz-PhQ) (1.07 g, 2.27 mmol) and IrCl3$H2O (0.32 g,
0.91 mmol) were added to a mixture of 2-ethoxyethanol and water
Fig. 6. Current density-voltage-luminance (a) and luminance and power efficiency-
current density (b) characteristics of (EO-CVz-PhQ)2Ir(prz) and (EO-CVz-PhQ)2Ir
(mprz).

M. Song et al. / Journal of Organometallic Chemistry 696 (2011) 2122e2128
2127
(80 mL, 3:1 v/v). The reaction mixture was stirred at 140 ꢀC for 20 h
and a brown precipitate was obtained after cooling at room
temperature. The precipitate was collected and washed with
deionized water (80 mL) and methanol (40 mL). Subsequently, the
cyclometalated iridium dimer was dried under vacuum to afford
a brown solid. Cyclometalated iridium dimer (0.6 g, 0.26 mmol) and
2-pyrazinecarboxylic acid (0.16 g, 1.28 mmol) or 5-methyl-2-pyr-
azinecarboxylic acid (0.18 g, 1.28 mmol) were mixed with Na2CO3
(0.27 g, 2.56 mmol) in 2-ethoxyethanol (30 mL). The mixture was
refluxed for 12 h under a N2 atmosphere. After cooling at room
temperature, the crude solution was poured onto water and
extracted with ethyl acetate, dried over anhydrous MgSO4 and
evaporated in a vacuum. The residue was purified by silica gel
chromatography (hexane:ethyl acetate ¼ 1:4) as an eluent and
further purified by recrystallization twice using dichloromethane/
hexane mixture to afford a red solid complex (0.25 g, 78%).
deposited with effective area of 4 mm2 at a pressure 5 ꢂ 10ꢁ6 Torr.
The film thickness was measured by using a-Step IQ surface profiler
(KLA Tencor, San Jose, CA). EL spectra and current density-voltage-
luminance (J-V-L) characteristics of PhOLEDs were measured with
a programmable Keithley model 236 power source and spectrascan
CS-1000 photometer, respectively. All measurements were carried
out at room temperature under an ambient atmosphere without
encapsulation.
Acknowledgements
This work was supported by National Research Foundation of
Korea (NRF) grant funded from the Ministry of Education, Science
and Technology (MEST) of Korea (No. M10600000157-06J0000-
15710) and Basic Science Research Program through the National
Research Foundation of Korea (NRF) grant funded from the Ministry
of Education, Science and Technology (MEST) of Korea for the
Center for Next Generation Dye-sensitized Solar Cells (No. 2010-
0001842). This work was supported by the New & Renewable
Energy program of the Korea Institute of Energy Technology Eval-
uation and Planning (KETEP) grant (No. 20103020010050) funded
by the Ministry of Knowledge Economy, Republic of Korea.
4.2.1. (EO-CVz-PhQ)2Ir(prz)
1H NMR (300 MHz, CDCl3):
d (ppm): 9.04 (s, 1H), 8.78 (d, 2H),
8.64 (s, 1H), 8.55 (s, 1H), 8.27 (s, 1H), 8.13 (d, J ¼ 7.8 Hz, 1H), 8.04
(d, 1H), 7.82e7.61 (m, 12H), 7.51e7.12 (m, 12H), 7.03 (s, 1H), 6.86
(t, 1H), 6.32 (s, 1H), 4.09e3.77 (m, 4H), 3.58e3.25 (m, 4H),
3.15e2.88(m, 14H) 13C NMR (300 MHz, CDCl3):
d (ppm): 170.8,
170.2,168.5,150.6,150.5,148.6,147.9,147.1,147.0,146.6,144.0,142.8,
142.2, 140.4, 140.3, 139.2, 138.5, 137.6, 137.5, 137.3, 129.5, 129.1,
129.0, 128.8, 128.7, 126.4, 126.1, 125.5, 125.2, 125.1, 125.0, 124.6,
123.9, 123.8, 119.7, 119.3, 119.1, 119.0, 118.9, 118.0, 117.0, 115.2, 113.6,
109.0, 108.7, 71.6, 71.3, 70.2, 70.1, 68.0, 67.5, 59.0, 58.9, 41.9. Anal.
Calcd. For C69H57N6O6Ir: C, 65.85; H,4.57; N,6.68; found: C,66.15;
H,4.63; N,6.56.
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OXD-7 (20 nm)/Ba (3 nm)/Al (100 nm). The indium tin oxide (ITO)
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U per square was
washed in turn with a substrate-cleaning detergent, deionized
water, acetone, and isopropyl alcohol, and finally treated with
UVeozone chamber for 20 min. The PEDOT:PSS (40 nm, CLEVIOS P
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