Abstract:
The present invention includes infrared emitting materials and infrared emitting devices. The present invention demonstrates 1.54 micron infrared PL and EL emission from an organic complex. This provides a very simple way to obtain a light source at 1.54 micron wavelength that may be both optically and electrically pumped.

Description:
[0001]    This application claims the benefit of Provisional application Ser. No. 60/187,278 filed Mar. 6, 2000, which is incorporated herein by reference. 
     
    
       [0002] The present invention arose through work supported in part by DARPA through a grant monitored by the Army Research Office. The United States Government may have certain rights to this invention under 35 U.S.C. Section 200 et seq. 
     
    
     
       TECHNICAL FIELD OF THE INVENTION  
         [0003]    This invention relates to light-emitting devices driven by an electric field or light source and which are commonly referred to as electroluminescent devices.  
         BACKGROUND OF THE INVENTION  
         [0004]    Conjugated polymer based light-emitting devices have become a topic of great interest since the report of electroluminescent properties in poly(phenylene vinylene) (PPV). A large variety of polymers, copolymers, and their derivatives have been shown to exhibit electroluminescent properties. The configurations of these devices may consist of a simple single layer, bilayers, or blends used to enhance efficiency and tune the emission wavelength, or multilayers that may allow the device to operated under an applied voltage. Typical single layer polymer LEDs are constructed by sandwiching a thin layer of luminescent conjugated polymer between two electrodes, an anode and a cathode, where at least one electrode is either transparent or semi-transparent.  
           [0005]    Trivalent erbium ions in different host environments emit photons at several wavelengths, for instance, green emission at 545 nm, and infrared emission at 1.54 and 2.94 microns. This green emission has attracted attention for applications such as fabrication of electroluminescent (EL) devices for use in display technologies. The infrared emission at 1.54 microns is of high interest for optical communication, as this wavelength coincides with the minimum-loss transmission window of silica-based fibers, and the narrow line width of this emission at room temperature also offers high bandwidth capacity in fiber optical communication. Recently, erbium-doped silicon has become a very active field of research for its possible use as electrically pumped light emitters for 1.54 micron wavelength devices. The light emitting devices are based on erbium-doped inorganic materials and prepared by ion implantation, molecular beam epitaxy (MBE), or ion beam epitaxy (IBE) methods. Sharp electroluminescence is observed for these devices at room temperature. Trivalent neodymium and trivalent holmium, when excited, also emit at infrared wavelengths.  
           [0006]    Rapid progress has been made in the field of organic EL devices ever since efficient electroluminescence was demonstrated from organic molecular materials. Organic fabrication techniques provide simple and easy methods to construct EL devices with high efficiency and low operating voltages. A variety of organic materials including metal complexes, polymers, and fluorescent dyes have been employed to the fabrication demonstrating different emission colors in the visible wavelength region. Among them, metal complexes such as aluminum tris(8-hydroxyquinoline) are widely used as emitting materials in sublimed molecular film-based EL devices. When coordinated with rare-earth ions, metal complexes exhibit extremely sharp EL emission bands due to the  4   f  electrons of the ions. Since  4   f  orbitals are effectively shielded from the influence of external forces by the overlying  5   s   2  and  5   p   6  orbitals, the states arising from the f n  configurations are split by external fields by only about 100 cm −1 . Moreover, as the central metal ions are excited via intramolecular energy transfer (IMET) from the triplet excited states of the ligand, the EL devices based on metal complexes can be very efficient in principle due to the contribution of triplet states.  
           [0007]    It is thus an object of the present invention to develop a cheap, simple electroluminescent or photoluminescent device that demonstrates peak infrared emissions at room temperature.  
           [0008]    Although described with respect to the field of light-emitting devices driven by an electric field or optical source, it will be appreciated that similar advantages of infrared emission, as well as other advantages, may obtain in other applications of the present invention. Such advantages may become apparent to one of ordinary skill in the art in light of the present disclosure or through practice of the invention.  
         SUMMARY OF THE INVENTION  
         [0009]    The present invention includes polymer devices capable of peak emissions in the infrared spectrum. A first device has a cathode layer in contact with a hole transporting polymer layer. The hole transporting polymer may be any polymer adapted to act as a hole transporter, such as those selected from the group consisting of poly(vinylcarbazole)s, poly(diphenylacetylene)s (PDPAs), carbazole substituted PDPAs, poly(fluorene)s, poly(triphenylamine)s, copolymers, or mixtures thereof. An anode layer contacts the hole transporting polymer layer, opposing the cathode layer.  
           [0010]    The anode and cathode materials may be selected from any appropriate conductive materials known and used in the field of light emitting polymers. For instance, the cathode material may selected from relatively lower work function metals and semiconductors, such as aluminum, lithium-doped aluminum, calcium, magnesium, and alloys thereof; n-doped conjugated polymers such as n-doped polyacetylene, and n-doped inorganic semiconductors such as n-doped silicon and gallium arsenide.  
           [0011]    Examples of typical anode materials include indium tin oxide (ITO), gold, p-doped polymers such as camphor sulfonate acid-doped polyaniline, p-doped polythiophene (PT), or p-doped polypyrrole or their derivatives on ITO, or hole-injecting metal such as gold, or a p-doped inorganic semiconductor such as p-doped silicon and gallium arsenide.  
           [0012]    The anode, cathode and optional substrate materials may be selected so that the light generated from the device may egress from either side of the device or along the edge of the device, where the device is constructed as a layered device. For instance, infrared-transparent or semi-infrared-transparent anode and cathode materials may be used to allow the light to egress from one or both sides of the device where a layered construction is provided. As an alternative, infrared-reflective anode and cathode materials may be used so as to allow the light only to egress from the edge of the device where a layered construction is used.  
           [0013]    At least one electron transporting molecule is located within the device, the electron transporting molecule having energy levels capable of peak emission(s) within the infrared spectrum. The molecule or moiety may be any molecule comprising a metal-containing molecule capable of such infrared emissions upon excitation. Examples of such electron transporting molecules include metal-containing organic compounds such as metal chelates. Examples of chelates that may be used in accordance with the invention include those of erbium, holmium or neodynium.  
           [0014]    Suitable such compounds include tris(acetylacetonato) (1,10-phenathroline) erbium, tris(acetylacetonato) (1,10-phenathroline) neodymium, and tris(acetylacetonato) (1,10-phenathroline) holmium (abbreviated Er(acac) 3 (phen), Nd(acac) 3 (phen), and Ho(acac) 3 (phen) ). The electron-transporting molecule may also have substitutions on the chelating ligands.  
           [0015]    The electron-transporting molecule may be contained in the hole-transporting polymer itself, or in a separate material layer, such as in a separate polymeric layer. Where the electron-transporting molecule is contained in the hole-transporting polymer itself, the electron-transporting molecule may be blended into the hole-transporting polymer, or be included either in the main chain of the hole-transporting polymer, or as a covalently bound substituent group on the main chain of the hole-transporting polymer. Where the electron-transporting molecule is contained in the hole-transporting polymer itself, it is preferred that the electron-transporting molecule the present in an amount of from about 10 percent to about 80 percent by weight of the hole-transporting polymer.  
           [0016]    The light emitting polymeric devices of the prevention may also be provided with a substrate support to provide additional dimensional stability, such as those materials known and used in the art. These materials may include opaque, transparent or semi-transparent materials as the particular application requires. Examples include glass and plastic materials, such as infrared-transmitting glass, non-infrared transmitting glass, infrared-transmitting plastic and non-infrared transmitting plastic. The substrates may be flexible, non-flexible, or conformable depending upon the desired application.  
           [0017]    The light emitting polymeric devices of the present invention may be driven by any appropriate source of voltage, such as are known and used in the art. These sources may include line current sources, batteries, etc. Where the light emitting polymeric devices of the present invention are electrically driven, the electron driving force may be modulated to generate a frequency- and/or amplitude-modulated infrared source. Modulated electron driving forces may allow light emitting polymeric devices of the present invention to find application in communications, such as is described herein.  
           [0018]    It is preferred that the peak emissions of the present invention be relatively discrete 1.54 micron, 1.2 micron, or 2.9 micron emissions, with minimal full-width half-height measurements and no substantial secondary peaks in the infrared region. Other preferred peaks are within the range of 0.5 to 5.0 microns in the near infrared region.  
           [0019]    The polymer device may be constructed so as to have at least one edge adapted to allow infrared emission. An optical fiber, such as a silica based optical fiber, may be placed in contact with the light emitting polymeric device in any fashion adapted to allow the optical fiber to receive light emitted from the device. For instance, the optical fiber may be placed against the face or against the edge of a light emitting polymeric device where that device is created as a layered device. Where the optical fiber receives the light from the edge of the light emitting polymeric device, the edge of the hole-transporting polymer may be beveled so as to focus and direct the light into the optical fiber. The fiber may be connected via any appropriate method known in the art, but as by the use of index-matching optical cement.  
           [0020]    Light emitting polymeric devices of the present invention may also be constructed using a so-called “SCALE” structure that allows for bipolar operation, as described in U.S. Pat. Nos. 5,663,573 and 5,858,561, hereby incorporated herein by reference.  
           [0021]    The present invention also includes light emitting devices featuring the hole-transporting polymer and electron-transporting molecule as described herein, where the device may be optically driven through light excitation (in which case the anode and cathode portions are unnecessary). A source of optical energy is positioned sufficiently near and directed toward the polymer so as to irradiate the polymer, adapted to stimulate infrared emissions from the electron-transporting moiety. A preferred optical source generates photons of approximately 0.9 microns. It is preferred that the electron transporting molecule and hole transporting polymer be as described above.  
           [0022]    One edge of the device may be beveled so as to focus the infrared emissions. At this or any appropriate edge/face of the device may be attached an optical fiber, adapted to guide light emitted by the device. The fiber may be attached by any appropriate means.  
           [0023]    Another device of the present invention utilizes two layers. The first layer comprises an electron transporting layer having energy levels capable of generating peak emissions in the infrared spectrum. The second layer is a hole transporting polymer layer in contact with the electron transporting layer. The device includes a cathode in contact with the electron transporting layer and an anode in contact with the hole transporting polymer layer. These electrodes may then have a voltage applied so as to generate infrared emissions.  
           [0024]    Another two-layer device of the present invention comprises similar electron transporting and hole transporting polymer layers. This device utilizes a source of optical energy in contact with the hole transporting polymer and electron transporting layers to stimulate infrared emissions in the polymer device.  
           [0025]    In another embodiment, a light emitting polymeric device of the present invention may be in the form of a cylindrical body or fiber, so as to be able, in one embodiment to be connected to an optical fiber. This construction allows for the production of a source of communication light that may be transmitted down the fiber. The optical fiber comprises a hole transporting polymer and an electron transporting molecule. The device utilizes a source of optical energy in contact with the optical fiber, adapted to stimulate infrared emissions along the fiber. The device may also have an optical receiver in contact with the optical fiber, positioned opposite the optical energy source. An optical receiver may also be placed in contact with the optical fiber opposite the optical energy source. It is preferred that the hole transporting polymer and electron transporting molecule are as described herein.  
           [0026]    In another embodiment of the present invention, the optical fiber again comprises a hole transporting polymer and an electron transporting molecule as described herein. An anode layer in placed in contact with the optical fiber, extending over a portion of the circumference of the fiber and running along a portion of the length of the fiber. A cathode layer is placed in contact with the optical fiber, extending over a portion of the circumference of the fiber preferably opposite the anode layer, running along a portion of the length of the fiber similar to the portion of the anode layer. Here, the hole-transporting polymer and electron-transporting molecule may be formed by extrusion, and the anode and cathode materials may be deposited on the sides of the fiber so formed. The deposition may be by any appropriate method, such as silk-screening or vacuum deposition. It is preferred that the hole transporting polymer and electron transporting molecule are selected from those described previously.  
           [0027]    Also included in the present invention is an infrared laser device that incorporates a photonic band gap material, the material comprising a semitransparent matrix of periodic hollow cells, such as periodic hollow sphere-like cells. It is preferred that the diameter of each sphere be approximately that of the wavelength of light emitted by the device. A hole transporting polymer is contained in the matrix of periodic hollow cells. An electron transporting molecule, having energy levels capable of peak emissions in the infrared spectrum, is utilized in the device. The device also includes a source of optical energy in contact with the photonic band gap material, adapted to stimulate infrared emissions. It is preferred that the hole transporting polymer and electron transporting molecule be selected from those described herein.  
           [0028]    Another embodiment of an infrared laser utilizes the same hole transporting polymer and electron transporting molecule, but uses a pair of mirrors to stimulate lasing. Two parallel mirrors are placed on opposing sides of the hole transporting polymer layer containing the electron transporting I.R. emitting molecule, the reflective side of each mirror facing the polymer layer. The mirrors are preferably planar, and one of the mirrors should be semi-transparent in the infrared spectrum. A source of energy, electrical or optical, is then placed in electrical or optical contact with the polymer layer so as to stimulate infrared emissions that are primarily orthogonal to the planes of the mirrors. The elements should be arranged such that light emitted from the electron transporting molecule will be reflected back and forth between the two mirrors through the polymer layer until passing through the infrared semi-transparent mirror. The effect of several similar emissions is that the emitted light will begin to lase. The laser may be placed in contact with optical fibers for communication purposes or to transmit the light.  
           [0029]    The present invention also includes a communication system. An optical fiber network, comprising at least one polymer fiber, is used to send optical signals. The polymer fiber comprises a hole transporting polymer and an electron transporting molecule, the molecule having energy levels capable of peak emissions in the infrared spectrum. It is preferred that the emissions be at 1.54 or 2.9 microns. A transmission device is placed in contact with the polymer fiber, the transmission device adapted to stimulate infrared emissions in that fiber. An infrared reception device is then placed in contact with the optical fiber network, the infrared reception device adapted to receive infrared emissions propagated along the fiber network. It is preferred that the hole transmitting polymer and electron transporting molecule are selected from those described above.  
           [0030]    The present invention also includes optical amplifiers that may be made using the arrangements of the present invention. Optical amplifiers may be used to in high-speed optical transmission systems. Optical amplifiers may span portions of the lightwave spectrum, and may be used to increase the optical bandwidth of amplifiers used in commercial wavelength-division multiplexed (WDM) communications systems. WDM is a technique for transmitting a mix of voice, data and video, in the ones and zeros of digital information, over various wavelengths, or colors, of light. The present invention may be used to provide optical amplifiers capable of operating in the infrared spectrum.  
           [0031]    The present invention may also include lasers made using the polymers and arragements of the present invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0032]    [0032]FIG. 1 shows an example of host polymers of the present invention.  
         [0033]    [0033]FIG. 2 shows an example of a rare earth (RE) chelate complex of the present invention.  
         [0034]    [0034]FIG. 3 shows examples of rare earth chelate complex containing polymers of the present invention.  
         [0035]    [0035]FIG. 4 shows a single layer infrared light emitting electroluminescent device of the present invention.  
         [0036]    [0036]FIG. 5 shows a two layer infrared light emitting electroluminescent device of the present invention.  
         [0037]    [0037]FIG. 6 shows a three layer infrared light emitting electroluminescent device of the present invention.  
         [0038]    [0038]FIG. 7 shows an embodiment of an infrared light emitting electroluminescent device of the present invention wherein the electroluminescent device is operated on a surface.  
         [0039]    [0039]FIG. 8 shows an infrared light emitting electroluminescent fiber of the present invention.  
         [0040]    [0040]FIG. 9 shows plots of absorbance and photoluminescence for Er(acac) 3 (phen) used in accordance with the present invention.  
         [0041]    [0041]FIG. 10 shows a current vs. voltage curve for ITO:PVK: Er(acac) 3 (phen)/Li:Al and an inset plot of the electroluminescence of such a device used in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0042]    In accordance with the foregoing summary, the following present a detailed description of the preferred embodiment of the invention that is currently considered to be the best mode.  
         [0043]    The erbium complex, Er(acac) 3 (phen), may be synthesized by the conventional method. To fabricate the EL devices from the compound, poly(N-vinylcarbazole) (PVK) may be employed as a host polymer. Thin films of PVK doped with Er(acac) 3 (phen) are preferably prepared by spin coating from PVK:Er(acac) 3 (phen) (10:8 by weight ratio) dichloroethane solution. The sheet resistance of ITO (indium-tin oxide coated glass) substrate may be 20 Ω/square. The thickness of the Er(acac) 3 (phen)-doped PVK film is preferably about 80 nm. A single-layer type of EL devices may be constructed as ITO/PVK:Er(acac) 3 (phen)/Al:Li/Ag (bi-layer devices with separate PVK and Er(acac) 3 (phen) layers have also been fabricated, showing similar EL). The metal cathode Al:Li(0.1%) (100 nm) may be thermally deposited in a vacuum chamber of 1×10 −7  torr. The luminescent area of the devices for test purposes may be 0.25 mm 2 . The PL and EL spectra may be recorded with a spectrometer, such as a Bruker IFS66/S, at room temperature. The EL devices are preferably driven by a DC bias in air. FIG. 4 shows a schematic structure of an EL device  1  of the present invention. The EL device  1  shown has an ITO/PVK:Er(acac) 3 (phen)/Al:Li/Ag structure, where the PVK:Er(acac) 3 (phen) EL polymer layer  3  is coated onto an indium-tin oxide  4  coated glass substrate  5  (the anode) and then coated by an Al:Li/Ag layer  2  (the cathode). A source of electrical energy  6  may then be connected to the anode  4  and cathode  2 . FIG. 1 shows the chemical structure of a preferred PVK material of the present invention. FIG. 2 shows the chemical structure of preferred RE chelate complexes. FIG. 3 shows the chemical structure of preferred RE chelate complexes containing polymers.  
         [0044]    Another preferred embodiment of a device  7  is shown in FIG. 5, where a layer of electron transporting RE chelate complex containing polymer  9  and a hole transporting polymer layer  10  are placed between the anode  11  and cathode  8 . FIG. 6 shows an embodiment of a preferred device  12  wherein a first layer  14  and second layer  16  of a conducting polymer material are placed between a blended electron transporting infrared emitting moiety and hole transporting host layer  15  and the electrodes  13  and  17 . FIG. 7 shows an embodiment of a preferred planar device  18  wherein the anode  19 , polymer blend  20 , and cathode  21  all lie in a plane on the surface of a substrate  22 . FIG. 8 shows an embodiment of a preferred device  23  wherein a hole transporting polymer fiber  25 , shown in side view, incorporates an infrared emitting electron transporting molecule, or is surrounded by a transparent or semi-transparent electron transporting infrared emitting molecule layer  24 . Optical energy may then be directed at the fiber  25  so as to stimulate infrared emission.  
         [0045]    The absorption spectrum of Er(acac) 3 (phen) vacuum-evaporated film (about 20 nm) on a quartz substrate is shown in FIG. 9. The absorption of Er(acac) 3 (phen) in the wavelength region of 600 to 350 nm is due to the charge transfer states formed between the ligands and the central Er ion. The relatively sharp absorption peak at 285 nm is from the phenanthroline ligand. There is no absorption from the Er ion because of the shielding effect of the ligands surrounding the erbium ion. The PL spectrum of Er(acac) 3 (phen) excited by the 350 nm excitation line, as shown in FIG. 9, exhibits a sharp emission peak at 1.54 microns. The luminescence originates from the  4 | 13/2 → 4 | 15/2  transition of the partially filled  4   f  shell. Because the  4   f  shell is well shielded by the outer  5   s  and  5   p  orbits, the energy of this transition is relatively independent of the ligands and ambient temperature. Er(acac) 3 (phen) shows no emission in the visible range when excited by the 350 nm light, which suggests that the  4 | 13/2 → 4 | 15/2  transition might be the most favorable transition. The absolute PL efficiency of Er(acac) 3 (phen) remains unknown. However, the other lanthanide complexes such as Eu(acac) 3 (phen) and Tb(acac) 3 (phen) with the same ligands and lanthanide electronic configurations, which emit in the red and green wavelength ranges, show PL efficiencies as high as 10%.  
         [0046]    There are at least two methods to form erbium-ion containing polymer films: a first method is to covalently bind an erbium ion complex to a polymer main chain; and a second is to blend an erbium complex into a host polymer. As Er(acac) 3 (phen) is soluble in some common solvents such tetrahydrofuran and chloroform, one may select the latter method and use poly(N-vinylcarbazole) (PVK) as a host polymer. The Er(acac) 3 (phen)-doped PVK when excited with a 350 nm source shows the same 1.54 micron PL emission as that in FIG. 9. This method of doping erbium complexes into polymer matrices may provide a way to fabricate optically pumped infrared emitters for possible applications. The host polymer is not limited only to PVK. As an example, the erbium complex may also be doped into poly(methyl methacrylate) (PMMA), which is a common polymer matrix material.  
         [0047]    A single-layer EL device may be prepared using PVK:Er(acac) 3 (phen) as an emitting layer. The weight ratio of Er(acac) 3 (phen) and PVK is preferably 8:10. PVK is a hole-transporting polymer and emits blue light. Therefore, its excitation energy may be efficiently transferred to Er(acac) 3 (phen), the absorption of which is within 350 to 600 nm. Er(acac) 3 (phen) itself is an electron transporting molecule via the phenanthroline ligand. The EL of ITO/PVK:Er(acac) 3 (phen)/Al:Li/Ag device may be observed under 10 V DC bias (using ITO as the anode). FIG. 10 shows the current-voltage (I-V) curve and the EL spectrum (inset) in the infrared region, which is from the Er ion peaked at 1.54 microns. Since ITO-coated glass used as the anode transmits only about 20% light around 1.5 microns, it may limit the output of the infrared light. In the visible range, there may be a weak broad band EL emission of the background from 480 to 610 nm which is proposed to originate from an exciplex formed by PVK and Er(acac) 3 (phen), as neither PVK and Er(acac) 3 (phen) may emit at this region. The EL quantum yield (photons/ electron) in this device is still unknown. The EL efficiency of a terbium complex Tb(acac) 3 (phen) with the similar device structure, i.e. ITO/PVK:Tb(acac) 3 (phen)/Mg, is about 0.1%.  
         [0048]    One possible electroluminescence excitation process of PVK:Er(acac) 3 (phen) blends may be accomplished as follows. When a bias is applied to the device, holes may be injected and transported into the EL layer via PVK (through the carbazole side group) and electrons via phenathroline ligands of Er(acac) 3 (phen); the holes and electrons may then recombine to generate excitons. The energy of excitons may be transferred to the ligands of Er(acac) 3 (phen) through Förster energy transfer and finally to the erbium ion. The radiative transition of  4 | 13/2 → 4 | 15/2  of the excited erbium ion may lead to infrared light emission at 1.54 microns.  
         [0049]    The preferred embodiments herein disclosed are not intended to be exhaustive or to unnecessarily limit the scope of the invention. The preferred embodiments were chosen and described in order to explain the principles of the present invention so that others skilled in the art may practice the invention. Having shown and described preferred embodiments of the present invention, it will be within the ability of one of ordinary skill in the art to make alterations or modifications to the present invention, such as through the substitution of equivalent materials or structural arrangements, or through the use of equivalent process steps, so as to be able to practice the present invention without departing from its spirit as reflected in the appended claims, the text and teaching of which are hereby incorporated by reference herein. It is the intention, therefore, to limit the invention only as indicated by the scope of the claims and equivalents thereof.  
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