A low-cost phenylbenzoimidazole containing electron transport material for efficient green phosphorescent and thermally activated delayed fluorescent OLEDs

Article abstract of DOI:10.1039/c5tc00350d

A new phenylbenzoimidazole-based electron-transport material (ETM), 2,4,6-tris(2-phenyl-1H-benzo[d]imidazol-1-yl)benzonitrile (iTPBI-CN), is designed and synthesized through a simple low-cost one-step C-N coupling reaction by using 2,4,6,-trifluorobenzonitrile and 2-phenyl-1H-benzo[d]imidazole as the starting materials. In comparison with the four step synthesis of commercial ETM of 2,2,2-(1,3,5-phenylene)-tris(1-phenyl-1H-benzimidazole) (TBPI), the introduction of a cyano moiety into iTPBI-CN greatly simplifies the synthetic procedure and allows for an isomerized linkage of phenylbenzoimidazole. The glass transition temperature increases from 124 °C of TPBI to 139 °C of iTPBI-CN. Both compounds show similar HOMO levels of ～5.9 eV and a triplet energy of ～2.6 eV. The deeper LUMO level of iTPBI-CN (2.79 eV) than TPBI (2.38 eV) allows for more efficient electron-injection and a much higher device efficiency. Solution-processed green phosphorescent OLEDs with the structure of ITO/PEDOT:PSS/host:Ir(mppy)₃/iTPBI-CN versus TPBI/LiF/Al show maximum current and power efficiencies of 37.7 cd A^-1 and 29.0 lm W^-1versus 26.1 cd A^-1, 12.2 lm W^-1 in the CBP host and 31.3 cd A^-1 and 23.9 lm W^-1versus 20.6 cd A^-1, 7.4 lm W^-1 in the mCP host. Furthermore, the superior device performance of iTPBI-CN over TPBI is also found in both CBP and mCP hosted green thermally activated delayed fluorescence (TADF) devices by using 2,3,5,6-tetracarbazole-4-cyano-pyridine (4CzCNPy) as a dopant.

Full text of DOI:10.1039/c5tc00350d

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Materials Chemistry C
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A low-cost phenylbenzoimidazole containing
electron transport material for efficient green
phosphorescent and thermally activated delayed
fluorescent OLEDs
Cite this:DOI: 10.1039/c5tc00350d
Fangfang Wang,ab Jia Hu,a Xudong Cao,a Tao Yang,c Youtian Tao,*a Ling Mei,a
Xinwen Zhang*c and Wei Huang*ac
A new phenylbenzoimidazole-based electron-transport material (ETM), 2,4,6-tris(2-phenyl-1H-benzo[d]-
imidazol-1-yl)benzonitrile (iTPBI-CN), is designed and synthesized through a simple low-cost one-step
C–N coupling reaction by using 2,4,6,-trifluorobenzonitrile and 2-phenyl-1H-benzo[d]imidazole as the
starting materials. In comparison with the four step synthesis of commercial ETM of 2,2,2-(1,3,5-
phenylene)-tris(1-phenyl-1H-benzimidazole) (TBPI), the introduction of a cyano moiety into iTPBI-CN
greatly simplifies the synthetic procedure and allows for an isomerized linkage of phenylbenzoimidazole.
The glass transition temperature increases from 124 1C of TPBI to 139 1C of iTPBI-CN. Both compounds
show similar HOMO levels of B5.9 eV and a triplet energy of B2.6 eV. The deeper LUMO level of iTPBI-
CN (2.79 eV) than TPBI (2.38 eV) allows for more efficient electron-injection and a much higher device
efficiency. Solution-processed green phosphorescent OLEDs with the structure of ITO/PEDOT:PSS/
host:Ir(mppy)3/iTPBI-CN versus TPBI/LiF/Al show maximum current and power efficiencies of 37.7 cd Aꢀ1
and 29.0 lm Wꢀ1 versus 26.1 cd Aꢀ1, 12.2 lm Wꢀ1 in the CBP host and 31.3 cd Aꢀ1 and 23.9 lm Wꢀ1 versus
20.6 cd Aꢀ1, 7.4 lm Wꢀ1 in the mCP host. Furthermore, the superior device performance of iTPBI-CN over
TPBI is also found in both CBP and mCP hosted green thermally activated delayed fluorescence (TADF)
devices by using 2,3,5,6-tetracarbazole-4-cyano-pyridine (4CzCNPy) as a dopant.
Received 5th February 2015,
Accepted 24th April 2015
DOI: 10.1039/c5tc00350d
www.rsc.org/MaterialsC
phosphorescent triplet emitters are normally doped into suitable
organic host materials.4
1. Introduction
Organic light-emitting diodes (OLEDs) have become of significant
The basic structure of an OLED is multilayered, typically
commercialization interest for their potential applications in full consisting of a transparent metal oxide anode, a hole-transport
color flat-panel displays and solid state lightings.1,2 Compared to layer (HTL), an emissive layer (EML), an electron-transport layer
the 25% of internal quantum efficiency (IQE) of conventional (ETL), and a metal cathode. A high hole mobility of up to almost
fluorescent OLEDs, phosphorescent OLEDs based on heavy metal 10ꢀ2 cm2 Vꢀ1 sꢀ1 has already been achieved in most widely
complexes which can utilize both singlet and triplet excitons reported hole-transport materials, such as 1-bis[4-[N,N-di(4-tolyl)-
through intersystem crossing to theoretically approach 100% IQE amino]phenyl]-cyclohexane (TAPC) and N,N-dinaphthyl-N,N0-
have drawn much attention from both academia and industry.3 To diphenylbenzidine (NPB).5 On the other hand, the electron
reduce concentration quenching and triplet–triplet annihilation, mobility of excellent electron-transport materials is at least
10-fold lower than that of hole mobility of a HTL in devices.4a
As charge transport properties of electron- and hole-transport
materials are imbalanced, the electron feeding from the cathode into
a Key Lab for Flexible Electronics & Institute of Advanced Materials,
the emissive layer dominates the device characteristics of OLEDs.6
Jiangsu National Synergistic Innovation Center for Advanced Materials (SICAM),
Therefore, the development of efficient organic electron-transport
materials is a challenge of high priority.7
Nanjing Tech University, 30 South Puzhu Road, Nanjing, 211816, P. R. China.
E-mail: iamyttao@njtech.edu.cn, iamwhuang@njtech.edu.cn
b College of Chemistry and Chemical Engineering, Inner Mongolia University,
Hohhot 010021, P. R. China
c Key Laboratory for Organic Electronics and Information Displays and Institute of
Advanced Materials, Nanjing University of Posts and Telecommunications,
9 Wenyuan Road, Nanjing 210046, P. R. China.
An appropriate electron-transport material should possess
low-cost synthesis, high electron mobility, decent chemical and
thermal stability, low-lying highest occupied molecular orbital
(HOMO) energy levels to block hole leakages from the emissive
layer and low-lying lowest unoccupied molecular orbital (LUMO)
E-mail: iamxwzhang@njupt.edu.cn
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energy levels to facilitate electron injection to the emissive layer, 2. Experimental
as well as high enough triplet energy levels to confine triplet
2.1 General information
excitons within the emissive layer.8 2,9-Dimethyl-4,7-diphenyl-
1H NMR and 13C NMR spectra were measured on a MECUYR-
VX300 spectrometer. Elemental analyses of carbon, hydrogen,
and nitrogen were performed on a Vario EL III microanalyzer.
Mass spectra were measured on a Bruker autoflex matrix-assisted
laser desorption/ionization time-of-flight (MALDI-TOF). UV-Vis
absorption spectra were recorded on a Shimadzu UV-2500 recording
spectrophotometer. PL spectra were recorded on a Hitachi F-4500
fluorescence spectrophotometer. Differential scanning calorimetry
(DSC) was performed on a NETZSCH DSC 200 PC unit at a heating
rate of 10 1C minꢀ1 from ꢀ40 1C to 300 1C under argon. The glass
transition temperature (Tg) was determined from the second heating
scan. Thermogravimetric analysis (TGA) was carried out using a
NETZSCH STA 449C instrument. The thermal stability of the
samples under a nitrogen atmosphere was determined by
measuring their weight loss while heating at a rate of 20 1C minꢀ1
from 25 to 600 1C. Cyclic voltammetry (CV) was measured in
nitrogen-purged dichloromethane for oxidation scan and tetra-
hydrofuran for reduction scan, using a CHI voltammetric
analyzer. Tetrabutylammonium hexafluorophosphate (TBAPF6)
(0.1 M) was used as a supporting electrolyte. The conventional
three-electrode configuration consists of a platinum working
electrode, a platinum wire auxiliary electrode, and an Ag wire
pseudo-reference electrode with ferrocenium–ferrocene (Fc+/Fc)
as the internal standard. Cyclic voltammograms were obtained
at a scan rate of 100 mV sꢀ1. Formal potentials are calculated as
the average of cyclic voltammetric anodic and cathodic peaks.
The onset potential was determined from the intersection of
two tangents drawn at the rising and background current of the
cyclic voltammogram.
1,10-phenanthroline (BCP) and tris(8-hydroxyquinoline)aluminum
(Alq3) which are the most widely used relatively cheaper electron-
transport materials were normally combined together as the
hole blocking and electron transporting materials in green to red
phosphorescent OLEDs due to their rather low triplet energy
of 2.09 and 2.5 eV,10 respectively. 2,2,2-(1,3,5-phenylene)-tris(1-
phenyl-1H-benzimidazole) (TBPI)11 is another widely used electron
transporting material in blue to red phosphorescent OLEDs due to
its relatively higher triplet energy levels (2.6–2.74 eV) and electron
mobility (3.3–8 ꢁ 10ꢀ5 cm2 Vꢀ1 sꢀ1 at an electric field of 4.7–7 ꢁ
105 V cmꢀ1).12 In our previous work, because of one order higher of
electron mobility, higher triplet energy and better matched LUMO
energy levels, TPBI was used instead of BCP/Alq3 to simplify the
device architectures and improve electron injection and transport,
as well as confine triplet excitons in the emissive layer, and thus
to significantly enhance the device performances in green phos-
phorescent OLEDs.13 Besides, oxadiazole containing 2-(4-biphenylyl)-
5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD)14 and 1,3-bis[(4-tert-
butylphenyl)-1,3,4-oxadiazolyl]phenylene (OXD-7)15 are mostly
employed as ETLs in solution-processable phosphorescent polymer
light-emitting devices, other electron-withdrawing 1,2,4-triazole,
pyridne and phosphine oxide derivatives such as 1,3,5-tri(m-pyrid-
3-yl-phenyl)benzene (Tm3PyPB),16 3-(biphenyl-4-yl)-5-(4-tert-butyl-
phenyl)-4-phenyl-4H-1,2,4-triazole (t-Bu-TAZ)17 and dibenzo[b,d]-
thiophene-2,8-diylbis(diphenylphosphine oxide) (PO15)18 have also
been widely used as HBLs/ETLs in blue PhOLEDs due to their high
triplet energies and favorable energy levels. However, their design
and synthesis rely mainly on a multi-step strategy, which involves in
noble metal catalyzed Ullman and Suzuki cross coupling reactions,
or rigorous conditions and/or the use of harsh reagents such as
thionyl chloride, sulfuric acid and phosphorus (oxy) chloride.
Therefore, in order to reduce the cost for the commercialization
of OLEDs, from the materials point of view, it is highly desirable
to develop organic materials exhibiting an encouraging device
performance with the simplest synthetic chemistry by directly using
commercially available inexpensive starting materials.19
Computational details. The geometrical and electronic properties
of both compounds were performed using the Gaussian 09 program
package. The calculation was optimized by means of the B3LYP
(Becke three parameters hybrid functional with Lee–Yang–Perdew
correlation functionals) with the 6-31G(d) atomic basis sets The
triplet states DE (T1 ꢀ S0) were calculated using time dependent
density functional theory (TD-DFT) calculations with B3LYP/
6-311+g(d). Molecular orbitals were visualized using Gauss view.
Based on this concept, in this study, a novel phenylbenz-
imidazole-derivative is designed as an electron transport material
in comparison with TPBI. By introducing the electron-withdrawing
cyano moiety into the central phenyl ring, the new material
2.2 Synthesis of 2,4,6-tris(2-phenyl-1H-benzo[d]imidazol-
1-yl)benzonitrile (iTPBI-CN)
2,4,6-tris(2-phenyl-1H-benzo[d]imidazol-1-yl)benzonitrile (iTPBI-CN) A mixture of 2-phenylbenzimidazole (1.02 g, 5.2 mmol), 2,4,6-
can be readily synthesized through a simple low-cost one-step C–N trifluorobenzonitrile (0.25 g, 1.6 mmol), and K2CO3 (0.99 g,
coupling reaction by using 2,4,6-trifluoro-benzonitrile and 2-phenyl- 7.2 mmol) in dimethyl sulfoxide (DMSO) (8 mL) was stirred at
1H-benzo[d]imidazole as the starting materials. It should be noted 150 1C for 12 h under an argon atmosphere. After cooling to
that the synthesis of iTPBI-CN shows significant advantage of room temperature, the mixture was poured into water, filtered,
general four steps of 1-phenyl-1H-benzo[d]imidazol-2-yl based TPBI and then purified by column chromatography over silica gel
or its derivatives with TPBI as a core central structure. Moreover, the with CH2Cl2/ethyl acetate (10 : 1) as an eluent to afford a white
1
structure–property relationships between TPBI and iTPBI-CN are solid (0.95 g, 88%). H NMR (300 MHz, (CD3)2SO): d ppm 8.34
systematically investigated both through experimental and theore- (s, 2H), 7.91–7.88 (m, 3H), 7.72–7.61 (m, 12H), 7.48–7.38 (m, 9H),
tical methods. Solution processible green phosphorescent and 7.29–7.27 (d, 3H, J = 6 Hz); 13C NMR (75 MHz, (CD3)2SO,): d ppm
thermally activated delayed fluorescent devices by using the new 155.2, 155.1, 145.8, 145.7, 145.6, 144.3, 138.8, 133.3, 133.1,
iTPBI-CN instead of TPBI as an ETL, both CBP and mCP as hosts, 132.9, 132.6, 132.3, 132.2, 132.0, 126.9, 126.6, 122.8, 122.7, 115.2,
exhibited much better device performances.
114.9, 114.1, 113.5, 113.4, 113.3; MALDI-TOF (m/z): 679.637 [M+];
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anal. calcd for C46H29N7: C 81.28, H 4.30, N 14.42; found:
C 81.35, H 4.09, N 14.58.
2.3 Device fabrication and measurements
The patterned indium tin oxide (ITO) glass substrates were
ultrasonically cleaned with detergent, alcohol and acetone,
deionized water and then dried at 120 1C in a vacuum oven
for more than one hour. After ultraviolet (UV)-ozone treating for
4 min, a 40 nm PEDOT:PSS was spin-coated on the ITO
substrate and dried at 120 1C in a vacuum oven for 15 min to
remove the residual solvent. Afterward, the emissive layers
(EMLs) were spin-coated on top of PEDOT:PSS from chloro-
benzene and then annealed using a hot plate at 80 1C for 20 min to
extract the residual solvent. The thickness of the emissive layer is
about 40–45 nm. Following that, the samples were transferred into
a thermal evaporator chamber. Then ETLs (60 nm), LiF (0.5 or
0.8 nm), and Al (100 nm) were deposited by thermal evaporation
under a pressure of 5 ꢁ 10ꢀ4 Pa, respectively. The active area of the
device is 13.5 mm2.
Scheme 1 Synthetic routes for literature reported commercial TPBI
(ref. 20) and the new compound iTPBI-CN.
The luminance–current–voltage characteristics of the devices
were recorded using a combination of a Keithley source-meter
(model 2602) and a calibrated luminance meter. Electrolumines-
cence (EL) spectra and Commission international de l’eclairage
(CIE) coordinates were obtained using a spectra-scan PR655
spectrophotometer. The thickness of the organic films was
measured using a spectroscopic ellipsometer (a-SE, J. A. Wollam
Co. Inc.). All the measurements were carried out at room
temperature under ambient conditions.
functions as a nitrogen source in the C–N coupling reaction,
thus the nitrogen atom in benzoimidazole (2-phenyl-1H-benzo-
[d]imidazol-1-yl) is linked to the C atom at the central phenyl
ring. However, in TPBI, the carbon atom in benzoimidazole
(1-phenyl-1H-benzo[d]imidazol-2-yl) is linked to the central
phenyl. Therefore, the phenyl-benzo[d]imidazole unit exhibited
an isomerized structure between TPBI and iTPBI-CN. The new
1
compound iTPBI-CN was fully characterized by H and 13C NMR,
mass and elemental analysis. The optimized geometries are
shown in Fig. 1. The dihedral angles involving the C–N linkage
between phenyl and benzoimidazole rings are much larger
than the C–C linkage in both compounds. Therefore, at the
same position, the dihedral angles between the central phenyl
and benzoimidazole ring for iTPBI-CN (62.7–74.21) are B301 larger
than those in TPBI (37.11), suggesting a more twisted structure
of iTPBI-CN due to the isomerized linkage for phenyl-1H-
benzo[d]imidazole as well as the substitution of cyano moieties
to the central phenyl core.
3. Results and discussion
Scheme 1 illustrates the chemical structure and synthetic
routes for TPBI and the new compound iTPBI-CN studied in
this work. As shown, superior advantage for the synthesis of
iTPBI-CN is demonstrated. The preparation of TPBI follows a
four-step literature method.12b,20 Firstly, the reaction of 1-fluoro-2-
nitrobenzene and aniline in the presence of potassium fluoride at
170–180 1C for 72 h gave 2-nitro-N-phenylaniline, followed by the
reduction of the nitro group with stannous chloride dihydrate,
then the resulting diamine derivative reacted with 1,3,5-benzene-
tricarbonyl trichloride to produce the corresponding triamide,
after the final condensation reaction at 250 1C for 3 h, TPBI was
obtained with a total yield of B40%.20 On the other hand, iTPBI-CN
can be facilely synthesized through a one-step aromatic nucleophilic
substitution reaction by using 2-phenyl-1H-benzo[d]imidazole
as a nitrogen nucleophile and electron-withdrawing cyano activated
trifluorobenzene as an electrophile under the conditions of K2CO3
base and DMSO solvent. A high yield of 88% was obtained. More
importantly, all the starting materials are cheap and easily
accessible from commercial resources. Different from our pre-
vious work on this simple one-step catalyst free C–N coupling
reaction employing the electron-donation 9H-carbazole or its alkyl
substituted derivatives,21 in this work, we expanded to the electron-
withdrawing 2-phenyl-1H-benzo[d]imidazole as a nitrogen nucleo-
phile. In the synthesis of iTPBI-CN, phenyl-benzo[d]imidazole
The thermal properties of the new compound were measured
by thermogravimetric analysis (TGA) and differential scanning
calorimetry (DSC) (Fig. 2). The thermal decomposition tempera-
ture (Td, corresponding to 5% weight loss) determined from TGA
Fig. 1 Optimized geometries and corresponding dihedral angles for TPBI
and iTPBI-CN.
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for iTPBI-CN was as high as 410 1C, and the glass transition
temperatures (Tg) observed from DSC curves was 139 1C. Under a
similar star-shaped molecular skeleton, the introduction of one
cyano unit into iTPBI-CN makes its Tg 15 1C higher than
literature record of 124 1C for TPBI.22 The high thermal stability
of the compound could be attributed to the rigid and bulky
configuration.
The UV-Vis absorption and photoluminescence (PL) spectra
at room temperature for both benzoimidazole-based compounds
are shown in Fig. 3, and the peak maxima are summarized
in Table 1. The absorption peak for iTPBI-CN was observed at
278 and 282 nm in chloroform and the film state, respectively,
which can be attributed to p–p* transitions of the molecule,
these data were red-shifted to 304 and 314 nm for TPBI. The
blue-shifted higher energy absorption of iTPBI-CN is clearly
a consequence of steric interactions that prevents effective
conjugation between the phenylbenzimidazole groups and the
central cyanobenzene core.23 However, iTPBI-CN exhibited
significantly red-shifted emission than TPBI at room temperature,
with photoluminescence peaks at 455 and 432 nm in chloroform
and the film state respectively, compared to 364 and 381 nm for
TPBI, which might be induced by the introduction of a strong
electron-withdrawing cyano-moiety. The singlet (ES) and triplet
energy (ET) determined by the highest-energy vibronic sub-band
of the 77 K fluorescence and phosphorescence spectra (Fig. 4) is
TPBI 3.55 and 2.63 eV, respectively, which is in accordance with
previous report.12b Due to the red-shifted emission, the singlet
energy of iTPBI-CN is greatly lowered to 2.99 eV, however, the triplet
energy only slightly reduced to 2.60 eV, indicating that the
introduction of an electron-withdrawing cyano moiety and the
Fig. 2 TGA (a) and DSC (b) curves of iTPBI-CN.
Fig. 4 Solid-state fluorescence and phosphorescence spectra of TPBI
and iTPBI-CN at 77 K.
Fig. 3 Normalized UV-Vis absorption and PL spectra of TPBI and iTPBI-
CN in CHCl3 and the film state at room temperature.
Table 1 Physical properties of TPBI and iTPBI-CN
g
a
b
a
b
c
d
e
Tg/Td [1C]
labs [nm]
labs [nm]
lem [nm]
lem [nm]
ES [eV]
ET [eV]
ET [eV]
HOMO/LUMO f [eV]
TPBI
iTPBI-CN
124/ꢀ
304
278
314
282
364
455
381
432
3.55
2.99
2.63
2.60
2.84
2.95
ꢀ5.89/ꢀ2.38
ꢀ5.90/ꢀ2.79
139/410
a
b
c
Measured in CHCl3 solution at room temperature. Measured in the film state at room temperature. Singlet energy was calculated from low
d
e
temperature (77 K) fluorescence spectra. Triplet energy was calculated from low temperature (77 K) phosphorescence spectra. Triplet energy
was obtained from DFT calculation. Measured from the on-set of oxidation and reduction curves of CV. Tg of TPBI was recorded from ref. 22.
f
g
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Fig. 5 HOMOꢀ1, HOMO, LUMO and LUMO+1 distribution, highest occupied and lowest unoccupied natural transition orbitals (HONTO and LUNTO,
respectively) of lowest singlet (S1) and triplet excited (T1) states for compounds TPBI and iTPBI-CN.
isomerization structure of phenylbenzoimidazole to the central
phenyl core show little influence on triplet energy. ET of the
new low-cost iTPBI-CN is much higher than that of the green
phosphorescent emitter (B2.4 eV), implying that it may act as
an appropriate electron transport material to confine triplet
excitons in the emissive layer for green to red phosphorescent
OLEDs.24
To gain insight into the electronic properties of the com-
pounds, DFT calculations on the frontier molecular orbitals are
performed. Fig. 5 shows the ground state highest occupied
molecular orbitals (HOMO), the lowest unoccupied molecular
orbitals (LUMO), the next highest occupied molecular orbitals
Fig. 6 Cyclic voltammetry for TPBI and iTPBI-CN in CH2Cl2 (oxidation)
and THF (reduction).
(HOMOꢀ1) and the next lowest unoccupied molecular orbitals
(LUMO+1), as well as the highest occupied natural transition
orbitals (HONTO) and lowest unoccupied natural transition
orbitals (LUNTO) of singlet and triplet excited states for both levels are greatly lowered from 2.38 eV of TPBI to 2.79 eV of iTPBI-
TPBI and iTPBI-CN. The two occupied orbitals, HOMO and CN, due to the introduction of a strong electron-withdrawing cyano
HOMOꢀ1 for iTPBI-CN are mainly located on the 1-phenyl-1H- group stabilizing the molecular orbitals.
benzo[d]imidazol-2-yl side group, and the lowest unoccupied
In order to investigate the electron transporting ability of the
orbital is mainly located on the electron-deficient cyanophene- two phenylbenzoimidazolyl containing electron-transport
nyl core, while the LUMO+1 only on the central phenyl core materials, electron-only devices based on an ETL thickness of
with negligible contribution from phenylbenzoimidazolyl side 60 nm with the structure of ITO/LiF (0.8 nm)/TPBI or iTPBI-CN
group but no contribution from the cyano group. The results (60 nm)/LiF (0.8 nm)/Al (100 nm) were fabricated. Data were
for the star-shaped TPBI show that all the HOMO and LUMO obtained in 4 batches of experiments with an average of 16
orbitals are distributed partly over the phenylbenzoimidazolyl devices. From their current density–voltage ( J–V) characteristics
side groups and the central phenenyl group. Similar distribu- (Fig. 7), it is observed that the new compound iTPBI-CN
tions with the ground state HOMO and LUMO distributions can exhibited significant higher current and lower voltage than
also be found in their HONTO and LUNTO singlet excited states TPBI based devices, indicating the more efficient electron-
for both compounds. Both the HONTO and LUNTO triplet- injection from the cathode to the ETL in the iTPBI-CN device.
excited states for TPBI and iTPBI-CN are well controlled and As the LUMO energy levels of iTPBI-CN (2.8 eV) are much lower
located at the 1-phenyl-1H-benzo[d]imidazol-2-yl moiety. As the than TPBI (2.4 eV), the electron injection barrier from cathode
maximum conjugation length is the prior impact factor deter- LiF/Al (2.9 eV)25 to the ETL of iTPBI-CN is 0.1 eV, lower than
mining the triplet energy levels, it is at the same scale of 0.5 eV in TPBI. On the other hand, the higher current at the
phenylbenzoimidazolyl for these two molecules. Therefore, same voltage of iTPBI-CN might suggest better electron trans-
similar ET values are obtained which is consistent with the port properties of iTPBI-CN.
experimental results.
To evaluate the performance of the low cost iTPBI-CN as an
The electrochemical properties of the compounds were probed electron transport material, solution processed green phosphor-
by cyclic voltammetry (Fig. 6). Under the same conditions, TPBI and escent organic light-emitting devices A–D with the structure of
iTPBI-CN showed quasi-reversible oxidation and reduction behavior. ITO/PEDOT:PSS (40 nm)/EML (40–45 nm)/ETL (60 nm)/LiF
Their HOMO and LUMO energy levels were determined from the (0.5 nm)/Al were fabricated. Poly(3,4-ethylenedioxythiophene)
onsets of the oxidation and reduction potentials (relative to the polystyrene sulfonate (PEDOT:PSS) and LiF were used as the
vacuum level), respectively (Table 1). In response to their ground hole- and electron-injecting materials, respectively, CBP or mCP
state HOMO level distributions which are mainly delocalized hosts doped with 6 wt% of tris[2-(p-tolyl)pyridine]iridium(III)
through the phenylbenzoimidazolyl skeleton, both compounds [Ir(mppy)3] were spin-coated as the emissive layers, and the
exhibited similar HOMO levels of B5.9 eV. However, the LUMO newly designed iTPBI-CN was vacuum deposited as the electron
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3.6 V was achieved for both devices, with a maximum luminance
(Lmax) of 36 315 and 26 250 cd mꢀ2, respectively. The maximum
current (Zc,max) and power efficiency (Zp,max) for TPBI based
device A was 26.1 cd Aꢀ1 and 12.2 lm Wꢀ1, which greatly
increased to 37.7 cd Aꢀ1 and 29.0 lm Wꢀ1 in device B by using
iTPBI-CN as an electron transport material, with an Zc,max
improvement of 44% and Zp,max of 134%. Similar trends
can be observed in mCP hosted devices C and D with an ETL.
And maximum current and power efficiency of 20.6 cd Aꢀ1 and
7.4 lm Wꢀ1 for TPBI based on device C and 31.3 cd Aꢀ1 and
23.9 lm Wꢀ1 for iTPBI-CN based on device D were obtained.
In order to further verify the superior performance of iTPBI-
CN over commercial TPBI, devices E–H by using our recently
developed thermally activated delayed fluorescence (TADF) dye
2,3,5,6-tetracarbazole-4-cyano-pyridine (4CzCNPy) were also
investigated.21d The device configuration was slightly changed
as ITO/PEDOT:PSS (40 nm)/host:4CzCNPy (8 wt%, 40–45 nm)/
ETL (60 nm)/LiF (0.8 nm)/Al. The key device data and L–V–J,
Fig. 7 Current density–voltage curves of electron-only devices for TPBI
and iTPBI-CN with the structure of ITO/LiF (0.8 nm)/TPBI or iTPBI-CN
(60 nm)/LiF (0.8 nm)/Al (100 nm), data are obtained from an average of
16 devices.
transporting layer. Simultaneously, the widely used commercial efficiency vs. Luminance curves as well as the EL spectra can be
electron transport material TPBI based devices were also demon- found in Table 2 and Fig. 9(a–d) respectively. As shown, similar
strated for comparison. The luminance–voltage–current density results with phosphorescent devices were observed. Both CBP
(L–V–J) characteristics, current and power efficiency versus lumi- and mCP hosted devices with the new iTPBI-CN as an ETL
nance curves and the EL spectra of the devices are shown in exhibited obvious lower turn-on voltages and higher device
Fig. 8(a–d), and the key device data are summarized in Table 2. efficiencies. Devices E (CBP host) and G (mCP host) based on
As shown, iTPBI-CN exhibited significantly higher device effi- TPBI showed turn-on voltages of 5.1 and 5.6 V, respectively,
ciencies than TPBI in both CBP and mCP hosted devices. In CBP which were reduced to 3.6 and 4.1 V in iTPBI-CN based devices
hosted devices A and B, a relatively lower turn-on voltage (Von) of F and H. The current efficiency in TPBI based devices E and G
Fig. 8 Current density–voltage–luminance (a), current efficiency–luminance (b), power efficiency–luminance curves (c) and EL spectra at 9 V (d) for
green phosphorescent devices A–D.
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Table 2 Electroluminescent properties of the green phosphorescent devices
Lmax (cd mꢀ2
)
Zc,max (cd Aꢀ1
)
Zp,max (lm Wꢀ1
)
CIE (x,y) at 9 V
a
b
c
d
Host/Dopant
ETL
Von (V)
A
B
C
D
E
F
CBP/Ir(mppy)3
CBP/Ir(mppy)3
mCP/Ir(mppy)3
mCP/Ir(mppy)3
CBP:4CzCNPy
CBP:4CzCNPy
mCP:4CzCNPy
mCP:4CzCNPy
TPBI
iTPBI-CN
TPBI
iTPBI-CN
TPBI
iTPBI-CN
TPBI
3.6
3.6
5.2
4.1
5.1
3.6
5.6
4.1
36 315
26 250
13 954
9545
11 166
26 590
21 911
22 830
26.1
37.7
20.6
31.3
9.3
23.4
15.7
20.2
12.2
29.0
7.4
23.9
3.5
10.0
5.7
6.9
(0.32,0.62)
(0.29,0.62)
(0.31,0.61)
(0.28,0.61)
(0.35,0.59)
(0.36,0.60)
(0.34,0.59)
(0.34,0.61)
G
H
iTPBI-CN
a
b
c
d
Turn-on voltage. Maximum luminance. Maximum current efficiency. Maximum power efficiency.
Fig. 9 Current density–voltage–luminance (a), current efficiency–luminance (b), power efficiency–luminance curves (c) and EL spectra at 9 V (d) for
green TADF devices E–H.
was 9.3 and 15.7 cd Aꢀ1, respectively, these values for iTPBI-CN iTPBI-CN to be synthesized directly through a one-step low-cost
containing F and H were increased to 23.4 and 20.2 cd Aꢀ1 catalyst free C–N coupling reaction by using cheap starting
respectively.
materials. Compared to the four step synthesis of the widely
Due to their similar HOMO energy levels to block holes, the reported commercially available electron transporting TPBI,
much better device performances for iTPBI-CN based devices iTPBI-CN exhibited red-shifted emission, similar HOMO levels
may be attributed to the more efficient electron-injection and triplet energies, deeper LUMO and efficient electron injec-
behavior and electron transport properties than in TPBI based tion. By using iTPBI-CN as an electron transportingꢀ1layer,
devices, as discussed in the electron only devices.
maximumꢀc1urrent and power efficiencies of 37.7 cd A and
29.0 lm W have been achieved for CBP and 31.3 cd Aꢀ1 and
23.9 lm Wꢀ1 for mCP hosted solution processed green phos-
phorescent OLEDs. These values in TPBI based devices lower to
26.1 cd Aꢀ1, 12.2 lm Wꢀ1 and 20.6 cd Aꢀ1, 7.4 lm Wꢀ1. And a
4. Conclusion
In conclusion, we have demonstrated a new phenylbenzoimi- superior device performance of the new electron-transport
dazolyl containing electron transport material iTPBI-CN. The iTPBI-CN over TPBI has also been achieved in green TADF
introduction of an electron-withdrawing CN unit has spurred devices. Further work on vacuumed deposited devices as well as
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Journal of Materials Chemistry C
10 V. I. Adamovich, S. R. Cordero, P. I. Djurovich, A. Tamayo,
M. E. Thompson, B. W. D’Andrade and S. R. Forrest, Org.
Electron., 2003, 4, 77.
lifetime for the comparison of iTPBI-CN and TPBI is underway.
The much easier low-cost synthesis and higher device efficiency
suggest great potential of the new electron-transport iTPBI-CN
material for commercial applications.
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Acknowledgements
This work was supported in part by the Funding from 973
Program (No. 2014CB648300), National Natural Science Foun-
dation of China (61474062), Natural Science Foundation of
Jiangsu Province (BK20130919 and BM2012010) and the Foun-
dation for the Author of National Excellent Doctoral Disserta-
tion of China (FANEDD) (201436).
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