Towards high efficiency and low roll-off orange electrophosphorescent devices by fine tuning singlet and triplet energies of bipolar hosts based on indolocarbazole/1, 3, 5-triazine hybrids

Article abstract of DOI:10.1002/adfm.201303926

Orange-emitting phosphorescent organic light-emitting diodes (PHOLEDs) are drawing more and more attention; however, high-performance hosts designed for orange PHOLEDs are rare. Here, four indolocarbazole/1, 3, 5-triazine hybrids are synthesized to optimize the singlet and triplet energies, as well as transporting properties, for ideal orange PHOLEDs. By introducing moieties with different electronegativity, a graded reduction of the singlet and triplet energies is achieved, resulting in minimum injection barrier and minimum energy loss. Besides, the charge transporting abilities are also tuned to be balanced on the basis of the bipolar features of those materials. The optimized orange PHOLED shows a maximum external quantum efficiency (EQE) of 24.5% and a power efficiency of 64 lm W^-1, both of which are among the best values for orange PHOLEDs. What is more, the efficiency roll-off is extremely small, with an EQE of 24.4% at 1000 cd m^-2 and 23.8% at 10 000 cd m^-2, respectively, which is the lowest efficiency roll-off for orange PHOLEDs to date, resulting in the highest EQE for orange PHOLEDs when the luminance is above 1000 cd m^-2. Besides the balanced charges, the small roll-off is also attributed to the wide recombination zone resulting from the bipolar features of the hosts. A series of indolocarbazole/1, 3, 5-triazine hybrids are designed and prepared as hosts for phosphorescent orange-emitting diodes. Singlet-triplet splitting and carrier injection/transporting are adjusted by introducing moieties with different electronegativity. By harmonizing the optical and electrical properties, high efficiency with ultra-low roll-off is realized in the device.

Full text of DOI:10.1002/adfm.201303926

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Towards High Efficiency and Low Roll-Off Orange
Electrophosphorescent Devices by Fine Tuning
Singlet and Triplet Energies of Bipolar Hosts Based on
Indolocarbazole/1, 3, 5-Triazine Hybrids
Dongdong Zhang, Lian Duan,* Yilang Li, Haoyuan Li, Zhengyang Bin, Deqiang Zhang,
Juan Qiao, Guifang Dong, Liduo Wang, and Yong Qiu
and solid-state lighting and thus being
Orange-emitting phosphorescent organic light-emitting diodes (PHOLEDs)
widely researched.[5–8] Therefore, high per-
are drawing more and more attention; however, high-performance hosts
designed for orange PHOLEDs are rare. Here, four indolocarbazole/1, 3,
5-triazine hybrids are synthesized to optimize the singlet and triplet energies,
as well as transporting properties, for ideal orange PHOLEDs. By introducing
moieties with different electronegativity, a graded reduction of the singlet
and triplet energies is achieved, resulting in minimum injection barrier and
minimum energy loss. Besides, the charge transporting abilities are also
tuned to be balanced on the basis of the bipolar features of those mate-
rials. The optimized orange PHOLED shows a maximum external quantum
efficiency (EQE) of 24.5% and a power efficiency of 64 lm W–1, both of which
are among the best values for orange PHOLEDs. What is more, the efficiency
roll-off is extremely small, with an EQE of 24.4% at 1000 cd m–2 and 23.8%
at 10 000 cd m–2, respectively, which is the lowest efficiency roll-off for orange
PHOLEDs to date, resulting in the highest EQE for orange PHOLEDs when
the luminance is above 1000 cd m–2. Besides the balanced charges, the small
roll-off is also attributed to the wide recombination zone resulting from the
bipolar features of the hosts.
formance orange PHOLEDs are urgently
desired. To get high performance phos-
phorescent devices, hosts and phosphors
are equally crucial. However, although
orange phosphors have been widely
reported,[9–13]
researches
exclusively
focused on hosts for orange PHOLEDs are
rarely reported and their performances are
far from satisfaction.
A suitable host material for PHOLEDs
should possess both a first triplet energy
level (T1) high enough to fit the excited
level of the emitting phosphors, and a
first singlet energy level (S1) low enough
to improve carrier injecting into the emit-
ting layers (EMLs) and reduce the driving
voltage.[14–16] Besides, to obtain balanced
charge transporting, hosts with bipolar
features are highly desired.[17] Materials
possessing small singlet-triplet splits
(ΔEST) show the potential to satisfy those
demands through carefully design. One
strategy to achieve small ΔEST is to use exciplex-forming co-
hosts,[18–21] which possess a small ΔEST via the energetically
degenerated intermolecular charge transfer states. Since the
co-host incorporates an electron transporting material and a
hole transporting material in one layer, reduced injection bar-
riers and balanced charge transport can also be anticipated.
Using this co-host strategy, Kim et al. demonstrated an orange
PHOLED with ultimate performance: a high external quantum
efficiency (EQE) of 25%, a low turn-on voltage of 2.4 V and
low efficiency roll-off.[20] A small ΔEST can also be achieved via
intramolecular charge transfer states, which is attainable in
systems containing spatially separated highest occupied molec-
ular orbital (HOMO) and lowest unoccupied molecular orbital
(LUMO). In fact, ΔESTs of these materials can be low enough
to boost thermally activated delayed fluorescence (TADF).[22–25]
Moreover, bearing donor and acceptor moieties, such mol-
ecules also possess bipolar features and thus balanced charge
transporting properties. Recently we have demonstrated by
using a indolocarbazole/ 1,3,5-triazine hybrid host (PIC-TRZ)
with small ΔEST of 0.11 eV as the host for a green-emitter,
1. Introduction
Besides blue, green, and red phosphorescent organic light-
emitting diodes (PHOLEDs), orange PHOLEDs are drawing
more and more attention due to their potential applications
in lithography labs, traffic and signal lights, and high quality
RGBY-TV. Moreover, orange PHOLEDs can also be used to
fabricate white organic light-emitting diodes (WOLEDs),[1–4]
which show their potential applications for flat panel displays
D. Zhang, Dr. L. Duan, Y. Li, H. Li, Z. Bin, Dr. D. Zhang,
Dr. J. Qiao, Dr. G. Dong, Prof. L. Wang. Prof. Y. Qiu
Key Lab of Organic Optoelectronics and
Molecular Engineering of Ministry of Education
Department of Chemistry, Tsinghua University
Beijing 100084, China
E-mail: duanl@mail.tsinghua.edu.cn
DOI: 10.1002/adfm.201303926
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tris(2-phenylpyridinato-C2,N)iridium(III) (Ir(ppy)3), extremely
low voltage OLEDs can be achieved with small efficiency roll-
off, indicating that indolocarbazole/1,3,5-triazine hybrids
are promising to construct well performed orange PHOLED
hosts.[26] Therefore, by fine tuning singlet and triplet energies
of the materials, ideal hosts with low driving voltages for emit-
ters with different colors can be anticipated.[15]
In this work, to gain insight into the relationship between
the bipolar molecular structures and the optoelectronic prop-
erties, and to reveal the influence of ΔEST on the performance
of PHOLEDs, moieties with different electron-withdrawing
and -donating properties were introduced in the indolocarba-
zole/1, 3, 5-triazine hybrids to tune the energy level, the trans-
porting ability and the ΔEST of the hosts. Among those deriva-
tives, an electron-donating phenoxy unit makes shallower but
better overlapped HOMO and LUMO levels and thus a larger
ΔEST. While an electron-withdrawing cyanophenyl unit makes
deeper but more separated HOMO and LUMO, thus a small
ΔEST. Orange PHOLEDs based on the four materials as hosts
were fabricated. A maximum EQE of 24.5% and a power effi-
ciency of 64 lm W–1 were obtained using 2-phenyl-4,6-bis(12-
phenylindolo[2,3-a] carbazole-11-yl)-1,3,5-triazine (PBICT) as
the host, comparable with the best co-host device in a simplified
structure. What is more, the efficiency roll-off was extremely
small, with an EQE of 24.4% at 1000 cd m–2 and 23.8% at
10 000 cd m–2, respectively, which was the lowest efficiency
roll-off for orange PHOLEDs to date. Even when the brightness
reaches 30 000 cd m–2, the EQE is still 21.6%. To the best of
our knowledge, these efficiencies at high luminances (above
1000 cd m–2) are the highest reported for orange OLEDs.
2. Results and Discussion
2.1. Synthesis and Characterization
The compounds (as is shown in Figure 1), 2- phenoxy-4,6-bis(12-
phenylindolo[2,3-a] carbazole-11-yl)-1,3,5-triazine (POBICT),
4,6-bis(12-phenylindolo[2,3-a] carbazole-11-yl)- 1,3,5-triazine
(BICT), PBICT and 2-benzenecyano-4,6-bis(12-phenylindolo[2,3-a]
carbazole-11-yl)-1,3,5-triazine (BBICT), were facilely prepared
through aromatic nucleophilic substitution reactions. All of the
1
compounds were fully characterized using H NMR spectros-
copy, mass spectrometry and elemental analysis. The strategy
Figure 1. The energy-transfer process in phosphorescent doping systems and molecular structures. The arrows on the structures of molecules indicate
the orientation where the property of electron withdrawing enhances. The darker parts of the molecules represent the electron-donating units while the
brighter parts are the electron-withdrawing ones. FET stands for forward energy transfer while BET means back energy transfer.
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for molecular design is based on the introduction of moieties
with different electronegativity to tune the electronic proper-
ties of the molecular backbone of BICT, whose π-conjugation
is significantly distorted by steric hindrance introduced through
bulky substituents and thus helpful to separate HOMO and
LUMO distribution to reduce the ΔEST of the molecule.
2.2. Theoretical Calculations
To understand the structure-property relationship of the com-
pounds at the molecular level, the geometrical and electronic
properties of the compounds were studied using density func-
tional theory (DFT) and time-dependent DFT (TDDFT) calcu-
lations using the B3LYP hybrid functional (for details, see the
Experimental Section).
As can be seen from Figure 2, according to the DFT cal-
culations, all the compounds show twisted structures, with
the dihedral angles between the indolocarbazole unit and the
1, 3, 5- triazine ring of 43.94°, 43.13°, 40.96°, and 40.75° for
POBICT, BICT, PBICT, and BBICT, respectively. Besides, the
two indolocarbazole units are nearly parallel in all molecules,
suggesting the possibility of intramolecular π–π stacking
in single crystals. These results are corresponding to the
experimental values from the single-crystal X-ray diffraction
analysis.
The HOMO and LUMO distribution of the hosts are largely
different with overlapped distribution for POBICT and BICT
while separated ones for PBICT and BBICT, indicating the
donating or withdrawing properties of the moieties have great
influence on the distribution of the frontier energy levels. The
compounds except BICT can be treated as three-arm structures
with triazine as the core. With all the three arms of POBICT
showing electron-donating effect, there is no enough electron-
withdrawing spaces for the LUMO to locate, resulting in over-
lapped HOMO and LUMO distribution. On the contrary, a neu-
tral moiety of benzene in PBICT and an electron withdrawing
moiety of cyanophenyl in BBICT provide large spaces for
LUMOs to locate, while the HOMOs locate on those electron-
donating indolocarbozole arms, resulting in separated HOMO
and LUMO distribution. Herein, since the ΔEST is proportional
to the exchange energy, which is proportional to the overlap
between HOMO and LUMO,[22] it is well assumed that PBICT
and BBICT have smaller ΔEST than POBICT and BICT. From
The molecular structures of POBICT, BICT and BBICT
were further determined by single-crystal X-ray crystal-
lographic analysis (see Supporting Information Figure
S1). The dihedral angles between the indolocarbazole unit
and the 1, 3, 5-triazine ring were measured to be 44.994°,
47.499°, and 34.391° for POBICT, BICT and BBICT, respec-
tively, resulting in twisted structures as expected. Intramo-
lecular π–π stacking formed between the two indolocarbazole
units can be seen in all the three single-crystal structures.
The centroid-to-centroid distance between the π-stacking
five-membered rings containing N7 and N5 is measured
to be 3.589 Å, 3.779 Å, and 3.503 Å for POBICT, BICT, and
BBICT, respectively. Moreover, for BICT, the intermolecular
π–π stacking is also formed between indolocarbazole units
with a shortest centroid-to-centroid distance of 3.621 Å. The
intermolecular π–π stacking facilitates the hole transporting,
however, to keep uniform films, strong intermolecular
forces have to be avoided. Usually, the approach to prevent
such forces is the introduction of bulky substituents, which
leads to a larger intermolecular distance and a hindrance in
packing and therefore an amorphous behavior. But on the
other hand, the increased distance separates the conducting
units from each other and results in decreased charge car-
rier mobility.[27] Here, the introduction of flexible phenoxy
unit and cyanophenyl unit in POBICT and BBICT prevents
the intermolucular molecular π–π stacking. At the same time,
the distances between the conducting units are not notice-
ably affected because of the special “Y”-like structures of the
hosts, as can be seen from Figure 1, resulting in high charge
transport abilities.
Figure 2. Calculated spatial distributions of the HOMO and LUMO levels of the hosts.
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the triplet energy and the singlet energies calculated, ΔESTs are
in the order of BICT (0.51 eV) ≈ POBICT (0.49 eV) > PBICT
(0.15 eV) > BBICT (0.07 eV), in good agreement with expec-
tation. The strong withdrawing property of cyano unit renders
BBICT with more separated HOMO and LUMO distribution
than PBICT, and thus the smallest ΔEST.
well with the absorption spectrum of 12-phenylindolo[2,3-
a] carbazole (PIC) (see Supporting Information Figure S3).
Therefore, these peaks can be attributed to the intrinsic n–π*
or π–π* transition of 12-phenylindolo[2,3-a]carbazole. Besides
those peaks, all hosts show a longer-wavelength absorption at
378 nm, possibly due to the charge transition from the electron-
donating indolo[2,3-a]carbazole moiety to the electron-accepting
1, 3, 5- triazine moiety. The assignment is manifested by the
fact that the intensity of the charge transition for PBICT and
BBICT is weaker in comparison to that of POBICT and BICT,
which may be ascribed to the suppression of intramolecular
charge transfer owing to the small overlapping between PBICT
and BBICT. In addition, even longer-wavelength absorption
between 400–450 nm can be seen from the absorption spectra
of PBICT and BBICT, resulting from the long conjugation of 2-
phenyl-1, 3, 5-triazine and 2- cyanophenyl-1, 3, 5-triazine.
The emission peaks of the compounds (Figure 3) are in a
range from deep blue to green with emission peaks of POBICT
at 412 nm, BICT at 425 nm, PBICT at 469 nm and BBICT at
516 nm, respectively, measured in the solution of dichlo-
romethane. The electron-donating effect of phenoxy unit makes
the emission of POBICT hypsochromic shift, while the elec-
tron-withdrawing inductive effect of cyanophenyl unit makes
that of BBICT bathochromic shift. The triplet energies, deter-
mined by the highest-energy vibronic sub-band of the phos-
phorescence spectra at 77 K with a delay time of 20 µs, follow
2.3. Thermal Properties
The good thermal stability of the compounds (Supporting
Information Figure S2) is indicated by their high decomposi-
tion temperatures, Td (corresponding to 5% weight loss), in the
range of 390–428 °C, determined through thermogravimetric
analysis (TGA). Their glass-transition temperatures (Tg), deter-
mined through differential scanning calorimetry, are between
164 °C and 202 °C, which are much higher than those of car-
bozole-based compounds, resulting from the bulky substituent
of indolocarbazole and the non-planar structures. These values
indicate that these compounds can form uniform amorphous
films upon evaporation, which is highly important in improving
the efficiency and lifetime of OLEDs.
2.4. Photophysical Properties
As can be seen from Figure 3, all the compounds show absorp-
tion peaks around 258 nm, 289 nm, and 325 nm, corresponding
Figure 3. UV-vis absorption spectra and photoluminescence (PL) spectra of hosts in dichloromethane solution at room-temperature (RT), as well as
their PL spectra in dichloromethane solution with 20 µs at 77 K for a) POBICT, b) BICT, c) PBICT, d) BBICT.
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Figure 4. Cyclic voltammogram of hosts in CH2Cl2, measured with tetrabutylammonium hexafluorophosphate (TBAPF6) as a supporting electrolyte
at a scan rate of 100 mV s–1.
the sequence of BICT (2.70 eV) ≈ POBICT (2.70 eV) > PBICT
(2.66 eV) > BBICT (2.47 eV), which are the inverse of the
π-conjugation degrees and are close to the calculation ones. The
introduction of phenoxy unit does not change the π-conjugation
degree of BICT, thus the T1 of POBICT is almost the same with
BICT. From the florescence and the phosphorescence spectra,
the ΔESTs of the hosts were calculated to be 0.34 eV, 0.28 eV,
0.10 eV, and 0.06 eV for POBICT, BICT, PBICT, and BBICT,
respectively. Those values spread in a wide range and are helpful
to reveal the influence of ΔEST on the device performance.
energy. Compared with the LUMO, the HOMO levels of the
hosts are only slightly different (Table 1). The reason is that the
introduction of benzene, phenyl or cyanophenyl greatly influ-
ence the distribution of LUMO levels, but the HOMO levels
mainly locate on the indolo[2,3-a]carbazole unity. The LUMO
energy level of BBICT was also calculated using the reduction
diagram to be 2.84 eV, which is in good agreement with the one
calculated from the energy gap.
2.6. Carrier Transporting Properites
2.5. Electrochemical Properties
To evaluate the carrier transporting characters of the hosts,
single-carrier devices of the host films with the structures
[indium tin oxide (ITO)/ 1,4,5,8,9,11-hexaazatriphenylene
hexacarbonitrile (HATCN) (10 nm)/host (100 nm)/HATCN
(10 nm)/Al (150 nm)] for the hole-only device, and [ITO/
bathophenanthroline (Bphen) (10 nm)/host (100 nm)/Bphen
(10 nm)/LiF (0.5 nm)/Al (150 nm)] for the electron-only device,
were fabricated. For the hole-only devices, due to the large elec-
tron injection barriers between HATCN (LUMO = 5.58 eV)
and the hosts, only holes can be injected from the anode to the
organic layers. In the electron-only devices, an additional thin
Bphen layer, which has a low-lying HOMO level of 6.4 eV, was
inserted as a hole-blocking layer to prevent the injection of
holes from ITO (4.8 eV) to the organic layers and ensure a pure
electron current in the device. The I–V characteristics of single-
carrier devices (Figure 5) document that all the hosts are capable
The electrochemical properties of the compounds were probed
by cyclic voltammetry. As can be seen from Figure 4, all the
compounds show quasi-reversible oxidation peaks but only
BBICT shows a reversible reduction peak, which can be attrib-
uted to the introduction of electron-withdrawing cyano moie-
ties. The HOMO energy levels were determined from the
onsets of the oxidation diagrams and the LUMO levels were
calculated from the energy gaps, which were determined from
the onset of the absorption bands. Compared with the neu-
tral benzene group in PBICT (HOMO = 5.52 eV, LUMO =
2.55 eV), an electron-donating effect of phenyl in POBICT
makes the HOMO (5.43 eV) and LUMO (2.23 eV) shallower
while an electron-withdrawing inductive effect of benzonitrile
in BBICT deepens the HOMO (5.63 eV) and LUMO (2.80 eV)
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Table 1. Physical and electrochemical data of the hosts.
Compound
Tg
[°C]
Td
[°C]
Eg
Em F/P
HOMO/LUMO HOMO/LUMO
T1
ΔEST
[eV]a)
[nm]b)
exp [eV]c)
5.43/2.23
5.56/2.39
5.52/2.55
5.63/2.80
cal [eV]d)
5.20/1.30
5.29/2.09
5.19/1.85
5.34/2.45
[eV] exp/cal
2.70/2.82
2.70/2.81
2.66/2.75
2.47/2.40
[eV] exp/ cal[d]
POBICT
BICT
164
180
194
202
427
414
387
401
3.20
3.17
2.97
2.83
407/459
417/459
449/466
490/501
0.34/0.49
0.28/0.51
0.10/0.15
0.06/0.07
PBICT
BBICT
a)Calculated from the onset of the absorption spectrum; b)Fluorescence and phosphorescence at 77 K in dichloromethane; c)Determined from the onset of the oxidation
and the Eg; d)Values from DFT calculation.
of transporting both electrons and holes. The coexistence of
both hole and electron current is due to the bipolar features of
hosts. The I–V characteristic of PBICT shows the most similar
hole and electron current. The hole current of POBICT is much
larger than its electron current since the hole-transporting
ability of POBICT is enhanced by the phenoxy group with elec-
tron-donating nature, while the opposite case is observed for
BBICT due to the electron-withdrawing ability of cyano group.
The onset voltages of the I–V diagrams can reflect the injection
barriers for holes and electrons. Both the onset voltages of hole-
and electron-only devices of PBICT are small, indicating the
low energy barrier for both hole and electron injection and thus
balanced charge injection. The shallow LUMOs of POBICT
and BICT renders the electron-only devices with larger onset
voltages while the deep HOMO of BBICT makes larger onset
voltage of the hole-only device. The time-of-flight (TOF) tech-
nique was also utilized to measure the carrier mobilities of the
hosts (see Supporting Information Figure S4). Of the indolocar-
bazole/1, 3, 5-triazine hybrids, a similar trend is observed that
the electron transporting ability relative to its hole transporting
ability is gradually improved with the increase of electronega-
tivity of the substituent on the 2-position of triazine.
2.7. Phosphorescent OLEDs
Bearing donor/acceptor moieties and suitable frontier energy
levels, the indolocarbazole/triazine hybrids with triplet energy
higher than 2.2 eV are potentially ideal host materials for orange
PHOLEDs. To evaluate the performance of the hosts for orange
Figure 5. Current density versus voltage characteristics of the hole-only and electron-only devices for a) POBICT, b) BICT, c) PBICT, and d) BBICT.
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curves of efficiency versus current density for the devices, and the
EL data are summarized in Table 2 .
Form the electroluminescence spectra of devices in
Figure 7a, only the emission of PO-01 can be seen,[28] indicating
that no excitons are wasted for host emission. The lowest turn-
on voltage (at 1 cd m–2) of 2.76 V was achieved for the device
using PBICT as the host, while the highest value of 3.22 V was
obtained from device hosted by BICT. The same trend can be
observed from the current density-voltage characteristics of
devices (Figure 7b). Moreover, as can be seen from Figure 7c,
the device hosted by PBICT achieves a highest EQE of 24.5%,
higher than devices hosted by POBICT (22.0%), BICT (16.8%),
and BBICT (13.7%). In addition, the low operation voltage and
the high EQE renders the devices with high power efficiency,
which can be seen from Figure 7d. The highest power effi-
ciency is obtained from the device using PBICT as the host with
a highest value of 64.5 lm W–1 while 57.3 lm W–1 at 1000 cd m–1
and 45.4 lm W–1 at 10 000 cd m–2, respectively. The EQEs of
orange PHOLEDs with single hosts are rarely higher than
20% and their power efficiency are usually low, much lagging
behind the co-host orange devices. In this work, the EQE and
the power efficiency of device hosted by PBICT are among the
highest ones for the orange devices, comparable with the best
co-host device in a simplified structure.[20] As we know, devices
with co-hosts usually show small roll-off due to their balanced
charge transporting. Here, the efficiency roll-off of the devices
was also studied. As is shown in Figure 7e, the EQE/EQEmax
verses current density is used to represent the efficiency roll-
off of devices. All the devices show small roll-off, even when
the luminance reaches 10 000 cd m–2, over 90% of their
initial EQE values is maintained. The reason can be attributed
to the wide charge recombination zone resulting from the
Figure 6. The device structure and energy level diagram of the materials
used.
phosphorescent guests, phosphorescent OLEDs were fabricated
with an orange emitter, iridium(III) bis(4-phenylthieno[3,2-c]
pyridinato-N,C2′) acetylacetonate (PO-01), as the guest. Devices
with the structure of ITO/ HATCN (5 nm)/4,4′-N,N′-bis[N-(1-
naphthyl)-N-phenylamino]biphenyl (NPB) (40 nm)/tris(4-(9H-
carbazol-9-yl)phenyl)amine (TCTA) (10 nm)/EML (30 nm)/Bphen
(40 nm)/LiF (0.5 nm)/Al (150 nm) were fabricated, as shown in
Figure 6. Where the EML stands for POBICT: 10 wt% PO-01,
BICT: 10 wt% PO-01, PBICT: 10 wt% PO-01, BBICT: 10 wt%
PO-01, respectively. NPB and Bphen were used as hole- and elec-
tron-transporting materials, respectively, TCTA was used as elec-
tron- and exciton-blocking layer, HATCN and LiF served as the
hole- and electron-injecting layers, respectively. Figure 7 shows
the current density-voltage-luminance (J–V–L) characteristics and
Figure 7. a) Brightness-voltage characteristics. b) Current density-voltage characteristics. c) EQE-brightness characteristics. d) Power efficiency-bright-
ness characteristics. e) The EQE of the devices normalized by the maximum EQE (EQEmax) of the devices vs brightness. f) The operation lifetime of
the OLEDs measured at a brightness of 1000 cd m–2 under constant current. The insert figure in (a) is the electroluminescence spectra of devices.
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Table 2. Electroluminescence properties of the devices.
Hosts
Voltage
[V]
EQE
(%)
Power Efficiency
[lm W–1
CIE
(x,y)
]
Turn-on
@1000
@10 000
Max
@1000
@10 000
Max
@1000 cd/m2
@10 000
[cd m–2
]
[cd m–2
]
[cd m–2
]
[cd m–2
]
[cd m–2
]
POBICT
BICT
2.93
3.22
2.75
3.15
4.14
5.41
22.0
16.8
24.5
13.7
21.7
21.4
52.6
38.4
64.5
19.7
49.2
34.1
57.3
19.0
37.5
(0.508, 0.488)
(0.507, 0.487)
(0.507, 0.488)
(0.507, 0.488)
4.54
5.90
16.6
15.4
24.4
PBICT
BBICT
3.96
5.06
24.2
23.8
45.4
4.63
6.35
13.6
13.1
12.7
bipolar features of the hosts since triplet-triplet annihilation and
exciton-polaron annihilation are easier to take place in narrow
recombination zone.[29] Another reason is the relatively bal-
anced charges in the EMLs also owing to the bipolar properties
of hosts. The smallest efficiency roll-off is observed from the
device hosted by PBICT, with an EQE of 24.4% at 1000 cd m–2
and 23.8% at 10 000 cd m–2. Even when the brightness reached
30 000 cd m–2, the EQE is still higher than 20%. To the best of
our knowledge, the efficiency roll-off is the smallest one even
when considering orange PHOLEDs with co-hosts, resulting in
the highest EQE reported to date under the luminance above
1000 cd m–2 for orange PHOLEDs. Besides, the CIE coordi-
nates of the OLEDs were only slightly varied from (0.508, 0.488)
at 1000 cd m–2 to (0.507, 0.488) at 10 000 cd m–2, which is very
important for WOLEDs to keep a stable white emission. All
the performances are helpful to achieve a high efficiency, high
luminance, stable CIE and low driving voltage WOLED oper-
ating under high luminance. The angle dependent emission
intensities of the devices were measured and can be fitted
by the lambertian distribution (see Supporting Information
Figure S6), so these performances are not overestimated.
Compared with the other three hosts, the efficiencies of
PBICT based device are higher, the roll-off and the turn-on
voltage are lower, as summarized in Table 2. The higher effi-
ciency can be attributed to the most balanced recombination
of electrons and holes in PBICT. As a merit of the relatively
smaller ΔEST compared with POBICT and BICT, the LUMO
and HOMO energy level of PBICT match better with the elec-
tron and the hole transporting layers, which leads to a lower
working voltage. Device based on BICT as the host shows lower
efficiency than POBICT and PBICT ones. The reason can be
attributed to the strong intermolecular π–π stacking in the
BICT films, which would adversely affect the film morphology.
The amorphous films can guarantee that the emitter stays uni-
formly diluted in the matrix to minimize the effect of concen-
tration quenching.[27]
the ability of the dopant to trap electrons is reduced. There-
fore, holes injected in to the emitting layer would preferably
recombine with the electrons on the BBICT host and the direct
recombination on the dopant is reduced, resulting in lower
efficiency[28a] and higher voltage.[30] Those results show that
small ΔEST are not always helpful to reduce the voltage. The
device performance depends on the whole energy levels of all
layers. Only when the small ΔEST properly change the frontier
levels of hosts according to other parts of the device, can the
voltage be reduced and the performances improved.
To study the stability of the host materials, the operation life-
time of the OLEDs were measured at a brightness of 1000 cd m–2
under constant current. As can be seen from Figure 7f, devices
with PBICT and BBICT showed much longer lifetimes than
POBICT and BICT based devices. Even though a relatively
unstable material of Bphen is used as the electron transporting
layer (ETL), extrapolated lifetimes of 955 and 1910 hours are
achieved in PBICT and BBICT based devices, respectively, sug-
gesting the excellent molecular stability of PBICT and BBICT.
To verify the triplet exciton confinement ability of the hosts,
the transient electroluminescence was measured (Figure 8a).
All the transient decay curves were observed at 566 nm. For
devices using POBICT, BICT and PBICT as hosts, clear mono-
exponential decay curves with relatively short lifetime of about
0.7 µs were obtained, indicating the triplet exciton are well
confined on the guest. However, for the device using BBICT
as the host, a complex decay curve was observed, which can be
well-fitted with a double exponential decay curve to give a short
first exponential component lifetime of τ1 = 1 µs and a long
second one of τ2 = 4 µs (the amplitudes are A1 = 806.2, A2 =
50.5, respectively). The complex decay curve can be attributed
to lower T1 of BBICT, which is not high enough than PO-01.
Thus reverse energy transfer from PO-01 to BBICT can take
place under thermal activation.[31] The ratio of the phospho-
rescence intensities of the two components becomes (τ1A1)/
(τ2A2) = 0.80/ 0.20 = 4.0, indicating the majority of PO-01 tri-
plets decay radiatively with a lifetime of 1 µs and the rest decay
radiatively or non-radiatively with a lifetime of 4 µs through the
multiple recycling process between BBICT and PO-01 T1 levels,
resulting in low device efficiency.
As can be seen from Table 2, although BBICT has the
smallest ΔEST, the voltage of the device based on it is not the
lowest one. From the energy diagram of the devices (Figure 6),
we can see that electron injection barrier between BBICT and
Bphen is small because of the deep LUMO energy level of
BBICT. This should result in more balanced charge injection
considering hole injection is usually easier than electron injec-
tion, and thus an improvement of the quantum efficiency of
the device. However, it is also important to consider the energy
levels of the dopant when discussing the performance of the
device. Since BBICT has lower LUMO level than that of PO-01,
In order to further reduce the voltage of device based on
PBICT as the host, OLEDs with different ETLs but the same
other layers were fabricated. A material, 9, 10-bis (3-(pyridin-
3-yl) phenyl) anthracene (DPyPA),[32] with high electron trans-
porting ability was used. Because the T1 of DPyPA is low,
Bphen is used as an exciton-blocking layer with different thick-
ness. Although the current efficiencies are a little lower than
©
8
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2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Funct. Mater. 2014,
DOI: 10.1002/adfm.201303926

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Figure 8. a) The electroluminescence transient decay curves of devices observed at 566 nm with a voltage of 6 V. b) The brightness-voltage character-
istics of device with different ETL.
electrochemical workstation, using Pt as working electrode, platinum wire
as auxiliary electrode, and a Ag wire as reference electrode standardized
against ferrocene/ferrocenium. The reduction/oxidation potentials were
the device using Bphen as the ETL (see Supporting Information
Figure S7), the voltages of those devices are significantly lower,
with 2.4 V at 1 cd m–2 and 3.1 V at 1000 cd m–2 (Figure 8b),
resulting in even higher power efficiency (the highest value is
68 lm W–1).
measured in dichloromethane (CH2Cl2) solution containing 0.1
M
n-Bu4NPF6 as supporting electrolyte at a scan rate of 100 mV s−1
.
Single-Crystal Structure: A single crystal suitable for X-ray structural
analysis was obtained by vacuum sublimation. Single crystals were
grown from slow evaporation of dichloromethane/petroleum ether (1:4)
solutions. The room temperature single-crystal X-ray experiments were
performed on a RIGAKU SATURN 724+ CCD diffractometer equipped
with a graphite monochromatized Mo Kα radiation. The structure was
solved by direct methods and refined with a full-matrix least-squares
technique based on F2 with the SHELXL-97 crystallographic software
package.[33] The corresponding CCDC reference numbers (CCDC:
971751–971753) and the data can be obtained free of charge from The
Cambridge Crystallographic Data Centre.[34]
Preparation of Materials: All commercially available reagents were
used as received unless otherwise stated. All reactions were carried
out under a nitrogen atmosphere. Cyanuric chloride is definitely an
excellent synthon for the straightforward preparation of highly structured
multitopic molecules. Indeed, each chloride atom of 2,4,6-trichloro-
1,3,5-triazine can be substituted by any nucleophile. The first
substitution is exothermic and so the reaction mixture must be cooled
down to 0 °C. The second chloride substitution can be performed at
room temperature. Finally, the third position is functionalized under
solvent reflux. Therefore, by carefully controlling the temperature,
2,4,6-trisubstituted triazines can be synthesized by sequential, very
selective addition of all kinds of nucleophiles. The key intermediate,
2-chloro-4, 6- bis(12-phenylindolo[2,3-a] carbazole-11-yl)-1,3,5-triazine
(S3) was synthesized according to the report.[22]
3. Conculsion
In conclusion, we have designed and synthesized a series of
indolocarbazole/1, 3, 5-triazine hybrids as bipolar hosts with a
gradient of ΔEST. The photophysical and electrochemical prop-
erties of the hybrids can be tuned through introduction of moi-
eties with different electronegativity. Balanced charge injection
and transporting are achieved using PBICT as the host, which
benefits from its small ΔEST. Devices using PBICT as the host
achieved an EQE of 24.5% and a power efficiency of 65 lm
W–1, which is among the highest values of orange PHOLEDs.
Besides, owing to the balanced charges and the wide recom-
bination zone in the EML, the efficiency roll-off is extremely
small, with an EQE of 24.4% at 1000 cd m–2 and 23.8% at
10 000 cd m–2, respectively. This work reveals that by finely
tuning the ΔEST and thus the frontier levels of hosts, orange
PHOLEDs with high efficiency can be achieved, showing the
potential to construct the well performed tandem WOLEDs.
Synthesis of POBICT: Under nitrogen atmosphere, phenol (0.94 g,
10 mmol) in dehydrated N,N-dimethylformamide (10 mL) was added
dropwise into a dehydrated N,N-dimethylformamide (10 mL) solution
containing sodium hydride (55%, 0.65 g, 15 mmol) for 15 min
and stirred for 1 h. Then, S3 (3.87 g, 5 mmol) in dehydrated N,N-
dimethylformamide (10 mL) was added dropwise for 15 min. Then the
solution was stirred for 5 h at 80 °C. After that, water (350 g) was added
into the solution and the precipitate was filtered and dried in vacuum.
The product was purified by column chromatography on silica gel,
resulted in the product POBICT (3.54 g, 4.25 mmol, 85%).
1H NMR (600 MHz, CDCl3) : δ = 8.134 (d, 2H), 8.085 (d, 2H), 7.790
(d, 2H), 7.607–7.578 (m, 4H), 7.495 (t, 2H), 7.439–7.378 (m, 3H),
7.313 (t, 2H), 7.217 (d, 2H), 7.187 (t, 2H), 7.138 (d, 2H), 6.962 (s, 2H),
6.828 (d, 2H), 6.765 (t, 2H), 6.538 (s, 2H), 5.932 (s, 2H), 5.616 (s, 2H).
ESI-MS m/z: 834 [M+H]+. Elemental analysis calcd for C57H35N7O: C
82.09, H 4.23, N 11.76; found: C 81.96, H 4.20, N 11.74.
4. Experimental Section
General Information: 1H NMR spectra were measured on a JEOLAL-
600 MHz spectrometer at ambient temperature with tetramethylsilane as
the internal standard. Mass spectra were recorded on a Bruker Esquire
iontrap mass spectrometer. Elemental analyses were performed on a
flash EA 1112 spectrometer. UV-vis absorption spectra were recorded by
a Agilent 8453 spectrophotometer. The emission spectra were recorded
by a fluorospectrophotometer (Jobin Yvon, FluoroMax-3). Differential
scanning calorimetry (DSC) measurements were performed on a DSC
2910 modulated calorimeter at a heating rate of 10 °C min−1 from 20
to 450 °C under a nitrogen atmosphere. Thermogravimetric analysis
(TGA) was performed on a STA 409PC thermogravimeter by measuring
the weight loss while heating at a rate of 10 °C min −1 from 25 to 800 °C
under nitrogen. Electrochemical measurement were performed with
a Potentiostat/ Galvanostat Model 283 (Princeton Applied Research)
Synthesis of BICT: S3 (10.0 g, 12.9 mmol), sodium hydride (55%,
0.65 g, 15 mmol), and Tetrakis(triphenylphosphine)palladium(0) (1.5 g,
©
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Adv. Funct. Mater. 2014,
DOI: 10.1002/adfm.201303926
2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
9

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1.3 mmol) were dissolved in N,N-dimethylformamide (20 mL). Then the
solution was stirred for 5 h at 85 °C. After that, water (350 g) was added
into the solution and the precipitate was filtered and dried in vacuum.
The product was purified by column chromatography on silica gel,
resulted in the product BICT (2.6 g, 3.5 mmol, 35%).
–3 V. The period is 50 µs, and delayed time is 25 µs while the duty cycle
is 30%. The decay curves of devices were detected using the Edinburg
FL920P transient spectrometer.
1H NMR (600 MHz, CDCl3) : δ = 8.610 (dd, 2H), 8.441 (s, 1H),
8.192–8.178 (m, 2H), 7.813 (d, 2H), 7.609 (d, 2H), 7.576–7.535 (m, 4H),
7.316 (t, 2H), 7.221 (d, 2H), 7.192 (t, 2H), 6.899 (s, 2H), 6.809 (d, 2H),
6.656 (t, 2H), 6.540 (s, 2H), 5.908 (s, 2H), 5.587 (s, 2H). ESI-MS m/z:
742 [M+H]+. Elemental analysis calcd for C51H31N7: C 82.57, H 4.21, N
13.22; found: C 82.55, H 4.26, N 11.19.
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
Synthesis of PBICT: S3 (10.0 g, 12.9 mmol), phenylboronic acid
(1.98 g, 16.4 mmol), and Tetrakis(triphenylphosphine)palladium(0)
(1.5 g, 1.3 mmol) were dissolved in the mixture of ethanol (50 mL)
and toluene (100 ml). The, sodium carbonate (6.5 g, 47.0 mmol) in
water (50 mL) was added into the solution and stirred for 5 h at 85 °C.
Upon cooling to room temperature, water (100 mL), dichloromethane
(100 mL) was added and stirred. After filtration of the solution, the
solution was partitioned between dichloromethane and water. The
product was extracted from the organic layer and evaporated under
reduced pressure. The product was purified by column chromatography
on silica gel, resulted in the yellow product PBICT (7.3 g, 8.9 mmol, 69%)
1H NMR (600 MHz, CDCl3) : δ = 8.742 (dd, 2H), 8.495 (dd, 2H),
8.203 (d, 2H), 7.823 (d, 2H), 7.685–7.612 (m, 5H), 7.593–7.550 (m,
3H), 7.346–7.296 (m, 3H), 7.238–7.174 (m, 3H), 6.850–6.739 (m, 5H),
6.593 (s, 2H), 6.503 (t, 2H), 5.840 (s, 2H), 5.610 (s, 2H). ESI-MS m/z:
818 [M+H]+. Elemental analysis calcd for C57H35N7: C 83.70, H 4.31, N
11.99; found: C 83.15, H 4.26, N 12.59.
Synthesis of BBICT: The BBICT was synthesized by a procedure similar
to that for PBICT except that 4-cyanophenylboronic acid was used as
the reactant instead of phenylboronic acid. The product was purified
by column chromatography on silica gel, resulted in the yellow product
BBICT (6.7 g, 8.0 mmol, 62%)
1H NMR (600 MHz, [D6]DMSO): δ = 8.971 (d, 2H), 8.911 (d, 2H),
8.723 (d, 2H), 8.423 (d, 2H), 8.026 (d, 2H), 7.976 (d, 2H), 7.812 (t, 2H),
7.750 (t, 2H), 7.386 (t, 2H), 7.303 (d, 2H), 7.246 (t, 2H), 6.855 (s, 2H),
6.603 (d, 2H), 6.433 (t, 4H), 5.419 (s, 2H), 5.151 (s, 2H). ESI-MS m/z:
843 [M+H]+. Elemental analysis calcd for C59H34N8: C 82.64, H 4.07, N
13.29; found: C 82.62, H 4.05, N 13.33.
Acknowledgements
The authors would like to thank the National Natural Science Foundation
of China (Grant Nos. 50990060, 51173096, 21161160447, and 61177023)
for financial support. The authors would also like to thank Prof. Z.X.W.
for his advice about the measurement of the angle dependent emission
intensities of the devices.
Received: November 21, 2013
Revised: January 13, 2014
Published online:
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program package.[35] The calculation was optimized by means of B3LYP
(Becke three parameters hybrid functional with Lee–Yang–Perdew
correlation functionals) with the 6–31G(d) atomic basis set.32. The
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Device Fabrication and Measurement: Before device fabrication, the
ITO glass substrates were pre-cleaned carefully. Then the sample was
transferred to the deposition system. The devices were prepared in
vacuum at a pressure of 5 × 10−5 Torr. The hole-injection material
HATCN, hole-transporting material NPB, exciton blocking material
TCTA, and hole-blocking and electron-transporting material Bphen
were thermally evaporated at a rate of 1.0 Å s−1. After the organic film
deposition, 0.5 nm of LiF and 150 nm of aluminum were thermally
evaporated onto the organic surface. All of the organic materials
used were purified by a vacuum sublimation approach. The electrical
characteristics of the devices were measured with a Keithley 2400 source
meter. The electroluminescence spectra and luminance of the devices
were obtained on a PR650 spectrometer. All the device fabrication and
characterization steps were carried out at room temperature under
ambient laboratory conditions. Current–voltage characteristics of
single-carrier devices were measured using the same semiconductor
parameter analyzer as for PHOLED devices. The single-carrier device
measurements were performed under dark and ambient conditions.
For measurement of the transient electroluminescence characteristics,
short-pulse excitation with a pulse width of 15 µs was generated using
Agilent 8114A. The amplitude of the pulse is 9 V, and the baseline is
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Adv. Funct. Mater. 2014,
DOI: 10.1002/adfm.201303926
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