Phosphorescent OLEDs with extremely low efficiency roll-off enabled via rationally designed benzimidazole-based bipolar hosts

Article abstract of DOI:10.1016/j.dyepig.2020.108477

Phosphorescent organic light-emitting diodes (PhOLEDs) have manifested enormous potential for display and lighting applications because of their high internal and external quantum efficiencies, nevertheless, severe efficiency roll-off at high luminance remains a major bottleneck in their large-scale commercialization. Herein, two new bipolar host compounds, DCbz-Ph-Bz and DDPA-Ph-Bz, based on carbazole/diphenylamine as the donor and benzimidazole as the acceptor, are successfully obtained. By employing these materials as hosts, extremely low efficiency roll-off PhOLEDs without sacrificing the efficiency are realized. Especially, green PhOLED based on DDPA-Ph-Bz as the host and GD001 as the dopant demonstrated a low turn-on voltage of 2.6 V, a peak current efficiency (CE) of 62.2 cd/A and a maximum luminance of 39,370 cd/m². Importantly, our green device exhibited a very low efficiency roll-off as the peak external quantum efficiency (EQE) of 18.3% at 1000 cd/m² was still retained at 17.6% even at a high luminance of 10,000 cd/m². Likewise, red PhOLED with DCbz-Ph-Bz as the host and RD001 as the dopant displayed a high maximum EQE of 20.1% and a CE of 24.5 cd/A. High efficiency with low efficiency roll-off in our devices reveals the potential of bipolar molecules as excellent hosts to achieve high-performance PhOLEDs.

Full text of DOI:10.1016/j.dyepig.2020.108477

Dyes and Pigments 180 (2020) 108477
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: http://www.elsevier.com/locate/dyepig
Phosphorescent OLEDs with extremely low efficiency roll-off enabled via
rationally designed benzimidazole-based bipolar hosts
,
Tian Zhanga, Jingsheng Miaoa, Muhammad Umair Alia b, Ming Shia, Yaowu Hea,
,
*
Tianchen Fua, Hong Menga
a School of Advanced Materials, Peking University Shenzhen Graduate School, Peking University, Shenzhen 518055, PR China
b Department of Materials Science and Engineering, College of Engineering, Peking University, Beijing 100871, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Phosphorescent organic light-emitting diodes (PhOLEDs) have manifested enormous potential for display and
lighting applications because of their high internal and external quantum efficiencies, nevertheless, severe ef-
ficiency roll-off at high luminance remains a major bottleneck in their large-scale commercialization. Herein, two
new bipolar host compounds, DCbz-Ph-Bz and DDPA-Ph-Bz, based on carbazole/diphenylamine as the donor
and benzimidazole as the acceptor, are successfully obtained. By employing these materials as hosts, extremely
low efficiency roll-off PhOLEDs without sacrificing the efficiency are realized. Especially, green PhOLED based
on DDPA-Ph-Bz as the host and GD001 as the dopant demonstrated a low turn-on voltage of 2.6 V, a peak current
efficiency (CE) of 62.2 cd/A and a maximum luminance of 39,370 cd/m2. Importantly, our green device
exhibited a very low efficiency roll-off as the peak external quantum efficiency (EQE) of 18.3% at 1000 cd/m2
was still retained at 17.6% even at a high luminance of 10,000 cd/m2. Likewise, red PhOLED with DCbz-Ph-Bz as
the host and RD001 as the dopant displayed a high maximum EQE of 20.1% and a CE of 24.5 cd/A. High ef-
ficiency with low efficiency roll-off in our devices reveals the potential of bipolar molecules as excellent hosts to
achieve high-performance PhOLEDs.
High efficiency
Low efficiency roll-off
Bipolar host
PhOLEDs
1. Introduction
and TPA, emitters should be mixed in a host matrix, which plays an
important part in achieving PhOLEDs with high efficiency and low ef-
Organic light-emitting diodes (OLEDs) demonstrate exceptional po-
tential for display and lighting applications on account of their excellent
characteristics, including good color quality, low power consumption
and easy fabrication of large area devices [1]. In particular, phospho-
rescent OLEDs (PhOLEDs) exhibit superior performance and have been
widely used in smart phones. However, despite the enormous progress
made so far, several challenges and issues still exist in currently
commercialized PhOLEDs, such as their low efficiency, short lifetime,
complicated fabrication and so on [2–4]. Usually, OLEDs work stably at
low luminance ranging between 100 and 400 cd/m2 [5], which is below
the requirements of general illumination purposes (i.e. 5000 cd/m2 to
10,000 cd/m2). While, their efficiency tends to decrease as the lumi-
nance increases, especially at high luminance, which is called efficiency
roll-off [5,6]. Efficiency roll-off in PhOLEDs is generally induced by
various factors, such as triplet-triplet annihilation (TTA) between the
emitters and triplet–polaron annihilation (TPA). In order to avoid TTA
ficiency roll-off.
The choice of suitable host is a key to attain PhOLEDs with high
efficiency as hosts are responsible for balancing the charge transport and
confinement of electron-hole pairs within the emitting layer (EML)
[7–9]. As reported to date, three kinds of hosts are considered to realize
efficient PhOLEDs, including monopolar hosts, bipolar hosts and
co-hosts. Conventional monopolar hosts, such as mCP and CBP generally
show severe efficiency roll-off because of their unipolar
charge-transporting ability which results in narrow recombination area
in the EML. Although, co-hosts can result in balanced carrier mobility,
they increase the complexity of the devices. Whereas, in case of bipolar
hosts, electron donor and acceptor units are linked together in a single
small molecule, which not only make the electric flux high and balanced
but also simplify the device structure and hence, the fabrication. Owing
to these characteristics, bipolar hosts have gained considerable attention
in PhOLEDs fabrication. For instance, Cheng’s group reported a highly
* Corresponding author.
E-mail address: menghong@pku.edu.cn (H. Meng).
https://doi.org/10.1016/j.dyepig.2020.108477
Received 11 December 2019; Received in revised form 16 March 2020; Accepted 23 April 2020
Available online 28 April 2020
0143-7208/© 2020 Elsevier Ltd. All rights reserved.

T. Zhang et al.
Dyes and Pigments 180 (2020) 108477
Scheme 1. Synthetic routes of DCbz-Ph-Bz and DDPA-Ph-Bz.i) t-BuOK, DMSO, 120 �C, 40 min ii) PdCl2(dppf), KOAc, 4,4,40,40,5,5,50,50-octamethyl-2,20-bi(1,3,2-
dioxaborolane), 1,4-dioxane, 90 �C,12 h iii) t-BuNa, tris(dibenzylideneacetone)dipalladium, anhydrous toluene, 90 �C, 2.5 h iv) Pd(PPh3)4, 2 M K2CO3, toluene/
ethanol, 110 �C, 24 h.
efficient universal bipolar host, BCPO, which consists of carbazole and
phosphine oxide moieties [10]. D. Ma et al. integrated mCP with benz-
imidazole moiety on different positions of N-phenyl [11], and applied
the resulting materials as universal phosphorescent hosts in OLEDs
which demonstrated substantially low efficiency roll-off at high lumi-
nance. Among their reported hosts, the devices based on o-mCPBI
showed the best characteristics with EQEs of over 20% for white, red,
green, blue and yellow PhOLEDs. Sue et al. linked TP and carbazole to
synthesize bipolar host for TADF and green PhOLEDs [12], which
revealed exceptional capability to attain high efficiency with low
roll-off. TP26DCbz-Ph-Bz2-host based PhOLEDs developed in their work
exhibited the maximum EQE of 25.6%, which was maintained at 25.2%
even at a high luminance of 5000 cd/m2.
NMR spectrometer. Thermal gravimetric analysis (TGA) was conducted
using a TA Instruments Thermogravimetric Analyzer, TA2950 at a
heating rate of 10 �C/min under nitrogen flow rate of 50 cm3/min.
Differential scanning calorimetry (DSC) experiments were performed
using a TA Instruments Q1000 DSC at a heating or cooling rate of 5 �C/
min under nitrogen flow. UV–Vis absorption spectra of vacuum depos-
ited thin films were recorded by PerkinElmer Lambda 750 Spectrometer.
The fluorescence spectra were obtained with a Hitachi F-7000 Spec-
trometer at room temperature and the phosphorescence spectra were
measured with a Hitachi F-7000 Spectrometer at 77 K in toluene. Cyclic
voltammetry (CV) measurements were conducted using a Gamry Inter-
face 1000 Potentiostat Electrochemical Work Station in DMF solution
containing 0.1 M/L tetra-nbutylammonium (TBAPF6). A glassy carbon
electrode, a working electrode, a Pt wire counter electrode and an Ag
wire reference electrode were employed for the CV measurements along
with ferrocene/ferrocenium (Fc/Fcþ) as the internal reference. Theo-
retical calculations were performed using PBE0 functional.
In this work, we successfully developed two new bipolar hosts, 9,90-
(40-(1-phenyl-1H-benzo[d]imidazol-2-yl)-[1,10-biphenyl]-3,5-diyl)bis
(9H-carbazole) (DCbz-Ph-Bz) and N3,N3,N5,N5-tetraphenyl-40-(1-
phenyl-1H-benzo[d]imidazol-2-yl)-[1,10-biphenyl]-3,5-diamine
(DDPA-Ph-Bz), using benzimidazole as the acceptor moiety and carba-
zole/diphenylamine (DPA) as the donor unit. Carbazole and DPA are
selected because of their attractive hole mobilities [13], and benzimid-
azole unit is employed as acceptor for its high electron injection and
2.2. PhOLED fabrication and characterizations
Pre-cleaned ITOs with a sheet resistance of 12 Ω/□ were used as the
substrates. First, the substrates were transferred into a vacuum chamber
(Super-SPECTROS 200, Kurt J. Lesker) to deposit the organic layers with
a base pressure of <10À 6 Torr. Then, the device fabrication was
completed via thermal deposition of Liq (2.5 nm) followed by capping
the underlayers with Al metal (100 nm) as the cathode. The CIE co-
ordinates and current densityÀ voltageÀ luminance (JÀ VÀ L) curves of
the fabricated PhOLEDs were measured using a luminance clorimeter
(BM7A, TOPCON, Inc.), and the EL spectra of the devices were recorded
with a spectroradiometer (PR-788, Photo Research, Inc.).
transport capability [14]. By linking the two moieties with
a
size-appropriate biphenyl -conjugated bridge, we mainly focused on
π
exploring the effect of benzimidazole-based compounds as bipolar hosts
in PhOLEDs. As expected, both of our synthesized compounds exhibited
suitable energy levels, high triplet energies (ET), good thermal stability
and considerable glass transition temperature (Tg). In order to explore
the potential of these compounds as hosts, we fabricated green and red
PhOLEDs, and achieved impressive electroluminescence (EL) perfor-
mance. Especially, DDPA-Ph-Bz hosted green PhOLEDs demonstrated
quite low efficiency roll-off as the EQE was retained at 18.3% and 17.7%
at corresponding luminance of 1000 and 10,000 cd/m2, respectively.
Red PhOLEDs containing DCbz-Ph-Bz as host showed a high EQE of
20.1% with corresponding power efficiency (PE) of 28.5 lm/W and CE of
24.5 cd AÀ 1. It is worth mentioning that the efficiency roll-off in our
devices (3.3%) at 10,000 cd/m2 is the best value ever reported for bi-
polar host-based green PhOLEDs (see Table S1). Our results demonstrate
that the development of benzimidazole-based bipolar hosts is an effec-
tive way to acquire high-efficiency PhOLEDs with low efficiency roll-off.
2.3. Materials synthesis
(i) DCbz-Ph-Bz
Synthesis routes for DCbz-Ph-Bz and DCbz-Ph-Bz are presented in
Scheme 1. For the synthesis of DCbz-Ph-Bz, compound 4 (10.8 mmol),
2-(4-bromophenyl)-1-phenyl-1H-benzo[d]imidazole
(9.8
mmol),
ethanol (40 mL) and toluene (150 mL) were added into a 350 mL flask
followed by the addition of aqueous potassium carbonate (2 M). After
that, nitrogen was purged into the mixture for 20 min, and a small
quantity of Pd(PPh3)4 was added as the catalyst. Afterwards, the mixture
was left for reaction at 110 �C for 20 h. After the temperature of the
mixture droped to room temperature, it was poured into methanol (350
mL). Water and methanol were used to wash the precipitates which were
obtained though filtration. At last, the raw product was dried under
2. Experimental section
2.1. General information
1H NMR and 13C NMR spectra were acquired on a Bruker Avance 300
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T. Zhang et al.
Dyes and Pigments 180 (2020) 108477
Fig. 1. (a) TGA and (b) DSC curves of DCbz-Ph-Bz and DDPA-Ph-Bz.
vacuum and further purified via sublimation for four times. 1H NMR
(300 MHz, DMSO‑d6): δ 8.27 (d, J ¼ 7.6 Hz, 4H), 8.12 (d, J ¼ 1.8 Hz,
2H), 7.95 (d, J ¼ 8.6 Hz, 2H), 7.86 (t, J ¼ 1.8 Hz, 1H), 7.81 (dd, J ¼ 6.9,
1.5 Hz, 1H), 7.70–7.54 (m, 9H), 7.52–7.45 (m, 6H), 7.37–7.25 (m, 6H),
7.19 (dd, J ¼ 7.0, 1.6 Hz, 1H). MS (þESI, m/z) (M þ H)þ calculated for
677.2661, found 677.2702.
DMSO‑d6): δ 7.77 (d, J ¼ 7.1 Hz, 1H), 7.60–7.45 (m, 5H), 7.39 (dd, J ¼
7.6, 1.7 Hz, 2H), 7.35–7.20 (m, 12H), 7.16 (d, J ¼ 7.3 Hz, 1H),
7.05–6.96 (m, 12H), 6.75 (d, J ¼ 1.9 Hz, 2H), 6.61 (d, J ¼ 1.7 Hz, 1H).
MS (þESI, m/z) (M þ H)þ calculated for 681.2974, found 681.3013.
3. Results and discussion
5-chloro-N1,N1,N3,N3-tetraphenylbenzene-1,3-diamine (7).
The compounds, 1,3-dibromo-5-chlorobenzene (2.70 g,10 mmol)
and diphenylamine (3.55 g, 21 mmol), t-BuNa(2.98 g, 31 mmol) along
with 35 mL anhydrous toluene were poured into a 350 mL flask. Later,
nitrogen was purged into the mixture for 20 min, and dichlorobis[di-tert-
3.1. Synthesis and characterization
The chemical structures and synthetic routes of the target pruducts,
DCbz-Ph-Bz and DDPA-Ph-Bz can be found in Scheme 1. The inter-
mediate compound 4 was obtained by adopting a previously reported
method [1]. The target compounds, DCbz-Ph-Bz and DDPA-Ph-Bz were
attained via palladium catalyzed Suzuki–Miyaura coupling reaction of
compounds 4 and 8 with 2-(4-bromophenyl)-1-phenyl-1H-benzo[d]
imidazole, respectively. Both of the final products were further purified
through column chromatography and then processed via
temperature-gradient vacuum sublimation before employing in PhO-
LEDs. The material’s purity was further confirmed through nuclear
magnetic resonance (NMR) and high-resolution mass spectrometery
(HRMS). The Experimental Section and Supporting Information contain
the details of synthetic procedures and characterization results.
butyl(4-diMethylaMinophenyl)
phosphine]palladium(II)
(Pd
(AMPHOS)2Cl2) (0.07 g, 0.1 mmol) was added. Afterwards, the reaction
was kept at 90 �C for 3 h. Then, after cooling, some water was poured
into the flask, and chloroform was used to extract the organic phase,
which was then treated with anhydrous MgSO4. To remove the solvent,
residual was purified through column chromatography using dichloro-
methane (DCM)/petroleum at 1:9 (v:v) as the eluent. Finally, the inter-
mediate product in the form of white solid was obtained (Yield: 73%).
1H NMR (400 MHz, CDCl3): δ 7.24–7.17 (m, 8H), 7.05 (dt, J ¼ 8.8, 1.7
Hz, 8H), 7.02–6.96 (m, 4H), 6.65 (t, J ¼ 2.0 Hz, 1H), 6.57 (d, J ¼ 2.0 Hz,
2H). 13C NMR (400 MHz, CDCl3) δ 149.50, 147.05, 135.24, 129.41,
124.78, 123.53, 116.69, 115.90. HRMS(APCI): m/z calculated for
C
30H24ClN2 [MþH]þ 447.1628, found 447.1621.
3.2. Thermal properties
N1,N1,N3,N3-tetraphenyl-5-(4,4,5,5-tetramethyl-1,3,2-dioxabor-
olan-2-yl)benzene-1,3-diamine (8).
TGA and DSC were used to evaluate the thermal properties of the
newly developed materials. As can be observed from the obtained curves
(Fig. 1), both materials indicate great thermal stability: the decompo-
sition temperatures (Td) that refers to the 5% weight loss are estimated
to be 508 �C and 456 �C, while clear Tg values are measured to be 156 �C
and 113 �C, respectively for DCbz-Ph-Bz and DDPA-Ph-Bz. DCbz-Ph-Bz
has higher Td and Tg values with lower molecular weight compared with
DDPA-Ph-Bz, which is caused by the carbazole group. Carbazole is a
well-known chemically and morphologically stable building block. Also,
its good coplanrity can increase the molecular rigidity and reduce tor-
sion resistance. Both of our materials exhibit substantially higher Td and
Tg than those for previously reported compounds, such as for mCP [15]
or similar benzimidazole compounds (o-mCPBI, m-mCPBI, p-mCPBI)
[11]. Overall, the improved thermal properties are beneficial for PhO-
LEDs as they ensure the formation of stable amorphous thin films and
maintain high operational stability.
A mixture of anhydrous 1,4-dioxane (35 mL), 7 (2.23 g, 5 mmol), tris
(dibenzylideneacetone)dipalladium (0.10 g, 0.1 mmol), 2-di-tert-butyl-
phosphino-20,40,60-trIIsopropylbiphenyl (0.08 g, 0.2 mmol), bis(pina-
colato)diboron (1.40 g, 5.5 mmol) and potassium acetate (1.47 g, 15
mmol) was refluxed at 90 �C in a nitrogen atmosphere for 12 h. When
the temperature dropped to 25 �C, the mixture was treated with satu-
rated sodium chloride solution and chloroform sequentially. After
separating out the organic layers, the mixture was further treated with
anhydrous MgSO4. To remove the solvent, the solid was purified
through column chromatography with petroleum ether acted as the
eluent and a white powder was obtained (Yield: 82%). 1H NMR (400
MHz, CDCl3) δ 7.23–7.16 (m, 10H), 7.07–7.01 (m, 8H), 6.94 (dd, J ¼
10.5, 4.2 Hz, 5H), 1.27 (s, 12H). 13C NMR (400 MHz, CDCl3) δ 148.37,
147.77, 129.20, 126.08, 125.17, 123.77, 122.44, 83.96, 67.24, 24.94.
HRMS(APCI): m/z calculated for C36H36BN2O2 [MþH]þ 539.2870,
found 539.2872.
(ii) DDPA-Ph-Bz
3.3. Photophysical properties
The synthesis method for DDPA-Ph-Bz was the same as that for
DCbz-Ph-Bz (except using compound 8 instead of compound 4) and the
material was attained with a yield of 56.8%. 1H NMR (300 MHz,
The UV–vis absorption and photoluminescence (PL) spectra (at 25
�C) of thin films, as well as PL spectra (at 77 K) in toluene of DCbz-Ph-Bz
3

T. Zhang et al.
Dyes and Pigments 180 (2020) 108477
Fig. 2. (a) Absorption and fluorescence spectra of DCbz-Ph-Bz (black lines) and DDPA-Ph-Bz (red lines) films and (b) corresponding phosphorescence spectra in
toluene at 77 K. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
Table 1
Physical properties of DCbz-Ph-Bz and DDPA-Ph-Bz.
Compound
λabs [nm]
λPL [nm]
HOMOa [eV]
LUMO [eV]
Ebg [eV]
S1 [eV]
T1c [eV]
△Est [eV]
Tdd [�C]
Tdg [�C]
DCbz-Ph-Bz
DDPA-Ph-Bz
317, 326, 340
313
404
435
¡5.60
¡5.50
¡2.21
¡2.53
3.39
2.97
3.42
3.10
2.80
2.83
0.62
0.27
508
456
156
114
a
c
Deduced from CV. bElectrochemical band-gaps calculated according to the absorption spectra of films using △Eg ¼ 1240/λonset
.
Measured in toluene at 77 K dMeasured by TGA and DSC.
and DDPA-Ph-Bz are displayed in Fig. 2, and the obtained values are
summarized in Table 1. Both of the molecules possess similar absorption
spectra since they contain nearly identical molecular skeleton and
functional groups. The absorption peaks for DCbz-Ph-Bz and DDPA-Ph-
Bz are observed at around 317 and 313 nm, respectively, which could
have resulted from the
π→π* transition corresponding to the benz-
imidazole group and adjacent benzene ring. While in the absorption
spectra of DCbz-Ph-Bz, other absorption peaks can be seen at 324 nm
and 340 nm, corresponding to the n→π* electron transitions of the
carbazole unit [9]. PL emission peaks of these materials are found at 404
nm and 435 nm in the form of neat films, while at 419 nm and 461 nm in
dilute DCM solution for DCbz-Ph-Bz and DDPA-Ph-Bz, respectively. As
can be observed in the PL spectra, the corresponding peaks of both of
these materials in the form of neat films are blue-shifted by 15–26 nm
compared with those in DCM, possibly due to the different dielectric
surroundings. Compared with DDPA-Ph-Bz, DCbz-Ph-Bz has short
emission wavelength, which is attributed to its improved non-planarity
between the middle biphenyl unit. As a result, the twisted biphenyl core
limits π-conjugation length, as shown in Fig. 2. The dihedral angle is
37.0� for DCbz-Ph-Bz and 35.8� for DDPA-Ph-Bz as calculated by
density functional theory (DFT). According to the onset of absorption
and emission wavelengths of the spectra in the form of films, the optical
energy gap (Eg)/singlet energy (S1) values are estimated to be 3.39/3.42
eV and 2.97/3.10 eV for DCbz-Ph-Bz and DDPA-Ph-Bz, respectively,
revealing well-matched S1 and Eg values. Based on the onset of phos-
phorescence spectra [16], ET values are evaluated to be 2.80 and 2.83 eV
for DCbz-Ph-Bz and DDPA-Ph-Bz, respectively. Though, these materials
have different singlet energies, their triplet energies are almost the same.
The similar triplet energies can be explained by the low ET values of the
benzimidazole group, since phosphorescence is derived from the triplet
emission of the benzimidazole with the lowest ET values; both of these
materials behave analogously. Considering CbzNBI, Mcpnbi [17] and
CbPr-3, Bim-4 [18] for example, the PL spectra of DCbz-Ph-Bz and
DDPA-Ph-Bz is consistent with the phosphorescent emission of benz-
imidazole derivatives. The ET values of DCbz-Ph-Bz and DDPA-Ph-Bz
are sufficiently high, making them suitable to adopt as hosts for
Fig. 3. CV curves of DCbz-Ph-Bz and DDPA-Ph-Bz in dilute DCM.
phosphorescent emitters.
3.4. Electrochemical properties
To understand the electrochemical behavior of these newly devel-
oped materials, CV analysis was performed and the obtained results are
depicted in Fig. 3. According to the onsets of the oxidation, the HOMOs
of DCbz-Ph-Bz and DDPA-Ph-Bz are estimated to be À 5.60 and À 5.50
eV, respectively, and the corresponding LUMOs are À 2.21 eV and À 2.53
eV, as calculated from the HOMO levels and Eg values obtained from the
absorption spectra. DCbz-Ph-Bz exhibits irreversible oxidation curves
like most of the carbazole derivatives since their radical cations are
unstable [19]. The almost same HOMOs indicate that the donor
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T. Zhang et al.
Dyes and Pigments 180 (2020) 108477
PhOLEDs.
3.5. Theoretical calculations
To further explore the structure-property relationship of these ma-
terials at the molecular level, density functional theory(DFT) calcula-
tions were performed. The HOMO–LUMO levels, energy gaps and
frontier-molecular-orbital distributions estimated via PBE0 calcula-
tions are presented in Fig. 4 which revealed that DCbz-Ph-Bz and
DDPA-Ph-Bz show similar orbital distributions. Their HOMOs are
scattered among DPA or carbazole which are the electron-donating
groups, while their LUMOs are located in the benzimidazole due to
the electron affinity of nitrogen. The separated HOMOs and LUMOs
suggest that holes and electrons can be efficiently transported by these
molecules, which could improve the device performance. Moreover,
there is a little overlap at the benzene rings between carbazole or DPA,
and an appropriate overlap between LUMO and HOMO offers an effec-
tive intramolecular charge transfer (ICT) transition emission and a
higher ET.
Fig. 4. HOMO–LUMO levels, energy gaps and frontier-molecular-orbital dis-
tribution of DCbz-Ph-Bz (Left) and DDPA-Ph-Bz (Right).
3.6. Single carrier devices
To investigate the charge-transporting performance of the synthe-
sized compounds, single carrier devices were developed. The hole-only
devices with a structure of ITO/NPB (10 nm)/Host (80 nm)/NPB (10
nm)/Al (100 nm), and the electron-only devices with a configuration of
ITO/TPBI (10 nm)/Host (80 nm)/TPBI (10 nm)/Liq (2.5 nm)/Al (100
nm) were fabricated. To block electron injection from anode to the EML,
electronic units have similar oxidation potential, and between the donor
and acceptor units, there is an interaction which results in similar
HOMOs of both of these compounds. In addition, they exhibit deep
LUMOs, potentially due to the increased electron-withdrawing benzoi-
midazole units, that could improve the electron-injection to the EML in
Fig. 5. Current density versus voltage (J-V) characteristics of the hole-only and electron-only devices for (a) DCbz-Ph-Bz and (b) DDPA-Ph-Bz, (c) Absorption
spectra of GD001 and RD001, and fluorescence spectra of DCbz-Ph-Bz and DDPA-Ph-Bz films.
5

T. Zhang et al.
Dyes and Pigments 180 (2020) 108477
Fig. 6. Energy-level diagram of the fabricated PhOLEDs and the molecular structures of constituent materials.
evaluate their performance as hosts in PhOLEDs.
we used NPB as an electron-blocking layer in hole-only devices. While,
in case of electron-only devices, TPBI palys a part in transporting the
electrons and blocking the holes. As can be seen in Fig. 5a and b, it is
clear that the current densities are quite low which indicates that the
charge-transporting property of these hosts is poor. The low current
desnities in these single carrier devices is ascribed to the twisted mo-
lecular conformation of hosts which resulted in an inapproperiate mo-
lecular stacking order in the neat films. As mentioned above, the
dihedral angle of the two middle benzene rings in case of DCbz-Ph-Bz is
bigger than that of DDPA-Ph-Bz. Bigger dihedral angle suggests weaker
coplanar configuration and less ordered molecular arrangement, which
undoubtedly contributes to rough or poor charge transportation, leading
to lower charge-transporting property of DCbz-Ph-Bz. Nevertheless, the
charge-transporting characteristics of the hosts are well-balanced; the
low but balanced charge-transporting property of the hosts could
enlarge the recombination area of the EML and reduce the efficiency
roll-off. Compared with the hole-only devices, the corresponding
electron-only devices show slightly higher current densities, indicating
that both of the materials are bipolar hosts.
First, we fabricated green-emitting devices, G1 and G2, by utilizing
DCbz-Ph-Bz and DDPA-Ph-Bz as green hosts for PhOLEDs with GD001
as the dopant in a device configuration of ITO/HATCN (5 nm)/LHT002
(30 nm)/LB002 (10 nm)/Host: Dopant (50 nm,10%)/LET003 (30 nm)/
Liq (2.5 nm)/Al (100 nm). The used materials, including LHT002,
LEB002, LET003, LGD001 and LRD001 were synthesized according to
previously reported methods [20–22]. The molecular structures of these
organic materials and corresponding energy-levels diagram of the
fabricated devices are depicted in Fig. 6. The key-paramters of green
PhOLEDs can be seen in Table 2. The turn-on voltage, defined as the
voltage at which a device achieves a luminance of 1 cd/m2, is quite low,
i.e. 2.6 V for both of the devices. Such low turn-on voltage is ascribed to
the well-matched energy-levels of the hosts with adjacent layers [23].
Device G1 using DCbz-Ph-Bz as the host rendered a maximum PE of
66.2 lm/W, the highest CE of 57.2 cd/A and a peak EQE of 16.1%. De-
vice G2 hosted by DDPA-Ph-Bz exhibited superior performance with
44.4 lm/W, 62.2 cd/A and 18.9% as the maximum PE, CE and EQE
values, respectively. As evident in Fig. 7, device G2 demonstrated a
considerably higher maximum luminance of 39,370 cd/m2 at 7 V than
12,890 cd/m2 for G1, which could have resulted from different
charge-transporting performance of the used host materials.
DDPA-Ph-Bz has better carrier transport capability resulting in PhO-
LEDs with higher current density and hence, higher luminance. Obvi-
ously, in case of green PhOLEDs, DDPA-Ph-Bz outperformed
3.7. PhOLEDs characterization
The excellent thermal stability, suitable energy levels, bipolar charge
transport capability, and especially, extremely high ET values of our
newly developed benzimidazole-based materials motivated us to
Table 2
Electroluminescence characteristics of PhOLEDs with DCbz-Ph-Bz and DDPA-Ph-Bz as hosts.
Device
Host
Vaon [V]
CEb [cd/A]
PEc [lm/w]
EQE (%)
Maximum
@1000 cd/m2
@10,000 cd/m2
G1
G2
R1
R2
DCbz-Ph-Bz
DDPA-Ph-Bz
DCbz-Ph-Bz
DDPA-Ph-Bz
2.6
2.6
2.7
2.6
57.2
62.2
24.5
16.5
69.2
44.4
28.5
16.3
16.1
18.9
20.1
16.2
13.3
18.3
18.4
15.5
12.6
17.7
13.4
11.8
aVon, the turn-on voltage. bThe maximum values of CE. cThe maximum values of PE.
6

T. Zhang et al.
Dyes and Pigments 180 (2020) 108477
Fig. 7. (a) Current densityÀ voltageÀ luminance (JÀ VÀ L) characteristics, (b) CE-L-EQE curves, and (c) EL spectra of green PhOLEDs at 6 V. (For interpretation of the
references to color in this figure legend, the reader is referred to the Web version of this article.)
DCbz-Ph-Bz by demonstrating superior EQE possibly due to three po-
tential reasons: (1) the slightly higher ET of DDPA-Ph-Bz (2.83 eV) than
that of the DCbz-Ph-Bz (2.80 eV), which could decrease the energy
transfer loss between the host and dopant. Further, DDPA-Ph-Bz’s
deeper S1 of 3.10 eV is also beneficial on account of lower stress of the
host in the process of light emission [24]. (2) DDPA-Ph-Bz has
well-matched energy-levels, i.e. its HOMO is 5.5 eV, shallower than
DCbz-Ph-Bz, which is good for hole transport. DDPA-Ph-Bz’s LUMO is
2.5 eV, which is the same as that of the adjacent electronic transport
layer, resulting in reduced barrier for electron transfer. (3)
DDPA-Ph-Bz’s higher charge-transporting property also plays an
important role in superior device performance.
further fabricated red PhOLEDs, R1 and R2, in a device structure of ITO/
HATCN (5 nm)/LHT002 (20 nm)/LEB002 (10 nm)/Host: Dopant (30
nm, 3%)/LET003 (50 nm)/Liq (2.5 nm)/Al (100 nm), where RD001 is
employed as the dopant while DCbz-Ph-Bz and DDPA-Ph-Bz are used as
hosts, respectively. Both of the red PhOLEDs exhibited high efficiency,
among which, the device R1 reached a higher maximum luminance of
14,140 cd/m2 at 7 V, and maximum values of 24.5 cd/A, 28.5 lm/W,
20.1% as CE, PE and EQE, respectively, accompanied by CIE coordinates
of (0.66, 0.33). While, the device R2 showed inferior performance as it
could attain a maximum luminance of 12,800 cd/m2 at 7 V (and
maximum efficiencies of 16.5 cd/A and 16.3 lm/W, 12.7%) with CIE
coordinates of (0.65, 0.34).
Interestingly, both of the devices revealed extremely low efficiency
roll-off. In case of device G1, for example, the EQE was 13.3% at a
luminance of 1000 cd/m2 which still remained nearly unchanged at
12.6% when the luminance reached up to 10,000 cd/m2. Likewise, for
device G2, the EQEs at 1000 and 10,000 cd/m2 are observed to be 18.3%
and 17.7%, respectively. The high EQEs with extremely low efficiency
roll-off in these devices are ascribed to the two reasons, including: (1)
balanced electron and hole transport characteristics of the constituent
materials which effectively confined the electron-hole pairs in the EML,
and (2) high ET values of our newly developed hosts which could avoid
reverse energy that transfers from the guest to the host. Furthermore, it
is worth mentioning that green PhOLEDs with our newly developed
compounds outperformed the counterpart adopting CBP, a commonly
used host, both in terms of efficiency and the efficiency roll-off (see
Fig. S5 and Table S1), thereby indicating superior potential of
benzimidazole-based bipolar hosts over conventional materials.
Inspired by the decent characteristics of our green PhOLEDs, we
In case of red devices, the results are opposite to those acheived in
case of green devices where DCbz-Ph-Bz exhibited better performance.
This could be ascribed to the utilization of efficient energy transfer from
host material to the phosphorescent dopant. The EL spectra of the green
devices showed only the emission from green dopant, suggesting that
the EL is resulted from sufficient exciton transformation from the host to
the dopant. In red devices, as shown in Fig. 8 (c), there is another small
peak in device R2’s EL spectrum at 439 nm, which is the emission from
DDPA-Ph-Bz itself, indicating that the energy transfer from the host to
the dopant is incomplete. Moreover, DDPA-Ph-Bz’s PL and red dopant’s
absorption spectra do not overlap well with each other. Also, the dopant
has wider absorption in DCbz-Ph-Bz’s emission range, so R2’s efficiency
is lower than that of the R1.
4. Conclusions
In summary, two new bipolar host compounds, DCbz-Ph-Bz and
7

T. Zhang et al.
Dyes and Pigments 180 (2020) 108477
Fig. 8. (a) JÀ VÀ L characteristics, (b) CE-L-EQE curves, and (c) EL spectra of red PhOLEDs at 6 V. (For interpretation of the references to color in this figure legend,
the reader is referred to the Web version of this article.)
DDPA-Ph-Bz, based on benzimidazole moiety, are successfully synthe-
sized which demonstrated excellent performance in PhOLEDs. The
application of benzimidazole unit to develop novel hosts resulted in
materials with high thermal stability and well-balanced charge fluxes
and high ET values. PhOLEDs employing these materials as hosts showed
decent characteristics and a low turn-on voltage of about 2.6 V with high
efficiency. Importantly, PhOLEDs containing DDPA-Ph-Bz as green host
showed extremely low efficiency roll-off at high luminance as the peak
EQE of 18.9% at 2920 cd/m2 was retained at 18.3% and 17.7% at cor-
responding luminance of 1000 and 10,000 cd/m2, respectively. In
addition, red PhOLEDs with DCbz-Ph-Bz as the host exhibited a
maximum CE of 24.5 cd/A and a peak EQE of 20.1%. These results
reveal the potential of benzimidazole derivatives as hosts for red and
green PhOLEDs with excellent performance. Our findings demonstrate
that a host adopting benzimidazole unit could be an appropriate choice
for the design of high efficiency PhOLEDs with low efficiency roll-off.
Acknowledgments
This work was financially supported by the Guangdong Key Research
Project (No. 2019B010924003), the Shenzhen Engineering Laboratory
(Shenzhen Development and Reform Commission [2018]1410), the
Shenzhen Science and Technology Research Grant (JCYJ2018030
2153451987, JCYJ20190808182803805, JCYJ20170818085627721)
and Shenzhen Peacock Plan (KQTD2014062714543296).
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.dyepig.2020.108477.
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