Organic electroluminescent cells having a tunnel injection cathode

[57] ABSTRACT

An electroluminescent cell comprises an anthracene layer having an anode and cathode thereon. The cathode of the novel cell is of the tunnel injection type. A typical cathode consists of a 10-100 A. thick layer of silicon dioxide deposited over a degenerate n type silicon body. The silicon dioxide layer of the cathode is placed in contact with the anthracence layer. The anode should provide hole injection into the anthracence layer. A preferred anode is a film of copper oxide and copper iodide.

Claims, 4 Drawing Figures

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PATENTED JAN 91973

3,710,167

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ORGANIC ELECTROLUMINESCENT CELLS HAVING A TUNNEL INJECTION CATHODE

BACKGROUND OF THE INVENTIONO

This invention relates to improved electroluminescent cells useful as simple light sources or in display devices. It more particularly relates to electroluminescent cells which employ organic phosphors as the luminescent material.

Organic phosphor electroluminescent cells are known in the art. These prior art cells are formed from admixtures of anthracene or other organic phosphor compositions with electrically conductive materials such as metals, graphite or other forms of carbon. The admixture is deposited as a thin layer on a conductive surface. The conductive surface is typically a tin oxide coating applied to a glass substrate. The admixture is coated with an insulating layer such as glycerin and a metal electrode is provided to make electrical contact with the glycerin. Examples of these prior art cells can be found with reference to U.S. Pat. No. 3,173,050 issued to E. F. Gurnee.

Although the voltage and frequency requirements for operating the cells as described above are stated therein to offer an improvement in the then existing art, this improvement has not been sufficient to result in low cost, low voltage cells emitting a brightness which would approach commercial utility.

In order to approach commercial utility the electroluminescent cell should have a long life, improved brightness, simple construction free of shorts and for many device applications, it should be compatible with silicon technology.

SUMMARY OF THE INVENTION

An electroluminescent cell comprises a layer of an organic phosphor composition, an anode upon one surface of the layer and a tunnel-injection type cathode upon the other surface of the layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a novel electroluminescent cell.

FIG. 2 is a front elevational view of an embodiment of an electroluminescent cell having a desired luminescent pattern therein.

FIG. 3 is a cross-sectional view of the embodiment shown in FIG. 2 taken along line 3—3 thereof.

FIG. 4 is a cross-sectional view of an electroluminescent cell employing a glass support for the anode.

DETAILED DESCRIPTION OF THE INVENTION

The organic phosphor layers utilized in the present invention comprise a conjugated organic compound of condensed benzene rings. Examples of useful phosphors are anthracene, naphthalene, methyl derivatives of anthracene and anthracene doped with tetracene or other similar fluorescent dyes.

Preferably, the phosphor should be of high purity. For example, a preferred phosphor is anthracene having an electron trap density in the order of &lt;10¹⁴traps/cc. The thickness of the phosphor layer should be less than 10 micrometers (/un) and preferably between 1 and 5 gm.

Unlike the cited prior art devices of Gurnee, the phosphor layer of the novel device is free of dispersed conductive particles. Such particles, if they were to exist in the novel device, would induce shorts and absorb light from the phosphor and thereby reduce the efficiency and life of the novel device.

Preferably, the phosphor layer comprises phosphor grains which are mono-crystalline in the direction of current flow. This results in a more efficient cell.

A feature of the novel cells is the use of a tunnel injection cathode for injecting electrons into the phosphor. Such a cathode comprises a base electrode material having a dielectric film of a thickness of less than about 100 A., and generally between 10-100 A., on the surface thereof. The dielectric film is in contact with the phosphor layer of the cell. Preferably the dielectric material has a dielectric constant of less than 5, but higher dielectric constant films are also suitable.

A preferred tunnel injection cathode has as the electrode material a wafer of n type silicon, preferably degenerate n type silicon. The dielectric film of this cathode is a 10-100 A. thick silicon dioxide layer on a surface of the silicon wafer. This cathode is hereinafter designated as n type Si—SiO₂. Another useful tunnel injection cathode consists of aluminum, such as an evaporated layer of aluminum, having a 10-100 A. thick aluminum oxide layer thereon. This cathode is hereinafter designated as Al—A1₂O₃.

The use of such cathodes results in devices which are easy to construct, are compatible with silicon technology, are long-lived and have improved brightness.

The anode of the device, which is a hole injecting contact, may consist of a conductive oxide, such as tin oxide, indium oxide, or a mixture of cuprous oxide and cuprous iodide; a metal, such as gold or platinum; an alloy, such as selenium-tellurium alloy; or a tunnel injecting contact such as p type silicon having a 10-100 A. thick layers of silicon dioxide thereon. Preferred anodes comprise a transparent copper oxide and copper iodide layer or a conductive oxide such as tin oxide or indium oxide coated with a very thin transparent layer of Te. A Te layer which is a few atoms thick is sufficient.

Although edge emission is possible, for most purposes and in order for the device to emit a greater amount of light, it is preferred that either the anode or the cathode or both be transparent.

Examples of a transparent anode are films of tin oxide coated with a very thin layer of tellurium (preferably a few atoms thick) or tin oxide coated with a thin layer of a copper oxide-copper iodide mixture. A transparent electrode can be obtained by forming it in a mesh-like configuration so that light is freely transmitted through the spaces in the mesh.

In order to control the thickness of the phosphor layer, spacers defining this thickness may be employed. The novel devices make possible the use of spacers which are an integral part of the electrode structure. For example, the silicon dioxide layer of the tunnel injecting cathode can be built up around the periphery of the electroluminescent cell so as to provide a spacer of several microns in thickness which is integral with the tunnel injection area (less than 100 A. thick) of the cell.

3,710,167

Alternatively, the cathode or anode may be formed on a glass support, either with or without a transparent conductive coating thereon, and the spacer evaporated, sputtered or otherwise formed on this structure.

Referring now to FIG. 1 there is shown a preferred novel electroluminescent cell 4. The novel cell 4 comprises an anthracene layer 6 sandwiched between a cathode 8 and an anode 10. The cathode comprises a degenerate n type silicon wafer 12 having a silicon dioxide coating 14 thereon. The degenerate silicon wafer typically has a resistivity in the range of 6X10^-4 to lxl0ohm-cm. The silicon dioxide coating, which may be formed by any of the techniques which are well known in the art, is provided with a thin central region 16 where electron injection occurs by a tunneling mechanism. This thin region is in the order of 10 to 100 A. thick and is preferably less than 50 A. thick. The outer periphery 18 of the silicon dioxide layer is thicker than the central portion 16. This outer periphery 18, which has a typical uniform thickness of from 1 to 5 pm., functions as a spacer to determine the thickness of the anthracene phosphor layer 6 of the cell and to prevent shorts. A preferred technique for forming the silicon dioxide coating for tunnel injection comprises the steps of (1) cleaning the silicon wafer in a boiling ammoniated solution of hydrogen peroxide, (2) etching the cleaned wafer in concentrated HF, (3) oxidizing the etched wafer in a steam atmosphere at 1,100° C., (4) recleaning and re-etching the oxidized wafer, (5) heating the wafer for several minutes at 150° C., (6) oxidizing the wafer with water vapor at 600° C., and (7) annealing the oxidized wafer at 500° C. in a pure hydrogen atmosphere. This technique is the subject of a U.S. Patent Application filed by Alvin M. Goodman and James M. Breece.

The anthracene layer 6 may be formed by melting finely divided anthracene on the central portion 16 of the silicon dioxide layer 14 and allowing this molten material to crystallize.

The anode 10, is provided on the anthracene layer 6 on the side opposite the cathode 8. The anode 10 consists of a thin coating Of a copper oxide-copper iodide film. This film may be deposited directly on the anthracene or alternatively it is deposited on one surface of a conducting glass flat 20 as shown. The copper oxide-copper iodide film is less than 100 A. thick. If the copper oxide-copper iodide film exceeds about 500 A. in thickness, the transmission of light therethrough is reduced such that it is preferably formed in a mesh or crossed-grid pattern so as to allow light to be efficiently emitted from the cell.

In operation, the silicon wafer 12 is connected to negative terminal of a battery and the copper oxidecopper iodide anode layer 10 is connected to the positive terminal of the battery. Electrons are injected into the anthracene layer 6 through the silicon dioxide layer 14 and holes are injected into the anthracene layer 6 from the copper oxide-copper iodide layer 10. Recombination of electron-hole pairs occurs in the anthracene layer 6 resulting in the emission of blue light.

Although the injecting tunnel cathode is shown to comprise n type silicon, p type silicon can also be used. In fact, p type silicon has the advantage that it is easier to deposit p type silicon on glass than n type silicon, and, hence, may offer a cost advantage where the cathode is formed on a glass substrate. There is however, the disadvantage that it requires a somewhat higher voltage to operate a cell of a given configuration where the silicon is p type as compared with n type.

In FIGS. 2 and 3 there is shown another embodiment of the invention. In this embodiment, an electroluminescent cell 30 comprises a grid-like anode 32 formed on a transparent glass support 34, and a silicon dioxide spacer 36 around the periphery of the anode support 34. A 2-3 μιη layer 38 of anthracene phosphor is disposed on the anode 32. A cathode structure 40 is positioned over the anthracene to form the complete cell. This cathode structure 40 consists of a wafer 42 of n type silicon having a 2-3 pm thick SiO₂ layer 44 thereon. The SiO₂ layer 44 is in contact with the anthracene layer 38. The SiO₂ layer 44 has 1 mil. diameter plugs 46 of the n type silicon extending completely therethrough. A 10-50 A. thick dielectric SiO₂ layer 48 is provided on the outer surface of each of the silicon plugs 46 so as to contact the anthracene layer 38. The plugs are formed in a desired pattern, for example, to portray RCA as shown in FIG. 2. Electron injection into this device takes place by tunneling through the dielectric SiO₂ layer 48 on each of the silicon plugs 46. An alternative and somewhat similar structure to that described with reference to FIGS. 2 and 3 is one in which the silicon dioxide layer 36 has an array of holes therein extending therethrough rather than silicon plugs extending through the SiO₂ layer as shown in the FIGURES. In this alternative structure, the Si layer is provided with a thin SiO₂ coating (10-100 A.) in the region of each hole and the anthracene then fills each hole.

The cell 50 shown in FIG. 4 employs an Al—A1₂O₃cathode structure 52. The cathode may be formed by vacuum evaporating a layer 54 of aluminum onto a glass substrate 56 and anodizing the surface of the aluminum layer 54 to form an A1₂O₃ layer 58 which is preferably from 10-50 A. thick. The cell 50 is also provided with an anode 60 having spacers 62 the same as that described with reference to FIG. 3. An organic phosphor layer 64 is disposed between and in contact with the anode and cathode.

Where one desires light emission from the cathode side of the cell, the cathode can be formed in a thin line or mesh pattern. Such a pattern can be formed, for example, by evaporating the cathode material onto a glass substrate by standard flash evaporation or electron beam evaporation techniques through an appropriate mask.

Although the examples indicate tunnel injection cathodes comprising Al—A1₂O₃ and Si—SiO₂, it should be understood that other metal-metal oxide tunnel injection cathodes may also be useful, for example, tantalum-tantalum oxide and magnesium-magnesium oxide.

Electroluminescent cells made in accordance with the invention comprising anthracene containing tetracene therein in an amount not exceeding 5 ppm were operated at applied average fields of only approximately 2X10⁵ volts/cm. across the phosphor layer. Light emission was observed at wavelengths between 4,100-5,400 A. These cells comprised a cathode consisting of a fine grid of aluminum having about a 50 A.

3,710,167 thick film of A1₂O₃ thereover and an anode consisting of either colloidal platinum paste, copper-copper iodide paste, silver-silver iodide paste, an evaporated conductive selenium-tellurium alloy or a conductive arsenic-tellurium alloy containing a small percentage of selenium therein. The choice of anode did not have a major effect on the efficiency of light emission from the cell but it does affect the speed of response to the applied field. For the applied fields stated above, current densities ranging from 5X1 O'³ to 5x1 O’² amp/cm² have been observed. The higher current densities correspond to measured surface brightnesses of 60 footlamberts. This may be compared to a brightness of only 10 foot-lamberts for the blue light of a commercial color television kinescope. The quantum efficiency of these cells is between about 0.01-0.04 photons/electron.

Cells having a cathode consisting of n⁺ silicon, either as a wafer or a vacuum deposited film, covered with a 20-40 A. thick film of SiO₂ and an evaporated transparent Cu₂O—Cui anode were also operated at an average field of about 2.5X10⁵ volts/cm. These cells had a current density of 1.8X10² amp./cm.² and a quantum efficiency of from about 0.01-0.03 photons/electron. The surface brightness of these cells was estimated to exceed 10 foot-lamberts.