WO2017001760A1

WO2017001760A1 - Light-emitting semiconductor device including a structured photoluminescent layer

Info

Publication number: WO2017001760A1
Application number: PCT/FR2016/051592
Authority: WO
Inventors: Marianne Consonni; Abdenacer Ait-Mani; Pascal BOURION; Adrien Gasse
Original assignee: Commissariat à l'énergie atomique et aux énergies alternatives; Best
Priority date: 2015-06-30
Filing date: 2016-06-28
Publication date: 2017-01-05
Also published as: FR3038452A1

Abstract

The invention relates to a light-emitting optoelectronic device (1) including: - at least one light-emitting diode (10) having an emission surface (11); and - a photoluminescent layer (30), which at least partially coats the emission surface, including at least one first luminophore (Le) suitable for absorbing incident light radiation emitted by the light-emitting diode and for emitting in response light radiation at a first wavelength, and including at least one cavity (33i) formed from a side (32) of the photoluminescent layer that is opposite the emission surface. At least one second luminophore (Li) is placed in said cavity (33i), the second luminophore being suitable for absorbing incident light radiation and for emitting in response light radiation at a second wavelength different from the first wavelength.

Description

SEMICONDUCTOR ELECTROLUMINESCENT DEVICE HAVING A LAYER

STRUCTURED PHOTOLUMINESCENT

TECHNICAL AREA

The invention relates to a semiconductor light-emitting device, that is to say an optoelectronic device comprising at least one light-emitting diode, adapted to emit light radiation according to desired colorimetric characteristics, and also relates to a method for producing a light-emitting diode. such an electroluminescent device. The invention finds application particularly in lighting systems, in particular those emitting a white light.

STATE OF THE PRIOR ART

In order to obtain a white light, it is known to use an optoelectronic device comprising a light-emitting diode whose emission surface is covered with a layer of photoluminescent material adapted to convert at least a portion of the light radiation. emitted by the light-emitting diode into radiation of another wavelength.

By way of example, mention may be made of devices comprising a light-emitting diode based on gallium nitride (GaN) adapted to emit blue light, that is to say whose emission spectrum has a peak of intensity. around 440nm at approximately 490nm, combined with a photoluminescent layer of yttrium aluminum garnet (YAG, for Yttrium Aluminum Garnet) activated by the cerium ion, called YAG: Ce, adapted to convert a portion of the blue light in a yellow light, that is to say whose emission spectrum has a peak intensity between 560nm and 590nm approximately. The white light is thus obtained by superposition of the blue light flux emitted by the light-emitting diode and not converted by the photoluminescent layer and the yellow light flux emitted by the photoluminescent layer. Such a semiconductor light-emitting device can be characterized electrically and optically in particular by its light output, that is to say the ratio between the luminous flux emitted by the device and the electrical power injected at the source. In addition, the colorimetric properties of the device can be characterized in particular in terms of color temperature, that is to say the black body temperature whose emitted radiation has a substantially identical spectral distribution, ie a shade similar to that device, and color rendering index that describes the ability of the device to render the different colors of an object. Thus, by way of example, a GaN-based light-emitting diode coated with a YAG-based photoluminescent layer has a color temperature ranging from 4000 K to a so-called hot white color at 6500 K for a so-called cold white color. The color rendering index is usually greater than 80 (on a scale of 0 to 100) and a light output of the order of 150 Im / W.

It may be desired to adjust the colorimetric properties of the electroluminescent device, for example so that the emitted light has a warm white, that is to say with a color temperature of the order of 2500K to 3500K. For this, it is known to modify the spectral distribution of the light radiation emitted by the device by covering the photoluminescent layer with a second photoluminescent layer made of a material adapted to absorb a portion of the blue light and to emit light in response. red example. The wavelength spectrum of the radiation emitted by the device then has a proportion of decreased blue light and a new red component. The color temperature can then be of the order of 2500K to 3500K and the perceived color is said to be hot.

Document US2009 / 0221106 describes an example of a semiconductor light-emitting device that has a structured photoluminescent layer so as to adjust the intensity and / or the color of the emitted light radiation. This device comprises a plurality of light-emitting diodes assembled on a first face of an optically transparent layer whose opposite face is covered with a photoluminescent layer. The photoluminescent layer has different adjacent areas which differ from each other by the addition or removal of photoluminescent material. More specifically, the photoluminescent layer is formed of the stack of two elementary layers made of a different photoluminescent material, wherein a pad of a third photoluminescent material is disposed on certain areas of the photoluminescent layer while other areas have a cavity more or less deep formed in the photoluminescent layer. The arrangement of the different areas and the depth of the cavities depend on the desired intensity and color of the light radiation.

US2013 / 0126918 discloses an optoelectronic device comprising a LED chip (for Light Emitting Diode, in English) partially covered by a light conversion wall. This defines a through cavity opening on the chip. The document US2006 / 0099449 describes an optoelectronic device of which an LED chip is covered by a photoluminescent layer. This has a concavity at which it is coated by a second photoluminescent layer. US2015 / 0194579 discloses an optoelectronic device having a plurality of diodes covered by a glass color conversion element with cavities filled with phosphors. The document WO2013 / 038579 describes an optoelectronic device comprising a plurality of diodes covered by a photoluminescent layer with non-through cavities.

There is however a need to further adjust the colorimetric properties of the light radiation emitted by the light emitting device. There is also a need to improve the light output of this type of electroluminescent device.

STATEMENT OF THE INVENTION

The object of the invention is to remedy at least in part the drawbacks of the prior art, and more particularly to propose an optoelectronic device with a light-emitting diode whose colorimetric characteristics can be precisely adjusted. Another objective of the invention is to propose an optoelectronic device with a light-emitting diode whose luminous efficiency is improved.

Another object of the invention is to provide a particularly compact optoelectronic device with a light-emitting diode.

For this, the object of the invention is an optoelectronic light emitting device, comprising:

at least one light-emitting diode having an emission surface;

a photoluminescent layer, which at least partly covers the emission surface, comprising at least a first phosphor adapted to absorb at least part of an incident light radiation emitted by the light-emitting diode and to emit light radiation at a first wavelength, and having at least one cavity formed from a face of the photoluminescent layer opposite to the emission surface.

According to the invention, at least one second phosphor is disposed in said cavity, the second phosphor being adapted to absorb at least part of incident light radiation and to emit in response light radiation at a second wavelength different from the first one. wave length.

In addition, the cavity is non-traversing so that the photoluminescent layer locally has a cavity bottom portion located under said cavity. Furthermore, the photoluminescent layer comprises a plurality of cavities each housing at least one phosphor pad different from the first phosphor. It is then possible to adjust the colorimetric properties of the optoelectronic device at the emission face of the light-emitting diode, along an axis substantially orthogonal to the emission surface by adjusting the depth of the non-cavity or cavities. through their phosphor studs, as well as in a plane parallel to the emission surface by means of cavities and their luminescent pads. This adjustment of the colorimetric properties can be obtained without causing any degradation of the emission surface of the light-emitting diode. Some preferred but non-limiting aspects of this optoelectronic device are the following:

The photoluminescent layer may be in contact with the emission surface of the light emitting diode.

The emission surface may be a face of a semiconductor layer doped with a PN junction of said light emitting diode. It may alternatively be a transparent passivation layer covering and in contact with a semiconductor layer doped with a PN junction of the light emitting diode.

Each cavity of the photoluminescent layer may be non-through.

The cavity bottom portion may have an average thickness less than or equal to 10% of a minimum thickness of photoluminescent layer necessary to absorb substantially all the incident light radiation emitted by the light emitting diode.

The cavity bottom portion may have an average thickness less than or equal to ΙΟμιη.

Said phosphor pads are adapted to each emit a light selected from blue, red, yellow, green and orange.

A part of the cavities may not have a phosphor pad, the corresponding cavity bottom portion may have an average thickness less than or equal to 10% of a minimum photoluminescent layer thickness necessary to absorb substantially all the incident light radiation emitted by the light-emitting diode.

The optoelectronic device may comprise a single light-emitting diode whose emission surface is covered by the photoluminescent layer.

The light-emitting diode may be based on a III-V compound and preferably III-N. The invention also relates to a method for producing an optoelectronic device having any of the preceding features, comprising the steps in which:

depositing, on an emission surface of at least one light-emitting diode, a photoluminescent layer comprising at least a first phosphor;

at least one cavity is made from a face of the photoluminescent layer opposite to the emission surface, said cavity being non-through so that the photoluminescent layer locally has a cavity bottom portion situated under said cavity;

at least one pad of second phosphor is deposited in said cavity, the photoluminescent layer comprising a plurality of cavities each housing at least one phosphor pad different from the first phosphor.

Said cavity may be formed by laser ablation.

The photoluminescent layer may be obtained from a mixture of the first phosphor with a solvent, the step of depositing said photoluminescent layer comprising a phase of deposition of said mixture on the emission surface followed by a phase of evaporation of solvent.

The photoluminescent layer may be obtained from a mixture of a first phosphor powder with a binder matrix of a transparent and optically inert material, the step of depositing said photoluminescent layer comprising a phase of depositing said mixture on the surface of emission followed by a phase of sedimentation of the powder of first phosphor within the matrix binder

BRIEF DESCRIPTION OF THE DRAWINGS

Other aspects, objects, advantages and characteristics of the invention will appear better on reading the following detailed description of preferred embodiments thereof, given by way of non-limiting example, and with reference to the accompanying drawings. on which ones : FIGS. 1a and 1b are diagrammatic sectional views of an optoelectronic light-emitting diode device according to one embodiment, comprising a structured photoluminescent layer (FIG. 1a) and phosphor pads disposed in cavities of the structured photoluminescent layer (FIG. lb);

FIGS. 2a and 2b are diagrammatic views from above of an optoelectronic light-emitting diode device according to an embodiment in which FIG. 2a represents a spatial distribution of the phosphor pads in the structured photoluminescent layer, and FIG. 2b represents the color. corresponding light emitted by the pads and the photoluminescent layer;

FIGS. 3a to 3d are diagrammatic cross-sectional views of a light-emitting diode optoelectronic device part according to one embodiment, for different steps of the production method.

DETAILED PRESENTATION OF PARTICULAR EMBODIMENTS

In the figures and in the remainder of the description, the same references represent identical or similar elements. In addition, the various elements are not represented on the scale so as to favor the clarity of the figures.

The invention relates to an optoelectronic light-emitting diode device comprising a photoluminescent structure. By photoluminescent structure is meant here a structure comprising at least two photoluminescent materials each adapted to absorb light at a first wavelength and to emit in response light radiation at a wavelength greater than the first length of light. wave. The two photoluminescent materials are different from each other in the sense that their photoluminescence emission spectrum is different from each other. These materials are generally called phosphors (phosphor), and, for illustrative purposes, can be adapted to emit: in the green, that is to say that the emission spectrum has a peak intensity between 495nm and 560nm approximately, and can be for example made based on SrSi ₂ 0 ₂ N ₂ : Eu ²⁺ , α-sialon: Eu ²⁺ , or Sr ₃ Si ₃ Al ₂ O ₂ N ₂ : Eu ²⁺ ;

in the yolk, that is to say that the emission spectrum has a peak intensity of between 560nm and 580nm approximately, and can be for example made from YAG: Ce, Sr ₃ B ₂ 0 ₆ Eu ²⁺ , Ca ₃ Si ₂ 0 ₇ : Eu ²⁺ , Sr ₂ SiO ₄ : Eu ²⁺ , or even a mixture of YAG: Ce and Sr 3 SiOs: Eu ²⁺ ;

in the orange, that is to say that the emission spectrum has a peak intensity of between 580 nm and 600 nm approximately, and can be made for example based on a material of silicate, nitride and / or sulphide;

in the red, that is to say that the emission spectrum has a peak intensity of between 600 nm and 650 nm approximately, and can be for example made based on M ₂ SÎ5N8: Eu ²⁺ where M is selected from Ca, Sr, Ba, Sr _x Cai _x AISiN3: Eu ²⁺ , Sr ₂ Si ₅ N ₈ : Eu ²⁺ , or (Ca, Sr) SiO ₄ : Eu ²⁺ .

The photoluminescent material may also be in the form of quantum dots (quantum dots), that is to say in the form of semiconductor nanocrystals, whose mean size may be between 0.2 nm and 1000 nm, for example example between 1nm and 100nm, and especially between 2nm and 30nm. The semiconductor material of the nanocrystals may be chosen in particular from cadmium selenide (CdS), indium phosphorus (InP), cadmium sulphide (CdS), zinc sulphide (ZnS), cadmium oxide (CdO ), zinc and cadmium selenide (CdZnSe), or among other semiconductor materials that may be suitable. The semiconductor nanocrystals may be dispersed in a binder matrix, for example silica.

The phosphors may be in the form of a layer or a pad. By layer is meant a range of phosphor whose thickness is less, for example ten times or even twenty times, to its longitudinal dimensions of width and length. The layer is produced by depositing, on the light-emitting diode, at least one material comprising a luminophore and is therefore not a layer, possibly self-supporting, previously produced and then assembled to the light-emitting diode. By plot, we mean a volume of phosphor whose thickness is less than, equal to or greater than its longitudinal dimensions of width and length, and whose longitudinal dimensions are smaller than those of the phosphor layer. A plot can take the form of a drop or even a structured volume.

The phosphor layer or pad may also comprise, especially when the phosphor is in the form of a grain or a powder, a binding matrix in the form of a transparent and optically inert material providing a binder function with respect to the phosphor for example silicone. Transparency means a material which transmits at least 50% of the incident light, and preferably at least 80%. By optically inert is meant a material that does not emit light in response to its incident light absorption.

Here, the quantum efficiency, or conversion efficiency η, specific to each phosphor, is defined as the ratio between the number of photons converted by the phosphor to the number of photons absorbed by it. The conversion efficiency is relative to the type of phosphor and does not depend substantially on the dimensional characteristics of the phosphor portion considered. The conversion efficiency of the phosphors listed above is of the order of 70% to 98%.

The absorption rate i _a bs, specific to a phosphor portion, is also defined as being the ratio between the number of photons absorbed by the phosphor portion over the number of incident photons. The absorption rate thus depends, as a first approximation, on the dimensional characteristics of the phosphor portion, in particular its thickness, and on the phosphor volume fraction (ratio of the volume of phosphor in the portion under consideration to the total volume of this phosphor. portion). It is understood that the absorption rate can be adjusted according to the phosphor thickness in the portion in question and / or its volume fraction.

Finally, the conversion rate τ∞ην, specific to a phosphor portion, is defined as the ratio between the number of photons converted by the phosphor portion to the number of incident photons. The conversion rate τ∞ην can be expressed as the product of the conversion efficiency η and the absorption rate i _a bs. In the remainder of the description, the terms "substantially", "approximately", "approximately" extend "to within 10%". In addition, the terms "between ... and ...", "from ... to ..." mean that the terminals are included unless otherwise stated.

FIG. 1a partially and schematically illustrates an optoelectronic device 1 with a light-emitting diode 10 according to one embodiment, comprising a photoluminescent structure 20 with a structured photoluminescent layer 30.

A three-dimensional orthonormal reference is defined here and for the rest of the description, in which the X and Y axes are oriented along the longitudinal dimensions of the light-emitting diode and the Z axis is oriented along its transverse dimension, that is to say according to its thickness.

The optoelectronic device 1 comprises at least one light-emitting diode 10 having a transmission surface 11 through which light radiation is intended to be emitted. The optoelectronic device 1 here comprises only one light-emitting diode 10 but, as mentioned hereinafter, the optoelectronic device may comprise a plurality of light-emitting diodes arranged with respect to one another so as to define a surface of emission common to all diodes.

In a conventional manner, the light-emitting diode 10 comprises, arranged on a support 2, a stack formed of a doped semiconductor portion 12 of a first type of conductivity, for example of the P type, of an active zone 13 from which is emitted the light radiation of the light emitting diode, and a doped semiconductor portion 14 of a second conductivity type opposite to the first type of conductivity, for example of type N.

The light-emitting diode 10 may be made based on III-V semiconductor material, that is to say mainly comprising at least one group III element and a group V element. More precisely, the diode can be made on the basis of a III-N compound, such as, for example, GaN, InGaN, AIGaN, AlN, InN and AllnGaN. The active zone 13 may comprise at least one quantum well made from a material semiconductor having a band gap energy lower than doped semiconductor portions 12 and 14. The doped semiconductor portions 12, 14 are here made of GaN and the active zone 13 comprises an alternation of unintentionally doped semiconductor layers forming barrier layers based GaN, and at least one quantum well for example based on InGaN. The diode is here adapted to emit a blue light, that is to say whose emission spectrum has a peak intensity around 440nm to 490nm approximately.

The thickness of the doped semiconductor portion P may be between 50 nm and 20 μm; that of the active zone 13 may be between 10 nm and 500 nm; and that of the semiconductor portion 14 doped N may be between Ο, ΐμιη and 20μιη. The light emitting diode 10 may have dimensions along the X axis and / or Y between 200μιη and 5mm. Preferably, the diode has a square surface of 1mm side. The emitting surface 11 of the light-emitting diode forms a substantially flat surface (by virtue of the surface micro-structuring facilitating the extraction of light) which extends along the plane (X, Y). It corresponds here to the upper face along the Z axis of the N doped semiconductor portion 14, or even to the upper face of a thin passivation layer (not shown) covering the semiconductor portion 14.

Figure 1b partially and schematically illustrates the optoelectronic device shown in Figure la where the structured photoluminescent layer 30 is partially covered with phosphor pads P 1.

With reference to FIGS. 1a and 1b, the optoelectronic device 1 thus comprises a photoluminescent structure 20 which at least partially covers the emission surface 11 of the light-emitting diode 10. The photoluminescent structure 20 comprises a structured photoluminescent layer 30 formed of minus a first phosphor (Figure la) and at least a second phosphor different from the first phosphor (Figure lb). The photoluminescent layer 30 extends between a lower face 31 facing the light emitting diode 10 and an upper face 32 opposite the lower face. In this example, the photoluminescent layer 30 is in contact with the light emitting diode 10, that is to say that there is no additional intermediate element between the photoluminescent layer and the light emitting diode. Thus, the emitting surface 11 of the light-emitting diode (for example the upper face of the doped semiconductor portion 14 or the upper face of a passivation layer) is in contact with the lower face 31 of the photoluminescent layer.

The photoluminescent layer 30 comprises at least a first phosphor Le, adapted to convert at least a portion of the light emitted by the light-emitting diode 10 into a light radiation of wavelength greater than that of the absorbed radiation. In the case of a light-emitting diode emitting a blue light, that is to say whose emission spectrum comprises a peak of intensity centered between 440 nm and 490 nm, the phosphor Le may be adapted to absorb emitted light by the diode and to emit in response yellow, green, orange, red, or other light. Preferably, the phosphor Le is adapted to emit yellow light and is for example made from YAG: Ce. The average thickness of the photoluminescent layer 30 may be between ΙΟμιη and 500μιη, preferably between ΙΟμιη and ΙΟΟμιη, for example equal to approximately 30μιη or approximately 50μιη.

The photoluminescent layer 30 may be adapted to absorb all or part of the incident light emitted by the light-emitting diode 10 as a function of the desired colorimetric properties, as a function, in particular, of the volume fraction of the phosphor Le and the average thickness e _c of the layer. By way of example, a layer of YAG: Ce with a high volume fraction, for example greater than or equal to 30%, and with an average thickness e _c greater than or equal to 50 μm, leads to an absorption rate i _a bs approximately greater than or equal to 90%, or even close to 100% vis-à-vis the incident radiation emitted by a light emitting diode emitting blue light. The light flux emitted by the optoelectronic device therefore essentially has a yellow color. Thus, the average thickness e _c of the layer Photoluminescent as well as the type of phosphor Le and its volume fraction can be adjusted according to the desired colorimetric properties of the optoelectronic device.

The photoluminescent layer 30 is said to be structured in the sense that it comprises at least one cavity 33 at its upper face 32. The optoelectronic device here comprises a plurality of cavities 33i, 33 ₂ , 333 separated from each other by a so-called Zc zone. The term "cavity" means a recess extending from the upper face 32 of the photoluminescent layer 30 in the direction of the depth of the photoluminescent layer 30.

The cavities 33i, 33 ₂ , 333 may have a horizontal profile, that is to say in the plane (X, Y) and / or a vertical profile, that is to say in a plane containing the axis Z, of any shape, for example triangular, square, rectangular and more generally polygonal, even circular, oval, oblong, or other. The transverse dimensions along X and Y cavities may be between a few microns and a few millimeters, for example between 50μιη and 500μιη, and preferably between ΙΟΟμιη and 300μιη. The transverse dimensions of the cavities as well as their surface density, that is to say their number per surface unit of the photoluminescent layer, can be adjusted according to the desired colorimetric properties of the optoelectronic device.

The cavities 33 each comprise a lateral wall 34, which extends towards the emission surface 11 and laterally delimits the cavity volume. In the case of a triangular vertical profile, the side wall 34 may terminate at a point or a bottom line of the cavity. In the case of a rectangular vertical profile, as shown in Figures 1a and 1b, the side wall 34, joins a bottom wall 35, cavity, which extends here substantially parallel to the underside of the photoluminescent layer.

The cavities 33 are advantageously non-through in the sense that they do not open onto the emitting surface 11 of the light-emitting diode. Thus, the photoluminescent layer 30 locally comprises a cavity bottom portion Q delimited by the bottom wall 35, and the lower face 31. The bottom portion C, of cavity C, has an average thickness e _C i measured along the axis Z.

As illustrated in FIG. 1b, the photoluminescent structure 20 further comprises at least a second phosphor L, and preferably a plurality of pads P, of second phosphor, arranged in the cavities 33,. The second phosphor L is different from the phosphor Le of the photoluminescent layer 20 in the sense that it is adapted to absorb light and to emit spectrum light in response different from that of the first phosphor. The volume of the phosphor L thus forms a pad which may or may not fill the interior space of the cavity 33 in which it is arranged. It is thus possible to define an average thickness ep, along the Z axis of the pad P, of phosphor.

By way of example, the phosphor L can be adapted to emit a yellow, green, red orange or other light, and be made, for example, based on a luminescent material mentioned above. In the case where the phosphor Le of the photoluminescent layer 30 is adapted to emit yellow light, the phosphor L is preferably adapted to emit green, red or orange light. Each pad P, phosphor may comprise a phosphor different from that of other pads, and have a thickness ep, different from each other. Thus, each phosphor pad may be characterized by the type of phosphor and the volume fraction thereof, as well as by transverse and thickness dimensions, the choice of which makes it possible to adjust the desired colorimetric properties of the optoelectronic device.

The photoluminescent structure 20 thus has a plurality of portions Z, of colored light, juxtaposed from each other, thus forming a set of pixels Z, of color whose characteristics can be adjusted so as to obtain the desired colorimetric properties of the optoelectronic device, while optimizing the light output of it.

The portions Z, of colored light, or pixels of color, are light conversion zones formed portions of the photoluminescent layer 30 having cavities 33, at least some of which comprise P-pads, phosphor. They are separated from each other by the intermediate zone Zc of photoluminescent layer located between two neighboring color pixels. Color pixels are used in the sense that these portions have colorimetric properties different from those of the photoluminescent layer intermediate zone.

In the example of FIG. 1b, three color pixels Z1, Z2, Z3 are shown, separated from each other by the intermediate zone Zc of photoluminescent layer.

The color pixel ZI comprises a cavity 33i, whose depth defines a bottom portion C1 of thickness eci comprising the phosphor Le. This cavity houses a plot PI of a luminophore L1 of average thickness e _P i. The color pixel ZI is therefore characterized by two pairs of conversion efficiency and absorption rate (r | i_ _c , Tabs, ci) and (r | _L i, iabs, pi). The absorption rate T _a bs, ci of the bottom portion C1 and the absorption rate T _a bs, pi of the pad PI depend in particular, respectively, on the thickness eci and the thickness epi. Thus, as a first approximation, the luminous flux cp _c , i_c converted by the phosphor Le depends on the absorption rate T _a bs, ci, of the conversion efficiency r | _Lc and incident light flux CPLED, ZI emitted by the diode. In addition, the luminous flux cp _c , i_i converted by the luminophore L1 depends on the absorption rate T _a bs, pi, the conversion efficiency r \, and the luminous flux (1- T _a bs, ci) x cpi_ED, zi emitted by the diode and not absorbed by the bottom portion Cl. The luminous flux cpzi emitted by the portion ZI, that is to say the luminous flux from the portion ZI, whether transmitted or emitted by photoluminescence , is formed by the superposition of the luminous flux cp _c , i_c converted by the phosphor Le, the luminous flux cp _c , i_i converted by the luminophore L1, and a part of the luminous flux emitted by the light-emitting diode and not absorbed by the Ll and Le phosphors. Thus, the colorimetric properties of the ZI portion can be adjusted by the choice of phosphors, their respective volume fraction and the dimensions of the bottom portion C1 and the pad Pl.

However, the luminous flux emitted by a color pixel may be modified in the event of reabsorption, that is to say absorption by the phosphor L1 of at least a portion of the luminous flux cp _c , i_c converted by the luminophore The. More specifically, the phosphor L1 may also be adapted to absorb light radiation emitted by the phosphor Le and to emit radiation in response to its luminescence wavelength. This double absorption phenomenon at a color pixel occurs in particular when the luminescence wavelength of the phosphor Le is lower than that of the phosphor L1, for example when the phosphor Le emits in the yellow and the phosphor L1 emits in the Red. As a result, the spectral distribution of the luminous flux emitted by the pixel ZI has a reduced proportion of yellow and a greater proportion of red. In addition, besides the fact that the colorimetric properties are modified, this reabsorption decreases the conversion rate insofar as a fraction of the luminous flux emitted by the pixel ZI depends on ηι_ι x ηι _, ο which consequently impacts the overall efficiency of the optoelectronic device.

The color pixel Z2 can be sized in order to reduce the reabsorption phenomenon and thus optimize the overall efficiency of the optoelectronic device. It comprises here a cavity 33 ₂ whose depth defines a bottom portion C2 of average thickness e _C2 comprising the phosphor Le, in which is located a pad P2 of a phosphor L2 of average thickness ep ₂ . The color pixel Z2 is therefore characterized by two pairs of the conversion efficiency and the absorption rate (ηΐ, Tabs, c ₂ ) and (ηι_ ₂ , Tabs, p ₂ ). The thickness ec ₂ of the bottom portion C2 is such that the absorption rate T _a bs, c ₂ by the phosphor Le is less than or equal to 10%, and preferably less than or equal to 5%, so as to to limit the absorption by the phosphor of incident light emitted by the light-emitting diode. For this, the thickness e _C2 of the bottom portion is less than or equal to 10%, and preferably less than or equal to 5%, of the thickness of the photoluminescent layer that would be necessary to obtain an absorption rate approximately equal to 100%. By way of illustration, in the case of a photoluminescent layer based on YAG: Ce, a thickness of 50 μm for a phosphor volume fraction of greater than or equal to 30% may be sufficient to convert approximately all the incident light emitted by the light into yellow light. light emitting diode. The thickness e _C2 of the bottom portion C2 can thus be less than or equal to 5μιη, for example equal to 2μιη. It is also advantageous that the volume fraction of phosphor is less than 30% so as to further reduce the absorption rate tabs, C2- The photoluminescent structure 20 here comprises a third color pixel Z3 having a cavity 33 ₃ whose depth defines a bottom portion C3 of thickness ec3 comprising the luminophore Le. Unlike the pixels Z1 and Z2, the pixel Z3 has no phosphor pad in the cavity. The pixel Z3 is therefore characterized by the conversion efficiency and the absorption rate (ηΐ, Tabs, c3). As for the pixel Z2, the thickness e _C3 of the bottom portion is such that the absorption rate T _a bs, c3 by the first phosphor Le is less than or equal to 10%, and preferably less than or equal to 5%, so as to limit the absorption by the phosphor Le of incident light emitted by the light-emitting diode. For this, the thickness ec3 of the bottom portion is less than or equal to 10%, and preferably less than or equal to 5% to the thickness of the photoluminescent layer that would be necessary to obtain an absorption rate approximately equal to 100%. It is also advantageous that the volume fraction of phosphor is less than 30% so as to further reduce the absorption rate Tabs, c3. A pixel is thus obtained whose emitted light comes essentially from the light-emitting diode, this emitting for example a blue light.

In FIG. 1b, three examples of pixels are represented for purely illustrative purposes and other examples are feasible, which differ in particular from the depth of the cavity, the type of phosphors, as well as from the number and thickness of studs. phosphor present in each cavity.

Thus, the luminous flux emitted by the optoelectronic device corresponds to the superposition of the luminous flux emitted by the intermediate zone Zc of the photoluminescent layer and the luminous flux emitted by the different pixels Z, of color. The colorimetric properties of the optoelectronic device thus depend on those of the intermediate zone Zc and the various pixels Z ,, and can be adjusted as a function of the surface density and the dimensions of the pixels, and the dimensions of the phosphor pads and the phosphor type. present or not in the cavities. In addition, the overall efficiency of the optoelectronic device is optimized by the presence of pixels sized to limit the reabsorption phenomena. FIG. 2a illustrates an example of an optoelectronic device 1 in plan view where the photoluminescent structure is represented. The photoluminescence structure 20 here has a cross section in the plane (X, Y) of square profile and comprises a matrix of 3 × 3 pixels of color Z, separated from each other by an intermediate zone Zc of photoluminescent layer 30. In this example, the photoluminescent layer 30 is adapted to absorb incident light emitted by the light emitting diode and to emit in response a yellow light indicated in the figure by the letter Y (for yellow, in English).

The photoluminescent structure 20 comprises three color pixels Z _4, each comprising a phosphor pad P4 adapted to absorb incident light and to emit green light, indicated in the figure by the letter G (for green, in English). ; three color pixels Z5 each comprising a phosphor pad P5 adapted to absorb incident light and to emit red light in response, indicated in the figure by the letter R (for red in English). The photoluminescent structure also comprises three color pixels Ze each having a cavity 33e having no phosphor pad and whose depth is adapted so that the absorption rate by the cavity bottom portion is less than 10% and preferably less than or equal to 5%. Thus, the light emitted by these pixels comes essentially from the light, for example blue indicated in the figure by the letter B (for blue, in English), emitted by the light-emitting diode and transmitted through the cavity bottom portion. .

In this example, the photoluminescent structure has a square section of 1mm side. The photoluminescent layer is made based on YAG: Ce and has an average thickness of 30μιη with a phosphor volume fraction of 30%. The color pixels have a square section of 200μιη and are separated from each other by an intermediate photoluminescent layer area of average width of ΙΟΟμιη.

FIG. 2b illustrates the light emitted locally by the optoelectronic device according to the example of FIG. 2a. In this example, the average thickness e _c of photoluminescent layer 30, associated with the volume fraction of phosphor Le, is insufficient. to absorb most of the incident light emitted by the light-emitting diode. Thus, the light emitted by the luminescent layer at the intermediate zone Zc is the superposition of a portion of blue light emitted by the diode and yellow light emitted by the photoluminescent layer, so that the perceived light is white, indicated in the figure by the letter W (white, in English).

Thus, the color pixels, distributed in the plane (X, Y) of the photoluminescent structure, make it possible to finely adjust the colorimetric properties of the optoelectronic device, in particular the color temperature and the color rendering index, by the adjustment of the parameters such as the surface density and the dimensions of the color pixels, the type of luminophores used, their volume fraction, and the dimensional and colorimetric characteristics of the different color pixels. In addition, while adjusting the colorimetric properties of the optoelectronic device, the overall efficiency of the latter can be optimized by sizing color pixels whose absorption rate by the first phosphor Le is less than 10% so as to reduce the reabsorption phenomenon.

In addition, unlike the example of the prior art mentioned above, the fact that the photoluminescent layer is in contact with the light emitting diode makes it possible to improve the extraction efficiency, defined as the ratio between the luminous flux received. by the photoluminescent layer on the light flux emitted by the light emitting diode, and thus to further optimize the overall efficiency of the optoelectronic device. Indeed, an intermediate transparent layer between the light-emitting diode and the photoluminescent layer can reduce the transmission of the light flux emitted by the diode by total internal reflection in the transparent layer, thereby reducing the extraction efficiency.

An exemplary embodiment method is now described with reference to FIGS. 3a to 3d. FIG. 3a shows a light-emitting diode according to the VTF (Vertical Thin Film) configuration. Other diode configurations electroluminescent can be used, such as the configuration TFFC (for Thin Film Flip Chip, in English).

The light-emitting diode 10 comprises the stack formed of the doped semiconductor portions 12 and 14 and of the active zone 13. This stack rests on a support 2, more precisely on an electrically conductive layer 3 intended to form the electrode P. In addition, an electrically conductive strip 4 intended to form the electrode N, preferably made of a material transparent to the light radiation emitted by the light-emitting diode, is disposed on the upper face of the N-doped semiconductor portion 14, part of which is illustrated on FIG. FIG. 3a, situated on the edge of the upper face of the N-doped semiconductor portion 14. An electrical connection pad 5 also rests on the upper face of the support 2, so as to ensure the electrical connection of the N electrode. The upper face the stack forms the emission surface 11 through which the light radiation is intended to be emitted. This configuration, conventional for those skilled in the art, is not described in more detail.

In FIG. 3b, the photoluminescent layer 30 is deposited on the light-emitting diode, more precisely on the whole of the emission surface 11 thereof. Preferably, it is in contact with the emitting surface 11 of the light emitting diode. It has an average thickness e _c substantially constant over its entire surface area. The deposition may be performed by a conventional technique known to those skilled in the art, such as screen printing, dispensing, electrophoresis, spin coating, or the like. The photoluminescent layer 30 is made based on at least one phosphor material Le, for example in the form of a powder, and may comprise a binder matrix of a transparent and optically inert material, for example silicone.

It may be advantageous to produce a photoluminescent layer with a high luminophore volume fraction, for example greater than or equal to 20% and preferably greater than or equal to 30%. A high volume fraction indeed contributes to increasing the absorption rate and therefore the conversion rate of the photoluminescent layer. She can also help to improve the heat dissipation of the heat produced by the light-emitting diode. It can finally simplify and make more robust the cavity forming step when it is performed by laser ablation.

According to a first variant, a mixture of phosphor Le, a binder and a solvent is prepared beforehand. By way of example, the phosphor Le may be YAG: Ce in powder form, the silicone binder and the glycerol solvent. The phosphor volume fraction is chosen less than or equal to 25%, and preferably less than or equal to 5%. The volume fraction of binder is chosen less than or equal to 30%, and preferably less than or equal to 15%. For example, for a light emitting diode surface of 1 mm ² side, the phosphor may have a mass of between 100 .mu.g and 200 .mu.g and the binder a mass of about 100 .mu.g. The proportion of solvent can be adjusted depending on the viscosity of the desired mixture, to facilitate the deposition of the layer. The mixture thus obtained is deposited on the emitting surface 11 of the light-emitting diode 10, and then the solvent is evaporated so as to obtain in fine a photoluminescent layer 30 where the arrangement of the phosphor grains leads to a volume fraction. high and homogeneous. The evaporation is carried out for a sufficiently long time to prevent any convection movement in the mixture, for example for several minutes, between 120 ° C. and 150 ° C. In the example given above, here we obtain a photoluminescent layer 30 of constant average thickness, less than ΙΟΟμιη and between about 30μιη and 50μιη, with a volume fraction of phosphor greater than or equal to 20%.

According to a second variant, a mixture of luminophores is produced in a large quantity of binder, without solvent, with a phosphor volume fraction typically less than 20% of phosphor and preferably less than 5%, for example 100 μg of phosphor in 1 mg of phosphor. binder. The mixture is then deposited on the emission surface of the light emitting diode and the luminophore is allowed to settle, for example for 10 h at 50.degree. A photoluminescent layer having an average thickness of 30 μm with a phosphor volume fraction greater than or equal to 20% covered with a layer containing mainly binder is thus obtained. According to a third variant, a mixture comprising mainly the phosphor and the binder is deposited on the emission surface of the light-emitting diode, with a phosphor volume fraction greater than or equal to 20%, and preferably less than 30% to adjust the phosphor. viscosity and facilitate the dispensation.

Once the photoluminescent layer 30 has been obtained on the emission surface 11 of the light-emitting diode, cavities 33i, 33 ₂ , 333 are made from the upper face 32 of the photoluminescent layer. This step can be carried out by thin-film material removal techniques known to those skilled in the art, such as laser ablation, dry etching of the plasma etching or RIE etching type, or even photolithography.

It is particularly advantageous to make the cavities 33 ₇ , 33 ₈ , 33 ₉ by laser ablation, to precisely control the depth of the cavities and thus the thickness ea of the bottom portion C, cavity. This thus makes it possible to finely control the colorimetric properties of the color pixel in question, but also to avoid damaging the emitting surface 11 and therefore the doped semiconductor portion 14 of the light emitting diode.

The laser may be chosen from lasers emitting in the UV range, for example between 193 nm and 355 nm, in the visible range, for example in the red at 633 nm or the green at 532 nm or even in the infrared, for example between 9 μm and ΙΟ μm ( laser C0 ₂ ), or between 0.9μιη and 1.4μιη (YAG laser). The power of the laser can be chosen between a few milliJoules or less and a few joules, depending in particular on the hardness of the layer to be etched and the expected accuracy of the etching. In order to obtain an etching accuracy as regards both the depth of the cavity and the lateral profile of the cavity, the power of the laser is advantageously chosen to be of the order of a few milli-joules, and several passes of the laser are made. to engrave the same cavity.

For example, a cavity 200 de330μιη of side and 25μιη deep can be made in a photoluminescent layer, having a mass fraction of 68% of YAG: Ce and 32% of silicone, using a laser UV pulsed at 50Hz, wavelength 355nm and power 0.2mJ. Several passages are made in order to obtain the desired dimensions. The size of the focusing spot of the laser beam can be adapted according to the area of the cavity to be formed. Thus, a spot of 2χ2μιη can be used to form the profile of the cavity and a spot of larger size, for example 45χ90μιη can be used to form the interior of the cavity.

In the VTF configuration of the light-emitting diode as illustrated in FIG. 3c, a cavity 33io is made facing the electrode N 4, in a through manner to open locally on the electrode N 4. The absence, at this stage, of wire 6 for electrical connection between the electrode N 4 and the electrical connection pad 5 makes it possible to prevent the deposition of the photoluminescent layer 30 from being disturbed by the presence of the wire 6. In addition, the electrode portion 4 forms an etch stop layer with respect to laser etching when a UV laser in the range of 255nm to 355nm and low power is used.

The phosphor studs P are then deposited in all the cavities 33, or preferably in certain cavities so as to leave empty cavities of phosphor studs, with the exception of the through cavity 33io. The deposition of the phosphor pads P may be effected by dispensing small drops whose volume depends on the desired dimensions of the pad. By way of example, for cavities with a depth of between approximately 30μιη and 50μιη and transverse dimensions between ΙΟΟμιη and 300μιη, the drop volume deposited can be between 10 ^{~ 4} mm ³ and 10 ^{~ 2} mm ³ .

Each cavity 33 may receive one or more pads P, the size and type of phosphor depends on the desired colorimetric properties. Thus, several different phosphor pads can be deposited superimposed in the same cavity. Furthermore, the phosphor pad may have a smaller volume, equal to or even greater than the volume of the cavity receiving it.

The deposited drops intended to form the phosphor pads may be prepared according to the same examples of preparation of the photoluminescent layer, for example with a solvent intended to be evaporated or with a high proportion of binder and a step of sedimentation of the luminophores so as to form the plot.

Finally, the conventional steps of producing an electroluminescent component are then carried out. Thus, an electrically conductive wire 6 is placed so as to connect electrically the electrode 4 to the electrical connection pad 5. Finally, a protective layer or dome of the optoelectronic device (not shown), made of a transparent and optically inert material, for example silicone, can be deposited on the optoelectronic device so as to cover the photoluminescent structure. These steps are known to those skilled in the art and are not detailed further.

Specific embodiments have just been described. Various variations and modifications will occur to those skilled in the art.

In the embodiments described above, the optoelectronic device comprises a single light-emitting diode whose emission face is covered by the structured photoluminescent layer. Alternatively, the optoelectronic device may comprise a plurality of light emitting diodes arranged mutually so as to define an emitting surface common to the diodes, preferably plane. According to a first example, the light emitting diodes have the structure described in the patent application FR 14 50077 filed January 7, 2014, where the light emitting diodes have a mesa structure, that is to say that the active zone of the diodes is located protruding above the substrate following a step of etching its flanks, and comprise a common cathode. According to a second example, the light-emitting diodes have a structure described in the publication of Fan et al entitled lll-nitride micro-emitter arrays development and applications,]. Phys. D: Appl. Phys. 41 (2008) 094001.

Furthermore, the embodiments described above show a photoluminescent layer advantageously disposed in contact with the emission surface of the light emitting diode. Alternatively, an intermediate layer, made of a transparent and optically inert material, may be present between the emission surface and the photoluminescent layer. This transparent intermediate layer may provide additional protection for the light-emitting diode during the cavity-forming step, in particular by laser ablation. Moreover, the cavities described above are not through, so that a cavity bottom portion is present between the cavity and the emission surface of the light emitting diode. Alternatively, at least a portion of the cavities may be through and lead to the emission surface of the light emitting diode.

Finally, the photoluminescent layer may be formed of a single layer of phosphor or a stack of several photoluminescent layers having different optical properties. Thus, the stack may comprise a first elementary layer based on a first type of phosphor covered with one or more elementary layers made on the basis of different luminophores, that is to say with a wavelength of photoluminescence emission is different from that of the phosphor of other elementary layers.

Claims

An optoelectronic light emitting device (1), comprising:

at least one light-emitting diode (10) having an emission surface

(he) ;

a photoluminescent layer (30), which at least partially covers the emission surface, comprising at least a first phosphor (Le) adapted to absorb at least part of an incident light radiation emitted by the light-emitting diode and to emit in response a light radiation at a first wavelength, and having at least one cavity (33) formed from a face (32) of the photoluminescent layer opposite the emission surface;

characterized in that said cavity (33) is non-through so that the photoluminescent layer (30) locally has a bottom portion (C) of cavity located under said cavity (33), at least a second phosphor (U) ), adapted to absorb at least part of incident light radiation and to emit in response light radiation at a second wavelength different from the first wavelength, being disposed in said cavity (33,),

and in that the photoluminescent layer (30) comprises a plurality of cavities (33) each housing at least one pad (P) of phosphor (L) different from the first phosphor (Le).

Optoelectronic device (1) according to claim 1, comprising a single light-emitting diode (10) whose emission surface (11) is covered by said photoluminescent layer (30).

Optoelectronic device (1) according to claim 1 or 2, the emission surface (11) is a face of a semiconductor layer doped with a PN junction of said light emitting diode (10).

Optoelectronic device (1) according to any one of claims 1 to 3, wherein the photoluminescent layer (30) is in contact with the emission surface (11) of the light emitting diode (10).

5. Optoelectronic device (1) according to any one of claims 1 to 4, each cavity of the photoluminescent layer (30) being non-traversing.

Optoelectronic device (1) according to any one of claims 1 to 5, wherein the cavity bottom portion (C) has an average thickness (e) less than or equal to 10% of a minimum layer thickness. photoluminescent device (30) necessary to absorb substantially all the incident light radiation emitted by the light-emitting diode (10).

7. Optoelectronic device (1) according to any one of claims 1 to 6, wherein the cavity bottom portion (C) has an average thickness (ea) less than or equal to ΙΟμιη.

Optoelectronic device (1) according to any one of claims 1 to 7, wherein said phosphor pads (P,) are adapted to each emit a light selected from blue, red, yellow, green and yellow. 'orange.

9. Optoelectronic device (1) according to any one of claims 1 to 8, wherein a portion of the cavities (33 ₃ , 33 ₆ , 33 ₈ ) has no phosphor pad, and whose cavity bottom portion corresponding has an average thickness less than or equal to 10% of a minimum thickness of photoluminescent layer necessary to substantially absorb all the incident light radiation emitted by the light emitting diode.

10. Optoelectronic device (1) according to any one of claims 1 to 9, wherein the light emitting diode (10) is made based on a compound III-V and preferably III-N.

11. A method of producing an optoelectronic device (1) according to any one of claims 1 to 10, comprising the steps in which:

depositing, on an emission surface (11) of at least one light-emitting diode (10), a photoluminescent layer (30) comprising at least a first phosphor (Le);

at least one cavity (33,) is made from a face (32) of the photoluminescent layer opposite to the emission surface (11), said cavity (33) being non-through so that the photoluminescent layer ( 30) locally has a bottom portion () of cavity located under said cavity (33,); at least one pad (P) of second phosphor (U) is deposited in said cavity (33), the photoluminescent layer (30) comprising a plurality of cavities (33) each housing at least one pad (P) of phosphor (U) different from the first phosphor (Le).

The method of realization of claim 11, wherein said cavity (33) is formed by laser ablation.

13. Production method according to claim 11 or 12, wherein the photoluminescent layer (30) is obtained from a mixture of the first phosphor (Le) with a solvent, the step of depositing said photoluminescent layer comprising a phase depositing said mixture on the emission surface (11) followed by a solvent evaporation phase.

The production method according to claim 11 or 12, wherein the photoluminescent layer (30) is obtained from a mixture of a first phosphor powder (Le) with a binder matrix of a transparent and optically inert material, the step of depositing said photoluminescent layer comprising a deposition phase of said mixture on the emission surface (11) followed by a sedimentation phase of the first phosphor powder (Le) within the binder matrix.