Efficient deep-blue emitters based on triphenylamine-linked benzimidazole derivatives for nondoped fluorescent organic light-emitting diodes

Article abstract of DOI:10.1016/j.orgel.2013.06.022

Two triphenylamine-substituted benzimidazole derivatives were synthesized for use as efficient deep-blue emitters in nondoped fluorescent organic light-emitting diodes (FLOLEDs). The molecular design of 40,40 0-(1H-benzo[d]imidazole-1,2-diyl)bis(N,N-diphenylbiphenyl-4-amine) (T2B) to limit the molecular packing density enabled T2B-based devices to suppress the exciton quenching by a bulky three-dimensional structure. Nondoped FLOLEDs fabricated using T2B as a blue emitter exhibited an external quantum efficiency of 4.67% with color coordinates of (0.15, 0.08).

Full text of DOI:10.1016/j.orgel.2013.06.022

Organic Electronics 14 (2013) 2497–2504
Contents lists available at SciVerse ScienceDirect
Organic Electronics
journal homepage: www.elsevier.com/locate/orgel
Efficient deep-blue emitters based on triphenylamine-linked
benzimidazole derivatives for nondoped fluorescent organic
light-emitting diodes
⇑
⇑
Seongjin Jeong, Myoung-Ki Kim, Sung Hyun Kim , Jong-In Hong
Department of Chemistry, College of Natural Sciences, Seoul National University, Seoul 151-747, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Two triphenylamine-substituted benzimidazole derivatives were synthesized for use as
efficient deep-blue emitters in nondoped fluorescent organic light-emitting diodes (FLOL-
Received 22 March 2013
Received in revised form 12 June 2013
Accepted 24 June 2013
0
00
EDs). The molecular design of 4 ,4 -(1H-benzo[d]imidazole-1,2-diyl)bis(N,N-diphenylbi-
phenyl-4-amine) (T2B) to limit the molecular packing density enabled T2B-based devices
to suppress the exciton quenching by a bulky three-dimensional structure. Nondoped
FLOLEDs fabricated using T2B as a blue emitter exhibited an external quantum efficiency
of 4.67% with color coordinates of (0.15,0.08).
Available online 6 July 2013
Keywords:
Deep-blue emitter
Fluorescent organic light-emitting diodes
Nondoped system
Ó 2013 Elsevier B.V. All rights reserved.
Packing density
1
. Introduction
ative as the host and a diphenylamine-substituted spiro-
benzofluorene derivative as the dopant showed an EQE of
Deep-blue fluorescent organic light-emitting diodes
FLOLEDs) possessing high efficiency and pure Commission
5.4% with CIE coordinates of (0.13,0.15) [9]. Deep-blue
FLOLEDs based on spirocyclic aromatic hydrocarbon deriv-
atives and purine derivatives exhibited CIE coordinates of
(0.16,0.08) with 1.1 cd/A and CIE coordinates of
(0.15,0.06) with an EQE of 3.1%, respectively [10,11].
A nondoped system has an advantage of a simple device
structure that employs only a single emitting material
without using a complicated host–dopant codeposition
process [12–16]. However, in this type of system, excimers
are usually induced by intermolecular interactions be-
tween emitters, or excitons are readily quenched, resulting
in a red-shift of the CIE coordinates and/or low quantum
efficiency because emitters tend to be closely packed in
the EML [17]. Relatively little attention has been paid to
the synthesis of deep-blue emitters because EQEs > 2%
and CIEy < 0.10 in the deep-blue nondoped system are dif-
ficult to obtain simultaneously. Therefore, it is still chal-
lenging to design deep-blue emitters for reliable device
performance.
(
Internationale de l’Eclairage (CIE) chromaticity coordinates
are a prerequisite for full-color displays and lighting de-
vices. There have been many studies on improving the de-
vice performance by optimally designing the emitters
using doped or nondoped systems [1–3]. Nonetheless, ow-
ing to the intrinsic wide bandgap nature of deep-blue
emitters, only a few examples of high-performance deep-
blue FLOLEDs have been reported, in contrast to red and
green fluorescent devices [4–7].
Doping of emitters into host materials in the emitting
layer (EML) was mainly involved for the development of
high-efficiency deep-blue FLOLEDs. Chen et al. reported
an external quantum efficiency (EQE) of 2.3% with CIE
coordinates of (0.15,0.11) based on mono(styryl)amine-
based deep-blue dopants in FLOLEDs [8]. FLOLED devices
fabricated using a 9-anthracene-spirobenzofluorene deriv-
In this study, we synthesized deep-blue fluorescent-
⇑
Corresponding authors. Tel.: +82 2 880 6682; fax: +82 2 889 1568.
E-mail addresses: shkim75@snu.ac.kr (S.H. Kim), jihong@snu.ac.kr (J.-I.
0
emitting
materials,
N,N-diphenyl-4 -(1-phenyl-1H-
Hong).
benzo[d]imidazol-2-yl)biphenyl-4-amine)
(T1B)
and
1
566-1199/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.orgel.2013.06.022

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S. Jeong et al. / Organic Electronics 14 (2013) 2497–2504
0
00
4
,4 -(1H-benzo[d]imidazole-1,2-diyl)bis(N,N-diphenylbi-
MALDI-TOF MS: calcd for C12
290.37.
9 2 2
H BrN O 291.98, found
phenyl-4-amine) (T2B), for the development of highly effi-
cient, nondoped FLOLEDs with deep-blue CIE coordinates,
where the correlation between the device performance
and the molecular packing density was investigated. FLOL-
EDs using sterically bulky T2B as an emitter without a dop-
ant showed deep-blue emissions with a CIEy of 0.08 and an
EQE of 4.67% at 100 cd/m , which is better than the values
obtained from T1B owing to more efficient inhibition of
exciton quenching.
2.2.3. Synthesis of N-(4-bromophenyl)-1,2-phenylenediamine
0
A
suspension of N-(4-bromophenyl)-N -(2-nitro-
phenyl)amine (2.00 g,6.85 mmol) and stannous chloride
dihydrate (7.73 g,34.25 mmol) in ethanol was refluxed
for 24 h. The reaction mixture was concentrated in vacuo
and neutralized with aqueous sodium bicarbonate solu-
tion. The resulting solution was extracted with ethyl ace-
tate, dried using anhydrous sodium sulfate, and
concentrated in vacuo to afford the crude product. The
product was isolated by column chromatography on silica
2
2
. Materials and methods
gel using ethyl acetate and hexane (v/v 1:3) as the eluent
2.1. Materials
1
(
1.78 g,99%).
3
H NMR (400 MHz, CDCl ) d 7.26 (d,
J = 11.4 Hz, 2H), 7.08–7.01 (m, 2H), 6.80–6.73 (m, 2H),
N-phenyl-o-phenylenediamine,
1-fluoro-2-nitroben-
1
3
6
.58 (d, J = 8.2 Hz,2H), 5.16 (s, 1H), 3.71 (s, 2H). C NMR
CDCl 100 MHz) 144.53, 142.12, 132.04, 127.70,
26.28, 125.26, 119.17, 116.62, 116.22, 110.95. MALDI-
11BrN 262.01, found 262.32.
zene, 4-bromobenzaldehyde, 4-(diphenylamino)phenylbo-
ronic acid, tetrakis(triphenylphosphine) palladium, and
sodium carbonate were purchased from Aldrich and used
without purification.
(
3
,
d
1
TOF MS: calcd for C12
H
2
2
.2.4. Synthesis of 1,2-bis(4-bromophenyl)-1H-
2
2
.2. Synthetic procedure
benzo[d]imidazole (B2)
Acetic acid (20 mL) was added to a flask containing
N-(4-bromophenyl)-1,2-phenylenediamine (1.0 g,5.43 mmol)
and 4-bromobenzaldehyde (1.21 g,6.51 mmol). After the
mixture was refluxed for 12 h, distilled water was added
and the organic layer was extracted with dichloromethane.
The organic layer was washed with aqueous sodium bicar-
bonate solution and brine and dried using anhydrous so-
dium sulfate. The filtrate was concentrated in vacuo to
give a crude mixture, which was then subjected to column
chromatography on silica gel using ethyl acetate and hexane
.2.1. Synthesis of 2-(4-bromophenyl)-1-phenyl-1H-
benzo[d]imidazole (B1)
Acetic acid (20 mL) was added to a flask containing N-
phenyl-o-phenylenediamine (1.50 g,8.14 mmol) and 4-
bromobenzaldehyde (1.66 g,8.96 mmol). After the mixture
was refluxed for 12 h, distilled water was added and the or-
ganic layer was extracted with dichloromethane. The organ-
ic layer was washed with sodium bicarbonate and brine and
dried using anhydrous sodium sulfate. The filtrate was con-
centrated in vacuo to give a crude mixture, which was then
subjected to column chromatography on silica gel using
ethyl acetate and hexane (v/v 1:20) as the eluent. Analyti-
cally pure 2-(4-bromophenyl)-1-phenyl-1H-benz[d]imid-
azole was isolated as a white solid (1.39 g,49%). 1H NMR
(
v/v 1:6) as the eluent to afford analytically pure 1,2-bis(4-
1
bromophenyl)-1H-benzo[d]imidazole (1.11 g,47%).
NMR (400 MHz, CDCl ) d 7.86 (d, J = 7.8 Hz,1H), 7.63 (d,
J = 7.9 Hz,2H), 7.43 (dd, J = 8, 12 Hz, 4H), 7.34 (t, J = 7 Hz,
1
1
1
1
4
H
3
13
3
H), 7.32–7.16 (m, 4H). C NMR (CDCl , 100 MHz) d
(
400 MHz, CDCl
3
) d 7.87 (d, J = 8.4 Hz, 1H), 7.53–7.42 (m, 7
51.09, 142.95, 136.91, 135.78, 133.29, 131.74, 130.83,
28.87, 128.57, 124.29, 123.82, 123.40, 122.65, 120.08,
1
3
H), 7.35–7.21 (m, 5H). C NMR (CDCl
1
1
3
, 100 MHz) d 151.21,
42.90, 137.25, 136.75, 131.55, 130.84, 130.01, 128.90,
28.77, 127.37, 124.04, 123.58, 123.15, 119.90, 110.48.
13BrN 348.03, found 349.20.
10.22. MALDI-TOF MS: calcd for C19
27.23.
2 2
H12Br N 425.94, found
MALDI-TOF MS: calcd for C19
H
2
0
2
.2.5. N,N-diphenyl-4 -(1-phenyl-1H-benzo[d]imidazol-2-
2.2.2. Synthesis of N-(4-bromophenyl)-2-nitroaniline
yl)biphenyl-4-amine (T1B)
A mixture of 1-fluoro-2-nitrobenzene (5.00 g,35.436
A
mixture of compound B1 (0.70 g,2.00 mmol),
mmol), 4-bromoaniline (12.19 g,70.87 mmol), and potas-
sium fluoride (2.57 g,44.30 mmol) was heated at 170–
4-(diphenylamino)phenylboronic acid (0.70 g,2.40 mmol),
tetrakis(triphenylphosphine)palladium (0.12 g,0.10 mmol),
and aqueous sodium carbonate (2 M,10 mL) in tetrahydro-
furan (40 mL) and methanol (10 mL) was heated to reflux
in a nitrogen atmosphere for 24 h. After the reaction mix-
ture was concentrated in vacuo, the resulting mixture
was extracted with dichloromethane. The organic layer
was washed with water and brine and dried using anhy-
drous sodium sulfate. The filtrate was concentrated in va-
cuo to give a crude mixture, which was then subjected to
column chromatography on silica gel using dichlorometh-
ane and hexane (v/v 1:1) as the eluent to afford analytically
1
80 °C for 72 h. The resulting mixture was extracted with
dichloromethane, and the organic layer was washed with
water and brine and dried using anhydrous sodium sulfate.
The filtrate was concentrated in vacuo and was then sub-
jected to column chromatography on silica gel using
dichloromethane as the eluent. The resulting red solid
was recrystallized from methanol to afford N-(4-bromo-
0
phenyl)-N -(2-nitrophenyl)amine as orange needles
1
(
3.10 g,30%). H NMR (400 MHz, CDCl
3
) d 9.37 (s, 1H),
8
.19 (d, J = 8.6 Hz,1H), 7.51 (d, J = 8.6 Hz,2H), 7.37 (t,
0
J = 7.8 Hz,1H), 7.24–7.14 (m, 3H), 6.80 (t, J = 7.4 Hz, 1H).
pure N,N-diphenyl-4 -(1-phenyl-1H-benzo[d]imidazol-2-
1
3
C NMR (CDCl
3
, 100 MHz) d 142.36, 137.93, 135.73,
yl)biphenyl-4-amine as
H NMR (400 MHz, CDCl ) d 7.88 (d, J = 8 Hz, 1H), 7.61
a
yellow solid (0.78 g,76%).
1
1
33.50, 132.80, 126.74, 125.70, 118.40, 118.04, 115.94.
3

S. Jeong et al. / Organic Electronics 14 (2013) 2497–2504
2499
(
2
d, J = 8.4 Hz, 2H), 7.52 (d, J = 8 Hz, 4H), 7.45 (d, J = 8.8 Hz,
H), 7.35 (d, J = 7.8 Hz, 2H), 7.27–7.23 (m, 8H), 7.10 (dd,
calorimetry (DSC), respectively. The TGA and DSC analyses
ꢀ1
were performed at 10 °C min under a nitrogen atmo-
sphere. T was determined from the third heating scan.
The electrochemical properties of T1B and T2B were stud-
ied using cyclic voltammetry (CV) in CH Cl solutions
(1.00 mM) with 0.1 M tetra-n-butylammoniumhexafluoro-
phosphate (TBAPF ) as the supporting electrolyte. A glassy
13
J = 8.8,2 Hz,6H), 7.02 (t, J = 7.2 Hz, 2H). C NMR (CDCl
3
,
g
1
1
1
1
5
C
00 MHz) d 152.18, 147.65, 147.51, 141.39, 137.29,
37.09, 133.66, 129.89, 129.79, 129.28, 128.57, 127.62,
27.47, 127.24, 124.53, 123.62, 123.28, 123.09, 122.98,
2
2
19.75, 110.38. MALDI-TOF MS: calcd for
C
37
H
27
N
3
6
+
13.22 g/mol, found 513.26 g/mol. HRMS (FAB ) [M + H:
]: calcd for 514.2283, found 514.2283. Anal.
carbon electrode was employed as the working electrode
and referenced to a Ag reference electrode. All potential
values were calibrated against the ferrocene/ferrocenium
37
28 3
H N
Calcd. For C37
8
H
27
N
3
: C, 86.52; H, 5.30; N, 8.18. Found: C,
+
6.05; H, 5.18; N, 8.11.
(Fc/Fc ) redox couple. The onset potential was determined
from the intersection of two tangents drawn at the rising
and background current of the cyclic voltammogram.
0
00
2
.2.6. Synthesis of 4 ,4 -(1H-benzo[d]imidazole-1,2-
diyl)bis(N,N-diphenylbiphenyl-4-amine) (T2B)
Using an approach similar to that used for the synthesis
2
.4. Computational calculation
of T1B by the Suzuki coupling reaction, a white powder
1
was obtained (0.65 g,74%). H NMR (400 MHz, CDCl
3
)
The optimized geometries and frontier molecular orbi-
d 7.89 (d, J = 8.4 Hz,1H), 7.70 (d, J = 8.4 Hz,2H), 7.66 (d, J =
.4 Hz,2H), 7.52 (dd, J = 8.4, 2 Hz,4H), 7.46 (d, J =
tal energy levels for T1B and T2B were obtained using den-
sity functional theory (DFT) calculations with the Gaussian
0
the Becke 3-parameter Lee–Yang–Parr (B3LYP) functional
with the 6-311G-level atomic basis set.
8
8
7
1
1
1
1
1
7
C
.4 Hz,2H), 7.38 (d, J = 8.4 Hz, 2H), 7.34–7.23 (m, 11H),
9 program package. The geometries were optimized using
.17–7.09 (m, 12H), 7.03 (q, J = 8.4 Hz,4H). 13C NMR (CDCl
,
3
00 MHz) d 152.19, 147.85, 147.65, 147.51, 147.48, 143.09,
41.40, 140.81, 137.32, 135.59, 133.67, 133.17, 129.83,
29.34, 129.28, 128.20, 127.78, 127.72, 127.63, 126.28,
24.61, 124.52, 123.62, 123.28, 123.21, 123.08, 122.99,
2.5. Deep-blue FLOLEDs fabrication and electro-optical
characterization
19.78, 110.46. MALDI-TOF MS: calcd for
56.33 g/mol, found 756.77 g/mol. HRMS (FAB )[M + H:
]: calcd for 757.3331, found 757.3331. Anal. Calcd.
: C, 87.27; H, 5.33; N, 7.40. Found: C, 86.72; H,
.28; N, 7.30.
55 40 4
C H N
+
0
0
N,N -di(1-naphthyl)-N,N -diphenylbenzidine
(NPB),
55
41 4
H N
0
00
4
,4 ,4 -tris(N-carbazolyl)triphenylamine (TCTA), and 1,3,5-
40 4
For C55H N
tris(N-phenylbenzimidazol-2-yl)benzene (TPBI) were pur-
chased from commercial sources and used without purifica-
tion. Fabrication of organic light-emitting diodes (OLEDs)
5
2
.3. Measurements
ꢀ6
was conducted through high-vacuum (2 ꢁ 10 Torr) ther-
1H and 13C NMR spectra were recorded using an Agilent
mal evaporation of the organic materials onto indium tin
1
oxide (ITO)-coated glass (sheet resistance: 15 X/square;
Applied Film Corp.). Glass substrates with patterned ITO
electrodes were washed with isopropyl alcohol and then
4
00 MHz Varian spectrometer in CDCl
3
. H NMR chemical
shifts in CDCl
3
were referenced to CHCl
3
(7.27 ppm) and
1
3
C NMR chemical shifts in CDCl
3
were reported relative
2
cleaned through O plasma treatment. OLED devices were
3
to CHCl (77.23 ppm). UV–visible spectra were recorded
fabricated with a configuration of ITO/NPB/TCTA/T1B or
T2B/TPBI/LIF/Al. NPB and TCTA as hole transporting layers,
TPBI as an electron transporting layer, LiF and Al electrodes
were deposited on the substrate in the order of sequence.
The emission properties were determined using a PR-650
SpectraScan SpectraColorimeter as a source meter. Cur-
rent–voltage characteristics were measured using
programmable electrometer equipped with current and
voltage sources (Keithley 2400).
on a Beckman DU650 spectrophotometer. Fluorescence
spectra were recorded on a Jasco FP-6500 spectrophotom-
eter. Fluorescent quantum yields in the solid films were
recorded on an Otsuka electronics QE-2000 spectropho-
tometer. For time-resolved PL spectroscopy, a femtosecond
pulsed Ti:sapphire laser and a nanosecond pulsed Nd:YAG
laser were employed as excitation light sources for a
prompt component and a delayed component, respec-
tively, and a monochromator-attached photomultiplier
tube was used as an optical detection system. Mass spectra
were obtained using matrix-assisted laser desorption–ion-
ization time-of-flight mass spectrometry (MALDI-TOF MS)
from Bruker. High-resolution masses were measured by
either fast atom bombardment (FAB) or electron ionization
a
3. Results and discussion
3.1. Computational calculation
(
EI) using a JEOL HP 5890 series. Analytical thin-layer chro-
To predict the optimized geometries and frontier
molecular orbital energy levels for T1B and T2B, DFT calcu-
lations were carried out with the B3LYP/6-311G level using
Gaussian ’09. The highest occupied molecular orbital
(HOMO) and lowest unoccupied molecular orbital (LUMO)
distributions of T1B and T2B are shown in Fig. 1. The
HOMOs of T1B and T2B were localized on the triphenyl-
amine (TPA) unit, whereas the LUMOs of T1B and T2B were
dispersed over the benzimidazole (BI) unit. DFT analysis
matography was performed using Kieselgel 60F-254 plates
from Merck. Column chromatography was carried out on
Merck silica gel 60 (70–230 mesh). All solvents and re-
agents were commercially available and used without fur-
ther purification unless otherwise noted. Decomposition
temperatures (T ) and glass transition temperatures (T )
d g
were obtained using Q-5000-IR thermogravimetric
analysis (TGA) and DSC-Q-1000 differential scanning

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S. Jeong et al. / Organic Electronics 14 (2013) 2497–2504
indicates that the HOMO and LUMO energy levels of T1B
and T2B are similar (5.23 eV and 1.60 eV, respectively, for
T1B; 5.21 eV and 1.58 eV, respectively, for T2B) [18].
DFT analysis of the molecular conformation revealed
that the phenyl ring at the 1-position of BI was twisted
with respect to the BI plane, with a dihedral angle of
by column chromatography and vacuum sublimation. T1B
and T2B were characterized by H NMR, C NMR, and low-
and high-resolution mass data.
1
13
3.3. Thermal properties
2
7.9°, whereas the phenyl ring at the 2-position of BI was
The thermal properties of T1B and T2B were investigated
twisted with respect to the BI plane with a dihedral angle
of 59.3°. Furthermore, the conjugation length of T2B was
similar to that of T1B because the phenyl moiety at the
with TGA and DSC (Fig. 3). T , defined as the temperature at
which 5% mass loss occurs, was measured by TGA. The Td
values of T1B and T2B were 362 and 475 °C, respectively,
d
2
-position of BI was almost perpendicular to the TPA moi-
whereas their T values were 97 and 137 °C, respectively.
g
ety, thereby increasing the steric bulkiness in T2B as com-
pared to T1B. In general, bulky, nonplanar structures
suppress the molecular packing through increased steric
hindrance between molecules, thus decreasing the charge
carrier transport and the excimer formation. Therefore,
T2B-based devices would have a higher efficiency than
T1B-based devices [18–22].
Tg of T2B was increased by 40 °C by the additional TPA unit.
This indicates that a T2B emitter would be much better than
a T1B emitter in terms of thermal stability [25].
3.4. Photophysical properties
Fig. 4 depicts the normalized UV–vis absorbance (at
3
00 K) and photoluminescence (PL) spectra (at 300 K and
3.2. Synthesis
77 K) of T1B and T2B. The absorption bands around
3
n–
00 nm for T1B and T2B originate from the TPA-centered
ꢂ
The synthetic routes and molecular structures of T1B
p
transition [18]. The absorption bands around
and T2B are illustrated in Fig. 2. T1B and T2B comprise
two main components: TPA as the donor moiety and BI
as the acceptor moiety; these two components were con-
nected through a phenyl linkage by the Suzuki coupling
reaction [23,24]. In T1B, the TPA unit was directly linked
to the para position of the 2-phenyl moiety of 1,2-diphe-
nylbenzimidazole. However, in T2B, the two TPA units
were attached to the para positions of the two pendant
phenyl moieties of 1,2-diphenylbenzimidazole, which
would suppress exciton quenching by limited molecular
packing owing to the more bulky three-dimensional struc-
ture. Synthetic yields of the Suzuki reactions leading to T1B
and T2B were 76% and 74%, respectively, after purification
350 nm are attributed to the intramolecular charge-trans-
fer transition from the electron-donating TPA moiety to the
electron-accepting BI moiety [18,26,27]. The absorption
edges of the UV–vis spectra were found to be 420 nm,
which corresponds to the optical bandgap of 2.95 eV. From
the optical bandgap and the HOMO energy level of 5.33 eV
estimated from CV, the LUMO energy level was calculated
to be 2.38 eV.
The PL kmax peaks of T1B and T2B were observed around
440 nm without any vibration peaks. Although an extra
TPA unit was introduced at the para position of the pen-
dant 2-phenyl group in T2B, the emission spectra of T1B
and T2B display the identical maxima in both the solution
Fig. 1. DFT calculations of T1B and T2B were performed at B3LYP/6-311G level: (a) chemical structures, (b) three-dimensional optimized geometries, and
c) HOMO and LUMO electron density map.
(

S. Jeong et al. / Organic Electronics 14 (2013) 2497–2504
2501
Fig. 2. Synthetic routes of T1B and T2B.
Fig. 3. TGA thermograms of T1B and T2B (inset: DSC analysis of T1B and T2B).
and the solid states. These results demonstrate that the
additional TPA moiety connected to the para position of
the pendant 2-phenyl group does not contribute to the
extension of the conjugation length [19]. The nonplanar
structures of T1B and T2B facilitate their use as emitters
in nondoped deep-blue FLOLEDs owing to the efficient
inhibition of
deposition.
p–p stacking in the film state by vapor
On going from singlet (300 K) to triplet (77 K) states in
PL spectra, a red shift of 80 for T1B and 79 nm for T2B is ob-
served, indicating the energy gap between the lowest
excited singlet state and the lowest excited triplet state

2
502
S. Jeong et al. / Organic Electronics 14 (2013) 2497–2504
Fig. 4. UV–vis absorption and photoluminescence (PL) spectra of (a) T1B and (b) T2B. (Dashed lines perpendicular to the x-axis correspond to kmax of PL
spectra.)
(
D
E
ST) of 0.44 and 0.43 eV, respectively. It has been known
that a small ST in aromatic organic compounds would
facilitate the reverse intersystem crossing (RISC) even
without heavy metals [28–31]. These ST values predict
(20 nm)/LiF (1 nm)/Al (100 nm). Two different blue FLOL-
EDs, one using T1B and the other using T2B as a deep-blue
emitter, were fabricated and the performance of each de-
vice was evaluated (Table 1).
D
E
D
E
that the surrounding thermal energies may enable RISC
in T1B and T2B, which could contribute to the increased
device efficiency over the intrinsic limitations of fluores-
cent device performances. We investigated the emission
lifetimes of T1B and T2B by using transient PL decay mea-
surements in order to evaluate the possibility of RISC in
terms of delayed fluorescence (Fig. S1). PL decay lifetimes
of T1B and T2B are 1.72 ns and 1.75 ns for prompt fluores-
Fig. 5a shows current density–voltage characteristics of
deep-blue fluorescent devices with T1B and T2B. The cur-
rent density of the device with T1B was higher than that
of the device with T2B, whereas the luminance values of
the two devices with T1B and T2B emitters are similar at
the same driving voltage as shown in Fig. 5b. The current
density mainly depends on the differences in the energy
levels of the organic thin layers in OLEDs [33–35]. Despite
little difference in the energy levels of both deep-blue
emitters, there is a discrepancy in the current densities at
the same voltage. It is known that efficient packing of
semiconductor molecules usually facilitates better current
flow through the devices [18]. Thus, the alternation of the
packing densities in the solid films derived from the
structurally different T1B and T2B would result in different
current densities; the current density of the device with
the structurally less bulky T1B was higher than that of
the device with the structurally more bulky T2B. These
results are consistent with our simulation using DFT
calculations. Unlike the current density characteristics
of T1B- and T2B-based devices, similar luminance
values of the two devices indicate that exciton formation
and light emission are more efficient in T2B-based devices
cence, and 8.33 ls and 8.52 ls for delayed fluorescence,
respectively. Therefore, it is clear from these data that
the delayed fluorescence originates from the RISC process
despite the relatively large
DEST values.
The photoluminescence quantum yields (PLQYs) of T1B
and T2B in solution were measured to be 0.72 and 0.66,
respectively, using 9,10-di(naphth-2-yl)anthracene (AD-
N,PLQY = 0.57) as a reference at room temperature [32].
The PLQYs of T1B and T2B in the solid state are 0.50 and
0
.60, respectively. T2B showed only a 9.1% reduction in
the PLQYs upon going from the solution to solid states,
comparing with a 30.6% reduction in the case of T1B. These
results indicate that T2B suppresses exciton quenching by
limited molecular packing owing to the increased steric
bulkiness of T2B compared to T1B.
(Fig. 5b).
3.5. Electroluminescence properties
Quantum efficiency–luminance of two devices was
plotted in Fig. 5c based on current density and luminance
of devices. The T2B-based device shows a better maximum
EQE of 4.67% compared with that of 3.05% of the T1B-based
Nondoped FLOLEDs were fabricated using T1B and T2B
as emitters in the EML, with a device configuration of ITO/
NPB (40 nm)/TCTA (20 nm)/EML (T1B or T2B, 40 nm)/TPBI
2
device at 100 cd/m . In the case of T2B-based devices, EQE

S. Jeong et al. / Organic Electronics 14 (2013) 2497–2504
2503
Table 1
Electroluminescence (EL) device performance of T1B and T2B.
At 100 cd/m2
Current density (mA/cm2)
Quantum efficiency (%)
CIE (x,y)
EL kmax
T1B
T2B
5.12
3.13
3.05
4.67
0.15, 0.08
0.15, 0.08
452
452
Fig. 5. Device performance of deep-blue FLOLEDs with T1B and T2B. (a) current density–voltage characteristics (inset: energy level diagram of the
FLOLEDs), (b) luminance–voltage characteristics, (c) quantum efficiency–luminance characteristics and (d) electroluminescence spectra. (For interpretation
of the references to color in this figure legend, the reader is referred to the web version of this article.)
is about 1.4 times higher than the theoretical maximum
EQE of about 3.3%, assuming that the EL quantum effi-
ciency is the same as the PL quantum efficiency of 66%. Rel-
atively high EQEs of the T1B- and T2B-based devices are
attributed to thermally activated delayed fluorescence
density, and then appeared to be saturated at high current
density as shown in Fig. S2. Consequently, enhanced effi-
ciency by delayed fluorescence is not due to the TTA, but
due to the TADF. In addition, as expected from the molec-
ular packing density of DFT simulations and the solid
PLQYs of T1B and T2B, the T2B-based device has relatively
fewer sites for exciton recombination with respect to
molecular concentrations, resulting in a smaller exciton
density as compared to the T1B-based device. However,
concentrated excitons caused by more efficient packing
would be more easily quenched in T1B. Therefore, the
device performance based on T2B is superior to that of
T1B-based devices.
(
TADF) by RISC that converts triplet excitons into singlet
excited states [30,36]. It has been known that such an unu-
sual increase in EQE originates from the triplet–triplet
annihilation (TTA) and TADF [37,38]. For the TTA, the lumi-
nance is known to increase more than linearly with an in-
crease in the current density [39]. However, in the case of
the TADF, which occurs in a fluorescent molecule contain-
ing a sufficiently separated HOMO and LUMO [31], the
luminance increases linearly or less than linearly with an
increase in the current density [30]. In the current work,
the luminance showed a linear increase at very low current
Fig. 5d shows normalized EL spectra of the deep-blue
fluorescent devices. A blue emission with a peak wave-
length at 452 nm, which originated from the singlet

2
504
S. Jeong et al. / Organic Electronics 14 (2013) 2497–2504
excitons of blue emitters without any other emission from
the adjacent layers, was observed in all devices. The EL
spectra are essentially stable against changes in the molec-
ular structure.
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4
. Summary
Two triphenylamine-attached benzimidazole deriva-
tives (T1B and T2B) were designed and synthesized for
use as deep-blue emitters in efficient nondoped FLOLEDs.
The fluorescent devices fabricated using T2B exhibited a
high EQE of 4.67%, which is 1.4 times higher than the the-
oretical maximum EQE of 3.3%, along with the CIE coordi-
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