Carbazole-benzimidazole hybrid bipolar host materials for highly efficient green and blue phosphorescent OLEDs

Article abstract of DOI:10.1039/c1jm12598b

In this study, we synthesized a series of bipolar hosts (CbzCBI, mCPCBI, CbzNBI, and mCPNBI) containing hole-transporting carbazole and electron-transporting benzimidazole moieties and then examined the morphological, thermal, and photophysical properties

Full text of DOI:10.1039/c1jm12598b

View Article Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links <
C
Journal of
Materials Chemistry
Cite this: J. Mater. Chem., 2011, 21, 14971
www.rsc.org/materials
PAPER
Carbazole–benzimidazole hybrid bipolar host materials for highly efficient
green and blue phosphorescent OLEDs†
a
b
You-Ming Chen, Wen-Yi Hung, Hong-Wei You,b Atul Chaskar,a Hao-Chun Ting,a Hsiao-Fan Chen,a
*
Ken-Tsung Wong and Yi-Hung Liua
a
*
Received 8th June 2011, Accepted 25th July 2011
DOI: 10.1039/c1jm12598b
In this study, we synthesized a series of bipolar hosts (CbzCBI, mCPCBI, CbzNBI, and mCPNBI)
containing hole-transporting carbazole and electron-transporting benzimidazole moieties and then
examined the morphological, thermal, and photophysical properties and carrier mobilities of these
bipolar host materials. Altering the linking topology (C- or N-connectivity of the benzimidazole)
changed the effective conjugation length and led to different excited-state solvent relaxation behavior.
The N-connected compounds (CbzNBI, mCPNBI) possessed higher triplet energies (ET) than those of
their C-connected analogues (CbzCBI, mCPCBI) by 0.23 eV. The higher values of ET of CbzNBI and
mCPNBI endowed them with the ability to confine triplet excitons on the blue-emitting guest. A blue
PhOLED device incorporating mCPNBI achieved a maximum external quantum efficiency, current
efficiency, and power efficiency of 16.3%, 35.7 cd Aꢀ1, and 23.3 lm Wꢀ1, respectively; confirming the
suitability of using N-connected bipolar hosts for the blue phosphor. The donor/acceptor interactions
of the C-connected analogue resulted in a lower triplet energy, making it a suitable bipolar host for
green phosphors. A green-phosphorescent device incorporating CbzCBI as the host doped with
(PBi)2Ir(acac) achieved a maximum external quantum efficiency, current efficiency, and power
efficiency of 20.1%, 70.4 cd Aꢀ1, and 63.2 lm Wꢀ1, respectively.
development of host materials possessing bipolar properties.
1. Introduction
Tailor-made bipolar host materials can be obtained through
Phosphorescent organic light-emitting diodes (PhOLEDs) have
judicious selection of functional sub-structures featuring linked
attracted much attention because they can achieve 100% internal
hole-transporting (donor) and electron-transporting (acceptor)
quantum efficiency when they incorporate triplet emitters.1 To
moieties; the resulting emitting layer can, therefore, achieve
reduce concentration quenching (triplet–triplet annihilation) of
balanced charge fluxes, resulting in high device performance.5 In
PhOLEDs, phosphorescent metal-complex emitters are usually
this regard, many classes of bipolar host materials have been
doped into a suitable host material.2 To ensure exothermic
prepared with various combinations of hole- and electron-
energy transfer and well-confined triplet exciton formation on
transporting moieties. For example, carbazoles and arylamines
the dopant, the triplet energy (ET) of the host material must be
are frequently used as hole-transporting moieties because they
possess attractive hole mobilities (mh ca. 10ꢀ3 to 10ꢀ5 cm2 Vꢀ1
materials typically require good energy level match-ups with the
sꢀ1);8 typical electron-transporting materials feature molecular
higher than that of the dopant. For efficient PhOLEDs, host
neighboring functional layers.3 In addition, host materials with
structures based on oxadiazole, triazine, phenanthroline, and
good carrier transporting properties are required to increase the
benzimidazole units.5b,6,7 Benzimidazole groups are used widely
opportunity of electron and hole recombination within the
as functional groups that facilitate electron injection and trans-
emitting layer.4 Recent research trends have drifted to the
port.5e,7,8 Recently, Ma and Yang et al. reported a series of host
materials, incorporating benzimidazole/carbazole hybrids, for
aDepartment of Chemistry, National Taiwan University, Taipei, 106,
Taiwan. E-mail: kenwong@ntu.edu.tw; Fax: + 886-2-33661667; Tel:
+886-2-33661665
green phosphors; the maximum external quantum efficiency,
current efficiency, and power efficiency obtained from these
devices were 18.7%, 70.2 cd Aꢀ1, and 73.4 lm Wꢀ1, respectively.9
Anzenbacher et al. used the same p/n-type [1,3,5-tris(N-phenyl-
benzimidazol-2-yl)benzene/carbazole and diphenylamine] moie-
ties in their design of bipolar host materials for green PhOLEDs,
achieving a maximum external quantum efficiency, current effi-
ciency, and power efficiency of 14%, 48.2 cd Aꢀ1, and
46.0 lm Wꢀ1, respectively.10 Carbazoles are widely used key
bInstitute of Optoelectronic Sciences, National Taiwan Ocean University,
Keelung, 202, Taiwan. E-mail: wenhung@mail.ntou.edu.tw
† Electronic supplementary information (ESI) available: Experimental
procedure and spectral analysis. Crystal data for CbzCBI and synthetic
procedures and spectroscopic characterizations of CbzCBI, CbzNBI,
mCPCBI, and mCPNBI. CCDC reference number 829029. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/c1jm12598b
This journal is ª The Royal Society of Chemistry 2011
J. Mater. Chem., 2011, 21, 14971–14978 | 14971

View Article Online
building blocks for hole-transporting-type host materials
because they possess high triplet energies and excellent hole-
transport properties.11 For example, the most commonly used
carbazole-based material, 4,40-bis(9-carbazolyl)-2,20-biphenyl
(CBP), possessing a triplet energy (ET) of 2.56 eV, has been used
prominently as a host for green and red phosphorescent emit-
ters.12–18 A structurally modified variant of this host material,
1,3-bis(9-carbazolyl)benzene (mCP), behaves as a superior
phosphorescent host because of its higher triplet energy (ET ¼ 2.9
eV).15b Although CBP and mCP possess appropriate triplet
energies, their relatively low thermal and morphological stabili-
ties [e.g., glass transition temperatures (Tg) of 62 and 60 ꢁC,
respectively],14,15a,19 and low electron drift mobilities20,21 hinder
their applications as host materials in OLEDs. In this study, we
combined versatile carbazole core units (N-phenylcarbazole and
mCP) with an electron-deficient benzimidazole moiety to prepare
new bipolar materials, applying a concise strategy to manipulate
the physical properties of the bipolar host materials by changing
the linking topology (C- or N-connectivity of the benzimidazole
unit). We investigated the thermal, photophysical, electro-
chemical, and carrier-transport properties of the host materials
with respect to their molecular structures. These bipolar host
materials have potential practical use as hosts for highly efficient
green and blue PhOLED devices.
2. Results and discussion
2.1 Synthesis
Scheme 1 presents the synthetic routes that we followed to
prepare the bipolar host materials CbzCBI, CbzNBI, mCPCBI,
and mCPNBI. We obtained CbzCBI and CbzNBI in 67% and
62% yields, respectively, through Negishi couplings of the
organozinc intermediate 3, prepared through lithium/bromine
exchange of 2-bromo-N-phenylcarbazole (2) followed by treat-
ment with zinc(II) chloride, with the brominated diphenylbenzi-
midazole derivatives 45c and 5,5c respectively. During the
preparation of this manuscript, CBzCBI used to serve as the
green host is reported by Park et al.22 We isolated mCPCBI and
mCPNBI in 54% and 45% yields, respectively, after Negishi and
Suzuki couplings, respectively. We obtained (in 84% yield) the
key intermediate for both of these products, 1-(3-bromocarba-
zolyl)-3-carbazolylbenzene (7), through treatment of 1,3-dicar-
bazolylbenzene (6) with N-bromosuccinimide (NBS). The
organozinc intermediate 8 (boronic ester 9) used for the Negishi
(Suzuki) coupling was prepared through treatment of 7 sequen-
tially with n-BuLi and zinc(II) chloride (isopropyl pinacol
borate).
Scheme 1 Synthetic routes toward bipolar host materials, CbzCBI,
CbzNBI, mCPCBI, and mCPNBI and their chemical structures.
temperatures of the carbazole–benzimidazole hybridized bipolar
hosts having the same molecular weight (CbzCBI and CbzNBI;
mCPCBI and mCPNBI) were identical, regardless of the linking
topology (C- or N-connectivity of the benzimidazole unit to the
carbazole moiety). In contrast, the C-linked species (CbzCBI,
mCPCBI) possessed slightly higher thermal stabilities than those
of their N-connected counterparts (CbzNBI, mCPNBI). A high
glass transition temperature is desirable for a host material to be
used in a PhOLED because it suppresses the formation of
aggregates and prevents morphological changes of the amor-
phous organic layer upon heating.23 The high values of Tg and Td
for these compounds imply that they are capable of enduring not
only vacuum thermal sublimation—a general requirement for
PhOLED fabrication—but also the inevitable joule heating that
would occur during device operation.
2.2 Thermal analysis
We used differential scanning calorimetry (DSC) and thermog-
ravimetric analysis (TGA) to characterize the thermal properties
of CbzCBI, CbzNBI, mCPCBI, and mCPNBI (Table 1). The
mCP-linked compounds (mCPCBI, mCPNBI) exhibit much
higher decomposition temperatures (Td, corresponding to 5%
weight loss; 432–435 ꢁC) and glass transition temperatures (Tg ¼
151 ꢁC) relative to those of the N-phenylcarbazole-based
analogues (CbzCBI, CbzNBI), consistent with their greater
molecular weights. Interestingly, the glass transition
2.3 Photophysical properties
Fig. 1 displays UV-Vis absorption and photoluminescence (PL)
spectra (recorded at room temperature) of these compounds in
14972 | J. Mater. Chem., 2011, 21, 14971–14978
This journal is ª The Royal Society of Chemistry 2011

View Article Online
Table 1 Physical properties of CbzCBI, mCPCBI, CbzNBI, and mCPNBI
Tg/ꢁC
Tm/ꢁC
Td/ꢁC
Eox/Va
E1/2red/Va
labs/nm, sol.a/film
lPL/nm, sol.b/film
ET/eV
(HOMOc/LUMO/Eg)/eV
CbzCBI
mCPCBI
CbzNBI
mCPNBI
117
151
117
151
286
371
435
351
432
1.21
1.24
1.31
1.28
ꢀ2.00
ꢀ1.99
ꢀ2.15
ꢀ2.17
304, 327/309, 332
293, 325/297, 331
297/300
402/425
402/420
352/390
360/392
2.49
2.50
2.72
2.73
ꢀ5.52/–2.32/3.2
ꢀ5.6/–2.42/3.18
ꢀ5.62/–2.22/3.4
ꢀ5.8/–2.36/3.44
n.d.d
n.d.
n.d.
292/296
a
On-set potential. b In CH2Cl2, PL spectra were obtained by the excitation at the absorption maxima in solution. c Determined by AC-2 measurements.
d
LUMO ¼ HOMO + Eg. The value of Eg was calculated from the absorption onset of the solid film. Not detected.
various solutions and in the form of solid thin films, as well as
their phosphorescence spectra in EtOH at 77 K; Table 1
summarizes the data. The absorption, fluorescence, and phos-
phorescence maxima depended on the N-phenylbenzimidazole
linking topology (C- or N-connectivity) to the carbazolyl moiety.
Remarkable red-shifts appeared in the spectra of C-connected
compounds (CbzCBI, mCPCBI) relative to those of their
N-connected counterparts (CbzNBI, mCPNBI).
states are subjected to rather small changes in dipole moment
with respect to the ground states.24 The corresponding PL
spectra, however, revealed pronounced solvent-dependent emis-
sion signals, especially for the C-linked bipolar molecules
CbzCBI and mCPCBI. The different levels of dependence of the
emission spectra of CbzCBI and mCPCBI on the solvent polarity
relative to those of CbzNBI and mCPNBI imply different
behaviors of the excited states in these two sets of bipolar
molecules. In the compounds featuring carbazole moieties linked
to the N-phenylbenzimidazole acceptor via C-connections, the
emission spectral shift was proportional to the solvent polarity,
suggesting a mechanism involving rapid photo-induced electron
transfer (PET) within the bipolar molecule.25 The PET process
would lead to a large change in the dipole moment in the excited
state, rendering the excited state highly sensitive to the dielectric
environment, thereby providing fluorescence solvatochromism.
On the other hand, the spectra of the N-connected analogues
CbzNBI and mCPNBI featured broad emission peaks near
450 nm, rather than red-shifting of the emission maxima, upon
increasing the solvent polarity. This solvent relaxation process
involving two emitting species is consistent with a reversible
excited-state two-step reaction, in which the emission is
contributed from the locally (Frank–Condon) excited (LE) and
charge transfer (CT) emissions. The overlapped LE and CT
emissions are probably due to weak orbital interactions between
the donor and acceptor units in the N-connected bipolar mole-
cules, resulting in lower electron transfer rate constants. Notably,
the triplet energies (ET) of CbzCBI and mCPCBI (2.49 and
2.50 eV, respectively), determined from the highest energy
vibronic sub-band of the phosphorescence spectra, were signifi-
cantly lower than those of CbzNBI and mCPNBI (2.72 and
2.73 eV, respectively). The values of ET of CbzCBI and mCPCBI
are sufficiently high for them to host the green phosphorescent
For example, the absorption spectrum of CbzCBI displays an
additional broad peak centered at 327 nm, whereas its N-linked
counterpart CbzNBI provided a peak centered at 297 nm. We
ascribe this long wavelength p–p* electronic transition of
CbzCBI to the extended p conjugation resulting from C-
connection between the N-phenylbenzimidazole acceptor and the
carbazole donor. We assign the absorption peak centered near
300 nm to the p–p* electronic transition of the donor moiety
(N-phenylcarbazole). The signals in the PL spectra of these four
compounds in the form of neat films were red-shifted by 20–
30 nm relative to those in CH2Cl2 solution, presumably because
of different dielectric surroundings in the solid state. Fig. 1 also
displays the UV and PL spectra of CbzCBI, mCPCBI, CbzNBI,
and mCPNBI in various organic solvents. The absorption
spectra of these four compounds in cyclohexane, toluene,
CH2Cl2, and MeCN are nearly identical. Such solvent-indepen-
dent absorption spectra suggest that the Franck–Condon excited
emitter
bis(2-phenylpyridinato)iridium(III)
acetylacetonate
[(PPy)2Ir(acac)], which exhibits a value of ET of 2.3 eV.25 We
suspected that CbzNBI and mCPNBI, with their high values of
ET, might serve as hosts0 for the blue emitter bis[4,6-(difluor-
ophenyl)-pyridinato-N,C2 ]picolinate (FIrpic), which has a value
of ET of 2.62 eV.26
2.4 Crystal structure
We obtained crystals of CbzCBI suitable for X-ray diffraction
analysis, through slow evaporation of the solvent from a CHCl3/
hexane solution. The crystal data were summarized in Table S1
(ESI†). In the crystals, two CbzCBI molecules were found to pair
up with opposite orientations due to the donor/acceptor inter-
actions. There are two sets of face-to-face packings, one is
Fig. 1 Room-temperature absorption and PL spectra of CbzCBI,
mCPCBI, CbzNBI, and mCPNBI in various solutions and in neat films,
as well as phosphorescence (Phos) spectra recorded from their EtOH
solutions at 77 K.
This journal is ª The Royal Society of Chemistry 2011
J. Mater. Chem., 2011, 21, 14971–14978 | 14973

View Article Online
Fig. 3 Cyclic voltammograms of CbzCBI, CbzNBI, mCPCBI, and
mCPNBI; 0.1 M TBAP (reduction) in DMF and 0.1 M TBAPF6
(oxidation) in CH2Cl2 were used as supporting electrolytes. A glassy
carbon electrode was used as the working electrode; scan rate: 100 mV
sꢀ1
.
molecules CbzCBI and mCPCBI exhibited quasi-reversible
reduction waves, arising from their N-phenylbenzimidazole
segments, in which slightly extended p-conjugations as N-phe-
nylbenzimidazole connected to the C3 of carbazole are expected.
Obviously, the reduction on-sets of CbzNBI and mCPNBI are at
higher potentials as compared to those of C-connected coun-
terparts, suggesting the electron distributions on their lowest
unoccupied molecular orbitals (LUMOs) are solely localized on
the N-phenylbenzimidazole moieties. Because of the lack of
reversibility of the oxidation scans, we determined the energy
levels of highest occupied molecular orbitals (HOMOs) using
photoelectron yield spectroscopy (Riken AC-2); we calculated
the LUMO energy levels from the HOMO energy levels using the
equation LUMO ¼ HOMO + Eg, where Eg is the optical band
gap determined from the onset wavelength of the absorption
band. Table 1 summarizes the calculated HOMO and LUMO
energies. The lower energy gaps of CbzCBI, mCPCBI as compare
to their counterparts CbzNBI, mCPNBI are accounted for their
shallow HOMO and lower LUMO energy levels.
Fig. 2 Selected molecular packing in the unit cell and molecular struc-
ture of CbzCBI. Selected dihedral angles: C(16)–C(17)–C(20)–C(21),
37.6ꢁ; N(1)–C(13)–C(14)–C(15), 28.7ꢁ.
between the phenylene rings bridging carbazole and benzimid-
ꢀ
azole with a closest point-to-point distance of ca. 3.65 A, another
one is between the N-phenyl groups of benzimidazole with
ꢀ
a closest point-to-point distance of ca. 3.75 A, suggesting negli-
gible intermolecular p–p interactions (Fig. 2). This large inter-
molecular separation resulted from the molecule’s twisted
conformation; the dihedral angles of the carbazole/phenylene
linker and phenylene linker/benzimidazole units of 37.6ꢁ and
28.7ꢁ, respectively, render the molecule rather bulky. A lack of
intermolecular interactions in the solid state would benefit the
formation of an amorphous thin film by vacuum deposition.
2.6 Charge carrier mobility
2.5 Electrochemical properties
To further understand the charge-carrier transport properties of
CbzCBI, mCPCBI, CbzNBI, and mCPNBI, we used the time-of-
flight (TOF) technique27 to evaluate the carrier mobilities. Fig. 4
(a) displays representative TOF transient for holes of CbzCBI.
The transit time, tT, can be evaluated from the intersection point
of two asymptotes in the double-logarithmic representation of
the TOF transient. It is then used to determine the carrier
mobilities, according to the equation m ¼ d2/VtT, where d is the
sample thickness and V is the applied voltage. Fig. 4(b) reveals
that the field dependence of the hole mobility follows the nearly
universal Poole–Frenkel relationship, with values in the range
from 3 ꢂ 10ꢀ5 to 8 ꢂ 10ꢀ6 cm2 Vꢀ1 sꢀ1. In contrast, the TOF
transient for electrons exhibited strong dispersive photocurrents,
which could not be used to determine the electron mobility. For
the other three compounds, the transient photocurrent signals
were too weak for us to evaluate their mobilities using the TOF
technique.
We used cyclic voltammetry (CV) to probe the electrochemical
properties of the bipolar compounds CbzCBI, CbzNBI,
mCPCBI, and mCPNBI, with tetra(n-butyl)ammonium hexa-
fluorophosphate (TBAPF6) in CH2Cl2 and tetra(n-butyl)
ammonium perchlorate (TBAP) in DMF as supporting electro-
lytes for the oxidation and reduction scans, respectively. Table 1
summarizes the measured redox potentials. The irreversible
oxidations during the first CV scan of the four bipolar molecules
were due to the dimerization or electropolymerization of the
carbazole units at the C3 and/or C6 positions. The oxidation on-
sets of CbzCBI, CbzNBI, mCPCBI, and mCPNBI occurred at
1.21, 1.31, 1.24, and 1.28 V (vs. AgCl), respectively (Fig. 3). Once
again, these values closely correlate to the degree of p-conjuga-
tion, in which the higher degree of p-conjugation in the C-con-
nected bipolar molecules CbzCBI and mCPCBI impart lower
oxidation potentials. In the reduction scans, C-connected
14974 | J. Mater. Chem., 2011, 21, 14971–14978
This journal is ª The Royal Society of Chemistry 2011

View Article Online
device incorporating CbzCBI, indicating that the most favorable
hole-transport occurred in the CbzCBI thin film, probably
because preferential molecular alignment in the solid state
created suitable transport channel for holes. To assess the elec-
tron transport properties, we fabricated electron-only devices
having the device structure ITO/BCP (30 nm)/compounds
(60 nm)/TPBI (20 nm)/LiF/Al (Fig. 5(b)). Here, BCP served as
the hole-blocking layer to impede the hole current in the devices.
The similar turn-on voltages for the devices featuring each of the
four compounds suggest that they have comparable electron-
transport abilities, presumably because of their similar electron-
transport structures.
2.7 OLED characteristics
In view of the favorable triplet energy gaps of CbzCBI (2.49 eV)
and mCPCBI (2.50 eV), we selected bis(2-phenylpyridinato)
iridium(III) acetylacetonate (PPy)2Ir(acac)25 and bis(2,N-diphe-
nylbenzimidazolito) iridium(III) acetylacetonate (PBi)2Ir(acac)28
(see Scheme 2 for structures) as green emitters for electro-
phosphorescent devices having the configuration ITO/poly-
Fig. 4 (a) Representative TOF transients for CbzCBI (thickness: 1.1 mm;
E ¼ 5.4 ꢂ 105 V cmꢀ1). Inset: double-logarithmic plot. (b) The hole
ethylenedioxythiophene:polystyrenesulfonate
(PEDOT:PSS)
(30
nm)/9,9-di[4-(di-p-tolyl)aminophenyl]fluorene
(DTAF)
mobility of CbzCBI plotted with respect to E1/2
.
(40 nm)/host:dopant
8
wt% (30 nm)/1,3,5-tris(N-phenyl-
benzimidizol-2-yl)benzene (TPBI) (30 nm)/LiF/Al. Here, we used
the conducting polymer PEDOT:PSS as the hole injection layer
(HIL) and bulky DTAF as the hole transporting layer (HTL) to
facilitate hole transport, due to its good hole injection and
transporting capability with limited intermolecular interactions
in the solid state.29 We expected the low HOMO/LUMO energy
levels of TPBI30 to benefit hole blocking and facilitate electron
injection and transport. LiF and Al served as the electron-
injection layer and cathode, respectively. Table 2 summarizes the
EL characteristics of these PhOLEDs.
Fig. 6 presents the current density–voltage–luminance (I–V–L)
characteristics, device efficiencies, and EL spectra of the devices
incorporating CbzCBI and mCPCBI doped with green phos-
phors (devices A1, A2, B1, and B2). These devices exhibited turn-
on voltages of 2.0–2.5 V (defined as the voltage at which EL
became detectable), high external quantum efficiencies (17.8–
20.1%, 66.9–70.4 cd Aꢀ1), high power efficiencies (59.3–67 lm
Wꢀ1), and maximum brightnesses of 125 700–152 000 cd mꢀ2. In
the series of devices A and B, the turn-on voltages of the devices
incorporating CbzCBI (which featured a lower injection barrier
for holes at the DTAF–CbzCBI interface) as host were slightly
lower (ca. 2 V) than those of the devices incorporating mCPCBI.
Furthermore, the CbzCBI-based devices required lower driving
voltages (4.7–4.9 V at a practical brightness of 1000 cd mꢀ2) than
those of the corresponding mCPCBI-based devices, consistent
with our results obtained using the hole-only single-carrier
Fig. 5 Current density–voltage (I–V) characteristics of (a) hole- and (b)
electron-only devices.
Next, we fabricated carrier-only devices to investigate the
effect of the compounds on the transport characteristics. We
prepared hole-only single-carrier devices, having the device
structure indium tin oxide (ITO)/a-NPD (20 nm)/compounds (60
nm)/a-NPD (20 nm)/LiF/Al, to evaluate the hole transport
behavior of CbzCBI, mCPCBI, CbzNBI, and mCPNBI (Fig. 5
(a)).
With its high-lying LUMO, we used a-NPD to limit the elec-
tron carrier. The lowest turn-on voltage was that provided by the
Scheme 2 Structures of the heavy metal complexes used in this study.
J. Mater. Chem., 2011, 21, 14971–14978 | 14975
This journal is ª The Royal Society of Chemistry 2011

View Article Online
Table 2 EL performance of devices incorporating various hosts and emitters
Device Host:dopanta
Vonc/V L ¼ 1000 nit (V, %) Lmax/cd mꢀ2
Imax/mA cmꢀ2 hext max (%, cd Aꢀ1
)
hp max/lm Wꢀ1 CIE1931 (x, y)
A1
A2
B1
B2
C3
D3
CbzCBI:PPy2Iracac 2.0
4.7, 17.7
4.9, 19.1
5.2, 17.5
5.7, 19.5
7, 13
152 000 (9.5 V) 1800
150 000 (9.5 V) 2100
138 000 (10.5 V) 1250
125 700 (11.5 V) 1780
38 500 (14 V) 680
47 200 (14.5 V) 1310
17.8, 67.6
59.3
63.2
67
63.5
25.8
23.3
0.33, 0.64
0.38, 0.60
0.34, 0.63
0.38, 0.59
0.16, 0.36
0.17, 0.37
CbzCBI:PBi2Iracac 2.0
mCPCBI:PPy2Iracac 2.5
mCPCBI:PBi2Iracac 2.5
20.1, 70.4
17.9, 66.9
19.6, 67.8
13.5, 28.7
16.3, 35.7
CbzNBI:FIrpicb
mCPNBI:FIrpicb
2.5
3.5
8.1, 15.8
a
Device configuration: ITO/PEDOT (30 nm)/DTAF (40 nm)/host:dopant 8 wt% (30 nm)/TPBI (30 nm)/LiF/Al. b Device configuration: ITO/PEDOT
c
(30 nm)/DTAF (40 nm)/host:FIrpic 12 wt% (30 nm)/TAZ (30 nm)/LiF/Al. Turn-on voltage at which emission became detectable (10ꢀ2 cd mꢀ2).
densities. Therefore, the use of a bipolar host material can provide
balanced charge fluxes within the emitting layer, rendering devices
exhibiting only a limited external quantum efficiency roll-off.
When compared with devices A1 and A2, devices B1 and B2 with
mCPCBI as the host, we achieved similar device efficiencies (Fig. 6
(b)). Next, we doped the host materials with higher triplet energy
gaps, CBzNBI (2.72 eV) and mCPNBI (2.73 eV), with a commer-
cially available triplet blue emitter, bis(4,6-difluorophenylpyr-
idinato-N,C2) picolinatoiridium(III) (FIrpic),26 to prepare devices
C3 and D3 with the configuration ITO/PEDOT:PSS (30 nm)/
DTAF (40 nm)/host:dopant 12 wt% (30 nm)/3-(4-biphenylyl)-4-
phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole (TAZ, 30 nm)/LiF/Al.
Fig. 6 (a) I–V–L characteristics, (b) plots of EL efficiency versus
brightness, and (c) EL spectra for devices incorporating CbzCBI and
mCPCBI doped with green phosphors.
devices (Fig. 5(a)). In devices A1 and A2, we employed CbzCBI
as the host for (PPy)2Ir(acac) and (PBi)2Ir(acac), respectively.
The (PBi)2Ir(acac)-based device A2 exhibited maximum effi-
ciencies (20.1%, 70.4 cd Aꢀ1, 63.2 ml Wꢀ1) that were superior to
those of the (PPy)2Ir(acac)-based device A1 (17.8%, 67.6 cd Aꢀ1
,
59.3 ml Wꢀ1). The maximum external quantum efficiency (hext) of
20.1% was equal to the theoretical limit (20%) for a PhOLED.
When the brightness reached 1000 cd mꢀ2, the value of hext
remained high (19.1%; retaining >95% of the external quantum
efficiency), suggesting that balanced injection and transport of
holes and electrons in the device occurred even at high current
Fig. 7 (a) I–V–L characteristics, (b) plots of EL efficiency versus
brightness, and (c) EL spectra for devices incorporating CbzNBI and
mCPNBI doped with 12 wt% FIrpic.
14976 | J. Mater. Chem., 2011, 21, 14971–14978
This journal is ª The Royal Society of Chemistry 2011

View Article Online
2 (a) M. A. Baldo, C. Adachi and S. R. Forrest, Phys. Rev. B: Condens.
Matter, 2000, 62, 10967; (b) S.-J. Su, H. Sasabe, T. I. Takeda and
J. Kido, Chem. Mater., 2008, 20, 1691.
3 (a) M. A. Baldo, S. Lamansky, P. E. Burrows, M. E. Thompson and
S. R. Forrest, Appl. Phys. Lett., 1999, 75, 4; (b) M. H. Lu,
M. S. Weaver, T. X. Zhou, M. Rothman, R. C. Kwong, M. Hack
and J. J. Brown, Appl. Phys. Lett., 2002, 81, 3921.
4 (a) G. Hughes and M. R. Bryce, J. Mater. Chem., 2005, 15, 94; (b)
C. T. Chen, Y. Wei, J. S. Lin, M. V. R. K. Moturu, W. S. Chao,
Y. T. Tao and C. H. Chien, J. Am. Chem. Soc., 2006, 128, 10992;
(c) N. J. Lundin, A. G. Blackman, K. C. Gordon and D. L. Officer,
Angew. Chem., Int. Ed., 2006, 45, 2582.
5 (a) X. Cai, A. B. Padmaperuma, L. S. Sapochak, P. A. Vecchi and
P. E. Burrows, Appl. Phys. Lett., 2008, 92, 083308; (b) Y. Tao,
Q. Wang, C. Yang, Q. Wang, Z. Zhang, T. Zou, J. Qin and D. Ma,
Angew. Chem., Int. Ed., 2008, 47, 8104; (c) Z. Ge, T. Hayakawa,
S. Ando, M. Ueda, T. Akiike, H. Miyamoto, T. Kajita and
M.-A. Kakimoto, Adv. Funct. Mater., 2008, 18, 584; (d) Z. Ge,
T. Hayakawa, S. Ando, M. Ueda, T. Akiike, H. Miyamoto,
T. Kajita and M.-A. Kakimoto, Org. Lett., 2008, 10, 421; (e)
M. Y. Lai, C. H. Chen, W. S. Huang, J. T. Lin, T. H. Ke,
L. Y. Chen, M. H. Tsai and C. C. Wu, Angew. Chem., Int. Ed.,
2008, 47, 581.
6 (a) H.-F. Chen, S.-J. Yang, Z.-H. Tsai, W.-Y. Hung, T.-C. Wang and
K.-T. Wong, J. Mater. Chem., 2009, 19, 8112; (b) Z. Ge,
T. Hayakawa, S. Ando, M. Ueda, T. Akiike, H. Miyamoto,
T. Kajita and M. Kakimoto, Org. Lett., 2008, 10, 421.
7 Y. Li, M. K. Fung, Z. Xie, S.-T. Lee, L.-S. Hung and J. Shi, Adv.
Mater., 2002, 14, 1317.
8 C.-H. Chen, W.-S. Huang, M.-Y. Lai, W.-C. Tsao, J. T. Lin,
Y.-H. Wu, T.-H. Ke, L.-Y. Chen and C.-C. Wu, Adv. Funct.
Mater., 2009, 19, 2661.
9 S. Gong, Y. Zhao, C. Yang, C. Zhong, J. Qin and D. Ma, J. Phys.
Chem. C, 2010, 114, 5193.
10 S. Takizawa, V. A. Montes and P. Anzenbacher, Jr, Chem. Mater.,
2009, 21, 2452.
11 M.-H. Tsai, H.-W. Lin, H.-C. Su, T.-H. Ke, C. Wu, F.-C. Fang,
Y.-L. Liao, K.-T. Wong and C.-I. Wu, Adv. Mater., 2006, 18,
1216.
12 G. Zhou, W.-Y. Wong, B. Yao, Z. Xie and L. Wang, Angew. Chem.,
Int. Ed., 2007, 46, 1149.
13 C.-L. Ho, W.-Y. Wong, Z.-Q. Gao, C.-H. Chen, K.-W. Cheah,
B. Yao, Z. Xie, Q. Wang, D. Ma, L. Wang, X.-M. Yu, H.-S. Kwok
and Z. Lin, Adv. Funct. Mater., 2008, 18, 319.
14 S. Tokito, T. Iijima, Y. Suzuri, H. Kita, T. Tsuzuki and F. Sato, Appl.
Phys. Lett., 2003, 83, 569.
15 (a) V. Adamovich, J. Brooks, A. Tamayo, A. M. Alexander,
P. I. Djurovich, B. W. D’Andrade, C. Adachi, S. R. Forrest and
M. E. Thompson, New J. Chem., 2002, 26, 1171; (b)
R. J. Holmes, S. R. Forrest, Y. J. Tung, R. C. Kwong,
J. J. Brown, S. Garon and M. E. Thompson, Appl. Phys. Lett.,
2003, 82, 2422.
16 H.-H. Chou, H.-H. Shih and C.-H. Cheng, J. Mater. Chem., 2010, 20,
798.
17 D. Hu, P. Lu, C. Wang, H. Liu, H. Wang, Z. Wang, T. Fei, X. Gu and
Y. Ma, J. Mater. Chem., 2009, 19, 6143.
This device structure is similar to that of device A, except for the
presence of TAZ as the ETL. The triplet energies of DTAF (2.87
eV)29 and TAZ (2.70 eV)31 were sufficiently high to prevent any
possible luminescence quenching by the carrier-transporting
layers and to confine the triplet excitons in the EML. Fig. 7(a)
displays the I–V–L characteristics of these blue light-emitting
devices. The CBzNBI-based device C3 exhibited significantly
lower turn-on and operation voltages than did the mCPNBI-
based device D3. We attribute the enhanced device current to the
facilitation of hole injection when using CBzNBI as the host
material, as revealed by its relatively high HOMO energy level and
hole transporting properties (Fig. 5(a)). Table 2 and Fig. 7(b)
reveal that the mCPNBI-based device D3 exhibited maximum
efficiencies (16.3%, 35.7 cd Aꢀ1, 23.3 ml Wꢀ1) that were superior to
those of the CBzNBI-based device C3 (13.5%, 28.7 cd Aꢀ1, 25.8 ml
Wꢀ1), presumably because mCPNBI possesses relatively poor
hole injection and transport behavior, leading to a balanced hole
and electron in the EML. These values are comparable to those
that we reported previously for a device based on CzSi,11 con-
firming the suitability of using N-connected bipolar hosts for the
blue phosphor.
Conclusions
We have synthesized four bipolar hosts, CbzCBI, mCPCBI,
CbzNBI, and mCPNBI, containing hole-transporting carbazole
and electron-transporting benzimidazole moieties, for the reali-
zation of highly efficient green and blue electrophosphorescent
devices. Different linking topologies between the benzimidazole
group (C- or N-connectivity) and the carbazole donor resulted in
different effective conjugation lengths and different excited-state
solvent relaxation processes. Each of the bipolar hosts exhibited
high morphological and thermal stability, suitable energy levels,
and balanced electron/hole transporting characteristics, all of
which are necessary for high-performance PhOLEDs. A green-
phosphorescent device incorporating CbzCBI as host doped with
(PBi)2Ir(acac) achieved a maximum external quantum efficiency,
current efficiency, and power efficiency of 20.1%, 70.4 cd Aꢀ1
and 63.2 lm Wꢀ1
respectively (device A2). The bipolar
,
,
compounds featuring N-connected benzimidazole moieties
(CbzNBI, mCPNBI) possessed the higher triplet energies
required to host blue phosphors. A blue PhOLED device (device
D3) incorporating mCPNBI as the host achieved a maximum
external quantum efficiency, current efficiency, and power effi-
ciency of 16.3%, 35.7 cd Aꢀ1, and 23.3 lm Wꢀ1, respectively. Our
results suggest that varying the molecular linking topology is
a possible strategy toward designing useful bipolar host mate-
rials; we hope that this approach will trigger the molecular design
of novel host materials for highly efficient PhOLEDs.
18 N. Chopra, J. Lee, Y. Zheng, S. H. Eom, J. Xue and F. So, Appl. Phys.
Lett., 2008, 93, 143307.
19 M.-H. Tsai, Y.-H. Hong, C.-H. Chang, H.-C. Su, C.-C. Wu,
A.
Matoliukstyte,
J.
Simokaitiene,
S.
Grigalevicius,
J. V. Grazulevicius and C.-P. Hsu, Adv. Mater., 2007, 19, 862.
20 K.-K. Kim, B.-S. Moon and C.-S. Ha, J. Appl. Phys., 2006, 100,
064511.
21 N. Matsusue, Y. Suzuki and H. Naito, Jpn. J. Appl. Phys., 2005, 44,
3691.
22 J. H. Park, E. K. Kim, I. M. El-Deeb, S. J. Jung, D. H. Choi,
D.-H. Kim, K. H. Yoo, J. H. Kwon and S. H. Lee, Bull. Korean
Chem. Soc., 2011, 32, 841.
23 (a) E. Han, L. Do, Y. Niidome and M. Fujihira, Chem. Lett., 1994, 23,
969; (b) S. Tokito and Y. Taga, Appl. Phys. Lett., 1995, 66, 673; (c)
S. Liu, F. He, H. Wang, H. Xu, C. Wang, F. Li and Y. Ma, J.
Mater. Chem., 2008, 18, 4802.
24 F.-M. Hsu, C.-H. Chien, C.-F. Shu, C.-H. Lai, C.-C. Hsieh,
K.-W. Wang and P.-T. Chou, Adv. Funct. Mater., 2009, 19,
2834.
Acknowledgements
The authors gratefully acknowledge the financial support from
National Science Council and Ministry of Economic Affairs of
Taiwan.
Notes and references
1 M. A. Baldo, D. F. O’Brien, Y. You, A. Shoustikov, S. Sibley,
M. E. Thompson and S. R. Forrest, Nature, 1998, 395, 151.
This journal is ª The Royal Society of Chemistry 2011
J. Mater. Chem., 2011, 21, 14971–14978 | 14977

View Article Online
25 C. Adachi, M. A. Baldo, M. E. Thompson and S. R. Forrest, J. Appl.
Phys., 2001, 90, 5048.
26 C. Adachi, R. C. Kwong, P. Djurovich, V. Adamovich, M. A. Baldo,
M. E. Thompson and S. R. Forrest, Appl. Phys. Lett., 2001, 79,
2982.
27 P. M. Borsenberger and D. S. Weiss, Organic Photoreceptors for
Imaging Systems, Marcel Dekker, New York, 1993.
28 (a) W.-S. Huang, J. T. Lin, C.-H. Chien, Y.-T. Tao, S.-S. Sun and
T.-S. Wen, Chem. Mater., 2004, 16, 2480; (b) W.-S. Huang,
J. T. Lin and H.-C. Lin, Org. Electron., 2008, 9, 557.
29 (a) W.-Y. Hung, L.-C. Chi, W.-J. Chen, Y.-M. Chen, S.-H. Chou and
K.-T. Wong, J. Mater. Chem., 2010, 20, 10113; (b) M.-T. Kao,
W.-Y. Hung, Z.-H. Tsai, H.-W. You, H.-F. Chen, Y. Chi and
K.-T. Wong, J. Mater. Chem., 2011, 21, 1846; (c) W.-Y. Hung,
H.-W. You, F.-C. Fan, H.-F. Chen and K.-T. Wong, Org.
Electron., 2011, 12, 575.
30 S.-J. Yeh, M.-F. Wu, C.-T. Chen, Y.-H. Song, Y. Chi, M.-H. Ho,
S.-F. Hsu and C. H. Chen, Adv. Mater., 2005, 17, 285.
31 L. Xiao, S. J. Su, Y. Agata, H. Lan and J. Kido, Adv. Mater., 2009, 21,
1271.
14978 | J. Mater. Chem., 2011, 21, 14971–14978
This journal is ª The Royal Society of Chemistry 2011