Efficient and stable deep-red phosphorescent organic light-emitting diodes based on an iridium complex containing a benzoxazole-substituted ancillary ligand

Article abstract of DOI:10.1002/asia.201300671

In the deep red: A biscyclometalated deep-red phosphorescent dopant containing a new ancillary ligand, iridium(III) bis(1-phenylisoquinolinato-N, C²′) (2-(benzo[d]oxazol-2-yl)phenol) (Ir(piq)₂bop) has been designed and synthesized. B

Full text of DOI:10.1002/asia.201300671

COMMUNICATION
DOI: 10.1002/asia.201300671
Efficient and Stable Deep-Red Phosphorescent Organic Light-Emitting
Diodes Based on an Iridium Complex Containing a Benzoxazole-substituted
Ancillary Ligand
Chuan Wu,[a, b] Silu Tao,*[a] Mingming Chen,[a] Fu-Lung Wong,[b] Yi Yuan,[b, c]
Hin-Wai Mo,[b] Weiming Zhao,[d] and Chun-Sing Lee*[b]
In the past two decades, organic light-emitting devices
(OLEDs) have drawn considerable attention as a promising
technology for practical optoelectronic applications.[1] Cyclo-
prove due to the energy gap law and the drop in luminous
flux in the saturated-red region.[7a]
Okada et al. synthesized a widely used IrIII
ACHTUNGTRENNUNG
metalated iridiumACHTUNGTRENNUNG(III) complexes incorporated into OLEDs
are one of the most studied class of compounds.[2] Their
heavy metal core results in the mixing of singlet and triplet
manifolds, which enables harvesting of both singlet and trip-
let excitons.[3] Furthermore, the iridium complexes are suita-
ble for phosphorescent OLED (PHOLED) applications due
to their relatively short phosphorescence lifetimes, easily
sublimable feature, and excellent color tunability.[4] For
PHOLEDs to be useful in display and lighting applications,
red, green, and blue emitters with sufficient luminous effi-
ciencies, saturated color chromaticities, and adequate life-
times are required.[5] The advances in green phosphors ach-
ieved in the past couple of decades is particularly impres-
sive.[6] While the development of red phosphors has also
gone through several evolutional steps,[7] their performances
are still considerably behind those of green phosphors. For
example, reports on high-performance red PHOLEDs with
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
its a peak emission at 623 nm and a maximum powereffi-
ciency of 8.0 LmWꢁ1 (9.3 cdAꢁ1) at 100 cdmꢁ2.[8] Sun et al.
reported an iridium complex, Ir
quinoline as the main ligand and acetylacetone as ancillary
ligand. The reported Ir(piq)2acac-based device exhibits
a peak emission at 624 nm and a maximum current efficien-
cy of 7.83 cdAꢁ1 [9]
In recent work, Kwon et al. reported an
ACHTUNGRTEN(NUNG piq)2acac, with 1-phenyliso-
AHCTUNGTRENNUNG
;
iridium complex using a new main ligand (2-(3,5-dimethyl-
phenyl)-4-methylquinoline); a device based on this complex
reached a maximum power efficiency of 32 LmWꢁ1.[10] So
far, most red iridium complexes reported were obtained by
changing the main ligands, and there are few reports on the
modification of the ancillary ligands.[11]
In this study, a deep-red emitter containing a new ancil-
a Commission internationale de l’ꢀclairage
(CIE) coordinate
lary
ligand,
G
iridium
(III)
bis(1-phenylisoquinolinato-
(piq)2bop) has
xꢀ0.67 are still rare because the efficiency and brightness
N,C2’)(2-
of saturated-red phosphorescent OLEDs are difficult to im-
been designed and synthesized. The use of a bulky 2-(ben-
zo[d]oxazol-2-yl)phenol ancillary ligand is expected to sup-
press molecular aggregation and enhance the performance
by reducing self-quenching.[10] By doping Ir
ACTHNUGTRNE(UNG piq)2bop into
[a] C. Wu, Dr. S. Tao, M. Chen
School of Optoelectronic Information
University of Electronic Science and Technology of China (UESTC)
Chengdu, 610054 (China)
Fax : (+86)28-83201745
E-mail: silutao@uestc.edu.cn
Bebq2 (Bebq2 =bis(10-hydroxybenzo[h] quinolinato)berylli-
um), an efficient deep-red OLED with a luminous efficiency
of 9.1 LmWꢁ1 (10.3 cdAꢁ1) can be obtained at 100 cdmꢁ2,
along with CIE coordinates of (0.67, 0.33). Furthermore, we
found that the device has an extrapolated lifetime of over
30000 h at an initial luminance of 100 cdmꢁ2. Due to the
high efficiency and stability of the device, even in a simple
[b] C. Wu, Dr. F.-L. Wong, Y. Yuan, Dr. H.-W. Mo, Prof. C.-S. Lee
Center of Super-Diamond and Advanced Films (COSDAF) & De-
partment of Physics and Materials Science
City University of Hong Kong
Hong Kong Special Administrative Region
ACHTUNGTRENNUNG(China)
device structure, we conclude that IrACTHNUTRGNEUNG(piq)2bop is a promising
organic phosphor for practical OLED applications.
The ligand 1-phenylisoquinoline was prepared through
a Suzuki coupling reaction between 1-chloroisoquinoline
and phenylboronic acid. Synthesis of the final Ir complex in-
volved two key steps: In the first step, IrCl3 was reacted
with an excess of the synthesized ligand (1-phenylisoquino-
line) to produce a chloro-bridged Ir dimer. This dimer can
Fax : (+852)27847830
E-mail: apcslee@cityu.edu.hk
[c] Y. Yuan
Department of Chemistry
Shantou University
Guangdong, 515063 (China)
[d] Dr. W. Zhao
be easily converted into the monomeric complex IrACTHUNRGTNEUNG(piq)2bop
by replacing the bridging chlorides with 2-(benzo[d]oxazol-
Dongguan LITEWELL (OLED) Technology Incorporation
Dongguan, 523656 (China)
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Chun-Sing Lee et al.
2-yl)phenol in the presence of Na2CO3. The synthetic path-
way towards the preparation of Ir(piq)2bop is outlined in
Scheme 1. The molecular structure of Ir(piq)2bop has been
confirmed by H NMR spectroscopy, mass spectrometry, and
elemental analysis.
tris(2-phenylpyridine) iridium(Ir
ACHTUNGTRENNUNG
U
dard.[14] The HOMO level of Ir
ACHTUNGTRENNUNG
G
using ultraviolet photospectroscopy (UPS), is 5.2 eV, while
the LUMO level is 2.8 eV, as determined from the HOMO
level and the optical absorption edge. It should be pointed
out that the HOMO energy level of the iridium complex
corresponds to the 5d orbital of the iridium atom with sub-
stantial mixing with the p orbitals of the 1-phenylisoquino-
line ligands, while the LUMO level is determined by the p*
orbitals of the 1-phenylisoquinoline ligand and independent
of the 2-(benzo[d]oxazol-2-yl)phenol ligand.[15]
1
To understand the electroluminescent (EL) properties of
Ir
ACHTUNGTRENNUNG
NPB (40 nm)/Bebq2:5% IrAHCTUNTGRENNUNG
LiF (1 nm)/Al (100 nm) was fabricated, where ITO (indium
tin oxide) and LiF/Al are the anode and the cathode respec-
tively, 4,4’-bis[N-(1-naphthyl)-N-phenyl amino] biphenyl
(NPB) is the hole-transporting layer (HTL), and Bebq2,
which possesses LUMO and HOMO energy levels of 2.8
and 5.5 eV, respectively,[16] was used as host material and the
electron transporting layer (ETL) (Figure 2). The device
shows almost unchanged EL spectra centered at 624 nm at
different luminances, without any undesired emission
(Figure 3), which indicates that an effective charge capture
and complete energy transfer occurred between Bebq2 and
Scheme 1. Synthesis of IrACTHNUTRGNEUNG(piq)2bop.
Figure 1 shows UV/Vis absorption and photoluminescence
(PL) spectra of Ir(piq)2bop in a dilute dichloromethane
(CH2Cl2) solution at room temperature. From the absorp-
ACHTUNGTRENNUNG
Ir
tra suggests that the EL is indeed due to the emission from
Ir(piq)2bop.
ACHTUNGRTEN(NGNU piq)2bop. The similarity between the PL and the EL spec-
AHCTUNGTRENNUNG
Figure 1. Absorption and photoluminescence (PL) spectra of Ir
in dilute dichloromethane solution at room temperature. Inset: Structure
of Ir(piq)2bop.
ACHTUNGRTEN(NUNG piq)2bop
ACHTUNGTRENNUNG
Figure 2. Schematic cross-section of the PHOLED device and molecular
structure of Bebq2.
tion spectra, the high intensity peaks at 267 and 328 nm are
attributed to the spin-allowed ligand centered p–p* transi-
tion localized on the 1-phenylisoquinoline ligand, while the
weak, low-energy absorption shoulders located at 382 and
1
480 nm, respectively, likely originate from the S0! MLCT
3
(metal-to-ligand charge transfer) and S0! MLCT transi-
tions.[12] These features are analogous to other (CN)2Ir(L)
complexes as reported previously.[13] Strong spin-orbital cou-
pling was confirmed, where the oscillator strengths of singlet
and triplet MLCTs are similar (i.e., less than a factor of 2 in
their extinction coefficients), and thus result in strong phos-
phorescence.[13e] Ir
ACHTUNGTRENNUNG(piq)2bop in a dilute CH2Cl2 solution ex-
hibits a strong deep-red emission peaked at 614 nm, thus in-
dicating that the lowest excited triplet state is dominated by
the 3MLCT excited state. The PL quantum efficiency in
CH2Cl2 solution was determined to be 0.49, by using fac-
Figure 3. EL spectra of the PHOLED device at different luminances.
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Chem. Asian J. 2013, 8, 2575 – 2578

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Chun-Sing Lee et al.
phenylisoquinolinato-N,C2’) (2-(benzo[d]oxazol-2-yl)phenol)
(Ir(piq)2bop) has been designed and synthesized. By doping
AHCTUNGTRENNUNG
the new complex into Bebq2, a saturated red-emitting device
was obtained. At a luminescence of 100 cdmꢁ2, the device
shows efficiencies of 10.3 cdAꢁ1 and 9.1 LmWꢁ1 with CIE
coordinates of (0.67, 0.33). The device also shows a good
projected lifetime of over 30000 h at an initial brightness of
100 cdmꢁ2. Due to the reasonable good efficiency and stabil-
ity of the device even in a simple device configuration, we
believe that Ir
ACHTUNGRTENN(UNG piq)2bop is a promising organic phosphor for
applications in OLED technology.
Figure 4. Current efficiency and power efficiency as a function of current
density of the PHOLED with 5% IrACTHNUTRGNE(UNG piq)2bop. The inset depicts the J–V–
Experimental Section
L characteristics of the device.
Absorption and fluorescence spectra were recorded with a Perkin–Elmer
Lambda 2S UV-Vis spectrophotometer and a Perkin–Elmer LS50B lumi-
nescence spectrophotometer, respectively. The highest occupied molecu-
lar orbital (HOMO) value of the Ir complex was measured by ultraviolet
photoelectron spectroscopy using a VG ESCALAB 220i-XL ultrahigh
vacuum (UHV) surface analysis system. The lowest unoccupied molecu-
lar orbital (LUMO) value was determined from the difference between
the HOMO energy and the energy gap determined from the optical ab-
sorption edge.
Figure 4 shows efficiencies and J–V–L (inset) characteris-
tics of the PHOLED device with 5% Ir(piq)2bop doped into
AHCTUNGTRENNUNG
Bebq2. Owing to the matched energy levels, the energy bar-
rier for hole injection from NPB to Bebq2 is only 0.1 eV.
The device exhibits low operating voltages of 2.6, 3.6, and
4.6 V to give luminance values of 1, 100, and 1000 cdmꢁ2, re-
spectively. Considering that the HOMO and LUMO energy
Indium tin oxide (ITO)-coated glass substrates with a sheet resistance of
30 W/square were used as anodes for OLEDs, and all layers were deposit-
ed by vacuum thermal evaporation. Before successive deposition, the
substrates were patterned by traditional lithography, cleaned and dried in
an oven at 1208C. The ITO substrates were then treated with UV light-
ozone before loading into a deposition chamber. All organic layers and
the cathode were sequentially deposited onto the ITO substrates under
a pressure of 10ꢁ6 Torr. Immediately after preparation, the device was en-
capsulated under nitrogen atmosphere using a two-component epoxy
glue and a glass lid. Current density–voltage–luminance (J–V–L) charac-
teristics, CIE coordinates, and electroluminescent (EL) spectra were
measured with a programmable Keithley model 237 source and a Photo-
research PR650 photometer.
levels of Ir
(piq)2bop can act as traps for both electrons and holes. This
suggests that exciton formation on Ir(piq)2bop can occur
ACHTUNGTRENNUNG(piq)2bop are embedded by those of Bebq2, Ir-
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
both through direct charge trapping and energy transfer
from the host material, which may result in a high efficiency
of the OLED. The device shows an efficiency of 9.1 LmWꢁ1
(10.3 cdAꢁ1) at 100 cdmꢁ2. The device was then simply en-
capsulated by using a two-component epoxy glue and
a cavity glass lid without using any desiccant. The encapsu-
lated device shows a projected lifetime of over 30000 h and
600 h at an initial brightness of 100 cdmꢁ2 and 1000 cdmꢁ2,
respectively (Figure 5.[17] Notably, the performance of the Ir-
Acknowledgements
G
ACHTUNGERTN(NUNG piq)3-
This work was supported by a grant from the Research Grants Council
of the Hong Kong Special Administrative Region, China (Project No.
T23-713/11) and the Science and Technology Planning Project of Guang-
dong Province, China via Skyworth Flat Panel Display Technology Co.,
Ltd. (Project No. 2011A091200002). S.L.T. acknowledges support from
the National Natural Science Foundation of China (NSFC Grant
21002011), “Program for New Century Excellent Talents in the Universi-
ty of the Chinese Ministry of Education (NCET-11-0067)”, “the special
fund project of industry, university and research institute collaboration of
Guangdong province and Ministry of Education of P. R. China”
(2011B090400464), and the Science and Technology Program for Dong-
guanꢂs Higher Education, Science and Research, and Health Care Insti-
tutions (2011108101006).
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG(piq)3-based device. In addition, it shows a lifetime of about
150 h at 1000 cdmꢁ2.[16]
In summary, a biscyclometalated deep-red phosphorescent
dopant containing a new ancillary ligand, iridiumACTHNUTRGENUG(N III) bis(1-
Keywords: deep-red emission
PHOLEDs · long lifetime
·
doping
·
iridium
·
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Received: May 16, 2013
Revised: June 6, 2013
Published online: July 11, 2013
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