JPH08213167A - Edge emitter with cap - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve light emitting efficiency of an edge emitter. SOLUTION: A cap 112 is arranged on a transparent electrode of an edge emitter by laminating insulating films 120 and 116 by interposing/inserting a reflector 122 and an active film 118 and a transparent electrode 114. The cap 112 has the almost same refractive index, a smaller attenuation factor and a larger thickness as compared with the active film 118. A light emitting side surface 138 of the cap 112 is larger in transmissivity than the other side surface.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION This invention relates to electroluminescent light, and more particularly to edge emitters that emit electroluminescent light.

[0002]

2. Description of the Related Art Edge emitters that emit light by electroluminescence usually have a structure known as a thin film electroluminescent stack, which has a plurality of films. This stack is usually
It has five films: a conductive electrode, an insulating film, an active film, another insulating film, and another electrode. The basic idea is to excite the dopant ions in the active film. Light is generated when the excited dopant relaxes. An electric field for excitation is generated by the voltages of the upper and lower electrodes.

The insulating film and the active film are usually formed in a sheet shape, and the upper and lower electrodes are formed as strips E1 and E2. FIG. 1 shows a plan view of this structure. A pixel is formed by the intersection of two strips. In this example, the pixel length is l,
The width is represented by w. The width w is much smaller than the length l. The film assembly is configured to expose one end in the width direction to form the edge of the edge emitter. This is intended to increase the light emitting area and reduce the edges to reduce the emitted light rays.

[0004]

Light is generated in the active membrane between the two strips. Most of the light generated propagates laterally across the sheet within the film stack by total internal reflection. Preferably all of the light generated is directed to the narrow edges. However, due to the shape of this film stack, the percentage of light exiting the edge is very small.

For example, the edge emitter is 600 dots /
When used for the print head of inch (dpi) printer,
Many pixels must be adjacent in one row. Each pixel is one dot of this printer. In this example, the width of each dot is about 0.035 mm. If the pixel length is 3 mm and normal edge emitter material is used, 20,000 nW for each pixel
The optical output of can be generated. The power generated is high, but only about 70 nW of light is coupled from the edges. Therefore, the optical efficiency is 0.35%.

One method of increasing the light output is to increase the area of a pixel to increase the amount of light generated.
In order to reduce the rays from the edges, the width must be reduced. Therefore, the pixel length must be increased in order to increase the area. But,
There is damping in the membrane stack. The measured decay lengths for conventional membrane stacks range from 0.07 to 0.5 mm. The output does not increase even if the pixel length is increased.

From the above, it is clear that it is necessary to improve the optical efficiency of the edge emitter. There must be a high percentage of the light generated at each pixel that is directed out the edge and out of it, rather than propagating in the other direction on the sheet.

[0008]

SUMMARY OF THE INVENTION The present invention provides an edge emitter with greatly improved optical efficiency.
According to the invention, the percentage of the light generated in each pixel towards the corresponding edge is significantly higher.

According to the invention, a cap is provided on top of the thin film electroluminescent stack. This cap collects, guides, and directs most of the light generated to the edge of the edge emitter.

Structurally, the stack has an upper transparent electrode, a lower reflective electrode, and an active film disposed therebetween. The upper electrode is provided directly under the cap. The cap is preferably thicker than the thin film electroluminescent stack. The cap has less attenuation than the thin film stack and is made of a material having an index of refraction that closely matches that of the active film. At least all sides of the cap except the top side and the sides adjacent to the edges are preferably smooth reflective surfaces. The light generated preferably exits the surface adjacent to this edge, ie the emitting side surface.

In operation, a significant amount of the light generated in the active region does not propagate laterally through the thin film stack, but to the cap. The light inside the cap reflected by the reflecting surface is guided out of the cap and exits from the light emitting side surface. If the thickness of the cap is greater than the thickness of the thin film stack, the total internal reflection before the generated light is emitted from the edge and the light emitting side surface is reduced. As a result, light attenuation is reduced. If the attenuation of the cap is less than that of the thin film stack, the light attenuation is even less.

The present invention is implemented as described above. The material of the cap does not exactly match the material of the active film,
One has a refractive index of about 2.3 and the other has a refractive index of about 1.5, but the optical efficiency is improved by 100%.

Other features and advantages of the present invention will become apparent upon reading the following detailed description, which illustrates the principles of the invention, with reference to the accompanying drawings.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 2 illustrates a system 100 having an array 106 of edge emitters (108, etc.) of the present invention. Array
106 is provided on the glass substrate 102. The system also includes a driver 104 having a multiplexer that drives the edge emitters. The driver 104 having a multiplexer is well known to those skilled in the art, and therefore its explanation is omitted. When driven appropriately, each edge emitter emits electromagnetic radiation. For example, emitter 108
Emits radiation 109.

FIG. 3A shows a cross-sectional view of one embodiment of the edge emitter 110 of the present invention. To make the explanation easier to understand,
The glass substrate 102 is not shown. This emitter consists of a cap 112 overlying the thin film electroluminescent stack, which forms an improved edge emitter. This improved edge emitter is also referred to herein simply as an edge emitter. The thin film electroluminescent stack is preferably a transparent electrode 114, two insulating films 116, 120 sandwiching an active film 118,
And a reflective electrode 122. These electrodes are electrically conductive.

An electric field is applied to the active film 118, for example by connecting a voltage source 124 to the two electrodes. The electric field applied to the active film excites the dopant ions in the active film 118, relaxes the excited dopant, and generates radiation. The manufacturing method of the embodiment is well known to those skilled in the art and will not be described.

In Example 100, the transparent electrode and the reflective electrode are well aligned vertically. In other embodiments, the width of one electrode can be much larger than the width of the other. Excitation and recombination occur only in the overlapping part.

Structurally, the reflective electrode 122, the two insulating films 1
20, 116 and the transparent electrode 114 are very thin and the active film 118
Are thicker than those, but the cap 112 is even thicker than the active film. In the illustrated embodiment, the insulating film 120 over the reflective electrodes fills the gap, such as region 140, between adjacent reflective electrodes. In another embodiment, the reflective electrode 12
2 is much wider than the transparent electrode 114.

Usually, the refractive index of the active film 118 is different from that of the insulating film. However, the insulating film is very thin with respect to the wavelength of the radiation generated in the active film. Therefore, the influence of the mismatch of the refractive indexes of the active film and the insulating film is very small.

The material of the cap is selected based on its electromagnetic properties. This property includes the index of refraction, which should be similar to the index of refraction of the active film to improve the coupling of radiation generated in the active film 118 to the cap 112. The cap must be made of a material that has a lower attenuation per unit length of generated radiation than the active film 118. Of further concern is whether the cap can be fabricated to the desired dimensions on the transparent electrode. This includes how smooth the surface can be. This is an important issue, as we will see later.

The cap 112 has four sides 132, 134, 136,
138 and upper surface 130. Preferably, the radiation is directed out of one of these sides (light emitting side 132) and edge 142 of active film 118. In this example, the radiation is preferably emitted in the X direction.

In order to improve the directivity of the radiation, the light emitting side surface 132 has a higher transmittance than the other side surfaces (reflection side surfaces 134, 136 and 138) and the upper surface 130.

FIG. 3B shows another embodiment 150 of the present invention.
FIG. 3B shows structures 152 and 154 having two caps. The reflective electrode 156 is shared by both structures. But,
In each cap structure, the cap 154 is the transparent electrode 15
8. The cap 152 has its own transparent electrode such as the transparent electrode 162.

There are various methods to give each surface a different transmittance. Six such methods will be described. The method described here is only given as an example, and other methods can be used. The first method is to smooth the upper surface and the reflective side surface. Optically, this means that these surfaces have a high finesse. On the other hand, the light emitting side surface is roughly finished by sandblasting or the like.

The second method is to metalize the top and reflective sides. The third method is to coat the reflective side surface and the upper surface with a film of a material having a refractive index much lower than that of the cap. Due to the refractive index mismatch, incident radiation is reflected at these surfaces. The fourth method combines the second and third methods by first forming a low refractive index material on these surfaces and then metallizing it. In particular, when the cap is much thicker than the active film, it may be better to ignore the edge 142 of the active film 118 and focus only on the light emitting side surface 132. It should also be noted that the light emitting side 132 of the cap may not coincide with the edge 142 of the stack and may extend beyond the edge. This facilitates manufacturing of the edge emitter. In one embodiment, if the emitting side 132 extends beyond the edge 142 of the stack, the transparent electrode 114 also extends beyond the edge with the cap.

FIG. 4 shows another embodiment of the present invention and shows a sixth method for increasing the reflection on the upper surface. The top surface 170 of the cap 172 is provided with a groove for directing light passing through the transparent electrode 174 of the thin film electroluminescent stack at a small angle to the light emitting side surface 176. If the top surface is simply smooth and no other measures are taken to improve its reflectivity, much of the incident radiation will be reflected except for those inside the cone of light. The angle of the cone is the critical angle of the cap material. A cap material with a refractive index of 2.3 reflects about 90% of the incident light. In addition, this grooved structure directs a significant portion of the radiation within the cone to the emitting side. Therefore, radiation passing through the transparent electrode at a small angle is also directed to the emitting side.

Embodiments of the sixth method described above are well known to those skilled in the art and will not be described herein.

FIG. 5 is a ray diagram comparing the optical path of the generated radiation in the case of the prior art and in the case of the present invention on a perspective view of the embodiment 100 of the present invention. This perspective shows the reflective side 134
Is a cross section parallel to.

In a typical conventional edge emitter, the radiation produced at 200 passes through active film 118 before being emitted from edge emitter 110. This orientation is accomplished by various total internal reflections as shown by optical path 202.

In the present invention, the radiation generated at 200 propagates to the cap 112. The cap is thicker than the active film 118. As a result, total internal reflection before the radiation hits the light emitting side surface 132 is reduced. In this example, the radiation generated at 200 travels along optical path 204 and undergoes total internal reflection once before exiting emission side 132. Attenuation per unit length of cap 112 is active membrane 1
Less than 18 attenuation. Therefore, the present invention increases the percentage of the generated light that is emitted from the emitter 110. In general, a thicker cap will better guide the radiation towards the emitting side. This is because thicker caps reduce the number of total internal reflections between the top surface and the thin film electroluminescent stack prior to radiation. However, depending on the application, the cap may not be too thick. This is because the thicker the cap, the larger the beam size of the radiation emitted from the emitter.

FIG. 6 is a ray diagram for comparing the optical path of the generated radiation with the case of the prior art and the case of the present invention on a perspective view of the embodiment 110 of the present invention. This perspective view shows the light emitting side 132
It is one cross section. In the prior art, the radiation generated at 250 is guided in the optical path 252 along the plane of the active film by multiple total internal reflections. Such radiation propagates along the thin film electroluminescent stack and usually disappears and is not emitted from the edge 142 of the active film 118.

In the present invention, this generated radiation travels along the optical path 254 in the cap. Within the cap, the light emitting side has a higher transmissivity than all other side and top surfaces. Unless the radiation path 254 is perfectly parallel to the emitting side 132, eventually the radiation will exit the emitting side 132 unless it is attenuated. This is accomplished by total internal reflection along the "helical" optical path.

[Example of Operation] The present invention will be described in more detail with reference to the following example. The following examples are merely provided as examples of embodiments of the present invention.

The reflective electrode 122 is made of aluminum. Two
The two insulating films are made of silicon oxynitride, which has a refractive index of about 1.7. The active film is made of manganese-doped zinc sulfide. The refractive index of the active film is about 2.3. The transparent electrode is made of indium tin oxide and has a refractive index of about 2. Zinc oxide can also be used as the transparent electrode. The cap is made of acrylic with a refractive index of 1.56.

The thickness of the reflective electrode (Y direction) is about 1000 angstroms, the thickness of the two insulating films is about 2000 angstroms, the thickness of the active film is about 1 micron, and the thickness of the transparent electrode is about 2000 angstroms.

The cap has a thickness (Y direction) of about 10 microns, a width (Z direction) of about 0.07 mm, and a length (X direction) of about 3 mm.
Is. The reflective side and top surfaces are metallized with aluminum about 1000 angstroms thick. By providing a film having a lower refractive index than the cap such as cryolite between the cap and the metallized surface, the reflection performance can be further improved. Cryolite has a refractive index of 1.33.

The electroluminescent radiation is yellow and has a wavelength of 600 nm. The optical efficiency of the above structure is improved by about 100% compared to a similar structure without the cap 112. It should be noted that in the above structure the cap material is not perfectly matched to the active film material.

[Conclusion] The thin-film electroluminescent stack described here has five films: a transparent electrode, a first insulating film, an active film, a second insulating film, and a reflective electrode. However, other thin film stacks can be used. For example, the insulating film of this stack may be only the first insulating film,
It is also possible to use only the second insulating film. In other embodiments, the reflective electrode is not reflective and is merely an electrode. In this example. The edge emitter is not efficient because some radiation propagates in the "reflecting electrode".

For the cap, other materials such as chalcogen glass having a refractive index of 2.1 to 2.6 and BCB which is a known plastic material having a refractive index of 1.5 can be used. Like acrylic, BCB can be easily formed into a cap structure. Other materials with different refractive indices can also be used. Generally, the better the matching of the refractive index between the cap and the active film, the more efficient the edge emitter can be obtained.

The present invention has been described as using a light emitting side that has a higher transmissivity than other aspects. In another embodiment, the light emitting side may be more transmissive than at least one side such as the opposing surface 136 in FIG. 3A. This can be achieved, for example, by roughening the emitting side or by making one side reflective. With such a structure, the generated radiation has a suitable directivity, and more radiation is emitted from this rough surface than the reflective surface. Alternatively, more radiation propagates away from the reflecting surface.

As an improvement over the present embodiment, the light emitting side surface of the cap can be curved so that the lens structure serves as a lens to improve the outcoupling and / or angular distribution of the emitted radiation. Similar results can be obtained by providing Fresnel grooves on the light emitting side surface. Methods of performing such bending and grooving are well known to those skilled in the art and will not be described here.

In the present invention, the cap has been described as a rectangular block. The cap can have other configurations with more sides or curved surfaces. An example of this is shown in FIG.
Shown in The cap 402 has a curved top surface on the thin film stack 406. In this example, side 404 has a higher transmission coefficient than the other sides.

FIG. 8 shows another embodiment of the present invention. In this example, the cap 502 contains the electroluminescent film stack 504. Both the cap and the membrane stack are provided on the substrate 506. In this embodiment, the bottom surface 508 of the cap 502 is also a reflective surface.

These embodiments can be used in a printer as shown in FIG. 9 and the thin film electroluminescent source acts as a pixel lighting device.

From the above description, it can be seen that the cap collects and guides the generated radiation and directs it towards the edge. This greatly improves the efficiency of the edge emitter. Those of ordinary skill in the art will be able to devise other embodiments by considering the specification and the embodiments described herein. The specification and examples are merely examples, and the spirit and scope of the present invention are defined in the claims.

[0046]

[Brief description of drawings]

FIG. 1 is a plan view of a conventional edge emitter.

FIG. 2 is a perspective view of a system having an array of edge emitters of the present invention.

FIG. 3A is a perspective view of an embodiment of the edge emitter of the present invention.

FIG. 3B is a perspective view of an embodiment of the edge emitter of the present invention.

FIG. 4 is a cross-sectional view of another embodiment of an edge emitter that enhances top surface reflection.

FIG. 5 is a ray diagram comparing the case of the prior art of the optical path of the generated light and the case of the present invention in a cross section of an embodiment of the present invention.

FIG. 6 is a ray diagram for comparing the case of the prior art and the case of the present invention of the optical path of the generated light.

FIG. 7 is a diagram showing another embodiment of the present invention.

FIG. 8 is a diagram showing still another embodiment of the present invention.

FIG. 9 is a perspective view of a printer using an edge emitter embodying the present invention.

[Explanation of symbols]

 100: system 102: glass substrate 104: driver 106: array of edge emitters 108: edge emitter 109: radiation 110: edge emitter 112: cap 114: transparent electrode 116, 120: insulating film 118: active film 122: reflective electrode 124: Voltage source 130: Top surface 132, 134, 136, 138 of cap 112: Side surface of cap 112 140: Region 142: Edge of active film 118 150: Examples 152, 154: Structure with cap 156: Reflective electrode 158: Transparent electrode 162: Electrode 170: Cap 172 upper surface 172: Cap 174: Transparent electrode 176: Light emitting side surface 200: Radiation generation point 202: Optical path 204: Optical path 250: Radiation generation point 252: Optical path 254: Optical path 402; Cap 404: Thin film Stack 502: Cap 504: Electroluminescent film stack 506: Substrate 508: Bottom surface of cap 502 E1, E2: Strips l: Pixel length w: Pixel width

Claims (22)

[Claims] 1. A thin film electroluminescent stack having an upper transparent electrode, a lower electrode, and an active film generating electroluminescent radiation between the two electrodes, and a plurality of side surfaces provided on the transparent electrode. And a cap having a transmission coefficient of radiation on one of the side surfaces higher than that of the other side surface, and the side surface with a high transmission coefficient is a light emitting side surface. The low coefficient surface is a reflective surface and the radiation entering the cap is emitted at a higher percentage from the emitting side surface than the reflective surface. 2. The edge emitter according to claim 1, wherein the thin film stack further includes a first insulating film provided between the upper transparent electrode and the active film, or the active film and the lower film. A second insulating film provided between the electrodes, or a first and a second insulating film provided at the position, and the transmission coefficient of the light emitting side surface is greater than the transmission coefficient of the other side surface and the upper surface. 2. The edge emitter according to claim 1, wherein all the side surfaces except the upper surface and the light emitting side surface are the reflective surfaces. 3. The edge emitter of claim 1, wherein the cap is thicker than all other films and electrodes. 4. The edge emitter according to claim 3, wherein the thickness of the cap is about 10 times the thickness of the active film. 5. The edge emitter according to claim 1, wherein the refractive index of the cap substantially matches the refractive index of the active film. 6. The edge emitter according to claim 1, wherein attenuation per unit length of the electroluminescent radiation is higher in the active film than in the cap. 7. The edge emitter according to claim 2, wherein the wavelength of the electroluminescent radiation is greater than the thickness of any element selected from the electrodes and films except the active film. 8. The edge emitter according to claim 1, wherein a groove is provided in the upper surface of the cap, and the generated electroluminescent radiation is reflected by the cap and directed to the light emitting side surface. 9. The edge emitter according to claim 1, wherein a refractive index of the cap is larger than a refractive index of the active film. 10. The edge emitter according to claim 1, wherein the cap is chalcogen glass. 11. The edge emitter according to claim 1, wherein the reflective surface of the cap is metallized. 12. The reflective surface of the cap is coated with a material having a lower refractive index than the cap.
Edge emitter described in. 13. The edge emitter of claim 12, wherein the coated material is metallized. 14. The edge emitter of claim 1 wherein the light emitting surface is roughened to increase emission of radiation from this surface. 15. The edge emitter according to claim 1, wherein the light emitting surface is curved to increase the emission of radiation to cause a lens movement to guide the radiation emitted from the surface. 16. The edge emitter according to claim 1, wherein a Fresnel groove is provided on the light emitting surface to increase the emission of radiation to cause a lens operation and guide the radiation emitted from the surface. 17. The edge emitter according to claim 1, wherein the radiation is light. 18. The edge emitter according to claim 1, wherein the cap has four sides. 19. The edge emitter according to claim 3, wherein a refractive index of the cap substantially matches a refractive index of the active film. 20. The edge emitter according to claim 19, wherein the radiation is light. 21. The edge emitter of claim 20, wherein the decay per unit length of electroluminescent radiation in the cap is less than that in the active film. 22. A printer having an edge emitter according to claim 1.