Multifunctional iridium complexes based on carbazole modules as highly efficient electrophosphors

Article abstract of DOI:10.1002/anie.200602906

(Figure Presented) Getting the green light: Highly efficient organic light-emitting diodes (OLEDs) have been synthesized from robust green-electrophosphorescent Ir^III complexes based on carbazole derivatives (see picture, HT = hole transporting, EL = electroluminescence). The combination of short triplet lifetime, high emission efficiency, and improved charge-transporting properties allows these OLEDs to achieve peak efficiencies of 12% photons per electron and 38 cd A^-1.

Full text of DOI:10.1002/anie.200602906

Communications
desirable that the designed phosphor shows improved charge-
balancing features and permits a complete transfer of energy
between the host and the dopant in the device. Interestingly,
small-molecule IrIII complexes derived from hole-transport-
ing (HT) carbazole units have not been reported to date.
Many carbazole derivatives are known to transport predom-
inantly positive charge carriers in organic systems. For
example, poly(9-vinylcarbazole) (PVK) is often described as
being a unipolar hole transporter whereas 4,4’-N,N’-dicarb-
azolebiphenyl (CBP) is recognized to have a more bipolar
transport character.[4] Typically, purely organic carbazole-
based compounds are high-mobility HT materials with a
tunable and high triplet energy level, and they are widely used
as the host materials for electrophosphors emitting different
colors.[5] The integration of HT carbazole modules and
emissive Ir complexes seems to be a wise choice for
vacuum-evaporated OLED materials with multicomponent
molecules, which are essential for more efficient charge
transport in the electroluminescence process.[6] We report
here two robust IrIII triplet emitters of green light that are
based on the rigid 2-[3-(N-arylcarbazolyl)]pyridine system
and possess the dual functions of light emission and hole
transportation, and also show improved charge-transporting
properties and morphological stability.
Organic Light-Emitting Diodes
DOI: 10.1002/anie.200602906
Multifunctional Iridium Complexes Based on
Carbazole Modules as Highly Efficient
Electrophosphors**
The green-light-emitting multifunctional metal chelates 1
and 2 can be synthesized in moderate to good yields by a one-
step cyclometalation of L1 and L2 with [Ir(acac)3] (acac =
acetylacetone) in refluxing glycerol (Figure 1a).[3f] Purifica-
tion of the mixture by preparative thin-layer chromatography
(TLC) on silica gel furnished homoleptic fac-[Ir(X-Cz-py)3]
(X = H, 1; X = F, 2; Cz = carbazolyl, py = pyridyl) as air-stable
yellow powders in high purity. Their 1D 1H NMR spectra are
consistent with a facial geometry around the Ir center, which
was also confirmed by a single-crystal X-ray analysis of 1
(Figure 1b).[7] Both 1 and 2 show excellent thermal stability,
with their onset decomposition temperatures (Td) greater
than 4208C, as revealed by thermogravimetric analysis (TGA,
Table 1), and with 1 being thermally much more stable than 2
and fac-[Ir(ppy)3] (4238C). Furthermore, both of them can be
vacuum-evaporated without decomposition and show good
film-forming qualities.
Wai-Yeung Wong,* Cheuk-Lam Ho, Zhi-Qiang Gao,
Bao-Xiu Mi, Chin-Hsin Chen, Kok-Wai Cheah, and
Zhenyang Lin
Since the seminal work on organic green-light-emitting diodes
using fac-[Ir(ppy)3] (Hppy = 2-phenylpyridine),[1] intensive
research effort has been directed towards the development
of efficient phosphorescent materials for potential use in full-
color flat-panel-display technology.[2] Although the emission
efficiency and the color of Ir-based organic light-emitting
diodes (OLEDs) can be tuned by structural modifications of
the ligand chromophores,[3] the performance of the device is
greatly influenced by the charge balance between the
electrons and holes from opposite electrodes. It is highly
The nature of the X group on the carbazole ring does not
have a great influence on the absorption and photolumines-
cence (PL) spectra of 1 and 2 (Table 1). The first two bands
appear to be ligand-based transitions that closely resemble
the spectra of the free ligands (labs: ca. 252 and 297 nm).
Somewhat weaker bands are observed at lower energies in the
visible region which correspond to electronic excitations to
the allowed and spin-forbidden metal-to-ligand charge-trans-
[*] Dr. W.-Y. Wong, C.-L. Ho
Department ofChemistry
Hong Kong Baptist University
Waterloo Road, Kowloon Tong, Hong Kong (P.R. China)
Fax: (+852)3411-7348
E-mail: rwywong@hkbu.edu.hk
Dr. Z.-Q. Gao, Dr. B.-X. Mi, Prof. C.-H. Chen, Prof. K.-W. Cheah
Centre for Advanced Luminescence Materials
Hong Kong Baptist University
3
fer states (1MLCT and MLCT, respectively). These assign-
Waterloo Road, Kowloon Tong, Hong Kong (P.R. China)
ments were supported by DFT calculations on 1, which show
that the highest occupied molecular orbitals (HOMOs) are
derived from the out-of-phase combinations of the Ir “t2g” set
of d orbitals and the p orbitals of the three Ir-bonded phenyl
Dr. Z. Lin
Department ofChemistry
The Hong Kong University ofScience and Technology
Clearwater Bay, Hong Kong (P.R. China)
À
rings of the three N C bidenate ligands. In each of the first
[**] This work was supported by a CERG Grant from the Hong Kong
Research Grants Council (HKBU 2022/03P) and a Faculty Research
Grant from the Hong Kong Baptist University (FRG/04-05/II-59).
three HOMOs, the Ir(d) orbitals make an approximate 20%
contribution, with the N(amine) pp orbitals also contributing
to the HOMOs. The lowest unoccupied molecular orbitals
(LUMOs) are the p* orbitals of the three pyridine rings. Both
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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FP in CH2Cl2 of these triplet emitters of green light are
approximately 0.33–0.43 relative to a fac-[Ir(ppy)3] standard
(FP = 0.40).[8] The triplet lifetimes tP are very short at 0.46–
0.54 ms for 1 and 2 in the solid state. Accordingly, the triplet
radiative lifetimes (tr) deduced from tr = tP/FP are 1.07–
1.64 ms, which are shorter than that of fac-[Ir(ppy)3]
(ca. 4.75 ms)[3f] and other IrIII complexes.[3]
Cyclic voltammetric studies show that both 1 and 2
undergo a reversible one-electron oxidation of the Ir–
ox
1=2
carbazolyl ring system (E 0.07, 1; 0.13 V, 2). The attachment
of electron-withdrawing F substituents in 2 causes the
oxidation to occur at a more anodic potential, which would
have a major impact on the device properties (see below).
Both 1 and 2 were oxidized at less-positive potentials than fac-
[Ir(ppy)3], thus indicating a greater propensity for 1 and 2 to
lose an electron.[3k,5b] Cathodic sweeps in THF reveal an
irreversible wave at À2.49 to À2.56 V. There is subtle changes
in the HOMO and LUMO levels arising from the presence of
different X groups in 1 and 2, and we were able to tune the
HOMO level without influencing the triplet energy of these Ir
phosphors. The HOMO levels are significantly elevated
(À4.87, 1; À4.93 eV, 2) relative to those of fac-[Ir(ppy)3]
(À5.50 eV).[3k,5b] This finding indicates that they have a lower
ionization potential (IP) than fac-[Ir(ppy)3], thus leading to a
better HT ability in 1 and 2 which favors the transport of
holes. The LUMO levels of 1 and 2 are comparable to that of
the widely used electron-transporting/hole-blocking 2-(4-
biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole
(À2.4 eV). Our complexes show T1 levels well below the level
of those of the CBP host (2.56 eV) which prevents reverse
energy transfer from the guest back to the host, and confines
triplet excitons to the guest molecules.
The optoelectronic properties of both IrIII complexes were
characterized, and efficient OLEDs I and II were fabricated
using 1 and 2, respectively, as phosphorescent dopant emitters
in a four-layer structure of ITO/NPB (40 nm)/6% 1 or 2 in
CBP (20 nm)/BCP (8 nm)/Alq3
Figure 1. a) Synthesis of 1 and 2; b) ORTEP plot of 1, with the
hydrogen atoms omitted for clarity.
(20 nm)/Mg:Ag (200 nm) (ITO =
indium tin oxide, NPB = 4,4’-bis[N-
(1-naphthyl)-N-phenylamino]bi-
[8C][f] phenyl,
BCP = 2,9-dimethyl-4,7-
Table 1: Photophysical, electrochemical, and thermal data for 1 and 2.
Absorption[a]
Emission
E
E
HOMO
[eV]
LUMO
[eV]
Td
ox
1=2
[V][d]
red
1=2
[V][e]
labs [nm]
lem [nm][b]
tP [ms][c]
diphenyl-1,10-phenanthroline,
Alq3 = tris(8-hydroxyquinoline)).
CBP acts as the host material for
the electrophosphor and NPB as a
hole-transport layer. A thin hole-
blocking barrier layer of BCP
inserted between the CBP and the
electron-transporting Alq3 was nec-
essary to confine excitons within the
luminescent zone and hence main-
tain high efficiencies. The electro-
luminescence (EL) spectra consist
only of a green phosphor emission
1
2
291 sh (4.69),
316 (4.69),
398 sh (0.20)
281 sh (4.89),
315 (4.85),
515
(0.43)
0.46
0.07
À2.56
À4.87
À4.93
À2.24
À2.31
477
421
514
(0.33)
0.54
0.13
À2.49
400 sh (0.16)
[a] In degassed CH2Cl2 at 293 K; loge values are shown in parentheses; sh=shoulder. [b] FP values are
shown in parentheses versus fac-[Ir(ppy)3] (FP =0.40), lex =380 nm. [c] Triplet lifetimes at 293 K.
[d] 0.1m [Bu4N]PF6 in CH2Cl2, versus the ferrocene/ferrocinium (Fc/Fc+) couple. [e] 0.1m [Bu4N]PF6 in
THF, versus Fc/Fc+ couple. [f] At a heating rate of 20 8CminÀ1 under N2.
complexes emit strong phosphorescence (lem ca. 514 nm,
Stokes shifts ca. 200 nm) at room temperature in CH2Cl2.
The triplet energy levels (T1: ca. 2.41 eV) of the two
complexes are very similar and also similar to that of fac-
[Ir(ppy)3] (2.43 eV).[1a] The phosphorescence quantum yields
without any residual emission from the host and/or adjacent
layers, even at high drive currents—an indication of complete
energy and/or charge transfer from the host exciton to the
phosphor upon electrical excitation. The EL spectra and
Commission Internationale de LꢀEclairage (CIE) coordinates
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ox
1=2
for the devices exhibited no significant change on variation of
consequence of the lower E value of 1. As the HOMO of 1
the current and operating bias voltages from 5 V to 12 V
(Figure 2a), and both of the devices I and II show a sharp EL
peak at 508 nm. The turn-on voltages (defined as the bias at a
is positioned higher in energy than that of 2 (relative to the
CBP level of 6.3 eV), the efficiency of recombination by
sequential charge trapping is increased in 1. The performance
parameters of device I (maximum external quantum effi-
ciency (hext): 11.6% photons/electron, luminance efficiency
(hL): 38 cdAÀ1, power efficiency (hP): 24 LmWÀ1) clearly
outweigh those of a prototypical green-light-emitting device
of ITO/NPB/fac-[Ir(ppy)3]:CBP/BCP/Alq3/Mg:Ag (maxi-
mum hext: 7.5%, hL: 26 cdAÀ1, hP: 19 LmWÀ1) in a device of
similar structure, and this represents a tremendous improve-
ment of about 55% in the hext value.[1a] The maximum
luminance of device I is 14730 cdmÀ2 at 12 V, with the full
width at half maximum (fwhm) only 56 nm at 8 V. With 2 as
the dopant, device II has
a maximum brightness of
19360 cdmÀ2 and a luminance efficiency of approximately
22 cdAÀ1, which corresponds to a hext value of 6.7% and a
hP value of approximately 13 LmWÀ1. We can surmise that it
is probably the carbazolyl moiety that brings about a more
balanced electron and hole recombination in the host matrix
of CBP, and/or that the rigid carbazole spacers play a crucial
role in alleviating the self-quenching of the [Ir(X-Cz-py)3]
luminophores, and consequently increase the emission effi-
ciency. Typical efficiency roll-off at higher currents, attribut-
able to a combination of triplet–triplet annihilation and field-
induced quenching effects, is also observed here, but at the
reference luminance of 100 cdmÀ2 (6.5 V) the efficiencies
remain high at 10.6%, 35 cdAÀ1, and 17 LmWÀ1 for device I.
These values correspond to a loss of only 8% from the
maximum hext value. Likewise, a loss of 7% was noted in the
hext value for device II. Although the performance of these
devices has yet to be optimized, it is our intention to highlight
the potential merits of this prominent class of carbazole-based
Ir phosphors in high-efficiency OLED applications.
In summary, we have reported some triplet-harvesting
carbazolyl-substituted multicomponent iridium complexes,
and this approach can offer advantages in terms of lowering
the first IP, facilitating hole transport, and enhancing EL
efficiencies relative to the prototypical fac-[Ir(ppy)3]. Given
the ease of synthesis and performance advantages inherent to
these carbazole-based phosphors, extension of the system to
other emission colors warrants further examination.
~
~
*
Figure 2. a) The EL spectra ofdevice I at 5 ( ), 8 ( ), 10 ( ), and
*
12 V ( ) and b) current–voltage–luminance (J-V-L) characteristics for
device I.
brightness of 1 cdmÀ2) are 4.4 and 4.8 V for devices I and II,
respectively, with the CIE color coordinates being (0.24, 0.63)
and (0.27, 0.60), which correspond to the bright green region
of the chromaticity diagram. There is a close resemblance
between the EL and PL spectra in each case.
Phosphors 1 and 2 show outstanding EL performance,
presumably because of the short tP value and good hole-
transporting properties of carbazole, which may help diminish
the quenching of the triplet exciton. Figure 2b shows the
current density–voltage–luminance (J-V-L) curve of device I.
Complex 1 was shown to be a more efficient electrophosphor
than 2, which suggests that the F substituents are rather
unimportant here in relieving intermolecular interactions in
Received: July 20, 2006
Published online: October 24, 2006
Keywords: carbazoles · iridium · optical properties ·
.
phosphorescence · structure elucidation
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