Monomers, polymers incorporating said monomers and their use...

Details Monomers, polymers incorporating said monomers and their use... Monomers, polymers incorporating said monomers and their use...

2002-03-14

2004-03-02

Pezzuto, Helen L. (Department: 1713)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Polymers from only ethylenic monomers or processes of...

C526S256000, C526S260000, C526S265000, C526S279000, C526S280000, C526S284000, C526S310000, C526S312000, C568S038000

Reexamination Certificate

active

06699954

ABSTRACT:

CROSS REFERENCE TO RELATED APPLICATIONS
This application is a continuation of PCT application PCT/FR00/02538 filed Sep. 14, 2000 designating the United States, and published in French as WO 01/19765 on Mar. 21, 2001. PCT/FR00/02538 claimed the priority of French application FR 99/11702 filed Sep. 15, 1999. The entire disclosures of both are incorporated herein by reference.
The invention relates to a novel monomer and to the polymer (homopolymer or copolymer) incorporating said monomer, and to their use in organic electroluminescent devices.
Among organic electroluminescent devices, a distinction is made between electroluminescent diodes, also known by the term LED, signifying light emitting diode, and electroluminescent electrochemical cells, also denoted LEC, signifying light emitting electrochemical cell.
LEDs and LECs are well known and correspond to emitters consisting of a thin polymer layer defining a so-called active zone, which is sandwiched between two electrodes, a cathode and an anode respectively, of which at least one is transparent or semitransparent so as to facilitate observation of the luminous emission. The thin film structure of these emitters is particularly advantageous for diffuse lighting and flat screens.
In practice, the semitransparent anode is made of ITO, signifying indium tin oxide, resulting from the doping of In
2
O
3
with SnO
2
, or of tin oxide doped with antimony or fluorine (SnO
2
:Sb or SnO
2
:F), while the cathode is made of aluminum.
The polymers employed within the active zone are so-called conjugated polymers, i.e., polymers whose constituent monomers exhibit an alternation of single bonds, called &sgr; bonds, and double bonds, called &pgr; bonds, leading to a system of &pgr; electrons which is highly delocalized along the carbon chain.
Compounds of this kind are derived principally from polyacetylene or else obtained by catenating aromatic nuclei such as benzene, naphthalene, pyrrole, thiophene, pyridine, quinoline, anthracene, carbazole, and fluorene.
In some cases, to allow the steric stresses to be reduced and so to maintain the aromatic entities within the same plane, the abovementioned nuclei are coupled by way of a —(CH═CH)—vinylene bond.
From an electrochemical standpoint, the conjugated polymers can be oxidized or reduced for a structural rearrangement of the alternation of their bonds. More specifically, oxidation, i.e., the loss of an electron, can be interpreted as the making of a hole in the valence band (HOMO), or “p” doping of the material. Similarly, reduction, which consists in adding an electron, can be interpreted as being the provision of an electron in the conduction band (LUMO), or “n” doping. Consequently, the difference between the oxidation potential and reduction potential of the conjugated polymer can be analyzed as corresponding to the gap of the polymer as a semiconductor material.
Owing to its semiconducting properties, this type of polymer can be employed in the abovementioned organic electroluminescent devices, the phenomenon of electroluminescence resulting from the radiative recombination of an electron from the conduction band and a hole from the valence band, with the carriers (holes and electrons) being injected into the active layer when a potential is applied between the two electrodes.
LEDs and LECs differ in the composition of their active zone.
Accordingly, while the active zone of LEDs consists exclusively of a polymer containing no conductivity additive, which is obtained in film form from a solution in a solvent or by evaporation under vacuum, the layer of the LECs also comprises a salt and a solvent for this salt which is mixed with the conjugated polymer or grafted onto it.
The effect of this difference in composition is to impart different operating voltages to these two devices. Thus with regard to LEDs it is necessary to apply a high operating voltage of more than 10 volts, whereas an operating voltage of between 3 and 4 volts is sufficient for LECs.
The high voltage required for operating LEDs is due, to the very low conductivity of the polymers, which are used in the undoped state, and to the nonohmic nature of the contacts between polymer and electrodes, this nonohmic nature resulting from the existence of potential barriers.
The addition of a salt to the thin layer of the LECs allows the height of this potential barrier to be reduced in accordance with the following phenomenon.
When a low potential is applied between the anode and the cathode, the ions from the dissociated salt migrate toward the electrodes in question to form two fine, charged layers at the interface of the active medium and the electrodes, and these layers will promote the injection of electrons and holes, making the polymer conductive at the polymer/electrode interfaces. When the applied potential exceeds the threshold voltage, the electrons and the holes are injected respectively into the anode/active layer and cathode/active layer interfaces, thereby initiating the formation of a p-n junction. When the voltage is increased again, the additional electrons and holes injected migrate under the effect of this potential excess toward the cathode and the anode. The radiative recombination of these carriers within the space charge zone constitutes the origin of the phenomenon of electroluminescence.
When doping is sufficient, contact between the electrodes and the polymer is ohmic in character, such that the operating voltages are reduced considerably and are close to the theoretical value, viz. the energy difference between the valence band (HOMO) and the conduction band (LUMO) of the polymer.
However, producing the thin layer or active zone of LECs is not simple, on account of the fact that said layer is likely to contain a hydrophobic polymeric material, which is virtually nonpolar, and an ionic species, which undergoes appreciable dissociation only in the presence of a polar solvent.
In order to allow the mixing of the hydrophobic polymer with the hydrophilic salt, it has been proposed that cations and anions resulting from the dissolution of the salt be solvated.
To do this, one first solution consists in solvating the salt, such as the lithium salt, for example, of trifluoromethanesulfonic acid (LiCF
3
SO
3
) by means of a solvating polymer of the poly(ethylene oxide) type. However, the two polymers are immiscible, and even less compatible in the presence of salt, so that the mixture obtained is in heterogeneous form.
Another method, which is described in the document WO 97/33326, consists in grafting solvating segments of oligo(ethylene oxide) type onto the frame of a conjugated polymer of the fluorene type. Although this type of compound does make it possible to solve the problem of inhomogeneity of the polymer mixture (phase microseparation), the conjugation which exists between the various monomer units is liable, however, to be modified.
In effect, the solvating segments of oligo(ethylene oxide) type induce a local disorder which—necessary for ionic conduction—is manifested in a loss of coplanarity of the units which allow the &pgr; conjugation. In addition, the local disorder is further accentuated by the addition of an ionic compound, the lateral chains being arranged in priority in order to ensure the salvation of the Li
+
cations. Moreover, the energy employed by this process (greater than 60 KJ) is markedly greater than the energy gain obtained by virtue of the extension of the &pgr; conjugation (greater than 10 KJ).
Thus, for example, polythiophenes substituted in position 3 by oligo(ethylene oxide) groups with a mass close to 200 change their optical absorption spectrum by passing from the violet to the yellow in the presence of alkali metal cations, thereby demonstrating the loss of the &pgr; conjugation.
In parallel, the reduction in conjugation is detrimental to the electronic conductivity, which requires a local order, and to the fluorescence and luminescence.
Furthermore, the reduction in conjugation makes things more difficult for “n” and “p” doping, which takes place at potentials

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