FR3150042A1 - LIGHT EMITTING DIODE DISPLAY SCREEN WITH IMPROVED LIGHT EXTRACTION AND DISPLAY PIXELS FOR SUCH A SCREEN - Google Patents

Abstract

LIGHT-EMITTING DIODE DISPLAY SCREEN WITH IMPROVED LIGHT EXTRACTION AND DISPLAY PIXELS FOR SUCH A SCREEN The present description relates to a display pixel (3') comprising a light-emitting zone (8) comprising at least one light-emitting diode, a support (9) transparent to the radiation emitted by the light-emitting zone (8), the height of the support (9) being greater than 5 µm, and an interlayer (12) transparent to the radiation emitted by the light-emitting zone (8) and interposed between the light-emitting zone (8) and the support (9), the refractive index of the interlayer (12) being strictly less than 1.5 and being less than the refractive index of the support (9) with a refractive index difference greater than or equal to 0.2. Figure for the abstract: Fig. 3

Description

LIGHT EMITTING DIODE DISPLAY SCREEN WITH IMPROVED LIGHT EXTRACTION AND DISPLAY PIXELS FOR SUCH A SCREEN

The present description relates generally to display screens comprising display pixels comprising light-emitting diodes based on semiconductor materials and their manufacturing methods.

It is known to produce a display screen comprising display pixels, each display pixel comprising at least one light-emitting diode, for example a semiconductor material based on a compound predominantly comprising at least one element from group III and one element from group V (for example gallium nitride GaN), hereinafter called a III-V compound.

The light extraction efficiency (LEE) of a display screen is generally defined as the ratio of the number of photons that escape from the display screen to the number of photons emitted by the light-emitting diodes in the display pixels. It is desirable that the extraction efficiency of a display screen be as high as possible.

An example of a method of manufacturing a display screen includes placing display pixels on a panel and depositing a planarization layer to obtain a substantially planar emitting face.

A disadvantage of existing display screens is that a fraction of the photons emitted by each display pixel do not escape the display screen.

One embodiment overcomes all or part of the drawbacks of known display screens.

One embodiment provides a display pixel comprising:
- an electroluminescent zone comprising at least one light-emitting diode;
– a support transparent to the radiation emitted by the electroluminescent zone, the height of the support being greater than 5 µm; and
- an interlayer transparent to the radiation emitted by the electroluminescent zone and interposed between the electroluminescent zone and the support, the refractive index of the interlayer being strictly less than 1.5 and being less than the refractive index of the support with a refractive index difference greater than or equal to 0.2.

The interlayer makes it possible to reflect, towards the electroluminescent zone, the light rays emitted by the electroluminescent zone whose incidence is high and to allow only the light rays emitted by the electroluminescent zone whose incidence is low to pass. The light rays reflected by the interlayer towards the electroluminescent zone, by diffusion and reflection phenomena occurring within the electroluminescent zone, are then returned towards the interlayer and can pass through the interlayer when their incidence is low. The interlayer therefore advantageously makes it possible to increase the number of light rays emitted by the display pixel with a low incidence. This advantageously makes it possible to increase the light extraction efficiency of a display screen comprising such display pixels since the reflections of light rays on the emission face of the display screen are reduced.

According to one embodiment, the support is a single-piece glass support. This is advantageously a material having good mechanical strength and good optical transparency properties.

According to one embodiment, the display pixel further comprises a bonding layer interposed between the support and the interlayer. This advantageously makes it possible to facilitate the manufacture of the display pixel.

According to one embodiment, the light-emitting zone comprises light-emitting diodes comprising wire, conical, or truncated semiconductor elements.

According to one embodiment, the light-emitting diode comprises a textured surface. This advantageously makes it possible to increase the diffusion of the radiation emitted by the light-emitting diode.

According to one embodiment, the electroluminescent zone further comprises, for at least one electroluminescent diode, an electrically insulating block covering the electroluminescent diode and interposed between the interlayer and the electroluminescent diode. The block makes it possible in particular to protect the electroluminescent diodes, in particular when the electroluminescent diodes comprise wire, conical or truncated semiconductor elements, to obtain a flat upper face, and possibly to adjust the emission wavelength of the display pixel.

According to one embodiment, the block is photoluminescent.

According to one embodiment, the block is diffusing for the radiation emitted by the light-emitting diode. This advantageously makes it possible to increase the diffusion of the radiation emitted by the light-emitting diode.

According to one embodiment, the block is transparent to the radiation emitted by the light-emitting diode.

According to one embodiment, the electroluminescent zone further comprises a reflective layer opposite the interlayer, the block being interposed between the reflective layer and the interlayer. This advantageously makes it possible to prevent light rays from escaping from the electroluminescent zone on the side opposite the support.

According to one embodiment, the electroluminescent zone further comprises reflective walls surrounding the block. This advantageously prevents light rays from escaping over the lateral edges of the electroluminescent zone.

According to one embodiment, the display pixel comprises electrically conductive fixing pads on a face opposite the support. This advantageously makes it possible to fix the fixing pads of the display pixel on a panel by manipulating the display pixel by the support.

One embodiment also provides a display screen comprising:
- a slab;
- display pixels as defined above, the supports of the display pixels being located on the side opposite the panel; and
- a planarization layer covering the panel and the display pixels.

The interlayer of each display pixel advantageously allows the light extraction efficiency of the display screen to be increased since the reflections of light rays on the emission face of the display screen are reduced.

An embodiment also provides a method of manufacturing the display pixels as defined above, comprising the following steps:
- formation on a semiconductor plate of several copies of the electroluminescent zone of the display pixel;
– formation of the interlayer on the electroluminescent areas; and
- fixing, on the intermediate layer, a plate of the material making up the support; and
- separation of display pixels.

One embodiment also provides a method of manufacturing a display screen, comprising the following steps:
- formation of display pixels as defined above;
- the individual placement and fixing of each display pixel on a panel; and
- formation of a planarization layer covering the display pixels and the panel between the display pixels.

The display pixel support allows individual manipulation of the display pixel during the step of placing the display pixel on the panel.

According to one embodiment, the manipulation of each display pixel during the step of individually placing and fixing each display pixel on the panel comprises the use of a gripper manipulating the display pixel by the support of the display pixel.

These and other features and advantages will be set forth in detail in the following description of particular embodiments given without limitation in connection with the attached figures, among which:

there represents, in a partial and very schematic way, an example of a display screen;

there illustrates the light ray paths for the display screen of the ;

there represents, in a partial and very schematic manner, an embodiment of a display screen;

there illustrates the light ray paths for the display screen of the ;

there schematically illustrates the paths of light rays and for a display pixel of the display screen of the on the left side and for a display pixel of the display screen of the partly right;

there represents, in a partial and schematic manner, an embodiment of a display pixel of the display screen of the ;

there represents, in a partial and schematic manner, another embodiment of a display pixel of the display screen of the ;

there represents, in a partial and schematic manner, a more detailed embodiment of the display pixel represented in ;

there represents an embodiment of a three-dimensional light-emitting diode; and

there represents another embodiment of a three-dimensional light-emitting diode.

The same elements have been designated by the same references in the different figures. In particular, the structural and/or functional elements common to the different embodiments may have the same references and may have identical structural, dimensional and material properties.

For the sake of clarity, only the steps and elements useful for understanding the embodiments described have been shown and are detailed.

Unless otherwise specified, when referring to two elements connected together, this means directly connected without intermediate elements other than conductors, and when referring to two elements connected (in English "coupled") together, this means that these two elements can be connected or be connected by means of one or more other elements.

In the following description, when reference is made to absolute position qualifiers, such as the terms "front", "back", "top", "bottom", "left", "right", etc., or relative position qualifiers, such as the terms "above", "below", "upper", "lower", etc., or to orientation qualifiers, such as the terms "horizontal", "vertical", etc., reference is made, unless otherwise specified, to the orientation of the figures or to a ... in a normal position of use.

Unless otherwise specified, the expressions "approximately", "approximately", "substantially", and "of the order of" mean to within 10%, preferably to within 5%.

In the remainder of the description, the internal transmittance of a layer corresponds to the ratio between the intensity of the radiation leaving the layer and the intensity of the radiation entering the layer. The absorption of the layer is equal to the difference between 1 and the internal transmittance. In the remainder of the description, a layer is said to be transparent to radiation when the absorption of the radiation through the layer is less than 60%. In the remainder of the description, a layer is said to be absorbent to radiation when the absorption of the radiation in the layer is greater than 60%. When radiation has a spectrum of general "bell" shape, for example of Gaussian shape, having a maximum, the wavelength of the radiation, or central or main wavelength of the radiation, is called the wavelength at which the maximum of the spectrum is reached. In the remainder of the description, the refractive index of a material corresponds to the refractive index of the material for the wavelength range of the radiation emitted by the optoelectronic device. Unless otherwise stated, the refractive index is considered to be substantially constant over the wavelength range of the useful radiation, for example equal to the average of the refractive index over the wavelength range of the useful radiation. The refractive index is a dimensionless number that characterizes the optical properties of a medium, including absorption and scattering. The refractive index is equal to the real part of the complex optical index. The refractive index can be determined, for example, by ellipsometry.

Further, the terms "insulator" and "conductor" are herein considered to mean "electrically insulating" and "electrically conducting", respectively.

There is a partial and very schematic sectional view of an example of display screen 1.

The display screen 1 comprises a panel 2 and display pixels 3 attached to the panel 2, a single display pixel 3 being shown in . For certain applications, it is desirable for the emission face 6 of the display screen 1 to be substantially planar. For this purpose, the display screen 1 comprises a planarization layer 4 covering the display pixels 3 and the panel 2 between the display pixels 3 and a stack 5 of layers covering the planarization layer 4 and delimiting the emission face 6 of the display screen 1. The planarization layer 4 makes it possible in particular to encapsulate and protect the display pixels 3. The stack 5 of layers makes it possible in a known manner to improve certain optical properties of the display screen 1. The stack 5 comprises, for example, anti-reflective coatings.

The display pixel 3 comprises a substrate 7 on which an electroluminescent zone 8 is formed. The display pixel 3 is fixed to the panel 2 on the side of the substrate 7. The display pixel 3 further comprises a support 9, for example a glass block, so as to allow the display pixel 3 to be manipulated. The electroluminescent zone 8 is then interposed between the substrate 7 and the support 9.

There illustrates the paths of light rays, obtained by simulation, during operation of the display screen 1 of the . In , the electroluminescent zone 8 is here simulated by a point light source. As shown in this figure, some rays emitted by the light source are reflected at the interface between the stack 5 of layers and the air and remain trapped in the screen 1, leading to a reduction in the extraction efficiency of the emitted rays.

There is a partial and very schematic sectional view of an embodiment of a display screen 10.

The display screen 10 shown in the includes all the elements of the display screen 1 shown in the , except that the display pixels 3 are replaced by display pixels 3', each display pixel 3' comprising all of the elements of the display pixels 3 and further comprising an interposed layer 12 interposed between the electroluminescent zone 8 and the support 9.

The interlayer 12 is a transparent layer for the radiation emitted by the electroluminescent zone 8. The refractive index of the interlayer 12 is as close as possible to 1, preferably strictly less than the refractive index of the support 9 and strictly less than the refractive index of the material or materials making up the electroluminescent zone 8 in contact with the interlayer 12. According to one embodiment, the refractive index of the interlayer 12 is strictly less than 1.5, preferably in the range from 1 to 1.3. According to one embodiment, the interlayer 12 is made of a material selected from the group comprising magnesium fluoride (MgF ₂ ) and organic polymers, in particular acrylates. The fact that the interlayer 12 corresponds to a layer of a solid material advantageously makes it possible to facilitate the connection between the support 9 and the electroluminescent zone 8. According to one embodiment, the thickness of the interlayer 12 is less than or equal to 2 μm, preferably less than or equal to 1 μm, and more preferably less than or equal to 500 nm. According to one embodiment, the thickness of the interlayer 12 is greater than or equal to 10 nm. According to one embodiment, the interlayer 12 comprises a lower face 13 and an upper face 14 that are opposite and flat. According to one embodiment, the faces 13 and 14 are parallel to the emission face 6. According to one embodiment, the interlayer 12 is a layer deposited according to a conformal deposition. According to one embodiment, the interlayer 12 is made of a material allowing spin coating.

The support 9 is transparent to the radiation emitted by the electroluminescent zone 8. According to one embodiment, the support 9 is made of a material selected from the group comprising glass and sapphire. According to one embodiment, the height of the support 9 is greater than 5 µm, preferably between 20 µm and 500 µm. The thickness of the support 9 is sufficiently large so that the support 9 can ensure mechanical strength of the assembly comprising the support 9, the interlayer 12, the electroluminescent zone 8, and the substrate 7 during a handling step. For comparison, the thickness of the electroluminescent zone 8 may be greater than 500 nm, preferably between 1 µm and 50 µm, the thickness of the interlayer 12 may be less than or equal to 2 µm, preferably less than or equal to 1 µm, and more preferably less than or equal to 500 nm, as indicated above, and the thickness of the substrate 7, when present, may be less than 100 µm. The aspect ratio of the support 9, i.e. the ratio between the height and the maximum width of the support 9, may be between 0.01 and 10, preferably between 0.05 and 2. The refractive index of the support 9 is strictly greater than 1.3, preferably in the range of 1.4 to 1.6. Preferably, the refractive index difference between the refractive index of the support 9 and the refractive index of the interlayer 12 is greater than 0.2. According to one embodiment, the support 9 comprises an upper face 15, on the side opposite the electroluminescent zone 8. Preferably, the upper face 15 is flat.

According to one embodiment, the maximum height of the display pixel 3' is between 20 µm and 500 µm. The aspect ratio of the display pixel 3', i.e. the ratio between the height and the maximum width of the display pixel 3', may be between 0.01 and 10, preferably between 0.05 and 2.

One embodiment of a method of manufacturing a 3' display pixel comprises forming a plate comprising a plurality of the 3' display pixel followed by separating the 3' display pixels. In particular, one embodiment of a method of manufacturing a 3' display pixel comprises the following steps:
- formation on a semiconductor plate of several copies of the electroluminescent zone 8;
- formation of the interlayer 12 on the electroluminescent zones 8; and
- fixing, on the intermediate layer 12, a plate of the material making up the support 9;
- 3' display pixel separation.

The separation of the 3' display pixels can be achieved by cutting the plate, in particular by mechanical sawing or by laser cutting.

An embodiment of a method for manufacturing the display screen 10 comprises the individual placement and fixing of each display pixel 3' on the panel 2, followed by the formation of the planarization layer 4 covering the display pixels 3' and the panel 2 between the display pixels 3' and, when present, the stack 5 of layers. In particular, according to one embodiment, the manipulation of each display pixel 3' during the step of individual placement and fixing of each display pixel 3' on the panel 2 comprises the use of a gripper manipulating the display pixel 3' by the support 9. According to one embodiment, the gripper exerts suction on the upper face 15 of the support 9. For this purpose, advantageously, the upper face 15 of the support 9 is planar.

There illustrates the light ray paths, obtained by simulation, for the display screen 10 of the . In , the light-emitting area 8 is simulated here by a point light source. The light extraction efficiency of the display screen 10 is increased compared with the light extraction efficiency of the display screen 1. It can be estimated by simulation that the light extraction efficiency of the display screen 10, with an interlayer 12 of refractive index of 1.2, can be increased by 30% to 40% compared with the light extraction efficiency of the display screen 1.

There schematically illustrates on the left side the paths of light rays R1 and R2 for a display pixel 3 of the display screen 1 of the and on the right side the paths of light rays R1' and R2' for a display pixel 3' of the display screen 10 of the . For the purpose of illustration, the refractive indices of the planarization layer 4, the support 9, the electroluminescent zone 8, and the stack 5 of layers are substantially equal in .

Without an interlayer 12, the light rays R2 emitted by the electroluminescent zone 8 whose incidence is significant relative to the emission face 6 are reflected on the emission face 6 which corresponds to the interface between the stack 5 of layers and the air. The emission face 6 only lets through the light rays R1 emitted by the electroluminescent zone 8 whose incidence is low relative to the emission face 6.

The interlayer 12 makes it possible to reflect, towards the electroluminescent zone 8, the light rays R2’ emitted by the electroluminescent zone 8 whose incidence is high relative to the emission face 6 and to allow only the light rays R1’ emitted by the electroluminescent zone 8 whose incidence is low relative to the emission face 6 and which will, for the most part, manage to escape from the display screen 10. The light rays R2’ reflected by the interlayer 12 towards the electroluminescent zone 8, by diffusion and reflection phenomena taking place within the electroluminescent zone 8, are then returned towards the interlayer layer 12 and can pass through the interlayer layer 12 when their incidence is low relative to the emission face 6. The total number of light rays finally reaching the emission face 6 with a low incidence is thus increased, so that the efficiency light extraction from the display screen 10 is increased. In other words, the interlayer 12 thus makes it possible to reduce, or even eliminate, the reflections of the rays emitted by the electroluminescent zone 8 at the interface between the stack 5 of layers and the air, thus creating a sort of recycling of the light rays R2’ at the level of the electroluminescent zone 8.

According to one embodiment, the light-emitting area 8 of each display pixel 3' comprises at least one light-emitting diode, preferably sets of light-emitting diodes. Each light-emitting diode may be a three-dimensional light-emitting diode or a planar light-emitting diode. According to one embodiment, the light-emitting diodes LED comprise semiconductor layers made of III-V compounds, for example GaN, AlN, InN, InGaN, AlGaN or AlInGaN, or of II-VI compounds.

A three-dimensional light-emitting diode comprises a three-dimensional semiconductor element, for example a wire, conical, truncated cone, or pyramidal element, in particular a microwire or a nanowire, on which the active zone of the light-emitting diode extends. The term "microwire" or "nanowire" designates a three-dimensional structure of elongated shape in a preferred direction of which at least two dimensions, called minor dimensions, are between 5 nm and 2.5 µm, preferably between 50 nm and 2.5 µm, the third dimension, called major dimension, being at least equal to 1 time, preferably at least 2 times, the largest of the minor dimensions. In certain embodiments, the minor dimensions may be less than or equal to approximately 3 µm, preferably between 100 nm and 3 µm, more preferably between 1 µm and 1.5 µm. In some embodiments, the height of each microwire or nanowire may be greater than or equal to 500 nm, preferably between 1 µm and 50 µm.

There is a partial and schematic sectional view of an embodiment of a display pixel 3' of the display screen 10 shown in wherein the light-emitting zone 8 comprises nanowire or microwire LED light-emitting diodes.

The electroluminescent zone 8 comprises from bottom to top in :
- a reflective layer 16 for the radiation emitted by the light-emitting diodes LED;
- sets of light-emitting diodes LED (two sets of eight light-emitting diodes being shown schematically as an example), each light-emitting diode LED here having the general shape of a nanowire or a microwire;
- insulating blocks 18 here resting on the reflective layer 16, each block 18 being located opposite one of the LED light-emitting diodes or a set of LED light-emitting diodes and completely surrounding the LED light-emitting diode(s), each block 18 being a block transparent to the radiation emitted by the LED light-emitting diodes, and/or a block diffusing the radiation emitted by the LED light-emitting diodes, and/or a photoluminescent block; and
- walls 22 between the blocks 18, each wall 22 being opaque to the radiation emitted by the light-emitting diodes LED.

The 3' display pixel includes from bottom to top in :
- the substrate 7 comprising a lower face 23 and an upper face 24 opposite the lower face 23, the upper face 24 preferably being flat at least at the level of the light-emitting diodes LED;
- the electroluminescent zone 8, the reflective layer 16 of the electroluminescent zone 8 resting on the upper face 24;
- the intermediate layer 12 covering the electroluminescent zone 8, in particular the blocks 18 and the walls 22;
- optionally, a color filter 26 or more than one color filter, covering at least some of the blocks 18, a single filter 26 covering a block 18 being shown as an example, the interlayer 12 being interposed between the blocks 18 and the filter 26 or filters;
- a bonding layer 28; and
- support 9.

There is a partial and schematic sectional view of another embodiment of a display pixel 3' of the display screen 10 shown in wherein the light-emitting area 8 comprises at least one planar light-emitting diode (LED). A planar light-emitting diode, also called a two-dimensional light-emitting diode, is manufactured by forming a stack of substantially planar semiconductor layers on a substrate, followed by delineating the light-emitting diode, for example by etching trenches in the stack of semiconductor layers.

The 3' display pixel represented in includes all the elements of the 3' display pixel represented in except that the light-emitting diode LED comprises a stack of substantially flat layers. According to one embodiment, the light-emitting diode LED comprises an upper face 29 which is diffusing for the radiation emitted by the light-emitting diode LED.

In FIGS. 6 and 7, the substrate 7 may correspond to a single-block structure. It may not be present. The substrate 7 may correspond to an integrated circuit configured to control the light-emitting diode LED or the light-emitting diodes of the light-emitting zone 8 and comprising electronic components, in particular insulated gate field effect transistors, also called MOS transistors, or thin-film transistors, also called TFT transistors (English acronym for Thin-Film Transistor). We then speak of smart 3' display pixels. When the substrate 7 is absent (not shown), the light-emitting diode(s) can be attached to an integrated circuit, in particular a control circuit, comprising electronic components, and the connection can be made via conductive pads (not shown) made in the lower part of the display pixel 3', the lower part then being that opposite the support 9 relative to the light-emitting zone 8. The conductive pads are then electrically connected with an electrode layer described later.

In Figures 6 and 7, the reflective layer 16 is shown continuously on the upper face 24 of the substrate 7. In practice, the reflective layer 16 can be interrupted to allow the connection of the light-emitting diodes LED to the substrate 7.

In the embodiments described above, the reflective layer 16 may be a conductive layer, in particular a metal layer, for example made of iron, copper, aluminum, tungsten, silver, titanium, hafnium, zirconium or a combination of at least two of these compounds. Preferably, the reflective layer 16 is formed of a material compatible with the manufacturing methods implemented in microelectronics. Preferably, the reflective layer 16 is formed of aluminum or silver. According to one embodiment, the thickness of the reflective layer 16 is between 100 nm and 300 nm. As a variant, the reflective layer 16 may correspond to a Bragg mirror comprising a stack of layers of different refractive indices. In this case, the reflective layer 16 is composed of dielectric materials, such as silicon oxide (SiO ₂ ), silicon nitride (SiN), titanium oxide (TiO ₂ ) or any other oxide or nitride transparent to the wavelength of the radiation emitted by the electroluminescent zone 8. In the case where the reflective layer 16 corresponds to a Bragg mirror, it can have a thickness of up to 10 µm.

The bonding layer 28 allows the support 9 to be fixed to the intermediate layer 12. According to one embodiment, the bonding layer 28 is made of a polymer configured to harden when exposed to radiation, for example ultraviolet radiation. The bonding layer 28 may correspond to an optical adhesive, for example an optical adhesive marketed by the company Norland under the name NOA. According to one embodiment, the thickness of the bonding layer 28 is greater than 1 µm, preferably between 5 µm and 30 µm. The refractive index of the bonding layer 28 is strictly greater than 1.3, preferably in the range of 1.4 to 1.6. According to one embodiment, the refractive index of the intermediate layer 12 is strictly less than the refractive index of the bonding layer 28, in particular with a difference greater than 0.2. The refractive index of the bonding layer 28 may be substantially equal to the refractive index of the support 9.

The aspect ratio of each block 18, i.e. the ratio between the height and the maximum width of the block 18, may be between 0.01 and 10, preferably between 0.05 and 2. The height of each block 18, measured perpendicular to the upper face 24, may be between 500 nm and 15 µm. The refractive index of each block 18 is between 1.4 and 2. According to one embodiment, the refractive index of the interlayer 12 is strictly lower than the refractive index of each block 18, in particular with a difference greater than 0.2. According to one embodiment, the upper face of the block 18 is flat and parallel to the upper face 24 of the substrate 7. According to one embodiment, the side wall(s) of the block 18 are perpendicular to the upper face 24. As a variant, the side wall(s) of the block 18 may be inclined relative to the upper face 24.

According to one embodiment, when the block 18 is a photoluminescent block, it comprises phosphors adapted, when excited by the light emitted by the associated light-emitting diode LED, to emit light at a wavelength different from the wavelength of the light emitted by the associated light-emitting diode LED.

In one embodiment, each photoluminescent block 18 comprises particles of at least one photoluminescent material, for example in a transparent matrix. An example of a photoluminescent material is trivalent cerium ion-activated yttrium aluminum garnet (YAG), also referred to as YAG:Ce or YAG:Ce ³⁺ . The average particle size of conventional photoluminescent materials is typically greater than 5 µm.

According to one embodiment, each photoluminescent block 18 comprises a matrix of an inorganic or organic material in which are optionally dispersed nanometric-sized monocrystalline particles of a semiconductor material, also called semiconductor nanocrystals or nanoluminophore particles hereinafter. The internal quantum efficiency QY _int of a photoluminescent material is equal to the ratio between the number of photons emitted and the number of photons absorbed by the photoluminescent substance. The internal quantum efficiency QY _int of the semiconductor nanocrystals is greater than 5%, preferably greater than 10%, more preferably greater than 20%.

According to one embodiment, when the blocks 18 comprise phosphors, different phosphors can be provided according to the sets of light-emitting diodes.

According to one embodiment, the average size of the nanocrystals is in the range of 0.5 nm and 1000 nm, preferably from 0.5 nm to 500 nm, even more preferably from 1 nm to 100 nm, in particular from 2 nm to 30 nm. For dimensions less than 50 nm, the photoconversion properties of the semiconductor nanocrystals depend essentially on quantum confinement phenomena. The semiconductor nanocrystals then correspond to quantum dots (in the case of confinement in three dimensions) or to quantum wells (in the case of confinement in two dimensions).

According to one embodiment, the semiconductor material of the semiconductor nanocrystals is selected from the group comprising cadmium selenide (CdSe), indium phosphide (InP), cadmium sulfide (CdS), zinc sulfide (ZnS), zinc selenide (ZnSe), cadmium telluride (CdTe), zinc telluride (ZnTe), cadmium oxide (CdO), zinc cadmium oxide (ZnCdO), zinc cadmium sulfide (CdZnS), zinc cadmium selenide (CdZnSe), silver indium sulfide (AgInS ₂ ), perovskites of the PbScX ₃ type, where X is a halogen atom, in particular iodine (I), bromine (Br) or chlorine (Cl), and a mixture of at least two of these compounds. According to one embodiment, the semiconductor material of the semiconductor nanocrystals is selected from the materials cited in the publication in the name of Le Blevenec et al. of Physica Status Solidi (RRL) - Rapid Research Letters Volume 8, No. 4, pages 349–352, April 2014.

According to one embodiment, the dimensions of the semiconductor nanocrystals are chosen according to the desired wavelength of the radiation emitted by the semiconductor nanocrystals. For example, CdSe nanocrystals whose average size is of the order of 3.6 nm are suitable for converting blue light into red light and CdSe nanocrystals whose average size is of the order of 1.3 nm are suitable for converting blue light into green light. According to another embodiment, the composition of the semiconductor nanocrystals is chosen according to the desired wavelength of the radiation emitted by the semiconductor nanocrystals.

The matrix is made of a material at least partly transparent to the radiation emitted by the photoluminescent particles and/or the LED light-emitting diodes, preferably more than 80%. The matrix is, for example, made of silica. The matrix is, for example, made of any polymer at least partly transparent, in particular silicone, epoxy, acrylic resin of the poly(methyl methacrylate) (PMMA) type, or polyacetic acid (PLA). The matrix may in particular be made of an at least partly transparent polymer used with three-dimensional printers. The matrix may correspond to a glass deposited by centrifugation (SOG, acronym for Spin On Glass), photosensitive or non-photosensitive. According to one embodiment, the matrix contains from 2% to 90%, preferably from 10% to 60%, by weight of nanocrystals, for example approximately 30% by weight of nanocrystals.

The height of the photoluminescent blocks 18 depends on the concentration of nanocrystals and the type of nanocrystals used. The height of the photoluminescent blocks 18 is preferably greater than the height of the light-emitting diodes when they are wire-shaped and less than or equal to the height of the walls 22. In top view, each photoluminescent block 18 may correspond to a square, a rectangle, an "L"-shaped polygon, etc., the area of which may be equal to the area of a square having a side measuring from 1 µm to 100 µm, preferably from 3 µm to 15 µm.

According to one embodiment, when the block 18 is a diffusing block, it comprises particles adapted to diffuse the radiation emitted by the associated light-emitting diode LED which are distributed in a transparent matrix. The particles are for example particles of titanium oxide (TiO2), zirconium oxide ( _ZrO2 ), zinc sulfide (ZnS), or lead sulfide (PbS). The average size of the diffusing particles is generally in the range of 100 nm to 300 nm. The matrix is at least partly transparent to the radiation emitted by the light-emitting diodes LED, preferably more than 80%. The matrix may be made of one of the materials described above for the photoluminescent blocks 18. The diffusing block 18 may comprise quantum dots or not comprise quantum dots.

According to one embodiment, when the block 18 is a photoluminescent block, it is also a diffusing block for the radiation emitted by photoconversion. The photoluminescent block 18 may be diffusing by the presence of the phosphors. As a variant, the photoluminescent block 18 may, in addition, comprise diffusing particles. The diffusing nature of the block 18 may be characterized by the bidirectional scattering distribution function (BSDF), including in particular the bidirectional reflectance distribution function (BRDF) and the bidirectional transmittance distribution function (BTDF). The bidirectional scattering distribution function may be determined by a dedicated measuring instrument.

According to one embodiment, when the block 18 is a transparent block, it is composed of a material which can be one of the materials described previously for the matrix of the photoluminescent blocks 18.

According to one embodiment, the opaque walls 22 are reflective for the radiation emitted by the light-emitting diodes LED. The walls 22 are at least partly made of a reflective material. The reflective material may be a metallic material, in particular iron, copper, aluminum, tungsten, silver, titanium, hafnium, zirconium or a combination of at least two of these compounds. Preferably, the walls 22 are formed of a material compatible with the manufacturing methods implemented in microelectronics. Preferably, the walls 22 are formed of aluminum or silver.

The height of the walls 22, measured in a direction perpendicular to the upper face 24, is in the range of 300 nm to 200 µm, preferably 3 µm to 15 µm. The thickness of the walls 22 measured in a direction parallel to the upper face 24, is in the range of 100 nm to 50 µm, preferably 0.5 µm to 10 µm.

According to one embodiment, the walls 22 may be formed of a reflective material or each wall 22 comprises reflective walls by being covered with a coating reflecting at the wavelength of the radiation emitted by the photoluminescent blocks 18 and/or the light-emitting diodes LED, for example polymer with TiO ₂ particles.

According to one embodiment, each wall 22 comprises opaque walls. For example, each wall 22 may comprise walls covered with a layer of black-colored resin. This resin is preferably suitable for absorbing electromagnetic radiation over the spectral range including the emission spectrum of the electroluminescent zone 8 and that of the phosphors when they are present. According to another embodiment, each wall 22 is made of a resin that is partially transparent to visible light. According to one embodiment, the walls 22 are not made entirely of black-colored resin.

Preferably, the walls 22 surround the blocks 18. The walls 22 then reduce crosstalk between adjacent blocks 18. Preferably, the interlayer 12 is in contact with the walls 22. In the embodiments illustrated in FIGS. 6 and 7, the interlayer 12 is shown completely covering the walls 22. Alternatively, the walls 22 may pass through all or part of the interlayer 12.

The display pixel 3' optionally comprises one, two or three color filters 26, for example a single yellow filter, two filters, the first being a yellow filter and the second being a red filter, or three filters, the first being a red filter, the second being a green filter and the third being a blue filter, covering at least some of the blocks 18. The filter 26 may correspond to a colored layer or to a stack of layers of different refractive indices forming a Bragg filter. According to one embodiment, each filter 26 may comprise one or more layers adapted to absorb and/or reflect the radiation emitted by the electroluminescent zone 8.

When the upper surface 29 of the light-emitting diode LED is a diffusing surface, it may have a texturing allowing the diffusion of the radiation emitted by the light-emitting diode LED. The texturing of the upper face 29 may be carried out by chemical etching or by physical etching. When the diffusion obtained by the upper face 29 is sufficient, the block 18 covering the upper face 29 may be a block transparent to the radiation emitted by the light-emitting diode LED.

There is a partial, schematic, sectional view of a more detailed embodiment of a portion of the 3' display pixel shown in .

In this embodiment, the electroluminescent zone 8 comprises from bottom to top:
- a germination layer 30 made of a material promoting the growth of threads and arranged on the upper face 24 of the substrate 7;
- an insulating layer 32 covering the seed layer 30 and comprising openings 34 exposing portions of the seed layer 30;
- light emitting diodes LED (six light emitting diodes being shown), each light emitting diode LED being in contact with the seed layer 30 through one of the openings 34;
- an insulating layer 36 extending over the lateral flanks of a lower portion of the light-emitting diode LED and extending over the insulating layer 32 between the light-emitting diodes LED;
- an electrically conductive layer 38 forming an electrode covering each light-emitting diode LED and further extending over the insulating layer 36 between the light-emitting diodes LED;
- a reflective layer 16 corresponding here to a conductive layer extending over the electrode layer 38 between the light-emitting diodes LED, the reflective layer 16 being able, as a variant, to be interposed between the electrode layer 38 and the insulating layer 36 between the light-emitting diodes LED;
- a dielectric protection layer 40 extending over layers 38 and 16;
- 18 blocks covering the sets of LED light-emitting diodes;
- an insulating layer 42 covering the upper face of each block 18, or only of some of the blocks 18, the insulating layer 42 possibly not being present;
- a protective layer 44 covering the insulating layers 42, the side faces of the blocks 18 and the electrode layer 38 between the blocks 18;
- the walls 22 between the blocks 18, each wall 22 comprising a core 46 surrounded by a reflective coating 48; and
- a protective layer 49 covering the entire structure, and covered with the intermediate layer 12, not shown in .

There is a partial and schematic sectional view of an embodiment of the LED light-emitting diodes of the display pixel 3'. According to one embodiment, each LED light-emitting diode comprises a wire 50 in contact with the seed layer 30 through one of the openings 34 and a shell 52 comprising a stack of semiconductor layers covering the side walls and the top of the wire 50. Such a configuration is called radial. The assembly formed by each wire 50 and the associated shell 52 constitutes the LED light-emitting diode.

The shell 52 may comprise a stack of several layers including in particular an active layer 54 and a bonding layer 56. The active layer 54 is the layer from which the majority, preferably all, of the radiation provided by the light-emitting diode LED is emitted. According to one example, the active layer 54 may comprise confinement means, such as a single quantum well or multiple quantum wells. The bonding layer 56 may comprise a stack of semiconductor layers of the same III-V material as the wire 50 but of the opposite conductivity type to the wire 50.

There is a partial, schematic, sectional view of another embodiment of the LED light-emitting diodes of the 3' display pixel. The LED light-emitting diode shown in includes all the elements of the light-emitting diode LED shown in except that the shell 52 is only present at the top of the wire 50. Such a configuration is called axial.

The formation of the LEDs as shown in FIGS. 9 and 10, i.e. the growth of the wires 50 in the openings 64, and the formation of the shells 52 covering the wires 50 can be carried out for example by metal-organic chemical vapor deposition (MOCVD) or any other suitable method.

The seed layer 30 is made of a material that promotes the growth of the wires. For example, the material composing the seed layer 30 may be a nitride, a carbide or a boride of a transition metal from column IV, V or VI of the periodic table of elements or a combination of these compounds. According to another embodiment, the seed layer 30 may not be present. According to another embodiment, the seed layer 30 may be replaced by seed pads, for example formed at the bottom of the openings 34.

Each insulating layer 32, 36, 40, 42, 44, 50 and the cores 46 may be made of a dielectric material, for example silicon oxide (SiO ₂ ), silicon nitride (Si _x N _y , where x is approximately equal to 3 and y is approximately equal to 4, for example Si ₃ N ₄ ), silicon oxynitride (in particular of general formula SiO _x N _y , for example Si ₂ ON ₂ ), aluminum oxide (Al ₂ O ₃ ), hafnium oxide (HfO ₂ ), titanium dioxide (TiO ₂ ) or diamond. Each insulating layer 32, 36, 40, 42, 44, 50 may have a single-layer structure or correspond to a stack of two layers or more than two layers.

The electrode layer 38 is at least partially transparent so as to allow the electromagnetic radiation emitted by the light-emitting diodes to pass through. The material forming the electrode layer 38 may be a transparent and conductive material such as indium-tin oxide (or ITO, an acronym for Indium Tin Oxide), zinc oxide doped with aluminum or gallium, or graphene. The thickness of the electrode layer 38 may be between 0.01 µm and 10 µm.

Various embodiments and variations have been described. The person skilled in the art will understand that certain features of these various embodiments and variations could be combined, and other variations will occur to the person skilled in the art. Finally, the practical implementation of the described embodiments and variations is within the ability of the person skilled in the art from the functional indications given above.

Claims (16)

Display pixel (3') comprising:
- an electroluminescent zone (8) comprising at least one light-emitting diode (LED);
– a support (9) transparent to the radiation emitted by the electroluminescent zone (8), the height of the support (9) being greater than 5 µm; and
- an interlayer (12) transparent to the radiation emitted by the electroluminescent zone (8) and interposed between the electroluminescent zone (8) and the support (9), the refractive index of the interlayer (12) being strictly less than 1.5 and being less than the refractive index of the support (9) with a refractive index difference greater than or equal to 0.2. A display pixel according to claim 1, wherein the support (9) is a single-piece glass support. A display pixel according to claim 1 or 2, further comprising a bonding layer (28) interposed between the support (9) and the interlayer (12). A display pixel according to any one of claims 1 to 3, wherein the light-emitting area (8) comprises light-emitting diodes (LEDs) comprising wire, conical, or frustoconical semiconductor elements. A display pixel according to any one of claims 1 to 3, wherein the light emitting diode (LED) comprises a textured surface (29). Display pixel according to any one of claims 1 to 5, in which the electroluminescent zone (8) further comprises, for at least one light-emitting diode (LED), an electrically insulating block (18) covering the light-emitting diode (LED) and interposed between the interlayer (12) and the light-emitting diode (LED). A display pixel according to claim 6, wherein the block (18) is photoluminescent. A display pixel according to claim 6, wherein the block (18) is diffusing for the radiation emitted by the light emitting diode (LED). A display pixel according to claim 6, wherein the block (18) is transparent to radiation emitted by the light emitting diode (LED). A display pixel according to any one of claims 6 to 9, wherein the light-emitting area (8) further comprises a reflective layer (16) opposite the interlayer (12), the block (18) being interposed between the reflective layer (16) and the interlayer (12). A display pixel according to any one of claims 6 to 10, wherein the light emitting area (8) further comprises reflective walls (22) surrounding the block (18). Display pixel according to any one of claims 1 to 11, comprising electrically conductive fixing pads on a face opposite the support (9). Display screen (10) comprising:
- a slab (2);
- display pixels (3') according to any one of claims 1 to 12, the supports (9) of the display pixels (3') being located on the side opposite the panel (2); and
- a planarization layer (4) covering the panel (2) and the display pixels (3'). Method of manufacturing display pixels (3') according to claims 1 to 12, comprising the following steps:
- formation on a semiconductor plate of several copies of the electroluminescent zone (8) of the display pixel (3');
– formation of the interlayer (12) on the electroluminescent zones (8); and
- fixing, on the intermediate layer (12), a plate of the material making up the support (9); and
- separation of display pixels (3'). A method of manufacturing a display screen (10), comprising the following steps:
- formation of display pixels (3') according to any one of claims 1 to 12;
- the placement and individual fixing of each display pixel (3') on a panel (2); and
- formation of a planarization layer (4) covering the display pixels (3') and the panel (2) between the display pixels (3'). A method according to claim 15, wherein the manipulation of each display pixel (3') during the step of individually placing and fixing each display pixel (3') on the panel (2) comprises the use of a gripper manipulating said display pixel (3') by the support (9) of said display pixel (3').