Solution processed organic double light-emitting layer diode based on cross-linkable small molecular systems

Article abstract of DOI:10.1002/anie.201303031

Stacks of potential: A cross-linkable host-guest system for state-of-the-art multi-layer device stacks is developed. The spirobifluorene-based host materials were co-cross-linked with green emitting phosphorescent (fac)-Ir^III complexes. As a result a five-layered organic light-emitting diode (OLED) could be fabricated in which each organic layer has been deposited from solution. Copyright

Full text of DOI:10.1002/anie.201303031

Angewandte
Chemie
Communications
OLEDs
Stacks of potential: A cross-linkable host–
guest system for state-of-the-art multi-
layer device stacks is developed. The
spirobifluorene-based host materials
were co-cross-linked with green emitting
phosphorescent (fac)-IrIII complexes. As
a result a five-layered organic light-emit-
ting diode (OLED) could be fabricated in
which each organic layer has been
deposited from solution.
G. Liaptsis, D. Hertel,
K. Meerholz*
&&&& — &&&&
Solution Processed Organic Double
Light-Emitting Layer Diode Based on
Cross-Linkable Small Molecular Systems
Angew. Chem. Int. Ed. 2013, 52, 1 – 6
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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DOI: 10.1002/anie.201303031
OLEDs
Solution Processed Organic Double Light-Emitting Layer Diode Based
on Cross-Linkable Small Molecular Systems**
Georgios Liaptsis, Dirk Hertel, and Klaus Meerholz*
Dedicated to the Bayer company on the occasion of its 150th anniversary
State-of-the-art organic light-emitting diodes (OLEDs) con-
sist of a multi-layer device structure to ensure higher power
conversion efficiencies and extended lifetimes compared to
their single-layer counterparts.[1] The use of phosphorescent
emitters based on transition metal complexes such as PtII [2] or
IrIII compounds[3] has led to quantum yields close to unity
through their effective inter-system crossing rate from excited
singlet states to highly emissive triplet states.[4] Losses in
radiative decay caused by triplet–triplet annihilation and
aggregation quenching of excited states can be suppressed by
the introduction of a host–guest system as the emitting layer
(EML).[5] The energy-level arrangement of the host and the
guest must energetically fit in regards to charge-carrier
injection and Fçrster[6] and/or Dexter[7] type energy transfer
from excited host molecules to the emitting guest molecules.
Thus, efficient triplet harvesting on the emitting molecules is
accomplished.[8] An advanced concept to enhance the effi-
ciency of phosphorescent OLEDs (PHOLEDs) is the imple-
mentation of a charge-carrier accumulating interface inside
the EML. This idea is realized by so-called “double-emission
layers” (DEL) which utilize two different host materials with
the same guest emitter. They can reduce losses of triplet
excitons into regions which are not doped by the phosphor-
escent material.[9]
To allow for cost-effective OLED production for solid-
state lighting or display applications, solvent-based fabrica-
tion techniques, such as roll-to-roll or ink-jet printing, are
favoured over more expensive vacuum-based deposition
techniques.[10] When creating a multi-layer OLED from
solution, it is important to prevent interface mixing and
erosion during deposition of the subsequent layers. Besides
orthogonal solvent processing[11] a well-established approach
to prepare solvent-based multi-layer devices is the chemical
functionalization of the organic semiconductors with reactive
moieties for thermally polymerization[12] and/or cross-link-
ing.[13] We have reported on cross-linkable oxetane-function-
alized semiconductors, that can be cross-linked by cationic
ring opening polymerization (CROP), which is induced by
a photo acid generator (PAG), yielding an insoluble film after
deposition and curing without degradation of the optoelec-
tronic properties.[14] Herein, we present the first DEL–
OLEDs fabricated entirely from solution using both, novel
oxetane-functionalized host and phosphorescent guest mate-
rials. We report on the materialsꢀ synthesis, their optoelec-
tronic characterization, and finally their use in OLED devices.
Regarding phosphorescent emitters, we focused on easy-
to-synthesize, highly soluble, and cross-linkable tris-cyclo-
metallated homo- and heteroleptic (fac)-IrIII complexes
derived from the established emitter fac-[Ir(mppy)3], which
is commonly used in OLEDs.[2a] We propose to replace the
methyl group on the p-methylphenyl pyridine ligand by
a diarylamine, yielding a pyridine-substituted triarylamine
(TPA-Py) ligand. By doing so, the hole-trapping properties
are improved. At the same time, the attachment of the cross-
linkable oxetane unit by a C6-spacer dramatically increases
the solubility of the resulting complexes, reducing the
tendency to self-aggregate and, thus, self-quench.
Synthesis, electrochemical, and spectroscopic character-
ization of the new emitters (X-IrGn, see Figure 1) are
described in the Supporting Information. In short, all the
complexes feature reversible oxidation (Figure S1), and the
HOMO energy decreases with increasing number n of X-
TPA-Py ligands from À5.25 eV for the reference compound
fac-[Ir(mppy)3] (n = 0) to À5.14 eV for X-IrG3 (n = 3),
respectively (Table 1). This trend is caused by the increasing
amount of donor-substituted ligands connected to the metal
center. Compared to fac-[Ir(mppy)3] the donor effect of the
diphenylamine group increases the absorption coefficient of
the metal-to-ligand charge transfer (MLCT) state and shifts
the photoluminescence bathocromically.
The suitability of the new oxetane-functionalized com-
plexes as emitters in OLEDs was first tested in solution-
processed multi-layer PHOLEDs (see Figure 1 for structural
formula and Figure 2 for device setup).[15] These devices are
referred to as P1–P3 (containing X-IrG1–G3, respectively).
For comparison, devices using the same layer sequence, but
fac-[Ir(mppy)3] as the emitter were fabricated (Figure 3 left;
inverted triangles; device P0). As shown in Figure 3 left,
device P1 performed best, reaching a luminous efficiency at
a luminance of 1000 CdmÀ2 (LE1000) of 45.9 CdAÀ1, followed
[*] Dr. G. Liaptsis, Dr. D. Hertel, Prof. Dr. K. Meerholz
Chemistry Department, University of Cologne
Luxemburger Strasse 116, 50939 Cologne (Germany)
E-mail: klaus.meerholz@uni-koeln.de
Homepage: http://www.meerholz.uni-koeln.de
[**] We acknowledge funding by the State of Northrhine-Westfalia and
the Europꢀischer Fonds fꢁr Regionale Entwicklung (EFRE) through
the PROTECT project, which is part of the Centre of Organic
Production Technologies COPT.NRW.
by P2 (LE1000 = 35.2 CdAÀ1), and finally P3 (LE1000
22.9 CdAÀ1). The reference device P0 reached slightly
=
better performance under these conditions (LE1000
=
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201303031.
25.3 CdAÀ1, however, at lower light output all novel com-
2
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Figure 1. Structural formula of compounds used in this study. The top row: the novel cross-linkable host materials X-H1 and X-H2 as well as the
phosphorescent green emitting materials fac-[Ir(mppy)3] and X-IrG1–3. Bottom row: the additional compounds used in the devices P0–3, S1–2,
and D, these are the cross-linkable hole conductor X-TAPC, the electron conductors 1,3-bis(5-(4-tert-butylphenyl)-1,3,4-oxadiazol-2-yl)benzene
(OXD-7) and 1,3,5-tris(1-phenyl-1H-benzo[d]imidazol-2-yl)benzene (TPBI), as well as the host material poly(N-vinylcarbazole) (PVK).
transport and hole trapping on the complexes.[15b]
Compared to fac-[Ir(mppy)3] which has no X-TPA-
Py ligand, X-IrG1 traps holes more efficiently and
the attraction of electrons followed by the formation
of excitons is improved.[17] On the other hand, the
decreasing device performance with increasing
amount of triphenylamine (TPA) subunits of X-
IrG1–3 is explained by the competition between
their hole trapping ability and their hole transport
ability. An increase of TPA subunits enhances the
hole mobility in the emitting layer, which has
Table 1: Summary of physical properties.[a]
Compound
EOx
[V]
EHOMO
[eV]
ledge
[nm]
ELUMO
[eV]
S1
[eV]
T1
[eV]
DEST
[eV]
X-H1
X-H2
0.93
0.08
0.17
0.11
0.05
0.04
–
À5.94
À5.17
À5.25
À5.20
À5.15
À5.14
À5.90[27]
À6.30[27]
À5.94
À5.27
372
435
500
500
500
500
–
À2.61
À2.32
À2.61
À2.72
À2.77
À2.78
À2.20[27]
À2.40[27]
À2.23
À2.14
3.06
2.87
–
–
–
–
–
–
–
2.27
2.26
2.45[15b]
2.34
2.34
2.34
2.75[28]
2.70[27]
–
0.79
0.61
–
–
–
–
–
–
–
[Ir(mppy)3]
X-IrG1
X-IrG2
X-IrG3
PVK
OXD-7
H1
H2
–
–
–
0.93[20]
0.19[29]
a
negative impact on the device performance
396[29]
–
–
–
within this device series.[18] This result is confined
by hole-only devices (Figure S5). More detailed
device performance parameters are listed in the
Supporting Information (Table S2).
[a] EOx is the oxidation potential determined by cyclic voltammetry referenced to the
ferrocene/ferrocenium redox couple. EHOMO is the HOMO energy calculated
according to Equation (1). ledge is the bathochromic edge of absorption determined
in THF solution and extracted on a logarithmic scale of the absorption data. ELUMO is
the LUMO energy. S1 and T1 are the first excited singlet and triplet states,
respectively, wherein DEST is the difference between these values. The data of the
parent host materials without cross-linkable side chains is mentioned for
comparison. The HOMO energies were calculated as mentioned in the Supporting
Information using the first oxidation potential versus the Ferrocene/Ferrocenium
redox couple published in literature (Note that we used 0.48 Vas potential for Fc/Fc+
vs. Ag/AgCl electrode). The LUMO of H1 was calculated by the addition of the
difference between the first oxidation and first reduction potential to the HOMO
level. The LUMO of H2 was calculated by adding the band-gap energy to the HOMO
level.
In view of the excellent electroluminescent
properties of the new emitters compared to a stan-
dard emitter such as fac-[Ir(mppy)3], we searched for
novel cross-linkable, bipolar host materials to enable
the fabrication of multi-emitting-layer devices from
solution. Compounds containing spirobifluorene as
central building block appear to be promising
candidates,[19] offering bipolar transport properties[20]
in addition to small singlet–triplet splitting energies
(DEST).[19c,21,22] A high-lying triplet state of the host is
important to prevent the exothermic energy back
plexes clearly outperform the reference fac-[Ir(mppy)3],
which is also reflected in the turn-on voltage, decreasing in
the sequence P1 < P2 < P3 ! P0. This improvement relative
to the parent compound is clearly attributed 1) to an
improved dispersion of the complexes within the matrix
through higher solubility[16] and 2) to the improved hole
transfer from the emissive triplet state of the emitting
transition-metal complex to the non-emissive triplet state of
the hydrocarbon host material during OLED operation.
Synthesis, electrochemical, and spectroscopic character-
ization of the new host materials X-H1 and X-H2 (see
Figure 1) are described in Supporting Information. In short,
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Figure 2. Energy level arrangement of the optimized devices S1 (left), S2 (middle), and D (right). The device S1 consists of X-TAPC (20 nm)//X-
H1- X-IrG3 (8%) (40 nm)//OXD-7: PMMA (75:25) (40 nm)//CsF (2 nm)//Al (100 nm). The device S2 consists of X-TAPC (20 nm)//X-H2- X-IrG3
(8%) (40 nm)//TPBI (60 nm)//CsF (2 nm)//Al (100 nm). The device D consists of X-TAPC (10 nm)//X-H2- X-IrG3 (8%) (20 nm)//X-H1- X-IrG3
(8%) (20 nm)//TPBI (40 nm)//CsF (2 nm)//Al (100 nm).
the energy levels allow efficient charge injection and sub-
sequent trapping in the IrIII emitter. The materials form
homogeneous films with a high photoluminescence quantum
yield (PLQY) upon doping with the IrIII complexes
(Table S1).
best performance, followed by devices S2 and S1, respectively.
With increasing light output, the devices reach their max-
imum luminous efficiencies of LEmax = 34 (29) CdAÀ1 for the
devices S2 and D (S1, respectively), however, the voltage to
achieve this increases in the sequence D < S2 < S1. At the
reference brightness of 1000 CdmÀ2 the three devices per-
formed similarly, reaching LE = (27.5 Æ 1) CdAÀ1. Finally, at
higher brightness LE drops down dramatically, which is
commonly observed in many triplet-based OLEDs and
attributed to triplet–triplet annihilation[23] or triplet–polaron
quenching.[24] The best performing device is D (12 CdAÀ1 at
8500 CdmÀ2), followed by S2 and S1, respectively, resulting
from enhanced recombination of charge carriers at the
interface between the two emission layers[9] and, as a result,
a widening of the emission zone (EMZ) profile (see
Table S2). It is important to note, that all cross-linked devices
outperform the reference device P3, despite the fact that
some of the emitter had degraded (ca. 40%; Figure S4). Thus,
Multi-layer OLEDs with state-of-the-art device stacks
were fabricated. These devices were optimized towards high
luminous efficiency by varying the emitter (X-IrG1–3) and
the host (X-H1 or X-H2), the doping concentration of the
emitter in the host (between 2.5 and 15.0 mole%), the cross-
linking method (Figure S4) as well as the choice of electron-
transport/hole-blocking material (TPBI or OXD-7 in
PMMA; see Figure 1). As the hole-transport/electron-block-
ing layer we used X-TAPC, since its HOMO energy fits
perfectly between the work function of PEDOT:PSS and the
HOMO level of X-IrG3 (the performance of devices without
X-TAPC was strongly reduced). Finally, the layer thicknesses
d of all individual layers were optimized (0 < d < 100 nm). X-
IrG3 generally performed slightly
better than X-IrG1 and X-IrG2
because it has the lowest quenching
rate of all three emitters after the cross-
linking. Therefore, we focus on devices
using X-IrG3 as the emitter. The opti-
mum initial doping concentration of X-
IrG3 in the emission layer was found to
be 8 mole%. The structure of the
optimized
single-emission-layer
OLEDs (SEL-OLED) based on the
matrix materials X-H1 or X-H2,
respectively (referred to as devices S1
and S2), and DEL–OLEDs (referred to
as device D) are described in detail in
Figure 2.
The OLED performance of the
three devices is shown in Figure 3 right,
more details are listed in Table S2. All
devices turn on at rather low voltage
(2.75–3.00 V). At low light output
(under 10 CdmÀ2) device D shows the
Figure 3. OLED Performance: Luminous efficiency (LE) as a function of luminance (L). Left:
Devices P0–P3 consisting of PEDOT:PSS (35 nm)//X-TAPC (20 nm)//EML (80 nm)//CsF
(2 nm)//Al (100 nm). The EML consisted of non-cross-linked X-IrG1 (squares: P1), X-IrG2 (cir-
cles: P2), X-IrG3 (up triangles: P3), fac-[Ir(mppy)3] (down triangles: P0) as emitter in PVK/OXD-
7 (5/67/28 wt%). Right: Devices S1 (closed, squares), S2 (closed, circles), and D (closed, up
triangles); all devices contain cross-linked X-IrG3 as the emitter in the respective matrix (see
Figure 2 for layer stack). For direct comparison, we have included the data for device P3 here as
well (open, up triangles; same data is in Figure 3a).
4
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2899; d) J. S. Chen, C. S. Shi, Q. Fu, F. C. Zhao, Y. Hu, Y. L.
our results show that the DEL concept works even in
solution-processed devices.
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The electroluminescence (EL) spectra (Figure S6) of the
optimized devices show their maximum at 530 nm in all cases;
slight differences occur in the intensity of the vibronic
shoulder, leading to slight variations of the CIE color
coordinates (Table S2). This is probably caused by different
location, shape, and/or width of the emission zone (EMZ)
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constant at all performance levels which indicates bipolar
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In conclusion, we report the synthesis and characteriza-
tion of new cross-linkable green-emitting IrIII complexes,
which outperform solution-based polymeric PHOLEDs
based on the commonly used reference compound fac-
[Ir(mppy)3] as a result of improved solubility and hole-
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50 CdAÀ1 and PE = 30 LmWÀ1 (device P1, X-IrG1 in PVK/
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Published online: && &&, &&&&
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Keywords: cross-links · double-emission layers ·
.
iridium complexes · organic light-emitting diodes ·
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