Novel benzothiadiazine 1,1-dioxide based bipolar host materials for efficient red phosphorescent organic light emitting diodes

Article abstract of DOI:10.1016/j.orgel.2021.106104

In this work, three novel bipolar host materials TPA-SA, 3CBZ-SA and 4CBZ-SA have been designed and synthesized by incorporating triphenyl amine and carbazole as donor and benzothiadiazine 1,1-dioxide as an acceptor. These molecules exhibit moderately high triplet energies and bipolar carrier transport characteristics (ambipolarity) which is useful for the exothermic energy transfer to the dopants and also for the balanced carrier injection/transport in the emissive layers. These materials exhibited good performances in PhOLEDs and furnished external quantum efficiency in the range of 10.0–15.0%. Notably, a red phosphorescent device using TPA-SA as the host doped with Ir(pq)₂(acac) exhibited a maximum EQE, power efficiency and current efficiency of 15.0%, 16.0 lm/W, and 25.3 cd A^?1, respectively.

Full text of DOI:10.1016/j.orgel.2021.106104

Organic Electronics 92 (2021) 106104
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: http://www.elsevier.com/locate/orgel
Novel benzothiadiazine 1,1-dioxide based bipolar host materials for
efficient red phosphorescent organic light emitting diodes
a
b
a
c
a
Jatin Lade , Nian-Yun Lee , Bhausaheb Patil , Yogita Y. Deshpande , B. Pownthurai ,
d,
e
c
,
***
d
,
e,
**
a,*
Chung-An Hsieh , Subhash S. Pingale
, Li-Yin Chen
, Atul Chaskar
a
National Centre for Nanosciences and Nanotechnology, University of Mumbai, Mumbai, 400098, India
Department of Photonics, National Sun Yat-Sen University, Kaohsiung, 804, Taiwan
Department of Chemistry, Savitribai Phule Pune University, Pune, 411007, India
b
c
d
e
Department of Photonics, National Yang Ming Chiao Tung University, Hsinchu 30010, Taiwan
Department of Photonics, National Chiao Tung University, Hsinchu 30010, Taiwan
A R T I C L E I N F O
A B S T R A C T
Keywords:
In this work, three novel bipolar host materials TPA-SA, 3CBZ-SA and 4CBZ-SA have been designed and syn-
thesized by incorporating triphenyl amine and carbazole as donor and benzothiadiazine 1,1-dioxide as an
acceptor. These molecules exhibit moderately high triplet energies and bipolar carrier transport characteristics
Benzothiadiazine1,1-dioxide
Bipolar host materials
Carbazole
(
ambipolarity) which is useful for the exothermic energy transfer to the dopants and also for the balanced carrier
Triphenylamine
Red PhOLED
injection/transport in the emissive layers. These materials exhibited good performances in PhOLEDs and fur-
nished external quantum efficiency in the range of 10.0–15.0%. Notably, a red phosphorescent device using TPA-
2
SA as the host doped with Ir(pq) (acac) exhibited a maximum EQE, power efficiency and current efficiency of
1
5.0%, 16.0 lm/W, and 25.3 cd A 1, respectively.
ꢀ
1
. Introduction
balanced charge transport properties is very crucial for making highly
efficient phosphorescent organic light-emitting diodes (PhOLEDs) [4].
Moreover, the host material must possess high triplet energy as
compared to guest so that reverse energy transfer from the guest back to
the host can be prevented and excitons would remain confined on guest
molecule. Nevertheless, they should be capable of suppressing the un-
favorable effects such as aggregation caused quenching and triplet–-
triplet annihilation of phosphorescent emitters [5].
The phenomenon of electroluminescence in organic compound was
first observed in acridine orange thin film upon applying high voltage
1]. Subsequently, Tang et al. demonstrated first practical green organic
light emitting diode comprised of Tris(8-hydroxyquinolinato)
aluminium (Alq ) sandwiched between a cathode and an anode [2].
[
3
Initially, fluorescent organic materials were used as emitting layer, but
their internal quantum efficiency could not be extended beyond 25% as
per spin statistics rule which limited their applications in display tech-
nologies. In fluorescent OLED’s only 25% of singlet excitons are utilized
while 75% triplet excitons get wasted in the form of heat or other
non-radiative decays. As per the spin selection rule, transition from
triplet excited state to the singlet ground state is forbidden but use of
organometallic complexes comprising heavy metals such as Ir, Pt, Re
and Os induces a spin – orbit coupling which opens a radiative path for
the transition and promote intersystem crossing [3].
In this context, different classes of bipolar host materials have been
designed and synthesized with abundant combinations of hole- and
electron transporting moieties. Typically, carbazole and arylamine are
frequently used as hole-transporting motifs owing to its excellent hole
ꢀ
3
ꢀ 5
2
ꢀ 1 ꢀ 1
mobilities (
μ
h
ca. 10 to 10 cm
V s ) [6] whereas, oxadiazole,
triazine, phenanthroline, and benzimidazole are commonly used as
electron-transporting moieties [7–13].
More recently, Jia et al. [14] reported two host materials DAMP and
DCMP incorporating carbazole and triphenyl amine for hosting green
(EQE = 19.5% and 22.2%) and red (EQE = 15.3% and 20.0%)
The designing of tailor-made bipolar host materials which contains
*
Corresponding author.
*
*
* Corresponding author. Department of Photonics, National Yang Ming Chiao Tung University, Hsinchu 30010, Taiwan.
** Corresponding author.
E-mail addresses: subhash.pingale@unipune.ac.in (S.S. Pingale), lychen@nycu.edu.tw (L.-Y. Chen), achaskar25@gmail.com, achaskar@nano.mu.ac.in
(
A. Chaskar).
https://doi.org/10.1016/j.orgel.2021.106104
Received 18 January 2021; Received in revised form 31 January 2021; Accepted 3 February 2021
Available online 20 February 2021
1
566-1199/© 2021 Elsevier B.V. All rights reserved.

J. Lade et al.
Organic Electronics 92 (2021) 106104
phosphorescent OLED’s. Two novel bipolar host materials M1 and M2
induce bipolar characteristic in molecules. Albeit, benzothiadiazine 1,
1-dioxide is not explored so far as electron acceptor unit in bipolar
host materials. Strong electron accepting nature of sulfone moiety makes
them an attractive candidate to be used in bipolar host material as an
acceptor unit. In last few years, some reports were published which
utilizes sulfone core as an electron acceptor unit to fabricate electrolu-
minescent devices. [23]. Their performance as bipolar hosts are still
limited and need to be explored to the full potential. In this context, we
here with report synthesis and characterization of novel and high triplet
energy bipolar host materials, TPA-SA, 3CBZ-SA and 4CBZ-SA for the
application in red phosphorescent OLED.
based on carbazole/diphenyl quinoxaline were synthesized by Gao et al.
[
15] and tested for red phosphorescent OLEDs and have shown EQE of
4.66% and 15.07% respectively. In addition, Lee et al. [16] designed
1
and synthesized a series of bipolar host materials with dipyridyl amine
and carbazole linked to triazine core (p-BCTPy, m-BCTPy and 3-BCTPy)
and tested for green PhOLEDs. Amongst three, 3- BCTPy shown rela-
tively high EQE of 18.9%.
This clearly emphasized that carbazole and triphenyl amine are very
important moieties in bipolar host molecules owing to its high triplet
energy and excellent hole transporting properties. Various electron
acceptor units such as pyrrolo[1,2-a]quinoxaline [17], triazolopyridine
[
18], triazole [19], triazine [20], oxadiazole [21], phosphine oxide [22],
etc. were used in combination with carbazole and triphenyl amine to
Scheme 1. Synthesis of TPA-SA, 3CBZ-SA and 4CBZ-SA.
2

J. Lade et al.
Organic Electronics 92 (2021) 106104
2
. Result and discussion
solvent-independent nature of absorbance shows that the
Franck–Condon transitions leads to small change in dipole moment at
the ground state. The PL spectra in toluene solution were recorded at
2
.1. Synthesis
2
98 K. TPA-SA, 3CBZ-SA and 4CBZ-SA shows emission maxima at 470,
Synthetic pathway for the TPA-SA, 3CBZ-SA and 4CBZ-SA is
396, 434 nm respectively, which correspond to singlet emission. The
significant solvent-dependent emission was observed for 3CBZ-SA. The
emission spectral shift proportional to solvent polarity suggested rapid
photo-induced electron transfer (PET) which led to a noticeable change
in the dipole moment in the excited state. Further, red-shifted PL spectra
observed for neat film of 3CBZ-SA exhibiting different dielectric sur-
depicted in Scheme 1. The intermediates 1, 3 and 4 were synthesized
using previously reported methods [24–26]. Intermediate 1 is obtained
by the Vilsmeier-Haack reaction on triphenyl amine. Carbazole on
N-arylation using iodobenzene gave N-phenyl carbazole (2) which
3
subsequently formylated using DMF/POCl to form 3. Intermediate 4
was synthesized via reaction of carbazole with 4-bromobenzaldehyde in
presence of CuI as a catalyst. The condensation reaction of 1, 3, and 4
with 2-amino benzenesulfonamide in the presence of sodium bisulphite
resulted in the formation of TPA-SA, 3CBZ-SA and 4CBZ-SA, respec-
tively. The products were purified by trituration in diethyl ether and
rounding in the solid state. The triplet energies (E
T
) of 2.43–2.64 eV
were observed for TPA-SA, 3CBZ-SA and 4CBZ-SA from the first phos-
phorescence emission peak of low temperature PL spectrum recorded at
77 K (Table 1).
1
13
characterized by H, C NMR and mass spectroscopy.
2
.4. Electronic properties
2
.2. Thermal properties
The electrochemical behavior of TPA-SA, 3CBZ-SA and 4CBZ-SA
were probed by using cyclic voltammetry measurements. 3-Electrode
cell system encompassing a glassy carbon electrode as working elec-
trode, silver/silver chloride (Ag/AgCl) as reference electrode and a
platinum wire as counter electrode was used. Oxidation scans were
performed using 0.1 M tetra(n-butyl)ammonium hexafluorophosphate
TPA-SA, 3CBZ-SA and 4CBZ-SA exhibited excellent morphological
◦
stabilities with decomposition temperatures (T
d
) of 394 C for TPA-SA,
◦
◦
4
12 C for 3CBZ-SA and 417 C for 4CBZ-SA, corresponding to 5%
weight loss, calculated using thermogravimetric analysis (TGA). Glass
transition (T ) and melting temperature (T ) were determined using
differential scanning calorimetry (DSC). TPA-SA, 3CBZ-SA and 4CBZ-
g
m
6 2 2
(TBAPF ) in CH Cl and reduction scans were performed using tetra(n-
butyl)ammonium perchlorate (TBAP) in DMF as supporting electrolytes.
All of these compounds displayed quasi-irreversible oxidation as well as
reduction potentials which may be arise due to their triphenylamine,
9H-phenylcarbazole and benzothiadiazine 1,1-dioxide segments,
respectively. The oxidation peak potentials observed at 1.15, 1.16 and
◦
◦
◦
SA displayed glass transition temperature of 267 C, 235 C and 231 C,
◦
◦
◦
and melting temperature of 351 C, 381 C and 344 C, respectively.
Such significantly high values of T and T for these compounds suggests
g
d
that they can be sublimed using vacuum thermal unit, also they are
capable of resisting inevitable joule heating which occur during device
operation.Fig. 1.
+
1.21 eV (vs. Fc/Fc ) for TPA-SA, 3CBZ-SA and 4CBZ-SA, respectively.
These values are meticulously associated to the degree of
TPA-SA, 3CBZ-SA impart lower oxidation potentials as compare to
CBZ-SA. The HOMO energy levels of these bipolar hosts were calcu-
lated using HOMO (eV) = - (Eox onset - EFc/Fc+ onset) - 4.80 eV i.e -5.55,
5.56 and ꢀ 5.61 eV for TPA-SA, 3CBZ-SA and 4CBZ-SA, respectively.
While, LUMO energy levels of TPA-SA, 3CBZ-SA and 4CBZ-SA were
, where E is an optical
π
-conjugation,
4
2
.3. Photophysical properties
ꢀ
The electronic absorption (UV–vis) and photoluminescence (PL)
spectra (recorded at 298 K) of TPA-SA, 3CBZ-SA and 4CBZ-SA in
various solvents and in the form of solid thin films are depicted in Fig. 2.
The data is summarized Table 1. TPA-SA showed absorbance at
evaluated using the formula LUMO = HOMO + E
g
g
bandgap, calculated using absorption onset. 3CBZ-SA and 4CBZ-SA
exhibited higher values of HOMO energy level compared to N-phenyl
carbazole (ꢀ 5.88 eV) which helps to prevent the formation of exciplexes
with neighboring electron donor unit.Fig. 3.
2
81–292 nm and 353–373 nm, 3CBZ-SA displayed absorption peaks in
the range of 289–299 nm and 325–341 nm while 4CBZ-SA exhibited
absorption peak at 282–293 nm and 335–358 nm. The solvent polarity
independent absorption band from 281 to 299 nm are ascribed to the
π
–
π
* transition while peaks in the range of 325–373 nm are due to n–
π
*
2.5. Theoretical calculations
transition of the carbazole moiety. The higher wavelength absorption is
due to the occurrence of intramolecular charge transfer between
carbazole(donor) and benzothiadiazine 1,1-dioxide (acceptor). The
The geometrical structures of the TPA-SA, 3CBZ-SA and 4-CBZ-SA
molecular systems were optimized by the density functional theory
Fig. 1. TGA (a) and DSC (b) of TPA-SA, 3CBZ-SA and 4CBZ-SA.
3

J. Lade et al.
Organic Electronics 92 (2021) 106104
Fig. 2. Photophysical properties of TPA-SA, 3CBZ-SA and 4CBZ-SA in various solvents.
4

J. Lade et al.
Organic Electronics 92 (2021) 106104
Table 1
Physical properties of TPA-SA, 3CBZ-SA and 4CBZ-SA.
o
Ox
a
a
(
T /T
g m
d
/T )/
C
(E1/2 /V
λ
abs sol/nm
λabs film/nm
λ
PL sol/nm
λ
PLfilm/nm
E
T
/eV
g
(HOMO/LUMO/E d)/eV
TPA-SA
267/351/394
235/381/412
231/344/417
1.15
1.16
1.21
362
353
340
370
340
350
470
396
434
530
450
500
2.43
2.64
2.64
ꢀ 5.55/-2.60/2.95
ꢀ 5.56/-2.32/3.24
ꢀ 5.61/-2.56/3.05
3
4
CBZ-SA
CBZ-SA
a
In toluene solution (1 × 10ꢀ 5 M).
structures the UV–Vis absorption spectral properties in DMSO solvent
were calculated by time-dependent density functional theory (TD-DFT)
[
28] associating with the polarized continuum model (PCM). [29,30].
The calculated plots of the frontier molecular orbitals (HOMO and
LUMO) were visualized using the UNIVIS software [31]. All the calcu-
lations were performed with GAUSSIAN09 [32] program.
The theoretically calculated λmax of TPA-SA in DMSO solvent at
CAM-B3LYP/6-31 + G(d,p) level of theory is observed at 329.9 nm with
oscillator strength (f) value of 0.98 (See Table 2). For 3CBZ-SA, the
respective λmax and corresponding f values are observed to be 293.4 nm
and 0.20, whereas, for 4CBZ-SA, these values turned out to be 304.1 nm
and 0.61 respectively. The corresponding to λmax values in all the three
systems are dominated by HOMO to LUMO transitions. The HOMO and
LUMO diagrams in Fig. 4 clearly show that the HOMOs of all the mol-
ecules are located on donor triphenyl amine or carbazole and the LUMOs
are located on benzothiadiazine 1,1-dioxide acceptor parts. This clearly
highlights the bipolar nature of the molecules where the intramolecular
charge transfer occurs from donating group (triphenyl amine or carba-
zole) to accepting (benzothiadiazine 1,1-dioxide) ones.
The calculated λmax values of all the three molecules are slightly
lower than the respective experimentally observed ones (cf. Table 2).
However, the trend of the calculated λmax values correlated well with the
respective experimental ones having a correlation coefficient of 0.99 (cf.
Fig. 5). The TPA-SA shows highest λmax value, whereas, that of the
3
CBZ-SA becomes the lowest one.
The HOMO-LUMO energy gap (EHL) calculated at CAM-B3LYP/6-31
+
G(d,p) level of theory are tabulated in Table 2. In comparison with the
experimentally obtained energy band gap (E ) in Table 2, theoretically
g
calculated EHL values are observed to be overestimated. This discrep-
ancy can be attributed to the different platform used to obtain the
values. Despite of this, the obtained trend of Eg and EHL is quite similar.
The normalized values of EHL to TPA-SA i.e. ENHL are matching very well
with Eg values.
3
. Devices fabrication and EL performance
Fig. 3. Cyclic voltammogram of TPA-SA, 3CBZ-SA and 4CBZ-SA.
3
.1. Device fabrication and characterization
(
DFT) [27]. CAM-B3LYP functional with 6-31 + G(d,p) basis set was
Before depositing organic layers, Indium-tin-oxide (ITO) glass sub-
employed for the optimization of all the systems. The optimized geom-
etries are taken for the frequency calculation and verified that the ob-
tained structures are minima on the potential energy surface with zero
imaginary frequencies. On the basis of the optimized geometry
ꢀ 1
strates (sheet resistance ~15 Ω sq ) were cleaned in an ultrasonic bath
using deionized water and 2-propanone in succession for 20 min each,
followed by warmed in methanol for 10 min. The cleaned ITO glass
Table 2
The calculated maximum absorption wavelength (λmax) value, oscillator strength (f) with respective most probable transition at CAM-B3LYP/6-31 + G(d,p) level of
theory along with experimental λmax of the studied compounds in DMSO. EHL is the HOMO-LUMO energy gap, ENHL is the normalized EHL calculated at CAM-B3LYP/6-
3
1 + G(d,p) level of theory and Eg is the experimental band gap.
a
b
Systems
Theoretical λmax (nm)
f (Transition)
EHL (eV)
E
NHL (eV)
Experimental λmax (nm)
Eg (eV)
TPA-SA
329.9
0.98 (HOMO
LUMO)
6.044
6.618
6.199
2.95
3.23
3.03
358
2.95
3.24
3.05
3
4
CBZ-SA
CBZ-SA
293.4
304.1
0.20 (HOMO
LUMO)
331
341
0.61 (HOMO
LUMO)
a
b
E
NHL = EHL/2.95, The values of respective HOMO and LUMO energies are given in supporting information Table S3.
See Table 1 for details.
5

J. Lade et al.
Organic Electronics 92 (2021) 106104
Fig. 4. Schematic representations of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) having isosurface value 0.030
a.u. of the studied compounds.
2
hole-transporting layer. Ir(pq) (acac) (bis(2-phenylquinoline) (acetyla-
cetonate)iridium(III)) was used as the red dopant of the PhOLEDs.
′
TmPyPB (1,3,5-tris[(3-pyridyl)-phen-3-yl]benzene) or TPBi (2,2 ,2"-
(
1,3,5-Benzinetriyl)-tris(1-phenyl-1-H-benzimidazole)) were chosen as
the electron transporting layer. The chemical structures of the materials
used for device fabrication are shown in Fig. 6b. All the organic mate-
rials were deposited via a vacuum evaporation technique under a base
ꢀ
6
ꢀ 1
pressure of 1 × 10 Torr with a deposition rate of 0.1 nm s . Finally,
LiF and Al cathodes were deposited without breaking the vacuum at a
ꢀ
1
ꢀ 1
rate of 0.01 nm s and 0.5 nm s , respectively. The thicknesses of
organic layers were acquired with a SOPRA ellipsometer. The current
density–voltage–luminance (J–V–L) characteristics of the devices were
measured simultaneously in a nitrogen filled glove-box using a Keithley
2
400 source meter and a Keithley 6485 picoammeter equipped with a
calibrated silicon photodiode. The EL spectra of the devices were
recorded using an Ocean Optics QE65000 spectrometer.
3
.2. Thin film and device characterization
Fig. 5. Graph of maximum absorption wavelength (λmax) value calculated at
CAM-B3LYP/6-31 + G(d,p) level of theory against respective experimental one
of the studied compounds in DMSO.
On the basis of the favorable results from physical study and pho-
tophysical investigation it is clear that all the materials possess prom-
ising characteristics that can allow them to serve as a host of
phosphorescent dopants due to their appropriate HOMO/LUMO energy
level and moderate triplet energy. After estimation of the HOMO and
LUMO levels of the sulfonamide based materials, we fabricated three
PhOLEDs with the following optimized architectures (Fig. 6a): (I) ITO/
substrates were then treated with UV-ozone to remove residual organic
′
′
contamination. NPB (N,N -bis(naphthalene-1-yl)-N,N -bis(phenyl)-
benzidine), TAPC (di-{4-N,N-ditolyl-aminophenyl}cyclohexane) or
′
′′
TCTA (4,4 ,4 -tris(N-carbazolyl)-triphenylamine) were chosen as the
6

J. Lade et al.
Organic Electronics 92 (2021) 106104
Fig. 6. (a) Energy level diagram of materials used in the TPA-SA, 3CBZ-SA and 4CBZ-SA devices; (b) chemical structures of the materials used for device fabrication.
TAPC (60 nm)/TPA-SA with 12% Ir(pq)
LiF (0.5 nm)/Al (130 nm); (II) ITO/TAPC (20 nm)/TCTA (20 nm)/3CBZ-
SA with 12% Ir(pq) (acac) (20 nm)/TmPyPB (50 nm)/LiF (1 nm)/Al
130 nm); (III) ITO/NPB (40 nm)/TCTA (20 nm)/4CBZ-SA with 6% Ir
pq) (acac) (20 nm)/TmPyPB (60 nm)/LiF (1 nm)/Al (130 nm).
The current density-voltage-luminance (J-V-L), current efficiency-
2
(acac) (20 nm)/TPBi (60 nm)/
turn-on voltage. The high turin-on voltage exhibited in the TPA-SA
device might due to its shallow LUMO level, which make the energy
barrier between the emission layer and electron transporting layer
slightly higher in the TPA-SA device compared to other devices. The
high EQE of TPA-SA can be attributed to the large spectral overlap be-
2
(
(
2
2
tween the absorption of Ir(pq) (acac) and the emission of TPA-SA.
luminance-power efficiency and EQE-luminance characteristics and
the EL spectrum plots for the devices are shown in Fig. 7, and the per-
formance parameters are compiled in Table 5. All devices emit the
4. Conclusions
characteristic red light of Ir(pq)
superimposable with the PL spectrum, indicating that light emission
originates from Ir(pq) (acac). The TPA-SA device shows the highest
luminance (33484 cd/m ), maximum external quantum efficiency
15.0%) and maximum current efficiency (25.3 cd/A). However, TPA-
SA device shows lower power efficiency than 4CBZ-SA due to its higher
2
(acac), and the EL spectrum is nearly
In summary, we have designed and synthesized three novel benzo-
thiadiazine 1,1-dioxide based bipolar host materials TPA-SA, 3CBZ-SA
and 4CBZ-SA. All the three materials were used for fabrication of the red
phosphorescent OLED and amongst them TPA-SA showed good device
performance with maximum external quantum efficiency, current effi-
ciency and power efficiency of 15.0%, 25.3 cd/A, 16.0 lm/W,
2
2
(
7

J. Lade et al.
Organic Electronics 92 (2021) 106104
Fig. 7. (a) Current density-voltage-luminance (J-V-L) characteristics, (b) power and current efficiency, (c) EQE, and (d) emission spectra of the OLED devices for the
three compounds.
Table 5
Electroluminescent characteristics of the devices.
2
Material
V
turn-on (V)
L
max (cd/m )
ηext.max (%)
η
ext@1000cd/m2 (%)
η
c,max (cd/A)
η
c@1000cd/m2 (cd/A)
ηp.max (lm/W)
η
p@1000cd/m2 (lm/W)
TPA-SA
4.6
2.7
2.9
33484
21394
21429
15.0
10.0
12.7
11.9
6.1
25.3
15.2
20.6
20.1
11.3
12.6
16.0
17.6
22.5
5.8
6.0
6.2
3
4
CBZ-SA
CBZ-SA
8.3
2
V
turn-on = turn-on voltage; Lmax = maximum luminance;
η
ext.max = maximum external quantum efficiency;
η
ext@1000cd/m2 = external quantum efficiency at 1000 cd/m ;
p@1000cd/m2 = power efficiency at 1000
2
η
c.max = maximum current efficiency;
η
c@1000cd/m2 = current efficiency at 1000 cd/m ;
η
p.max = maximum power efficiency;
η
2
cd/m ; Vturn-on was obtained from the x-intercept of log (luminance) vs. applied voltage plot.
respectively. We think that the present study of benzothiadiazine-1,1-
dioxide based bipolar host materials will open a new avenue for the
molecular designing in the area of organic LED’s.
India for providing a CSIR-SRF fellowship (09/019(0013)/2020-EMR-I).
B.N.P. thanks UGC for providing SRF fellowship. A.C.C. thanks Defence
Research and Development Organization (ERIP/ER/1503212/M/01/
1
666), New Delhi, India for financial support. L.-Y. Chen thanks the
funding support from Ministry of Science and Technology (MOST) under
the grant number MOST-109-2221-E-009-166. SSP and YYD are
thankful to UPE Phase II project of Savitribai Phule Pune University for
financial assistance.
Declaration of competing interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Appendix A. Supplementary data
Acknowledgements
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.orgel.2021.106104.
J.J.L. thanks Council of Scientific & Industrial Research, New Delhi,
8

J. Lade et al.
Organic Electronics 92 (2021) 106104
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