WO2019075215A1

WO2019075215A1 - Nanocone metasurface for omni-directional detectors and photovoltaics

Info

Publication number: WO2019075215A1
Application number: PCT/US2018/055437
Authority: WO
Inventors: Connie Chang-Hasnain; Jipeng QI; Indrasen BHATTACHARYA
Original assignee: The Regents Of The University Of California
Priority date: 2017-10-12
Filing date: 2018-10-11
Publication date: 2019-04-18

Abstract

Anti-reflection coatings and coated solar cells are provided with a base layer of high refractive material and an array of nanocones or tapered nanopillars that form a metasurface that operates as an omni-directional anti-reflection coating allowing the collection of light from all directions with little reflection. Changes to nanocone height, width, fill ratio base thickness and materials will tune the properties of the coatings. The anti-reflection coatings will improve the light absorption capability and conversion efficiency of a solar cell. The anti-reflection coatings are particularly suited for solar cell, photodetector sensor and high power optical applications. The textured anti-reflection coating also substantially reduces the reflection of light under diffuse illumination, thus greatly improving light scavenging from indoor illumination sources.

Description

NANOCONE METASURFACE FOR OMNI-DIRECTIONAL

DETECTORS AND PHOTOVOLTAICS

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to, and the benefit of, U.S. provisional patent application serial number62/571 ,636 filed on October 12, 2017, incorporated herein by reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0001] Not Applicable

INCORPORATION-BY-REFERENCE OF

COMPUTER PROGRAM APPENDIX

[0002] Not Applicable

NOTICE OF MATERIAL SUBJECT TO COPYRIGHT PROTECTION

[0003] A portion of the material in this patent document is subject to

copyright protection under the copyright laws of the United States and of other countries. The owner of the copyright rights has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the United States Patent and Trademark Office publicly available file or records, but otherwise reserves all copyright rights whatsoever. The copyright owner does not hereby waive any of its rights to have this patent document maintained in secrecy, including without limitation its rights pursuant to 37 C.F.R. § 1.14.

BACKGROUND

[0004] 1 . Technical Field

[0005] The technology of this disclosure pertains generally to light

anti reflective coatings and methods of fabrication, and more particularly to omni-directional anti-reflection nanocone meta-surfaces for improved efficiency of solar cells and photodetectors etc.

[0006] 2. Background Discussion

[0007] Solar energy is a renewable resource that is needed to meet

increasing domestic energy demands and global environmental concerns.

Solar panel performance is measured in terms of its efficiency in turning sunlight into electricity. To maximize performance, the percentage of absorption of incident light should be maximized. However, a bare silicon solar cell absorbs only two-thirds of the light that shines on it and the rest is reflected away and lost.

[0008] Reducing reflection from surfaces is very important for improving the efficiency of solar cells and photodetectors, producing improved optical displays with less glare as well as coatings for high power optical applications. Anti-Reflection coatings may be applied to reduce reflection and to increase light absorption of solar cells and thus increase cell performance. Without anti-reflection (AR) coatings, semiconductor surfaces reflect 30% to 40% of incident light and even glass reflects 10% to 20% at normal incidence, which is very significant for the above

applications. The reflection can be >70% with large incidence angles.

[0009] The traditional method of anti-reflection is through the application of a thin film AR coating layer. The traditional AR coating is designed to be a quarter-wavelength in thickness (typically 50 nm to 100 nm) and has refractive index equal to the geometric mean of the two refractive indices of the media between which anti-reflection is desired. Anti-reflection is achieved using destructive interference that is necessarily a narrow-band and narrow-angle effect. The anti-reflective performance deteriorates (reflection increases) as incidence angle increases and is particularly severe beyond 40 to 50 degrees. This is a major issue in the presence of diffuse light, which is the case in any realistic environment. If reduced reflectivity across a broad range of wavelengths and viewing angles is desired, the traditional quarter-wave AR coating is ineffective.

[0010] One conventional approach for reducing reflectivity across multiple wavelengths and angles is to texture or roughen the surface of the cell, producing a continuation gradation in refractive index between the two media that prevents an abrupt reflection. The roughening approach that achieves a gradient in index has been termed ^'black silicon'. Typically, this gradient index is achieved using a damaging etching process that leads to a high dark current and degrades the optoelectronic performance of the device by significantly reducing the open circuit voltage of the photovoltaic device. The large dark current is particularly detrimental for dim light level applications (such as indoor lighting) where the carrier density is low to begin with, and the device is especially sensitive to poor surface quality. In addition, the roughening process is not always feasible for low thickness (thin film) photovoltaic devices as well as in non-silicon materials that are used in some applications and so the approach is limited in scope.

[0011] Another approach for producing a quasi anti-reflection effect is

based on coating with a film of carbon nanotubes that will absorb all the incident light. However, this is limited in applicability since no light can reach the underlying surface or exit from the underlying surface since the intervening carbon nanotubes completely absorb it. It would not be suitable for use with solar panels.

[0012] Accordingly, there is a need for an anti-reflection coating that will improve the light trapping capability and conversion efficiency of a solar cell that has reduced reflectivity across multiple wavelengths and angles of incidence that is durable and has controllable features.

BRIEF SUMMARY

[0013] The present technology provides a nanocone or tapered nanopillar metasurface that operates as an omni-directional anti-reflection coating allowing the collection of light from all directions with little reflection. The anti-reflection coatings are particularly suited for solar cell, photodetector sensor and high power optical applications.

[0014] In the solar cell embodiments, it is possible to directly absorb light with the nanocones or nanopillars of the anti-reflection coating allowing for much greater absorption. Solar cells made with a metasurface of tapered nanopillars or nanocones that has completely angle insensitive absorption should produce a steady power output without the need of an expensive solar tracking system to adjust for sunlight angle of illumination changes during the day and throughout seasons.

[0015] This textured anti-reflection coating is also expected to significantly reduce the reflection of light under diffuse illumination, thus greatly improving light scavenging from indoor illumination sources. The available energy source from ambient illumination is roughly 100-1000 times lower in intensity compared to solar illumination, leading to a significantly lower carrier concentration compared to photovoltaic operation under the solar AM1 .5G spectrum. In order to achieve high power conversion efficiencies at such low intensity angularly diffuse light, it is essential to have both excellent angle-insensitive light absorption as well as high

photoluminescence external quantum efficiency (EQEPL) at these low illumination intensities. Photovoltaic (PV) powering based on energy harvested from indoor or ambient illumination would be sustainable, less wasteful and potentially produce sufficient power to render small devices completely autonomous.

[0016] The passive nanocone/nanopillar metasurface provides an effective antireflection texture that can be adapted to a variety of different underlying substrates that is angle insensitive. This differentiates the nanopillar antireflection texture from a traditional quarter-wave antireflection (AR) coating. The traditional AR coating is designed to operate via destructive interference of a coherent incident plane wave with itself. However, both the thickness and refractive index of the traditional thin film coating needs to be chosen carefully such that cancellation occurs. For normal incidence, this is achieved by using a film with a refractive index that is the geometric mean of the two indices of refraction between which antireflection is desired. Further, the thickness of the AR coating is required to be a quarter of the wavelength (in the medium of the AR coating). Such traditional thin film AR coatings can only operate well within a narrow range of wavelengths around the designed wavelength, and similarly for angles of incidence. The nanocone metasurface addresses the high angle and broad wavelength anti-reflection limitations of the traditional thin film AR coatings.

[0017] A thin film solar cell adaptation is used to illustrate the structure and functionality of the anti-reflective metasurface coating. The solar cell embodiment has an indium gallium phosphide absorber with the anti- reflective coating on the top surface of the absorber and a back reflector on the bottom surface. The nanocone metasurface incorporates an array of nanocones or tapered nanopillars on a base layer such as silicon nitride. The structure of the anti-reflective coating has an array of nanocones or tapered (i.e. width varies along the length) nanopillars and a fill factor. The nanocones/nanopillars have a height and width and the base has a thickness that can be manipulated along with the base area fill factor and choice of materials to create coatings with the desired properties.

[0018] According to one aspect of the technology, an anti-reflection coating embodiment is provided that have pillar/cone dimensions and spacing that have been chosen to index match an underlying quarter wave AR coating.

[0019] According to another aspect of the technology, a gradient index

structure is provided that has the effect of bending even the glancing incidence rays such that the angle of incidence at the AR coating is close to the normal. With incidence angle closer to the normal, the AR coating functions better and leads to improved anti-reflectivity.

[0020] Another aspect of the technology is to provide an omni-directional anti-reflection coating for solar cells, photodetectors and high-power optics where absorption characteristics are maintained at oblique angles of incidence.

[0021] Further aspects of the technology described herein will be brought out in the following portions of the specification, wherein the detailed description is for the purpose of fully disclosing preferred embodiments of the technology without placing limitations thereon. BRIEF DESCRIPTION OF THE SEVERAL VIEWS

OF THE DRAWINGS

[0022] The technology described herein will be more fully understood by reference to the following drawings which are for illustrative purposes only:

[0023] FIG. 1A: Device schematic with silicon nitride nanocone metasurface on an InGaP absorber thin film solar cell according to one embodiment of the technology.

[0024] FIG. 1 B: is a detail view of the nanocone metasurface of the solar cell structure. The nanocone metasurface serves as an omni-directional anti-reflection coating.

[0025] FIG. 2: Gradient index equivalent illustration of a nanocone

metasurface that bends light from all incident angles to normal direction.

[0026] FIG. 3: is a graph of calculated reflectivity vs. incidence angle using a ray optics formulation.

[0027] FIG. 4 is a graph of calculated reflectivity vs. incident angle using Rigorous Coupled Wave Analysis (RCWA) demonstrating that the nanocone metasurface significantly reduces reflection especially at high incident angles. DETAILED DESCRIPTION

[0028] Referring more specifically to the drawings, for illustrative purposes, embodiments of apparatus, system and methods for a gradient index structures with anti-reflection nanocone/nanopillar metasurfaces allowing for omnidirectional light absorption are generally shown. Several embodiments of the technology are described generally in FIG. 1 A to FIG. 4 to illustrate the characteristics and functionality of the devices, methods and systems within the context of a solar cell adaptation. It will be appreciated that the methods may vary as to the specific steps and sequence and the systems and apparatus may vary as to structural details without departing from the basic concepts as disclosed herein. The method steps are merely exemplary of the order that these steps may occur. The steps may occur in any order that is desired, such that it still performs the goals of the claimed technology.

[0029] Turning now to FIG. 1 A and FIG. 1 B, one embodiment of a thin film solar cell 10 with a nanocone array anti-reflection coating structure is shown schematically. The embodiment of solar cell 10 shown generally in FIG. 1A, has at least one planar absorber layer 12 with a preferably transparent anti-reflection coating layer 14 with a nanocone/nanopillar array on the top surface of the absorber layer 12 and an optional silver back reflector 16 on the bottom surface of the absorber 12. Light rays 26 and light rays 28 with different angles of incidence can be absorbed by the anti- reflection coating 14 as shown in FIG. 1A.

[0030] Referring also to the detailed view of FIG. 1 B, the structure of the anti-reflection coating 14 has a base layer 18 and a plurality of generally vertical AR structures 20. The AR structures 20 are nanocones in the embodiment shown in FIG. 1 A and FIG. 1 B. However, the AR structures 20 can also be tapered nanopillars.

[0031] The base layer 18 is preferably made of the same high index

material as the nanocones 20. Silicon nitride is a particularly preferred material for the base layer 18 and nanocones 20 in this illustration. The base layer 18 covers, and is functionally coupled to, the upper surface of the absorber 12. The array 14 has one or more AR structures 20, each with a height 22 and a width or diameter 24, and a volume fill ratio of AR structures on the surface of base layer 18.

[0032] It can be seen that the overall anti-reflective characteristics of the coating 14 can be adapted or modulated with the selection of the volume fill ratio, the height 22 and width 24 of the AR structures 20, the thickness of the base layer 18 and the materials that are used. The selection of the materials used for the absorber 12 and the thickness of the absorber may also influence efficiency of the cell. Generally, the height 22 of the cone or pillar AR structure 20 should be greater than half of the wavelength of light to be absorbed. In one embodiment, the heights of the nanocones or tapered nanopillars are varied and are not uniform.

[0033] The preferred materials that are selected may depend on the adaptation, but are generally high refractive index materials that are preferably transparent such as ΤΊΟ2, AgTe, AgSe, silicon nitride, niobium nitride, titanium nitride etc. The fill ratio of the array generally refers to the percentage of surface coverage measured at the base of the nanocones. Preferred fill ratios range from about 5% to about 70%. However, essentially any fill ratio may be selected, with an appropriate base layer thickness being chosen accordingly. Each AR structure of the array generally comprises a nanocone or tapered nanopillar having a height from about 50 nm to about 1000 nm, and a base diameter or width of about 15 nm to about 450 nm.

[0034] The tapered nanopillar or conical shape allows for both high

absorption efficiency and omnidirectional absorption. This allows for diffuse and angled sunlight to be absorbed and has implications for harvesting ambient light energy. Further, the sub-wavelength size near the tip of the nanopillar allows for efficient extraction of generated photons, thus increasing EQEPL and the open circuit voltage of the photovoltaic device.

[0035] The anti-reflective nanocone metasurface structure can be fabricated using standard clean room processes. For example, a silicon nitride layer base layer 18 can be deposited using chemical vapor deposition. The AR structures 20 such as nanocones can be fabricated by using an array of polymer spheres as an etch mask for an isotropic etch process. An alternative method to fabricating nanopillars is through a bottom-up metal organic chemical vapor deposition, in which case the nanopillars can themselves be made from IMA material. It should be emphasized that the underlying surface is protected and passivated by the nanopillar AR coating, as opposed to the damaging etch process used conventionally in the art.

[0036] One feature of the gradient index texture of the AR coating 14 is that the nanopillars or nanocones absorb light leading to near unity absorption by the coating even with a minimal volume fill ratio. One explanation for the observed minimal reflection of the AR coating 14 is that the tapered nanopillars or cones act as an impedance matching layer between air and the lll/V semiconductor. The gradual change in impedance feature can more generally be used to construct a textured anti-reflection coating, allowing for a greater degree of freedom when it comes to the PV absorber.

[0037] A key benefit of this kind of a passive nanopillar or nanocone

metasurface coating 14 is the ability to select dimensions to tailor an effective anti-reflection texture for a variety of different underlying

substrates in a manner that is angle insensitive. This differentiates the nanopillar antireflection texture from a traditional quarter-wave anti- reflection (AR) coating.

[0038] The advantage of a graded refractive index layer is that it can be used to bend the angle of incidence of rays closer to the normal as they impinge on the absorbing medium. As illustrated in FIG. 2, the gradient index structure coating 14 has the effect of bending even the glancing incidence rays 28 such that the angle of incidence at the AR coating is close to the normal 30. This is predicted simply from Snell's law: the angle of refraction becomes smaller when rays enter a high index medium. With incidence angle closer to the normal, the AR coating functions better and leads to improved anti-reflectivity.

[0039] An effective medium approach can be used to describe how the

nanocone metasurface coating 14 is designed. Suppose each nanocone can be divided vertically into multiple layer stacks. Each layer will have a different refractive index. This will generate a refractive index profile n(z) in the vertical direction corresponding to the equation:

n(z) = ri_j + f x (n_f - η;) where rij and n_f are the refractive indices above and below the nanocone metasurface and f is the area fill factor of each layer.

[0040] With the refractive index profile generated, the incoming light wave 26, 28 can be treated as gradually being "bent" following the refractive index profile from all incident angles into normal 30 incident case as seen in

FIG. 2.

[0041] It is important to emphasize that the angle of incidence at the underlying AR coating is close to the normal even for high incidence angles. The refractive index at the base of the nanocones (n (0)) is determined by the fill factor and may be chosen such that the underlying quarter wave AR coating has refractive index that is the geometric mean of n (0) and the refractive index of the substrate, similar to a traditional AR coating. Since traditional AR coatings are designed to operate at normal incidence, this has the effect of significantly reducing the reflectance for light incident at high angles in air.

[0042] Therefore, the bending feature can be used to markedly improve the effectiveness of a quarter wave coating. For instance, if the quarter wave coating refractive index is 2.1 (silicon nitride), and the underlying IMA solar cell substrate has a refractive index of 3.5, then the bottom of the gradient index structure needs to have an effective medium refractive index of 1.26 in order for the anti-reflection condition to be met. The gradient index layer can then be graded from the refractive index of the surrounding medium (air = 1 ) to 1 .26 and will prevent abrupt discontinuities that cause reflection.

[0043] The superior anti-reflection properties as illustrated are not only

limited to silicon nitride nanocone and ΙΙΙΛ/ solar cell applications. In fact, a wide range of materials choices and designs are available, with a choice of fill factor and refractive index material evaluations as well as a choice of nanocone or tapered nanopillar dimensions and light ranges. It can be seen that high index semiconductor subwavelength metastructures have been shown to exhibit extraordinary capabilities to manipulate optical properties of a surface, such as wide bandwidth reflection, omni-directional reflection and anti-reflection. With proper design, this type of

nanostructured surface can be applied to many surfaces which require low to little reflection for a broad range of incidence angles.

[0044] It will be appreciated that the technology can be used in a variety of applications, including but not limited to (a) Indoor light harvesting for Internet of Things; (b) Utility-scale solar cells; (c) Building integrated or attached photovoltaics; (d) Smartphone displays; (e) Head mounted displays for augmented/virtual/mixed reality; (f) High power optical coatings; (g) Semiconductor photodetectors; and (h) LIDAR systems (Light Detection and Ranging).

[0045] The technology described herein may be better understood with reference to the accompanying examples, which are intended for purposes of illustration only and should not be construed as in any sense limiting the scope of the technology described herein as defined in the claims appended hereto.

[0046] Example 1

[0047] To demonstrate the operational principles of the apparatus and

methods, anti-reflective IMA based solar cells of a structure shown in FIG.

1 A were simulated and evaluated. In this example, the nanocone array was made of silicon nitride sitting on a 50 nm thin silicon nitride layer. This underlying layer is similar to a traditional thin film AR coating. The nanocone height is 300 nm and the base diameter is 1 15 nm with an area fill factor about 20%. Underneath the nanocone structures and nitride layer was the 1 urn thick indium gallium phosphide absorber. The nanocone metasurface was shown to function as an omnidirectional anti-reflection coating that helps collect light from all directions.

[0048] A nanopillar based implementation of the gradient index structure illustrated in FIG. 1A to allow omnidirectional anti-reflection for a lll/V based solar cell was also designed and fabricated. The pillar dimensions and spacing were chosen to index match the underlying quarter-wave AR coating, which is the same material as the nanopillars (silicon nitride). The case when the nanopillar and AR coating are the same material as the underlying substrate was also evaluated.

[0049] The requirement for having a refractive index of 1 .26 at the base was maintained by using an appropriate pitch and diameter for the nanopillars. For ease of fabrication, the nanopillars can be fabricated using the same material as the underlying quarter wave layer. Then, the area fill factor at the base of the nanopillars was set to be 60%. The diameter and pitch of the nanopillars should be chosen to be small enough so that an effective medium approach is valid. A pitch of around 100 nm, with a base diameter of 87.5 nm, for example, satisfied these requirements.

[0050] The gradient index principle is only effective when the index changes sufficiently slowly. In other words, the nanopillars need to be sufficiently tall for the impedance matching effect to be observed. A rough prescription for the height of the pillars is that it should be half the vacuum wavelength of light being considered. In this case, antireflection for indoor illumination around 400-600 nm was considered, so the nanopillar texture was selected with a height of 200-300 nm. Along with the roughly 50 nm quarter wave layer, this amounted to a total of 250-350 nm thick anti-reflection layer. The deposition process needed to be optimized in order to deposit sufficiently thick films.

[0051] Example 2

[0052] To demonstrate the operational principles of the apparatus and

methods, simulations of a gradient index structure configured with an effective medium approach using effective index equivalents to

demonstrate the principles of anti-reflection were performed.

[0053] The graph of FIG. 3 shows a ray-optics based analytical calculation where the angle dependence is calculated from Snell's law and the reflectivity is calculated using interference between the multiple bounces within the quarter wave film. It can be seen that the quarter wave coating works well at incidence angles below 40°. However, this rapidly degrades at higher incidence angles. Adding the gradient index nanopillar texture significantly reduces the reflectivity at higher incidence angles, simply due to the impedance matching effect and reduced angle of incidence at the quarter wave film. The quasi ray optics simulations were repeated at both incidence polarizations.

[0054] Example 3

[0055] To further demonstrate the operational principles of the apparatus and methods, a full vectoral wave-optics framework was prepared to model the gradient index more accurately.

[0056] This framework also accounted for the "abruptness" of the gradient index layer coating, in case the height was too small compared to the incidence wavelength. A rigorous coupled wave analysis (RCWA) technique was used to simulate both the case of no gradient index layer (quarter wave AR only), as well as the situation with the gradient index layer fabricated on top of the quarterwave coating ("optimized nanocone"), as shown on the graph of FIG. 4. It can be seen that the nanopillar gradient index layer indeed leads to a significant reduction in reflection for angles of incidence above 40°, and that this effect is maintained up to an 80° incidence angle.

[0057] Finite difference time domain (FDTD) simulations also provided an indication of the behavior of the actual nanocone/nanopillar structure. It can be noted that FDTD suggests that a 300 nm tall nanopillar structure already reduces the amount of reflected light at 85° by nearly a factor of two compared to the thin-film only case. This was further improved by the use of 900 nm tall nanocones.

[0058] From the description herein, it will be appreciated that that the

present disclosure encompasses multiple embodiments which include, but are not limited to, the following:

[0059] 1 . An antireflective surface coating, comprising: (a) a base layer of high refractive index material with an upper surface and a lower surface configured to couple to a surface of substrate; and (b) a metasurface layer of an array of nanocones or tapered nanopillars on the upper surface of the base layer.

[0060] 2. The coating of any preceding or following embodiment, wherein each nanocone or nanopillar has a height that is greater than half of a wavelength of light to be absorbed by the coating.

[0061] 3. The coating of any preceding or following embodiment, wherein the base layer has a thickness that is about one quarter of a wavelength of light to be absorbed by the coating.

[0062] 4. The coating of any preceding or following embodiment, wherein the metasurface layer has a fill ratio of between 5% and 70%.

[0063] 5. The coating of any preceding or following embodiment, wherein each nanocone or nanopillar of the metasurface layer array comprises a nanocone or nanopillar having a height from about 50 nm to about 1000 nm.

[0064] 6. The coating of any preceding or following embodiment, wherein each nanocone or nanopillar of the metasurface layer array comprises a nanocone or nanopillar having a base diameter of about 15 nm to about

450 nm.

[0065] 7. The coating of any preceding or following embodiment, wherein the nanocones or nanopillars comprise a first high refractive index material; and the base layer comprises a second high refractive index material.

[0066] 8. The coating of any preceding or following embodiment, wherein the base layer and the metasurface layer are made of the same high refractive index material.

[0067] 9. The coating of any preceding or following embodiment, wherein the high index material is a material selected from the group of materials consisting of T1O2, AgTe, AgSe, silicon nitride, niobium nitride and titanium nitride.

[0068] 10. A photovoltaic cell device, comprising: (a) an optical absorber; and (b) an antireflective surface coating on the absorber layer, the coating comprising: (i) a base layer of high refractive index material with an upper surface and a lower surface configured to couple to the absorber; and (ii) a metasurface layer of an array of nanocones or tapered nanopillars on the upper surface of the base layer.

[0069] 1 1 . The device of any previous or following embodiment, further comprising: a back reflector mounted to the absorber on an opposite side from the surface coating.

[0070] 12. The device of any previous or following embodiment, wherein the optical absorber is from about 300 nm to about 3 pm in thickness.

[0071] 13. The device of any previous or following embodiment, wherein the optical absorber comprises an indium gallium phosphide absorber.

[0072] 14. The device of any previous or following embodiment, wherein each nanocone or nanopillar has a height that is greater than half of a wavelength of light to be absorbed by the coating. [0073] 15. The device of any previous or following embodiment, wherein the base layer has a thickness that is about one quarter of a wavelength of light to be absorbed by the coating.

[0074] 16. The device of any previous or following embodiment claim 10, wherein the metasurface layer has a fill ratio of between 5% and 70%.

[0075] 17. The device of any previous or following embodiment, wherein each nanocone or nanopillar of the metasurface layer array comprises a nanocone or nanopillar having a height from about 50 nm to about 1000 nm and a base diameter of about 15 nm to about 450 nm.

[0076] 18. The device of any previous or following embodiment, wherein the high index material of the coating is a material selected from the group of materials consisting of T1O2, AgTe, AgSe, silicon nitride, niobium nitride and titanium nitride.

[0077] 19. A solar cell device, comprising: (a) a plurality of silicon nitride (SiN) nanopillars disposed on a silicon nitride layer; and (b) an InGaP solar cell beneath the silicon nitride layer; wherein the plurality of silicon nitride nanopillars and silicon nitride layer form a nanocone metasurface that serves as an omni-directional anti-reflection coating.

[0078] 20. The device of claim 19: wherein the silicon nitride layer is about 50 nm thick; wherein the nanopillars have a height of about 300 nm, a base diameter of about 1 15 nm, and an area fill factor of about 20%; and wherein the InGaP solar cell has a thickness of about 1 pm.

[0079] 21 . A device comprising a silicon nitride nanocone metasurface disposed on an InGaP absorber thin film solar cell.

[0080] 22. The device of any preceding or subsequent embodiment,

wherein the nanocone metasurface serves as omni-directional anti- reflection coating.

[0081] 23. The device of any preceding or subsequent embodiment,

wherein the silicon nitride nanocone metasurface comprises a plurality of silicon nitride (SiN) nanopillars disposed on a silicon nitride layer.

[0082] 24. A device comprising: a plurality of silicon nitride (SiN)

nanopillars disposed on a silicon nitride layer; and an InGaP solar cell beneath the silicon nitride layer; wherein said plurality of silicon nitride nanopillars and silicon nitride layer form a nanocone metasurface that serves as an omni-directional anti-reflection coating.

[0083] 25. The device of any preceding or subsequent embodiment:

wherein the silicon nitride layer is about 50 nm thick; wherein the

nanopillars have a height of about 300 nm, a base diameter of about 1 15 nm, and an area fill factor of about 20%; and wherein the InGaP solar cell has a thickness of about 1 urn.

[0084] 26. An antireflective material, comprising: (a) a nanocone array layer; and (b) an optical absorber layer; (c) wherein the combination of said nanocone array layer and said optical absorber layer form a nanocone metasurface as an omni-directional anti-reflection coating which increases the level of light collection upon a structure upon which the antireflective material is disposed.

[0085] 27. The material of any preceding or subsequent embodiment, wherein said nanocone array comprises a silicon nitride material.

[0086] 28. The material of any preceding or subsequent embodiment, wherein each nanocone of said nanocone array comprises a cone having a height from about 50 to 1000 nm, and a base diameter of about 15 to 450 nm.

[0087] 29. The material of any preceding or subsequent embodiment, wherein each nanocone of said nanocone array comprises a cone having a height of about 300 nm with a base diameter of about 1 15 nm.

[0088] 30. The material of any preceding or subsequent embodiment, wherein said optical absorber layer comprises an indium gallium phosphide absorber.

[0089] 31 . The material of any preceding or subsequent embodiment, wherein said optical absorber layer is from about 300 nm to about 3 urn in thickness.

[0090] 32. The material of any preceding or subsequent embodiment, wherein said optical absorber layer is about 1 urn in thickness.

[0091] 33. The material of any preceding or subsequent embodiment, wherein said antireflective material is configured for fabricated upon or coupled to solar cells and photodetectors toward increasing light collection.

[0092] 34. The material of any preceding or subsequent embodiment,

wherein said antireflective material is configured for fabricated upon or coupled to a traditional anti-reflective surface to augment anti-reflection properties.

[0093] 35. The material of any preceding or subsequent embodiment,

wherein said antireflective material is configured for being fabricated upon, or coupled to an optical display toward increasing optical display visibility by reducing glare effects.

[0094] As used herein, the singular terms "a," "an," and "the" may include plural referents unless the context clearly dictates otherwise. Reference to an object in the singular is not intended to mean "one and only one" unless explicitly so stated, but rather "one or more."

[0095] As used herein, the term "set" refers to a collection of one or more objects. Thus, for example, a set of objects can include a single object or multiple objects.

[0096] As used herein, the terms "substantially" and "about" are used to describe and account for small variations. When used in conjunction with an event or circumstance, the terms can refer to instances in which the event or circumstance occurs precisely as well as instances in which the event or circumstance occurs to a close approximation. When used in conjunction with a numerical value, the terms can refer to a range of variation of less than or equal to ± 10% of that numerical value, such as less than or equal to ±5%, less than or equal to ±4%, less than or equal to ±3%, less than or equal to ±2%, less than or equal to ±1 %, less than or equal to ±0.5%, less than or equal to ±0.1 %, or less than or equal to ±0.05%. For example, "substantially" aligned can refer to a range of angular variation of less than or equal to ±10°, such as less than or equal to ±5°, less than or equal to ±4°, less than or equal to ±3°, less than or equal to ±2°, less than or equal to ±1 °, less than or equal to ±0.5°, less than or equal to ±0.1 °, or less than or equal to ±0.05°. [0097] Additionally, amounts, ratios, and other numerical values may sometimes be presented herein in a range format. It is to be understood that such range format is used for convenience and brevity and should be understood flexibly to include numerical values explicitly specified as limits of a range, but also to include all individual numerical values or sub-ranges encompassed within that range as if each numerical value and sub-range is explicitly specified. For example, a ratio in the range of about 1 to about 200 should be understood to include the explicitly recited limits of about 1 and about 200, but also to include individual ratios such as about 2, about 3, and about 4, and sub-ranges such as about 10 to about 50, about 20 to about 100, and so forth.

[0098] Although the description herein contains many details, these should not be construed as limiting the scope of the disclosure but as merely providing illustrations of some of the presently preferred embodiments. Therefore, it will be appreciated that the scope of the disclosure fully encompasses other embodiments which may become obvious to those skilled in the art.

[0099] All structural and functional equivalents to the elements of the

disclosed embodiments that are known to those of ordinary skill in the art are expressly incorporated herein by reference and are intended to be encompassed by the present claims. Furthermore, no element,

component, or method step in the present disclosure is intended to be dedicated to the public regardless of whether the element, component, or method step is explicitly recited in the claims. No claim element herein is to be construed as a "means plus function" element unless the element is expressly recited using the phrase "means for". No claim element herein is to be construed as a "step plus function" element unless the element is expressly recited using the phrase "step for".

Claims

CLAIMS What is claimed is:

1 . An anti reflective surface coating, comprising:

(a) a base layer of high refractive index material with an upper surface and a lower surface configured to couple to a surface of substrate; and

(b) a metasurface layer of an array of nanocones or tapered nanopillars on the upper surface of the base layer.

2. The coating of claim 1 , wherein each said nanocone or nanopillar has a height that is greater than half of a wavelength of light to be absorbed by the coating.

3. The coating of claim 1 , wherein said base layer has a thickness that is about one quarter of a wavelength of light to be absorbed by the coating.

4. The coating of claim 1 , wherein said metasurface layer has a fill ratio of between 5% and 70%.

5. The coating of claim 1 , wherein each nanocone or nanopillar of said metasurface layer array comprises a nanocone or nanopillar having a height from about 50 nm to about 1000 nm.

6. The coating of claim 5, wherein each nanocone or nanopillar of said metasurface layer array comprises a nanocone or nanopillar having a base diameter of about 15 nm to about 450 nm.

7. The coating of claim 1 , wherein:

said nanocones or nanopillars comprise a first high refractive index material; and

said base layer comprises a second high refractive index material.

8. The coating of claim 1 , wherein said base layer and said

metasurface layer are made of the same high refractive index material.

9. The coating of claim 1 , wherein said high index material is a material selected from the group of materials consisting of T1O2, AgTe, AgSe, silicon nitride, niobium nitride and titanium nitride.

10. A photovoltaic cell device, comprising:

(a) an optical absorber; and

(b) an antireflective surface coating on said absorber layer, said coating comprising:

(i) a base layer of high refractive index material with an upper surface and a lower surface configured to couple to said absorber; and

(ii) a metasurface layer of an array of nanocones or tapered nanopillars on the upper surface of the base layer.

1 1 . The device of claim 10, further comprising:

a back reflector mounted to said absorber on an opposite side from said surface coating.

12. The device of claim 10, wherein said optical absorber is from about 300 nm to about 3 pm in thickness.

13. The device of claim 10, wherein said optical absorber comprises an indium gallium phosphide absorber.

14. The device of claim 10, wherein each said nanocone or nanopillar has a height that is greater than half of a wavelength of light to be absorbed by the coating.

15. The device of claim 10, wherein said base layer has a thickness that is about one quarter of a wavelength of light to be absorbed by the coating.

16. The device of claim 10, wherein said metasurface layer has a fill ratio of between 5% and 70%.

17. The device of claim 10, wherein each nanocone or nanopillar of said metasurface layer array comprises a nanocone or nanopillar having a height from about 50 nm to about 1000 nm and a base diameter of about 15 nm to about 450 nm.

18. The device of claim 10, wherein said high index material of said coating is a material selected from the group of materials consisting of T1O2, AgTe, AgSe, silicon nitride, niobium nitride and titanium nitride.

19. A solar cell device, comprising:

(a) a plurality of silicon nitride (SiN) nanopillars disposed on a silicon nitride layer; and

(b) an InGaP solar cell beneath the silicon nitride layer;

wherein said plurality of silicon nitride nanopillars and silicon nitride layer form a nanocone metasurface that serves as an omni-directional anti-reflection coating.

20. The device of claim 19:

wherein the silicon nitride layer is about 50 nm thick;

wherein the nanopillars have a height of about 300 nm, a base diameter of about 1 15 nm, and an area fill factor of about 20%; and

wherein the InGaP solar cell has a thickness of about 1 pm.