A new benzothienoindole-based bipolar host material for efficient green phosphorescent organic light-emitting diodes with extremely small efficiency roll-off

Article abstract of DOI:10.1016/j.orgel.2019.04.006

In this contribution, two new hole-transport units, 10H-benzo[4,5]thieno[3,2-b]indole (BTI) and 10H-benzofuro[3,2-b]indole (BFI), were developed to design and synthesize bipolar host materials namely 10-(3-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-10H-benzo[4,5]thieno[3,2-b]indole.(mBTITrz) and 10-(3-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-10H-benzofuro[3,2-b]indole.(mBFITrz), for phosphorescent organic light-emitting diodes (PHOLEDs). The effect of heteroatoms in the hole-transport unit on the physicochemical and electroluminescence properties of the hosts were investigated in detail. Interestingly, the phosphorescence of the compounds was highly dependent on the hole-transport unit, because it originated from their local excited state (³LE), which was attributed to the interrupted strong electronic communication between donor and acceptor units by meta-mode of conjugation on the phenyl linker. Consequently, the mBTITrz showed high triplet energy (E_T) of ～2.88 eV compared to its counterpart mBFITrz (E_T ～ 2.65 eV). Both compounds displayed marked thermal stability with high thermal decomposition temperatures of above 410 °C and glass-transition temperatures of above 103 °C. In addition, the single carrier device studies revealed a bipolar charge transporting character for the compounds. Furthermore, the compounds were evaluated as bipolar hosts for green PHOLEDs by employing Ir(ppy)₃ dopant. The mBTITrz-hosted device demonstrated much better performance than did the mBFITrz-hosted device, with a maximum external quantum efficiency of 21.3% and maximum current efficiency of 76.0 cd/A. Notably, the mBTITrz-based device exhibited excellent efficacy stability with uncompromised efficiency roll-off at 1000 cd/m² and an extremely low efficiency roll-off of 3.2% at 5000 cd/m² and 9.0% at 10,000 cd/m².

Full text of DOI:10.1016/j.orgel.2019.04.006

Accepted Manuscript
A new benzothienoindole-based bipolar host material for efficient green
phosphorescent organic light-emitting diodes with extremely small efficiency roll-off
Rajendra Kumar Konidena, Kyunghyung Lee, Jun Yeob Lee, Wan Pyo Hong
PII:
S1566-1199(19)30171-5
DOI:
https://doi.org/10.1016/j.orgel.2019.04.006
ORGELE 5200
Reference:
To appear in: Organic Electronics
Received Date: 5 February 2019
Revised Date: 24 March 2019
Accepted Date: 6 April 2019
Please cite this article as: R.K. Konidena, K. Lee, J.Y. Lee, W.P. Hong, A new benzothienoindole-based
bipolar host material for efficient green phosphorescent organic light-emitting diodes with extremely
small efficiency roll-off, Organic Electronics (2019), doi: https://doi.org/10.1016/j.orgel.2019.04.006.
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ACCEPTED MANUSCRIPT
A New Benzothienoindole-based Bipolar Host Material for Efficient Green
Phosphorescent Organic Light-Emitting Diodes with Extremely Small
Efficiency Roll-Off
Rajendra Kumar Konidena1, Kyunghyung Lee1 and Jun Yeob Lee1*, Wan Pyo Hong2*
1School of Chemical Engineering, Sungkyunkwan University, 2066 Seobu-ro, Jangan-gu,
Suwon, Gyeonggi 440-746, Korea
2LG Chem, Ltd, LG Science Park, 30, Magokjungang 10-ro, Gangseo-gu, Seoul, 07796,
Republic of Korea
Email: leej17@skku.edu, whongw@lgchem.com
TOC
Abstract
In this contribution, two new hole-transport units, 10H-benzo[4,5]thieno[3,2-b]indole (BTI)
and 10H-benzofuro[3,2-b]indole (BFI), were developed to design and synthesize bipolar host
materials namely 10-(3-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-10H-benzo[4,5]thieno[3,2-
b]indole (mBTITrz) and 10-(3-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-10H-benzofuro[3,2-

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b]indole (mBFITrz), for phosphorescent organic light-emitting diodes (PhOLEDs). The
effect of heteroatoms in the hole-transport unit on the physicochemical and
electroluminescence properties of the hosts were investigated in detail. Interestingly, the
phosphorescence of the compounds was highly dependent on the hole-transport unit, because
it originated from their local excited state (3LE), which was attributed to the interrupted
strong electronic communication between donor and acceptor units by meta-mode of
conjugation on the phenyl linker. Consequently, the mBTITrz showed high triplet energy (ET)
of ~ 2.88 eV compared to its counterpart mBFITrz (ET ~ 2.65 eV). Both compounds
displayed marked thermal stability with high thermal decomposition temperatures of above
410oC and glass-transition temperatures of above 103oC. In addition, the single carrier device
studies revealed a bipolar charge transporting character for the compounds. Furthermore, the
compounds were evaluated as bipolar hosts for green PhOLEDs by employing Ir(ppy)3
dopant. The mBTITrz-hosted device demonstrated much better performance than did the
mBFITrz-hosted device, with a maximum external quantum efficiency of 21.3% and
maximum current efficiency of 76.0 cd/A. Notably, the mBTITrz-based device exhibited
excellent efficacy stability with uncompromised efficiency roll-off at 1000 cd/m2 and an
extremely low efficiency roll-off of 3.2% at 5000 cd/m2 and 9.0% at 10,000 cd/m2.
Introduction
During the past three decades, organic light-emitting diodes (OLEDs) received enormous
impetus from both the academic and industrial laboratories due to their versatile applications
in next generation energy saving displays and solid state lightings.1-9 Also, OLEDs possess
numerous attractive features such as ultra-thinness, inexpensive, light weight, flexible and
low power consumption compared to traditional display technologies.1-9 In the OLEDs, as per
the spin statistics the recombination of charge carriers generates singlet and triplet excitons in

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1:3 ratio.4-6 Conventional fluorescent materials can harvest only singlet excitons for light
emission due to the spin forbidden transition rules, which limited their internal quantum
efficiency to 25%.4-6 In order to resolve this, Forrest et al. developed heavy metal-based
phosphorescent emitters (PhOLEDs) to exploit both singlet and triplet excitons for light
emission, which enabled 100% IQE.4,5 After the pioneering work, tremendous research
efforts have been dedicated to improve the device performance of PhOLEDs.10-23 In general,
PhOLEDs constructed by the host-guest device architecture by doping the phosphorescent
emitter in a suitable host to alleviate the unwanted concentration quenching and triplet-triplet
annihilation.10-15 As the host materials occupy the major portion of the emitting layer, they
plays an important role in controlling the charge injection and transportation, location of
charge recombination zone and device stability.10-15 Nevertheless, engineering of the host
materials is as crucial as the dopant emitters for fabricating efficient PhOLEDs. In general, an
ideal host should have sufficiently high triplet energy compared to dopant, suitable HOMO
and LUMO energy levels with neighboring layers, high thermal stability and a broad
recombination zone of the charge carriers in the EML.10-15
It has been demonstrated that the bipolar host materials consisting of both the hole-
transporting (electron donor) and electron-transporting (electron acceptor) units can improve
the balanced charge transport in emitting layer and thus are good for promoting the device
performance and reducing the efficiency roll-off at high brightness compared to the unipolar
hosts.10-15 Previous reports illustrated that the choice of donor and acceptor units and their
structural combination of bipolar host materials should be examine carefully because they
dominates the charge-transport ability and overall device performance in many cases.10-15, 24-
29. Therefore, the understanding of the structure and property relationships of bipolar host
materials is highly desirable for good device performances. Until now, several electron-
deficient units, such as triazine, cyano, triazole, pyrazole, pyridine, triazolopyridine,

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phosphine oxide, benzimidazole, and oxadiazole, has been examined as electron transporting
units for developing bipolar host materials for green/blue PhOLEDs.10-15 However, the
potential donors are limited to carbazole and its derivatives because of their excellent hole-
transporting property, thermal stability, and high triplet state.10-15,24-44 Thus, the development
of novel hole-transport units with the aforementioned criteria is highly desirable to execute
new designs of high-quality bipolar host materials for PhOLEDs. Besides, the green
PhOLEDs have surpassed the maximum theoretical limit of EQE (20%).33-43 However,
typical PhOLEDs encounter significant efficacy roll-off issues at high brightness (> 1000
cd/m2) in spite of high maximum EQE (ηmax), which is a challenging issue of green
PhOLEDs. For example, recently, Su et al. reported a new bipolar host featuring
triazolopyridine electron-transport unit and demonstrated ηmax of 25.0% and reduced efficacy
roll-off of 7.6% at 1000 cd/m2.34 Wei et al. developed an indenocarbazole-based bipolar host
and demonstrated ηmax of 23.6% and reduced efficacy roll-off of 8.9% at 1000 cd/m2.35
Similarly, Liu et al. reported highly efficient triazole-featured bipolar host and found a ηmax of
28.0% and suppressed efficiency roll-off of 6.0% at 1000 cd/m2.36 However, the efficiency
roll-off of the aforementioned devices is still large at practical luminance range of 5000 cd/m2.
Therefore, the development of proficient bipolar hosts with extremely low efficiency roll-off
for PhOLEDs remained as challenging work for researchers.
Inspired by the above discussions and with our continuing interest in the development of
new bipolar hosts for PhOLEDs,44-47 herein we report two new hole-transport units, 10H-
benzo[4,5]thieno[3,2-b]indole (BTI) and 10H-benzofuro[3,2-b]indole (BFI), for designing
bipolar
host
materials,
10-(3-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-10H-
and 10-(3-(4,6-diphenyl-1,3,5-triazin-2-
benzo[4,5]thieno[3,2-b]indole
(mBTITrz)
yl)phenyl)-10H-benzofuro[3,2-b]indole (mBFITrz), by integration with a moderate
aryltriazine electron-transport unit for efficient green PhOLEDs, respectively. Although some

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synthetic attempts have been documented for BTI and BFI in the literature, their potential as
hole-transport units in developing functional materials for electroluminescent applications
was not explored in the literature.48 To this end, here we explored their potential applications
as hole-transport units in developing bipolar hosts for OLED applications. The donor and
acceptor units were connected in a meta-conjugation mode through a phenyl linker to avoid
the extension of conjugation and maintain high triplet energy for the host materials. The
effect of heteroatoms in the hole-transport unit on photophysical and device performances
was evaluated in detail. Interestingly, the compounds showed similar behavior in
fluorescence spectra, whereas their phosphorescence spectra were dominated by the hole-
transport units. The phosphorescence of the compounds predominantly originated from the
local excited state (3LE) of donor units, and their corresponding triplet energies were 2.88 eV
for mBTITrz and 2.65 eV for mBFITrz. Both compounds exhibited high thermal stability,
with a decomposition temperature (Td) above 410°C and a glass-transition temperature (Tg)
above 103°C. In addition, the single carrier device studies revealed a bipolar charge-
transporting feature for the compounds. Furthermore, the compounds were evaluated as
bipolar host materials for green PhOLEDs by employing Ir(ppy)3 dopant. The mBTITrz-
hosted device demonstrated much better performance than did the mBFITrz-hosted device,
with a maximum EQE of 21.3% and a maximum current efficiency of 76.0 cd/A. Notably, the
mBTITrz-hosted device exhibited excellent efficiency stability, with an uncompromised
efficiency roll-off at 1000 cd/m2 and an extremely small efficiency roll-off of 3.2% at 5000
cd/m2 and 9.0% at 10,000 cd/m2.

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Scheme 1. Synthetic scheme of the compounds mBTITrz and mBFITrz.
Results and Discussion
Synthesis and Characterization
Our molecular design was mainly focused on the study of the effect of heteroatoms (oxygen
or sulfur) in the hole-transport unit on physicochemical and electroluminescent properties of
the host materials. We envisaged that the integration of electron-donating BTI or BFI units
with an aryltriazine acceptor through a phenyl linker via a meta-conjugation mode would
impart a bipolar charge-transport feature for mBTITrz and mBFITrz, and maintain their
triplet energy sufficiently higher than the common green phosphorescent emitter Ir(ppy)3. The
key precursors, BFI and BTI, were synthesized according to the procedure in the literature.48
The aforementioned donors was reacted with 2-(3-bromophenyl)-4,6-diphenyl-1,3,5-triazine
under a palladium-catalyzed Buchwald-Hartwig reaction49 to accomplish the desired
materials, mBFITrz and mBTITrz, in reasonable yields. A synthetic scheme for mBFITrz
and mBTITrz is shown in Scheme 1. The chemical structures of the compounds were
confirmed by the nuclear magnetic resonance spectroscopy (1H and 13C NMR) and mass
spectral analysis. Also, the two host materials were purified by the vacuum train sublimation
process before applying in the OLED devices.
Thermal properties
Thermal stability of the host materials is one of the important parameters in fabricating
efficient OLED devices with higher operational stability. The thermogravimetric analysis

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(TGA) and differential scanning calorimetry (DSC) measurements were employed to
investigate the thermal stability of the compounds mBTITrz and mBFITrz. The
corresponding TGA and DSC traces of the compounds are portrayed in Figure 1, and
pertinent data were shown in Table 1. Thermal decomposition temperatures corresponding to
the 5% weight loss (Td) of the compounds mBTITrz and mBFITrz are of 414°C and 413°C,
respectively indicates their excellent thermal stabilities. Such superior thermal stability of the
compounds mBTITrz and mBFITrz is ascribed to the robust chemical structures of the BTI,
BFI, and aryltriazine units, and aromatic unit based backbone structures. Furthermore, the
DSC curves showed the definite glass transition temperatures (Tg) of 106°C for mBTITrz
and 103°C for mBFITrz during a second heating cycle. Evidently, these materials showed
significantly better Tg than that of the conventional host materials namely 1,3-bis(N-
carbazolyl)-benzene (mCP; 60°C) and 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP; 62°C).50
The results suggests that the compounds mBTITrz and mBFITrz possess excellent thermal
stabilities and tolerate high temperatures during the vacuum evaporation process of OLED
device fabrication and device operation.
a)
100
mBTITrz
mBFITrz
80
60
40
20
0
50 100 150 200 250 300 350 400 450 500 550 600
Tempreture (oC)

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b)
mBTITrz
mBFITrz
106 oC
103 oC
60
70
80
90
100
110
120
130
Tempreture (oC)
Figure 1. a) TGA and b) DSC traces of the hosts mBTITrz and mBFTTrz.
Electrochemical properties
To investigate the electrochemical behavior of the new compounds mBTITrz and
mBFTTrz, the cyclic voltammetry measurements were collected in dilute dichloromethane
solution. The oxidation potentials of the compounds was referenced to Fc/Fc+ redox couple,
and the relevant data were tabulated in Table 1. In general, the electron-donating strength of
the chromophores can be evaluated based on their oxidation propensity.51 Therefore, to
understand the effect of the heteroatoms in the hole-transport units on their electron donating
strength, the oxidation potentials of the BTI and BFI were collected. Interestingly, the BTI
possessed a smaller oxidation potential (+ 0.90 V) than did the BFI (+ 0.95 V), which
indicates its strong electron donor character, which probably results because the BTI unit
contains a sulfur atom, which is less electronegative than is the oxygen atom in the BFI
unit.52 Indeed, mBTITrz showed more facile oxidation than that of mBFITrz because of

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having a BTI donor that is stronger than the relatively weak BFI donor in the latter. Upon
reduction sweep, the compounds showed irreversible reduction waves with almost similar
reduction potentials attributed to the presence of same aryltriazine acceptor in both the
compounds. The corresponding HOMO and LUMO energy levels of the compounds was
calculated from their onset potentials of oxidation and reduction peaks, respectively.
Accordingly, the HOMO/LUMO energy levels of the compounds were -5.78/-3.22 eV for
mBTITrz and -5.83/-3.19 eV for mBFITrz. The HOMO levels of the mBTITrz and
mBFITrz were shallower than those of common hole-transport type hosts such as CBP (-6.0
eV) and mCP (-6.1 eV), suggesting good hole-related properties.50 Additionally, the deep
LUMO levels propose good electron-related properties. Therefore, good bipolar properties
are expected from the two hosts. The HOMO level became shallow and resulted in a smaller
HOMO-LUMO gap (Eg = 2.56 eV) in the mBTITrz than in the mBFITrz (Eg = 2.64 eV).
Theoretical calculation
Density functional theoretical (DFT) calculations was performed on the model compounds
of the new host materials mBTITrz and mBFITrz using the B3LYP/6-31G* basis set on the
Gaussian 09 program package to understand their electronic structure and frontier molecular
orbital distributions (HOMO and LUMO). The optimized geometrical structures of the
compounds and their HOMO/LUMO orbitals are displayed in Figure 2. In both the
compounds mBTITrz and mBFITrz, the HOMO orbitals are exclusively localized on the
BTI and BFI electron donating moieties with little contributions from the phenyl linker,
whereas the LUMO orbitals are delocalized on the aryltriazine acceptor unit and extended
into phenyl linker. These HOMO and LUMO distribution results suggest that the BTI and
BFI would function as the hole-transporting units, whereas the aryltriazine moiety would
behave as the electron-transporting unit. As shown in the optimized geometries, both

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compounds exhibited non-planar geometries, with a large twisting between the hole-transport
unit and phenyl linker with a large dihedral angle of above 51o. The calculated HOMO and
LUMO energies and band gap (Eg) of the host materials are of -5.14 eV, -2.03 eV, and 3.11
eV for mBTITrz and -5.17 eV, -2.00 eV and 3.17 eV for mBFITrz, respectively. The trends
in the computed HOMO, LUMO and band gap (Eg) values are in good agreement with the
experimentally deduced values.
Figure 2 a) Optimal geometrical configurations and b) orbital distributions of the mBTITrz
and mBFITrz estimated by DFT methods.
Photophysical properties
The photophysical properties of the compounds was analyzed by absorption and emission
spectral measurements using 10-5 M THF solutions and the corresponding spectral profiles
are shown in Figure 3 and the pertinent data were fitted in Table 1. Both compounds showed
multiple absorption bands in the range of 260-375 nm. The high energy absorption bands
seeming below ~ 335 nm correspond to the localized π-π* electronic transitions of different
aromatic conjugative segments in the compounds. The long-wavelength absorption band with

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mediocre intensity appearing above ~ 345 nm is assigned to the intramolecular charge
transfer (ICT) transitions from the BTI or BFI donor to the aryltriazine acceptor. The
observed weak intensity of ICT band is attributed to the interrupted strong electronic
communication between the BTI or BFI donor and aryltriazine acceptor by virtue of non-
conjugative meta-mode of linkage between them. The dilute solutions of the compounds
showed green fluorescence under the visible light. Both compounds showed featureless
emission profiles, with an emission maximum of 538 nm for mBTITrz and 534 nm for
mBFITrz, showing that the singlet transition originating from the CT states (vide infra).
Comparatively, that mBTITrz exhibited a slight bathochromic shift with a broader emission
profile than that of its counterpart mBFITrz is attributed to a relatively stronger CT excited
state generated by the charge transfer from the BTI donor to the aryltriazine acceptor.
Furthermore, the excited state CT of these compounds is certified by their observed
bathochromic shift in emission profiles form non-polar toluene (TOL) to polar THF solution
with a large full width at half maximum (FWHM). Such a red-shifted emission with
broadened spectral profiles in polar solvents is a feature of CT induced electronic
perturbations in the excited state of bipolar compounds.51 However, the extent of CT is
minimal in these compounds according to the small bathochromic shift (∆λem > 48 nm) from
TOL to THF solvents caused by the meta-conjugation linkage between the donor and
acceptor moities.53 The low-temperature PL spectra (LTPL) of the compounds were recorded
in a frozen THF at 77 K. In contrast to the fluorescence spectra, the phosphorescence spectra
were largely dominated by the donor units in the compounds. As shown in Figure 3, the
phosphorescence spectra of the mBTITrz and mBFITrz were almost indistinguishable in
both the profile and the peak wavelength from those of BTI and BFI units, respectively. This
clearly indicates that the phosphorescence in these compounds stems from the local triplet
excited states (3LE) of the hole-transport units, perhaps because of the poor electronic

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coupling between the hole-transport and electron-transport units resulting from the non-
conjugative meta-mode linkage between them.54,55 The triplet energy (ET) of the mBTITrz
and mBFITrz were calculated from the onset wavelengths of the phosphorescence spectra
and are of 2.88 eV and 2.65 eV, respectively. The ET of these compounds is sufficiently
superior than the common green Ir(ppy)3 phosphor to ensure efficient forward energy transfer
and prevent back energy transfer when employed as hosts for green PhOLEDs.
80000
70000
60000
50000
40000
30000
20000
10000
0
1.1
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
mBTITrz
mBFITrz
mBTITrz TOL
mBFITrz TOL
mBTITrz THF
mBFITrz THF
mBTITrz LTPL
mBFITrz LTPL
BTI LTPL
BFI LTPL
245 290 335 380 425 470 515 560 605 650 695 740
Wavelength (nm)
Figure 3. UV-visible Absorption recorded in THF and emission spectra collected in THF and
TOL solutions and phosphoresce spectra measured at 77K (LTPL) of the compounds.
Table 1. Physicochemical and thermal properties of the compounds
Compound
λabs, nm (εmax, λem
ET
HOMO/LUM Eg
T5d
Tg
M-1 cm-1 ×103)a
(nm)a
(eV)b
O (eV)c
(eV) (oC)e
(oC)f
d
350 (7.12), 313 538
(sh), 261 (74.0)
349 (sh), 332 (s 534
h), 319 (33.1), 2
68 (66.3)
2.88
2.65
-5.78/-3.22
-5.83/-3.19
2.56 414
2.64 413
106
103
mBTITrz
mBFITrz

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c
aMeasured in THF solution. bEstimated from onset of phosphorescence spectra. HOMO and
d
LUMO estimated from the cyclic voltagrams. Band gap estimated from the electrochemical
measurements. eThermal decomposition temperature (T5d). fGlass transition temperature (Tg).
Carrier transport properties
The bipolar charge-transport character of the host materials is one of the most essential
parameters to facilitate the injection and transport of charge carriers, balance the hole-
electron distribution, increase the recombination probability, and achieve the superior device
performance with low efficiency roll-off in the PhOLEDs. As discussed above,
electrochemical and theoretical studies was indirectly demonstrated the bipolar charge-
transport feature of mBTITrz and mBFITrz. Furthermore, single carrier devices were
fabricated to certify this. The electric-field-dependent current-density data of the single
carrier devices were displayed in Figure 4. The hole-only (HOD) and electron-only (EOD)
devices of the hosts mBTITrz and mBFITrz exhibited significantly superior current
densities compared to the conventional host material 3,3-di(9H-carbazol-9-yl)biphenyl
(mCBP), suggesting that the new host materials possess appropriate hole and electron-
transporting ability and have the bipolar charge-transport feature. It is noted that the two
compounds mBTITrz and mBFITrz exhibited superior hole current density than the mCBP,
shows that the new hole-transport units (BTI and BFI) are more efficient for hole-transport
than is carbazole. Comparing the two host, the mBTITrz displayed a hole current density
higher than that of the mBFITrz because the BTI has better hole-transport properties than
does the BFI hole-transport unit. Besides, the EOD have comparable current-densities,
indicating that the two hosts have comparable electron-transport properties, because the same
electron-transport units were used in the two hosts. From these results, it is expected that the
mBTITrz and mBFITrz work well as the host balancing carriers because of the bipolar
charge-transport property.

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Figure 4. I-V characteristics of a) hole-only and b) electron-only devices of mBTITrz,
mBFITrz and mCBP.
Electroluminescence properties
Figure 5. Energy level alignment and materials chemical structures used in the devices.
The high triplet energy, bipolar charge-transport character, and excellent thermal stability of
these compounds motivated us to explore their potential applicability as bipolar host
materials for green PhOLEDs using a well-known green Ir(ppy)3 dopant. The emitting layer
(EML) was composed by doping of 5wt% of Ir(ppy)3 in either mBTITrz (Device G1, G
denotes green phosphorescence) or mBFITrz (Device G2) hosts. The energy-level alignment

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and the chemical structures of each layer used in the device were displayed in Figure 5. The
EL spectra, current density (J)-voltage (V)-luminance (L) and EQE vs luminance plots of the
green devices are shown in Figure 6, and their detailed EL parameters are tabulated in Table 2.
In EL spectra (Figure 6a), both G1 and G2 devices emitted pure green emission corresponds
to the Ir(ppy)3 dopant, with peak wavelengths of 517 nm and 514 nm and CIE coordinates of
(0.30, 0.63) and (0.31, 0.63), respectively. The absence of any other emission peaks
corresponding to the hosts or other layers from the devices, suggesting efficient energy
transfer from the hosts mBTITrz or mBFITrz to the Ir(ppy)3 dopant and exciton
imprisonment in the EML. Comparatively, the mBTITrz-hosted device G1 showed slightly
higher current-density and luminance than did the mBFITrz-based G2 device because of its
good hole-transport property, as shown in the single carrier device data. The maximum EQE
and current efficiency (CE) of the devices are of 21.3% and 76.0 cd/A for device G1, and
15.7% and 56.3 cd/A for device G2. The superior performance of the mBTITrz-hosted
device G1 compared to that of the mBFITrz hosted device G2 is attributed to its sufficiently
higher triplet energy than the dopant for efficient energy transfer and good bipolar charge-
transporting property for balanced charge carrier injection and transport. The EQE vs
luminance plots of the compounds are portrayed in Figure 6c. Notably, the device G1
exhibited extremely low efficacy roll-off at high brightness. At the luminance of 1000 cd/m2,
the device G1 exhibited EQE of 21.3%, which is almost unchanged compared to their
maximum EQE value. Even at the high brightness level of 5000 cd/m2, the device G1
revealed excellent efficiency stability (EQE = 20.6%) with a small efficacy roll-off of 3.2%.
Furthermore, even at 10,000 cd/m2, the mBTITrz-hosted G1 device maintained a high EQE
of 19.4%. The efficiency drop at 10,000 cd/m2 was only 9.0%, indicating its excellent
efficiency stability at a practical brightness range. Undoubtedly, this is attributed to the
balanced charge transport of the mBTITrz host. From these results, it is evident that the BTI

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unit is a potential hole-transport unit for developing highly efficient bipolar hosts for
PhOLEDs.
Figure 6. a) EL plot, b) J-V-L plots, c) EQE brightness and d) CE vs luminance plots of the
green PhOLED devices.

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Table 2. Summarized EL performances of the devices
Host
Voltage (Von)a
EQE (%)
Max.
CE (cd/A)
EL peak (n CIE (x, y)
m)
@1000 cd/m2
@5000 cd/m2
Max. @1000 c @5000 c
d/m2
75.9
56.3
d/m2
73.3
51.3
3.9
4.0
21.3
15.7
21.3
15.6
20.6
14.3
76.0
56.3
515
517
0.30, 0.63
0.31, 0.63
mBTITrz
mBFITrz
aturn-on voltage (V) at 1 cd/m2

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Conclusions
In summary, we have designed and synthesized two new bipolar host materials viz.,
mBTITrz and mBFITrz, using BTI or BFI as the hole-transport units and aryltriazine as the
electron-transport unit for green PhOLEDs. The introduction of the BTI and BFI hole-
transport units allowed a high triplet energy over 2.65 eV, a high thermal decomposition
temperature over 410 oC and a glass-transition temperature above 103 oC. DFT computations
and single carrier device studies revealed bipolar charge transporting feature for the
compounds. Further, the green PhOLEDs were fabricated using these new compounds as
bipolar host materials by employing Ir(ppy)3 emitter. The mBTITrz-hosted device worked
better than the mBFITrz-hosted device, with maximum EQE of 21.3% and CE of 76.0 cd/A.
Moreover, the mBTITrz-hosted device maintained uncompromised EQE of 21.3% at 1000
cd/m2 and high EQE of 20.6% at 5000 cd/m2 with an extremely low efficiency roll-off of
3.2%. These results suggest that BTI is a potential hole-transport unit for developing highly
proficient bipolar hosts for PhOLEDs and expands the library of donor units.
Acknowledgement
This work was supported by the Basic Science Research Program (Grant No.
2016M3A7B4909243 and 2018R1D1A1B07048498) through the National Research
Foundation (NRF) of Korea funded by the Ministry of Science, ICT, and Future Planning.
Key words: organic light-emitting diodes (OLEDs); green PhOLEDs; bipolar host; hole-
transport unit; efficiency roll-off.

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Bipolar hosts with new hole transport units for high external quantum efficiency
Small efficiency roll-off in green phosphorescent organic light-emitting diodes using
the bipolar hosts
-
Effect of heteroatom in the hole transport unit on the physical and device
performances of the hosts