Electroluminescent device

Abstract

A device includes first and second electrodes that are at least partially transparent in a spectral domain; an electroluminescent layer that lies between the first and second electrodes suitable for emitting electromagnetic radiation in the spectral domain, the electromagnetic radiation being circularly polarized in a first polarization direction; a structured substrate, the first electrode lying between the structured substrate and the electroluminescent layer, the structured substrate including features that are reflective in the spectral domain, and that possess a hollow geometric shape configured so that electromagnetic radiation that passes through the first electrode is reflected from the reflective features while preserving the first polarization direction, a filler material that is transparent in the spectral domain and that is arranged to fill the reflective features so that the structured substrate has a planar surface.

Claims

1. An electroluminescent device, comprising: first and second electrodes that are at least partially transparent in a spectral domain; an electroluminescent layer that lies between the first and second electrodes and that is suitable for emitting electromagnetic radiation in the spectral domain, the electromagnetic radiation being circularly polarized in a first polarization direction; a structured substrate, the first electrode lying between the structured substrate and the electroluminescent layer, the structured substrate comprising: features that are reflective in the spectral domain, and that possess a hollow geometric shape configured so that electromagnetic radiation that passes through the first electrode is reflected from the reflective features while preserving the first polarization direction, a filler material that is transparent in the spectral domain and that is arranged to fill the reflective features so that the structured substrate has a planar surface.

2. The device according to claim 1, wherein the hollow geometric shape of the reflective features is configured so that electromagnetic radiation that passes through the first electrode is reflected from the reflective features N times, N being a non-zero even integer number.

3. The device according to claim 1, wherein the reflective features have at least one V-shaped profile.

4. The device according to claim 1, wherein the reflective features comprise inclined flanks that make an angle comprised between 30° and 60° with respect to the normal to the planar surface of the structured substrate.

5. The device according to claim 1, wherein the reflective features are of concave shape, and the structured substrate comprises a matrix array of microlenses forming the filler material.

6. The device according to claim 1, wherein the reflective features are periodically distributed in a direction perpendicular to the normal to the planar surface of the structured substrate.

7. The device according to claim 6, wherein the electromagnetic radiation possesses a dominant wavelength, denoted λ, and the reflective features have a spatial period longer than 5λ, and preferably longer than 10λ.

8. The device according to claim 1, wherein the electromagnetic radiation possesses a dominant wavelength, denoted λ, and the reflective features have a depth larger than 5λ.

9. The device according to claim 8, wherein the depth, denoted H, and the spatial period, denoted P, respect H=P/2.

10. The device according to claim 1, wherein the reflective features are made of a metal chosen from Ag and Al.

11. The device according to claim 1, wherein the electroluminescent layer is made of a chiral organic material.

12. The device according to claim 1, wherein the first and second electrodes are made of a transparent conductive oxide.

13. The device according to claim 1, wherein the spectral domain is chosen from: the visible domain with wavelengths comprised between 400 nm and 780 nm, the UV-A domain with wavelengths comprised between 315 nm and 400 nm, the near-infrared domain with wavelengths comprised between 780 nm and 3 μm.

14. The device according to claim 1, wherein the reflective features comprise inclined flanks that make an angle comprised between 40° and 50° with respect to the normal to the planar surface of the structured substrate.

15. The device according to claim 1, wherein the electromagnetic radiation possesses a dominant wavelength, denoted λ, and the reflective features have a depth larger than 10λ.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Other features and advantages will become apparent from the detailed description of various embodiments of the invention, the description containing examples and references to the appended drawings.

(2) FIG. 1 is a schematic cross-sectional view of a prior-art electroluminescent device.

(3) FIG. 2 is a schematic cross-sectional view of an electroluminescent device according to the invention.

(4) FIGS. 3a and 3b are schematic views, in perspective and in cross section, respectively, of a structured substrate (in the absence of a filler material) belonging to an electroluminescent device according to the invention.

(5) FIGS. 4a to 4h are schematic cross-sectional views illustrating a first process for fabricating an electroluminescent device according to the invention.

(6) FIGS. 5a to 5c are schematic cross-sectional views illustrating a second process for fabricating an electroluminescent device according to the invention.

(7) FIGS. 6a to 6f are schematic cross-sectional views illustrating a third process for fabricating an electroluminescent device according to the invention.

(8) FIG. 7 is a graph the x-axis of which represent wavelength (in nm) and the y-axis of which represents photoluminescence intensity (arbitrary units), for a structured substrate (B) and for a planar substrate (A) exposed to ultraviolet radiation. The structured substrate (B) is a silicon substrate comprising reflective features, of hollow geometric shape, that are made of Ag (V-shaped profile, inclined flanks that make an angle of 45°) and that are filled with a transparent resist. The substrate B is a silicon substrate on which a planar Ag layer is formed. The substrates (A, B) are coated with a helicene organic chiral layer. This graph illustrates the influence of a structured substrate (B) of a device according to the invention—with respect to a planar substrate (A)—on the intensity of the circularly polarized electromagnetic radiation output from the organic chiral layer.

DETAILED DESCRIPTION OF EMBODIMENTS

(9) For the sake of simplicity, elements that are identical or that perform the same function in the various embodiments are designated with the same references.

(10) One subject of the invention is an electroluminescent device, comprising: first and second electrodes E.sub.1, E.sub.2 that are at least partially transparent in a spectral domain; an electroluminescent layer EL that lies between the first and second electrodes E.sub.1, E.sub.2 and that is suitable for emitting electromagnetic radiation in the spectral domain, the electromagnetic radiation being circularly polarized in a first polarization direction; a structured substrate 1, the first electrode E.sub.1 lying between the structured substrate 1 and the electroluminescent layer EL, the structured substrate 1 comprising: features 10 that are reflective in the spectral domain, and that possess a hollow geometric shape configured so that electromagnetic radiation that passes through the first electrode E.sub.1 is reflected from the reflective features 10 while preserving the first polarization direction, a filler material 11 that is transparent in the spectral domain and that is arranged to fill the reflective features 10 so that the structured substrate 1 has a planar surface.
First and Second Electrodes

(11) For a bottom-emitting architecture, the second electrode E.sub.2 is transparent, with an intensity transmission coefficient averaged over the spectral domain higher than or equal to 70%, preferably higher than or equal to 80%, and more preferably higher than or equal to 90%. The first electrode E.sub.1 is preferably transparent, with an intensity transmission coefficient averaged over the spectral domain higher than or equal to 70%, preferably higher than or equal to 80%, and more preferably higher than or equal to 90%. The first and second electrodes E.sub.1, E.sub.2 may be made of a transparent conductive oxide. The second electrode E.sub.2 may be made of indium-tin oxide (ITO).

(12) For a top-emitting architecture, the second electrode E.sub.2 is semi-transparent, with an intensity transmission coefficient averaged over the spectral domain comprised between 30% and 70%. The second electrode E.sub.2 may be made of a metal such as Ag or Al. The first electrode E.sub.1 is preferably transparent, with an intensity transmission coefficient averaged over the spectral domain higher than or equal to 70%, preferably higher than or equal to 80%, and more preferably higher than or equal to 90%. The first electrode E.sub.1 may be made of a transparent conductive oxide.

(13) Electroluminescent Layer

(14) The electroluminescent layer EL is preferably organic. The organic electroluminescent layer EL is advantageously made of a chiral organic material. By way of non-limiting examples, the chiral organic material may be: a helicene, such as a platinahelicene, chiral poly(fluorene-alt-benzothiadiazole) (c-PFBT)—where “alt” designates an alternating copolymer, a lanthanide complex, an iridium (III) complex.

(15) Other examples of organic materials suitable for emitting circularly polarized electromagnetic radiation are given in the document J. Han et al., “Recent Progress on Circularly Polarized Luminescent Materials for Organic Optoelectronic Devices”, Advanced Optical Materials, vol. 6, 17, 2018.

(16) According to one alternative, the electroluminescent layer EL may be inorganic, so as to obtain a spin-LED. By way of non-limiting examples, the inorganic electroluminescent layer EL may take the form of a quantum well, for example InGaN/GaN or AlGaAs/GaAs. Where appropriate, the second electrode E.sub.2 is made of a magnetic material so as to circularly polarize the light emitted by the inorganic electroluminescent layer EL, the magnetic material possibly for example being MgO/FeCo. Where appropriate, the structured substrate 1 is preferably made of silicon.

(17) The electromagnetic radiation emitted by the electroluminescent layer EL may possess a dominant wavelength, denoted λ. The spectral domain of the electromagnetic radiation emitted by the electroluminescent layer EL is advantageously chosen from: the visible domain with wavelengths comprised between 400 nm and 780 nm, the UV-A domain with wavelengths comprised between 315 nm and 400 nm, the near-infrared domain with wavelengths comprised between 780 nm and 3 μm.

(18) The electroluminescent layer EL preferably does not make contact with the first and second electrodes E.sub.1, E.sub.2. The device may for example comprise (electron and hole) transport layers and (electron and hole) injection layers lying between an electrode E.sub.1, E.sub.2 and the electroluminescent layer EL (which is also called the emissive layer).

(19) Structured Substrate

(20) The hollow geometric shape of the reflective features 10 is advantageously configured so that electromagnetic radiation that passes through the first electrode E.sub.1 is reflected from the reflective features N times, N being a non-zero even integer number.

(21) The reflective features 10 advantageously have at least one V-shaped profile. The reflective features 10 advantageously comprise inclined flanks that make an angle θ comprised between 30° and 60°, preferably comprised between 40° and 50°, and more preferably equal to 45°, with respect to the normal to the planar surface of the structured substrate 1. The influence of a substrate structured with V-shaped reflective features (inclined flanks that make an angle of 45°) is shown by FIG. 7. The inventors have observed an increase in the intensity of the polarized electromagnetic radiation of the order of a factor of 3 with respect to a planar substrate.

(22) The reflective features 10 are advantageously periodically distributed in a direction perpendicular to the normal to the planar surface of the structured substrate 1. The reflective features 10 advantageously have a spatial period longer than 5λ, and preferably longer than 10λ. The reflective features 10 advantageously have a depth larger than 5λ, and preferably larger than 10λ. The depth, denoted H, and the spatial period, denoted P, advantageously respect the following relationship: H=P/2.

(23) The reflective features 10 are advantageously made of a metal, which is preferably chosen from Ag and Al. The structured substrate 1 may be made of plastic or of silicon. The features of the structured substrate 1 may be metallized so as to obtain the reflective features 10.

(24) According to one embodiment, the reflective features 10 are of concave shape, and the structured substrate 1 comprises a matrix array of microlenses forming the filler material 11. According to one alternative, the filler material 11 may be a composite (oxide/photo-polymerizable polymer) material formed by a sol-gel process.

(25) The filler material 11 advantageously makes contact (i.e. direct contact) with the first electrode E.sub.1 in the sense that there is no element between the filler material 11 and the first electrode E.sub.1.

(26) Process for Fabricating the Device for a Top-Emitting Architecture

(27) As illustrated in FIGS. 4a to 4h, a process for fabricating an electroluminescent device according to the invention comprises the following steps: a) providing a structured substrate 1 comprising features possessing a hollow geometric shape; b) metallizing the features so as to obtain reflective features 10; c) forming a filler material 11 that is transparent in a spectral domain inside the reflective features 10 so that the structured substrate 1 has a planar surface; d) forming a first electrode E.sub.1 that is transparent in the spectral domain on the planar surface of the structured substrate 1; e) forming pixel regions ZP on the first electrode E.sub.1 via etching of an electrically insulating resist RI; f) forming a stack 2 of layers on the pixel regions ZP of the first electrode E.sub.1, the stack 2 comprising an electroluminescent layer EL suitable for emitting electromagnetic radiation in the spectral domain, the electromagnetic radiation being circularly polarized in a first polarization direction; g) forming a second electrode E.sub.2 that is semi-transparent in the spectral domain on the stack 2; h) forming a first encapsulation layer 3 on the second electrode E.sub.2; i) forming first and second contact pads 4a, 4b electrically connected to the first and second electrodes E.sub.1, E.sub.2, respectively; j) forming a second encapsulation layer 5 on the first encapsulation layer 3.

(28) Step a) may be executed using a grooved plastic film, for example a BEF II film of the Vikuiti™ trademark. The grooved plastic film may have a thickness of about 150 μm. The hollow geometric shape of the features of the structured substrate 1 provided in step a) is configured so that electromagnetic radiation that passes through the first electrode E.sub.1 is reflected from the reflective features 10 while preserving the first polarization direction. Step b) may be executed via wafer-level metallization, so as to form a thin metal layer on the features, for example one made of silver, of a thickness comprised between 50 nm and 100 nm. Step c) may be executed by slot die coating of a composite organic/inorganic resist, followed by cross-linking under the action of UV radiation. Step c) is preferably followed by a step c.sub.1) of chemical-mechanical polishing. The first encapsulation layer 3 formed in step h) may be made of SiO. The second encapsulation layer 5 formed in step j) may be deposited by atomic layer deposition (ALD).

(29) Process for Fabricating the Device for a Bottom-Emitting Architecture

(30) As illustrated in FIGS. 5a to 5c, a process for fabricating an electroluminescent device according to the invention comprises the following steps: a′) providing a light-emitting diode comprising, in succession: a glass substrate S.sub.0, an electrode (second electrode E.sub.2) that is transparent in a spectral domain, a stack 2 of layers, comprising an electroluminescent layer EL suitable for emitting electromagnetic radiation in the spectral domain, the electromagnetic radiation being circularly polarized in a first polarization direction, an electrode (first electrode E.sub.1) that is transparent in a spectral domain, an encapsulation layer 5′; b′) providing a structured substrate 1, formed according to steps a), b) and c) described above; c′) covering the structured substrate 1 with a tri-layer structure 6 so as to encapsulate the structured substrate 1, the tri-layer structure 6 possibly being of the Al.sub.2O.sub.3/polymer/Al.sub.2O.sub.3 type, the tri-layer structure 6 having a thickness smaller than 1 μm; d′) transferring the structured substrate 1 to the encapsulation layer 5′ via the tri-layer structure 6, step d′) possibly being executed by adhesive bonding/rolling.
Process for Fabricating the Device for a Display

(31) As illustrated in FIGS. 6a to 6f, a process for fabricating an electroluminescent device according to the invention comprises the following steps: a″) providing a complementary-metal-oxide-semiconductor (CMOS) substrate 1′ covered with an electrically insulating resist RI, the substrate 1′ comprising a CMOS circuit with a plurality of metallization levels M.sub.1, M.sub.2, and with pairs of metal-oxide-semiconductor field-effect transistors (MOSFETs), a single transistor being shown in FIGS. 6a to 6f, each thereof comprising a source S, a gate G and a drain D; b″) etching the electrically insulating resist RI so as to form pixel regions; step b″) being executed so as to obtain features possessing inclined flanks that make an angle θ comprised between 30° and 60°, and preferably comprised between 40° and 50°, with respect to the normal to the substrate 1′; the CMOS substrate 1′ thus being structured at the end of step b″); c″) metallizing the features so as to obtain reflective features 10; d″) forming a filler material 11 that is transparent in a spectral domain inside the reflective features 10 so that the structured substrate 1′ has a planar surface; e″) forming a first electrode E.sub.1 that is transparent in the spectral domain on the planar surface of the structured substrate 1′ facing the pixel regions; f″) forming an additional electrically insulating resist RI′ on the planar surface of the structured substrate, between the pixel regions, so as to prevent short-circuits between the pixels; g″) forming a stack 2 of layers on the first electrode E.sub.1, the stack 2 comprising an electroluminescent layer EL suitable for emitting electromagnetic radiation in the spectral domain, the electromagnetic radiation being circularly polarized in a first polarization direction; h″) forming a second electrode E.sub.2 that is semi-transparent in the spectral domain on the stack 2; i″) forming an encapsulation layer (not illustrated) on the second electrode E.sub.2.

(32) The electrically insulating resist RI may be a dielectric layer made of SiN or SiO.sub.2. The electrically insulating resist RI preferably has a thickness larger than 5 μm. The features obtained in step b″) have a hollow geometric shape configured so that the electromagnetic radiation that passes through the first electrode E.sub.1 is reflected from the reflective features 10 while preserving the first polarization direction. In step b″), the last metallization level M.sub.2 plays the role of etch-stop layer. Step d″) may be executed by slot die coating of a composite organic/inorganic resist, followed by cross-linking under the action of UV radiation. Step d″) is preferably followed by a step of chemical-mechanical polishing.

(33) The invention is not limited to the described embodiments. Those skilled in the art will be able to consider technically workable combinations thereof, and to substitute equivalents therefor.