Efficient multilayer and single layer phosphorescent organic light-emitting devices using a host with balanced bipolar transporting properties and appropriate energy level

Article abstract of DOI:10.1016/j.orgel.2017.07.034

Efficient multilayer (ML) and single layer (SL) phosphorescent organic light-emitting devices (PHOLEDs) with a bipolar host 9,9′-[4′-(2-ethyl-1H-benzimidazol-1-yl) [1,1′-biphenyl]-3,5-diyl] bis-H-Carbazole (EBBPC) are fabricated. The photophysical, electrochemical and carrier transporting properties of EBBPC host are investigated. The maximum external quantum efficiency (EQE) can reach 14.9%, 15.6%, 16.0% and 15.4% for red (R), green (G), blue (B), and white ML-PHOLEDs, respectively. And the maximum EQE can also reach 10.1%, 14.6% and 9.8% for R, G, and B SL-PHOLEDs, respectively. Besides, the SL-PHOLEDs show low efficiency roll-off due to the broader exciton formation zone in SL OLEDs. The excellent performance of ML- and SL-PHOLEDs could be attributed to the balanced bipolar transporting properties and appropriate energy level of the host.

Full text of DOI:10.1016/j.orgel.2017.07.034

Accepted Manuscript
Efficient multilayer and single layer phosphorescent organic light-emitting devices
using a host with balanced bipolar transporting properties and appropriate energy
level
Chunxiu Zang, Xiaomei Peng, Hui Wang, Ziwei Yu, Letian Zhang, Wenfa Xie, Hongyu
Zhao
PII:
S1566-1199(17)30366-X
10.1016/j.orgel.2017.07.034
ORGELE 4226
DOI:
Reference:
To appear in: Organic Electronics
Received Date: 12 June 2017
Revised Date: 23 July 2017
Accepted Date: 23 July 2017
Please cite this article as: C. Zang, X. Peng, H. Wang, Z. Yu, L. Zhang, W. Xie, H. Zhao, Efficient
multilayer and single layer phosphorescent organic light-emitting devices using a host with balanced
bipolar transporting properties and appropriate energy level, Organic Electronics (2017), doi: 10.1016/
j.orgel.2017.07.034.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Efficient multilayer and single layer phosphorescent organic
light-emitting devices using a host with balanced bipolar
transporting properties and appropriate energy level
a
a
a
a
a
a,
Chunxiu Zang , Xiaomei Peng , Hui Wang , Ziwei Yu , Letian Zhang , Wenfa Xie *,
b,
Hongyu Zhao *
a
State key Laboratory on Integrated Optoelectronics, College of Electronics
Science and Engineering, Jilin University, Changchun, 130012, People’s
Republic of China.
b
Beijing Tuocai Optoelectronics Technology CO. LTD, Beijing, 100086, People’s
Republic of China.
*
Corresponding author: xiewf@jlu.edu.cn, zhy_china@126.com
Abstract:
Efficient multilayer (ML) and single layer (SL) phosphorescent organic light-emitting
devices (PHOLEDs) with a bipolar host 9,9'-[4'-(2-ethyl-1H-benzimidazol-1-yl)
[
1,1'-biphenyl]-3,5-diyl] bis-H-Carbazole (EBBPC) are fabricated. The photophysical,
electrochemical and carrier transporting properties of EBBPC host are investigated.
The maximum external quantum efficiency (EQE) can reach 14.9%, 15.6%, 16.0%
and 15.4% for red (R), green (G), blue (B), and white ML-PHOLEDs. And the
maximum EQE can also reach 10.1%, 14.6% and 9.8% for R, G, and B SL-PHOLEDs.
Besides, the SL-PHOLEDs show low efficiency roll-off due to the broader exciton
formation zone in SL OLEDs. The excellent performance of ML- and SL-PHOLEDs
could be attributed to the balanced bipolar transporting properties and appropriate
energy level of the host.
Keywords: bipolar; organic light-emitting device; single-layer; roll-off
1

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1
. Introduction
Phosphorescent organic light-emitting diodes (PHOLEDs) have received
tremendous scientific and industrial interest in recent years for the high efficiency as
they could achieve 100% internal quantum efficiency in principle by utilizing both the
singlet and triplet excitons [1-8]. In the meantime, the PHOLEDs reported generally
adopt multilayer configuration in which the emission layers are sandwiched within
many functional layers to obtain balanced charge fluxes and improve the device
efficiency. In these complicated structures OLEDs, however, the presence of
heterojunction interfaces in the multilayer structure is detrimental to the device
stability due to the accumulation of charges, exciplex formation and the generation of
a higher electric field at the interfaces [9, 10]. Therefore, special attention must be
paid to match the appropriate energy levels of adjacent layers and to avoid the above
phenomena. However, the employing of these complicated device configurations will
doubtlessly increase the complexity of the manufacture and the fabrication cost. This
presents an intense motivation to develop single-layer OLEDs (SLOLEDs), as much
fewer types of material are required and simpler structure is adopted.
Moreover, in PHOLEDs, as triplet excitons have a long lifetime by nature,
triplet-triplet annihilation (TTA) and triplet-polaron quenching (TPQ) caused by high
concentration of excitons are the dominating loss mechanisms result in efficiency
roll-off [11, 12]. Thus, the phosphorescent materials are often doped into a suitable
host material that can disperse triplet emitters to reduce aggregation quenching and
TTA of guest triplet emitters and plays an indispensable role in energy transfer and
carrier transport at the same time [6, 13]. As a high-performance host material, it must
fulfill the some certain requirements including a high triplet energy level suitable for
dopants to facilitate energy transfer, similar mobility for both electrons and holes (i.e.
bipolar materials) to ensure a broad exciton recombination zone, and the energy level
[
(
highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital
LUMO)] of the host material should be matched with neighboring layers in order to
achieve both an effective carrier injection and a good carrier confinement [3, 14-18].
Thus, many hosts do not suit for different guests, e. g., 4,4'-N,N'-dicarbazole- biphenyl
2

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(CBP) [19] and poly-(N-vinylcarbazole) (PVK) [20] are unsuitable for blue
PHOLEDs, as the blue phosphorescent dye in common use is bis
2
[
(4,6-difluorophenyl)-pyridinato-N,C '] (picolinato) Ir(III) (FIrpic) with the triplet
level of 2.62 eV, the triplet levels of CBP (2.56 eV) and PVK (2.5 eV) are lower than
that of FIrpic; hence the energy-transfer process from CBP or PVK to FIrpic is
endothermic, which limits the device efficiency, operating temperature and long-term
operational stability [18, 21].
More importantly, many host materials could not be used as host in SLOLEDs
because of the unsuitable HOMO/LUMO level for electrode or unipolar transport
property. In SLOLEDs, Poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate)
(PEDOT:PSS) modified Indium tin oxide (ITO) is usually used as anode and excellent
hole injection is easier to achieve for most hosts, therefore, the hosts in the SLOLEDs
need to have better electron injection ability. In addition, in order to avoid TTA and
TPQ caused by the carrier accumulation at the electrode (especially cathode), the host
materials need to have balanced bipolar transport properties, excellent electron
transporting property in particular. As a result, there are relatively few reports on
well-performing SLOLEDs [22-26].
In this paper, efficient ML- and SL-PHOLEDs with bipolar
9
,9'-[4'-(2-ethyl-1H-benzimidazol-1-yl) [1,1'-biphenyl]-3,5-diyl] bis-H-Carbazole
(EBBPC) host is reported. For comparison, ML-PHOLEDs with CBP, 2,7-bis
(diphenylphosphoryl)-9-[4-(N,N-diphenylamino) phenyl]-9-phenylfluorene (POAPF)
and 2,6-bis(3-(carbazol 9,9'-[4'-(2-ethyl-1H-benzimidazol-1-yl) -9-yl) phenyl)pyridine
26DCzPPy) host are also fabricated. Iridium (III) bis-(2-methyldibenzo- [f, h]
quinoxaline) (acetylacetonate) [Ir(MDQ) (acac)], tris(2-phenylpyridine) iridium
(
2
[
Ir(ppy) ] and Firpic are used as the red, green and blue dopants. EBBPC host shows
3
balanced bipolar transporting properties and appropriate energy level which makes it
can be used as efficient host for most common phosphorescent dopants in ML- and
SL-PHOLEDs.
2
. Experimental section
2
.1. Materials and instruments
3

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Ultraviolet-visible (UV-vis) absorption and photoluminescence (PL) spectra at
room temperature are measured using Cary 500 UV-vis-NIR spectrophotometer and
FL-4600 FL spectrophotometer, respectively. The phosphorescence spectrum is
studied from the delayed emission of EBBPC at 77 K. Cyclic voltammetry (CV)
studies are recorded on a BAS 100 W instrument containing 0.1 M
tetrabutylammonium hexa-fluorophosphate in CH Cl as a supporting electrolyte with
2
2
a scan rate of 100 mV/s. All materials and solvents are commercially available and
used as received without further purification.
2
.2. Synthesis
Scheme1. Synthetic routes of EBBPC.
The synthetic route and structure of EBBPC is shown in Scheme 1. EBBPC can be
synthesized according to the method described in Ref. 27 and 28. 11.2 g (70 mmol)
carbazole (B), 10 g (31 mmol) 1,3,5-tribromobenzene (A), 4.9 g (35 mmol) K CO ,
2
3
0
.06 g (3 mmol) copper (I) iodide, and 0.06 g (3 mmol) 1,10-phenanthroline are added
to 100 mL of dried dimethyl formamide (DMF). The mixture is heated at 160 °C for
2 h under nitrogen atmosphere. After this mixture cooled to room temperature, the
1
reaction is quenched with 200 mL of water and extracted with 100 mL of
dichloromethane three times. Combining the organic layer and removing the solvent,
the residue is then purified by silica gel column chromatography with petroleum
1
ether/ethyl acetate (10:1) as the eluent to give 7.78 g of white power (C). H NMR
(
400 MHz, CDCl , d): 8.13 (d, 2 H), 7.75 (s, 1 H), 7.70 (s, 2 H), 7.54-7.25 (m, 6 H).
3
MS (ESI) [m/z]: calcd for C H Br N, 401.09; found, 401.00. Anal. Calcd for
1
8
11
2
C H Br N: C, 53.90; H, 2.76; N 3.49. Found: C, 53.82; H, 2.84; N 3.53%.
18
11
2
Under an argon atmosphere, 3.0 g (10 mmol) of intermediate C, 4.5 g (10 mmol) of
4

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intermediate D, 0.23 g (0.2 mmol) of Pd(PPh ) and 10 ml of an aqueous 2M sodium
3
4
carbonate solution are added to 50 ml of THF, and the mixture is heated and stirred
under reflux. To the resultant, menthol is added to precipitate solids. The solids is
collected and dissolved in dichloromethane to obtain an oily matter. The oily matter is
purified by means of silica gel column chromatography to obtain 4.3 g of EBBPC
(
yield 80%). As the eluent of silica gel column chromatography, a solvent of hexane:
dichloromethane=10:1, and a solvent of henane: dichloromethane=3:1 are used in this
1
sequence. MS (MALDI-TOF) [m/z]: calcd for C H N 628.76; found, 628.90. H
4
5
32
4
NMR (600 MHz, CDCl δ [ppm]): 8.18 (d, J = 7.8 Hz, 4H), 8.00 (d, J = 1.8 Hz, 2H),
3
7
.91 (d, J = 8.4 Hz, 2H), 7.89 (t, J = 1.8 Hz, 1H), 7.82 (d, J = 7.8 Hz, 1H), 7.63 (d, J =
.8 Hz, 4H), 7.51 (d, J = 8.4 Hz, 2H), 7.48 (t, J = 7.2 Hz, 4H), 7.34 (t, J = 7.2 Hz, 4H),
.30 (t, J = 7.2 Hz, 1H) 7.22 (t, J = 7.8 Hz, 1H), 7.16–7.18 (m, 1H),2.86–2.90 (m, 2H),
7
7
1
3
1
.40 (t, J = 7.2 Hz, 3H). C NMR (600 MHz, CDCl , δ [ppm]):156.08, 143.25,
3
1
42.38, 140.54, 140.12, 139.77, 136.35, 136.14, 129.00, 128.68, 128.19, 127.95,
26.26, 125.26, 124.49, 124.33, 123.73, 122.79, 122.55, 120.57, 120.54, 119.19,
09.85, 109.62, 77.21, 77.00, 76.78, 21.28, 12.06. Anal. Calcd for C H N : C, 85.96;
1
1
45
32
4
H, 5.13; N 8.91. Found: C, 85.73; H, 5.20; N 8.65%.
2
.3. Device fabrication and characterization
All the devices in this paper are manufactured on glass substrates coated with
conducting ITO. The substrates are cleaned with Decon 90 and deionized water, dried
in the oven and then handled in plasma for about 5 min. In advance of thermal vapor
deposition process, a PEDOT:PSS layer with the thickness to be approximately 40 nm
is spin-coated on the top of the ITO anode for SL-PHOLEDs. Finally, organic layers
and cathode materials are deposited on the substrates sequentially under vacuum
–
4
(
~5.0×10 Pa). A shadow mask is used to define the cathode and to make four
2
identical 10 mm devices on each substrate. Luminance-current-voltage
characteristics and spectra of unpackaged devices are measured simultaneously using
Goniophotometric Measurement System based on spectrometer (GP-500, Otsuka
Electronics Co. Osaka, Japan) in air at room temperature. The impedance studies are
carried out using an Impedance Analyzer (TH2829C, Tonghui Co. Changzhou, China).
5

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6
This setup allows frequency sweeps covering a range from 20 to 10 Hz. The ac
oscillator level is set to 100 mVrms for all measurements and the superimposed dc
bias is varied between 0 and +10 V.
3
3
. Results and Discussion
.1. Photophysical properties
Figure 1 (a) shows the UV-vis absorption and emission spectra of EBBPC in
CH Cl . Fig.1 (b) and (c) depicts the cyclic voltammogram, thermogravimetric
2
2
analysis (TGA) and differential scanning calorimetry (DSC) curves of EBBPC. The
key parameters of EBBPC are summarized in Table 1. The optical energy band gap
(Eg) of EBBPC is calculated to be 3.35 eV from the onset of the absorption spectrum
according to the UV-vis absorption curve. The fluorescence emission peak of EBBPC
is 400 nm and its singlet state (S ) is estimated to be 3.1 eV accordingly. The
1
phosphorescence spectrum is obtained from the delayed emission of EBBPC at 77 K
and the triplet energy (T ) is estimated to be 2.74 eV (453 nm). The HOMO level of
1
EBBPC is estimated to be -5.71 eV through the cyclic voltammogram measurement
and LUMO level is deduced to be -2.36 eV from HOMO and E . The
g
oxidation signal of EBBPC exhibit quasi-reversible oxidation, we believed that it is
due to that EBBPC is a carbazole-containing complex [29]. At the same time, EBBPC
exhibits high glass transition temperature (T ) of 142.6 °C. From the structure of
g
EBBPC we can see that EBBPC contains two carbazole and one
benzimidazole groups. Such a molecular configuration renders the molecule in a
non-planar structure and which is strongly beneficial to the thermal stability. The
carbazole group has high triplet energy and hole transporting property, while the
benzimidazole is an electron transporting group. The incorporation of ethyl at the
2
-position of the benzimidazole ring limits the extent of conjugation which further
keeps the triplet energy gap at a very high level. Thus, EBBPC has high triplet energy,
excellent charge transport properties and high thermal stability.
6

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Fig.1. (a) Normalized absorption spectrum (Abs), photoluminescence spectrum at 298 K (PLRT),
phosphorescence spectrum at 77 K (PLLT) of EBBPC, Abs of FIrpic and PLRT of POAPF in CH Cl . (b)
2
2
Cyclic voltammogram of EBBPC. (C) Thermogravimetric analysis (TGA) [Inset is differential scanning
calorimetry (DSC) curve] of EBBPC.
Table 1 Photophysical characteristics of EBBPC, POAPF, CBP and 26DCzPPy
a
a,b
c
d
e
f
g
λabs,max
nm)
40
λPL,max
S1
(eV) (eV) (eV)
3.1 2.74 3.35
T1
Eg
HOMO LUMO
Tg
Td
Complex
(
(nm)
(eV)
(eV)
(°C)
(°C)
2
400, 483
-5.71
-2.36
142.6 431.2
EBBPC
h
h
i
h
h
h
h
h
POAPF
CBP
294
510, N.A. N.A. 2.75
2.86
3.10
3.40
-5.26
6.0
-2.40
2.90
129
466
i
i
i
i
341
355, 485
3.49 2.56
62
N.A.
N.A.
j
j
j
2
6DCzPPy
N.A.
N.A. 457 N.A. 2.71
6.05
2.65
N.A.
a
-5
b
c
Measured in CH Cl (10 M) at 298 K and 77 K. Estimated from the emission spectrum at 298
2
2
d
e
K. Estimated from the emission spectrum at 77 K. Estimated from the UV-vis absorption
f
g
h
spectrum. Calculated from onset oxidation potential. Deduced from HOMO and E . Obtained
g
i
j
from [30]. Obtained from[31]. Obtained from [32].
3
.2. Carrier transportation properties
Admittance spectroscopy is a powerful method to infer charge transport properties
of the material by analyzing the frequency-dependent capacitance of a device [33-35].
The average transit time τ of the carriers can be extracted from a plot of the negative
dc
differential susceptance against the frequency [33] or the frequency of imaginary part
of impedance [ImZ(f)] [35]. Here, the frequency dependence [ImZ(f)] is used to
7

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determine the mobility of EBBPC and the carrier mobility could be obtained from the
2
equation of ꢀ=f d /(V·κ), where f , κ, ꢀ, d and V represents the peak frequency,
p
p
constant numerical factor, carrier mobility, the thickness of organic material and the
applied voltage, respectively. Device A with structure of ITO/TAPC (3 nm)/EBBPC
(
50 nm)/TAPC (3 nm)/MoO (3 nm)/Ag (100 nm) and device B with structure of
3
ITO/LiF (1 nm)/TmPyPB (3 nm)/EBBPC (70 nm)/Bphen (3 nm)/LiF (1 nm)/Ag (100
nm) is fabricated to measure the frequency dependence of imaginary part of
impedance.
Where
TAPC,
TmPyPB,
and
Bphen
are
1
,1-bis-(4-bis(4-tolyl)-aminophenyl) cyclohexene, 1,3,5-tri [(3-pyridyl)-phen-3-yl]
benzene, and 4,7-diphenyl-1,10-phenanthroline, respectively. TAPC (TmPyPB and
Bphen) and MoO (LiF) are inserted between electrodes and EBBPC to facilitate the
3
injection of hole (electron) and block the transport of electron (hole) for device A (B),
and the time that carrier transport through the insert layer could be neglected since the
insert layers are thin enough. Hole- and electron-only devices also are fabricated to
give a qualitative explanation. The configuration of the hole-only device is
ITO/MoO (3 nm)/TAPC (40 nm)/EBBPC(20 nm)/TAPC(40 nm)/MoO (3
3
3
nm)/Ag(120 nm)，while the electron-only device has the structure of ITO/LiF(0.5
nm)/Bphen(65 nm)/EBBPC(20 nm)/Bphen(40 nm)/Ag(120 nm). As the work function
of MoO being high enough to block electron injection [36]，and Bphen is used as
3
electron-transporting/hole-blocking layer [37], so it can be assumed that only single
carriers can be injected and transported through the devices.
Fig.2 (a) and (b) are the frequency dependence of imaginary part of impedance [ImZ(f)] for different
voltage of device A and B, respectively. (c)Current density-voltage characteristics of the hole-only and
electron-only devices based on EBBPC, hole and electron mobility for EBBPC calculated from ImZ(f).
8

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Fig.2 (a) and (b) show the ImZ(f) of device A and B at different DC voltage with
frequency. Thus, the hole and electron mobility of EBBPC could be calculated and is
-5
2
-1 -1
shown in Fig. 2 (c). The electron mobility of the EBBPC is ~2×10 cm v ·s and
this value is about twice of its hole mobility. Fig.2 (c) depicts the current
density-voltage curves of the single-carrier devices. We also could see that the current
density of the electron-only device is higher than that of the hole-only device. Above
results indicated that EBBPC is a bipolar transport material with better electron
transporting property.
3
.3. Monochromatic ML-PHOLEDs with EBBPC host
Fig.3 (a) The device structures of PHOLEDs with different hosts and guests. (b) Chemical structures of part
of organic materials.
To confirm the effectiveness of EBBPC as host for PHOLEDs, red (R), green (G)
and blue (B) ML-PHOLEDs whose structures are shown in Fig.3 have been fabricated
successfully. They share the similar structure: ITO/MoO (3 nm)/TAPC (35
3
nm)/TCTA (5 nm)/emitting layer (EML, 20 nm)/Bphen (50 nm)/LiF (0.5 nm)/Mg:Ag
(15:1, mass ratio, 120 nm), the EML is the host materials doped with different
phosphorescent dyes. Where TCTA is 4,4',4"-tris (carbazol-9-yl) triphenylamine.
TAPC, TCTA are hole transporting layers (HTL), Bphen is electron transporting layer
(ETL). For example, the blue PHOLEDs are demonstrated in detail. And the
corresponding devices are marked as device B (EBBPC host), device B (POAPF
1
2
host), device B (CBP host) and device B (26DCzPPy host). Previous literatures
3
4
showed that POAPF can be used as bipolar host in blue ML-PHOLED and
SL-PHOLED [30, 38]. Thus, the devices with POAPF host are used as references. Fig.
4
shows the normalized electroluminescence (EL) spectra of the devices B and B at
1 2
9

ACCEPTED MANUSCRIPT
6
V. As can be seen, both the devices exhibit the typical FIrpic EL characteristics with
a maximum intensity at ca. 471 nm. No extra emission is observed, indicating that
efficient energy transfer from host to guest has been realized.
Fig.4. Normalized EL spectra of device B and B at 6 V [Inset: External quantum efficiency (EQE)-current
1
2
density curves].
Fig. 5 (a) depicts the current density-voltage-luminance curves of the blue devices.
The current density of device B is higher than that of device B , and the turn-on
2
1
2
voltage (driving voltage at a brightness of 1 cd/m ) of device B (ca. 2.5 V) is also
2
lower than that of device B (ca. 2.9 V). The low driving voltage and high current
1
density of device B could be attributed to the low HOMO level of POAPF. The
2
HOMO/LUMO of EBBPC and POAPF is 5.71/2.26 eV and 2.4/5.26 eV, respectively.
While the HOMO of TcTa (5.7 eV) is higher than that of POAPF and the electron
injection barrier between Bphen and POPAF (0.4 eV) is smaller than that between
Bphen and EBBPC (0.54 eV) as the LUMO of Bphen is 2.8 eV, which results in an
easier injection of carrier in devices used POAPF host and a lower turn-on voltage.
Although the current density of device B is lower than that of device B2, the
1
maximum brightness and current efficiency (CE) of device B is higher than those of
1
2
device B1. The maximum brightness of device B (22050 cd/m ) is about twice of
1
2
device B (11320 cd/m ). As could be seen in Fig.1 (a), there is a larger spectra
2
overlap between absorption band of FIrpic and fluorescence band of EBBPC than
POAPF, which means there should be a better energy transfer between FIrpic and
EBBPC compared to POAPF and we can obtain a higher efficiency blue OLED using
FIrpic as dopant and EBBPC host than using POAPF host. Fig.5 (b) shows the
1
0

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efficiency-luminance characteristics of devices B and B . At brightness of 100, 1000
1
2
2
and 10000 cd/m , the CE of device B are 28.7, 22.5 and 13.8 cd/A, while those of
1
device B are 21.1, 19.7 and 5.8 cd/A, and power efficiencies (PE) of B are 26.2,
2
1
1
5.7 and 6.3 lm/W, while those of B are 22.1, 15.5 and 2.5 lm/W, respectively. The
2
similar results are also observed for the devices using Ir(ppy) or Ir(MDQ) (acac) as
3
2
dopant. The characteristics of the devices are summarized in Table 2. As can be seen,
the devices based on EBBPC host show higher efficiencies than those using POAPF
as host at the same brightness.
Fig.5 (a) Current density-voltage-luminance curves of blue devices B and B . (b) Current efficiency-power
1
2
efficiency-luminance characteristics of blue devices B and B .
1
2
Table 2. EL characteristics of the devices.
L
max
η
c.max, ηc.1000
,
η
p.max, ηp.1000
,
EQEmax
(%)
CIE(x,y)
@6V
Device
V
on (V)
2
(
cd/m )
22050
11320
16080
30640
η
c.5000 (cd/A)
ηp.5000 (lm/W)
B
1
B
2
B
3
B
4
2.9
2.5
2.8
3.4
31.9, 22.5, 17.3
21.2, 19.7, 12.1
15.5, 10.8, 7.3
22.9, 21.7, 18.9
33.4, 15.7, 9.3
22.2, 15.5, 6.9
16.2, 7.2, 3.6
20.5, 13.8, 9.9
16.0
10.4
7.9
(0.16,0.32)
(0.16,0.33)
(0.16,0.32)
(0.16,0.33)
PHOLEDs
11.2
with
G
1
2.8
81160
54.1, 53.9, 50.2 54.5, 43.1, 32.4
15.6
(0.32,0.61)
EBBPC,
POAPF,
CBP and
DCzPPy
host
G
G
G
2
3
4
2.4
2.9
3.4
3.2
2.9
3.0
3.8
3.6
3.4
3.4
3.4
3.4
49120
73930
63020
39100
16970
32450
25440
60480
54630
63080
48930
37800
50.0, 48.5, 43.6 52.4, 37.5, 27.2
44.5, 44.0, 41.9 46.5, 32.3, 24.8
42.1, 42.0, 39.4 31.3, 25.0, 19.6
14.5
13.0
12.6
14.9
10.5
15.0
7.8
(0.32,0.61)
(0.31,0.62)
(0.30,0.62)
(0.61,0.39)
(0.61,0.38)
(0.61,0.39)
(0.60,0.39)
(0.31,0.61)
(0.31,0.61)
(0.31,0.61)
(0.31,0.61)
(0.34,0.42)
R
1
R
2
R
3
R
4
23.5, 18.7, 15.3
14.7, 11.6, 8.3
23.7, 21.7, 17.4
12.6, 12.1, 9.6
21.1, 11.0, 7.1
14.1, 7.3, 4.1
21.3,14.8, 9.3
7.9, 6.3, 4.0
PHOLEDs
with
G-a
G-b
G-c
G-d
W
50.2, 49.7, 44.6 38.5, 28.5, 21.2
43.6, 43.5, 38.5 33.4, 27.6, 20.2
48.8, 47.0, 40.8 38.3, 31.2, 22.8
43.0, 39.5, 42.3 27.0, 26.6, 24.1
40.2, 35.0, 29.6 36.1, 20.0, 13.7
14.6
12.7
14.2
12.4
15.4
different
ETL
WOLED
1
1

ACCEPTED MANUSCRIPT
To investigate the above phenomena, the energy level of the materials is shown in
Fig.6 (a). The HOMO and LUMO energy level of POAPF are -2.40 and -5.26 eV [30],
respectively. Thus, the E value of POAPF is 2.86 eV. It could be seen that EBBPC
g
has better energy levels matching with the adjacent organic layer (TCTA and BPhen),
especially the HOMO, which results in that there is less energy loss in the charge
transporting of the devices using EBBPC host than those with POAPF host, as a result,
device R , G , and B show a higher luminance than device R , G , and B ,
1
1
1
2
2
2
respectively.
Fig.6 (a) Scheme of energy level of the materials. (b) EQE performance of devices based on different
2
hosts.(for example, REQE5000 means the EQE of red OLED at 5000 cd/m )
The high efficiency of the devices possibly results from the precise confinement of
triplet excitons within the emissive layers. Triplet excitons generally have long
diffusion lengths [39]. Good confinement of triplet excitons within the emitting layer
(EML) can be achieved when all corresponding materials, i.e., the host materials and
the adjacent HTL and ETL have higher E than that of the triplet emitter [40]. The ET
T
of POAPF and EBBPC are 2.75 [30] and 2.74 eV, respectively, which are higher
compared to FIrpic (2.62 eV), Ir(ppy) (2.42 eV) [41], and Ir(MDQ) acac (2.05 eV)
3
2
[
42]. As we mentioned above, the electron-transporting character of EBBPC is
superior to its hole-transporting character, so the triplet excitons mainly formed at the
interface of EML/HTL. The E of TCTA (2.85 eV) [43] and Bphen (2.6 eV) [44] are
T
high enough to confine the formed triplet excitons at the interface. As is shown in
Fig.6 (a), the LUMO levels of the dopants are lower than the hosts (EBBPC and
POAPF), so the electrons transport mainly on the dopants. The HOMO of POAPF is
1
2

ACCEPTED MANUSCRIPT
deeper than that of dopants while the HOMOs of Ir(ppy) and Ir(MDQ) acac are
3
2
deeper than that of EBBPC and there is only a small energy gap (0.09 eV) between
FIrpic and EBBPC, which results in that the holes transport mainly on POAPF in B2,
G and R while in G and R it mainly transport on dopants, so it’s more difficult for
2
2
1
1
the formation of the excitons in B , G and R than B , G and R because that the
2
2
2
1
1
1
hole and electron transport on different molecules. As a result, the efficiencies of
devices with EBBPC host are higher than those of devices with POAPF host.
The EQE characteristics of the devices based on different hosts and dopants are
summarized in Fig.6 (b). As can be seen, CBP is unsuitable for blue PHOLED, while
POAPF and 26DCzPPy are unsuitable for red PHOLED. Thus, as a host material for
R, G, and B ML-PHOLEDs, EBBPC shows a relatively excellent performance
compared to POAPF, CBP and 26DCzPPy. It should be attributed to that EBBPC
possesses balanced bipolar transport characteristic, well-matched HOMO/LUMO
levels with adjacent layers and high E at same time.
T
3
.4. ML-PHOLEDs with EBBPC as ETL
Fig.7 (a) Current density-voltage-luminance curves of device G-a to G-d. (b) Efficiency-luminance
characteristics of device G-a to G-d.
Inspired by the better electron-transporting properties of EBBPC, it could be an
excellent electron transport material for OLEDs. Thus, the green OLEDs employing
EBBPC (G-a) as ETL and the control devices based on Bphen (G-b), TmPyPB (G-c)
or 1,3-Bis[3,5-di(pyridin-3-yl)phenyl]benzene (BmPyPhB) (G-d) are fabricated to
convince the feasibility of EBBPC as ETL in OLEDs. The current
density-voltage-luminance curves, current efficiency-power efficiency-luminance
characteristics and EQE-current density curves of the devices are depicted in Fig. 7
1
3

ACCEPTED MANUSCRIPT
and Fig. 8, respectively. EBBPC has a higher LUMO level (-2.36 eV) compared to the
other ETLs (-2.8 eV for Bphen, -2.7 eV for TmPyPB, -2.6 eV for BmPyPhB). As a
result, device G-a shows lower current density than devices G-b to G-d under the
same voltage. However, the maximum luminance of device G-a can still reach 60480
2
2
2
2
cd/m , which is 54630 cd/m , 63080 cd/m and 48930 cd/m for device G-b to device
G-d, respectively. Besides, the maximum current efficiency is 50.2, 43.6, 48.8, and
4
3
3.0 cd/A for device G-a to G-d, and the maximum power efficiency is 38.5, 33.4,
8.3, and 27.0 for device G-a to G-d, respectively. It means that device G-a shows
comparable or even better performance to the other three devices. All above results
confirmed that it is feasible to employ EBBPC as ETL in OLEDs.
Fig. 8. EQE-current density curves of device G-a to G-d (The inset is the EL spectra at 6V).
3
.5. White ML-PHOLEDs with EBBPC as host and ETL
The excellent performances of monochrome devices employing EBBPC as host or
ETL provide the possibility of its applications in high efficiency white OLEDs
WOLEDs). WOLED (marked as device W) with EBBPC as host and electron
(
transport layer is fabricated. Efficient WOLED with complementary colors of blue
and yellow is achieved by carefully adjusting the thickness of each emitting layers.
Fig.9 (a) shows the normalized EL spectra of WOLED under different biases. The
spectra almost has no change under the biases from 5 to 10 V, which means that
device W possess excellent color stability. The CIE color coordinates just change
from (0.341, 0.424) at 5 V to (0.335, 0.424) at 10 V, which should be attribute to a
stable recombination zone in the emitting layer. As is known, the probability of
1
4

ACCEPTED MANUSCRIPT
excitons formation is proportional to the density of electrons and holes. Thus, there is
usually potential barrier between EML and HTL or ETL, which results that carriers
accumulate at the interface of EML and HTL or ETL, and the excitons often formed
at these interfaces, this contribute to a narrow recombination zone and it may move its
position with the rise of voltage and lead to the change of spectra for multi-layer
WOLEDs. Since we employing the same material-EBBPC as ETL and host, there
should be no barrier at the interface of EML and ETL and the same situation occurs
between EML and HTL as is depicted in Fig.5, which brings about little carriers
accumulation and that most carriers are injected to EML, this lead to a wide
recombination zone even cover the whole EML and generate a stable spectra, finally.
Fig. 9 (a) Normalized EL spectra of the white OLEDs (Inset: External quantum efficiency-current density
curves). (b) Efficiency-luminance characteristics of WOLED (Inset is current density-voltage-luminance
curves of the device).
Fig. 9 (b) depicts the efficiency-luminance and current density- voltage-luminance
characteristics of device W. It can be seen that device W also possesses a low turn-on
voltage of 3.4 V. The driving voltages are 4.3 and 5.5 V at brightness of 100 and 1000
2
2
cd/m , and the maximum brightness of the device can reach 37800 cd/m (at 11 V).
The maximum efficiencies of the device can reach 40.2 cd/A and 36.1 lm/W. As is
shown in Fig. 9 (b), the device also shows a relatively low efficiency roll-off. At a
2
luminance of 1000 cd/m , the current efficiency is 12.9% off its peak values, and the
power efficiency also can reach 20 lm/W.
3
.6. Single-layer PHOLED with EBBPC host
Considering the balanced hole and electron transport properties and suitable LUMO
energy level, EBBPC is adopted to realize efficient SL-PHOLEDs as host materials,
1
5

ACCEPTED MANUSCRIPT
and blue (marked as device SL-B), green (marked as device SL-G) and red (marked
as device SL-R) SL-PHOLEDs is fabricated successfully, and device SL-G with a
configuration of ITO/ITO (40 nm)/EBBPC:10wt%Ir(ppy) (90 nm)/LiF(0.5 nm)/
3
Mg:Ag(15:1, mass ratio, 120 nm) is demonstrated in detail.
Fig.10 (a) Current density-voltage-luminance curves. (b)Current efficiency-luminance-power efficiency
characteristics of SL OLED (Inset: External quantum efficiency-current density curves).
The current density-voltage-luminance characteristics of device SL-G is shown in
Fig.10 (a), it could be seen that the turn-on voltage of SL-G is ~3.4 V, which indicates
an efficient injection of electrons and holes in the devices. At a low current density of
2
2
5
mA/cm , the SL-G depicts a high brightness of 2580 cd/m , and the peak brightness
2
of the device can reach 28990 cd/m (at 10.5 V). Such results demonstrate that the
charges could transport well in the devices and the excitons could recombine
efficiently. We attribute above results to the balanced bipolar transporting properties
and well-matched HOMO/LUMO levels with electrodes, and also the high E of
T
EBBPC, which paves the way to achieve excellent energy transfer between EBBPC
and guest material.
As shown in Fig.10 (b), the maximum EQE, current and power efficiencies of
device SL-G can reach 14.6%, 52.3 cd/A and 35.4 lm/W, respectively. The current
efficiency and EQE of device SL-G are compared with those of device G . But, the
1
power efficiency of Device SL-G is lower due to relatively high drive voltage.
However, device SL-G shows a low efficiency roll-off. At a luminance of 1000 and
2
5
000 cd/m , the current efficiency is 0.5% and 4.9% off its peak values, respectively,
2
and the power efficiency also can reach 23.3 lm/W at 5000 cd/m . The performances
1
6

ACCEPTED MANUSCRIPT
of the single-layer devices based on EBBPC host are shown in Table 3. Due to the
low current density, the maximum luminances of SL-PHOLEDs are lower than those
of ML-PHOLEDs. Although the maximum EQEs of blue and red SL-PHOLEDs are
much lower than those of corresponding multilayer devices, the maximum EQEs of
SL-B and SL-R also can reach 9.8% and 10.1%, respectively. We think that the lower
EQEs of single-layer devices are due to the fact that the guests destroy the balanced
carrier transporting property of EBBPC. However, the efficiency roll-off of
SL-PHOLED is lower than that of ML-PHOLEDs. Table 4 lists the efficiency roll-off
of ML- and SL-PHOLEDs. As can be seen, the efficiency roll-off of ML-PHOLEDs
is more serious than that of corresponding SL-PHOLED. For example, the EQE
2
roll-off of blue ML-PHOLED is 45.6% @5000 cd/m , while that is only 19.4% for
blue SL-PHOLED. We attributed it to the broader exciton formation zone in SL
OLEDs
.
Table 3. EL characteristics of the single-layer devices.
L
max
η
c.max, ηc.1000
,
ηp.max, ηp.1000,
EQEmax CIE(x,y)
Device Von (V)
2
(
cd/m )
η
c.5000 (cd/A)
η
p.5000 (lm/W)
(%)
@8V
SL-B
SL-G
SL-R
5.3
3.4
7.7
7130
21.6, 20.8, 17.6
10.4, 8.2, 5.4
9.8
(0.15,0.39)
(0.30,0.64)
(0.62,0.38)
28990 52.3, 52.0, 49.7 35.4, 30.8, 23.3
4920 16.6, 13.6, N.A. 5.5, 3.2, N.A.
14.6
10.1
Table 4. Efficiency roll-off of ML- and SL-PHOLEDs.
CERO, PERO and EQERO are CE, PE and EQE roll-off.
CERO(%)
1000, 5000 cd/m
29.5, 45.8
3.7, 18.5
PERO(%)
@1000, 5000 cd/m
EQERO(%)
@1000, 5000 cd/m
29.4, 45.6
Device
ML
EQEmax
2
2
2
@
16.0%
9.8%
53.0, 72.2
21.2, 48.0
20.9, 40.6
13.0, 34.2
48.8, 66.4
41.8, N.A.
Blue
Green
Red
SL
ML
SL
ML
SL
4.1, 19.4
15.6%
14.6%
14.9%
10.1%
0.4, 7.2
0.6, 7.7
0.6, 5.0
0.7, 4.8
20.4, 34.9
18.1, N.A.
24.2, 38.3
18.8, N.A.
4
. Conclusions
In summary, efficient multilayer and single-layer phosphorescent OLEDs have
17

ACCEPTED MANUSCRIPT
been demonstrated based on bipolar EBBPC host. For example, the maximum
external quantum efficiency of the green ML- and SL-PHOLEDs with EBBPC host
can reach 15.6% and 14.6%, respectively. Besides, the device with ETL of EBBPC
shows comparable efficiency with the devices with ETL of Bphen, TmPyPB, or
BmPyPhB. Furthermore, efficient WOLED applying EBBPC as host and ETL is also
realized. The WOLED shows maximum external quantum efficiency of 15.4% and
good color stability. The excellent performance of the devices should be attribute to
that EBBPC possesses balanced bipolar transport characteristic, well-matched
HOMO/LUMO levels with adjacent layers and high E at same time.
T
Acknowledgements
This work is supported by the National Natural Science Foundation of China (Grant
Nos. 61474054, 61475060).
References:
[
1] M.A. Baldo, D. O'brien, Y. You, A. Shoustikov, S. Sibley, M. Thompson, S.
Forrest, Highly efficient phosphorescent emission from organic electroluminescent
devices, Nature. 395 (1998) 151.
[
2] D. O'brien, M. Baldo, M. Thompson, S. Forrest, Improved energy transfer in
electrophosphorescent devices, Appl. Phys. Lett. 74 (1999) 442.
3] M. Baldo, S. Lamansky, P. Burrows, M. Thompson, S. Forrest, Very
[
high-efficiency green organic light-emitting devices based on electrophosphorescence,
Appl. Phys. Lett. 75 (1999) 4.
[
4] M. Baldo, M. Thompson, S. Forrest, High-efficiency fluorescent organic
light-emitting devices using a phosphorescent sensitizer, Nature. 403 (2000) 750.
5] R. Kwong, S. Lamansky, M. Thompson, Organic light-emitting devices based on
phosphorescent hosts and dyes, Adv. Mater. 12 (2000) 1134.
6] C. Adachi, M. A. Baldo, M. E. Thompson, S. R. Forrest, Nearly 100% internal
phosphorescence efficiency in an organic light-emitting device, J. Appl. Phys. 90
[
[
(2001) 5048.
[
7] T. Tsuzuki, N. Shirasawa, T. Suzuki, S. Tokito, Color tunable organic
1
8

ACCEPTED MANUSCRIPT
light-emitting diodes using pentafluorophenyl-substituted iridium complexes, Adv.
Mater. 15 (2003) 1455.
[
8] B. Chen, L Zuo, A. Li, R. Sheng, Y. Duan, Y. Zhao, P. Chen, High-efficiency
orange and white phosphorescent organic light-emitting diodes based on
homojunction structure, Org. Electron. 44 (2017) 183.
[
9] H. Aziz, Z. D. Popovic, N. X. Hu, A. M. Hor, G. Xu, Degradation mechanism of
small molecule-based organic light-emitting devices, Science. 283 (1999) 1900.
10] Z. Wang, Y. Lou, S. Naka, H. Okada, Direct comparison of solution-and
[
vacuum-processed small molecular organic light-emitting devices with a mixed single
layer, ACS Appl. Mate. Interfaces. 3 (2011) 2496.
[
11]M. A. Baldo, S. R. Forrest, Transient analysis of organic electrophosphorescence:
I. Transient analysis of triplet energy transfer, Phys. Rev. B. 62 (2000) 10958.
12] M. A. Baldo, C. Adachi, S. R. Forrest, Transient analysis of organic
[
electrophosphorescence. II. Transient analysis of triplet-triplet annihilation, Phys. Rev.
B. 62 (2000) 10967.
[
13]D. S. Leem, S. O. Jung, S. O. Kim, J. W. Park, J. W. Kim, Y. S. Park, Y. H.
Kim,S. K. Kwon, J. J. Kim, Highly efficient orange organic light-emitting diodes
using a novel iridium complex with imide group-containing ligands, J. Mater. Chem.
1
9 (2009) 19.
[
14]R. J. Holmes, S. R. Forrest, Y. J. Tung, R. C. Kwong, J. J. Brown, S. Garon, M. E.
Thompson, Blue organic electrophosphorescence using exothermic host-guest energy
transfer, Appl. Phys. Lett. 82 (2003) 82.
[
15]S. J. Yeh, M. F. Wu, C. T. Chen, Y. H. Song, Y. Chi, M. H. Ho, S. F. Hsu, C. H.
Chen, New dopant and host materials for blue light-emitting phosphorescent organic
electroluminescent devices, Adv. Mater. 17 (2005) 285.
[
16]X. F. Ren, J. Li, R. J. Holmes, P. I. Djurovich, S. R. Forrest, M. E. Thompson,
Ultrahigh energy gap hosts in deep blue organic electrophosphorescent devices, Chem.
Mater. 16 (2004) 4743.
[
17]C. Adachi, M. A. Baldo, S. R. Forrest, S. Lamansky, M. E. Thompson, R. C.
Kwong, High-efficiency red electrophosphorescence devices, Appl. Phys. Lett. 78
1
9

ACCEPTED MANUSCRIPT
(
2001) 1622.
18] S. H. Cheng, W. Y. Hung, M. H. Cheng, H. F. Chen, G. H. Lee, C. L. Chung, T. C.
[
Yeh, W. C. Tang, S. L. Huang, K. T. Wong, Highly twisted carbazole-oxadiazole
hybrids as universal bipolar host for high efficiency PhOLEDs. Adv. Electron. Mater.
2
(2016) 1500241.
[
19] C. Adachi, R. C. Kwong, P. Djurovich, V. Adamovich, M. A. Baldo, M. E.
Thompson, S. R. Forrest, Endothermic energy transfer: A mechanism for generating
very efficient high-energy phosphorescent emission in organic materials, Appl. Phys.
Lett. 79 (2001) 2082.
[
20] Y. Kawamura, S. Yanagida, S. R. Forrest, Energy transfer in polymer
electrophosphorescent light emitting devices with single and multiple doped
luminescent layers, J. Appl. Phys. 92 (2002) 87.
[
21] G. Lei, L. Wang, L. Duan, J. Wang, Y. Qiu, Highly efficient blue
electrophosphorescent devices with a novel host material, Synth. Metals. 144 (2004)
49.
22] H-H Chang, W-S Tsai, C-P Chang, N-P Chen, K-T Wong, W-Y Hung, S-W Chen,
2
[
A new tricarbazole phosphine oxide bipolar host for efficient single-layer blue
PhOLED, Org. Electron. 12 (2011) 2025.
[
23] Y. Liu, L-S Cui, M-F Xu, X-B Shi, D-Y Zhou, Z-K Wang, Z-Q Jiang, L-S Liao,
Highly efficient single-layer organic light-emitting devices based on a bipolar
pyrazine/carbazole hybrid host material, J. Mater. Chem. C. 2 (2014) 2488.
[
24] X. Qiao, Y. Tao, Q. Wang, D. Ma, C. Yang, L. Wang, J. Qin, F. Wang, Controlling
charge balance and exciton recombination by bipolar host in single-layer organic
light-emitting diodes, J. Appl. Phys. 108 (2010) 034508.
[
25] Y. Yin, X Piao, Y. Li, Y. Wang, J. Liu, K. Xu, W. Xie, High-efficiency and
low-efficiency-roll-off single-layer white organic light emitting devices with a bipolar
transport host, Appl. Phys. Lett. 101 (2012) 063306.
[
26] L. Zuo, G. Han, R. Sheng, K. Xue, Y. Duan, P. Chen, Y. Zhao, Dramatic
efficiency improvement in single-layer orange phosphorescent organic light-emitting
devices with suppressed efficiency roll-off. RSC Advances. 6 (2016) 55017.
2
0

ACCEPTED MANUSCRIPT
[
27] S. Satoshi, S. Satoko; M. Miyako, H. Naoaki, I. Takahiro, T. Kyoko, S. Hiromi,
Heterocyclic compound, light-emitting element, light-emitting device, electronic
device, and lighting device, Patent US2013082591A1.
[
28]K. Yoshida, T. Iwakuma, H. Nagashima, R. Hashimoto, Organic
electroluminescence device, Patent US20140027744A1.
29]X. S. Zeng, M. Tavasli,I. F. Perepichka, A. S. Batsanov, M. R. Bryce, C. J.
Chiang, C. Rothe, A. P. Monkman, Cationic bis-cyclometallated
[
iridium(III)phenanthroline complexes with pendant fluorenyl substituents: synthesis,
redox, photophysical properties and light-emitting cells, Chem. Eur. J. 14 (2008) 933.
[
30]F. M. Hsu, C. H. Chien, C. F. Shu, C. H. Lai, C. C. Hsieh, K. W. Wang, P. T.
Chou, A Bipolar host material containing triphenylamine and diphenylphosphoryl
substituted fluorene units for highly efficient blue electrophosphorescence. Adv.
Funct. Mater. 19 (2009) 2834.
[
31]M. H. Tsai, Y. H. Hong, C. H. Chang, H. C. Su, C. C. Wu, A. Matoliukstyte, J.
Simokaitiene, S. Grigalevicius, J. V. Grazulevicius, C. P.
Hsu,3-(9-Carbazolyl)carbazoles and 3,6-Di(9-carbazolyl)carbazoles as Effective Host
Materials for Efficient Blue Organic Electrophosphorescence. Adv. Mater. 19 (2007)
8
62.
[
32]C. Cai, S. J. Su, T. Chiba, H. Sasabe, Y. J. Pu, K. Nakayama, J. Kido,
High-efficiency red, green and blue phosphorescent homojunction organic
light-emitting diodes based on bipolar host materials. Org. Electron. 12 (2011) 843.
[
33]S. W. Tsang, S. K. So, J. B. Xu, Application of admittance spectroscopy to
evaluate carrier mobility in organic charge transport materials. J. Appl. Phys. 99
(2006) 013706.
[
34] Y. P. Wang, J. S. Chen, J. Y. Huang, D. G. Ma, L. S. Dong, H. Chen, Electron
transport properties of bis[2-(2-hydroxyphenyl)-pyridine]beryllium investigated by
impedance spectroscopy, J. Appl. Phys. 115 (2014) 223707.
[
35]D. C. Tripathi, A. K. Tripathi, Y. N. Mohapatra, Mobility determination using
frequency dependence of imaginary part of impedance (ImZ) for organic and
polymeric thin films, Appl. Phys. Lett. 98 (2011) 033304.
2
1

ACCEPTED MANUSCRIPT
[
36]P. C. Kao, J. H. Lin, J. Y. Wang, C. H. Yang, S. H. Chen, Li CO as an n-type
2
3
dopant on Alq -based organic light emitting devices, J. Appl. Phys. 109 (2011) 109.
3
[
37]M. Cai, T. Xiao, E. Hellerich, Y. Chen, R. Shinar, J. Shinar, High efficiency
solution-processed small molecule electrophosphorescent organic light-emitting
diodes, Adv. Mater. 23 (2011) 3590.
[
(
[
38]Y. M. Yin, X. M. Wen, J. Yu, L. T. Zhang, W. F. Xie, IEEE Photonic Tech L 25
2013) 2205.
39]Y. Sun, N. C. Giebink, H. Kanno, B. Ma, M. E. Thompson, S. R. Forrest,
Management of singlet and triplet excitons for efficient white organic light-emitting
devices, Nature 440 (2006) 908.
[
40]S. J. Su, E. Gonmori, H. Sasabe, J. Kido, Highly efficient organic blue-and
white-light-emitting devices having a carrier- and exciton-confining structure for
reduced efficiency roll off, Adv. Mater. 20 (2008) 4189.
[
41]X. Yang, X. Du, S. Tao, Y. Huang, X. Ding, R. Xue, Efficient hole-transporter for
phosphorescent organic light emitting diodes with a simple molecular structure, Org.
Electron. 26 (2015) 481.
[
42]Y. L. Chang, Z. Wang, M. Helander, J. Qiu, D. Puzzo, Z. Lu, Enhancing the
efficiency of simplified red phosphorescent organic light emitting diodes by exciton
harvesting, Org. Electron. 13 (2012) 925.
[
43] W. Y. Hung, G. C. Fang, Y. C. Chang, T. Y. Kuo, P. T. Chou, S. W. Lin, K. T.
Wong, Highly efficient bilayer interface exciplex for yellow organic light-emitting
diode, ACS Appl. Mater. Interfaces 5 (2013) 6826.
[
44]S. Hofmann, M. Furno, B. Lüssem, K. Leo, M. C. Gather, Investigation of triplet
harvesting and outcoupling efficiency in highly efficient two-color hybrid white
organic light-emitting diodes, Phys. Status. Solidi. A 210 (2013) 1467.
2
2

ACCEPTED MANUSCRIPT
►
ML- and SL-PHOLEDs with a bipolar host EBBPC are fabricated. ►The
maximum EQE are 14.9%, 15.6%, 16.0% and 15.4% for R, G, B, and white
ML-PHOLEDs.►The maximum EQE are 10.1%, 14.6% and 9.8% for R, G, and B
SL-PHOLEDs. ►It could be attributed to the balanced bipolar transporting properties
and appropriate energy level of EBBPC.