Tailoring electronic structure of organic host for high-performance phosphorescent organic light-emitting diodes

Article abstract of DOI:10.1016/j.orgel.2014.08.006

We investigated highly efficient phosphorescent organic light-emitting diodes (PHOLEDs) based on three novel 1,3,5-triazine derivatives as the host materials and two kinds of iridium complexes as the guests, respectively. For comparison, the devices using a common phosphorescent host 4,4'-N,N'-dicarbazolebiphenyl (CBP) have also been fabricated. Results show that the devices using 9-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-9H-carbazole (PTC) and 4-(4,6-diphenoxy-1,3,5-triazin-2-yl)-N,N-diphenylaniline (POTA) as host have better performance than that of CBP. In comparison with the PHOLEDs based on CBP host, PTC- and POTA-based PHOLEDs show significantly lower driving voltages and higher power efficiencies. The high bipolar carrier mobility of the host is found to be critical to this kind of doping system, which would balance the injection of both carriers and improve efficiency.

Full text of DOI:10.1016/j.orgel.2014.08.006

Organic Electronics 15 (2014) 2763–2768
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Tailoring electronic structure of organic host for high-performance
phosphorescent organic light-emitting diodes
Jing Xiao a, , Xiao-Ke Liu b, Xin-Xin Wang c, Cai-Jun Zheng b, Feng Li a
⇑
a College of Physics and Electronic Engineering, Taishan University, Taian, Shandong 271021, PR China
b Nano-Organic Photoelectronic Laboratory and Key Laboratory of Photochemical Conversion & Optoelectronic Materials, Technical Institute of Physics &
Chemistry, Chinese Academy of Sciences, Beijing 100190, PR China
c Institute of Functional Nano & Soft Materials (FUNSOM) and Collaborative Innovation Center of Suzhou Nano Science & Technology, Soochow University, Suzhou,
Jiangsu 215123, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 7 March 2014
Received in revised form 28 July 2014
Accepted 3 August 2014
Available online 15 August 2014
We investigated highly efficient phosphorescent organic light-emitting diodes (PHOLEDs)
based on three novel 1,3,5-triazine derivatives as the host materials and two kinds of irid-
ium complexes as the guests, respectively. For comparison, the devices using a common
phosphorescent host 4,40-N,N0-dicarbazolebiphenyl (CBP) have also been fabricated.
Results show that the devices using 9-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-9H-car-
bazole (PTC) and 4-(4,6-diphenoxy-1,3,5-triazin-2-yl)-N,N-diphenylaniline (POTA) as host
have better performance than that of CBP. In comparison with the PHOLEDs based on CBP
host, PTC- and POTA-based PHOLEDs show significantly lower driving voltages and higher
power efficiencies. The high bipolar carrier mobility of the host is found to be critical to this
kind of doping system, which would balance the injection of both carriers and improve
efficiency.
Keywords:
Phosphorescent organic light-emitting
diodes
Novel 1,3,5-triazine derivatives
Iridium complexes
Energy-transfer
Bipolar carrier mobility
Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction
[17,18]. However, for most of PHOLEDs, the device quan-
tum efficiency drops rapidly with increasing current den-
Organic light-emitting diodes (OLEDs) possess high
potential in the development of full-color flat-panel dis-
plays and lighting [1–6]. In this regard, fabrication of
OLEDs with higher power efficiency becomes essential.
Phosphorescent organic light-emitting diodes (PHOLEDs)
have attracted considerable attention as they can harvest
both singlet and triplet excitons and achieve, in theory,
100% internal quantum efficiency, corresponding to a four-
fold improvement in efficiency relative to that achievable
in single-harvesting fluorescent OLEDs [7–13]. In general
phosphorescence emitters are doped into a host [14–16],
and the dopant molecules usually get excited through
direct charge trapping or energy transfer from the host
sity, as triplet excitons relax more slowly and the
emission inevitably reaches saturation through a quench-
ing mechanism involving triplet–triplet annihilation [19].
In order to alleviate the problem, the host materials are
significant to possess high bipolar carrier mobility and
suitable highest occupied molecular orbital (HOMO)/low-
est unoccupied molecular orbital (LUMO) levels that match
the neighboring layers [20,21]. To achieve this goal, tre-
mendous effort has been devoted. In general, the bipolar
host materials possess electron-transporting groups (elec-
tron acceptors) and hole-transporting units (electron
donors) [22]. The donor–acceptor interaction of bipolar
host materials is very critical, which have an influence on
triplet energies. In addition, the HOMO–LUMO energy
band gap (Eg) and triplet level (ET) of the host materials
should be higher than those of phosphorescent dopants,
⇑
Corresponding author.
E-mail address: xiaojingzx@163.com (J. Xiao).
http://dx.doi.org/10.1016/j.orgel.2014.08.006
1566-1199/Ó 2014 Elsevier B.V. All rights reserved.

2764
J. Xiao et al. / Organic Electronics 15 (2014) 2763–2768
which can prevent reverse energy transfer from the guest
back to the host. Nevertheless, a host material with a high
ET often has a wide Eg as a result of its typically large sin-
When cooled to room temperature, the mixture was
extracted with dichloromethane and dried over Na2SO4.
After the solvent had been removed under reduced pres-
sure, the residue was purified by column chromatography
on silica gel using dichloromethane–petroleum ether (1:1)
as the eluent to give a light yellow solid (0.547 g, 34%). 1H
NMR (400 MHz, DMSO-d6) d (ppm) 8.89 (d, J = 8.6 Hz, 6H),
8.29 (d, J = 7.7 Hz, 6H), 7.94 (d, J = 8.6 Hz, 6H), 7.58 (d,
J = 8.2 Hz, 6H), 7.53–7.45 (m, 6H), 7.35 (t, J = 7.4 Hz, 6H).
13C NMR spectra was unavailable due to the poor solubil-
ity. MS-MALDI [M + H]+: calcd 805.957. Found: 805.032.
PTC: This compound was synthesized according to the
synthesis procedure of TCT from the intermediates of 2-
chloro-4,6-diphenyl-1,3,5-triazine and 4-(9H-carbozol-9-
yl)phenylboronic acid. 1H NMR (400 MHz, CDCl3) d (ppm)
9.03 (d, J = 8.5 Hz, 2H), 8.82 (dd, J = 11.6, 10.0 Hz, 4H),
8.18 (d, J = 7.7 Hz, 2H), 7.83 (d, J = 8.5 Hz, 2H), 7.70–7.53
(m, 8H), 7.46 (t, J = 7.3 Hz, 2H), 7.34 (t, J = 7.3 Hz, 2H). 13C
NMR (100 MHz, CDCl3) d (ppm) 170.48, 170.15, 150.67,
136.70, 129.97, 133.25, 130.22, 129.41, 128.54, 128.38,
127.98, 124.33, 123.42, 120.97, 121.22, 110.41. MS-EI
[M]+: calcd 474.1844. Found: 474.1440.
glet–triplet energy difference (DEST) [23]. The host material
with large Eg usually leads to direct charge trapping within
the emitting layer (EML), which will finally produce poor
conductivity in host material and high operation voltages
in phosphorescent devices as result [24]. Host materials
with narrow Eg are reported to have better performances
[25–27]. In this case, design of bipolar host material with
high ET and narrow Eg is important.
In this study, we report a new series of 1,3,5-triazine
derivatives, 2,4,6-tris(4-(9H-carbazol-9-yl)phenyl)-1,3,5-
triazine (TCT), 9-(4-(4,6-diphenyl-1,3,5- triazin-2-yl)
phenyl)-9H-carbazole (PTC) and 4-(4,6-diphenoxy-1,3,
5-triazin-2-yl)-N, N-diphenylaniline (POTA), which contain
1,3,5-triazine as the electron-accepting core and carbazole-
containing electron acceptors. The three compounds were
used as hosts for green and red iridium complex guests
for highly efficient OLEDs. Since Baldo et al. developed
the concept of PHOLEDs [28], 4,40-N,N0-dicarbazolebi-
phenyl (CBP) has been used widely as a host material for
PHOLEDs because of its suitable ET and good hole-trans-
porting ability [29–32]. We compared the electrolumines-
cent (EL) properties of OLEDs based on the three 1,3,
5-triazine derivatives with the devices using CBP as the
host. We also investigated the effect of structural change
on the performance of host: dopant system.
POTA was synthesized following the reported proce-
dure [33]. The molecular structures of these three com-
pounds are shown in Fig. 1.
2.3. Device fabrication and measurement
Patterned indium tin oxide (ITO, 6
X/sq) substrates (Lum
2. Experimental
Tech), organic (Nichem Fine Tech) and inorganic materials
(Alfa Aesar), were commercially purchased. After the rou-
tine cleaning and ultraviolet (UV) ozone treatment, the ITO
substrates were introduced into a high-vacuum deposition
chamber (Trovato MFG, base pressure about 1 ꢁ 10ꢂ6 Torr)
with multiple thermal evaporation sources, where film
thickness was monitored in situ with a calibrated quartz
crystal microbalance. N,N0-diphenyl-N,N0-bis(1-naphthyl)-
(1,10-biphenyl)-4,40-diamine (NPB), 4,40,400-tris(N-carbazol-
yl)-triphenyl amine (TCTA), host-dopant composition,
1,3,5-tris(phenyl-2-benzimidazolyl)-benzene (TPBi), lith-
ium fluoride (LiF), and aluminum (Al) were evaporated on
the substrates in turn by vacuum deposition. Phosphores-
2.1. General information
NMR spectra were determined on a Bruker Advance-400
spectrometer with chemical shifts reported as ppm. Mass
spectrum data were obtained with a Finnigan 4021C GC–
MS spectrometer. Phosphorescence spectra at 77 K were
recorded with a Hitachi fluorescence spectrometer F-4600.
The film UV–vis absorption spectra were performed with
Lambda 750, PerkinElmer. Ultraviolet photoemission spec-
troscopy (UPS) was measured in a Kratos AXIS Ultra DLD
ultrahigh vacuum surface analysis system. UPS analysis
was performed with an unfiltered He I (ꢀ21.2 eV) gas dis-
charge lamp and a total instrumental energy resolution of
100 meV. The Fermi level (EF) is referred to as the zero bind-
ing energy (BE) in the UPS spectra.
cent
bis(2-phenylpyridine)iridium
acetylacetonate
[Ir(ppy)2(acac)] and tris(2-phenylquinoline) iridium(III)
[Ir(2-phq)3] were used as dopants for green and orange–
red emissions, respectively. The comparative devices were
fabricated under identical experimental conditions. The EL
spectra and the current density–voltage–luminance (J–V–
L) characteristics of the corresponding devices were
measured simultaneously with a Photo Research PR-655
spectrometer and a computer controlled programmable
Keithley model 2400 power source. All measurements were
carried out at room temperature under ambient conditions
after the devices have been encapsulated in a glove box.
2.2. Synthesis
Starting chemicals and reagents were commercially
available and used without further purification. Compounds
TCT, PTC and POTA were synthesized via palladium-cata-
lyzed suzuki reactions by using chlorine-substituted 1,3,5-
triazine units with 4-(9H-carbozol-9-yl)phenylboronic acid
or 4-(diphenylamino)phenylboronic acid.
TCT: Toluene (12 ml), ethanol (6 ml), and 2 M aqueous
Na2CO3 (9 ml) were added to a mixture of 2,4,6-trichloro-
1,3,5-triazine (0.368 g, 2 mmol), 4-(9H-carbozol-9-yl)
phenylboronic acid (1.894 g, 6.6 mmol), and tetrakis
(triphenylphosphine)platinum (0.234 g, 0.2 mmol). The mix-
ture was refluxed for 26 h under a nitrogen atmosphere.
3. Results and discussion
The UV–vis absorption spectra of neat TCT, PTC, and
POTA films are shown in Fig. 2(a). From the onsets of the
absorption spectra, narrow Eg values are obtained for

J. Xiao et al. / Organic Electronics 15 (2014) 2763–2768
2765
Fig. 1. The molecular structures of TCT, PTC, and POTA.
TCT, PTC and POTA, which are 3.15, 3.00 and 2.89 eV,
respectively. Fig. 2(b) shows the He I UPS spectra of TCT,
PTC, and POTA layers on the underlying Si substrate.
According to their secondary electron cutoff, the work
function of TCT, PTC and POTA films are 4.61, 4.12, and
3.93 eV. On the other hand, the valence band maximum
(VBM) of TCT, PTC and POTA can be obtained by the inter-
cept of the tangent of the leading edges of the low BE fea-
ture and the background level, and locate at 1.54, 1.78 and
1.73 eV below EF, respectively. The HOMO energy levels are
determined to be ꢂ6.15, ꢂ5.90 and ꢂ5.66 eV for TCT, PTC
and POTA by the energy difference between the HOMO
edge and vacuum level (VL) onset. In consideration of Eg,
the LUMO energy levels of TCT, PTC and POTA are deduced
to be ꢂ3.00, ꢂ2.98, and ꢂ2.77 eV, respectively.
Fig. 3 shows the phosphorescence spectra of TCT, PTC
and POTA in 2-methyltetrahydrofuran at 77 K. TCT and
PTC show similar characteristic vibrational structure for
carbazole around 458 and 485 nm, whereas POTA has a
quite red-shifted single emission with maximum of
508 nm. The triplet energy of TCT, PTC and POTA are esti-
mated to be 2.71, 2.71 and 2.44 eV from the highest-energy
peak of their phosphorescence spectra, which are suffi-
ciently high to excite green and red phosphorescent
dopants.
To confirm the bipolar properties of TCT, PTC and POTA,
hole-only and electron-only devices based on them are
investigated. The hole-only device has the structure of
ITO/MoO3 (12 nm)/TCT or PTC or POTA (30 nm)/MoO3
(12 nm)/Al, and that is ITO/TPBi (12 nm)/TCT or PTC or
POTA (30 nm)/TPBi (12 nm)/LiF (1 nm)/Al for the elec-
tron-only device. It can be assumed that electron (hole)
injection and transportation is prevented by MoO3 (TPBi)
in these devices because the LUMO (HOMO) energy level
of MoO3 (TPBi) is high (low) enough to block electron
(hole) injection. The current density–voltage (J–V) curves
of these hole-only and electron-only devices are shown
in Fig. 4. The remarkable hole and electron current densi-
ties in these devices demonstrate that the three com-
pounds are capable of transporting both hole and
electron, and exhibit bipolar character.
To understand the EL properties of the 1,3,5-triazine
derivative hosts, we fabricated green and red OLEDs using
TCT, PTC and POTA as host materials with the configuration
of ITO/NPB (30 nm)/TCTA (10 nm)/G-EML (or R-EML)
Fig. 2. (a) UV–vis absorption spectra and (b) UPS spectra of TCT, PTC, and
Fig. 3. The phosphorescence spectra of TCT, PTC and POTA in 2-
POTA films deposited on ITO and Si substrates, respectively.
methyltetrahydrofuran at 77 K.

2766
J. Xiao et al. / Organic Electronics 15 (2014) 2763–2768
The EL performances of those devices are summarized
in Table 2. Devices GB and GC using PTC and POTA as hosts
have higher current and power efficiencies than device GD
based on CBP host. Among these devices GA–GD, device GC
exhibits the highest current efficiency of 62.52 cd/A, the
highest power efficiency of 52.31 lm/W, and a maximum
brightness of 46,100 cd/m2 (0–100 mA/cm2). This phenom-
enon can also be observed during compare the current effi-
ciency of devices RA–RD. Device RC displays the highest
current efficiency of 37 cd/A, the highest power efficiency
of 28.29 lm/W, and a maximum brightness of 24,790 cd/
m2 (0–100 mA/cm2). Obviously, the device GA (or RA)
using TCT as host has the worst performances.
Fig. 5 depicts the current density–voltage (J–V) charac-
teristics of green OLEDs (devices GA–GD) and red OLEDs
(devices RA–RD). As observed in Fig. 5(a), the driving volt-
age of devices GB and GC remarkably decreased compared
with device GD based on CBP host. This phenomenon can
also be observed when comparing current density under
the same applied voltage of red devices in Fig. 5(b). Accord-
ing to the energy level diagram (Fig. 6), the hole-injection
barrier from TCTA and the electron-injection barrier from
TPBi to PTC or POTA layer were both considerably lower
than those to the CBP layer. As a result, the presence of
PTC or POTA facilitates hole- and electron-injection, as
observed in the J–V plots. We ascribe the reduced driving
voltage of PTC and POTA based devices to facile hole/elec-
tron-injections and less-effective charge trapping within
these two novel bipolar hosts. And the electron-injection
barrier from TPBi to TCT layer was also lower than that
to the CBP layer, which is beneficial to the electronic injec-
tion. So the driving voltage of device GA based on TCT was
slightly lower than that of device GD.
The current efficiency versus the current density of
green and red OLEDs using three different hosts with the
same doping concentrations is plotted in Fig. 7. For green
PHOLEDs, devices GB and GC using PTC and POTA as hosts
have higher current efficiency than that of device GD based
on CBP host. For example, at 10 mA/cm2 used for illumina-
tion applications, device GB shows an efficiency of
52.41 cd/A, 58.96 cd/A for device GC, and only 50.33 cd/A
for device GD. This phenomenon can also be observed
when comparing the current efficiency of red PHOLEDs.
At 10 mA/cm2, the efficiency of device RB is 25.77 cd/A,
35.66 cd/A for device RC, and 29.30 cd/A for device RC.
Since new host materials PTC and POTA have high bipolar
carrier mobility, the better balance between hole and elec-
tron injection in the device may explain the better effi-
ciency of the device (Fig. 4). At all current densities, the
TCT-composed devices showed the lowest efficiency of
all, perhaps due to the improper HOMO/LUMO energy
levels.
Fig. 4. Current density–voltage (J–V) curves of the (a) hole-only and (b)
electron-only devices based on TCT, PTC, and POTA.
(30 nm)/TPBi (30 nm)/LiF (1 nm)/Al (100 nm), where G-
EML and R-EML refer to the 1,3,5-triazine derivatives layer
doped with Ir(ppy)2(acac) and Ir(2-phq)3, respectively.
Two sets of devices were fabricated in our experiment.
The device architectures of set 1 and 2 are listed in Table 1.
The optimized doping concentration of Ir(ppy)2(acac) is
7 wt.%, and 6 wt.% for Ir(2-phq)3. For comparison, we also
fabricated reference devices employing CBP as the host
with the same configuration.
Table 1
Layer structures of the OLED devices, where G-EML and R-EML refer to
green and red electroluminescence units, respectively.
Devices or units
Layer structures
Device GA
Device GB
Device GC
Device GD
Device RA
Device RB
Device RC
Device RD
G-EML1
G-EML2
G-EML3
G-EML4
R-EML1
ITO/NPB/TCTA/G-EML1/TPBi/LiF/Al
ITO/NPB/TCTA/G-EML2/TPBi/LiF/Al
ITO/NPB/TCTA/G-EML3/TPBi/LiF/Al
ITO/NPB/TCTA/G-EML4/TPBi/LiF/Al
ITO/NPB/TCTA/R-EML1/TPBi/LiF/Al
ITO/NPB/TCTA/R-EML2/TPBi/LiF/Al
ITO/NPB/TCTA/R-EML3/TPBi/LiF/Al
ITO/NPB/TCTA/R-EML4/TPBi/LiF/Al
TCT:Ir(ppy)2(acac)(7%; 30 nm)
PTC:Ir(ppy)2(acac)(7%; 30 nm)
POTA:Ir(ppy)2(acac)(7%; 30 nm)
CBP:Ir(ppy)2(acac)(7%; 30 nm)
TCT:Ir(2-phq)3(6%; 30 nm)
Besides the high bipolar carrier mobility of host, emis-
sive layer energy architecture plays an important role in
the resultant current efficiency. Fig. 6 presents the energy
level diagram of green and red phosphorescent devices.
HOMO (or valence band) positions for TCT, PTC and POTA
are extracted by adding the charge-transport gaps relative
to the HOMO energy levels. The HOMO of TCTA is ꢂ5.8 eV,
while those of TCT, PTC, POTA and CBP are ꢂ6.2 eV,
ꢂ5.9 eV, ꢂ5.7 eV, and ꢂ6.0 eV, respectively. The devices
R-EML2
PTC:Ir(2-phq)3(6%; 30 nm)
R-EML3
POTA:Ir(2-phq)3(6%; 30 nm)
R-EML4
CBP:Ir(2-phq)3(6%; 30 nm)

J. Xiao et al. / Organic Electronics 15 (2014) 2763–2768
2767
Table 2
Device parameters at current density of 10 mA/cm2 and maximum current and power efficiencies of green devices GA–GD, and red devices RA–RD.
Device name
Voltage
(V)
Luminance
(cd/m2)
Current efficiency
(cd/A)
Maximum current efficiency
(cd/A)
Maximum power efficiency
(lm/W)
GA
GB
GC
GD
RA
RB
RC
RD
6.04
3.99
4.70
6.17
5.11
4.88
4.93
6.15
4209
5241
5896
5033
1570
2577
3566
2435
42.09
52.41
58.96
50.33
15.70
25.77
35.66
24.35
44.94
55.60
62.52
50.33
17.00
28.72
37.00
25.80
25.40
51.33
52.31
26.14
9.65
22.85
28.29
17.33
Fig. 5. Current density–voltage (J–V) curves of (a) green devices GA–GD
Fig. 7. Current efficiency as a function of current density for (a) green
and (b) red devices RA–RD.
devices GA–GD and (b) red devices RA–RD.
Fig. 6. Energy level diagram of materials investigated in green and red OLEDs, where the HOMO (or VBM) positions of TCT, PTC, and POTA are derived from
the UPS measurements, and the LUMO (or CBM) is derived from optical gaps plus HOMO. Energy levels of Ir(ppy)2(acac) (dashed line) and Ir(2-phq)3 (dotted
line) are also shown.

2768
J. Xiao et al. / Organic Electronics 15 (2014) 2763–2768
(GC and RC) with POTA have the highest efficiency, which
may be attributed to their zero barriers for injection of
holes from the HTL to the EML, since the hole is the dom-
inant carrier in typical OLED devices [34]. Replacing the
host POTA with CBP or TCT, gives a hole-injection barrier
of 0.2 or 0.4 eV, which would reduce the number of
injected holes, as evidenced by a decrease in the current
density (Fig. 5).
References
[1] T. Fuhrmann, J. Salbeck, MRS Bull. 28 (2003) 354.
[2] F. So, J. Kido, P. Burrows, MRS Bull. 33 (2008) 663.
[3] J. Peng, X.X. Wang, J. Liu, X.D. Huang, J. Xiao, S.D. Wang, H.Q. Wang,
W.L. Ma, J. Mater. Chem. C 2 (2014) 864.
[4] X.X. Wang, J. Xiao, X. Gao, X.H. Zhang, S.D. Wang, AIP Adv. 3 (2013)
102124.
[5] J. Xiao, X.X. Wang, H. Zhu, X. Gao, Z.H. Yang, X.H. Zhang, S.D. Wang,
Appl. Phys. Lett. 101 (2012) 013301.
[6] J. Xiao, H. Zhu, X.X. Wang, X. Gao, Z.H. Yang, X.H. Zhang, S.D. Wang, J.
Appl. Phys. 112 (2012) 014513.
[7] S. Lamansky, P. Djurovich, D. Murphy, F. Abdel-Razzaq, H.-E. Lee, C.
Adachi, P.E. Burrows, S.R. Forrest, M.E. Thompson, J. Am. Chem. Soc.
123 (2001) 4304.
[8] C. Ulbricht, B. Beyer, C. Friebe, A. Winter, U.S. Schubert, Adv. Mater.
21 (2009) 4418.
[9] Y. You, S.Y. Park, J. Am. Chem. Soc. 127 (2005) 12438.
[10] S. Reineke, F. Linder, G. Schwartz, N. Seidler, K. Walzer, B. Lüssem, K.
Leo, Nature 459 (2009) 234.
[11] Y.H. Lan, C.H. Hsiao, P.Y. Lee, Y.C. Bai, C.C. Lee, C.C. Yang, M.K. Leung,
M.K. Wei, T.L. Chiu, J.H. Lee, Org. Electron. 12 (2011) 756.
[12] H. Sasabe, K. Minamoto, Y.J. Pu, M. Hirasawa, J. Kido, Org. Electron.
13 (2012) 2615.
[13] D.D. Zhang, L. Duan, D.Q. Zhang, J. Qiao, G.F. Dong, L.D. Wang, Y. Qiu,
Org. Electron. 14 (2013) 260.
[14] R. Wang, D. Liu, H. Ren, T. Zhang, H. Yin, G. Liu, J. Li, Adv. Mater. 23
(2011) 2823.
[15] R. Wang, D. Liu, R. Zhang, L. Deng, J. Li, J. Mater. Chem. 22 (2012)
1411.
[16] C. Adachi, R.C. Kwong, P. Djurovich, V. Adamovich, M.A. Baldo, M.E.
Thompson, S.R. Forrest, Appl. Phys. Lett. 79 (2001) 2082.
[17] R.J. Holmes, S.R. Forrest, Y.J. Tung, R.C. Kwong, J.J. Brown, S. Garon,
M.E. Thompson, Appl. Phys. Lett. 82 (2003) 2422.
[18] Y. Kawamura, K. Goushi, J. Brooks, J.J. Brown, H. Sasabe, C. Adachi,
Appl. Phys. Lett. 86 (2005) 071104.
[19] M.A. Baldo, C. Adachi, S.R. Forrest, Phys. Rev. B 62 (2000) 10967.
[20] W.Y. Hung, T.C. Tsai, S.Y. Ku, L.C. Chi, K.T. Wong, Phys. Chem. Chem.
Phys. 10 (2008) 5822.
According to the energy level diagrams of the EML with
neighboring hole-transporting layer (HTL) and electron-
confining layer (ETL), we also investigated green or red
OLEDs with a carrier- and exciton-confining structure.
The PTC-composed devices and the POTA-composed
devices possess excellent hole-confinement properties,
enhancing the recombination probability, thus resulting
in a better device performance. The HOMO level of TPBi
is 0.3 and 0.54 eV deeper than those of PTC and POTA,
respectively, confining holes within the emissive layers.
In contrast, the energy-barrier for holes at the CBP/TPBi
interface is only 0.2 eV, which does not favor hole-confine-
ment and causes lower device efficiency. For the TCT-
composed devices, their electron-confinement is rather
poor, which produced worse results. Moreover, the LUMO
level of TCTA is 0.58 and 0.37 eV higher than those of
PTC and POTA, respectively, confining electrons within
the emissive layers. The lower LUMO levels of PTC and
POTA provide better electron-confinement within the
emissive-layer, which also effectively enhances the recom-
bination probability and improves the device efficiency.
[21] H. Fukagawa, N. Yokoyama, S. Irisa, S. Tokito, Adv. Mater. 22 (2010)
4775.
4. Conclusions
[22] A. Chaskar, H.F. Chen, K.T. Wong, Adv. Mater. 23 (2011) 3876.
[23] H. Bassler, V.I. Arkhipov, E.V. Emelianova, A. Gerhard, A. Hayer, C. Im,
J. Rissler, Synthetic. Met. 135 (2003) 377.
[24] R.F. Chen, G.H. Xie, Y. Zhao, S.L. Zhang, J. Yin, S.Y. Liu, W. Huang, Org.
Electron. 12 (2011) 1619.
In summary, green and red PHOLEDs using three 1,3,
5-triazine derivatives as the host and Ir(ppy)2(acac),
Ir(2-phq)3 as the guest, respectively, were studied. Impres-
sively, PTC- and POTA-based PHOLEDs show significantly
lower driving voltages and higher power efficiencies com-
pared with the PHOLEDs based on CBP host. The higher
current efficiencies for the devices based on POTA host
can be attributed to the high bipolar nature of POTA and
a good energy level match between guest and host.
[25] T.J. Park, W.S. Jeon, J.J. Park, S.Y. Kim, Y.K. Lee, J. Jang, J.H. Kwon, R.
Pode, Appl. Phys. Lett. 92 (2008) 113308.
[26] Z.Q. Gao, M. Luo, X.H. Sun, H.L. Tam, M.S. Wong, B.X. Mi, P.F. Xia,
K.W. Cheah, C.H. Chen, Adv. Mater. 21 (2009) 688.
[27] S.J. Su, C. Cai, J. Kido, Chem. Mater. 23 (2011) 274.
[28] M.A. Baldo, D.F. O’Brien, Y. You, A. Shoustikov, S. Sibley, M.E.
Thompson, S.R. Forrest, Nature 395 (1998) 151.
[29] C.H. Yang, C.C. Tai, I.W. Sun, J. Mater. Chem. 14 (2004) 947.
[30] C.L. Li, Y.J. Su, Y.T. Tao, P.T. Chou, C.H. Chien, C.C. Cheng, R.S. Liu, Adv.
Funct. Mater. 15 (2005) 387.
Acknowledgements
[31] S. Takizawa, Y. Sasaki, M. Akhtaruzzaman, H. Echizen, J. Nishida, T.
Iwata, S. Tokito, Y. Yamashita, J. Mater. Chem. 17 (2007) 841.
[32] C.L. Ho, W.Y. Wong, Z.Q. Gao, C.H. Chen, K.W. Cheah, B. Yao, Z. Xie, Q.
Wang, D. Ma, L. Wang, X.M. Yu, H.S. Kwok, Z. Lin, Adv. Funct. Mater.
18 (2008) 319.
[33] X.K. Liu, C.J. Zheng, M.F. Lo, J. Xiao, Z. Chen, C.L. Liu, C.S. Lee, M.K.
Fung, X.H. Zhang, Chem. Mater. 25 (2013) 4454.
[34] G. Hughes, M.R. Bryce, J. Mater. Chem. 15 (2005) 94.
This work was supported by the National Basic
Research Development Program of China (973 Program,
Nos. 2010CB934503 and 2011CB808404), the National
Natural Science Foundation of China (No. 61204051), the
Natural Science Foundation of Shandong Province (No.
ZR2010FQ006) and the Technology Planned Project of
Tai’an (No. 20122053).