Efficient blue organic light-emitting diodes based on triphenylimidazole substituted anthracene derivatives

Article abstract of DOI:10.1016/j.orgel.2015.02.018

A series of new blue emissive materials based on the conjugates of highly fluorescent diaryl anthracene and electron-transporting triphenylimidazole moieties: 2-(4-(anthracen-9-yl)phenyl)-1,4,5-triphenyl-1H-imidazole (ACBI), 2-(4-(10-(naphthalen-1-yl)anthracen-9-yl)phenyl)-1,4,5-triphenyl-1H-imidazole (1-NaCBI), 2-(4-(10-(naphthalen-2-yl)anthracen-9-yl)phenyl)-1,4,5-triphenyl-1H-imidazole (2-NaCBI) were designed and synthesized successfully. These materials exhibit good film-forming properties and excellent thermal stabilities. Meanwhile, the decreased π-conjugation in these compounds compared with phenanthroimidazole derivatives leads to obvious hypsochromic shift. To explore the electroluminescence properties of these materials, typical three-layer organic light-emitting devices were fabricated. With respect to the three layer device 2 using 1-NaCBI as the emitting layer, its maximum current efficiency reaches 3.06 cd A^-1 with Commission Internationale del'Eclairage (CIE) coordinates of (0.149, 0.092). More interestingly, sky blue doped device 5 based on 1-NaCBI achieved a maximum current efficiency of 15.53 cd A^-1 and a maximum external quantum efficiency of 8.15%, high EQE has been proved to be induced by the up-conversion of a triplet excited state.

Full text of DOI:10.1016/j.orgel.2015.02.018

Organic Electronics 21 (2015) 9–18
Contents lists available at ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Efficient blue organic light-emitting diodes based on
triphenylimidazole substituted anthracene derivatives
Guangyuan Mu a, Shaoqing Zhuang a,b, Wenzhi Zhang a, Yixin Wang a, Bo Wang a, Lei Wang a,
,
⇑
Xunjin Zhu b,
⇑
a Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan 430074, People’s Republic of China
b Department of Chemistry, Hong Kong Baptist University, Waterloo Road, Hong Kong, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of new blue emissive materials based on the conjugates of highly fluorescent diaryl
anthracene and electron-transporting triphenylimidazole moieties: 2-(4-(anthracen-9-
yl)phenyl)-1,4,5-triphenyl-1H-imidazole (ACBI), 2-(4-(10-(naphthalen-1-yl)anthracen-9-
yl)phenyl)-1,4,5-triphenyl-1H-imidazole (1-NaCBI), 2-(4-(10-(naphthalen-2-yl)anthracen-
9-yl)phenyl)-1,4,5-triphenyl-1H-imidazole (2-NaCBI) were designed and synthesized
successfully. These materials exhibit good film-forming properties and excellent thermal
Received 25 February 2014
Received in revised form 19 January 2015
Accepted 16 February 2015
Available online 23 February 2015
Keywords:
Blue material
stabilities. Meanwhile, the decreased
p-conjugation in these compounds compared with
phenanthroimidazole derivatives leads to obvious hypsochromic shift. To explore the elec-
troluminescence properties of these materials, typical three-layer organic light-emitting
devices were fabricated. With respect to the three layer device 2 using 1-NaCBI as the
emitting layer, its maximum current efficiency reaches 3.06 cd Aꢀ1 with Commission
Internationale del’Eclairage (CIE) coordinates of (0.149, 0.092). More interestingly, sky blue
doped device 5 based on 1-NaCBI achieved a maximum current efficiency of 15.53 cd Aꢀ1
and a maximum external quantum efficiency of 8.15%, high EQE has been proved to be
induced by the up-conversion of a triplet excited state.
Anthracene
1,4,5-Triphenyl-1H-imidazole
Organic light-emitting devices
Triplet–triplet annihilation
Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction
be utilized to generate light of other colors by energy cas-
cade to lower energy fluorescent to realize white light. Red
In the past decade, organic light-emitting diodes
(OLEDs) have attracted much scientific and commercial
interest because of their potential application in full-color
displays and large-area, flexible, light-weight light sources
[1]. Full-color displays require red, green, and blue emis-
sions of relatively equal stability, efficiency, and color puri-
ty. Specifically, highly efficient blue electroluminescent
devices with Commission Internationale del’Eclairage
(CIE) y-coordinate value <0.10, can not only effectively
reduce the power consumption of a full-color OLED but
and green phosphorescent electroluminescent devices
with high efficiencies, long lifetimes, and proper CIE coor-
dinates have been well developed. However, blue phos-
phorescent devices are still the bottleneck for the high
CIE coordinates (y-coordinate value >0.30), high cost and
short device lifetime [2]. To address the problem, a con-
tinuous effort has been contributed to develop highly effi-
cient blue-light emitters with good color purity and high
efficiency [3].
Anthracene derivatives have been studied extensively as
blue-light-emitting materials in OLEDs because of their
excellent photoluminescence (PL) and electroluminescence
(EL) properties [4]. For example, Li’ group [5] have synthe-
sized some fluorene derivatives with naphthylanthracene
⇑
Corresponding authors. Tel./fax: +86 27 87793032 (L. Wang).
E-mail addresses: wanglei@mail.hust.edu.cn (L. Wang), xjzhu@hkbu.
edu.hk (X. Zhu).
http://dx.doi.org/10.1016/j.orgel.2015.02.018
1566-1199/Ó 2015 Elsevier B.V. All rights reserved.

10
G. Mu et al. / Organic Electronics 21 (2015) 9–18
endcaps and achieved device efficiency of 2.10 lm Wꢀ1, col-
or coordinates (0.16, 0.09). Shu’s group [6] have developed
2-tert-butyl-9,10-bis[40-(diphenylphosphoryl)phenyl]an-
thracene (POAn) with a maximum luminance efficiency of
2.9 cd Aꢀ1, CIE coordinates of (0.15, 0.07). On the other
hand, phenanthroimidazole derivatives have attracted
great attention as electroluminescent materials because of
their high thermal stability and efficient electron
transporting ability. Huang’s group [7] exploited
bis(phenanthroimidazolyl)biphenyl derivatives as excel-
lent non-doped blue emitting materials with optimized
device efficiency of 7.3 lm Wꢀ1, CIE coordinates of (0.15,
0.14). Using phenanthroimidazole as a building block for
luminescent materials, Ma’s group [8] fabricated pure blue
devices with luminance efficiency of 6.87 cd Aꢀ1, CIE coor-
dinates of (0.15, 0.21) and a turn on voltage of 2.8 V. The
impressive device performances of 2.63 cd Aꢀ1 CIE (0.15,
0.09) and 5.66 cd Aꢀ1, (0.15, 0.11) based on donor-linker-
acceptor structural deep-blue emitting phenanthroimida-
zole derivatives have been reported by Tong and Ma’s
group, respectively [9]. Very recently, an promising device
performance with EQE of 7.8% and current efficiency of
10.4 cd Aꢀ1 has been reported based on an excellent blue
dopant PPIE containing n-type imidazole moiety [10]. In
the subsequent research, the high EQE is proved to be
induced by the triplet–triplet annihilation (TTA). In spite
of their successful performance in a given aspect, satisfacto-
ry blue luminescent materials are still rare.
In our previous work, some hybrid bipolar phosphores-
cent hosts for green and orange OLEDs have been synthe-
sized by conjugating carbazole moiety to the rigid
skeleton of 1,2-diphenyl-1H-phenanthro[9,10-d]imidazole
[11]. We also developed some asymmetric phenan-
throimidazole C2-phenyl or N1-phenyl position substituted
anthracene derivatives for efficient blue OLEDs [12]. The
introduction of anthracene moieties can effectively increase
the electron injection and transport ability and also finely
adjust the ionization potentials (Ip) of the compounds,
resulting in reduced hole injection barrier and balanced
recombination ability. In this paper, we introduced the
bulky non-planar triphenylimidazole instead of phenan-
throimidazole moiety to obtain deeper blue fluorescent
materials. As expected, these new materials show excellent
thermal stabilities, proper HOMO levels and excellent EL
performances with low onset voltages. Three layer device
2 using 1-NaCBI as the emitting layer, achieved a maximum
current efficiency of 3.06 cd Aꢀ1 with Commission
Internationale del’Eclairage (CIE) coordinates of (0.149,
0.092). In addition, device 5 based on doping BUBD-1 in
1-NaCBI, the maximum current efficiency reaches
15.53 cd Aꢀ1 with maximum external quantum efficiency
of 8.3%. The high EQE has been proved to be induced by
the up-conversion of a triplet excited state.
1H NMR spectra were recorded on a Bruker-AF301 at
400 MHz. Mass spectra were carried out on an Agilent
MALDI-TOF. Elemental analyzes of carbon, hydrogen, and
nitrogen were performed on an Elementar (Vario Micro
cube) analyzer. Fluorescence spectra and transient fluores-
cence decay lifetimes were obtained on Edinburgh instru-
ments (FLSP920 spectrometers) and UV–Vis absorption
spectra were measured using a Shimadzu UV–Vis–NIR
Spectrophotometer (UV-3600). The differential scanning
calorimetry (DSC) analyzes were performed under a nitro-
gen atmosphere at a heating rate of 10 °C/min using a PE
Instruments DSC 2920. Thermogravimetric analyzes
(TGA) were undertaken using a PerkinElmer Instruments
(Pyris1 TGA) under nitrogen atmosphere at a heating rate
of 10 °C/min. Atomic force microscopies (AFMs) were
measured using Shimadzu (SPM9700). To measure the
photoluminescence (PL) quantum yields (UF), degassed
solutions of the compounds in CH2Cl2 were prepared. The
concentration was adjusted so that the absorbance of the
solution would be between 0.05 and 0.1. The excitation
was performed at 330 nm and 9,10-diphenylanthracene
(DPA) in cyclohexane (
U = 0.9 in cyclohexane) was used
as a standard [13]. Cyclic voltammetry (CV) measurements
were carried out in a conventional three electrode cell
using a Pt button working electrode of 2 mm in diameter,
a
platinum wire counter electrode, and
a Ag/AgNO3
(0.1 M) reference electrode on computer-controlled
a
EG&G Potentiostat/Galvanostat model 283 at room tem-
perature. Oxidations of all compounds were performed in
dichloromethane containing 0.1 M tetrabutylammonium
hexafluorophosphate (Bu4NPF6) as supporting electrolyte.
The onset potential was determined from the intersection
of two tangents drawn at the rising and background
current of the cyclic voltammogram and calibrated to the
ferrocene/ferrocenium (Fc|Fc+) redox couple. DFT calcula-
tions were performed to characterize the 3D geometries
and the frontier molecular orbital energy levels of ACBI,
1-NaCBI and 2-NaCBI at the B3LYP/6-31G⁄ level by using
the ADF2009.01 program.
2.2. Device fabrication and measurement
The EL devices were fabricated by vacuum thermal
evaporation technology according to the methods modified
from our previous approach [11,12]. Before the deposition
of an organic layer, the clear ITO substrates were treated
with oxygen plasma for 5 min. The deposition rate of
organic compounds was 0.9–1.1 Å sꢀ1. Finally, a cathode
composed of cesium pivalate (2 nm) and aluminum
(100 nm) was sequentially deposited onto the substrate
in the vacuum of 10ꢀ5 Torr. The L–V–J curves of the devices
were measured with a Keithley 2400 Source meter and
PR655. All measurements were carried out at room tem-
perature under ambient conditions.
2. Experimental section
3. Results and discussion
2.1. Material and methods
3.1. Synthesis
All reagents and solvents were used as purchased from
Aldrich without further purification. Most of experiment
methods were according to our published results [11,12].
The structures and synthetic routes of the three well-de-
fined compounds are shown in Scheme 1. Anthracen-9-yl-

G. Mu et al. / Organic Electronics 21 (2015) 9–18
11
NH2
H
C O
O
O
N
N
N
+
+
i) NH4Ac
Br
ArB(OH)
2
ii)
c
HA
N
Pd(PPh3)4
Br
1
ACBI
ii
ii
N
N
N
N
1-NaCBI
2-NaCBI
Scheme 1. Synthetic routes for ACBI, 1-NaCBI and 2-NaCBI. Reagents and conditions: (i) CH3COONH4, CH3COOH, reflux; (ii) toluene, K2CO3, ethanol, aryl
boric acid, Pd(PPh3)4, reflux.
Table 1
The optical, photophysical and thermal properties of ACBI, 1-NaCBI, and 2-NaCBI.
Compound HOMO/LUMO (eV)a HOMO/LUMO (eV)b UF
Eg (eV) Eox (V) PL (nm)
Solution (FWHMs) Film (FWHMs)
Abs (nm)
Tg/Tm/Td (°C)
ACBI
1-NaCBI
2-NaCBI
ꢀ5.54/ꢀ2.47
ꢀ5.56/ꢀ2.56
ꢀ5.53/ꢀ2.55
ꢀ5.18/ꢀ2.06
ꢀ5.17/ꢀ2.23
ꢀ5.14/ꢀ2.28
0.70 3.07
0.65 3.00
0.56 2.98
0.88
0.90
0.87
420 (57)
428 (54)
434 (53)
443 (76)
439 (57)
445 (51)
350, 368, 388 123/282/402
358, 377, 398 165/NA/412
359, 378, 398 161/336/420
a
Determined from the onset of oxidation potentials and the Eg = HOMO–LUMO.
Values from DFT calculation.
b
boronic acid, 9-(1-naphthyl)anthracene-10-boronic acid,
and 9-(2-naphthyl)anthracene-10-boronic acid were syn-
thesized according to the literature [14]. The intermediate
2-(4-bromophenyl)-1,4,5-triphenyl-1H-imidazole was
synthesized in high yield [15]. The target products were
prepared through the palladium-catalyzed Suzuki coupling
reactions between the intermediate and the corresponding
aryl boronic acid, then purified by column chromatography
on silica gel using petroleum ether-dichloromethane as the
eluent. Repeated temperature-gradient vacuum sublima-
tion are required for further purification of these materials
used in OLEDs. All the new compounds were fully charac-
terized by 1H and 13C NMR, mass spectrometry, and ele-
mental analysis. The detailed procedure, 1H, 13C NMR and
MS spectra for preparation of compounds are summarized
in Supporting Information.
and 336 °C, respectively. From ACBI to 2-NaCBI, the
enhancement of Tg can be attributed to the increase of effi-
cient molecular weight. High Tg and Td values indicate that
these compounds are stable and have the potential to be
fabricated into devices by vacuum thermal evaporation
technology, which is highly desirable for high performance
OLED applications.
3.3. Morphology properties
Since good film-forming properties of light emitting
materials are crucial for the total performance of the
devices, the surface morphologies of vacuum-deposited
thin films of the three new compounds, ACBI, 1-NaCBI
and 2-NaCBI, were studied by atomic force microscopy
(AFM). For a direct comparison, we prepared both unan-
nealed films and thermally annealed samples films at
120 °C for 2 h under an N2 atmosphere. As shown in Fig. 2,
the unannealed film samples exhibit a root-mean-square
(RMS) roughness of 0.816, 0.706 and 0.667 nm for ACBI,
1-NaCBI and 2-NaCBI, respectively. The annealed film exhi-
bits a smooth surface morphology with a root-mean-square
(RMS) roughness of 0.477, 0.575 and 0.373 nm for ACBI, 1-
NaCBI and 2-NaCBI, respectively. The small RMS difference
between unannealed and annealed film samples suggests
their excellent thermal and amorphous stability, which
indicates that attaching triphenyl-1H-imidazole moiety to
anthracence’s 9-position may influence the arrangement
of the molecules in the thin films and makes them
desirable for good performance OLEDs. After annealing,
3.2. Thermal properties
The thermal properties of the three compounds were
investigated by thermal gravimetric analysis (TGA) and dif-
ferential scanning calorimetry (DSC) under a nitrogen
atmosphere, and the related data are listed in Table 1.
Compounds ACBI, 1-NaCBI and 2-NaCBI exhibit good ther-
mal stability with decomposition temperatures (Td, 5%
weight loss) at 402, 412 and 420 °C, respectively. As shown
in Fig. 1, Obvious glass transition temperatures (Tg) are
observed at 123, 165, 161 °C for ACBI, 1-NaCBI and 2-
NaCBI, respectively, while only ACBI and 2-NaCBI show
endothermic melting transition temperatures (Tm) at 282

12
G. Mu et al. / Organic Electronics 21 (2015) 9–18
100
(b)
Tc 260 oC
(a)
Tc 195 oC
ACBI
1-NaCBI
2-NaCBI
90
80
Tm 336 oC
70
ACBI
1-NaCBI
2-NaCBI
Tm 282 oC
60
50
40
30
20
10
0
Tg 161 oC
Tg 123 oC
Tg 165 oC
150
120
180
100 150 200 250 300 350 400
100
200
300
400
500
600
700
Temperature (oC)
Temperature (oC)
Fig. 1. (a) DSC curves of ACBI, 1-NaCBI and 2-NaCBI; (b) TGA curves of ACBI, 1-NaCBI and 2-NaCBI.
ACBI
1-NaCBI
2-NaCBI
Fig. 2. AFM topographic images of the three compounds ACBI, 1-NaCBI and 2-NaCBI in unannealed and annealed films.
the roughness of the three films become a little smaller, it
can be induced by the very slightly rearrangement of mole-
cular morphology.
ACBI. Also the energy levels of HOMO is about 0.08–
0.12 eV higher than those of the phenanthroimidazole-
substituted anthracene derivatives [12].
3.4. Theoretical calculations
3.5. Photophysical properties
The three-dimensional geometries and the frontier
molecular orbital energy levels of these compounds were
calculated using density functional theory. It was obvious
that the planarities of those phenanthroimidazole-substi-
tuted anthracene derivatives were broken by the bulky
and non-planar triphenylimidazole moiety. The resulted
HOMO and LUMO distribution are depicted in Table 1. As
shown in Fig. 3, the spatial distributions of electron densi-
ties of HOMO and LUMO for all the three compounds are
mostly localized on the anthracene units, it implies that
the absorption and emission process may mainly be
Fig. 4(a) shows the UV–Vis spectra of compounds ACBI,
1-NaCBI and 2-NaCBI at room temperature in a CH2Cl2 solu-
tion. And Fig. 4(b) shows the fluorescence spectra of com-
pounds ACBI, 1-NaCBI and 2-NaCBI at room temperature
for solutions in CH2Cl2 and solid film. The related photo-
physical properties are summarized in Table 1. All of these
compounds have similarly structured absorption spectra
in the range of 340–425 nm in CH2Cl2, which are assigned
to the p–
p⁄ transition of the characteristic vibrational struc-
tures of anthracene groups [12]. The 10 nm red shift for 1-
NaCBI and 2-NaCBI compared to ACBI was observed and
attributed to the
p–
p⁄ transition centered at the anthra-
cene moiety, which is the same with the compound ACPI,
1-NaCPI and 2-NaCPI [12]. The HOMO level of ACBI, 1-
NaCBI and 2-NaCBI locate at ꢀ5.18, ꢀ5.17 and ꢀ5.14 eV,
respectively. As a comparison with ACBI, the HOMO levels
for 1-NaCBI and 2-NaCBI increase in a value of about 0.01–
attributed to the increased p-conjugation. And upon excita-
tion at 330 nm, the maximum emission wavelengths of
ACBI, 1-NaCBI and 2-NaCBI in CH2Cl2 were observed at
420, 428 and 434 nm, respectively. Compared with ACPI,
1-NaCPI and 2-NaCPI, hypsochromic shifts of 3–7 nm were
attributed to the decreased conjugation by the bulky and
nonplanar triphenylimidazole moiety. The maximum
0.04 eV for the elongation of
p-conjugation when introduc-
ing naphthyl group on the 9-position of anthracene of

G. Mu et al. / Organic Electronics 21 (2015) 9–18
13
ACBI
1-NaCBI
2-NaCBI
HOMO
LUMO
Fig. 3. Three-dimensional structures and calculated HOMO and LUMO density maps of ACBI, 1-NaCBI and 2-NaCBI.
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
(a)
ACBI film
1-NaCBI film
2-NaCBI film
ACBI
1-NaCBI
2-NaCBI
(b)
ACBI
1-NaCBI
2-NaCBI
300
350
400
450
400
450
500
550
600
Wavelength (nm)
Wavelength (nm)
Fig. 4. (a) Normalized absorption spectra of solutions in dichloromethane and (b) normalized photoluminescence spectra of solutions in dichloromethane
and solid films for ACBI, 1-NaCBI and 2-NaCBI.
emission peaks of ACBI, 1-NaCBI and 2-NaCBI thin films are
red-shifted by 23, 19, and 19 nm compared with those of
solutions in CH2Cl2, respectively, which are slightly lower
than the corresponding values of 29, 24 and 20 nm for ACPI,
1-NaCPI and 2-NaCPI, respectively [12]. In addition, the
FWHMs of 1-NaCBI and 2-NaCBI are much shorter than
ACBI. These features in film state are reasonable for
decreased aggregation by greater steric hindrance of
1-naphthalene moiety and non-plannarity property of
triphenylimidazole moiety. Using 9,10-diphenylanthracene
as a standard, the PL quantum yields (UF) were determined
as high as 0.70, 0.65 and 0.56, for ACBI, 1-NaCBI and 2-
NaCBI, respectively. The high PL quantum yield along with
good film-forming property and excellent thermal stability
indicate those compounds are excellent candidates as effi-
cient emitting materials in OLEDs.
3.6. Electrochemical properties
To further evaluating the feasibility of those new mate-
rials for optoelectronic applications, the electrochemical
behaviors of ACBI, 1-NaCBI and 2-NaCBI were examined
by cyclic voltammetry (Fig. 5). The onset oxidation peaks
(Eox) for ACBI, 1-NaCBI and 2-NaCBI are 0.88, 0.90, and
0.87 eV versus the Ag/Ag+ redox couple, respectively. And
the HOMO levels were calculated to be ꢀ5.54, ꢀ5.56 and
ꢀ5.53 eV for ACBI, 1-NaCBI and 2-NaCBI, respectively
(Table 1) by the method ꢀ(Eox + 4.66) eV calibrated to the
Ag/Ag+ redox couple. The HOMO levels for ACBI, 1-NaCBI
and 2-NaCBI are higher than that of ACPI, 1-NaCPI and
2-NaCPI, respectively [12]. It could be rationalized that
the decreased conjugation might reduce the oxidation
potentials of the molecule and increase the HOMO energy

14
G. Mu et al. / Organic Electronics 21 (2015) 9–18
host exhibit a maximum brightness of 9007, 6372, 11,520
and 5831 cd mꢀ2, luminous efficiency of 2.94, 3.06, 4.49
and 3.03 cd Aꢀ1, low threshold voltage of 2.92, 2.63, 2.62
and 3.62 V, respectively (see Table 2). Compared with the
high turn-on voltage of MADN and previous reported
phenanthroimidazole-substituted anthracene derivatives
[12], the lower turn on voltages of the devices based on
the three compounds can be attributed to the matched
HOMO levels of the hole-transporting layer and the
emissive layer. The EL spectra of 1, 2 and 3 show blue light
emission with peaks at 460, 448 and 452 nm, respectively
(Fig. 7(a)). Narrow EL peaks in the ACBI, 1-NaCBI and 2-
NaCBI devices with FWHMs of 71, 58 and 61 nm, respec-
ACBI
1-NaCBI
2-NaCBI
tively, are observed. At a current density of 20 mA cmꢀ2
,
0.0
0.2
0.4
0.6
0.8
1.0
1.2
+
1.4
1.6
Potential (V vs Ag/Ag )
the CIE coordinates of devices 1, 2 and 3 are (0.152,
0.137), (0.149, 0.092) and (0.161, 0.150), respectively (see
Table 2). Although ACBI exhibits the shortest emission in
solution, aggregation induces bathochromic shift in EL
spectrum. Compared with ACPI, 1-NaCPI and 2-NaCPI with
almost the same device structure, ACBI, 1-NaCBI and
2-NaCBI exhibit lower CIEy. The results demonstrated that
the introduction of bulky and non-planar triphenylimida-
Fig. 5. The CV measurement of ACBI, 1-NaCBI and 2-NaCBI.
level. This trend is in good agreement with the calculated
values and confirms the above hypothesis. Higher HOMO
levels reduce the energy barriers between hole transport
and emitting layers and will facilitate hole injection into
the emission layer.
zole moiety with decreased p-conjugation is beneficial for
achieving deeper blue emitting material. Moreover, device
2 using 1-NaCBI as emitting layers shows the shortest
emission peak, the narrowest FWHMs and the lowest value
of CIEy, which is consistent with its PL of solid thin film.
Take the widely used deep blue fluorescent material MADN
as reference, the device performance of 1-NaCBI is superior
in turn-on voltage and efficiency based on the same device
structure.
3.7. Electroluminescence
To evaluate the EL performances of ACBI, 1-NaCBI and 2-
NaCBI as blue emitters, devices with three layers: anode/
HTL/emitter/ETL/cathode were fabricated. In these devices,
indium tin oxide (ITO) was used as the anode, 4,40,400-tris[N-
(1-naphthyl)-N-phenylamino]triphenylamine (2-TNATA)
was selected as the hole-injection layer, 4,4-bis[N-(1-naph-
thyl)-N-phenyl-l-amino]biphenyl (NPB) was used as the
hole transporting layer (HTL), 1,3,5-tris(1-phenyl-1H-
benzimidazol-2-yl)benzene (TPBI) was used as the electron
transporting layer (ETL), and (CH3)3CCOOCs was used as the
electron injecting layer [16]. The relative HOMO/LUMO
energy levels of the materials are illustrated in Scheme 2,
and detailed structures and key performance of these
devices are summarized in Table 2.
Non-doped devices 1–4 were fabricated with typical
configuration as follows: ITO/2-TNATA(60 nm)/NPB
(10 nm)/emitting layer (30 nm)/TPBI (10 nm)/(CH3)3
CCOOCs (1 nm)/Al (100 nm). The current density–voltage–
luminance (I–V–L) and current density–current efficiency
characteristics are shown in Fig. 6. Simple devices 1–4
utilizing compounds ACBI, 1-NaCBI, 2-NaCBI and reference
2-methyl-9,10-bis(naphthalen-2-yl)anthracene (MADN) as
Inspired by the impressive device performance of the
non-doped device 2, we further explore the possibility of
using this new blue emissive material as host in doped
sky blue device. For this purpose, we fabricated device 5
utilizing 1-NaCBI as the host and a sky blue fluorescent
material BUBD-1 as the dopant with the following struc-
ture: ITO/2-TNATA(60 nm)/NPB (10 nm)/emitting layer
(30 nm):3% BUBD-1/TPBI (10 nm)/(CH3)3CCOOCs (1 nm)/
Al (100 nm). The detailed device performances and EL
emission characteristics are summarized in Table 2. Appar-
ently, the EL spectrum of device 5 shows a similar blue
light emission to the emitter BUBD-1 [17] with peaks at
472 and 496 nm and CIE of (0.151, 0.299) at a current den-
sity of 20 mA cmꢀ2 (see Fig. 7(b)), indicating an efficient
energy transfer from the host to the dopant. As described
in Fig. 8, device 5 shows a maximum brightness of
10,570 cd mꢀ2, luminous efficiency of 15.53 cd Aꢀ1, and
maximum external quantum efficiency (EQE) of 8.15 %.
The efficiency of device 5 using 1-NaCBI as the host is
much better than the published literature values [12,17].
Devices 6 and 7 with 2-NaCBI and ACBI as the host were
also fabricated, which exhibit a maximum external quan-
tum efficiency of 6.75% and 5.89%, respectively. The typical
emission from BUBD-1 means that the energy transfer
from the hosts to the dopant is complete.
2
2.3
2.5
2.47
6.20
2.56
2.55
2.7
LUMO
H )
s
3CCOOC /
Al
(
C
3
4
6
ITO
N P B
A C B I
T P B I
1 - N a C B I
2 - N a C B I
2 T N A T A
HOMO
It is worth noting that the maximum EQE of all doped
devices are above the classical estimate for the maximum
EQE of fluorescent OLEDs without enhanced optical out
coupling [18], especially for the device 5. Two possible
mechanisms for the up-conversion of a triplet excited state
5.10
5.56
5.2
5.3
5.53
5.54
6.2
Scheme 2. Energy-level diagram of the materials used in the devices.

G. Mu et al. / Organic Electronics 21 (2015) 9–18
15
Table 2
EL performance of devices 1–7.
Device
Von (V)a
Lmax (voltage)
(cd mꢀ2) (V)b
gP (lm Wꢀ1)c/d
gC (cd Aꢀ1)e/f
gext (%)g/h
L (voltage)
kem (FWHM)
CIE (x,y)k
(cd mꢀ2) (V)i
(nm)j
1
2
3
4
5
6
7
2.80
2.63
2.62
3.62
2.61
2.70
4.70
9007 (9.00)
6372 (9.50)
1.70 (2.11)
1.94 (2.52)
2.91 (3.88)
1.32 (1.88)
3.88 (7.41)
4.57 (7.21)
2.59 (4.79)
2.90 (2.94)
3.03 (3.06)
4.30 (4.49)
2.59 (3.03)
12.78 (15.53)
12.37 (12.58)
9.91 (11.41)
2.59 (2.66)
3.49 (3.50)
3.54 (3.55)
3.15 (3.67)
7.92 (8.15)
6.66 (6.74)
5.15 (5.89)
594 (5.33)
620 (4.89)
910 (4.79)
514 (6.23)
2592 (10.33)
2322 (8.50)
1878 (12.00)
460 (71)
448 (58)
452 (61)
456 (66)
472 (57)
468 (53)
468 (55)
(0.152, 0.137)
(0.149, 0.092)
(0.161, 0.150)
(0.144, 0.092)
(0.151, 0.299)
(0.162, 0.274)
(0.158, 0.296)
11,520 (7.75)
5831 (9.90)
10,570 (13.50)
15,640 (11.5)
18,170 (15.00)
a
b
c
d
e
f
Von: turn-on voltage.
Lmax: maximum luminance. Voltage: voltage at the maximum luminance.
P: power efficiency measured at 20 mA cmꢀ2
Maximum power efficiency.
C: current efficiency measured at 20 mA cmꢀ2
Maximum current efficiency.
g
.
g
.
g
h
i
g
ext: external quantum efficiency measured at 20 mA cmꢀ2
Maximum external quantum efficiency.
L: luminance measured at 20 mA cmꢀ2. Voltage: voltage at the luminance.
.
j
V: FWHM: full width at half maximum at 20 mA cmꢀ2
.
k
kem: CIE at 20 mA cmꢀ2
.
500
450
400
350
300
250
200
150
100
50
5
4
3
2
1
0
5
4
3
2
1
0
ACBI
ACBI
(a)
(b)
10000
1000
100
1-NaCBI
2-NaCBI
MADN
1-NaCBI
2-NaCBI
MADN
-1
10
-2
-3
0
2
3
4
5
6
7
8
9
10 11
0
100 200 300 400 500 600 700 800 900
Current Density (mA/cm2)
Voltage (V)
Fig. 6. (a) Voltage–current density–luminance (V–J–L) characteristics and (b) efficiency curves for blue devices 1–4.
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
1
2
3
4
(b)
(a)
5
350 400 450 500 550 600 650 700 750
350 400 450 500 550 600 650 700 750
Wavelength(nm)
Wavelength (nm)
Fig. 7. (a) EL spectra of devices 1–4 and (b) EL spectrum of device 5.
(T1) into a singlet excited state (S1) have already been pro-
posed to improve EQE of fluorescent OLEDs. One is triplet–
triplet annihilation (TTA) and the other is thermally acti-
vated delayed fluorescence (TADF) [19]. For TADF, the
up-conversion of exciton occurs through reverse intersys-
tem crossing (RISC) process, thus small
DEST is an essential
factor. Yet to achieve small EST, a small overlap between
D
HOMO and LUMO is considered to be necessary. Here, we
tried to test the triplet energy level of these compounds
in 77 K, but failed. It should be noted that the
DEST of all

16
G. Mu et al. / Organic Electronics 21 (2015) 9–18
12
10
8
18
350
300
250
200
150
100
50
5
6
7
(b)
15
5
6
7
10000
(a)
12
1000
9
6
6
100
3
4
0
10
1
2
-3
0
0
2
3
4
5
6
7
8
9
10 11 12 13 14 15 16
0
40
80
120
160
200
240
Current Density (mA/cm2)
Voltage (V)
Fig. 8. (a) Voltage–current density–luminance (V–J–L) characteristics and (b) efficiencies curves for devices 5, 6 and 7.
reported anthracene derivatives was very large. Mean-
while, to achieve small EST, a small overlap between
1-NaCBI/2-NaCBI/ACBI
D
HOMO and LUMO is considered to be necessary [20].
However, a well overlap between HOMO and LUMO of
ACBI, 1-NaCBI and 2-NaCBI shown above excludes the
assumption (see Fig. 3). In addition, transient fluorescence
decay lifetime excited at 330 nm in a film of 1-NaCBI (3%
doped) was also measured. The fluorescence emission
displays a dual-exponential decay with s1 of 0.759 ns and
TTA
2ET
ES
energy transfer
ES
ET
BUBD-1
s2 of 2.488 ns, respectively (Fig. 9(a)). The short decay life-
excitation
light emission
time indicates that the singlet excitons involved in emis-
sion are not generated through TADF process, otherwise a
l
s tail should be observed in the decay curve. On the other
Scheme 3. The process of TTA and energy transfer.
hand, transient electroluminescence response of the 3%
doping blue emitting device 5 is shown in Fig. 9(b). The
EL intensity rapidly decreased in the sub-microseconds at
the end of the excitation pulse. It is clear that the TTA
occurs in the electroluminescence process. After TTA, the
energy of new produced singlet excitons were transferred
to BUBD-1. The energy transfer process of devices have
been demonstrated in Scheme 3. The ratio of the delayed
component depicted in Fig. 9(b) show a nonmonotonic
transformation, which was increased with the excitation
pulse voltage from 5.8 V to 6.5 V and then decreased with
more higher driving voltage. These results can be
explained by the two competitive processes in electrolumi-
nescence. At the low current density, the contribution from
the TTA process dominates the delayed fluorescence. While
with the increasing operating voltage, the initially bal-
anced charge injection will be changed, and the surplus
charge will react with the triplet exciton and triplet-charge
annihilation (TCA) process will be unfavorable for delayed
fluorescence emission through TTA [21]. The regular is also
in accordance with the efficiency roll-off curve.
In order to further investigate the high EQE of device 5,
we also fabricated electron and hole only device with the
following structure: ITO/LiF (1 nm)/TPBi (40 nm)/1-NaCBI
(30 nm):0–3%
BUBD-1/TPBi
(40 nm)/LiF
(1 nm)/Al
1
(a)
(b)
5.8V
6.5V
7.5V
8.5V
3%
1000
100
10
1
0.1
0
2
4
6
8
10 12 14 16 18 20
0
20
40
Time (µsec)
Time (ns)
Fig. 9. (a) Transient fluorescence decay lifetime of doping film in 1-NaCBI; (b) transient electroluminescence decay lifetime of device 5 with various
excitation pulse voltages.

G. Mu et al. / Organic Electronics 21 (2015) 9–18
17
1000
800
600
400
200
0
1000
(a)
(b)
0% BUBD-1
3% BUBD-1
0% BUBD-1
3% BUBD-1
800
600
400
200
0
2
4
6
8
10 12 14 16 18
8
10
12
14
16
18
20
Voltage (V)
Voltage (V)
Fig. 10. (a) Voltage–current density characteristics for electron-only devices and (b) hole-only devices.
(100 nm) and ITO/MoO3 (10 nm)/NPB (10 nm)/1-NaCBI
Appendix A. Supplementary material
(30 nm):0–3% BUBD-1/NPB (40 nm)/ MoO3 (10 nm)/Al
(100 nm). For the electron-only devices (see Fig. 10(a)),
no obvious current density change was observed between
doped and undoped device. However, dramatic decrease in
current density exhibited from doped device to undoped
device for hole-only devices. It can be attributed to the hole
capture of BUBD-1. The large energy level offset between
HOMO of NPB and BUBD-1 would trap charge carriers
and thus reduce current density. On one hand, moderate
trap effect of the dopant further balances the charge in
emission layer and increases the efficiency. On the other
hand, regrettably, the more trapped holes will lead to sin-
glet (or triplet)-charge annihilation and induce the effi-
ciency roll-off when the current density is increased.
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/
j.orgel.2015.02.018.
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