A host material with a small singlet-triplet exchange energy for phosphorescent organic light-emitting diodes: Guest, host, and exciplex emission

Article abstract of DOI:10.1016/j.orgel.2012.06.009

A host material containing a triazine core and three phenylcarbazole arms, called 2,4,6-tris(3-(carbazol-9-yl)phenyl)-triazine (TCPZ), was developed for phosphorescent organic light-emitting diodes (OLEDs). Ultra-low driving voltages were achieved by util

Full text of DOI:10.1016/j.orgel.2012.06.009

Organic Electronics 13 (2012) 1937–1947
Contents lists available at SciVerse ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
A host material with a small singlet–triplet exchange energy
for phosphorescent organic light-emitting diodes: Guest, host,
and exciplex emission
Shi-Jian Su a,b, , Chao Cai c, Junichi Takamatsu c, Junji Kido c,
⇑
⇑
a State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China
b Institute of Polymer Optoelectronic Materials and Devices, South China University of Technology, Guangzhou 510640, China
c Department of Organic Device Engineering, Graduate School of Science and Engineering, Yamagata University, 4-3-16 Jonan, Yonezawa,
Yamagata 992-8510, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
A host material containing a triazine core and three phenylcarbazole arms, called 2,4,6-
tris(3-(carbazol-9-yl)phenyl)-triazine (TCPZ), was developed for phosphorescent organic
light-emitting diodes (OLEDs). Ultra-low driving voltages were achieved by utilizing TCPZ
Received 24 March 2012
Received in revised form 26 May 2012
Accepted 8 June 2012
as the host due to its decreased singlet–triplet exchange energy (DEST) and low-lying low-
Available online 21 June 2012
est unoccupied molecular orbital (LUMO) energy level. Interaction between the RGB triplet
emitters and TCPZ were studied in both photoluminescent and electroluminescent
processes. Transient photoluminescence (PL) measurement of the co-deposited film of
fac-tris(2-phenylpyridine) iridium (Ir(PPy)3):TCPZ exhibits a shoulder at 565 nm whose
lifetime is about two times longer than that of the Ir(PPy)3 triplet excitons and can be
attributed to the triplet exciplex formed between Ir(PPy)3 and TCPZ. Such exciplex was also
found for the green phosphorescent OLED, giving the most efficient phosphorescent OLED
with triplet exciplex emission hitherto. Different from the PL process, a broad featureless
Keywords:
Host material
Singlet–triplet exchange energy
Phosphorescent
Organic light-emitting diode
Exciplex
band with a maximum at 535 nm was found for the OLED based on an EML of iridium(III)
0
bis(4,6-(di-fluorophenyl)pyridinato-N,C2 )picolinate (FIrpic):TCPZ, which can be attributed
to the emission from the singlet excited state of TCPZ formed by direct hole-electron
recombination. A multi-emitting-layer white OLED was also fabricated by utilizing FIrpic
and tris(1-phenylisoquinolinolato-C2,N)iridium(III) (Ir(piq)3) as the complementary triplet
emitters and TCPZ as the host. Different from most of ever reported white OLEDs fabricated
with blue/red complementary triplet emitters that exhibit color rendering index (CRI)
lower than 70, a high CRI of 82 is achieved due to the combination of blue and red phos-
phorescence emissions from FIrpic and Ir(piq)3, and the emerging green fluorescence emis-
sion from TCPZ.
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
100% internal quantum efficiency of electroluminescence
has been realized by harvesting both electro-generated
Since the development on phosphorescent transition
metal complexes by Forrest and Thompson’s groups, nearly
singlet and triplet excitons by fast intersystem crossing
for phosphorescence emission from their triplet states
[1]. For those phosphorescent organic light-emitting
diodes (OLEDs), phosphorescent emitters are normally
doped into an appropriate host material to avoid self-
quenching. As thus, it is essential that the triplet energy
(ET) of the host is higher than that of the emitter in order
⇑
Corresponding authors. Address: State Key Laboratory of Luminescent
Materials and Devices, South China University of Technology, Guangzhou
510640, China. Tel.: +86 20 87114346; fax: +86 20 87110606 (S.-J. Su).
E-mail addresses: mssjsu@scut.edu.cn (S.-J. Su), kid@yz.yamagata-
u.ac.jp (J. Kido).
1566-1199/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.orgel.2012.06.009

1938
S.-J. Su et al. / Organic Electronics 13 (2012) 1937–1947
to achieve an efficient energy transfer from the host to the
guest and also to prevent energy back transfer from the
guest to the host [2]. In addition, an optimized carrier
recombination in the emitting layer (EML) is also indis-
pensable to give a high efficiency. In general, the electron
mobility of many hosts is much lower than the hole mobil-
ity because of the fact that they mainly consist of strong
electron donors like aromatic amines or carbazoles [3]. Be-
sides the building blocks of electron donors like aromatic
amines or carbazoles, introduction of electron deficient
heterocycles, like pyridine [4], oxadiazole [5], and phe-
nathroline [6], was proven to be an effective route to give
host materials with improved bipolarity. Triazine has an
electron affinity larger than those of other typical electron
deficient heterocycles (e.g., pyridine, pyrimidine), and its
derivatives are known to be good electron transport mate-
rials in OLEDs [7]. Besides, some triazine derivatives were
also reported as electron-transport type hosts. Adachi
and co-workers established a series of 1,3,5-triazine deriv-
atives as hosts for the green phosphor of fac-tris(2-phenyl-
pyridine) iridium (Ir(PPy)3), resulting in an external
energy, were thoroughly characterized. Transient photolu-
minescence (PL) decays of the co-deposited films with red,
green, and blue (RGB) phosphors of tris(1-phenylisoquino-
linolato-C2,N)iridium(III) (Ir(piq)3), Ir(PPy)3, and FIrpic
were measured for the study of their interaction in photo-
luminescence process. In addition, phosphorescent OLEDs
were also fabricated with those RGB triplet emitters as
the guest and TCPZ as the host for the study of their inter-
action in electroluminescence process. Moreover, a multi-
emitting-layer white OLED was fabricated by utilizing FIr-
pic as a blue emitter and Ir(piq)3 as a complementary red
emitter. In contrast to most of ever reported white OLEDs
fabricated with blue/red phosphorescent emitters that give
CRI lower than 70, a high CRI of 82 is achieved due to the
combination of the phosphorescence emissions from
Ir(piq)3 and FIrpic and the emerging fluorescence emission
from TCPZ.
2. Experimental
quantum efficiency (
g
ext) of 10.2% and a power efficiency
2.1. General
(g
P) of 14 lm Wꢀ1 [8]. Wong and co-workers demonstrated
a series of 1,3,5-triazine derivatives as hosts for the green
phosphors of Ir(PPy)3 and (PPy)2Ir(acac), providing maxi-
mum gext and gP of 17.5% and 59.0 lm Wꢀ1 [9]. Strohriegl
and co-workers demonstrated a series of donor substituted
1,3,5-triazine derivatives as hosts for the blue phosphor of
iridium(III) bis(4,6-(di-fluorophenyl)pyridinato-N,C )pico-
linate (FIrpic), yielding a maximum current efficiency up
to 21 cd Aꢀ1 [3]. Our previous reports demonstrated that
All the reagents were purchased from Sigma–Aldrich
and were used without further purification. 2,4,6-Tris(3-
bromophenyl)-triazine (TBrPZ) was synthesized according
to the literature procedure [7c]. The developed TCPZ was
purified by silica gel chromatography and then repeated
thermal gradient vacuum sublimation before characteriza-
tion and device fabrication. 1H and 13C NMR spectra were
recorded on Varian 500 (500 MHz) spectrometer. Mass
spectra were obtained using a JEOL JMS-K9 mass spec-
trometer. Differential scanning calorimetry (DSC) was
2
0
the energy difference (DEST) between the singlet and trip-
let emission transitions decreases with introducing elec-
tron deficient heterocycles combined with electron
donors like carbazole, giving host materials as attractive
candidates to reduce the driving voltage of OLEDs [10]. It
is anticipated that the bipolar host materials consisting of
electron acceptor like triazine should also possess a small
performed using
a Perkin–Elmer Diamond DSC Pyris
instrument under nitrogen atmosphere at a heating rate
of 10 °C min–1. Thermogravimetric analysis (TGA) was
undertaken using a SEIKO EXSTAR 6000 TG/DTA 6200 unit
under nitrogen atmosphere at a heating rate of 10 °C min–
1. UV–Vis absorption spectrum of the neat film was
measured with a Shimadzu UV-3150 UV–vis–NIR spectro-
photometer. Room temperature steady-state photolumi-
nescent (PL) spectrum of the neat film was obtained with
a FluroMax-2 (Jobin–Yvon–Spex) luminescence spectrom-
eter. Ionization potential was determined by atmospheric
ultraviolet photoelectron spectroscopy (Rikken Keiki AC-
3). Time-resolved emission spectra of the neat film were
obtained at T = 4.2 K under excitation by a nitrogen laser
(k = 337 nm, 50 Hz, 800 ps) combined with a streak scope
C4334 (Hamamatsu) and a synchronous delay generator
C4792-02) (Hamamatsu). In comparison, transient PL de-
cays and the corresponding simultaneous PL spectra of
the phosphorescent emitter-doped films were recorded at
room temperature. PL quantum efficiencies of the co-
deposited films were measured by using an integrating
sphere under nitrogen gas flow at room temperature. Den-
sity functional theory (DFT) calculations were performed
by using the Gaussian suite of programs (Gaussian 03 W).
For the calculation of HOMO and LUMO energy levels,
the ground state structures were optimized at the re-
stricted B3LYP/6-31G(d) level, and the single-point ener-
gies were calculated at the restricted B3LYP/6-311+G(d,p)
D
EST to give a reduced driving voltage of OLEDs. However,
the introduction of both the electron donor and acceptor to
the host material may also lead to an intramolecular
charge transfer, resulting in the reduction of the energy
band-gap (Eg) of the molecule, and the effect may be much
more serious for a triazine-containing bipolar host mate-
rial because of its higher electron affinity than most of
other electron deficient heterocycles. Moreover, as a host
material co-deposited with triplet emitters in phosphores-
cent OLEDs, the interaction between the host material and
the triplet emitters and thus the origin of the emitting state
are critical factors for the device performance. To date, few
works reported on such an interaction, especially for blue,
green, and red phosphorescent emitters, which is of impor-
tance to realize highly efficient phosphorescent OLEDs for
applications to full color flat-panel displays and lighting.
In this article, we report on a bipolar host material
composed of 1,3,5-triazine as the core and three phenylc-
arbazoles as the arms, called 2,4,6-tris(3-(carbazol-9-yl)-
phenyl)-triazine (TCPZ). Its carrier mobility and energy
levels, including the highest occupied molecular orbital
(HOMO), the lowest unoccupied molecular orbital (LUMO),
singlet and triplet energies, and singlet–triplet exchange

S.-J. Su et al. / Organic Electronics 13 (2012) 1937–1947
1939
level. The ground state (S0) structure optimization and sin-
gle-point energy calculation were performed at the B3LYP/
6-31G(d) level. In contrast, the lowest-energy triplet ex-
cited state (T1) structure optimization and single-point en-
ergy calculation were performed at the unrestricted
B3LYP/6-31G(d) level [11].
benzene (BmPyPB) [14] (40 nm) were deposited as the HTL
and ETL, respectively. A multi-emitting-layer comprising
TCPZ:FIrpic (11 wt.%) (4.75 nm), TCPZ:Ir(piq)3 (4 wt.%)
(0.5 nm), and TCPZ:FIrpic (11 wt.%) (4.75 nm) were succes-
sively deposited between the HTL and ETL. Cathodes
consisting of a 0.5-nm-thick layer of LiF followed by a
100-nm-thick layer of Al were patterned using a shadow
mask with an array of 2 ꢂ 2 mm openings. The electrolu-
minescent (EL) spectra were recorded by an optical mul-
ti-channel analyzer, Hamamatsu PMA 11. The current
density and luminance versus driving voltage characteris-
tics were measured by Keithley source-measure unit
2400 and Konica Minolta chroma meter CS-200, respec-
tively. gext is calculated from the luminance, current den-
sity, and EL spectra, assuming a Lambertian distribution.
For the carrier mobility measurement, the neat film of
2.2. Synthesis of TCPZ
A
mixture of TBrPZ (0.979 g, 1.8 mmol), carbazole
(1.05 g, 6.3 mmol), PdCl2 (28.7 mg, 0.162 mmol), tris(tert-
butyl)phosphine (131 mg, 0.648 mmol), and sodium tert-
butoxide (0.778 g, 8.1 mmol) in anhydrous o-xylene
(100 mL) was stirred at 120 °C for 17 h under nitrogen
atmosphere. After cooling to room temperature, the mix-
ture was poured into water and then extracted with chlo-
roform. The combined organic phase was washed with
brine and dried over MgSO4. The subjection of the crude
mixture to silica gel chromatography (chloroform/n-hex-
ane = 2/1) afforded TCPZ (1.14 g, 79%) as white powders.
1H NMR (500 MHz, CDCl3): d (ppm) 8.93 (t, J = 2.0 Hz,
3H), 8.74 (d, J = 8.0 Hz, 3H), 8.15 (d, J = 8.0 Hz, 6H), 7.78
(d, J = 8.0 Hz, 3H), 7.73 (t, J = 8.0 Hz, 3H), 7.42 (d,
J = 8.0 Hz, 6H), 7.33 (t, J = 7.0 Hz, 6H), 7.28 (t, J = 7.0 Hz,
6H). 13C NMR (500 MHz, CDCl3): d (ppm) 171.26, 140.76,
138.26, 137.84, 131.30, 130.30, 128.06, 127.60, 126.08,
123.42, 120.32, 120.11, 109.60. MS (EI): m/z 806 [M+H+]
(calcd m/z 804.94). Anal. Calc. for C57H36N6 (%): C, 85.05;
H, 4.51; N, 10.44. Found: C, 85.10; H, 4.70; N, 10.14.
TCPZ (ꢁ1.6
lm) was prepared on the ITO-coated sub-
strates. A semi-transparent Al layer was patterned using
a shadow mask with an array of 2 ꢂ 2 mm openings. The
hole/electron mobility was measured by using a conven-
tional photo-induced time-of-flight (TOF) technique. A
nitrogen laser was used as the excitation source (k =
337 nm) and was incident on the sample through the ITO
or semi-transparent Al electrode.
3. Results and discussion
As shown in Scheme 1, TCPZ was synthesized by a Pal-
ladium catalyzed amination reaction between TBrPZ and
carbazole. UV–vis absorption and steady-state PL spectra
of the vacuum-deposited film (60 nm) of TCPZ on quartz
substrate were measured at room temperature (Fig. 1a).
The absorption peaks and shoulders at 283, 296, 329, and
2.3. Device fabrication and characterization
Phosphorescent OLEDs were grown on glass substrates
precoated with a ꢁ110-nm-thick layer of indium-tin oxide
(ITO) having a sheet resistance of 15 O/h. The substrates
were cleaned with ultra-purified water and organic sol-
vents and then dry-cleaned for 20 min by exposure to an
UV-ozone ambient. To improve the hole injection from
the anode, poly(arylene amine ether sulfone)-containing
tetraphenylbenzidine (TPDPES) doped with 10% (by
weight) tris(4-bromophenyl) aminium hexachloroantimo-
nate (TBPAH) was spun onto the precleaned substrate from
its dichloroethane solution to form a 20-nm-thick polymer
buffer layer [12]. For the red phosphorescent OLEDs, a 35-
nm-thick 1,1-bis(4-(N,N-di(p-tolyl)-amino)phenyl)cyclo-
hexane (TAPC) was deposited onto the buffer layer as a
hole-transport layer (HTL). Then, 4% (by weight) Ir(piq)3
was co-deposited with TCPZ to form a 10-nm-thick EML.
343 nm can be attributed to the
bazole chromophore of TCPZ. The absorption peaks at 244
and 267 nm can be attributed to the n–p⁄ and p⁄ transi-
p–
p⁄ transitions of the car-
p–
tions of the central 2,4,6-triphenyl-1,3,5-triazine skeleton,
respectively. Different from the previously reported carba-
zole substituted 1,3,5-triazine derivatives where the carba-
zole is directly combined with the triazine through its
9-position (kabs = 289 and 333 nm) [3,8], there is a little
bathochromic shift in absorption peaks of carbazole, and
it can be attributed to the insertion of phenyl between
the triazine core and the carbazole periphery that induces
an elongated
p conjugation although they are combined
with each other at the meta positions. In addition, another
pronounced phenomenon is that absorption at wave-
Finally,
a 65-nm-thick electron-transport layer (ETL)
of 2,4,6-tris(30(pyridin-2-yl)biphenyl-3-yl)-1,3,5-triazine
(TmPyBPZ) [7c] was deposited to block holes and to con-
fine excitons in the emissive zone. For the green phospho-
rescent OLEDs, TAPC (30 nm), Ir(PPy)3 (8 wt.%):TCPZ
(10 nm), and TmPyBPZ (50 nm) were successively depos-
ited as the HTL, EML, and ETL, respectively. 2,20-Bis(m-di-
p-tolylaminophenyl)-1,10-biphenyl (3DTAPBP) (30 nm),
FIrpic (11 wt.%):TCPZ (10 nm), and 3,5,30,50-tetra(m-pyrid-
3-yl)phenyl-(1,10)-biphenyl (BP4mPy) [13] (40 nm) were
successively deposited as the HTL, EML, and ETL, respec-
tively, for the blue phosphorescent OLEDs. For the white
OLEDs, TAPC (30 nm) and 1,3-bis(3,5-dipyrid-3-yl-phenyl)
Scheme 1. Synthetic route of TCPZ. (i) CF3SO3H, anhydrous chloroform,
0–25 °C; (ii) PdCl2, tris(tert-butyl)phosphine, sodium tert-butoxide, anhy-
drous o-xylene, 120 °C.

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S.-J. Su et al. / Organic Electronics 13 (2012) 1937–1947
to be 0.14 eV. In contrast,
any phenyl bridges is 0.35 eV, which is much larger than
that of TCPZ. Moreover, EST of TCPZ is also much smaller
DEST of the analogues without
D
than those of the previously reported host materials
[10,15], indicating that TCPZ may be an attractive candi-
date for the host of phosphorescent OLEDs giving reduced
driving voltages. Most recently, a triazine derivative of 2-
biphenyl-4,6-bis(12-phenylindolo[2,3-a]carbazole-11-yl)-
1,3,5-triazine was reported with a
However, it was used as an emitting dopant to achieve
an efficient singlet emission, and the effect of EST on the
DEST of 0.11 eV [16].
D
driving voltage of the device was not demonstrated.
HOMO energy level of TCPZ determined by atmospheric
photoelectron spectroscopy is 6.18 eV. Its LUMO energy le-
vel estimated from the HOMO energy level and Eg is
3.23 eV. Compared with a general host material of N,N0-
dicarbazolyl-4,40-biphenyl (CBP) without any electron defi-
cient heterocycle (LUMO = 2.7 eV) [15], much lower-lying
LUMO energy level is achieved by introducing triazine with
high electron affinity as the core. Its LUMO energy level is
also much lower-lying than that of the analogues without
any phenyl bridges (LUMO = 2.6 eV) due to the reduced Eg
[8], indicating a relatively lower electron-injection barrier
from the electron-transport layer.
In addition, TCPZ shows a glass transition temperature
(Tg) as high as 162 °C and a melting temperature (Tm) of
323 °C. In comparison, CBP shows a crystalline behavior,
and no Tg was detected in its DSC thermogram [17]. No
Tg was also found for the carbazole substituted triazine
derivatives without any phenyl bridges [3,8]. The high Tg
of TCPZ proves the high morphologic stability of the amor-
phous phase in a deposited film, which is a prerequisite for
its applications in OLEDs.
DFT calculations were performed using the Gaussian
suite of programs (Gaussian 03 W) for the structure of
TCPZ. As shown in Fig. 2, HOMO of TCPZ is mainly located
at the outer carbazoles. In contrast, its LUMO is mainly lo-
cated at the central 2,4,6-triphenyl-1,3,5-triazine skeleton
among the outer carbazoles, giving a low-lying LUMO en-
ergy level due to the strong electron affinity of triazine.
In addition, the calculated spatial distributions show lim-
ited overlap between its HOMO and LUMO, further indicat-
ing localized HOMO and LUMO energy levels on the
carbazole and triphenyltriazine, respectively.
Fig. 1. (a) Room temperature UV/vis absorption and steady-state photo-
luminescence spectra for the vacuum-deposited neat film of TCPZ on
quartz substrate. (b) Time-resolved emission spectra for the vacuum-
deposited neat film of TCPZ excited by a nitrogen laser (k = 337 nm, 50 Hz,
800 ps) at T = 4.2 K. Black line: emission spectrum of the prompt
component (0.23 ms < t < 0.60 ms), mostly composed of fluorescence.
Grey line: emission spectrum of the delayed component (1 ms < t <
10 ms), also called phosphorescence.
lengths longer than 350 nm emerges in contrast to the
other donor substituted triazines. It can be attributed to
the strong electron affinity of triazine and intramolecular
charge transfer between the central triazine and the outer
carbazole through the phenyl bridges. From the absorption
edge, Eg of TCPZ can be assumed to be 2.95 eV, which is
much narrower than that of the analogues without any
phenyl bridges (Eg = 3.4 eV) [3,8]. Similar to the absorption
spectrum, PL spectrum of TCPZ is also shifted to the longer
wavelength in comparison with the analogues without any
phenyl bridges (kPL = 392 nm) to give a featureless emis-
sion peak at 467 nm. It also shows a larger Stokes shift
compared with the analogues without any phenyl bridges,
further indicating that the origin of the emission from TCPZ
is based on the intramolecular charge transfer between the
central triazine and the outer carbazole.
To obtain ET of TCPZ, extremely long decay components
of its transient photoluminescence lasting for ꢁ10 ms were
recorded at T = 4.2 K. Fig. 1(b) shows two time-resolved
emission spectra. One is the emission spectrum of the
prompt component (0.23 ms < t < 0.60 ms), mostly com-
posed of singlet emissions, also called fluorescence spec-
trum, which is almost consistent with the steady-state PL
spectrum that measured at room temperature. From the
Its triplet energy is also calculated by the difference of
their ground state (S0) and triplet excited state (T1) ener-
gies, and the calculated triplet energy (T1 ꢀ S0) of TCPZ is
2.72 eV. The energy difference
(DE) between Eg and
v
¼0
v
highest energy fluorescence peak S0 S1¼0, singlet en-
ergy (ES) of TCPZ can be estimated as 2.66 eV. The other
is the emission spectrum of the delayed component
(1 ms < t < 10 ms), generally consisting of only triplet emis-
sions, also called phosphorescence spectrum. TCPZ exhibits
(T1 ꢀ S0) is taken as an estimate for the energy difference
between the singlet and triplet transitions, and E of
0.39 eV is obtained for TCPZ. Our previously demonstrated
bipolar host materials containing a pyridine core have
of 0.83–1.06 eV, and the bipolar host materials with a
pyrimidine core have E of 0.53–0.57 eV [10]. Since the
electron affinity of triazine is stronger than that of pyridine
and pyrimidine, a reduced E seems to be achieved for the
D
D
E
v
¼0
v¼0
the highest energy phosphorescence peak S0 T1 at
492 nm, corresponding to ET of 2.52 eV. Generally, a favor-
able exchange energy as small as possible is requested for a
host material to allow for both charge injection into the
host and efficient triplet emission from a dispersed triplet
emitter. Here, the energy difference between the singlet
and triplet emission transitions is taken as an estimate
D
D
bipolar host materials by introducing a heterocycle with
stronger electron affinity. This is also proved by the exper-
imental result that
DEST of TCPZ is much smaller than
those of the host materials containing heterocycles like
pyridine and pyrimidine [10].
for the exchange energy, and
DEST of TCPZ was estimated

S.-J. Su et al. / Organic Electronics 13 (2012) 1937–1947
1941
Fig. 2. The calculated spatial distributions and energy levels of HOMO and LUMO for TCPZ.
As former mentioned, as a host material for a triplet
emitter, its triplet excited state should be higher than that
of the triplet emitter to prevent reverse energy transfer
from the guest to the host. Compared with the bipolar host
material containing a pyridine or pyrimidine core, de-
creased ET is obtained for TCPZ. It seems in contradiction
with this requirement. To verify its triplet exciton confine-
ment ability, well-known red, green, and blue triplet emit-
ters of Ir(piq)3, Ir(PPy)3, and FIrpic were co-deposited with
TCPZ for transient PL decay measurements.
Since ET of TCPZ is slightly lower than that of FIrpic
(2.61 eV), it is anticipated that there should be a back en-
ergy transfer from FIrpic to TCPZ. As shown in Fig. 3, FIrpic
(3 wt.%) doped into TCPZ exhibits a complex decay curve,
which can be well-fitted with a third exponential decay
curve to give a short first exponential component lifetime
thus a non-radiative deactivation. The simultaneous PL
spectrum features emission bands at 475 and 510 nm,
which are identical to the PL bands of FIrpic (Fig. 4), indi-
cating all the emissions originate from the triplet excited
state of FIrpic.
For the co-deposited film of Ir(PPy)3 (8 wt.%):TCPZ, it
also shows a complex decay curve with a short first expo-
nential component lifetime of 0.56 ls and a relatively low
gPL of (69 1)%. It is of interest that its PL spectrum is
slightly different from that of Ir(PPy)3 with improved
shoulder at 560 nm (Fig. 4), and the PL spectrum in the
whole range (0 < t < 20 ls) can be divided into that from
Ir(PPy)3 and another new band with a maximum peak at
565 nm and a shoulder at 530 nm (Fig. 4, grey lines), and
the emission components from Ir(PPy)3 and the new band
are quite similar. To study the origin of this new band, the
of 0.45
l
s. In addition, its PL quantum efficiency (
g
PL) is
emissions in the range of 1.5–2.5
20 s (Fig. 5c – c3) were recorded in combination of the
emissions in the whole range (0<t<20 s) (Fig. 5c – c1),
ls (Fig. 5c – c2) and 10–
(66 1)%. The complex decay curve and the relatively
low gPL can be attributed to lower ET of TCPZ than FIrpic
and thus reverse energy transfer from FIrpic to TCPZ and
l
l
as indicated in Fig. 5(a). It can be clearly seen that the
Fig. 3. Black solid lines: transient photoluminescence decay curves for
the red (Ir(piq)3, 4 wt.%), green (Ir(PPy)3, 8 wt.%), and blue (FIrpic, 3 wt.%)
triplet emitters co-deposited with TCPZ. Grey dotted lines: the exponen-
tially fitted decay curves for the corresponding films.
Fig. 4. Photoluminescence spectra for the triplet emitters (FIrpic (3 wt.%),
Ir(PPy)3 (8 wt.%), and Ir(piq)3 (4 wt.%)) co-deposited with TCPZ. The
photoluminescence spectrum of TCPZ:Ir(PPy)3 is divided into the emis-
sion bands from Ir(PPy)3 and the triplet exciplex (Grey lines).

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S.-J. Su et al. / Organic Electronics 13 (2012) 1937–1947
and the transient decay of b1 was well-fitted by:
emission spectrum in the range of 1.5–2.5
identical to that of Ir(PPy)3, and the emission spectrum in
the range of 10–20 s is quite similar to the new band that
ls is almost
s1
s2
s3
IPLðtÞ ¼ A1eꢀt= þ A2eꢀt= þ A3eꢀt=
ð2Þ
l
shown in Fig. 4. Their different lifetimes and emission
spectra suggest that the emitting state of the new band is
not the Ir(PPy)3 triplet excited state. Due to its relatively
longer lifetime, the new band can be assumed to be origi-
nated from the triplet exciplex formed between TCPZ and
Ir(PPy)3. Although emissions from Ir(PPy)3 and the triplet
exciplex are covered with each other between 500 and
650 nm, most of emissions at the wavelengths shorter than
500 nm should be originated from the relaxation of Ir(P-
Py)3 triplet excited state, and most of emissions at the
wavelengths longer than 650 nm should be originated
from the triplet exciplex. Their corresponding decay curves
are shown in Fig. 5(b) as b2 and b3, respectively, in com-
parison with that of the emissions in the range of 400–
700 nm b1, as indicated in Fig. 5(a). It can be clearly seen
that the lifetime of b2 is much lower than that of b3. The
transient decays of b2 and b3 were well-fitted by:
where IPL is the photoluminescence intensity, A1, A2, and A3
are quantities of the corresponding emission components, t
is decay time, and s1
responding emission components. As shown in Table 1, the
first exponential component lifetime ( 1) is 0.65 s for b2,
, s2, and s3 are the lifetimes of the cor-
s
l
which is about half of that for b3. Although the emissions
in the range of 400–700 nm are comprised of emissions
from Ir(PPy)3 and the triplet exciplex, it shows a short s1
of 0.56 ls, which is quite similar to that for b2, indicating
its first exponential component is mainly originated from
the triplet excited state of Ir(PPy)3. This further proves that
the emission lasting for a short time is identical to that of
Ir(PPy)3. In addition, it also shows s2 of 1.82 ls, which is
slightly longer than s1 for b3, indicating the second
exponential component is most likely originated from the
triplet exciplex. Moreover, the quantities of the emission
components (A1 and A2 for b1) are similar to each other,
further proving that the emission components from
Ir(PPy)3 and the triplet exciplex are similar. The elongated
s1
s2
IPLðtÞ ¼ A1eꢀt= þ A2eꢀt=
ð1Þ
Fig. 5. (a) Image of transient photoluminescence decay for the co-deposited film of TCPZ:Ir(PPy)3 (8 wt.%). (b) Transient photoluminescence decay curves
according to the emissions in the range of 400–700 nm (b1), 400–500 nm (b2), and 650–700 nm (b3), as indicated in (a). (c) The emission spectra in the
range of 0–20
ls (c1), 1.5–2.5 ls (c2), and 10–20 ls (c3), as indicated in (a). (d) Scheme of the singlet and triplet energy levels for TCPZ and Ir(PPy)3 and the
triplet exciplex formed between triplet excited states of TCPZ and Ir(PPy)3.

S.-J. Su et al. / Organic Electronics 13 (2012) 1937–1947
1943
Table 1
Exponentially fitted lifetimes (s1, s2, and s3) and quantities (A1, A2, and A3) of the corresponding emission components for the transient photoluminescence
decay of the co-deposited films of TCPZ:FIrpic (3 wt.%), TCPZ:Ir(PPy)3 (8 wt.%), and TCPZ:Ir(piq)3 (4 wt.%).
Guest
gPL (%)
Wavelength (nm)
350–650
s1
(
l
s)
A1
s1
(
l
s)
A2
s3
(
l
s)
A3
FIrpic
66
69
1
1
0.45
0.50
1.92
0.49
6.58
0.10
Ir(PPy)3
b1
b2
b3
400–700
400–500
650–700
0.56
0.65
1.29
0.54
5.78
1.43
1.82
3.50
4.74
0.44
0.19
0.49
5.90
–
–
0.10
–
–
Ir(piq)3
54
1
520–820
1.15
1.10
–
–
–
–
lifetime for b3 indicates the new emission band should
arise from the triplet exciplex formed between the triplet
excited states of TCPZ and Ir(PPy)3. After the formation of
the TCPZ and Ir(PPy)3 triplet excited states, there should
be a charge transfer in the Ir(PPy)3:TCPZ exciplex, giving
an elongated lifetime (Fig. 5d). As shown in Fig. 6, the
emission intensity at longer wavelengths increases with
the increment of Ir(PPy)3 concentration, further proving
that the emission at longer wavelengths originates from
the Ir(PPy)3:TCPZ triplet exciplex. Note that an exciplex is
an excited state whose wave function straddles two dis-
similar molecules, one a net electron donor and the other
an acceptor, and strong spin–orbital coupling of Ir presum-
ably leads to the formation of a triplet exciplex [18]. In this
system, TCPZ more likely behave as an electron acceptor
due to the strong electron affinity of its triazine core.
For Ir(piq)3 with the lowest triplet energy among these
phosphorescent emitters, the co-deposited film of Ir(piq)3
(4 wt.%):TCPZ exhibits a clear mono-exponential decay
energy is well-confined on the Ir(piq)3 molecules due to its
higher ET than the guest.
Since the charge carriers are recombined in the EML in
an OLED, balanced carrier injection and transport into the
EML is a prerequisite for improved device performance.
As thus, different from the hole-transport and electron-
transport materials, balanced hole and electron mobility
is ideal for a host material. Hole/electron mobility of TCPZ
was measured by using a conventional photo-induced TOF
technique using a nitrogen laser as an excitation source
(k = 337 nm). Fig. 7 shows its hole (lh) and electron (le)
mobility plotted as a function of the square root of the
electric field (E). lh of TCPZ lies in the range from 3.8 ꢂ
10ꢀ5 to 4.6 ꢂ 10ꢀ5 cm2 Vꢀ1 sꢀ1 at electric field between
8.1 ꢂ 105 and 1.0 ꢂ 106 V cmꢀ1, which is quite similar to
its
l
e, ranging from 2.7 ꢂ 10ꢀ5 to 5.4 ꢂ 10ꢀ5 cm2 Vꢀ1 sꢀ1
at electric field between 6.4 ꢂ 105 and 1.0 ꢂ 106 V cmꢀ1
.
Different from many carbazole-based host materials that
exhibit higher lh than le due to the well-known electron
donor carbazole moiety, the similarity of lh and le sug-
gests that TCPZ behaves more like a bipolar host material,
further proving the validity of the molecular design [10].
To valuate TCPZ as a host material for phosphorescent
emitters, OLEDs were fabricated by doping Ir(piq)3,
Ir(PPy)3, and FIrpic into TCPZ as the EMLs, respectively.
The Ir(piq)3-based red phosphorescent OLED shows a
turn-on voltage (Von) of 2.3 V for electroluminescence
(luminance of 1 cd mꢀ2 was detected) and driving voltages
curve with relatively long lifetime of about 1.15 ls and radi-
ative and nonradiative rate constants (kr and knr) of 4.7 ꢂ 105
and 4.0 ꢂ 105 sꢀ1, respectively. Its
gPL is (54 1)%, which is
one of the highest values for the Ir(piq)3-doped films. The
simultaneous PL spectrum features emission bands at 627
and 683 nm, which are identical to the photoluminescent
bands of Ir(piq)3 (Fig. 4), indicating all the emissions origi-
nate from the triplet excited state of Ir(piq)3. The transient
PL observation indicates that the triplet energy transfer
from Ir(piq)3 to TCPZ is completely suppressed and the
Fig. 7. Hole (lh) (open circles) and electron (le) (closed circles) mobility
for the neat film of TCPZ plotted as a function of the square root of the
electric field (E). Inset: typical transient photocurrent signals for electron
and hole.
Fig. 6. Photoluminescence spectra for the co-deposited films of
TCPZ:Ir(PPy)3 at different Ir(PPy)3 concentrations.

1944
S.-J. Su et al. / Organic Electronics 13 (2012) 1937–1947
of 3.02 and 4.25 V achieved at 100 and 1000 cd mꢀ2
,
based red phosphorescent OLEDs (Table 2) [10,19,20], indi-
cating a well-balanced carrier is achieved in the EML due to
the improved bipolarity of TCPZ. The emission color is deep
red with an EL emission peak at 621 nm and Commission
Internationale de L’Eclairage (CIE) color coordinates of
(0.67, 0.33) (Fig. 9). This proves the emission directly from
the triplet emitter of Ir(piq)3 due to the good triplet exciton
confinement on the phosphor molecules.
For the green phosphorescent OLED, ultra-low driving
voltages were also achieved with a threshold voltage
(Vth) of 2.0 V for current and a Von of 2.11 V for electrolumi-
nescence, which are even 0.1 V lower than the lowest driv-
respectively (Fig. 8), which are the lowest driving voltages
hitherto [10] and are even lower than those of the devices
based on a p-i-n architecture [19]. The low driving voltages
achieved here can be attributed to the low-lying LUMO en-
ergy level and the small
DEST of TCPZ, giving an improved
carrier injection into the EML. Note that the concentration
of Ir(piq)3 is as low as 4 wt.%. At low current density, carri-
ers may be injected into the EML through the Ir(piq)3 dop-
ant molecules because of its lower-lying LUMO (3.4 eV)
and higher-lying HOMO (5.4 eV) energy levels. But at high
current density, more carriers must be injected into the
EML through the host molecules. The reduced driving volt-
age achieved by utilizing TCPZ proves that more carriers
must be injected into the EML through the host rather than
direct carrier trap by the dopant. Aside from the low driv-
ing voltages previously reported in
a similar device
architecture based on CBP (Vth = 2.1 V, Von = 2.22 V) [7](c).
In addition, driving voltages as low as 2.47 and 2.98 V were
achieved at 100 and 1000 cd mꢀ2, respectively. The driving
voltages are also much lower than those of the previously
reported green phosphorescent OLEDs based on the other
triazine derivatives as the hosts [8,9], and the reduced
driving voltage can be attributed to the lower-lying LUMO
ing voltage, a very high maximum gext of 19.1% and a max-
imum gP of 20.5 lm Wꢀ1 were achieved at low current
density. The efficiencies are 17.9%, 15.8 lm Wꢀ1 and
14.1%, 8.85 lm Wꢀ1 at 100 and 1000 cd mꢀ2, respectively,
which is one of the highest efficiencies for the Ir(piq)3-
energy level and the reduced exchange energy
DEST that
give an improved carrier injection into the EML. Similar
to the PL spectrum of the co-deposited film of TCPZ:Ir(P-
Py)3, EL spectrum of the device based on an EML of
TCPZ:Ir(PPy)3 is featureless with a broad band between
500 and 600 nm, indicating there is also a triplet exciplex
emission in the electroluminescent process. Compared
with the PL spectrum, the emission band at 560 nm is
slightly higher in intensity. The EL spectrum was fitted to
the emissions corresponding to Ir(PPy)3 and the triplet
exciplex. The emission corresponding to the triplet exci-
plex in the electroluminescence process is slightly stronger
than that in the photoluminescence process, indicating
more fraction of the triplet exciplex emission. In this sys-
tem, since some carriers must be injected into the EML
through the TCPZ molecules, the hole-electron recombina-
tion should lead to both Ir(PPy)3 and TCPZ molecules in
their excited states. Small difference of their triplet excited
states (ꢁ0.1 eV) may facilitate charge transfer between the
excited states, resulting in relaxation into the exciplex
state from the excited states of Ir(PPy)3 and TCPZ. It is gen-
erally thought that the exciplex emission shows rather low
efficiency. As an example, phosphorescent OLEDs based on
Ir(PPy)3 that yield broad structureless triplet exciplex
emission between Ir(PPy)3 and the host of N-2,6-dibrom-
ophenyl-1,8-naphthalimide (niBr) show a maximum gext
as low as 0.3% [18]. In contrast, although the formation
of the triplet exciplex, a maximum gext of 19.6% and a max-
imum gP of 87.8 lm Wꢀ1 were achieved in this work, and
the efficiencies roll very slightly to 19.1%, 85.0 lm Wꢀ1
and 19.1%, 70.4 lm Wꢀ1 at 100 and 1000 cd mꢀ2, respec-
tively (Table 2). In addition, the efficiencies are also much
higher than the previously reported Ir(PPy)3-based green
phosphorescent OLEDs utilizing the other triazine deriva-
tives as the hosts [8,9]. Although the mechanism is slightly
different, platinum complexes like platinum(II)[2-(40,
Fig. 8. (a) Current density (open) and luminance (closed) vs driving
voltage and (b) external quantum efficiency (
gext) (closed) and power
efficiency ( P) (open) vs luminance characteristics of the phosphorescent
g
2
60-difluorophenyl)pyridinato-N,C )](2,4-pentanedionato)
(FPt) was known to have a triplet excimer emission origi-
nated from the interaction between the same molecules
[21], and the optimized highest gext is near 18% that is
achieved at a low brightness of 1 cd mꢀ2 [21b]. For com-
0
OLEDs in structures: ITO/TPDPES:TBPAH (20 nm)/TAPC (35 nm)/
TCPZ:4 wt.% Ir(piq)3 (10 nm)/TmPyBPZ (65 nm)/LiF (0.5 nm)/Al (100 nm)
(sd); ITO/TPDPES:TBPAH (20 nm)/TAPC (30 nm)/TCPZ:8 wt.% Ir(PPy)3
(10 nm)/TmPyBPZ (50 nm)/LiF (0.5 nm)/Al (100 nm) (hj); ITO/TPD-
PES:TBPAH (20 nm)/3DTAPBP (30 nm)/TCPZ:11 wt.% FIrpic (10 nm)/
BP4mPy (40 nm)/LiF (0.5 nm)/Al (100 nm) ( .).
r

S.-J. Su et al. / Organic Electronics 13 (2012) 1937–1947
1945
Table 2
Performance data of the phosphorescent OLEDs using TCPZ as the host material.
Guest
Von
(V)
Maximum efficiency
gext (lm Wꢀ1/%)
At 100 cd mꢀ2
V/gP
gext (V/lm Wꢀ1/%)
At 1000 cd mꢀ2
V/gP
gext (V/lm Wꢀ1/%)
gP
/
/
/
Ir(piq)3
Ir(PPy)3
FIrpic
Ir(piq)3 and FIrpic
2.3
2.1
3.0
3.0
20.5/19.1
87.8/19.6
62.0/18.5
31.2/13.7
3.02/15.8/17.9
2.47/85.0/19.1
3.74/25.4/10.1
3.91/9.45/5.39
4.25/8.85/14.1
2.98/70.4/19.1
4.72/17.3/8.67
5.36/5.95/4.76
from TCPZ was found, the new band emerging during the
electroluminescent process may be attributed to the TCPZ
singlet exciton formed by direct carrier recombination on
the TCPZ molecules. It further proves that the carriers
should be partly injected into the EML through TCPZ. Com-
pared with the PL spectrum of the neat TCPZ film, a bath-
ochromic shift in EL spectrum was found, and it may be
attributed to its aggregation and thus the excimer
emission.
Besides RGB OLEDs for display application, white OLEDs
have also attracting particular attention because of its po-
tential application in solid-state lighting. As definition,
white emission requires the mixture of complementary
(e.g., blue and yellow or orange) or primary colors (red,
green, and blue). Generally, the white OLEDs fabricated
with complementary colors exhibit color rendering index
(CRI) lower than 70 due to the incomplete coverage of the
whole visible region [14,23]. The emerging new band at
535 nm for the TCPZ:FIrpic-based device just covers the
green-light region that lacks in a general blue/red two-color
white OLED. It is thus potential to fabricate a white OLED
with improved CRI by utilizing only two kinds of comple-
mentary phosphorescent emitters. To confirm this idea, a
multi-emitting-layer device was fabricated in the structure
of ITO/TPDPES:TBPAH (20 nm)/TAPC (30 nm)/TCPZ:11 wt.%
FIrpic (4.75 nm)/TCPZ:4 wt.% Ir(piq)3 (0.5 nm)/TCPZ:11
wt.% FIrpic (4.75 nm)/BmPyPB (40 nm)/LiF (0.5 nm)/Al
(100 nm). As shown in the inset of Fig. 10(a), both its EL
spectra recorded at 100 and 1000 cd mꢀ2 comprise an ex-
pected yellowish green band at ꢁ540 nm combined with
the bands of FIrpic and Ir(piq)3. The CIE coordinates are
(0.39, 0.45) at 100 cd mꢀ2, and shift slightly to (0.42, 0.46)
at 1000 cd mꢀ2. The shift in CIE coordinates at different
brightness is due to the dominate emission from TCPZ at
brighter luminance, and it can be attributed to more singlet
excitons of TCPZ formed by direct carrier recombination on
the host molecules at higher current density. Nevertheless,
although the thickness of the red-light-emitting layer is
only a twentieth to that of the total emissive layers, inser-
tion of an ultra-thin red-light-emitting layer induced suffi-
cient red-light emission to achieve a white emission.
Moreover, an improved CRI of 82 was achieved at the appli-
cable brightness of 100 and 1000 cd mꢀ2. Although it might
not be appropriate to be called a two-color white OLED
since there is a green-light-emission from the host TCPZ,
the current findings indicate that a white OLED with CRI
higher than 80 can be achieved with two kinds of phospho-
rescent emitter, FIrpic for blue and Ir(piq)3 for red. Consid-
ering the utilization of singlet emission from TCPZ, the
current white OLED can be also called a fluorescent/phos-
phorescent hybrid white OLED, from which emission is
Fig. 9. Electroluminescent (EL) spectra of the phosphorescent OLEDs in
structures: ITO/TPDPES:TBPAH (20 nm)/TAPC (35 nm)/TCPZ:4 wt.%
Ir(piq)3 (10 nm)/TmPyBPZ (65 nm)/LiF (0.5 nm)/Al (100 nm) (s); ITO/
TPDPES:TBPAH (20 nm)/TAPC (30 nm)/TCPZ:8 wt.% Ir(PPy)3 (10 nm)/
TmPyBPZ (50 nm)/LiF (0.5 nm)/Al (100 nm) (h). The dotted lines are the
fitted emissions corresponding to Ir(PPy)3 and the triplet exciplex. ITO/
TPDPES:TBPAH (20 nm)/3DTAPBP (30 nm)/TCPZ:11 wt.% FIrpic (10 nm)/
BP4mPy (40 nm)/LiF (0.5 nm)/Al (100 nm) (5). The dotted lines are the
fitted emissions corresponding to FIrpic and TCPZ.
parison, to our best knowledge, the current work gives the
most efficient phosphorescent OLED with triplet exciplex
or excimer emission hitherto.
As anticipated, different from the green and red phos-
phorescent OLEDs, efficiency of the phosphorescent OLED
based on an EML of TCPZ:FIrpic is lower than that of the
previously reported devices in a similar architecture be-
cause of the lower triplet energy of TCPZ than FIrpic, giving
a
maximum gext of 18.5% and a maximum gP of
62.0 lm Wꢀ1 [4]. Since FIrpic is more like an electron-trans-
porter and the concentration of FIrpic is higher than that of
Ir(PPy)3 and Ir(piq)3, effect of the host on current density is
slighter than that in green and red devices. Nevertheless,
current density for this device is the highest level of the
previously reported blue phosphorescent OLEDs based on
FIrpic. The efficiencies roll to 10.1% and 25.4 lm Wꢀ1 at
100 cd mꢀ2 (Table 2), and the reduced efficiency can be
partially attributed to the reduced carrier balance because
of electron-transport property of FIrpic and low electron-
injection barrier from the electron transport layer when
TCPZ is utilized as the host. Different from the EL spectra
of the most devices based on FIrpic reported previously
[22],
a broad featureless band with a maximum at
535 nm was found in addition to the FIrpic band for the de-
vice based on TCPZ (Fig. 9). Different from the PL spectrum
of the co-deposited FIrpic:TCPZ film where no emission

1946
S.-J. Su et al. / Organic Electronics 13 (2012) 1937–1947
achieved. Compared with the slightly reduced ET, the sig-
nificantly decreased EST achieved by introducing triazine
D
as the core indicates that introducing appropriate electron
donors and acceptors as building blocks of a host material
is an effective route to give attractive candidates for phos-
phorescent OLEDs with ultra-low driving voltage as well as
good triplet exciton confinement. In addition, improved
bipolarity is also achieved by introducing triazine as the
core and carbazole as the periphery. Transient PL measure-
ment of Ir(PPy)3:TCPZ films exhibits a shoulder at 565 nm
whose lifetime is about two times longer than that of Ir(P-
Py)3 triplet excitons and can be attributed to the triplet
exciplex formed between Ir(PPy)3 and TCPZ. Such triplet
exciplex was also found for the green phosphorescent
OLED giving the most efficient phosphorescent OLED with
triplet exciplex or excimer emission hitherto. Different
from the PL process, a broad featureless band with a max-
imum at 535 nm was found for the phosphorescent OLED
based on an EML of FIrpic:TCPZ, which can be attributed
to the emission from the TCPZ singlet excited state formed
by direct hole-electron recombination. On this base, in
contrast to most of ever reported fluorescent/phosphores-
cent hybrid white OLEDs that utilize green and red (or or-
ange) phosphorescence and blue fluorescence, a unique
multi-emitting-layer fluorescent/phosphorescent hybrid
white OLED was fabricated by introducing Ir(piq)3 as a
complementary red phosphorescent emitter to give a high
CRI of 82. It also shows us an alternative route to achieve
high quality white light emission by the utilization of the
guest, host, and exciplex emission.
Fig. 10. (a) Current density (open) and luminance (closed) vs driving
voltage characteristics and electroluminescent (EL) spectra recorded at
Acknowledgements
100 and 1000 cd mꢀ2 (Inset) and (b) external quantum efficiency (gext
)
(closed) and power efficiency (
the white OLED in structure: ITO/TPDPES:TBPAH (20 nm)/TAPC (30 nm)/
TCPZ:11 wt.% FIrpic (4.75 nm)/TCPZ:4 wt.% Ir(piq)3 (0.5 nm)/
gP) (open) vs luminance characteristics for
S.J.S. greatly appreciates the financial support from the
National Natural Science Foundation of China (51073057),
the Ministry of Science and Technology (2009CB930604
and 2011AA03A110), the Ministry of Education (NCET-
11-0159), and the Fundamental Research Funds for the
Central Universities (2011ZZ0002). The authors acknowl-
edge Chemipro Kasei Kaisha limited for TPDPES.
TCPZ:11 wt.% FIrpic (4.75 nm)/BmPyPB (40 nm)/LiF (0.5 nm)/Al (100 nm).
composed of blue and red phosphorescence and green fluo-
rescence. It is unique since most of ever reported fluores-
cent/phosphorescent hybrid white OLEDs utilize green
and red (or orange) phosphorescence and blue fluorescence
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