Non-interlayer and color stable WOLEDs with mixed host and incorporating a new orange phosphorescent iridium complex

Article abstract of DOI:10.1016/j.orgel.2014.08.034

In this work, we have synthesized a new phosphorescent iridium complex (Bppya)₂Ir(acac), as an orange dopant for organic light-emitting diodes (OLEDs). The device achieved an external quantum efficiency (EQE) of 22.4%, a current efficiency of 49.5 cd A^-1and a power efficiency of 38.9 lm W^-1, which are among the highest values for orange OLEDs. Furthermore, color stable and high efficiency phosphorescent white OLED (WOLED) was demonstrated by utilizing a mixed-host in the emissive layers (EMLs) composed of a blue phosphor and the new orange phosphorescent emitter, as well as by avoiding the use of interlayers that commonly exist between different EMLs in WOLEDs. Our WOLED presented decent white emission with maximum efficiencies of 25.3 cd A^-1and 12.6%. Furthermore, the 1931 Commission Internationale de l'Eclairage color coordinates exhibited extremely small variation of (0.3940 ± 0.0102, 0.4323 ± 0.0046) in a wide luminance range from 49 to 38,035 cd m^-2when driving voltages increased from 4 V to 12 V. The root cause for this excellent color stability is the utilization of the mixed-host to obtain bipolar transport properties in EMLs as well as the eliminating of interlayer between different EMLs, which, on one hand, effectively broaden the exciton recombination zone; on the other hand, reduce the accumulation of triplet excitons at the emissive interface.

Full text of DOI:10.1016/j.orgel.2014.08.034

Organic Electronics 15 (2014) 2964–2970
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Non-interlayer and color stable WOLEDs with mixed host and
incorporating a new orange phosphorescent iridium complex
⇑
Li-Yuan Guo, Xun-Lu Zhang, Min-Jie Zhuo, Chen Liu, Wang-Yang Chen, Bao-Xiu Mi ,
Juan Song, Yong-Hua Li, Zhi-Qiang Gao
⇑
Key Laboratory for Organic Electronics & Information Displays (KLOEID), Institute of Advanced Materials (IAM), Nanjing University of Posts &
Telecommunications, Nanjing 210023, China
Jiangsu Engineering Centre for Plate Displays & Solid State Lighting, Institute of Advanced Materials (IAM), Nanjing University of Posts & Telecommunications,
Nanjing 210023, China
School of Material Science and Engineering, Nanjing University of Posts & Telecommunications, Nanjing 210023, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 16 July 2014
Received in revised form 16 August 2014
Accepted 22 August 2014
Available online 2 September 2014
In this work, we have synthesized a new phosphorescent iridium complex (Bppya)2Ir(acac),
as an orange dopant for organic light-emitting diodes (OLEDs). The device achieved an
external quantum efficiency (EQE) of 22.4%, a current efficiency of 49.5 cd Aꢀ1 and a power
efficiency of 38.9 lm Wꢀ1, which are among the highest values for orange OLEDs. Further-
more, color stable and high efficiency phosphorescent white OLED (WOLED) was demon-
strated by utilizing a mixed-host in the emissive layers (EMLs) composed of a blue
phosphor and the new orange phosphorescent emitter, as well as by avoiding the use of
interlayers that commonly exist between different EMLs in WOLEDs. Our WOLED
presented decent white emission with maximum efficiencies of 25.3 cd Aꢀ1 and 12.6%.
Furthermore, the 1931 Commission Internationale de l’Eclairage color coordinates exhib-
ited extremely small variation of (0.3940 0.0102, 0.4323 0.0046) in a wide luminance
range from 49 to 38,035 cd mꢀ2 when driving voltages increased from 4 V to 12 V. The root
cause for this excellent color stability is the utilization of the mixed-host to obtain bipolar
transport properties in EMLs as well as the eliminating of interlayer between different
EMLs, which, on one hand, effectively broaden the exciton recombination zone; on the
other hand, reduce the accumulation of triplet excitons at the emissive interface.
Ó 2014 Elsevier B.V. All rights reserved.
Keywords:
WOLED
Color stability
Mixed host
Orange
Iridium
1. Introduction
molecular white-emitter, WOLEDs are generally obtained
by incorporating either three primary color emitters (red,
White organic light-emitting devices (WOLEDs) have
drawn intensive attention due to their potential applica-
tion as a solid-state flat light source and a back light unit
for liquid crystal displays (LCDs). Except utilizing single
green, and blue) or two complementary emitters (e.g.,
sky blue and yellow) in a single- or multi-layer(s) struc-
ture. Although it is theoretically simple for three-color
based WOLEDs to achieve the standard white light with
the Commission Internationale de l’Eclairage (CIE) coordi-
nates of (0.33, 0.33) because of the simultaneous presence
of three primary colors, this method usually results in pro-
cessing complexity and high operational voltages [1–4].
Accordingly, the strategy of two emitters with comple-
mentary colors may solve the problem and has ascended
⇑
Corresponding authors at: Key Laboratory for Organic Electronics &
Information Displays (KLOEID), Institute of Advanced Materials (IAM),
Nanjing University of Posts
China.
& Telecommunications, Nanjing 210023,
E-mail addresses: iambxmi@njupt.edu.cn, iamzqgao@njupt.edu.cn
(B.-X. Mi).
http://dx.doi.org/10.1016/j.orgel.2014.08.034
1566-1199/Ó 2014 Elsevier B.V. All rights reserved.

L.-Y. Guo et al. / Organic Electronics 15 (2014) 2964–2970
2965
to be one important method for the fabrication of WOLEDs
[5–7].
recombination zone; on the other hand, reduce the accumu-
lation of triplet excitons at the emissive interface.
To achieve high efficiency WOLED devices, the incorpo-
ration of phosphorescent emitter(s) is almost a prerequi-
site, due to its (their) ability of harvesting the electrically
generated 25% singlet and 75% triplet excitons. However,
efficiency roll-off needs to be sincerely considered mainly
because triplet exciton has long life time, causing severe
exciton quenching at high exciton density with the
increase of driving voltages. In this aspect, cohost for phos-
phor was proved to be an effective way not only for reduc-
ing roll-off, but also for high device-efficiency, mainly
because of the expansion of recombination zone [8–10].
Besides efficiency, color stability is another big chal-
lenge in WOLEDs. The main causes leading to color shift
in WOLED devices with multiple emitting layers (EMLs)
include alteration of recombination zone [11,12], variation
of energy transfer situation [13], changing of exciton
quenching rate [14,15], as well as electric field or temper-
ature dependent charge mobility [16]. For example, it has
been reported that the recombination zone will shift with
increasing driving voltage in multi-EML structure when it
has a different field-dependent mobility between the hole
transporting layer (HTL) and the electron transporting
layer (ETL), and/or different carrier trapping situation at
the EML [12]. To date, many efforts have been taken to
improve the color stability in multi-EML WOLED devices.
Among them, the carrier/exciton balance/confinement is
a popular approach. It was proved that the sandwiched
EMLs with the narrow band-gap emitting layer inserted
between wide band-gap ones were beneficial to color sta-
bility [17,18]. In addition, EMLs containing mixed hosts
with both hole-rich and electron-rich features were dem-
onstrated to broaden recombination zone and facilitate
color stabilization [19]. However, depending on the
employed functional materials, device structure needs to
be carefully designed. Another general strategy to achieve
color stable WOLEDs is to insert an interlayer between dif-
ferent emitting layers [20,21]. Unfortunately, the power
efficiency generally decreases because of the carrier-hurdle
by the additional interlayer(s). Meanwhile, such interlay-
ers complicate fabrication procedure.
In this work, we demonstrated color stable and non-
interlayer full phosphorescent WOLEDs by introducing a
mixed-host in the EML consisting of a blue phosphor FIr-
pic{iridium(III)bis[4,6-(difluorophenyl)pyridinato-N,C20]
picolinate} and a new orange phosphorescent emitter. Spe-
cifically, the new phosphor is a heteroleptic iridium com-
plex with main ligand of C^N = N type [22,23]. By device
physics study and optimization, WOLED with maximum
efficiencies of 25.3 cd Aꢀ1 and 12.6% was achieved. Further-
more, the emission color was very good and stable in a wide
luminance range. When voltages increased from 4 V to 12 V,
the CIE coordinates only varied from (0.4042, 0.4369) to
(0.3838, 0.4277), with the brightness changing from 49 to
38,035 cd mꢀ2. Considering the good CIE color, the device
efficiencies are decently high. We demonstrated that the
employment of the mixed-host with bipolar transport
properties as well as the eliminating of interlayer between
different EMLs was accounted for this good device perfor-
mance, which, on one hand, effectively broaden the exciton
2. Experimental
2.1. Material characterization
The 1H NMR spectra were recorded on a Bruker Ultra
Shield Plus 400 MHz spectrometer. UV–vis absorption
spectra were recorded using a Shimadzu UV-3600 spectro-
photometer. Photoluminescence (PL) spectra at room tem-
perature were measured on
a Shimadzu RF-5301PC
fluorophotometer. The PL quantum efficiency and lifetime
were performed with an Edinburgh Instruments FLS920
spectrometer. Electrochemical analysis was performed on
a Bioanalytical Systems CHI660E operating in cyclic vol-
tammetry (CV) mode. Glassy carbon, platinum wire, and
Ag/AgNO3 (0.01 mol Lꢀ1) were employed as working, aux-
iliary, reference electrode, respectively. Tetrabutylammo-
nium perchlorate (0.1 M) dissolved in dichloromethane
(DCM) for oxidation solutions was used as electrolyte.
The scan rate was 100 mV sꢀ1. High resolution mass spec-
trometry (HRMS) data were measured on an Ion Spec 4.7
Tesla FTMS instrument, operating at MALDI/DHB mode.
2.2. OLED device fabrication and characterization
The OLED devices were fabricated on indium tin oxide
(ITO) coated glass substrates with a sheet resistance of
25
X
squareꢀ1. Before device fabrication, ITO glass sub-
strates were precleaned carefully with de-ionized water,
acetone, and ethanol, and then treated with O2 plasma
for 30 s. All layers were grown in succession by thermal
evaporation without breaking vacuum (<3.0 ꢁ 10ꢀ3 Pa).
The current efficiency–luminance–power efficiency char-
acteristics and electroluminescence (EL) characteristics of
the devices were measured with a computer-controlled
Keithley 2400 source meter and fiber spectroscopy cali-
brated by a PR655 spectrophotometer at room tempera-
ture. The device emission area (0.1 cm2) was defined by
the overlapping of ITO anode and Al cathode. All measure-
ments were carried out under ambient conditions without
device encapsulation.
2.3. Material synthesis
The new iridium complex, bis[3,6-bis(phenyl)-pyridazi-
nato]iridium(acetylacetonate) [(Bppya)2Ir(acac)], was syn-
thesized following the procedures depicted in Scheme 1.
2.3.1. Synthesis of Ligand HBppya
Trans-1,2-Dibenzoylethylene (0.75 g, 3.18 mmol) was
dissolved in acetic acid (20 mL) by heating. After cooling
to room temperature, an excess amount of hydrazine
monohydrate (13 mL) was added dropwise. The reaction
mixture was kept stirring at room temperature for another
hour, and then refluxed overnight. After cooling down, the
solution was poured into ice to give a white precipitate.
Subsequent recrystallization from chloroform yielded the
pure product (0.28 g, 38%).

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L.-Y. Guo et al. / Organic Electronics 15 (2014) 2964–2970
Scheme 1. Synthesis of iridium(III) complex (Bppya)2Ir(acac).
2.3.2. Synthesis of (Bppya)2Iridium(acac)
IrCl3ꢂ3H2O (1 mmol) and the ligand HBppya (2.5 mmol)
were added to the mixture of 2-ethoxyethanol and water
(v/v = 3:1, 16 mL). After refluxing under nitrogen for 24 h,
the reaction mixture was cooled to room temperature.
The orange precipitate was collected by filtration and
washed with water, ethanol and hexane successively. The
solid was then pumped dry completely to give the crude
chloro-bridged dimer complex. Without further purifica-
tion, the dimer was added to a mixture of Na2CO3 (5 mmol),
acetyl acetone (1.5 mmol), and 2-ethoxyethanol (10 mL).
After refluxing under nitrogen overnight, the solution was
cooled to room temperature. The orange precipitate was
filtered off and washed with water, ethanol and hexane.
The crude product was purified by chromatography on
silica gel using petroleum ether/dichloromethane as
mobile phase. (Bppya)2Iridium(acac): Yield 52%. 1H NMR
(400 MHz, CDCl3) d 7.98 (d, J = 7.8 Hz, 4H), 7.50 (d, J = 14.0,
6.5 Hz, 3H), 7.38 (d, J = 8.0, 5.4 Hz, 7H), 7.05 (d, J = 9.1 Hz,
2H), 6.76 (t, J = 7.5 Hz, 2H), 6.56 (t, J = 7.6 Hz, 2H), 5.99
(d, J = 7.8 Hz, 2H), 2.05 (s, 6H). HRMS (FAB, m/z): calcd for
C
37H29IrN4O2 752.1898, found M+ 752.1891.
3. Results and discussion
As shown in Fig. 1a, (Bppya)2Ir(acac) reveals intense UV
absorption bands between 225 and 320 nm (extinction
coefficient
tributed from the spin-allowed
e
of 40,000–55,000 Mꢀ1 cmꢀ1), which are con-
1
(p–
p⁄) transitions of the
coordinated 3,6-bis(phenyl)-pyridazinato group. The less
intense bands between 350 and 440 nm
(e = 5000–
Fig. 1. (a) Absorption and emission spectra of (Bppya)2Ir(acac) in CH2Cl2
12,000 Mꢀ1 cmꢀ1) can be assigned to an inter ligand charge
transfer (1LLCT) and a metal-to-ligand charge transfer
(S0 ? 1MLCT) [5,13,14]. The iridium complex also shows
and (b) cyclic voltammogram of (Bppya)2Ir(acac) recorded in CH2Cl2, scan
rate: 100 mV sꢀ1
.
weak absorption bands above 450 nm
(e
< 5000 Mꢀ1
cmꢀ1). These absorptions are likely attributed to
photoluminescence of (Bppya)2Ir(acac) exhibits almost
single-exponential decay with emission lifetime of
0.57 ls, indicating the triplet nature of the emission.
Cyclic voltammetry was used to investigate the electro-
chemical behavior of the iridium(III) complex and deter-
mine its energy levels, as shown in Fig. 1b. The HOMO
energy level was calculated from the oxidation potential
S0 ? 3MLCT and
3
(p–
p⁄) transitions due to the effect of
strong spin–orbit coupling in iridium complex. The
(Bppya)2Ir(acac) shows intense orange photoluminescence
with the emission peak of 582 nm. Its photoluminescence
quantum yield (PLQY) is 0.31, measured in a degassed
CH2Cl2 solution at room temperature. In addition, the

L.-Y. Guo et al. / Organic Electronics 15 (2014) 2964–2970
2967
(Eox), while the LUMO energy level was derived by combin-
ing the HOMO level with the optical bandgap obtained
from the absorption edge [24,25]. To determine the oxida-
tion potentials, the reference electrode was calibrated
using
a ferrocene/ferrocenium salt couple, which is
assumed to have a redox potential with an absolute energy
level of ꢀ4.80 eV in vacuum. The HOMO energy value
was calculated using the following equation: EHOMO
=
ꢀ(4.8 + Eox ꢀ EFc) eV, where Eox and EFc are the oxidation
potentials (vs. Ag/Ag+) of the complex and ferrocene,
respectively [26]. The HOMO level of (Bppya)2Ir(acac) is
ꢀ5.41 eV and its LUMO level is ꢀ2.80 eV.
Based on this new material, we studied both the mono-
chromic and white OLED devices. The related material
structures and energy levels are depicted in Fig. 2 [27–
31]. Firstly, monochromic device with a traditional device
configuration of ITO/NPB (4,40-bis[N-(1-naphthyl)-N-
phenylamino]-biphenyl) (40 nm)/(Bppya)2Ir(acac): CBP
(6 wt%, 30 nm)/BCP (2,9-dimethyl-4,7-diphenyl-1,10-phe-
nanthroline) (8 nm)/Alq3 (tris(8-hyroxyquinolinato)alumi-
num(III)) (30 nm)/LiF (1 nm)/Al (100 nm) was fabricated.
As shown in Fig. 3, this device exhibited strong orange
(peaked at 580 nm) electroluminescence (EL) with an
external quantum efficiency (EQE) of 22.4%, a current effi-
ciency of 49.5 cd Aꢀ1 and a power efficiency of 38.9 lm Wꢀ1
under driving voltage of 4 V, which are among the highest
values for orange OLEDs. Additionally, the broad EL band
from 520 nm to 748 nm is highly desired for WOLEDs.
Furthermore, device efficiencies were very stable, at a high
luminance of 1000 cd/m2, these values still remained as
Fig. 3. Current efficiency–luminance–power efficiency characteristics for
the monochromic device. The inset is normalized EL spectra at different
voltages.
With regards to WOLED devices employing (Bppya)2
Ir(acac), a traditional blue phosphor FIrpic was selected
and a series of devices both with (W1 and W2) and without
(W3) interlayers between the orange and blue EMLs were
fabricated. For W1 series, the device structure was ITO/
NPB(40 nm)/TCTA(5 nm)/(Bppya)2Ir(acac):CBP (6 wt%,
10 nm)/mCP (2 nm in device W1–1 or 5 nm in device W1–2)/
FIrpic:mCP (6 wt%, 10 nm)/TPBi (30 nm)/LiF (1 nm)/Al.
Here, mCP (3,50-N,N0-dicarbazole-benzene) serves as host
for FIrpic and the interlayer material, TPBi acts as the elec-
tron-transporting layer. TCTA [4,40,400-tri(N-carbazolyl)tri-
phenylamine] was inserted between NPB and CBP for the
high as 45.7 cd Aꢀ1
, , and 17.5%, further
22.3 lm Wꢀ1
showing its potential for constructing WOLED.
Fig. 2. The energy levels and structures of related material used in the monochromic and white OLEDs.

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L.-Y. Guo et al. / Organic Electronics 15 (2014) 2964–2970
purpose of flattening the HOMO injection gap between NPB
(HOMO = 5.4 eV) and CBP (HOMO = 6.0 eV). The HOMO of
TCTA (5.7 eV) is located in between NPB and CBP, which
can provide a buffer gradient for hole injection and reduce
the energy barrier effectively. On the other hand, the LUMO
of TCTA (2.3 eV) is the same as NPB and higher than CBP
(2.9 eV), which can help to block electron drifting out of
CBP to NPB layer, making the recombination of excitons in
EML maximally. The two emitters are arranged with
sequence of orange to blue from anode to cathode.
It can be seen from Fig. 4a that with the interlayer of
mCP = 2 nm, device W1–1 exhibited white color with max-
imum efficiency of 5.0 cd Aꢀ1 and good CIExy of (0.3550,
0.3813). This low efficiency is inconsistent with the mono-
chromic device employing (BPPya)2Iracac, which shows
high efficiency of 49.5 cd Aꢀ1 (see above). Therefore, we
suspect that in W1–1, the emission of orange color should
be mainly produced by energy transfer from FIrpic EML
due to the thinner interlayer of 2 nm, but not direct recom-
bination in that layer. To verify this, we increased the inter-
layer to 5 nm (device W1–2, Fig. 4b). As a result, due to the
block of energy transfer by thicker interlayer, the FIrpic
emission predominated in device W1–2, confirming that
in device W1 series the recombination zone locates in the
blue-emitter region near the cathode. Therefore, due to
the intrinsic low electroluminescent efficiency of FIrpic
based device [19], as well as the quenching of this blue
EL by the orange EML, the efficiencies of W1 series are low.
For the purpose of enlarging orange emission by direct
recombination in the orange-emitting zone, electron
transporting toward orange-emitting layer must be
enhanced. Therefore, device W2 series were made with
structure of ITO/NPB (40 nm)/TCTA (5 nm)/(Bppya)2
Ir(acac):CBP (6 wt%, 10 nm)/CBP:TPBi (5 nm)/FIrpic:TPBi
(6 wt%, 10 nm)/TPBi(30 nm)/LiF (1 nm)/Al. Here beside
using electron transport TPBi instead of hole transport
dominated mCP to host FIrpic in order to improve electron
transport toward anode, the electron transport property of
the interlayer was also enhanced by using mixture of
CBP:TPBi. The ratios of CBP:TPBi are 1:1 (W2–1), 2:1
(W2–2) and 4:1 (W2–3). As expected (Fig. 5a), due to the
direct recombination in both orange- and blue-EMLs, blue
and orange emissions were realized with greatly improved
efficiencies. For example, the maximum efficiency of
Fig. 4. (a and b) are current efficiency–luminance–power efficiency
characteristics for devices W1–1 and W1–2, respectively. The insets on
the left are EMLs of devices W1–1 and W1–2, Orange stands for
(Bppya)2Ir(acac) based EML, Blue stands for FIrpic based EML. The insets
on the right are normalized EL spectra at 4 V. (For interpretation of the
references to colour in this figure legend, the reader is referred to the web
version of this article.)
Fig. 5. (a) Current efficiency–luminance characteristics for device W2
series, the insets are EL spectra at 4 V and the EML structure and (b)
normalized EL intensity–wavelength characteristics at different voltages
for device W2–3
.

L.-Y. Guo et al. / Organic Electronics 15 (2014) 2964–2970
2969
W2–3 (20.5 cd Aꢀ1) increased almost four folds compared to
device W1 series (5.0 cd Aꢀ1). Nevertheless, due to the
tradeoff between exciton formation in blue and the orange
EMLs, the EL spectra were not always white, varying with
the ratio of the interlayer components (inset of Fig. 5a),
e.g., the blue emission increased with the increase of CBP
in the interlayer of CBP:TPBi. Among these devices, W2–3
(with a ratio of 4:1 in CBP:TPBi interlyer) achieved a warm
white emission with a maximum current efficiency of
20.5 cd Aꢀ1 and CIE coordinate of (0.3814, 0.4207) at 4 V.
However, as shown in Fig. 5b, the white color only pre-
sented at low driving voltage. With the increase of voltage,
orange emission gradually dominated.
Seeking device mechanisms in device W2 series, energy
levels and the transport properties of materials in the EMLs
and the interlayer were examined. Since CBP and TPBi are
mainly hole and electron transporting materials, respec-
tively, at the double interfaces (Orange-CBP/CBP:TPBi/
Blue-TPBi) in the EML of W2 series, the holes will transport
across Orange-CBP and accumulate near the interface of
CBP:TPBi/Blue-TPBi; accordingly, the electrons will mainly
fill the LUMO of TPBi, reaching the interface of Orange-
CBP/CBP:TPBi. This will lead to the interface dominated
orange and blue exciton generation hence narrow recom-
bination zone (see inset of Fig. 5a). Additionally, consider-
ing the relative position of LUMOs and HOMOs in CBP and
TPBi, there was no barrier for electron to inject from blue
doped TPBi into orange doped CBP, while there was about
0.3 eV energy barrier for hole injection from orange doped
CBP to blue doped TPBi. Therefore with the increase of
voltage, the unbalanced electron/hole injection into the
blue and orange EMLs should be account for the unstable
white spectra in W2–3.
In order to overcome this problem, we fabricated devices
(W3 series) employing CBP and TPBi as mixed-host for both
(Bppya)2Ir(acac) and FIrpic to allow for bipolar charge
transporting in the whole EML, thus broadening the recom-
bination zone. In the meantime, the interlayer between dif-
ferent EMLs was eliminated and the balanced white
emission is expected achievable by varying the thickness
of the blue and orange EMLs. The device structure was
ITO/NPB(40 nm)/TCTA(5 nm)/FIrpic:CBP:TPBi/(Bppya)2Ir(acac):
CBP:TPBi/FIrpic:CBP:TPBi/TPBi(30 nm)/LiF/Al. Here, the
ratio of CBP:TPBi is 4:1, and the concentration of the blue
and the orange phosphors is always 6%. We adopted the
EML pattern of blue/orange/blue, considering that, on one
hand, the efficiency of FIrpic based device is generally lower
than that of (Bppya)2Ir(acac) one; on the other hand, as
demonstrated in W1 series that the excited state energy of
the blue emitter can partially transfer to that of orange
emitter without or with thin interlayer. Two blue EMLs
can enhance blue emission, which will be good for balanced
white emission. The thickness of the EML (blue/orange/
blue) was chosen as 10 nm/10 nm/10 nm for W3–1 and
12 nm/6 nm/12 nm for W3–2 (see Fig. 6a). As a result,
devices W3–1 and W3–2 exhibited good white emission with
maximum efficiencies of 25.3 cd Aꢀ1 (12.6%) and
20.4 cd Aꢀ1 (10.4%), and color rendering indexes (CRI) of
63 and 61, respectively (Fig. 6b and c). At brightness of
1000 cd mꢀ2, their efficiencies still remained as 19.4 cd Aꢀ1
and 15.9 cd Aꢀ1, indicative of small efficiency roll-off.
Fig. 6. (a) EML structures of W3–1 and W3–2, (b) the current efficiency–
luminance–power efficiency curves for device W3–1 and W3–2 and (c) the
normalized EL intensity–wavelength characteristics for device W3–1 and
W3–2, the insets are CIE coordinates under different voltages.
Particularly, the white colors were very stable as shown
in Fig. 6c. For example, when voltages increased from
4 V to 12 V, the CIE coordinates showed small alteration
of (0.3940 0.0102, 0.4323 0.0046) for W3–1
(0.3353 0.0032, 0.4252 0.0048) for W3–2 in a wide lumi-
nance range of 49–38,035 cd mꢀ2 and 47–30,253 cd mꢀ2
,
and
,
respectively. Evidently the utilization of CBP and TPBi as
mixed-hosts greatly improved device performance in com-
parison with only CBP- or TPBi-hosted devices. It can be
attributed to bipolar transport of the mixed-hosts, which
could effectively broaden the exciton recombination zone
as well as reduce the accumulation of triplet excitons at
the emissive interface. Additionally, this is also account
for the low efficiency roll-off.
4. Conclusions
In conclusion, we have developed a new iridium com-
plex (Bppya)2Ir(acac), which is capable of producing highly
efficient orange and white OLEDs. For the orange OLEDs, a
high external quantum efficiency of 22.4% and a current
efficiency of 49.5 cd Aꢀ1 can be achieved. More importantly,

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a series of color stable and non-interlayer full phosphores-
cent WOLEDs were fabricated by introducing a mixed-host
in the emissive layer (EML) composed of a blue phosphor
FIrpic and
a
new orange phosphorescent emitter.
Result shows that the maximum device efficiencies are
25.3 cd Aꢀ1/20.4 cd Aꢀ1 and 12.6%/10.4%, respectively.
When voltages increase from 4 V to 12 V, the CIE
coordinates exhibit small alteration of (0.3940 0.0102,
0.4323 0.0046)/(0.3353 0.0032, 0.4252 0.0048) in a
wide luminance range of 49–38,035 cd mꢀ2/47–30,253 cd mꢀ2
.
The root cause for excellent color stability is the employ-
ment of the mixed-host with bipolar transport properties
as well as the eliminating of interlayer between different
emission zone, which, on one hand, effectively broaden
the exciton recombination zone; on the other hand, reduce
the accumulation of triplet excitons at the emissive inter-
face. The mixed-host strategy is a promising way for devel-
opment of high color stable OLED light sources for use in
LCD backlights and indoor planar lighting applications.
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This work was financially supported by the National
Natural Science Foundation of China (Nos. 61077021,
61076016, 61474064), the Natural Science Foundation of
Jiangsu Province (Nos. BK20141425), and funded from
Nanjing University of Posts and Telecommunications
(Nos. NY212076, NY212050).
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