Patent Application
20060113895
Existing light emitting diodes (“LEDs”) can emit light in the ultraviolet (“UV”), visible or infrared (“IR”) wavelength range. These LEDs generally have narrow emission spectrum (approximately ±10 nm). As an example, a blue InGaN LED may generate light with wavelength of 470 nm±10 nm. As another example, a green InGaN LED may generate light with wavelength of 510 nm±10 nm. As another example, a red AlInGaP LED may generate light with wavelength of 630 nm±10 nm.
However, in some applications, it is desirable to use LEDs that can generate broader emission spectrums to produce desired color light, such as white light. Due to the narrow-band emission characteristics, these monochromatic LEDs cannot be directly used to produce broad-spectrum color light. Rather, the output light of a monochromatic LED must be mixed with other light of one or more different wavelengths to produce broad-spectrum color light. This can be achieved by introducing one or more photoluminescent materials into the encapsulant of a monochromatic LED to convert some of the original light into longer wavelength light through photoluminescence. The combination of original light and converted light produces broad-spectrum color light, which can be emitted from the LED as output light. The most common photoluminescent materials used to create LEDs that produce broad-spectrum color light are fluorescent particles made of phosphors, such as Garnet-based phosphors, Silicate-based phosphors, Orthosilicate-based phosphors, Sulfide-based phosphors, Thiogallate-based phosphors and Nitride-based phosphors. These phosphor particles are typically mixed with the transparent material used to form the encapsulants of LEDs so that original light emitted from the semiconductor die of an LED can be converted within the encapsulant of the LED to produce the desired output light.
Recently, quantum dots have also been used to create LEDs that produce broad-spectrum color light. Similar to phosphor particles, quantum dots are typically mixed with the transparent material used to form the encapsulants of LEDs. However, it is a challenge to use the proper types of quantum dots in proper proportions to produce the desired output light with respect to wavelength characteristics. In addition, quantum dots tend to agglomerate when mixed with the transparent material used to form the encapsulants of the LEDs. Thus, the output light color of the resulting LEDs may not be uniform. Furthermore, the intensity of the output light may be reduced due to the agglomeration of quantum dots.
In view of these concerns, there is a need for a light emitting device that produces output light using quantum dots that alleviates some or all of these concerns and method for making the device.
A light emitting device utilizes multiple layers of quantum dots to convert at least some of the original light emitted from a light source of the device to longer wavelength light to produce an output light. The light emitting device is made by forming the multiple layers of quantum dots over a light source and then forming an encapsulant over the multiple layers of quantum dots. The multiple layers of quantum dots can be used to produce broad-spectrum color light, such as white light.
A device in accordance with an embodiment of the invention comprises a light source that emits original light, multiple layers of quantum dots positioned over the light source, the multiple layers being positioned to receive the original light and to convert at least some of the original light to converted light, the converted light being a component of an output light, and an encapsulant positioned over the multiple layers of quantum dots, the output light being emitted from the encapsulant. Each of the multiple layers includes quantum dots of a predefined particle size range.
A method for making a light emitting device in accordance with an embodiment of the invention comprises providing a light source, forming multiple layers of quantum dots over the light source, each of the multiple layers including quantum dots of a predefined particle size range, the multiple layers being used to convert at least some of original light emitted by the light source to control characteristics of output light of the light emitting device, and forming an encapsulant over the multiple layers of quantum dots.
Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrated by way of example of the principles of the invention.
With reference to
The LED die 102 is a semiconductor chip that generates light of a particular peak wavelength. Thus, the LED die 102 is a light source of the LED 100. The LED die 102 may be a deep ultraviolet (UV), UV, blue or green LED die. Although the LED 100 is shown in
In this embodiment, the leadframe 104 includes a depressed region 116 at the upper surface, which forms a reflector cup in which the LED die 102 is mounted. Since the LED die 102 is mounted on the leadframe 104, the leadframe 104 can be considered to be a mounting structure for the LED die. The surface of the reflector cup 116 may be reflective so that some of the light generated by the LED die 102 is reflected away from the leadframe 104 to be emitted from the LED 100 as useful output light.
The LED die 102 is covered by the multi-layered region 110 of quantum dots, which is described in more detail below. The LED die 102 and the multi-layered region 110 are encapsulated in the encapsulant 112. The encapsulant 112 includes a main section 118 and an output section 120. In this embodiment, the output section 120 of the encapsulant 112 is dome-shaped to function as a lens. Thus, the light emitted from the LED 100 as output light is focused by the dome-shaped output section 120 of the encapsulant 112. However, in other embodiments, the output section 120 of the encapsulant 112 may be horizontally planar. The encapsulant 112 is made of an optically transparent substance so that light from the LED die 102 can travel through the encapsulant and be emitted out of the output section 120 as output light. As an example, the encapsulant 112 can be made of a host matrix, such as polymer (formed from liquid or semisolid precursor material such as monomer), polystyrene, epoxy, silicone, glass or a hybrid of silicone and epoxy.
In an embodiment, the encapsulant 112 may include non-quantum fluorescent material. The non-quantum fluorescent material included in the encapsulant 112 may be one or more types of non-quantum phosphors, such as Garnet-based phosphors, Silicate-based phosphors, Orthosilicate-based phosphors, Thiogallate-based phosphors, Sulfide-based phosphors and Nitride-based phosphors. The non-quantum phosphors may be phosphor particles with or without a silica coating. Silica coating on phosphor particles reduces clustering or agglomeration of phosphor particles when the phosphor particles are mixed with the host matrix to form the encapsulant 112. Clustering or agglomeration of phosphor particles can result in an LED that produces output light having a non-uniform color distribution.
The silica coating may be applied to synthesized phosphor particles by subjecting the phosphor particles to an annealing process to anneal the phosphor particles and to remove contaminants. The phosphor particles are then mixed with silica powders, and heated in a furnace at approximately 200 degrees Celsius. The applied heat forms a thin silica coating on the phosphor particles. The amount of silica on the phosphor particles is approximately 1% with respect to the phosphor particles. Alternatively, the silica coating can be formed on phosphor particles without applying heat. Rather, silica powder can be added to the phosphor particles, which adheres to the phosphor particles due to Van der Waals forces to form a silica coating on the phosphor particles.
The non-quantum fluorescent material included in the encapsulant 112 may alternatively include one or more organic dyes or any combination of non-quantum phosphors and organic dyes.
The multi-layered region 110 of quantum dots includes a number of interstitial layers 220 deposited on the LED die 102, as illustrated in
The quantum dots included in the interstitial layers 220 of the multi-layered region 110 may be quantum dots made of CdS, CdSe, CdTe, CdPo, ZnS, ZnSe, ZnTe, ZnPo, MgS, MgSe, MgTe, PbSe, PbS, PbTe, HgS, HgSe, HgTe and Cd(Si1-xSex), or made from a metal oxides group, which consists of BaTiO3, PbZrO3, PbZrzTi1-zO3, BaxSr1-x TiO3, SrTiO3, LaMnO3, CaMnO3, La1-xCaxMnO3. These quantum dots may or may not be coated with a material having an affinity for the host matrix. The coating passivates the quantum dots to prevent agglomeration or aggregation to overcome the Van der Waals binding force between the quantum dots.
The coating on the quantum dots can be (a) organic caps, (b) shells or (c) caps made of glass material, such as Si nanocrystals. Organic caps can be formed on quantum dots using Ag2S and Cd(OH)2, which may preferably be passivated with Cd2+ at high pH. A surface modification of the quantum dots is then performed by attaching dyes to the surface of the quantum dots. As an example, CdSe surface surfactant is labile and can be replaced by sequential addition of Se+ and Cd2+, which can grow to make a seed (quantum dot) larger. For Cd2+ rich surface, the surface can be treated with Ph—Se− and an organic coating is covalently linked to the surface. This isolation of molecular particles is referred to as “capped”. Types of known capping molecules include Michelle liquids (Fendler), Tio-terminations (S-based) (Weller-Hamburg), Phosphate termination (Berwandi-MIT), Nitrogen termination (pyridine, pyrazine) and Dendron caps (multi-stranded ligands) (Peng).
Shells are coatings on inner core material (quantum dots). Generally, coating material that forms the shells can be oxide or sulfide based. Examples of shell/core are TiO2/Cds, ZnO/CdSe, ZnS/Cds and SnO2/CdSe. For CdSe core, it can also be coated with ZnS, ZnSe (selenide based) or CdS, which improves the efficiency of the CdSe dramatically.
The quantum dots included in the interstitial layers 220 of the multi-layered region 110 may also be coated with a material having affinity for the host matrix to uniformly suspend the quantum dots in the host matrix. This coating material could be organic or inorganic based. As an example, the coating material may be an adhesion promoter material, such as silane. The quantum dots can be coated with the adhesion promoter material by adding the quantum dots into an adhesion promoter solution and stirring well the solution with the quantum dots to ensure that the quantum dot surfaces are completely wetted by the adhesion promoter solution. The solution is then heated to evaporate the adhesion promoter solution, leaving a thin coating of adhesion promoter on the surface of the quantum dots. The coated quantum dots are then mixed into the host matrix.
Another technique to suspend the quantum dots in the host matrix is by adding organic or inorganic dispersants into the host matrix and stirring well the host matrix until the dispersants are homogenously dispersed in the host matrix. The quantum dots are then added to the host matrix. One example of an inorganic material that can be used is silica or silica-based suspension agent.
Each interstitial layer 220 of the multi-layered region 110 includes only quantum dots of a particular particle size range. Thus, the quantum dots can be selectively positioned within the multi-layered region 110 with respect to their particle size. Different sized quantum dots can be positioned at different interstitial layers 220 within the multi-layered region 110 in a predefined order to produce output light having desired wavelength characteristics. The thickness of each interstitial layer 220 can be varied, depending on the desired wavelength characteristics of the output light and the type of light source(s) included in the LED 100. The thickness of some of the interstitial layers 220 can be as thin as the diameter of the largest quantum dots included in that interstitial layer, e.g., approximately 5 microns thick. Alternatively, the thickness of some of the interstitial layers can be hundreds of microns thick. As an example, the total thickness of the multi-layered region 110 may be equal to or less than 100 microns.
As an example, the quantum dots can be arranged within the multi-layered region 110 from smallest to largest in the direction away from the LED die 102, as illustrated in
As another example, the quantum dots can be arranged within the multi-layered region 110 in an alternating fashion between smaller-sized quantum dots and larger-sized quantum dots, as illustrated in
Although the multi-layered region 110 is shown in
In operation, the non-quantum fluorescent material included in the encapsulant 112, if any, absorbs some of the original light emitted from the LED die 102, which excites the atoms of the non-quantum fluorescent material, and emits longer wavelength light. Similarly, the quantum dots included in the multi-layered region 110 absorb some of the original light emitted from the LED die 102, which excites the quantum dots, and emit longer wavelength light. The wavelength of the light emitted from the quantum dots partly depends on the size of the quantum dots. In an implementation, the light emitted from the non-quantum fluorescent material and/or the light emitted from the quantum dots are combined with unabsorbed light emitted from the LED die 102 to produce broad-spectrum color light such as white light, which is emitted from the light output section 120 of the encapsulant 112 as output light of the LED 100. In another implementation, virtually all the light emitted from the LED die 102 is absorbed and converted by the non-quantum fluorescent material and/or the quantum dots. Thus, in this implementation, only the light converted by the non-quantum fluorescent material and/or the quantum dots is emitted from the light output section 120 of the encapsulant 112 as output light of the LED 100.
The combination of the light emitted from the non-quantum fluorescent material and the quantum dots of the LED 100 can produce broad-spectrum color light that has a higher CRI than light emitting using only non-quantum fluorescent material or using only quantum dots. The broad-spectrum color output light of the LED 100 can be adjusted by using one or more different LED dies, using one or more different non-quantum fluorescent materials, using one or more different types of quantum dots and/or using different sized quantum dots. In addition, the broad-spectrum color output light of the LED 100 may also be adjusted using non-quantum fluorescent material of phosphor particles with or without a silica coating, using quantum dots with or without a coating and/or using different type of coating on the quantum dots. Furthermore, the ratio between the non-quantum fluorescent material and the quantum dots included in the LED 100 can be adjusted to produce output light having desired color characteristics.
The type(s) of quantum dots included in the multi-layered region 110 may partly depend on the wavelength deficiencies of the non-quantum fluorescent material. As an example, if the non-quantum fluorescent material produces an output light that is deficient at around 600 nm, then a particular type of quantum dots can be selected that can produce converted light at around 600 nm to compensate for the deficiency, which will increase the CRI of the output light.
The encapsulant 112 of the LED 100 may include dispersant or diffusing particles that are distributed throughout the encapsulant. The diffusing particles operate to diffuse light of different wavelengths emitted from the LED die 102, the non-quantum fluorescent material of the encapsulant 112 and/or the quantum dots of the multilayered region 110 so that color of the resulting output light is more uniform. The diff-using particles may be silica, silicon dioxide, aluminum oxide, barium titanate, and/or titanium oxide. The encapsulant 112 may also include adhesion promoter and/or ultraviolet (UV) inhibitor.
The process for fabricating the LED 100 in accordance with an embodiment of the invention is now described with reference to
Turning now to
Turning now to
Turning now to
In other embodiments, as illustrated in
Still in other embodiments, as illustrated in
Although the invention has been described with respect to LEDs, the invention can be applied to other types of light emitting devices, such as semiconductor lasing devices. In these light emitting devices, the light source can be any light source other than an LED die, such as a laser diode.
A method for fabricating a light emitting device, such as an LED, in accordance with an embodiment of the invention is described with reference to the process flow diagram of
Although specific embodiments of the invention have been described and illustrated, the invention is not to be limited to the specific forms or arrangements of parts so described and illustrated. The scope of the invention is to be defined by the claims appended hereto and their equivalents.
