Organic electroluminescent component with charge transport layer

An organic electroluminescent component with a layer arrangement includes a first electrode layer, an inorganic layer which conducts electrons, one or several optoelectronically active layers with at least one light-emitting layer which comprises an organic emitter, and a second electrode layer. The inorganic layer which conducts electrons is an N-type conducting oxide of a transition metal chosen from the group consisting of zirconium oxide, hafnium oxide, vanadium oxide, barium titanate, barium-strontium titanate, strontium titanate, calcium titanate, calcium zirconate, potassium tantalate, and potassium niobate.

BACKGROUND OF THE INVENTION 
The invention relates to an organic electroluminescent component, in 
particular a light-emitting diode (LED) for luminous signs, luminaires, 
solid-state image intensifiers, or picture screens, with a layer 
arrangement comprising a first electrode layer, an inorganic layer which 
conducts electrons, one or several optoelectronically active layers with 
at least one light-emitting layer which comprises an organic emitter, and 
a second electrode layer. 
Prior-art LEDs are usually inorganic semiconductor diodes, i.e. diodes 
whose emitter material is an inorganic semiconductor such as doped zinc 
sulphide, silicon, germanium, or III-V semiconductors, for example InP, 
GaAs, GaAlAs, GaP, or GaN with suitable dopants. 
Work has been going on for several years in the development of luminescent 
radiation sources in which the emitter material is not an inorganic 
semiconductor but an organic electrically conductive material. 
Electroluminescent components with light-emitting layers built up from 
organic materials are clearly superior to light sources made from 
inorganic materials in a number of respects. An advantage is their easy 
plasticity and high elasticity which opens new possibilities for 
applications such as luminous signs and picture screens. These layers may 
readily be manufactured as large-area, flat, and very thin layers which in 
addition require little material. They excel through their remarkably high 
brightness accompanied by low operating voltages. 
In addition, the color of the emitted light can be varied over a wide range 
from approximately 400 nm up to approximately 650 nm through the choice of 
the luminescent material. These colors have a striking luminance. 
Such organic electroluminescent components may be built up in various ways. 
They all have in common that one or several optoelectronically active 
organic layers, among which the light-emitting layer, are arranged between 
two electrode layers to which the voltage necessary for operating the 
component is applied. At least one of the electrode layers is transparent 
to visible light so that the emitted light can emerge to the exterior. The 
entire layer construction is usually provided on a substrate which is also 
transparent to visible light if the emitted light is to issue from the 
side facing the substrate. 
The layer sequence of the optoelectronically active organic layers is known 
in several variations. For example, the light-emitting layer comprising a 
thin stratum of organic pigment molecules and possibly conductive organic 
polymers may be embedded between two further electrically conductive 
organic layers which transport charge carriers from the two electrodes to 
the light-emitting layer. The electrically conductive organic layer 
between the light-emitting layer and the cathode conducts electrons 
whereas the corresponding layer between the light-emitting layer and the 
anode conducts holes. 
The use of such organic charge carrier transport layers, however, also 
involves problems. The thermal load on the layers during operation and 
material interactions between the electrode layers and the transport 
layers lead to a deterioration of the luminous efficacy of the component 
in the course of time. The useful life of the organic electron-conducting 
layer is very short in this case, in particular when strongly reducing 
metals such as calcium or magnesium, which have a particularly low work 
function for the electrons, are used as the cathode material in order to 
achieve a high luminous efficacy. 
It is accordingly proposed in U.S. Pat. No. 5,128,587 to choose a 
composition of an organic or alternatively inorganic semiconductor for the 
charge transport layer which transports electrons and lies between the 
electrode with low work function and the luminescent film. Inorganic 
semiconductors proposed here are Ge, Si, Sn, SiC, AlSb, BN, BP, GaN, GaSb, 
GaAs, GaP, InSb, InAs, InP, CdSe, CdTe, ZnO, ZnS, or ZnSe. The 
semiconducting layer may be amorphous or crystalline and it may be an 
N-type doped semiconductor or an intrinsic semiconductor. 
A disadvantage of a component having a charge transport layer with an 
inorganic semiconductor of the kind mentioned above is that this layer 
absorbs light in the visible spectrum range. 
SUMMARY OF THE INVENTION 
It is accordingly an object of the invention to provide an organic 
electroluminescent component with improved properties. 
According to the invention, this object is achieved by means of an organic 
electroluminescent component with a layer structure comprising a first 
electrode layer, an inorganic layer which conducts electrons, one or 
several optoelectronically active layers with at least one light-emitting 
layer which comprises an organic emitter, and a second electrode layer, 
which is characterized in that the inorganic layer which conducts 
electrons is an N-type conducting oxide of a transition metal chosen from 
the group comprising zirconium oxide, hafnium oxide, vanadium oxide, 
barium titanate, barium-strontium titanate, strontium titanate, calcium 
titanate, calcium zirconate, potassium tantalate, and potassium niobate. 
Such inorganic layers conducting electrons have a high thermal and chemical 
stability and achieve a very good electron contact to the emitter 
molecules. They may be manufactured with ceramic surfaces of defined 
roughness which render it possible to accommodate more emitter molecules 
and thus to increase the active surface area. The oxides do not absorb 
light in the visible range and are transparent in thin layers. 
It is preferred within the scope of the present invention that the N-type 
conducting oxide chosen from the group comprising zirconium oxide, hafnium 
oxide, vanadium oxide, barium titanate, barium-strontium titanate, 
strontium titanate, calcium titanate, calcium zirconate, potassium 
tantalate, and potassium niobate is a doped oxide. 
It may also be preferred that the oxide of the transition metal is 
niobium-doped strontium titanate SrTiO.sub.3. Strontium titanate 
SrTiO.sub.3 doped with niobium is particularly photoactive. 
It may also be preferable that the organic emitter is a rare earth metal 
complex with a ligand comprising a carboxylate or a phosphonate group. 
The light-emitting complex is grafted to the oxide surface by these 
ligands. An interlocking group is formed thereby between the rare earth 
metal ion and the charge transport layer, which achieves a close 
electronic coupling between the two. 
The bonding of the emitter molecules to the oxide layers, which have a high 
refractive index, in addition reduces the life of the excited state of the 
emitter molecule and thus the fluorescence decaying time. This is 
advantageous for display applications because the detrimental afterglow of 
moving objects in the picture (comet tail effect) is eliminated.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The organic electroluminescent component according to the invention 
comprises a layer structure with a first electrode layer (cathode), an 
inorganic layer 1 which conducts electrons, one or several 
optoelectronically active layers with at least one light-emitting layer 
comprising an organic emitter 4, and a second electrode (anode). 
This two-layer arrangement is shown in FIG. 2. A three-layer arrangement 
according to FIG. 1 is also possible, however, where the layer arrangement 
comprises a further charge transport layer 3 which conducts holes between 
the optoelectronically active layers 2 and the anode. 
The cathode is usually made from a metal or an alloy with a low work 
function, for example Mg, MgAg, Li, Al, Na, K, Ca, Rb, Sr, Ce, rare earth 
metals, as well as alloys thereof comprising antimony or indium. 
The inorganic layer which conducts electrons comprises an oxide of a 
transition metal chosen from the group comprising zirconium oxide 
ZrO.sub.2, hafnium oxide HfO.sub.2, vanadium oxide V.sub.2 O.sub.5, barium 
titanate BaTiO.sub.3, barium-strontium titanate (Ba, Sr)TiO.sub.3, 
strontium titanate SrTiO.sub.3, calcium titanate CaTiO.sub.3, calcium 
zirconate CaZrO.sub.3, potassium tantalate KTaO.sub.3, and potassium 
niobate KNbO.sub.3. 
The generation of the N-type conductivity in the oxides may be achieved, 
for example, by means of a suitable dopant, by a thermal aftertreatment in 
an inert or reducing atmosphere, or by a combination of these measures. 
The inorganic layer which conducts electrons may be a layer of the ceramic 
type, i.e. a layer made from powder particles, which is subsequently 
sintered. It may also be a layer of the nanocrystalline type made from 
very small powder particles which have a particle size between 1 and 100 
nm and which form a porous network. Nanocrystalline layers may be 
manufactured, for example, in a sol-gel process. The layer may 
alternatively be amorphous, colloidal, or microcrystalline and be 
manufactured by a vacuum deposition technique such as vaporizing, PCVD, 
MOCVD, etc. Layer thicknesses of less than 100 nm up to an upper limit of 
approximately 5 mm are possible, depending on the manufacturing technique 
and conductivity of the oxide material. 
Various arrangements are possible for the sequence of the 
optoelectronically active layers. For example, it may comprise only a 
single organic layer. This may be formed by a conductive organic polymer 
which itself is capable of light emission. It may alternatively comprise 
one or several conductive organic polymers and one or several organic 
pigment compounds, while the polymers and the pigment compounds in the 
layer may either be physically mixed or chemically bound. 
The optoelectronically active layers may comprise one or several 
organometallic complexes of rare earth metals with organic oxygen, 
sulphur, or nitrogen ligands as the organic emitter. Organometallic 
complexes are understood to be complexes with said organic ligands in 
which the bonds are achieved by means of the hetero atoms within the scope 
of the present invention. Depending on the desired color of the emitted 
light, several rare earth metal complexes may also be used. Rare earth 
metal complexes may also be used which are not capable of sublimation or 
which are not electrically conductive. 
The rare earth metal ion may be, for example, Eu.sup.2+, Eu.sup.3+, 
Tb.sup.3+, Tm.sup.3+, Dy.sup.3+, Sm.sup.3+ or Pr.sup.3+. 
Red fluorescence can be generated with europium and samarium complexes, 
green fluorescence with the terbium complexes, and blue fluorescence with 
the thulium and dysprosium complexes. 
Particularly suitable rare earth metal complexes having the general 
composition SE[L.sub.1 ].sub.3 [L.sub.2 ].sub.n. SE here is a trivalent 
rare earth metal cation, L.sub.1 is an anionic ligand which may be 
monodentate or bidentate, and L.sub.2 is a neutral ligand which may be 
monodentate or bidentate. n is chosen such that all coordination locations 
of the rare earth ion are saturated, so that n may assume the values 0, 1, 
2, 3, and 4. L.sub.1 and L.sub.2 are always two different ligands. The 
corresponding formula for Eu.sup.2+ is Eu[L.sub.1 ].sub.2 [L.sub.2 
].sub.n. 
Particularly suitable for the ligand L.sub.1 are the beta-diketonates 
R.sub.1 C(OH)CHCOR.sub.2. The rests R.sub.1 and R.sub.2 may be F.sub.3 
C--, thenoyl C.sub.4 H.sub.3 S--, furanoyl C.sub.4 H.sub.3 O--, t-butyl, 
and perfluororo-n-propyl C.sub.3 F.sub.7 --. If R.sub.1 and R.sub.2 are 
CF.sub.3 -- rests, the beta-diketonate hexafluoroacetylacetonate (hfa) is 
obtained. If R.sub.1 and R.sub.2 are a t-butyl rest, the beta-diketonate 
2,2,6,6-tetramethyl-3,5-heptandione (thd) is obtained. If R.sub.1 is a 
thenoyl rest and R.sub.2 a CF.sub.3 -rest, the beta-diketone 
thenoyltrifluoroacetylacetonate (ttfa) is obtained. If R.sub.1 is a 
furanoyl rest and R.sub.2 a Cf.sub.3 -rest, the beta-diketone 
furanolyltrifluoroacetylacetonate (ftfa) is obtained. If R.sub.1 is a 
t-butyl rest and R.sub.2 is a perfluoro-n-propyl rest, the beta-diketone 
7,7-dimethyl-1,1,1,2,2,3,3-heptafluoro-4,6-octandione (FOD) is obtained. A 
further beta-diketone which is suitable as a ligand is 
3-(trifluoromethylhydroxymethylene)-1-camphor. 
Particularly efficient are the rare earth chelate complexes with ligands 
L.sub.1, the anions of aromatic carbonic acids such as benzoic acid, 
dimethylpyridine acid, and methylpyridine acid. 
The ligands L.sub.2 are neutral ligands which may be monodentate or 
multidentate. The monodentate ligands may be pyridine and its derivatives, 
trialkylphosphinoxide, alkylphenylphosphinoxide, and 
triphenylphosphinoxide, dialkylsulphoxide, alkylphenylsulphoxide and 
diphenylsulphoxide, alkylamine, alkylphenylamine, and phenylamine, as well 
as alkylphosphate, alkylphenylphosphate, and phenylphosphate. 
Pluridentate ligands are 2,2'bipyridine, 2,2',6,2"terpyridine, 
1,10-phenanthroline, and N,N,N',N'-tetramethylethylenediamine and its 
derivatives. 
Particularly suitable ligands L.sub.2 are those with a phosphonate or 
caboxylate group, for example phosphonated or carboxylated polypyridyl 
ligands such as 2,2':6',2"-terpyridine-4'-phosphonate (4'-PO.sub.3 H.sub.2 
-terpy) or 2,2'-bipyridyl-4,4'-dicarboxylate. These ligands at the same 
time form a light-emitting chelate complex with the rare earth metal ions 
while they are adsorbed to the oxide of the layer which conducts electrons 
via the phosphonate or caboxylate group. The rare earth metal complexes 
are bonded to the surface of the layer which conducts electrons in this 
manner. The adsorbed complex acts as a charge transfer sensitizer, and the 
electron transition from the conduction band of the oxide into the excited 
state of the organic emitter takes place ultra fast and with a high 
quantum efficiency. 
Suitable materials for the anode from which holes are injected into the 
optoelectronically active layers are metals, metal oxides, and 
electrically conductive organic polymers with a high work function for 
electrons. Examples are thin, transparent layers of indium-doped tin oxide 
(ITO), gold, and polyaniline. 
At least one of the electrodes, usually the anode, is transparent to 
visible light so that the emitted light can emerge to the exterior. The 
entire layer structure is provided on a substrate which should also be 
transparent to visible light if the emitted light is to issue from the 
side which faces the substrate. 
Given such a construction of the electroluminescent organic component, with 
a layer of material which conducts electrons but which does not or 
substantially not conduct holes arranged between the cathode and the 
optoelectronically active layers, the electrons coming from the cathode 
can only reach the optoelectronically active layer but not the anode. 
Inversely, holes from the anode can only reach the optoelectronically 
active layer but not the cathode. This charge carrier confinement achieves 
that the leakage currents through the component are small and the 
optoelectronic efficiency of the component is increased, because many 
charge carriers are forced to remain in the vicinity of the 
optoelectronically active layer and accordingly transmit their energy to 
the emitter. 
The component according to the invention in addition has a very good charge 
carrier balance. Since the injection of electrons from the cathode and 
holes from the anode may have strongly different efficiencies, the ratio 
of electrons to holes in the optoelectronically active layer may differ 
considerably from the ideal value 1 in components without one-way 
impervious transport layers. Since the energy transfer to the luminous 
centers in the optoelectronically active layer is based on the 
recombination of pairs of electrons and holes, this reduces the efficiency 
of the component. In the component according to the invention, the excess 
charge carriers remain confined in the corresponding boundary layers of 
the optofunctional layer and generate space charge zones depending on 
their charge which reinforce the injection of the minority charge carriers 
of opposite charge. The ratio of the charge carriers in the component 
according to the invention is thus better balanced and the efficiency of 
the component is enhanced. 
Embodiment 1: 
200 g SrCO.sub.3, 110 g TiO.sub.2, and 540.2 mg Nb.sub.2 O.sub.5 are mixed 
with distilled water and milled in the wet state for 24 hours. The 
suspension thus obtained is dried and the resulting powder is calcinated 
at 1100.degree. C. for 4 hours. Then the calcinated powder is pressed into 
2-3 mm thick slices of 10 mm diameter under a pressure of 5 tons. The 
slices are subsequently sintered in the air at 1200.degree. C. for 2 hours 
and then at 1350.degree. C. for 4 hours. Nitrogen/hydrogen in a ratio of 
3:1 is passed over the slices during the cooling-down phase after the last 
heating step. The cooling rate is kept constant at 7.degree. C. min.sup.-1 
during this. Finally, one side of each slice is coated with indium metal 
vapour in vacuo (contact layer) (Sundaram, S. K.; J. Mater. Sci. Mater. 
Electron. 5 (1994) 344-346). A solution of 2.5% by weight of 
poly(vinylcarbazol) and 0.1% by weight of Eu(Ttfa).sub.3 phen 
(Eu=europium; Ttfa=1-(2-thienyl)-4,4,4-trifluoro-1,3-butandione, 
phen=1,10-phenanthroline) in THF is provided on the non-contacted slice 
side from the solution by means of a spinning process. A thin, transparent 
gold film is vapor-deposited as the anode. The luminescent diode shows a 
red fluorescence. 
Embodiment 2: 
A solution of 2.5% by weight of polyvinylcarbazol and 0.01% by weight of 
Coumarin-6 (laser pigment, Lambada Physics) in a 1:1 mixture of THF and 
1,1,1-trichloroethane is provided in a spinning process on an uncovered, 
contacted n-strontium titanate substrate in accordance with embodiment 1. 
A thin, transparent gold film is vapor-deposited as the anode. The 
luminous diode shows a green fluorescence.