CHIN. PHYS. LETT. Vol. 29, No. 12 (2012) 127801
A Novel Efficient Red Emitting Iridium Complex for Polymer Light Emitting
Diodes *
HU Zheng-Yong(胡峥勇)** , YANG Jian-Kui(杨建奎), LUO Jing(罗景), LIANG Min(梁敏), WANG Jing(王景)
Science College, Hunan Agricultural University, Changsha 410128
(Received 5 June 2012)
Photo-physical properties of iridium complexes bis(1-(2’,4’-difluorobiphenyl -4-yl)isoquinoline)iridium(III)(5-(4(bis(4-methoxyphenyl)amino)phenyl)picolinic acid) used as phosphorescent dopant in polymer light-emitting devices with a blend of poly(9,9-dioctylfluorene) and 2-tert-butyl-phenyl-5-biphenyl-1,3,4-oxadiazole as a host matrix
are investigated. The iridium complex exhibits distinct UV-vis absorption bands around 300–450 nm and intense
red photoluminescent emissions peaked at around 618 nm in dichloromethane. The devices display a maximum
external quantum efficiency of 4.8% and luminous efficiency of 3.1 cd·A−1 at current density of 3.2 mA·cm−2 with
a dominant red emission peak around 620 nm and a shoulder around 660 nm. At 100 mA·cm−2 , the devices still
display a maximum external quantum efficiency as high as 3.9%.
PACS: 78.60.Fi, 85.60.Jb
DOI: 10.1088/0256-307X/29/12/127801
Polymer light-emitting devices (PLEDs) have attracted a great deal of attention owing to their potential applications in large-area flat panel displays.
Considerable progress has been achieved in phosphorescence PLEDs employing cyclometalated iridium complexes as dopants and polymers as host
matrices.[1−13] The high external quantum efficiency
of 10.4% photons/electron (ph/el) and luminance efficiency of 36 cd/A for the iridium complex-doped
PLEDs with green emission were reported by Gong
et al.[1] Highly efficient red polymer light-emitting devices doped with a novel iridium(III) complex containing a phenyl quinazoline ligand have been achieved
with a maximum luminance of 7107 cd·m−2 and a
maximum external quantum efficiency of 18.44% by
Mei et al.[7] The highly efficient single-layer white
PLEDs based on polytriphenylamine featured iridium
dendrimers were achieved with the EQE of 18.5% by
Zhu et al.[8] It is obvious that the emission efficiencies from the devices are strongly dependent on the
choice of the dopants. Therefore, it is very important to develop new red-emitting iridium complexes
for phosphorescent PLEDs with high efficiency.
Up to now, most of high-efficiency heteroleptic
cyclometalated iridium complexes have been obtained
by modifying the structure of their cyclometalating ligands because both the efficiency and emission
wavelength are strongly dependent on their structure. However, few heteroleptic cyclometalated iridium complexes were reported by tuning the structure of their anionic ancillary ligands. In the past
few years, researchers have developed heteroleptic
cyclometalated iridium complexes by changing the
𝛽-diketonate.[14−16] In our previous work, we exhibited an enhanced red electrophosphorescence from
isoquinoline-functionalized iridium complexes.[17] As
incorporated arylamine units into the cyclometalated
ligand, these iridium complexes have exhibited improved optoelectronic properties. This is mainly attributed to the excellent hole-transportation property of arylamine units. Therefore, we expect that
the incorporation of arylamine units into the anionic ancillary ligands of picolinic acids can also improve the hole-transporting, optoelectronic properties of their resulting cyclometalated iridium complexes. In addition, we expect that incorporating non-plane diphenylamine units into the ancillary ligand can benefit suppression of the molecular aggregation of the iridium complex due to the
space effect of the triphenylamino unit, which finally results in decreased concentration and emission quenching. In this Letter, we report the design
and synthesis of a novel iridium complex bis(1-(2’,4’difluorobiphenyl- 4-yl)isoquinoline)iridium(III)(5-(4(bis(4-methoxy-phenyl)amino)phenyl)picolinic acid)
(FPiq)2 Ir (MeOTPAPic) containing an ancillary ligand of triarylamine functionalized picolinic acids. The
properties of the novel iridium complex and the iridium complex-doped PLEDs are initially investigated.
The chemical structure of the triarylamine functionalized iridium complex and the device configuration are shown in Fig. 1. This red-emitting phosphorescent iridium complex was synthesized and purified
according to the reported procedure.[18] All fabricated
PLEDs have a device configuration of ITO/PEDOT
(50 nm)/PVK (40 nm)/Ir-complex (𝑥%): PFO-PBD
(30%) (75 nm)/Ba (4.5 nm)/Al (100 nm), in which
an iridium complex was employed as an emitting
guest and the PFO-PBD blend was used as a host
matrix.
The doping concentrations of the irid-
* Supported
by the Science and Technology Program of Educatioal Committee of Hunan Province under Grant No 10C0818.
author. Email: huzy2000@yahoo.cn
© 2012 Chinese Physical Society and IOP Publishing Ltd
** Corresponding
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CHIN. PHYS. LETT. Vol. 29, No. 12 (2012) 127801
ium complex were 1%, 2%, 4% and 8%, respectively. The mass ratio of PBD was 30% in the
PFO-PBD blend. PFO with a molecular weight
of 100000, poly(3,4-ethylenedioxythiophene)/poly(4styrenesulfonate) (PEDOT/PSS), polyvinyl carbazole
(PVK) and PBD were purchased from American Dye
Source Inc.
directions in the integrating sphere. Current-voltage
(𝐼–𝑉 ) data were collected with a computerized Keithley 236 source measure unit. Luminance was measured with a Si photodiode and calibrated by using
a PR-705 Spectrascan spectrophotometer (Photo Research).
1.4
OCH3
H3CO
Normalized PL
intensity (arb. units)
N
Ba (4.5 nm)/Al(100 nm)
Ir-complexes:PFO-PBD (75 nm)
N
PVK (40 nm)
PEDOT (50 nm)
N
Ir C
2 O
O
F
ITO/Glassn Substrate
F
(FPiq)2Ir(MeOTPAPic)
Fig. 1. Molecular structure of the iridium complexes and
their doped device configuration.
1.2
1.0
0.8
0.6
0.4
8%
4%
0.2
2%
1%
0.0
1.2
PL of (Fpiq)2Ir(DMeOTPAPic)
PL of PFO+PBD
300
1.0
1.0
0.8
0.8
356
265
0.6
0.6
332
0.4
0.4
0.2
0.2
0.0
0.0
300
400
500
600
700
600
700
800
Fig. 3. PL spectra of the iridium complexes-doped PFOPBD films in different doping concentrations.
1.4
1%
2%
4%
8%
1.2
Normalized EL
intensity (arb. units)
1.2
500
Wavelength (nm)
Normalized PL intensity (arb. units)
Normalized UV intensity (arb. units)
400
UV of (Fpiq)2Ir(DMeOTPAPic)
Wavelength (nm)
Fig. 2. UV-vis absorption and PL spectra of iridium complexes in CH2 Cl2 , and PL spectra of the PFO–PBD film.
1.0
0.8
0.6
0.4
0.2
0.0
400
Thermo-gravimetric analysis (TGA) was carried
out with a NETZSCH STA 449 from 25 to 700∘C at
a heating rate of 20∘C × min−1 under nitrogen atmosphere. UV-vis absorption spectra were recorded on
a PerkinElmer Lambda 25 UV-vis absorption spectrophotometer. Electrochemical data were examined
using a CHI660A electrochemical work station. A conventional three-electrode configuration consisting of a
working electrode of glassy carbon, a reference electrode of Ag/Ag+ and a Pt-wire counter electrode was
used. The solvent was CH3 CN and the supporting
electrolyte was 0.1 M [Bu4 N]PF6 in electrochemical
measurements. Cyclic voltammetry was carried out
at a scan rate of 50 mV/s at room temperature under argon protection. The energy levels of the highest
occupied molecular orbital (HOMO) and the lowest
unoccupied molecular orbital (LUMO) for these iridium complexes were calculated by cyclic voltammetric
data. Photoluminescence (PL) spectra under excitation of 325 nm line of a He-Cd laser (OmniChrome
Co.) and EL spectra were recorded with an Insta-Spec
IV CCD system (Oriel). External quantum efficiency
was obtained by measuring the total light output in all
500
600
700
800
Wavelength (nm)
Fig. 4. EL spectra of the iridium complexes-doped PFO–
PBD devices in different dopings.
The iridium complex exhibited high thermal stability. The onset decomposition temperatures (Td) is
343∘C. The UV-visible absorption spectra and the PL
spectra of the iridium complex in dichloromethane and
the PL spectrum of the PFO-PBD blend are shown in
Fig. 2. The multiple absorption bands of the iridium
complex are displayed in the UV spectral profile. The
former intense absorption band around 300 nm is assigned to a typical spin-allowed 1𝜋–𝜋 * transition of
the (2’,4’-difluorobiphenyl-4-yl)isoquinoline) FPiq ligand. The next lower-lying absorption band in the region 356–475 nm attributed to the spin-allowed and
spin-forbade metal to ligand charge-transfer (1MLCT
and 3MLCT) transitions. Three absorption bands
from the MLCT transitions are observed, indicating
the presence of multiple close lying MLCT states in
this complex.[19] A reasonable overlap between the
emission profile of the PFO-PBD blend and the absorption bands of this iridium complex is observed in
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CHIN. PHYS. LETT. Vol. 29, No. 12 (2012) 127801
-4.7 eV
-5.5 eV
3.0
-2
PFO
PBD
-1.498
-5.8 eV
1.0
0.5
HOMO
-6.2 eV
2.8
2.6
-0.731
2.4
2.0
1.5
0.0
-0.5 -1.0 -1.5 -2.0
Voltage (V)
Fig. 5. The energy diagram (inset) and cyclic voltammogram of (FPiq)2 Ir(MeOTPAPic).
Figure
3
shows
the
PL
spectra
of
(FPiq)2 Ir(MeOTPAPic) doped into the PFO-PBD
blend at different doping ratios under excitation of
325 nm line from a He-Cd laser. The extent of
energy transfer from the PFO-PBD blend to the
(FPiq)2 Ir(MeOTPAPic) dopant is presented in the
PL spectral profiles at the dopant concentrations of
1–8%. The peaks around 430 nm and 450 nm, which
originate from the emission of the PFO, and the PL
peak of PFO+PBD at ∼510 nm are originated from
exciplex of PFO+PBD blend, another component
peak at 615 nm with a shoulder around 670 nm corresponds to the triplet emission of the iridium complex
(FPiq)2 Ir(MeOTPAPic). Complete quenching of the
PFO-PBD emission is not observed even at the dopant
concentration of 8%. This implies that energy transfer from the PFO-PBD blend to the Ir(tpaiq)2 (acac)
complex is inefficient under photo-excitation.
In contrast,
the EL emissions of the
(FPiq)2 Ir(MeOTPAPic)-doped PFO-PBD devices are
completely dominated by the dopant emission at the
dopant concentrations of 1–8%, as shown in Fig. 4.
PFO-PBD emission is not exhibited in the EL spectral profiles of the devices. This indicates that charge
300
250
200
150
100
5
4
3
2
1
00
1%
2%
4%
8%
1%
2%
4%
8%
50 100 150 200
2
Current density (mA/cm )
50
0
0
4000
3500
3000
2500
2000
1500
1000
500
0
-500
-1000
2
3.2
ITO
-1.052
Ba
Luminance (cd/m )
PVK
3.4
(FPiq)2Ir(MeOTPAPic)
3.6
-2.6 eV-2.7 eV
-5.13 eV
Normalized current (A)
-3.35 eV
LUMO
Quantum efficiency (%)
-2.1 eV
-2.0 eV
3.8
2
4.0
trapping and the short-range Dexter triplet energy
transfer may be due to the predominant mechanism
of EL in these devices.[20] The electrochemical property of the iridium complex can confirm this suggestion to some extent. The energy diagram (inset)
and cyclic voltammogram of (FPiq)2 Ir(MeOTPAPic)
are revealed in Fig. 5. As the HOMO energy level
is −5.13 eV and the LUMO energy level is −3.35 eV
for this iridium(III) complex, as well as the HOMO
and LUMO energy levels of PFO are −5.80 eV and
−2.10 eV, respectively, the HOMO energy level of the
iridium complex is 0.67 eV higher than that of PFO
host, and the LUMO energy level of the iridium complex is 1.25 eV lower than PFO host. According to
this diagram, the HOMO and LUMO energy level
of the iridium complex fall within those of the PFO,
implying that the iridium complex could function
as trap for both electrons and holes under electrical
excitation.[21] At the same time, we note that the
HOMO energy level of the iridium complex is 0.37 eV
higher than that of the hole-transporting layer PVK,
thus facilitating the transport of holes to the iridium
complex.
Current density (mA/cm )
Fig. 2, which enables an efficient Förster energy transfer from the singlet-excited state of the host to the
MLCT states of the iridium complex.
5
10
15
20
Voltage (V)
Fig. 6. Current density-voltage-brightness characteristics
and external quantum efficiency–current density characteristics of the iridium complexes doped PFO-PBD devices.
Table 1. Device performances of the iridium complex doped PLEDs at different dopant concentrations with 𝐵, 𝜂max , 𝜅max
representing the brightness, the maximal luminance efficiency and the maximal luminous efficiency.
Dopant ratio
(wt%)
1
2
3
4
𝑉turn−on
(V)
6.3
8.3
9.6
10.0
𝐵
(cd m−2 )
3210(263 mA/cm2 )
2615(119 mA/cm2 )
3222(249 mA/cm2 )
2754(110 mA/cm2 )
The current density–voltage–brightness characteristics of the (FPiq)2 Ir (MeOTPAPic)-doped PFOPBD devices at the dopant concentrations of 1–
8% are plotted in Fig. 6. The driving voltages increase as the dopant concentrations increase from
𝜂max
(%)
4.7(1.9 mA/cm2 )
4.1(21 mA/cm2 )
4.8(3.1 mA/cm2 )
4.2(62 mA/cm2 )
𝜅max
(cd A−1 )
3.0
2.7
3.1
2.7
𝜆max
(nm)
620
620
621
622
CIE
0.66,
0.67,
0.67,
0.67,
0.33
0.33
0.33
0.33
1% to 8%. This implies that the EL process of
the (FPiq)2 Ir(MeOTPAPic)-doped devices are dominated by short-range Dexter triplet energy transfer rather than charge trapping process.
This
is contrast to described previously.[22−24] Table 1
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CHIN. PHYS. LETT. Vol. 29, No. 12 (2012) 127801
summarizes the luminous performance of the devices. The best device performance was observed
for (FPiq)2 Ir(MeOTPAPic)-doped PFO±PBD (30%)
device with 4% doping concentration with a turnon voltage of 9.6 V and the maximum brightness of
3222 cd·m−2 at 21.6 V. The maximal external quantum efficiency 𝜂max = 4.8% ph/el and the maximal
luminous efficiency 𝜅max = 3.1 cd·A−1 were achieved
at a current density of 3.1 mA·cm−2 . The efficiency of
this device remained as high as 𝜂ext =3.3% ph/el and
𝜅 = 2.1 cd·A−1 at a luminance of 2366.8 cd·m−2 and
current density of 100 mA·cm−2 . These luminous performances are comparable with those reported in the
Ir(piq)(acac)-doped PLEDs.[25] The maximum external quantum efficiency (EQE)-current density characteristics of the (FPiq)2 Ir(MeOTPAPic)-doped PFOPBD devices at the dopant concentrations of 1–8% are
not changed obviously (inset in Fig. 6). This may imply that the non-plane triarylamine unit can suppress
the molecular aggregation of the iridium complex.
In summary, we have demonstrated efficient
red-emitting PLEDs using an isoquinoline-based
iridium(III) complex with a modified ancillary ligand
as a dopant and a PFO-PBD blend as a host matrix. The bright electroluminescence emission with
a maximum at 622 nm and CIE color coordinates of
(0.67, 0.33) are presented in the devices. The presented triarylamino functionalized iridium complex is
a promising organic phosphor for use in high-efficiency
red-emitting PLEDs.
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