Broadband light emitting device with grating-structured electrode
A light emitting device includes a substrate layer, a first electrode layer, a light emitting layer, and a patterned second electrode layer. The patterned second electrode layer includes a periodic grating structure having a grating period λg less than or equal to 200 nm and the patterned second electrode layer and the light emitting layer are separated by at most 100 nm.
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This application is a reissue application of U.S. Pat. No. 9,865,836, filed as U.S. patent application Ser. No. 16/199,607 on Jul. 9, 2018 as a national stage entry of International Patent Application No. PCT/US2014/065016, filed on Nov. 11, 2014, and claims priority from U.S. Provisional Patent Application No. 61/902,734, filed on Nov. 11, 2013.
BACKGROUNDEnergy consumption of lighting systems is presently a major societal concern. This concern has even led to some governments to encourage the development of more energy efficient light sources and smart controls for those light sources. Some methods of improving light efficiency rely on improvements of the underlying materials and structures of the lighting devices themselves. Other methods involve designing customized spectral distributions that satisfy color quality and luminous efficacy. Still other methods require designing light management systems with sophisticated sensors and algorithms to control the intensity of lights depending on ambient light, occupancy, and usage.
For the specific case of multi-layered surface light emitting devices/diodes, including inorganic light emitting diodes (LEDs) and organic light emitting devices (OLEDs), only a small fraction of the generated light often leaves the device. This inefficiency is because much of the generated light becomes trapped in the device, e.g., due to total internal reflection at a layer interfaces as well as at output surface, in addition to absorption of the light by the metal electrode. For example, for a typical OLED comprised of several organic material layers sandwiched by two electrode layers, anode and cathode, only about 20% of the light may be directly emitted into the air. The rest of the light is either trapped inside the substrate (as a substrate mode), inside the organic layers (as a waveguide mode), or absorbed by the metal cathode (usually called a plasmon mode).
Among these many light loss channels, the plasmon loss (may also be termed “plasmonic loss”) may amount to about 40% of the total emitted light and thus, the elimination of plasmon loss represents an ongoing challenge in improving the light extraction of (O)LEDs. It is thought that plasmon loss occurs mainly through the coupling of the emitted light into surface plasmon polariton (SPP) waves. SPP waves are surface waves confined to propagating along the interface between the metal electrode layer and the emitting material layer. Energy in the SPP waves is eventually absorbed by the metal electrode and thus leads to inefficient operation of the (O)LED.
Plasmon loss in light emitting devices may be reduced by using a periodic grating-structured electrode having a period that is comparable to the wavelength of the emitted light so as to induce Bragg scattering of the SPP wave into free light. However, existing grating structures are only effective for plasmon loss reduction in a very narrow wavelength range (i.e., monochromatic sources) and none are effective for broadband, i.e., white light, sources such as white OLEDs.
SUMMARY OF INVENTIONIn general, in one aspect, the invention relates to a light emitting device that includes a substrate layer, a first electrode layer, a light emitting layer, and a patterned second electrode layer. The patterned second electrode layer includes a periodic grating structure having a grating period λg less than or equal to 200 nm, and the patterned second electrode layer and the light emitting layer are separated by at most 100 nm.
In general, in another aspect, the invention relates to a light emitting device that includes a substrate layer, a first electrode layer, and a broadband light emitting layer that emits a broadband light having a discrete spectral power distribution of a plurality of emission peaks at a plurality of emission wavelengths. The light emitting device also includes a patterned second electrode layer that includes an optimized periodic grating structure. A plasmonic loss spectrum of the light emitting device due to the patterned second electrode includes a plurality of plasmonic loss peaks at a plurality of wavelengths and the plurality of emission peaks are at different wavelengths from the plurality of wavelengths of the plurality of plasmonic loss peaks.
Other aspects of the invention will be apparent from the following description and the appended claims.
Specific embodiments of the invention will now be described in detail with reference to the accompanying figures. Like elements in the various figures are denoted by like reference numerals for consistency. Further, the use of “Fig.” in the drawings is equivalent to the use of the term “Figure” in the description.
In the following detailed description of embodiments of the invention, numerous specific details are set forth in order to provide a more thorough understanding of the invention. However, it will be apparent to one of ordinary skill in the art that the invention may be practiced without these specific details. In other instances, well-known features have not been described in detail to avoid unnecessarily complicating the description.
In general, embodiments of the invention relate to a light emitting device with a periodic grating structure engineered to impair the formation of surface plasmon polariton waves across the entire range of the visible spectrum (e.g., 430 nm-700 nm). In one or more embodiments of the invention, a multi-layered LED includes a periodic grating structure on a metal electrode layer that inhibits the energy from coupling into the electrode and forming surface plasmons in the metal electrode. In one or more embodiments, the periodic grating structure may be one or two dimensional in nature. The characteristics of the grating structure, such as the size and shape of the grating, are selected to reduce coupling into surface plasmons for a broad wavelength range. In accordance with one or more embodiments of the invention, the multi-layered LED may also include other grating structures to inhibit losses in the LED device at boundaries other than the metal electrode, for example to prevent total internal reflection, formation of waveguide and/or substrate modes.
In one or more embodiments of the invention, the layers of the LED device may be selected based on a plasmonic loss spectrum. For example, the emitting layer wavelength emission may be selected to avoid any wavelengths in a plasmonic loss spectrum. In other words, the size, shape, and material of the layers in the multi-layered LED device may be selected based on the size, shape, and material of one or more grating structures in the LED device.
One of ordinary skill will appreciate that the layers of the device may be made from a number of different materials. For example, the substrate layer may be made of glass and the anode layer may be made of indium tin oxide (ITO). The hole transport layer may be made of NBP, the emission layer may be made of fluorescent or phosphorescent material, and the electron transport layer may be made of Alq3. Furthermore, one of ordinary skill will appreciate that the metal electrode may be made of any suitable metal, e.g., silver, gold, aluminum, etc. In addition, one of ordinary skill will appreciate that the EML layer may itself be formed of several layers, each having a different band of emission wavelength, e.g., a red emission layer, a green emission layer, and a blue emission layer (not shown). Furthermore, one of ordinary skill will appreciate that the multiple layers in the device may be manufactured by any known OLED manufacturing process. For example, vacuum deposition, solution processes, or lithography techniques.
Returning to
For example,
While the above examples were focused on the optimization of the shape of the grating structure one or more embodiments of the invention may also be implemented by employing a light source design that is optimized per a particular plasmon loss spectrum of a particular grating-structured electrode.
In other words, the wavelength peaks in the source spectral power distribution should not be at the same wavelengths as the peaks in the plasmonic loss spectrum in accordance with one or more embodiments of the invention. Such designer sources may be manufactured according to known methods, for example using quantum dots or optical microcavities, and when used in conjunction with that disclosed herein may operate at increased efficiency when compared to standard broadband (O)LED systems.
While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.
Claims
1. A light emitting device comprising:
- a substrate layer;
- a first electrode layer;
- a light emitting layer that emits light in the visible spectrum; and
- a patterned second electrode layer comprising a periodic grating structure having a grating period λg less than or equal to 200 nm, and
- wherein the patterned second electrode layer and the light emitting layer are separated by at most 100 nm.
2. The light emitting device of claim 1, wherein the periodic grating structure of the patterned second electrode layer comprises a plurality of trapezoidal protrusions.
3. The light emitting device of claim 2, wherein each one of the plurality of trapezoidal protrusions comprises:
- a top portion having a length Lt;
- a bottom portion having a length Lb; and
- a height h, wherein: h is greater than or equal to 30 nm and less than or equal to 70 nm; Lt/λg is greater than or equal to 0 and less than or equal to 0.4; and Lb/λg is greater than 0.5 and less than 1.
4. The light emitting device of claim 3, wherein:
- λg is less than 120 nm,
- h is greater than or equal to 40 nm and less than or equal to 60 nm,
- Lt/λg is greater than or equal to 0 and less than or equal to 0.2, and
- Lb/λg is greater than or equal to 0.6 and less than or equal to 0.8.
5. The light emitting device of claim 1, wherein the grating structure comprises periodicity in two dimensions.
6. The light emitting device of claim 5, wherein the grating structure is arranged in a square lattice structure.
7. The light emitting device of claim 5, wherein the grating structure is arranged in a hexagonal lattice structure.
8. The light emitting device of claim 1, wherein the grating structure comprises periodicity in only one dimension.
9. The light emitting device of claim 1, wherein the periodic structure of the patterned second electrode layer comprises a plurality of conical protrusions.
10. The light emitting device of claim 1, wherein the periodic structure of the patterned second electrode layer comprises a plurality of circular protrusions.
11. The light emitting device of claim 1, wherein the periodic structure of the patterned second electrode layer comprises a plurality of rectangular protrusions.
12. The light emitting device of claim 1, wherein the periodic structure of the patterned second electrode layer comprises a plurality of square protrusions.
13. The light emitting device of claim 1, wherein the light emitting layer comprises a red light emitting layer, a green light emitting layer, and a blue light emitting layer.
14. A light emitting device comprising:
- a substrate layer;
- a first electrode layer;
- a broadband light emitting layer configured to emit broadband light having a discrete spectral power distribution comprising a plurality of emission peaks at a plurality of emission wavelengths;
- a patterned second electrode layer comprising an optimized periodic grating structure,
- wherein a plasmonic loss spectrum of the light emitting device due to the patterned second electrode comprises a plurality of plasmonic loss peaks at a plurality of wavelengths, and
- wherein the plurality of emission peaks are at different wavelengths from the plurality of wavelengths of the plurality of plasmonic loss peaks.
15. The light emitting device of claim 14, wherein the plurality of emission wavelengths are within a range of 430 nm to 700 nm.
16. The light emitting device of claim 14, wherein the plurality of emission wavelengths comprises a red wavelength, a green wavelength, and a blue wavelength.
17. A light emitting device comprising:
- a substrate layer;
- a first electrode layer;
- a light emitting layer that emits light in the visible spectrum; and
- a patterned second electrode layer comprising a periodic grating structure having a grating period λg less than or equal to 200 nm, wherein
- the patterned second electrode layer and the light emitting layer are separated by at most 100 nm, and
- the periodic grating structure of the patterned second electrode layer comprises a plurality of trapezoidal protrusions, wherein each one of the plurality of trapezoidal protrusions comprises: a top portion having a length Lt; a bottom portion having a length Lb; and a height h, wherein: h is greater than or equal to 30 nm and less than or equal to 70 nm; Lt/λg is greater than or equal to 0 and less than or equal to 0.4; and Lb/λg is greater than 0.5 and less than 1.
18. The light emitting device of claim 17, wherein:
- λg is less than 120 nm,
- h is greater than or equal to 40 nm and less than or equal to 60 nm,
- Lt/λg is greater than or equal to 0 and less than or equal to 0.2, and
- Lb/λg is greater than or equal to 0.6 and less than or equal to 0.8.
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Type: Grant
Filed: Nov 26, 2018
Date of Patent: Apr 5, 2022
Assignees: KONICA MINOLTA BUSINESS SOLUTIONS U.S.A., INC. (San Mateo, CA), THE BOARD OF TRUSTEES OF THE LELAND STANFORD JUNIOR UNIVERSITY (Stanford, CA)
Inventors: Leiming Wang (Foster City, CA), Mark Brongersma (Menlo Park, CA), Majid Esfandyarpour (Stanford, CA), Jun Amano (Hillsborough, CA)
Primary Examiner: Leonardo Andujar
Application Number: 16/199,607
International Classification: H01L 27/32 (20060101); H01L 51/52 (20060101); H01L 51/50 (20060101); H01L 33/38 (20100101); H01L 33/50 (20100101); H01L 33/40 (20100101);