Ideal bipolar host materials with bis-benzimidazole unit for highly efficient solution-processed green electrophosphorescent devices

Article abstract of DOI:10.1021/ol502531y

An ideal host material with high triplet energy, suitable HOMO energy level, excellent thermal and electrochemical stability, and bipolar charge carrier transport ability was synthesized. A high external quantum efficiency of 13.7% and a luminance e ffici

Full text of DOI:10.1021/ol502531y

Letter
pubs.acs.org/OrgLett
Ideal Bipolar Host Materials with Bis-benzimidazole Unit for Highly
Efficient Solution-Processed Green Electrophosphorescent Devices
Wei Jiang,† Jinan Tang,† Xinxin Ban,† Yueming Sun,*,† Lian Duan,*,‡ and Yong Qiu‡
†School of Chemistry and Chemical Engineering, Southeast University, Nanjing, Jiangsu 211189, P. R. China
‡Key Lab of Organic Optoelectronics and Molecular Engineering of Ministry of Education, Department of Chemistry, Tsinghua
University, Beijing 100084, P. R. China
S
* Supporting Information
ABSTRACT: An ideal host material with high triplet energy,
suitable HOMO energy level, excellent thermal and electro-
chemical stability, and bipolar charge carrier transport ability
was synthesized. A high external quantum efficiency of 13.7%
and a luminance efficiency of 48.2 cd A−1 with low efficiency
roll-off were achieved in solution-processed green electro-
phosphorescent devices.
hosphorescent organic light-emitting diodes (PhOLEDs)
employing triplet emitters doped into a host material have
mCP and CBP) for vacuum deposition are almost unsuitable
for the solution process.
P
attracted significant attention because of their high efficiencies
as a result of the utilization of singlet and triplet excitons for
emission.1 While vacuum deposited PhOLEDs with very high
efficiencies have been demonstrated, solution-based PhOLEDs
are economically highly desired to low-cost and large-area
lighting devices.2 To suppress severe concentration quenching
and triplet−triplet annihilation, the triplet emitter is usually
dispersed into an appropriate host. Thus, promotion of host
materials is the key to highly efficient PhOLEDs. An ideal host
material for solution-processed PhOLEDs should satisfy the
following requirements: (1) high triplet energy, (2) good
charge carrier transport property, (3) appropriate high
occupied molecular orbital (HOMO) and lowest unoccupied
molecular orbital (LUMO) levels, (4) high solubility and film-
forming ability. To date, a large number of solution-processed
PhOLEDs with poly(N-vinylcarbazole) (PVK) as the host
material have been reported.3 It is known that PVK-based
devices usually have relatively high operational voltages owing
to its low HOMO level and a large barrier with the poly(3,4-
ethylenedioxythiophene):poly(styrenesulfonate) (PE-
DOT:PSS) layer.4 For purposes of balancing charge fluxes in
the emitting layer (EML), a large amount of electron-
transporting material must be admixed with PVK to improve
electron transporting, such as 2-(4-biphenylyl)-5-(4-tert-butyl-
phenyl)-1,3,4-oxadizole (PBD) and 1,3-bis[4-tert-butylphenyl-
1,3,4-oxidiazolyl]phenylene (OXD-7), which may lead to phase
separation.2b Considering the high purity and structural
uniformity features of small molecules, solution-processed
PhOLEDs using small molecules as the host material may
serve as an approach to resolving such a problem. However,
due to poor film morphology and high crystallization tendency,
the widely used small-molecule-based host materials (such as
In this communication, we report two novel bipolar host
materials, 9-(3′,5′-bis(1-phenyl-1H-benzo[d]imidazol-2-yl)-
[1,1′-biphenyl]-4-yl)-3,6-ditert-butyl-9H-carbazole (Cz-BBPI)
and 9′-(3′,5′-bis(1-phenyl-1H-benzo[d]imidazol-2-yl)-[1,1′-bi-
phenyl]-4-yl)-3,3″,6,6″-tetra-tert-butyl-9′H-9,3′:6′,9″-tercarba-
zole (3Cz-BBPI), which use carbazole units as the electron
donor and benzimidazole units as the electron acceptor. As
shown in Scheme 1, the newly synthesized compounds meet
almost all the requirements of an ideal host material for
solution-processed PhOLEDs. First, the twisted configuration
and meta-position linkage mode keep the molecules at high
triplet energy levels and enhance the thermal stability with high
glass transition temperatures. Second, carbazole units and
benzimidazole units are introduced to the molecular structure
to achieve bipolar transporting, which can balance charge
refluxes and broaden the recombination zone, thereby reducing
the efficiency roll-off. In addition, the bipolar transport nature
of host materials avoids the need of a mixed host system and
prevents phase separation in EML. Third, tert-butyl groups
improve the solubility and film-forming properties of
compounds in common solvents and protect the electrochemi-
cally active sites of carbazole units, keeping hosts stable when
they transport positive charge carriers in devices. Finally, the
HOMO energy levels can be adjusted to approach the work
function of PEDOT:PSS, facilitating the injection of holes to
EML and reducing the turn-on voltage.
The synthetic routes are illustrated in Scheme 1. The
reaction of 5-bromoisophthaloyl dichloride N-phenylbenzene-
Received: August 27, 2014
Published: September 26, 2014
© 2014 American Chemical Society
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Letter
To understand the electronic states of host molecules,
density functionalized theory (DFT) calculations are per-
formed. The results reveal that the HOMO and LUMO
electron density distributions of Cz-BBPI and 3Cz-BBPI are
similar mainly on the electron-donating carbazole and the
electron-accepting benzimidazole fragments. The separation of
HOMO and LUMO is believed to benefit the charge transport
properties. For Cz-BBPI, the HOMO is mainly concentrated in
the 9-phenylcarbazole unit, while the HOMO of 3Cz-BBPI is
distributed over the whole tricarbazole moiety. Thus, the
calculated HOMO value of 3Cz-BBPI is 0.25 eV higher than
that of Cz-BBPI, which better matches the work function of
PEDOT:PSS for the injection of holes. As shown in Figure 1b,
Scheme 1. Sythetic Routes toward Cz-BBPI and 3Cz-BBPI
Figure 1. (a) Electrochemical properties of Cz-BBPI and 3Cz-BBPI.
(b) Calculated HOMO and LUMO density maps for Cz-BBPI and
3Cz-BBPI.
the dihedral angles of the biphenyl unit in Cz-BBPI and 3Cz-
BBPI are 54.0° and 55.8°, respectively. The highly twisted
configurations of host molecules allow the separation of
HOMO and LUMO, which is the key point for high triplet
energies. The cyclic voltammetry is used to investigate
electrochemical properties of host materials. Cz-BBPI and
3Cz-BBPI exhibit similar reversible oxidation and irreversible
reduction processes, which arise from their electron-donating
carbazole unit and electron-accepting benzimidazole unit.
These behaviors indicate that Cz-BBPI and 3Cz-BBPI possess
bipolar carrier transport characteristics, which are further
confirmed by hole-only and electron-only devices. As shown
in Figure S11 in Supporting Information, the hole current
density of 3Cz-BBPI device is much higher than that of Cz-
BBPI, while the electron current density of 3Cz-BBPI and Cz-
BBPI are similar. There is a slight difference of the hole and
electron current density in 3Cz-BBPI, which indicated the
charge transport property of 3Cz-BBPI is more balanced. It is
worth noting that the oxidation curves show no change during
the repeated anodic scans, which can be attributed to the
blocking of electrochemically active sites of carbazole units,
improving the electrochemical stability of the host materials.7
On the basis of the onset potentials for oxidation and
reduction, the HOMO and LUMO values of Cz-BBPI and
3Cz-BBPI are estimated to be −5.68, −2.51, −5.33, and −2.52
eV, respectively.
The newly synthesized host materials exhibit almost the same
photophysical properties. The UV−vis absorption and photo-
luminescence (PL) spectra of Cz-BBPI and 3Cz-BBPI are
shown in Figure 2. In the UV−vis spectra, the strong
absorption bands observed around 296 nm for Cz-BBPI and
3Cz-BBPI can be assigned to the n−π* transitions of the
carbazole moiety, while the longer-wavelength weak absorption
at around 347−353 nm could be attributed to the π−π*
transitions from the electron-donating carbazole moiety to the
electron-accepting benzimidazole moiety.8 Cz-BBPI and 3Cz-
BBPI show deep blue emission with peaks at 414 and 418 nm,
1,2-diamine gave 2,2′-(5-bromo-1,3-phenylene)bis(1-phenyl-
1H-benzo[d]imidazole) (a) in 60% yield, which then converted
to the aryboronic ester of 2,2′-(5-(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan-2-yl)-1,3-phenylene)bis(1-phenyl-1H-benzo[d]-
imidazole) (b) catalyzed by PdCl2(dppf) in the presence of
KOAc. 9-(4-Bromophenyl)-3,6-di-tert-butyl-9H-carbazole and
9-(4-bromophenyl)-3,6-bis(3,6-di-tert-butyl-carbazol-9-yl)-car-
bazole were prepared following the literature method.5 Finally,
Cz-BBPI and 3Cz-BBPI can easily be synthesized by a Suzuki
cross-coupling reaction with moderate yields. The structures of
hosts are confirmed by 1H NMR, 13C NMR, mass
spectrometry, and elemental analysis, as described in the
experimental section. As shown in Figure S9 in Supporting
Information, Cz-BBPI and 3Cz-BBPI both exhibit excellent
thermal stability at the decomposition temperatures (Td, 5%
weight loss) of 417 and 514 °C, with the glass transition
temperatures (Tg) of 161 and 228 °C, respectively. With two
extra carbazole units in the structure, 3Cz-BBPI showed much
higher Td and Tg values than Cz-BBPI. The thermal properties
of Cz-BBPI and 3Cz-BBPI are substantially superior to those of
analogous carbazole-based host materials, such as mCP and
CBP.6 The high quality of spin-coated thin films of Cz-BBPI
and 3Cz-BBPI doped with 8% Ir(mppy)3 is investigated by
AFM with the surface roughness of 0.56 and 0.32 nm,
respectively. No phase separation or any aggregation is
observed. The results demonstrate that these host materials
are capable of forming uniform amorphous films, which are one
of the key factors for highly efficient solution-processed
PhOLEDs.
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Organic Letters
Letter
12.1% and 13.7%, for Cz-BBPI and 3Cz-BBPI, respectively. The
luminance efficiency of our devices is higher than the values
reported previously with polymer and small molecule hosts.11
For example, Ge et al.11a reported triphenylamine-benzimida-
zole-based bipolar hosts for solution-processed green PhO-
LEDs with a turn on voltage of 6.9 V and the maximum LE
value of 27.3 cd A−1, and Li et al.11b reported green PhOLEDs
using carbazole-based dendritic as hosts with a turn on voltage
of 4.5 V and the maximum LE value of 38.7 cd A−1. The
electroluminescence spectra of devices is green emission with
the Commission Internationale de L’Eclairage coordinates of
(0.34, 0.60), corresponding to the emission of Ir(mppy)3. No
additional emission from the host materials is observed, which
indicated efficient energy transfer from the hosts to the triplet
emitter.
It is noteworthy that these devices exhibit rather low
efficiency roll-off. Even though the LE values decrease gradually
as the current density increases, the luminance efficiency is still
36.3 and 42.1 cd A−1 with a brightness of 1000 cd m−2 for Cz-
BBPI and 3Cz-BBPI, respectively. The high efficiencies and low
efficiency roll-off at high brightness for PhOLEDs can be
mainly attributed to the use of the bipolar host materials, which
balance charge fluxes and broaden the recombination region on
the emitting layer. The improvement of hole and electron
injection and excellent thermal and electrochemical stability
also make considerable contributions to the device perform-
ance.
In summary, we have designed and synthesized two novel
bipolar host materials Cz-BBPI and 3Cz-BBPI for solution-
processed green PhOLEDs. These host materials satisfy the
requirements of an ideal host material for solution-processed
green PhOLEDs, because they have a high triplet energy level,
excellent thermal and electrochemical stability, bipolar charge
transport character, solubility and film-forming property, and
appropriate HOMO energy level at the same time. When these
new compounds are used as host materials, solution-processed
green PhOLEDs with Ir(mppy)3 as a dopant show a maximum
current efficiency of 48.2 cd A−1 and a maximum external
quantum efficiency of 13.7%. The performance of Cz-BBPI-
and 3Cz-BBPI-based devices are among the highest for
solution-processed green PhOLEDs. Our work paves the way
to designing ideal host materials for solution-processed
PhOLEDs.
Figure 2. (a) Absorption and emission spectra in CH2Cl2 and
phosphorescent spectra in 2-methyltetrahydrofuran at 77 K. (b)
Transient photoluminescence decay curves of FIrpic and Ir(mppy)3
doped films monitored at 470 and 520 nm, respectively.
respectively. The triplet energies (ET) of Cz-BBPI and 3Cz-
BBPI determined by the phosphorescence spectra at 77 K are
both 2.67 eV, which is higher than that of Ir(mppy)3 (2.40 eV)
and Firpic (2.62 eV), implying that they might be used as host
materials for green and blue triplet emitters. To further confirm
their exciton confinement property, transient photolumines-
cence decays of Cz-BBPI and 3Cz-BBPI films doped with 5%
FIrpic and Ir(mppy)3 are measured. As shown in Figure 2b, the
decay curves of FIrpic-doped films are multiexponential with
long decay components, which demonstrate the exothermic
back energy transfer from FIrpic to host materials, while
Ir(mppy)3-doped films exhibit monoexponential decay curves,
indicating that the triplet energy transfer from Ir(mppy)3 to
hosts are completely suppressed.9 The results indicate that the
ET’s of Cz-BBPI and 3Cz-BBPI are not high enough to confine
the triplet excitons on FIrpic molecule and unsuitable to serve
as the host for blue PhOLEDs.10 Thus, Cz-BBPI and 3Cz-BBPI
are expected to be used as appropriate hosts for green
PhOLEDs.
To investigate the utility of Cz-BBPI and 3Cz-BBPI as host
materials for solution-processed green PhOLEDs, devices with
the configuration ITO/PEDOT:PSS/Hosts:Ir(mppy)3 (8 wt
%)/TPBi/Cs2CO3/Al have been fabricated by spin coating
method. The conducting polymer PEDOT:PSS was used as the
hole injection layer, Cs2CO3 is used as the electron-injection
layer, and TPBi serves as the hole/exciton blocking and
electron transport layers simultaneously. According to the J−V
curves, the introduction of two carbazole units leads to a slight
increase in the current density, with a decrease of turn-on
voltage (corresponding to 1 cd m−2) of 5.4 and 4.5 V for Cz-
BBPI and 3Cz-BBPI, respectively. As shown in Figure 3a, the
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental and characterization details of the new com-
pounds. This material is available free of charge via the Internet
at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Authors
Figure 3. (a) Schematic energy-levels diagram of the devices. (b)
Luminance efficiency versus current density plots and EL spectra for
the devices.
*E-mail: 101011462@seu.edu.cn.
*E-mail: duanl@mail.tsinghua.edu.cn.
Notes
The authors declare no competing financial interest.
lower turn-on voltage of 3Cz-BBPI is mainly due to its high-
lying HOMO level, which better matches the work function of
the PEDOT:PSS layer and improves hole injection of the
device. Furthermore, these devices exhibited excellent perform-
ance with a maximum luminance efficiency (LE) of 42.6 and
48.2 cd A−1 and a maximum external quantum efficiency of
ACKNOWLEDGMENTS
■
The authors thank the National basic Research Program China
(2013CB932902) and National Natural Science Foundation of
China (51103023, 21173042) for their financial support.
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Letter
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