Pigmented and scattering particles in side coating materials for LED applications

Abstract

Phosphor-converted LED side reflectors disclosed herein comprise pigments that are photochemically stable under illumination by light from the pcLED. The pigments absorb light in at least a portion of the spectrum of light emitted by the first phosphor converted LED. The side reflector may also comprise light scattering particles and/or air voids. The pigments, light scattering particles and/or air voids may be homogeneously distributed in the reflector. Alternatively the side reflector may be layered, with the pigments, light scattering particles and/or air voids inhomogeneously distributed in the reflector. The side reflector may comprise phosphor particles.

Claims

1. A light emitting device comprising: a substrate; a first phosphor converted LED pixel comprising a first LED disposed on the substrate, a first wavelength converting structure disposed on a surface of the first LED with the first LED between the first wavelength converting structure and the substrate, a first light emitting external surface located with the first phosphor converted LED pixel between the first light emitting external surface and the substrate, and a first set of side walls extending from the substrate to the first light emitting external surface; a second phosphor converted LED pixel comprising a second LED disposed on the substrate, a second wavelength converting structure disposed on a surface of the second LED with the second LED between the second wavelength converting structure and the substrate, a second light emitting external surface located with the second phosphor converted LED pixel between the second light emitting external surface and the substrate, and a second set of side walls extending from the substrate to the second light emitting external surface, the second phosphor converted LED pixel being spaced apart from the first phosphor converted LED pixel by a street defined by the substrate and corresponding side walls of the respective first and second sets that face each other across the street; and a side reflector disposed in the street on the corresponding side wall of the first set, the side reflector including one or more pigments that absorb light in at least a portion of a spectrum of light emitted by the first phosphor converted LED pixel.

2. The light emitting device of claim 1, wherein one or more of the pigments absorb blue light.

3. The light emitting device of claim 2, wherein one or more of the pigments absorb green light and transmit or reflect red light.

4. The light emitting device of claim 2, wherein one or more of the pigments absorb red light and transmit or reflect green light.

5. The light emitting device of claim 1, wherein one or more of the pigments reflect or transmit blue light and absorb green light, absorb red light, or absorb green light and red light.

6. The light emitting device of claim 1, wherein the one or more pigments are dispersed in a transparent binder material.

7. The light emitting device of claim 6, wherein the transparent binder material includes air voids.

8. The light emitting device of claim 1, wherein the side reflector includes light scattering particles.

9. The light emitting device of claim 8, wherein the one or more pigments and the light scattering particles are distributed homogenously in a transparent binder material.

10. The light emitting device of claim 9, wherein the transparent binder material is in contact with the corresponding side wall of the second set and fills the street.

11. The light emitting device of claim 9, the side reflector including phosphor particles distributed homogeneously in the transparent binder material.

12. The light emitting device of claim 11, wherein the transparent binder material is in contact with the corresponding side wall of the second set and fills the street.

13. The light emitting device of claim 8, wherein: the light scattering particles are dispersed in a first layer of transparent binder material disposed on the corresponding side wall of the first set; and the one or more pigments are dispersed in a second layer of transparent binder material disposed on the first layer with the first layer between the second layer and the corresponding side wall of the first set.

14. The light emitting device of claim 13, the side reflector further including one or more additional pigments disposed in the first layer.

15. The light emitting device of claim 8, wherein: a first plurality of the light scattering particles is dispersed in a first layer of transparent binder material disposed on the corresponding side wall of the first set; a second plurality of the light scattering particles is dispersed in a second layer of transparent binder material disposed on the corresponding side wall of the second set; the one or more pigments are disposed in a third layer of transparent binder material disposed between the first layer and the second layer; and the side reflector fills the street.

16. The light emitting device of claim 15, the side reflector further including a first plurality of phosphor particles dispersed in the transparent binder material of the first layer and a second plurality of phosphor particles dispersed in the transparent binder material of the second layer.

17. A composition of matter for use as a reflective side coat on LEDs, the composition of matter comprising: a transparent liquid or gel binder material; light scattering particles dispersed in the binder material; one or more pigments dispersed in the binder material; and phosphor particles dispersed in the binder material, one or more of the pigments absorbing light at a wavelength emitted by the phosphor particles.

18. The composition of matter of claim 17, wherein the one or more pigments absorb blue light and green light and transmit or reflect red light.

19. The composition of matter of claim 17, wherein the one or more pigments absorb blue light and red light and transmit or reflect green light.

20. The composition of matter of claim 17, wherein the one or more pigments reflect or transmit blue light and absorb green light, absorb red light, or absorb green light and red light.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a schematic cross-sectional view of a portion of an example pcLED array according to the invention.

(2) FIG. 2A shows reflection spectra of side coat material samples comprising different concentrations of a blue absorbing pigment.

(3) FIG. 2B shows corresponding transmission spectra for the samples of FIG. 2A.

(4) FIG. 3A shows color point coordinates u′ and v′ for the output of pcLEDs having reflective side walls formed with the sample side coat materials of FIGS. 2A and 2B.

(5) FIG. 3B compares total light output from the pcLEDs of FIG. 3A.

DETAILED DESCRIPTION OF THE DRAWINGS

(6) The following detailed description should be read with reference to the drawings, in which identical reference numbers refer to like elements throughout the different figures. The drawings, which are not necessarily to scale, depict selective embodiments and are not intended to limit the scope of the invention. The detailed description illustrates by way of example, not by way of limitation, the principles of the invention.

(7) FIG. 1 shows a schematic cross-sectional view of two adjacent individual pcLED pixels 10A, 10B on a shared substrate 14 in a pcLED array. The pcLED array may comprise many more such pcLED pixels arranged on a shared substrate, but only two are shown here for simplicity of presentation. Each pcLED pixel comprises an LED 12 disposed on substrate 14, and a wavelength converting structure 16 disposed on the LED. The pcLEDs are spaced apart from each other by a street (i.e., trench or gap) having a width W. The streets may have widths W of 5 to 30 microns, for example, or any other suitable width. An optional off state white layer, not shown, may be disposed on the top light output surface of each wavelength converting structure. Off-state white layers typically strongly scatter ambient light and consequently appear white when the pcLED is not operating, but become less scattering at the elevated temperatures resulting from pcLED operation.

(8) Although in FIG. 1 the streets are shown as being defined by straight parallel pcLED side walls oriented perpendicularly to the substrate, in other variations the LED side walls, the wavelength converter sidewalls, or both the LED side walls and the wavelength converter side walls may be angled with respect to the substrate. The streets may therefore by narrower toward the top light emitting surface of the pcLEDs than near the substrate, or alternatively wider near the top light emitting surface of the pcLEDs than near the substrate.

(9) The array of pcLED pixels may be formed using any suitable methods. The streets between pixels may be formed for example by photoresist patterning, mechanical sawing, or laser patterning. The wavelength converting structures may be formed directly on the LEDs, or an array of wavelength converters may be separately formed and then attached to an array of LEDs to form the array of pcLED pixels.

(10) Each LED 12 comprises an active region disposed between n-type and p-type layers. Application of a suitable forward bias across the diode structure results in emission of light from the active region. The LEDs may be, for example III-Nitride LEDs that emit blue, violet, or ultraviolet light. LEDs formed from any other suitable material system and that emit any other suitable wavelength of light may also be used. Other suitable material systems may include, e.g. III-Phosphide materials, III-Arsenide materials, and II-VI materials.

(11) Wavelength converting structures 16 may comprise, for example, phosphor particles dispersed in a binder. Suitable binders may include silicones and solgels, for example. Alternatively, wavelength converting structures 16 may comprise ceramic phosphor structures, formed for example by sintering phosphor particles. The wavelength converting structures may comprise phosphors of only a single composition, or phosphors of two or more different compositions. Phosphors in wavelength converting structure 16 may, for example, convert blue light to red, yellow, green, or cyan light. Any suitable phosphor materials may be used, depending on the desired optical output from the pcLED.

(12) Reflective side coats 18 are disposed on the sides of the pcLEDs, in the streets between adjacent pcLEDs. Reflective side coats 18 may fill the streets as shown in FIG. 1. Alternatively adjacent pcLEDs may each have a side coat 18 in the street between the pixels, without completely filling the street.

(13) As summarized above, reflective side coats 18 are formed from a material comprising pigments that are photochemically stable under illumination by light from the LED and from the wavelength converting structure. These pigments absorb light from the pcLED, decreasing light leakage through the side walls. This allows effective isolation of adjacent pcLED pixels with much thinner reflective side walls than would be necessary without the pigments. Because the pigments can be chosen to selectively absorb only portions of the spectrum of light from the pcLED, they also may help tailor the color point of the pcLED.

(14) Suitable inorganic green pigments (absorbing blue and red light) may include, for example, Co.sub.2TiO.sub.4, Zn.sub.2TiO.sub.4, Ni.sub.2TiO.sub.4, or (Co, Zn, Ni).sub.2TiO.sub.4. Suitable inorganic red pigments (absorbing blue and green light) may include, for example, BiO.sub.4V (bismuth vanadate). Suitable inorganic blue pigments (absorbing green and red light) may include, for example, CoAl.sub.2O.sub.4 or YInMn-blue. Suitable organic blue and green pigments may include, for example, phtalocyanines. Suitable organic red pigments may include, for example, perylenes.

(15) Also as summarized above, the pigments may be dispersed with light scattering particles in a binder, and optionally with phosphor particles in the binder as well. An advantage of including phosphor particles is that when adjacent pcLED pixels are outputting light there would be less of a dark gap between the operating pixels, without use of external optics to blur the resolution. The phosphor particles in the reflective side coat may, for example, convert blue light from the LED to red, yellow, green, or cyan light.

(16) The side coating material may be applied to the pcLED array to fill the streets by, for example, a dispensing, spraying, or molding process. Excess side coat material, for example disposed on the light output top surface of a wavelength converting structure or on the light output top surface of an off-state white layer disposed on the wavelength converting structure, can be removed by mechanical abrasion, planarization, polishing or grinding, for example.

(17) After deposition, the reflective side coat 18 may comprise, for example, TiO.sub.2 light scattering particles, air voids, inorganic or organic pigments, and optionally phosphor particles, all dispersed homogeneously in a silicone or solgel binder matrix. The air voids may act as light scatterers, and may also reduce the refractive index of the binder matrix. In this variation the difference between the refractive index of the phosphor particles and the surrounding matrix is typically homogeneous throughout the reflector.

(18) Alternatively, the reflective side coat may comprise two or more layers oriented parallel to the sides of the pcLEDs. A first layer, disposed on the side of the pcLED, may comprise light scattering particles (and optionally air voids) dispersed in a transparent silicon or solgel matrix. A second layer, disposed on the first layer and spaced apart from the pcLED by the first layer, may comprise pigments dispersed in a transparent silicon or solgel matrix. The separate layers may comprise different binder matrices. Such a layered structure maximizes the light scattering/reflecting effect of the first (e.g., TiO.sub.2) layer while maintaining the absorption/light extinguishing effect of the pigment containing layer.

(19) The layered structure can be made by sequential application of the first layer with a thickness less than the street width, followed by application of the second layer. The layering can also be performed by first filling the street with side coat material for the first layer, then sawing a smaller street within the first layer, then filling the new street with the material for the second layer.

(20) The resulting layered reflective side coat structure between two adjacent pcLED pixels may comprise, for example, a first light scattering particle layer disposed on the side wall of one of the pcLED pixels, a second light scattering particle layer disposed on the side wall of the other of the adjacent pcLED pixels, and one or more pigment layers disposed between the first and second light scattering layers. Reflective side coat 18 may comprise additional layers between the two adjacent pcLED pixels, as suitable. In such a layered structure, phosphor particles may optionally be dispersed in the light scattering particle layer, in the pigment layer, or in both the light scattering layer and the pigment layer.

(21) In a layered side coat reflective structure as just described, the difference between the refractive index of the phosphor particles and the surrounding matrix may vary with distance from the pcLED side wall (e.g., have a gradient) because the matrix is layered rather than homogeneous and the matrix index of refraction may be different in different layers. Depending on the layer structure, the refractive index may for example vary gradually, or more strongly as for example a step function.

(22) FIG. 2A and FIG. 2B show, respectively, reflection and transmission spectra of side coat material samples comprising different concentrations of approximately 0.5 to 1 micron diameter particles of blue absorbing pigment Fe.sub.2O.sub.3 uniformly dispersed in a TiO.sub.2/silicone mixture. In both figures, referring to the curves from top (first) to bottom (fifth), the first curve is for a sample with no pigment, the second curve is for a sample comprising 0.01% pigment, the third curve is for a sample comprising 0.02% pigment, the fourth curve is for a sample comprising 0.05% pigment, and the fifth curve is for a sample comprising 0.5% pigment (all % by weight).

(23) FIG. 3A shows color point coordinates u′ and v′ for the output of pcLEDs having reflective side walls formed with these sample side coat materials, and demonstrates that increasing sidewall light absorption of blue light by sidewalls of increasing pigment concentration moves the color point of the pcLEDs to higher u′ and v′ color points. FIG. 3B shows that the increasing pigment concentrations reduces the total light output from the pcLEDs. Images of these pcLEDs in operation (not shown) demonstrate that side light leakage from the pcLEDs decreases with increasing pigment concentration in the side wall reflectors.

(24) This disclosure is illustrative and not limiting. Further modifications will be apparent to one skilled in the art in light of this disclosure and are intended to fall within the scope of the appended claims.