Efficient single-layer electroluminescent device based on a bipolar emitting boron-containing material

Article abstract of DOI:10.1039/b513918j

A novel multifunctional 1,6-bis(2-hydroxyphenyl)pyridine boron bis(4-n-butyl-phenyl)phenyleneamine compound in which the hole-transporting (HT), electron-transporting (ET), and emitting (EM) components are integrated into a single molecule was synthesized

Full text of DOI:10.1039/b513918j

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Efficient single-layer electroluminescent device based on a bipolar
emitting boron-containing material{
Hongyu Zhang, Cheng Huo, Jingying Zhang, Peng Zhang, Wenjing Tian and Yue Wang*
Received (in Cambridge, UK) 30th September 2005, Accepted 28th October 2005
First published as an Advance Article on the web 2nd December 2005
DOI: 10.1039/b513918j
on the structure and properties of 1,6-bis(2-hydroxyphenol)pyr-
idinel boron bis(4-n-butyl-phenyl)phenyleneamine ((dppy)BTPA)
by incorporating a light-emitting center, hole-transporting and
electron-transporting fragments into one molecule. This new
compound is stable with respect to sublimation and thus is
suitable for vacuum deposition in OLEDs.
A
novel multifunctional 1,6-bis(2-hydroxyphenyl)pyridine
boron bis(4-n-butyl-phenyl)phenyleneamine compound in
which the hole-transporting (HT), electron-transporting (ET),
and emitting (EM) components are integrated into a single
molecule was synthesized and used as an emitting material to
fabricate an efficient single-layer electroluminescent device.
The pyridine–phenol ligand H2dppy was synthesized according
to the literature procedure26 and the boron compound was
prepared following the synthetic route shown in Scheme S1(see
ESI{).{ The X-ray diffraction study shows that compound
(dppy)BTPA in the solid state has a tetrahedral structure,§ typical
of boron compounds (Fig. 1). The conformation of the dppy
moiety is slightly distorted from planarity with inter-ring torsion
angles of 10.5 and 16.7u between the pyridyl ring and the phenolate
rings, while the triphenylamine moiety is greatly distorted from
planarity with dihedral angles 68.1, 68.9 and 76.37u between the
three phenyl rings. Therefore, the introduction of the bulky
triphenylamine group decreases the symmetry of the compound
and the resultant compound (dppy)BTPA has a non-planar
molecular structure, which can easily form uniform thin films. The
crystal packing diagram given in Fig. 2 shows that in the crystal
(dppy)BTPA molecules display a molecular chain arrangement.
Moreover, the compound is observed to self-assemble into a
supramolecular zig-zag motif by intermolecular hydrogen-bonding
The development of organic electroactive and photoactive
materials has been greatly progressed due to their potential
applications in optoelectronic devices, such as electroluminescent
(EL) devices, photovoltaic devices, thin film transistors, and solid-
state lasers.1–17 Development of high-performance organic EL
devices requires various materials that perform different roles,
including hole-transporting and electron-transporting materials, as
well as light-emitting materials, and typically devices commonly
contain three layers of organic materials. However, fabrication of
multilayer devices is often tedious, difficult and more expensive
than single-layer devices. In view of this, one of the key challenges
on the path of developing the next generation of high-performance
OLEDs is the design and synthesis of molecules that are capable of
functioning as efficient emitters as well as charge-transport
materials. Therefore, materials for use in the emitting layer should
meet the requirements of energy levels matching the injection and
transport of both holes and electrons, and hence should desirably
possess bipolar character to permit the formation of both stable
cation and anion radicals. To endow a single organic material with
multifunctions, which generally have to be realized by at least two
kinds of molecules, is very difficult.18–24 Stable multifunctional
materials remain to be the most challenging and most sought-after
materials in OLEDs.
…
˚
interaction. The distance of C–H O hydrogen bond is 2.568 A
with an angle of 157.4u. The special 1-dimensional feature of the
(dppy)BTPA molecules in solid state may enhance the charge-
transport ability.
Fig. 3 shows the cyclic voltammograms of (dppy)BTPA. The
compound showed reversible redox properties, exhibiting one
In our previous work, we have reported white light emission EL
devices which come from exciplex25–26 by introducing mixed
phenol–pyridine functional groups to tetrahedral coordinated
boron compounds that exhibited electron mobility and emitting
properties. To improve the performance of the boron compounds,
we modified the boron molecule by attaching the triphenylamine
group to the boron center by a B–C bond to produce a novel
molecule. The triphenylamine group is known to have a superior
hole mobility,27–28 therefore, the proposed molecule may have
bipolar transport character and thus offer good recombination
sites for hole and electron charge carriers. In this paper, we focus
Key Laboratory for Supramolecular Structure and Materials of Ministry
of Education, College of Chemistry, Jilin University, Changchun,
130012, P. R. China. E-mail: yuewang@jlu.edu.cn;
Fax: 86-431-5193421; Tel: 86-431-5168484
{ Electronic supplementary information (ESI) available: details of
synthesis and Scheme S1. See DOI: 10.1039/b513918j
Fig. 1 The molecular structure of (dppy)BTPA.
Chem. Commun., 2006, 281–283 | 281
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Fig. 2 Molecular chain (left); zig-zag motif of (dppy)BTPA along side
(middle) and axis (right) directions. Purple, C; yellow, B; red, O; blue, N;
white, H; H-bond contacts, broken black lines. For middle and right: most
of the atoms are omitted for clarity, and the only H atoms are those for
H-bond contacts.
Fig. 4 Diagram showing the energy level of (dppy)BTPA in the single-
layer device.
photoluminescent (PL) spectrum of (dppy)BTPA thin film (Fig. 5).
The quite low turn-on voltage of 3.8 V (defined as the voltage
required to give a luminance of 1 cd m22) and low operation
voltages suggest that (dppy)BTPA function well for charge
transport. The results demonstrate that (dppy)BTPA is capable
of functioning as a hole-transporting layer (HTL), an electron-
transporting layer (ETL) and an emitting layer (EML) in EL
device, consistent to the results of the cyclic voltammogram. The
yellow light emission from the single-layer device is stable and its
EL spectra are insensitive to driving voltage. The typical current–
light–voltage (I–L–V) characteristics of device 1 is inset in Fig. 5.
We have not yet studied the lifetime of the device, but the
unencapsulated device was stable, and the emission color and the
I–L–V did not change during the measurement of EL data in air.
In order to compare, two double-layer devices and one three-
layer device were fabricated. The device structures are [ITO/
NPB(50 nm)/(dppy)BTPA(50 nm)/LiF(1 nm)/Al(200 nm)] (device
2), [ITO/(dppy)BTPA(50 nm)/Alq3(50 nm)/LiF(1 nm)/Al(200 nm)]
(device 3), and [ITO/NPB(50 nm)/(dppy)BTPA(40 nm)/
Alq3(10 nm)/LiF(1 nm)/Al(200 nm)] (device 4). Device 2 with
NPB as HTL and (dppy)BTPA as EML and ETL exhibits the
maximum luminescence of 800 cd m22 at 13.5 V, and the
maximum efficiency of 1.5 cd A21 (at 7 V). Device 3 with Alq3 as
ETL produces a green EL emission with a spectrum typical of
Fig. 3 Cyclic voltammogram of (dppy)BTPA in DMF at a scan rate of
100 mV s21
.
anodic and cathodic wave on oxidation and reduction, respec-
tively. Based on the electrochemical measurement the HOMO and
LUMO energy levels of (dppy)BTPA were calculated to be
25.3 eV and 22.9 eV, respectively. The LUMO level of
(dppy)BTPA is comparable to that of tri(8-hydroxyquinolino)-
aluminium (Alq3 23.0 eV),29 that was demonstrated to be a good
electron injection and transport material. Simultaneously,
the HOMO level of (dppy)BTPA is close to that of
N,N9-diphenyl-N,N9-bis(1-naphthyl)-(1,19-biphenyl)-4,4-diamine
(NPB, 25.4 eV),29 which was often used as a hole injection and
transport material. A compound that can stabilize the formation
of both cation and anion radicals is probably promising bipolar
(electron-transport (n-type) and hole-transport (p-type)) material
and suggested to be beneficial to OLEDs.30–31 Therefore, we
anticipated that (dppy)BTPA may be able to transport holes and
electrons in OLEDs. The energy levels of electrodes and organic
material are shown in Fig. 4.
The energy level alignments of (dppy)BTPA enlighten us to
fabricate
a
single-layer device with structure of [ITO/
(dppy)BTPA(100 nm)/LiF(1 nm)/Al(200 nm)] (device 1)." This
single-layer device reached a maximum efficiency of 5.2 cd A21
(3.6 lm W21) and a maximum brightness of 2654 cd m22. To our
knowledge, the efficiency is the highest value for small molecule
single-layer EL devices that have been achieved until now. The
single active layer device reported previously displayed a maximum
Fig. 5 EL and PL spectra for [ITO/(dppy)BTPA/LiF/Al] and
(dppy)BTPA thin films, respectively; the inset shows the I–V ($) and
L–V (&) characteristics for the device.
efficiency of 1.2 cd A21 and
1065 cd m22 32
The EL emission of device 1 is identical to the
a maximum brightness of
.
282 | Chem. Commun., 2006, 281–283
This journal is ß The Royal Society of Chemistry 2006

View Article Online
ratio. The structure was solved with direct methods and refined with a full-
matrix least-squares technique, using the SHELXIL programs. Hydrogen
atoms were assigned isotropic displacement coefficients. CCDC 285222.
For crystallographic data in CIF or other electronic format see DOI:
10.1039/b513918j
" The device of [ITO/(dppy)BTPA/LiF/Al] was prepared in vacuum at a
pressure of 5 6 1026 Torr, and the organic materials were deposited onto
an indium–tin–oxide at a deposition rate of 1–2 A s21. Aluminium
˚
electrode was thermally evaporated onto the organic surface
resulting in active areas of y5 mm2. The thickness of the organic material
and the cathode layers were controlled using a quartz crystal thickness
monitor.
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Fig. 6 EL spectra of devices 2–4.
Alq3. This indicates that the emission takes place in the Alq3 layer.
For device 3, the maximum luminescence is 730 cd m22 at 13.0 V,
and the maximum efficiency is 1.1 cd A21 (at 8.0 V) with turn-on
voltage 5.0 V. Device 4 is a typical three-layer device which exhibits
a green-yellow EL light emission, suggesting that there is a
contribution from the Alq3 emission in device 4. The EL spectra of
devices 2–4 are shown in Fig. 6. Device 4 shows the maximum
brightness of 2773 cd m22 at 12.0 V and the maximum efficiency
of 6.8 cd A21 (4.3 lm W21). The single-layer device 1 displays
higher efficiency and brightness than the double-layer devices 2–3.
The EL performance of device 1 is similar to the three-layer device
4. Above results demonstrated that (dppy)BTPA really has
multifunctional characteristics.
In conclusion, a novel boron compound (dppy)BTPA combin-
ing a light-emitting center, hole- and electron-transporting groups
in one molecule, was synthesized and demonstrated to be a
promising yellow-light emitting material in an EL device. The
incorporation of electron-rich TPA and electron-deficient phenol–
pyridine–boron groups leads to the formation of bipolar material.
Efficient single-layer and non-doping EL device based on
(dppy)BTPA is constructed. Although the EL performance of
the single layer device is lower than that of multilayer devices, the
device structure fabrication processes are dramatically simplified.33
This work was supported by the National Natural Science
Foundation of China (50225313 and 50520130316), the Major
State Basic Research Development Program (2002CB613401), Jilin
Province Science Foundation (20050120) and the Program for
Changjiang Scholars and Innovative Research Team in University
(IRT0422).
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Notes and references
{ Mass, 1H NMR spectroscopic and elemental analysis: (dppy)BTPA:
EIMS: m/z 628. dH(CDCl3, 500 MHz) 8.371 (t, J = 8.0 Hz, 1H), 8.317 (d,
J = 7.0Hz, 2H), 8.040 (d, J = 6.5 Hz, 4H), 7.398 (m, 4H), 7.203–7.143 (m,
4H), 7.035–6.992 (m, 4H), 6.961–6.929 (m, 4H), 2.454 (m, J = 7.0 Hz, 4H),
1.478 (m, J = 4.0 Hz, 4H), 0.867 (t, J = 7.5 Hz, 6H). Anal. Calcd. for
C43H41BN2O2: C, 82.16; H, 6.57; N, 4.46. Found: C, 82.05; H, 6.85; N,
4.29%.
§ Crystal data: (dppy)BTPA: C43H41BN2O2, Mr = 629, monoclinic, space
˚
32 W. Y. Wong, Z. He, S. K. So, K. L. Tong and Z. Y. Lin,
Organometallics, 2005, 24, 4079.
group P21/c, a = 16.815(3), b = 10.182(2), c = 20.772(4) A, b = 99.93(3)u,
3
V = 3503.3(2) A , Z = 4, Dc = 1.192 g cm23, F(000) = 1336. 8315
˚
reflections measured, 7814 unique (Rint = 0.089). Structure diffraction in
tensities were collected on a Rigaku RAXIS-PRID diffractometer using the
˚
v-scan technique with graphite-monochromated MoKa (c = 0.071073 A)
33 K. Q. Ye, J. Wang, H. Sun, Y. Liu, Z. C. Mu, F. Li, S. M. Jiang,
J. Y. Zhang, H. X. Zhang, Y. Wang and C. M. Che, J. Phys. Chem. B,
2005, 109, 8008.
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Chem. Commun., 2006, 281–283 | 283