Multifunctional phosphorescent iridium (III) complexes based on 2-phenylbenzothiazole derivative for highly efficient organic light-emitting diodes

Article abstract of DOI:10.1016/j.dyepig.2014.02.025

Phosphorescent robust heteroleptic iridium (III) [Ir(III)] complexes based on 2-(4-(9H-carbazol-9-yl)phenyl)benzothiazole integrated with three auxiliary ligand were designed, synthesized and used to fabricate organic light-emitting diodes (OLEDs). The Ir(III) complexes emitted intense phosphorescence with yellow or orange color at room temperature with high quantum efficiencies and relatively short lifetimes. The auxiliary ligand greatly influenced the electronic and photophysical properties of their corresponding heteroleptic Ir(III) complexes. The complexes showed reversible redox characteristics. Hole-only single-carrier devices indicated that three complexes all possessed excellent hole-transporting ability. The phosphorescent OLEDs using three Ir(III) complexes as dopants exhibited turn-on voltages of ca. 4 V, maximum current efficiencies of 22.3-44.2 cd A^-1 and maximum power efficiencies of 18.3-23.3 lm W^-1, which were superior to the (bt)₂Ir(acac)-based device (19.1 cd A^-1, 9.3 lm W ^-1) under the same condition. The elevated devices' performances based on the new Ir(III) complexes as emitters were probably attributed to the hole-transporting rigid carbazole moiety resulting in more balanced electron and hole transportation and recombination in emissive layer.

Full text of DOI:10.1016/j.dyepig.2014.02.025

Dyes and Pigments 106 (2014) 51e57
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Multifunctional phosphorescent iridium (III) complexes based on
2-phenylbenzothiazole derivative for highly efficient organic
light-emitting diodes
,
a
a
a
a
b,
*
Xiao Li a , Tie-Nan Zang , Hai-Jun Chi , Yan Dong , Guo-Yong Xiao , Dong-Yu Zhang
*
a School of Chemical Engineering, University of Science and Technology Liaoning (USTL), Anshan 114051, People’s Republic of China
b Suzhou Institute of Nano-Tech and Nano-Bionics (SINANO), Chinese Academy of Sciences, Suzhou 215123, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
Phosphorescent robust heteroleptic iridium (III) [Ir(III)] complexes based on 2-(4-(9H-carbazol-9-yl)
phenyl)benzothiazole integrated with three auxiliary ligand were designed, synthesized and used to
fabricate organic light-emitting diodes (OLEDs). The Ir(III) complexes emitted intense phosphorescence
with yellow or orange color at room temperature with high quantum efficiencies and relatively short
lifetimes. The auxiliary ligand greatly influenced the electronic and photophysical properties of their
corresponding heteroleptic Ir(III) complexes. The complexes showed reversible redox characteristics.
Hole-only single-carrier devices indicated that three complexes all possessed excellent hole-transporting
ability. The phosphorescent OLEDs using three Ir(III) complexes as dopants exhibited turn-on
voltages of ca. 4 V, maximum current efficiencies of 22.3e44.2 cd Aꢀ1 and maximum power effi-
Received 4 November 2013
Received in revised form
17 February 2014
Accepted 24 February 2014
Available online 11 March 2014
Keywords:
Iridium complex
2-Phenylbenzo[d]thiazole
Carbazole
Electroluminescence
Photoluminescence
OLEDs
ciencies of 18.3e23.3 lm Wꢀ1, which were superior to the (bt)2Ir(acac)-based device (19.1 cd Aꢀ1
,
9.3 lm Wꢀ1) under the same condition. The elevated devices’ performances based on the new Ir(III)
complexes as emitters were probably attributed to the hole-transporting rigid carbazole moiety resulting
in more balanced electron and hole transportation and recombination in emissive layer.
Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction
stabilities, adequate triplet lifetimes, and strong spin-orbit coupling
effects.
Organic light-emitting diodes (OLEDs) have attracted enormous
attention in the scientific community due to their potential appli-
cations for full-color flat-panel display and large-area solid-state
lighting. Research on novel phosphorescent materials based on
transition-metal [such as Ir(III) [1e6], Pt(II) [7e12], and Re(I) [13e
15]] complexes has played a key role in developing OLEDs. Singlet
and triplet states in phosphorescent complexes are mixed
adequately owing to strong spin-orbital coupling of transition-
metal ions, thus both singlet and triplet excitons can be fully uti-
lized resulting in the internal quantum efficiency of 100% theoret-
ically. Among those phosphorescent emitters, Ir(III) complexes are
considered to be the most successful and prime candidates for
highly efficient emissive dopants in OLEDs because of high photo-
luminescence (PL) efficiency, flexible color tenability, thermal
2-Phenylbenzo[d]thiazole (bt) is a general ligand framework to
construct Ir(III) complexes such as typical yellow bis(2-
0
phenylbenzothiozolato-N,C2 )iridium(acetylacetonate)
[(bt)2Ir(-
acac)] which is particularly promising to develop two-color white
OLEDs with simple architectures [16,17]. Up to now, many
(bt)2Ir(acac) derivatives by structure modifications on bt ligand
have exhibited versatile properties such as adjustable color, high
luminescent efficiency and EL performances [18e20]. For example,
Lu et al. [21,22] reported that fine tuning of emission color of Ir(III)
complexes via substituent effect on bt ligand. Recently, Li et al.
[23,24] reported that the incorporation of substituent on the benzo
[d]thiazole ring of the bt ligand did not change significantly the
phosphorescence energy and emission color of the Ir(III) com-
plexes, but had a strong influence on their phosphorescence
quantum yield and EL performance, mainly depending on the
electronic nature of the substituent. Ma and Yang et al. [25]
employed the rational device structures to obtain high power ef-
ficiency yellow phosphorescent OLEDs by using Ir(III) complexes
with halogen-substituted bt ligands. Liu et al. [26] reported very
* Corresponding authors. Tel.: þ86 412 5929881; fax: þ86 412 5929627.
E-mail addresses: aslixiao@gmail.com (X. Li), dyzhang2010@sinano.ac.cn
(D.-Y. Zhang).
http://dx.doi.org/10.1016/j.dyepig.2014.02.025
0143-7208/Ó 2014 Elsevier Ltd. All rights reserved.

52
X. Li et al. / Dyes and Pigments 106 (2014) 51e57
high-efficiency red-electroluminescence devices based on a bt-
ligated iridium complex [(bt)2Ir(dipba)] by use of an easily avail-
able amidinate as auxiliary ligand.
1H), 7.78 (d, J ¼ 8.5 Hz, 2H), 7.59e7.55 (t, 3H), 7.49e7.46 (m, 3H),
7.36e7.33 (t, 2H); 13C NMR (125 MHz, CD2Cl2, TMS): 166.91, 154.04,
140.48, 140.22, 135.15, 132.22, 129.05, 127.19, 126.52, 126.15, 125.45,
123.67, 123.15, 121.77, 120.38, 120.30, 109.83.
In addition, the performance of the device is greatly influenced
by the charge balance between the electrons and holes from
opposite electrodes. Therefore, it’s highly desirable that the
designed phosphor shows the improved charge-transporting fea-
tures. Carbazole unit as a successful example has been introduced
into transition-metal-based complexes to promote their charge
carrier injection and transport ability [27e33]. Lately, we intro-
duced carbazole unit into bt ligand by flexible alkoxy chain forming
functional (cbbt)2Ir(acac) complex, which exhibited improved
electrophosphorescent performances [34]. Unfortunately, due to
the twisting motion of the flexible alkoxy chain which facilitated
the nonradiative decay channels, (cbbt)2Ir(acac) didn’t serve as very
efficient yellow phosphors compared with those reported yellow or
orange Ir(Ⅲ)-doped OLEDs [20,23,35].
With the aim of continuing effort, we reported herein three
robust Ir(III) complexes which were based on the rigid ligand 2-(4-
(9H-carbazol-9-yl)phenyl)benzo[d]thiazole (cbt) system with three
auxiliary ligand, namely (cbt)2Ir(acac) (acac ¼ acetylacetone),
(cbt)2Ir(tmd) (tmd ¼ 2,2,6,6-tetramethylheptane-3,5-dione) and
(cbt)2Ir(pic) (pic ¼ picolinic acid), and possessed the dual functions
of light emission and hole transportation. The photophysical,
electrochemical and electroluminescent (EL) properties of the Ir(Ⅲ)
complexes were also investigated in detail. The OLEDs based on
these Ir(III) complexes as doped emitters exhibited attractive
electrophosphorescent performances.
2.2.2. Synthesis of (cbt)2Ir(acac)
Under N2 atmosphere, a mixture of ligand cbt (0.8 g, 2.14 mmol),
IrCl3$3H2O (0.352 g, 1 mmol), 2-ethoxyethanol and water (16 mL,
3:1, v/v) were heated to 110e120 ꢂC for 30 h. Then the reaction
mixture was cooled to RT and water was added. The brick red dimer
was obtained after vacuum filtration and drying.
A mixture of the above resulting dimer, acetylacetone (Hacac)
(2.2 mL, 2 mmol), potassium carbonate (0.69 g, 5 mmol) and 2-
ehoxyethanol (12 mL) was heated to 110e120 ꢂC for 24 h under
N2 atmosphere. After cooling the mixture to RT and the addition of
water, the resulting precipitate was collected by vacuum filtration
and drying. The orange-red product was obtained by column
chromatography on silica gel using petroleum and ethyl acetate as
eluents. Yield: 71%. MS (ESI): m/z 1043.2 [M þ Hþ]. 1H NMR
(500 MHz, CD2Cl2, TMS):
d
8.34 (d, J ¼ 8.5 Hz, 2H), 8.018e8.016 (m,
4H), 8.002e7.913 (m, 2H), 7.88 (d, J ¼ 8.0 Hz, 2H), 7.69 (d, J ¼ 8.5 Hz,
2H), 7.57 (t, J ¼ 7.5 Hz, 2H), 7.247e6.710 (m, 14H), 6.706 (s, 2H),
5.340 (s, 1H), 1.952 (s, 6H); 13C NMR (125 MHz, CD2Cl2, TMS):
186.31, 179.18, 150.58, 149.65, 140.56, 139.79, 138.36, 131.79, 131.69,
128.74, 127.63, 126.80, 125.64, 125.56, 123.44, 122.81, 119.90, 119.85,
119.84, 118.88, 110.23, 101.70, 29.67, 28.08, 18.86. Elemental analysis
for C55H37IrN4O2S2. Calcd: C, 63.38; H, 3.58; N, 5.38; Found: C 63.22,
H 3.65, N 5.33.
2. Experimental
2.2.3. Synthesis of (cbt)2Ir(tmd)
The complex (cbt)2Ir(tmd) was prepared according to the same
procedure as the complex (cbt)2Ir(acac). Orange powder. Yield:
87%. MS (ESI): m/z 1126.8 [M þ Hþ]. 1H NMR (500 MHz, CD2Cl2,
2.1. Reagents and physical measurements
Commercially available reagents and starting materials were
used for synthesis of the Ir(Ⅲ) complexes without further purifi-
cation. Solvents were purified and dried by standard procedures
prior to use. NMR spectra were recorded on a Bruker AC 500
spectrometer with tetramethylsilane (TMS) as an internal refer-
ence. Mass spectroscopy (MS) was performed on AB SCIEX API 3200
spectrometer. The melting point (mp) was measured on WRS-2A
numeral melting point instrument. Elemental analysis was per-
formed on Vario EL III CHNS instrument. UVevis absorption and PL
spectra of the Ir(Ⅲ) complexes in CH2Cl2 solution of
1.0 ꢁ 10ꢀ5 mol Lꢀ1 were completed on a PerkinElmer Lambda 900
spectrophotometer and LS 55 fluorescence spectrophotometer,
respectively. The luminescent lifetimes of the Ir(Ⅲ) complexes in
CH2Cl2 were detected by a system equipped with a TDS 3052 digital
TMS):
d
8.25 (d, J ¼ 8.0 Hz, 2H), 8.023e8.006 (m, 4H), 7.918e7.873
(m, 4H), 7.591e5.561 (m, 2H), 7.52 (t, J ¼ 7.0 Hz, 2H), 7.27 (t, 4H),
7.215e6.839 (m, 10H), 6.836 (s, 2H), 5.557 (s, 1H), 0.953 (s, 18H); 13
C
NMR (125 MHz, CD2Cl2, TMS): 195.67, 179.05, 151.12, 150.61, 140.81,
139.98, 138.07, 132.36, 131.29, 127.12, 126.49, 125.58, 125.34, 123.35,
122.43, 120.13, 119.83, 119.76, 118.62, 110.26, 91.52, 41.15, 27.71.
Elemental analysis for C61H49IrN4O2S2. Calcd: C, 65.04; H, 4.38; N,
4.97; Found: C 64.96, H 4.35, N 4.92.
2.2.4. Synthesis of (cbt)2Ir(pic)
The complex (cbt)2Ir(pic) was prepared according to the same
procedure as the complex (cbt)2Ir(acac). Orange powder. Yield:
53%. MS (ESI): m/z 1066.5 [M þ Hþ]. 1H NMR (500 MHz, CD2Cl2,
TMS):
d
8.75 (d, J ¼ 8.0 Hz,1H), 8.22 (t, J ¼ 8.5 Hz, 2H), 8.033e7.9934
phosphor oscilloscope pulsed Nd:YAG laser with
a
Third-
(m, 7H), 7.841e7.796 (m, 2H), 7.727 (d, J ¼ 7.5 Hz, 1H), 7.582 (d,
J ¼ 6.5 Hz, 1H), 7.516 (d, J ¼ 8.0 Hz, 1H), 7.392e6.892 (m, 16H), 6.625
(s, 1H), 6.622 (s, 1H), 6.348 (d, J ¼ 8.5 Hz, 1H); 13C NMR (125 MHz,
CD2Cl2, TMS): 180.55, 178.45, 172.49, 153.32, 150.95, 149.87, 149.83,
149.74, 148.72, 139.82, 139.79, 139.76, 139.58, 139.33, 139.25, 138.47,
131.99, 131.63, 131.19, 130.79, 128.39, 128.35, 127.88, 127.44, 127.41,
126.99,125.93,125.68, 125.30,123.60,123.47,123.44,122.71,120.76,
120.13, 119.99, 119.91, 119.82, 119.77, 119.13, 117.67, 110.20, 110.10.
Elemental analysis for C56H34IrN5O2S2. Calcd: C, 63.14; H, 3.22; N,
6.57; Found: C 63.06, H 3.15, N 6.52.
Harmonic-Generator (THG) 355 nm output. Cyclic voltammetry
experiments were conducted using a CHI 660D electrochemical
analyzer with a scan rate of 100 mV sꢀ1. All measurements were
carried out at room temperature (RT) unless otherwise specified.
2.2. Synthesis of the Ir(Ⅲ) complexes
2.2.1. Synthesis of cbt
A
mixture of 4-(9H-carbazol-9-yl)benzaldehyde (1.626 g,
6 mmol), 2-aminothiophenol (0.825 g, 6.6 mmol) and DMSO
(24 mL) was heated to 150e160 ꢂC in oil bath for 3.5 h. The mixture
was cooled to RT and poured into sodium chloride aqueous solution
and then the resulting precipitate was filtered. The light-yellow
crystal was obtained after recrystallization with ethyl acetate as
solvent. Yield: 65.6%. Mp: 223.3e223.6 ꢂC. MS (ESI): m/z 377.3
2.3. Device fabrication and EL measurements
OLEDs were fabricated through vacuum deposition of the ma-
terials at about 1 ꢁ 10ꢀ6 Torr onto ITO-coated glass substrates with
a sheet resistance of 25
U
sqꢀ1. The ITO-coated substrates were
[M þ Hþ]. 1H NMR (500 MHz, CD2Cl2, TMS):
d
8.39 (d, J ¼ 8.5 Hz, 2H),
routinely cleaned by ultrasonic treatment in solvent and then
cleaned by exposure to a UV-ozone ambient. All organic layers were
8.20 (d, J ¼ 7.5 Hz, 2H), 8.14 (d, J ¼ 8.5 Hz, 1H), 8.01 (d, J ¼ 8.0 Hz,

X. Li et al. / Dyes and Pigments 106 (2014) 51e57
53
deposited in succession without breaking vacuum. The devices
were prepared with the following structures of ITO/m-MTDATA
(20 nm)/NPB (20 nm)/CBP: Ir(Ⅲ) complexes (20 nm)/BmPyPb
(30 nm)/Alq3 (10 nm)/LiF (1 nm)/Al (150 nm), in which m-MTDATA
[4,40,400-Tris(N-3-methylphenyl-N-phenylamino)triphenylamine ],
NPB [4,40-bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl], CBP
(4,40-N,N0-dicarbazolebiphenyl), BmPyPb [1,3-bis(3,5-dipyrid-3-yl-
phenyl)benzene], and Alq3 [tris(8-hydroxyquinoline) aluminum]
were used as hole injection layer, hole transporting layer, host,
exciton blocking layer and electron transporting layer, respectively,
and LiF/Al as the composite cathode. Deposition rates and thick-
nesses of the layers were monitored in situ using oscillating quartz
monitors. Thermal deposition rates for organic materials, LiF, and Al
were w1, w1, and w10 A sꢀ1, respectively. The active area of the
ꢀ
fabricated devices is 2.0 ꢁ 2.0 mm2. EL spectra were measured by a
PR 655 spectra scan spectrometer. The luminanceecurrentevoltage
(LeIeV) characteristics were recorded simultaneously with the
measurement of EL spectra by combining the spectrometer with a
Keithley 2400 source meter. All measurements were carried out at
RT under ambient conditions.
Fig. 1. UVevis absorption and PL spectra of the Ir(III) complexes in CH2Cl2.
3.2. Photophysical properties
The UVevis absorption and PL spectra of three Ir(III) complexes
in CH2Cl2 are shown in Fig. 1 and all data are summarized in Table 1.
As most Ir(III) complexes, the absorption spectra could be divided
into two regions. The dominant absorption bands at 240e400 nm
3. Results and discussion
3.1. Synthesis and characterization
were assigned to spin-allowed 1p
ep* transitions from the cyclo-
The synthetic routes of the Ir(Ⅲ) complexes are outlined in
Scheme 1. The 4-(9H-carbazol-9-yl)benzaldehyde was synthesized
according to the literature method [36]. Condensation of 2-
aminobenzenethiol with 4-(9H-carbazol-9-yl)benzaldehyde gave
the crude ligand which was further purified via recrystallization
with ethyl acetate in 65.6% yield. The synthesis of Ir(Ⅲ) complexes
was accomplished in two step sequence, in which a chloride-
bridged dimer, formed by the reaction of excess cbt with
IrCl3$3H2O in the first step, could be readily converted to the
monomeric complex by replacing the bridging chlorides with
auxiliary ligand (acac, tmd and pic) in the presence of potassium
carbonate. The Ir(III) complexes were obtained in satisfactory yields
after purification by column chromatography. The ligand and Ir(III)
complexes were stable under air and well soluble in common
metalating ligand cbt. At lower energies, moderate and not clearly
resolved absorption bands extending into the visible region from
430 to 520 nm were observed, which corresponded to electronic
excitations to the metaleligand charge transfer of the spin-allowed
transition (1MLCT) and spin-forbidden bands of 3MLCT. With
respect to the MLCT transition bands, the absorption bands of
(cbt)2Ir(pic) were slightly blue-shifted relative to (cbt)2Ir(acac) and
(cbt)2Ir(tmd), indicative of strong ligand-field strength from the pic.
Upon irradiation with 365 nm light, (cbt)2Ir(acac), (cbt)2Ir(tmd) and
(cbt)2Ir(pic) all exhibited strong yellow or orange phosphorescence
with maximum emission peaks at 570, 570, 550 nm and shoulder
peaks at 600, 603, 590 nm, respectively, which could be assigned to
predominantly 3MLCT state radiative transitions. It should be
noticed that introducing pic as auxiliary ligand led to a blue shift of
20 nm among the PL spectra of the Ir(III) complexes, which was
organic solvents and so easily characterized by MS, 1H NMR and 13
NMR, which were consistent with the proposed structures.
C
Scheme 1. The synthetic routes of the Ir(III) complexes.

54
X. Li et al. / Dyes and Pigments 106 (2014) 51e57
Table 1
Photophysical and electrochemical data of the Ir complexes.
Ox
Ir-complex
labs [nm]
lem [nm]
4
s
[m
s]
E
[V]
HOMO [eV]
LUMO [eV]
Eg [eV]
onset
(cbt)2Ir(acac)
(cbt)2Ir(tmd)
(cbt)2Ir(pic)
240, 370, 485
240, 360, 485
240, 377,460
570 (600)
570 (603)
550 (590)
0.35
0.25
0.31
1.2
1.9
1.5
1.07
1.12
0.83
ꢀ5.37
ꢀ5.42
ꢀ5.13
ꢀ2.97
ꢀ3.04
2.62
2.40
2.38
2.51
Ox
potential of the first oxidation ðE
Þ and the absorption edge data,
onset
the HOMO energy levels were estimated from the formula:
Ox
EHOMO ¼ ꢀðE
þ 4:3Þ, and then the LUMO energy levels were
onset
obtained from ELUMO ¼ EHOMO þ Eg (Eg, the optical energy gap ob-
tained from the singlet MLCTabsorption threshold). The HOMO and
LUMO energy levels of the Ir(III) complexes were calculated to
be ꢀ5.13 to ꢀ5.42 eV and ꢀ2.62 to ꢀ3.04 eV, respectively. The
electrochemical data of the Ir(III) complexes were listed in Table 1.
3.4. Electrophosphorescent properties
In order to study the electrophosphorescent properties of the
Ir(III) complexes, conventional multilayer devices with the struc-
ture of ITO/m-MTDATA (20 nm)/NPB (20 nm)/CBP: Ir(Ⅲ) complexes
(20 nm)/BmPyPb (30 nm)/Alq3 (10 nm)/LiF (1 nm)/Al (150 nm)
were fabricated. Complex (cbt)2Ir(acac) was firstly used as an
example to optimize the doping concentration for perfect device
performance. All data are summarized in Table 2. As depicted in
Fig. 3, the presence of the residue emission of CBP at about 425 nm
implied that the 5% doping concentration was low. With increasing
the doping concentration to 8% and 10%, the residue emission from
the host disappeared and the EL spectra contained exclusively the
emission from (cbt)2Ir(acac). It could be speculated that the energy
was completely transferred from host to (cbt)2Ir(acac) at high
doping concentration. That inconspicuous emission from CBP also
supported a model of charge trapping [38]. The maximum EL
emissions of different (cbt)2Ir(acac)-doped devices were almost
same, which appeared at 565 nm with the shoulder peak of 608 nm
and were insensitive to the applied voltages in the range of 4e11 V.
The LeIeV characteristics of (cbt)2Ir(acac)-doped devices are
plotted in Fig. 4. The devices exhibited low turn-on voltages of 3.8e
4.1 V. The luminance increased with increasing injection current as
well as voltage in those devices. The device at 8% (cbt)2Ir(acac)-
doped concentration achieved the maximum luminance of
70925 cd mꢀ2 at the voltage of 10 V. The device efficiency varied
with the doping concentration of (cbt)2Ir(acac), as shown in Fig. 5.
Device with 8% (cbt)2Ir(acac)-doped concentration showed the
highest current efficiency of 44.2 cd Aꢀ1 at the current density of
Fig. 2. CV curves of the Ir(III) complexes in CH2Cl2 with a scan rate of 100 mV sꢀ1
.
consistent with the absorption spectra. Using (ppy)2Ir(acac) as a
standard reference [37], the emission quantum yields (4p) of
(cbt)2Ir(acac), (cbt)2Ir(pic) and (cbt)2Ir(tmd) in solution were
determined as 0.35, 0.31 and 0.25, respectively. The phosphores-
cent lifetimes of these complexes were in the range of 1.2e1.9 ms,
which was indicative of strong spin-orbit coupling leading to effi-
cient intersystem crossing from the singlet to the triplet state.
3.3. Electrochemical properties
The electrochemical behavior of the Ir(III) complexes was
investigated by CV which was shown in Fig. 2. The electrochemical
characteristics meant that all of these complexes showed reversible
redox peaks, suggesting that these Ir(III) complexes stabilized the
formation of both cation and anion radicals in OLEDs, which was
necessary to determine their electronic energy levels. Upon the
anodic sweep in deaerated CH2Cl2 solutions, one or two reversible
peaks for the Ir(III) complexes were observed, which should be
assigned to the metal-centered Ir(III)/Ir(IV) oxidation couple or the
possible oxidation of the auxiliary ligand. The redox potentials of
the Ir(III) complexes were obtained relative to an internal ferrocene
reference. As shown in Fig. 2, the oxidation potentials were
observed in the range of 0.83e1.12 V. On the basis of the onset
28 mA cmꢀ2, corresponding to the power efficiency of 23.3 lm Wꢀ1
.
Table 2
EL performances of different ratio (cbt)2Ir(acac)-doped OLEDs.
a
b
c
d
e
f
g
Ratio
lmax
/
Von
V
/
Lmax
/
hL
/
hL
/
hL
/
hp /
nm
cd mꢀ2
cd Aꢀ1
cd Aꢀ1
cd Aꢀ1
lm Wꢀ1
5%
8%
10%
565 (608)
565 (608)
565 (608)
4.1
3.8
3.9
58725
70925
56900
42.2
44.2
37.8
35.6
40.3
33.8
24.5
27.5
23.2
22.8
23.3
20.1
a
Maximum emission wavelength. Values in parentheses are shoulder peaks.
b
c
d
e
f
Turn-on voltage.
Maximum luminance.
Maximum current efficiency.
Current efficiency at 20 mA cmꢀ2
.
Current efficiency at 100 mA cmꢀ2
Maximum power efficiency.
.
g
Fig. 3. EL spectra of (cbt)2Ir(acac)-doped devices.

X. Li et al. / Dyes and Pigments 106 (2014) 51e57
55
Fig. 4. LeIeV characteristics of (cbt)2Ir(acac)-doped devices.
Fig. 6. EL spectra of 8% different Ir(III)-doped devices.
At the high current density of 100 mA cmꢀ2, the device efficiencies
remained to be very high among 23e28 cd Aꢀ1. The high efficiency
of the device was considered to arise from the following three as-
pects. Firstly, the high 4p of (cbt)2Ir(acac) contributed to the high
device efficiency; secondly, the good hole-transporting ability of
(cbt)2Ir(acac) could facilitate the injection and transport of elec-
trons and holes (See below); thirdly, the energy levels of (cbt)2Ir(-
acac) (ꢀ5.37/ꢀ2.97) were entirely embraced by host CBP (ꢀ5.9/
ꢀ2.6), which induced the effective traps of the dopant for both
electrons and holes injected into the host. These devices witnessed
gradual efficiency decay with increasing current density that was
frequently observed for electrophosphorescent OLEDs, which
typically resulted from a combination of tripletetriplet annihilation
and field-induced quenching effects [39e41].
It was well noted that the overall performances of (cbt)2Ir(acac)-
doped devices outperformed those of our previously reported de-
vices based on similar Ir(Ⅲ) complex (dpabt)2Ir(acac) containing 4-
arylamine-2-phenylbenzo[d]thiazole [42], which were probably
attributed to the hole-transporting rigid carbazole moiety resulting
in a more balanced electron and hole transportation and recom-
bination in emissive layer.
luminescence (PL and EL) should be from the same excited state.
The introduction of carbazole moiety did not alter EL emission
between (cbt)2Ir(acac) and (bt)2Ir(acac) emitters, as shown in Fig. 6.
The EL peak was also greatly dependent on the auxiliary ligands.
Especially, the EL spectrum of (cbt)2Ir(pic)-doped device was blue-
shifted by ca. 20 nm compared with that of (cbt)2Ir(acac) or
(cbt)2Ir(tmd)-doped devices.
The LeIeV characteristics and current efficiency-current density
curves of 8% different Ir(III)-doped devices are illustrated in Figs. 7
and 8, respectively. The (bt)2Ir(acac)-doped device exhibited gen-
eral efficiencies with a turn-on voltage of 4.1 V, a maximum current
efficiency of 19.1 cd Aꢀ1 and a peak power efficiency of 9.3 lm Wꢀ1
.
Obviously, the performances of the (cbt)2Ir(acac)-doped device
(44.2 cd Aꢀ1, 23.3 lm Wꢀ1) are better than those of (cbt)2Ir(pic)/
(cbt)2Ir(tmd)-doped devices (36.4 cd Aꢀ1, 14.8 lm Wꢀ1 and
22.2 cd Aꢀ1, 12.3 lm Wꢀ1) probably due to much higher 4p of
(cbt)2Ir(acac) compared with (cbt)2Ir(pic) and (cbt)2Ir(tmd). For
(cbt)2Ir(acac)/(cbt)2Ir(pic)-doped devices, the efficiency roll-off was
only 37% and 43% at 100 mA cmꢀ2. The slow efficiency roll-off
further suggested that the positive and negative charges were
better balanced to guarantee efficient charge recombination in the
emissive layer. As shown in inset of Fig. 8, the low-lying LUMO
(2.6 eV) of CBP perfectly matched with exciton blocking layer
BmPyPb. There was almost no injection barrier between the CBP
and BmPyPb interface. However, at the hole injection side, the deep
HOMO of CBP induced an injection barrier of 0.5 eV. In addition, the
energy levels of three phosphorescent dopants were all embraced
by host CBP, indicating that carrier trapping maybe play an
In view of (cbt)2Ir(acac), the devices based on (cbt)2Ir(tmd) and
(cbt)2Ir(pic) as emitters were fabricated at a doping concentration
of 8%. For comparison, the 8% (bt)2Ir(acac)-doped device was also
fabricated. The EL spectra for these devices are provided in Fig. 6. All
devices exhibited yellow or orange phosphorescent emissions. It
could be found that the EL spectra of these devices were almost
identical to their corresponding PL spectra, indicating that the
Fig. 5. Current efficiencyecurrent density curves of (cbt)2Ir(acac)-doped devices.
Fig. 7. LeIeV characteristics of 8% different Ir(III)-doped devices.

56
X. Li et al. / Dyes and Pigments 106 (2014) 51e57
efficient yellow or orange emissions, these Ir(III) complexes are
necessarily promising phosphorescent dopants in simple white
OLEDs for practical application in flat panel display and solid-state
lighting.
Acknowledgments
Authors gratefully acknowledge the supports from the National
Natural Science Foundation of China (Grant No. 61204021), the
Project from Department of education of Liaoning Province (Grant
No. L2012094) and Anshan Science and Technology Foundation.
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