Processor polymers for the electrochemical deposition of electrically conducting polymer films

#The present invention relates to polymer-based electroluminescent devices, and more particularly, to a class of precursors for generating the electrically conducting polymers that provide electroluminescent layer in such devices.

#Polymer-based electroluminescent devices (PLEDs) have the potential for providing inexpensive alternatives to alpha-numeric displays and x-y addressable displays. PLEDs also have the potential to provide an alternative to back lighted, liquid crystal displays. A simple PLED may be constructed from an electroluminescent layer sandwiched between an electron injection electrode and a hole injection electrode. The electroluminescent layer is typically constructed by depositing a conjugated or conductive polymer on one of the electrodes. Devices based on poly(p-phenylenevinylene) (PPV), or derivatives thereof, have been demonstrated with sufficient quantum yields to be commercially attractive. More complicated devices utilize electron and hole transport layers between the above mentioned electrodes and the electroluminescent layer. The electroluminescent layer generates light when holes and electrons recombine in the layer.

#The deposition and patterning of the electroluminescent layer present significant technical problems that must be overcome before economically attractive devices can be fabricated. If the conjugated polymer is soluble in a solvent, a thin film can be made by the spin-coating of a polymer solution. While spin-coated polymer films having good electro-optical properties can be obtained in this manner, the adhesion of spin-coated film to the underlying layer is often insufficient. In addition, many attractive polymers are not sufficiently soluble to be applied via spin-coating.

#FIG. 1 illustrates the polymerization of monomer units via two different processes.

#FIG. 2 is an example of a flexible spacer cross-linking between two chains of monomers.

#FIG. 3 is an example of a cross-linking between two chains of monomers via an electroluminescent active site.

#The manner in which the present invention achieves its advantages may be more easily understood with reference to FIG. 1, which is a schematic drawing of the polymerization of monomer units 11 via two different processes. Monomer 11 is chosen such that it has four functional groups that can be utilized to form polymers. Groups 12 and 13 can be linked with the aid of a spacer 16 to form a polymer 21 that is soluble in a predetermined solvent, but which is neither conducting nor electroluminescent. Groups 14 and 15 can be linked via electrochemical polymerization to form either an electroluminescent or electrically conducting polymer 22, which is insoluble in the solvent. Since polymer 22 is insoluble, the number of units that can be linked electrochemically is limited. As a result, the quality of polymer films obtained by electrochemical polymerization groups 14 and 15 leaves much to be desired. In many cases, the film consists of a layer of powdery material that is precipitated in the vicinity of the electrode rather than being deposited onto the electrode. This precipitation problem results from the poor solubility of polymer units having a few monomers that are electrochemically linked together that are formed in the vicinity of the electrode. Attempts to overcome this problem by utilizing parent monomers with flexible-side groups to increase the solubility of short chain polymer units have not yielded films of a quality needed for electronic devices. In addition, this approach also has a disadvantage of reducing the concentration of electroactive functional groups in the film structure, and hence, the efficiency of light generation by the films.

#The present invention avoids the problems associated with polymer 22 by utilizing polymer 21 as the precursor for forming an electrically conducting or electroluminescent polymer 23 by electrochemical polymerization of polymer 21. The spacers 16 in polymer 21 provide a high degree of connectivity to the moieties contain groups 13 and 14, and hence, result the deposited film having a higher degree of mechanical strength and uniformity than the films obtained by direct electrochemical polymerization of the unlinked moieties. As a result, the polymer is deposited on the electrode and can grow into a high quality film.

#The film formed during the electrochemical polymerization may be two-dimensional, or even, three-dimensional because of the cross-linking of the various one-dimensional polymer chains. The cross-linking occurs through the sites used to link the flexible spacers or through the electroluminescently active sites. An example of a flexible spacer cross-linking between two chains 31 and 32 is shown in FIG. 2. The cross-link is shown at 33. This type of cross-linked structure is formed during the preparation of the precursor polymers. In this case, the pi-conjugated bonds are formed during electrochemical polymerization between monomers in the same chain.