Abstract:
A method for depositing an electrically conducting film on an electrode and the film resulting from that method. An electrically conducting film according to the present invention is deposited by immersing the electrode in a solution of a precursor polymer in a predetermined solvent. The precursor polymer includes a plurality of electrochemical polymerizable monomers. Each monomer has first and second polymer-forming active sites that can be joined by electrochemical polymerization and third and fourth polymer-forming active sites that can be joined chemically in solution. The precursor polymer is constructed from the monomers joined by the third and fourth polymer-forming active sites. The precursor polymer is soluble in the solvent whereas a polymer formed by electrochemical polymerization of the first and second polymer-forming active sites is insoluble in the solvent. An electrical potential is applied to the electrode to cause monomers of the precursor polymer molecules to be joined electrochemically by their first and second polymer-forming active sites.

Description:
CROSS REFERENCE TO RELATED APPLICATION(S) 
     This is a divisional of application Ser. No. 09/401,666 filed on Sep. 22, 1999 now U.S. Pat. No. 6,294,245. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to polymer-based electrically conducting or electroluminescent films, and more particularly, to films constructed from a class of precursor polymers that are electrochemically polymerized to form the electrically conducting film. 
     BACKGROUND OF THE INVENTION 
     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. 
     Spin-coating and other processes in which the entire substrate is coated, present additional problems in multi-color displays in which different “pixels” must be coated with different polymers. The deposition of each layer requires a three-step procedure consisting of a masking step to protect areas that are not to be coated, the spin-coating step, and a mask removal step. In addition to the increased complexity of the masking steps, the solvents utilized with conventional masking systems are often incompatible with the polymers being deposited. Accordingly, it would be advantageous to provide a system that does not require such masking operations. 
     Broadly, it is the object of the present invention to provide an improved method for depositing an electrically conducting or electroluminescent film. 
     It is another object of the present invention to provide a method that may be utilized with materials that cannot be spin-cast. 
     It is further object of the present invention to provide a method that can selectively deposit such films without the use of the masking operations discussed above. 
     These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings. 
     SUMMARY OF THE INVENTION 
     The present invention is a method for depositing an electrically conducting film on an electrode and the film resulting from that method. An electrically conducting film according to the present invention is deposited by immersing the electrode in a solution of a precursor polymer in a predetermined solvent. The precursor polymer includes a plurality of electrochemical polymerizable monomers. Each monomer has first and second polymer-forming active sites that can be joined by electrochemical polymerization and third and fourth polymer-forming active sites that can be joined chemically in solution. The precursor polymer is constructed from the monomers joined by the third and fourth polymer-forming active sites. The precursor polymer is soluble in the solvent whereas a polymer formed by electrochemical polymerization of the first and second polymer-forming active sites is insoluble in the solvent. An electrical potential is applied to the electrode to cause monomers of the precursor polymer molecules to be joined electrochemically by their first and second polymer-forming active sites. Precursor polymers may be constructed from monomers such as flourene, thiophene, pyrrole, biphenyl, poly(vinyl carbazole) or poly (vinyl oxy thiophene). Precursor polymers may also be constructed from dimers constructed from monomers chosen from this group. The monomers may include a spacer group bonded to one of the first or second active sites. Exemplar spacer groups include (CH 2 ) n , (OCH 2 ) n , or (OCH 2 CH 2 ) n , where 1≦n≦20. The solution may also include non-polymerized monomers or other compounds that can be electrochemically lined to the first and second polymer-forming active sites of the monomers. The concentration of the monomers by molar percent of monomeric repeat unit of polymer is between 0.01 and 99.99%. These monomeric units are joined to the first and second polymer-forming active sites of the monomers in the precursor polymers, and to each other, during the electrochemical polymerization thereby forming chains of the monomeric units that are linked to the precursor polymers by the first or second polymer-forming active sites of the monomers in the precursor polymers. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     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. 
     FIG. 4 shows some examples of monomeric functional groups that may be utilized o form the precursor polymers of the present invention. 
     FIG. 5 illustrates the preparation of one class of electroluminescent polymers according to the present invention. 
     FIG. 6 is a schematic representation of two possible polymers formed from monomers labeled A and B. 
     FIG. 7 illustrates the electrochemical polymerization of a precursor polymer having the configuration shown at  61  in FIG. 6 on an electrode. 
     FIG. 8 is an example of the formation of an electroluminescent polymer layer based on a dimer unit such as shown in FIG.  6 . 
     FIG. 9 illustrates the various types of chemical structures that are obtained when a solution having precursor polymers and various monomers is electrochemical polymerized. 
     FIG. 10 is a top view of a display according to the present invention. 
     FIG. 11 is a cross-sectional view of the display shown in FIG.  10  through line  251 - 252 . 
     FIGS. 12-14 are top views of a display  200  at various stages in the fabrication process. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     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 unit  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 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 flexibility to the moieties containing groups  13  and  14 , and hence, result in 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. 
     An example of a cross-linking between two chains  41  and  42  via a pi-conjugated bond  43  is shown in FIG.  3 . In this case, the chains are linked during the electrochemical polymerization. It will also be apparent from the previous discussion that more complex arrangements may be formed in which two or three-dimensional chains are further cross-linked during either the original polymerization using the flexible spacers or the electrochemical polymerization. 
     Some examples of monomeric functional groups that may be utilized to form the precursor polymers are shown in FIG. 4 at  301 - 306 . Compounds  301 - 306  are fluorene, thiophene, pyrrole, biphenyl, poly(vinyl carbazole) and poly (vinyl oxy thiophone), respectively. These groups may be joined by spacers of the form (CH 2 ) n , (OCH 2 ) n , (OCH 2 CH 2 ) n , . . . , where 1≦n≦20. 
     Refer now to FIG. 5, which illustrates the preparation of one class of electroluminescent polymers according to the present invention. A series of precursor polymers was prepared by dilithiation of fluorene with n-butyllithium, followed by the reaction with α,ω-dibromoalkanes (n=4,6,8,10,12). The formed precursor polymers are soluble in common organic solvents such as chloroform, dichloromethane or toluene. 
     Electrochemical polymerization is carried out using solutions of precursor polymers at a concentration of 50 mM with electrolyte (tetrabutylamionium tetrafluoroborate: 100 mM) in dichloromethane as a solvent. Conductive glass substrates with a thin layer of indium-tin oxide may be used as working electrodes. Cyclic voltammetry is performed for each precursor polymer solution, between −200 mV and +2000 mV (versus Ag/Ag+reference electrode). At the first cycle, the oxidation of the fluorene group is observed at +1350 mV. At the following cycles, reversible oxidation and reduction is observed at around +1200 mV, and the change in color of the material on the electrode is observed (light brown at low potential, bright red at high potential). After the film has reached the desired thickness, the electrodes are rinsed with toluene, which is a good solvent for these precursor polymers to remove any non-electrochemically polymerized precursor polymers. A thin film of a light-brown material remains on the electrode surface. This film exhibits an absorbence spectra that is very close to that of poly(fluoren-2,7-diyl). 
     The precursors of the present invention can also be utilized to form films of copolymers from monomers having different electronic characteristics, such as emission spectra and energy bands. Refer now to FIG. 6, which is a schematic representation of two possible polymers formed from monomers labeled A and B. To simplify the discussion, unit A is assumed to be the same as monomer unit  11  shown in FIG.  1 . Unit B, which is shown at  51 , also has two active sites,  52  and  53 , that may be used to polymerize the monomers to form a soluble chain that does not emit light and two active sites,  54  and  55 , that can be joined via electrochemical polymerization to form a light emitting polymer that is insoluble in the solvent. The monomeric units can be combined to form a soluble copolymer precursor in a number of different configurations. For example, a precursor block copolymer  61  can be generated by combining the A units via spacer  56  to form a poly-A precursor that is then joined to a poly-B precursor formed by joking the B monomers via spacers  59 . Similarly, the two sub-units can be joined via a spacer  57  to form the alternating copolymer precursor. This second configuration for the electrochemical polymerization precursor is shown at  62 . In this case, a dimer consisting of one A-unit and one B-unit is first formed. The dimers are then polymerized with the aid of spacers  58  to form the alternative copolymer precursor. 
     While the examples shown in FIG. 6 utilize different spacers, it will be obvious to those skilled in the art from the preceding discussion that the monomers may be chosen such that a single spacer unit can be utilized for all of the sub-unit attachments. A random copolymer precursor polymer may be constructed by applying the solution polymerization process to a solution containing both monomers. It will also be apparent form the preceding discussion that precursors having more than two types of monomers may also be constructed in this manner. For example, the monomeric functional groups can be two or more of any electrochemically polymerizable group such as fluorene, thiophene, pyrrole, phenylene, pyridine, or triptycene or one of the derivatives of the above compounds. Component of each species can be between 0.001% and 99.999%. 
     The precursor copolymer can have any form of structure including alternative copolymer, block copolymer, graft copolymer and random copolymer. Furthermore, the precursors can form complex two and three-dimensional structures as discussed above. The functional groups can be either linked by a spacer or simply attached to a polymer backbone. The spacer or polymer backbone can be any flexible molecular chains such as oligomethylene units or oligooxymethylene units, or it may contain functional groups such as phenyl group or amino group. The number of repeating units either in the spacer or between electroactive functional groups is preferably between 1 and 20. 
     Once the precursor polymer is prepared, the precursor polymer is dissolved in a solvent with an electrolyte. A cyclic voltammogram or a bulk electrolysis can be carried out for this solution. By choosing the appropriate potential of the electrode for each functional unit, the precursor polymer is polymerized and deposited on the electrode. Refer now to FIG. 7, which illustrates the electrochemical polymerization of a precursor polymer having the configuration shown at  61  in FIG. 6 on an electrode  70 . For the purposes of this discussion, it will be assumed that the A-subunit has a lower oxidation potential than the B-subunit. Hence, the A-subunits will be joined at the lower potential difference between the electrode on which the film is deposited and the reference electrode. This leads to a film in which the A-subunits are polymerized as shown at  72  in FIG.  7 . When all of the A-subunits have been polymerized, the B-subunits will then be oxidized to form the polymer shown at  73 . 
     An electroluminescent polymer layer based on a dimer unit such as shown in FIG. 6 at  62  may be formed in the manner shown in FIG. 8. A precursor polymer  81  is prepared by dilithiation of fluorene with n-butyllithium, followed by the reaction with 3,4 bis(ω-bromoalkyl) thiophene. Polymers with n=4,6,8,10,12 can be utilized; however, precursors with other n values may also be useful. The formed precursor polymers are soluble in common organic solvents such as chloroform, dischloromethane, or toluene. 
     Electrochemical polymerization can then be carried out using solutions of precursor polymers at a typical concentration of 50 mM in dichloromethane as a solvent to which an electrolyte (tetrabutylammonium tetrafluoroborate: 100 mM) has been added. Conductive glass substrates with a thin layer of indium-tin oxide can be used as working electrodes. The electrochemical polymerization is carried out via cyclic voltammetry typically performed between −200 mV and +2000 mV (versus Ag/Ag+reference electrode). At the first cycle, the oxidation will be observed, due to oxidation of thiophene and fluorene, respectively. At the following cycles, reversible oxidation and reduction will be observed at around +1000 mV and +1200 mV, and the change in color of the material on the electrode will also be observed. After the polymerization is completed, the electrodes are rinsed with toluene, to remove any unpolymerized precursor polymers. A thin film of a reddish material will remain on the electrode surface. This thin film should exhibit an absorbency spectra that has the combined characteristics of poly(thiophen-2,5diyl) and poly(fluoren-1,7diyl). 
     It should be noted that the precursor polymers described above may also be used as “seed” polymers for growing additional electroluminescent or conducting chains. It has been found experimentally that the seed polymers may be mixed with the monomers used to form the precursor as well as other monomers that have only electrochemical polymerization active sites. Refer now to FIG. 9, which illustrates the various types of chemical structures that are obtained when a solution having precursor polymers and various monomers is electrochemical polymerized on an electrode  91 . In this example, the precursor polymers are constructed from a monomeric unit A, an exemplary precursor polymer being shown at  96 . Two types of monomers are assumed to be present in the solution, monomers of type A as shown at  94  and monomers of type D shown at  95 . For example, the D monomers may be dyes that determine the emission characteristics of the spectral lines emitted by the final electroluminescent layer that is deposited on electrode  91 . 
     As noted above, the precursors form an insoluble material by forming bonds between adjacent A&#39;s in the precursor such as shown at  99  and by cross-linking two precursor polymers such as precursor polymers  92  and  93 . The various monomers can be electrochemical polymerized onto any of the open active sites on the precursor polymers that permit such polymerization. Once a monomer has bonded to a precursor polymer, it can provide the starting point for a chain of monomers such as shown at  97  and  98 . 
     A high quality electrically conduction and/or electroluminescent layer can be obtained in this manner with a relatively low percentage of precursor polymers It has been found experimentally that precursor polymer concentrations as low as 1% can provide high quality films in the presence of 99% monomers. For example, a high quality conducting film was deposited in this manner by the electrochemical polymerization of a solution having 100 Moles of oxlyithiophene per mole of the repeating unit in the polyvinyl carbazole. 
     The films generated by the electrochemical polymerization of the precursor polymers in the presence of the monomer have two advantages over films obtained by the electrochemical polymerization of just precursor polymers. First, the resultant polymer can have a higher density of electroactive active sites than that obtained without including the monomer units, and hence, provides superior light output. Films may be deposited with as little as 0.01% precursor polymer and 99.99% monomer. Second, the cost of the film is substantially reduced, since the cost of making the precursor can be quite high. 
     It should be noted that the film can be deposited in two steps. In the first step, the precursor polymers are deposited via electrochemical polymerization forming a three-dimensional mesh. In the second step, the monomers, including any dyes are deposited by electrochemical polymerization on the precursor mesh. In this embodiment of the present invention, the surface with the precursor deposited film can be provided as a starting material that is customized for a particular application by second electrochemical polymerization. 
     The precursor polymers of the present invention are well suited for constructing multi-color displays such as the pixelated display shown in FIGS. 10 and 11. FIG. 10 is a top view of a display  250 , and FIG. 11 is a cross-sectional of display  250  view through line  251 - 252 . Display  250  is composed of a number of pixels arranged in a rectangular array having rows and columns defined by horizontal electrodes and vertical electrodes. An exemplary horizontal electrode is shown at  253 , and an exemplary vertical electrode is shown at  254 . An electroluminescent polymer  255  is sandwiched between the horizontal and vertical electrodes at each intersection point. An individual pixel is activated by applying a voltage between the row and column electrodes that define that pixel. In the example shown in FIGS. 10-11, the vertical electrodes are deposited on a substrate  256  by patterning a conducting layer that is deposited on the substrate. 
     The manner in which the precursors of the present invention are used to construct a pixelated multi-colored display such as that shown in FIGS. 10-11 may be more easily understood with reference to FIGS. 12-14, which are top views of a display  200  at various stages in the fabrication process. In this example, the display is constructed from red, blue, and green pixels that are addressed by the row-column addressing scheme discussed above. To simplify the following discussion, it will be assumed that each pixel consists of an electroluminescent polymer layer sandwiched between the two electrodes. The pixels are fabricated on the column electrodes by depositing the electroluminescent polymer layers constructed from precursors according to the present invention for the various colors on the corresponding column electrodes electrodes. The row electrodes are then deposited. For the purposes of this discussion, it will be assumed that the column electrodes have already been patterned on a substrate  201  as shown in FIG.  12 . For example, the column electrodes can be patterned from indium tin oxide that is lithographically patterned on a glass substrate. It will also be assumed that any additional layers such as an electron transport layer that are common to all pixels have also been deposited. 
     The deposition of the electroluminescent material is carried out in three separate electrochemical deposition steps, one for each color. The column electrodes that are to receive the red, green, and blue-emitting materials are shown at  202 - 204 , respectively. The red-emitting polymer layer is deposited by connecting all of the red columns  202  together and placing the substrate in a deposition bath  210  that contains the precursor polymers and dyes for the red-emitting layer as shown in FIG.  12 . The precursors and monomers are polymerized as discussed above. During the deposition of the red polymer, the green and blue columns are not energized; hence, material is only deposited on the red columns  202 . At the end of the deposition process, the red-emitting columns are coated with a polymer film  212  as shown in FIG.  11 . 
     The process is then repeated by connecting columns  203  together and utilizing an electrochemical bath having the precursor polymers and dyes for the green-emitting layer. Finally, the process is repeated by connecting columns  204  together and utilizing an electrochemical bath having the precursor polymers and dyes for the blue-emitting layer. 
     It should be noted that the column electrodes can be patterned such that all of the electrodes that are to receive a particular color are connected by bus lines that run over a portion of the substrate that is not utilized in the final display. These bus lines can then be removed after the electrochemical deposition is complete by removing that portion of the substrate or by breaking the connections. For example, the traces can be broken by laser ablation of the indium tin oxide in selected areas. 
     The thickness of the film determines both the final light output of the device and, to some extent, the color of the emitted light. In the present invention, the thickness of the deposited film can be controlled for each film layer by monitoring the thickness during deposition by optical measurements such as low coherence reflectometry or by observing the current flowing through the electrode during the deposition. In addition, the deposition system can be calibrated by using different deposition times and then measuring the light output of the resulting devices. 
     Various modifications to the present invention will become apparent to those skilled in the art from the foregoing description and accompanying drawings. Accordingly, the present invention is to be limited solely by the scope of the following claims.