Borazine materials for organic optoelectronic applications

Article abstract of DOI:10.1039/b504510j

Borazine materials have been demonstrated to be a new class of multifunctional and thermally stable materials with high electron (10 ^-3 cm² V^-1 s^-1) and moderate hole (10^-4 cm² V^-1

Full text of DOI:10.1039/b504510j

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Borazine materials for organic optoelectronic applications{
Iona H. T. Sham, Chi-Chung Kwok, Chi-Ming Che* and Nianyong Zhu
Received (in Cambridge, UK) 31st March 2005, Accepted 16th May 2005
First published as an Advance Article on the web 9th June 2005
DOI: 10.1039/b504510j
where R1 5 R3 5 R5 5 phenyl group and R2 5 R4 5 R6 5 H; (3)
Borazine materials have been demonstrated to be a new class of
multifunctional and thermally stable materials with high elec-
2,4,6-tris(diphenylamino)borazine (3) where R1 5 R3 5 R5 5 H
and R2 5 R4 5 R6 5 diphenylamino group; (4) 1,3,5-tris(4-tert-
butylphenyl)borazine (4) where R1 5 R3 5 R5 5 4-t-butylphenyl
group and R2 5 R4 5 R6 5 H; (5) 1,3,5-tris(1-naphthyl)borazine
tron (1023 cm2 V21 s21) and moderate hole (1024 cm2 V21 s21
)
mobilities for applications in electroluminescent devices.
(5) where R1
5 5 5 1-naphthyl group and
R3 R5
An important focus of research in organic optoelectronics is to
develop robust materials with high thermal and thin film
morphological stability. We envisage that such materials can be
achieved, without sacrificing the ease of device fabrication via
vacuum deposition, by incorporation of inorganic elements into
the materials. The inorganic analogue of benzene, borazine (Fig. 1),
is a class of compounds that has been known for decades. There is,
however, rarely any investigation on its applications except for its
use as ceramic BN precursors.1–3 When compared with benzene,
the partial ionic character of the BN bond due to the different
electronegativities of B and N would result in the borazine ring
behaving differently in intermolecular dipolar interactions.
Borazine derivatives also undergo addition reactions at the N–H
bonds leading to functionalization of the aromatic ring more
readily than its benzene analogues.4 Coˆte´ et al. have previously
shown, using theoretical calculations, that boron nitride polymers
may be suitable for applications in electronic devices.5 Here, we
report that borazine derivatives could provide an entry to a new
class of multifunctional and thermally stable materials with high
charge mobilities.
R2 5 R4 5 R6 5 H, used are depicted in Fig. 1. The crystal
structures{ show that 211 and 312 are non-planar; the angles
between the phenyl rings with the borazine ring are 42–48u and 56u
for 2 and 3 respectively. This lack of planarity would disfavour
p-stacking of the molecules in the solid state, which could be a
problem of concern when p-conjugated organic materials are used
in materials studies. The physical parameters of 1–5 are presented
in Table 1.
The thermal behaviour of 1–5 was studied using thermogravi-
metric analysis (TGA) and differential scanning calorimetry
(DSC).13 The thermal stability of these compounds could be
revealed by their decomposition temperature (Td, 230–340 uC
under nitrogen atmosphere) as depicted in Table 1. No decom-
position was observed for 1–5 after exposure to air for weeks; their
stability against hydrolysis and oxidation could be indicated by
their Td in air, which ranges from 204 uC for 4 to 238 uC for 1. The
charge mobilities of 1–4 (Fig. 2) were measured using the time-of-
flight (TOF) method14,15 on thin film samples: Au (20 nm)/n-
or p-type Si/borazine derivative (200 nm)/Au (100 nm).
Compounds 1–4 possess both favourable hole and electron
mobilities (1026–1024 and 1026–1023 cm2 V21 s21 respectively),
making them suitable for applications as hosts for dopants. Their
electron mobilities are comparable to that of Alq3 (tris(8-
quinolinolato)aluminum, 1026 cm2 V21 s21 at 4 6 105 V cm21
or 6 6 102 V1/2 cm21/2). Although their hole mobilities are lower
than that of NPB (4,49-bis[N-(1-naphthyl)-N-phenylamino]-
biphenyl, 1024–1023 cm2 V21 s21),16 these compounds are still
good enough for use as hole transporting materials (HTMs) in
organic light-emitting devices (OLEDs) since their hole mobilities
are of the same order of magnitude as the electron mobility of
Alq3, which is a common electron transporting material (ETM).
The electrochemical properties of 1, 2, 4 and 5 were studied
using cyclic voltammetry on samples dissolved in
N,N-dimethylformamide (DMF).17 As an example, the voltam-
mogram of 4 shows an irreversible oxidation wave with Epa at
0.76 V versus [Cp2Fe]+/0. The onset potential for oxidation was
estimated to be at 0.48 V. Compounds 1, 2 and 4 show an
absorption band with lmax at 264, 231 and 234 nm respectively
(Table 1). The emission of 1 and 4 is in the UV region (lmax at 288
and 362 nm accordingly) while no emission was observed for 2
(Table 1). Since the absorption and emission are in the UV region,
any visible light emitted from the device would not be reabsorbed
and there would be no unwanted visible emission from these
The five borazine derivatives,6–10 (1) 2,4,6-triphenylborazine (1)
where the substituents R1
5
R3
5
R5
5
H
and
R2 5 R4 5 R6 5 phenyl group; (2) 1,3,5-triphenylborazine (2)
Fig. 1 General formula of
a
borazine derivative (R1–R6 denote
substituents).
{ Electronic supplementary information (ESI) available: thermograms,
DSC plots, cyclic voltammograms, UV-Vis, PL and EL spectra as well as
CIE coordinates. See http://dx.doi.org/10.1039/b504510j
Department of Chemistry and the HKU-CAS Joint Laboratory on New
Materials, The University of Hong Kong, Pokfulam Road, Hong Kong
Special Administrative Region, China. E-mail: cmche@hku.hk;
Fax: +852 2857 1586; Tel: +852 2859 2154
*cmche@hku.hk
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Chem. Commun., 2005, 3547–3549 | 3547

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Table 1 Physical parameters for 1–5
1
2
3
4
5
Decomposition Temperature (Td)/ uCa
Hole mobility/cm2 V21 s21
Electron mobility/cm2 V21 s21
Band gap/eV
278
238
340
1025
230
248
N/Ab
1026–1024
1026–1025
4.3
1026–1025
1026
4.5
1025 –1024
1025–1023
3.9
1026–1025
—
—
N/Ab
3.3
c
c
Absorption lmax/nmd
264
288d
231
234
362d
284, 330
417d,, 429f
0.29d
c
Emission lmax/nm
Quantum yield/W
n. o.e
N/Ab
—
N/Ab
c
Under nitrogen atmosphere. N/A 5 not applicable. No measurement was made due to the poor solubility of 3 in various solvents. In
N/Ab
N/Ab
a
b
d
e
f
dichloromethane solution. n. o. 5 not observed; 2 is non-emissive. In the solid state.
Fig. 3 The current density–voltage–brightness (J–V–B) characteristic
curves of device A.
Fig. 2 Hole and electron mobilities for 1–4 at various electric fields.
compounds. In this regard, the employment of borazine derivatives
in OLEDs is advantageous over Alq3 and NPB; the latter two give
yellow and blue emission respectively even though they both
absorb UV light only.18 The band gap for 1, 2, 4 and 5 is estimated
from the tails of the absorption bands (4.3, 4.5, 3.9 and 3.3 eV
from 288, 278, 315 and 380 nm respectively).
luminance at high voltages could be due in part to the high thermal
stability of borazine compounds, which possess higher decom-
position temperatures than the common HTMs, biarylamines.
White OLED B and blue OLED C were prepared using 4 as the
host; the dopant was [Zn4OL6]22–24 where L is 3-(9-anthracenyl)-7-
azaindolate in device B and 7-azaindolate in device C. The
structure of device B was ITO/copper phthalocyanine (CuPc,
15 nm)/4:[Zn4OL6] (L 5 3-(9-anthracenyl)-7-azaindolate, 5%,
40 nm)/2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP,
10 nm)/Alq3 (30 nm)/LiF (0.5 nm)/Al (100 nm).22 White emission
The borazine core could also modulate the emission energy of
fluorescent organic molecules. Some borazine derivatives had
previously been reported to be fluoresecent in the UV region
(lmax 300–400 nm).19 The emission of 5 at 417 nm in
dichloromethane solution (with a quantum yield of 0.29) or
429 nm in a thin film is red-shifted from that of 1-naphthylamine
at lmax 5 400 nm.
´
at 1931 Commission Internationale de l’Eclairage (CIE_1931)
coordinates of (0.36, 0.43) was obtained at 21 V; the EL spectra
(Fig. 4) and CIE coordinates at different applied voltages were
similar. Since the host, 4, does not emit in the visible region, it can
be concluded that the white emission of device B comes solely from
the dopant, [Zn4OL6] (L 5 3-(9-anthracenyl)-7-azaindolate).22
Borazine derivative 4 played a significant role in this study,
since host materials that are non-emissive in the visible region
(l , 400 nm) are rare. The employment of a single emitting
component is an advancement for white OLEDs;25,26 Li et al. have
recently reported white EL from a dendritic europium complex.27
In device C with configuration ITO/4 (30 nm)/4:[Zn4OL6] (L 5
7-azaindolate,23 5%, 20 nm)/BCP (10 nm)/Alq3 (30 nm)/LiF
(0.5 nm)/Al (100 nm), 4 acted as the HTM in addition to being the
host. A blue emission solely from [Zn4OL6] (L 5 7-azaindolate)
with CIE_1931 of (0.19, 0.22) was obtained at 20 V.
OLEDs were prepared to illustrate the use of borazine
derivatives in electroluminescent (EL) devices. Compound 4 was
chosen as the HTM for device A, in which the configuration is
ITO (20 V square21)/NPB (6 nm)/4 (35 nm)/NPB (6 nm)/Alq3
(40 nm)/LiF (0.5 nm)/Al (100 nm), while Alq3 was the ETM. The
device gave yellow EL emission characteristic of Alq3. Device A
was designed such that (i) 4 with bulky 4-t-butylphenyl groups was
used to minimize recrystallization of the film; (ii) a ‘‘step’’ (NPB,
6 nm) was placed next to the ITO electrode to facilitate hole
injection into 4; (iii) a thin layer (6 nm) of NPB was inserted
between 4 and Alq3 to enhance hole injection into Alq3 and to
block electron and exciton leakage.20,21 If the two thin layers of
NPB on each side of 4 were not used, the device performance was
poor; a maximum luminance of only 290 cd m22 was obtained at
11 V. For device A, a maximum luminance of 6200 cd m22 was
achieved at 21 V (Fig. 3). The ability of the device to maintain high
In summary, we have found that borazine compounds
constitute a new class of thermally stable and multifunctional
materials for device applications. Metal complexes with high
3548 | Chem. Commun., 2005, 3547–3549
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¯
12 3: C36H30B3N6, formula weight 5 579.09, rhombohedral, R3c,
˚
3
˚
˚
a 5 14.818(2) A, c 5 24.856(5) A, V 5 4726.5(13) A , Z 5 6, T 5
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23
˚
˚
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emit at different colours, in contrast to the well-documented host
materials, [Ru(bpy)3]2+ (bpy 5 bipyridine) and Alq3, which cannot
host blue emitters.16 The colour purity of the device can be
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peripheral substituents on the borazine core, this class of
compounds could provide a new entry to multifunctional materials
with tunable physical and chemical properties.
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.
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.
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22 [Zn4OL6] (L 5 3-(9-anthracenyl)-7-azaindolate): zinc acetate (0.25 g,
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We gratefully acknowledge financial support by University
Development Fund (Nanotechnology Research Institute,
00600009) of The University of Hong Kong and Innovation and
Technology Commission of The Government of Hong Kong
Special Administrative Region (HKSAR), China (Project No. ITS/
53/01). We would also like to thank Sunic System Ltd. for support
on fabrication techniques.
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This journal is ß The Royal Society of Chemistry 2005
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