Thin-film electroluminescence apparatus including optical interference filter

Abstract: An electroluminescence apparatus in which a voltage is applied to a lamination of a fluorescent material layer and a dielectric layer through a pair of electrodes one of which is light-transmissible to extract fluorescence. A reflecting mirror layer is provided on the lamination while a particular relationship is established between the refractive index and the thickness of the lamination, thereby improving the efficiency with which the fluorescence is extracted.

Description: BACKGROUND OF THE INVENTION This invention relates to thin-film electroluminescence apparatus and, more particularly, to a thin-film electroluminescence apparatus suitable for thin-film flat displays for use with information terminal of office automation systems. A display based on a thin-film electro-luminescence (hereinafter referred to simply as "thin-film EL") apparatus has been proposed which has a construction described below. Fig. 1 shows a structure in which dielectric layers 4 and 6 are provided on two sides of a fluorescent material layer 5, and these layers are interposed between a transparent electrode 2 and a back electrode 7. A corresponding structure is disclosed in DE2 260 205 B2 Thin-film EL displays in which ZnS: Tb, F for green luminescence or ZnS: Mn for orange luminescence is used for the fluorescent material layer 5 are known. In all cases, emitted light is extracted through a glass surface on one side of the layers where the transparent electrode is provided, and the intensity of light thereby extracted is at most about 10% of that of the light emitted from the emission center of the fluorescent material layer. This cause is based on the Fresnel's law, that is 90% or more of the light emitted from the emission center of the fluorescent material layer is reflected by the interface between the fluorescent material layer and the dielectric layer or between the latter and the transparent electrode. This is because the angle of total reflection to the emission wavelength is considerably small, that is, it is about 25°. On the other hand, a method is known in which a Fabry-Perot interferometer is used for selecting the wavelength of light emitted from a light source having a wide range of emission wavelength. The Fabry-Perot interferometer allows transmission of light only when the light satisfies the following optical interference condition: L·q = K·π (π: circular constant) where L represents the distance between a pair of reflecting mirrors 8 disposed parallel to each other as shown in Figs. 2a and 2b, q represents the number of waves between the reflecting mirrors, and K is a positive integer. It has been actually found that as the reflectivity R of the reflecting mirrors is increased, the half width of the spectrum of light becomes narrower, as shown in Figs. 3a and 3b. This phenomenon is described on pages 51 to 56 of Laser Physics Nyumon (Introduction to Laser Physics) written by Khoichi Shimota (published on Apr. 22, 1983 by Iwanami Shoten). It is also known that this interferometer can be used as a laser resonator if a laser medium is inserted in the interferometer. A thin film interposed between repetition multilayer films (multilayer-film optical interference filter) has a structure such as that shown in Fig. 4. It has been revealed that the interference characteristics of a thin film having this type of structure including reflecting layers formed on two sides of the film and having a high reflectivity ensure the same effects as the Fabry-Perot interferometer, as shown in Fig. 5. This type of thin film is formed by laminating optical thin films having different refractive indexes while setting the film thicknesses so as to satisfy the conditions for prevention of reflection with respect to the emission wavelength λ, that is, (n·d = (1/4 + m/2)·λ where n represents the refractive index, d represents the film thickness, and m = 0, 1, 2 ...). Explanations relating to this thin film are found on pages 30 to 34 and 98 to 129 of Optical Thin Film edited by Shiro Fujiwara (published on Feb., 25, 1985 by Kyoritsu Shuppan). The thin-film EL apparatus shown in Fig. 1 has an advantage in being easily manufactured, and thin-film EL displays based on this apparatus have been put to practical use. However, colors of these displays are limited to orange based on the use of ZnS: Mn for the fluorescent material layer and green based on the use of ZnS: Tb. To manufacture a thin-film EL display capable of displaying three elementary colors, materials for the fluorescent material layer are required which enable emission of light having red and blue emission colors with a high emission efficiency, but fluorescent layer materials have been not yet developed for realization of a practical display. Further it has been very important to improve the emission efficiency. SUMMARY OF THE INVENTION The present invention is devised in view of the above-mentioned problems sticking to the prior art electroluminescent apparatus, and accordingly, a main object of the present invention is to provide a thin-film electroluminescence apparatus which can produce bright light of three elementary colors with a high degree of luminescent efficiency. According to the present invention there is provided a thin-film electroluminescence apparatus including an optical interference filter, comprising: a light-transmissible electrode layer; a light reflecting electrode layer; a fluorescent material layer or a laminated structure of a fluorescent material layer and a dielectric material layer, a voltage being applied to the fluorescent material layer or the laminated structure through the electrode layers; and a multilayer-film optical interference filter capable of selectively transmitting light emitted from the fluorescent material layer and having an arbitrary wavelength λ, the optical interference filter being provided on a light extraction side of the fluorescent material layer or the laminated structure, the optical interference filter being formed of at least one first dielectric film having a smaller refractive index and at least one second dielectric film having a larger refractive index, the first and second dielectric films being alternately laminated based on an equation λ/4 = film thickness × refractive index in the order of the second dielectric film and the first dielectric film, the fluorescent material layer or the laminated structure being formed by laminating a fluorescent material layer having a refractive index larger than that of the second dielectric film based on an equation λ/2*N = film thickness × refractive index (where N is an integer equal to or larger than 1). In this construction, a means which has the same function as a Fabry-Perot interferometer is provided in the thin-film EL apparatus, and light spontaneously emitted from the fluorescent material layer can be extracted while the direction of transmission is uniformly set with respect to an emission wavelength selected as desired. Light which is emitted from the emission center in the fluorescent material layer and which has a desired wavelength can therefore be extracted through the display surface at an improved efficiency, thereby obtaining three elementary colors, red, blue and green, with an emission efficiency ten times higher than that attained by the conventional apparatus. The structure of the multilayer-film optical interference filter thus restricted makes it possible to effectively apply an electric field to the fluorescent material layer. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a cross-sectional view of the structure of a conventional thin-film EL apparatus; Figs. 2a and 2b are diagrams of a Fabry-Perot interferometer; Figs. 3a and 3b are diagrams of the principle of a function of the Fabry-Perot interferometer; Fig. 4 is a diagram of a multilayer-film optical interference filter; Fig. 5 is a diagram of a basic characteristic of the multilayer-film optical interference filter; Fig. 6 is a cross-sectional view of the basic construction of a thin-film EL apparatus which represents a first embodiment of the present invention; Fig. 7 is a diagram of luminance-voltage characteristics of the thin-film EL apparatus in accordance with the embodiment; Figs. 8 to 10 are diagrams of spectra of light emitted by the thin-film EL apparatus which represents the first embodiment of the present invention; and Fig. 11 is a cross-sectional view of the basic construction of a thin-film EL apparatus which represents a second embodiment of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Explanation will be made of preferred embodiments of the present invention with reference to the drawings. Embodiment 1 A first embodiment of the present invention will be described below with reference to the accompanying drawings. Fig. 6 shows in section a basic construction of a thin-film EL apparatus in accordance with the sixth embodiment of the present invention. A transparent electrode 52 is formed on a glass substrate 51, and a first dielectric layer (a) 54a having a refractive index nl of about 2.4 with respect to the emission wavelength and having a dielectric constant εl and a thickness dl is formed on the electrode 52. An optical thin film having a refractive index n2 of about 1.5 and a thickness d2 (e.g., film of MgF 2 (n1 = 1.38) or SiO 2 (n1 = 1.52)) is formed as a second dielectric layer (b) 54b on the first dielectric layer (a) 54a. Another dielectric thin film identical with the first dielectric layer (a) is successively superposed as a first dielectric layer (c) 54c, and a still further dielectric layer (d) 54d having the refractive index n2 and the thickness d2 is successively superposed. A. fluorescent material layer 55 having refractive index n3 of about 2.4 and a thickness d3 is formed on the dielectric layer (d) 54d, and a dielectric thin film having a refractive index n4 of about 2.4 ± 0.2 close to n3 and having a thickness d4 is formed as a second dielectric layer 56 is formed on the fluorescent material layer 55. Back electrodes 57 having the function of a reflecting mirror layer as well as the function of an electrode layer are formed on the second dielectric layer 56. A thin-film EL apparatus having this structure was manufactured and the refractive indexes nl, n2, n3, and n4 of the first dielectric layers (a) to (d), the fluorescent material layer and the second dielectric layer with respect to an emission wavelength λ0 were measured with an ellipsometer. The thicknesses dl, d2, and d4 of the dielectric layers and the thickness d3 of the fluorescent material layer were determined so as to satisfy the following equations based on the multilayer-film optical interference filter design method: ni·di = λ0/4 (i = 1, 2) n3·d3 = λ0/2·N n4·d4 = λ0/2·N (N: positive number (1, 2, 3 ...)) That is, an EL device having the function of electroluminescence as well as the function of an optical interference multilayer-film filter was formed. It was confirmed that the thin-film EL apparatus of this embodiment shown in Fig. 6 had a voltage-luminance characteristic such as that shown in Fig. 7(b), and that the luminance from the fluorescent material layer could be efficiently extracted through the luminescence surface. The fluorescent material layer was formed by using a fluorescent material selected from the group consisting of ZnS: Mn which emits orange light with a main emission wavelength of 580 nm, ZnS: Tb, F or ZnS: Tb, P which emits green light with a main emission wavelength of 544 nm, CaS: Eu or ZnS: Sm which emits red light with a main emission wavelength of 650 nm, and SrS: Ce or ZnS: Tm which emits blue light with a wavelength of about 480 nm. The materials of the first dielectric films (a) to (d) and the second dielectric film were selected from yttrium oxide, tantalum oxide, aluminum oxide, silicon oxide, silicon nitride and perovskite-type oxide dielectric materials represented by strontium titanate, barium tantalate and the like in consideration of the refractive index with respect to the emission wavelength. Table 1 shows the characteristics of the dielectric films used for the present invention. Constituent material Dielectric breakdown field strength Dielectric constant n SiO 2 6 ~ 10 3.9 ~ 1.4 Al 2 O 3 2 ~ 8 8.5 ~ 1.5 Ta 2 O 5 0.5 ~ 4 25 ~ 2.3 HfO 2 0.2 ~ 4 16 ~ 2.2 Y 2 O 3 0.5 ~ 4 10 ~ 14 ~ 2.0 Si-O-N 5 ~ 8 4 ~ 1.5 Si 3 N 4 7 6.8 ~ 2.0 PbTiO 3 0.5 30 ~ 200 ~ 2.5 a-BaTiO 3 3 ~ 5 10 ~ 40 ~ 2.2 SrTiO 3 0.5 ~ 3 20 ~ 16 ~ 2.5 Ba(Sn, Ti)O 3 1 ~ 6 20 ~ 16 ~ 2.5 Sr(Zr, Ti)O 3 1 ~ 6 20 ~ 16 ~ 2.5 BaTa 2 O 6 3 ~ 5 22 ~ 2.3 PbNb 2 O 6 1.5 40 ~ 60 ~ 2.4 The thickness of each of the dielectric layers and the fluorescent material layer of this embodiment was determined by using the equations (1), (2), and (3) and values of the emission wavelength λ0 and the refractive index n of the dielectric layers and the fluorescent material layer determined by the ellipsometer and by measurement of optical transmittance with respect to the wavelength of light emitted from the fluorescent material layer. It was confirmed that the present invention enabled manufacture of a thin-film EL apparatus capable of emitting light with a desired emission wavelength with a high efficiency. The increase in the emission efficiency was greater as the half width with respect to the selected emission wavelength was reduced. The reflectivity of the reflecting mirror layer formed of the optical interference multilayer-film filter where the luminescence was extracted was set to be smaller than that of the reflectivity of the back electrodes. Figs. 8, 9, and 10 show spectra of a thin-film EL apparatus manufactured by using ZnS: Tb, F, ZnS: Sm, and SrS: Ce for the fluorescent material layer. It was demonstrated that the present invention enabled manufacture of a thin-film EL apparatus capable of emitting light of a desired wavelength with an efficiency which is 5 to 80 times higher than that attained by the conventional thin-film EL apparatus having no multilayer-film optical interference filter and no reflecting mirror layer, and also capable of selecting desired luminescence colors that is, capable of emitting three elementary colors, green, red and blue. These effects were improved as the value of K was reduced, and the increase in emission efficiency was remarkably large when the half width with respect to the selected emission wavelength was reduced. The reflectivities of the two reflecting mirror layers, i.e., those of the optical interference filter and the metallic electrodes were selected in such a manner that the reflectivity of the optical interference filter on the luminescence extraction side was smaller. The construction in which an optical interference filter is used to constitute one of the two reflecting mirror layers ensures a reduction in the half width with respect to the emission wavelength as well as an increase in the optical amplification as compared with the case where the two reflecting mirror layers are single-layer films formed of metallic thin films or the like. Embodiment 2 A second embodiment of the present invention will be described below with reference to the accompanying drawings. Fig. 11 shows in section a basic construction of a thin-film EL apparatus in accordance with the ninth embodiment of the present invention. A transparent electrode 82 is formed on a glass substrate 81, and an optical thin film having a refractive index nl of about 1.5 with respect to the emission wavelength and having a dielectric constant ε1 and a thickness dl (e.g., film of MgF 2 (nl = 1.38) or SiO 2 (nl = 1.52)) is formed as a first dielectric layer 83 on the electrode 82. A fluorescent material layer 84 having a refractive index n3 of about 2.4 and a thickness d3 is formed on the first dielectric layer 83, and another dielectric thin film equal to the first dielectric layer is successively superposed as a second dielectric layer 85. Another fluorescent material layer 86 also having the refractive index n3 of about 2.4 and the thickness d3 is formed on the second dielectric layer 85, still another dielectric thin film identical with the first dielectric layer is successively superposed as a third dielectric layer 87 on the fluorescent material layer 86, and still another fluorescent material layer 88 having the refractive index n3 of about 2.4 and a thickness d4 (twice as large as d3) is formed on the third dielectric layer 87. Similarly, on the fluorescent material layer 88 are successively formed a fourth dielectric layer 89 which is the same dielectric thin film as the first dielectric layer, a fluorescent material layer 90 having the refractive index n3 of about 2.4 and the thickness d3, a fifth dielectric layer 91 which is the same dielectric thin film as the first dielectric layer, a fluorescent material layer 92 having the refractive index n3 of about 2.4 and the thickness d3, and a sixth dielectric layer 93 which is the same dielectric thin film as the first dielectric layer. Back electrodes 94 having the function of a reflecting mirror layer as well as the function of an electrode layer are formed on the sixth dielectric layer 93. A thin-film EL apparatus having this structure was manufactured and the refractive indexes nl and n3 of the first dielectric layer, the fluorescent material layer and the second dielectric layer with respect to an emission wavelength λ0 were measured with an ellipsometer. The thicknesses dl of the first and second dielectric layers and the thickness d3 of the fluorescent material layers were determined so as to satisfy the following equation based on the nultilayer-film optical interference filter design method: n1·d1 = n3·d3 = λ0/4 That is, an EL device having the function of electroluminescence as well as the function of an optical interference multilayer-film filter was formed. It was confirmed that the thin-film EL apparatus of this embodiment shown in Fig. 11 had a voltage-luminance characteristic such that light of the emission wavelength λ0 could be efficiently extracted from the fluorescent material layer through the luminescence surface. The fluorescent material layer was formed by using a fluorescent material selected from the group consisting of ZnS: Mn which emits orange light with a main emission wavelength of 580 nm, ZnS: Tb, F or ZnS: Tb, P which emits green light with a main emission wavelength of 544 nm, CaS: Eu or ZnS: Sm which emits red light with a main emission wavelength of 650 nm, and SrS: Ce or ZnS: Tm which emits blue light with a wavelength of about 480 nm. Yttrium oxide films, tantalum oxide films, aluminum oxide films, silicon oxide films, silicon nitride films or perovskite-type oxide dielectric films represented by a strontium titanate film were used for the first and second dielectric films. The characteristics of the dielectric films used for the invention are shown in Table 1. In accordance with the present invention, thin film EL apparatus capable of emitting light of the desired wavelengths at an improved efficiency are manufactured, thereby realizing full-color flat displays used as OA system terminals, TV image display units, view finder units and so on.

Claims: A thin-film electroluminescence apparatus including an optical interference filter, comprising: a light-transmissible electrode layer formed on a glass substrate; a light reflecting electrode layer; a fluorescent material layer or a laminated structure of a fluorescent material layer and a dielectric material layer, a voltage being applied to said fluorescent material layer or said laminated structure through said electrode layers; and a multilayer-film optical interfence filter capable of selectively transmitting light emitted from said fluorescent material layer and having an arbitrary wavelength λ, said optical interference filter being provided on a light extraction side between said fluorescent material layer or said laminated structure of fluorescent and dielectric material layers and said light transmissible electrode layer, said optical interference filter being formed of at least one first dielectric film having a larger refractive index and at least one second dielectric film having a smaller refractive index, said first and second dielectric films being alternately laminated based on an equation λ/4 = film thickness x refractive index and said fluorescent material layer having a refractive index larger than that of said second dielectric film based on an equation λ/2 * N = film thickness x refractive index (where N is an integer equal to or larger that 1). A thin-film electroluminescence apparatus including an optical interfernce filter according to Claim 1, wherein an oxide having a refractive index of 2 or larger in a visible region and including a perovskite-type oxide or tantalum and an oxide or nitride having a refractive index larger than 1 and smaller than 2 are used for said dielectric material layers.

Inventors: ABE ATSUSHI, KUWATA JUN, ABE, ATSUSHI, KUWATA, JUN
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1990
Main classification: H05B33
IPC codes: H05B33