Aluminium-salen luminophores as new hole-blocking materials for phosphorescent OLEDs

Article abstract of DOI:10.1039/b717754b

The organic light-emitting diodes (OLEDs) employing complex [salen( ^tBu)₄Al(OC₆H₄-p-C₆H ₅)] (4) as a hole-blocking layer produced stable green EL emission of Ir(ppy)₃ irrespectiv

Full text of DOI:10.1039/b717754b

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www.rsc.org/dalton | Dalton Transactions
Aluminium–salen luminophores as new hole-blocking materials for
phosphorescent OLEDs†‡
Kyu Young Hwang,a Min Hyung Lee,a Hyosook Jang,a Yeunjoo Sung,a Jong Soon Lee,b Se Hun Kimb and
Youngkyu Do*a
Received 16th November 2007, Accepted 22nd January 2008
First published as an Advance Article on the web 15th February 2008
DOI: 10.1039/b717754b
The organic light-emitting diodes (OLEDs) employing com-
plex [salen(tBu)4Al(OC6H4-p-C6H5)] (4) as a hole-blocking
layer produced stable green EL emission of Ir(ppy)3 irrespec-
tive of changing current density and showed higher brightness
and device efficiency than the BAlq-based device.
relatively low efficiency but a fairly long lifetime delivered by high
stability.3b Since the lifetime is a key factor for device fabrication,
it is still of great interest to search for new penta-coordinated alu-
minium systems as hole-blocking materials that can produce high
efficiency of phosphorescent emitters in conjunction with a long
lifetime. In this regard, we now set out to determine if the penta-
coordinated aluminium systems based on Schiff-base salen [N,Nꢀ-
bis(3,5-di-tert-butyl-salicylidene)-ethylenediamine] can be utilized
as promising hole-blocking materials. Although tetradentate-salen
derivatives have been widely used as anchoring ligands in various
inorganic and organometallic fields such as catalysis,8 sensors,9
and molecular magnets10 owing to their strong chelating ability,
facile introduction of various subsitituents and easy synthesis,
the photophysical properties and further applications to the
OLED materials of metal-salen compounds have rarely been
investigated.9a,11 The only example of the use of bis(salen)Zn
as an emitting material has recently been reported.12 Herein,
the brief accounts of the synthesis, structure, and novel OLED
characteristics of salen-based aryloxy aluminium complexes 1–4
as hole-blocking materials are described.
Phosphorescent organic light-emitting diodes (PhOLEDs) based
on heavy metal complexes have recently received great attention
due to their high internal quantum efficiency, up to 100% of a
theoretical value, from contribution of both singlet and triplet ex-
citons for light-emission.1,2 Nonetheless, PhOLEDs have a barrier,
inter alia, to overcome: triplet excitons can diffuse to the adjacent
layers due to their long lifetime,1a leading to undesired emission or
quenching of light. Thus, the confinement of triplet excitons within
an emitting layer (EML) is necessary to deploy phosphorescent
emitters, which requires an additional hole-blocking layer (HBL)
between an EML and an electron-transporting layer (ETL).2 The
HBL could also be advantageous to control of charge balance by
blocking hole-diffusion from the EML into the ETL.
In order to achieve high device performance, such hole-blocking
materials are required to have a deep highest occupied molecular
orbital (HOMO) energy level, a large energy band gap, and a
lowest unoccupied molecular orbital (LUMO) energy level close to
that of an ETL. Hole-blocking materials that are developed up to
date include quinolinol-containing complexes,3 multi-phenylene
systems,4 oxadiazole or triazol-containing compounds,5 phenan-
throline derivatives,2,6 and spiro-type diazafluorene systems.7
Among them are 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline,
known also as bathocuproine (BCP),2,6a and bis(2-methyl-8-
quinolinolato)-mono(4-phenylphenolato)aluminium (BAlq)3 that
have been the commonly used hole-blocking materials. OLEDs
employing BCP that has a deep HOMO energy level with a large
energy band gap showed high efficiency but a short device lifetime
due to low stability while OLEDs employing BAlq displayed
Overall synthetic procedures for 1–4 are outlined in Scheme 1.
Salen ligand was prepared by the reaction of 3,5-di-tert-butyl-2-
hydroxybenzaldehyde and ethylenediamine in a 2 : 1 molar ratio
as reported previously.13 Treatment of the salen with trimethyla-
luminium afforded the precursor14 that in turn gave the desired
penta-coordinated aluminium complexes 1–4 in 72–80% yield as
yellow solids in the presence of the corresponding phenol.15
aDepartment of Chemistry, School of Molecular Science BK-21 and Center
for Molecular Design and Synthesis, KAIST, Daejeon 305-701Korea.
E-mail: ykdo@kaist.ac.kr; Fax: +82 42 869 2810; Tel: +82 42 869 2829
Scheme 1 Synthesis of the salen-based aluminium complexes.
bR & D Center, Dongwoo FineChem Co., Ltd 1177, Pyeongtaek-Si, Kyunggi-
Do, 451-764, Korea
The molecular structure of complex 4 illustrated in Fig. 1
reveals distorted penta-coordination geometry around Al ranging
between trigonal bipyramidal and square pyramidal. Geometry
analysis with the index of the degree of ‘trigonality’ s = (b −
a)/60 (s = 0 for perfectly square pyramidal geometry and s =
1 for perfectly trigonal bipyramidal geometry)16 for 4 affords a
s value of 0.61, indicating that the coordination geometry of 4
has a more trigonal bipyramidal nature. The Al–O3 bond length
† Crystal Data for 4: C44H55AlN2O3, Mw = 686.88, monoclinic, P21/c, a =
˚
20.263(8), b = 11.792(4), c = 18.060(9) A, a = 90.00, b = 114.585(12), c =
◦
3
˚
90.00 , V = 3924(3) A , Z = 4, T = 293(2) K, measd reflns = 22 087, unique
reflns = 8569, Rint = 0.0482, reflns used for refinement = 8569, R1 = 0.0522,
wR2 = 0.1137. CCDC reference number 656227 (4). For crystallographic
data in CIF or other electronic format see DOI: 10.1039/b717754b
‡ Electronic supplementary information (ESI) available: Syntheses and
characterization for 1–4, X-ray data for compound 4, and fabrication of
OLEDs. See DOI: 10.1039/b717754b
◦
˚
[1.7609(16) A] and the Al–O3–C33 [136.49(14) ] angle of the
1818 | Dalton Trans., 2008, 1818–1820
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Fig. 2 UV-Vis absorption and photoluminescence (PL) spectra of 1.
imum wavelengths at 364 nm and 480 nm, respectively, are very
similar regardless of the type of the ancillary aryloxy ligand. The
fluorescence quantum efficiencies of complexes 1–4 are in the range
0.31–0.41.20 The HOMO–LUMO energy gaps of all complexes
1–4 were estimated as 2.9 eV from the absorption edges of the
optical absorption spectra, indicating that the ancillary ligand
attached to the aluminium–salen luminophore has little effect on
the energy band gap. On the other hand, the HOMO energy levels
are determined from the cyclic voltammetric oxidation potential21
and thereby the LUMO energy levels are slightly dependent on the
ancillary ligand.
In order to evaluate the OLED characteristics of the forego-
ing salen-based aryloxy aluminium complexes as hole-blocking
materials, 4 was selected since it shows the highest thermal
stability and has the HOMO–LUMO energy levels comparable to
those (5.2–2.2 eV HOMO–LUMO levels) of BAlq. Two OLEDs
(OLED-4 and OLED-BAlq) were fabricated by vacuum deposi-
tion with the structure of ITO (anode)/CuPc (HIL, 10 nm)/a-
NPD (HTL, 40 nm)/8wt% Ir(ppy)3 in CBP (EML, 50 nm)/4 or
BAlq (HBL, 10 nm)/Alq3 (ETL, 30 nm)/LiF (EIL, 1 nm)/Al
(cathode, 100 nm) (CuPc = copper phthalocyanine, HIL = hole-
injection layer, a-NPD = 4,4ꢀ-bis(N-1-naphthyl-N-phenylamino)-
biphenyl, HTL = hole-transporting layer, CBP = 4,4ꢀ-bis(N-
carbazolyl)biphenyl, Ir(ppy)3 = tris(2-phenyl-pyridine)-iridium,
Alq3 = tris(8-hydroxyquinolinolato)aluminium, EIL = electron-
injection layer).
Fig. 1 Molecular structure of 4.† Thermal ellipsoids are drawn at the
30% probability level. Hydrogen atoms are omitted for clarity. Selected
◦
˚
bond lengths (A) and angles ( ): Al–O1 1.7900(17), Al–O2 1.7763(16),
Al–O3 1.7609(16), Al–N1 1.9848(19), Al–N2 2.0093(19), O3–C33 1.339(3),
O1–Al–O2 94.53(7), O1–Al–O3 97.33(8), O2–Al–O3 118.31(8), N1–Al–O1
89.26(8), N1–Al–O2 129.65(8), N1–Al–O3 110.86(8), N2–Al–O1
166.18(8), N2–Al–O2 88.18(7), N2–Al–O3 93.28(8), N2–Al–N1 78.66(8),
C33–O3–Al 136.49(14).
Al–aryloxide moiety are comparable to those found in the similar
(salen)Al(OC6H2Me3-2,4,6) compound.17
All the complexes 1–4 are air-stable both in the solid and so-
lution states. Penta-coordinated aluminium complexes containing
salentendtoshow low chemicalstabilityand readilydissociate into
other species in the air or in the presence of Lewis base especially
when an alkyl or halide unit is employed as an ancillary ligand.18
Such enhancement of stability can be ascribed to the presence
of tert-butyl groups in the salen ligand and aryloxide ancillary
ligand, affording the encapsulation of the metal center and
stronger bonding with Al, respectively. In addition, the presence
of tert-butyl groups increases the solubility of the complexes in
common organic solvents. The increased chemical stability seems
to lead to the increased thermal stability as indicated by 5 wt%
decomposition temperatures, Td5 of 230, 270, 280 and 330 ◦C
for complexes 1–4, respectively. These results also indicate that
thermal stability can be modified dramatically by change of the
ancillary ligand.
Both devices emitted bright green light contributed only from
Ir(ppy)3 and their EL spectra were not significantly changed
upon varying the applied voltage. The CIE color coordinate was
changed slightly from (0.27, 0.64) to (0.27, 0.63) in the case of
OLED-BAlq as the current density increased from 10 mAcm−2 to
100 mAcm−2 while OLED-4 maintained the constant CIE color
coordinate at (0.27, 0.63). These results indicate that complex 4
functions as a stable hole-blocking layer material. Fig. 3a displays
current density as a function of applied voltage and luminance as a
function of current density. The turn-on voltages of OLED-4 and
OLED-BAlq are 5.4 and 5.2 V, respectively. The current density
and the brightness of OLED-4 were measured as 70 mAcm−2 at
13 V and 16 200 cdm−2 at 100 mAcm−2, respectively. OLED-BAlq
showed a current density of 116 mAcm−2 and a brightness of
13 000 cdm−2 under the same conditions. As shown in Fig. 3b,
OLED-4 gave a luminous efficiency (gL) of 17.1 cdA−1 and a power
efficiency (gp) of 4.7 lmW−1 at 20 mAcm−2 that can be compared
The absorption and emission spectra of complex 1 in chloroform
are shown in Fig. 2 and the spectroscopic and energy level data for
all the complexes are summarized in Table 1.19 The spectroscopic
properties of complexes 1–4, the absorption and emission max-
Table 1 Spectroscopic and energy level data for complexes 1–4
Complex kabs a/nm (log e) kema/nm (grel)b HOMOc/eV LUMO/eV
1
2
3
4
363 (4.01)
364 (3.98)
366 (3.92)
364 (4.04)
480 (0.35)
479 (0.41)
479 (0.40)
480 (0.31)
5.3
5.0
5.4
5.2
2.4
2.1
2.5
2.3
b
a Chloroform solution (1.0 × 10−5 M). grel is the relative PL efficiency
with respect to that (0.55) of quinine sulfate.20 c HOMO energy levels were
determined from the oxidation potentials measured by cyclic voltammetry
in CH2Cl2.
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Notes and references
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with gL = 12.3 cdA−1 and gp = 3.6 lmW−1 for OLED-BAlq. Overall,
complex 4 led to much improved OLED performance with higher
brightness and efficiency as compared to BAlq as hole-blocking
materials.
In summary, a set of new stable aluminium–salen luminophores
was developed and tested selectively as hole-blocking layer mate-
rials for a green phosphorescent OLED. The OLED-employing
complex 4 produced stable green EL emission of Ir(ppy)3 irrespec-
tive of changing current density and showed higher brightness
and device efficiency than the BAlq-based device, implying that
aluminium–salen based luminophores would constitute a promis-
ing class of hole-blocking materials for phosphorescent OLEDs.
Currently, various synthetic modifications based on molecular
orbital calculations are in progress to establish the scope of salen-
based aluminium complexes as OLED materials.
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Acknowledgements
The authors gratefully acknowledge financial support from
CMDS, KOSEF(R01-2005-000-10229-0) and BK 21 project.
1820 | Dalton Trans., 2008, 1818–1820
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The Royal Society of Chemistry 2008
©