JP2008159338A - Inorganic electroluminescent device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an inorganic electroluminescent device capable of making improvement of luminous efficiency by raising luminance with low voltage driving. <P>SOLUTION: A light reflecting electrode 2 having a shape following the shape of the projecting and recessed surface 10A of a base material 1 and contacting with the base material 1, a first insulating layer 3 contacting with the light reflecting electrode 2, an inorganic EL (luminescent) layer 4 contacting with the insulating layer 3, and a second insulating layer 5 contacting with the inorganic EL layer 4 are laminated in this order with an almost uniform thickness kept for each of them. Especially, the top face of the inorganic EL layer 4 is formed into a projecting and recessed surface 10D having a conical projecting part 10a<SB>4</SB>, and is formed as an inclined surface with respect to a flat light emitting surface 8 from which light emitted from the transparent electrode 6 is extracted. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an inorganic electroluminescent device suitable as a display device such as a self-luminous display.

  Inorganic electroluminescent devices (inorganic EL devices) are more stable than organic EL devices and are easy to manufacture, so they have been used for backlights of watch dials, etc. It is used for lighting and the like, and is practically superior to organic EL devices.

  For example, Patent Documents 1 and 2 below describe inorganic EL devices using a light-emitting material using ZnS as a base material and a substance serving as a light emission center added.

FIG. 7 is a schematic view showing a cross section of a thin-film inorganic EL element in Patent Document 1 (Japanese Patent Laid-Open No. 9-129374). According to this inorganic EL element, a first transparent electrode (first electrode) 36 made of optically transparent ZnO (zinc oxide) is formed on a glass substrate 31 that is an insulating substrate, and an optical surface is formed on the upper surface thereof. First transparent insulating layer 35 made of transparent Ta ₂ O ₅ (tantalum pentoxide), light emitting layer (inorganic EL layer) 34 made of ZnS added with Tb fluoride as the light emission center, optically transparent Ta ₂ O _A second insulating layer 33 made of ₅ and a second transparent electrode (second electrode) 32 made of optically transparent ZnO are formed.

FIG. 8 is a cross-sectional view schematically showing the configuration of the thin-film inorganic EL element in Patent Document 2 (Japanese Patent Laid-Open No. 2003-157972). In this inorganic EL element, a transparent electrode pattern 46 is wired with a transparent electrode film made of indium tin oxide (ITO) on a translucent insulating substrate 41 such as a glass substrate, on which SiO ₂ , Si _A first insulating layer 45 made of ₃ N ₄ , Y ₂ O _3, Ta ₂ O ₅ or the like is formed by a sputtering method, and a light emitting layer (inorganic EL layer made of a material such as ZnS: Mn) is formed on the first insulating layer 45. ) 44 is formed, and a second insulating layer 43 made of the same material as that of the first insulating layer 45 is formed thereon, thereby forming a so-called double insulating structure in which the light emitting layer is sandwiched between the insulating layers. Further, a back electrode pattern 42 made of a metal electrode film is disposed on the second insulating layer 43 so as to be orthogonal to the transparent electrode pattern 46, and the light emitting layer sandwiched between the selection lines of both electrode patterns is insulated from the light emitting layer. By applying an alternating voltage through the layers, only a selected dot is caused to emit light, thereby realizing a dot matrix display.

  In such a double insulating structure, the insulating layers 43 and 45 limit the current flowing through the light emitting layer 44, thereby improving the operation stability and light emitting characteristics of the EL element, and the light emitting layer 44 that is a hygroscopic material. There is also an effect of imparting element reliability by passivating (this is also the case with the conventional example of FIG. 7).

Insulating layer materials are required to have a high dielectric breakdown voltage and a large capacitance. From this point, the value of dielectric breakdown voltage (E _B ) x relative dielectric constant (ε) determines the material characteristics. Al ₂ O ₃ , SiO ₂ , Si ₃ N ₄ , Ta ₂ O _5, etc. are practical.

JP-A-9-129374 (column 4, lines 33-43, FIG. 1) Japanese Patent Laid-Open No. 2003-157972 (second column, lines 18 to 47, FIG. 8)

Inorganic EL devices require a high electric field for light emission (approximately 10 ⁶ V / cm is required), but have a problem of low brightness. That is, an inorganic EL device emits light by exciting a base material with the energy of electrons accelerated by an electric field, and transmitting the energy of the excited base material to an additive substance that is an emission center. There is an electric field threshold value for light emission, and generally about 10 ⁶ V / cm is required. For this reason, when the light emitting layer is formed thin, the voltage required to start light emission can be kept low. On the other hand, since the light emitting layer becomes thin, there is a trade-off that the luminance itself obtained is lowered.

  In view of the above circumstances, an object of the present invention is to provide an inorganic electroluminescent device capable of improving luminance and improving luminous efficiency and being compatible with low voltage driving.

  That is, the present invention relates to a first electrode (for example, a light reflective electrode), an insulating layer (for example, a first insulating layer and a second insulating layer described later), and an inorganic light emitting layer (for example, ZnS as a base material). And an inorganic electroluminescent device (inorganic EL device or element) in which a second electrode (for example, a light transmissive electrode) is stacked and an inorganic electroluminescent device in which a substance serving as a light emission center is added) The surface of the lower layer has, for example, an uneven shape having a substantially triangular cross section, and the light emitting layer is formed on the uneven surface so as to follow the uneven shape, and the light emitting layer is formed. is there.

  In the present invention, the term “inorganic electroluminescent device” means not only an aggregate of light emitting elements composed of the above-mentioned laminates (a product device such as a surface light source and a display) but also a device composed only of the light emitting elements. To do.

  According to the present invention, the stability is high and the fabrication is easy. The insulation layer existing between the light emitting layer and the electrode improves the breakdown voltage (withstand voltage) and supplies the light to the light emitting layer and the light emitting layer. In addition to the features of inorganic EL devices that can improve reliability by passivation, the light-emitting layer is formed in a concavo-convex shape that follows the concavo-convex shape of the lower layer. The light emitting layer has an inclined shape, and the thickness of the light emitting layer in the light extraction direction (that is, the apparent effective thickness that contributes to light emission) is larger than the thickness of the light emitting layer itself (film formation thickness). As a result, when the thickness of the light emitting layer itself is equivalent to the conventional thickness, the apparent effective thickness that contributes to light emission is large, so that the luminance of light emission increases even at the same voltage, and the thickness of the light emitting layer itself is reduced. When it is made smaller, the voltage required for light emission can be lowered while maintaining a relatively high luminance, and both high luminance and low voltage can be achieved.

  In the present invention, in order to reliably and sufficiently exert the above-described effects, it is preferable that the light emitting layer is formed on the uneven surface with a substantially uniform thickness.

  In addition, it is desirable that the concavo-convex shape of the light emitting layer has a repeated continuous shape (for example, a cone or a pyramid shape) of convex portions and concave portions having a substantially triangular cross section, and the apex angle of the convex portions is 30 degrees or more. Moreover, the height of the convex part is preferably about 300 to 800 nm.

  Further, in order to suppress a reduction in efficiency due to light reflection between the respective layers, one of the first electrode and the second electrode is a light reflective electrode and the other is a light transmissive electrode. It is desirable that the refractive index difference Δn between adjacent layers existing between the light reflecting surface of the light reflective electrode and the light extraction surface of the light transmissive electrode is 0.2 or less.

  Also, in order to prevent uneven brightness due to a non-uniform electric field that tends to occur in the vicinity of the convex portion, one of the first electrode and the second electrode is a light reflective electrode and the other is a light transmissive electrode. The light extraction surface of the light transmissive electrode is preferably subjected to light diffusion treatment.

  More specifically, the light emission is in contact with the first insulating layer on the uneven surface of the first electrode or on the uneven surface of the first insulating layer in contact with the first electrode. A layer is formed, and a structure in which the second insulating layer and the second electrode are sequentially formed on the uneven surface of the light emitting layer having a shape following the uneven shape is preferable.

  Hereinafter, an inorganic electroluminescent device according to a preferred embodiment of the present invention will be described with reference to the drawings.

Structure example 1 of inorganic EL device
FIG. 1 shows a cross-section of the main part of Structural Example 1 of the inorganic EL device.

According to this inorganic EL device (or element), the upper surface is processed into a substantially triangular cross section, whereby the base material 1 that forms the concave-convex surface 10A having a conical convex portion 10a ₁ and concave portion 10b ₁ is formed. A light reflecting electrode 2 as a first electrode (lower electrode) in contact with the substrate 1 in a shape following the uneven shape of the uneven surface 10A of the substrate 1, and a first in contact with the light reflecting electrode 2 The insulating layer 3, the inorganic EL (light emitting) layer 4 in contact with the insulating layer 3, and the second insulating layer 5 in contact with the inorganic EL layer 4 are sequentially stacked with a substantially uniform thickness, and the second electrode A transparent electrode 6 as (upper electrode) is laminated on the insulating layer 5. The upper surface of the transparent electrode 6 is flat and serves as a light emitting surface 8 through which emitted light is extracted. The light emitting surface 8 is subjected to a light diffusion treatment, and a light diffusion structure 7 made of a light diffusion material made of transparent beads, for example, is covered. It is worn.

In the laminated structure described above, irregular surface 10A of the substrate 1 is also held in the upper layer as it is, to follow the respective layers of the uneven shape, the upper surface of the light reflective electrode 2 is uneven surface by the convex portion 10a ₂ and the recesses 10b ₂ 10B, the upper surface of the insulating layer 3 is an uneven surface 10C formed by the convex portions 10a ₃ and the concave portions 10b ₃ , the upper surface of the inorganic EL layer 4 is an uneven surface 10D formed by the convex portions 10a ₄ and the concave portions 10b _4, and the upper surface of the insulating layer 5 is The protrusions 10a ₅ and the recesses 10b ₅ are respectively formed on the uneven surface 10E. In particular, as will be described later, it is important that the inorganic EL layer 4 is formed in an uneven shape that follows the uneven shape of the lower layer and has a substantially uniform thickness.

  When describing each layer of the above-described laminated structure in detail, the material of the substrate 1 is not particularly limited, and includes rigid bodies such as glass, ceramics, plastic plates, PET (polyethylene terephthalate), PEN (polyethylene naphthalate), A flexible film such as TAC (triacetyl cellulose) may be used. However, when the inorganic EL layer 4 needs to be heated for crystallization, it is necessary to select a material having sufficient heat resistance and a low thermal expansion coefficient as the substrate 1.

The lower electrode 2 serves to supply a voltage and reflect light rays forward. For this purpose, a material having high light reflectance such as Ag, an Ag compound, and Al is desirable. The electrode 2 has a concavo-convex structure formed of conical convex portions 10a ₂ by a technique such as nanoimprint described later. The electrode 2 can be formed by a technique such as vacuum deposition or sputtering of an electrode material.

The insulating layer 3 and the insulating layer 5 have a role of supplying electrons to the base material of the inorganic EL layer 4 as well as ensuring a withstand voltage (see Patent Document 2 described above). Any of them can be formed by vacuum vapor deposition or sputtering, and Nb ₂ O ₅ , TiO ₂ , Ta ₂ O ₅ , ZrO ₂ , ZnO or the like can be used as a material. The film thickness is generally about 100 nm to 1 μm.

The inorganic EL layer 4 can contain ZnS or ZnO as a base material and rare earth fluorides such as Mn, Al, Cu, TbF _{3 and the} like as luminescence centers. These materials can be deposited on the inorganic EL layer 4 by vacuum deposition or sputtering in the same manner as the above layers, and the film thickness is desirably 500 nm or more. Moreover, the height h of the convex part in this layer 4 is good to set it as 300-800 nm (the convex part of another layer is also the same).

The upper electrode 6 needs to be transparent in order to form the light emitting surface 8, and ITO is suitable as a typical material, but SnO ₂ or ZnO can also be used. The upper electrode 6 is also formed so as to fill the lower EL structure configured in a concavo-convex shape including the above-mentioned conical convex portion, and the upper surface is desirably as flat as possible.

  A light diffusing structure 7 is applied to the upper surface of the upper electrode 6 by applying a light diffusing material such as glass beads or styrene beads.

  This laminated structure can be manufactured using the nanoimprint method shown in FIG. For example, in a state where plastic is applied to a substrate (not shown) and the plastic layer is softened by heating, the mold having the concave / convex surface 10A and the mold having the concave / convex transfer surface having the reverse pattern is brought into contact with and pressed, thereby the plastic layer. The plastic layer is cured by cooling while keeping the pressed state. Then, after the plastic layer is sufficiently cured, the mold is separated, the unevenness of the mold is transferred to the plastic layer, and the substrate 1 made of the plastic layer having the uneven surface 10A is formed as shown in FIG. .

Then, on the uneven surface 10A of the base material 1, as shown in FIG. 4B, Al or the like having a high light reflectance is deposited to a uniform film thickness by vacuum evaporation or sputtering, and the uneven surface 10A of the substrate 1 is deposited. to form a light reflective electrode 2 having an irregular surface 10B consisting of the convex portion 10a ₂ and the concave portion 10b ₂ follow the.

  Thereafter, the above-described layers 3, 4 and 5 and the electrode 6 are sequentially deposited by vacuum evaporation or sputtering. At this time, the layers 3, 4 and 5 are deposited to a uniform film thickness, and the electrode 6 is deposited to a sufficient thickness. Furthermore, a light diffusing material 7 such as glass beads is applied on the flat light emitting surface 8 of the electrode 6 to deposit the light diffusing structure 7.

  Next, with reference to FIG. 2, the reason why the inorganic EL device according to the first structural example described above can achieve both high luminance and low voltage (however, in FIG. Omitted).

As described above, according to this structure, a structure inclined in a conical shape, for example, is formed between the electrodes 2-6 with respect to the light emitting surface 8, and the electric field E is applied perpendicularly to the conical surface. On the other hand, the projected thickness l of the light emitting layer (inorganic EL layer) 4 with respect to the light emitting surface 8 is as follows: d is the thickness of the light emitting layer 4 and θ is the apex angle of the cone.
l = d / sin (θ / 2)
It can be expressed as

  For this reason, for example, when the apex angle is 90 degrees, the apparent thickness l of the light emitting layer 4 is l = d / sin 45 ° ≈1.4 d, that is, about 1.4 times, and thereby the emission luminance is also increased. The thickness is improved by about 1.4 times (at this time, the thickness d of the light emitting layer 4 is equal to that of the conventional one).

  In this case, even if the thickness d of the light emitting layer 4 is reduced, the luminance can be increased as compared with the conventional example in which l = d. For example, when d is reduced to 4/5, l≈1. 4 × (4/5) d = 1.12d, and the luminance is 1.12 times. If the luminance is equivalent to that of the conventional example, the thickness d can be reduced by several tens of percent to d / 1.4≈0.71d.

  From this, the inorganic EL device according to the structural example 1 has improved luminance and can maintain high luminance even when the thickness of the inorganic EL layer is reduced, so that both high luminance and low voltage can be sufficiently achieved. be able to.

  From this point of view, it is desirable that the apex angle θ is as small as possible in order to keep the apparent thickness of the inorganic EL layer large. However, when considering a realistic configuration, the refractive index of a transparent conductive film such as ITO or AZO, which is assumed as an electrode material on the light extraction side, is about 1.8 to 2.1. On the other hand, since the refractive index range of the insulating film located in the lower layer is 1.47 to 2.3, when a material of 2.3 is selected as the refractive index of the insulating film, total reflection is required unless the apex angle is 30 degrees or more. The light extraction efficiency may be significantly reduced depending on the conditions. For this reason, the lower limit of the apex angle is desirably 30 degrees, and 40 degrees is desirable assuming the maximum refractive index difference. Further, when the apex angle is 90 degrees, the effect of the present invention is 1.4 times based on the equation shown in [0033], and the effect is further reduced when the apex angle is 90 degrees or more. For this reason, in order to ensure an effect of 1.4 times or more, it is desirable to set it to 90 degrees or less. Therefore, from the above consideration, the apex angle range is desirably 30 degrees or more and 90 degrees or less, and considering the maximum refractive index difference, 40 degrees or more and 90 degrees or less are appropriate.

In particular, the uneven shape of the inorganic EL layer (light emitting layer) 4 may be various shapes illustrated in FIG. As shown in FIG. 1 (1), it is desirable that the concave / convex surface 10D has a conical convex portion 10a ₄ , but the shape is a regular quadrangular pyramid, regular hexagonal pyramid or other various shapes shown in FIG. 2 (2). Alternatively, the convex portions may not be distributed in an island shape, and the cross section may be, for example, a substantially triangular shape extending in a stripe shape as shown in FIG.

  If the problem of this structural example 1 is enumerated, since the above-mentioned convex part is conical, for example, if there is a mismatch in the refractive index between the layers constituting the laminated structure, reflection at the interface will increase. In some cases, the critical angle is exceeded, and there is light that cannot come out.

  Therefore, since the refractive index of, for example, ZnS, which is the base material of the light emitting layer 4, is about 2.2, the material of each layer is set so that the refractive index difference Δn with other layers is suppressed to 0.2 or less as follows. By selecting, the regular reflectance at the interface can be suppressed to about 0.2% or less.

  Further, for example, there is a possibility that a large amount of tunnel current flows near the apex of the conical convex portion due to electric field concentration. For this reason, since uneven brightness is expected to occur in the vicinity thereof, a light diffusing material such as acrylic beads, styrene beads, glass beads or the like is applied to the surface of the upper electrode 6 (light emitting surface 8). A structure 7 that relaxes or disperses may be formed. However, application of the light diffusing material may be omitted if the above-described processing for removing the apex of the convex portion is performed.

Structure example 2 of inorganic EL device
FIG. 5 shows a cross section of the main part of Structural Example 2 of the inorganic EL device. Compared with Structural Example 1 described above, the light-reflecting electrode 12 has a sectional shape equivalent to that of the above-described base material 1, and the base material 11. Is a flat shape, and the other components are the same.

  In this structural example 2, a more rigid and flat material such as a glass plate can be used as the base material 11, but the light-reflective electrode 12 can be formed by plating or the like, and then the base material 11 can be adhered. . Since other components are the same as those in Structural Example 1, the description thereof is omitted.

Structural example 3 of inorganic EL device
FIG. 6 shows a cross section of an essential part of Structural Example 3 of the inorganic EL device. Compared with Structural Example 1 described above, the insulating layer 23 has a sectional shape equivalent to that of the substrate 1 described above, and the light reflective electrode 22. And the base material 21 differs in a flat shape, and other components are the same.

  In this structural example 3, a more rigid and flat material such as a glass plate can be used as the base material 21, and the light reflective electrode 22 can also be formed as a flat film by vacuum deposition or the like, and then the insulating layer 23. Can be formed by transferring the unevenness of the mold described above. Since other components are the same as those in Structural Example 1, the description thereof is omitted.

  In this structural example 3, since the layer thickness of the insulating layer 23 can be increased, there is an advantage that the withstand voltage can be increased. However, in order to suppress a decrease in the withstand voltage due to a decrease in the capacitance, the dielectric constant is reduced. It is desirable to form the insulating layer 23 with a large material and keep the capacitance large (see the above-mentioned Patent Document 2).

  As described above, the present invention has been variously illustrated based on the embodiment, but it goes without saying that these examples can be appropriately modified without departing from the gist of the present invention.

  For example, the layer configuration and the material thereof, the film formation or formation method, and the like may be variously changed, including the above-described uneven shape type and pattern.

  Moreover, about the electrode mentioned above, what is necessary is just to make one side a light reflective electrode and the other a light transmissive electrode (transparent electrode), Therefore The taking-out direction of emitted light may be a base material side contrary to what was mentioned above. . Or you may take out light from both surfaces as a light-transmissive electrode in both electrodes.

  In addition, each structural example described above shows the main part of the element. However, depending on the emission color of the inorganic EL layer or a combination thereof, it may be applied to a backlight, illumination, or a monochrome or color display in which a large number of elements are arranged. Is possible.

  As a high-intensity, low-voltage inorganic electroluminescent device, it can be applied to various fields such as lighting devices and display devices.

It is principal part sectional drawing of the structural example 1 of the inorganic electroluminescent apparatus based on this invention. It is principal part sectional drawing for demonstrating a performance and a physical property similarly. It is a top view or perspective view showing various surface shapes of an inorganic EL layer. It is principal part sectional drawing which shows a manufacture example in the order of a process equally. It is principal part sectional drawing of the structural example 2 of the inorganic electroluminescent apparatus based on this invention. It is principal part sectional drawing of the structural example 3 of the inorganic electroluminescent apparatus based on this invention. It is sectional drawing of the inorganic electroluminescent element of the prior art example 1. FIG. It is sectional drawing of the inorganic electroluminescent element of the prior art example 2.

Explanation of symbols

1, 11, 21 ... base material, 2, 12, 22 ... (light reflective) electrode, 3, 23 ... insulating layer,
4 ... inorganic EL layer, 5 ... insulating layer, 6 ... (transparent) electrode, 7 ... light diffusion structure, 8 ... light emitting surface,
10A ... Uneven surface, 10a ₁ ... convex part, 10a ₂ ... convex part, 10a ₃ ... convex part, 10a ₄ ... convex part,
10a ₅ ... convex part, 10B ... uneven surface, 10C ... uneven surface, 10D ... uneven surface, 10E ... uneven surface, E ... electric field direction, L ... light emission direction

Claims (6)

  In the inorganic electroluminescent device in which the first electrode, the insulating layer, the inorganic light emitting layer, and the second electrode are laminated, the surface of the lower layer of the light emitting layer has an uneven shape, on the uneven surface. Further, the light emitting layer is formed in a concavo-convex shape following the concavo-convex shape.   The inorganic electroluminescent device according to claim 1, wherein the light emitting layer is formed with a substantially uniform thickness on the uneven surface.   3. The inorganic according to claim 1, wherein the concavo-convex shape of the light emitting layer has a continuous continuous shape of a convex portion and a concave portion having a substantially triangular cross section, and the apex angle of the convex portion is 30 degrees or more and 90 degrees or less. Electroluminescent device.   One of the first electrode and the second electrode is a light reflective electrode and the other is a light transmissive electrode, and the light reflective surface of the light reflective electrode and the light transmissive electrode The inorganic electroluminescence device according to claim 1 or 2, wherein a refractive index difference Δn between adjacent layers existing between the light extraction surface and the light extraction surface is 0.2 or less.   One of the first electrode and the second electrode is a light reflective electrode, and the other is a light transmissive electrode, and a light diffusing treatment is applied to a light extraction surface of the light transmissive electrode. Or the inorganic electroluminescent apparatus described in 2.   The light emitting layer is formed in contact with the first insulating layer on the uneven surface of the first electrode or on the uneven surface of the first insulating layer in contact with the first electrode. The inorganic electroluminescent device according to claim 1, wherein the second insulating layer and the second electrode are sequentially formed on the uneven surface of the light emitting layer having a shape following the shape.

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