1,3,5-Triazine derivatives as new electron transport-type host materials for highly efficient green phosphorescent OLEDs

Article abstract of DOI:10.1039/b913423a

We have synthesized three star-shaped 1,3,5-triazine derivatives - 2,4,6-tris(biphenyl-3-yl)-1,3,5-triazine (T2T), 2,4,6-tris(triphenyl-3-yl)-1,3, 5-triazine (T3T), and 2,4,6-tris(9,9′-spirobifluorene-2-yl)-1,3,5-triazine (TST) - as new electron transport

Full text of DOI:10.1039/b913423a

View Article Online / Journal Homepage / Table of Contents for this issue
PAPER
www.rsc.org/materials | Journal of Materials Chemistry
1,3,5-Triazine derivatives as new electron transport–type host materials for
highly efficient green phosphorescent OLEDs†
a
a
b
b
Hsiao-Fan Chen, Shang-Jung Yang, Zhen-Han Tsai, Wen-Yi Hung, Ting-Chih Wanga
*
a
*
and Ken-Tsung Wong
Received 7th July 2009, Accepted 27th August 2009
First published as an Advance Article on the web 21st September 2009
DOI: 10.1039/b913423a
We have synthesized three star-shaped 1,3,5-triazine derivatives—2,4,6-tris(biphenyl-3-yl)-1,3,
5-triazine (T2T), 2,4,6-tris(triphenyl-3-yl)-1,3,5-triazine (T3T), and 2,4,6-tris(9,90-spirobifluorene-2-yl)-
1,3,5-triazine (TST)—as new electron transport (ET)-type host materials for green phosphorescent
organic light-emitting devices. The morphological, thermal, and photophysical properties and the
electron mobilities of these ET-type host materials are influenced by the nature of the aryl substituents
attached to the triazene core. The meta–meta linkage between the 1,3,5-triazine core and the peripheral
aryl moieties in T2T and T3T limited the effective extension of their p conjugation, leading to high
triplet energies of 2.80 and 2.69 eV, respectively. Time-of-flight mobility measurements revealed the
good electron mobilities for these compounds (each > 10ꢀ4 cm2 Vꢀ1 sꢀ1), following the order T3T > TST
> T2T. The device incorporating T2T as the host, doped with (PPy)2Ir(acac) and 1,3,5-tris(N-
phenylbenzimidizol-2-yl)benzene (TBPI) as the ET layer, achieved a high external quantum efficiency
(hext) of 17.5% and a power efficiency (hp) of 59.0 lm Wꢀ1. For the same device configuration, the
T3T-based device provided values of hext and hp of 14.4% and 50.6 lm Wꢀ1, respectively; the TST-based
device provided values of 5.1% and 12.3 lm Wꢀ1, respectively. We ascribe the superior performance of
the T2T-based devices to balanced charge recombination; we ascribe the poor efficiencies of the
TST-based devices to its relatively low triplet energy (2.54 eV), which did not allow efficient
confinement of the triplet excitons on the green phosphorescent emitter (PPy)2Ir(acac).
and (iv) decent charge carrier transport properties (to achieve
Introduction
a low operating voltage). In the development of organic materials
used for optoelectronics, it is natural to adopt a high-triplet-
energy host with preference for hole transport (HT) rather than
electron transport (ET). One common example is the use of
carbazole-based derivatives,3b–c,4 which usually possess high
triplet energies and good HT ability.4a,5 Thus, most host materials
that have been reported previously for use in PhOLEDs exhibit-
ing excellent device characteristics possess the structural features
of a carbazole. Progress in the development of ET-type host
materials has lagged far behind that of their HT counterparts.
Poor charge carrier mobility and unbalanced charge recom-
bination in the emitting layer are detrimental to the power effi-
ciency of OLEDs.6 To improve the carrier drift mobility and
achieve good charge balance, ET-type high-triplet-energy host
materials with low electron injection barriers, capable of hole-
blocking, and high electron mobilities are promising candidates
for reducing the operating voltage and improving the device
performance. In addition to the triplet energy, suitable thermal
stability and energy levels are also critical when designing ET
materials. The designed molecules should form morphologically
stable and uniform amorphous films when using typical
processing techniques. The use of electron-deficient heteroarene-
embedded small molecules has proved very useful for improving
balanced charge injection and subsequent recombination in
OLEDs.7 Triazine has an electron affinity (EA) larger than
those of other typical electron-deficient heteroaromatic rings
(e.g., pyridine, pyrimidine). In addition, it is reasonably
Phosphorescent light-emitting diodes (PhOLEDs) have attracted
considerable attention because they effectively harvest electro-
generated singlet and triplet excitons to achieve nearly 100%
internal quantum efficiency.1 The prospective use of PhOLEDs in
display and lighting technologies has led to a tremendous amount
of research effort being devoted to the development of novel
materials and sophisticated device configurations. One strategy
toward highly efficient PhOLEDs is to suppress the detrimental
effects of transition metal-centered phosphors, such as aggrega-
tion quenching and triplet–triplet annihilation, by employing
suitable host materials that can accommodate the emitters
homogeneously.2 Regardless of the emitting colors, the host
materials should possess (i) triplet energies higher than those of
the guest molecules (to prevent exothermic reverse energy trans-
fer),3 (ii) good charge balance properties (to effectively confine
triplet excitons within the emitting layer), (iii) energy levels
appropriately aligned with those of the neighboring active layers,
aDepartment of Chemistry, National Taiwan University, Taipei, 106,
Taiwan. E-mail: kenwong@ntu.edu.tw; Fax: +886 2 33661667; Tel:
+886 2 33661665
bInstitute of Optoelectronic Sciences, National Taiwan Ocean University,
Keelung, 202. E-mail: wenhung@mail.ntou.edu.tw; Fax: +886
24634360; Tel: +886 2 24622192 ext. 6718
† Electronic supplementary information (ESI) available: Synthetic
2
1
procedure and spectroscopic characterizations, H, 13C NMR spectra of
compounds T2T, T3T, and TST; device characteristics of T2T, T3T
and TST-based PhOLEDs. See DOI: 10.1039/b913423a
8112 | J. Mater. Chem., 2009, 19, 8112–8118
This journal is ª The Royal Society of Chemistry 2009

View Article Online
straightforward to structurally modify the 1,3,5-triazine core. As
a result, triazine derivatives are electron-deficient heterocycles
that are employed widely as active materials in OLEDs.8 For
example, Pang et al. reported that the star-shaped compounds
2,4,6-tris(di-2-pyridylamino)-1,3,5-triazine and 2,4,6-tris[p-(di-2-
pyridylamino)phenyl]-1,3,5-triazine provide blue emissions when
incorporated in electroluminescent devices.9 Burn et al. observed
weak electroluminescence (EL) from a tetrakis(triazine) deriva-
tive; a single-layer OLED emitted blue light with an external
quantum efficiency (EQE) of 0.003%.10 Fink et al. discovered
that the use of dimeric 1,3,5-triazine ethers possessing high glass
transition temperatures (Tg) as hole-blocking and ET layers led
to increased device efficiency.11 More recently, Kang et al.
reported a hole- and exciton-blocking material possessing silane
and triazine moieties (DTBT) for use in an Ir(PPy)3-doped CBP
device (PPy ¼ phenylpyridine; CBP ¼ 4,40-bis(N-carbazolyl)
biphenyl), which exhibited a maximum EQE of 17.5% and
In this paper, we report the syntheses and physical properties
of a new series of star-shaped molecules—2,4,6-tris(biphenyl-3-
yl)-1,3,5-triazine (T2T), 2,4,6-tris(triphenyl-3-yl)-1,3,5-triazine
(T3T), and 2,4,6-tris(9,90-spirobifluorene-2-yl)-1,3,5-triazine
(TST)—and their applications as ET-type host materials in green
PhOLEDs. We introduced the various aryl substituents into
the triazine core to probe their structure–property relation-
ships. These new ET-type host materials exhibit reasonably
high triplet energies (>2.54 eV) and good ET properties (me ¼ ca.
10ꢀ4 cm2 Vꢀ1 sꢀ1), rendering them suitable for hosting green
phosphorescent emitters to realize highly efficient, low-driving
voltage PhOLEDs. When incorporating typical ET layers
[e.g., 1,3,5-tris(N-phenylbenzimidizol-2-yl)benzene (TBPI) or
3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole
(TAZ)] to facilitate electron injection and hole blocking, green
PhOLEDs featuring T2T as the host material provided high
EQEs of 17.5% for (PPy)2Ir(acac) and 17.4% for Ir(PPy)3. The
highly twisted homologue T3T, which exhibited superior
morphological stability and electron mobility, but a slightly
lower triplet energy, served as an effective ET-type host for
Ir(PPy)3, providing a green PhOLED having an EQE of 16.2%.
a maximum power efficiency of 47.8 lm Wꢀ1 12
.
Klenker
et al. reported triaryl-1,3,5-triazine derivative (BTB)
a
possessing an electron mobility of 7.2 ꢁ 10ꢀ4 cm2 Vꢀ1 sꢀ1 at a field
of 8 ꢁ 105 V cmꢀ1; OLEDs incorporating BTB as the ETL
exhibited lower driving voltages and a higher efficiencies relative
to those incorporating Alq3.13 Adachi et al. established a series of
1,3,5-triazine derivatives (TRZ) as ET-type hosts.7c In particular,
the use of 2,4,6-tris(carbazolo)-1,3,5-triazine (TRZ2) as a host
for tris(2-phenylpyridine)iridium [Ir(PPy)3] resulted in an EQE
Results and discussion
Scheme 1 outlines the syntheses of T2T, T3T, and TST. The key
intermediate, 1,3,5-tris(3-bromophenyl)triazine (1), was synthe-
sized in 83% yield according to a modification of published
of 10.2% and a power efficiency of 14 lm Wꢀ1
.
Scheme 1 Synthetic routes toward T2T, T3T, and TST.
This journal is ª The Royal Society of Chemistry 2009 J. Mater. Chem., 2009, 19, 8112–8118 | 8113

View Article Online
procedures.14 We obtained T2T and T3T in isolated yields of
99 and 98%, respectively, through Suzuki coupling of 1 with
phenylboronic acid and biphenyl-2-boronic acid, respectively,
in the presence of a catalytic amounts of Pd(PPh3)4 and
PtBu3. We obtained TST in 63% yield after treating 2-cyano-9,
90-spirobiflouorene (2)15 with
a catalytic amount of tri-
fluoromethanesulfonic acid.
We used differential scanning calorimetry (DSC) and ther-
mogravimetric analysis (TGA) to study the morphological
properties and thermal stabilities, respectively, of T2T, T3T, and
TST (Table 1). Because of their starburst structural features,16 we
observed well-defined glass transition temperatures for T2T
(95 ꢂC) and T3T (108 ꢂC). In addition, no crystallization was
evident for T3T during the second DSC scan; in contrast, we
observed a crystallization peak and a melting peak for T2T. For
TST, there is no distinct phase transition detected up to 400 ꢂC.
Compounds T2T, T3T, and TST exhibited good thermal
stabilities with decomposition temperatures (Td, corresponding
to 5% weight loss) within the range 352–478 ꢂC. A high glass
transition temperature is desirable for host materials in PhO-
LEDs because it prevents morphological changes and suppresses
the formation of aggregates upon heating.
Fig. 1 Room-temperature absorption and emission (PL) spectra of T2T,
T3T, and TST in CH2Cl2 solutions and as neat films and corresponding
phosphorescence (Phos) spectra recorded from their EtOH solutions at
77 K.
Fig. 1 presents the electronic and photoluminescence (PL)
spectra of T2T, T3T, and TST in solution (CH2Cl2) and as solid
films on a quartz slide. The photophysical data are also
summarized in Table 1. T2T exhibits a broad absorption band
centered at 270 nm in solution and as a solid film. The absorption
maximum of T3T was slightly red-shifted (lmax ¼ 275 nm)
because of the three additional peripheral phenyl rings.
The dramatically red-shifted absorption maximum of TST
(lmax ¼ 355 nm), relative to those of T2T and T3T, is consistent
with its extended p-conjugation through coplanar fluorene
moieties. Compounds T2T, T3T, and TST exhibit deep blue
emissions in CH2Cl2 solutions with PL peaks located in the range
380–393 nm. The PL spectra of T2T, T3T, and TST in their neat
films were slightly red-shifted by 10–20 nm, presumably because
of the different dielectric surroundings in the solid state. Fig. 1
displays the phosphorescence spectra of these three ET-type host
materials in EtOH at 77 K. We calculated the triplet energies (ET)
of T2T, T3T, and TST to be 2.80, 2.69, and 2.54 eV, respectively;
these values are sufficiently high for hosting green phosphores-
cent emitters, preventing possible reverse energy transfer.
corresponding HOMO/LUMO energy levels for T2T, T3T, and
TST to be ꢀ5.64/ꢀ2.08 eV, ꢀ5.71/ꢀ2.15 eV, and ꢀ5.69/ꢀ2.47 eV,
respectively (Table 1).
We performed time-of-flight (TOF) measurements to evaluate
the electron mobilities of T2T, T3T, and TST. Fig. 2(a)–(c)
display typical room-temperature TOF transients of electron for
these materials under an applied electric field; in each case, we
observe dispersive ET behavior. In the double-logarithmic
representations [insets to Fig. 2(a)–(c)], the carrier-transit time,
tT, required to determine the carrier mobilities, was extracted
from the intersection point of the two asymptotes. In Fig. 2(d),
the field-dependent electron mobilities existed in the range from
5 ꢁ 10ꢀ5 to 9 ꢁ 10ꢀ4 cm2 Vꢀ1 sꢀ1 for fields varying from 1.6 ꢁ 105
to 5.9 ꢁ 105 V cmꢀ1. The observed electron mobility (me ¼ ca.
8.7 ꢁ 10ꢀ4 cm2 Vꢀ1 sꢀ1) of T3T was higher than those of TST
(me ¼ ca. 6.8 ꢁ 10ꢀ4 cm2 Vꢀ1 sꢀ1) and T2T (me ¼ ca. 1.2 ꢁ 10ꢀ4 cm2
Vꢀ1 sꢀ1) under the same electric field (E ¼ 3.6 ꢁ 105 V cmꢀ1).
These electron mobilities are similar to those of other 1,3,
13
5-triazine derivatives (e.g., BTB: me ¼ ca. 10ꢀ4 cm2 Vꢀ1 sꢀ1
)
and are 10-fold higher than that of the widely used ET material
tris(8-hydroxyquinoline) aluminium (Alq3). The combination of
high electron mobilities and reasonably high triplet energy levels
for T2T, T3T, and TST indicate that they have potential to serve
as ET-type host materials for green PHOLEDs.
We used photoelectron yield spectroscopy (Riken AC-2) to
determine the HOMO energy levels of T2T, T3T, and TST; we
estimated the LUMO energy levels from the HOMO values and
the optical band gaps (Eg) by using the equation LUMO ¼
HOMO + Eg, where the values of Eg of T2T, T3T, and TST,
determined from the onset wavelengths of the absorption bands,
were 3.56, 3.56, and 3.22 eV, respectively. We calculated the
To evaluate the ability of the ET-type host materials T2T,
T3T, and TST to confine triplet excitons in conjunction with
corresponding dopants having values of ET of less than 2.54 eV,
Table 1 Physical Properties of T2T, T3T, and TST
a
HOMO/LUMO/Eg (eV)
b
Tg/Tc/Tm (ꢂC)
Td/ꢂC
labs/nm [sol./film]
lPL/nm [sol./film]
ET/eV
me /cm2 Vꢀ1 sꢀ1
T2T
T3T
TST
95/137/204
108/ n.d.c/ n.d.
N.d./ n.d./ n.d.
352
390
478
270/270
275/275
355/360
380/400
381/400
393/410
2.80
2.69
2.54
5.64/2.08/3.56
5.71/2.15/3.56
5.69/2.47/3.22
1.2 ꢁ 10ꢀ4
8.7 ꢁ 10ꢀ4
6.8 ꢁ 10ꢀ4
a
HOMO energy level determined using photoelectron yield spectroscopy (AC-2). LUMO ¼ HOMO + Eg; value of Eg calculated from the absorption
b
c
. not detected.
onset of the solid film. Value of me determined through TOF measurement at E ¼ 3.6 ꢁ 105 V cmꢀ1
8114 | J. Mater. Chem., 2009, 19, 8112–8118
This journal is ª The Royal Society of Chemistry 2009

View Article Online
leakages and operating voltages, limiting the overall efficiency.
To optimize the performance, we also fabricated devices
incorporating an additional hole- and exciton-blocking layer
of 1,3,5-tris(N-phenylbenzimidizol-2-yl)benzene (TPBI)4c or
3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole
(TAZ).19 The low HOMO/LUMO energy levels of TPBI and
TAZ were beneficial for blocking holes and facilitating electron
injection/transport, thereby enhancing the device performance
(Fig. 3). Table 2 summarizes the electroluminescence data of the
devices.
Fig. 4 presents the current density–voltage–luminance (I–V–L)
characteristics and EL efficiencies of the devices A1–3 [T2T
doped with (PPy)2Ir(acac) and featuring various ET layers
(ETLs)]. The I–V and L–V curves of devices A2 and A3 feature
steep increases, indicating that the TPBI or TAZ layer blocked
holes and improved the electron injection/transport properties.
Their device performances improved by a factor of 80% with
respect to that of device A1 incorporating T2T as the ETL. The
best performance was obtained for device A2 when using TPBI as
the ETL; the maximum luminance, external quantum efficiency
(hext), and power efficiency (hp) were 109,000 cd mꢀ2, 17.5%, and
59 lm Wꢀ1, respectively. The value of hext approaches the theo-
retical limit of 20% expected for phosphorescence-based OLEDs.
At a brightness of 1000 cd mꢀ2, the device efficiency remained
high (hext ¼ 16% at 5.6 V). We attribute this improved perfor-
mance to the more-balanced charge flux and better exciton
confinement within the emission layer. It appears that the
introduction of TPBI as the ET layer and the exciton blocker in
this multilayer device reduced the possibility of triplet–triplet
annihilation and, therefore, improved the efficiency and reduced
the efficiency roll-off.
Fig. 2 TOF transients for electrons: (a) T2T (thickness: 1.9 mm);
E ¼ 3.2 ꢁ 105 V cmꢀ1; (b) T3T (thickness: 2.6 mm); E ¼ 2.7 ꢁ 105 V cmꢀ1
;
(c) TST (thickness: 1.87 mm); E ¼ 3.7 ꢁ 105 V cmꢀ1. Insets: Double-
logarithmic plots of (a)–(c). (d) Electron mobilities plotted with respect
to E1/2
.
we adopted bis(2-phenylpyridinato)iridium(III) acetylacetonate
[(PPy)2Ir(acac)] and tris(2-phenylpyridinato)iridium(III)
[Ir(PPy)3] as green guests.17 Initially, we employed a relatively
simple four-layer configuration to fabricate the green PHOLED
devices: indium tin oxide (ITO)/polyethylene dioxythiophene:
poly(styrene sulfonate) (PEDOT:PSS, 30 nm)/4,40-bis[N-(1-
naphthyl)-N-phenyl]biphenyldiamine (a-NPD, 20 nm)/4,40,
400-tri(N-carbazolyl)triphenylamine (TCTA, 5 nm)/1,3,5-triazine
derivative:dopant (25 nm)/1,3,5-triazine derivative (50 nm)/LiF
(0.5 nm)/Al. Here, we inserted a thin layer of TCTA, which has
a high triplet energy (ET ¼ ca. 2.76 eV), as an exciton-blocking
layer18 at the a-NPD-1,3,5-triazine derivative interface as
a means of preventing exciton diffusion and, hence, resulting in
higher device efficiency. LiF and Al served as the electron-
injection layer and cathode, respectively. In this device configu-
ration, the 1,3,5-triazine derivative functioned as both host and
ET materials. Because triazine derivatives do not exhibit hole-
blocking effects, according to the energy level alignments in
Fig. 3, and possess high LUMO energies, which could impede
electron injection from the cathode, we anticipated that this
device configuration might function with relatively high current
Device A3 incorporating TAZ as the ETL exhibited a turn-on
voltage of 3 V and a driving voltage of 6.6 V at 1000 cd mꢀ2. We
attribute the increased driving voltage, relative to that of device
A2, to the lower electron mobility of TAZ, leading to lower
power efficiency. Device A3 also exhibited a rather high quantum
efficiency of 17.4% and a power efficiency of 48 lm Wꢀ1. Fig. 5
presents corresponding EL spectra that reveal a pure green
emission arising solely from (PPy)2Ir(acac), matching perfectly
with that reported previously,20 indicating the effective confine-
ment of carriers within the emissive layers. The (PPy)2Ir(acac)-
based green-emitting device using T2T as host possesses superior
EL performance, particularly in terms of external quantum
efficiency (hext), relative to those of our and other groups’
recently reported results (Table 3).
In addition to its (PPy)2Ir(acac)-based devices, we also used
T2T as a host material for the Ir(PPy)3-based devices B1–3 (see
Fig. S1 and S2 in the ESI†). The device B1 incorporating Ir(PPy)3
as the dopant exhibited higher efficiency (hext ¼ 15.7%) relative
to that of device A1. The devices B2 and B3 incorporating TPBI
and TAZ as ETLs, respectively, also exhibited rather high
quantum efficiencies (hext ¼ 16.3 and 15.3%, respectively) and
power efficiencies (hp ¼ 54 and 47 lm Wꢀ1, respectively).
In the series of devices C and D, we employed T3T as the host
for the (PPy)2Ir(acac)- and Ir(PPy)3-based devices, respectively
(see Fig. S3 and S4 in the ESI†). The injection barrier for
electrons at the T3T–TPBI interface (0.55 eV) was less than
that at the T2T–TPBI interface (0.62 eV), as indicated in
Fig. 3. Furthermore, the observed electron mobility (me ¼ ca.
Fig. 3 Energy levels of the HTLs, EMLs, and ETLs used in this study.
This journal is ª The Royal Society of Chemistry 2009
J. Mater. Chem., 2009, 19, 8112–8118 | 8115

View Article Online
Table 2 EL Performance of devices as a function of the emitter and ETL
Device Emittera
T2T:
ETL
T2T
Vonb/V 1000 nit (V, %) Lmax/cd mꢀ2 Imax (V)/mA cmꢀ2 hext (%, cd Aꢀ1
)
hp/lm Wꢀ1 CIE1931(x, y)
A1
A2
A3
B1
B2
B3
C1
C2
C3
D1
D2
D3
E1
E2
E3
F1
F2
F3
2.5
10.5, 8.5
5.6, 16
6.6, 16
9.5, 10.1
5.2, 14.8
5.7, 14.1
8, 7.9
49,500
109,000
85,000
68,700
97,600
97,000
90,200
162,000
165,000
34,000
105,000
80,600
44,000
124,000
84,000
23,400
27,600
31,200
630 (18)
9.7, 36.3
17.5, 56
17.4, 54
15.7, 54
16.3, 55
15.3, 52.8
8, 30.3
14.7
59
0.32, 0.64
0.33, 0.62
0.32, 0.63
0.31, 0.62
0.34, 0.61
0.32, 0.62
0.31, 0.64
0.33, 0.63
0.33, 0.63
0.35, 0.60
0.38, 0.58
0.38, 0.58
0.36, 0.62
0.36, 0.62
0.37, 0.61
0.44, 0.54
0.44,0.54
0.44, 0.54
(PPy)2Ir(acac) TPBI 2.5
10 wt%
T2T:
Ir(PPy)3
10 wt%
T3T:
(PPy)2Ir(acac) TPBI 2.5
10 wt%
T3T:
Ir(PPy)3
10 wt%
TST:
(PPy)2Ir(acac) TPBI 2.5
10 wt%
TST:
1060 (13)
920 (14)
690 (17)
TAZ
T2T
TPBI 2.5
TAZ
T3T
3
2.5
48
56
54
47
1050 (13)
950 (13)
3
2
1050 (15.5)
1830 (12)
1000 (12)
1020 (12)
2000 (10.5)
970 (11)
1220 (13)
3400 (11.5)
1300 (11.5)
780 (19.5)
740 (15)
16.2
50.6
65.2
15.6
20.8
25.8
10.3
12.3
18.9
16.2
14.5
12.1
4.6, 14.1
5.2, 15.7
8.3, 4.2
4.6, 8
5.2, 8.8
7.2, 3.5
5.1, 5
5.5, 6.1
11, 4.2
7.5, 5.1
8.4, 4.6
14.4, 53.1
16.2, 62.3
5.6, 18.4
8.3, 27.2
9.1, 30.1
4, 15.1
TAZ
T3T
TPBI 2.5
TAZ
TST
2.5
2.5
3
2
5.1, 19.4
6.1, 23.1
5.55, 16.6
5.7, 17.1
5.3, 15.6
TAZ
TST
TPBI
TAZ
2.5
2.5
3
Ir(PPy)3
10 wt%
3.5
830 (15.5)
a
Device configuration: ITO/PEDOT:PSS/a-NPD (20 nm)/TCTA (5 nm)/emitter (25 nm)/ETL (50 nm)/LiF (0.5 nm)/Al (100 nm). b Turn-on voltage at
which emission became detectable (10ꢀ2 cd mꢀ2).
Fig. 5 EL spectra of T2T doped with (PPy)2Ir(acac) and featuring
various ETLs.
Table 3 Performance of recent (PPy)2Ir(acac)-based green electro-
phosphorescent devices
hext
(%)
hp/lm
Wꢀ1
Host
Type
Lmax/cd mꢀ2
Ref.
T2T
SInF3
T2N
TPBI-Da
TPBI
CBP
ETa
109 000 (13 V)
255 000 (9.5 V)
69 000 (17 V)
48 400 (16.5 V)
78 200
17.5
15.8
12.3
15
5.7
12.3
19
59
63
38
70
33
38
60
This work
HTb
ET
21
7e
22
22
17
23
Bipolar
ET
Bipolar
ET
Fig. 4 (a) I–V–L characteristics and (b) plots of EL efficiency versus
brightness for devices incorporating T2T doped with (PPy)2Ir(acac) and
featuring various ETLs.
32 500
—
TAZ
a
electron-transporting. b hole-transporting.
8.7 ꢁ 10ꢀ4 cm2 Vꢀ1 sꢀ1) of T3T is higher than that of T2T (me ¼ ca.
1.2 ꢁ 10ꢀ4 cm2 Vꢀ1 sꢀ1) under the same electric field (E ¼ 3.6 ꢁ
105 V cmꢀ1). Thus, the T3T-based devices required lower driving
voltages than their corresponding T2T-based devices. The
devices C2 and D2 (Table 2) exhibited significantly reduced turn-
on voltages (ca. 4.6 V) required to reach a luminance of 1000 cd
mꢀ2. In contrast, all of the devices (E and F series) incorporating
TST as the host exhibited lower efficiencies and red-shifted
emissions in their EL spectra (see Figs. S5 and S6 in the ESI†).
We attribute this behavior to the lower triplet energy of TST
(ET ¼ ca. 2.54 eV), relative to those of T2T and T3T, leading to
reverse exothermic energy transfer from the guest triplet states to
the host, resulting an inefficient triplet energy confinement on the
Ir-based emitters within the emissive layers.
Fig. 6 presents the I–V–L characteristics and EL efficiencies of
devices incorporating T2T, T3T, and TST as host materials
doped with (PPy)2Ir(acac). The energy levels, triplet energies
(ET), and electron mobilities (see Table 1) of T2T, T3T, and TST
affected the device performance dramatically. The I–V charac-
teristics revealed that the device incorporating T2T as the host
8116 | J. Mater. Chem., 2009, 19, 8112–8118
This journal is ª The Royal Society of Chemistry 2009

View Article Online
in green electrophosphorescent devices. Our molecular design
imparted these new host materials with high morphological and
thermal stabilities, good electron charge transport properties
(me ¼ ca. 10ꢀ4 cm2 Vꢀ1 sꢀ1), appropriate HOMO/LUMO energy
levels, and triplet energies suitable for the formation and
confinement of excitons in the emitting layer. We employed
(PPy)2Ir(acac) and Ir(PPy)3 as green emitters dispersed into the
ET-type host materials T2T, T3T, and TST. We incorporated
various types of ETLs into the following general device config-
uration: ITO/PEDOT:PSS/a-NPD (20 nm)/TCTA (5 nm)/host
doped with green emitter (25 nm)/ETL (50 nm)/LiF (0.5 nm)/Al
(100 nm). In general, the devices incorporating (PPy)2Ir(acac) as
the emitter and TPBI as the ETL exhibited superior efficiencies.
Among these three new ET-type host materials, the devices
featuring T2T, which provided much better balanced electron/
hole recombination within the emitting layer, as the host material
exhibited superior device characteristics. The best green
PhOLED device achieved a high external quantum efficiency of
17.5% and a power efficiency of 65.2 lm Wꢀ1
.
Experimental
Fig. 6 (a) I–V–L characteristics and (b) plots of EL efficiency versus
Device fabrication and measurement
brightness for host:PPy2Ir(acac) devices incorporating TAZ as the ETL.
All chemicals were purified through vacuum sublimation prior to
use. The OLEDs were fabricated through vacuum deposition of
the materials at 10ꢀ6 torr onto ITO-coated glass substrates
having a sheet resistance of 15 U ,ꢀ1. The ITO surface was
cleaned ultrasonically—sequentially with acetone, methanol, and
deionized water—and then it was treated with UV-ozone. The
ꢀ1
ꢀ
deposition rate of each organic material was ca. 1–2 A s
.
ꢀ1
ꢀ
Subsequently, LiF was deposited at 0.1 A s and then capped
with Al (ca. 5 A sꢀ1) through shadow masking without breaking
ꢀ
the vacuum.
The I–V–L characteristics of the devices were measured
simultaneously using a Keithley 6430 source meter and a Keith-
ley 6487 picoammeter equipped with a calibration Si-photodiode
in a glovebox system. EL spectra were measured using an Ocean
Optics spectrometer.
respectively, for device structure C3. The TST host possesses
extended p-conjugation through its coplanar fluorene moieties,
resulting in a relatively low triplet energy (ET ¼ ca. 2.54 eV),
which could not confine the triplet exciton efficiently because of
reverse exothermic energy transfer. We measured the absolute
PL quantum efficiency (FPL) using a calibrated integration
sphere (HAMAMATSU C9920). It was confirmed that a neat
film (100 nm) of CBP doped with 10 wt% Ir(PPy)3 exhibited FPL
of 64% in our system. The green phosphorescent thin films
(100 nm) of T2T and T3T doped with 10 wt% Ir(PPy)2(acac)
exhibited FPL of 52% and 49%, respectively, which are consistent
with our observed excellent device efficiencies for devices A3 (hext
¼ 17.4%) and C3 (hext ¼ 16.2%). In contrast to T2T and T3T, the
thin film (100 nm) of TST doped with 10 wt% Ir(PPy)2(acac)
exhibited a rather low FPL of 28%, which indicates that the poor
capability of TST for effectively confining triplet energy of
Ir(PPy)2(acac) due to its relatively lower ET (2.54 eV). Accord-
ingly, device E3 exhibited poor device performance, with values
of hext and hp of 6.1% of 23.1 lm Wꢀ1, respectively.
Time-of-flight mobility measurements
The samples for the TOF measurements were prepared through
vacuum deposition in the configuration glass/Ag (30 nm)/organic
(2–3 mm)/Al (150 nm); they were then placed inside a cryostat and
maintained under vacuum. The thickness of the organic film was
monitored in situ using a quartz crystal sensor and calibrated
using a thin film thickness measurement system (K-MAC
ST2000). A pulsed nitrogen tunable dye laser was used as the
excitation light source (to match the absorption of the organic
film) through the semitransparent electrode (Ag)-induced pho-
togeneration of a thin sheet of excess carriers. Under an applied
dc bias, the transient photocurrent was swept across the bulk of
the organic film toward the collection electrode (Al); it was then
recorded using a digital storage oscilloscope. Depending on the
polarity of the applied bias, the selected carriers (holes or elec-
trons) were swept across the sample with a transit time of tT. For
an applied bias V and a sample thickness D, the applied electric
field E is equal to V/D; the carrier mobility is then given by the
equation
In conclusion, we have synthesized three new star-shaped ET-
type host materials, stemming from 1,3,5-triazine cores, for use
This journal is ª The Royal Society of Chemistry 2009
J. Mater. Chem., 2009, 19, 8112–8118 | 8117

View Article Online
m ¼ D/(tT E) ¼ D2/(VtT)
B. A. Bean and S. A. Jenekhe, Macromolecules, 2004, 37, 3554; (c)
H. Inomata, K. Goushi, T. Masuko, T. Konno, T. Imai, H. Sasabe,
J. J. Brown and C. Adachi, Chem. Mater., 2004, 16, 1285; (d)
S. Oyston, C. Wang, G. Hughes, A. S. Batsanov, I. F. Perepichka,
M. R. Bryce, J. H. Ahn, C. Pearson and M. C. Petty, J. Mater.
Chem., 2005, 15, 194; (e) T.-Y. Hwu, T.-C. Tsai, W.-Y. Hung, S.-
Y. Chang, Y. Chi, M.-H. Chen, C.-I. Wu, K.-T. Wong and L.-
C. Chi, Chem. Commun., 2008, 4956; (f) S. Chen, X. Xu, Y. Liu,
W. Qiu, G. Yu, X. Sun, H. Zhang, T. Qi, K. Lu, X. Gao, Y. Liu
and D. Zhu, J. Mater. Chem., 2007, 17, 3788.
8 (a) A. P. Kulkarni, C. J. Tonzola, A. Babel and S. A. Jenekhe, Chem.
Mater., 2004, 16, 4556; (b) G. Hughes and M. R. Bryce, J. Mater.
Chem., 2005, 15, 94.
9 J. Pang, S. Freiberg, X.-P. Yang, M. D’Iorio and S. Wang, J. Mater.
Chem., 2002, 12, 206.
from which the carrier transit times, tT, can be extracted from the
intersection points of the two asymptotes to the plateau and tail
sections in the double-logarithmic plots.
Acknowledgements
We thank Dr Teng-Chih Chao from Material and Chemical
Research Laboratories, Industrial Technology Research Insti-
tute (ITRI) for the aid in Riken AC-2 to determine the HOMO
energy levels. The work was supported by the financial aid of the
National Science Council of Taiwan.
10 J. M. Lupton, L. R. Hemingway, I. D. W. Samuel and P. L. Burn, J.
Mater. Chem., 2000, 10, 867.
11 R. Fink, Y. Heischkel, M. Thelakkat and H.-W. Schmidt, Chem.
Mater., 1998, 10, 3620.
References
1 M. A. Baldo, D. F. O’Brien, Y. You, A. Shoustikov, S. Sibley,
M. E. Thompson and S. R. Forrest, Nature, 1998, 395, 151.
2 S.-J. Su, H. Sasabe, T. I. Takeda and J. Kido, Chem. Mater., 2008, 20,
1691.
3 (a) C. Adachi, R. C. Kwong, P. Djurovich, V. Adamovich,
M. A. Baldo, M. E. Thompson and S. R. Forrest, Appl. Phys. Lett.,
2001, 79, 2082; (b) R. J. Holmes, S. R. Forrest, Y.-J. Tung,
R. C. Kwong, J. J. Brown, S. Garon and M. E. Thompson, Appl.
Phys. Lett., 2003, 82, 2422; (c) S. Tokito, T. Iijima, Y. Suzuri,
H. Kita, T. Tsuzuki and F. Sato, Appl. Phys. Lett., 2003, 83, 569;
(d) K. Goushi, R. Kwong, J. J. Brown, H. Sasabe and C. Adachi, J.
Appl. Phys., 2004, 95, 7798.
4 (a) M. A. Baldo, S. Lamansky, P. E. Burrows, M. E. Thompson and
S. R. Forrest, Appl. Phys. Lett., 1999, 75, 4; (b) X. Gong,
M. R. Robinson, J. C. Ostrowski, D. Moses, G. C. Bazan and
A. J. Heeger, Adv. Mater., 2001, 14, 581; (c) S.-J. Yeh, M.-F. Wu,
C.-T. Chen, Y.-H. Song, Y. Chi, M.-H. Ho, S.-F. Hsu and C.-
H. Chen, Adv. Mater., 2005, 17, 285; (d) K.-T. Wong, Y.-M. Chen,
Y.-T. Lin, H.-C. Su and C.-C. Wu, Org. Lett., 2005, 74, 5361; (e)
M.-H. Tsai, H.-W. Lin, H.-C. Su, T.-H. Ke, C.-C. Wu, F.-C. Fang,
Y.-L. Liao, K.-T. Wong and C.-I. Wu, Adv. Mater., 2006, 18, 1216;
(f) P.-I. Shih, C.-L. Chiang, A. K. Dixit, C.-K. Chen, M.-C. Yuan,
R.-Y. Lee, C.-T. Chen, E. W.-G. Diau and C.-F. Shu, Org. Lett.,
2006, 8, 2799; (g) M.-H. Tsai, Y.-H. Hong, C.-H. Chang, H.-C. Su,
C.-C. Wu, A. Matoliukstyte, J. Simokaitiene, S. Grigalevicius,
J. V. Grazulevicius and C.-P. Hsu, Adv. Mater., 2007, 19, 862; (h)
D. Tanaka, Y. Agata, T. Takeda, S. Watanabe and J. Kido, Jpn. J.
Appl. Phys., 2007, 46, L117.
12 J.-W. Kang, D.-S. Lee, H.-D. Park, Y.-S. Park, J. W. Kim, W.-
I. Jeong, K.-M. Yoo, K. Go, S.-H. Kim and J.-J. Kim, J. Mater.
Chem., 2007, 17, 3714.
13 R. A. Klenkler, H. Aziz, A. Tran, Z. D. Popovic and G. Xu, Org.
Electron., 2008, 9, 285.
14 S. Hayami and K. Inoue, Chem. Lett., 1999, 7, 545.
15 K.-T. Wong, S.-Y. Ku, Y.-M. Cheng, X.-Y. Lin, Y.-Y. Hung, S.-
C. Pu, P.-T. Chou, G.-H. Lee and S.-M. Peng, J. Org. Chem., 2006,
71, 456.
16 (a) N. Tamoto, C. Adachi and K. Nagai, Chem. Mater., 1997, 9, 1077;
(b) Z. Gao, C. S. Lee, L. Bello, S. T. Lee, R. M. Chen, T. Y. Luh,
J. Shi and C. W. Tang, Appl. Phys. Lett., 1999, 74, 865; (c)
Y. Shirota, K. Okumoto and H. Inada, Synth. Met., 2000, 111, 387;
(d) S. Berleb, W. Brutting, M. Schwoerer, R. Wehrmann and
A. Elschner, J. Appl. Phys., 1998, 83, 4403; (e) K. Itano,
T. Tsuzuki, H. Ogawa, S. Appleyard, M. R. Willis and Y. Shirota,
IEEE Trans. Electron Devices, 1997, 44, 1218; (f) Y. Kuwabara,
H. Ogawa, H. Inada, N. Noma and Y. Shirota, Adv. Mater., 1994,
6, 677; (g) I. Y. Wu, J. T. Lin, Y. T. Tao and E. Balasubramaniam,
Adv. Mater., 2000, 12, 668; (h) E. Ueta, H. Nakano and Y. Shirota,
Chem. Lett., 1994, 2397; (i) Y. Shirota, J. Mater. Chem., 2000, 10, 25.
17 S. Lamansky, P. Djurovich, D. Murphy, F. Abdel-Razzaq, H.-E. Lee,
C. Adachi, P. E. Burrows, S. R. Forrest and M. E. Thompson, J. Am.
Chem. Soc., 2001, 123, 4304.
18 S.-J. Su, E. Gonmori, H. Sasabe and J. Kido, Adv. Mater., 2008, 20,
4189.
19 J. Kido, M. Kimura and K. Nagai, Science, 1995, 267, 1332.
20 C. Adachi, M. A. Baldo, M. E. Thompson and S. R. Forrest, J. Appl.
Phys., 2001, 90, 5048.
21 L.-C. Chi, W.-Y. Hung, H.-C. Chiu and K.-T. Wong, Chem.
Commun., 2009, 3892.
22 S.-Y. Takizawa, V. A. Montes and P. Anzenbacher Jr., Chem. Mater.,
2009, 21, 2452.
5 S. Lamansky, P. I. Djurovich, F. Abdel-Razzaq, S. Garon,
D. L. Murphy and M. E. Thompson, J. Appl. Phys., 2002, 92, 1570.
6 H. H. Fong, C. H. Wallace Choy, K. N. Hui and Y. J. Liang, App.
Phys. Lett., 2006, 88, 113510.
7 (a) C. Wang, G. Y. Jung, Y. Hua, C. Pearson, M. R. Bryce,
M. C. Petty, A. S. Batsanov, A. E. Goeta and J. A. K. Howard,
Chem. Mater., 2001, 13, 1167; (b) C. J. Tonzola, M. M. Alam,
23 C. Adachi, M. A. Baldo, M. E. Thompson and S. R. Forrest, J. Appl.
Phys., 2000, 90, 5058.
8118 | J. Mater. Chem., 2009, 19, 8112–8118
This journal is ª The Royal Society of Chemistry 2009