BILAYER POLYMER ELECTROLUMINESCENT DEVICE FEATURING INTERFACE ELECTROLUMINESCENCE

Abstract: An electroluminescent device includes a first polymeric layer (1) as a hole transport/electron blocking layer and a second polymeric layer (2) as an electron transporting emissive layer, wherein the first layer and the second layer are in electrical contact so as to form an interface being capable of producing an exciplex-like emission upon a current being passed through the interface. The first layer has a greater hole transport capability than the second layer, and the second layer having greater electron transport capability than the first layer.

Description: BILAYER POLYMER ELECTROLUMINESCENT DEVICE FEATURING INTERFACE ELECTROLUMINESCENCE Related Application Data This application claims the benefit of U.S. provisional application No. 60/036,232 filed on March 12, 1997, which is incorporated herein by reference. Technical Field This invention relates to light-emitting devices driven by an electric field and which are commonly referred to as electroluminescent devices. Background Conjugated polymer based light-emitting devices have become a topic of great interest since the report of electroluminescent (EL) properties in poly(phenylene vinylene) (PPV). A large variety of polymers, copolymers, and their derivatives have been shown to exhibit EL properties, including a relatively new class: polvpyridines, and poly(pyridyl vinylene)s. 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 AC applied voltage. In single layer devices, the low efficiency frequently is due to the imbalance of- electrons and holes. Inserting a hole-transport (electron blocking) or electron- transport (hole-blocking) layer provides a means to enhance minority carriers and block the majority carriers and confine them to the emitter layer, which increases the probability of recombination. Poly(N-vinylcarbazole) (PVK) has been used as a hole transport layer and occasionally in blends with the emitter polymer. PVK is a well studied photoconductive polymer which often forms exciplexes with organic molecules, e.g., dimethyl terephthalate. Recently, there has been interest in exciplex formation between PVK and conjugated polymers. Osaheni and Jenekhe have reported PL due to exciplex formation in bilayers of poly(p-phenylene benzobisoxazole) (PBO) and tris(p- tolyl)amine, but not EL, although they suggest exciplexes may be important in light- emitting devices. Even though many groups have studied bilayer and multilayer devices, EL due to exciplex formation until now has not been reported. For example, in highly efficient bilayer devices of CN-PPV and PPV and of PPV and 2-(4- biphenylyl)-5(-4-tert-butyphenyl)-l,3,4-oxadiazole exciplex formation is not observed. It is thus an object of the present invention to provide light emission in polymer-based light-emitting devices that occurs relatively further away from the operating electrodes which tend to quench luminescence. It is also an object of the present invention to provide polymeric light-emitting devices with increased probability of electron-hole recombination, and thereby increased attendant efficiency. Another object of the present invention is to provide polymeric light-emitting- devices which provide enhanced protection of the emission from environmental degradation, such as that due to exposure to oxygen. Finally, it is also an object of the present invention to provide for polymeric light-emitting devices that may optionally be used to produce laser light. In view of the present disclosure and the practice of the present invention, other advantages of the present invention may become apparent. Summary Of The Invention In general terms, the present invention includes an electroluminescent device comprising (a) a first polymeric layer adapted to act as a hole transport/electron blocking layer; and (b) a second polymeric layer adapted to act as electron transporting emissive layer, wherein the first polymeric layer and the second polymeric layer are in electrical contact so as to form an interface, the interface being capable of producing an exciplex-like emission upon a current being passed through the interface. An exciplex is a transient donor-acceptor complex between the excited state of the donor and ground state of the acceptor. The first polymeric layer has a greater hole transport capability than the second polymeric layer, and the second polymeric layer having greater electron transport capability than the first polymeric layer. The Hole Transport/Electron Blocking Layer Referring to Figure 1 , the hole transport/electron blocking layer 5 is shown in electrical contact with electrode 1 and electron transporting layer 4. This layer may comprise any appropriate polymer, copolymer or oligomer, or derivative thereof. An example of the polymeric hole transport/electron blocking layer may be any carbazole-coating polymer, such as poly(vinyl carbazole) (PVK). Figure 2 shows the chemical structure of the repeating units of polymers that may be used in accordance with one embodiment of the present invention, structure (b) being that of a repeating unit of poly(vinyl carbazole) (PVK). The Electron Transporting Layer Also shown in Figure 1 is the electron transporting layer 4 shown in electrical contact with electrode 3 and hole transport/electron blocking layer 5. The polymeric layer acting as the electron transporting emissive layer may comprise any appropriate polymer, copolymer or oligomer, or substituted or wrapped derivatives thereof, such as poly(pyridyl vinylene phenylene vinylenes), poly(dithienylene phenylenes), such as shown in the examples below. This layer may comprise, for instance, a poly(pyridyl vinylene phenylene vinylene) (PPy VPV) polymer, copolymer or oligomer, or derivative thereof, such as shown in structure (a) of Figure 2, being that of a repeating unit of a poly(pyridyl vinylene phenylene vinylene) (PPy VPV) derivative (i.e., PPyVP(COOC 12 H 25 ) 2 V). Figure 7(a), 7(b) and 7(c) show other examples of copolymers and their derivatives that may be used as the electron transporting emissive layer. The electron- withdrawing nature of the side groups, such as those in the copolymer PPyVP(COOC 12 FI 25 ) 2 V, render the copolymer more resistive to oxidation than the unsubstituted copolymer, and are thus preferred. The copolymers are soluble in common organic solvents such as ~ tetrahydrofuran (TFIF), xylene, and chloroform. Hole-Injecting Electrodes With respect to such alternative materials and referring to Figure 1, the hole- injecting electrode 1 may be of any appropriate material. Electrodes may be fashioned from any suitable conductive material including metals, degenerate semiconductors, and conducting polymers including, but not limited to, a wide variety of conducting materials such as (1) indium-tin-oxide ("ITO"), (2) metals such as gold, aluminum, silver, copper, indium and magnesium, (3) alloys such as magnesium- silver, (4) conducting fibers such as carbon fibers, and (5) highly-conducting organic polymers such as highly-conducting doped polyaniline, highly-conducting doped polypyrrole, polyaniline salt (such as PAN-CSA) or other doped pyridyl nitrogen- containing polymer, such as polypyridylvinylene. Electron-Injecting Electrodes With respect to such alternative materials and referring to Figure 1, the electron-injecting electrode 3 may be of any appropriate material. Electrodes may be fashioned from any suitable conductive material including metals, degenerate semiconductors, and conducting polymers including, but not limited to, a wide variety of conducting materials such as (1) metals such as aluminum, calcium, silver, copper, indium and magnesium, (2) alloys such as magnesium-silver and lithium-aluminum, (3) conducting fibers such as carbon fibers, and (4) highly-conducting organic polymers such as highly-conducting doped polyaniline, highly-conducting doped polypyrrole, polyaniline salt (such as PAN-CSA) or other doped pyridyl nitrogen- containing polymer, such as polypyridylvinylene. In typical applications where the device is used for illumination and_ display, " at least one of the electrodes may be fashioned from a transparent material such as ITO or a partially transparent material such as highly-conducting doped polyaniline. Partially transparent electrodes may be used to advantage to filter or clip unwanted frequencies of light coming from the light-emitting material. It is noted that it is not necessary that the electrode material be transparent or even partially transparent. In cases where the electrode materials are opaque to the emitted light, light emission from the edge of the device may be utilized in, for example, edge-lighted displays or in coupling applications such as in coupling the device to an optical fiber. Substrate For ease of manufacture and safety purposes, it is often desirable to form the device on a substrate which also serves to protect and often to insulate (both physically and electrically) the device during use. The substrate layer may be any appropriate material, such as glass or clear electrically insulating plastic substrates which are preferred when the device is used for lighting and display purposes. A DC driven device is especially suitable for light emission from both sides of the device in which case electrode materials, as well as any protective substrates that may be used with one or both electrodes, are at least partially transparent. The substrate layer is shown in Figure 1 as substrate layer 2. The Source of Electrical Energy The devices of the present invention may be operated by any appropriate source of electrical energy 8 shown in Figure 1. The first electrode and the second electrode are electrically connected to-a potential difference. For instance, the first electrode can be connected to a positive potential (anode) while the second electrode is connected to a negative potential (cathode) The electrodes 1 and 2 are connected to a voltage source 8 by means of suitable electrical connectors or contacts. Such electrical connectors and contacts are conventional in the art and may include wire leads, printed circuit connectors, spring clips, snaps, solder, wrapped posts, conducting glues, etc. It is also to be understood that the electrical connector or contact can be the electrodes 1 and 3 themselves. That is, the potential difference from voltage source 8 may be applied directly to the electrodes in which case electrodes 1 and 3 may become the electrical contact or connector. The devices of the present invention may feature a relatively low turn-on and operating DC voltage of less than about 24 volts, depending upon polymeric thickness. More preferably, a turn-on and operating voltage of less than about 12, and even less than about 5 volts may be achieved. Such low voltages make these devices particularly advantageous for use in toys, as commercial light strips such as found on airplanes and in theaters, as signs, and as flat panel displays for computer and television use. Devices of the present invention may be operated with AC current in which case the device will operate when current is flowing in the forward direction. Advantages of the devices of the present invention include that the light emission occurring relatively further away from the electrodes (i.e., such as the Al and ITO electrodes), which may tend to quench luminescence. Another advantage is that the charge is confined at the polymer/polymer — interface by the electron blocking nature of the electron blocking layer (i.e., such as the PVK layer). This leads to an increased probability of electron-hole recombination due to the density of electrons and holes at the interface. The devices of the present invention also feature a sequestered polymer/polymer interface that protects the emission from degradation due to oxygen that tends to change the vinylene units to carbonyl units which in turn quenches the luminescence. The devices of the present invention may also be used to produce electrically or optically pumped laser light. By using a polymeric layer arrangement of the present invention, it is possible to concentrate the energy spatially at the interface. This would allow attainment of lasing at relatively low pump thresholds, whether the device is electrically or optically pumped. Optically pumped lasing may be attained by irradiating the interfacing polymer layers with sufficient light intensity at sufficiently short wavelength to cause photoexcitation of the exciplex. Electrically pumped lasing may be attained by supplying enough current density to create a critical density of exciplexes sufficient to cause lasing, which will depend upon geometrical factors understood in the art. The foregoing and other advantages of the invention will become apparent from the following disclosure in which one or more preferred embodiments of the invention are described in detail and illustrated in the accompanying drawings. It is contemplated that variations in procedures, processing, structural features, arrangement of parts, experimental design, ingredients, compositions, compounds, and elements may occur to a person skilled in the art without departing from the scope- of or sacrificing any of the advantages of the invention. Brief Description of the Drawings Figure 1 is a general schematic of a light-emitting device of the present invention. Figure 2 shows the chemical structure of the repeating units of polymers that may be used in accordance with one embodiment of the present invention; structure (a) being that of a repeating unit of a poly(pyridyl vinylene phenylene vinylene) (PPy VPV) derivative (i.e., PPyVP(COOC 12 H 25 ) 2 V), and structure (b) being that of a repeating unit of poly(vinyl carbazole) (PVK). Figure 3 is a graph showing the photoluminesence spectra of PPyVP(COOC 12 H 25 ) 2 V at 2.8 eV excitation energy (— ), a bilayer of PVK and PPyVP(COOC 12 H 25 ) 2 V at 3.6 eV excitation energy (D) and 2.8 eV excitation energy (0), and PVK at 3.6 eV excitation energy (...), all on quartz substrates, in accordance with one embodiment of the present invention. Figure 4 is a graph showing the photoluminesence spectra of a bilayer of PVK and the copolymer PPyVP(COOC| 2 H 25 ) 2 V in a light-emitting device in accordance with one embodiment of the present invention, as a function of both excitation and emission energy. Figure 5 shows graphs of the absorption and photoluminesence excitation (PLE) spectra of (a) a single layer of the copolymer PPyVP(COOC, 2 H 25 ) 2 V ( ), (b) a single layer of PVK ( . . . . ), and (c) a bilayer of PVK and the copolymer PPyVP(COOC| 2 H 25 ) V ( — ) in a light-emitting device in accordance with one — embodiment of the present invention. Figure 6 shows graphs of the electroluminesence and photoluminesence spectra of a light-emitting device in accordance with one embodiment of the present invention. Also shown are the current density vs. voltage and brightness vs. voltage data. Figure 7 shows the chemical structures (a) - (e) of the repeating units of copolymers that may be used in accordance with several embodiments of the present invention; structure (a) shows three alternative derivatives according to variations in moiety R, such that R, = OC, 6 H 33 , C 12 H 25 or COOC 12 H 25 , designated structures "ax," "ay" and "ex," respectively; structure (b) shows an unsubstituted "wrapped" copolymer, designated wPPyVPV; structure (c) shows three alternative derivatives of "wrapped" copolymers according to variations in moiety R, such that R, = OC 16 H 33 , C 12 H 25 or COOC 12 H 25 , designated wPPyVPV(ax), wPPyVPV(ay), and wPPyVPV(cx), respectively . Structure (d) of Figure 7 is designated wPTP and structure (e) is designated wPDTP. Figures 8a through 8e are graphs showing the photoluminesence and PLE spectra of several polymer/copolymer materials that may be used in accordance with several embodiments of the present invention. Figure 9 is a graph showing the photoluminesence spectra of several wPPyVPV(ax)/PVK blends in accordance with several embodiments of the present invention; graph (a) showing PL film efficiency as a function of wPPyVPV(ax) content, and graph (b) showing PL relative intensity and normalized PL intensity as a function of energy. Figures 10 and 1 1 show the PL spectra of single layers of PVK and each of the copolymers along with the corresponding bilayers. Figure 10 shows (a) the film PL spectra of PPy VPV (structure ay; solid lines), PVK (♦) and a bilayer of PVK/PPyVPV (structure ay) (dashed lines); and (b) the film PL spectra of PPy VPV (structure ex; solid lines), PVK (♦) and a bilayer of PVK/PPy VPV(structure ex) (dashed lines). Figure 11 shows the PL for single layer films (solid lines), bilayer films (dashed lines) and PVK( ), for three copolymers wPPyVPV(ax), wPPyVPV(ay), and wPPyVPV. Figure 12 shows the EL of two bilayer devices (see caption) along with the corresponding PL results. The EL and PL of these bilayer devices are substantially the same, demonstrating that the EL originates from the exciplex states formed at the interface between the hole- and electron-transporting layers. Detailed Description of the Preferred Embodiments In accordance with the foregoing summary of the invention, the following describes preferred embodiments of the present invention which are presently considered to be the best mode of the invention. Example of a Sample Preparation A conjugated polymer light-emitting device consists of an emitting material (layer) sandwiched between two electrodes, one of which is preferably transparent. Indium-tin-oxide was used as the positive transparent electrode (anode) and aluminum was most often used as the negative electrode (cathode). The ITO-coated glass (commercially available from Donnelly Applied Films or Delta Technologies Ltd.) was purchased in large sheets 12" x 12". The ITO-coated glass was cut into — appropriate size pieces (typically 2 cm x 2 cm) by the glass shop. Each individual substrate then was etched before use. The etching was done with a solution of 20 % HC1, 5% HN0 3 , and 75% distilled water, by volume, heated to -50 to 60° C. After etching, the ITO substrates were cleaned. The emitting polymer was then spin-coated onto the clean etched ITO substrate or on top of a previously spin coated layer of PVK from the appropriate solvents. PPy was cast from formic acid solution and the copolymers of PPy V and PPV were cast from tetrahydrofuran (THF), xylene or chloroform. Solution concentrations of the copolymer were typically s-10 mg/ml. The powders were weighed on a balance after which the appropriate solvent was added. The solutions were stirred with a spin bar for at least 1 hour or until the powders were almost completely dissolved. The solutions were filtered with either a 1 micron or 0.2 micron pore filter and stored in a hood until used. In the class 100 cleanroom, the films were made by dropping 3 - 5 drops of solution from a pipette on to the ITO glass substrate. The substrate was then immediately spun at speeds ranging from 1000 to 2000 rpm. Following spin-coating the top electrode (Al or Au) was vacuum deposited (evaporated) at pressures below -10 " torr. A mask was used to evaporate the appropriate electrode pattern. To prevent damage due to heating during evaporation, the substrates were mounted on a cold water cooled stage during deposition. In addition, evaporation rates were -0.5-1.4 A/s for the first 100 A of deposition and then increased to -3-5 A/s until the desired thickness was reached (usually 1000 to 2000 A). The device performance was improved by inserting either a hole-transporting — layer between the anode and emitter or an electron-transporting layer between the emitter and cathode. The most commonly used hole-transporting layer, poly(9-vinyl carbazole) (PVK), was cast from THF (10 mg/ml) onto the ITO at 3000 rpms. When fabricating multilayer devices, the choice of solvents is critical. The second layer should not dissolve the original layer and the solvents should be compatible enough to make uniform films. Each of the copolymers was treated identically although the solution concentration may have been slightly different. For the PL experiments the Al electrodes were not evaporated. After fabrication some devices are annealed at 80° C for 2 hours. The photoluminesence and electroluminesence results were as follows: Figure 3 is a graph showing the photoluminesence spectra of PPyVP(COOC 12 H 25 ) 2 V at 2.8 eV excitation energy (— ), a bilayer of PVK and PPyVP(COOC, 2 H 25 ) 2 V at 3.6 eV excitation energy (D) and 2.8 eV excitation energy (O), and PVK at 3.6 eV excitation energy (...), all on quartz substrates, in accordance with one embodiment of the present invention. The PL of single PVK layers excited at 3.6 eV has a peak emission energy at 3.05 eV, similar to previous reports of the PL of PVK. The PL for single layer copolymer films excited at 3.1 eV shows an emission peak at 2.05 eV. The bilayer when excited at an energy less than the absorption edge of the PVK, but greater than the absorption edge of the copolymer, shows PL peaked at the same energy as for the copolymer along with a low intensity tail to the blue side. When the bilayer was excited at energy equivalent to the excitation energy for the single PVK layer (3.6 eV), the PL emission spectrum contains contributions from both single layers (3.05 eV and 2.05 eV), as well as from- a completely new species, which may be identified with an exciplex. To the low energy side of the exciplex PL is a weak shoulder near the PL energy for the single layer of the copolymer. Figure 4 is a graph showing the photoluminesence spectra of a bilayer of PVK and the copolymer PPyVP(COOC 12 H 25 ) 2 V in a light-emitting device in accordance with one embodiment of the present invention, as a function of both excitation and emission energy. The 3D plot shows three prominent features: a peak due to the PVK (excitation energy from 3.6 to 4.2 eV, emission energy 2.8 to 3.4 eV), a peak due to the copolymer (excitation energy from 2.4 to 3.0 eV, emission energy 1.8 to 2.2 eV), and the exciplex peak (excitation energy from 3.6 to 4.2 eV, emission energy 2.2 to 2.8 eV). At excitation energies above 3.6 eV the PL due to the exciplex and PVK are apparent, but if the excitation energy is lowered below 3.4 eV these peaks have essentially disappeared. As the excitation energy is further lowered into the peak absoφtion of the copolymer, PL from the copolymer strongly predominates (excitation energy 2.6 to 3.0 eV and principal emission energy 1.8 to 2.2 eV). The 3D plot shows three prominent features: a peak due to the PVK (excitation energy from 3.6 to 4.2 eV, emission energy 2.8 to 3.4 eV), a peak due to the copolymer (excitation energy from 2.4 to 3.0 eV, emission energy 1.8 to 2.2 eV), and the exciplex peak (excitation energy from 3.6 to 4.2 eV, emission energy 2.2 to 2.8 eV). Figure 5 shows graphs of the absorption and photoluminesence excitation (PLE) spectra of (a) a single layer of the copolymer (b) a single layer of PVK, and (c) a bilayer of PVK and the copolymer PPyVP(COOCι 2 H 25 ) 2 V, on quartz substrates, in a light-emitting device in accordancσ with one embodiment of the present invention. The PLEs were recorded at 2.05, 3.05, and 2.55 eV, respectively. The copolymer absorption is 5 times less than shown. The absorption and photoluminescence excitation (PLE) spectra are shown in Figures 5a and 5b. The onset of the absorption of the single PVK layer is at about 3.5 eV and shows two spectral features at 3.6 and 3.75 eV similar to previous reports. The PLE of PVK follows the absoφtion showing nearly identical features. The absorption and PLE of the copolymer peak at 2.95 eV, with the onset at about 2.4 eV. The absorption of the bilayer is the sum of the single PVK layer absoφtion and the single copolymer absorption and shows both the copolymer peak at 2.95 eV and the PVK peaks at 3.6 and 3.75 eV. The PLE of the bilayer is also the sum of the PVK PLE and the copolymer PLE and shows both the PVK spectral features and the copolymer peak, although the copolymer peak is shifted to slightly higher energy. The lack of any new absorption or PLE features in the bilayer film implies that the new species is not directly accessible from the ground state of the copolymer or PVK. and is thus consistent with the assignment of an exciplex. The PL, PLE and absorption were measured on the same films making it possible to estimate the relative PL quantum efficiencies of the copolymer emission and the exciplex emission. The copolymer absolute PL efficiency was reported previously to be 18%. A lower bound on the quantum efficiency of the exciplex was calculated to be 15-20%, nearly the same as the copolymer efficiency. Figure 6 shows graphs of current-voltage( — ) and brightness-voltage (G) characteristics of a typical bilayer light-emitting device. Inset: PL (...) and EL ( — ) of a bilayer light-emitting device, in accordance with one embodiment of the present" invention. Bilayer devices were fabricated using ITO as the anode and aluminum as the cathode. The inset of Figure 6 shows the EL spectrum of a typical device with the PL spectrum from the same device. The devices can easily be seen in a bright lit room, appear bright green to the eye, and have internal quantum efficiencies of - 0.1-0.5%. Although the PL efficiencies are comparable, the EL efficiency of the bilayer configuration ~ 0.1-0.5% is much greater than for a single layer device which has an EL efficiency of less than 0.0001%. The similarity between the PL and EL of the bilayer device demonstrates that the exciplex is responsible for the EL emission. Figure 6 shows the current density-voltage and brightness-voltage characteristics for a typical bilayer device. The turn-on voltage of the bilayer devices depends on the thickness of the polymer layers and in this case is ~ 18 volts, with the brightness following the current. The generality of this concept has been demonstrated using several other pyridine-based copolymers. Through the use of polyaniline network electrodes, the threshold voltage was lowered to below 5 volts while maintaining the same efficiency. The increase in efficiency of the bilayer device compared to the single layer device appears to be due primarily to charge confinement at the PVK/copolymer interface. The electrons are injected from the Al electrode into the conduction band of the copolymer, but are confined when they reach the electron blocking PVK. Also, the holes are injected into the valence band of the PVK and are confined at the interface. The electron and hole blocking at the interface enhances exciplex emission. That the electron and holes are unable to easily conduct through both layers leads to a small current density (< or ~ ImA/mm ) and hence a greatly increased efficiency. In- addition, the buried interface implies that most of the radiative recombination will occur at the interface and away from the EL quenching electrodes. A wide range of devices were fabricated with a variety of different emitting polymers and hole transport layers, including bilayer devices with the following emitter layers: poly(pyridyl vinylene), PPyVP(R) 2 V (R=OC 16 H 33 , R=C 12 H 25 , R=COOC 12 H 25 ), a strapped copolymer PPyVP(R) 2 V (with RAH, R=OC 16 H 33 , C| 2 H 5 ), poly(thienylene phenylene) with a strap, and poly(dithienylene phenylene) with a strap. The molecular repeat unit of these copolymers is shown in Figure 7, structures (a) - (e). Figures 8a through 8e are graphs showing the electroluminesence and photoluminesence spectra of several polymer/copolymer materials that may be used in accordance with several embodiments of the present invention. In Figure 8(a) the polymer referred to as "C 12 H 25 " is that shown in Figure 7(a) where R=C 12 H 25 . In Figure 8(b), the polymer is that shown in Figure 7(a) wherein R=COOC 12 H 25 . In Figure 8(c), the polymer referred to as "D40" is that shown in Figure 7(c) wherein R=OCι 6 H 33 . In Figure 8(d) the polymer referred to as "D41" is that shown in Figure 7(c) wherein R= C 12 H 25 . In Figure 8(e). the polymer referred to as "Dl 12" is that shown in Figure 7(b). In each case, the PVK was used as the hole-transporting layer. Blends of these polymers/copolymers with PVK show PL emission due to- exciplex formation (Figure 9) and are expected to show EL emission from exciplexes. The generality of using pyridyl containing polymers as the emitting polymer thus has been shown and it is expected that the other light-emitting polymers such as polyparaphyenylene (PPP), polyparaphenylene vinylenes (PPV), polythiophenes (PT) and their derivatives and/or copolymers to behave similarly based on chemical and electronic similarities. Other hole-transporters such as PPPs, PPVs, polymethyl methacrylate, polystyrene, polyethylene, poly(ethylene teraphthalate) and blends of these materials are expected to behave similar to PVK. Figure 9 is a graph showing the photoluminesence spectra of several wPPy VPV(ax)/PVK blends in accordance with several embodiments of the present invention; graph (a) showing PL film efficiency as a function of wPPyVPV(ax) content, and graph (b) showing PL relative intensity and normalized PL intensity as a function of energy. Figures 10 and 1 1 show the PL spectra of single layers of PVK and each of the copolymers along with the corresponding bilayers. Figure 10 shows (a) the film PL spectra of PPy VPV (structure ay; solid lines), PVK (♦) and a bilayer of PVK/PPyVPV (structure ay) (dashed lines); and (b) the film PL spectra of PPy VPV (structure ex; solid lines), PVK (♦) and a bilayer of PVK/PPyVPV(structure ex) (dashed lines). Figure 11 shows the PL for single layer films (solid lines), bilayer films (dashed lines) and PVK( ), for three copolymers wPPyVPV(ax), wPPyVPV(ay), and wPPyPV. Figure 12 shows the EL of two bilayer devices (see caption) along with the corresponding PL results. The EL and PL of these bilayer devices are substantially the same, demonstrating that the EL originates from the exciplex states formed at the — interface between the hole- and electron-transporting layers. In summary, heteroj unctions of PVK and PPyVP polymers show a strong photoluminescence and electroluminescence feature due to exciplex emission at the interface. The absoφtion and PLE spectra and have shown that the exciplex is not directly accessible from the ground state. The exciplex is also the primary species of electroluminescence emission in the bilayer devices. The efficiency of the bilayer devices is greatly enhanced over single layer devices due to charge confinement and exciplex formation and emission at interface. Thus, the bilayer devices of the described embodiment of the present invention use PVK as the hole transport layer and a derivative of the copolymer PPy VPV as the emitter layer. Absorption, photoluminesence and electroluminesence results are consistent with emission due to exiplex formation between the PVK and PPy VPV copolymer. The PL and EL of bilayer films are dramatically different from that of the single layer. 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, so as to be able to practice the present invention without departing from its spirit as reflected in the appended claims. It is the intention, therefore, to limit the inventiorr only as indicated by the scope of the claims.

Claims: What is claimed is: — 1. An electroluminescent device comprising: (a) a first polymeric layer adapted to act as a hole transport/electron blocking layer; and (b) a second polymeric layer adapted to act as an electron transporting emissive layer, said first polymeric layer and said second polymeric layer being in electrical contact so as to form an interface, said interface capable of producing an exciplex light emission upon a current being passed through said interface. 2. An electroluminescent device according to claim 1 wherein said first polymeric layer comprises a carbazole-containing polymer. 3. An electroluminescent device according to claim 1 wherein said first polymeric layer comprises a poly(vinyl carbazole). 4. An electroluminescent device according to claim 1 wherein said second polymeric layer is selected from the group consisting of a poly (pyridyl vinylene phenylene vinylene), poly (thienyl phenylene) and poly (dithienyl phenylene) polymers, copolymers and oligomers, and substituted and wrapped derivatives thereof. 5. An electroluminescent device according to claim 1 additionally comprising a source of direct electric current adapted to provide a flow of electrons through said second polymeric layer to said interface. 6. An electroluminescent device comprising: (a) a first polymeric layer adapted to act as a hole transport/electron blocking layer; and (b) a second polymeric layer adapted to act as an electron transporting — emissive layer, said first polymeric layer having greater hole transport capability than said second polymeric layer, and said second polymeric layer having greater electron transport capability than said first polymeric layer, said first polymeric layer and said second polymeric layer being in electrical contact so as to form an interface, said interface capable of producing an exciplex light emission upon a current being passed through said interface. 7. An electroluminescent device according to claim 6 wherein said first polymeric layer comprises a carbazole-containing polymer 8. An electroluminescent device according to claim 6 wherein said first polymeric layer comprises a poly(vinyl carbazole). 9. An electroluminescent device according to claim 4 wherein said second polymeric layer is selected from the group consisting of a poly (pyridyl vinylene phenylene vinylene), poly (thienyl phenylene) and poly (dithienyl phenylene) polymers, copolymers and oligomers, and substituted and wrapped derivatives thereof. 10. An electroluminescent device according to claim 6 additionally comprising a source of direct electric current adapted to provide a flow of electrons through said second polymeric layer to said interface. 1 1. A laser light generating device comprising: (a) a first polymeric layer adapted to act as a hole transport/electron blocking layer; and (b) a second polymeric layer adapted to act as an electron transporting emissive layer, said first polymeric layer and said second polymeric layer being in electrical contact so as to form an interface, said interface capable of producing optically pumped laser light upon being irradiated with light so as to create a critical density of exciplex light emission at said interface; and (c) a source of light of sufficient intensity and of sufficiently short wavelength o as to be capable of irradiating said first and second polymeric layers with light so s to create a critical density of exciplex light emission at said interface. 12. A laser light generating device comprising: (a) a first polymeric layer adapted to act as a hole transport/electron blocking layer; and (b) a second polymeric layer adapted to act as an electron transporting emissive layer, said first polymeric layer and said second polymeric layer being in electrical contact so as to form an interface, said interface capable of producing electrically pumped laser light upon being supplied with a sufficient amount of current so as to create a critical density of exciplex light emission at said interface; and (c) a source of electrical current adapted to supply said sufficient amount of current.

Inventors: EPSTEIN ARTHUR J, GEBLER DARREN D, WANG YUNZHANG, EPSTEIN, ARTHUR, J., GEBLER, DARREN, D., WANG, YUNZHANG
Applicants: UNIV OHIO STATE RES FOUND, THE OHIO STATE UNIVERSITY RESEARCH FOUNDATION
Application year: 1998
Main classification: H01J1
IPC codes: C09K11, H01L33, H01L51, H01S3, H05B33