The invention relates generally to phosphor-converted light emitting diodes.
Semiconductor light emitting diodes and laser diodes (collectively referred to herein as “LEDs”) are among the most efficient light sources currently available. The emission spectrum of an LED typically exhibits a single narrow peak at a wavelength determined by the structure of the device and by the composition of the semiconductor materials from which it is constructed. By suitable choice of device structure and material system, LEDs may be designed to operate at ultraviolet, visible, or infrared wavelengths.
LEDs may be combined with one or more wavelength converting materials (generally referred to herein as “phosphors”) that absorb light emitted by the LED and in response emit light of a longer wavelength. For such phosphor-converted LEDs (“pcLEDs”), the fraction of the light emitted by the LED that is absorbed by the phosphors depends on the amount of phosphor material in the optical path of the light emitted by the LED, for example on the concentration of phosphor material in a phosphor layer disposed on or around the LED and the thickness of the layer.
Phosphor-converted LEDs may be designed so that all of the light emitted by the LED is absorbed by one or more phosphors, in which case the emission from the pcLED is entirely from the phosphors. In such cases the phosphor may be selected, for example, to emit light in a narrow spectral region that is not efficiently generated directly by an LED.
Alternatively, pcLEDs may be designed so that only a portion of the light emitted by the LED is absorbed by the phosphors, in which case the emission from the pcLED is a mixture of light emitted by the LED and light emitted by the phosphors. By suitable choice of LED, phosphors, and phosphor composition, such a pcLED may be designed to emit, for example, white light having a desired color temperature and desired color-rendering properties.
This specification discloses LEDs and pcLEDs comprising a top light emitting surface, an oppositely positioned bottom surface, and side walls connecting the top and bottom surfaces. Sidewall reflector structures disposed on the sidewalls comprise a thin specular reflection layer and a light scattering layer disposed between the sidewall and the specular reflection layer.
These sidewall reflector structures are more diffusively reflective than a specular reflector, yet maintain high reflectivity. LEDs and pcLEDs comprising such sidewall reflector structures can exhibit a narrower and more Lambertian radiation output pattern from the top (light emitting surface) than devices employing only specularly reflective sidewall reflectors. In addition, such sidewall reflector structures may be thinner than conventional volume scattering sidewall reflectors, allowing LEDs and pcLEDs employing the sidewall reflector structures disclosed herein to be placed closer to each other than would be the case if volume scattering sidewall reflectors were used.
These LEDs and pcLEDs may be advantageously employed in arrays of closely spaced devices, for example in automobile headlights.
Other embodiments, features and advantages of the present invention will become more apparent to those skilled in the art when taken with reference to the following more detailed description of the invention in conjunction with the accompanying drawings that are first briefly described.
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.
The LED may be, for example, a III-Nitride LED that emits 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, for example, III-Phosphide materials, III-Arsenide materials, and II-VI materials.
Any suitable phosphor materials may be used, depending on the desired optical output from the pcLED.
As shown in
Individual pcLEDs may optionally incorporate or be arranged in combination with a lens or other optical element located adjacent to or disposed on the phosphor layer. Such an optical element, not shown in the figures, may be referred to as a “primary optical element”. In addition, as shown in
As summarized in the summary section above, this specification discloses thin sidewall reflector structures for LEDs and pcLEDs that can provide an improved radiation output pattern compared to devices employing only specularly reflective sidewall reflectors, yet maintain high reflectivity with a relatively thin reflector structure.
Light emitted by the semiconductor diode or by the wavelength converting structure that is incident on a side wall 510 of the wavelength converting structure (e.g., light ray 514) or on a sidewall 512 of the semiconductor diode and transparent substrate is generally diffusively reflected by volume scattering sidewall reflectors 516 back into and/or out of the device (e.g., light rays 518), so that essentially all light output by pcLED 500 is output through top surface 508.
Light emitting semiconductor diode 502 may be, for example, a III-Nitride LED that emits 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, for example, III-Phosphide materials, III-Arsenide materials, and II-VI materials.
Transparent substrate 503 is transparent to light emitted by semiconductor diode 502 and to light emitted by wavelength converting structure 506. Transparent substrate 503 may be a sapphire substrate, for example.
Wavelength converting structure 506 may comprise any suitable phosphor material, and may for example be a ceramic phosphor plate or other ceramic phosphor structure formed by sintering phosphor particles. Such a ceramic phosphor plate or structure may exhibit little or no scattering of light emitted by the semiconductor diode or emitted by the wavelength converter.
Volume scattering reflectors 516 may comprise Titanium Oxide particles dispersed in a silicone binder, for example. Volume scattering reflectors 516 may have a thickness of about 30 microns to about 300 microns, for example, measured perpendicularly to the sidewalls.
An advantage of using volume scattering sidewall reflectors as in device 500 is that they are diffusively reflected as illustrated in
An advantage to using thin film side coat reflectors 616 is that they may be sufficiently thin that the lateral dimensions of device 600 are not significantly larger than those of the light emitting semiconductor diode. This allows for closer spacing of such devices than is possible for devices employing volume scattering sidewall reflectors.
A disadvantage is that such thin film side coat reflectors are typically specularly reflective rather than diffusively reflective—they reflect light rays at an angle of reflection equal to the angle of incidence (e.g., ray 614). As a consequence, the output radiation pattern from top surface 608 of the device is typically wider than the preferred Lambertian distribution, especially if the wavelength converting structure (for example, a ceramic phosphor structure) does not strongly scatter light. The wider output radiation pattern is commonly disadvantageous when the device is combined with external optics causing low coupling efficiency, meaning increasing unused light. One way of improving the radiation pattern is to increase volume scattering in the wavelength converting structure, but this method typically increases scattering loss in the wavelength converter.
A further disadvantage of using such thin film side coat reflectors is that for light generated in a low-scattering wavelength converter such as a ceramic phosphor platelet, the light can become trapped circulating in an optical cavity due to the specularly reflective nature of the sidewall reflectors.
A difficulty with using thin film side coat reflectors is that typically they must be formed on a smooth supporting surface. This is particularly true for Distributed Bragg Reflectors, which must be formed on a sufficiently smooth supporting surface to minimize transmission loss.
The combination of scattering layer 716A and specularly reflective layer 716B maintains high reflectivity while providing a more diffusive reflector, allowing for light to be directed in a random direction or preferentially forward (towards the light output surface) direction. This improves the radiation output pattern through top surface 708 of the device, making the radiation pattern more Lambertian, and also helps prevent optical cavity trapping of light in the wavelength converter.
Specularly reflective layer 716B may be or comprise a reflective metal layer or a Distributed Bragg Reflector, for example. Scattering layer 716A may be or comprise, for example, a porous columnar structure that provides optically diffusive characteristics in the plane perpendicular to the columns, while maintaining a physically smooth outer layer. Alternatively, scattering layer 716A may be or comprise, for example, Titanium Oxide particles dispersed in a silicone binder.
As shown in
Referring now to
Referring now to
In other variations a diffusive light scattering layer 716A is applied to a wavelength converting structure by vapor phase deposition, for example pulsed laser deposition, of a dielectric layer comprising light-scattering (e.g., air-filled) voids. The outer surface of layer 716A is made smooth, as described above. Similarly to as shown in
Saw cuts through a ceramic phosphor plate typically leave rough edges, but the saw cuts through the filled streets that define layers 716A leave sufficiently smooth outer surfaces on these layers to support deposition of specular reflective layers 716B, which may be subsequently deposited.
Sidewall reflectors 716 may be formed on a wavelength converting structure 506, and then the wavelength converting structure subsequently attached to a light emitting semiconductor diode or to a transparent substrate on a light emitting semiconductor diode.
Alternatively, sidewall reflectors 716 may be formed on a wavelength converting structure after the wavelength converting structure is attached to a light emitting semiconductor diode or to a transparent substrate on a light emitting semiconductor diode. In that case the sidewall reflectors 716 may be formed to extend to cover sidewalls of the transparent substrate and the light emitting semiconductor diode, as shown for example in
Sidewall reflectors 716 may also be formed on sidewalls of a light emitting semiconductor diode and/or side walls of a transparent substrate attached to the light emitting semiconductor diode with no wavelength converting structure attached to the device.
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.