Orthogonally Substituted Benzimidazole-Carbazole Benzene As Universal Hosts for Phosphorescent Organic Light-Emitting Diodes

Article abstract of DOI:10.1021/acs.orglett.5b03631

The novel ambipolar hosts of o-CbzBz and o-DiCbzBz contain carbazole and benzimidazole through an ortho-connection. The orthogonal conformations cause the triplet state to be confined at the carbazole units to secure efficient energy transfer. The phosphorescent organic light-emitting diodes (PhOLEDs) show a high current efficiency, power efficiency, and low efficiency roll-off. o-DiCbzBz can be used as a host for sky-blue, green, and orange-red PhOLEDs, giving 57.5, 78.4, and 60.3 cd/A, respectively. (Figure Presented).

Full text of DOI:10.1021/acs.orglett.5b03631

Letter
pubs.acs.org/OrgLett
Orthogonally Substituted Benzimidazole-Carbazole Benzene As
Universal Hosts for Phosphorescent Organic Light-Emitting Diodes
Jau-Jiun Huang,† Yu-Hsiang Hung,∥ Pei-Ling Ting,† Yu-Ning Tsai,† Huan-Jie Gao,∥ Tien-Lung Chiu,*,∥
Jiun-Haw Lee,§ Chi-Lin Chen,† Pi-Tai Chou,† and Man-kit Leung*,†,‡
†Department of Chemistry, ‡Institute of Polymer Science and Engineering, §Graduate Institute of Photonics and Optoelectronics and
Department of Electrical Engineering, National Taiwan University, 1 Roosevelt Road Section 4, Taipei 106, Taiwan, R.O.C.
∥Department of Photonics Engineering, Yuan Ze University, 135 Yuan-Tung Road, Taoyuan 32003, Taiwan, R.O.C.
S
* Supporting Information
ABSTRACT: The novel ambipolar hosts of o-CbzBz and o-DiCbzBz contain
carbazole and benzimidazole through an ortho-connection. The orthogonal
conformations cause the triplet state to be confined at the carbazole units to secure
efficient energy transfer. The phosphorescent organic light-emitting diodes
(PhOLEDs) show a high current efficiency, power efficiency, and low efficiency
roll-off. o-DiCbzBz can be used as a host for sky-blue, green, and orange-red
PhOLEDs, giving 57.5, 78.4, and 60.3 cd/A, respectively.
Scheme 1. Molecular Structures and the Synthetic Routes of
o-CbzBz and o-DiCbzBz
hosphorescent organic light-emitting diodes (PhOLEDs)
Phave high potential in white light illuminating technology
due to their high energy conversion efficiency. However, the
nonideal device performance and the short lifetime of blue
PhOLEDs, which prohibit them from commercialization, have
become the last pieces of the puzzle to be solved.1 To achieve
high PhOLED efficiency, triplet−triplet annihilation (TTA)
between phosphorescent emitters has to be avoided.2 This
requires that emitters be isolated by a host matrix that meets
several requirements: (i) high triplet energy (ET) to prevent
reverse energy transfer from the emitter to the hosts, (ii)
suitable energy level matching for charge injection, (iii)
balanced charge transport properties to confine the electron−
hole recombination within the emitting layer and to reduce the
efficiency roll-off, and (iv) high thermal stability.3 It would be
ideal if the host is universal for blue, green, and red emitters.4
Devices from ambipolar hosts usually exhibit good electrical
performance.5 However, intramolecular charge transfer (ICT)
leads to a narrower band gap and lower triplet energy. To
reduce the ICT effects, disruption of the π-conjugation by
introducing nonconjugated linkers such as tetraphenylsilane,6 a
meta-connecting benzene ring, or a sterically twisted structure
has been studied.7 Outstanding hosts for a blue PhOLED such
as mNBICz (54.5 cd/A, 52.2 lm/W, and 26.2%), mCPCN
(58.7 cd/A, 57.6 lm/W, and 26.4%), CbBPCb (53.6 cd/A, 50.6
lm/W, and 30.1%), and ortho-(1,2,4-triazole) (carbazole)
substituted benzene (52.1 cd/A, 46.1 lm/W, and 24.4%) have
been reported.8
transport into the emitting layer. Unlike N,N-dicarbazolyl-3,5-
benzene (mCP), steric interactions between the substituents in
o-CbzBz and o-DiCbzBz keep the substituents perpendicular
to the central benzene ring; ICT through π-conjugation is
therefore largely reduced so that high T1 energy could be
maintained to fit blue phosphorescent emission. In particular,
the sky-blue PhOLEDs device using o-DiCbzBz for a host gives
a maximum current efficiency of 57.5 cd/A, with the EQE up to
27%. o-DiCbzBz can also be utilized as a universal host for
green and orange-red PhOLEDs with maximum efficiency up
to 78.4 and 60.3 cd/A, respectively.
The routes to obtain o-CbzBz and o-DiCbzBz are displayed
in Scheme 1. The details are reported in the Supporting
Information (SI, pp S2−S4). Condensation of 2-fluoro- and
2,6-difluorobenzoyl chloride with 2-PhNH(C6H4)NH2 under
acid catalyzed conditions afford 1 and 2, followed by
nucleophilic aromatic substitution with 9H-carbazole in
Benzimidazole is known as an electron-transporting moiety
for OLED.9 Herein we incorporate benzimidazole (Bz)/
carbazole (Cbz) in an ortho-substituted fashion to give two
novel ambipolar hosts denoted as o-CbzBz and o-DiCbzBz
(Scheme 1; Table 1) that facilitate the hole and electron
Received: December 22, 2015
© XXXX American Chemical Society
A
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Letter
Table 1. Physical Properties of o-CbzBz and o-DiCbzBz
a
b
c
λabs
(sol./film, nm)
FL λmax (nm)
ET (eV)
Eg (sol./film, eV)
HOMO/LUMO (eV)
Td/Tg /Tm (°C)
(onset)
o-CbzBz
352/350
342/354
396
2.9
3.1
3.5/3.5
5.9/2.4
5.8/2.3
323/77/289
o-DiCbzBz
418
3.6/3.5
381/117/295
a
b
c
Measured in 2-MeTHF at 77 K; ET = 1240.8/λPh (onset). Eg = 1240.8/λabs (onset). Determined through AC-II measurement with thin film (50 nm)
and UV absorption spectra; LUMO = HOMO − Eg.
DMSO to give o-CbzBz and o-DiCbzBz. Any DMSO residue
in the last step was removed by extraction with aqueous NH4Cl
and NaOH.
X-ray crystallography (Figure 1) revealed dihedral angles of
51.02° for C(31)−N(3)−C(19)−C(18) and 52.16° for N(1)−
Figure 2. Absorption (― ―), fluorescence (− − −), low-
temperature fluorescence (··· ···), and low-temperature phosphor-
escence spectra (− · −) of (a) o-CbzBz and (b) o-DiCbzBz at 77 K.
The excitation wavelength is 290 nm.
Figure 1. ORTEPs of o-CbzBz and o-DiCbzBz.
calculations, which clearly shows separation of the HOMO
and LUMO in o-CbzBz and o-DiCbzBz.
C(13)−C(14)−C(19) for o-CbzBz. On the other hand, o-
DiCbzBz shows dihedral angles of 86.79° and 81.60° for
C(43)−N(4)−C(19)−C(18) and C(20)−N(3)−C(15)−
C(14) of the outer Cbz units, indicating that intersubstituent
steric interactions are essential to control the conformation of
the structures. The dihedral angles of 69.91° for N(1)−C(13)−
N(14)−C(19) of the central Bz group is large in comparison to
that of o-CbzBz. The 4.30 Å centroid distance between Bz and
Cbz’s in o-CbzBz and 4.37 Å/4.25 Å in o-DiCbzBz,
respectively, are out of the range of strong ground state π−π
interactions.
The thermal stability of o-CbzBz and o-DiCbzBz was first
illustrated by differential scanning calorimetry (DSC) with a 10
°C/min scanning rate (Figure S5), in which high glass
transition temperatures (Tg) at 77 and 117 °C were found,
respectively (Table 1). These were far higher than that of mCP
(60 °C), a common host for PhOLED, and could help to
prevent the Ph emitters from phase segregation. A high
crystallization temperature (Tc) of 178 °C for o-DiCbzBz again
indicates the stability of the amorphous state under normal
device operation conditions. In addition, high decomposition
temperatures (Td: corresponding to 5% weight loss) of 323 and
381 °C were found in the thermogravimetric analysis (TGA);
this ensures that the hosts can tolerate the vacuum thermal
evaporation conditions in the device fabrication process.
The photophysical properties of o-CbzBz and o-DiCbzBz
were characterized in tetrahydrofuran (THF) or in 2-methyl
tetrahydrofuran (2-MeTHF). The weak absorption at 310−350
nm in THF and the fine vibronic structures are characteristic
for the n−π* transitions of the Cbz units (Figure 2). The
absorption bands that peaked at around 300 nm arise from the
π−π* transitions of the Cbz and Bz chromophores. The spectra
are nearly identical with the overlay spectra of N-phenyl-
carbazole (NPC) and Bz, indicating that the ICT in their
ground states is weak (Figure S6, measured in THF). The
interruption of the π-conjugation by the orthogonal alignment
of the aromatic rings is further supported by the DFT
At 77 K, both compounds show structured fluorescence (FL)
spectra at 350−400 nm in a 2-MeTHF frozen grassy matrix. On
the other hand, at room temperature (26 °C) in THF, the
structureless FL spectrum peaking at 396 nm for o-CbzBz and
418 nm for o-DiCbzBz was respectively observed. The
emission maxima show solvatochromism from cyclohexane to
MeOH and are shifted up to 50 nm for o-CbzBz and 35 nm for
o-DiCbzBz (Figure S7). These results indicate that there are at
least some degrees of ICT in their excited state. Since the
conformation of the donor−acceptor substituents are restricted
and the π-conjugation effects should be small in o-CbzBz and
o-DiCbzBz, the solvato red shifts may arise from the solvent
relaxation effects. The actual origins of the ICT characters are
pending. However, our X-ray crystallography analysis disclosed
that the distance between donor and acceptor is ca. 4.3 Å which
may allow charge transfer through space in their S1 excited
states.
The low-temperature phosphorescence (Ph) spectra are
quite unexpected. While the observed Ph vibronic pattern of o-
DiCbzBz is similar to that of mCP, the structureless pattern of
o-CbzBz is similar to that of N-phenylbenzimidazole (BImP).9b
We attribute this to the coplanar conformation of the Bz unit in
o-CbzBz in the triplet state T1. As shown in Figure 3, the Bz
unit is sterically allowed to stay coplanar with the central
benzene unit, leading to an extension of the π-conjugation with
Figure 3. Possible conformation of o-CbzBz and o-DibzBz in their T1
states.
B
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a lower ET of 2.9 eV being recorded. The Ph spectrum of o-
CbzBz therefore shows the characteristic of the Bz unit. On the
other hand, in o-DiCbzBz, the 1,2,3-ortho-substituent array
prohibits the Bz unit from being coplanar with the central
benzene ring. In this situation, the π-conjugation of the Bz unit
becomes interrupted, and hence the T1 state falls onto the Cbz
units, with a higher ET of 3.0 eV being recorded. Nevertheless,
the ET values of both compounds are high enough to prevent
reverse energy transfer from FIrpic. Accordingly they can be
used as hosts in this study.
o-CbzBz and o-DiCbzBz are electrochemically active and
showed oxidation and reduction waves in their cyclic
voltammetry (CV) and differential-pulse voltammetry (DPV)
(Figure S8). This observation is in good agreement with our
prediction of the ambipolar properties. In the oxidation scan in
MeCN, o-CbzBz and o-DiCbzBz show an irreversible
oxidation wave with EDPV of 0.83−0.84 V with respect to the
ferrocene/ferrocenium (Fc/Fc+) couple. The oxidation poten-
tial is very close to that of NPC, indicating that the oxidation
should occur on the carbazole units. On the other hand,
reduction took place at around −2.60 to −2.70 V in DMF,
which is close to that of BImP (−2.55 V), suggesting that the
Bz group functions as an electron-accepting unit. In reference
to the redox couple of Fc/Fc+, the HOMO/LUMO levels of
5.8/2.2 eV for o-CbzBz and 5.8/2.3 eV for o-DiCbzBz were
estimated from the DPV data, using the Forrest approach.10
The HOMO in the solid thin film was further confirmed by
AC-II and the LUMO was estimated according to the equation
of LUMO = HOMO − Eg, where Eg is the optical band gap
determined from the onset of the lowest absorption band
(Figure S9). The HOMO/LUMO of 5.9/2.4 eV for o-CbzBz
and 5.8/2.3 eV for o-DiCbzBz solid film are close to those of
common hole and electron transport layers. This will help to
reduce the charge injection energy barrier.
To show the potential of materials as hosts for PhOLED, a
series of devices were fabricated with configuration of ITO
glass/TAPC (50 nm)/mCP (10 nm)/hosts: X% dopant (30
nm)/DPPS (35 nm for o-CbzBz and 45 nm for o-DiCbzBz
after optimization)/LiF (0.9 nm)/Al (120 nm). The molecular
structures of TAPC, DPPS, TAZ, and the dopants including
FIrpic, Ir(PPy)3, and (Bt)2Ir(acac) and their energy level
alignment are shown in Figure S11. TAPC and DPPS were
used respectively as the hole- and electron-transport layer.
Although the actual role of mCP is pending, mCP may probably
act as an exciton block.11 Without being helped by the mCP
layer, the device efficiency will significantly drop.
Figure 4. Performances of PhOLEDs with o-DiCbzBz as the universal
host. (a) The J−L−V curves. (b) The CE (square), PE (cycle), and
EQE (triangle) of the PhOLEDs: FIrpic-doped (blue and empty);
Ir(ppy)3-doped (green and filled); (Bt)2Ir(acac)-doped (orange-red
and half-filled). (c) The EL spectra. (d) The plots of current density
versus applied electrical voltage for the hole-only (empty) and
electron-only (filled) devices: intrinsic (square); FIrpic-doped (cycle),
Ir(ppy)3-doped (triangle), and (Bt)2Ir(acac)-doped (inverted-trian-
gle).
the CE of 55.4 cd/A and EQE of 26.0%. Since o-DiCbzBz
showed better performance in the sky-blue PhOLED, it was
selected for further research in a high efficiency universal host.
In these devices, Ir(ppy)3 (6%) as the green emitter and
(Bt)2Ir(acac) (0.5%) as the orange-red emitter (device D) were
respectively doped in o-DiCbzBz matrix. The Ir(ppy)3-doped
device C exhibited Lmax = 22790 cd/m2, CEmax = 78.4 cd/A,
PEmax = 66.5 lm/W, and EQEmax = 20.4% (Figure 4). The
(Bt)2Ir(acac)-doped device D exhibited Lmax = 34 600 cd/m2,
CEmax = 60.3 cd/A, PEmax = 54.1 lm/W, and EQEmax = 20.1%.
The EL spectra for devices B, C, and D are shown in Figure 4c.
To understand the charge-carrier transporting properties of
o-DiCbzBz and the emitter doped (6%) o-DiCbzBz, the
electron-only device (EOD) and hole-only device (HOD) were
studied. Figure 4d shows the plots of current density arising
from single-charge carriers versus applied electrical voltage of
nondoped o-DiCbzBz and doped o-DiCbzBz. The hole and
electron mobility of o-DiCbzBz was estimated by trap-free
Mott−Gurney law and the trap-relative Poole−Frenkel mode.
The hole mobility (μ0 = 3 × 10−6 cm2/(V s)) is about 50 times
higher than the electron mobility (μ0 = 6 × 10−8 cm2/(V s)),
indicating the intrinsic bipolar property of o-DiCbzBz.
FIrpic, a sky-blue emitter, with the doping level of 12% in o-
CbzBz (device A) and 6% in o-DiCbzBz (device B), was
adopted. The current density−voltage−luminance (J−V−L)
characteristics, efficiency performances, and spectra are shown
in Figure 4, Figures S12−S17 and summarized in Table 2. The
devices, in general, exhibited low turn-on voltage at 3.5−4.0 V
(defined as the voltage at 10 cd/m2). Maximum luminescence
(Lmax) of 14 850 and 11 160 cd/m2 were achieved at 11.0 and
10.5 V for devices A and B, respectively. However, the
performance of device A was inferior to that of device B in
many dimensions. For example, although device A showed
reasonably good performance with the maximum current
efficiency (CEmax) of 49.5 cd/A, maximum power efficiency
(PEmax) of 43.6, and maximum external quantum efficiency
When doped with FIrpic, the hole and electron mobilities
drop by a similar order of magnitude, which is unexpected.
FIrpic is a deep electron-trapping center in common host
matrix due to the large difference in their LUMO levels. Since
the LUMO level difference of 0.6 eV was found between o-
(EQEmax) of 22.2%, device B achieved even better CEmax, PEmax
,
and EQEmax (57.5 cd/A, 48.9 lm/W, and 27.0%). Amazingly,
the performance of device B at 1000 cd/m2 remained high, with
C
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Letter
Table 2. EL Characteristics of Various PhOLEDs Devices
a
b
b
b
device
host
Von (V)
Lmax (cd/m2)
CE (cd/A)
PE (lm/W)
EQE (%)
c
A: 12% FIrpic
o-CbzBz
3.5
3.7
3.8
3.7
14 850@11 V
11 160@10.5 V
22 790@12 V
34 600@12 V
49.5, 47.3
57.5, 55.4
78.4, 60.6
60.3, 52.0
43.6, 26.4
48.9, 30.0
66.5, 29.4
54.1, 29.1
22.2, 21.3
27.0, 26.0
20.4, 14.3
20.1, 17.3
c
B: 6% FIrpic
o-DiCbzBz
o-DiCbzBz
o-DiCbzBz
d
C: 6% Ir(ppy)3
c
D: 0.5% (Bt)2Ir(acac)
a
b
c
Turn-on voltage at 10 cd/m2. The maximum value and the values at 1000 cd/m2. ITO glass/TAPC (50 nm)/mCP (10 nm)/hosts: X% dopant
d
(30 nm)/DPPS (35 nm for o-CbzBz and 45 nm for o-DiCbzBz)/LiF (0.9 nm)/Al (120 nm). 50 nm for DPPS.
DiCbzBz (2.3 eV) and FIrpic (2.9 eV), the electron-trapping
mechanism is expected. In contrast, FIrpic should act as a
shallow hole trap because of a small difference in HOMO levels
between FIrpic (5.8 eV) and o-DiCbzBz (5.8 eV).12 The
observation of the hole-mobility drop in the FIrpic doped o-
DiCbzBz is quite unexpected. Nevertheless, the relatively high
hole mobility in comparison to the electron mobility secures
the position of the recombination zone nearby the cathode.
This is important for the high EL efficiency because the exciton
quenching phenomenon is usually significant at the mCP
interface.
In the (Bt)2Ir(acac) doped o-DiCbzBz, due to the higher
HOMO of (Bt)2Ir(acac), the hole current drops significantly in
comparison to that of the FIrpic doped device. Nevertheless,
the apparent hole mobility is still higher than the electron
mobility to secure the recombination zone nearby the cathode.
On the other hand, for the Ir(ppy)3 doped devices, although
the hole current is higher than the electron current in the low
voltage region, the curves cross over at 2 V and the electron
current in the EOD becomes slightly higher than that of the
HOD at the high voltage region, indicating that the
recombination zone may migrate toward the anode under
high electrical voltage conditions. This reflects on the relatively
large roll-off of the device efficiency.
155-028-MY3, 104-2221-E-002-156-MY3, and NSC 102-2221-
E-002-182-MY3). Prof. Yu-Tai Tao, Institute of Chemistry,
Academia Sinica, for discussions and AC-II measurements.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.5b03631.
■
S
Synthetic procedures, spectral data, AC-II measurements,
energy diagram, device performance (PDF)
X-ray data for o-CbzBz (CIF)
X-ray data for o-DiCbzBz (CIF)
AUTHOR INFORMATION
Corresponding Authors
(10) (a) D’Andrade, B. W.; Datta, S.; Forrest, S. R.; Djurovich, P.;
Polikarpov, E.; Thompson, M. E. Org. Electron. 2005, 6, 11−20.
(b) Djurovich, P. I.; Mayo, E. I.; Forrest, S. R.; Thompson, M. E. Org.
Electron. 2009, 10, 515−520.
■
*E-mail: tlchiu@saturn.yzu.edu.tw (T.-L.C.)
*E-mail: mkleung@ntu.edu.tw (M.-k.L)
Notes
(11) Mamada, M.; Ergun, S.; Per
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10651−10660. (b) Matsusue, N.; Ikame, S.; Suzuki, Y.; Naito, H. J.
Appl. Phys. 2005, 97, 123512.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Ministry of Science and Technology, R.O.C. (MOST 104-
2119-M-002-023, 101-2113-M-002-010-MY3, 103-3113-E-155-
001, 104-3113-E-155-001, 105-3113-E-155-001, 103-2221-E-
D
DOI: 10.1021/acs.orglett.5b03631
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