EP0826248A1 - Electrochemical light-emitting devices - Google Patents

Abstract

Electrochemical light emitting devices (10) are disclosed which include an electroluminescent material (16) in contact with two electrodes (12, 14). The electroluminescent material (16) can be a composite material which is an admixture of ionic species and an 'immobile' semiconductor. The semiconductor is capable of supporting both p- and n-type carriers and having a doping profile which can be dynamically changed in a controlled fashion through reversible electrochemical oxidation and reduction. These devices (10) can be in a sandwich configuration or can be a surface cell configuration with both electrodes on the same side of the electroluminescent material (16) which is present as a layer. Devices (10) having this structure may be used to generate electrochemically induced p-n junctions, thereby providing a new means of exploiting the light emitting properties of such junctions under an applied voltage. Systems and methods for generating useful levels of light employing these devices (10) are also disclosed.

Description

ELECTROCHEMICAL LIGHT-EMTTTING DEVICES

Field of the Invention

The present invention relates to electrochemical light emitting devices and their use to generate useful levels of light and to such devices having a surface cell configuration.

Background of the Invention

The semiconductor light emitting diode is a p-n junction specially fabricated to emit light under an applied voltage. In conventional inorganic semiconductor devices a p-n junction consists of a single crystal semiconductor formed such that part of the crystal is doped with positively charged carriers called /Hype and the other part is doped with negatively charged carriers called n-type. It is a basic feature of all such junctions that their chemical composition and, hence, doping profile are static, or fixed in position in the host crystal. During operation in a device charge carriers are injected into, or removed from, the junction through electrical contacts external to the junction region. Abrupt junctions, in which the transition between the n- and p-type regions is relatively narrow, are typically fabricated by alloying a solid impurity (for example, a metal) with the semiconductor, or by one of a number of epitaxial growth techniques on crystalline semiconductor substrates; graded junctions, in which the transition region is relatively broader, are produced by diffusion or ion implantation of impurities into the host semiconductor. These technologically demanding manufacturing processes make it both difficult and expensive to fabricate large area displays. Moreover, such devices are inherently brittle and lack the mechanical and processing advantages generally associated with organic, and especially polymeric materials. For these reasons, there has been considerable interest for many years in the development of suitable organic materials for use as the active (light-emitting) components of light emitting diodes.

More recently, a number of workers have disclosed electroluminescent devices using organic materials as the active light emitting layer in sandwich architecture devices. For example, S. A. Van Slyke and C. W. Tang in U. S. Patent No. 4,539,507 disclosed a device consisting of a bilayer of two vacuum sublimed films of small organic molecules sandwiched between two contacts, while R. H. Friend et al. in U. S. Patent No. 5,247,190 disclosed a device consisting of a thin dense polymer film comprising at least one conjugated polymer sandwiched between two contacts. Because these are electric field driven devices, the active electroluminescent layer must be very thin (about 1000 angstroms thick or less) and uniform. In these devices excess charge carriers are injected through the contacts into the light emitting semiconductor layer by processes well known in the study of metal-semiconductor interfaces [see, e.g. , M. A. Lampert and P. Mark, Current Injection in Solids, Academic Press, NY, 1970]. Dissimilar metals were used for the contacts to facilitate the injection of electrons at one contact and of holes at the other. As a result the current-voltage characteristic curves of these devices show a pronounced asymmetry with respect to the polarity of the applied voltage, like that typical of the response of diodes. Hence, the rectification ratio of such devices is high, typically greater than 10³, and light is emitted for only one polarity of the applied voltage. Among other drawbacks, the devices disclosed by S. A. Van Slyke and C. W. Tang and by Friend et al. suffer from the need to use metals of relatively low work function to inject sufficient numbers of electrons into the active layers to produce efficient light output at low drive voltages. Because such metals are readily oxidized, they are a source of device degradation in ambient conditions and require passivating packaging.

Electrochemistry provides a convenient means of reversibly doping a number of semiconductors with n- and /Hype carriers. This carrier injection mechanism is physically distinct from that in the sandwich architecture electroluminescent devices disclosed by S. A. Van Slyke and C. W. Tang and by Friend et al. In particular, in the case of electrochemical doping the charge carriers generated are compensated by counter-ions from the electrolyte. However, the mobility of these carriers is often too low for practical use. This is believed to be due to the fact that in a semiconductor in contact with an electrolyte, subsequent to an electrochemical oxidation or reduction reaction, the charge carrying species generated are typically ionically bound to a counter- ion from the electrolyte. The electrochemical generation of the charge carriers therefore necessarily also involves incorporation of the compensating counter- ions within the semiconductor. The often dense morphology of many semiconductors inhibits the diffusion of the counter-ions, leading to slow doping and undoping kinetics.

Semiconducting polymers offer particular advantages as electronic materials. These materials exhibit the electrical and optical properties of semiconductors in combination with the processing advantages and mechanical properties of polymers. Inorganic crystalline devices, in contrast with many polymer materials and objects, are mechanically brittle. Semiconducting polymers can often be doped by chemical means with relative ease, and the dopant species can often diffuse into the anisotropic polymer structure at room temperature. Alternatively, the doping can often be carried out electrochemically as a redox reaction, and the doping level controlled by the applied electrochemical potential with respect to a counter-electrode. Although useful as electrodes in battery applications, electrochromic devices, and the like, such electrochemically doped materials have not been considered suitable for semiconductor device applications since the dopant species are mobile at room temperature. As a result, any doping profile (such as that needed for forming a p-n junction) is necessarily transient.

The electrochemiluminescent cell, a device for generating light using the reversible oxidation-reduction reactions of organic or metallo-organic species in an electrochemical cell, is disclosed by A. J. Bard et al. in U. S. Patent No. 3,900,418. As in the present invention, electrochemiluminescent devices produce light by electron transfer reactions between electrogenerated species. The devices disclosed by Bard et al. rely on an organic solvent containing the electrolyte to transport the oxidized or reduced light emitting molecules themselves, rather than the charge carriers, between the electrodes. The oxidized and reduced species react with each other to form the original organic or metallo-organic species in an electronically excited state which may subsequently decay radiatively. N. Levantis and M. S. Wrighton in U. S. Patent No. 5,189,549 disclose electrochemiluminescent displays in which the electrochemiluminescent substance is dissolved in a solid electrolyte. Again, N. Levantis and M. S. Wrighton disclose that after generation of the oxidized and reduced species, said species diffuse away from their generation sources (i.e. the electrodes) and eventually meet somewhere between the electrodes.

Alternatively, other workers have found that an electrochemiluminescent material can be fixed on one of the electrodes in an electrochemical cell and cyclically reduced and oxidized by an alternating potential. A direct current potential can be used only if the cell contains an additional species which serves to interact with the luminescing material in such a way as either to oxidize it at the same potential at which it is electrochemically reduced or to reduce it at the same potential at which it is electrochemically oxidized [see, e.g. , M. M. Richter et al , to be published in Chem. Phys. Lett. 226, 115 (1994)]. A key disadvantage of many of these electrochemiluminescent devices is the large volume of organic solvent relative to the quantity of the electrochemiluminescent material, said volume of organic solvent being a source of reactants whose electrochemical side reaction products act to quench the recombination radiation. The use of the solvent is also a disadvantage from the point of view of the fabrication and packaging of such devices. In solid electrolytes, such as polymer electrolytes, the diffusion rate of the oxidized and reduced species are substantially lower, which compromises device performance. Electrochemical light emitting devices that include a composite material in contact with two electrodes have also been proposed. The composite material is an admixture of ionic species and an "immobile" semiconductor. The semiconductor is capable of supporting both p- and n-type carriers and having a doping profile which can be dynamically changed in a controlled fashion through reversible electrochemical oxidation and reduction. Devices having this structure may be used to generate electrochemically induced p-n junctions. These electrochemical cell devices of the past have generally been thin film devices in a sandwich (multi-layer) configuration comprising an active luminescent, semiconducting material laminated between two electrodes. One of the electrodes is made semi-transparent, thereby allowing the emitting light to exit from the device. Both inorganic materials such as ZnS.Mn and organic materials such as organic dye molecules [C.W. Tang and S.A. Van Slyke, Appl. Phys. Lett. 51 (1987) 913; J. Appl. Phys. 65 (1989) 3610] and conjugated polymers [H. Burroughes, D.D.C. Bradley, A.R. Brown, R.N. Marks, K. Mackay, R.H. Friend, P.L. Burns and A.B. Holmes, Nature 347

(1990) 530; D. Braun and A.J. Heeger, Appl. Phys. Lett. 58 (1991) 1982] have been used in this general type of electroluminescent device.

The sandwich type EL devices have attracted attention because of their potential advantages in developing large size, flat panel displays. With a conjugated polymer as the active material, such as, for example, poly(2- methoxy-5-(2'ethyl-hexyloxy)-l,4-phenylenevinylene), MEH-PPV, a semiconducting polymer with energy gap E_g - 2.1 eV [F. Wudl, P.M. Allemand, G. Srdanov, Z. Ni, and D. McBranch, in Materials for Nonlinear Optics:Chemical Perspectives, Ed. S.R. Marder, J.E. Sohn and G.D. Stucky (American Chemical Society, Washington DC, 1991, pp. 683-686], flexible light-emitting diodes and large size, flexible segmented displays have been demonstrated using transparent electrodes such as conducting polyaniline (PANT) and/or indium/tin-oxide (ITO) on glass or on flexible plastic substrates [G. Gustafsson, Y.Cao, G.M. Treacy, F. Klavetter, N. Colaneri, and A.J. Heeger, Nature, 357 (1992) 477]. Such devices have demonstrated colors throughout the visible spectrum and have achieved luminance intensities of several hundred cd/m² at 3-5 V with external quantum efficiencies of 1.5-4 % ph/el.

Summary of the Invention The present invention improves electrochemical composite light emitting devices. The improvement increases the efficiency of such devices. One aspect involves incorporating a minor amount of a gel-promoting material into the composite. The gel-promoting material is a liquid material. It is high- boiling and has a polar region covalently bonded to a nonpolar region, with the polar region conesponding to a parent compound having a dielectric constant greater than about 10.

Another aspect involves a new device configuration. This is a surface cell configuration having a film of electroluminescent material having two sides and with at least one anode and at least one cathode in contact with the same side of the film.

In some embodiments, the electroluminescent material can be an inorganic semiconductor material but more typically is an organic semiconductor material, especially a polymeric material and particularly a conjugated organic polymeric material. In especially advantageous embodiments the electroluminescent material is an electrochemically dopable conjugated polymer so as to create an electrochemical cell.

(The adjectives 'a' or 'an' are employed when describing elements and components of the invention. This is done merely for convenience and to give a general sense of the invention. Certainly, mixtures of materials may be used and this description should be read to include one or at least one and the singular also includes plural unless it is obvious that it is meant otherwise.)

The term 'immobile' is also employed. By 'immobile' is meant that the oxidized or reduced regions of the semiconductor do not physically move from one electrode to the other, but rather it is the p- and n-type carriers which move within the semiconductor.

Thus in one aspect this invention provides an electrochemical light emitting device made up of a pair of contact layers held in a spaced relationship to one another by an admixed composite layer. This composite layer contains an immobile semiconductor and an ionic species source. When a powering voltage is applied between the first and second contacts the semiconductor is electrochemically reduced at the cathode to form an n-type region containing negatively charged carriers and electrochemically oxidized at the anode to form a /Hype region containing positively charged carriers (holes). At the interface between the n-type and /Hype regions a p-n junction is formed. Under the influence of the applied voltage electrons from the n-type region and holes from the /Hype region combine in the junction to form charge carrier pairs which decay radiatively so that radiation is emitted from the layer. The gel-promoting additive increases the efficiency of the device. In one preferred aspect the immobile semiconductor is an organic semiconductor and the mixture of semiconductor and ionic species source is in the form of an open connected network of organic semiconductor filled with the ionic species source and admixed gel-promoting liquid.

The gel-promoting material, in preferred aspects can be described as a two part compound A-B with A being the polar end covalently bonded to B, the nonpolar end. In these compositions, the semiconductor is an organic polymeric or oligomeric semiconductor, especially a conjugated material, although non-conjugated polymer comprising p-conjugated moieties and gel materials and laser dyes and polymers and oligomers containing laser dye moieties are within the contemplation of the invention.

An ionic species source is present in the composite layer of the devices of the invention. This can be admixed salt. This admixed salt can be alone but preferably is a polymeric electrolyte made up of a mixture of electrolyte polymer and salt. Thus in some preferred embodiments the layer of composite material may include a polymer electrolyte and an organic semiconductor selected from the group consisting of organic small molecules, monomers, prepolymers, oligomers and polymers, and mixtures thereof.

In yet another aspect the composite layer contains a block or random copolymer containing segments which provide immobile semiconductor regions and segments which are an ionic species source.

In yet another preferred embodiment of the light emitting device, the layer of composite material includes a semiconductor containing at least one conjugated polymer or co-polymer which contains segments of β-conjugated moieties. In another still further preferred embodiment, the layer of composite material is a mixture of polymer electrolyte and organic semiconductor or semiconducting polymer, the organic semiconductor or semiconducting polymer forming an open connected network of low density within the supporting polymer solid electrolyte. In another aspect this invention provides devices of the type described wherein at least one of the contact layers is substantially transparent at the wavelength at which the device emits light and the light is emitted through this layer.

In another aspect, the devices are configured as surface cells with both anode and cathode being on the same side of the film. In some embodiments the anode and cathode are interdigitated.

In most embodiments of the invention the device is in contact with and supported by a supporting substrate. The device can take the form of a thin film or a filament or the like. In addition to being embodied as a light-emitting device this invention can take the form of a system for light emission. This system includes a device of the type described in combination with a power supply applying an effective powering voltage across the pair of anode and cathode contact layers. The effective powering voltage reduces the immobile semiconductor at the cathode to form an n-type region containing negatively charged carriers (electrons) and oxidizes the immobile semiconductor at the anode to form a /Hype region containing positively charged carriers (holes). These negatively charged electrons and positively charged holes propagate in the layer under the influence of the applied voltage and combine to form charge carrier pairs which decay radiatively in the layer so that light is emitted from the layer.

In yet a further aspect, this invention provides a process for generating useful levels of light. This process involves applying an effective powering voltage across the electrodes of the system just described, causing the system to emit light and using the light so emitted for, for example, illumination or display purposes. The light so emitted and used can be polarized, if desired.

Brief Description of the Drawings

Fig. 1 is a graph that shows the electrochemically induced light emission as measured in Example 1 from a layer comprising the polymer poly(ethylene oxide), the conjugated polymer poly(p-phenylene vinylene), and the salt lithium triflate, said layer sandwiched between electrodes of indium tin-oxide and aluminum, and a voltage of +5 V or -5 V applied between said electrodes.

Fig. 2 is a graph that shows the intensity of light measured outside the electrochemical light emitting device of Example 1 for a range of voltages applied between the electrodes. Fig. 3 is a graph that shows the current drawn by the electrochemical light emitting device of Example 1 for a range of voltages applied between the electrodes.

Fig. 4 is a graph that shows the intensity of light measured outside the electrochemical light emitting device of Example 1 for a range of voltages applied between the electrodes, said measurement being made as the applied voltage was swept at a rate of 67 mV/s either from 0 V to 5 V or from 5 V to 0 V, the direction indicated by the arrowheads on the graph.

Fig. 5 is a graph of data determined in Example 2 which shows the electrochemically induced light emission from a layer comprising the polymer poly(ethylene oxide), the conjugated polymer poly[2-methoxy,5-(2'-ethyl- hexyloxy)-l,4-phenylene vinylene], and the salt lithium triflate, said layer sandwiched between electrodes of indium tin-oxide and aluminum, and a voltage of +4 V or -4 V applied between said electrodes. Fig. 6 is a graph that shows the intensity of light measured outside the electrochemical light emitting device of Example 2 for a range of voltages applied between the electrodes.

Fig. 7 is a graph that shows the cunent drawn by the electrochemical light emitting device of Example 2 for a range of voltages applied between the electrodes.

Fig. 8 shows the structure of the bilayer two-color device described in Example 6.

Fig. 9 shows the I vs V and L vs V characteristics of the two-color rrO/PPV/MEH/Al devices described in Example 6. ITO is the anode and Al is the cathode.

Fig. 10 shows the EL spectra of the bilayer ITO/PPV/MEH-PPV/A1 LEC in forward and in reverse bias, as described in Example 6. Both spectra are clean spectra with no "cross-talk" in EL color. The different spectra imply different color emission. The inset shows the device structure of this LEC. Fig. 11 shows the cunent vs voltage and light vs voltage of a device containing the octylcyanoacetate additive, as described in Example 9 .

Fig. 12 shows the response time (turn-on time) and the light output (at 2.7 V) for devices with and without the octylcyanoacetate additive as described in Example 9. Fig. 13 shows the stress life at constant voltage (initial brightness of 300 cd/m²) of the device containing the octylcyanoacetate additive described in Example 9.

Fig. 14 shows current vs voltage (I-V) and light intensity vs voltage (I_L vs V) for the light-emitting electrochemical cell fabricated with a polyaniline electrode (conducting emeraldine salt form); see Example 11. Fig. 15 shows the quantum efficiency for electroluminescence from light-emitting electrochemical cells fabricated with poyaniline (in the conducting emeraldine salt form) as one electrode; see Example 11.

Fig. 16 shows the quantum efficiency for electroluminescence from light-emitting electrochemical cells fabricated with polyaniline (in the conducting emeraldine sale form) as one electrode; see Example 14.

Fig. 17 is a schematic prospective view of an elementary surface cell device.

Fig. 18 is a schematic perspective view of another embodiment of the surface cell device.

Fig. 19 is a schematic representation of a process used to prepare the surface cell devices.

Fig. 20 and Fig. 21 are graphs of luminescence vs voltage for the surface cell devices. Fig. 22 is a current vs voltage curve for a surface cell device.

Fig. 23 is a graph of emitted light intensity as a function of cunent for a surface cell device.

Fig. 24 is a graph of electroluminescent efficiency as a function of current for such a device. Fig. 25 is a graph of electroluminescence intensities collected in front of and in back of a device with a transparent substrate as a function of drive cunent.

Fig. 26 is a graph showing the good shelf-life achieved with the surface cell devices.

Detailed Description of the Invention

It has been discovered that incorporating a gel-forming liquid into a blend of an 'immobile' semiconducting material with ionic species improves the performance of that blend as an active layer in a light-emitting device. When such a layer is supported between two electrodes and a small voltage is applied, the layer functions as a p-n junction and may emit light. This observation is a completely unexpected result, as one skilled in the art would anticipate that such a device would cause the semiconducting polymer to be doped to a conducting state, effectively quenching its luminescence. For example, J. Heinze [Synth. Met. 43:2805 (1991)] reviews the well-known electrochemistry of conjugated polymers, which has demonstrated that in the presence of an electrolyte and an applied voltage such polymers can be reversibly doped with charge carriers whose charge is compensated by counterions from the electrolyte. A number of researchers, including S. Hayashi et al. [Solid State Commun. 61:249 (1987)], have noted that even at relatively low levels of a few mole percent, such electrochemical doping efficiently quenches the luminescence of the polymer.

Upon application of a voltage between the two electrodes, the semiconductor in the layer of composite material is electrochemically oxidized to form /Hype carriers at the anode. Said /Hype carriers then propagate within the layer, under the influence of the field due to the applied voltage, toward the cathode. Similarly, the semiconductor in the layer of composite material is electrochemically reduced to form n-type carriers at the cathode which then propagate within the layer, under the influence of the field due to the applied voltage, toward the anode. Between the regions of n- and /Hype carriers, the /Hype carriers propagating toward the cathode and the n-type carriers propagating toward the anode meet in a region which defines the electrochemically induced p-n junction. The width of the electrochemically induced p-n junction may be either equal to or less than the thickness of the ionically conducting layer. Within the region of the electrochemically induced p-n junction, the n- and /Hype carriers combine in the layer to form charge carrier pairs which decay radiatively to the neutral ground state of the semiconductor.

The ability to generate a p-n junction electrochemically, in-situ, offers particular advantages. Since the device is fabricated with the semiconductor in its neutral form and the electrochemically induced p-n junction is generated in- situ electrochemically, manufacturing and packaging are simpler and less demanding. When it is operated as a light emitting p-n junction, the dynamic electrochemically generated p-n junction is bipolar, by which is meant that reversing the applied voltage reverses the doping profile. The bipolar character, thus, enables the device to operate under either forward or reversed polarity. This is reflected in a comparison of the current-voltage characteristic curves of conventional static p-n junctions, well known in the art [see, e.g. , S. M. Sze, Physics of Semiconductor Devices, Wiley & Sons, N.Y., 1981], with those of the present invention. The conventional static p-n junction is a diode, meaning that its current response to a swept applied voltage is asymmetric with respect to the polarity of the applied voltage. The result is a large rectification ratio (ratio of forward to reverse cunent), typically of order 10³ - 10\ By contrast, the cunent response of the bipolar electrochemical light emitting device is more nearly symmetric, as demonstrated in Figures 3 and 7, and its rectification ratio is near unity.

The Composite Layer

The composite is a mixture of immobile semiconductor and ionic species source. The composite material is preferably in the form of a layer of substantially uniform thickness. This is generally a thin layer and can range in thickness from about 5 nm to about 1 cm or more. In most applications thicknesses from about 5 nm to about 1 mm and particularly 10 nm to about 0.1 mm are employed. As a general rule, thinner la yers are more commonly employed than thicker layers. Layers in the form of films, fibers, or other more complex shapes may be used including shapes whose dimensions fall outside this range.

The proportions of semiconductor and ionic species source can vary throughout the range which permits light to be emitted, for example from about 0.1:0.9 to about 0.9:0.1 on a weight basis. At the present time ranges of from about 0.2:0.8 to about 0.8:0.2 and especially from about 0.3:0.7 to about 0.7:0.3 are preferred. The amount of high-boiling gel-forming liquid which is admixed with the semiconductor and ionic species source is from about 0.1 to about 3 and especially 9.15 to 2 parts by weight, based on the total weight of semiconductor plus ionic species source. It should be emphasized that these numbers represent the proportion of gel-forming liquid added to the composite material. It should be understood that some amounts of the gel-forming liquid may be lost to vaporization during fabrication of the layer from the mixture. We estimate that these losses could range from 0 to 33% of the gel-forming liquid additive initially incorporated into the composite-layer-forming liquid. This could cause the actual lower limits and preferred lower limit on the proportion of additive present in the final products to drop to 0.06 and 0.1 respectively.

The composite material may also comprise other materials or additives useful for facilitating the processing of said composite material or for enhancing its function in electrochemical light emitting devices. Such additives may include, for example, stabilizers, plasticizers, dyes, colorants, fillers, mechanical spacer particles, and the like. These materials are optional, and so can range from 0 to about 75% of the total of the layer. The physical shape, form, or state of the matter of the semiconductor is not critical and may include homogeneous and heterogeneous blends with an ionic medium, mixtures and admixtures with an ionic medium, interpenetrating networks within an ionic medium, solid solutions with an ionic medium, gels swollen by a solvent which comprises ionic species, foams or porous structures further comprising an ionic medium, and the like, provided that said semiconductor is 'immobile' and further provided that said semiconductor supports both p- and n-type carriers which can propagate throughout the semiconductor and that the doping profile of the semiconductor can be dynamically changed in a controlled fashion through reversible electrochemical oxidation and reduction. The electrochemical light emitting device may comprise more than one spatially separated region, each region further comprising different semiconductors, such that by changing the operating parameters of a device incorporating said separated regions the position of the p-n junction within the device may be moved among said regions. Thus, for example, in a light emitting structure the junction might be moved between regions comprising semiconductors with differing energy gaps so that light of different colors may be emitted.

The Gel-Forming Liquid The composite layer includes a high-boiling gel-forming liquid. This liquid is furtehr characterized by being made up of a bifunctional liquid compound which has a polar region covalently bound to a nonpolar region. It may include mixtures of such compounds. The nonpolar region is an organic moiety and is selected from hydrocarbyl and substituted hydrocarbyl materials having from about 2 to about 30 carbons; especially from about 3 to 24 carbons and more particularly 5 to 20 carbons. These nonpolar regions can be simple hydrocarbyls including aliphatic and aromatic groups. These may be alkyls and alkenyls, both linear and branched, cycloalkyls, and aryl-containing groups ranging from aralkyls to aryls to alkaryls. These groups may also be substituted with heteroatom groups which do not significantly interfere with the nonpolar quality of this region. Typical substituents include ether oxygens, sulfide sulfurs, tertiary amine nitrogens, halogens, and the like.

The polar region of the gel-forming liquid conesponds to a parent compound having a dielectric constant greater than 10. For example, if the polar region is cyanoactetate, the corresponding parent compund is cyanoacetic acid, a compound having a dielectric constant of 33.4. Other representative materials are shown in Table 1. In this table, "A" is the polar region and "B" is the nonpolar region. The types of groups typically present in the polar region include cyanos, nitros, ketones, aldehydes, carboxyls, primary or secondary amines, amides, carbonates, lactones, pynolidinones, oxalidinones, sulfoxides, sulfolanes, phosphoramides and the like which meet the dielectric constant requirements. The gel-forming liquid is also high-boiling. That is, it has a boiling point above 240 °C at atmospheric pressure or above 75 °C at 1 mm Hg.

Suitable gel-forming liquids include, for example, hexylcyanoacetate, octylcyanoacetate, decylcyanoacetate, dodecylcyanoacetate, 2- ethylhexylcyanoacetate, N, N-dimethyldodecanamide, N,N-diethyldecanamide, N,N-diethyldodecanamide, myristamide, palmitamide, stearamide, arachidamide, decanolactone, dodecanolactone, tetradecanolactone, N- octylpyrrolidinone, N-dodecylpynolidinone, 1-methyl-dodecylpynolidinone, 2- tert-butylcyclohexanone, 2-tert-amylcyclohexanone, 4-tert-butylcyclohexanone and the like. The amount of gel-forming liquid can vary. Typically the amont incorporated into the composite ranges from about 10% to about 300 on a weight basis compared to the weight of organic semiconductor plus ionic species source. Larger or smaller concentrations may be used if desired. Prefened use level range from about 15 to about 200% by weight on the same basis.

It will be appreciated that these amounts may be difficult to determine in a finished polymer system due to analytical difficulties inherently posed by polymers. Thus, these amounts are the amounts added to the mixture during forming. Solvents are often removed from the mixture during processing. This could have the effect of stripping some of the high-boiling liquid additive out of the mixture. However, these losses are usually minor and do not significantly affect the proportion of high-boiling liquid in the final product, that is by more than about 33%.

Although not understood with certainty, it is believed that the gel- forming liquid improves device performance by increasing the mobility of ion in the composite layer. The Semiconductor

The layer includes a semiconductor. In some of the preferred embodiments, the semiconductor is a polymer, said polymer further preferably comprising at least one conjugated polymer or a co-polymer which contains segments of p p-conjugated moieties. Conjugated polymers are well known in the art [see, e.g. , Conjugated Polymers, J.-L. Bredas and R. Silbey eds., Kluwer Academic Press, Dordrecht, 1991]. Suitable choices include, but are in no way limited to (i) poly(p-phenylene vinylene) and its derivatives substituted at various positions on the phenylene moiety;

(ii) poly(/ phenylene vinylene) and its derivatives substituted at various positions on the vinylene moiety;

(iii) poly(arylene vinylene), where the arylene may be such moieties as naphthalene, anthracene, furylene, thienylene, oxadiazole, and the like, or one of said moieties with functionalized substituents at various positions;

(iv) derivatives of poly(arylene vinylene), where the arylene may be as in (iii) above, substituted at various positions on the arylene moiety;

(v) derivatives of poly(arylene vinylene), where the arylene may be as in (iii) above, substituted at various positions on the vinylene moiety;

(vi) co-polymers of arylene vinylene oligomers with non-conjugated oligomers, and derivatives of such polymers substituted at various positions on the arylene moieties, derivatives of such polymers substituted at various positions on the vinylene moieties, and derivatives of such polymers substituted at various positions on the arylene and the vinylene moieties;

(vii) poly(p-phenylene) and its derivatives substituted at various positions on the phenylene moiety, including ladder polymer derivatives such as poly(9,9- dialkyl fluorene) and the like;

(viii) poly(arylenes) and their derivatives substituted at various positions on the arylene moiety; (ix) co-polymers of oligoarylenes with non-conjugated oligomers, and derivatives of such polymers substituted at various positions on the arylene moieties;

(x) polyquinoline and its derivatives; (xi) co-polymers of polyquinoline with -phenylen e and moieties having solubilizing function;

(xii) rigid rod polymers such as poly(p-phenylene-2,6-benzobisthiazole), poly (p-phenylene-2 , 6-benzobisoxazole) , poly(p-phenylene-2 , 6-benzimidazole) , and their derivatives; and the like.

Other suitable choices for the semiconducting component of the invention are small organic molecules, monomers, prepolymers, or oligomers which are semiconducting in their solid state and which can be processed into suitable blends or composites with an electrolyte, said small organic molecules being 'immobile' in the blends in the sense defined above. Laser dyes are one type of small molecule which has worked well in this application. These laser dyes can be present as small molecules or as polymer materials in which they are chemically bound or otherwise tethered to an immobilizing polymer.

The immobilization may be effected, for example, by forming a solid mixture of the semiconducting molecules, monomers, prepolymers, or oligomers in a suitable solid medium comprising ionic species, or by forming a physically or covalently cross-linked network of the semiconducting molecules or oligomers in a suitable medium comprising ionic species, or by physically or covalently bonding the semiconducting molecules or oligomers to a suitable immobile network comprised within the layer further comprising ionic species.

The Ionic Species Source

Ionic species may be provided by incorporation in the composite material of a salt, by which is meant ionic positively charged cations and negatively charged anions which in a solvent or other suitable medium to render said solvent or medium electrically conducting.

In these electrochemical light emitting devices, suitable representative salts have cations which include the singly ionized alkali metals lithium, sodium, potassium, rubidium, and cesium; ions of the metals aluminum, silver, barium, calcium, cobalt, copper, chromium, zinc, and the like; metallo-orgamc cations; and organic cations such as ammonium, tetramethyl ammonium, tetraethyl ammonium, tetrapropyl ammonium, tetrabutyl ammonium, tetradecyl ammonium, tribenzyl ammonium, phenyl ammonium, and the like. Representative choices of anion include the singly ionized halogens fluorine, chlorine, bromine, and iodine; inorganic anions such as perchlorate, bromate, nitrate, sulfate, phosphate, tetrafluoroborate, hexafluorophosphate, aluminum tetrachlorate, and the like; and organic anions such as n-butyrate, trifluoromethane sulfonate, trifluoromethane sulfonamide, acetate, trifluoroacetate, tetraphenylborate, toluene sulfonate, dodecylbenzene sulfonate, camphor sulfonate, picrate, salicylate, thiocyanate, benzoate and the like.

Mixtures comprising more than one cationic species and more than one anionic species may also be suitable for use in electrochemical light emitting devices.

In a preferred embodiment, the ionic species are provided by a polymer electrolyte. Polymer electrolytes are well known in the art [see, e.g. , K. M. Abraham, Highly Conductive Polymer Electrolytes in Applications of

Electroactive Polymers, B. Scrosati ed., Chapman & Hall, London, 1993] and consist of salt complexes of high polymers containing optimally spaced electron donor atoms or moieties which coordinately bond with the cation of the salt. Suitable examples of such polyionomers or polyionophores include poly (ethylene oxide), polyφropylene oxide), poly (dimethyl siloxane), polyoxymethylene, poly(epichlorohydran), poly hosphazene), poly[bis- (methoxyethoxyethoxy)phosphazene], polyoxetane, polytetrahydrofuran, poly(l,3-dioxolane), poly(ethylene imine), poly(ethylene succinate), poly(ethylene sulfide), poly(propylene sulfide), poly[oligo(oxyethylene) oxymethylene], poly[oligo(oxyethylene) methacrylate], poly[oligo(oxyethylene) cyclotrisphosphazene] and the like, and mixtures thereof.

In another embodiment, the light emitting layer of an electrochemical light emitting device employs the salt of a conjugated polymer covalently bonded to a self-doping moiety. Suitable choices of such polymers include the salts of sulfonate and alkylsulfonate substituted polyarylenes, polyarylenevinylenes, and the like, such as poly(p-phenylene sulfonate), poly(3- alkylsulfonate-thiophene), poly(N-alkylsulfonate pyrrole), and the like, where alkyl is one of propyl, butyl, pentyl, hexyl, heptyl, octyl, decyl, undecyl, dodecyl, etc. In another embodiment the electrochemical light emitting device may include a block or graft copolymer further comprising segments which function as polymer electrolytes and segments which contain p-conjugated moieties, such as oligoarylenes, oligoarylenes substituted at various positions on the arylene moiety, oligoarylene vinylenes, oligoarylene vinylenes substituted at various positions on the arylene moiety, oligoarylene vinylenes substituted at various positions on the vinylene moiety, oligoarylene vinylenes substituted at various positions on the arylene and the vinylene moiety, di-arylene polyenes, di- arylene polyenes substituted at various positions on the arylene moiety, di- arylene polydienes, di-arylene polydienes substituted at various positions on the arylene moiety, and the like.

In yet another embodiment, the ionic species may be generated in-situ by the ultraviolet radiation induced photolysis of suitable salts incorporated within the composite layer. Examples of such salts, commonly used in resist imaging applications, include aryldiazonium salts, diaryliodonium salts, and triarylsulfonium salts.

Contact Layers

The devices have contact layers. These layers serve as electrodes for applying the powering voltage across the composite layer. These electrodes may be placed in contact with the layer of composite material by processing the admixture of the ionic species and the semiconducting material into a layer on a substrate bearing one electrode and then depositing a second electrode on the free surface of the layer.

Any material capable of forming an electrical pathway for the powering voltage to the composite layer may be employed as these contact layers. Typical materials include metals such as aluminum, silver, platinum, gold, palladium, tungsten, indium, copper, iron, nickel, zinc, lead and the like; metal oxides such as lead oxide, tin oxide, and the like; graphite; doped inorganic semiconductors such as silicon, germanium, gallium arsenide, and the like; and doped conducting polymers such as polyaniline, polypynole, polythiophene, and the like.

In a prefened embodiment of electrochemical light emitting devices at least one electrode consists of a semitransparent conductive material such as indium tin oxide, tin oxide, nickel, gold, doped polyaniline, doped polypynole, and the like. Preferably, such electrodes have a sheet resistance of about 10 to 1000 ohms per square and an optical transmittance of about 80% for wavelengths longer than about 400 nm.

Surface Cell Configuration

In a surface cell configuration, electrodes may be pre-prepared onto a substrate with a gap in between them. For example, a simple surface cell configuration is shown in Fig. 17. Device 10 includes anode 12 and cathode 14 in contact with semiconducting electroluminescent and ionic layer 16 (preferably made of a conducting polymer) carried by substrate 18. The metal electrodes 12 and 14 can be fabricated on the substrate 18 by, for example, thermal evaporation, electron beam evaporation, sputtering, chemical deposition, printing and so on. The electrodes can be patterned. This can be done either with a shadow mask or by means of photolithographic techniques well known in the art. To improve the photosensitivity (i.e., to increase the length/ width ratio of the gap between electrodes), the electrodes 12 and 14 can be fabricated in an interdigitated pattern as shown in b of Fig. 18 where they are shown coupled to switched power supply 20. This interdigitated electrode configuration has been used in polymer field effect transistors to increase the source-to-drain current density for a given gate electrode area [for example, A. Assadi, C. Svensson, M. Wilander and O. Inganas, Appl. Phys. Lett. 53

(1988) 195; F. Gamier, G. Horowitz, X. Peng and D. Fichou, Adv. Mater. 2 (1990) 592].

The interdigitated surface cell electrode configuration has not been used in thin film electroluminescent devices heretofore for a number of reasons. Conventional organic or inorganic semiconducting materials have high bulk resistivities; thus requiring sub-micron dimensions. Different metals are required as the carrier-injecting electrodes. Moreover, since the thin film organic EL devices require high electric fields for carrier injection p.D. Parker, J. Appl. Phys. 75 (1994) 1656], the use of a surface cell configuration might be considered to require dangerously high voltage operation. For example, a polymer light-emitting diode which turns on at 5 volts in sandwich cell with film thickness of 1000 A would require approximately 250 volts in a surface cell with interdigitated electrode spacing of 5 μm.

As light-emitting electrochemical cells, the devices turn on at applied voltages comparable with the energy gap of the luminescent, semiconducting material. Such surface configuration light-emitting electrochemical cells enable a new type of flat panel display which, can be viewed from both sides and which can be made with plastic materials such that they are fully flexible. In contrast to the conventional light emitting diodes, the thickness and the film qualities (surface roughness, pinholes etc.) of the luminescent semiconducting material are not critical in the surface cell configuration. Moreover, in the surface cell configuration, light-emitting electrochemical cell devices are fabricated in a single step by depositing the semiconducting luminescent material (for example by casting from solution) onto pre-pattemed electrodes with no need for subsequent deposition of electrodes after casting the polymer film. Thus, the surface cell configuration offers significant manufacturing advantages over the prior art.

The invention provides a device configuration such as shown in Fig.s 17 and 18 for implementation of the light-emitting electrochemical cell in a surface cell configuration and a method of obtaining light from said surface light- emitting electrochemical cell.

In one embodiment, the electrodes are interdigitated metal electrodes patterned on the substrate by vacuum deposition through a shadow mask.

In an alternative embodiment, the interdigitated electrodes are patterned onto the substrate using photolithography. The use of photolithographic methods is particularly important when high resolution is required.

In yet another embodiment, the interdigitated electrodes are patterned onto the substrate using the methods of screen printing.

Film thicknesses for the active semiconducting, luminescent and ionically conducting material are not critical and range from a few hundred Angstrom units to a few microns, or even thicker. A limitation on the thickness is set by the transparency of the active material at the wavelength of emission. For efficient external generation of light the film thickness should be such that the optical density at the emission wavelength is less than unity (OD < 1).

For devices used for viewing in both front and back directions, it is desirable that the device be semi-transparent throughout the visible spectrum. Thus, for active luminescent materials with absorption gap less than 2.3 eV, film thicknesses should be limited such that the optical density (OD) at the absorption peak < 3; typically less than 6000 A for most conjugated polymers. For large band gap, blue-emitting devices with low absorption throughout the visible spectrum, transparency does not limit the thickness.

Substrates

Suitable substrates may be rigid or mechanically flexible and include glass, metals and alloys, and plastics. Typically the surface of the substrate which carries the thin layer has been coated with an appropriate electrode material. Admixtures may be processed from solutions of the ionic species and semiconductor in one or more solvents or mixtures of solvents, or from a thermally processible blend of the two materials, or by using an appropriate vacuum deposition technique. A second electrode may be placed in contact with the layer of composite material by deposition of a suitable solvent-borne material, or by thermal evaporation of a suitable material, or by electroless deposition of a suitable metal or alloy. Alternatively, both suitable electrodes may be placed in contact with a free-standing layer of the composite material by lamination.

In an embodiment of the present invention in which mechanically flexible substrates, electrodes, and packaging materials have been used, the electrochemical light emitting device may be mechanically flexible. Such devices may be useful for displays mounted on curved surfaces, such as are found on a number of appliances, automotive instrument panels, consumer electronics and the like. If it is compatible with the physical properties of the layer, post-shaping operations such as vacuum-forming may be used.

In a prefened embodiment comprising a polymer electrolyte and a semiconducting polymer, these polymers are preferably processible from the same organic solvent or a suitable mixture of solvents. Alternatively, the semiconducting polymer may be prepared by means of a chemical or thermal treatment of a processible precursor polymer. In a further preferred embodiment the solution processible precursor polymer is a polyelectrolyte, which may enhance intimate mixing of said precursor polymer with the polymer electrolyte. Suitable co-solutions of the components of the composite material may be processed into substantially uniform layers of appropriate thickness using a number of established techniques including spin coating, roll coating, screen printing, flexographic printing, meniscus coating, and the like. In another prefened embodiment comprising a polymer electrolyte and a semiconducting polymer, said polymers are preferably processible from the melt such as, for example, by extrusion.

In another prefened embodiment comprising a semiconducting polymer and an electrolyte dissolved in a solvent, the semiconducting polymer forms a gel incorporating said electrolyte solution. Such gels may be formed, for example, by semiconducting polymers with moieties which may be physically or chemically cross-linked to form a network.

To construct displays of higher information content, electrochemical light emitting devices may be fabricated as arrays of individually addressable electrochemical light emitting devices. Such arrays may readily be produced by suitable patterning of either of the electrodes or of the layer of composite material.

When the electrodes are in a flat-panel configuration that are often patterned by laser ablation or by UV degradation, a continuous manufacturing process (using linear coating technology for the semiconducting, luminescent and ionically transporting material) can be envisioned in a "roll to roll" process as shown schematically in Fig. 19. As shown in Fig. 19, a device of this invention can be continuously prepared by passing a sheet of flexible substrate 18 from feed spool 22 to take-up spool 24. As substrate 18 passes under station 26, a layer of conductive electrode-forming material is deposited. At station 28, this layer is patterned with U.V. light to give use to a pattern of interdigitated electrodes. At station 30 a layer of electroluminescent material in deposited onto the interdigitated electrodes. Thereafter, at station 32 an optional protective overlayer is applied. In steps not shown, the continuous sheet is cut into individual devices. An important advantage is that vacuum evaporation equipment need not be involved in the manufacturing line.

Optional Elements

Electrochemical light emitting devices may further comprise additional materials or layers, for example, such as to exclude oxygen or moisture from the composite material, or for support, mounting, or decorative purposes, or to adjust the apparent color of the emitted light.

If desired, one or each of the electrodes can include, in addition to the electrode material, a further layer or layers which serve to control the electrochemical potential of the reduction or oxidation reactions at the electrode and the double layer potential at the electrode. Such layers would serve to control the relative populations of n- and Hype carriers in the layer, thereby optimizing the doping profile of the electrochemically induced p-n junction upon application of a voltage. The following Examples are set forth to further illustrate the present invention and should not be construed as limiting the spirit or scope of the invention.

Example 1 Electrochemical light emitting devices were fabricated in the following way. A co-solution of the polymer poly(ethylene oxide), the polyelectrolyte poly(p-xylylidene tetrahydrothiophenium chloride), and the salt lithium triflate (LiCF₃SO₃) in a mixture of either acetonitrile and water (5:1 by volume) or acetonitrile and methanol (2:1 by volume) was prepared. The molar ratio of the CH₂CH₂O moieties in the poly(ethylene oxide) to the salt was approximately 20:1. Thin films of a 1:1 (by weight) blend of the two polymers were then deposited onto indium tin-oxide coated glass substrates either by spin coating or drip coating of the solution onto the substrates. The transparent electrically conducting layer of indium tin-oxide on the substrate served as one contact to the resulting ionically conducting layers. The thickness of the layers formed in this way ranged between about 500 angstroms and 1 mm, with excellent devices produced from films of about 5000 angstroms thickness. The layers were subsequently heated to approximately 100°C for about 3 hours on a hot plate in a nitrogen atmosphere, in order to convert the poly(p-xylylidene tetrahydrothiophenium chloride) to the conjugated polymer poly(p-phenylene vinylene) through the thermal elimination of tetrahydrothiophene and hydrochloric acid. The coated substrates were then transferred into a thermal evaporator, in which a second contact was formed by evaporation of a 400 angstrom aluminum layer onto the surface of the ionically conducting layer opposite the substrate. Because the indium tin-oxide covered the entire surface of the substrates, the aluminum contacts, in the form of thin strips approximately 0.3 cm x 1 cm, effectively defined the active light emitting area of the devices.

Upon application of a voltage between the indium tin-oxide and aluminum contacts, emission through the transparent contact of yellow-green light from the layer became visible in a lighted room at a voltage whose magnitude was less than 5 V. While this demonstrated that the devices will turn on at quite low effective powering voltages, higher voltages such as up to and including 10 or 20 volts or even unreduced line voltages of as much as 110 volts or even 220 volts can be used if desired. Fig. 1 also shows the spectrally resolved light output from the device upon application of a voltage of either polarity, demonstrating that the device is bipolar. The spectrum of emitted light is seen to be the same in either case, indicating that the emission is independent of which electrode serves as the anode and which serves as the cathode. Fig. 2 shows the applied voltage dependence of the light output from the device, while Figure 3 shows the cunent drawn by the device. For positive voltages the indium tin-oxide contact served as the anode and the aluminum contact as the cathode, whereas for negative voltages the role of the two contacts was reversed. This further demonstrates the bipolar nature of these devices. The cunent versus voltage curve is nearly symmetric. The extemal quantum efficiency (photons emitted per electron supplied by the power source) of these devices was essentially independent of the voltage polarity. The device was operated in a dry and oxygen-free argon atmosphere under a continuous d.c. bias voltage of 5 V with light output observed over a period of several days. Devices using layers with a variety of thicknesses in the range from 500 angstroms to 1 mm showed differences in performance. The switching time and apparent threshold operating voltage increased with increasing layer thickness. The device current-voltage characteristics showed both a voltage scan rate dependence and hysteresis as a consequence of the dynamic nature of the electrochemical generation of the p-n junction. Fig. 4 shows the current- voltage characteristics for a scan rate of about 67 mV/s, both when the voltage is scanned from 0 to 5 V and when it is scanned from 5 to 0 V. For faster scan rates the apparent threshold voltage for light emission increases for increasing voltage scans and decreases for decreasing voltage scans. In addition, the perceived switching time varies depending on the cycle history of the device: the speed with which the light output appears in response to a transient voltage of a given polarity is slower if the device has been previously operated with a voltage of the opposite polarity than if it has been previously operated with one of the same polarity. These phenomena are all a result of the kinetics of the electrochemical generation and transport of n- and /Hype carriers in the ionically conducting layer.

Example 2 Electrochemical light emitting devices were fabricated in the following way. A co-solution in pyridine of the polymer poly (ethylene oxide), the conjugated polymer poly[2-methoxy,5-(2'-ethyl-hexyloxy)-l ,4-phenylene vinylene], and the salt lithium triflate (LiCF₃SO₃) was prepared. The molar ratio of the CH₂CH₂O moieties in the poly(ethylene oxide) to the salt was approximately 20: 1. Thin films of a 1: 1 (by weight) blend of the two polymers were then deposited onto indium tin-oxide coated glass substrates either by spin coating or drip coating of the solution onto the substrates. The transparent electrically conducting layer of indium tin-oxide on the substrate served as one contact to the resulting ionically conducting layers. The thickness of the layers formed in this way ranged between about 500 angstroms and 1 mm. The coated substrates were transferred into a thermal evaporator, in which a second contact was formed by evaporation of a 400 angstrom aluminum layer onto the surface of the ionically conducting layer opposite the substrate. Because the indium tin-oxide covered the entire surface of the substrates, the aluminum contacts, in the form of thin strips approximately 0.3 cm x 1 cm, effectively defined the active light emitting area of the devices.

Upon application of a voltage between the indium tin-oxide and aluminum contacts, emission through the transparent contact of yellow-orange light from the layer became visible in a lighted room at a voltage whose magnitude was less than 4 V. Fig. 5 shows the spectrally resolved light output from the device upon application of a voltage of either polarity, demonstrating that the device is bipolar. The spectrum of emitted light is seen to be the same in either case, indicating that the emission is independent of which electrode serves as the anode and which serves as the cathode.

Fig. 6 shows the applied voltage dependence of the light output from the device, while Fig. 7 shows the cunent drawn by the device. As for the devices described in Example 1 , these data demonstrate the bipolar nature of the present devices. The cunent versus voltage curve is again nearly symmetric. The extemal quantum efficiency (photons emitted per electron supplied by the power source) of these devices was essentially independent of the voltage polarity.

Example 3 Electrochemical light emitting devices were prepared from a co-solution of a complex of the salt lithium triflate with the polymer poly(ethylene oxide) and the laser dye Rhodamine 6G chloride in acetonitrile. Thin films of a 1.6: 1 (polymeπdye by weight) blend of the polymer and the dye were then deposited onto indium tin-oxide coated glass substrates by spin coating the solution onto the substrates. In this Example, the network of dye aggregates within the host poly(ethylene oxide) layer served as the semiconductor. The transparent electrically conducting layer of indium tin-oxide on the substrate served as one contact to the ionically conducting polymer layer. The thickness of the layers formed in this way ranged between about 1000 angstroms and 1 mm. Following deposition of the layers, the coated substrates were transferred into a thermal evaporator, in which a second contact was formed by evaporation of an aluminum layer onto the top surface of the layer.

Upon application of a voltage between the indium tin-oxide and aluminum contacts, emission through the transparent contact of light from the layer was detected at a voltage whose magnitude was less than 15 V.

Example 4

Electrochemical light emitting devices were prepared from a co-solution of a complex of the salt lithium triflate with the polymer poly(ethylene oxide) and the laser dye Kiton red 620 in acetonitrile. Thin films of a 1.6:1 (polymeπdye by weight) blend of the polymer and the dye were then deposited onto indium tin-oxide coated glass substrates by spin coating the solution onto the substrates. In this Example, as in Example 2, the network of dye aggregates within the host poly(ethylene oxide) layer served as the semiconductor. The transparent electrically conducting layer of indium tin- oxide on the substrate served as one contact to the ionically conducting polymer layer. The thickness of the layers formed in this way ranged between about 1000 angstroms and 1 mm. The coated substrates were then transfened into a thermal evaporator, in which a second contact was formed by evaporation of an aluminum layer onto the top surface of the layer.

Upon application of a voltage between the indium tin-oxide and aluminum contacts, emission through the transparent contact of light from the layer was detected at a voltage whose magnitude was less than 6 V when indium tin-oxide was connected as the anode, and less than 14 V when aluminum was connected as the anode. The current-voltage characteristic curves showed a more significant asymmetry than those measured for the devices described in Examples 1 and 2. Light was detected for applied voltages of either polarity.

Example 5 Electrochemical light emitting devices were prepared from a co-solution of 15 mg of the tetrabutyl ammonium salt of dodecylbenzene sulfonic acid and 20 mg of the conjugated polymer poly[2-methoxy,5-(2'-ethyl-hexyloxy)-l,4- phenylene vinylene] in 3 ml of xylenes. Thin films of a 1.3: 1 (polymeπsalt by weight) blend of the polymer and the salt were then deposited onto indium tin- oxide coated glass substrates by spin coating. In this Example, the conjugated polymer layer served both as the semiconductor and as the ionically conducting matrix for the tetrabutyl ammonium cations and dodecyl benzene sulfonate anions. The transparent electrically conducting layer of indium tin-oxide on the substrate served as one contact to the ionically conducting polymer layer. The thickness of the layers formed in this way ranged between about 1000 angstroms and 1 mm. The coated substrates were then transfened into a thermal evaporator, in which a second contact was formed by evaporation of an aluminum layer onto the top surface of the layer.

Upon application of a voltage between the indium tin-oxide and aluminum contacts, emission through the transparent contact of red orange light from the layer became visible in a lighted room at a voltage whose magnitude was less than 5 V. The cunent-voltage characteristic curves were nearly symmetric, as in the case of the devices described in Example 1. Light was detected for applied voltages of either polarity.

Example 6 Light-emitting electrochemical cell (LEC) devices were fabricated with two separated layers, each layer comprising different semiconductors. By changing the polarity of the bias voltage of these bilayer devices, the position of 001 PCMJS96/072

-33-

the p-n junction within the device was moved from one layer of the bilayer to the other. Since the two layers comprise semiconductors with differing energy gaps light of different colors was emitted.

The device structure is shown in Fig. 8; the polymer with larger band- gap must be next to the transparent electrode, so that the emitted light from the smaller band-gap material will not be absorbed.

Two-layer LECs with TTO/PPV/MEH-PPV/A1 structure were fabricated. In these devices, both the PPV layer and MEH-PPV layer are blended with PEO and Li salt. The ratio of polymer to PEO to Li salt is 1:1:0.1. Forward bias is defined such that ITO is wired as positive (the anode) and Al is wired as negative (the cathode). This two-color device shows typical LEC I vs V characteristics, i.e. light is emitted in both forward and reverse bias. However, since the emitting junction is close to one electrode (in this case the cathode), this LEC emitted two completely different colors in forward and reverse bias. The device IV and LV characteristics are shown in Fig. 9.

Under forward bias (+5 V), the emission is orange, characteristic of Meh-PPV. Under reverse bias (-4 V) the emission is green, characteristic of PPV. The EL spectra obtained under forward bias and under reverse bias are shown in Fig. 10.

Example 7

Light-emitting electrochemical cell (LEC) devices have been fabricated which emit white light, and red-green-blue full color generation has been demonstrated with these devices. The luminescent semiconducting polymers used were poly(9,9-dialkyl fluorene), PAF, and poly(decyloxy-phenelene), DO- PPP. Blends for the LECs were prepared with PEO:Li in pyridine with a ratio of EL polymer to PEO to Li (i.e. Li triflate) of 1:1:0.2.

The light emmitted from LECs made from PAF was white; fresh devices were "paper-white" turning to greenish-white after stressing for a few minutes. The devices were operated in the range from 3-6 V. The quantum efficiency of the PAF LECs was 0.2-0.3 % photons per electron. The light intensity reached 150-200 cds/m² at 5.6 V, bright enough for indoor displays. The turn-on time was 30 - 60 seconds, similar to that observed in other LECs made the same solid electrolyte from pyridine solutions. To enhance the blue component of the emission, LECs were fabricated with PAF/DO-PPP blends (2:1 and 1:1).

Red, green and blue emission was generated by using color filters to separate the different colors from the broad-band white light emission.

This Example demonstrates that LECs are excellent candidates for use as backlights in full color liquid crystal displays; the emitted light covers the entire visible spectrum and the backlight can be operated at low voltage.

Example 8 Full color capability was demonstrated for LECs; devices were fabricated which emitted red light, devices were fabricated which emitted green light, and devices were fabricated which emitted blue light. Thus, LECs were demonstrated to have the capabiity of use in three-color displays.

The red LECs were made from MEH-PPV:PEO:Li. The devices had efficiency ranging from 0.5 to 2% photon/electron. Typical luminance at 3 V was 200 - 250 cd/m². The stress life of these red devices was greater than 200 hours.

The green LECs were made from PPV:PEO:Li at ratios of 1: 1:0.2 (again, Li triflate was used). The efficiency was 0.2 - 0.5% photon/electron. The stresslife was greater than 72 hours at 3 V.

The blue LECs were made from DO-PPP; poly(decyloxy para- phenylene) at ratio of DO-PPP:PEO:Li of 1: 1:0.2 (again, Li triflate was used). The efficiency was 0.2 - 0.4% photon/electron. The brightness reached 200 cd/m² at 6 V.

Example 9 In order to enhance the ion mobility in the active layer of the light- emiting electrochemical cell (LEC), a liquid additive was included in the active layer. The goal was to create a gel which would significantly enhance the ion mobility. The additive was chosen to fulfill following requirements: The additive was a solvent for the Li PEO complex;

The additive had moderate to high dielectric constant (in order to solvate the ions);

The semiconducting and luminescent polymer must swell in in the presence of the additive, ideally forming a physical gel. The additive had high boiling point. The high boiling point enabled the removal of allother volatiles while leaving the selected additive in the film.

Ten grams of a 5 % w/w solution of polyethylene oxide (M.W. 400,000, Polysciences) solution in cyclohexanone was mixed into a lg solution of Li trifluoromethanesulfonate 0-i Triflate) (Aldrich) in cyclohexanone. 6 g of the resulting Li Triflate/PEO solution was mixed with 6 g of 0.5 % (w/w) MEH-PPV in cyclohexanone. The additive, 0.06 g octylcyanoacetate, was added into the solution. This provided a starting weight ratio of additive to total semiconductor plus ion source of 1:1.

The resulting solution was stined at RT for two hours. The solution was then spin-coated onto an ITO glass substrate at 3000 rpm spin speed inside the dry box. The coated substrate was heated on on a hot plate at 60-70°C for 10 hours inside dry box (filled by nitrogen). Finally an Al electrode, 1,000- 1,500 Angstrom thick, was deposited on the top of MEHPPV/Li Triflate/PEO layer in the vacuum evaporator. For comparison, devices were fabricated as in Example 1, but without adding the octyl cyanoacetate into the EL layer.

The I-V characteristic and light intensity vs voltage were measured (in a controlled atmosphere dry box) for devices fabricated with the octylcyanoacetate, additive; the same measurements were carried out on the device with no octylcyanoacetate. The data for the device containing the additive are shown in Fig. 11; the devices reached a brightness of approximately 400 cd/m² at 3 V. As shown in Fig. 12, the response time (turn- on time) was considerably improved, and the light output (at 2.7 V) was significanty greater for the device with the octylcyanoacetate additive. As 5 demonstrated in Fig. 13, the stress life at constant voltage (initial brightness of 300 cd/m²) was in excess of 83 hours.

Example 10 Devices were fabricated as in Example 9, but octylcyanoacetate was replaced by N-dodecyl pynolidinone, N,N-diethyldodecanamide, stearamide, 10 (+) dodecanolactone, 2-tert-butylcyclohexanone and propylene carbonate. The cunent, light intensity and quantum efficiency obtained with these devices at 3 V bias voltage are summarized in Table 2.

Table 2:

Device performance in the presence of different additives

Additive Device characteristic name bp Voltage Current Light Int. Efficienc additive (°C/mm Hg) (V) (mA) (Cd/m²) %

Octylcyanoacetate 95/0.1 3 6.9 472 1.89

N. ,N-diethyldodecananide 166/2 3 3.3 117 0.45

StSflramide 274/100 3 2.0 136 0.81

( ± )-Dodecanolactone 126/1 3 0.49 49 1.4

2-tert-butylcyclohexanone 62.5/4 3 3.2 240 0.92

N-dodecylpynolidinone 202/11 3 8.0 44 0.66 bis-(2-ethylhexyl)phthalate* 241/760 3 7.1 325 0.55

25

Dielectric constant of bis-(2-ethylhexyl)phthalate: e=5.8, so this is for comparison. To characterize the role of the additive in the morphology of the blend, a thin film (100-200A) of MEH-PPV + PEO/Li + additive (octylcyanoacetate) in the ratio 1:1: 1 was cast from solution and thoroughly dried. The film wa characterized by transmission electron microscopy (TEM). TEM micrographs such as Fig. 14 show that the morphology of the phase separated composite material is that of a bicontinuous three-dimensional interpenetrating network. The open immobile network of semiconducting polymer is filled with PEO/Li; typical dimensions are 10-lOθA.

Due to the bifunctional character of the additive (A:B, polar-nonpolar), the additive worked like a surfactant. During the process of liquid-liquid phase separation of the semiconducting polymer (for example, MEH-PPV) and the electrolyte (for example, PEO plus Li), the bifunctional A-B additive served to control the morphology of the phase separated composite material. Because of the surfactant-like character, a high surface area bicontinuous network moφhology was formed. Such a network morphology is ideal for the light- emitting electrochemical cell for the following reasons: (i) The semiconducting polymer forms a continuous network phase enabling electronic transport of injected electrons and holes, (ii) The electrolyte forms a continuous network phase enabling ion transport. (iii) The intimate intermixing of the bicontinuous network enables relatively rapid transport of ions from the electrolyte into the semiconducting polymer during electrochemical doping.

Example 11 Example 9 was repeated with a 1: 1 weight ratio of (PEO/Li) to MEH- PPV and weight ratio of octylcyanoacetate to total MEH-PPV plus PEO/Li varying among 0.1, 0.5, 1, 2, 4 and 6 respectively. The current, light intensity and quantum efficiency obtained with these devices at 3V bias voltage are summarized in Table 3. Table 3

Device performance iwth different Octylcyanoacetate to

(MEH-PPV+PEO/Li', weight ratio

OCA content Voltage Cunent Light Int. Efficiency

(w/w)* (V) (mA) (cd/m²) (%)

2 3 11.2 117 0.13

1 3 10.8 309 0.49

0.5 3 8.1 490 1.02

0.25 3 10.0 427 0.50

0.1 3 7.1 243 0.58

* Ratio of Octylcyanoacetate to total amount of polymers (MEH-PPV + PEO)

Example 12

Example 9 was repeated varying the weight ratio of PEO-Li to MEH-PPV with constant content of octylcyanoacetate [1 part octylcyanoacetate to 1 part w/w (PEO/Li + MEH-PPV)]. The cunent, light intensity and quantum efficiency obtained with these devices at 3V bias voltage are summarized in

Table 4.

Table 4

Device performance with different PEO/Li to MEHPPV weight ratios PEO/Li to MEH-PPV Voltage Cunent Light Int. Efficiency (w/w) (V) (mA) (cd/m²) (%)

2 3 1.8 107 0.71

1 3 8.4 470 0.68

0.5 3 5.7 437 0.93 0.25 3 6.6 248 0.45

Example 13

Example 9 was repeated with Au, Ag, Cu as cathode instead Al. The current, light intensity and quantum efficiency obtained with these devices at

3V bias voltage are summarized in Table 5. Table 5

Device performance with different cathodes Cathode Voltage Cunent Light Int. Efficiency (V) (mA) (cd/m²) (%)

Au 3 10.1 337 0.75

Ag 3 10.6 564 0.64

Cu 3 8.2 420 0.62

Al 3 6.8 428 0.76

These data indicate that excellent device performance can be obtained with a wide range of metals as cathode.

Example 14 Light-emitting electrochemical cells (LECs) were fabricated using the conducting emeraldine salt form of polyaniline 0?ANI) asone of the electrodes. In this example, the devices were made on felxible plastic substrates Q?ET) coated with a transparent conducting layer of indium/tin oxide 0"TO). A semitransparent layer of a blend of PANI and a low molecular weight polyester 0?ES), PAN PES 25% w/w, [Y. Yang, E. Westerweele, C. Zhang, P. Smith and A.J. Heeger, J. of Applied Physics 77:694 (1995)] were spin cast from solution in m-cresol (concentration 2%) onto the ITO. The MEH-PPV :PEO(Li) layer was cast from CHO solution onto the PANI(PES) electrode. In this example, aluminum was used as the the second electrode.

The results of cunent vs voltage (I-V) and light intensity vs voltage (I_L vs V) are plotted in Fig.s 15 and 16. Both the current and the electroluminescent emission turn on shaφly at V >2.1 V. Since the electronic energy band gap of MEH-PPV is approximately 2.1 eV, the device turned on when the applied voltage was sufficient to initiate electrochemical doping (p-type on one side and n-type on the opposite side of the active layer). As shown in Fig. 15, the luminance at 5V is approximately 400 cd/m². Other devices fabricated in the same way (data not shown) yielded luminances as high as 100 cd/m² at 4 V. The electroluminescence efficiency was measured; the quantum efficiency was 0.4 - 0.5 % photons/electron for I> 40 mA/m². Typical efficiency vs cunent data are shown for three separate devices (open circles, solid dots, and solid squares) in Fig. 16. Gel-forming liquid could advantageously be added to the composite layer, as set forth herein.

Example 15 Surface light-emitting electrochemical (LEEC) cell devices were fabricated by drop casting an active blend of semiconducting luminescent polymer (MEH-PPV) and ionic species source from solution onto a substrate with interdigitated gold electrodes fabricated by photolithographic methods. The gap size between the interstitial electrodes was 5 μ .; the total device size was 5mmx5mm (see Fig. 18). This interdigitated electrode pattern is equivalent to a single gap surface cell (see Fig. 17) with length of

500x5mm =2.5m and width of 5 μm. The equivalent length (L) to width (W) ratio (L/W) is 5x10^s. These values are exemplary. Typical gap widths are from about 1 μm to about 100 μm and preferably 1.5 μm to 50 μm and especially 2 μm to 40 μm. Electrode widths can be similar widths although wider electrodes can also be used.

Electrical data were obtained with a Keithley 236 Source-Measure Unit. The light illumination intensity and luminance were monitored with a Si photodiode calibrated with an integrating sphere.

Fig. 20 shows the luminance as a function of bias voltage for a 5μm gap device. Light was detected for V 5V. The emitted light reached an intensity of approximately 100 cd/m² at ~ 15 V. The color of the emitted light was characteristic of the emission from MEH-PPV and was similar to that observed in light-emitting electrochemical cell devices fabricated in the sandwich cell configuration. This example demonstrates low-voltage light emission from the light- emitting electrochemical cell in the surface cell configuration. Radiative recombination of the injected carriers occurs in the semiconducting, luminescent and ionically conducting blend; said radiation originating from the conjugated polymer as the luminescent medium. The turn-on voltage was somewhat greater than the energy gap of the luminescent polymer (the turn on was approximately 3 V, see Fig. 21; the energy gap of MEH-PPV is approx. 2.1 V). The operating voltage can be further reduced by fabricating polymer bhlend materials with hiher ionic conductivity and/or by utilizing interdigitated electrodes with nanower inter- electrode spacing.

Example 16 Surface light-emitting electrochemical cell devices were fabricated by drop casting the active blend from solution onto a glass substrate with interdigitated gold electrodes fabricated by photolithographic methods. The gap size between the interdigitated electrodes was 30 μm and the width of the individual electrodes was 10 μm. The total equivalent length of the electrode is 22.5 cm. The L/W ration is 550. The lateral dimensions of the device were 3mmx3mm; i.e. with a total area of 0.09 cm². The thickness of the drop cast film used in this example was greater than one micron.

A typical cunent© vs voltage (V), I-V dependence obtained from a 30 μm gap device is shown in Fig. 22. This asymmetric I-V curve (solid dots) was recorded after biasing the device in a given polarity for about one minute. The rectification ration is > 100 at 90 V, corresponding to a field strength of 3x10* V/cm. In contrast, the common field strength used for polymer LEDs in the sandwich configuration is ~ 10⁶V/cm.

This rectifying diode characteristic was erased by applying an alternating field with gradually-attenuated magnitude. The device was switched into the opposite direction by first applying the extemal field in the reverse direction. Also plotted in Fig. 22 is the I-V characteristic of the same device after applying field in reverse direction (open dots) for ~ 5 minutes. Nearly identical rectifying behavior is created in the opposite direction. The switching time can be reduced significantly with reduced gap size or with blends of higher ionic mobility. This example demonstrates that a rectifying diode can be generated in-situ and that said diode behavior can be erased in the surface configuration electrochemical cell with the application of extemal bias; i.e., the in-situ electrochemical doping results in the formation of a dynamic p-n junction within the polymer blend. This rectifying diode can be switched to the opposite direction with reversed bias.

This example and the results of Example 15 demonstrate that the operating voltage can be adjusted by changing the gap size; for large gaps, the high series resistance increases the turn-on and the operating voltage. Since the electroluminescent intensity is proportional to the total length of the electrode, an increase in the number of the interdigitated electrodes (achieved by reducing the gap size and the electrode width) results in an increase in the device luminance. Thus, the surface interdigitated electrode geometry provides two independent parameters which enable separate control of the operating voltage and the device brightness. The results of Examples 15 and 16 demonstrate that by means of geometric design of the electrode, one can achieve a desired luminescent pattern at a given bias.

Example 17 Surface light-emitting electrochemical cell devices were fabricated by drop casting the active blend from solution onto a substrate with interdigitated gold electrodes fabricated by photolithographic methods. The gap size between the interstitial electrodes was 30μm and the width of the individual electrodes was 10μm, as described in detail in Example 16.

The emitted light intensity was proportional to the injected current as demonstrated in Fig. 23. The EL efficiency of this device was measured to be ~0.3% photons/electron, as shown in Fig. 24. These data were collated only from light emitted from the front of the device. The EL efficiency in reference light emitting electrochemical cells in the sandwich configuration fabricated with the same LEEC blends was ~ 0.5-0.7% ph/el (with the blend thickness of ~ 1500A). Thus, including the EL emission in both front and back directions, the total EL efficiency in the surface cell was similar to that in sandwich cell device.

This example demonstrates that the electroluminescence efficiency (and thus, the probability of radiative recombination) in the surface electrochemical cell is similar to that in sandwich cells, although the gap sizes are quite different.

Example 18 Light-emitting electrochemical cells were fabricated in the surface cell configuration (30μm gap) utilizing transparent substrates. Light was emitted and observed from both front and back directions. The EL intensities collected in both front and back directions are plotted as a function of the drive cunent in Fig. 25. The semiconducting, luminescent and ionically conducting blend was drop-cast with thickness ~ 1 μm. Although relatively thick (with OD > 3 at 500nm), the blend film was transparent at the emission wavelength (below the polymer absorption edge). Conecting for the residual absorption by the polymer blend, the emission intensities into both front and back directions were similar. With blend films less than OD=3 at the absorption peak (i.e. with thicknesses less than 3000A), there will be no significant difference in light intensity emitted in the two directions. This example demonstrates a novel feature of the surface cell configuration. The emitted light can be viewed from both the front and the back side of the display; an obvious advantage for display applications.

Example 19 The shelf life of the surface EL cells was monitored for a period in excess of eight months. The device used in this Example was a 30μm gap surface cell; the device was stored in a controlled atmosphere (Argon) dry box during the entire test period. In Fig. 26, the efficiency at 1= 10 μA is plotted as function of storage time. The EL efficiency remains at the level of 0.3% ph/el during the entire period; no degradation of EL efficiency was observed.

Black spot formation has become one of the key issues limiting the shelf life of conventional organic and polymer LEDs. This black spot formation is associated with deterioration of the cathode/polymer interface. Since the light- emitting electrochemical cell is fabricated with the semiconductor polymer in its neutral form, since stable metals are used for the contacting electrodes, and since the electrochemically induced p-n junction is generated in-situ, all components are environmentally stable in the quiescent "off state. As a result, light-emitting electrochemical cells exhibit intrinsically longer shelf life when compared to conventional polymer light-emitting diodes. Indeed, light-emitting electrochemical cells in the surface cell configuration, fabricated with either Au or PANI electrodes, showed no black spots after storage for 8 months.

Claims

What is claimed is:

1. An electrochemical light emitting device comprising a pair of electrical contacts in spaced relationship to one another and in contact with an admixed composite layer comprising an immobile open connected network of organic semiconductor filled with ionic species source and gel-forming liquid.

2. The device of claim 1 wherein the gel-forming liquid is a high-boiling liquid compound having a polar region covalently bonded to a nonpolar region, the polar region conesponding to a parent compound having a dielectric constant greater than 10.

3. The device of claim 2 wherein the nonpolar region is a 2 to 30 carbon organic moiety.

4. The device of claim 3 wherein the organic moiety has from 3 to 24 carbons and is selected from unsubstituted alkyl, alkenyl, cycloalkyl, aralkyl, aryl and alkaryls, and substituted akyl, alkenyl, cycloalkyl, aralkyl, aryl and alkaryl.

5. The device of claim 3 wherein the polar region contains a group selected from cyano, nitro, amino, amide, ketone, aldehyde, carbonate, ester, lactones, pyrrolidinones, oxazolidinones, sulfoxide, sulfolane, phosphoramide.

6. The device of claim 2 wherein the high boiling liquid has a boiling point above 240 C at atmospheric temperature.

7. The device of claim 2 wherein the gel forming liquid is present in a gel-forming concentration of from about 0.1 to about 3 parts by weight, based on the total weight of organic semiconductor plus ionic species source.

8. The device of claim 7 wherein the organic semiconductor is polymeric/oligomeric and selected from conjugated polymers, conjugated oligomers, nonconjugated polymers having pi conjugated moieties and nonconjugated oligomers having pi conjugated moieties and mixtures thereof.

9. The device of claim 7 wherein the organic semiconductor is selected from laser dyes and polymers and oligomers containing laser dye moieties.

10. The device of claim 7 wherein the ionic species source comprieses admixed salt.

11. The device of claim 7 wherein the ionic species source comprises polymeric electrolyte.

12. The device of claim 7 wherein the ionic species source comprises a mixture of polyionimer or polyionophore and salt.

13. The device of claim 1 wherein the contacts are held in spaced relationship to one another by the composite layer.

14. The device of claim 13 wherein the electrodes are on opposite sides of the composite layer.

15. An electrochemical light-emitting device comprising an anode and a cathode in electrical contact with a film of electroluminescent material, said film having two sides with both of the anode and cathode in contact with the same side of the film thereby providing a surface cell configuration.

16. The device of claim 15 wherein the electroluminescent material is a composite of semiconductor and ionic species source.

17. The device of claim 16 wherein the semiconductor is organic semiconductor.

18. The device of claim 15 wherein the anode and cathode are present as interdigitated electrodes.

19. The device of claim 15 wherein there are a plurality of anodes and a plurality of cathodes all in contact the same side of the film.

20. The device of claim 19 wherein the plurality of anodes and plurality of cathodes are present as interdigitated electrodes.

21. The device of claim 20 wherein the interdigitated electrodes are pattemed with closely spaced anode-cathode pairs separated from other closely spaced pairs by a larger distance.

22. The device of claim 1 wherein the electrodes are anode and cathode on the same side of the composite layer.

23. The device of claim 1 wherein the anode and cathode are present as interdigitated electrodes.

24. The device of claim 1 wherein there are a plurality of anodes and a plurality of cathodes all in contact the same side of the composite layer.

25. The device of claim 24 wherein the plurality of anodes and plurality of cathodes are present as interdigitated electrodes.

26. The device of claim 25 wherein the interdigitated electrodes are pattemed with closely spaced anode-cathode pairs separated from other closely spaced pairs by a larger distance.

27. An electrochemical light emitting device comprising a pair of contact layers contact with an admixed composite layer comprising immobile conjugated polymeric or oligomeric organic semiconductor, polymeric or oligomeric electrolye ionic species source and a gel-forming concentration of high-boiling gel-forming liquid.

28. The device of claim 27 wherein the weight proportions of immobile semiconductor and ionic species source in the admixed composite layer range are from about 0.1:0.9 to about 0.9:0.1 and the gel-forming liquid is present in an amount of from about 0.1 to 3 parts by weight based on the total weight of organic semiconductor plus ionic species source.

29. The device of claim 27 wherein at least one of the contact layers is substantially transparent at the wavelength at which the device emits light.

30. The device of claim 27 wherein both contact layers are in contact with a supporting substrate.

31. A system for light emission comprising a power supply applying an effective powering voltage between 0.1 and 110 volts across a pair of anode and cathode contact layers in contact with an admixed composite layer comprising immobile semiconductor, ionic species source and a gel-forming concentration of gel-forming liquid, the effective powering voltage thereby reducing the immobile semiconductor at the cathode to form an n-type region containing negatively charged carriers and oxidizing the immobile semiconductor at the anode to form a /Hype region containing positively charged carriers, said negatively charged carriers and positively charged carriers propagating in the composite layer under the influence of the applied voltage and combining to form charge carrier pairs which decay radiatively in the composite layer so that light is emitted from the layer.

32. The system of claim 31 wherein the gel-forming liquid is a high- boiling liquid compound having a polar region covalently bonded to a nonpolar region, the polar region corresponding to a parent compound having a dielectric constant greater than 10.

33. The system of claim 32 wherein the composite layer is from about 5 nm to about 10 um thick, the admixed composite is an open connected network of immobile conjugated organic semiconductor filled with the ionic species source and gel-forming liquid and the weight proportions of immobile conjugated organic semiconductor and ionic species source in the admixed composite thin layer range are from about 0.1:0.9 to about 0.9:0.1, with the proportion of gel-forming liquid being from about 0.1 to about 3 parts by weight based on the total weight of organic semiconductor plus ionic species source.

34. The system of claim 33 wherein the ionic species source comprises admixed salt.

35. The system of claim 33 wherein the ionic species source comprises polymeric electrolyte.

36. The system of claim 33 wherein the ionic species source comprises a mixture of polyionimer or polyionophore and salt.

37. The system of claim 33 wherein the powering voltage is less than 5 volts.

38. A process for generating useful levels of light comprising applying an effective powering voltage across a pair of anode and cathode contact layers in contact with an admixed composite layer comprising immobile semiconductor and ionic species source and gel-forming liquid, the effective powering voltage thereby reducing the immobile semiconductor at the cathode to form an n-type region containing negatively charged carriers and oxidizing the immobile semiconductor at the anode to form a p-typ region containing positively charged carriers, said negatively charged carriers and positively charged carriers propagating in the composite layer under the influence of the applied voltage and combining to form charge carrier pairs which decay radiatively in the composite layer so that light is emitted from the layer and using the emitted light.

39. A light-emitting electrochemical cell in a surface cell configuration comprising a substrate having a substrate surface, at least one anode and at least one cathode, present as an interdigitated pattern upon said substrate surface, and a film of electroluminescent material, itself comprising conjugated organic polymer admixed with ionic species source, in contact with said substrate surface and the interdigitated pattern of anode and cathode thereupon.

40. A method for preparing a light-emitting device comprising depositing a pattern of at least one anode and at least one cathode onto one side of a two- sided film of electroluminescent material.

41. A method for preparing a light-emitting device comprising a. depositing a pattern of at least one anode and at least one cathode onto a substrate surface thereby creating an anode and cathode-bearing substrate surface and b. applying a layer of electroluminescent material to the anode and cathode- bearing substrate surface.

42. The method of claim 41 wherein the anode and cathode are metal and the metal electrodes are deposited by vacuum deposition.

43. The method of claim 41 wherein the anode and cathode are metal, the pattem of anodes and cathodes is an interdigitated pattem and the depositing is vacuum depositing through a shadow mask.

44. The method of claim 41 wherein said depositing is screen print depositing.

45. The method of claim 41 wherein said depositing is photolithographic depositing.

46. The method of claim 41 wherein the interdigitated electrodes are fabricated by laser ablation.