Patent Publication Number: US-2012031487-A1

Title: Nanoscale High-Aspect-Ratio Metallic Structure and Method of Manufacturing Same

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS 
     This patent application is a Continuation-in-Part of co-pending U.S. patent application Ser. No. 13/026,637, filed Feb. 14, 2011, which claims the benefit of U.S. Provisional Patent Application No. 61/307,620, filed Feb. 24, 2010, the entire teachings and disclosure of which are incorporated herein by reference thereto. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT 
     This invention was made in part with Government support under Grant Numbers DE-ACO2-07CH11358 awarded by the Department of Energy. The Government has certain rights in this invention. 
    
    
     FIELD OF THE INVENTION 
     This invention generally relates to nanoscale high-aspect ratio metallic structures for use in solar cells and solid-state lighting devices, including organic light-emitting diodes. 
     BACKGROUND OF THE INVENTION 
     Since the turn of this century, awareness of climate change, the search for clean energy, and the need for utilizing energy efficiently have been primary topics for both industry and academic research. Such interests have spurred developments in organic solar cells (OSCs) and organic light-emitting diodes (OLEDs). The advancements in organic solar cells and OLEDs are largely processing advantages including lower production costs, and simple fabrication methods when compared to their inorganic counterparts. Furthermore, OSCs and OLEDs offer the possibility of device fabrication on flexible substrates over large areas with higher throughput, which could greatly improve both their functionality and economy. 
     As a result of the above-mentioned developments, cost-effective solar-electric energy conversion is becoming increasingly important for the world. This is evidenced by the fact that direct solar-electric energy conversion using photovoltaic (solar cell) technology has grown exponentially over the last few years, as the costs of producing that energy have decreased from approximately $100/W in the late 1960&#39;s to the current level of approximately $3.50/W. This translates into electric energy generation costs of approximately 20-25 cents/kW hour (kWh). The current worldwide production of solar cells is approximately 3.4 gigawatts (GW)/year. This is equivalent to the power produced by almost four nuclear power plants in a single year. To compare, not a single nuclear plant has been ordered in the United States in the last thirty years. 
     Solar cell panel production has been growing at an annual growth rate of approximately 40%/year over the last ten years, and the current worldwide revenue from photovoltaic (PV) systems is about $17.8 billion/year. The solar cell industry raised nearly $10 billion dollars worldwide in 2007 to build their plants, with almost $5.3 billion dollars coming as equity contribution. As these numbers demonstrate, the solar cell industry is a major growth industry worldwide. 
     Indeed, the demand for solar cells to produce electric power is being driven both by market pull because of government subsidies (as in Germany) and by its improving economic competitiveness with conventional power, particularly where sun shines brightly and power costs are high, e.g., California. In California, entire new housing developments have solar cells built-in on their roofs, with the cells providing excess power during daytime which is sold to the grid, and with the grid providing nighttime power to the homes. The daytime tariffs for electricity consumption in California are very high (approximately 15-20 cents/kWh), because the peak power produced during daytime relies on very expensive natural gas, which is now costing upward of $10.00/MMBTU. Unfortunately, the costs of solar cell panels, after continuously reducing for approximately 20 years, have recently started to increase. One reason is due to the cost of the silicon wafers, which typically use a very expensive feedstock made of purified polysilicon. Polysilicon currently costs about $110-120/kg. 
     A typical solar-to-electric conversion efficiency for conventional silicon solar cells of approximately 15% means that a one square meter panel produces about 150 W. Silicon wafers used in solar cell panels are typically about 270-300 micrometers thick. Taking into account material lost during cutting and processing, silicon having a thickness of approximately a 600 micrometers is needed to make a conventional silicon solar cell. A 600-micrometer-thick silicon translates into 10 kg of silicon per kW of power produced, or at $120/kg, approximately $1,200/kW for the silicon alone. This is one reason the retail cost of the finished panel, which includes solar cells, encapsulation, front glass window, frame, etc., are now averaging about $4,800/kW. At these costs, electricity produced in sunny climates costs about 20-25 c/kWh, which is much too high to compete against power produced by conventional means, e.g., from coal. Therefore, the solar energy industry has been exploring a variety of ways to reduce the cost of the producing solar cells that make up the bulk of the cost of a typical solar panel. 
     Another factor contributing to the high cost of solar cells is the cost associated with the fabricating solar cell electrodes. Currently, most solar cells, and even most solid-state lighting (SSL) devices, employ indium tin oxide (ITO) coated substrates as their electrodes on the front side because of their relatively high transparency to visible light and low electrical sheet resistance. However, there is concern about the rising cost of ITO due to the limited supply of indium. Further, ITO electrodes can be relatively brittle with limited mechanical stability and limited chemical compatibility with active organic materials. Recently, there have been reports of investigations into carbon nanotube networks, random silver metal nanowire meshes, and patterned metal nanowire grids using nanoimprint lithography techniques in search of the replacement for ITO substrates. While the carbon nanotube networks and the silver metal nanowire meshes have equivalent optical transparencies as ITO substrates, their electrical conductivities are still inferior to the ITO substrates, and they suffer from current shunt due to the random nature of nanotube and nanowire networks. 
     The use of carbon nanotube networks and silver metal nanowire meshes as electrodes for organic solar cells and organic LEDs is described in a paper by Jung-Yong Lee, Stephen T. Connor, Yi Cui, and Peter Peumans, entitled “Solution-Processed Metal Nanowire Mesh Transparent Electrodes” published in The American Chemical Society publication, Nano Letters, Vol. 8, No. 2 pp. 689-692 (2008), the teachings and disclosure of which are incorporated in their entireties by reference thereto. The patterned metal nanowire grids show good visible transparency, however, the small line-width and thickness for the patterned metals lead to high sheet resistance as well as concerns about possible deterioration of the conductivity of the system with use. The use of patterned metal nanowire is described in a paper by L. Jay Guo and Myung-Gyu Kang entitled “Nanostructured Transparent Metal Electrodes for Organic Solar Cells” published by SPIE Newsroom, DOI: 10.1117/2.1200904.1364 (2009), the teachings and disclosure of which are incorporated in their entireties by reference thereto. Nanoimprinting of patterned metal nanowire grids for organic solar cells is described in a paper by Myung-Gyu Kang, Myung-Su Kim, Jinsang Kim, and L. Jay Guo entitled “Organic Solar Cells Using Nanoimprinted Transparent Metal Electrodes” published by Advanced Materials, DOI: 10.1002/adma.200800750 (2008), the teachings and disclosure of which are incorporated in their entireties by reference thereto. Nanoimprinting of patterned metal nanowire grids for organic LEDs is described in a paper by Myung-Gyu Kang and L. Jay Guo entitled “Nanoimprinted Semitransparent Metal Electrodes and Their Application in Organic Light-Emitting Diodes” published by Advanced Materials, DOI: 10.1002/adma.200700134 (2007), the teachings and disclosure of which are incorporated in their entireties by reference thereto. 
     It would therefore be desirable to have a solar cell electrode which has a relatively high transparency for light and a low electrical sheet resistance, the fabrication of which results in an electrode less expensive to manufacture than conventional ITO electrodes. Embodiments of the invention described herein provide such electrodes and such methods of fabrication. These and other advantages of the invention, as well as additional inventive features, will be apparent from the description provided herein. 
     BRIEF SUMMARY OF THE INVENTION 
     In view of the above, embodiments of the present invention provide a new and improved solar cell electrode and method of fabricating solar cell electrodes that overcome one or more of the problems existing in the art. More specifically, embodiments of the present invention provide new and improved method utilizing nano-scale high-aspect-ratio metallic structures that can be used to enhance the performance of solar cells and LEDs and structures resulting therefrom. These nano-scale metallic structures may also be used as infrared control filters due to their ability to reflect a high amount of infrared radiation. In other embodiments, the nano-scale metallic structures may also include interdigitated conductors allowing realization of multiple potentials and use of switching signals for applications such as lateral photovoltaic cells. 
     In one aspect, embodiments of the invention provide a nanoscale electrode that includes a substrate transparent to visible light. An embodiment of the invention also includes a first metal rail spaced apart from, and parallel to, a second metal rail. In this embodiment, the two metal rails are supported by, and affixed to, a polymer bar disposed entirely between the first and second metal rails. Further, in an embodiment of the invention, the polymer bar is attached to the substrate. 
     In another aspect, embodiments of the invention provide a method of fabricating a nanoscale electrode that includes the steps of forming a material into a bar, and affixing the material to a transparent substrate. In an embodiment of the invention, the method also includes depositing a metal coating over the exposed side and top portions of the material, and removing the metal coating from a top portion of the material. In another embodiment, the method includes applying a grating mask on one end of the bars, depositing the metal coating in a first direction, applying a grating mask on the other end of the bars, and depositing the metal coating in a second direction. Thereafter the metal coating from a top portion of the material is removed resulting in interdigitated electrodes. 
     In accordance with an embodiment described herein, a method of manufacturing a nanoscale electrode includes the steps of filling a plurality of grooves of an elastomeric mold with a first polymer that can be UV cured. Each groove in the plurality of grooves in are parallel with each other. The first polymer is partially cured, and a second polymer is then coated on the first polymer, resulting in a filled elastomeric mold. The first and second polymers are suitable polymers of appropriate viscosity and with physical and chemical properties that allow the building of a layered structure and cured via UV light exposure. A transparent substrate is placed on the filled elastomeric mold, and the filled elastomeric mold and substrate are exposed to UV light. The filled elastomeric mold is peeled away from the first polymer and the second polymer such that the first polymer and second polymer form a polymer layer of polymer bars on the substrate. 
     The plurality of bars are then metal coated by oblique angle deposition. This is done to address the unique need for transparency that is met by using an oblique angle deposition method. Specifically, to maintain transparency, the substrate between the bars cannot have metal deposited thereon. As such, the oblique angle deposition method allows only the sides and the top of the bars to be coated, while leaving the substrate between the bars free of metal. In at least one embodiment, the metal coating on the top of the bars or bars is then removed by argon ion milling of the metal coating off of the top of the bars. In an alternate embodiment of the invention, the metal on top of the bars is removed by reactive ion etching. 
     In one embodiment, the metal deposition is performed such that metal film is also deposited on the substrate around the outside edges of the bars to electrically connect the vertical metal coatings on the sides of the bars to form a single potential electrode. In another embodiment, a mask is used to prevent metal from being deposited on one end of the bars and that end of the substrate during a first deposition, and to prevent metal from being deposited on an opposite end of the bars and substrate during a second deposition such that electrical connection between alternate vertical metal coatings on the sides of the bars are electrically isolated from one another to form a multiple-potential electrode with interdigitated electrode fingers. 
     In another embodiment, encapsulation is used with the structures to improve optical transparency and transparency at high angles. In such an embodiment, once the base structure is completed, a drop of polyeutherane (PU) liquid prepolymer is placed on top of the etched structure and UV cured, and a second glass substrate is placed on top to encapsulate the entire structure. The additional PU fills in the air channels bewteen the metal sidewalls and also forms a layer over the entire structure to reduce the diffraction effect from the grating pattern. Such a technique is particularly applicable to large area samples (2 in×2 in, or bigger, 6 in ×6 in, etc.). 
     In yet another embodiment, an inverted structure is utilized to facilitate fabrication of a solar cell or other device on the back-side of the completed structure. In this embodiment, the PU grating is fabricated on a water-soluble sacrificial layer coated glass substrate. After metal deposition and argon ion milling, a small droplet of PU prepolymer is placed on the sample to fill in the trenches of the grating structure. The PU prepolymer also serves to glue a second glass substrate onto the sample. After the PU filling is ultraviolet cured and solidified, the structure is submerged in distilled water to dissolve the sacrificial layer, and the original glass substrate is detached. Upon the separation of the original glass substrate, the bottom part of the structure is exposed and the structure is inverted with respect to the original structure. The active materials of a solar cell and the other electrode can be fabricated on this transparent electrode substrate. 
     In a further embodiment, a sandwich structure, i.e. multiple layered electrodes, are formed such that an active layer is sandwiched between two conductive layers. Once the PU grating is fabricated, metal angle-deposition is used to coat the top and one sidewall of the PU grating. Then, a dielectric layer, such as silicon dioxide, is also deposited onto the metal layer from the same side and deposition angle. A second layer of metal (same material as the first metal, or different metal) is angle deposited onto the dielectric layer. Lastly, the low angle argon ion milling is performed to remove all three layers on top of the PU grating, leaving a sandwiched (metal/dielectric/metal) structure on one sidewall of the PU grating pattern. 
     In yet a further embodiment, a structure with layered electro-active layer for use as a smart window (where the structure is encapsulated between glass to modify the incoming light is formed. Once the PU grating is fabricated, metal angle deposition is performed for one side. Then a second metal is angle deposited from the other side. The top metal layers are removed by low angle argon ion milling or other process. Lastly, an electrically responsive material is filled into the channels of the structure. This structure can be sandwiched between panes of glass for use as a ‘smart window’. 
     In further embodiments of the present invention, both quasi-2D and actual 2D structures are formed. 
     Other aspects, objectives and advantages of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings incorporated in and forming a part of the specification illustrate several aspects of the present invention and, together with the description, serve to explain the principles of the invention. In the drawings: 
         FIGS. 1A-1F  are schematic diagrams that illustrate the steps of a two-polymer microtransfer molding process, according to an embodiment of the invention; 
         FIG. 2  is a is a schematic diagram showing the angle deposition of metals on a one-layer polyurethane grating, according to an embodiment of the invention; 
         FIGS. 3A and 3B  are schematic diagrams illustrating the argon ion milling of metals on a one-layer polyurethane grating, according to an embodiment of the invention; 
         FIG. 4  is a pictorial illustration of exemplary electrodes constructed in accordance with an embodiment of the invention; 
         FIG. 5  is a graphical representation of the percentage of light transmitted to the solar cell by wavelength for an exemplary electrode constructed in accordance with an embodiment of the invention; 
         FIGS. 6A-E  are simplified illustrations of the multi-step angle deposition process of metals on a one-layer polyurethane grating and a top view illustration of a resulting electrode structure, according to an embodiment of the invention; 
         FIGS. 7A-B  are simplified illustrations of the multi-step angle deposition process of metals on a one-layer polyurethane grating resulting in interdigitated electrodes, according to an alternate embodiment of the invention; 
         FIG. 8  is a is a pictorial illustration of exemplary interdigitated electrodes constructed in accordance with an embodiment of the invention; 
         FIGS. 9A-D  are simplified illustrations of the encapsulation of the one-layer polyurethane grating to improve optical transparency in accordance with an embodiment of the present invention; 
         FIGS. 10A-D  are simplified illustrations of an inversion of the one-layer polyurethane grating to allow fabrication of a solar cell on a transparent electrode in accordance with an embodiment of the present invention; 
         FIGS. 11A-D  are simplified illustrations of the fabrication process to produce a sandwiched metal/dielectric/metal structure on the one-layer polyurethane grating in accordance with an embodiment of the present invention; 
         FIGS. 12A-D  are simplified illustrations of the fabrication process to produce a structure with active layer filling on the one-layer polyurethane grating to enable use as a smart window in accordance with an embodiment of the present invention; 
         FIG. 13  is a schematic illustration of a smart window constructed in accordance with the process of  FIGS. 12A-D ; 
         FIG. 14  is a schematic illustration of a quasi-2D structure in accordance with an embodiment of the present invention; 
         FIG. 15  is a top view schematic illustration of a 2D structure in accordance with an embodiment of the present invention; 
         FIG. 16  is a perspective view schematic illustration of the 2D structure of  FIG. 15 ; 
         FIG. 17  is a SEM image of an embodiment of a PU one-layer grating having a lower aspect ratio structure; and 
         FIG. 18  is a schematic illustration of an embodiment of the lower angle metal deposition used to construct a lower aspect ratio structure having metal sidewalls. 
     
    
    
     While the invention will be described in connection with certain preferred embodiments, there is no intent to limit it to those embodiments. On the contrary, the intent is to cover all alternatives, modifications and equivalents as included within the spirit and scope of the invention as defined by the appended claims. 
     DETAILED DESCRIPTION OF THE INVENTION 
       FIGS. 1A-1F  are schematic diagrams that illustrate the steps of a two-polymer microtransfer molding (2-P μTM) process used in manufacturing an embodiment of the invention. Such a two-polymer microtransfer molding process is described in U.S. Pat. No. 7,625,515, entitled Fabrication of Layer-By-Layer Photonic Crystals Using Two Polymer Microtransfer Molding, to Lee et al., and assigned to the assignee of the instant application, the teachings and disclosure of which are hereby incorporated in their entireties herein by reference thereto. In at least one embodiment, the nanoscale metallic structures described herein are configured to provide plasmonic light concentration to enhance light absorption in solar cells, while also reflecting high amounts of infrared radiation. 
     As shown in  FIGS. 1A-F , the photonic structure is prepared in a multiple stage process. PDMS (polydimethylsiloxane) or other suitable elastomeric molds  30  cast from a master pattern out of a photoresist relief pattern on a silicon wafer are used in the manufacture of the photonic structures. Typically, the PDMS mold is created from a master pattern that usually only has parallel lines. However, it should be recognized that any pattern may be used for the master pattern. In one embodiment, the master pattern is made by spinning on a layer of photoresist on a silicon wafer. In some embodiments of the invention, photolithography or e-beam lithography is used to generate a multiple line pattern on the resist-covered wafer and the resist is developed, resulting in the master pattern. In an alternate embodiment, two-beam laser holography is used to is used to generate a multiple line pattern on the resist-covered wafer. The PDMS mold is obtained by pouring PDMS on the master pattern. After the elastomeric mold  30  is cured, it is peeled off of the master pattern, resulting in an elastomeric mold  30  having channels  32  reflecting the structure of the master pattern. 
     A drop of a first prepolymer  34 , such as polyurethane (PU), is placed just outside of a patterned area on a PDMS mold and dragged at a constant speed across the PDMS mold  30  with a blade  36  (see  FIG. 1A ). The blade  36  is not in contact with the PDMS mold  30 . In one embodiment, the blade  36  is a metal blade controlled by mechanical actuators. After dragging through the patterned area, the prepolymer  34  only fills in the channels without any residue (see  FIG. 1B ). This filling method is referred to as “wet-and-drag” (WAD). In one embodiment, the speed for a forward movement (i.e. wetting) is around 0.5 mm/sec. The speed for a backward movement (i.e., dewetting) is around 30 μm/sec to achieve flat meniscus of the prepolymer  34  after filling while minimizing swelling of the PDMS mode by the prepolymer  34 . Other speeds may be used. 
     The filled prepolymer  34  is partially cured for approximately four minutes so it solidifies. In one embodiment, an ultraviolet (UV) dose for the partial curing of prepolymer  34  is within the range of 0.45 to 2.7 J/cm 2 . Then, a second WAD is performed to apply a second prepolymer  38 , such as polymethacrylate, which only wets the top surface of the polyurethane prepolymer  34  but not the PDMS mold  30  (see  FIG. 1C ), resulting in a filled PDMS mold  30  (see  FIG. 1D ). In one embodiment, the speed for a forward movement is around 0.5 mm/sec. The speed for a backward movement is around 100 μm/sec to minimize swelling of the PDMS mold  30  by the prepolymer  38 . Other speeds may be used. 
     By placing a substrate  40  on the mold  30  (see  FIG. 1E ) and exposing them to UV light for approximately three hours, the filled microstructure grating of polymer bars  35  formed from prepolymer  34  and prepolymer  38  adheres to the substrate  40 . In at least one embodiment, the substrate  40  is a transparent material such as glass or sapphire. The PDMS mold  30  is then peeled away leaving a single-layer polyurethane grating structure of the polymer bars  35  on the substrate  40  (see  FIG. 1F ). 
     In an embodiment of the invention, therefore, the polymer grating structure shown in  FIG. 1F  is fabricated by the two-polymer microtransfer molding technique to form the micron or submicron scale gratings of bars  35 . For the fabrication, the visibly transparent substrate  40 , e.g. glass or sapphire, is cleaned ultrasonically in distilled water so that it is without dust and residue on the surface. In this two-polymer microtransfer molding process discussed above, two pre-polymers  34 ,  38  are used, one as the filler, and the other as the adhesive to enhance the bonding strength between the first layer and the substrate. In at least one embodiment, the filler is UV-curable polyurethane and the adhesive is polymethacrylate. 
     After the polyurethane gratings of bars  35  are fabricated, in one embodiment a thin layer of metal (e.g., 80-100 nanometers), such as gold, silver, copper, etc., is angle deposited onto the polyurethane bars  35  by thermal evaporation, as shown in the simplified schematic diagram of  FIG. 2 . Since metal deposition at the normal incidence not only coats the polyurethane bars  35  but also the exposed substrate surface  42  in between each polyurethane bar  35 , a stationary sample holder with a tilted angle, e.g., at 45 degrees (shown by arrows  44 ), is used so that the metal is only deposited on the sidewalls and the top of polyurethane bars  35 . To coat both sides of the polyurethane bars  35 , two separate angle depositions of metal film are done (each of arrows  44 ) to cover the side walls and the top surface of the bars  35 . The angle at which the deposition is done is determined by the gap between two adjacent bars  35  and the height of each of the bars  35  for the grating. In the case where the bar gap is same as the bar height as shown in  FIG. 2 , a 45 degree angle of deposition is used. For other dimensions, the angle can be adjusted accordingly such that only the sides and top of the bars  35  are coated, but not the substrate surface  42  between the bars  35 . Depositing the metal in this manner is advantageous because the space between each polyurethane bar  35  is not covered by metal and therefore remains transparent, enhancing the optical transmission of the overall structure. 
     In an embodiment of the invention, the optical transparency of the structure may be improved further when the metal layer on top  50  of the polyurethane bars  35  is removed by, e.g., argon ion milling. In an alternate embodiment of the invention, the metal layer on top  50  of the polyurethane bars  35  may be removed by reactive ion etching. In yet another embodiment of the invention, the metal layer on top  50  of the polyurethane bars  35  may be removed by argon plasma sputtering. 
     Turning specifically to  FIG. 3A  there is illustrated a schematic diagram of the argon ion milling of metal from the top  50  of the one-layer polyurethane grating, in accordance with an embodiment of the invention. In one embodiment the parameters for the argon ion milling power are 3 kV and 1 mA. In this process, the sample is positioned with the ion gun beam direction being aligned parallel to the direction of the grating so that the metal on the sides of the bars  35  is not affected by the argon ions. The ion beam is positioned at a low incoming angle, e.g., at 10 degrees (shown by arrows  46 ), so the ion beam etches the metal from the top  50  surface at a controllable rate, and so that the ion beam impacts a larger surface area. 
     In at least one embodiment, after the ion milling (or reactive ion etching, or argon plasma sputtering) has removed the top metal layer from the bars  35 , the metal on the sidewalls of the bars  35  is left intact to form metal rails  48  as shown in  FIG. 3B . The polyurethane bars  35  may be partially etched by the argon ions as well, but this does not affect the optical transparency of the resultant structure. In at least one embodiment, after removal of the metal layer on top of the polyurethane bars  35 , oxygen plasma etching or reactive ion etching is used to remove a portion of the exposed polyurethane bar  35  to improve light transmission through the polyurethane and to reduce absorption of UV by the polyurethane. 
     As may be seen in this  FIG. 3B , a plurality of parallel structures, each including a pair of parallel metal rails  48  separated by and affixed to a polyurethane bar  35  is formed by the process discussed above. In one embodiment of the invention, the plurality of parallel structures are spaced evenly, that is, at a fixed distance from adjacent parallel structures. The spacing between these parallel structures in certain embodiments may range from 0.75 micrometers to 3 micrometers. 
       FIG. 4  illustrates an exemplary embodiment of a nanoscale high-aspect-ratio metallic electrode constructed in accordance with the teachings of the present invention. The polyurethane grating structure in such an embodiment may have at least two different periodicities, 2.5 micrometers and 1 micrometer. In the specific embodiment of  FIG. 4 , the polyurethane bars  35  have a trapezoidal cross-section, wherein this trapezoidal shape replicates the master used in the fabrication process. In an embodiment having the 2.5-micrometer-periodicity structure, the polyurethane bar  35  height from the substrate surface  40  is approximately 1.25 micrometers, and the top and bottom widths are 0.85 micrometer and 1.35 micrometers, respectively. In the illustrated embodiment, the base angle for this 2.5-micrometer version of the polyurethane bars  35  is about 12 degrees. In an embodiment having 1-micrometer-periodicity gratings, the polyurethane bar  35  height from the substrate surface  42  is approximately 570 nm, and the top and bottom widths are 330 nm and 580 nm, respectively. In such an embodiment, the base angle for this 1-micrometer version of the polyurethane bars  35  is about 15 degrees. 
     In each of these exemplary embodiments, the metal rails  48  formed from a metal such as gold, silver, copper, etc., have heights estimated to be the same as that of PU bars  35  (approximately 1.2 μm). The thickness of the metal rail  48  formed as discussed above is approximately 70 nm. As such, the metal rails  48  are effectively nanowires with a high 17:1 aspect ratio. Since a metal such as gold was deposited on both sidewalls of the bars  35 , the periodicities of the gold nanowire patterns (metal rails  48 ) are reduced by half to around 1.2-1.3 μm. 
       FIG. 5  is a graphical representation of the percentage of light transmitted to a solar cell by wavelength for an exemplary electrode constructed in accordance with an embodiment of the invention. The range of wavelengths along the x-axis of the graph corresponds to wavelengths for visible light. The graph shows the percentage of light transmission for polyurethane bars  35  (see  FIG. 4 ) spaced at 2.5 micrometers having with 100 nm-thick gold rails  48  shown by trace  52  or 100 nm-thick copper rails  48  shown by trace  54  on a glass substrate  40 . As can be seen from the graph, the percentage of light transmitted through the polyurethane grating is always greater than 60%, but transmission rates approaching 80% are also achievable. 
     When using the metal deposition method discussed above, a metal film  60  may also be deposited on the substrate  40  outside of or around the grating structure of the bars  35 . This may be seen from an inspection of  FIGS. 6A-E , which illustrate the metal deposition process illustrated briefly in  FIG. 2  in a step-by-step fashion, including a top view illustration of the resulting structure in  FIG. 6E  (scale exaggerated to allow better understanding). 
     As shown in  FIG. 6A , the grating structure of bars  35  on a substrate  40  ready for metal deposition is shown in an end view.  FIG. 6B  illustrates the angled deposition (arrow  44 ) of metal on the grating structure. In this first angled deposition, metal  60  is deposited on a leading portion of the substrate  40  before the first bar  35  (the left side of  FIG. 6B ), on one side of bars  35 , on the top  50  of the bars  35 , and beyond the last bar  35  of the grating (shown by metal  60  on the right side of  FIG. 6B ). Due to the angled deposition (arrow  44 ), no metal is deposited on the substrate surface  42  between the bars  35 . 
     As shown in  FIG. 6C , the second angled deposition (arrow  44 ) of metal on the grating structure is performed. In this second angled deposition, metal  60  is deposited on a leading portion (right side of  FIG. 6C ) of the substrate  40  before the first bar  35  (viewed from arrow  44 ), on the other side of bars  35 , again on the top  50  of the bars  35 , and beyond the last bar  35  of the grating (shown by metal  60  on the left side of  FIG. 6C ). Due to the angled deposition (arrow  44 ), no metal is deposited on the substrate surface  42  between the bars  35  during this second deposition step. 
       FIG. 6D  shows the grating structure after the step of ion milling or etching has taken place to remove the metal from the top  50  (see  FIGS. 6B-C ) of the bars  35 . As discussed above, this operation leaves the metal rails  48  attached to the sides of the bars  35 . It also leaves the metal  60  on the substrate  40  around the bars  35 . As shown from the top view illustration of  FIG. 6E , in embodiments wherein the bars  35  do not extend to the edge of the substrate, the metal deposition steps of  FIGS. 6B-C  also deposits metal  60  on the substrate at either end of the bars  35 . 
     In the embodiment shown in  FIG. 6E , the substrate extends beyond the bars  35  on all sides and is coated with metal  60 . In such an embodiment, this metal  60  may serve as an electrical connection point when the structure is used as an electrode. This is possible because there is no electrical isolation between the vertical metal rails  48  deposited on each side of each of the PU bars  35 . In other words, in the illustrated embodiment the metal  60  is electrically coupled to each metal rail  48 . 
     However, in an alternate embodiment of the present invention, a modified deposition scheme such as that illustrated in  FIGS. 7A-B , can provide electrical isolation between the two alternate vertical metal rails  48  on each bar  35 . As with the above described embodiment, a 1-D grating on a substrate  40  provides the basic structure. During the first angle deposition (arrow  44   − ) shown in  FIG. 7A , the lower part of the edge of the bars  35  and the substrate  40  are covered by a mask  62 . During this first deposition, the right side wall of the bars  35  not covered by the mask  62  are covered with the metal film  48   −  as is the unmasked portion of the substrate  60   −  beyond the top end of the bars  35  as oriented in  FIG. 7A . It is noted that the metal film on top of the grating is not shown to better illustrate the side wall structure (the top film is removed after all the depositions as discuss above). 
     The second angle deposition (arrow  44   + ) is performed with the top edge of the grating structure and the substrate previously coated with metal  60   −  covered by a mask  62  as shown in  FIG. 7B . During this second deposition, the left side wall of the bars  35  not covered by the mask  62  are covered with the metal film  48   +  as is the unmasked portion of the substrate  60   +  beyond the lower end of the bars  35  as oriented in  FIG. 7B . It is noted that the metal film on top of the grating is not shown to better illustrate the side wall structure (the top film is removed after all the depositions as discuss above). 
     After removing the top metal film with ion milling or etching, the resulting structure will look like that shown in  FIG. 8 . As may be seen, there is no electrical connection between the metal rails  48   − ,  48   +  on either side of each bar  35 . However, there is an electrical connection through the metal  60   − ,  60   +  on the top and bottom on the substrate  40  (as oriented in  FIG. 8 ) to each metal rail  48   − ,  48   + , respectively. This allows the metal  60   − ,  60   +  to be used as electrodes for separate electrical connection. The alternate fingers formed by rails  48   − ,  48   +  are isolated from each other and can be used as interdigitated electrodes for appropriate applications. 
     In one embodiment, the volume ( 42 ) between the interdigitated electrodes ( 48   − ,  48   + ) is filled with a material responsive to an applied field. In such an embodiment, the structure can be switched at will via a bias applied to the material within the structure by the interdigitated electrodes ( 48   − ,  48   + ). Such electrically active materials include liquid crystals phases, which can include a number of different morphologies and can be low melting inorganic phases or aromatic organics such as para-Azoxyanisole (PAA). In addition to liquid crystals, piezoelectric materials, photovoltaic materials, photo-luminescent materials, and organic (and inorganic) light emitting materials may also be used in further embodiments. Also, nonlinear materials could be used in other embodiments, but they are not always necessary for interdigitating. 
     Turning now to  FIGS. 9A-D , there are illustrated process steps that provide an encapsulation of the one-layer polyurethane grating to improve its optical transparency and its transparency at high angles, e.g. &gt;50°. Specifically, a PU grating of polyeurthane bars  35  is made by placing excess PU liquid prepolymer on a glass substrate  40 . Using a PDMS mold  30  (see  FIG. 1 ) with grating patterns or channels  32 , a direct stamping process is performed to transfer the pattern to the PU. After UV curing the PU is solidified, and the PDMS mold is removed. Because this process uses excess PU liquid prepolymer, there is an underlayer  64  of PU between the PU grating pattern (bars  35 ) and the glass substrate  40  as shown in  FIG. 9A . 
     Next, as shown in  FIG. 9B , a conformal coating  66  of metal is carried out by a sputtering process as illustrated by arrows  65 . As shown in  FIG. 9C , argon plasma etching illustrated by arrows  68  is performed to remove the metal on the top of PU bars  35  as well and in the channels of exposed substrate surface  42  in between each polyurethane bar  35 . The etching process is highly anisotropic so the metal sidewalls forming the vertical metal rails  48  are intact. Finally, as illustrated in  FIG. 9D , a drop of PU liquid prepolymer  72  is placed on top of the etched structure of  FIG. 9C  and UV cured, and a second glass substrate  70  is placed on top to encapsulate the entire structure. The additional PU liquid prepolymer  72  fills in the air channels between the metal sidewalls forming the vertical metal rails  48  and also forms a layer over the entire structure to reduce the diffraction effect from the grating pattern. 
     This technique is particularly well suited for application to large area samples (2 in ×2 in, or bigger, 6 in ×6 in, etc.) though it can also be used for smaller areas as well. The 2P-μTM technique of  FIG. 1  may also be used for large area samples (without PU underlayer  64 ), although the process would be slightly modified. Different periodicities and height of PU bars  35  can be made with both techniques. 
     Turning now to  FIGS. 10A-D , there are illustrated simplified process diagrams for constructing an inverted structure to facilitate fabrication of a solar cell or other device on the back-side of the one-layer polyurethane grating structure. Specifically, as illustrated in  FIG. 10A , the PU grating of bars  35  is fabricated on a water-soluble sacrificial layer  74  coated glass substrate  40 . After metal deposition and argon ion milling as described above, a small droplet of PU prepolymer  72  is placed on the sample to fill in the trenches of the grating structure between the polyurethane bars  35  and the vertical metal rails  48  as shown in  FIG. 10B . The PU prepolymer  72  also serves to glue a second glass substrate  70  onto the sample. 
     After the PU prepolymer  72  filling is ultraviolet cured and solidified, the sample is submerged in distilled water to dissolve the sacrificial layer  74 , and the original glass substrate  40  is detached as shown in  FIG. 10C . Upon the separation of the original glass substrate  40 , the bottom part of the structure is exposed and the sample is inverted with respect to the original structure. As shown in  FIG. 10D , the active materials of a solar cell and the other electrode (collectively illustrated as  76 ) can then be fabricated on this transparent electrode substrate. 
     Turning now to  FIGS. 11A-D , there are illustrated a method to fabricate a sandwich structure of multiple layered electrodes where an active layer is sandwiched between two conductive layers (metal/dielectric/metal structure) on the one-layer polyurethane grating in accordance with an embodiment of the present invention. Specifically, as illustrated in  FIG. 11A , metal angle-deposition represented by arrows  78  is used to coat the top and one sidewall of the PU grating polyurethane bars  35  with a first metal layer  80 . Then, as shown in  FIG. 11B , a dielectric layer  84 , such as silicon dioxide, is also angle deposited as illustrated by arrows  82  onto the metal layer  80  from the same side and deposition angle. As illustrated in  FIG. 11C , a second layer  88  of metal (same material as the first metal layer  80 , or a different metal) is angle deposited as shown by arrows  86  onto the dielectric layer  84 . As shown in  FIG. 11D , the low angle argon ion milling illustrated by arrows  90  is performed to remove all three layers  80 ,  84 ,  88  on top of the PU grating bars  35 , leaving a sandwiched (metal/dielectric/metal) structure on one sidewall of the PU grating pattern bars  35 . 
       FIGS. 12A-D  illustrate the fabrication process to produce a structure with active layer filling on the one-layer polyurethane grating to enable use as a smart window in accordance with an embodiment of the present invention. First, as shown in  FIG. 12A  metal angle deposition illustrated by arrows  92  is performed to deposit a first metal layer  94  on one side of the PU bars  35 . Second, as shown in  FIG. 12B  a second metal layer  98  is angle deposited as illustrated by arrows  96  from the other side of the PU bars  35 .  FIG. 12C  illustrates that the two metal layers deposited on the top of bars  35  are removed by low angle argon ion milling shown by arrows  100  or other process. As shown in  FIG. 12D , an electrically responsive material  102  (such as that discussed above with regard to  FIG. 8 ) is filled into the channels of the structure between bars  35 . As shown in  FIG. 13 , this structure can be sandwiched between panes of glass  40 ,  104  for use as a smart window to modify the incoming light. 
     To realize high IR reflection in both polarizations, the quasi-2D structure of  FIG. 14  was constructed. As may be seen, this quasi-2D structure is fabricated by including a 1D grating structure  106 ,  110  on each of two sides of a substrate  108 . Because glass substrates have some absorption in the mid-IR range, a 400 μm sapphire is used in a preferred embodiment as the substrate  108 . In this structure, two one-layer PU grating structures  106 ,  110  were fabricated on both sides of the substrate  108 , with the one-layer PU grating structures  106 ,  110  aligned orthogonally to each other at around 90°. 
     In one embodiment, the periodicity is approximately 2.5 μm, the width is approximately 1.2 μm, and the height is approximately 1.2 μm. Silver was deposited using the angle evaporation technique to coat the sidewalls as well as the top of the PU bars, and the metal on the PU top surface is removed by the argon ion milling as discussed above. 
     When a white light source was passed through the structure of  FIG. 14 , the transmitted light forms a 2D diffraction pattern. The average reflection intensity of both polarizations is about 80%, which shows that this quasi-2D structure is suitable to be used as hot mirrors in IR reflecting applications. 
     In another embodiment of a layer by layer (LBL) structure that provides high IR reflection in both polarizations, as illustrated in  FIGS. 15 and 16 , an actual 2D structure (rather than the quasi-2D structure of  FIG. 14 ) is shown. In one embodiment, the structure of  FIGS. 15 and 16  is fabricated with a different method using a top-down process such as reactive ion etching (RIE) to remove the metal on the PU top surface without removing the PU. This 2D structure has high reflection in the infrared range in both polarizations, and is a very good spectral reflector in at least the mid-IR range. In another embodiment, the periodicity is reduced to approximately 1 μm in order to use such structure for near-IR reflection. 
     As illustrated in  FIG. 17 , an alternative embodiment of the present invention utilizes a low aspect ratio structure. This embodiment utilizes the same periodicity as some to the other embodiments, but at a smaller height. In the illustrated embodiment a photoresist master with approximately 2.5 μm periodicity, approximately 1.2 μm width, and approximately 300 nm height was first fabricated on a silicon wafer. PDMS molds were made using the master, and 2P-μTM was used to make one-layer PU grating structures on a glass or sapphire substrates. The scanning electron microscope (SEM) images of the PU grating structure of  FIG. 17  show the periodicity is around 2400 nm and the height is around 300 nm. The total area is at 4×4 mm 2 . 
     In one embodiment as illustrated in  FIG. 18 , the structure has a higher transmission than that of  FIG. 17  with metal deposited on the PU sidewalls. Since the height of the structure is decreased to approximately 300 nm while the width is the same as some previous embodiments at approximately 1.2 μm, the aspect ratio of the PU bars is also changed from 1:1 to 1:4. If the angle of the metal evaporation were still kept at 45° as discussed above, the channels between adjacent PU bars would be coated with metal. This would greatly reduce the optical transmission since the additional metal coating in the channels could block additional light transmitted through the structure. As such, in one embodiment when the deposition angle was approximately 14° with respect to the sample surface, the metal was deposited only on the sidewalls and top of the PU bars as shown in  FIG. 18 . The samples were then ion milled after the metal deposition as discussed above. 
     All references, including publications, patent applications, and patents cited herein are hereby incorporated by reference to the same extent as if each reference were individually and specifically indicated to be incorporated by reference and were set forth in its entirety herein. 
     The use of the terms “a” and “an” and “the” and similar referents in the context of describing the invention (especially in the context of the following claims) is to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms “comprising,” “having,” “including,” and “containing” are to be construed as open-ended terms (i.e., meaning “including, but not limited to,”) unless otherwise noted. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention. 
     Preferred embodiments of this invention are described herein, including the best mode known to the inventors for carrying out the invention. Variations of those preferred embodiments may become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventors expect skilled artisans to employ such variations as appropriate, and the inventors intend for the invention to be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context.