Electroluminescent device

Details Electroluminescent device Electroluminescent device

2002-06-28

2004-07-06

Jones, Deborah (Department: 1775)

Stock material or miscellaneous articles

Composite

Of inorganic material

C428S917000, C428S704000, C313S504000, C313S506000, C257S040000, C257S103000

Reexamination Certificate

active

06759147

ABSTRACT:

The invention relates to an electroluminescent device comprising an active layer having an electroluminescent property, which layer comprises an organic compound in a polymer matrix and which is situated between two electrode layers, at least one of said electrode layers being transparent to the light to be emitted.
The active (or electroluminescent) layer and both electrode layers can jointly form one light-emitting diode (LED), but the EL device preferably comprises various LED's, for example in the form of a matrix of light-emitting surfaces, as intended for a display. An EL device emits light when an electric field is applied across the active or emissive layer. Such a device cannot only be used as a display but also, for example, as a light source.
The use of inorganic materials such as GaAs for the active layer has been known for a long time. Since a number of years also organic materials are known, in particular semiconductive organic polymers, which can be used for the active layer.
An EL device with a semiconductive polymer for the active layer is known from the international patent application WO 90/13148. The semiconductive polymer has a conjugated polymer chain. A particularly suitable polymer is, for example, poly(p-phenylene vinylene) (PPV), in particular 2,5-substitued PPV. Said patent application discloses also a mixture of PPV and another polymer. The band gap, the electron affinity and the ionization potential can be adjusted by choosing the proper conjugated polymer chain and proper side chains. Unlike electrically conducting polymers, these conjugated polymers are undoped. In addition, such polymers enable flexible substrates to be used. The active layer of an organic polymer is situated between two electrode layers of electrically conducting materials, namely one for the injection of holes and one for the injection of electrons into the active layer. If the electrode layers are connected to a voltage source, light is emitted from the polymer layer.
It is an object of the invention to provide an EL device, which has an alternative organic compound in the active layer, which shows a high electroluminescent intensity and has an increased stability, as compared to conventional aromatic compounds, with respect to the influence of oxygen and water. Moreover, the invention makes it possible to provide a blue emitting EL device.
This object is achieved by an EL device as described in the opening paragraph, which is characterized in accordance with the invention in that the organic compound consists of charge-transfer molecules having a donor-bridge-acceptor structure. Charge-transfer (CT) molecules used in accordance with the invention consist of a donor (D) moiety and an acceptor (A) moiety separated by a bridge, which either comprise a non-conjugated (D-&sgr;-A) or a conjugated (D-&pgr;-A) group. It was found that CT molecules in an active layer can be directly brought into the excited state by injection of holes and electrons from a contiguous anode and a cathode layer, respectively, provided that their concentration is sufficiently high to enable percolation of charge through the active layer, i.e. the percolation threshold is achieved. The residence of an injected hole and an electron on the same CT molecule is equivalent to the excited state of the molecule. This excited state relaxes by intramolecular charge recombination under the emission of light. The generation of the excited state is not necessarily due to energy transfer from the host polymer, so that directly a blue emitting species may be formed. Another advantage of the application of CT molecules is that they exhibit a large Stokes' shift, so that reabsorption and inner filter effects as well as Forster-type energy transfer are minimal. As a result, a high electroluminescent intensity can be generated.
The donor moiety comprises an electron-donating group, such as a nitrogen, oxygen, sulphur, or phosphorus atom. Examples are an amine-type structure, an aniline, a succinimidyl derivative or a diphenylcarbazole derivative. Preferably, the donor moiety comprises an aromatic moiety with an electron-donating group. The aromatic moiety is, preferably, phenyl or naphthyl, whereas the electron-donating group is, preferably, a nitrogen atom. A preferred donor moiety is the diphenylamino group.
The acceptor moiety is a preferably unsaturated unity with or without an electron-withdrawing group. Electron-withdrawing groups are e.g. nitrile, nitro, carbonyl, halogen, cyano, sulfoxide, —SO
2
R (R being an alkyl having 1 to 4 carbon atoms or aryl), —CF
3
, dicyanovinylidene, or tricyanovinylidene groups. Suitable aromatic groups are phenyl, pyrenyl, naphthyl, furanyl, and thienyl groups, which may be substituted or may contain hetero atoms (for phenyl and naphtyl groups). If unsaturated moieties contain more than one unsaturated bond, these bands are preferably conjugated bonds.
The bridge moiety provides sufficient electronic coupling between the donor and acceptor moieties to induce significant oscillator strength to the charge recombination process. Examples of suitable bridges are conjugated unsaturated or aromatic groups (D-&pgr;-A), But also nonconjugated bonds, in which an array of sigma bonds provides sufficient through-bond electronic couping (D-&sgr;-A). The latter example requires an array of sigma-bonds in the bridge aligned with the donor and acceptor orbitals in a continuous all-trans configuration or at least in a configuration that deviates litle from such an all-trans configuration.
An example of a &sgr;-bridge moiety is a tropane moiety (8-azabicyclo[3.2.1.]octane):
Other examples are:
Other suitable &pgr;-bridge moieties are the following structures:
Other suitable &pgr;-bridge moieties are:
wherein Y is hydrogen, CN, or halogen, and R is independently selected from (a), (b), (c), (d), (e), (f) and (g) above.
Some preferred examples of CT molecules to be used in an electroluminescent device in accordance with the invention are shown in FIG.
2
:
Formula (3): N-phenyl-3-[(4-cyano-1-naphthyl)methyl]-8-azabicyclo[3.2.1]octane,
Formula (4): p-amino-N-phenyl-3-[(4-cyano-1-naphthyl)methyl]-8-azabicyclo[3.2.1]octane, both having the tropane bridge, and
Formula (5): diphenylamino-p-tricyanovinylidenebenzene, having the phenyl bridge (a). Its synthesis is described in the international patent application WO-A-97/23807.
Some examples of other CT molecules, which can be used in an electroluminescent device in accordance with the invention, and which have one of the the above mentioned &pgr;-bridge moieties, are the N-donor-&pgr;-acceptor compounds disclosed in the above mentioned WO-A-97/23807. Some representatives are:
4-(diphenylamino)-4′-nitrostilbene (DPANS),
N,N-di-(2-hydroxyethyl)-tricyanovinylbenzenamine (TCVDEA),
4-tricyanovinyl-4′-[2-(hydroxymethyl)-3-hydroxypropyl]-triphenylamine (TCVDPA-1),
4-tricyanovinyl-4′-(2,3-dihydroxypropoxy)-triphenylamine (TCVDPA-2),
4,4′-dihydroxy-4″-nitrotriphenylamine (NTPA),
4,4′-dihydroxy-4″-cyanotriphenylamine.
Other CT molecules which can be used in an electroluminescent device in accordance with the invention, although less stable with respect to oxygen, are D-&pgr;-A NLO compounds having an acceptor group which is a sulphone group containing a substituent selected from the group formed by alkyl, hydroxyalkyl and alkyl(meth)acrylate, such as disclosed in the European patent application EP-A-396172, filed by Applicants.
The CT compound may be mixed with a transparent polymer in order to obtain an active layer in which the CT compound is present in a polymer matrix. These polymers may be electrically insulating, such as polystyrene, polycarbonates, polyurethanes, polyacrylates, polyimides, polyarylates, and polyethers. As these polymers are insulating, it is necessary that the concentration of the CT compound is sufficient to achieve the percolation threshold for charge transport in the active layer. It was found that 10 wt. % of a CT compound acco

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