Three-carbazole-armed host materials with various cores for RGB phosphorescent organic light-emitting diodes

Article abstract of DOI:10.1039/c2jm14151e

A series of three-carbazole-armed host materials containing various arylene cores, like benzene (1,3,5-tris(3-(carbazol-9-yl)phenyl)-benzene, TCPB), pyridine (2,4,6-tris(3-(carbazol-9-yl)phenyl)-pyridine, TCPY), and pyrimidine (2,4,6-tris(3-(carbazol-9-yl)phenyl)-pyrimidine, TCPM), were developed for red, green, and blue phosphorescent organic light-emitting diodes (OLEDs). An intramolecular charge transfer was observed for TCPY and TCPM with heterocyclic cores of pyridine and pyrimidine, giving bathochromic shifts in the photoluminescent spectrum and reduced energy band gaps in comparison with TCPB with a benzene core. In addition, lower energy singlet and triplet excited states, reduced lowest unoccupied molecular orbital (LUMO) energy level, smaller singlet-triplet exchange energy (ΔE_ST), and improved bipolarity were also achieved with introducing heterocycles of pyridine and pyrimidine instead of benzene. In contrast to the slightly decreased triplet energy (E_T), a significantly decreased ΔE_ST was achieved by introducing heterocycles of pyridine and pyrimidine as the core, and the more nitrogen atoms in the central heterocycle, the smaller ΔE _ST is achieved. Reduced driving voltages were achieved for the green and red phosphorescent OLEDs by utilizing TCPY and TCPM as the host due to their decreased ΔE_ST and lower-lying LUMO energy level, proving that more carriers must be injected into the emitting layer through the host molecules rather than direct carrier trapping by the dopant. Moreover, improved efficiency and suppressed efficiency roll-off were also achieved for the green and red phosphorescent OLEDs based on TCPY and TCPM due to their improved bipolarity and thus improved carrier balance.

Full text of DOI:10.1039/c2jm14151e

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PAPER
Three-carbazole-armed host materials with various cores for RGB
phosphorescent organic light-emitting diodes†
a
Shi-Jian Su, Chao Cai and Junji Kido
b
b
*
*
Received 24th August 2011, Accepted 23rd November 2011
DOI: 10.1039/c2jm14151e
A series of three-carbazole-armed host materials containing various arylene cores, like benzene (1,3,5-
tris(3-(carbazol-9-yl)phenyl)-benzene, TCPB), pyridine (2,4,6-tris(3-(carbazol-9-yl)phenyl)-pyridine,
TCPY), and pyrimidine (2,4,6-tris(3-(carbazol-9-yl)phenyl)-pyrimidine, TCPM), were developed for
red, green, and blue phosphorescent organic light-emitting diodes (OLEDs). An intramolecular charge
transfer was observed for TCPY and TCPM with heterocyclic cores of pyridine and pyrimidine, giving
bathochromic shifts in the photoluminescent spectrum and reduced energy band gaps in comparison
with TCPB with a benzene core. In addition, lower energy singlet and triplet excited states, reduced
lowest unoccupied molecular orbital (LUMO) energy level, smaller singlet–triplet exchange energy
(DEST), and improved bipolarity were also achieved with introducing heterocycles of pyridine and
pyrimidine instead of benzene. In contrast to the slightly decreased triplet energy (ET), a significantly
decreased DEST was achieved by introducing heterocycles of pyridine and pyrimidine as the core, and
the more nitrogen atoms in the central heterocycle, the smaller DEST is achieved. Reduced driving
voltages were achieved for the green and red phosphorescent OLEDs by utilizing TCPY and TCPM as
the host due to their decreased DEST and lower-lying LUMO energy level, proving that more carriers
must be injected into the emitting layer through the host molecules rather than direct carrier trapping
by the dopant. Moreover, improved efficiency and suppressed efficiency roll-off were also achieved for
the green and red phosphorescent OLEDs based on TCPY and TCPM due to their improved bipolarity
and thus improved carrier balance.
triplet–triplet annihilation. As a matrix material for the phos-
Introduction
phorescent emitters, it is essential that the triplet energy (ET) of
the host is higher than that of the emitter in order to facilitate
energy transfer from the host to the phosphorescent emitter and
to prevent energy back transfer from the guest to the host,
regardless of the emitting colors.5,6 To meet this requirement,
most effort have been made to develop host materials with
limited p-conjugation. For example, triphenyl silyl has been
introduced into the hosts as a building block to break their
p-conjugation,7–9 and several organosilicon compounds have
been reported having ultra-high triplet energies.10 However, the
charge hopping in the EML generally occurred between the
adjacent dopant molecules dueto their ultrawide energy band gaps
(Eg) to give increased operating voltage.11 As thus, another
requirement fora host materialisitscarriertransportproperty. An
optimized carrier recombination in the EML is indispensable to
give a high efficiency. Over the years, many host materials have
been described with buildingblocksofwell-knownelectrondonors
of carbazole12–16 and triphenylamine.17 In general, the electron
mobility of those hosts is much lower than the hole mobility
because of the fact that they mainly consist of strong electron
donors like aromatic amine or carbazole. Besides the building
blocks of electron donors, introduction of electron acceptors
like benzimidazole,18–21 phosphine oxide,22,23 triazine,24–26 and
Phosphorescent organic light-emitting diodes (OLEDs) have
attracted tremendous attention since nearly 100% internal
quantum efficiency of electroluminescence is readily achieved by
harvesting both electro-generated singlet and triplet excitons by
fast intersystem crossing for phosphorescence emission from the
triplet excited states of the transition metal-centered phos-
phors.1–4 For those phosphorescent OLEDs, phosphorescent
emitters are normally doped into an appropriate host material as
an emitting layer (EML) to avoid self-aggregation quenching and
aInstitute of Polymer Optoelectronic Materials and Devices, State Key
Laboratory of Luminescent Materials and Devices, South China
University of Technology, Guangzhou, 510640, China. E-mail: mssjsu@
scut.edu.cn
bDepartment of Organic Device Engineering, Graduate School of Science
and Engineering, Yamagata University, 4-3-16 Jonan, Yonezawa,
Yamagata, 992-8510, Japan. E-mail: kid@yz.yamagata-u.ac.jp
† Electronic supplementary information (ESI) available: Synthetic
procedure and spectrascopic characterizations of the compounds
TCPB, TCPY, and TCPM; molecular structures of DCzPB,
26DCzPPy, 24DCzPPy, 24DCzPPm, and 46DCzPPm; molecular
structures and acronyms of the materials used for device fabrication;
PL spectra of the co-deposited films; JVL and EL characteristics of the
RGB phosphorescent OLEDs. See DOI: 10.1039/c2jm14151e
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oxadiazole,27 was proven to be an effective route to give host
materials with improved electron mobility. Most recently, some
host materials with building blocks of both electron donor and
acceptor, called bipolar host materials, were developed with
improved bipolarity.28–34 However, few work reported on the
structure-property relationship according to the electron donor
and acceptor, especially for the phosphorescent OLEDs utilizing
blue, green, and red phosporescent emitters.
core, the absorptions in the range of 350 and 400 nm for TCPY
and TCPM slightly increase with introducing heterocycles of
pyridine and pyrimidine as the core instead of the central
benzene. This can be attributed to strong electron affinity of
pyridine and pyrimidine and thus intramolecular charge transfer
formed between the central heterocycle and the outer carbazole.
As a result, from their absorption edges, relatively narrower
energy band gaps (Eg) of 3.42 and 3.15 eV are obtained for TCPY
and TCPM, respectively, compared with 3.46 eV for TCPB
(Table 1). In addition, the more nitrogen atoms in the hetero-
cyclic core, the narrower Eg is obtained due to the improved
electron affinity of the central arylene and thus stronger intra-
molecular charge transfer.
In this article, we report on a series of three-carbazole-armed
host materials composed of carbazole as the periphery and tri-
phenyl arylene as the core. They are different from each other in
chemical structure and electron affinity of the central arylenes,
like benzene, pyridine, and pyrimidine. Their carrier mobilities
and energy levels, including the highest occupied molecular
orbital (HOMO), the lowest unoccupied molecular orbital
(LUMO), singlet and triplet energies, and their energy difference,
can be effectively tuned by the different central arylenes. Red,
green, and blue phosphorescent OLEDs were fabricated with
well-known triplet emitters of tris(1-phenylisoquinolinolato-
C2,N)iridium(III) (Ir(piq)3), fac-tris(2-phenylpyridine) iridium
(Ir(PPy)3), and iridium(III) bis(4,6-(di-fluorophenyl)pyridinato-
The PL spectrum of TCPB exhibits peaks at 351, 380 and 400
nm. In contrast, the PL spectra of TCPY and TCPM are
featureless and peak at 415 and 436 nm, respectively. The bath-
ochromic shift of the PL spectrum is clearly found by introducing
heterocycles of pyridine and pyrimidine as the core instead of the
central benzene of TCPB, the more nitrogen atoms present in the
heterocyclic core, the longer the bathochromic shift obtained.
This phenomenon can be also attributed to the intramolecular
charge transfer of the compounds containing a heterocyclic core.
HOMO energy levels of TCPB, TCPY, and TCPM which are
determined by atmospheric photoelectron spectroscopy are
slightly influenced by the change of the central arylene. In
contrast, lower-lying LUMO energy levels were achieved by
introducing heterocycles of pyridine and pyrimidine with
stronger electron affinities as the core instead of the central
benzene. The more nitrogen atoms present in the central
heterocyclic arylene, the stronger the electron affinity and the
lower-lying LUMO energy levels are achieved.
0
N,C2 ) picolinate (FIrpic), respectively. The effects of the central
arylenes on their photophysical properties, electron/hole mobil-
ities, and electroluminescence performances were studied
comprehensively. In addition, compared with the host materials
with the same arylene core and only two phenyl carbazole arms,
the current three-carbazole-armed host materials exhibit
improved bipolarity and thus an improved carrier balance and
device performance.
Results and discussion
To obtain the ETS of the developed host materials, extremely
long decay components of their transient photoluminescence,
lasting for ꢀ10 ms, were recorded at T ¼ 4.2 K. Fig. 1b shows
two time-resolved emission spectra for each film. One is the
emission spectrum obtained in the whole range (0 < t < 10 ms)
(black lines), comprising both singlet and triplet emissions. The
other is the emission spectrum of the delayed component (2 ms <
t < 10 ms) (grey lines), generally consisting of triplet emission,
also called phosphorescence spectrum of the corresponding host
material. TCPB exhibits the highest energy phosphorescence
peak (S0v ¼ 0 ) T1v ¼ 0) at 466 nm, corresponding to ET of 2.66 eV.
Compared with a general host material of N,N0-dicarbazolyl-
4,40-biphenyl (CBP) (ET ¼ 2.56 eV), the higher ET can be
attributed to the break of p conjugation when the central ary-
lenes are combined with each other at their meta-positions (refer
to the combination sites). With introducing pyrimidine instead of
the central benzene, slightly decreased ET of 2.64 eV is obtained
for TCPM with the highest energy phosphorescence peak at 470
nm. Different from the emission spectra of the delayed compo-
nents of TCPB and TCPM, there is a strong emission between
380 and 460 nm for the delayed component of TCPY, which can
be attributed to its delayed fluorescence. Besides the delayed
fluorescence, it also shows phosphorescence at wavelengths
longer than 460 nm with the highest energy phosphorescence
peak at 472 nm, corresponding to ET of 2.63 eV. Compared with
the host materials with the same arylene core and only two
phenyl carbazole arms,34 ET of the current hosts with three
phenyl carbazole arms is slightly lower, which can be attributed
Syntheses of 1,3,5-tris(3-(carbazol-9-yl)phenyl)-benzene (TCPB),
2,4,6-tris(3-(carbazol-9-yl)phenyl)-pyridine (TCPY), and 2,4,6-
tris(3-(carbazol-9-yl)phenyl)-pyrimidine (TCPM) are outlined in
Scheme 1. The intermediates of 1,3,5-tri(m-bromophenyl)
benzene (1),35 9-(3-bromophenyl)-carbazole (3),28 and 9-(3-
(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)-carbazole
(4)28 were synthesized according to the literature procedures.
2,4,6-Tris(3-bromophenyl)pyridine (2) was synthesized with
a condensation reaction of 3-bromobenzaldehyde and 3-bro-
moacetophenone in the presence of KOH and concentrated
NH4OH. TCPB and TCPY were synthesized with a palladium
catalyzed amination reaction in yields of 89% and 91%, respec-
tively. TCPM was synthesized with a palladium catalyzed Suzuki
coupling reaction in yield of 89%. All the developed host mate-
rials were purified by silica gel chromatography and then
repeated thermal gradient vacuum sublimation before charac-
terization and device fabrication.
UV-vis absorption and steady-state photoluminescence (PL)
spectra for the vacuum deposited films (60 nm) of the developed
host materials on quartz substrates were measured at room
temperature (Fig. 1a). Their UV-vis absorption spectra are quite
similar with peaks and shoulders at 285, 296, 330, and 345 nm
that can be attributed to the p–p* transitions of the carbazole
chromophore on the periphery. The absorption peaks at around
244 nm and the shoulders at 265 nm can be attributed to the
p–p* and n–p* transitions of the central triphenyl arylenes,
respectively. However, compared with TCPB with a benzene
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Scheme 1 Synthetic routes of TCPB, TCPY, and TCPM. Reagents and conditions: (i) SiCl4, anhydrous ethanol, 0–25 ꢁC; (ii) PdCl2, tris(tert-butyl)
phosphine, sodium tert-butoxide, anhydrous o-xylene, 120 ꢁC; (iii) KOH, ethanol, concentrated NH4OH, reflux; (iv) Cu, K2CO3, DMF, 130 ꢁC; (v)
bis(pinacolato)diboron, Pd2(dba)3, KOAc, PCy3, dioxane, 80 ꢁC; (vi) PdCl2(PPh3)2, 2 M K2CO3, 1,4-dioxane, 90 ꢁC.
to more delocalized p conjugation in these molecules. Different
from the fluorescence spectra, very slight bathochromic shift in
phosphorescence was found with introducing one or two
nitrogen atoms in the central arylene, resulting in slightly lower
ET. Since p conjugations of the current hosts are similar to each
other, compared with TCPB, the slightly reduced ET of TCPY
and TCPM can be attributed to the intramolecular charge
transfer formed between the heterocyclic core and the carbazole
periphery.
Compared with the phosphorescence spectra, the prompt and
stronger emissions at shorter wavelengths are due to the fluo-
rescence of the host materials, which are almost consistent with
those measured at room temperature. From the highest energy
v ¼ 0
fluorescence peaks (S0
) S1v ¼ 0), singlet energies (ES) of
TCPB, TCPY, and TCPM can be estimated as 3.26, 3.02, and
2.82 eV, respectively (Table 2). In general, a host material should
have an exchange energy as small as possible to allow for charge
injection into the host and efficient triplet emission from
a dispersed triplet emitter.16 Here, the energy difference (DEST
between the singlet and triplet emission transitions is taken as an
estimate for the exchange energy. Compared with TCPB (DEST
)
¼
Fig. 1 (a) Room temperature UV-vis absorption and steady-state pho-
toluminescence spectra for the vacuum-deposited films of TCPB, TCPY,
and TCPM on quartz substrates. (b) Time-resolved emission spectra for
the vacuum-deposited films of TCPB, TCPY, and TCPM excited by
a nitrogen laser (l ¼ 337 nm, 50 Hz, 800 ps) at T ¼ 4.2 K. Black lines:
Emission spectra in the whole range (0 < t < 10 ms). Grey lines: Emission
spectra of the delayed components, generally called phosphorescence
spectra (2 ms < t < 10 ms).
0.60 eV), a decrease of DEST by 0.21 eV was achieved for
TCPY by introducing a pyridine core instead of the central
benzene. In addition, with introducing one more nitrogen atom
in the central pyridine core, further decreased DEST of 0.18 eV
was achieved for TCPM, which is much smaller than those of
most previously reported host materials.16,34 Compared with the
slightly reduced ET, the significantly decreased DEST achieved
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Table 1 Physical and photophysical properties of TCPB, TCPY, and TCPM. Also shown is the data for the host materials with the same arylene core
and two phenyl carbazole arms, like DCzPB, 26DCzPPy, 24DCzPPy, 24DCzPPm, and 46DCzPPm.34 Their molecular structures are shown in Scheme
S1 (see ESI†)
Materials
Tm (ꢁC)
HOMOe (eV)
LUMOf (eV)
a
Tg (ꢁC)
a
b
Td (ꢁC)
c
lPL (nm)
d
Eg (eV)
TCPB
DCzPB
TCPY
26DCzPPy
24DCzPPy
TCPM
146
101
149
102
106
155
108
107
261
276
288
230
n.d.
310
185
175
518
452
511
455
466
509
454
466
351, 380, 400
352, 378, 400
3.46
3.48
3.42
3.40
3.38
3.15
3.17
3.13
6.16
6.08
6.19
6.05
6.15
6.17
6.19
6.19
2.70
2.60
2.77
2.65
2.77
3.02
3.02
3.06
415
387
401
436
426
439
24DCzPPm
46DCzPPm
a
Glass transition temperature (Tg) and melting temperature (Tm) obtained from differential scanning calorimetry (DSC) measurements.
Decomposition temperature (Td) obtained from thermogravimetric analysis (TGA). Photoluminescence (PL) spectra of vacuum-deposited films
b
c
d
on quartz substrates at room temperature. Energy band gap (Eg) from the lowest-energy absorption edge of the UV-vis absorption spectra.
e
f
HOMO energy level determined by atmospheric ultraviolet photoelectron spectroscopy (Rikken Keiki AC-3). LUMO energy level calculated
from HOMO and Eg.
with introducing heterocycles as the core indicates that intro-
ducing both electron donors and acceptors as building blocks of
a host material may be an effective route to give attractive
candidates for phosphorescent OLEDs with reduced driving
voltage as well as good triplet exciton confinement.
HOMO energy levels are slightly influenced by the change of the
central arylene. Moreover, the trends of the calculated energy
levels are conceivably consistent with the experimental ones,
although the exact values are somewhat different.
Their triplet energies are calculated from the difference of their
ground state (S0) and triplet excited state (T1) energies. The
calculated triplet energies (T1 ꢂ S0) of TCPB, TCPY, and TCPM
are 2.98, 2.87, and 2.81 eV, and it further proves that introduc-
tion of heterocycle as the core in combination of carbazole
periphery induces lower triplet energy. As shown in Table 2, the
energy difference (DE) between the calculated Eg and (T1 ꢂ S0) is
taken as an estimate for the energy difference between the singlet
and triplet transitions. Similar to the experimental DEST, DE of
TCPB with a benzene core is the largest one among these hosts,
and smaller DE is achieved with introducing one or two nitrogen
atoms in the central benzene. Moreover, consistent with the
experimental result, DE of the current hosts with three phenyl
carbazole arms is smaller than the hosts with the same arylene
core and two phenyl carbazole arms, and DE of TCPM is the
smallest one among these hosts.34
Besides photophysical properties, their thermal properties, like
glass transition temperatures (Tg) and melting temperatures (Tm)
are also influenced by the change of the arylene core. Improved
Tg and Tm are achieved for the host materials with one or two
nitrogen atoms in the central arylene, and it can be attributed to
the increase of polarity and thus stronger molecular interaction.
In addition, all they show a Tg higher than 140 ꢁC, which are over
40 ꢁC higher than those of the host materials with the same
arylene core and two phenyl carbazole arms (Table 1).34 In
comparison, a general host CBP shows a crystalline behavior,
and no Tg was detected in the DSC.36 The high Tg values of the
current three-carbazole-armed hosts prove the high morphologic
stability of the amorphous phase in a deposited film, which is
a prerequisite for their applications in OLEDs.
DFT calculations were performed using the Gaussian suite of
programs (Gaussian 03W) for the structures of TCPB, TCPY,
and TCPM. As shown in Fig. 2, HOMOs of all the molecules are
mainly located at the carbazole periphery. In contrast, their
LUMOs are mainly located at the central triphenyl arylene
skeleton among the outer carbazoles. As a result, lower-lying
LUMO energy levels are obtained by introducing heterocycles of
pyridine and pyrimidine as the core instead of the central
benzene, and the more nitrogen atoms in the heterocyclic, the
lower-lying LUMO energy level is obtained. In comparison, their
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. It seems in contra-
diction with this requirement since decreased ET is obtained by
introducing pyridine or pyrimidine as the core of the host in
comparison with the benzene core. To verify their triplet exciton
confinement ability, well-known red, green, and blue triplet
emitters of Ir(piq)3, Ir(PPy)3, and FIrpic were co-deposited with
the current hosts for transient PL decay measurements.
Table 2 Singlet energies (ES, measured as S0v ¼ 0 ) S1v ¼ 0 at 4.2 K), triplet energies (ET, measured as S0v ¼ 0 ) T1v ¼ 0 at 4.2 K), and singlet–triplet energy
difference (DEST ¼ ES ꢂ ET) for TCPB, TCPY, and TCPM
Materials
ES (eV)
ET (eV)
DEST (eV)
T1 ꢂ S0a (eV)
DEa (eV)
a
Eg (eV)
TCPB
TCPY
TCPM
3.26
3.02
2.82
2.66
2.63
2.64
0.60
0.39
0.18
4.03
3.59
3.24
2.98
2.87
2.81
1.05
0.75
0.43
a
Calculated energy band gaps (Eg ¼ HOMO ꢂ LUMO), triplet energies (T1 ꢂ S0), and energy differences between Eg and (T1 ꢂ S0) (DE ¼
Eg ꢂ (T1 ꢂ S0)).
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Fig. 2 The calculated spatial distributions and energy levels (eV) of
HOMO and LUMO for TCPB, TCPY, and TCPM. Also shown the
experimental HOMO and LUMO energy levels (eV).
For the FIrpic-doped films, it can be seen that FIrpic doped
into all the hosts exhibit nearly mono-exponential decay curves
(Fig. 3a) with relatively long lifetimes (s) between 1.38 and 1.60
ms (Table 3). For the host of TCPB that possess the highest ET
among these hosts, nonradiative rate constant (knr) of the co-
deposited film is 1.52 ꢃ 105 sꢂ1, which is the smallest one among
these doped films. In addition, it exhibits higher PL quantum
efficiency (hPL) of 77% than the other two, which can be attrib-
uted to more efficient energy transfer from the host to the guest
and good confinement of triplet energy on the FIrpic molecules.
Moreover, simultaneous PL spectra of all the co-deposited films
feature emission bands at 475 and 510 nm, which are identical to
the photoluminescent bands of FIrpic, indicating the emission
directly from the triplet excited state of FIrpic (Figure S1a†).
Ir(PPy)3 doped into TCPB, TCPY, and TCPM also exhibit
nearly mono-exponential decay curves with lifetimes between
1.17 and 1.26 ms and high hPL of about 84%. Their radiative rate
constants (kr) are between 6.67 ꢃ 105 and 7.18 ꢃ 105 sꢂ1, and knrs
are between 1.19 ꢃ 105 and 1.37 ꢃ 105 sꢂ1. Both are slightly
influenced by the different hosts. Photoluminescent spectra of
these co-deposited films feature an emission band at 520 nm with
a shoulder at 550 nm, which are identical to the photo-
luminescent bands of Ir(PPy)3 (Figure S1b†).
Fig. 3 Transient photoluminescence decay curves for the blue (FIrpic)
(a), green (Ir(PPy)3) (b), and red (Ir(piq)3) (c) triplet emitters co-deposited
with TCPB, TCPY, and TCPM.
general blue, green, and red phosphors to all the hosts is
successfully suppressed and the energy is well-confined on the
phosphor molecules due to higher ET of the hosts than the guests
although slightly decreased ET is obtained with introducing
heterocycles as the core.
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 or electron-transport materials,
balanced hole and electron mobility is ideal for a host material.
Hole/electron mobilities of the hosts were measured by using
Table 3 Phosphorescence lifetime (s), photoluminescence quantum
efficiency (hPL), radiative and nonradiative rate constants (kr and knr) of
FIrpic, Ir(PPy)3, and Ir(piq)3 co-deposited with TCPB, TCPY, and
TCPM
For Ir(piq)3 with the lowest triplet energy among these phos-
phorescent emitters, all the co-deposited films clearly exhibit
almost identical mono-exponential decay curves and relatively
long lifetimes of about 1.15 ms. They also show hPL of 54%, which
is the highest level for the Ir(piq)3-doped films reported previ-
ously. Their kr and knr are also slightly affected by the change of
the central arylene. In addition, they also show identical photo-
luminescent spectra (Figure S1c†), which originate from the
relaxation of the triplet excited states of Ir(piq)3. The transient
PL observation indicates that the triplet energy transfer from the
Guests
FIrpic
Hosts
s (ms) hPL (%) kr (ꢃ 105 sꢂ1
)
knr (ꢃ 105 sꢂ1
)
TCPB
TCPY
TCPM 1.38
1.51
1.60
77
72
72
84
85
83
54
54
53
5.08
4.50
5.22
7.18
6.73
6.67
4.72
4.69
4.67
1.52
1.75
2.03
1.37
1.19
1.37
4.03
3.99
4.15
Ir(PPy)3 TCPB
TCPY
1.17
1.26
TCPM 1.24
TCPB
TCPY
Ir(piq)3
1.14
1.15
TCPM 1.13
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a conventional photo-induced time-of-flight (TOF) technique
using a nitrogen laser as an excitation source (l ¼ 337 nm). Fig. 4
shows their hole (mh) and electron (me) mobilities plotted as
a function of the square root of the electric field (E). In general,
the electron mobility of many hosts is much lower than the
hole mobility because of the fact that they mainly consist of
strong electron donors, like aromatic amines or carbazoles. As
an example, mh of 1,3-bis(3-(carbazol-9-yl)phenyl)benzene
(DCzPB),34 a host material without any heterocycles, is one order
of magnitude higher than its me. Similarly, mh of the carbazole-
based host material TCPB lies in the range from 2.4 ꢃ 10ꢂ5 to 4.1
ꢃ 10ꢂ5 cm2 Vꢂ1 sꢂ1 at an electric field between 7.2 ꢃ 105 and 1.0 ꢃ
106 V cmꢂ1, which is five times higher than its me, ranging from 4.6
ꢃ 10ꢂ6 to 8.5 ꢃ 10ꢂ6 cm2 Vꢂ1 sꢂ1 at an electric field between 5.6 ꢃ
105 and 1.0 ꢃ 106 V cmꢂ1, which can be attributed to the
carbazole moiety. In contrast, with introducing a pyridine ring
instead of the central benzene, two orders of magnitude higher me
in the range from 3.8 ꢃ 10ꢂ4 to 4.7 ꢃ 10ꢂ4 cm2 Vꢂ1 sꢂ1 was
achieved for TCPY at an electric field between 6.4 ꢃ 105 and 1.0
ꢃ 106 V cmꢂ1. In addition, most importantly, its mh is quite
similar to its me, suggesting that TCPY behaves more like
a bipolar host material. Similar phenomenon was also found for
TCPM, but its mh and me are nearly one order of magnitude lower
than those of TCPY, but higher than those of TCPB. It is
anticipated that improved carrier injection, transport, and
recombination in the EML based on TCPY and TCPM should
be achieved due to their improved carrier mobility and bipo-
larity. Compared with the host materials with the same hetero-
cyclic core and two phenyl carbazole arms,34 hole/electron
mobilities of TCPY and TCPM are one order of magnitude
higher. Moreover, they behave more like a bipolar host material
because of their similar mh and me.
Fig. 5 External quantum efficiency (hext) (closed) and power efficiency
(hP) (open) vs. luminance (L) characteristics of the RGB phosphorescent
OLEDs in structures: (a) ITO/TPDPES:TBPAH (20 nm)/3DTAPBP
(30 nm)/host:11 wt% FIrpic (10 nm)/BP4mPy (40 nm)/LiF (0.5 nm)/Al
(100 nm); (b) ITO/TPDPES:TBPAH (20 nm)/TAPC (30 nm)/host:8 wt%
Ir(PPy)3 (10 nm)/TmPyBPZ (50 nm)/LiF (0.5 nm)/Al (100 nm); (c)
ITO/TPDPES:TBPAH (20 nm)/TAPC (35 nm)/host:4 wt% Ir(piq)3
(10 nm)/TmPyBPZ (65 nm)/LiF (0.5 nm)/Al (100 nm). Host: TCPB
(BC), TCPY (,-), and TCPM (>A).
RGB phosphorescent OLEDs were fabricated by doping
Ir(piq)3, Ir(PPy)3, and FIrpic into the developed host materials as
the EMLs, respectively. For the blue phosphorescent OLEDs
based on FIrpic, the highest efficiency was achieved for the device
based on TCPB, giving a maximum external quantum efficiency
(hext) of 26.0% and a maximum power efficiency (hP) of 65.4 lm
Wꢂ1, which rolls to 20.5%, 39.3 lm Wꢂ1 and 17.3%, 27.1 lm Wꢂ1
at 100 and 1000 cd mꢂ2, respectively (Table 4). In comparison,
slightly lower efficiencies of 19.5% and 36.2 lm Wꢂ1 were
obtained at 100 cd mꢂ2 for the device based on TCPY. For the
device based on TCPM with a pyrimidine core, the efficiencies at
100 cd mꢂ2 are 18.0% and 35.9 lm Wꢂ1. The reduced efficiency
may be partly attributed to the lower hPL of the co-deposited
films of TCPY:FIrpic and TCPM:FIrpic in comparison to
TCPB:FIrpic. Compared with the devices based on the corre-
sponding two-arm compounds in a similar device architecture,
the efficiencies of the devices based on TCPB and TCPY are
somewhat lower than those of the devices based on DCzPB and
26DCzPPy, and it can be attributed to the lower hPL of the co-
deposited films. However, the efficiencies of the devices based on
TCPY and TCPM are much higher than those of the devices
based on 24DCzPPy, 24DCzPPm, and 46DCzPPm due to the
improved bipolarity and thus the improved carrier balance.34
Considering the reduced LUMO energy level and improved
electron mobility of TCPY and TCPM, there should be a reduced
carrier balance for the devices based on TCPY and TCPM. Note
that FIrpic is a well-known electron-transport triplet emitter and
the concentration of FIrpic is relatively high (11 wt%). The
electrons may be partly injected into the EML through the FIrpic
molecules. It is shown in Figure S2a† that the current density for
Fig. 4 Hole (mh) (open) and electron (me) (closed) mobilities of the neat
films for TCPB (CB), TCPY (-,), and TCPM (A>) plotted as
a function of the square root of the electric field (E).
3452 | J. Mater. Chem., 2012, 22, 3447–3456
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Table 4 Performance data of the RGB phosphorescent OLEDs using TCPB, TCPY, and TCPM as the host materials
Maximum efficiency
at 100 cd mꢂ2 at 1000 cd mꢂ2
Guests
FIrpic
Hosts
PE (lm Wꢂ1
)
hext (%)
V (V)
PE (lm Wꢂ1
)
hext (%)
V (V)
PE (lm Wꢂ1
)
hext (%)
TCPB
TCPY
TCPM
TCPB
TCPY
TCPM
TCPB
TCPY
TCPM
65.4
56.2
51.9
107
119
116
19.8
19.3
18.7
26.0
24.4
21.2
23.4
26.0
25.6
18.6
18.4
18.2
3.68
3.72
3.69
2.76
2.71
2.66
3.83
3.54
3.08
39.3
36.2
35.9
92.3
108
109
7.3
20.5
19.5
18.0
22.5
25.7
25.6
10.2
15.5
17.8
4.49
4.56
4.49
3.21
3.19
3.18
5.71
5.60
4.25
27.1
26.5
24.9
68.2
84.8
89.1
3.83
6.72
9.80
17.3
17.5
15.2
19.4
23.7
24.9
7.93
13.8
15.4
Ir(PPy)3
Ir(piq)3
12.0
15.6
these devices is quite similar regardless of the change of the host.
The slightly reduced efficiency for the devices based on TCPY
and TCPM should also be attributed to the reduced carrier
balance recombined in the EML. EL spectra of the blue phos-
phorescent OLEDs based on TCPB, TCPY, and TCPM peak at
473 nm with a shoulder at 500 nm, which are identical to those
reported previously, indicating the emission directly from the
excited state of FIrpic (Figure S3a†).35,37
achieved at low driving voltages. With an increase of current
density, a steep decrease of hext was found for the device based on
TCPB. The steep decrease in hext can be attributed to the reduced
carrier balance at high current density. Considering relatively
low-lying LUMO (3.40 eV) and high-lying HOMO (5.40 eV)
energy levels of Ir(piq)3, it is generally thought that carriers are
injected into the EML through the Ir(piq)3 molecules. However,
note that the concentration of Ir(piq)3 is as low as 4 wt%. At low
current density, carriers may be injected into the EML through
the Ir(piq)3 molecules due to its lower-lying LUMO and higher-
lying HOMO energy levels compared with the hosts. But at high
current density, more carriers must be injected into the EML
through the host molecules. Relatively high-lying LUMO energy
level and low electron-mobility of TCPB indicate that TCPB
more likely behaves as a hole-transporter and thus facilitate hole-
injection and transport into the EML to give a reduced carrier
balance. The reduced carrier balance induces decreased carrier
recombination efficiency and thus reduced hext. In addition, the
imbalance of the carriers also causes domination of hole polaron
(h+), and a large number of holes and polarons accumulate at the
interface of EML and ETL and/or in the active layer, resulting in
triplet exciton–polaron quenching and thus further efficiency
roll-off.40 On this result, hext rolls off to 10.2% at 100 cd mꢂ2 and
7.93% at 1000 cd mꢂ2. Besides the reduced carrier balance,
relatively high driving voltages of 3.83 and 5.71 V were obtained
at 100 and 1000 cd mꢂ2, respectively, which can be attributed to
the high-lying LUMO energy level and low electron mobility of
TCPB and thus a high electron injection barrier from the electron
transport layer. Due to high driving voltages, hP further rolls off
to 7.30 lm Wꢂ1 at 100 cd mꢂ2 and 3.83 lm Wꢂ1 at 1000 cd mꢂ2. On
this reason, similar steep decreases in hext and hP have also been
found for the other host materials based on carbazole building
blocks, like CBP.34
For the green phosphorescent OLEDs, reduced driving volt-
ages were achieved for the devices based on TCPY and TCPM in
comparison to the device based on TCPB, especially at bright-
ness lower than 1000 cd mꢂ2. Considering the reduced LUMO
energy level and improved electron mobility of TCPY and
TCPM, it indicates more carriers should be injected into the
EML through the host molecules. In addition, the highest effi-
ciency was achieved for TCPM, exhibiting hext of 25.6% and hP
of 109 lm Wꢂ1 at 100 cd mꢂ2. Although the electron-transport
materials utilized are different to give a different driving voltage,
its hext values are higher than those of the devices based on the
corresponding two-arm compounds of 24DCzPPm and
46DCzPPm due to the improved bipolarity and thus the
improved carrier balance.34 In addition, hext values of the devices
based on TCPB and TCPY are also comparable to those of the
devices based on the corresponding two-arm compounds of
DCzPB, 26DCzPPy, and 24DCzPPy. Although hPL of Ir(PPy)3
doped into TCPB, TCPY, and TCPM are quite similar, the
improved efficiency achieved for the hosts of TCPY and TCPM
with heterocyclic cores can be attributed to their lower-lying
LUMO energy levels and improved bipolarity compared with
TCPB, leading to an improved electron injection and transport
and thus an improved carrier balance. In addition, besides the
high efficiency achieved by the host of TCPM, its hext roll very
slightly from a display-relevant luminance of 100 cd mꢂ2 to an
illumination-relevant luminance of 1000 cd mꢂ2. Even at a much
brighter luminance of 10 000 cd mꢂ2, hext remains 21.9% for the
device based on TCPM, compared with 12.1% and 19.6% for the
devices based on TCPB and TCPY, respectively (Fig. 5). Similar
to the blue phosphorescent OLEDs, EL spectra of the green
phosphorescent OLEDs based on TCPB, TCPY, and TCPM
peak at 515 nm with a shoulder at 550 nm, and they are identical
with those reported previously.38,39
Although efficiency roll-off was also found for the device
based on TCPY, it was significantly suppressed compared with
the device based on TCPB, giving hext values of 15.5% and 13.8%
and hP values of 12.0 and 6.72 lm Wꢂ1 at 100 and 1000 cd mꢂ2
,
respectively. Compared with the devices based on the host
materials containing a pyridine core and two phenyl carbazole
arms, efficiency roll-off was also significantly suppressed, giving
improved hext and hP (Figure S4a†). The suppressed efficiency
roll-off can be attributed to its lower-lying LUMO energy level
and improved bipolarity, giving improved carrier balance and
thus suppressed triplet exciton–polaron quenching at high
current density. Besides the improved efficiency, reduced driving
For the red phosphorescent OLEDs, all the devices show very
high maximum hext over 18% and maximum hP as ꢀ19 lm Wꢂ1 at
low current density. Considering the similar hPL values of the
co-deposited films of host:Ir(piq)3, similar carrier balances is
This journal is ª The Royal Society of Chemistry 2012
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voltage was also achieved for the device based on TCPY in
comparison with the device based on TCPB, which can be
were measured by using an integrating sphere under nitrogen gas
flow at room temperature.
attributed to its lower-lying LUMO energy level, smaller DEST
and increased carrier mobility (Figure S2c†).
,
Calculations
Efficiency roll-off was further suppressed for the device based
on TCPM with a pyrimidine core, giving hext values of 17.8% and
15.4% and hP values of 15.6 and 9.80 lm Wꢂ1 at 100 and 1000 cd
mꢂ2, respectively, which is one of the highest efficiencies for the
Ir(piq)3-based red phosphorescent OLEDs.41–44 Moreover,
further decreased driving voltages of 3.08 and 4.25 V were ach-
ieved at 100 and 1000 cd mꢂ2, respectively, which are even equal
to or lower than those of the devices based on a p-i-n architec-
ture,43,44 and it can be attributed to its further decreased LUMO
energy level, narrower Eg, and smaller DEST achieved with
introducing pyrimidine as the core. The reduced driving voltage
achieved with the change of host material further proves that
more carriers must be injected into the EML through the host
molecules rather than direct carrier trap by the dopant, especially
at high current density. In addition, the device based on TCPM
exhibits a lower driving voltage compared with the devices based
on the host materials containing a pyrimidine core and two
phenyl carbazole arms due to its high hole/electron mobilities.
Although a slightly lower hext is achieved at a luminance higher
than 800 cd mꢂ2, its hP is still higher than the other two due to the
reduced driving voltage (Figure S4b†). The emission color is deep
red with an EL emission peak at 621 nm and Commission
Internationale de L0Eclairage (CIE) color coordinates of (0.67,
0.33), independent of the choice of hosts. This proves the emis-
sion directly from the triplet emitter Ir(piq)3.
Density functional theory (DFT) calculations were performed by
using the Gaussian suite of programs (Gaussian 03W). For the
calculation of HOMO and LUMO energy levels, the ground
state structures were optimized at the restricted B3LYP/6-31G(d)
level, and the single-point energies were calculated at the
restricted B3LYP/6-311+G(d,p) level. The ground state (S0)
structure optimization and single-point energy calculation were
performed at the B3LYP/6-31G(d) level. In contrast, the lowest-
energy triplet excited state (T1) structure optimization and single-
point energy calculation were performed at the unrestricted
B3LYP/6-31G(d) level.45
Device fabrication and characterization
Phosphorescent OLEDs were grown on glass substrates pre-
coated with a ꢀ110-nm-thick layer of indium-tin oxide (ITO)
having a sheet resistance of 15 U/,. The substrates were cleaned
with ultrapurified water and organic solvents 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 hex-
achloroantimonate (TBPAH) was spun onto the precleaned
substrate from its dichloroethane solution to form a 20-nm-thick
polymer buffer layer.46 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 the host materials to form a 10-nm-thick
EML. 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)38
was deposited to block holes and to confine excitons in the
emissive zone. For the green phosphorescent OLEDs, TAPC
(30 nm), Ir(PPy)3 (7 wt%):host (10 nm), and TmPyBPZ (50 nm)
were successively deposited as the HTL, EML, and ETL,
Experimental
Characterization
All the developed host materials were purified by silica gel
chromatography and then repeated thermal gradient vacuum
1
sublimation before characterization and device fabrication. H
and 13C NMR spectra were recorded on a Varian 500 (500 MHz)
spectrometer. Mass spectra were obtained using a JEOL JMS-K9
mass spectrometer. Elemental analysis was performed on a Vario
EL elemental analysis instrument (Elementar Co.). Differential
scanning calorimetry (DSC) was performed using a Perkin-
Elmer Diamond DSC Pyris instrument under a nitrogen atmo-
sphere at a heating rate of 10 ꢁC minꢂ1. Thermogravimetric
analysis (TGA) was undertaken using a SEIKO EXSTAR 6000
TG/DTA 6200 unit under a nitrogen atmosphere at a heating
rate of 10 ꢁC minꢂ1. UV-Vis spectra were measured using a Shi-
madzu UV-3150 UV-vis-NIR spectrophotometer. Steady-state
PL spectra were obtained at room temperature using a Flur-
oMax-2 (Jobin-Yvon-Spex) luminescence spectrometer. Ioniza-
tion potentials were determined by atmospheric ultraviolet
photoelectron spectroscopy (Rikken Keiki AC-3). Time-resolved
emission spectra were obtained at T ¼ 4.2 K under excitation by
a nitrogen laser (l ¼ 337 nm, 50 Hz, 800 ps pulses) combined with
a streak scope C4334 (Hamamatsu) and a synchronous delay
generator C4792-02 (Hamamatsu). In comparison, transient PL
decays 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
respectively.
2,20-Bis(m-di-p-tolylaminophenyl)-1,10-biphenyl
(3DTAPBP) (30 nm), FIrpic (11 wt%):host (10 nm), and 3,5,30,50-
tetra(m-pyrid-3-yl)phenyl-(1,10)-biphenyl (BP4mPy)39 (40 nm)
were successively deposited as the HTL, EML, and ETL,
respectively, for the blue phosphorescent OLEDs. Molecular
structures and acronyms of the materials used for device fabri-
cation are shown in Scheme S2.† 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 mm ꢃ 2
mm openings. The electroluminescent (EL) spectra were recor-
ded by an optical multichannel analyzer, Hamamatsu PMA 11.
The current density and luminance versus driving voltage char-
acteristics were measured by Keithley source-measure unit 2400
and Konica Minolta chroma meter CS-200, respectively. hext is
calculated from the luminance, current density, and EL spectra,
assuming a Lambertian distribution.
For the carrier mobility measurement, neat films of host
materials (ꢀ10 mm) were vacuum-deposited on ITO-coated
substrates. A semi-transparent Al layer was patterned using
3454 | J. Mater. Chem., 2012, 22, 3447–3456
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a shadow mask with an array of 2 mm ꢃ 2 mm openings. The
hole/electron mobilities were measured by using a conventional
photo-induced TOF technique. A nitrogen laser was used as the
excitation source (l ¼ 337 nm) and was incident on the sample
through the ITO or semi-transparent Al electrode.
Matoliukstyte,
J.
Simokaitiene,
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Grigalevicius,
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29 Y. Tao, Q. Wang, C. Yang, Q. Wang, Z. Zhang, T. Zou, J. Qin and
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30 F.-M. Hsu, C.-H. Chien, P.-I. Shih and C.-F. Shu, Chem. Mater.,
2009, 21, 1017.
We report a series of star-shaped host materials containing
various arylene cores, like benzene, pyridine, and pyrimidine, for
RGB phosphorescent OLEDs. Reduced LUMO energy levels are
achieved by introducing heterocyclic cores of pyridine and
pyrimidine instead of benzene, and the more nitrogen atoms in
the heterocyclic core, the lower-lying LUMO energy level is
achieved. In addition, narrower energy band gaps, lower energy
singlet and triplet excited states, smaller energy difference DEST
,
and improved bipolarity are also achieved with the hosts con-
taining a heterocyclic core. Compared with the slightly reduced
ET, the significantly decreased DEST achieved with introducing
heterocyclic arylenes as the core indicates that introducing
appropriate electron donors and acceptors as building blocks of
a host material may be an effective route to give attractive
candidate for phosphorescent OLEDs with reduced driving
voltage as well as good triplet exciton confinement. As a result,
reduced driving voltage was achieved for the green and red
phosphorescent OLEDs based on TCPY and TCPM, giving
improved efficiency as well as suppressed efficiency roll-off.
Acknowledgements
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), and the Fundamental Research Funds for the
Central Universities (2011ZZ0002).
31 S. O. Jeon, K. S. Yook, C. W. Joo and J. Y. Lee, Adv. Funct. Mater.,
2009, 19, 3644.
32 S. O. Jeon, K. S. Yook, C. W. Joo and J. Y. Lee, Adv. Mater., 2010,
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33 H.-H. Chou and C.-H. Cheng, Adv. Mater., 2010, 22, 2468.
34 S.-J. Su, C. Cai and J. Kido, Chem. Mater., 2011, 23, 274.
35 S.-J. Su, T. Chiba, T. Takeda and J. Kido, Adv. Mater., 2008, 20,
2125.
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