Peripheral modification of 1,3,5-triazine based electron-transporting host materials for sky blue, green, yellow, red, and white electrophosphorescent devices

Article abstract of DOI:10.1039/c2jm31904g

A systematic comparison of the physical properties and the dual-role (host and electron transport) applications of star-shaped 1,3,5-triazine-based ET-type hosts (T2T, 3N-T2T, 3P-T2T, and oCF3-T2T) with differential peripheral groups was reported. The introduction of N-heterocyclic polar peripheries onto a 1,3,5-triazine core gave evident benefits to the electron injection/transport properties, rendering efficient PhOLEDs with a simpler device configuration feasible. Among these hosts, 3P-T2T can serve both as a promising host and electron-transport material for various Ir-based electrophosphorescence devices. These PhOLEDs configured with the same device structure exhibited low operation voltages with a maximum η_ext of 8%, 15.7%, 16.9%, 16.4% and 10.8% for sky blue (FIrpic), green [(PPy)₂Ir(acac)], yellow [(Bt)₂Ir(acac)], red [(Mpq)₂Ir(acac)] and white [FIrpic + 0.5 wt%(Mpq)₂Ir(acac)], respectively.

Full text of DOI:10.1039/c2jm31904g

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PAPER
Peripheral modification of 1,3,5-triazine based electron-transporting host
materials for sky blue, green, yellow, red, and white electrophosphorescent
devices†
a
a
a
b
Hsiao-Fan Chen, Ting-Chih Wang, Shih-Wei Lin, Wen-Yi Hung, Hung-Chi Dai,b Hao-Chih Chiu,b
*
Ken-Tsung Wong, Meng-Huan Ho,c Ting-Yi Cho,c Chieh-Wei Chenc and Chung-Chun Leec
a
*
Received 27th March 2012, Accepted 7th June 2012
DOI: 10.1039/c2jm31904g
A systematic comparison of the physical properties and the dual-role (host and electron transport)
applications of star-shaped 1,3,5-triazine-based ET-type hosts (T2T, 3N-T2T, 3P-T2T, and oCF3-T2T)
with differential peripheral groups was reported. The introduction of N-heterocyclic polar peripheries
onto a 1,3,5-triazine core gave evident benefits to the electron injection/transport properties, rendering
efficient PhOLEDs with a simpler device configuration feasible. Among these hosts, 3P-T2T can serve
both as a promising host and electron-transport material for various Ir-based electrophosphorescence
devices. These PhOLEDs configured with the same device structure exhibited low operation voltages
with a maximum hext of 8%, 15.7%, 16.9%, 16.4% and 10.8% for sky blue (FIrpic), green
[(PPy)2Ir(acac)], yellow [(Bt)2Ir(acac)], red [(Mpq)2Ir(acac)] and white [FIrpic + 0.5 wt%
(Mpq)2Ir(acac)], respectively.
reduction of production costs. One of the possible solutions is to
Introduction
design bifunctional host molecules. For example, host materials
Phosphorescent light-emitting devices (PhOLEDs) have received
with a preference for hole-transport (HT)4 or electron-transport
great attention for over a decade because they can effectively
(ET)5–7 are of great potential to achieve cost-effective PhOLEDs.
harvest electro-generated singlet and triplet excitons to achieve a
The advantage of using such host materials is that the device can
theoretical 100% internal quantum efficiency.1 One common
be free of a hole-transport layer (HTL) or an electron-transport
strategy utilized to obtain highly efficient PhOLEDs is to
layer (ETL). This strategy will reduce the complications of device
suppress the detrimental effects of transition metal-centered
fabrication and thus is beneficial to lower the production costs.
phosphors, such as aggregation quenching (triplet–triplet anni-
Along this line, some interesting host materials capable of
hilation), by adapting suitable host materials.2 Usually, the
transporting electrons have been developed and have served the
criteria for suitable host materials are (i) a higher triplet energy
dual-role of host as well as ETL.6 In addition to those excellent
than that of the guest molecule to prevent exothermic reverse
examples, we have previously reported a series of 1,3,5-triazine-
based ET-type hosts for PhOLEDs.7 With T2T (Scheme 1) as the
energy transfer enabling triplet excitons to be effectively confined
within the emitting layer;3 (ii) appropriately aligned energy levels
host for green phosphor, (PPy)2Ir(acac), and TPBI as the ETL,
relative to those of the neighboring active layers for better charge
the PhOLED exhibited an external quantum efficiency and
injection, giving devices with low operating voltage; (iii) excellent
power efficiency of 17.5% and 59 lm Wꢀ1, respectively. Since T2T
and balanced charge carrier transport properties to increase
possesses good electron mobility (me ¼ 1.2 ꢁ 10ꢀ4 cm2 Vꢀ1 sꢀ1), it
charge flux and thus enhance the propensity for electron–hole
was reasonably utilized as an ET material. However, the device
efficiencies dropped greatly (9.7% and 14.7 lm Wꢀ1) as T2T was
recombination, rendering high efficiency. Typically, the device
configuration of an efficient PhOLED comprising a judicious
used to replace the typical ET material TBPI. This result indi-
selection of active materials is very sophisticated, impeding the
cated that the ET and/or electron injection ability of T2T is
insufficiently good despite its high electron mobility. Taking the
aDepartment of Chemistry, National Taiwan University, Taipei 10617,
Taiwan. E-mail: kenwong@ntu.edu.tw; Fax: +886 2 33661667; Tel:
+886 2 33661665
bInstitute of Optoelectronic Sciences, National Taiwan Ocean University,
advantage of the high electron mobility of 1,3,5-triazine-centered
molecules, we report herein our peripherally modified 1,3,5-
triazine-based ET-type hosts by incorporating different polar
aromatic groups, namely 1-pyrazolyl and o-trifluorophenyl, onto
the three meta-positions of C3-symmetry 2,4,6-triphenyl-1,3,5-
triazine, giving 3P-T2T and oCF3-T2T, respectively. Another
heteroarene (2-pyridyl) substituted counterpart, (3N-T2T),5
Keelung, 20224, Taiwan. E-mail: wenhung@mail.ntou.edu.tw
cAdvanced Research Center, AU Optronics, Science-Based Industrial Park,
Hsinchu 30078, Taiwan
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c2jm31904g
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synthesized in 62% yield via Suzuki-coupling of 2-(tri-
fluoromethyl)phenylboronic acid9 and 1.
Thermal properties
The morphological properties and thermal stabilities of 3N-T2T,
3P-T2T, and oCF3-T2T were studied by differential scanning
calorimetry (DSC) and thermogravimetric analysis (TGA),
respectively. The data, together with previously reported T2T,
are summarized in Table 1. Notably, due to the highly twisted
nature, oCF3-T2T only exhibited a well-defined glass transition
temperature (Tg) at 75 ꢂC in the DSC scan up to 400 ꢂC. In
contrast, 3N-T2T and 3P-T2T showed no distinct phase transi-
ꢂ
tion below their melting points of 391 and 352 C, respectively.
The variation of their morphological properties might be closely
related to the different electronic natures of the molecules
imparted by the polar peripheral substituents. Compounds 3N-
T2T, 3P-T2T, and oCF3-T2T showed good thermal stabilities
with decomposition temperatures (Td, corresponding to 5%
weight loss) in the range of 313–391 ꢂC. A high durability toward
temperature is desirable for host materials in PhOLEDs because
it prevents morphological changes and suppresses the formation
of aggregates upon heating.
Photophysical and electrochemical properties
Scheme 1 Synthesis of 3P-T2T and oCF3-T2T and the chemical struc-
tures of T2T and 3N-T2T.
Fig. 1 presents the UV-visible absorption and photoluminescence
(PL) spectra of the solutions (CH2Cl2) and solid thin films. The
photophysical data are summarized in Table 1. In solution, all
compounds exhibited similar absorption maxima in a range of
266–274 nm while broadened spectra were observed in the solid
thin films, presumably due to the different dielectric surroun-
dings in the solid state. The similar absorption maxima indicate
that the different peripheries do not significantly alter the effec-
tive conjugation.
firstly reported by Kido et al. was also included in this contri-
bution for comparison. We anticipated that the introduction of
different aryl groups would allow the fine-tuning of the photo-
physical properties, triplet energies, energy levels, and electron
transport properties. We discovered that 3P-T2T can serve as a
universal host for various heavy metal-containing RGB phos-
phors as well as an efficient electron-transport material, giving
electrophosphorescence with remarkable efficiencies. The devices
using 3P-T2T as host and ETL achieved maximum external
quantum efficiencies (hext) up to 7.8% (3.7 V) for sky blue, 15%
(4.1 V) for green, 16.5% (4.1 V) for yellow, and 13.7% (4.6 V) for
red-emitting PhOLEDs. In addition, a two-emitter based white
OLED utilizing an ET-type host 3P-T2T achieved a maximum
hext of 10.5%. The practical uses of ET-type hosts in various
PhOLEDs were successfully demonstrated, giving devices with
high quantum efficiencies and low operating voltages at high
brightness even without using a complicated p–i–n junction
technique.
Compounds 3N-T2T, 3P-T2T, and oCF3-T2T exhibited deep
blue emissions in CH2Cl2 solutions with PL peaks located in the
range 374–409 nm. In thin films, no emission was detected for
3N-T2T, 3P-T2T, and oCF3-T2T, indicating that self-quenching
is severely enhanced by polar peripheries. The triplet energies
(ET) of 3N-T2T, 3P-T2T, and oCF3-T2T were calculated as 2.82,
2.80, and 2.79 eV, respectively, referring to the highest energy
vibronic sub-band of the phosphorescence spectra measured at
77 K (EtOH). The high triplet energies of these host materials
suggest their suitability for hosting various RGB phosphors. The
electrochemical properties of T2T, 3N-T2T, 3P-T2T, and oCF3-
T2T were examined by cyclic voltammetry (CV) as shown in
Fig. 2. The measurements were performed in THF with ferrocene
as the internal reference. All compounds exhibited reversible
reduction peaks which can be unambiguously assigned to the
reductions of 1,3,5-triazine cores. LUMO levels were obtained
from reduction potentials using published correlations,10
whereas HOMO levels were calculated via HOMO ¼ LUMO ꢀ
Etransport (where Etransport is the transport gap energy, obtained
from the optical gap with published correlation9). 3N-T2T, 3P-
T2T, and oCF3-T2T possess slightly stabilized LUMO levels as
compared with that of T2T, indicating the electron-withdrawing
nature of the peripheral substituents.
Results and discussion
Synthesis
Scheme 1 depicts the synthetic pathway toward new 1,3,5-
triazine-based ET-type host molecules, 3P-T2T and oCF3-T2T.
The bromo-substituted 1,3,5-triazine intermediate
1 was
obtained according to our previous report.7 The C–N bond
formation between 1H-pyrazole and 1 catalyzed by a copper/iron
co-catalyst gave 3P-T2T with 81% yield.8 oCF3-T2T was
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Table 1 Physical properties of T2T, 3N-T2T, 3P-T2T, and oCF3-T2T
labs
a
b
Tg/Tc/Tm (ꢂC) Td (ꢂC) E1/2
(V) (nm) [sol./film] lPL (nm) [sol./film] ET (eV) Egc (eV) HOMO/LUMOd/Etransport (eV)
Red
T2T
3N-T2T
3P-T2T
95/137/204
352
391
352
313
ꢀ2.13
ꢀ2.08
ꢀ2.06
ꢀ2.04
270/270
274/277
266/268
274/276
380/400
2.80
2.82
2.80
2.79
3.56
3.55
3.45
3.55
ꢀ6.74/ꢀ2.25/4.49
ꢀ6.77/ꢀ2.30/4.47
ꢀ6.67/ꢀ2.33/4.34
ꢀ6.82/ꢀ2.35/4.47
n.d.e/n.d./255
n.d./n.d./250
oCF3-T2T 75/n.d./n.d.
359, 374/n.d.
395/n.d.
409/n.d.
a
Reduction potential vs. ferrocene/ferrocenium redox couple. b The excitation wavelength was set to the absorption maximum. c Calculated from the
d
absorption onset of the solid film. LUMO energy level calculated from the reduction potential with published correlation. HOMO ¼ LUMO ꢀ
Etransport, where Etransport is obtained from Eg with published correlation. e Not detected.
Electron transport
T2T showed excellent electron mobility [me ¼ 1.2 ꢁ 10ꢀ4 cm2
Vꢀ1sꢀ1, determined by time-of-flight (TOF) technique]7 which is
10-fold higher than that of the widely used ET material tris(8-
hydroxyquinoline) aluminium (Alq3). We attempted to acquire
the electron mobilities of 3N-T2T, 3P-T2T, and oCF3-T2T via
the TOF technique. However, we could not observe transient
photocurrent signals to evaluate their electron mobilities.
Therefore, we fabricated electron-only devices with a device
structure of ITO/BCP (30 nm)/compound (50 nm)/LiF/Al to
investigate the electron-transport behaviour. Fig. 3 depicts the
current vs. voltage characteristics of the electron-only devices.
The introduction of BCP on top of ITO is to limit the injection of
hole carriers. Surprisingly, the turn-on voltages for the devices
with a 3N-T2T or 3P-T2T host were almost half those of the
T2T-based devices. The noteworthy decreases in driving voltages
indicate that 3N-T2T and 3P-T2T are even more efficient for
electron-transport and/or injection than T2T, which has been
shown to have high electron mobility in our previous study. The
great improvement of the electron-transport properties of 3N-
T2T and 3P-T2T is attributed to the introduction of N-hetero-
cyclic polar peripheries, pyridyl or pyrazolyl groups, which are
known as electron-withdrawing heteroaromatic groups.
Fig. 1 Room-temperature absorption and emission (PL) spectra of T2T,
However, simply attaching –CF3 to the outer phenylene rings
3N-T2T, 3P-T2T, and oCF3-T2T in CH2Cl2 solutions and as neat films
(oCF3-T2T) did not present any improvement in the electron-
only device, indicating that the introduction of N-heteroarene is
and the corresponding phosphorescence (Phos) spectra recorded from
their EtOH solutions at 77 K.
Fig. 2 Cyclic voltammograms of T2T, 3N-T2T, 3P-T2T, and oCF3-T2T
in THF and 0.1 M TBAClO4 was used as supporting electrolyte. A glassy
carbon electrode was used as the working electrode; scan rate 100 mV sꢀ1
Fig. 3 Current density–voltage (I–V) characteristics of electron-only
.
devices.
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advantageous for the electron-transport properties of the bulk
material over electron-withdrawing groups such as CF3.
Electroluminescent properties
We employed a relatively simple configuration for fabricating
OLED devices: ITO/polyethylene dioxythiophene: polystyrene
sulfonate (PEDOT:PSS, 30 nm)/4,40-bis[N-(1-naphthyl)-N-
phenyl]biphenyldiamine (NPB, 20 nm)11/4,40,400-tri(N-carbazolyl)-
triphenylamine (TCTA, 5 nm)12/1,3,5-triazine: dopant (25 nm)/
1,3,5-triazine (50 nm)/LiF (0.5 nm)/Al; NPB/TCTA functioned
as the HT layers, while the 1,3,5-triazine-based compound
functioned as the host and ET materials.0Four phosphors: blue
[bis(4,6-difluorophenyl)-pyridinato-N,C2 ]iridium(III)picolinate
(FIrpic),13 green bis(2-phenylpyridinato)iridium(III)(acetylaceto-
nate) [(PPy)2Ir(acac)],14 orange bis(2-phenylbenzothiazolato)iri-
dium(III)(acetylacetonate) [(Bt)2Ir(acac)],15 and red (Mpq)2Ir(acac),16
respectively, are used in this study (Scheme 2). A larger energy
gap between the HOMO energy levels of TCTA (ꢀ5.6 eV) and
1,3,5-triazine (ꢀ6.67 to ꢀ6.82 eV) suggests that holes would be
injected from TCTA into the dopants directly (Scheme 2). The
electrons injected from the 1,3,5-triazine layer were trapped by
the dispersed dopants, recombining with the holes to generate
excitons. The high triplet energy of the host allows the
efficient confinement of the emissive excitons within the emissive
layer.
The performance of green phosphorescent devices (devices
G1–G4) made up of (PPy)2Ir(acac) doped in host T2T, 3N-T2T,
3P-T2T and oCF3-T2T used as the emitting layer, respectively,
were examined. Fig. 4 shows the current density–voltage–
brightness (J–V–L) characteristics and efficiency versus bright-
ness of these devices, and the EL data are summarized in Table 2.
As shown in Fig. 4(a), the operating voltages of the devices based
on 3N-T2T (G2) and 3P-T2T (G3) are much lower than those of
the devices based on T2T (G1) and oCF3-T2T (G4), which are
consistent with the electron-only device results observed in
Fig. 3. The operating voltage at 1000 cd mꢀ2 of device G2 (3.5 V)
is slightly lower than that of the device G3 (4.3 V). Moreover,
Fig. 4 (a) J–V–L characteristics and (b) plots of EL efficiency versus
brightness for host: (PPy)2Ir(acac) devices.
device G2 achieved a maximum brightness (Lmax) of 97 600 cd
mꢀ2 at 7 V, a maximum external quantum efficiency (hext) of
11.3% corresponding to a current efficiency (hc) of 42.3 cd Aꢀ1
and a maximum power efficiency (hp) of 42.2 lm Wꢀ1, while
device G3 showed a Lmax of 103 700 cd mꢀ2 at 9.5 V, a maximum
hext of 15.7% corresponding to hc of 56.6 cd Aꢀ1 and a maximum
hp of 71.6 lm Wꢀ1 (Table 2). The higher electron injection/
transportation properties for 3N-T2T may be the reason leading
to the lower device performances because of the poor charge
balance in the emissive layer. The low operating voltage can be
attributed to the introduction of pyridine or pyrazoline as the
peripheries of the 1,3,5-triazine core, giving a low-lying LUMO
level. In addition, coordination effects between the pyridine or
pyrazoline peripheries and the Li cations may further reduce
their LUMO levels at the interface of the ETL and cathode,
giving excellent electron injection and thus reducing the oper-
ating voltages.17 Furthermore, the operating voltages of
conventional PhOLEDs can be reduced by using a well-designed
ET-type host (functions both as host and ETL) to lower the
electron injection barriers (EML/ETL) without using compli-
cated p–i–n junction techniques.
In addition, green PhOLEDs, 3N-T2T, 3P-T2T and oCF3-
T2T were also applied in different dopants (from sky blue to red).
The triplet energies of 3N-T2T (ET ¼ 2.82 eV), 3P-T2T (ET
¼
2.80 eV) and 3P-T2T (ET ¼ 2.79 eV) are higher than that of the
sky blue phosphor FIrpic (ET ¼ 2.62 eV) and other long-wave-
length phosphorescent dopants, implying effective confinement
of the triplet excitons on the guest by preventing the reverse
Scheme 2 Molecular structures of the dopants used in this study and an
energy level diagram of the device.
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Table 2 EL performance of devices as a function of the host and emitter
Imax/mA
cmꢀ2
hc/cd hp/lm
Wꢀ1
Host: 10% dopantb
Vonc/V Lmax/cd mꢀ2
hext/% hext at 103 nit/% Aꢀ1
CIE1931 (x, y)
G1a T2T: (PPy)2Iracac
2
2
2
2
2
2
2
2
2
2
2
3
3
3
3
49 500 (18 V)
21 900 (7.25 V)
97 600 (7 V)
630
600
9.7
8.3
11.3
10.4
7.7
8.5 (10.5 V)
8.2 (3.6 V)
10.8 (3.5 V)
10.2 (3.6 V)
7.6 (3.8 V)
9.9 (3.5 V)
7.8 (3.7 V)
15 (4.1 V)
16.5 (4.1 V)
13.7 (4.6 V)
10.5 (3.7 V)
2.6 (20 V)
36.3
21.3
42.3
27.8
8.8
16.3
21.0
56.6
41.6
15.8
22.3
20.5
61.1
33.7
8.7
14.7
20.0
42.2
28.8
10.8
16.9
19.1
71.6
45.8
20.0
23.2
13.9
32.6
13.9
3.6
0.32, 0.64
0.22, 0.45
0.34, 0.62
0.53, 0.47
0.65, 0.35
0.37, 0.41
0.29, 0.48
0.34, 0.62
0.53, 0.47
0.66, 0.34
0.40, 0.41
0.20, 0.35
0.32, 0.64
0.52, 0.48
0.64, 0.35
B2
G2
Y2
R2
W2
B3
G3
Y3
R3
W3
B4
G4
Y4
R4
3N-T2T: FIrpic
3N-T2T: (PPy)2Iracac
3N-T2T: (Bt)2Iracac
3N-T2T: (Mpq)2Iracac
3N-T2T: FIrpic + 0.5%(Mpq)2Iracac
3P-T2T: FIrpic
3P-T2T: (PPy)2Iracac
3P-T2T: (Bt)2Iracac
3P-T2T: (Mpq)2Iracac
3P-T2T: FIrpic + 0.5%(Mpq)2Iracac
oCF3-T2T: FIrpic
1130
1160
1030
1090
2250
1680
62 300 (7 V)
16 000 (7.25 V)
25 700 (7.5 V)
48 800 (9.5 V)
103 700 (9.5 V)
9.9
8.0
15.7
16.9
16.4
10.8
9.7
16.3
12.4
7.5
79 800 (10.25 V) 2100
23 700 (12.5 V)
45 100 (10 V)
4970 (29 V)
16 600 (29.5 V)
11 600 (30 V)
2200 (29.5 V)
1800
1900
220
100
120
oCF3-T2T: (PPy)2Iracac
oCF3-T2T: (Bt)2Iracac
oCF3-T2T: (Mpq)2Iracac
11.4 (17.5 V)
9.3 (18.5 V)
2.7 (23.5 V)
130
a
The notation 1–4 indicates the devices fabricated with host materials of T2T, 3N-T2T, 3P-T2T and oCF3-T2T, respectively. b Device configuration:
ITO/PEDOT:PSS (30 nm)/NPB (20 nm)/TCTA (5 nm)/host: dopant (25 nm)/host (50 nm)/LiF (0.5 nm)/Al (100 nm). c Turn-on voltage at a brightness of
10ꢀ2 cd mꢀ2
.
energy transfer from the guest to the host. Fig. 5 and 6 depict the
current density–voltage–luminance (J–V–L) characteristics,
device efficiencies, and EL spectra of the devices incorporating
3N-T2T and 3P-T2T, respectively. Table 2 summarizes the
electroluminescence data obtained. All of the devices in this
study exhibited a low turn-on voltage of 2 V (defined as the
voltage at a brightness of 10ꢀ2 cd mꢀ2). The devices hosted by 3N-
T2T with different dopants still exhibited significantly lower
operating voltages than that of 3P-T2T.
(PPy)2Ir(acac) film clearly exhibits a single exponential decay and
a relatively short lifetime of 0.5 ms, indicating that the triplet
energy transfer from (PPy)2Ir(acac) to 3P-T2T was completely
suppressed and the energy was well confined and thus an
improved performance of the green phosphorescent OLEDs.
In addition, yellow-emitting PhOLEDs were fabricated using
(Bt)2Ir(acac) as the dopant. Excellent performance of orange
electrophosphorescence was achieved for device Y3, which dis-
played substantially higher efficiencies (16.9%, 41.6 cd Aꢀ1, and
45.8 ml Wꢀ1) than those of device Y2 (10.4%, 27.8 cd Aꢀ1, and
28.8 ml Wꢀ1). Moreover, in red devices using (Mpq)2Ir(acac) as
the dopant, the device R3 also exhibited higher efficiencies
(16.4%, 15.8 cd Aꢀ1, and 20 lm Wꢀ1) relative to that of device R2
(7.7%, 8.8 cd Aꢀ1, and 10.8 lm Wꢀ1). It is worth mentioning that
the maximum efficiencies in our devices appeared to occur in the
more useful brightness range of 100–1000 cd mꢀ2, and low
operating voltages (<4.6 V) at 1000 cd mꢀ2. The devices displayed
relatively pure emission from green to red without long wave-
length emission tails as shown in Fig. 5(c) and 6(c), indicating
that 3N-T2T and 3P-T2T as ETL can effectively prevent exciton
diffusion except the blue ones. Therefore, 3N-T2T and 3P-T2T
can effectively contribute to give lower operating voltages and
higher efficiencies with simpler device structures, in which the
host materials also serve as the ETL for multi-colour PhOLEDs.
In contrast, all of the devices incorporating oCF3-T2T as the host
exhibited lower power efficiencies (see Fig. S1 in the ESI† and
Table 2), which is due to its low electron transport properties.
We also fabricated two-emitter WOLEDs (device W2 and W3)
utilizing 10 wt% FIrpic and 0.5 wt% (Mpq)2Ir(acac) co-doped
into either 3N-T2T or 3P-T2T as a single emissive layer. The
proportion of (Mpq)2Ir(acac) required to produce balanced
white emission in these devices was relatively low because the red
emission is from either efficient energy transfer from the blue
phosphor or direct exciton formation through charge-trapping
on the red dopant.20 Device W2 hosted by 3N-T2T achieved a
Lmax of 25 700 cd mꢀ2 at 7.5 V (1090 mA cmꢀ2), and EL effi-
ciencies of 9.9%, 16.3 cd Aꢀ1, and 16.9 lm Wꢀ1; device W3 hosted
The performances of the FIrpic-based sky blue devices were
then examined. In the blue devices, device B2 based on 3N-T2T
showed a Lmax of 21 900 cd mꢀ2 at 7.25 V (600 mA cmꢀ2), a
maximum hext of 8.3% corresponding to hc of 21.3 cd Aꢀ1 and hP
of 20 lm Wꢀ1, while device B3 based on 3P-T2T achieved a Lmax
of 48 800 cd mꢀ2 at 9.5 V (2250 mA cmꢀ2), a maximum hext of 8%
corresponding to hc of 21 cd Aꢀ1 and hP of 19.1 lm Wꢀ1 (Table 2).
In addition, device B2 and B3 at 1000 cd mꢀ2 exhibited a rather
low operating voltage and still retain a high efficiency of hext
¼
8.2% and hext ¼ 7.8% at 3.6 and 3.7 V, respectively, which are
much lower than previously reported values based on FIrpic.18
As depicted in Fig. 5(c) and 6(c), electroluminescence (EL)
spectra originated from the triplet emission of FIrpic exhibited
long tails as compared with the typical emission of FIrpic-based
devices. The long tail emission can be attributed to inefficient
energy transfer from the host to the blue phosphor and/or blue
exciton diffusion to the ETL. To further verify the exciton-
confinement behaviour of the host, transient photoluminescence
decays of the thin films with 10 wt% FIrpic dispersed in 3N-T2T
and 3P-T2T, and 10 wt% (PPy)2Ir(acac) doped into 3P-T2T were
measured (Fig. 7). The emissions of FIrpic-doped 3N-T2T and
3P-T2T films are not clear single exponential decays with life-
times of 1.08 and 1.05 ms, respectively, indicating the possibility
of triplet energy transferring from FIrpic to 3N-T2T or 3P-T2T
in the emissive layer.19 Hence, the long tail emission may be
attributed to exciton diffusion out of the emissive layer toward
the electron-transport layer of 3N-T2T or 3P-T2T in FIrpic-
doped blue phosphorescent OLEDs. In contrast, the 3P-T2T:
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Fig. 5 (a) Current density-voltage-luminance (J–V–L) characteristics,
(b) external quantum (hext) and power efficiencies (hP) as a function of
brightness, and (c) EL spectra for devices incorporating 3N-T2T as host.
Fig. 6 (a) Current density–voltage–luminance (J–V–L) characteristics,
(b) external quantum (hext) and power efficiencies (hP) as a function of
brightness, and (c) EL spectra for devices incorporating 3P-T2T as host.
by 3P-T2T achieved a Lmax of 45 100 cd mꢀ2 at 10 V (1900 mA
cmꢀ2), and EL efficiencies of 10.8%, 22.3 cd Aꢀ1, and 23.2 lm
Wꢀ1. The efficiencies of device W3 were slightly higher than those
of the device W2, which was probably because device W3
exhibited a higher ratio of red emission. In addition, when the
brightness reached 1000 cd mꢀ2, the efficiencies of devices W2
and W3 were still as high as 9.9 (3.5 V) and 10.5% (3.7 V),
respectively, with a rather low operating voltage.
Fig. 5(c) and 6(c) depict the EL spectra of device W2 and W3 at
different operation voltages. As the operation voltage increased,
the relative intensity of blue emission increased, leading to a
slight shift in the CIE coordinates. This is due to the low
concentration of (Mpq)2Ir(acac) in the blend, which results in the
partial saturation of emission from (Mpq)2Ir(acac) and the easier
formation of blue emission at higher voltage.21 The EL spectra of
devices W2 and W3 exhibited main emission peaks at 473, 501
and 610 nm, which were assigned to the emission from FIrpic and
(Mpq)2Ir(acac), respectively. Due to the tail emission of FIrpic,
Fig. 7 Transient photoluminescence decay of 3N-T2T and 3P-T2T:10
wt% FIrpic as well as 3P-T2T:10 wt% (PPy)2Ir(acac) under excitation by
a nitrogen laser (l ¼ 337 nm, 10 Hz, 700 ps pulses).
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the color-rendering indices (CRI) of devices W2 and W3 were
calculated to be as high as 75.5–70, which are higher than those
of typical two-colour white OLEDs.22
simultaneously in a glove-box using a Keithley 6430 source meter
and a Keithley 6487 picoammeter equipped with a calibration Si-
photodiode. EL spectra were measured using a photodiode array
(Ocean Optics USB2000).
Conclusions
In conclusion, we have examined a new series of 1,3,5-triazine-
based ET-type host materials containing different polar aromatic
groups as the peripheral groups. The relationships between their
molecular structures and thermal, electrochemical, and photo-
physical properties were addressed. Apparently, the introduction
of pyridine and pyrazoline as peripheries of 1,3,5-triazine showed
excellent electron injection/transport properties, which allow us
to adopt a relatively simple device architecture using 3N-T2T and
3P-T2T as host and ETL for different color electro-
phosphorescence devices. These devices exhibited very low
operation voltages and achieved high EL efficiencies. For those
PhOLEDs, the better efficiency was achieved by using 3P-T2T to
give a maximum hext of 8%, 15.7%, 16.9%, 16.4% and 10.8% for
sky blue (FIrpic), green [(PPy)2Ir(acac)], yellow [(Bt)2Ir(acac)],
red [(Mpq)2Ir(acac)] and white [FIrpic+ 0.5 wt%(Mpq)2Ir(acac)],
respectively, under the same device structure. Taking advantage
of the dual functions of ET-type host materials, PhOLEDs
possessing lower operating voltages and high efficiencies with a
simpler device structure can be successfully achieved. We believe
that our molecular design concept may trigger new ideas for the
development of bifunctional host materials, being beneficial for
suppressing the production costs of PhOLEDs.
Acknowledgements
We greatly appreciate the financial support from the National
Science Council of Taiwan (NSC 98-2119-M-002-007-MY3 and
100-2112-M-019-002-MY3).
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