Method of fabricating visible light emitting diodes from soluble semiconducting polymers

Field of the Invention

0001This invention relates generally to light-emitting diodes and their fabrication. More particularly, it concerns light-emitting diodes fabricated from semiconducting (conjugated) polymers which are soluble in common organic solvents, and yet more particularly to the fabrication of such diodes on flexible polymer substrates.

Background of the Invention

0002Solid-state light-emitting diodes (LEDs) have found widespread application in displays, as well as in a variety of less common applications. Currently, LEDs are fabricated from conventional semiconductors; for example, gallium arsenide (GaAs), typically doped with aluminum, indium, or phosphorus. Using this technology, it is very difficult to make large area displays. In addition, the LEDs made of these materials are typically limited to the emission of light at the long wavelength end of the visible spectrum. 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 LEDs. (See references 1-6). The need for relatively high voltages (i.e., voltages incompatible with digital electronics) for the onset of light emission has been a hindrance to the commercialization of LEDs fabricated from organic materials.

0003The utilization of semiconducting organic polymers (i.e., conjugated polymers) in the fabrication of LEDs expands the use of organic materials in electroluminescent devices and expands the possible applications for conducting polymers into the area of active light sources, (see Reference 7) with the possibility of significant advantages over existing LED technology. Controlling the energy gap of the polymer, either through the judicious choice of the conjugated backbone structure or through side-chain functionalization, should make possible the emission of a variety of colors throughout the visible spectrum.

0004In the prior art, Tomozawa et al (see Reference 8) disclosed diodes fabricated by casting semiconducting polymers from solution.

0005Also in the art, Burroughs et al (see Reference 7) disclosed a multi-step process in the fabrication of LED structures characterized as follows: <ul id="ul0001" list-style="none" compact="compact"><li>1) A glass substrate is utilized. The substrate is precoated with a transparent conducting layer of indium/tin oxide (ITO). This ITO coating, having high work function serves as the ohmic hole-injecting electrode.</li><li>2) A soluble precursor polymer to the conjugated polymer, poly(phenylene vinylene), PPV, is cast from solution onto the substrate as a thin, semi-transparent layer (approximately 100-200 nm).</li><li>3) The precursor polymer is converted to the final conjugated PPV by heat treating the precursor polymer (already formed as a thin film on the substrate) to temperatures in excess of 200°C while pumping in vacuum.</li><li>4) The negative, electron-injecting contact is fabricated from a low work function metal such as aluminum, or magnesium-silver alloy; said negative electrode acting as the rectifying contact in the diode structure.</li></ul> The resulting devices showed asymmetric current versus voltage curves indicative of the formation of a diode, and the diodes were observed to emit visible light under conditions of forward bias at bias voltages in excess of about 14 V with quantum efficiencies up to 0.05%.

0006The methods of Burroughs et al, therefore, suffer a number of specific disadvantages. Because of the use of a rigid glass substrate, the resulting LED structures are rigid and inflexible. The need for heating to temperatures in excess of 200°C to convert the precursor polymer to the final conjugated polymer precludes the use of flexible transparent polymer substrates, such as, for example, polyethyleneterephthalate, polystyrene, polycarbonate and the like, for the fabrication of flexible LED structures with novel shapes and forms. The need for heating to temperatures in excess of 200°C to convert the precursor polymer to the final conjugated polymer has the added disadvantage of possibly creating defects in the conjugated polymer and in particular at the upper surface of the conjugated polymer which forms the rectifying contact with the low work function metal.

0007Thus, the ability to fabricate light-emitting diodes from organic materials and in particular from polymers, remains seriously limited.

0008Tang et al (Reference 12) discloses an electroluminescent device comprised of, in sequence, an anode, an organic hole transporting zone, an organic electron transporting zone and a cathode. The cathode is composed of a layer of a plurality of metals other than alkali metals, at least one of said metals having a work function of less than 4 eV.

0009Nakano et al (Reference 13) is a European patent application that was published after the priority date of the present application but has a filing date before the priority date of the present application. The reference discloses an electroluminescence device having a light emitting layer and a charge transport layer disposed between a pair of electrodes at least one of which is transparent or semi-transparent, the light emitting layer comprising a conjugated polymer having a repeating unit of general formula (1):         -Ar₁-CH=CH-     (1) wherein Ar₁ represents an aromatic hydrocarbon group of 6 to 14 carbon atoms which is unsubstituted or substituted by one or two hydrocarbon or alkoxy groups of 1 to 22 carbon atoms.

References

0010<ul id="ul0002" list-style="none" compact="compact"><li>1. P.S. Vincent, W.A. Barlow, R.A. Hann and G.G. Roberts, Thin Solid Films, <u>94,</u> 476 (1982).</li><li>2. C.W. Tang, S.A. Van Syke, Appl. Phys. Lett. <u>51</u>, 913(1987).</li><li>3. C.W. Tang, S.A. Van Syke and C.H. Chen, J. Appl. Phys. <u>65</u>, 3610 (1989).</li><li>4. C. Adachi, S. Tokito, T. Tsutsui, and S. Saito, Appl. Phys. Lett. <u>55</u>, 1489 (1989).</li><li>5. C. Adachi, S. Tokito, T. Tsutsui, and S. Saito, Appl. Phys. Lett. <u>56</u>, 799 (1989).</li><li>6. M. Nohara, M. Hasegawa, C. Hosohawa, H. Tokailin, T. Kusomoto, Chem. Lett. 189 (1990).</li><li>7. J.H. Burroughs, D.D.C. Bradley, A.R. Brown, R. N. Marks, K. Mackay, R.H. Friend, P.L. Burns and A.B. Holmes, Nature <u>347,</u> 539 (1990).</li><li>8. H. Tomozawa, D. Braun, S.D. Phillips, R. Worland, A.J. Heeger, and H. Kroemer, Synth. Met. <u>28,</u> C687 (1989).</li><li>9. F. Wudl, P.-M. Allemand, G. Srdanov, Z. Ni, and D. McBranch, in Materials for Non-linear Optics: Chemical Perspectives (to be published in 1991).</li><li>10. S.M. Sze, Physics of Semiconductor Devices (John Wiley &amp; Sons, New York, 1981).</li><li>11. a. T.W. Hagler, K. Pakbaz, J. Moulton, F. Wudl, P. Smith, and A.J. Heeger, Polym, Commun. (in press). b. T.W. Hagler, K. Pakbaz, K. Voss and A.J. Heeger, Phys. Rev. B. (in press).</li><li>12. United States Patent No. 4,885,211 (Tang et al).</li><li>13. EP-A-443 861 (Nakano et al).</li></ul>

Summary of the Invention

0011It is accordingly an object of the present invention to overcome the aforementioned disadvantages of the prior art and, primarily, to provide light-emitting diodes fabricated from semiconducting polymers which are soluble in the conjugated form and therefore require no subsequent heat treatment at elevated temperatures.

0012It is additionally an object of the present invention to utilize the processing advantages associated with the fabrication of diode structures from soluble semiconductor polymers cast from solution to enable the fabrication of large active areas.

0013It is additionally an object of the present invention to provide light-emitting diodes fabricated from semiconducting polymers using flexible organic polymer substrates.

0014It is additionally an object of the present invention to provide methods for the fabrication of light-emitting diodes fabricated from semiconducting polymers which turn on at bias voltages compatible with digital electronics (i.e, at voltages less than 5 volts).

0015Additional objects advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art on examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.

0016In one aspect, this invention provides a process for fabricating light-emitting diodes (LEDs) which are able to emit visible light, which process comprises the steps of: <ul id="ul0003" list-style="none" compact="compact"><li>(i) precoating a substrate with a transparent conducting first layer, said first layer having high work function and being able to serve as an ohmic hole-injecting electrode;</li><li>(ii) casting directly upon said first layer directly from solution, a thin transparent layer of a soluble conjugated polymer; and</li><li>(iii) fabricating directly onto the conjugated polymer film from calcium or a lower work function metal, a negative electron-injecting contact, said contact being able to serve as the rectifying contact in the diode structure.</li></ul>

0017In another embodiment, an alternative process for fabricating light-emitting diodes which are able to emit visible light is provided. This process comprises the steps of: <ul id="ul0004" list-style="none" compact="compact"><li>(i) casting a free-standing semi-transparent film of a soluble conjugated polymer from solution, said film being able to serve as a luminescent semiconducting polymer and simultaneously as a substrate;</li><li>(ii) coating the free-standing, conjugated polymer film on one side with a transparent conducting first layer, said transparent conducting first layer having a hign work function and being able to serve as an ohmic hole-injecting electrode; and</li><li>(iii) fabricating onto the other side of the conjugated polymer film from calcium or a lower work function metal, a negative electron-injecting contact, said contact being able to serve as the rectifying contact in the diode structure.</li></ul>

0018In yet an additional embodiment, this invention provides a process for fabrication of a light emitting diode which is able to emit polarized visible light, said process comprising the steps: <ul id="ul0005" list-style="none" compact="compact"><li>(i) gel-processing a soluble conjugated polymer in admixture with an ultra-high molecular weight carrier polymer to provide an oriented free-standing film in which the conjugated polymer is chain-aligned, said film being able to serve as a luminescent semiconducting polymer which emits polarized visible light:</li><li>(ii) coating the free-standing conjugated polymer film on one side with a transparent conducting first layer, said transparent conducting first layer having a high work function and serving as an ohmic hole-injecting electrode; and</li><li>(iii) fabricating onto the other side of the conjugated polymer film from calcium or a lower work function metal, a negative electron-injecting contact, said contact being able to serve as the rectifying contact in the diode structure.</li></ul>

0019In another general aspect this invention provides the LEDs fabricated by any of these processes. These LEDs and their structures as herein defined (with and without a supporting substrate) is depicted schematically and in cross-section in Figures 1 and 2 of the drawing. In a more particular aspect of this invention, the LED devices employ poly(2-methoxy,5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene), MEH-PPV, as the conjugated polymer. MEH-PPV offers the advantage of being a conjugated polymer which is soluble in organic solvents. LED device fabrication is simplified because of the direct casting of the conjugated polymer from solution.

0020Surprisingly, it was found that by using calcium as the low work function rectifying contact, and by using ITO coated PET films as the substrate, flexible LED structures are fabricated which benefit from the excellent mechanical properties of both the polymer substrate and the conjugated polymer semiconducting layer and which exhibit the advantageous characteristics of a turn-on voltage reduced to 3-4 volts (i.e TTL compatible), and a quantum efficiency which is improved by more than an order of magnitude to values of approximately 1%.

0021Specific advantages of this invention over the prior art include the following: <ul id="ul0006" list-style="none" compact="compact"><li>(i) Because the luminescent semiconducting polymer is soluble in its final conjugated form, there is no need for heat treatment at elevated temperatures. This greatly simplifies the fabrication procedure and enables a continuous manufacturing process.</li><li>(ii) Since the luminescent semiconducting polymer layer can be cast onto the substrate directly from solution at room temperature, the LED structure can be fabricated on a flexible transparent polymer substrate. These polymer films are manufactured as large area continuous films. Thus, the use of flexible polymer films as substrate enables the fabrication of large area polymer LEDs using either a batch process or a continuous process.</li><li>(iii) The use of Calcium as the low work function contact onto MEH-PPV as the luminescent polymer leads to unexpected improvements in the efficiency of the device and in the compatibility of the device with modern digital electronic circuitry.</li></ul>

Detailed Description of the Invention

Brief Description of the Drawing

0022This invention is described with reference being made to the drawing in which <ul id="ul0007" list-style="none" compact="compact"><li>Figure 1 is a schematic cross-sectional view of a light-emitting diode of this invention having a supporting substrate; and</li><li>Figure 2 is a schematic cross-sectional view of a light-emitting diode of this invention not having a supporting substrate.</li></ul>

The Substrates

0023In some embodiments, the conjugated polymer-based LEDs are prepared on a substrate. The substrate should be transparent and nonconducting. It can be a rigid material such as a rigid plastic including rigid acrylates, carbonates, and the like, rigid inorganic oxides such as glass, quartz, sapphire, and the like. It can also be a flexible transparent organic polymer such as polyester - for example polyethyleneterephthalate, flexible polycarbonate, poly (methyl methacrylate), poly(styrene) and the like.

0024The thickness of this substrate is not critical.

The Conjugated Polymer

0025The invention provides LEDs based on conjugated polymers.

0026In one embodiment the conjugated polymer is cast directly from a solution onto a precooled substrate to form a substrate-supported film.

0027In another, the conjugated polymer is present as a free-standing film.

0028In a third embodiment, the conjugated polymer is present as a component of a gel-processed admixture with a carrier polymer and the film is formed from this admixture. This embodiment offers an easy way to obtain aligned conjugated polymer structures which lead to LEDs which can emit polarized light.

0029The conjugated polymers used herein include soluble conjugated polymers known in the art. These include, for example, poly(2-methoxy,5-(2'-ethyl-hexyloxy)-p-phenylenevinylene) or "MEH-PPV", P3ATs, poly (3-alkylthiophenes) (where alkyl is from 6 to 16 carbons), such as poly(2,5-dimethoxy-p-phenylene vinylene)-"PDMPV", and poly(2,5-thienylenevinylene); poly(phenylenevinylene) or "PPV" and alkoxy derivatives thereof; and polyanilines. Of these, the MEH-PPV materials are preferred. The preparation of MEH-PPV is given in the examples herein.

0030The conjugated polymer can be deposited or cast directly from solution. The solvent employed is one which will dissolve the polymer and not interfere with its subsequent deposition.

0031Typically, organic solvents are used. These can include halohydrocarbons such as methylene chloride, chloroform, and carbon tetrachloride, aromatic hydrocarbons such as xylene, benzene, toluene, other hydrocarbons such as decaline, and the like. Mixed solvents can be used, as well. Polar solvents such as water, acetone, acids and the like may be suitable. These are merely a representative exemplification and the solvent can be selected broadly from materials meeting the criteria set forth above.

0032When depositing the conjugated polymer on a substrate, the solution can be relatively dilute, such as from 0.1 to 20% w in concentration, especially 0.2 to 5% w. Film thicknesses of 50-400 and 100-200 nm are used.

The Carrier Polymer.

0033In some embodiments, the conjugated polymer is present in admixture with a carrier polymer.

0034The criteria for the selection of the carrier polymer are as follows. The material should allow for the formation of mechanically coherent films, at low concentrations, and remain stable in solvents that are capable of dispersing, or dissolving the conjugated polymers for forming the final film. Low concentrations of carrier polymer are preferred in order to minimize processing difficulties, i.e., excessively high viscosity or the formation of gross inhomogeneities; however the concentration of the carrier should be high enough to allow for formation of coherent structures. Preferred carrier polymers are high molecular weight (M.W. >100,000) flexible chain polymers, such as polyethylene, isotactic polypropylene, polyethylene oxide, polystyrene, and the like. Under appropriate conditions, which can be readily determined by those skilled in the art, these macromolecular materials enable the formation of coherent structures from a wide variety of liquids, including water, acids, and numerous polar and nonpolar organic solvents. Films or sheets manufactured using these carrier polymers have sufficient mechanical strength at polymer concentrations as low as 1%, even as low as 0.1%, by volume to enable the coating and subsequent processing as desired.

0035Mechanically coherent films can also be prepared from lower molecular weight flexible chain polymers, but generally, higher concentrations of these carrier polymers are required.

0036Selection of the carrier polymer is made primarily on the basis of compatibility of the conjugated polymer, as well as with the solvent or solvents used. For example, blending of polar conducting polymers generally requires carrier structures that are capable of co-dissolving with or absorbing polar reactants. Examples of such coherent structures are those comprised of poly(vinyl alcohol), poly(ethylene oxide), poly-para(phenylene terephthalate), poly-para-benzamide, etc., and suitable liquids. On the other hand, if the blending of the final polymer cannot proceed in a polar environment, nonpolar carrier structures are selected, such as those containing polyethylene, polypropylene, poly(butadiene), and the like.

0037Turning now to the issue of concentration, it is of crucial importance that the carrier structure formed have sufficient mechanical coherence for further handling during the formation of the final polymer blend. Therefore, the initial concentration of the carrier polymer generally is selected above 0.1% by volume, and more preferably above about 0.75% by volume. On the other hand, it is not desirable to select carrier polymer concentrations exceeding 90% by volume, because this has a diluting effect on the final conjugated polymer composite product. More preferably, the concentration of the carrier polymer in the solution is below 50% by volume, and still more preferably below 25% by volume.

0038Thus, solution is provided by dissolving a selected carrier polymer and conjugated polymer in a compatible solvent (or mixed solvents) to a predetermined concentration (using the aforementioned guidelines). In the present process the "compatible solvent" is a solvent system into which a desired quantity of soluble conjugated polymer (or soluble precursor polymer) can be dissolved. The solvent system is also one in which the carrier polymer is substantially soluble, and will not interfere with the subsequent structure formation process. The carrier solution is formed into selected shape, e.g. a fiber, film or the like, by extrusion or by any other suitable method. The solvent is then removed (through evaporation, extraction, or any other convenient method).

0039Gels can be formed from the carrier conjugate solution in various ways, e.g., through chemical crosslinking of the macromolecules in solution, swelling of crosslinked macromolecules, thermoreversible gelation, and coagulation of polymer solutions. In the present invention, the two latter types of gel formation are preferred, although under certain experimental conditions, chemically crosslinked gels may be preferred.

0040Thermoreversible gelation refers to the physical transformation of polymer solution to polymer gel upon lowering lower of the temperature of a homogeneous polymer solution (although in exceptional cases a temperature elevation may be required). This mode of polymer gelation requires the preparation of a homogeneous solution of the selected carrier polymer in an appropriate solvent according to standard techniques known to those skilled in the art. The polymer solution is cast or extruded into a fiber, rod or film form, and the temperature is lowered to below the gelation temperature of the polymer in order to form coherent gels. This procedure is well known and is commercially employed, e.g., for the formation of gels of high molecular weight polyethylene in decalin, paraffin oil, oligomeric polyolefins, xylene, etc., as precursors for high strength polyolefin fibers and films.

0041"Coagulation" of a polymer solution involves contacting the solution with a nonsolvent for the dissolved polymer, thus causing the polymer to precipitate. This process is well known, and is commercially employed, for example, in the formation of rayon fibers and films, and spinning of high-performance aramid fibers, etc.

0042Frequently, it is desirable to subject the carrier polymer/conducting polymer composite to mechanical deformation, typically by stretching, during or after the initial forming step. Deformation of polymeric materials is carried out in order to orient the macromolecules in the direction of draw, which results in improved mechanical properties. Maximum deformations of thermoreversible gels are substantially greater than melt processed materials.( P. Smith and P.J. Lemstra, Colloid and Polym. Sci., <u>258,</u> 891, (1980).) The large draw ratios possible with thermoreversible gels are also advantageous if composite materials may be prepared with materials limited in their drawability due to low molecular weights. In the case of conducting polymers, not only do the mechanical properties improve, but, more importantly, the electrical conductivity also often displays drastic enhancement by tensile drawing and the orientation of the conjugated polymer gives rise to LEDs which will emit polarized light because of the orientation.

The Transparent Conducting First Layer.

0043The conjugated polymer layer of the LEDs of this invention is bounded on one surface by a transparent conducting first layer.

0044When a substrate is present, this layer is between the substrate and the conjugated polymer layer. This first layer is a transparent conductive layer made of a high work function material, that is a material with a work function above 4.5 eV. This layer can be a film of an electronegative metal such as gold or silver, with gold being the preferred member of that group. It can also be formed of a conductive metal-metal oxide mixture such as indium-tin oxide.

0045These layers are commonly deposited by vacuum sputtering (RF and Magnetron) electron beam evaporation, thermal vapour deposition, chemical deposition and the like.

0046The ohmic contact layer should be low resistance: preferably less than 300 ohms/square and more preferably less than 100 ohms/square.

<u>The Electron Injecting Contact</u>

0047On the other side of the conjugated polymer film an electron-injecting contact is present. This is fabricated from calcium or a lower work function metal such as an alloy. Barium is a typical lower work function metal. These electrodes are applied by using methods well-known to the art (e.g. evaporated, sputtered, or electron-beam evaporation) and are able to act as the rectifying contact in the diode structure.

<u>Examples</u>

0048This invention will be further described by the following examples. These are intended to embody the invention but not to limit its scope.

Example 1

0049This example involves the preparation of poly (2-methoxy,5-(2'-ethylhexyloxy)-p-phenylenevinylene) "MEH-PPV".

Monomer Synthesis

1. Preparation of 1-Methoxy-4-(2-Ethyl-Hexyloxy) Benzene

0050A solution of 24.8 g (0.2 mole) of 4-methoxy phenol in 150 ml dry methanol was mixed under nitrogen with 2.5 M solution of sodium methoxide (1.1 equivalent) and refluxed for 20 min. After cooling the reaction mixture to room temperature, a solution of 2-ethylbromohexane (42.5 ml, 1.1 equivalent) in 150 ml methanol was added dropwise. After refluxing for 16 h, the brownish solution turned light yellow. The methanol was evaporated and the remaining mixture of the white solid and yellow oil was combined with 200 ml of ether, washed several times with 10% aqueous sodium hydroxide, H₂O and dried over MgSO₄. After the solvent was evaporated, 40 g (85%) of yellow oil was obtained. The crude material was distilled under vacuum (2.2 mm Hg, b.p. 148-149°C), to give a clear, viscous liquid. ¹H NMR (CDCl₃) δ 6.98 (4H, s, aromatics), 3.8 (5H, t, O-CH₂, O-CH₃), 0.7-1.7 (15 H, m, C₇H₁₅. IR (NaCI plate) 750, 790, 825, 925, 1045, 1105, 1180, 1235, 1290, 1385, 1445, 1470, 1510, 1595, 1615, 1850, 2030, 2870, 2920, 2960, 3040. MS. Anal. Calc. for C₁₅H₂₄O₂: C, 76.23; H, 10.23; O, 13.54. Found: C, 76.38; H, 10.21; O, 13.45.

2. Preparation of 2,5-bis(Chloromethyl)-1-Methoxy-4-(2-Ethyl-Hexyloxy)Benzene

0051To the solution of 4.9 g (20.7 mmoles) of compound (1) in 100 ml p-dioxane cooled down to 0-5°C, 18 ml of conc. HCI, and 10 ml of 37% aqueous formalin solution was added. Anhydrous HCI was bubbled for 30 min, the reaction mixture warmed up to R.T. and stirred for 1.5-2 h. Another 10 ml of formalin solution was added and HCI gas bubbled for 5-10 min at 0-5°C. After stirring at R.T. for 16 h, and then refluxed for 3-4 h. After cooling and removing the solvents, an off-white "greasy" solid was obtained. The material was dissolved in a minimum amount of hexane and precipitated by adding methanol until the solution became cloudy. After cooling, filtering and washing with cold methanol, 3.4 g (52%) of white crystalline material (mp 52-54°C) was obtained. ¹H NMR (CDCl₃) δ 6.98 (2H, s, aromatics), 4.65 (4H, s, CH₂-Cl), 3.86 (5H, t, O-CH₃, O-CH₂), 0.9-1.5 (15H, m, C₇H₁₅), IR (KBr) 610, 700, 740, 875, 915, 1045, 1140, 1185, 1230, 1265, 1320, 1420, 1470, 1520, 1620, 1730, 2880, 2930, 2960, 3050. MS. Anal. Calc. for C₁₇H₂₆O₂Cl₂: C, 61.26; H, 7.86; O, 9.60; Cl, 21.27. Found: C, 61.31; H, 7.74; O, 9.72; Cl, 21.39.

Polymerization

Preparation of Poly(1-Methoxy-4- (2 -Ethylhexyloxy-2,5-Phenylenevinylene) MEH-MPV

0052To a solution of 1.0 g (3 mmol) of 2,5-bis(chloromethyl)-methoxy-4-(2-ethylhexyloxy)benzene in 20 ml of anhydrous THF was added dropwise a solution of 2.12 g (18 mmol) of 95% potassium <u>tert</u>-butoxide in 80 ml of anhydrous THF at R.T. with stirring. The reaction mixture was stirred at ambient temperature for 24 h and poured into 500 ml of methanol with stirring. The resulting red precipitate was washed with distilled water and reprecipitated from THF/methanol and dried under vacuum to afford 0.35 g (45% yield). UV (CHCl₃) 500. IR (film) 695, 850, 960, 1035, 1200, 1250, 1350, 1410, 1460, 1500, 2840, 2900, 2940, 3040. Anal. Calc. for C₁₇H₂₄O₂: C, 78.46; H, 9.23. Found: C, 78.34; H, 9.26.

0053Molecular weight (GPC vs. polystyrene) 3 x 10⁵. Inherent viscosity - 5 dl/g (but time dependent due to the tendency to form aggregates). As is the case with a few other stiff chain polymers, the viscosity increases with standing, particularly in benzene. The resulting solution is therefore thixotropic.

0054The conjugated polymer is highly colored (bright red-orange).

Example 2

0055Preparation of MEH-PPV via a precursor polymer route.

Monomer Synthesis

0056The monomer synthesis is exactly the same as in Example 1.

Polymerization of the Precursor Polymer and Conversion to MEH-PPV

0057A solution of 200 mg (0.39 mmol) of the monomer salt of Example 1 in 1.2 ml dry methanol was cooled to 0°C for 10 min and a cold degassed solution of 28 mg (1.7 equivalents) of sodium hydroxide in 0.7 ml methanol was added slowly. After 10 min the reaction mixture became yellow and viscous. The above mixture was maintained at 0°C for another 2-3 h and then the solution was neutralized. A very thick, gum-like material was transferred into a Spectrapore membrane (MW cutoff 12,000-14,000) and dialyzed in degassed methanol containing 1% water for 3 days. After drying in vacuo, 70 mg (47%) of "plastic" yellow precursor polymer material was obtained. UV (CHCl₃) 365. IR (film) 740, 805, 870, 1045, 1075, 1100, 1125, 1210, 1270, 1420, 1470, 1510, 2930, 2970, 3020. Soluble in C₆H₅Cl, C₆H₃Cl₃, CH₂Cl₂, CHCl₃, Et₂O, THF. Insoluble in MeOH.

0058The precursor polymer was converted to the conjugated MEH-PPV by heating to reflux (approx. 214°C) in 1,2,4-trichlorobenzene solvent. The product was identical with the material obtained in Example 1.

Comparative Example 1

0059Light-emitting diodes (LEDs) were fabricated consisting of a rectifying indium (work function = 4.2 eV, Reference 10) contact on the front surface of an MEH-PPV film which is deposited by spin-casting from dilute tetrahydrofuran solution containing 1% MEH-PPV by weight onto a glass substrate. The resulting MEH-PPV films have uniform surfaces with thicknesses near 120nm. The glass substrate had been previously coated with a layer of indium/tin-oxide to form an "ohmic" contact. The Indium contact is deposited on top of the MEH-PPV polymer film by vacuum evaporation at pressures below 4x10^-7 Torr yielding active areas of 0.04 cm².

0060While ramping the applied bias, yellow-orange light becomes visible to the eye just below 9 V forward bias (no light is observed under reversed bias). Above 15 V, the rectification ratio of the diode exceeds 10⁴.

0061The EL spectra, obtained with 3 V AC superposed (at 681 Hz) on 13V forward bias, showed characteristic spectral features similar to those observed in the photoluminescence of MEH-PPV.¹¹ The room temperature electroluminescence peaks near 2.1 eV with a hint of a second peak above 1.9 eV. At 90K, the intensity increases and shifts to the red, and the two peaks become clearly resolved.

0062The electroluminecence intensity was measured as a function of current flow under increasing forward bias. The quantum efficiency was determined with a calibrated Silicon photodiode and corrected for the spectral response and the solid angle of the collecting optics. The measured quantum efficiency at 0.8 mA is ≈5x10^-4 photons per electron for Indium electrodes.

Example 3

0063Light-emitting diodes (LEDs) were fabricated consisting of a rectifying calcium (work function = 3 eV, Reference 10) contact on the front surface of an MEH-PPV film which is deposited by spin-casting from dilute solution onto a glass substrate. The resulting MEH-PPV films have uniform surfaces with thicknesses near 120nm. The glass substrate has been partially coated with a layer of indium/tin-oxide to form an "ohmic" contact. The calcium contact is deposited on top of the MEH-PPV polymer film by vacuum evaporation at pressures below 4x10^-7 Torr yielding active areas of 0.04 cm².

0064For the calcium/MEH-PPV diodes, rectification ratios as high as 10⁵ are achieved.

0065While ramping the applied bias, yellow-orange light becomes visible to the eye just above 3 V forward bias (no light is observed under reversed bias). The quantum efficiency was determined with a calibrated Silicon photodiode and corrected for the spectral response and the solid angle of the collecting optics. The measured quantum efficiency at 0.8 mA is =7x10^-3 photons per electron for calcium electrodes (i.e., nearly 1%!!). The emission from the Calcium/MEH-PPV LEDs is bright and easily seen in a lighted room at 4V forward bias.

Example 4

0066Light-emitting diodes (LEDs) were fabricated consisting of a rectifying calcium (work function = 3 eV, Reference 10) contact on the front surface of an MEH-PPV film which is deposited by spin-casting from dilute solution onto a flexible transparent polyethyleneterephthalate (PET) film (7 mils thickness) as substrate. The resulting MEH-PPV films on PET have uniform surfaces with thicknesses near 120nm. The PET substrate is precoated with a layer of indium/tin-oxide to form an "ohmic" contact. The calcium rectifying contact is deposited on top of the MEH-PPV polymer film by vacuum evaporation at pressures below 4x10^-7 Torr yielding active areas of 0.04 cm².

0067For the calcium/MEH-PPV diodes, rectification ratios as high as 10³ are achieved.

0068While ramping the applied bias, yellow-orange light becomes visible to the eye just above 9 V forward bias (no light is observed under reversed bias). The quantum efficiency was determined with a calibrated Silicon photodiode and corrected for the spectral response and the solid angle of the collecting optics. The measured quantum efficiency at 5.5 µA is 4 x 10^-3 photons per electron for calcium electrodes. The emission from the calcium/MEH-PPV LEDs is bright and easily seen in a lighted room at 4V forward bias.

Example 5

0069MEH-PPV is cast onto a film of pure UHMW-PE which has been stretched to a moderate draw ratio (e. g. draw ratio >20, Reference 11). The MEH-PPV is observed to orient spontaneously along the draw direction; both the photo-absorption and the photoluminescence spectra are highly anisotropic. Since the luminescence spectrum is polarized with electric vector along the chain alignment direction, light-emitting diodes can be fabricated which emit polarized light.

Example 6

0070MEH-PPV was gel-processed and chain oriented as a guest in UHMW-PE. The get-processing of a conjugated polymer as a guest in a gel-processed blend involves three steps: <ul id="ul0008" list-style="none" compact="compact"><li>A. Co-solution with a suitable carrier polymer</li><li>B. Carrier Structure Formation</li><li>C. Drawing of the Carrier/Polymer blend.</li></ul>

Carrier Solution Preparation, Film Formation, Gelation, and Drawing.

0071PE-MEH-PPV blends are prepared by mixing MEH-PPV (M_w=450,000) in xylene with UHMW polyethylene (Hostalen GUR 415; M_w=4x10⁶) in xylene such that the PE to solvent ratio was 0.75% by weight. This solution is thoroughly mixed and allowed to equilibrate in a hot oil bath at 126°C for one hour. The solution is then poured onto a glass surface to cool, forming a gel which was allowed to dry (into a film). Films were then cut into strips and tensile-drawn over a hot pin at 110-120°C. Once processed in this manner, the films are oriented. The high work function and low work function electrodes are offered as in Examples 3 and 4, and LEDs result.