Efficiency phosphorescent OLEDs with a low roll-off based on a hetero-triplet iridium complex

Article abstract of DOI:10.1016/j.dyepig.2014.10.001

In order to explore the triplet-triplet annihilation in cyclometalated iridium complexes, a hetero-triplet iridium complex [Ir(ppy)₂pbi] employing two 2-phenylpyridine (Hppy) ligands and one 1,2-diphenyl-1H-benzo[d]-imidazole (Hpbi) ligand with

Full text of DOI:10.1016/j.dyepig.2014.10.001

Dyes and Pigments 113 (2015) 649e654
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Efficiency phosphorescent OLEDs with a low roll-off based on a
hetero-triplet iridium complex
a
b
,
*
a
a
a, *
Liangliang Han , Dongyu Zhang , Jun Wang , Zhenggang Lan , Renqiang Yang
a
CAS Key Laboratory of Bio-based Materials, Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao 266101,
China
b
Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou 215123, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 4 June 2014
Received in revised form
In order to explore the tripletetriplet annihilation in cyclometalated iridium complexes, a hetero-triplet
iridium complex [Ir(ppy)
benzo[d]-imidazole (Hpbi) ligand with similar triplet energies was synthesized. A device based on
Ir(ppy) pbi exhibited high current efficiency 26.7 cd/A at high doping concentration 12 wt%. More
interestingly, the current efficiency was still maintained at 25.8 cd/A when the current density was above
2
pbi] employing two 2-phenylpyridine (Hppy) ligands and one 1,2-diphenyl-1H-
29 September 2014
2
Accepted 2 October 2014
Available online 12 October 2014
2
2
53.2 mA/cm . The efficiency roll-off was only about 3.4%, much lower than that of traditional cyclo-
metalated iridium complexes. It is proposed that the complicated energy level and unsymmetical ligand
Keywords:
2
in Ir(ppy) pbi may help to suppress the tripletetriplet annihilation.
Heterocyclometalated iridium complex
Hetero-triplet iridium complex
Tripletetriplet annihilation
Efficiency roll-off
©
2014 Elsevier Ltd. All rights reserved.
Exciton density
OLED
1. Introduction
which exhibited a less-pronounced efficiency roll-off at high current
densities. We also developed a novel phosphor with triplet lifetime of
2
Organic light-emitting devices (OLEDs) are currently an active
research area due to their potential applications in flat-panel displays
and solid state lighting [1e5]. Octahedral 4d and 5d metal complexes
based phosphorescent materials, especially the iridium complexes,
have attracted much attention for improving OLED efficiency because
3.92 ns, and the efficiency roll-off was only 28.2% at 100 mA/cm with
low doping concentration 2 wt% [18]. Although developing short
lifetime phosphors can effectively suppress the TTA, however, mate-
rials can only be designed by serendipity. Taking into account the
chemical structure of cyclometalated iridium complexes, three homo-
6
6
the close-lying
p
-
p
* and metal to ligand charge transfer (MLCT) states
cyclometalated ligands are always employed like Ir(ppy)
phenylisoquinoline]iridium [Ir(piq) ], commercialized materials for
green and red phosphorescent OLEDs. Lee [19] has reported a novel red
phosphorescent iridium complex Ir(ppy) piq by intramolecular en-
3
and tris[1-
in these complexes together with the heavy atom effect enhance the
spin-orbit coupling, which results in utilization of both singlet and
triplet excitons, causing nearly unity of internal quantum efficiency
3
2
[5e9]. However, tripletetriplet annihilation (TTA) exists in phospho-
ergy transfer from the MLCTof Ir-ppy to Ir-piq to improve the efficiency
rescent OLEDs because of the long lifetime of triplet exciton and high
by 20%, however, there was no improvement of the TTA. Thus, here we
triplet exciton concentration in the emitting layer [10e13]. Tris(2-
phenylpyridine)iridium [Ir(ppy) ], one of the most studied phospho-
3
rescent dyes frequently shows efficiency roll-off when used in phos-
phorescent OLEDs [14e16]. Though the TTA can be suppressed to a
design a new hetero-triplet iridium complex Ir(ppy)
ppy and pbi with similar triplet energy. Because of the different triplet
energies existed in Ir(ppy) pbi, we thought the TTA in the emitting
layer will be different from homo-triplet exciton complexes.
The emitting mechanisms of Ir(ppy) Ir(ppy) piq and
Ir(ppy) pbi were shown in Fig. 1. For Ir(ppy) , the green phos-
phorescence emitted from the decay of degenerate MLCT energy
level between iridium atom and ppy ligand, TTA became more
2
serious as the triplet exciton density increased. For Ir(ppy) piq,
2
pbi employing
2
certain extent by optimized device structure, the triplet lifetime 1.6
ms
3
,
2
of Ir(ppy) may be too long to avoid the TTA. Therefore, developing
3
2
3
3
short lifetime triplet exciton phosphorescent materials will be an
effective strategy to suppress the TTA. Shu et al. [17] developed a
cyclometalated osmium complex with short triplet lifetime 96 ns,
there was still no improvement about the TTA because of high
exciton concentration of Ir-piq caused by the intramolecular en-
ergy transfer from Ir-ppy to Ir-piq which resulted in a pure red
*
Corresponding authors.
E-mail addresses: dyzhang2010@sinano.ac.cn (D. Zhang), yangrq@qibebt.ac.cn
(
R. Yang).
http://dx.doi.org/10.1016/j.dyepig.2014.10.001
143-7208/© 2014 Elsevier Ltd. All rights reserved.
0

650
L. Han et al. / Dyes and Pigments 113 (2015) 649e654
Fig. 1. The emitting mechanism schematic diagrams of Ir(ppy)
3 2 2 3 2
, Ir(ppy) piq and Ir(ppy) pbi. Ir(ppy) : Irradiative decay from degenerated Ir-ppy; Ir(ppy) piq: Irradiative decay from
Ir-piq because of the intramolecular energy transfer; Ir(ppy) pbi: Irradiative decay from both Ir-ppy and Ir-pbi.
2
ꢀ
3
phosphorescence. For Ir(ppy)
ppy and Ir-pbi were similar, both Ir-ppy and Ir-pbi contribute to
the phosphorescence without intramolecular energy transfer.
2
pbi, the triplet energy levels of Ir-
compartment glass cell in dichloromethane containing 10
iridium complexes and 0.1 M tetra(n-butyl)ammonium hexa-
fluorophosphate (n-Bu NPF ) as a supporting electrolyte, where
M
4
6
Experiments show that Ir(ppy)
2
pbi exhibits low photo-
the scan rate was 100 mV/s, the HOMO and LUMO energy level was
calculated as the reference method [20].
luminescence quantum yield of 0.21 which may be caused by the
low triplet density in the emitting layer. Theoretical calculations
show that the highest occupied molecular orbital (HOMO) and the
lowest unoccupied molecular orbital (LUMO) energy levels of
2.2. Device fabrication and measurements
2 3
Ir(ppy) pbi were more complicated than that of Ir(ppy) because
the introduction of different cyclometalated ligands destroys the
high symmetry of cyclometalated complex. It supposed that the
complicated energy level and unsymmetical ligand play an
important role in suppression TTA. As a result, efficient green
phosphorescent OLEDs are obtained with efficiency roll-off of 3.4%
at high doping concentration 12 wt%.
OLEDs were prepared on a glass substrate pre-coated with a
130-nm-thick indiumetin-oxide (ITO) layer with a sheet resis-
~
tance of ~20
U/sq. Prior to organic layer deposition, the substrate
was degreased with solvents and cleaned for 5 min by exposure to
a UV-ozone ambient, then it was immediately loaded into the
ꢀ
4
evaporation system. With a base pressure of ~3 ꢁ 10 Pa, the
organic and metal cathode layers were grown successively by using
an in vacuo mask exchange mechanism without breaking the vac-
2
. Experimental
0
00
uum. Firstly, a 10-nm-thick 4,4 ,4 -tris(N-3-methylphenyl-N-phe-
2.1. Reagents and measurements
nylamino)triphenylamine (m-MTDATA) hole injection layer (HIL)
0
was deposited, followed by a 20-nm-thick 4, 4 -bis[N-(1-naphthyl)-
Hppy was purchased from Aldrich, all reactions were performed
N-phenylamino]biphenyl (NPB) hole transporting layer (HTL). Then
under argon. Solvents were carefully dried and distilled from
appropriate drying agents prior to use. Commercially available re-
agents were used without further purification unless otherwise
stated. All reactions were monitored by using thin-layer chroma-
tography (TLC) with Merck precoated glass plates. Compounds
were visualized with UV-light irradiation at 254 and 365 nm. High
resolution mass spectra (HRMS) were obtained using a Bruker
Maxis UHR-TOF, Ion Source: APCI system. Nuclear magnetic reso-
nance (NMR) spectra were measured in appropriate deuterated
a 30-nm-thick light-emitting layer (EML) consisting of various
weight ratio iridium complexes doped into a 4,4 -bis(9H-carbazol-
0
9-yl)biphenyl (CBP) host was prepared via thermal codeposition.
Next, a 30-nm-thick 1,3,5-tri(1-phenyl-1H- benzo[d]imidazol-2-yl)
phenyl (TPBi) layer was used to block hole and transport electron. A
shadow mask with rectangular 2 mm ꢁ 2 mm openings was used to
define the cathode consisting of a 1-nm-thick LiF layer, followed by
a 100-nm-thick Al layer. The electroluminescence spectra were
recorded with a PR655 spectrophotometer. The brightness-current-
voltage (BeIeV) characteristics of OLEDs were measured with
Keithley 2400. All the measurements were carried out at room
temperature under ambient conditions.
3 6
solvents (CDCl or DMSO-d ) on a Bruker AVANCE 600 MHz Fourier
transform NMR spectrometer; chemical shifts were quoted relative
1
to the internal standard tetramethylsilane for HNMR data. Ab-
sorption and photoluminescence spectra were measured with a
Hitachi U-4100 UVeViseNIR scanning spectrophotometer and
HORIBA JOBIN YVON FluoroMax-4 spectrophotometer, respec-
tively. The electrochemistry characteristics of the two complexes
were investigated using a cyclic voltammetry employing a glass
carbon electrode as working electrode, a platinum wire as counter
electrode and an Ag/AgCl (3.0 M KCl) electrode as reference elec-
trode. The cyclic voltammograms were obtained from a one-
2.3. Theoretical calculations
The molecular structures and corresponding molecular orbital
were obtained at the B3LYP [21,22]/def2-SVP [23] level. All the
calculations were performed using ORCA [24] program.

L. Han et al. / Dyes and Pigments 113 (2015) 649e654
651
2
.4. The general procedure for the synthesis of Ir(ppy)
2
pbi
2-ethoxyethanol as described in literature [25], after purified by
column chromatography using petroleum ether and ethyl acetic as
eluent, off white powder was obtained in 72% yield. The synthesis
Bis[1,2-diphenyl-1H-benzo[d]-imidazole](2-phenylpyridine)iridiu-
m(III) [Ir(ppy) pbi]: Hppy (0.31 g, 2.0 mmol) was dissolved into a
mixed solution of 2-ethoxyethanol (12 mL) and water (4 mL) in a
2
of Ir(ppy)
[26,27] as shown in Scheme 1. Hppy was used to react with
IrCl $3H to give cyclometalated Ir- -chloro-bridged dimer,
which could react with Hpbi to afford heterocyclometalated Ir(p-
py) pbi, in the presence of Na CO . More interestingly, Ir(ppy) can
also be obtained as a byproduct when we attempted to purify
Ir(ppy) pbi on a silica gel column with dichloromethane and hex-
2
pbi was carried out according to the reference method
2
3 2
5 mL round bottomed flask, and then IrCl $3H O (0.28 g, 0.8 mmol)
3
2
O
m
ꢂ
was added. The mixture was stirred under nitrogen at 120 C for 12 h.
The mixture was cooled to room temperature and the precipitate was
collected and washed with water, ethanol, and acetone, then dried in
2
2
3
3
vacuum to give a cyclometalated Ir-
m-chloro-bridged dimer. The
2
dimer complex, Hpbi (0.27 g, 1.0 mmol), Na
2
CO (0.34 g, 3.2 mmol)
3
ane as eluent, which was used for device fabrication directly. Both
the complexes show strong phosphorescence on silica gel plates
under the excitation of 365 nm.
and 5 mL glycerol were charged into a round bottomed flask, the
ꢂ
mixture was then heated to 240 C under an argon atmosphere for
another 12 h. After cooling to room temperature, the mixture was
poured into 100 mL water, yellow precipitate was filtered off and
washed with water, ethanol and ether. The crude product was puri-
fied by silica column chromatography with dichloromethane/pe-
3
.2. Photophysics characteristics
The UVevis absorption and photoluminescence spectra of
troleum ether (1:2, v/v) as the eluent to give 0.29 g Ir(ppy)
yellow powder. Yield: 47.5%, 0.14 g. Ir(ppy) can also be obtained as a
, 600 MHz): 7.90 (t, J ¼ 6.4 Hz,
H), 7.85 (d, J ¼ 4.8 Hz,1H), 7.77 (d, J ¼ 5.4 Hz,1H), 7.69e7.59 (m, 7H),
.50e7.47 (m, 2H), 7.06 (t, J ¼ 7.8 Hz, 1H), 6.97 (d, J ¼ 7.8 Hz, 1H),
.92e6.78 (m,10H), 6.72 (t, J ¼ 7.8 Hz,1H), 6.63 (d, J ¼ 7.8 Hz,1H), 6.53
2
pbi as a
3 2
complexes Ir(ppy) and Ir(ppy) pbi in dichloromethane solutions
3
were shown in Fig. 2. Both the complexes show similar absorp-
tion between 375 and 525 nm, however, a difference was obvious
in the region 325e375 nm because of the introduction of the pbi
ligand, from which we can conclude both the MLCT of ppy and
pbi were excited according to literature [19]. Ir(ppy)
py) pbi show strong green phosphorescence with
maximum wavelength at 514 and 517 nm with photo-
luminescence quantum yield (4) of 0.40 and 0.21, respectively,
calculated based on our previous report [28]. In the hetero-
cyclometalated complex Ir(ppy)
Ir-ppy and Ir-pbi were 2.41 and 2.37 eV, respectively, estimated
from the emission of Ir(ppy) [14] and tris(1,2-diphenyl-1H-
benzimidazol)iridium [Ir(pbi) ] [29]. Therefore the 0.04 eV dif-
1
byproduct, yield: 27.5%. HNMR (CDCl
3
2
7
6
3
and Ir(p-
peak
13
(
t, J ¼ 7.8 Hz, 1H), 6.05 (d, J ¼ 8.4 Hz, 1H); CNMR (CDCl
67.65, 166.68, 163.39, 162.61, 161.33, 148.06, 144.33, 143.92, 140.86,
37.90, 137.85, 136.91, 136.87, 136.32, 135.90, 135.80, 134.10, 130.38,
30.13, 129.89, 129.78, 129.74, 128.53, 128.17, 125.25, 123.94, 123.72,
23.23, 122.38, 122.00, 121.29, 119.66, 119.52, 119.09, 118.66, 118.54,
3
, 150 MHz):
2
a
1
1
1
1
2
pbi, the triplet MLCT state of
1
15.23, 110.53; HRMS (APCI): m/z calcd for C41
4
H29IrN : 770.2021;
þ
found: 771.2094 (M þ H) .
3
3
3
. Results and discussion
ference between the triplet MLCT state of Ir-ppy and Ir-pbi was
too small for the occurrence of efficient intramolecular energy
3
.1. Synthesis
transfer [30]. That is, the
donor was <<0 eV, so inefficient energy transfer should be
existed in Ir(ppy) pbi molecule. As shown in Fig. 1, both the
triplet of Ir-ppy and Ir-pbi contribute to the phosphorescence.
DG between the MLCT of acceptor and
The cyclometalated ligand Hpbi was synthesized by condensa-
tion of N-phenyl-o-phenylenediamine and benzaldehyde in
2
2
Scheme 1. Synthetic route of Ir(ppy) pbi.

652
L. Han et al. / Dyes and Pigments 113 (2015) 649e654
Table 1
Photophysics and electrochemistry characteristics of complexes Ir(ppy)
py) pbi in dichloromethane solution.
3
and Ir(p-
2
a
HOMOb LUMOc
Complexes Abs (nm)
PL
nm)
4
E
g
E
ox
(
(eV) (V)
(eV)
(eV)
Ir(ppy)
Ir(ppy)
3
345.8, 378.7, 405.7, 514
53.2, 485.4
pbi 374.8, 408.4, 454.5, 517
88.1
0.40 2.08 0.64 ꢀ4.92
0.21 2.07 0.61 ꢀ4.89
ꢀ2.84
4
2
ꢀ2.82
4
a
E
E
E
g
¼ 1240/
edge
l .
b
c
ðFc=FcþÞ
).
HOMO ¼ ꢀe(Eox þ 4.8 ꢀ E
1=2
LUMO ¼ EHOMO þ E
g
.
3.4. Electroluminescence characteristics
As is well known, the efficiency of phosphorescent OLEDs drops
markedly as the doping concentration increased because of the
strong concentration quenching, in addition, incomplete Dexter
energy transfer will exit as the doping concentration decreased
which cause strong NPB or CBP emission [18]. The electrolumi-
Fig. 2. UVevis absorption and photoluminescence spectra of Ir(ppy)
in dichloromethane solution (excited at 300 nm) at room temperature.
3 2
and Ir(ppy) pbi
nescence characteristics of Ir(ppy)
Fig. 4. The optimized doping concentration of Ir(ppy)
and the device showed the maximum current efficiency (
3
and Ir(ppy)
2
pbi are shown in
was 6 wt%,
) of
0.9 cd/A and maximum brightness of 50,340 cd/m , however,
because of the strong TTA, efficiency dropped badly to 23.4 cd/A at
3
.3. Electrochemistry characteristics
3
h
c
2
3
Fig. 3 depicts the cyclic voltammograms of both the com-
plexes. The onset oxidation (Eox) of Ir(ppy) and Ir(ppy) pbi
were 0.64 and 0.61 V, respectively, which were usually consid-
ered as the metal-centered couple. i. e. IrIII/IrIV of the com-
plexes [31,32]. The oxidation and reduction onset of ferrocence
3
2
2
2
15.3 mA/cm with an efficiency roll-off of 24.3%. Same phenom-
enon was obtained for Ir(ppy) pbi, at doping concentration 6 wt%,
the maximum was only about 19.5 cd/A, much lower than that of
Ir(ppy) , may be because of the lower 4 of Ir(ppy) pbi than Ir(ppy)
2
h
c
3
,
were 0.78 and 0.26 V, respectively, then the HOMO of Ir(ppy)
and Ir(ppy)
pbi were estimated to be ꢀ4.92 and ꢀ4.89 eV,
respectively, according to the previously reported method [33].
The optical band gaps (E ) of Ir(ppy) and Ir(ppy) pbi were
calculated about 2.08 and 2.07 eV, respectively, according to the
absorption edge in their dichloromethane solutions
Fig. 2). As a result the LUMO were calculated from HOMO and
3
3
2
and the efficiency roll-off was obviously as the current increased.
However, when the doping concentration was increased to 10 wt%,
2
both the maximum
4.1 cd/A and 58,170 cd/m , respectively. Interestingly, the
higher than that of Ir(ppy) when the current density increased to
95.8 mA/cm . The reason was ascribed to the lower efficiency in
the Ir(ppy) pbi device than the Ir(ppy) one caused by the low
exciton density, which was identical to the lower photo-
luminescence quantum yield of Ir(ppy) pbi than Ir(ppy) . In order
c
h and brightness were improved greatly to
g
3
2
2
2
c
h is
(
l
edge
)
3
2
1
(
E
g
, estimated to be ꢀ2.84 and ꢀ2.82 eV, respectively. By
2
3
replacing ppy with pbi, the HOMO and LUMO changed slightly
because of the similar electron deficient ability of benzoimida-
zole compared to pyridine. The photophysics and electrochem-
istry characteristics of complexes Ir(ppy)
summarized in Table 1.
2
3
to further investigate the TTA in this hetero-cyclometalated com-
plex, the doping concentration was increased to 12 wt%, as shown
3 2
and Ir(ppy) pbi were
2
in Fig. 4a, the efficiency was extremely stable from 3.5 mA/cm to
2
2
53.2 mA/cm , and the efficiency roll-off was only about 3.4% from
2
the maximum
c
h 26.7 cd/A to 25.8 cd/A at 253.2 mA/cm . At
2
103.1 mA/cm and above, this device was more efficient than that of
2
3
Ir(ppy) and the brightness was as high as 27,000 cd/m . We infer
that the suppressed TTA was ascribed to more complicated excited
states and the unsymmetrical ligand in Ir(ppy) pbi which lead to
low exciton density unlike Ir(ppy) or Ir(ppy) piq in which the
2
3
2
phosphorescence radiatively decayed only from the triplet exciton
of Ir-ppy or Ir-piq.
3.5. Theoretical calculations
To obtain a qualitative picture on the electronic property of
Ir(ppy)
and the results were shown in Fig. 5. The optimized structures of
Ir(ppy) and Ir(ppy) pbi were consistent with the experimental
results. For complex Ir(ppy) , because of the symmetrical structure
3 2
and Ir(ppy) pbi, theoretical calculations were carried out
3
2
3
1
(which was also proved by the HNMR data), a few HOMO (HOMO-
2
, HOMO-1, HOMO) and LUMO (LUMO, LUMO þ 1, LUMO þ 2) were
evenly distributed throughout the iridium atom and the three ppy
ligands. Moreover, due to high symmetry, three HOMOs become
degenerate and their energies are ꢀ4.30 eV. For LUMOs, their en-
ergies are closed, namely ꢀ1.96 eV (LUMO) and ꢀ1.88 eV
2 2 3
Fig. 3. Cyclic voltammetry curves of (cyclopbi) Iracac and (cyclopbi) IrCF in
dichloromethane solution.
(LUMO þ 1, LUMO þ 2). However, for complex Ir(ppy)
2
pbi, the

L. Han et al. / Dyes and Pigments 113 (2015) 649e654
653
Fig. 4. Electroluminescence characteristics of Ir(ppy)
3
and Ir(ppy)
2
pbi. a: Current efficiency and current density relationships of 6 wt% doped Ir(ppy)
and 6 wt%, 10 wt% and 12 wt% doped Ir(ppy) pbi.
3
and 6 wt%, 10 wt% and 12 wt%
doped Ir(ppy) pbi; b: Brightness and current density relationships of 6 wt% doped Ir(ppy)
2
3
2
energy levels were more complicated compared to Ir(ppy)
3
, the
complicated electron transitions may exist in the complex
Ir(ppy) pbi, because the introduction of different cyclometalated
ligands destroys the high symmetry of cyclometalated complexes.
As the result of the loss of electronic-state degeneracy, different
types of triplet excitons may be generated when Ir(ppy) pbi is
2
excited. Thus in the visible absorption range the distance between
the same kind of exciton becomes large, which results in
HOMO was concentrated in the metal and the electron-rich groups
of ppy and pbi with ꢀ3.97 eV, and the HOMO-1 and HOMO-2 were
mainly in the ppy and pbi ligand with different energy
levels ꢀ4.19 eV and ꢀ4.28 eV, respectively. The LUMO, LUMO þ 1
and LUMO þ 2 also show different energy levels ꢀ1.85 eV, ꢀ1.77 eV
and ꢀ1.74 eV, respectively. The calculations indicate more
2
3 2
Fig. 5. Optimized structures and plots of molecular orbital of Ir(ppy) and Ir(ppy) pbi with corresponding energies in Hartree unit.

654
L. Han et al. / Dyes and Pigments 113 (2015) 649e654
suppressed TTA. More detailed dynamic processes are still under-
way and the result will be published in due course.
[12] Wu C, Tao SL, Chen MM, Wong FL, Yang QD, Mo HW. Highly efficient blue and
white phosphorescent OLEDs based on an iridium complex. Dyes Pigments
2013;96:237e41.
[
13] Yang W, Chen YS, Jiang W, Ban XX, Huang B, Dai YQ, et al. A carbazole-based
dendritic host material for efficient solution-processed blue phosphorescent
OLEDs. Dyes Pigments 2013;97:286e90.
4
. Conclusion
[
14] Kajii H, Takahota N, Wang YD, Ohmori Y. Current-density dependence of
transient properties in green phosphorescent organic light-emitting diodes.
Jpn J Appl Phys 2011;50. 04DK05.
15] Yang XH, Jabbour GE. Efficient light emitting devices based on phosphores-
cent partially doped emissive layers. J Mater Chem C 2013;1:4663e6.
16] Reineke S, Walzer K, Leo K. Triplet-exciton quenching in organic phospho-
rescent light-emitting diodes with Ir-based emitters. Phys Rev B 2007;75:
In conclusion, high efficiency and efficiency-stable phospho-
rescent OLEDs were obtained based on the hetero-cyclometalated
iridium complex Ir(ppy) pbi. The maximum 26.7 cd/A was ob-
tained and the was still maintained at 25.8 cd/A when the current
[
2
h
c
h
c
[
2
density was above 253.2 mA/cm , only 3.4% efficiency roll-off. Both
the triplet state of Ir-ppy and Ir-pbi contributed to the phospho-
rescence because of the complicated energy level and unsymmet-
1
25328.
17] Wu CH, Shih PI, Shu CF, Chi Y. Highly efficient red organic light-emitting
devices based on fluorene-triphenylamine host doped with an Os(II)
[
[
a
ical ligand in Ir(ppy)
2
pbi, which was qualitatively proved by
phosphor. Appl Phys Lett 2008;92:233303.
18] Han LL, Yang DF, Li WL, Chu B, Chen YR, Su ZS, et al. The reduced triplet-triplet
annihilation of electrophosphorescent device doped by an iridium complex
with active hydrogen. Appl Phys Lett 2008;93:153303.
theoretical calculations. This result could be very interesting and
useful when designing efficiency phosphorescent materials with
lower efficiency decay.
[19] Lee JY. Intramolecular energy transfer in heteroleptic red phosphorescent
dopants. Appl Phys Lett 2006;89:223517.
[
20] Li YF, Cao Y, Gao J, Wang DL, Yu G, Heeger AJ. Electrochemical properties of
luminescent polymers and polymer light-emitting electrochemical cells.
Synth Met 1999;99:243e8.
Acknowledgments
The work was supported by National Natural Science Founda-
tion of China (21103213, 21274161, 51173199, 91233106 and
[21] Becke AD. Density-functional thermochemistry. III. The role of exact ex-
change. J Chem Phys 1993;98:5648.
[
22] Stephens PJ, Devlin FJ, Chabalowski CF, Frisch MJJ. Ab initio calculation of
vibrational absorption and circular dichroism spectra using density functional
force fields. J Phys Chem 1994;98:11623e7.
91123034), Outstanding Young Scientists Research Award Fund of
Shandong Province (BS2010NJ021), Shandong Provincial Natural
Science Foundation (ZR2011BZ007), Strategic Priority Research
Program of the Chinese Academy of Sciences (XDA09020201), and
National Science and Technology Ministry (2012BAF13B05-402).
[
23] Weigend F, Ahlrichs R. Balanced basis sets of split valence, triple zeta valence
and quadruple zeta valence quality for H to Rn: design and assessment of
accuracy. Phys Chem Chem Phys 2005;7:3297e305.
[24] Neese F. ORCA e an ab initio, density functional and semi empirical program
package V 2.9 development. D-45470 Muelheim/Ruhr, Germany: Max Planck
Institute for Bioinorganic Chemistry; 2012.
References
[
25] Huang WS, Lin JT, Chien CH, Tao YT, Sun SS, Wen YS. Highly phosphorescent
bis-cyclometalated iridium complexes containing benzoimidazole-based li-
gands. Chem Mater 2004;16:2480e8.
[1] Ho PKH, Kim JS, Burroughes JH, Becker H, Li SFY, Brown TM, et al. Molecular-
scale interface engineering for polymer light- emitting diodes. Nature
[26] You Y, Park SY. Inter-Ligand energy transfer and related emission change in
the cyclometalated heteroleptic iridium complex: facile and efficient color
tuning over the whole visible range by the ancillary ligand structure. J Am
Chem Soc 2005;127:12438e9.
[27] Kwon TH, Cho HS, Kim MK, Kim JW, Kim JJ, Lee KH, et al. Color tuning of
cyclometalated iridium complexes through modification of phenylpyrazole
derivatives and ancillary ligand based on ab initio calculations. Organome-
tallics 2005;24:1578e85.
[28] Han LL, Yang DF, Li WL, Chu B, Chen YR, Su ZS, et al. Intramolecular energy
transfer between the triplet of ancillary ligand and the metal to ligand charge
transfer state existed in heterocyclometalated iridium (III) complexes. Appl
Phys Lett 2009;94:163303.
[29] Ding JQ, Gao J, Cheng YX, Xie ZY, Wang LX, Ma DG, et al. Highly efficient
green-emitting phosphorescent iridium dendrimers based on carbazole
dendrons. Adv Funct Mater 2006;16:575e81.
2000;404:481e4.
[
[
[
2] Gu G, Parthasarathy G, Forrest SR. A metal-free, full-color stacked organic
light-emitting device. Appl Phys Lett 1999;74:305e7.
3] Wu CC, Chen CW, Cho TY. Three-color reconfigurable organic light-emitting
devices. Appl Phys Lett 2003;83:611e3.
4] Baldo MA, O'Brien DF, You Y, Shoustikov A, Sibley S, Thompson ME, et al.
Highly efficient phosphorescent emission from organic electroluminescent
devices. Nature 1998;395:151e4.
[
[
[
5] Adachi C, Baldo MA, Thompson ME, Forrest SR. Nearly 100% internal phos-
phorescence efficiency in an organic light-emitting device.
001;90:5048e51.
J Appl Phys
2
6] Ikai M, Tokito S, Sakamoto Y, Suzuki T, Taga Y. Highly efficient phosphores-
cence from organic light-emitting devices with an exciton-block layer. Appl
Phys Lett 2001;79:156.
7] Lamansky S, Djurovich P, Murphy D, Razzaq FA, Lee HE, Adachi C, et al. Highly
phosphorescent bis-cyclometalated iridium complexes: synthesis, photo-
physical characterization, and use in organic light emitting diodes. J Am Chem
Soc 2001;123:4304e12.
[30] Baldo MA, Adachi C, Forrest SR. Transient analysis of organic electro-
phosphorescence: I. Transient analysis of triplet energy transfer. Phys Rev B
2000;62:10958e66.
[31] Deng LJ, Zhang T, Wang RJ, Li JY. Diphenylphosphorylpyridine-functionalized
iridium complexes for high-efficiency monochromic and white organic light-
emitting diodes. J Mater Chem 2012;22:15910e8.
[
8] Wang Q, Chen YH, Chen JS, Ma DG. White top-emitting organic light-emitting
diodes employing tandem structure. Appl Phys Lett 2012;101:133302.
9] Baldo MA, Lamansky S, Burrows PE, Thompson ME, Forrest SR. Very high-
efficiency green organic light-emitting devices based on electro-
phosphorescence. Appl Phys Lett 1999;75:4.
[
[32] Su HC, Fang FC, Hwu TY, Hsieh HH, Chen HF, Lee GH, et al. Highly efficient
orange and green solid-state light-emitting electrochemical cells based on
III
cationic Ir complexes with enhanced steric hindrance. Adv Funct Mater
[
[
10] Baldo MA, Adachi C, Forrest SR. Transient analysis of organic electro-
phosphorescence. II. Transient analysis of triplet-triplet annihilation. Phys Rev
B 2000;62:10967e77.
11] Fukagawa H, Shimizu T, Ohbe N, Tokito S, Tokumaru K, Fujikake H. Anthra-
cene derivatives as efficient emitting hosts for blue organic light-emitting
diodes utilizing triplet-triplet annihilation. Org Electron 2012;13:1197e203.
2007;17:1019e27.
[33] Han LL, Zhang DY, Zhou YH, Yang Y, Woo HY, Bai FL, et al. Greenish yellow
organic light emitting devices based on novel iridium complexes containing
2-cyclohexenyl-1-phenyl-1H-benzo[d]imidazole. Dyes Pigments 2013;99:
1010e5.