Rational design and synthesis of solution-processable red-emitting Ir(III) complexes for phosphorescent organic light-emitting diodes

Article abstract of DOI:10.1080/15421406.2018.1452787

Here, we have rationally designed and synthesized multifunctional red-emitting Ir (III) complex, (TPQ)₂Ir(acac-OXD) based on thiophene-phenylquinoline (TPQ) as main ligand and acetyl acetonate-linked functional group (acac-OXD) as ancillary lig

Full text of DOI:10.1080/15421406.2018.1452787

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
Rational design and synthesis of solution-
processable red-emitting Ir(III) complexes for
phosphorescent organic light-emitting diodes
Woosum Cho, Taegyun Lee, Yeong-Soon Gal, Sung-Ho Jin & Jae Wook Lee
To cite this article: Woosum Cho, Taegyun Lee, Yeong-Soon Gal, Sung-Ho Jin & Jae Wook
Lee (2017) Rational design and synthesis of solution-processable red-emitting Ir(III) complexes
for phosphorescent organic light-emitting diodes, Molecular Crystals and Liquid Crystals, 659:1,
160-171, DOI: 10.1080/15421406.2018.1452787
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MOLECULAR CRYSTALS AND LIQUID CRYSTALS
, VOL. , –

https://doi.org/./..
Rational design and synthesis of solution-processable
red-emitting Ir(III) complexes for phosphorescent organic
light-emitting diodes
Woosum Cho , Taegyun Lee , Yeong-Soon Gal , Sung-Ho Jin a, and Jae Wook Leec
a
a
b
a
Department of Chemistry Education, Graduate Department of Chemical Materials, Institute for Plastic
b
Information and Energy Materials, Pusan National University, Busan, Republic of Korea; Polymer Chemistry
Laboratory, College of Engineering, Kyungil University, Gyeongsan, Republic of Korea; Department of
c
Chemistry, Dong-A University, Busan, Republic of Korea
ABSTRACT
KEYWORDS
Here, we have rationally designed and synthesized multifunctional
Benzothiophene; iridium(III)
complex; organic
light-emitting diodes;
phenylquinoline; red
PhOLEDs; solution-processed
red-emitting Ir (III) complex, (TPQ) Ir(acac-OXD) based on thiophene-
2
phenylquinoline (TPQ) as main ligand and acetyl acetonate-linked
functional group (acac-OXD) as ancillary ligands for highly efficient
phosphorescent organic light-emitting diodes (PhOLEDs). Synthesized
1
Ir(III) complexes were characterized by H NMR, mass spectroscopy
and their absorption, emission and cyclic voltammetry were steadily
investigated. The Ir(III) complex have shown high thermal stability,
and exhibited deep-red emission with high quantum yield. In par-
ticular, (TPQ) Ir(acac-OXD) has shown higher quantum yield than
2
(
TPQ) Ir(acac). When utilized these complex in solution-processable
2
PhOLEDs, they delivered an excellent device performance.
Introduction
In the fluorescent molecules, the molecular transition from excited to ground states has a
few strict rules related to quantum-mechanical conditions that must be satisfied. These rules
prevent every excited molecule from emitting a photon and returning to ground state. As
a result, fluorescent materials used in organic light-emitting diodes (OLEDs) often generate
heat instead of light. This phenomenon has been overcome by phosphorescent materials using
a heavy metal atom, such as iridium, bonded to the organic materials (typically comprising of
carbon, nitrogen and hydrogen) in an individual molecule [1-4]. Thus design and synthesis
of new phosphorescent materials are attracting great interest in the OLEDs for flat panel dis-
play. Particularly Iridium(III) complexes with nitrogen heterocycles such as phenylpyridine,
isoquinoline and phenylquinoline are well known materials which can exhibit high triplet
quantum yields due to mixing of the singlet and triplet excited state via spin–orbit coupling
[
5-7], which further leads to high phosphorescence yields, enhancing the triplet state. The
ultimate aim of new material design and synthesis in OLED research, is to achieve high power
CONTACT Prof. Sung-Ho Jin
shjin@pusan.ac.kr
Department of Chemistry Education, Graduate Department of
Chemical Materials, Institute for Plastic Information and Energy Materials, Pusan National University, Busan , Republic of
Korea; Prof. Jae Wook Lee
of Korea.
jlee@donga.ac.kr.
Department of Chemistry, Dong-A University, Busan –, Republic
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/gmcl.
©
 Taylor & Francis Group, LLC

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
161
efficiency and low power consumption when OLED display is used in real time applications
such as mobile appliance [8-10].
It is well known that the carbazole and arylamine units containing Ir(III) complexes are
good for hole injection and transportation, whereas the oxadiazole, organoboryl, diarylsul-
fone and triarylphosphine oxide moieties typically serve as promising electron-deficient com-
ponents. Here, we focus on the conceptual designs of some multifunctional iridium(III) com-
plexes with distinct C^N chelating ligand chromophores, which are carbazole and oxadizole
with phenylquinoline as main skeleton. The incorporation of electron donating units on the
C^N chelating ligands is another strategy in enhancing the hole-transporting properties of
iridium complexes and it can also induce the longer wavelength emission with pure color
quality. Two strategies can be adopted to red shift of phosphorescence cyclometalated Ir(III)
complexes. The first one is to decrease the energy of the emissive metal-to-ligand, intraligand
or ligand-to ligand charge transfer excited states by introducing electron-donating/electron-
withdrawing groups to the ligands and by disrupting communication between the ligands.
∗
The second way is to decrease the energy of the emissive π−π ligand-centered states by
extended conjugation in the ligand. In our previous report thiophene group has been attached
to the phenylquinoline moiety of the Ir(III) complexes to develop novel functionalized phos-
phorescent red emitters with electron transporting (ET) oxadiazole group [11]. Benefiting
from their ET ability, the TPQ-Ir(III) phosphorescent emitters can furnish attractive EL effi-
ciencies in homoleptic and heteroleptic red OLEDs. In addition, TPQ ligands bearing elec-
tron donating thiophene and electron accepting phenylquinoline groups have been employed
major role to get very high EL efficiencies to the concerned PhOLEDs [12]. In this extension,
we introduce the electron transporting and hole transporting moiety in thiophene core for
the synthesis of Ir(III) heteroleptic complexes.
Experimental section
General Information
All reactions were carried out in an inert and dry environment under anhydrous nitrogen (N2)
which was dried by passing through a column of calcium sulphate. All chemicals and reagents
and solvents were purchased from Aldrich Chemical Co. and Alfa Aesar and used without
1
further purification. (TPQ) Ir(acac) was synthesized using the reported procedures [13]. H
2
NMR spectra was recorded on a Varian Mercury plus 300 MHz spectrometer and chemi-
cal shifts were recorded in ppm units with CDCl as an internal standard. High-resolution
3
mass spectra data were obtained from the Korea Basic Science and Institute Daejeon Center
(
HR-ESI Mass). The UV-visible and PL spectra were recorded with JASCO V-570 and Hitachi
F-4500 spectrophotometers at room temperature (RT). The absorption and emission spectra
were measured by 10 mol/L in chloroform (CF) solution. Thermal analysis was carried out
−5
on Mettler Toledo TGA/SDTA 851e, and DSC 822e analyzer under N atmosphere at a heating
2
rate of 10°C/min. Cyclic voltammetry (CV) measurements were performed with a CHI 600C
potentiostat (CH Instruments) at a scan rate of 100 mV/s using anhydrous dichloromethane
(
CH Cl ) and 0.1 M tetrabutylammonium tetrafluoroborate (TBABF ) as the solvent and
2
2
4
electrolyte, respectively, at RT. A platinum disc was used as the working electrode, a plat-
inum wire as the counter electrode, and a Ag/AgCl as the reference electrode. The poten-
tials were referenced to the ferrocene/ferrocenium redox couple (Fc/Fc+). The sublimated
grade of 4,4’-cyclohexylidenebis[N,N-bis(4-methylphenyl)benzenamine] (TAPC), and 2,6-
bis(3-(9H-Carbazol-9-yl)phenyl)pyridine (26DCzPPy) were purchased from OSM and used

1
62
W. CHO ET AL.
as the host materials for emitting layer (EML). The sublimated grade of 1,3,5-tri[(3-pytidyl)-
phen-3-yl]benzene (TmPyPB) was purchased from OSM and used as electron transport layer
(ETL).
Synthesis of 4-methyl-N-(2-oxo-2-phenylethyl)benzamide (1)
A mixture of 4-methylbenzoyl chloride (1.00 g, 6.4 mmol) and NMP (25 mL) was stirred at
0
°C, and a solution of benzoyl hydrazide (0.87 g, 6.4 mmol) in 10 mL pyridine was added.
The reaction mixture was stirred at 40°C for 4 h, after which the reaction mixture was poured
into 200 mL distilled water. The precipitated product of 1 was collected by filtration (1.05 g,
1
7
2
6%). H NMR (300 MHz, Acetone-d ): δ 8.00-7.87 (m, 4H), 7.52-7.48 (m, 3H), 7.32-7.30 (d,
6
H), 2.03 (s, 3H).
Synthesis of 2-phenyl-5-tolyl-1,3,4-oxadiazole (2)
A mixture of 4-methyl-N-(2-oxo-2-phenylethyl)benzamide (1.00 g, 4.0 mmol) in 40 mL
POCl was stirred at 120°C for 7 h. After cooling to RT, the mixture was poured into ice
3
water. The precipitate was collected by filtration and washed twice with de-ionized water to
1
afford a white powder of 2-phenyl-5-tolyl-1,3,4-oxadiazole (0.56 g, 61%). H NMR (300 MHz,
CDCl ): δ 8.15-8.02 (m, 4H), 7.55-7.53 (m, 3H), 7.35-7.32 (d, 2H), 2.45 (s, 3H).
3
Synthesis of 2-(4-(bromomethyl)phenyl)-5-phenyl-1,3,4-oxadiazole (3)
2
1
-phenyl-5-tolyl-1,3,4-oxadiazole (3.00 g, 12.70 mmol), N-bromosuccinimide (2.4 g,
3.90 mmol), and benzoyl peroxide (0.3 g, 0.127 mmol) were dissolved in carbon tetrachlo-
ride (80 mL). The reaction mixture was heated to react at 85°C for 5 h. After the reaction was
completed, the product in solution was collected by filtration and purified by recrystallization
1
from a solution of CF/methanol (4:1) to give a white solid (2.40 g, 60%). H NMR (300 MHz,
CDCl ): δ 8.16-8.11 (m, 4H), 7.58-7.55 (m, 5H), 4.54 (s, 2H).
3
Synthesis of 6-(4-(5-phenyl-1,3,4-oxadiazol-2-yl)phenyl)hexane-2,4-dione (4)
Sodium hydride (50% in mineral oil, 48 mg, 2.0 mmol) was washed with THF (10 mL each)
three times. Acetyl acetone (150 mg, 1.5 mmol) was added dropwise to a stirred suspension of
o
sodium hydride in 50 mL THF at 0 C. The resultant mixture was stirred for a further 30 min
at 0°C. N-butyllithium (2.5 M in hexane, 0.6 mL, 1.5 mmol) was added dropwise at 0°C and
the resultant mixture was stirred for a further 30 min at 0°C. 2-(4-(bromomethyl)phenyl)-5-
phenyl-1,3,4-oxadiazole (150 mg, 0.5 mmol) dissolved in 20 mL THF was added dropwise to
the stirred solution at 0°C dropwise. The reaction was kept at 0°C for 1 h and a further 1 h
at RT, after which it was quenched with aqueous ammonium chloride (10%, 20 mL). Con-
centrated hydrochloric acid was added to the mixture until pH 1 was reached, and then the
aqueous phase was separated and extracted with diethyl ether. The combined organic phases
were washed with saturated sodium bicarbonate and dried over anhydrous sodium sulfate.
After removal of the solvent, the crude product was purified by silica gel column chromatog-
1
raphy with hexane/DCM (6:1 to 2:1) as eluent to give a white solid (66 mg, 42%). H NMR
(
8
300 MHz, CDCl ): δ 8.15-8.05 (m, 4H), 7.55-7.53 (m, 3H), 7.37-7.35 (d, 2H), 5.47 (s, 1H),
.16-8.11 (m, 4H), 3.05-3.00 (t, 2H),2.67-2.62 (t, 2H), 2.04 (s, 3H)
3

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
163
o
Figure . TGA curves of (TPQ) Ir(acac) and (TPQ) Ir(acac-OXD) measured at a scan rate of  C/min under


N atmosphere.
Synthesis of cyclometalated Ir(III) µ-chloride bridged dimer (5)
(
4-phenyl-2-(thiophen-2-yl)quinoline) (5 g, 17.40 mmol) and IrCl ·H O (2.08 g, 6.96 mmol)
3
2
were added to a mixture of 2-ethoxyethanol and water (80 mL, 3:1 v/v). The resulting mixture
was refluxed at 140 ºC for 20 h under an N atmosphere and cooled to RT. The formed red
2
solid was filtered, washed with water followed by methanol. Subsequently, the solid was dried
under vacuum at 120 ºC to afford cyclometalated Ir(III) μ-chloride bridged dimer as a red
1
solid (3.36 g, 60%). H NMR (300 MHz, CDCl ): δ (ppm) 8.19-8.16 (d, 1H), 8.06-8.05 (dd,
3
1
H), 7.90-7.86 (m, 2H), 7.74-7.69 (m, 2H), 7.56-7.51 (m, 5H), 7.48-7.43 (m, 2H)
Synthesis of bis[4-phenyl-2-(thiophen-2-yl)quinoline]iridium-6-(4-(5-phenyl-1,3,4-
oxadiazol-2-yl)phenyl)hexane-2,4-dione (6)
The cyclometalated iridium dimer (280 mg, 0.7 mmol) and 6-(4-(5-phenyl-1,3,4-oxadiazol-
2
0
-yl)phenyl)hexane-2,4-dione (150 mg, 0.45 mmol) were mixed with Na CO (47 mg,
2 3
.45 mmol) in 2-ethoxyethanol (50 mL). The mixture was refluxed for 12 h under a N atmo-
2
sphere. After cooling at RT, the crude solution was poured onto water and extracted with ethyl
acetate, dried over anhydrous MgSO and evaporated in a vacuum. The residue was purified
4
by silica gel chromatography (hexane:ethyl acetate 1:4) as an eluent and further purified by
recrystallization twice using dichloromethane/hexane mixture to afford a red solid complex
1
(
150 mg, 34%). H NMR (300 MHz, CDCl ): δ 8.42-8.38 (m, 2H), 8.10-8.08 (m, 2H), 7.85-
3
7
7
1
.78 (m, 3H), 7.74-7.72 (d, 3H), 7.66-7.60 (m, 6H), 7.55-7.41 (m, 9H), 7.35-7.30 (m, 3H),
.21-7.16 (m, 3H), 6.93-6.90 (d, 1H), 6.42-6.36 (d, 1H), 4.84 (s, 1H), 2.57-2.55 (t, 2H), 1.67-
.65 (t, 2H) Calcd for C H IrN O S : C, 62.43; H, 3.76; N, 5.10. Found: C, 62.37; H, 3.74;
58
41
+
4
3 2
N, 5.05. HRESI-MS [M+H] : m/z Found 1098.3265, Calcd for 1098.2240.
Results and discussion
The thermal stability of (TPQ) Ir(acac-OXD) was identified using a thermogravimetric ana-
2
lyzer (TGA) under N atmosphere at a heating rate 10°C/min (Fig. 1). The thermal decom-
2
position temperature (T ) of (TPQ) Ir(acac-OXD) at 5% mass loss was 316°C. (Fig. 1 and
d
2
Table 1).

1
64
W. CHO ET AL.
Table . Photophysical, electrochemical and thermal properties of (TPQ) Ir(acac) and (TPQ) Ir(acac-OXD).


Compound
Td
λ
λ
[nm]c
HOMO/LUMO
abs
em
[oC]
a
b
d
[nm]
[eV]
(
(
TPQ) Ir(acac)


, , , 
, , , 


-./-.
-./-.

TPQ) Ir(acac-OXD)

a
b
c
o
Temperature with % mass loss measure by TGA with a heating rate of  C/min under N .

−
Absorption maximums in CHCl solution at . ×  M concentration.

−
Maximum emission wavelength, measured in CHCl solution at . ×  M.

d
Determined from the onset of CV for oxidation and UV-visible absorption edge.
The photophysical properties of (TPQ) Ir(acac-OXD) was measured using UV-visible
2
absorption and PL spectra at RT. The UV-visible absorption spectrum of (TPQ) Ir(acac-
2
OXD) in CF solution (Fig. 2(a) and Table 1) showed absorption bands at 300–442 nm; these
1
∗
correspond to the spin-allowed π-π transitions of the cyclometalated ligands. A well-
resolved shoulder at 410 nm confirmed the presence of a spin-forbidden ligand-centered
3
∗
π-π transition, facilitated by spin-orbit coupling. A structure-less weak band at 605 nm
1
3
was assigned to admixture of some MLCT and MLCT states. The (TPQ) Ir(acac-OXD)
2
showed a PL emission maximum at 633 nm in CF solution (Fig. 2(b) and Table 1). Due
to the presence of donor/acceptor interactions between the electron donor thiophene and
Figure . (a) UV-visible absorption spectra and (b) PL spectra of (TPQ) Ir(acac) and (TPQ) Ir(acac-OXD).



MOLECULAR CRYSTALS AND LIQUID CRYSTALS
165
Figure . Cyclic voltammetry of (TPQ) Ir(acac) and (TPQ) Ir(acac-OXD).


electron acceptor quinoline (TPQ) groups in the cyclometalated ligand, the emission maxi-
mum of (TPQ) Ir(acac-OXD) complex was shifted to the bathochromic region. The donor-
2
acceptor character is caused by interaction between electron-rich thiophene and electron-
deficient quinoline [12]. CV analysis was carried out for (TPQ) Ir(acac-OXD) in anhydrous
2
CH Cl solvent with 0.1 M TBAClO as supporting electrolyte at a scan rate of 100 m/Vs to
2
2
4
find out the highest occupied molecular orbital (HOMO) energy level. The lowest unoccu-
pied molecular orbital (LUMO) energy level was calculated from the HOMO and optical band
opt
gap (Eg ), obtained from UV absorption edge value. The experimental HOMO and LUMO
energy levels of (TPQ) Ir(acac-OXD) are -5.43 eV and -3.41 eV, respectively (Fig. 3). Here,
2
opt
Eg of (TPQ) Ir(acac-OXD) is 2.02 eV. Both the HOMO and LUMO levels of (TPQ) Ir(acac-
2
2
OXD) are very close to the corresponding energy levels of (TPQ) Ir(acac) representing that
2
the introduction of either oxadiazole-based ET group on ancillary ligand does not much influ-
ence the frontier orbital energy levels of these Ir(III) complexes as similar to the photophysical
properties.
Before fabrication of the PhOLEDs with the new dopant, we measured the electron and
hole mobilities of the dopant [(TPQ) Ir(acac-OXD)] by using space-charge limited cur-
2
rent (SCLC) [14] method with the device configuration of ITO/ LiF (10 nm)/Ir(III) com-
plex (100 nm)/LiF (1 nm)/Al (100 nm) for electron-only device and ITO/PEDOT:PSS
(
40 nm)/Ir(III) complex (100 nm)/MoO (5 nm)/Al (100 nm) for hole-only device. The elec-
3
tron and hole mobilities of (TPQ) Ir(acac) and (TPQ) Ir(acac-OXD) were also measured by
2
2
2
the same method (Table 2). The J-V plots for the devices are shown in Fig. 4. The electron
−5
2
and hole mobilities of (TPQ) Ir(acac-OXD) are found to be 7.75 × 10 cm /V s and 1.63 ×
2
−5
2
1
0
cm /V s, respectively. This result clearly shows that tethering of OXD unit to the Ir(III)
complex improved the electron mobility as compared with (TPQ) Ir(acac).
2
In order to evaluate the performance of (TPQ) Ir(acac-OXD) as emitter, we fab-
2
ricated deep-red emitting PhOLEDs with a simple device structure containing a
solution-processed EML (Fig. 5). The device configuration adopted in this study is
Table . Mobility data for (TPQ) Ir(acac-OXD) in comparison with (TPQ) Ir(acac).


Ir(III) complex
Hole mobility
Electron mobility


(cm /V s)
(cm /V s)
−

−
−
(
(
TPQ) Ir(acac)
. × 
. × 

−

TPQ) Ir(acac-OXD)
. × 
. × 


1
66
W. CHO ET AL.

Figure . (a) Hole only device and (b) electron only device of J-V characteristics [ITO/LiF ( nm)/Ir(III) com-
plex ( nm)/LiF ( nm)/Al ( nm) for electron-only device and ITO/PEDOT:PSS ( nm)/Ir(III) complex
(
 nm)/MoO ( nm)/Al ( nm) for hole-only device].

Figure . Energy level diagram of the deep-red PhOLEDs.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
167
Figure . Characteristics of the deep-red PhOLEDs using (TPQ) Ir(acac) and (TPQ) Ir(acac-OXD) as dopants:


(a) J-V-L (b) CE-V-PE and (c) EQE-V.

1
68
W. CHO ET AL.
Figure . EL spectra of (a) deep-red PhOLEDs and (b) CIE coordinates using (TPQ) Ir(acac) and (TPQ) Ir(acac-


OXD) as dopants.
ITO/PEDOT:PSS/TAPC:26DCzPPy:12 wt% dopant/TmPyPB/LiF/Al. We have also fab-
ricated the control devices using (TPQ) Ir(acac) as dopant to compare the effect of OXD
2
group on device performance.
Data of the PhOLEDs are provided in Table 3. Fig. 6(a) & 6 (b) shows the current density-
voltage-luminance (J-V-L) and current efficiency-J-power efficiency (CE-J-PE) characteris-
tics. The CE and PE of TPQIr-HT are 12.76 cd/A and 3.44 lm/W, respectively, which are higher
than the CE (10.92 cd/A) and PE (2.45 lm/W) of (TPQ) Ir(acac). The high electron mobil-
2
ity of (TPQ) Ir(acac-OXD) relative to (TPQ) Ir(acac) results good charge balance in EML,
2
2
which enhances the performance of (TPQ) Ir(acac-OXD) in PhOLEDs.
2
All the PhOLEDs, irrespective of the dopant, HIL, and luminance range, exhibited the same
electroluminescent (EL) peak at 637 nm (Fig. 7(a)) and the CIE coordinates of (0.69, 0.30)
(Fig. 7(b)) representing the saturated red/ deep-red emission, which is highly sought-after as

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
169
O
Cl
O
O
N
NMP, K CO3
H
N
2
+
0 oC, 12 h
4
H
N
2
H
H
O
1
POCl3
NBS
85 oC, 5 h, CCl4
Br
O
O
1
20 oC, 7 h
N
2
N
N N
3
O
O
n-BuLi, NaH
THF
O
0 oC (2 h) - RT (1 h)
N
N
4
N
IrCl ืH O
Cl
Cl
S
3
2
N
Ir
Ir
1
40 oC, 20 h
S
N
S
2
2
5
S
N
Cl
DMF, 12 h
K CO
Ir
Ir
2
3
Cl
N
S
N
O
O
O
O
Ir
2
S
2
5
2
O
(TPQ) Ir(acac)
2
O
2
-ethoxyethanol
Reflux, 12 h
Na CO
O
2
3
N
N
N
O
Ir
O
S
O
2
N
N
6
(
TPQ) Ir(acac-OXD)
2
Scheme . Synthesis of Ir(III) complexes
like deep-blue emission. The color coordinates and the efficiency of (TPQ) Ir(acac-OXD) are
2
better than our previously reported solution-processed PhOLEDs containing diphenylquino-
line (DPQ) based and carbazole (CVz)-phenylquinoline(PQ) based deep-red emitting het-
eroleptic Ir(III) complexes (FPQ) Ir(pic-N-O) [15], and (EO-CVz-PhQ) Ir(acac) [16].
2
2

1
70
W. CHO ET AL.
Table . Device performance of the deep-red PhOLEDs with (TPQ) Ir(acac) and (TPQ) Ir(acac-OXD) as


dopants.
Vona
[V]
EQEb
[%]
Luminance
EL c
[nm]
CIEc
(x,y)
b
Compound
[cd/m]
(
(
TPQ) Ir(acac)
.
.
.
.




(., .)
(., .)

TPQ) Ir(acac-OXD)

a
b
c
Turn-on voltage measured at brightness of  cd/m.
Maximum efficiency.

At  mA/cm .
Conclusion
In summary, we attached a oxadizole based ET unit (OXD) to the heteroleptic Ir(III) complex,
TPQ) Ir(acac), to achieve a good charge balance in EML by improving the electron trans-
(
2
porting as well as hole transporting properties of (TPQ) Ir(acac-OXD) as compared with the
2
parent complex. Upon introduction of OXD moiety, carrier mobilities and the device perfor-
mance are improved relative to (TPQ) Ir(acac), without affecting the deep-red coordinates
2
(
0.67, 0.32). We achieved a high EQE of 14.18% for the PhOLED of (TPQ) Ir(acac-OXD),
2
which is superior to the performance of (TPQ) Ir(acac) (EQE = 12.42%). The overall results
2
of these deep-red PhOLEDs show that the introduction of new functional units is an effective
method to improve device performance.
Funding
This research was supported by the Dong-A University Research Fund.
ORCID
Sung-Ho Jin http://orcid.org/0000-0001-6631-983X
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