Solution-processable iridium complexes for efficient orange-red and white organic light-emitting diodes

Article abstract of DOI:10.1039/c1jm13846d

Two homoleptic and heteroleptic cyclometalated iridium complexes containing the 2-phenylbenzothiozole derivative as the main ligand, 1 and 2, have been synthesized and characterized as efficient orange-red phosphors. Triarylamine was introduced as an impo

Full text of DOI:10.1039/c1jm13846d

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PAPER
Solution-processable iridium complexes for efficient orange-red and white
organic light-emitting diodes†
a
Renjie Wang, Di Liu, Run Zhang, Lijun Deng and Jiuyan Li
b
b
a
a
*
Received 9th August 2011, Accepted 18th October 2011
DOI: 10.1039/c1jm13846d
Two homoleptic and heteroleptic cyclometalated iridium complexes containing the 2-
phenylbenzothiozole derivative as the main ligand, 1 and 2, have been synthesized and characterized as
efficient orange-red phosphors. Triarylamine was introduced as an important substituent into the 6-
position of the benzothiazole ring to tune the photonic and electronic properties of these complexes.
Different from most of small molecular iridium complexes, 1 and 2 are solution-processable and their
neat films can be obtained by a spin-coating method. Furthermore, their homogeneously dispersed
films in a small molecular matrix, 4,40-N,N0-dicarbazolebiphenyl (CBP), were successfully prepared by
solution method even with low doping levels. Organic light-emitting diodes (OLEDs) were fabricated
by solution processing the emitting layer containing 1 and 2 as doped emitters in the CBP host. Efficient
orange-red electroluminescence by using 5 wt% 2 as the dopant was realized with a maximum efficiency
of 14.49 cd Aꢀ1 (7.38 lm Wꢀ1 and 8.73%) and Commission Internationale de l’Eclairage (CIE)
coordinates of (0.60, 0.40), which are among the highest luminance efficiency ever reported for partially
solution-processed red and orange-red OLEDs so far. In addition, two-element white OLEDs were
achieved with these orange-red phosphors and the traditional blue emitter by spin coating the emission
layer. A maximum luminance efficiency of 8.97 cd Aꢀ1 and CIE of (0.33, 0.35) were realized.
Commission Internationale de l’Eclairage (CIE) coordinates of
Introduction
(0.68,0.33) have been realized using 9-arylcarbazolyl-isoquino-
line iridium complexes as the doped emitter.3 Cheng’s group
employed iridium complexes based on substituted pyrazines to
fabricate orange-red devices, (0.60, 0.39), with a high luminance
efficiency of 26.2 cd Aꢀ1.4 Kim and Yoon’s group also reported
a series of benzoylphenylpyridine Ir(III) complexes. Using these
Recently, organic light-emitting diodes (OLEDs) based on
phosphorescence materials have attracted great attention
because electrophosphorescence is easily generated from both
singlet and triplet excited states and can realize a theoretical
internal quantum efficiency of 100%.1 Great effort has been spent
investigating the third-row transition-metal complexes to
develop highly efficient phosphors that can emit various colors.2
Green and blue phosphorescent devices have achieved consid-
erably high external quantum efficiencies and power efficiencies,
yet the highest efficiency ever reported for red OLEDs is still
much lower. Red emission is not only an important mono-
chromatic light, but also a key component for full-color display
applications or high color-rendering index illumination. There-
fore, the demand for red-emitting materials with high efficiency
and brightness has been remarkably enhanced.
complexes as dopant materials, peak efficiencies of 15.6 cd Aꢀ1
,
10.1 lm Wꢀ1 and 10.5% with CIE coordinates of (0.61, 0.39) were
observed.5 Nevertheless, these highly efficient phosphorescent
OLEDs are fabricated by vacuum thermal evaporation, which
makes a large amount of organic materials wasted and leads to
relatively high fabrication costs. In comparison with the vacuum
evaporation, the solution processing technique to fabricate
OLEDs seems to be the most favorable method for large-scale
production of flat-panel displays and solid-state lighting, owing
to their simple processing route and low manufacturing cost. To
be solution-processable by itself is one important merit for light-
emitting phosphors since this means that they can be processed
by wet methods not only together with the polymer host but also
with small molecular hosts. Dendritic iridium complexes are such
a group of phosphorescent materials suitable for solution pro-
cessing techniques. High-efficiency red-emitting iridium den-
drimers with high efficiencies of 9.2 cd Aꢀ1 (12.8%)6 and 5.8 cd
Aꢀ1 (11.6%)7 in their OLEDs have been reported. As an alter-
native, Jin et al. developed a group of small molecular red iridium
To achieve this, several attempts have been made. Recently,
highly efficient (10.1 cd Aꢀ1 and 12%) red OLEDs with
aState Key Laboratory of Fine Chemicals, School of Chemical Engineering,
Dalian University of Technology, 2 Linggong Road, Dalian, 116024,
China. E-mail: jiuyanli@dlut.edu.cn; Fax: +86 411 84986233
bSchool of Chemistry, Dalian University of Technology, 2 Linggong Road,
Dalian, 116024, China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c1jm13846d
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complexes containing carbazole-phenylquinoline as the main
ligands and picolinic acid N-oxide as the ancillary ligand.8 A
maximum luminance efficiency of 8.89 cd Aꢀ1 with CIE coordi-
nates of (0.61, 0.38) was achieved in solution-processed OLEDs
using these iridium complexes as the doped emitter in small
molecular hosts. As far as we know, that was the highest effi-
ciency ever reported so far for solution-processable orange-red
OLEDs based on small molecular emitters. To further improve
the device performance, the design of a novel ligand framework
capable of giving highly efficient red emissions in OLEDs is quite
challenging.
the emitting layer. These devices exhibited a maximum forward
viewing luminance efficiency of 14.5 cd Aꢀ1 with CIE coordinates
of (0.60, 0.40). Furthermore, two-emitting-component white
OLEDs were prepared with a spin-coated emitting layer which
contains these orange-red iridium complexes and the traditional
blue-emitting iridium(III) bis(4,6-(difluorophenyl)pyridinato-N,
0
C2 )picolinate (Firpic) in a host. A maximum luminance effi-
ciency of 8.97 cd Aꢀ1 and a CIE coordinate of (0.33, 0.35) were
obtained for white light emission.
Results and discussions
In our previous reports, we studied two novel orange-emitting
iridium complexes based on substituted 2-phenylbenzothiozole
ligands.9 It was found that both the orange and the two-element
(blue-orange) white OLEDs containing these phosphors as
doped emitter exhibited good electroluminescent performances.
These results motivated us to design new ligands for iridium
complexes based on the 2-phenylbenzothiozole framework that
can lead to red-shifted emission. On the other aspect, it was well
demonstrated that incorporating some groups such as carba-
zoles, triarylamines with hole-injection and hole-transporting
functions into the ligand frame is beneficial to raising the HOMO
levels, reducing the barrier height for hole injection, decreasing
the triplet–triplet annihilation and finally improving the EL
performance of the resulting complexes.10 Along this line of
research, we report here the synthesis and luminescent properties
of two new homoleptic and heteroleptic Ir(III) complexes, 1 and 2
(Scheme 1), based on 2-phenylbenzothiazole ligand containing
hole-transporting triarylamines moieties. These complexes are
suitable for solution methods to prepare both neat films and
doped films due to the presence of bulky arylamino groups in the
ligands. Solution-processed orange-red OLEDs were successfully
fabricated using these complexes doped in a traditional host as
Synthesis and characterization
The cyclometalating ligand 6-(1-naphthalenylphenylamino)-2-
phenylbenzothiazole was synthesized in good yields by using the
general procedure shown in Scheme 1. The commercially avail-
able 4-bromoaniline waꢁs first treated with benzoic acid in the
presence of PCl3 at 120 C to afford N-(40-bromophenyl)benza-
mide.11 Then a mixture of the benzamide and Lawesson’s reagent
in dry toluene can obtain N-(40-bromophenyl)thiobenzamide
easily. Jocobson reaction12 of thiobenzamide with potassium
ferricyanide in the presence of sodium hydroxide resulted in the
formation of the benzothiazole skeleton. Finally, the desired
ligand was prepared by following a standard palladium-cata-
lyzed aromatic C–N coupling reaction13 of 6-bromo-2-phenyl-
III
benzothiazoles with 1-naphthylphenylamine. The homoleptic Ir
complex 1 was obtained by reaction of [Ir(acac)3] with the cor-
III
responding ligand in refluxing glycerol.14 The heteroleptic Ir
complex 2 was prepared by two steps that involved the prelimi-
nary synthesis of the corresponding dichloro-bridged dimer
complex and subsequent reaction of the intermediate with ace-
tylacetone in presence of Na2CO3.15 The synthetic pathways
Scheme 1 The synthetic routes of the new iridium(III) complexes 1 and 2.
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towards the preparation of complexes 1 and 2 are outlined in
Scheme 1. Both of them are found to be soluble in common
chlorinated hydrocarbon CH2Cl2 and stable in air in the form of
orange-red solid powder. These products are purified by silica-gel
column chromatography and were fully characterized by 1H and
13C NMR, MALDI-TOF or APCI-ES mass spectrometry, and
elemental analysis.
measured as 0.12 and 0.09, respectively. The room-temperature
emission lifetimes of complexes 1 and 2 in dichloromethane
solutions were determined as 2.2 ms and 1.4 ms, respectively,
which are consistent with the emission from the triplet excited
states.16b
Electrochemical analysis
The electrochemical properties of these complexes were investi-
gated by means of cyclic voltammetry (CV) measurements. The
cyclic voltammograms are provided in the Supporting Infor-
mation (Fig. S1†). During the anodic scan in deoxygenated
dichloromethane solution, both complexes underwent
a multiple-oxidation process. The onset potentials of the first
oxidation wave (Eooxnset) were at 0.68 and 0.76 V vs. the saturated
calomel electrode (SCE) for 1 and 2, respectively. On the basis of
Photophysical properties
The UV-visible and photoluminescence (PL) spectra of
complexes 1 and 2 in dilute dichloromethane solutions are dis-
played in Fig. 1. Both complexes show similar absorption
spectra, and they can be divided into two regions. The strong
absorption bands below 300 nm are assigned to the spin-allowed
1
ligand-centered p–p* transitions while the weak absorptions
the oxidation potentials (Eooxnset), the HOMO energy levels can be
starting from 330 to 550 nm belong to the spin-forbidden, ligand-
centered 3p–p* transition facilitated by spin–orbit coupling,
together with the metal-to-ligand charge-transfer transitions
(1MLCT and 3MLCT). These features are analogous to other Ir
(III) complexes as reported previously.16
onset
ox
estimated from the empirical formulate: EHOMO ¼ ꢀe(E
+
4.4)6 as ꢀ5.08 eV and ꢀ5.16 eV for 1 and 2, respectively. The
optical bandgaps (Eg) of these complexes was determined by the
absorption edge technique.20 The lowest unoccupied molecular
orbitals (LUMOs) were then obtained, by subtracting the
bandgaps from the energy of the HOMOs, as ꢀ2.96 eV and
ꢀ3.11 eV for 1 and 2, respectively. In comparison with the parent
complex (bt)2Ir(acac) (HOMO ¼ ꢀ5.27 eV), these two novel
complexes possess higher lying HOMO levels. This indicates that
the introduction of the electron-rich triarylamine moieties in
ligand can effectively raise the HOMO levels of the complexes 1
and 2. As a result it will be much easier for these complexes to get
excitation through a direct hole trapping mechanism in the
doped emitting layer. The theoretical quantum chemical calcu-
lation confirms this experimental result. It is well known that the
HOMO of the parent (bt)2Ir(acac) is located mainly on the
iridium center and the phenyl ring on the ligands, without
significant contribution from the benzothiazole moieties (figure
not shown). However, with introduction of the triarylamine
substituent on the benzothiazole ring, the HOMOs of these two
complexes (see Fig. S2 in the supporting information†) cover
a wide range including the iridium center, the whole framework
of 2-phenylbenzothiozole ligands, and the nitrogen atoms of the
triarylamine groups on the main ligands. Apparently the
contribution of these triarylamine groups to the HOMOs of these
complexes is attributed to their strong electron donating nature,
which finally results in the raising of the HOMO levels of these
complexes.
Upon photoexcitation at 400 nm, both complexes show
intense room-temperature phosphorescence with the two major
emission peaks in the PL spectra. As shown in Fig. 1, the
homoleptic complex 1 has a very similar emission pattern to the
heteroleptic complex 2 with the same N–C ligand. For 1 the
peaks are at 570 and 620 nm, while for 2 the emission peak is at
586 nm with two shoulders at 634 nm and 710 nm. The marked
red-shift in the PL spectra of heteroleptic complexes in
comparison to their homoleptic analogues were frequently
observed for iridium complexes and this is because of the
comparatively higher ligand field strength of the acetylacetonate
anion.17 Compared with the unsubstituted complex [(bt)2Ir
(acac)],18 the emission spectrum of 2 is significantly red-shifted by
about 30 nm. This indicates that incorporating 1-naphthylamine
segments onto the benzothiazole ring of the 2-phenyl-
benzothiazole backbone somehow causes significant bath-
ochromic shift of both the ligand-centered 3p–p* transition band
and MLCT transition band. Using Ir(ppy)3 as standard reference
(FP ¼ 0.40, Hppy ¼ 2-phenylpyridine),19 the phosphorescent
quantum yields of 1 and 2 in degassed CH2Cl2 solutions were
Electroluminescent properties
In order to evaluate the electroluminescent properties of these
iridium complexes, 1 and 2 were used as doped emitters to
fabricate OLEDs in which the emitting layer was prepared by
spin coating method. The carbazole derivative 4,40-N,N0-dicar-
bazolebiphenyl (CBP) has been widely used as a host material for
red and green phosphorescent OLEDs because of its suitable
triplet energy and good hole-transporting ability. However, CBP
itself cannot be spin-coated from solution to give a good-quality
thin film because of the limited molecular weight and solution
viscosity. Therefore, most electrophosphorescent OLEDs were
prepared by vacuum co-evaporation of the small molecular
Fig. 1 Electronic absorption and PL spectra of complexes 1 and 2 in
CH2Cl2 solutions at room temperature. For complex 1: log 3 ¼ 5.34 (262
nm), 4.54 (358 nm) and 4.00 (459 nm); for complex 2: log 3 ¼ 5.06 (267
nm), 4.49 (380 nm) and 4.09 (454 nm); 3 is the molar extinction
coefficient.
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iridium complexes and the CBP host. Fortunately, both
complexes 1 and 2 in present study can be well dissolved in many
common solvents such as p-xylene, chlorobenzene, and toluene
and can be spin coated into high-quality neat films from solu-
tions. Furthermore, these iridium complexes showed good
compatibility with the CBP host due to the presence of these
hole-transporting triarylamine groups. The homogeneously
distributed films of these complexes in the CBP host were
successfully prepared by spin coating their mixed solutions when
the doping concentration was over 3 wt%, as shown by their
atomic force microscope (AFM) topographs in Fig. S3 in the
supporting information.† It should be noted that these iridium
complexes only occupy the minority (#10 wt%) of the doped
film. Nevertheless, it is still possible to prepare the doped films by
the spin-coating technique. Therefore, the OLEDs were fabri-
cated by spin coating the blended iridium complexes in the CBP
host as the emitting layer. Under identical conditions, the parent
iridium complex (bt)2Ir(acac) was examined for comparison. It
was observed that neither the neat film nor the doped film of
(bt)2Ir(acac) in CBP was possible to be obtained by spin coating.
This indicates that the hole-transporting arylamine substituents
in complexes 1 and 2 indeed played an essential role to make
these complexes suitable for solution-processing techniques to
make OLEDs.
coordinates are at (0.60, 0.40), which correspond to the orange-
red region of the CIE chromaticity diagram. It is apparent that
the EL spectra of each complex matched well with their PL in
terms of spectral profile and position, confirming that the EL was
III
indeed generated from the triplet excited states of these Ir
complexes. A slight red-shift by around 5 nm was observed in the
EL spectra of both complexes in comparison with their PL. This
phenomena is frequently observed which is probably caused by
the effect of the electrical field on the excited states in OLEDs.21
Similar to the case in PL, the EL spectrum of 2 was also red-
shifted by ꢂ15 nm compared to that of 1. The absence of any
residue emission from CBP or TPBI even at high current densi-
ties implies a direct charge trapping mechanism by the iridium
complexes along with the effective energy transfer from CBP host
III
to the Ir complex. This also benefits from the effective hole-
blocking function of TPBI layer in addition to its electron-
transporting role. Furthermore, the EL spectra are almost
invariant of the applied voltages and also do not show any
concentration dependence.
Highly efficient solution-processable OLED devices have been
demonstrated with these new complexes and the best perfor-
mances are achieved at the doping level of 5 wt% for both 1 and
2. The decrease in luminance efficiency at higher doping
concentrations (7 and 10 wt%) probably results from the
increased self-quenching of the dopants and the possible triplet–
triplet annihilation effect. For complex 1, a maximum luminance
(Lmax) of 6798 cd mꢀ2 at 12 V, a maximum luminance efficiency
hL of 13.65 cd Aꢀ1 at 7 V and a maximum power efficiency hp of
6.56 lm Wꢀ1 at 6 V are achieved at a doping concentration of 5 wt
% in device A. The forward viewing external quantum efficiency
(hext) was calculated using the luminance efficiency, the EL
spectra, and the human photopic sensitivity. A peak external
quantum efficiency (hext) of 6.49% was obtained for complex 1.
The current density–voltage–luminance (L–V–J) characteristics
of OLEDs A–C and their efficiency curves were provided in
Fig. S4 and Fig. S5† of the supporting information, respectively.
The performance of the 2-doped device D is even better than the
1-doped devices with Lmax of 6613 cd mꢀ2, peak hL of 14.49 cd
Aꢀ1, hp of 7.38 lm Wꢀ1 and hext of 8.73% at a concentration of
5 wt% despite the lower phosphorescent quantum yields of 2 than
1. Fig. 4 presents current density and luminance versus bias
voltage (L–V–J) characteristics of devices D–F. The external
quantum and luminance efficiencies as a function of current
density for device D is depicted in Fig. 5. We attribute the good
performance of 1 and 2 to their excellent film-forming ability and
enhanced hole-injection/hole-transport properties brought by
the naphthalenylphenylamino groups in the ligands. It should be
noted that orange-red and red-emitting devices fabricated using
solution-processed Ir-doped systems were relatively rare in the
literature. In comparison with the partially solution-processed
red and orange-red phosphorescent OLEDs reported in litera-
tures,6–8,22 our present orange-red emitting devices, especially the
complex 2-based device D, exhibited comparable or even higher
EL performances, especially in terms of the luminance efficiency.
The device performance data of device A–F are listed in Table 1.
To further assess the electroluminescent properties of these
complexes, white OLEDs (WOLEDs) were fabricated using
these orange-red phosphors in combination with the blue emitter
(Firpic) as emissive dopants. Both complexes 1 and 2 exhibited
These iridium complexes based OLEDs have the configuration
as shown in Fig. 2, in which poly(3,4-ethylenedioxythiophene):
poly(styrenesulfonate) (PEDOT:PSS) acts as hole-injecting
layer, 1,3,5-tris[N-(phenyl)benzimidazole]-benzene (TPBI) as
both a hole-blocker and an electron-transporter, and LiF/Al as
the electron-injection layer and cathode. For optimization
purposes, the doping level of each iridium phosphor was varied
at three different concentrations (1: 5% device A, 7% device B,
10% device C; 2: 5% device D, 7% device E, 10% device F).
All these devices displayed intense orange-red emission. As
shown in Fig. 3, devices A–C, with complex 1 as the dopant,
show a strong EL peak at about 575 nm and a shoulder peak at
620 nm. The Commission International de L’Eclairage (CIE)
color coordinates of the three devices are (0.56, 0.44) and the
turn-on voltage (Vturn-on) are about 5 V. Devices D–F, with
complex 2 as the doped emitter, turned on at 4.8–5.5 V with an
EL peak at 590 nm and a shoulder peak at 640 nm. The CIE color
Fig. 2 Device architecture and chemical structures of the materials used
in these OLEDs.
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Fig. 3 EL spectra of (a) 1- and (b) 2-doped OLEDs at different doping concentrations (under 8 V).
orange-red EL in their single-color OLEDs, which are more
advantageous over orange emitters to lead to pure white light
emission if combined with blue emitter.
The WOLEDs have the structure of ITO/PEDOT:PSS (40
nm)/iridium 1 or 2 (x wt%): Firpic (8 wt%): CBP (60 nm)/TPBI
(45 nm)/LiF (1 nm)/Al (1 based WOLED: W1; 2 based WOLED:
W2). By fixing the concentration of Firpic at 8 wt% and then
tuning the blending ratio between Firpic/1 or 2, we have
successfully obtained white emissions with CIE coordinates of
(0.33, 0.36) (at 10 V) for 0.6 wt% 1-doped device W1 and (0.33,
0.35) (at 10 V) for 0.5 wt% 2-doped device W2, which are quite
close to the pure white light (0.33, 0.33). The corresponding EL
spectra for devices W1 and W2 are shown in Fig. 6. Two-emit-
ting-component WOLEDs using orange phosphors in combi-
nation with Firpic as emissive dopants have been widely studied
in the literatures.23 However, the colour purity of the most
Fig. 4 The luminance-voltage-current density (L–V–J) characteristics
for complex 2-based OLEDs D–F at different doping levels.
Fig. 6 EL spectra of the white OLEDs W1 (0.6 wt% of 1) and W2 (0.5 wt
% 2) at 10 V.
Fig. 5 The plots of luminance efficiency and external quantum efficiency
versus current density for 2-doped device D.
III
Table 1 Performance of Ir complexes-doped electrophosphorescent OLEDs
Device
Phosphor dopant
Vturn-on/V
Lmax/cd mꢀ2, Va
hext/%, Va
hL/cd Aꢀ1, Va
hP/lm Wꢀ1, Va
lem/nm
CIE (x, y)
A
B
C
D
E
F
1 (5 wt%)
1 (7 wt%)
1 (10 wt%)
2 (5 wt%)
2 (7 wt%)
2 (10 wt%)
5.5
5.0
5.0
5.5
5
6798, 12
6001, 12
3433, 10
6613, 13
8005, 14
6367, 12
6.49, 7
5.54, 6
5.10, 6
8.73, 7
8.35, 8
6.78, 6
13.65, 7
11.66, 6
10.73, 6
14.49, 7
13.85, 8
11.25, 6
6.56, 6
6.10, 6
6.01, 5.5
7.38, 6
6.67, 6
5.89, 6
574, 620 sh
575, 620 sh
575, 620 sh
590, 640 sh
590, 640 sh
590, 640 sh
(0.56, 0.44)
(0.56, 0.44)
(0.56, 0.43)
(0.60, 0.40)
(0.60, 0.40)
(0.60, 0.40)
4.8
a
Maximum values of the devices. Values in parentheses are the voltages at which they were obtained. sh ¼ shoulder.
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and orange-red OLEDs so far. More importantly, white OLEDs
were achieved using these orange-red phosphors and the tradi-
tional blue emitter by spin coating the emission layer. A
maximum luminance efficiency of 8.97 cd Aꢀ1 (5.49 lm Wꢀ1
,
4.90%) and a CIE of (0.33, 0.35) were realized. The bulky and
hole-transporting arylamine groups in the ligands are suggested
to be responsible for the solution-processable merit of these
complexes and their good compatibility with the small molecular
CBP host and thus the good EL performance. With the high
efficiency and easy fabrication process, these complexes are
promising phosphorescent dopants in large-area solution-pro-
cessed OLEDs.
Fig. 7 The plots of luminance efficiency versus current density for
devices W1 and W2.
Experimental
Synthesis and instrumentation are described in the Supporting
Information.†
devices was not satisfying despite of the good efficiencies. This
indicates that not only the emissive wavelength of the indepen-
dent emitter but also their contribution ratio in the blend are
essential factors which need to be carefully balanced so that the
collective integrated emission can approach the coordinates of
the standard white light point (0.33, 0.33). The advantageous
CIE coordinates of the present WOLEDs over those two-element
ones in literatures should benefit from the specific contribution of
these orange-red phosphors.
OLED fabrication and measurements
The pre-cleaned ITO glass substrates (30 U ,ꢀ1) were treated by
UV-ozone for 20 min. For all devices, a 40 nm thick PEDOT:PSS
film was first deposited on the ITO glass substrates, and baked at
120 ꢁC for 30 min in air. For the single-color OLEDs, the
emitting layer was then spin coated onto the PEDOT:PSS layer
from the mixed solution of the iridium complex 1 (or 2) and the
CBP host in chlorobenzene (at various doping concentrations).
For the white OLEDs, the emitting layer was spin-coated onto
the PEDOT:PSS layer from a mixed solution containing 0.6 wt%
1 (or 0.5 wt% 2), 8 wt% Firpic and CBP host in chlorobenzene.
After treating the films in vacuum oven at 50 ꢁC for half an hour
to remove residual solvent, the substrates were transferred into
the vacuum chamber for thermal deposition of the TPBI layer
and LIF/Al cathode with a base pressure less than 10ꢀ6 torr. The
EL spectra, CIE coordinates, and current–voltage–luminance
The turn-on voltage (to deliver a brightness of 1 cd mꢀ2) of
devices W1 and W2 are both about 4.3 V, and the maximum
luminance for device W1 and W2 are 2868 and 2815 cd mꢀ2
,
respectively. The L–V–J curves for W1 and W2 are shown in
Fig. S6 of supporting information.† Luminance efficiencies of the
emitted light in the forward direction of the WOLEDs versus
current density are plotted in Fig. 7. The peak EL efficiencies are
hL ¼ 8.26 cd Aꢀ1, hext ¼ 4.08%, hp ¼ 4.16 lm Wꢀ1 for device W1
and hL ¼ 8.97 cd Aꢀ1, hext ¼ 4.90%, hp ¼ 5.49 lm Wꢀ1 for device
W2. Apparently the EL efficiencies of W2 are higher than those
of W1. This should be because the orange-red phosphor 2
exhibited higher efficiencies than 1 in their single-color OLEDs.
The gradual efficiency roll-off was observed in these devices with
increasing current density, which can be typically attributed to
a combination of triplet–triplet annihilation and field-induced
quenching effects in electrophosphorescent devices. Although
the present device performances were not yet optimized, it is still
encouraging that an ideal white emission have been realized from
two-element WOLEDs containing a solution-processed emission
layer using these phosphorescent dyes with a CBP host.
characteristics were measured with
a computer-controlled
Spectrascan PR 705 photometer and a source-measure-unit
Keithley 236 under ambient conditions. The forward viewing
external quantum efficiency (hext) was calculated by using the
luminance efficiency, EL spectra and human photopic sensitivity.
Acknowledgements
We thank the National Natural Science Foundation of China
(20923006, 21072026, and 20704002), the Ministry of Education
for the New Century Excellent Talents in University (Grant
NCET-08-0074), the NKBRSF (2009CB220009), and the
Fundamental Research Funds for the Central Universities
(DUT10LK16) for financial support of this work.
Conclusions
In conclusion, we have successfully synthesized two new orange-
red phosphorescent iridium complexes based on the 2-phenyl-
benzothiazole ligand containing hole-transporting 1-naph-
thalenylphenylamino moieties. These complexes are suitable for
solution processing to make neat films and doped films in a small
molecular host material. Orange-red OLEDs were fabricated by
spin coating the emitting layer with these complexes doped in the
CBP host. The EL efficiency of 14.49 cd Aꢀ1, 7.38 lm Wꢀ1 and
8.73% with CIE coordinates of (0.60, 0.40) were obtained by
using 5 wt% 1 as the doped emitter, which are among the best
performance ever reported for partially solution-processed red
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