Novel benzimidazole derivatives as electron-transporting type host to achieve highly efficient sky-blue Phosphorescent organic light-emitting diode (PHOLED) device

Article abstract of DOI:10.1021/ol502602t

The development of benzimidazole substituted biphenyls as electron-transporting hosts for bis[2-(4,6-difluorophenyl)pyridinato-C²,N](picolinato)iridium-(III) is reported. Under the optimized conditions, the organic light-emitting diode (OLED) a

Full text of DOI:10.1021/ol502602t

Letter
pubs.acs.org/OrgLett
Novel Benzimidazole Derivatives as Electron-Transporting Type Host
To Achieve Highly Efficient Sky-Blue Phosphorescent Organic Light-
Emitting Diode (PHOLED) Device
†
,†,‡
,§
§
,†
Jau-Jiun Huang, Man-kit Leung,* Tien-Lung Chiu,* Ya-Ting Chuang, Pi-Tai Chou,*
and Yu-Hsiang Hung§
†
‡
Department of Chemistry and Institute of Polymer Science and 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, Chung-Li 32003, Taiwan, R.O.C.
*
S Supporting Information
ABSTRACT: The development of benzimidazole substituted biphenyls as electron-
2
transporting hosts for bis[2-(4,6-difluorophenyl)pyridinato-C ,N](picolinato)iridium-
(
(
5
III) is reported. Under the optimized conditions, the organic light-emitting diode
OLED) achieves the maximum current efficiency of 57.2 cd/A, power efficiency of
0.4 lm/W, and external quantum efficiency 25.7%.
1
0
hosphorescent organic light-emitting diodes (PHOLEDs)
have potential in display and lighting applications because of
EQE of 30%. In our present work, we have explored the
possibility of using N-phenylbenzimidazole (N-PBI) based
molecules, including BImBP and o-PhBz (Scheme 1), as
P
their high theoretical quantum efficiency, which can be, in
principle, four times higher than that of the fluorescent based
1
organic light-emitting diodes (OLEDs). To achieve high
Scheme 1. Chemical Structures of the N-PBI Derivatives
efficiency, the phosphorescence emitters have to be doped in a
host matrix to reduce the probability of triplet−triplet
annihilation. Therefore, development of host materials with
appropriate optoelectronic properties such as charge carrier
mobility, well-matched highest occupied molecular orbital and
lowest unoccupied molecular orbital (HOMO−LUMO) levels2,
optical band gap, morphology, and thermal stability is crucial.
With regards to the carrier behavior, the hosts can be at least
3
classified into four categories: (1) hole transporting; (2)
effective ETHs for FIrpic. A high current efficiency of 57.2 cd/
A with low efficiency roll-off, a power efficiency (η ) of 50.4 lm/
W, an EQE of 25.7% in the class, and a Commission International
4
5
6
7
bipolar; (3) nonpolar, and (4) electron transporting (ET).
p
Among many high performance blue phosphorescence emitters,
bis[2-(4,6-difluorophenyl)pyridinato-C ,N](picolinato)iridium-
2
de I’Eclairage coordinate (CIE) of (0.15, 0.38) were found. The
2
PHOLED can maintain a high η of 53 cd/A at 1000 cd/m .
(
III) (FIrpic) has been shown to exhibit high external quantum
c
To be an ideal host material in PHOLEDs, the host must have
efficiency (EQE) over 25%, strongly depending on the host
material. Therefore, many hole-transporting and bipolar hosts
such as 1,3-(dicarbazol-N-yl)benzene (mCP) and derivatives,
a triplet energy (E ) higher than that of the doped guest to
T
prevent reverse energy transfer. Moreover, to reduce the hole
and electron injection barriers, the HOMO/LUMO level of the
host should correspond with the HOMO/LUMO levels of the
hole- and ET-transporting layers, respectively. N-PBI derivatives
have been proven effective for electron injection and transport.
which achieved current efficiencies (η ) as high as 60 cd/A, have
c
8
been successfully developed. In addition, other bipolar hosts
have widely been published such as mCPCN, mNBICz, and
4
a,9
PPO27.
Conversely, examples of highly efficient electron-
transporting hosts (ETHs) for FIrpic are rare, except that Lee et
al. reported a pyridoindole-based host material named CbBPCb,
Received: September 5, 2014
Published: October 9, 2014
−1
which is ET dominating and obtained an η of 53.6 cd A with an
c
©
2014 American Chemical Society
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dx.doi.org/10.1021/ol502602t | Org. Lett. 2014, 16, 5398−5401

Organic Letters
Letter
For example, 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene
o-PhBz and BImBP show strong absorption at wavelengths
(λ) 270−335 nm that are characteristic for the π−π* transitions
of the N-PBI chromophores (Figure S6). Optical gaps of 3.78
and 3.76 eV (Table 1) were estimated respectively according to
(
TPBi) has been widely used as an ET layer in PHOLEDs.
However, to the best of our knowledge, N-PBI derivatives have
never been used as ETHs for FIrpic-based PHOLEDs. To
maintain the high E , we adopted a strategy of connecting the N-
T
PBI group at the 2,2′-position of biphenyl. We expect that the
inter-ring steric hindrance would force the aromatic rings aligned
in noncoplanar conformations and thus interrupt the π
conjugation. To further understand the electronic interactions
between the facing N-PBI group, the mono-N-PBI-substituted o-
PhBz has been prepared and studied for comparison.
Table 1. Electrochemical Data and Photophysical Data for
BImBPs and the References
onset
Max/onset
λ
λ
PH
abs
a
b
c
d
host
(nm)
(nm)
E /E (eV) EHOMO /ELUMO (eV)
T g
BImP
326
479/391
3.17/3.81 −6.22/−2.41
e
(−2.46)
The syntheses were facile with a high product total yield
greater than 54%), which started from 2-phenylbenzoic acid and
,2′-biphenyldicarboxylic acid that were converted to the
corresponding acyl chloride, followed by reaction with 2-
aminodiphenylamine under acid catalyzed dehydrative cycliza-
tion conditions to give o-PhBz and BImBP in multigram scale
Scheme S1 and synthetic procedures).
o-PhBz and BImBP have a high decomposition temperature
Td) of 267 and 350 °C respectively (Figure S5). However, o-
PhBz and BImBP show different morphological behavior. o-
o-PhBz
BImBP
328
330
480/403
550/430
3.08/3.78 −6.17/−2.39
e
(−2.44)
(
2
2.89/3.76 −6.12/−2.36
e
(−2.42)
f
−6.18/−2.36
a
Measured in THF (10−5 M). Measured in THF (10−6 M).
Estimated by CV with Bu NClO (0.1 M) as supporting electrolyte
vs Ag/AgCl. The E
Forrest approach. ELUMO = EHOMO + E . The E
b
c
4
4
(
was estimated in CH Cl according to the
HOMO
2
2
12
d
e
was estimated
g
LUMO
13 f
(
by CV in DMF. By AC2 and thin-film optical gap.
PhBz presents a low glass-transition temperature (T ) of 47 °C.
In contrast, BImBP shows dramatically different phase stability
g
their λonset values. The small spectral difference between BImP
(λmax of 295 nm), o-PhBz, and BImBP may be due to the
orthogonal conformation of the biphenyl bridge that interrupts
the π-conjugation in BImBP, causing the N-PBI units to function
as independent chromophores. On the other hand, the
fluorescence emissions, peaking at 385 and 378 nm and spanning
the range 350−430 nm, respectively, well overlap with the
MLCT absorption of FIrpic; this would allow effective energy
transfer from the excited state of o-PhBz and BImBP to that of
FIrpic. The triplet energies 3.08 and 2.89 eV were estimated on
with no T being observed below 270 °C; only an endothermic
g
peak associated with the melting point at 275 °C was observed in
the heating process while recrystallization was observed at 213
and 220 °C in the cooling process. This behavior indicates that
BImBP favors a native crystalline form. Nevertheless, to our
surprise, the high tendency of crystallization does not hamper the
PHOLED performances (vide infra).
X-ray crystallography, a useful tool for understanding material
1
1
properties, illustrated that molecular stacking of BImBP is very
different from that of o-PhBz. Perhaps due to the steric
repulsions between the N-PBI substituents and biphenyl
moieties, the biphenyl moiety of BImBP possesses noncoplanar
alignment with a C2 proper rotational axis perpendicular to the
biphenyl axis. The C2−C1−C1′−C2′ dihedral angle of 49.9° was
found for the biphenyl unit. Similar to BImBP, o-PhBz has the
C2−C1−C1′−C2′ dihedral angle of 45.3°. Both dihedral angles
are larger than 45°, indicating orthogonal alignments of the
aromatic units. This will strongly affect the electronic properties
of the molecules. For example, the lowest triplet state (T1)
therefore is maintained at high energy levels due to disruption of
the π-conjugations that is clearly observed in the phosphor-
escence measurement. Presumably the short face-to-face intra-
benzimidazole distance of 3.35 Å allows through-space
interactions between the benzimidazole units. Furthermore,
the short interbenzimidazole distance of 5.28 Å between the
adjacent molecules along the b axis of the unit cell also allows
effective electron hopping between the BImBPs units (Figure 1).
the basis of the phosphorescent λonset respectively. The E is high
enough to prevent the reverse energy transfer from the FIrpic
T
blue phosphor (E of 2.65 eV) to the host in the triplet manifold,
T
and hence in principle both are potentially useful host materials
for FIrpic in PHOLEDs.
The electrochemical properties were evaluated by cyclic
voltammetry (CV) and differential-pulse voltammetry (DPV) in
−1
DMF and CH Cl , respectively, using Bu NClO (10 M) as the
2
2
4
4
supporting electrolyte and Ag/AgCl as the reference electrode,
+
and calibrated against ferrocene/ferrocenium (Fc /Fc) (Figures
S7−S8). In the reduction scans, o-PhBz and BImBP exhibit
irreversible multireduction waves with the first DPV peak
+
potential at −2.56 and −2.59 V (vs Fc /Fc) respectively. In the
oxidation scans, o-PhBz shows a single irreversible oxidation
wave with the DPV peak potential at +1.14 V. On the other hand,
BImBP shows double oxidation waves peaking at +1.10 and 1.38
V. Double-oxidation waves indicated that the N-PBI substituents
of BImBP are electronically coupled in the cationic states; this
splits the oxidation wave into two with the first oxidation being
slightly stabilized in comparison to that of BImP and o-PhBz.
The CV analysis suggested that the presence of the N-PBI
functionality is beneficial for either electron or hole injection and
transport in the PHOLED.
The HOMO level of thin films of BImBP and o-PhBz was
further measured by AC-2 photoelectron spectrometry (Figure
S9). The HOMO level of −6.18 eV for BImBP was found
accordingly. Perhaps due to having a relatively low density of the
benzimidazole units in the matrix, the HOMO level of o-PhBz is
too deep and the work function cannot be accurately
extrapolated from the spectrum. However, the trend of the
curve suggests that the HOMO level is ∼0.25 eV deeper than that
Figure 1. Single crystal molecular packing of BImBPs.
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Organic Letters
Letter
of BImBP. The LUMO levels of −2.36 eV for BImBP was
density at the device-operation electrical voltage, indicating that
electrons are the major carriers in BImBP. Under normal
operating electrical voltage, the electron current density
overwhelms the hole-current density by orders of magnitude.
At 10 V, over 4 orders of magnitude discrepancy was recorded.
estimated according to the equation of E
= E
+ E ,
LUMO
HOMO g
where E is optical band gap of the thin film. The HOMO/
g
LUMO values are in good agreement with those of BImBP and
o-PhBz obtained from DPV, which were −6.12/−2.36 and
−5
2
The field independent mobilities of μ = 1.8 × 10 cm /(V s) for
−6.17/−2.39 eV respectively. The appropriate HOMO/LUMO
0
−10
2
the electron and μ = 6.2 × 10 cm /(V s) for the hole were
alignment not only benefits the hole and electron injection into
the layer but also allows BImBP to act as an effective host for
FIrpic (Figures S10, S13); these findings are very important
because the family of N-PBI are typically considered as ET
materials only.
0
found respectively (Figure S12). These observations suggest that
BImBP is an excellent ET material. During the measurement, it is
noteworthy that the device of o-PhBz is subject to damage at
higher applied electrical voltage due perhaps to the lower thermal
stability (vide supra); short circuiting of the device was observed
when the applied electrical voltage was >10 V. This observation
hampers the use of o-PhBz as a host for PHOLEDs.
To gain insight into the electronic properties, the hole and
electron mobility of BImBP and o-PhBz were examined by using
the hole- and electron-only devices. In these experiments, the
multilayer devices were prepared by thermal vapor deposition
methods under high vacuum conditions. The hole-transporting
The optimized device configuration is shown as follows: ITO/
4
,4′-cyclohexylidenebis[N,N-bis(4-methylphenyl)benzen-
amine] (TAPC, 45 nm)/mCP (10 nm)/BImBP: 12% FIrpic (35
nm)/diphenylbis[4-(pyridin-3-yl)phenyl]silane (DPPS, 55
nm)/LiF (0.9 nm)/Al (120 nm). Other data including the
energy level alignment, FIrpic concentration effects, and EL
spectral properties are shown in Figures S13−S16. To optimize
the recombination efficiency in the emitting layer, the
thicknesses of the DPPS and emitting layers were adjusted to
enable the hole−electron balance. Since BImBP shows superb
electron transport properties, TAPC, with high hole mobility and
14
properties were evaluated by a device of ITO/MoO (1 nm)/
3
host(100 nm)/MoO (1 nm)/Al(100 nm), and the ET proper-
3
ties were examined by a device of glass/Al(50 nm)/LiF(1 nm)/
host(100 nm)/LiF(1 nm)/Al(100 nm).
The layer thicknesses of BImBP and o-PhBz in both types of
devices were set to 100 nm for a fair comparison. The plot of
current density of single-charge versus applied electrical voltage
of BImBP in Figure 2a (Figure S11) shows that the electron
current density is much higher in general than the hole current
−2
2
high triplet energy (μ of 1.0 × 10 cm /(V s), E of 2.87 eV),
h
T
was employed as the hole-transporting layer. In addition, the
presence of the layer of mCP was found to be essential to
maintain the high efficiency. Figure 2b−2c show the current
density−voltage−luminance curves of the blue PHOLEDs, with
2
2
the luminance up to 9513 cd/m at 38.3 mA/cm , maximum η
p
up to 50.4 lm/W, maximum η up to 57.2 cd/A, and maximum
c
EQE up to 25.7%.
Despite BImBP being an ETH, we believe FIrpic dopant also
acts as an electron trap, which balances the hole and electron in
the emitting layer to increase the recombination efficiency. In
addition, using TAPC as the hole transport layer that has a high
E helps confine the charge and exciton inside the emitting layer
T
10,15
and hence enhances the performance of the BImBP device.
The blue PHOLED of BImBP exhibits maximum emission
peaking at 472 nm which is typical for the FIrpic emission, and its
CIE coordinate is (0.15, 0.38) at 4 V (Figure S17). No emission
from the host or the charge transport materials was observed,
indicating complete energy transfer from BImBP to FIrpic and
full charge confinement inside the emitting layer. This is further
supported by the fluorescence lifetime study, using a time-
correlated single photon counting technique. Instead of having a
long fluorescence lifetime τ of 3.57 ns in pure BImBP film, the
lifetime is significantly shortened (τ < 50 ps) by doping with
FIrpic (6%), indicating the effectiveness of the energy transfer
process (Figure S10).
In summary, we report herein the design of BImBP as well as
its synthesis, characterization, and application in an OLED. First,
the high production yield is important for scalable synthesis and
future application. Furthermore, the intramolecular electronic
interactions between the N-PBI units significantly reduce the
oxidation potential, leading to the HOMO level of −6.18 eV.
This permits effective hole injection into BImBP for exciton
recombination and light emission. In addition, the X-ray
crystallographic analysis illustrates that the packing of the
molecules allows effective electron hopping inside the matrix. By
employing BImBP as the host matrix for FIrpic, we obtained a
high current efficiency of 57.2 cd/A based on the ET host, with
Figure 2. (a) Plot of current density (I) versus applied electrical voltage
(V) for hole-only and electron-only device. (b) I−V−Luminance plot
and (c) EL efficiency versus brightness plot.
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Organic Letters
Letter
an EQE of 25.7%. The device performance of BImBP is
comparable to that of previously reported high-performing
devices with hole-transporting or bipolar hosts.
(9) (a) Pan, B.; Wang, B.; Wang, Y.; Xu, P.; Wang, L.; Chen, J.; Ma, D. J.
Mater. Chem. C 2014, 2, 2466−2469. (b) Son, H. S.; Seo, C. W.; Lee, J.
Y. J. Mater. Chem. 2011, 21, 5638−5644.
(
(
10) Lee, C. W.; Lee, J. Y. Adv. Mater. 2013, 25, 5450−5454.
11) Xu, X.; Yang, X.; Dang, J.; Zhou, G.; Wu, Y.; Li, H.; Wong, W.-Y.
ASSOCIATED CONTENT
■
Chem. Commun. 2014, 50, 2473−2476.
*
S
Supporting Information
(12) Andrade, B. W. D.; Datta, S.; Forrest, S. R.; Djurovich, P.;
Polikarpov, E.; Thompson, M. E. Org. Electron. 2005, 6, 11−20.
NMR data; UV−vis, FL, and phosphorescence spectra; CV, TGA
and DSC, CIF data; and device performance. This material is
available free of charge via the Internet at http://pubs.acs.org.
(13) Djurovich, P. I.; Mayo, E. I.; Forrest, S. R.; Thompson, M. E. Org.
Electron. 2009, 10, 515−520.
14) Lee, J.; Chopra, N.; Eom, S.-H.; Zheng, Y.; Xue, J.; So, F.; Shi, J.
Appl. Phys. Lett. 2008, 93, 123306(1−3).
15) Fan, C.; Zhu, L.; Liu, T.; Jiang, B.; Ma, D.; Qin, J.; Yang, C. Angew.
Chem., Int. Ed. 2014, 53, 2147−2151.
(
(
AUTHOR INFORMATION
■
Corresponding Authors
* E-mail: mkleung@ntu.edu.tw (M.-k.L.).
*E-mail: tlchiu@saturn.yzu.edu.tw (T.-L.C.).
*E-mail: chop@ntu.edu.tw (P.-T.C.).
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Ministry of Science and
Technology (MOST), R.O.C., under Grant MOST 101-2113-
M-002-010-MY3, 103-3113-E-155-001, 102-2221-E-155-047,
and 102-2622-E-155-008-CC3, and Bureau of Energy, Ministry
of Economic Affairs, R.O.C. under Grant No. 102-E0616. We
also thank Prof. Jiun-Haw Lee, Department of Electrical
Engineering, National Taiwan University, for his invaluable
opinion and suggestions and Prof. Yu-Tai Tao, Institute of
Chemistry, Academia Sinica, for AC-2 measurement.
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