ORGANIC LUMINESCENT ELEMENT AND ITS SUBSTRATE

Abstract: An electro-optic transducing element used in the field of information communication. This element has a semi-transparent reflecting layer (2) on a transparent substrate (1), a transparent conductive layer (3) on the semi-transparent reflection layer (2), a luminescent layer (5) of an organic thin film on the transparent conductive layer (3), and electrodes (7) formed on the layer (5). The semi-transparent reflective layer (2) transmits a part of the light emitted by the luminescent layer (5) toward the transparent substrate (1), and reflects a part of the light toward the luminescent layer (5). The semi-transparent reflection layer (2) forms resonators together with the electrodes (7).

Description: The present invention relates to an organic light emitting device suitable for use in display devices, light emitting devices for communications, information file reading/writing heads, printing apparatuses, and similar devices and apparatuses, and to a substrate plate for use in its manufacture. Description of the Prior Art Previously proposed organic light emitting devices include, for example, an organic electro-luminescent cell disclosed in the Japanese Patent Application Laid-Open No. 57-51781. The cell has a light emitting body of organic light emitting material and electrically insulating binder interposed between transparent substrates having transparent electrodes of indium tin oxide or the like. A compound layer of a porphyrin system is formed between the positive one of the transparent electrodes and the light emitting body. The organic electro-luminescent cell has a hole injecting body of the compound of a porphyrin system to inject holes on the basis of a signal voltage applied between the two electrodes, to cause light to be emitted. Such light emitting devices having an organic thin film used therein are advantageous in fabrication at low cost. However, their use is limited to display panels as their spectral half-widths are too wide. They are also available only for a single display color as the specific material can emit only one color of light. Object and Advantages It is an object of the present invention to provide an organic light emitting device having improved spectra width and light emitting characteristics. Another object of the present invention is to provide a substrate plate for use in manufacture of an organic light emitting device. SUMMARY OF THE INVENTION In a first aspect, the present invention provides an organic light-emitting device as set out in claim 1. In a second aspect the invention provides a substrate plate for use in the manufacture of the organic light emitting device of the invention, as set out in claim 12. In the organic light emitting device of the invention, preferably there is a hole-injecting layer between the transparent conductive layer and the light emitting layer. Preferably also there is an electron-injecting layer between the light emitting layer and the second electrode. Preferably the optical distance L between the semi-transparent reflective layer and the second electrode on the light emitting layer is given 0.9 n P < L < 1.1 n P where n is a positive integer and P is the peak wavelength of the light emitted by the device. The optical distance L between the semi-transparent reflective layer and the second electrode on the light emitting layer may be given by 0.9 P (integer - A/2π)/2 < L < 1.1 P (integer - A/2π)/2 where A is the shift of phase of the light reflected between the semi-transparent reflective layer and the second electrode in radians, and P is the peak wavelength of light emitted from the device. Preferably the semi-transparent reflective layer is formed of a plurality of films of dielectric material. Preferably the semi-transparent reflective layer is formed as a totally reflective metal film having a window for passage of the emitted light out. The reflectance of the semi-transparent reflective layer may be 50 to 99.9% or the transmittance thereof 50 to 0.1%. The substrate plate is preferably formed of quartz, glass, or a plastics material and the semi-transparent reflective layer may have the transparent conductive film patterned thereon. In the organic light emitting device of the present invention, the light micro-resonator may be achieved in that the semi-transparent reflective film is between the transparent electrode and the substrate plate and the optical distance between the reflective film and the rear electrode is made equal to or integer times the emitted light wavelength. The micro-resonator can make the half-width of the emitted light spectral narrow. Also, the micro-resonator can increase the light emission efficiency, generate coherent light, and improve the light emission characteristics. BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings: Fig. 1 is a cross-sectional structure illustrating an organic light emitting device in a first embodiment of the present invention; Fig. 2 is a cross-sectional structure illustrating a previous organic electroluminescent (EL) light emitting device; Fig. 3 is a comparison graph illustrating light emission spectra of the devices in Figs. 1 and 2; Fig. 4 is a graph illustrating a curve of the half-width of light emission spectra with respect to a resonating portion of a resonator of a light emitting device embodying the present invention; and Fig. 5 is a graph illustrating a curve of optical distance of the resonator with respect to a peak wavelength of the light emitted out of the device having the semi-transparent reflective film. DETAILED DESCRIPTION OF THE EMBODIMENT The following describes in detail an embodiment according to the present invention by reference to the accompanying drawings. Fig. 1 depicts a cross-sectional structure illustrating an organic light emitting device in a first embodiment of the present invention. The organic light emitting device has a glass substrate 1. The glass substrate 1 has a semi-transparent reflective film 2 which is a laminated TiO 2 film and SiO 2 film. On the semi-transparent reflective film 2 is formed successively a transparent conductive film 3 of indium tin oxide (abbreviated ITO), a hole injecting layer 4 of triphenyl diamine derivative (TAD), a light emitting layer 5 of aluminum chelate compound, an electron injecting layer 6 of oxydiazole derivative (PBD), and an electrode 7 of Ag-Mg. The sum of optical distances obtained in terms of products of thicknesses of the transparent conductive film 3, the hole injecting layer 4, the light emitting layer 5, and electron injecting layer 6 multiplied by their respective refractive indexes should be made to coincide with 530 nm which is a peak wavelength at which the aluminum chelate compound makes electroluminescent (EL) light emission. This enables the resonator of present invention to function. The hole injecting layer 4 and the electron injecting layer 6 as shown in Fig. 1 is not always needed if high performance characteristics are not required. The organic light emitting device can be used without either or both of them. With the omission of these layers, holes are injected into the transparent conductive film 3 from the transparent conductive film and electrons from the metal electrode, thereby completing the organic light emitting device. The device, however, should preferably have the hole injecting layer 4 and the electron injecting layer 6 provided therein. Reflectance of the semi-transparent reflective film 2 should be selected with respect to performance characteristics of the light emitting layer 5 and application of the device. An upper limit of the device is restricted in terms of the energy that a light resonator can store without self-destruction. The reflectance is 50 to 99.9 % while transmissivity of the semi-transparent reflective film 2 is 50 to 0.1 %. To obtain an light emission of at least 10 µW/cm 2 , the transmissivity of the light to be fed out cannot be made lower than 0.1 %. The reflectance is limited to 99.9 %. If the reflectance is made too low, the light resonator cannot function. If it is lower than 50 %, the spectra width cannot be made sufficiently narrow. The above-mentioned aluminum chelate compound used for the light emitting layer 5 may be alternatively replaced by perylene derivative, perinone derivative, naphthalene derivative, coumarin derivative, oxadiazole, bisbensoxazoline, aldazine, pyrazine derivative, distilbenzine derivative, polyphenyl derivative, bisstilanthracene derivative, and chelate metal complex. Any of the above-mentioned organic thin films can be formed by way of evaporation, coating, growing by chemical reaction, the Langmuir project method, or similar techniques. The thin films also can be formed of adequate mixtures of a plurality of organic materials. Fig. 2 depicts a cross-sectional structure illustrating a previous organic electroluminescent (EL) light emitting device. This previous device is formed with the semi-transparent reflective film 2 removed from the structure shown in Fig 1. Fig. 3 depicts a comparison graph illustrating light emission spectra of the devices in Figs. 1 and 2. We can see that a spectra curve A of the device of the present invention in Fig. 1 has narrower half-width than the previous device in Fig. 2. This is due to the fact that the semi-transparent reflective film 2 makes resonance of light emission in the device to generate an electromagnetic wave of a resonant frequency selectively. Such a light emission resonance provides effects that make narrow the half-width of the light emission spectra, increase light emission efficiency, and generate coherent light. The effects can be further increased by making the optical distance of a resonating portion of the resonator rather close to the light emission wavelength. Fig. 4 depicts a graph illustrating a curve of the half-width of the light emission spectra with respect to the resonating portion of the resonator. The curve is the results obtained by a measurement of the device with only a thickness of the hole injecting layer 4 changed in the device structure in Fig. 1. In the graph, the half-width without the semi-transparent reflective film 2 is 100 %. We can see that the half-width is least around 530 nm at which the optical distance coincides with the peak wavelength, and becomes wide quickly as the wavelength is apart from 530 nm. We can also see from curve B in Fig. 3 that the device having no semi-transparent reflective film 2 has wavelengths of 480 and 580 nm when the light emission intensity is a half of that at 530 nm. This range corresponds to the one in which the half-width in Fig. 4 is made narrow. Fig. 5 depicts a graph illustrating a curve of the optical distance of the resonator with respect to the peak wavelength of the light emitted out of the device having the semi-transparent reflective film 2. The resonance provides the phenomenon that if the optical distance is deviated from 530 nm, the peak wavelength of the emitted light also is deviated from 530 nm. If the deviation is too large to cause light emission due to resonance, the peak of the whole light emission becomes close to the peak wavelength of 530 nm when the semi-transparent reflective film 2 is not provided. We can see from the results in Fig. 5 that the resonance effect can be obtained when the optical distance is in a range of 0.9 to 1.1 times the peak wavelength of the emitted light. This range corresponds to the one in which the half-width in Fig. 4 is made narrow. The aluminum chelate used as the material for the light emitting layer in the first embodiment may be alternatively replaced by not only compounds similar to that, but also any of mixtures, laminated layers, and similar organic materials that can emit light by way of electron-hole coupling. It is important that the device should have an arrangement to keep the device temperature constant to stabilize the resonated light emission. Optimum transmissivity and reflectance may vary depending on the device structure and the material forming the semi-transparent reflective film 2, but the absorptance should be preferably as close as to 0. The semi-transparent reflective film 2 may be a totally metal reflective film having patterning processed and having a window open for passing light out. Also, the device structure make it hard for light to leak laterally to increase the light emission characteristics further. As described so far, embodiment 1 has the optical distance as resonator made to equal to the light emission wavelength. The embodiment, also, can theoretically provide similar effects of resonance if the optical distance is times of integers, such as two times, three times, ..., n times the light emission wavelength, and if the optical distance is times of half-integers, such as 1/2 times, 3/2 times, ..., n/2 times the light emission wavelength. Also, this embodiment has the total light phase shift made 0 or 1 wavelength by reflections on the upper and lower mirror surfaces. The embodiment, also, can make resonance for devices having the light shifted 1/2 wavelength on the metal electrode surface and not shifted on the semi-transparent reflecting mirror if the optical length is made 1/4, 3/4, 5/4, and so on, times the light emission wavelength. However, actual devices are involved in blunt sharpness of resonance due to fabrication of the devices themselves. The above-mentioned resonance effects therefore are keen as the film is thin, and the resonance is unclear as the number of times is large. For the reason, a practical number of times is limited to 10. The device of structure in this embodiment makes use of field light emission by electric charge injection and also, can be modified to have a transparent substrate plate to irradiate a light to make the light emitting layer generate a fluorescent light, thereby producing spectra of light emission of narrow half-width similar to that of the field light emission. In such a modified example, it is possible for the device to omit the transparent electrodes and the organic thin films other than the light emitting layer. As described above, the organic light emitting devices of the present invention provide such effects as increasing the light emitting characteristics, including making narrow the half-width of light emission spectra, increasing the light emission efficiency, and generating coherent light. The organic light emitting devices of the present invention can replace the previous light emitting diodes and semiconductor lasers formed of GaAs, SiC, ZnSe, and similar inorganic semiconductors. They are available for optical communication devices, information display panels, reading/writing heads for optical record file, and optical heads for laser printer.

Claims: An organic light emitting device, comprising a transparent substrate (1), a semi-transparent reflective layer (2) on the transparent substrate, a transparent conductive layer (3) serving as a first electrode on the semi-transparent reflective layer, a light emitting layer (5) in the form of an organic thin film on the transparent conductive layer, and a second electrode (7) serving as a reflecting mirror formed on the light emitting layer, wherein the semi-transparent reflective layer partially transmits the light emitted by the light emitting layer to the transparent substrate and partially reflects the light to the light emitting layer, and the semi-transparent reflective layer and the second electrode on the light emitting layer are arranged to act together as an optical resonator. An organic light emitting device according to claim 1, further comprising a hole injecting layer (4) disposed between the transparent conductive layer (3) and the light emitting layer (5), and an electron injecting layer (6) disposed between the light emitting layer (5) and the second electrode (7). An organic light emitting device according to claim 2, wherein sum of optical distances of the transparent conductive layer (3), the hole injecting layer (4), the light emitting layer (5), and the electron injecting layer (6) each given by the respective thickness multiplied by the respective refractive index is in the range 0.9 to 1.1 times the peak wavelength of the emitted light of the device. An organic light emitting device according to claim 1, further comprising a hole injecting layer (4) disposed between the transparent conductive layer (3) and the light emitting layer (4). An organic light emitting device according to claim 1, further comprising an electron injecting layer (6) disposed between the light emitting layer (5) and the second electrode (7). An organic light emitting device according to any one of claims 1 to 5, wherein the optical distance L between the semi-transparent reflective layer (2) and the second electrode (7) on the light emitting layer is given by L = (integer - A/2π) x P, provided that S < 2L < T where A is the shift of phase of the light reflected between the semi-transparent reflective layer and the second electrode in radians, P is the peak wavelength of light emitted from the device, and S and T are wavelengths at which in a light emission spectrum of a light emitting device having no semi-transparent reflective layer the intensity of the emitted light is one half of the peak intensity of the emitted light. An organic light emitting device according to any one of claims 1 to 5, wherein the optical distance L between the semi-transparent reflective layer (2) and the second electrode (7) on the light emitting layer is given by 0.9 n P < L < 1.1 n P where n is a positive integer, and P is the peak wavelength of light emitted from the device. An organic light emitting device according to any of claims 1 to 5, wherein an optical distance L between the semi-transparent reflective layer (2) and the second electrode (7) on the light emitting layer is given by 0.9 P (integer - A/2π)/2 < L < 1.1 P (integer - A/2π)/2 where A is the shift of phase of the light reflected between the semi-transparent reflective layer and the second electrode in radians, and P is the peak wavelength of light emitted from the device. An organic light emitting device according to any one of claims 1 to 8, wherein the semi-transparent reflective layer (2) is formed of a plurality of films of dielectric material. An organic light emitting device according to any one of claims 1 to 8, wherein the semi-transparent reflective layer (2) is formed as a totally reflective metal film having a window for passage of the emitted light out. An organic light emitting device according to any one of claims 1 to 9, wherein the reflectance of the semi-transparent reflective layer (2) is 50 to 99.9% or the transmittance thereof is 50 to 0.1%. A substrate plate suitable for use in the manufacture of an organic light emitting device according to any one of claims 1 to 11, comprising a transparent substrate plate (1), a semi-transparent reflective layer (2) on the transparent substrate plate formed of a plurality of films of dielectric material and capable of partially transmitting light and partially reflecting light, and a transparent conductive film (3) to serve as an electrode formed on the semi-transparent reflective layer. A substrate plate according to claim 12, wherein the reflectance of the semi-transparent reflective layer (2) is 50 to 99.9% or the transmittance thereof is 50 to 0.1%. A substrate plate according to claim 12 or 13, wherein the transparent substrate plate (1) is formed of quartz, glass, or a plastics material, and the transparent conductive film (3) is patterned on the semi-transparent reflective layer (2).

Inventors: HATTORI SHINTARO, ITO YUZO, KAKUTA ATSUSHI, NAKAYAMA TAKAHIRO, HATTORI, SHINTARO, ITO, YUZO, KAKUTA, ATSUSHI, NAKAYAMA, TAKAHIRO
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1993
Main classification: H05B33
IPC codes: H05B33, H01L51, H04N1, C09K11, H01S5, H01L33