A simple carbazole-N-benzimidazole bipolar host material for highly efficient blue and single layer white phosphorescent organic light-emitting diodes

Article abstract of DOI:10.1039/c3tc32388a

A new simple carbazole-N-benzimidazole bipolar luminogen mNBICz was constructed and utilized as a host for an FIrpic-doped blue device, and exhibited high efficiencies with a η_EQE,max of 26.2%, a η_C,max of 54.5 cd A^-1 and a η_P,max of 52.2 lm W^-1. Furthermore, a two-color, all-phosphor single layer white device displayed a η_EQE,max of 22.9%, a η_C,max of 62.5 cd A^-1 and a η_P,max of 60.4 lm W^-1.

Full text of DOI:10.1039/c3tc32388a

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DOI: 10.1039/C3TC32388A
Journal Name
RSCPublishing
COMMUNICATION
A simple carbazoleꢀNꢀbenzimidazole bipolar host
material for highly efficient blue and single layer white
phosphorescent organic lightꢀemitting diodes†
Cite this: DOI: 10.1039/x0xx00000x
Biao Pan,a Bo Wang,a Yixing Wang,a Peng Xu,a Lei Wang,*a Jiangshan Chen*b,
Dongge Mab
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
www.rsc.org/
separation of spatial distribution of electron density to increase
bipolar property and improve the triplet energy level.25 In this
communication, we demonstrate a new carbazoleꢀNꢀbenzimidazole
bipolar host material in which the carbazole unit is linked to the
benzimidazole segment via the meta position of the N1ꢀphenyl. The
synthetic procedure of mNBICz was illustrated in Scheme 1. 1ꢀ(3ꢀ
bromophenyl)ꢀ2ꢀphenylꢀ1Hꢀbenzo[d]imidazole (1) was synthesized
A new simple carbazoleꢀNꢀbenzimidazole bipolar luminogens
mNBICz was constructed and utilized as host for FIrpicꢀdoped
blue device, and exhibited high efficiencies with ηEQE,max of
26.2%, ηC
of 54.5 cd Aꢀ1 and ηP,max of 52.2 lm Wꢀ1.
max
Furthermore, a twoꢀcolor, allꢀphosphor single layer white device
displayed a ηEQE,max of 22.9 %, ηC max of 62.5 cd Aꢀ1 and ηP,max of
60.4 lm Wꢀ1.
according to the literature.22 All of the compounds were
1
In the past decades, organic lightꢀemitting diodes (OLED) have been characterized by H NMR, 13C NMR, elemental analysis, and mass
considered to be the next generation technology of fullꢀcolor flatꢀ data. Detailed synthetic procedures and characterization data were
panel displays and solidꢀstate lighting sources.1ꢀ4 However, blue described in ESI†.
PhOLEDs with high performance remains a challenge mainly due to
the lack of appropriate host materials.5 For efficient blue PhOLEDs
host materials, there are some basic principles such as: appropriate
HOMO/LUMO levels; higher triplet energy level than blue dopant
(for FIrpic, ET ≥ 2.7 eV); bipolar carrier transporting properties and
6
high thermal and morphological stability. Additionally, simple
structure and easy to prepare was an underlying request for industrial
manufacture. So it will be fascinating to develop a highly efficient
The threeꢀdimensional geometries and the frontier molecular
bipoalr host material with simple structure.
Scheme 1. The route of synthesis mNBICz
orbital energy levels of mNBICz were calculated using density
To achieve this goal, various hole and electron transporting
functional theory (Fig. 1). The spatial distribution of electron density
moieties have been developed to obtain bipolar host materials with
of HOMO is mainly localized on the carbazole unit and the LUMO
structural topology tuning of the connecting manner.7ꢀ12Among them,
orbital is dispersed in the benzimidazole moiety. The completely
carbazoleꢀbased materials possessing high triplet energy (> 2.9 eV)
separation of HOMO and LUMO can be rationalized by the broken
and excellent hole transporting (HT) properties were widely used as
conjugation at N1 position of benzimidazole. The separated FMOs
blue hosts.13ꢀ18 Benzimidazole groups were also used as functional
(Frontier Molecular Orbitals) indicates their potential ambipolar
units that facilitating electron injection and transport.15, 19 From this
characteristics. The calculated HOMO/LUMO energy levels of
point of view, many bipolar host materials incorporating these two
mNBICz were estimated to be ꢀ5.48 and ꢀ2.55 eV, respectively.
groups have been reported.20ꢀ22
Our group have reported a simple benzimidazoleꢀcarbazoleꢀbased
bipolar host mBICP, in which the carbazole unit is linked to the
benzimidazole segment via the meta position of the C2ꢀphenyl, for
high efficiency blue and white electrophosphorescent applications.23
Recently, Hung et al. reported an Nꢀconnected benzimidazole
compounds possessed a remarkable advantage compared with the Cꢀ
connected analogues.24 To our knowledge, the sp3 hybridization of
Fig.1 HOMO and LUMO distributions of mNBICz
N1 and meta position linking mode could provide much more
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VJieowuArrtnicalelONnlainme e
DOI: 10.1039/C3TC32388A
varying from 3.36×105 to 5.57×105 V cmꢀ1. Here, we also
compare the hole/electron mobility under same applied
electrical field (Fig. S4 in ESI†). Under a certain electron field
of 5.0×105 V cmꢀ1, the electron mobility was measured to be
6.12×10ꢀ5 cm2 Vꢀ1 sꢀ1, which was close to the value of the hole,
4.32×10ꢀ5 cm2 Vꢀ1 sꢀ1. All of these results showed that mobilities
of hole and electron are close to each other, indicated the
bipolar transporting properties of mNBICz.
Encouraged by its bipolar charge transport properties and
proper energy levels for charge injection, mNBICz was
evaluated as the host materials for blue PHOLEDs.
Phosphorescent OLEDs using FIrpic as dopants were fabricated
as follows: ITO/MoO3 (10 nm)/ TAPC (60 nm) / TCTA (5 nm)/
mNBICz: FIrpic (7 %, 20 nm)/ TmPyPB (35 nm)/ LiF(1 nm)/
Al(device A) (Fig. S5). Device B with mCP as host material
was applied as a comparision. TAPC and TmPyPB were
applied as the holeꢀ and electronꢀtransporting layers,
respectively; TCTA was chosen as the exciton blocker to
prevent the diffusion of excitons to the holeꢀtransporting layer.
Meanwhile, TCTA also can reduce the hole injection barrier
from TAPC to host; MoO3 and LiF served as holeꢀ and
electronꢀ injecting layer, respectively. Fig. 4 shows device
performance of blue PHOLEDs A and B.
The thermal properties have been investigated by thermal
gravimetric analysis (TGA) and differential scanning calorimetry
(DSC). mNBICz exhibits good thermal stability with decomposition
temperature (Td, 5% weight loss) at 349.3
and glass transition
temperature (Tg) at 86 (see Fig. S1 and S2). The electrochemical
properties were determined by the cyclic voltammetry (CV). The
HOMO /LUMO energy level of mNBICz were measured to be ꢀ
5.82/ꢀ2.25 eV, respectively. The HOMO level was calculated from
its oxidative onset potential by comparing with ferrocene (Fc)
(HOMO = − (Eox + 4.68) eV). The LUMO energy level was
calculated from the obtained HOMO level and optical bandgap (Fig.
S3 in the ESI†).
a
b
c
0.9
0.6
0.3
0.0
300
400
500
600
700
Wavelength (nm)
Fig.2 (a) UVꢀVis absorption spectrum of mNBICz in toluene; (b)
photoluminescence （PL） spectrum of mNBICz in toluene at 298 K; (c) PL
spectrum of mNBICz in 2ꢀmethyl tetrahydrofuran at 77 K.
100
10
1
100
10
1
a
Device A
Device B
The UV absorption, photoluminescence (PL), and lowꢀ
temperature PL emission spectra of mNBICz are shown in Fig. 2.
The maximum absorption peak at 292 nm can be assigned to the
carbazoleꢀcentered n–п* transition. The optical bandgap calculated
from the absorption edge of the absorption spectrum of mNBICz is
3.57 eV. PL emission of mNBICz in toluene at 298 K was observed
at 343 and 360 nm, which corresponds to the singlet emission. And
the triplet energy（ET）of mNBICz is 2.78 eV, obtained from the
first phosphorescent emission peak (λ = 446 nm) of the lowꢀ
temperature PL spectrum measured at 77 K. Detailed data are
summarized in Table S1.
b
Device A
Device B
1.0
0.8
0.6
0.4
0.2
0.0
10
400
500
600
700
800
Wavelength (nm)
1
101
102
103
104
10
100
1000
2)
Luminance cd/m
10000
2
(
Luminance
(
cd/m )
Fig 4. (a) Power and current efficiency–luminance and (b) quantum
efficiency ꢀluminance curves of device A and B. Inset: the EL spectra of the
device A (black line) and device B (red line)
In order to confirm the bipolar transporting properties of
mNBICz, we used timeꢀofꢀflight (TOF) techniques to measure
their carrier mobility based on the device configuration of
[ITO/ Host (um)/Al]. Fig.3 shows typical TOF transients of
hole/electron for mNBICz under different applied electrical
field. The hole mobility of mNBICz lay in the range of
3.34×10ꢀ5 and 5.72×10ꢀ5 cm2 Vꢀ1sꢀ1 for fields varying from
4.2×105 to 6.0×105 V cmꢀ1, which were colse to the electron
mobility between 4.2×10ꢀ5 and 7.0×10ꢀ5 cm2 Vꢀ1 sꢀ1 for fields
Device A shows a low turnꢀon voltage of 3.3 V, a high Lmax of
47995 cd mꢀ2 at 11.9 V and the maximum external quantum
efficiency, current efficiency, and power efficiency are 26.2 %,
54.5 cd Aꢀ1 and 52.2 lm Wꢀ1, respectively. Device B exhibits a
low turnꢀon voltage of 3.65 V, a high Lmax of 9093 cd mꢀ2 at
13.25 V and the maximum external quantum efficiency, current
efficiency, and power efficiency are 13.3 %, 27.7 cd Aꢀ1 and 24.7
lm Wꢀ1, respectively. The efficiency of device A is ca. 50%
higher than device B. The high efficiency of device A can be
attributed to the bipolar feature of mNBICz and matched
HOMO/LUMO to the adjacent layers. Meanwhile, device A
remained a competitive performance of 33.2 cd Aꢀ1 at a practical
brightness of 1000 cd mꢀ2, and exhibited low efficiency rollꢀoff.
In general, the outstanding performance of blue PHOLEDs
certified our purpose developing a highly efficient bipoalr host
material with a simple structure. All of the key performance
parameters of devices were summarized in Table 1.
1E-4
hole
electron
1E-5
550
600
650
700
750
800
E1/2 (V/cm)1/2
Fig.3 Electron mobilities plotted with respect to E1/2 of mNBICz.
2
| J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 2012

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Journal of Materials Chemistry C
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DOI: 10.1039/C3TC32388A
Table. 1 EL performance of device AꢀF
Device
Von
Lmax(cd mꢀ2) (V)a
ηC(cd Aꢀ1)b
ηP ( lm Wꢀ1)c
ηextmax(%)d
CIE [x, y]e
3.31
3.65
3.32
3.32
3.32
3.32
47995(11.9)
9093 (13.25)
48460 (13.9)
48380 (12.5)
48202 (12.5)
48504 (12.5)
54.5/33.2
27.7/24.6
46.9/34.0
54.7/45.7
62.5/49.9
55.3/47.7
52.2/19.6
24.7/10.1
44.6/17.4
48.7/23.4
60.4/27.4
52.7/24.7
26.2
13.3
17.2
17.5
22.9
20.0
(0.14, 0.32)
(0.15, 0.14)
(0.35, 0.44)
(0.39, 0.46)
(0.39, 0.45)
(0.44, 0.47)
A
B
C
D
E
F
Von: turnꢀon voltage at 1 cd mꢀ2. [a] Lmax: maximum luminance; V: voltage at the maximum luminance. [b] ηC: current efficiency at 1 cd mꢀ2
and 1000 cd mꢀ2. [c]ηP: power efficiency at 1 cd mꢀ2 and 1000 cd mꢀ2. [d] ηext.max: the maximum EQE. [e]Measured at 8.0 V.
at the brightness of 1.0 cd mꢀ2. Moreover, a maximum luminance
Inspired by the impressive results obtaining from the blue
of 48202 cd mꢀ2 at 12.5 V with CIE values of (0.39, 0.46) was
PhOLEDs, a single twoꢀcolor white phosphorescent organic
reached. These efficiency values are 46.9 cd Aꢀ1, 44.6 lm Wꢀ1, and
lightꢀemitting diodes (WOLEDs) adopting Firpic and the orange
17.2 % for device C, 54.7 cd Aꢀ1, 48.7 lm Wꢀ1and 17.5% for device
D, 55.3 cd Aꢀ1, 52.7 lm Wꢀ1 and 20 % for device F, respectively. The
relative high efficiencies of these devices can be attributed to the
bipolar characters of mNBICz which may result in balanced
charge fluxes and a broad distribution of recombination region
within the emitting layer. From the inset of Fig.5b, it can be
observed that the blue emission become stronger with the increased
biases. This is because that the FIrpic emission is mainly via host–
guest energy transfer, whereas the orange emission is due to the
direct chargeꢀcarrier trapping and then recombining on Ir(bt)2(acac)
molecules in the WOLED27. A decreased electric field is more
favorable for the holeꢀtrapping effect, which will lead to more
orange emission with respect to blue emission. Although the
emisssion of the blue changed with the biases, but the Internationale
de L’Eclairage (CIE) of the devices is in warm white light area. Take
device E for instance, the CIE coordinates are (0.40, 0.46) at 5 V and
shift to (0.38, 0.45) at 10V, and the CRI ranging from 60.3 to 62.4.
dopant Ir(bt)2(acac)26 with the configuration of ITO/MoO3
(10nm)/TAPC(60nm)/TCTA(5nm)/Host:FIrpic:Ir(bt)2(acac)(7
%:n%,20 nm)/TmPyPB (35 nm)/LiF(1 nm)/Al were fabricated
(C, D, E, F) corresponding to n=0.5, 1, 1.5, 2, respectively. Fig. 5
shows device performance of CꢀF.
a
100
100
10
1
C
E
D
F
10
1
101
102
103
104
2)
Luminance (cd/m
Additionally, device DꢀF exhibited
a significantly practical
application efficiency (>45 cd Aꢀ1) even at 1000 cd mꢀ2, which have
the potential to meet the requirements of daily life. All of these
results suggested that mNBICz was suitable for dopanting FIrpic
and Ir(bt)2(acac) for white OLEDs.
b
C
E
D
F
5
6
7
8
9
10
1.0
0.8
0.6
0.4
0.2
0.0
Conclusions
In conclusion, a simple new bipolar host material mNBICz for
phosphorescent OLEDs has been designed and synthesized by
attaching the carbazole unit to the benzimidazole segment via the
meta position of the N1ꢀphenyl. By using mNBICz as a host
material, we successfully developed highꢀperformance blue OLEDs
and single emission layer white OLEDs. To our knowledge, these
performances are among the best reported values.28ꢀ33 Our results
unambiguously demonstrate that a synthetic strategy combining
carbazole and benzimidazole subunits produces an effective
molecular framework for the preparation of high ET bipolar host
materials, with high quantum efficiencies above 25% achieved in
blue PhOLEDs.
400
500
600
700
800
100
Wavelength (nm)
101
102
103
104
2)
Luminance cd/m
(
Fig 5. (a) The current efficiency and power efficiency versus luminance curves. (b)
The external quantum efficiency (EQE)–luminance curves of white OLEDs CꢀF.
Inset: EL spectrum of Device E under different voltages.
As shown in Fig.5 and Table. 1, with the increase of the doping
concentration, the colour will gradually shift to red, but there will be
an increase of luminous efficiency. When the concentration of
orange dopant Ir(bt)2(acac) reached to 1.5%, the best performance
white device E were obtained. Device E achieves a ηc, max of 62.5 cd
Aꢀ1, ηp, max of 60.4 lm Wꢀ1, and ηext, max of 22.9% and exhibits 3.32 V
Acknowledgements
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DOI: 10.1039/C3TC32388A
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