Pure red phosphorescent organic light-emitting diodes made of iridium(iii) complex with thiophene-quinoline ligand

Article abstract of DOI:10.1166/jnn.2013.8152

A cyclometalated iridium(III) complex, bis(2-thiophen-2-yl-quinolinato)(acetoacetonate)iridium(III) [(tq)2Ir(III)(acac)] was synthesized for use in phosphorescent organic light-emitting diodes. The photophysical and electrochemical properties of the iridium(III) complex were characterized by UVvisible absorption, photoluminescence, and cyclic voltammetry. The maximum UV-visible absorption of (tq)2Ir(acac) was observed at 289 nm. (Tq)2Ir(acac) in dichloromethane showed its maximum photoluminescence (PL) emission at 629 nm. The optical band gap energy of (tq)2Ir(III)(acac) was measured to be 2.11 eV, and the HOMO energy level of (tq)2Ir(III)(acac) was calculated to be ?508 eV. The T1 state of (tq)2Ir(III)(acac), calculated from the PL emission maximum (2.01 eV), was well matched with the T1 level of CBP (2.6 eV). The phosphorescent organic light-emitting diode with a configuration of ITO/PEDOT:PSS/-NPD/TCTA/CBP:(tq)2Ir(III)(acac)(8 wt%)/BCP/Alq3/LiF/Al was fabricated and characterized. Light emission from the device was observed at a low turn-on voltage of 4.3 V. The device showed a maximum brightness of 24,000 cd/m2 at 16.3 V and an external quantum efficiency of 11.1% with a Commission Internationale de l'Eclairage (CIE) coordinate of (0.690, 0.310) Copyright

Full text of DOI:10.1166/jnn.2013.8152

Copyright © 2013 American Scientific Publishers
All rights reserved
Printed in the United States of America
Journal of
Nanoscience and Nanotechnology
Vol. 13, 8007–8010, 2013
Pure Red Phosphorescent Organic Light-Emitting
Diodes Made of Iridium(III) Complex with
Thiophene-Quinoline Ligand
Jong Min Lim1, Ji-Young Kwon2, Fei Xu1, Hee Un Kim1, and Do-Hoon Hwang1ꢀ∗
1Department of Chemistry, and Chemistry Institute for Functional Materials, Pusan National University,
Busan 609-735, Republic of Korea
2Department of Applied Chemistry, Kumoh National Institute of Technology, Kumi 730-701, Republic of Korea
A cyclometalated iridium(III) complex, bis(2-thiophen-2-yl-quinolinato)(acetoacetonate)iridium(III)
[(tq)2Ir(III)(acac)] was synthesized for use in phosphorescent organic light-emitting diodes. The
photophysical and electrochemical properties of the iridium(III) complex were characterized by UV-
visible absorption, photoluminescence, and cyclic voltammetry. The maximum UV-visible absorption
of (tq)2Ir(acac) was observed at 289 nm. (Tq)2Ir(acac) in dichloromethane showed its maximum
photoluminescence (PL) emission at 629 nm. The optical band gap energy of (tq)2Ir(III)(acac) was
measured to be 2.11 eV, and the HOMO energy level of (tq)2Ir(III)(acac) was calculated to be
−5ꢀ08 eV. The T1 state of (tq)2Ir(III)(acac), calculated from the PL emission maximum (2.01 eV),
was well matched with the T1 level of CBP (2.6 eV). The phosphorescent organic light-emitting diode
with a configuration of ITO/PEDOT:PSS/ꢁ-NPD/TCTA/CBP:(tq)2Ir(III)(acac)(8 wt%)/BCP/Alq3/LiF/Al
Delivered by Ingenta to: Chinese University of Hong Kong
was fabricated and characterized. Light emission from the device was observed at a low turn-on
IP: 195.34.79.136 On: Mon, 27 Jun 2016 17:53:37
voltage of 4.3 V. The device showed a maximum brightness of 24,000 cd/m2 at 16.3 V and an exter-
Copyright: American Scientific Publishers
nal quantum efficiency of 11.1% with a Commission Internationale de l’Eclairage (CIE) coordinate
of (0.690, 0.310).
Keywords: Red Phosphorescent Organic Light-Emitting Diode, Iridium Complex.
1. INTRODUCTION
bis(2-(2ꢀ-benzothienyl)pyridinato)(acetylacetonate) [(thp)2
Ir(III)(acac)] and iridium(III) bis(2-(2ꢀ-phenylpyridinato))
(acetylacetonate) [(ppy)2Ir(III)(acac)], respectively. On
comparison, the UV absorption spectra of the two com-
plexes were observed to be very similar; however, the
difference in photoluminescence emissions were ca. 50 nm
(562 nm for ((thp)2Ir(III)(acac) and 516 nm for (ppy)2Ir
(III)(acac)). This demonstrated that a cyclometalated irid-
ium(III) complex containing a thiophene moiety could
have a longer emission wavelength than a phenyl group
because the thiophene group has a better electron donating
ability. Consequently, quinoline could also give better red-
shifted emission than a pyridine ligand. Iridium(III)bis(2-
Recently many electrophosphorescent materials have been
actively investigated to overcome the quantum-efficiency
limit of fluorescence, because these materials can theo-
retically generate emission with 100% quantum efficiency
using both triplet and singlet excitons.1–4 By using triplet
emitters with heavy metal cores and various organic lig-
ands, strong spin-orbital coupling leads to enhanced mix-
ing of singlets and triplets, which in turn, induces efficient
intersystem crossing to overcome the internal quantum
yield.5–8 Many phosphorescent materials containing vari-
ous heavy metals such as Ru(II),9ꢀ10 Rh(III),11–14 Re(I),15
Ir(III),16 and Pt(II,IV)17–20 have been extensively developed
to demonstrate red, green, and blue electrophosphorescent
emission. In particular, cyclometalated iridium(III) com-
plexes show strong electrophosphorescent emission with a
high quantum efficiency.
(2ꢀ-phenylquinolyl))(acetylacetonate)
[(pq)2Ir(acac)]
showed a maximum PL emission at 597 nm, which is more
red-shifted than that for (ppy)2Ir(III)(acac) (516 nm).22
Huang et al. reported the synthesis of quinoline-
thiophene-based cyclometalated iridium(III) complex,
bis(2-thiophen-2-yl-quinolinato)(acetoacetonate)iridium(III)
[(tq)2Ir(III)(acac)],23 but detailed device results were not
provided in the report. In this study, we independently
Tsuboyama21 and Colombo21 investigated the opti-
cal properties of the iridium(III) complexes, iridium(III)
∗Author to whom correspondence should be addressed.
J. Nanosci. Nanotechnol. 2013, Vol. 13, No. 12
1533-4880/2013/13/8007/004
doi:10.1166/jnn.2013.8152
8007

Pure Red Phosphorescent Organic Light-Emitting Diodes Made of Iridium(III) Complex
Lim et al.
O
2.3. Synthesis of the Iridium Complex
S
KOH
OH
+
S
2.3.1. Synthesis of 2-Thiophen-2-yl-Quinoline (1)
NH2
N
1H-NMR (400 MHz, CDCl3, ꢁꢂ: 7.13 (t, 1 H), 7.45
(m, 2 H), 7.70 (m, 4 H), 8.03 (d, 1 H), 8.09 (d, 1 H);
13C-NMR (100 MHz, CDCl3, ꢁꢂ: 117.47, 125.78, 125.95,
127.02, 127.36, 127.98, 128.48, 129.05, 129.69, 136.48,
145.19, 147.90, 152.14.
1
1 + IrCl3
N
N
Cl
Cl
H2O
Ir
Ir
S
2-ethoxyethanol
2.3.2. Synthesis of Bis(2-Thiophen-2-yl-
Quinolinato)(Acetylacetonate)Iridium(III)
[(tq)2Ir(III)(acac)]
S
2
2
2
1H-NMR: (400 MHz, CDCl3, ꢁꢂ: 1.60 (s, 6H), 4.83
(s, 1 H), 6.25 (s, 2 H), 7.10 (d, 4 H), 7.40 (m, 4 H),
7.68 (m, 4 H), 7.98 (d, 2 H), 8.36 (d, 2 H); 13C-
NMR (100 MHz, CDCl3, ꢁꢂ: 29.98, 100.81, 156.93,
124.76, 125.77, 127.79, 128.79, 130.81, 134.34, 138.37,
139.38, 149.96, 153.48, 167.09, 185.73 Anal. Calcd for
C31H23IrN2O2S2 (%): C, 52.30; H, 3.26; N, 3.94; S, 9.01.
Found (%): C, 51.89; H, 3.16; N, 3.85; S, 8.77.
O
O
N
O
2 +
Ir
2-ethoxyethanol
O
S
2
(tq)2Ir(III)(acac)
Fig. 1. Synthetic route to (tq)2Ir(acac).
2.4. Fabrication of the EL Device
synthesized the [(tq)2Ir(III)(acac)] and evaluated its
luminescent properties as a red dopant for electrophos-
phorescent devices. The synthetic route and chemi-
cal structure of the [(tq)2Ir(III)(acac)] are shown in
A
phosphorescent OLED was fabricated to evalu-
ate the synthesized iridium complex. The device was
constructed with a configuration of ITO/PEDOT-PSS/
ꢃ-NPD/TCTA/CBP:iridium(III) complex (8 wt%)/BCP/
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Figure 1.
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Alq /LiF/Al. Indium-tin-oxide (ITO) was used as a trans-
Copyright: American Scientific Publishers
3
parent anode; PEDOT:PSS, as a hole injection layer;
2. EXPERIMENTAL DETAILS
ꢃ-NPD, as a hole transport layer; TCTA, as a hole trans-
port layer or electron blocking layer; CBP, as a host;
(tq)2Ir(III)(acac), as a triplet emitter; BCP, as a hole block-
ing layer; Alq3, as an electron transport layer; and LiF/Al,
as the compound cathode.
2.1. Materials
2-Aminobenzylalcohol and 2-acetylthiophene were pur-
chased from Aldrich. 2-Ethoxyethanol and potassium
hydroxide were purchased from JUNSEI. Iridium(III)
chloride was purchased from Alfa Aesar and 2,4-
pentanedione was purchased from ACROS. All chemicals
were used without further purification.
3. RESULTS AND DISCUSSION
3.1. Optical Properties
UV-visible absorption and PL emission spectra of
(tq)2Ir(III)(acac) were measured in dichloromethane. The
UV-visible spectrum of the Ir complex, including the
2-thiophene-2-quinolinato moiety, is presented in Figure 2.
2.2. Measurements
Elemental analysis was performed using an EA 1110
Fisons analyzer. UV-visible and PL emission spectra were
recorded using a Jasco V-530 and a Spex Fluorolog-3 spec-
trofluorometer. The film thickness was measured with a
TENCOR AlphaStep 500 Surface Profiler. OLED devices
were fabricated on glass substrates coated with indium-tin
oxide (ITO). EL spectra of the devices were obtained using
a Minolta CS-1000. Current–voltage–luminance (I–V –L)
characteristics were recorded simultaneously with the
measurement of the EL intensity by attaching the pho-
tospectrometer to a Keithley 238 and a Minolta LS-100
luminance detector. All measurements were carried out at
room temperature under ambient atmosphere.
1
The strongest absorption band, caused by the ꢄ–ꢄ∗ tran-
sition of the ligand attached to Ir(III), was observed at
289 nm. The medium band, shown at 352 nm, is from
the spin-allowed singlet metal-to-ligand charge transfer
(1MLCT). The weakest band of around 500 nm is caused
by the spin-forbidden triplet metal-to-ligand charge trans-
fer (3MLCT). The strong spin orbital coupling of Ir sup-
ports an absorbance in the triplet energy-transfer region
of around 500 nm. The PL spectrum of the iridium(III)
complex shows its emission maximum at 616 nm with full
width at half maximum (FWHM) (45 nm) as shown in
Figure 2.
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J. Nanosci. Nanotechnol. 13, 8007–8010, 2013

Lim et al.
Pure Red Phosphorescent Organic Light-Emitting Diodes Made of Iridium(III) Complex
PL
UV
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
300
400
500
600
Wavelength (nm)
Fig. 2. UV-visible absorption and PL spectra of (tq)2Ir(III)(acac) in
CH2Cl2.
3.2. Electrochemical Properties
The electrochemical analysis was performed using cyclic
voltammetry (CV) to determine the band gap energy, and
the HOMO and LUMO energy levels of (tq)2Ir(III)(acac).
According to the literature, the HOMO level can be esti-
mated from the onset oxidation potential using the follow-
ing equation:
Fig. 3. The fabricated EL device structure and the materials used for
each layer of the device.
EHOMO ꢅeVꢂ = −ꢅEoxꢀonset +E1/2Ferr
ꢂ
(1)
EHOMO represents the HOMO energy level of a specific
The electroluminescent (EL) spectra of the fabricated
device at different currents were measured to determine
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chemical species, Eoxꢀonset is the onset potential of that
Copyright: American Scientific Publishers
species, and E1/2ꢀFerr is the half oxidation potential of fer-
the spectral stability of the device. As shown in Figure 4,
no significant spectral change was observed in the current
range of 100 ꢆA to 1 mA. The fabricated phosphorescent
OLED showed very narrow FWHM (46 nm) as well as
very good color purity as a red emitter. The fabricated
device doped with 8 wt% of (tq)2Ir(III)(acac) showed the
Commission Internationale de l’Eclairage (CIE) coordinate
of (0.690, 0.310). This is close to that of standard red
(0.670, 0.330).
rocene. The energy levels of the LUMO and the HOMO-
LUMO energy gap can be determined from spectroscopic
and electrochemical data. The optical band gap of the
molecule could be determined from the onset shown in the
UV-PL spectra. CV data indicated that the onset oxida-
tion potential (Eoxꢀonsetꢂ of (tq)2Ir(III)(acac) was 0.35 eV.
From the oxidation onset and the half oxidation poten-
tial of ferrocene (4.73 eV), the HOMO energy level of
(tq)2Ir(III)(acac) was calculated to be −5.08 eV. The opti-
cal band gap energy of (tq)2Ir(III)(acac) was measured to
be 2.11 eV (586 nm). These frontier orbital levels closely
correspond to the energy levels for CBP (HOMO: −6.0 eV,
LUMO: −2.9 eV), so the electronic-structure requirements
for the OLED device could be satisfied. In addition, the
T1 state of (tq)2Ir(III)(acac), calculated from the PL emis-
sion maximum (2.01 eV), was well matched with the T1
level of CBP (2.6 eV),24 and this allows electrons to be
transferred from CBP to (tq)2Ir(III)(acac).
The current–voltage–luminance (I–V –L) characteristic
of the device was plotted in Figure 5 along with the current
2.0
(100 µA)
1.5
(500 µA)
(1 mA)
1.0
3.3. EL Properties of the Fabricated Device
0.5
To investigate the light-emitting properties of the
Ir(III) complex, phosphorescent OLEDs were fabricated
with a configuration of ITO/PEDOT:PSS/ꢃ-NPD/TCTA/
CBP:(tq)2Ir(III)(acac)(8 wt%)/BCP/Alq3/LiF/Al, as shown
in Figure 3.
0.0
500
600
700
800
Wavelength (nm)
Fig. 4. EL spectra of the fabricated devices at different currents.
J. Nanosci. Nanotechnol. 13, 8007–8010, 2013
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Pure Red Phosphorescent Organic Light-Emitting Diodes Made of Iridium(III) Complex
Lim et al.
1400
rendering index) funded by the Ministry of Knowledge
Economy (MKE, Korea), and a National Research Foun-
dation (NRF) grant funded by the Korean government
(MEST) (No. 2011-0011122).
Current Density
Luminance
25000
20000
15000
10000
5000
0
1200
1000
800
600
400
200
0
10
8
6
4
2
References and Notes
0
200
400
600
800
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Current Density (mA/cm2)
0
2
4
6
8
10 12 14 16 18 20
Voltage (V)
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Fig. 5. I–V –L characteristics of the device and current efficiency versus
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Table I. Electrophosphorescence data for the device doped with 8 wt%
Ir complex.
ꢇext
ꢇc
Vturn−on Luminance ꢈEL
(cd/m2ꢂb (nm)
Complex
(%) (cd/A) (V)a
CIE (x, y)
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(tq)2Ir(acac) 11.1 8.6 4.3
24,673 629 (0.690, 0.310)
Notes: aThis term was denoted as a bias at a brightness of 1 cd/m2. bThe maximum
luminance of the device at 16.3 V.
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cd/m2 at 16.3 V was achieved, and the peak current effi-
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tum efficiency of the device was 11.1% at 2.6 mA/cm2.
The performance of the fabricated phosphorescent OLED
is summarized in Table I.
Copyright: American Scientific Publishers
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4. CONCLUSIONS
In summary, we designed and synthesized a (tq)2Ir(III)
(acac) complex that emits pure red light. Efficient and
pure red organic phosphorescent devices using the iridium
complex were fabricated and characterized. The results
demonstrate that the iridium complex with the quinoline-
thiophene ligand could produce pure red phosphorescence
with a narrow FWHM.
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Acknowledgments: This work was supported by the
industrial strategic technology development program (No.
10039141, Development of core technologies for organic
materials applicable to OLED lighting with high color
Received: 1 May 2012. Accepted: 20 December 2012.
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J. Nanosci. Nanotechnol. 13, 8007–8010, 2013