Synthesis of New Heteroleptic Iridium(III) Complex Consisting of 2-Phenylquinoline and 2-[4-(Trimethylsilyl) phenyl]Pyridine for Red and White Organic Light-Emitting Diodes

Article abstract of DOI:10.1166/jnn.2017.14155

A novel red iridium(III) complex, (PQ)₂Ir(TMSppy), containing 2-phenylquinoline (PQ) as the main cyclometalated ligand and 2-[4-(trimethylsilyl)phenyl]pyridine (TMSppy) as the ancillary ligand, was synthesized for use in phosphorescent organic light-emitting diodes (OLEDs). (PQ)₂Ir(TMSppy) had a red emission with a maximum emission wavelength (λ_max) at 603 nm. To investigate the (PQ)₂Ir(TMSppy) as a red emitter in OLEDs, we fabricated a device with a multi-layer architecture. The (PQ)₂Ir(TMSppy) device showed an electroluminescence (EL) maximum emission peak at 612 nm and showed a maximum quantum efficiency (EQE_max) of 15.5% at a 10% doping concentration. In addition, white OLEDs, having three primary color components, made from (PQ)₂ Ir(TMSppy) with bis(4,6-difluorophenylpyridine)picolinate (Flrpic) and tris(2-phenylpyridinato-C²,N)iridium(III) (Ir(ppy)₃) gave the best performances, with an EQE_max of 18.1%, maximum power efficiency (PE_max) of 22.8 lm/w, and maximum current efficiency (CE_max) of 36.5 cd/A with Commission Internationale de LEclairage coordinates of (0.39,0.42) at a luminance of 1,000 cd/m².

Full text of DOI:10.1166/jnn.2017.14155

Article
Journal of
Nanoscience and Nanotechnology
Vol. 17, 5587–5592, 2017
Copyright © 2017 American Scientific Publishers
All rights reserved
Printed in the United States of America
www.aspbs.com/jnn
Synthesis of New Heteroleptic Iridium(III) Complex
Consisting of 2-Phenylquinoline and 2-[4-(Trimethylsilyl)
phenyl]Pyridine for Red and White Organic
Light-Emitting Diodes
1
1
1
2
1ꢀ∗
Hee Un Kim , Jae-Ho Jang , Hea Jung Park , Jun Yeob Lee , and Do-Hoon Hwang
1
Department of Chemistry, and Chemistry Institute for Functional Materials, Pusan National University, Busan 609-735, Republic of Korea
School of Chemical Engineering, Sungkyunkwan University 2066, Seobu-ro, Jangan-gu, Suwon, Gyeonggi 440-746, Republic of Korea
2
A novel red iridium(III) complex, (PQ) Ir(TMSppy), containing 2-phenylquinoline (PQ) as the main
2
cyclometalated ligand and 2-[4-(trimethylsilyl)phenyl]pyridine (TMSppy) as the ancillary ligand, was
synthesized for use in phosphorescent organic light-emitting diodes (OLEDs). (PQ) Ir(TMSppy)
2
had a red emission with a maximum emission wavelength (ꢀmax) at 603 nm. To investigate the
(
PQ) Ir(TMSppy) as a red emitter in OLEDs, we fabricated a device with a multi-layer archi-
2
tecture. The (PQ) Ir(TMSppy) device showed an electroluminescence (EL) maximum emission
2
peak at 612 nm and showed a maximum quantum efficiency (EQE ꢁ of 15.5% at a 10% dop-
max
IP: 93.179.90.191 On: Mon, 23 Apr 2018 21:08:37
ing concentration. In addition, white OLEDs, having three primary color components, made from
Copyright: American Scientific Publishers
(
PQ) Ir(TMSppy) with bis(4,6-difluorophenylpyridine)picolinate (FIrpic) and tris(2-phenylpyridinato-
2
Delivered by Ingenta
2
C ,Nꢁiridium(III) (Ir(ppy) ꢁ gave the best performances, with an EQE
of 18.1%, maximum power
3
max
efficiency (PEmaxꢁ of 22.8 lm/w, and maximum current efficiency (CEmaxꢁ of 36.5 cd/A with Commis-
2
sion Internationale de L’Eclairage coordinates of ꢂ0ꢃ39ꢄ0ꢃ42ꢁ at a luminance of 1,000 cd/m .
Keywords: Phosphorescence, Iridium(III) Complex, Organic Light-Emitting Diodes.
1
. INTRODUCTION
can achieve a theoretical internal quantum efficiency of
100%, due to generating light from both singlet and
Organic light-emitting diodes (OLEDs) have attracted
great attention during the last two decades, particularly
for applications such as full-color and flat-panel displays.
In these fields, OLEDs have many remarkable advantages,
such as low operating voltages for low power consump-
tion, high contrast and brightness with a wide viewing
angle, faster response times than liquid-crystal displays
7
–13
triplet excitons by intersystem crossing.
Among the
heavy metal complexes, iridium(III) complexes have
been employed in many phosphorescent emitters, such
∧
as homoleptic [(C N) Ir, CN; cyclometalated ligand] or
3
∧
heteroleptic [(C N) Ir(LX), LX; ancillary ligand] com-
2
plexes. These iridium(III) complexes exhibit good photo
and thermal stability, high photoluminescence (PL) quan-
(
LCDs), and a light weight for flexible displays and solid
1
–4
tum efficiency (ꢁ ꢂ, and relatively short excited-state
state lighting applications.
White OLEDs (WOLED),
PL
1
4ꢀ15
lifetimes in the microsecond range.
In particular,
using the three primary colors of OLEDs technique, are
ideal candidates for potentially cost-effective, environmen-
tally friendly, solid-state lighting sources. Thus, applica-
tions of the OLEDs technique are becoming diverse.
Phosphorescent OLEDs based on complexes containing
Ir(III), Pt(II), Os(II), and Ru(II) as typical heavy metals,
heteroleptic iridium(III) complexes can modify ancillary
ligands as well as cyclometalated ligands, which has
the advantages of easy control of photophysical and
electrochemical properties, and electroluminescent (EL)
performance. Typical ancillary ligands in heteroleptic
iridium(III) complexes are picolinic acid (pic), acety-
lacetone (acac), and 2,2,6,6-tetramethylheptane-3,5-dione
(tmd), and these have been the most widely used until
5
ꢀ6
∗
Author to whom correspondence should be addressed.
J. Nanosci. Nanotechnol. 2017, Vol. 17, No. 8
1533-4880/2017/17/5587/006
doi:10.1166/jnn.2017.14155
5587

Synthesis of New Heteroleptic Iridium(III) Complex
Kim et al.
O
O
2.2. Synthesis of Red Iridium(III) Complex
2-Phenylquinoline (PQ), 2-(4-(trimethylsilyl)phenyl)pyridine
TMSppy), and Ir(III)-ꢃ-chloro-bridged dimer ([Ir(PQ) IrCl)] ꢂ
O
O
Ir
2
acac
N
2
-ethoxyethanol
(
2
2
were synthesized in similar ways according to the methods
described in previous reports.
Cl
Cl
(PQ) Ir(acac)
2
IrCl -3H O
Ir
3
2
Ir
7
N -ethoxyethanol:water
N
N
2
(
3:1)
2
2
Si
N
N
PQ
Ir(III)-µ-chloro-bridgeddimer
TMSppy
2.2.1. Synthesis of 2-Phenylquinoline (PQ)
Ir
2
N
Silvertriflate,
-ethoxyethanol
2
-Chloroquinoline (5.0 g, 30.6 mmol), phenyl-
2
Si
boronic acid (4.47 g, 36.7 mmol), and tetra-
kis(triphenylphosphine)palladium(0) (1.01 g, 0.92 mmol)
(
PQ)2Ir(TMSppy)
Scheme 1. Synthetic routes for (PQ) Ir(TMSppy).
were dissolved in THF. A solution of 2 M K CO (100 ml)
2 3
2
and Aliquat 336 (1.23 g, 3.06 mmol) was added, and
the mixture was refluxed overnight under a nitrogen
atmosphere. The reaction mixture was cooled to room
temperature and extracted with ethyl acetate. The organic
now.16–18 In particular, acac is currently the most widely
used ancillary ligand in red-phosphorescent emitters,
such as bis(2-phenylquinoline)(acetylacetonate)iridium(III)
1
9
[
(pq) Ir(acac)], with excellent color purity. However,
layer was washed with water and dried over MgSO . The
2
4
acac as an ancillary ligand in heteroleptic iridium(III)
complexes causes decreased OLEDs performances due
to intrinsic problems such as low thermal stability and
concentration self-quenching or triplet–triplet annihila-
solvent was removed under reduced pressure to give a
crude residue. The crude product was purified by col-
umn chromatography on silica gel to obtain PQ (5.11 g,
1
81.5%). H NMR (300 MHz, CDCl ꢂ ꢄ (ppm): 8.30
3
tion at high doping concentrations in phosphorescent
(d, 1H), 8.24 (d, 2H), 8.11 (d, 1H), 7.82 (m, 3H), 7.59
OLEDs.2
0ꢀ21
To solve these problems, structural mod-
13
(m, 4H). C NMR (75 MHz, CDCl ꢂ ꢄ (ppm): 157.39,
3
ifications of various ligands in iridium(III) complexes
to prevent self-quenching have been suggested. Lee
et al. reported a fac-tris(2-phenylpyridine)iridium(III)
136.84, 129.75, 129.71, 129.36, 128.88, 128.16, 127.62,
127.49, 127.22, 127.20, 126.33, 119.04.
[
2
Ir(ppy) ] derivative containing a bulky pinene group in
3
2.2.2. Synthesis of 2-(4-(trimethylsilyl)phenyl)pyridine
2
2
-phenylpyridine (ppy). The bulky pinene group in the
(
TMSppy)
IP: 93.179.90.191 On: Mon, 23 Apr 2018 21:08:37
iridium(III) complex lead to minimum bimolecular inter-
Copyright: American Scientific Publishers
action and, therefore, significantly reducing self-quenching
at very high doping concentration by sterically hindered
Under a nitrogen atmosphere, n-BuLi (7.7 ml, 15.4 mmol,
.0 M solution in hexane) was added dropwise to
2
Delivered by Ingenta
2
-(4-bromophenyl)pyridine (3.0 g, 12.8 mmol) in THF at
78 C. The mixture was stirred at −78 C for 1 h,
the pinene spacers.
ꢀ
ꢀ
−
In this paper, we have designed an iridium(III) com-
and then chlorotrimethylsilane (2.45 ml, 19.2 mmol) was
added in one portion. The reaction mixture was allowed
to warm to room temperature overnight. After the reac-
tion mixture was quenched with methanol, it was extracted
with ethyl acetate and the organic layer was washed with
plex [(PQ) Ir(TMSppy)] based on 2-phenylquinoline (PQ)
as the main cyclometalated ligand and 2-[4-(trimethyl-
silyl)phenyl]pyridine (TMSppy) as a new ancillary ligand.
2
(
PQ) Ir(TMSppy) was able to reduce concentration self-
2
quenching at high doping concentrations and improve EL
performance in phosphorescent OLEDs due to the introduc-
tion of the bulky TMSppy moiety. The ppy core in TMSppy
has a high thermal stability, high phosphorescence (PL)
quantum yield, and a broad-band full width at half max-
imum (FWHM) range of emission.2 Moreover, the ster-
ically bulky trimethylsilyl group in TMSppy is electron
donating, chemical stable and have good solubility. The
synthetic routes and chemical structures of the heteroleptic
iridium complexes are outlined in Scheme 1.
water and dried over MgSO . The solvent was removed
4
under reduced pressure to give a crude residue. The crude
product was purified by column chromatography on sil-
1
ica gel to obtain TMSppy (1.86 g, 63.7%). H NMR
(
300 MHz, CDCl ꢂ ꢄ (ppm): 7.75 (d, 1H), 7.99 (d, 2H),
3
3
7
.73 (m, 2H), 7.65 (d, 2H), 7.22 (m, 1H), 0.30 (s, 9H,
13
CH ). C NMR (75 MHz, CDCl ꢂ ꢄ (ppm): 157.45,
3
3
1
1
49.72, 141.41, 139.70, 136.71, 133.80, 126.13, 122.16,
20.58, 1.08.
2
.2.3. Synthesis of Ir(III)-ꢃ-Chloro-Bridged Dimer
2
. EXPERIMENTAL DETAILS
([Ir(PQ)
The cyclometalated iridium(III)-ꢃ-chloro-bridged dimer
complex [Ir(PQ) Cl] , which was used directly in the next
2 2
IrCl)] )
2
.1. Materials
Phenylboronic acid, 2-chloroquinoline, 2-(4-bromophenyl)
pyridine, trimethylsilyl chloride, tetrakis(triphenylphosphine)
palladium(0), iridium(III) chloride hydrate, and silver
trifluoromethanesulfonate were purchased from Aldrich
and Alfa Aesar. All chemicals were used without further
purification.
2
2
step without further purification.
2.2.4. Synthesis of (PQ)
(PQ) Ir(TMSppy) was synthesized similarly with the
Ir(TMSppy)
2
2
2
4ꢀ25
previous reports.
The [Ir(PQ) Cl] dimer complex
2 2
5588
J. Nanosci. Nanotechnol. 17, 5587–5592, 2017

Kim et al.
Synthesis of New Heteroleptic Iridium(III) Complex
(
(
0.5 g, 0.39 mmol) and silver trifluoromethanesulfonate
0.21 g, 0.83 mmol) were placed in a 100 mL round-
PL_Solution
PL_Neat solid
UV
1.0
0.5
0.0
bottomed flask with acetonitrile (30 mL) and the mix-
ture was refluxed for 6 h. After cooling, the deep-red
solution was filtered to remove insoluble materials and
the solvent was removed in vacuo. TMSppy (0.20 g,
PL
UV
0
.88 mmol) and 2-ethoxyethanol (30 mL) were added to
+
the resulting solid, [Ir(PQ) (CH CN) trifluoromethane-
2
3
2
sulfonate], and the mixture was refluxed under a nitrogen
atmosphere for 12 h. After cooling to room temperature,
the crude solution was poured into water, extracted with
3
00
400
500
600
700
800
dichloromethane and brine, dried over MgSO , and con-
4
Wavelength (nm)
centrated in vacuo. The residue was purified using column
chromatography with dichloromethane as the eluent. The
resulting solid was recrystallized from dichloromethane
and methanol, and then the red solid was dried in vacuo.
Figure 1. UV-visible absorption and PL spectra of (PQ) Ir(TMSppy) in
solution and neat solid film state at 298 K.
2
The maximum PL emission wavelength (ꢆmaxꢂ of
(
5
PQ) Ir(TMSppy) was obtained as a red solid (0.17 g,
2
1
(PQ) Ir(TMSppy) in toluene solution at 298 K was
3.6%). H NMR (300 MHz, CDCl ꢂ ꢄ (ppm): 8.26
2
3
observed at 603 nm. This value was red-shifted compared
to that of a typical iridium(III) complex containing acac,
(
(
(
d, 1H), 8.17 (m, 3H), 7.97 (m, 3H), 7.64 (m, 4H), 7.57
d, 1H), 7.47 (t, 1H), 7.29 (d, 1H), 7.21 (m, 1H), 6.94
m, 4H), 6.77 (m, 8H), 0.02 (s, 9H). C NMR (75 MHz,
1
3
(PQ) Ir(acac) (598 nm at 298 K). Also, a red-shift of the
2
ꢆmax from solution to neat solid state in (PQ) Ir(TMSppy)
CDCl ꢂ ꢄ (ppm): 167.56, 166.12, 163.43, 160.90, 156.62,
2
3
(
614 nm) was observed, which may be related to aggrega-
tion effects between the iridium(III) complexes.
The PL quantum yields (ꢁ ꢂ of (PQ) Ir(TMSppy) in
1
1
1
1
1
7
49.15, 148.38, 147.82, 146.44, 146.23, 143.57, 141.50,
39.18, 137.25, 137.11, 136.57, 136.39, 135.67, 130.03,
29.60, 129.20, 128.68, 128.59, 128.07, 127.67, 127.58,
27.04, 126.02, 125.90, 125.60, 124.93, 124.72, 121.91,
20.15, 119.84, 118.70, 118.14, 117.98, 77.46, 77.03,
6.62, 1.30. Anal. Calcd (%) for C H IrN Si: C; 63.90,
PL
2
−5
dilute degassed 2-methyltetrahydrofuran solution (10 M)
was measured using (PQ) Ir(acac) as a standard. The solu-
2
IP: 93.179.90.191 On: Mon, 23 Apr 2018 21:08:37
tion state of ꢁ for (PQ) Ir(TMSppy) was calculated to
PL
2
4
4
36
3
Copyright: American Scientific Publishers
be 0.13. The ꢁ of (PQ) Ir(TMSppy) was slightly higher
H; 4.39, N; 5.08. Found: C; 63.84, H; 4.41, N; 5.11.
MALDI-TOF (M , C H IrN Si): calcd; 827.23, found;
PL
2
Delivered by Ingenta
2
6
+1
than that of (PQ) Ir(acac) as a standard (ꢁ = 0.10).
2
PL
4
4
36
3
8
4
27.35. (Purity: 99.7%, by HPLC). T (5% weight loss):
07 C.
d
ꢀ
3.2. Theoretical Calculation
To further understand the effects of TMSppy on the irid-
ium(III) complex, we performed density functional the-
2
.3. Measurements and Device Fabrication
2
7
ory (DFT) calculations using B3LYP/6-31G(d) basis sets.
As shown in Figure 2, the highest occupied molecu-
lar orbital (HOMO) of (PQ) Ir(TMSppy) was mainly
Measurements and devices fabrication were performed in
similar ways according to the methods described in previ-
ous report.7
2
3
. RESULTS AND DISCUSSION
3
.1. Photophysical Properties
Figure 1 shows the UV-visible absorption and phospho-
rescence (PL) spectra of (PQ) Ir(TMSppy) in both toluene
2
solution and the neat solid state at 298 K. The absorp-
tion bands at approximately 290–350 nm and weaker
absorption bands in the region above 390 nm, which
∗
corresponded to the spin allowed ꢅ–ꢅ transitions of
the cyclometalated main ligand, and spin-allowed admix-
ture of singlet and triplet metal-to-ligand charge transfer
1
3
(
MLCT, MLCT) transitions, respectively. These MLCT
bands were attributed to effective mixing between the
3
∗
1
3
ligand ꢅ–ꢅ state and spin-forbidden MLCT, and the
higher-lying MLCT transitions induced by strong spin-
orbit coupling effects. The optical energy bandgap of
Figure 2. Calculated HOMO and LUMO of (PQ) Ir(TMSppy) using
2
(
PQ) Ir(TMSppy) was calculated to be 2.16 eV.
DFT at the B3LYP/6-31G(d) level.
2
J. Nanosci. Nanotechnol. 17, 5587–5592, 2017
5589

Synthesis of New Heteroleptic Iridium(III) Complex
Kim et al.
Table I. Summary of performances of red-phosphorescent OLEDs.
a
2
b
b
b
Dopant
x%
ꢆmax (nm)
CIE ꢇxꢀyꢂ
Vt (V)
Lmax (cd/m )
EQE (%)
LE (cd/A)
PE (lm/W)
(
PQ) Ir(TMSppy)
3
5
600
603
611
ꢇ0ꢈ59ꢀ0ꢈ41ꢂ
ꢇ0ꢈ60ꢀ0ꢈ40ꢂ
ꢇ0ꢈ61ꢀ0ꢈ39ꢂ
3.0
3.0
3.0
12,204
21,911
29,092
15.1/14.2
14.5/14.2
15.5/15.1
27.5/26.0
24.8/24.2
24.1/23.5
28.8/21.0
23.7/18.9
25.1/18.4
2
10
(
PQ) Ir(acac)
5
597
ꢇ0ꢈ59ꢀ0ꢈ39ꢂ
3.5
10,450
14.9/13.9
22.3/21.1
15.0/9.7
2
Notes: aValues measured at a luminance of 1,000 cd/m2. bValues measured at maximum efficiency and luminance of 1,000 cd/m2.
localized at the phenyl group in PQ, the d-orbital of the
iridium atom, and the phenyl unit in TMSppy, whereas the
lowest unoccupied molecular orbital (LUMO) was largely
located on the quinolone unit in PQ. In general, typi-
cal ancillary ligands, such as acac, do not contribute to
both the HOMO and LUMO in iridium(III) complexes.
potential, was −5.08 eV. The HOMO energy level of
(PQ) Ir(TMSppy) was higher than in the well-known
2
(PQ) Ir(acac) complex (HOMO = −5ꢈ22 eV). The LUMO
2
energy level was calculated from differences between the
HOMO energy level and the optical bandgap energy. The
LUMO energy level of (PQ) Ir(TMSppy) was −2.93 eV.
2
Interestingly, TMSppy in (PQ) Ir(TMSppy) contributed to
These HOMO and LUMO energy levels are consistent
with DFT calculated energy levels.
2
the HOMO of the iridium(III) complex. The HOMO and
LUMO energy levels of (PQ) Ir(TMSppy) were −4.94
2
and −1.91 eV, respectively. The calculated HOMO and
LUMO energy levels of the iridium(III) complex with
TMSppy were slightly higher than those of the iridium(III)
complex containing acac (HOMO = −5ꢈ02 eV, LUMO =
3
.4. Electrophosphorescent OLEDs
To investigate (PQ) Ir(TMSppy) as a dopant in red-
2
phosphorescent OLEDs, we fabricated a device with
a multi-layer architecture containing ITO (50 nm)/
PEDOT:PSS (40 nm)/NPB (20 nm)/TCTA (10 nm)/
−
1ꢈ93 eV).
TCTA:TPBi:(PQ) Ir(TMSppy) (25 nm, x%)/TPBi
2
3
.3. Electrochemical Properties
(
35 nm)/LiF (1 nm)/Al (200 nm). The TCTA:TPBi (1:1)
The electrochemical behavior of (PQ) Ir(TMSppy)
2
was determined by cyclic voltammetry (CV). The
HOMO energy level, calculated from onset oxidation
IP: 93.179.90.191 On: Mon, 23 Apr 2018 21:08:37
(
a)
Copyright: American Scientific Publishers
Delivered by Ingent1a.0
W
(
a)
3
5
1
%
%
0%
3
2
1
00
00
00
0
10000
1000
100
0
0
.5
.0
4
00
500
600
700
800
10
Wavelength (nm)
0
2
4
6
8
Voltage (V)
2
2
1
1
5
0
5
0
5
0
25
(
b)
(
b) 30
30
20
15
10
5
2
0
0
0
20
10
0
1
3
5
1
%
%
0%
EQE
PE
0
1
0
100
1000
10000
10
100
1000
10000
Luminance (cd/m2)
Luminance (cd/m2)
Figure 3. (a) I–V –L and (b) EQE-L-CE curves of red OLED.
Figure 4. (a) EL spectra and (b) EQE-L-PE curves of WOLED.
J. Nanosci. Nanotechnol. 17, 5587–5592, 2017
5590

Kim et al.
Synthesis of New Heteroleptic Iridium(III) Complex
Table II. Summary of performances of WOLED.
2
a
a
a
b
b
WOLED
Vt (V)
.0
Lmax (cd/m )
EQE (%)
LE (cd/A)
PE (lm/W)
CIE ꢇxꢀyꢂ
CCT (K)
4
27,232
18.1/17.4
36.5/35.3
22.8/18.5
ꢇ0ꢈ39ꢀ0ꢈ42ꢂ
3,996
Notes: aValues measured at maximum efficiency and luminance of 1,000 cd/m2. bValues measured at a luminance of 1,000 cd/m2.
mixture was used as a mixed-host system for efficient
hole and electron injection. The device performances of
red-phosphorescent OLEDs were optimized by chang-
ing the doping concentration (3%, 5%, and 10%) of
maximum power efficiency (PEmaxꢂ of (PQ) Ir(TMSppy)
was 28.8 lm/W at a 3% doping concentration. The low
2
driving voltage of (PQ) Ir(TMSppy) at a 3% doping
2
concentration improved the PE value of the OLEDs.
(
PQ) Ir(TMSppy). The various doping concentrations and
We fabricated
a
primary three-color compo-
2
detailed characteristics of (PQ) Ir(TMSppy) are summa-
rized in Table I.
nent WOLED that incorporated bis[2-(4,6-difluoro-
phenyl)pyridinato-C ,N ](picolinato)iridium(III) [FIrpic]
2
2
The maximum emission peaks for a 3, 5, and 10%
as the blue emitter, Ir(ppy) as the green emitter, and
3
doping concentration for (PQ) Ir(TMSppy) appeared at
(PQ) Ir(TMSppy) as the red emitter with different doping
2
2
6
00, 603, and 611 nm, respectively. The maximum emis-
concentrations. EL spectra of the W device containing
sion peaks of (PQ) Ir(TMSppy) were slightly red-shifted
(PQ) Ir(TMSppy) are shown in Figure 4(a). The white EL
2
2
as the doping concentration increased from 3% to 10%,
because of the increased aggregation of dopant in the
devices at higher doping concentrations. The red-shift in
maximum emission peaks at high doping concentrations
spectra showed blue, green, and red peaks at 470, 502, and
602 nm, respectively; these matched well with the peak
wavelengths of FIrpic, Ir(ppy) , and (PQ) Ir(TMSppy),
3 2
respectively. The CIE color coordinate and correlated
color temperature (CCT) value for the W device were
ꢇ0ꢈ39ꢀ0ꢈ42ꢂ and 3996 K, respectively, at a luminance of
of (PQ) Ir(TMSppy) were consistent with the aggregation
2
effects on (PQ) Ir(TMSppy) neat solid films. The corre-
2
2
sponding Commission Internationale de L’Eclairage (CIE)
1,000 cd/m .
coordinates of (PQ) Ir(TMSppy) were ꢇ0ꢈ59ꢀ0ꢈ41ꢂ for 3%,
The EQE-L-PE curves for the W device containing
2
(
PQ) Ir(TMSppy) are shown in Figure 4(b), with the
ꢇ0ꢈ60ꢀ0ꢈ40ꢂ for 5%, and ꢇ0ꢈ61ꢀ0ꢈ39ꢂ for 10% at a lumi-
2
2
detailed characteristics summarized in Table II. The V and
nance of 1,000 cd/m .
t
IP: 93.179.90.191 On: Mon, 23 Apr 2018 21:08:37
2
L
values of the W device were 4.0 V and 27,232 cd/m ,
The current density–voltage–luminance (I–V –L) curves
Copyright: American Scientific Publishers
max
respectively. The EQE
and EQE for the W device at a
max
of (PQ) Ir(TMSppy) are shown in Figure 3(a). Th eD ce ul ir vr ee nr et d by Ingenta
2
2
luminance of 1,000 cd/m were 18.1% and 17.4%, respec-
tively, representing a low efficiency roll-off with increasing
luminance. Additionally, maximum current efficiency
density of the OLEDs increased with increasing concen-
tration of the (PQ) Ir(TMSppy) dopant, due to improved
2
2
8
charge hopping at higher doping concentrations. The
(
CEmaxꢂ and PEmax values for the W device were 36.5 cd/A
turn-on voltage (V , defined as the bias at a luminance of
t
2
and 22.8 lm/W, respectively. An EQEmax value of 18.1%
was obtained for the device with the structure TAPC/mCP/
1
(
cd/m ꢂ of all devices was 3.0 V, and the driving voltage
V , defined as the bias at a luminance of 1,000 cd/m ꢂ of
2
d
mCP:FIrpic:Ir(ppy) /TCTA:TPBi:(PQ) Ir(TMSppy)/TSPO1/TPBi.
the devices was 3.8 V at 3%, 3.9 V at 5%, and 4.1 V at
0% doping, respectively. In addition, the maximum lumi-
3
2
In particular, the exciton blocking layer (TSPO1) in the W
device structure improved both the electron injection and
charge balance of the emitting layer to give a high external
1
2
nance (Lmaxꢂ of (PQ) Ir(TMSppy) was 12,204 cd/m at
3
2
2
2
%, 21,911 cd/m at 5%, and 29,092 cd/m at 10% doping,
2
6
quantum efficiency WOLED. Also, we measured EL
spectra and CIE color coordinates for the fabricated W
respectively.
Figure 3(b) shows the external quantum efficiency-
luminance-power efficiency (EQE-L-PE) curves of
(
(
PQ) Ir(TMSppy). The external quantum efficiency of
2
1.0
PQ) Ir(TMSppy) was optimized at the 10% dopant
2
concentration and the maximum external quantum effi-
ciency (EQEmaxꢂ was 15.5%, slightly higher than that of
(
PQ) Ir(acac) (EQE = 14.9% at 5% doping concentra-
2
max
2
0.5
0.0
Voltage
tion). Also, the EQE value at a luminance of 1,000 cd/m
for (PQ) Ir(TMSppy) at a 10% doping concentration
was 15.1%. The EQE value of (PQ) Ir(TMSppy) had
a low efficiency roll-off with increasing luminance.
These results could be due to the sterically hindered
trimethylsilyl group in TMSppy preventing concentration
self-quenching at high doping concentrations and reducing
6
V (0.39,0.42)
2
7 V (0.39,0.41)
8 V (0.38,0.41)
2
9
1
V (0.37,0.41)
0 V (0.36,0.41)
500
600
700
800
Wavelength (nm)
2
9
triplet–triplet annihilation in the OLEDs. The optimized
Figure 5. (a) EL spectra of W device at different driving voltages.
J. Nanosci. Nanotechnol. 17, 5587–5592, 2017
5591

Synthesis of New Heteroleptic Iridium(III) Complex
Kim et al.
device at different driving voltages, from 6 to 10 V, to
test the color stability; the results are shown in Figure 5.
Despite the intensity of the red emission region in the
spectrum showing a slight decrease relative to the blue
and green regions, the CIE color coordinates of the device
were not significantly changed.
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2
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2
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nal quantum efficiency of the device fabricated using
2
(
PQ) Ir(TMSppy) were 29,092 cd/m and 15.5%, respec-
2
tively. The improved luminance and quantum efficiency for
PQ) Ir(TMSppy) were attributed to TMSppy in the irid-
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5, 1455 (2003).
1
(
2
1
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2
Also, the EQEmax and EQE at a luminance of 1,000 cd/m
1
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in the WOLED with (PQ) Ir(TMSppy) were 18.1 and
2
1
7.4%, respectively. The EQE curves of the WOLED with
1
(
PQ) Ir(TMSppy) showed a low efficiency roll-off with
2
increasing luminance. Therefore, TMSppy is a good can-
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IP: 93.179.90.191 On: Mon, 23 Apr 2018 21:08:37
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Acknowledgment: This research was supported by
the Ministry of Trade, Industry and Energy (MOTIE,
Korea) under Industrial Technology Innovation Program.
No. 10067715 and a National Research Foundation
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Received: 27 May 2016. Accepted: 27 August 2016.
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