Carbazole/triphenylamine-cyanobenzimidazole hybrid bipolar host materials for green phosphorescent organic light-emitting diodes

Article abstract of DOI:10.1016/j.orgel.2021.106090

With a view to attain balanced charge flux for higher device performance of PhOLEDs, we have used carbazole/triphenyl amine as hole transporting moiety and cyano along with benzimidazole as electron transporting core in 3-Cbz-ImdCN, 4-Cbz-ImdCN and TPA-Im

Full text of DOI:10.1016/j.orgel.2021.106090

Organic Electronics 92 (2021) 106090
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: http://www.elsevier.com/locate/orgel
Carbazole/triphenylamine-cyanobenzimidazole hybrid bipolar host
materials for green phosphorescent organic light-emitting diodes
Bhausaheb Patila, Jatin Ladea, Shian-Sung Chioub, Yen-Chia Chengb, Yi-Fan Linb,
,
,
,*
Yogesh Jadhava, Prabhakar Chettic **, Chih-Hao Changb ***, Atul Chaskara
a National Centre for Nanosciences and Nanotechnology, University of Mumbai, Mumbai, 400098, India
b Department of Electrical Engineering, Yuan Ze University, Chung-Li, 32003, Taiwan
c Department of Chemistry, National Institute of Technology, Kurukshetra, 136119, Haryana, India
A R T I C L E I N F O
A B S T R A C T
Keywords:
With a view to attain balanced charge flux for higher device performance of PhOLEDs, we have used carbazole/
triphenyl amine as hole transporting moiety and cyano along with benzimidazole as electron transporting core in
3-Cbz-ImdCN, 4-Cbz-ImdCN and TPA-ImdCN. Their thermal, photophysical and electrochemical properties have
been evaluated to shed light on structure-property-performance relationship. Good performances have been
exhibited by these bipolar host materials in green PhOLEDs with maximum external quantum efficiencies were
observed in the range of 5.3–11.5% using Ir(ppy)3 emitter. Further, 3-Cbz-ImdCN hosted orange and red PhO-
LEDs with the Ir(MDQ)2acac and Ir(piq)2acac emitters revealed the external quantum efficiencies of 5.1% and
6.3%, respectively. In all the devices pure emission was observed from dopants only which clearly implies that
the devices possess effective energy transfer from the host to the guest.
Donor-acceptor hybrid
Cyanobenzimidazole
Carbazole
Triphenylamine
Green phosphorescent organic light-emitting
diodes
1. Introduction
triplet energy than that of emitter to ensure exothermic energy transfer
and well confined triplet exciton formation on the dopant, good energy
Organic light-emitting diodes (OLED) perceived a massive heed in
the lighting, displays and printed electronic industries owing to their
low power consumption and roll to roll printing [1–5]. Hence, current
OLED research efforts are focused on enhancement of device stability
and efficiency with simplified device architecture. In conventional
OLED, light is emitted by singlet excitons leading to internal quantum
efficiency (IQE) up to 25% as exciton possess singlet: triplet ratio as 1:3.
The significant development in OLED research was design and devel-
opment of phosphorescent organic light-emitting diodes (PhOLEDs).
The electrophosphorescence can proficiently utilize both singlet as well
as triplet excitons to reach internal quantum efficiency (IQE) to ideal
level i.e., 100% [6–8]. The transition metal-centred phosphorescent
emitter used for this purpose convert singlet excitons to triplet excitons
through inter system crossing by means of spin-orbital interaction.
Albeit, phosphorescent emitter needs to be doped in suitable host ma-
terial to suppress the aggregation quenching and triplet-triplet annihi-
lation of emitters [9]. Nevertheless, the host materials must have higher
level alignment with the neighboring functional layers, it should pro-
mote balanced charge fluxes, and good thermal and morphological
stabilities to prevent morphological changes of the amorphous organic
layer upon heating [10–14]. Initially the unipolar hosts, viz.
hole-transporting material comprising electron-donating moiety and
electron-transporting material encompassing electron-withdrawing
moiety have been employed for hosting phosphorescent dopant. Un-
fortunately, they promoted unbalanced charge flux which leads to nar-
row charge recombination zone as recombination zone moves near to
hole transporting layer (HTL) or electron transporting layer (ETL)
resulted into exciton diffusion and triplet-triplet annihilation. Owing to
this researcher focused their attention on development of tailor-made
bipolar host materials by wise selection and proper linking of hole-
and electron-transporting moieties which could endorse balanced
charge flux to accomplish highly efficient phosphorescent organic
light-emitting diodes (PhOLEDs) [15–25]. The photophysical, electro-
chemical, and thermal properties of bipolar host materials could be
* Corresponding author.,
** Corresponding author.
*** Corresponding author.
E-mail addresses: chetti@nitkkr.ac.in (P. Chetti), chc@saturn.yzu.edu.tw (C.-H. Chang), achaskar25@gmail.com, achaskar@nano.mu.ac.in (A. Chaskar).
https://doi.org/10.1016/j.orgel.2021.106090
Received 10 January 2021; Received in revised form 22 January 2021; Accepted 25 January 2021
Available online 12 February 2021
1566-1199/© 2021 Elsevier B.V. All rights reserved.

B. Patil et al.
Organic Electronics 92 (2021) 106090
tuned by changing linking spacer, topology, and donor: acceptor ratio
[26–28].
efficiency (EQE) of 23.8% for fluorescent and 26.7% for PhOLEDs [58].
Liao L.-S. et al. demonstrated carbazole based novel host compounds i.e.,
CNPhCz and DCNPhCz which hold elevated energies and appropriate
HOMO/LUMO energy states, thus CNPhCz hosted devices revealed
maximum EQE of 17.1% and 24.4%, respectively [59]. Recently, same
group reported dibenzo[a,c]phenazine materials by introducing cyano
groups into dibenzo[a,c]phenazine and showed progressive perfor-
mance of device with the maximum EQE of 23.24% [60]. Although, the
host materials involving either heterocycles or cyano groups are now
much familiar, but the grouping of heterocycles with cyano as new
n-type building blocks are rarely reported [61]. Since last couple of
decades researchers across the globe put forward their efforts to develop
highly efficient PhOLEDs which led to the development of novel bipolar
host materials with reasonable thermal stability and high triplet energy
showing efficiency (EQE) up to 30% [62].
Hitherto, abundant classes of bipolar host materials have been re-
ported encompassing carbazole, indolocarbazole, and aryl amine moi-
eties as hole-transporting units owing to their excellent hole-mobilities
and favorable triplet energy (ET) [29–33] while oxadiazole, benzimid-
azole, pyridine, phenanthroline, phosphine oxide, phosphine sulfide,
etc. as electron-transporting units [34–47]. Notably, CBP is the most
frequently used carbazole-based host having a triplet energy (ET) of
2.56 eV. While, its structural analog mCP exhibited elevated triplet (ET)
of 2.9 eV which promoted it extensive use as host for all color emitters
for phosphorescent OLEDs [48–54]. More recently, Z. Gao et al. reported
diphenylquinoxaline-based host materials M1 and M2 for red phosphors
whereas, Su and coworkers developed new bipolar hosts o-PQPC,
m-PQPC, and p-PQPC which displayed maximum current of 21.9 cd/A,
power efficiency of 15.4 lm Wꢀ 1, and external quantum efficiency of
12.2% [55,56].
Herein, we wish to report novel bipolar hosts 3-Cbz-ImdCN, 4-Cbz-
ImdCN, and TPA-ImdCN comprising carbazole/triphenylamine as hole-
transporting functional unit (donor) and cyanobenzimidazole as an
electron-transporting moiety (acceptor). When cyano or benzimidazole
core used independently in bipolar hybrid their electron mobility failed
to match the hole mobility of donor core. Therefore, here we used
combination of two to enhance the electron mobility of bipolar host. A
concise strategy was applied to tune the photophysical, thermal and
Nevertheless, cyano moiety is also extensively used n-type electron-
transporting unit in the design and synthesis of bipolar host molecules.
Notably, Zhang and co-workers reported a bipolar unit CBP-CN which
showed enhanced activity for red and green PhOLEDs as compare to
commonly used green host CBP [57]. Lee et al. used a CN as a ubiquitous
unit for their molecules and they exhibited maximum external quantum
Scheme 1. Synthetic pathway and chemical structures of bipolar host materials 3-Cbz-ImdCN, 4-Cbz-ImdCN, and TPA-ImdCN.
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B. Patil et al.
Organic Electronics 92 (2021) 106090
electrochemical properties, and shed light on structure-property-
performance relationship. Eventually these molecules were used for
fabrication of green PhOLEDs and the external quantum efficiencies of
11.5%, 11.4% and 5.3% were observed for 3-Cbz-ImdCN, 4-Cbz-ImdCN,
and TPA-ImdCN hosted devices, respectively. Further, 3-Cbz-ImdCN
hosted orange and red PhOLED devices were also fabricated and they
revealed the external quantum efficiencies of 5.1% and 6.3%,
respectively.
of 3-Cbz-ImdCN, 4-Cbz-ImdCN, and TPA-ImdCN in dichloromethane
(DCM) at room temperature are displayed in Fig. 2. All these molecules
exhibit absorption peak around 325–375 nm owing to π-π* transition.
These long wavelength
π-π* electronic transition is attributed to
extended conjugation. Further, the red shift absorption in polar solvent
π
as compare to nonpolar solvent (Fig. 3) suggest that the Franck-Condon
excited states are subject to large changes in dipole moment with respect
to the ground state. The PL spectra of these molecules are observed
around 392–458 nm. The red shift emission in polar solvent (Fig. 3)
implies involvement of rapid photo-induced electron transfer (PET)
which leads to a large change in the dipole moment in the excited state.
Apparently, the triplet energies (ET) of 2.71, 2.62 and 2.46 eV were
determined for 3-Cbz-ImdCN, 4-Cbz-ImdCN, and TPA-ImdCN from the
highest energy peaks of the phosphorescence spectra (Fig. 4).
2. Results and discussion
2.1. Synthesis
Scheme 1 depicts the synthetic pathway we have followed to artic-
ulate the bipolar host materials 3-Cbz-ImdCN, 4-Cbz-ImdCN, and TPA-
ImdCN. 3-Formyl-N-phenylcarbazole synthesized from N-phenyl-
carbazole using Vilsmeier–Haack reaction [63,64], on condensation
2.4. Electrochemical properties
with
3,4-diaminobenzonitrile
offered
2-(9-phenyl-9-
Cyclic voltammetry measurements of 3-Cbz-ImdCN, 4-Cbz-ImdCN,
and TPA-ImdCN were performed using a CHI 619B potentiostat. 3-Elec-
trode cell system encompassing a glassy carbon electrode as working
electrode, silver/silver chloride (Ag/AgCl) as reference electrode and a
platinum wire as counter electrode was used. A ferrocene (Fc) was used
as an internal standard for cyclic voltammetry while tetrabutylammo-
nium hexafluorophosphate (TBAPF6 0.1 M) in DCM and tetrabuty-
lammonium perchlorate (TBAP, 0.1 M) in THF were used as the
supporting electrolyte for oxidation and reduction scan, respectively.
The oxidation potential arises from carbazole/triphenyl amine while
reduction potentials arise from cyanobenzimidazole (Fig. 5). The irre-
versible oxidations were observed owing to dimerization or electro-
polymerization of carbazole/triphenyl amine. The lower oxidation
potential detected for 3-Cbz-ImdCN as compare to 4-Cbz-ImdCN was
H-carbazol-3-yl)-1H-benzo[d]imidazole-5-carbonitrile (3-Cbz-ImdCN)
in 72% yield. Carbazole on reaction with 4-fluorobenzaldehyde in
presence of potassium tert-butoxide offered 4-(9H-carbazol-9-yl) benz-
aldehyde which on condensation with 3,4-diaminobenzonitrile yielded
2-(4-(9H-carbazol-9-yl) phenyl)-1H-benzo[d]imidazole-5-carbonitrile
(4-Cbz-ImdCN) in 68%. Triphenylamine of Vilsmeier–Haack reaction
gave 4-(diphenylamino) benzaldehyde which on condensation with 3,
4-diaminobenzonitrile furnished 2-(4-(diphenylamino) phenyl)-1-
H-benzo[d]imidazole-5-carbonitrile (TPA-ImdCN) in 78%. All the
products were purified by column chromatography and final three
molecules were further purified by sublimation for device fabrication.
The chemical structures of all the synthesized molecules were confirmed
by 1H NMR, 13C NMR spectra and mass spectral data.
assigned to higher degree of π-conjugation. Apparently, the reduction
on-set of 4-Cbz-ImdCN is at higher potential as compare to 3-Cbz-
ImdCN, implying the electron distribution on their lowest unoccupied
molecular orbital (LUMO) is solely confined on the cyanobenzimidazole.
2.2. Thermal analysis
Differential scanning calorimetry (DSC) and thermogravimetric
analysis (TGA) were employed to characterize the thermal properties of
3-Cbz-ImdCN, 4-Cbz-ImdCN, and TPA-ImdCN (Fig. 1, Table 1). The glass
transition temperature (Tg) of 89 ◦C and 78 ◦C were observed for 4-Cbz-
ImdCN and TPA-ImdCN, respectively. Whereas decomposition temper-
ature for 3-Cbz-ImdCN, 4-Cbz-ImdCN and TPA-ImdCN were found at
The HOMO energy levels were found out from the first oxidation peak of
onset
onset potential of (E
) by the mathematical equation of EHOMO = -e
ox
onset
(E
+ 4.4) [67,68] for 3-Cbz-ImdCN, 4-Cbz-ImdCN, and TPA-ImdCN
ox
is as ꢀ 5.65, ꢀ 5.71 and ꢀ 5.74 eV respectively. Similarly, the LUMO
energy levels were intended by the first reduction peak of onset potential
onset
onset
+ 4.4) [69] to be as
ox
◦
◦
◦
(E
) from the calculation by ELUMO = -e (E
390 C 218 C and 354 C, correspondingly. The high values of glass
transition (Tg) and decomposition temperature (Td) implies that they are
proficient of enduring vacuum thermal sublimation and suppress an
aggregation formation with preserving amorphous nature of thin film
[65,66].
red
ꢀ 3.31 eV, ꢀ 3.32 eV and ꢀ 3.68 eV. The band gap between the LUMO
and HOMO was found to be 2.28 eV, 2.39 eV, and 2.06 eV for these
molecules.
2.5. Theoretical studies
2.3. Photophysical properties
Molecular simulations for the reported molecules 3-Cbz-ImdCN, 4-
Cbz-ImdCN, and TPA-ImdCN were carried out using DFT and TDDFT
Electronic absorption (UV–Vis) and photoluminescence (PL) spectra
Fig. 1. TGA and DSC thermograms of 3-Cbz-ImdCN, 4-Cbz-ImdCN, and TPA-ImdCN observed at 10 ◦C minꢀ 1 heating rate.
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B. Patil et al.
Organic Electronics 92 (2021) 106090
Table 1
Physical properties of 3-Cbz-ImdCN, 4-Cbz-ImdCN and TPA-ImdCN.
λabs.a (nm)
λfluor.b (nm)
ETc (eV)
HOMO (eV)
LUMOd (eV)
Ege (eV)
Tdf (oC)
Tgg (oC)
3-Cbz-ImdCN
4-Cbz-ImdCN
TPA-ImdCN
344
356
366
392
434
458
2.71
2.62
2.46
ꢀ 5.65
ꢀ 5.71
ꢀ 5.74
ꢀ 3.31
ꢀ 3.32
ꢀ 3.68
2.28
2.39
2.06
390
218
354
–
89
78
a
Absorption maximum.
Fluorescence maximum.
Triplet energy at 77 K.
b
c
d
e
f
LUMO is find out via eq. LUMO = HOMO + Eg.
Estimated from the absorption onset;
Td, thermal decomposition temperature;
Tg, glass transition temperature.
g
Fig. 2. Room-temperature absorption and emission (PL) spectra of 3-Cbz-ImdCN, 4-Cbz-ImdCN, and TPA-ImdCN in DCM.
methods with G16 quantum chemical package [70,71] Ground state
molecular geometries are obtained at the B3LYP level with the standard
basis set 6-31G (d, p). The calculated vibrational spectrum in all the
optimized structures has no imaginary frequencies, which indicates that
all the optimized structures are located at the minimum point of the
potential energy surface. Electronic absorption properties were carried
out using the TDDFT (TD-B3LYP/6-31G (d, p) methodologies for the
optimized geometries and frontier molecular orbitals responsible for the
excitations leading to the absorption maximum, were pictorially visu-
alized from the population analysis.
excited state geometries of these molecules were optimized at TD-B3LYP
method with 6–31G (d, p) basis set. The dihedral angle between
carbazole and phenyl ring in ground and excited state for 3-Cbz-ImdCN
and 4-Cbz-ImdCN were found to be ꢀ 57.95, ꢀ 51.66 and ꢀ 49.29,
ꢀ 89.71, respectively. Whereas, in TPA-ImdCN those were observed as
ꢀ 33.07 and ꢀ 94.63, correspondingly (shown in Table 3). However, the
dihedral angle between donor carbazole/triphenyl amine and acceptor
cyanobenzimidazole in ground and excited state for 3-Cbz-ImdCN, 4-
Cbz-ImdCN, and TPA-ImdCN were found as ꢀ 171.18, ꢀ 08.00, ꢀ 07.56
and ꢀ 177.31, ꢀ 0.01, ꢀ 0.51, respectively. The difference between
dihedral angle was caused due to substitution pattern as well as steric
effect. The twisting between these three molecules and their non-planar
confirmations can efficiently hinder the unnecessary intramolecular
To get more insight in to the absorption energies, we have carried out
TDDFT calculations for the reported molecules. The calculated absorp-
tion energies along with major transitions are tabulated in Table 2. The
experimental absorption (λexp) of molecules 3-Cbz-ImdCN, 4-Cbz-
ImdCN, and TPA-ImdCN are 344 nm, 356 nm and 366 nm respectively;
while their calculated absorption energies are 330 nm, 379 nm and 372
nm respectively, which indicates a fair agreement of calculated results
with experimental one within the limitation. In all the three molecules
the excitations are from HOMO to LUMO. From FMO analysis, the
HOMO orbital of 4-Cbz-ImdCN is mainly distributed on carbazole where
as in 3-Cbz-ImdCN it is localised at carbazole with little contribution
from cyanobenzimidazole. Further in TPA-ImdCN it is situated at tri-
phenyl as well as benzimidazole. On the other hand, the LUMO of 4-Cbz-
ImdCN is populated on cyanobenzimidazole and phenyl ring of N-
phenyl carbazole while in 3-Cbz-ImdCN it is situated on cyanobenzi-
midazole and little on carbazole. However, in TPA-ImdCN LUMO energy
level is distributed on cyanobenzimidazole and phenyl ring linked to it
(Fig. 6). The clear spatial separation of HOMO and LUMO energy levels
in 4-Cbz-ImdCN implies that the HOMO-LUMO excitation would shift
the electron density distribution from carbazole to cyanobenzimidazole
which polarize excited state.
charge transfer and
π-π stacking, which are a typical requirement for
efficient OLEDs devices.
2.6. Reorganization energies
According to hopping model, charge transfer reaction is self-
exchange electron transfer reaction between neutral and radical cation
(hole transfer) shown in equation 1
M + M+→M+ + M
(1)
Marcus expresses the rate constant (K) for charge transfer reaction
between two molecules shown in equation (2) as [72,73].
(
)
(
)
4
π2
h
K =
H2(4
π
λkT)ꢀ 12exp ꢀ λ
(2)
/
4kT
Here, H is charge transfer integral matrix between neighboring
molecules [74,75], λ is reorganization energy, k is Boltzmann constant
and T is the temperature. Total reorganization energy is sum of internal
and external reorganization energies, however external λ is negligible as
Further, to get the geometrical parameters in its excited states,
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B. Patil et al.
Organic Electronics 92 (2021) 106090
Fig. 3. Room-temperature absorption and emission (PL) spectra of 3-Cbz-ImdCN, 4-Cbz-ImdCN, and TPA-ImdCN in DCM and toluene.
compared to internal λ. The calculation of internal λ is done by using the
neutral molecule Eꢀ (M0)) and energy of anion molecule in its lowest
energy geometry (Eꢀ (Mꢀ )), λ4 is energy difference between the energy
of neutral molecule (obtained at optimized anionic state E0(Mꢀ )) and
energy of neutral molecule in its lowest energy geometry (E0(M0))
(shown in equation (4)).
following equations [76,77].
[
]
λh = λ1 + λ2 = [E+(M0) ꢀ E+(M+)] + E0(M+) ꢀ E0(M0)
(3)
(4)
[
]
λe = λ3 + λ4 = [Eꢀ (M0) ꢀ Eꢀ (Mꢀ )] + E0(Mꢀ ) ꢀ E0(M0)
The calculated hole (λh) and electron (λe) reorganization energy for
the reported molecules are shown in Table 4. As evident from equation
(2), smaller reorganization energy led to the higher rate constant value
which in turns increases the charge transport properties in the molecule.
From Table 4, 4-Cbz-ImdCN is showing smallest λh value of 124 meV,
while 3-Cbz-ImdCN and TPA-ImdCN are showing 181 meV and 128 meV
respectively. Similar is the case for λe values, 4-Cbz-ImdCN is showing
313 meV and TPA-ImdCN being the highest (468 meV). Moreover, in all
Here, λ1 is the energy difference between the energy of cation (ob-
tained at ground state neutral molecule E+(M0)) and energy of cation
molecule in its lowest energy geometry (E+(M+)), λ is the energy dif-
ference between the energy of neutral molecule (obt2ained at optimized
cationic state E0(M+)) and energy of a neutral molecule in its lowest
energy geometry (E0(M0)) (shown in equation (3)). Similarly, λ3 is en-
ergy difference between the energy of anion (obtained at ground state
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B. Patil et al.
Organic Electronics 92 (2021) 106090
(TPD) which is typical hole transport material (λh = 290 meV) [78]. In
conclusion the theoretical calculations show that the studied isomers are
showing smaller hole reorganization energy than standard molecule
reported in literature. Also, the synthesized isomers are showing very
small hole reorganization energy suggesting these isomers are better
candidate for hole transport materials.
2.7. OLED device
In the context of promising physical properties, we decided to
investigate the bipolar host properties of newly synthesized carbazole/
triphenylamine-cyanobenzimidazole hybrid materials (3-Cbz-ImdCN,
4-Cbz-ImdCN, and TPA-ImdCN) for green PhOLEDs. We fabricated de-
vice with typical multi-layer architecture of ITO/TAPC doped with 20 wt
% MoO3 (10 nm)/TAPC (15 nm)/TCTA (5 nm)/Host doped with 8 wt%
Ir(ppy)3 (30 nm)/B3PyMPM (50 nm)/LiF (0.8 nm)/Al (120 nm). The
thicknesses of the p-doped hole-injection layer (HIL), HTLs, and ETLs in
the aforementioned devices were cautiously adjusted to improve the
radiation output and carrier balance. 3-Cbz-ImdCN was employed as a
host for device A, 4-Cbz-ImdCN for device B, and TPA-ImdCN for device
C while heavy-metal complex Ir(ppy)3 was used as an emitter. The p-
type conductivity dopant MoO3 was introduced into 1,1-bis[(di-4-tolyla-
mino)phenyl]cyclohexane (TAPC) to enhance conductivity and hole
injection, while intrinsic TAPC was used to prevent excitons quenching
by conductivity dopant [79]. 4,4′,4′′-tri(N-carbazolyl)triphenylamine
(TCTA) was used as the HTL [80], 4,6-Bis(3,5-di-3-pyridylphenyl)-2-me-
thylpyrimidine (B3PyMPM) was used as the ETL while LiF and Al were
used as the electron injecting layer and cathode, correspondingly.
Furthermore, TCTA was inserted between TAPC and the emitting layer
(EML) to reduce the excessive energy barrier between host material and
TAPC. The chemical structures of the materials used for device fabri-
cation and device architecture employed are shown in Fig. 7 while EL
spectra of devices, current density-voltage-luminescence characteristics,
external quantum efficiency, luminance efficiency, and power efficiency
versus luminance are revealed in Fig. 8. All the device performances are
summarized in Table 5.
Fig. 4. Room-temperature emission (PL) and low temperature Phosphores-
cence (Phos.) spectra of 3-Cbz-ImdCN, 4-Cbz-ImdCN, and TPA-ImdCN
in toluene.
The EL spectra of 3-Cbz-ImdCN, 4-Cbz-ImdCN, and TPA-ImdCN,
hosted devices showed pure emission from green dopant Ir(ppy)3 only
(Fig. 8a). The pure emission suggests that all tested devices possess
effective energy transfer from the host to the guest. Furthermore, the
diffusion of high-energy excitons located in the EML to adjacent layers
was significantly reduced. Current density-voltage-luminescence (J-V-L)
characteristics of devices A, B, and C are revealed in Fig. 8b. Clearly,
device C exhibits a higher current density than those of devices A and B,
which might the result of the relatively lower HOMO level of TPA-
ImdCN. Hence, the turn-on voltages of devices A and B are slightly
higher than that of device C. Nevertheless, the maximum luminance of
device A with 3-Cbz-ImdCN shows the highest value of 64303 cd mꢀ 2
,
which is about 2.5 times that of device C and about 1.4 times that of
device B. The much higher luminance obtained in device A illustrates
that 3-Cbz-ImdCN has great stability at high current density. Notably,
external quantum efficiencies and luminance/power efficiencies of the
three devices are shown in Fig. 8c and d, respectively. From Fig. 8 and
Table 5, the peak efficiencies accomplished are 11.5%, 39.0 cd Aꢀ 1, 43.3
lm Wꢀ 1 for device A, 11.4%, 39.3 cd Aꢀ 1, 46.8 lm Wꢀ 1 for device B, and
5.3%, 18.3 cd Aꢀ 1, 17.9 lm Wꢀ 1 for device C. Furthermore, at a high
luminance of 100 cd mꢀ 2, the external quantum efficiencies of devices A,
B, and C respectively maintained efficiency levels of 10.8%, 10.9%, and
5.3%; hence, they exhibited efficiency drops of only 6%, 4%, and 0%
from their respective peak values to those recorded at 100 cd mꢀ 2. All
tested devices retained high efficiency at high luminance levels and
showed negligible efficiency roll-off. Overall, the tested green PhOLEDs
presented great EL performance, which raises the feasibility of using the
carbazole/triphenylamine-cyanobenzimidazole hybrid bipolar host
materials in commercial applications.
Fig. 5. Cyclic voltammogram of 3-Cbz-ImdCN, 4-Cbz-ImdCN, and TPA-ImdCN.
Table 2
Calculated absorption in nm (λ), strength of oscillator (f), main transitions (MT),
and %weight (Ci) at B3LYP/6-31G (d, p) level.
Name
λCal
f
MT
%Ci
3-Cbz-ImdCN
4-Cbz-ImdCN
TPA-ImdCN
330
379
372
0.653
0.456
0.948
H→L
H→L
H→L
90
98
98
the three molecules λh< λe, suggesting the studied molecules are better
candidate for hole transport material rather than electron transport
materials. Also, The λh calculated for the isomers are smaller than that of
N, N′-diphenyl-N, N′-bis(3-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine
Further, 3-Cbz-ImdCN hosted orange and red PhOLEDs with the
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Organic Electronics 92 (2021) 106090
Fig. 6. Frontier molecular orbitals of 3-Cbz-ImdCN, 4-Cbz-ImdCN, and TPA-ImdCN calculated with DFT on a B3LYP/6-31G (d, p) level.
architecture of ITO/TAPC doped with 20 wt% MoO3 (10 nm)/TAPC (30
nm)/3-Cbz-imdCN with 8 wt% Ir(piq)2acac or 2 wt% Ir(MDQ)2acac (30
nm)/Bebq2 (50 nm)/LiF (0.8 nm)/Al (120 nm) were also fabricated
(Fig. 9). Fig. 10 shows the EL characteristics of the tested devices, and
Table 6 summarizes the corresponding numerical data. The EL spectra of
OLEDs showed emission from orange and red dopants (Fig. 10a) which
suggests that both the devices possess effective energy transfer from the
host to the guest. Nevertheless, the EL spectrum of the orange OLED
shows a small BeBq2 emission peak, indicating that the exciton forma-
tion zone is located near the EML/ETL interface. This phenomenon
might be the result of the low doping concentration of Ir(MDQ)2acac.
Current density-voltage-luminescence (J-V-L) characteristics of orange
and red devices are shown in Fig. 10b. As indicated, the low turn-on
voltage of 2.7 V obtained in the red device (Ir(piq)2acac) shows that
3-Cbz-ImCN has excellent carrier transport capability. The efficiencies
curves of the orange and red devices are shown in Fig. 10c and d. The
orange and red devices exhibited the maximum external quantum effi-
ciencies of 5.1% and 6.3%, respectively (Table 6). These results indicate
that 3-Cbz-ImdCN has application potential as the host material of or-
ange and red PhOLEDs.
3. Materials and methods
All the chemical reagents were brought from commercial resources.
All the reactions were performed in a Borosil round bottom flask and
monitored by TLC, aluminum plates (0.25 mm, E. Merck) precoated with
silica gel (Merck 60 F-254). TLC plates were envisaged under a short-as
well as long wavelength UV lamp. The reactions were escorted in open
air atmosphere in presence of solvents like MeOH, EtOH and DMF.
Yields mention to spectroscopically (1H, 13C NMR) homogeneous ma-
terial found after column chromatography implemented on silica gel
(100–200 mesh size) provided by S.D. Fine and Sigma Aldrich Chemicals
Limited, India. 1H and 13C NMR were recorded in CDCl3 as well as DMSO
solution with an Agilent 300/400 MHz spectrometer. Chemical shift (δ)
values were quoted in ppm, comparative to SiMe4 (δ = 0.0) as an in-
ternal standard. The number of protons (n) for a specified resonance is
indicated by nH. Peak multiplicities are labeled by the following ab-
breviations: s, singlet; d, doublet; t, triplet; q, quartet; m, multiplet; dd,
doublet of doublet; J, coupling constant in Hz (S6). Melting points were
recorded on a standard melting point apparatus from Gautam lab
equipment, Mumbai and are uncorrected. Electronic absorption spectra
values were note down on UV–Vis–NIR spectrophotometer. Emission
spectra were recorded on Varian Cary-Eclipse fluorescence spectro-
photometer. Thermal study was dignified on PerkinElmer system with a
7

B. Patil et al.
Organic Electronics 92 (2021) 106090
Table 3
Dihedral angles of the molecules in ground state and excited state.
Name
Ground State
Excited State
3-Cbz-ImdCN
4-Cbz-ImdCN
TPA-ImdCN
clear that all the materials possess promising characteristics that can
allow them to serve as a host of phosphorescent dopants due to their
appropriate HOMO/LUMO energy level.
Table 4
Calculated hole (λh) and electron (λe) reorganization energies (in meV).
Name
λh
λe
3-Cbz-ImdCN
4-Cbz-ImdCN
TPA-ImdCN
181
124
128
313
377
468
3.1. Preparation of 9-phenyl-9H-carbazole-3-carbaldehyde (2)
POCl3 (2.30 mL, 24.7 mmol) was added dropwise to DMF (14.6 mL,
189.0 mmol) under stirring at 0 ◦C. The resulting mixture was stirred at
0 ◦C for 4 h and then warmed to room temperature. To this mixture 9-
phenyl-9H-carbazole 1 (5.0 g, 20.5 mmol) was added. The reaction
mixture was heated to 90 ◦C and stirred for 7 h. The solution was poured
into an ice-bath, neutralized with sodium bicarbonate and extracted
with DCM (3 × 40 mL). The combined organic layer was dried with
anhydrous Na2SO4 and concentrated under reduced pressure. The res-
idue obtained was purified with column chromatography on 100: 200
mesh silica gel by using 20% ethyl acetate in n-hexane as the eluent to
afford compound 2 as a white solid (4.40 g, 79%).
20 ◦C/min heating rate and 20 mL/min N2 flow. Electrochemical read-
ings were carried out using CH electrochemical analyzer with a 100 mV/
s scan rate. HOMO and LUMO energy values were calculated from the
oxidation and reduction peak potentials.
1. Thin film and device fabrication
The indium-tin-oxide (ITO)-coated glass substrates (sheet resistance
~15 Ω sqꢀ 1) was cleaned in an ultrasonic bath by deionized water and
acetone in series for 20 min, followed by warmed in methanol for about
10 min. The washed ITO glass substrate was then used with UV-ozone to
eliminate residual organic impurities. Organic materials were purchased
from Shine Materials Technology. All organic materials were subjected
to temperature gradient sublimation. EL device was equipped by vac-
uum deposition in a base pressure of 1 × 10ꢀ 6 torr. The hole-
transporting, electron transporting, and emitting layers were placed
sequentially at a rate of 0.1 nm/s. The cathode, 0.8-nm-thick LiF, and
120-nm-thick Al was then set down at a rate of 0.02 nm/s and 0.5 nm/s,
respectively, through shadow mask (2 × 2 mm2) without breaking the
vacuum.
mp 108–110 ◦C; 1H NMR (500 MHz, CDCl3): δ = 7.36–7.41 (distorted
q, J = 8.0, 7.0 Hz, 2H), 7.43–7.49 (m, 2H), 7.52–7.56 (m, 3H), 7.65 (t, J
= 8.0, 7.5 Hz, 2H), 7.94–7.96 (dd, J = 9.0, 1.5 Hz, 1H), 8.21 (d, J = 8.0
Hz, 1H), 8.68 (s, 1H), 10.12 (s, 1H); 13C NMR (125 MHz, CDCl3): δ =
110.09, 110.40, 120.65, 121.18, 123.22, 123.58, 123.82, 126.98,
127.18, 127.44, 128.32, 129.45, 130.11, 136.71, 141.87, 144.50,
191.71; ESI-MS: m/z 272.02 [M + H]+.
3.2. Preparation of 4-(9H-carbazol-9-yl) benzaldehyde (5)
To a stirred solution of carbazole 3 (6.30 g, 37.7 mmol) in anhydrous
DMF (150 mL) was added potassium tert-butoxide (4.23 g, 37.7 mmol) at
room temperature under nitrogen atmosphere. The reaction mixture was
heated to 110 ◦C and then a solution of 4-fluorobenzaldehyde 4 (4.04
mL g, 37.7 mmol) in anhydrous DMF (10 mL) was added dropwise into
the mixture over 50 min. The mixture was kept at 110 ◦C for 36 h. Then
the mixture was cooled to room temperature and poured into 500 mL ice
water. The precipitate obtained was filtered, dried and recrystallized by
using acetone/water (v/v = 9:1) system to afford compound 5 as a pale
yellow solid (6.15 g, 60%).
2. Thin film and device characterization
The thicknesses of organic layers were acquired with a SOPRA
ellipsometer. Current density-voltage-luminance (J-V-L) characteristics
of the devices were performed in a glove-box using two Keithley 2401
source measure units equipped with a calibrated Si photodiode. EL
spectra of the devices were recorded using an Ocean Optics USB2000
spectrometer. On the basis of the favorable results from photophysical
investigation, thermal, electrochemical, DFT and physical study, it is
mp 156–158 ◦C; 1H NMR (500 MHz, CDCl3): δ = 7.33 (t, J = 7.0 Hz,
8

B. Patil et al.
Organic Electronics 92 (2021) 106090
Fig. 7. (a) Chemical structures of the materials used; (b) device architecture employed in this work.
2H), 7.44 (t, J = 8.0, 7.0 Hz, 2H), 7.51 (d, J = 8.0 Hz, 2H), 7.80 (d, J =
7.5 Hz, 2H), 8.15 (t, J = 7.5, 7.0 Hz, 4H), 10.12 (s, 1H); 13C NMR (125
MHz, CDCl3): δ = 109.75, 120.49, 120.80, 123.97, 126.27, 126.84,
131.37, 134.65, 140.07, 143.41, 190.95; ESI-MS: m/z 272.03 [M + H]+.
1H NMR (500 MHz, DMSO): δ = 13.54 (s, 1H), 9.14 (s, 1H),
8.37–8.31 (m, 1H), 8.27–8.21 (m, 1H), 8.14 (s, 1H), 8.04–7.84 (m, 1H),
7.78–7.69 (m, 2H), 7.64–7.59 (m, 2H), 7.56–7.49 (m, 2H), 7.43–7.35
(m, 2H), 7.30–7.25 (m, 1H), 7.22–7.13 (m, 1H); 13C NMR (125 MHz,
DMSO): 158.0, 155.8, 141.9, 141.4, 136.8, 130.8, 128.6, 127.5, 127.3,
127.1, 126.0, 125.8, 123.6, 123.1, 121.6, 121.3, 121.2, 120.6, 120.1,
119.3, 110.7, 110.5, 104.1; ESI-MS: m/z 384.4418 [M + H]+.
3.3. Preparation of 4-(diphenylamino) benzaldehyde (7)
To a stirred solution of triphenylamine 6 (8.0 g, 32.6 mmol) in DMF
(16 mL) was added dropwise POCl3 (15.2 mL, 163.0 mmol) at 0 ◦C. After
completion of addition, the reaction mixture was warmed to room
temperature and then heated to 45 ◦C for 2 h. The solution was poured
into an ice-bath and neutralized with sodium carbonate. The resulting
precipitate was filtered, dried and recrystallized from ethanol to afford
compound 7 as a pale yellow solid (8.20 g, 92%).
3.5. Preparation of 2-(4-(9H-carbazol-9-yl) phenyl)-1H-benzo[d]
imidazole-5-carbonitrile(4-Cbz-ImdCN)
To a stirring solution of 3,4-diaminobenzonitrile (8) (1.5 gm) in
MeOH, (100 mL) solution of 4-(9H-carbazol-9-yl) benzaldehyde (5) was
added dropwise through a dropping funnel. After complete addition the
reaction mass was refluxed for 10–12 h. After completion of the reaction
as indicated by TLC, the reaction mixture was cooled and poured into
water. The precipitate obtained was filtered, washed with water and
dried. The mixture was purified with column chromatography on 100:
200 mesh silica gel by using petroleum ether: DCM (9:1) to afford the
pure product 4-Cbz-ImdCN as a pale yellow solid (0.96 g, 68%).
1H NMR (500 MHz, DMSO): δ = 13.69 (s, 1H), 8.52 (d, J = 10 Hz,
1H), 8.33–8.21 (m, 3H), 8.11 (d, J = 10 Hz, 1H), 7.90 (d, J = 10 Hz, 1H),
7.81 (d, J = 10 Hz, 1H), 7.67–7.63 (m, 1H), 7.54 (d, J = 10 Hz, 1H),
7.50–7.46 (m, 2H), 7.40–7.31 (m, 3H), 7.15 (t, J = 10 & 15 Hz, 1H); ESI-
MS: m/z 384.4419 [M + H]+.
mp 120–122 ◦C; 1H NMR (500 MHz, CDCl3): δ = 7.02 (d, J = 8.5 Hz,
2H), 7.16–7.18 (m, 6H), 7.34 (t, J = 7.5 Hz, 4H), 7.68 (d, J = 8.0 Hz,
2H), 9.81 (s, 1H); 13C NMR (125 MHz, CDCl3): δ = 119.34, 125.09,
126.30, 129.11, 129.71, 131.28, 146.15, 153.35, 190.40; ESI-MS: m/z
274.13 [M + H]+.
3.4. Preparation of 2-(9-phenyl-9H-carbazol-3-yl)-1H-benzo[d]
imidazole-5-carbonitrile (3-Cbz-ImdCN)
To a stirring solution of 3,4-diaminobenzonitrile (8) (1.5 gm) in
MeOH, (100 mL), solution of 9-phenyl-9H-carbazole-3-carbaldehyde (2)
was added dropwise through a dropping funnel. After complete addition
the reaction mass was refluxed for 10–12 h. After completion of the
reaction, the reaction mixture was allowed to cool at room temperature
and poured into water. The precipitate obtained was filtered, washed
with water and dried. The solid was purified with column chromatog-
raphy on 100: 200 mesh silica gel by using Petroleum ether: DCM (9:1)
to afford the pure product 3-Cbz-ImdCN as a pale yellow solid (1.02 g,
72%).
3.6. Preparation of 2-(4-(diphenylamino) phenyl)-1H-benzo[d]
imidazole-5-carbonitrile (TPA-ImdCN)
To a stirring solution of 3,4-diaminobenzonitrile (8) (1.5 gm) in
MeOH (100 mL), solution of 4-(diphenylamino) benzaldehyde (7) was
added dropwise through a dropping funnel. After complete addition the
reaction mass was refluxed for 10–12 h. After completion of the
9

B. Patil et al.
Organic Electronics 92 (2021) 106090
Fig. 8. EL characteristics of the phosphorescent OLEDs: (a) normalized EL spectra; (b) current density-voltage-luminance (J-V-L) curves; (c) external quantum ef-
ficiency vs luminance; (d) luminance efficiency/power efficiency vs luminance for devices A, B, and C.
1H NMR (500 MHz, DMSO): δ = 13.3 (s, 1H), 8.07 (d, J = 5.0 Hz,
Table 5
1H), 7.88–7.83 (m, 1H), 7.73–7.56 (m, 1H), 7.44–7.34 (m, 3H),
EL Characteristics of green PhOLEDs with different host materials.
7.28–7.23 (m, 2H); 7.17–7.11 (m, 3H), 7.05–6.93 (m, 4H); ESI-MS: m/z
386.1533 [M + H]+.
Device
A
B
C
Host
3-Cbz-
ImdCN
4-Cbz-
ImdCN
TPA-ImdCN
4. Conclusion
External Quantum
Efficiency (%)
Peak
102 cd
mꢀ 2
11.5
10.8
11.4
10.9
5.3
5.3
We succeeded in design and synthesis of three bipolar host materials
3-Cbz-ImdCN, 4-Cbz-ImdCN, and TPA-ImdCN employing carbazole/
triphenylamine as donor and cyanobenzimidazole as an acceptor. Here
we have used cyano and benzimidazole as electron transporting moieties
because in most of the bipolar host materials hole mobility is always
higher than electron mobility when donor: acceptor ratio is 1:1. The
tuning in linking topology leads to alteration in photophysical, thermal,
and electrochemical properties which eventually affect the device per-
formance, as exhibited by 3-Cbz-ImdCN and 4-Cbz-ImdCN. Among the
three host materials, 3-Cbz-ImdCN hosted green device exhibited best
performance with external quantum efficiency of 11.5%, luminance
efficiency 39 cd Aꢀ 1 and power efficiency of 43.3 lm Wꢀ 1 with
maximum luminance of 64303 cd mꢀ 2 at 11.8 V. 3-Cbz-ImdCN hosted
orange and red PhOLED devices were also fabricated and interestingly
red device revealed the maximum external quantum efficiency of 6.3%.
We strongly believe that the present work will promote cyanobenzimi-
dazole as potential electron transporting motif in the molecular
designing of bipolar host materials for efficient phosphorescent organic
light-emitting diodes.
Luminance Efficiency
Peak
102 cd
mꢀ 2
39.0
36.7
39.3
37.5
18.3
18.2
(cd Aꢀ 1
)
Power
Peak
102 cd
mꢀ 2
43.3
30.3
46.8
33.5
17.9
16.8
Efficiency (lm Wꢀ 1
Turn-on Voltage (V)
)
1 cd
2.7
2.5
2.4
mꢀ 2
Maximum Luminance (cd mꢀ 2) [V]
64303
45713
25337
[11.8 V]
(0.37, 0.59)
[11.8 V]
(0.37, 0.59)
[10.4 V]
(0.36, 0.59)
CIE coordinate (x, y)
102 cd
mꢀ 2
103 cd
mꢀ 2
(0.37, 0.59)
(0.37, 0.59)
(0.36, 0.60)
reaction, the reaction mixture was cooled and poured into water. The
precipitate obtained was filtered, washed with water and dried. The
mixture was purified with column chromatography on 100: 200 mesh
silica gel by using petroleum ether: DCM (9:1) to afford the pure product
4-Cbz-ImdCN as a pale yellow solid (1.11 g, 78%).
10

B. Patil et al.
Organic Electronics 92 (2021) 106090
Fig. 9. (a) Chemical structures of the materials used; (b) device architecture of the orange and red OLEDs. (For interpretation of the references to color in this figure
legend, the reader is referred to the Web version of this article.)
Fig. 10. EL characteristics of the phosphorescent OLEDs: (a) normalized EL spectra; (b) current density-voltage-luminance (J-V-L) curves; (c) external quantum
efficiency vs luminance; (d) luminance efficiency/power efficiency vs luminance for devices O and R.
11

B. Patil et al.
Organic Electronics 92 (2021) 106090
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Table 6
EL Characteristics of orange and red phosphorescent OLEDs with 3-Cbz-imdCN
host.
Device
O
R
Emitter
Ir(MDQ)2acac
Ir(piq)2acac
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102 cd
mꢀ 2
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)
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22444 [15.6 V]
14999 [16.8
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102 cd
mꢀ 2
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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.
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Acknowledgements
[19] W.-Y. Hung, L.-C. Chi, W.-J. Chen, Y.-M. Chen, S.-H. Chou, K.-T. Wong, A new
benzimidazole/carbazole hybrid bipolar material for highly efficient deep-blue
electrofluorescence, yellow–green electrophosphorescence, and two-color-based
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c0jm02143a.
B.N.P. and J.J.L. thank University Grant Commission (UGC) and
Council of Scientific and Industrial Research (CSIR) for financial support
through award of research fellowships. A.C.C. greatly appreciates the
generous financial support from the Defense Research and Development
Organization (DRDO) (ERIP/ER/1503212/M/01/1666), New Delhi,
India. C.-H. Chang gratefully acknowledge financial support from the
Ministry of Science and Technology of Taiwan (MOST 109-2221-E-155-
052-).
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and red phosphorescent OLEDs, Adv. Mater. 22 (2010) 2468–2471, https://doi.
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light-emitting devices having a carrier- and exciton-confining structure for reduced
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NA-NA.
Appendix A. Supplementary data
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Supplementary data to this article can be found online at https://doi.
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