Patent Publication Number: US-2022216352-A1

Title: Nanostructures to reduce optical losses

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. § 119(a) on Patent Application No. 20210100005, filed in Greece on Jan. 4, 2021, the entire disclosure of which is incorporated herein by reference. 
     TECHNICAL FIELD 
     The present disclosure is directed to reducing reflective light losses on the top surface of solar cells. 
     BACKGROUND 
     Optical losses in solar cells have a significant impact on the electrical performance of solar cells. Top surface reflective optical losses have a remarkable impact on the absorption spectrum of the solar cell and as a result on the photo generated current output. Reducing reflective losses in planar solar cells can be achieved through chemical top surface etching. However, chemical etching is challenging for some materials used in solar cell, such as solution processed materials. Physical texturing of the top surface has been proposed as a solution. However, physical texturing may be uneconomical and not scalable. 
     SUMMARY 
     An embodiment disclosed in examples herein provides a method for fabricating sloped nanostructures on a surface of a substrate. The method includes placing a mask over the substrate, placing substrate with the mask in a deposition chamber, vaporizing material for deposition, and allowing clogging of openings in the mask during the deposition to form sloped shapes. 
     Another embodiment described in examples herein provides a solar cell. The solar cell includes an antireflective coating including sloped nanostructures formed in a vapor deposition process. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIGS. 1A and 1B  are schematic drawings of light reflecting from solar cells that are uncoated and coated with the sloped nanostructures described herein. 
         FIG. 2  is a schematic drawing of light transmitted through a structural coating of sloped shapes to the surface of an underlying solar cell. 
         FIG. 3  is a schematic drawing of light transmitted through a structural coating of sloped shapes wherein each of the shapes overlaps to form a base layer over the solar cell. 
         FIGS. 4A and 4B  are schematic diagrams of projected evaporation onto a substrate to form a sloped nanostructures. 
         FIG. 5  is a drawing of a mask that can be used to form pyramidal shapes on the substrate. 
         FIG. 6  is a drawing of a mask that can be used to form conical shapes on a substrate. 
         FIG. 7  is a process flow diagram of a method that can be used to form sloped nanostructures on a surface of a substrate. 
     
    
    
     DETAILED DESCRIPTION 
     As described herein, some types of solar cells are difficult to form with textured surfaces, such as solution processed solar cells. Embodiments described in examples herein provide a technique of fabricating three dimensional structures, termed sloped nanostructures, over a top surface of a solar cell to increase internal reflections and, thus, total absorption by the solar cell. A highly transparent and sloped three dimensional structure is patterned on the top surface of planar solar cells through projected physical evaporation. 
       FIGS. 1A and 1B  are schematic drawings of light reflecting from solar cells that are uncoated and coated with the sloped nanostructures described herein.  FIG. 1A  shows a solar cell  102 , for example, formed by a solution process technique, which does not have the antireflection structures described herein. An incident light ray  104  hits the surface of the solar cell  102 , and a portion of the photons are reflected away in a reflected light ray  106 . In the schematic drawings of  FIGS. 1A and 1B , the relative length of the reflected light ray  106  to the incident light ray  104  are used to provide a general indication of the number of photons that are reflected from the surface of the solar cells  102  and  108 . 
       FIG. 1B  shows a solar cell  108  that has the antireflection structures described herein formed on the surface. In this example, an incident light ray  110  is reflected from the surface of the solar cell  108 , forming a reflected light ray  112  that includes fewer photons than the incident light ray  106  impacting the smooth surface. 
       FIG. 2  is a schematic drawing of light transmitted through a structural coating  202  of sloped shapes to the surface  204  of an underlying solar cell  206 . Light projected on the solar cell is mostly transported through the structural coating  202  to the active layers of the solar cell  206 . The structure coating  202  is selected to be highly transparent material that has the same or higher transmission spectrum of the material forming the surface  204  of the solar cell  206 . Further, the transparent material may be selected to have a refractive index similar to the surface  204  of the solar cell  206 , and a transmittance spectrum similar or higher than the materials used to encapsulate the solar cell, which is often glass. In some embodiments, the evaporated material used for the structural coating  202  is silicon nitride. Silicon nitride is a highly transparent material that matches the transmittance spectrum of glass, which is often used as a top surface in the encapsulation of solution processed solar cells, e.g., over the structure coating  202 . 
     The structural coating  202  increases the total internal reflectance over the surface  204 , reducing interlayer refractive reflection losses. For example, a light ray  208  impinging on 1 of the structures is transmitted  210  towards the surface  204  of the solar cell  206 . In some cases, part of the light ray may be reflected  212  back from the surface  204 , for example, due to the incident angle or the refractive index mismatch of the different layers of the solar cell  206 . Therefore, it is highly advisable to match refractive index of the structural coating  202  to the solar cell  206 . The angled surface of the structure allows the reflected  212  light to impinge on the surface of the structure at a low angle causing an internal reflection  214 . The light ray reflected off the surface by internal reflection  214  impinges on another surface of the structure and is directed towards the surface by a second internal reflection  216 , and is then absorbed by the solar cell  206 . As another example, a second light ray  218  impinges on the surface of a structure at an oblique angle. A portion  220  of the light ray  218  is directed towards the surface  204  of the solar cell  206  where it is absorbed. Another portion  222  of the light ray  218  is reflected off the surface of the structure, impinging on a second structure. At the second structure, the portion  222  of the light ray  218  is transmitted  224  towards the surface  204  of the solar cell  206 , where it is absorbed. Accordingly, the deposition of the nanostructures reduces the light reflecting from the surface  204  of the solar cell  206 . As described with respect to  FIGS. 5 and 6  the structures may be conical pyramids or square pyramids, depending on the mask chosen to form the structures. 
     As the antireflection properties of the structural coating  202  is based on total internal reflectance, the properties may be adjusted by changing the angle  226  of the structures. This may be used, for example, to adjust the efficiency of the structural coating  202  to match the expected range of angles of light impinging on the solar cell  206 . 
       FIG. 3  is a schematic drawing of light transmitted through a structural coating of sloped shapes wherein each of the shapes overlaps to form a base layer  302  over the solar cell. Like numbered items are as described with respect to  FIG. 2 . As described herein, the process to form the shapes may be adjusted, for example, depending on such factors as the expected range of angles for the radiation to the solar cell, the materials of the solar cell, and the like. In the example of  FIG. 3 , each of the shapes overlap, creating a base layer  302  that is offset from the surface  204  of the solar cell  206 . The height  304  of the base layer  302  may be adjusted by the amount of overlap. Increasing the height  304  of the base layer  302  may be used to adjust the amount of total internal reflectance, for example, based on the expected range of angles for the radiation impinging on the solar cell. 
       FIGS. 4A and 4B  are schematic diagrams of projected evaporation onto a substrate  402  to form a sloped nanostructures  404 . As described herein the substrate  402  in some embodiments the substrate is a solar cell. In the projected evaporation process, material is evaporated from an evaporation source  406 . The evaporated material  408  passes through a mask  410  that comprises nanoscale openings  412 . The evaporated material  408  is directed at a deposition spot  414  that is directly below one of the nanoscale openings  412 , may be about the same size as the nanoscale opening  412  at the beginning of the process, decreasing as the nozzle clogs. The mask  410  is positioned at a distance  416  from the deposition spot  414 , which results in uneven deposition of the evaporated material  408 , e.g., higher at the center and lower at the sides of the deposition spot  414 . 
     The evaporated material  408  is also deposited on the mask  410  and at the edges of the nanoscale openings  412  in the mask  410 . Together with the migration effect the deposition of material at the edges of the nanoscale openings  412  slowly clogs the nanoscale openings  412  of the mask  410 . As the evaporative deposition proceeds, the clogging increases, which reduces the size of the nanoscale openings  412  in the mask  410 , and thus reduces the size of the deposition spot  414 . The resulting evaporated structure is a sloped nanostructure, such as a conical shape or a pyramidal shape, among others. 
     The distance  418  between the evaporation source  406  and the mask  410 , and the size of the evaporation source  406  determines the angle  226  ( FIG. 2 ) of the wall and the size of the base of the sloped nanostructure  404 . For example, as the distance  418  between the evaporation source  406  and the mask  410  is decreased, the base of the sloped nanostructure  404  and the angle  226  are increased, e.g., the sloped nanostructure  404  has less steep walls. The distance  418  is tuned to control the parameters of the sloped nanostructure  404 , for example, so that the angle  226  of the wall of the sloped nanostructure  404  is not large. The distance  420  between the nanoscale openings  412  on the mask  410  is generally selected to be equal to or less than the nanoscale openings  412 , for example, less than about 500 nm to ensure full surface deposition. This may be used to prevent gaps between sloped nanostructures  404 . However, the distance may be increased to change the shape of the sloped nanostructures  404 . A larger distance  416  increases the width of the area of projected evaporation and the size of the base of the sloped structure  414 . The evaporation rate, the distance  416  between the mask  410  and the deposition spot  414 , and the size of the nanoscale openings  412  in the mask  410  determines the height of the sloped nanostructure  404 . In some embodiments, the distance  416  is between about 1 μm and about 2 μm. This distance  416  promotes the formation of a thin structural coating  202  ( FIG. 2 ) and is far enough to protect the tip of the sloped nanostructure  404  from being damaged by the mask  410 , for example, as the mask  410  is removed. 
     The mask opening  412  could be made smaller or larger depending on the desired base area  414  of the sloped structure  404  and its height. In various embodiments, the openings are between about 100 nm and about 10 μm. In some embodiments, the size of the nanoscale openings  412  is about 500 nm. A narrow opening results in a short and small sloped structures, and a wide opening results in a large sloped structures. 
     The use of the clogging affect to form the sloped nanostructures is shown in  FIG. 4B . To promote the start of the clogging, in some embodiments, the design of the mask  410  includes edges  422  of the nanoscale openings  412  that are higher than the rest of the mask  410 . Creating tip-shaped corners at the nanoscale openings  412  promotes migration and the formation of clogging  424 . The clogging  424  is used to create the sloped nanostructures  404 . 
       FIG. 5  is a drawing of a mask  500  that can be used to form pyramidal shapes on the substrate. The mask  500  has square nanoscale openings  502 , which, as the clogging  424  ( FIG. 4 ) proceeds, creates sloped nanostructures that have a pyramidal shape. 
       FIG. 6  is a drawing of a mask  600  that can be used to form conical shapes on a substrate. The mask  600  has round nanoscale openings  602 , which, as the clogging  424  ( FIG. 4 ) proceeds, creates sloped nanostructures that have a conical shape. 
       FIG. 7  is a process flow diagram of a method  700  that can be used to form sloped nanostructures on a surface of a substrate. As described herein, in some embodiments, the sloped nanostructures are formed on the top surface of a solar cell to provide total internal reflections that decrease the amount of light reflecting off the top surface without being absorbed. In these embodiments, the tips face outward from the surface of the solar cell. In another embodiments, the sloped nanostructures are formed behind light sources used in a liquid crystal display panel to increase the amount of light from the light sources that is directed to the panel. In these embodiments, the tips face outward from the light sources to increase total internal reflection towards the liquid crystal panel. 
     The method begins at block  702 , with a determination of the separation of the mask and substrate. As described herein, the separation may be used to control the shape of the sloped nanostructures, including the angles between the walls, the widths of the base, and the overlapped between adjacent sloped nanostructures. 
     At block  704 , a determination is made of the separation of the mask in the evaporative source. As described herein, the separation between the mask and source also provides control over the shape of the sloped nanostructures. 
     At block  706 , the mask is placed over the substrate. As described herein, in some embodiments, the substrate is a solution process solar cell. In some embodiments, the mask is held apart from the substrate by an open frame with a thickness corresponding to the separation between the mask in the substrate. 
     At block  708 , the substrate with the mask is placed in a deposition chamber. The deposition chamber is generally a high vacuum chamber with a heated source providing the vapor for deposition. In some embodiments described herein, the separation between the source and the mask is adjustable, for example, to control the shapes of the sloped nanostructures. 
     At block  710 , the material is vaporized for deposition. This may be performed by heating a filament behind the material, or using other techniques to heed the material to vaporize it in the vacuum of the deposition chamber. 
     At block  712 , clogging of the mask is allowed during the deposition to form the sloped shapes. In contrast to other vapor deposition processes, the deposition may be allowed to proceed for a longer period of time, for example, at a lower rate. The resulting clogging leads to the formation of the sloped nanostructures, as described herein. 
     At block  714 , the mask and substrate are removed from the deposition chamber, for example, after the chamber is allowed to cool and the vacuum is released. In some embodiments, a continuous roll of material is treated in a deposition chamber. 
     An embodiment disclosed in examples herein provides a method for fabricating sloped nanostructures on a surface of a substrate. The method includes placing a mask over the substrate, placing substrate with the mask in a deposition chamber, vaporizing material for deposition, and allowing clogging of openings in the mask during the deposition to formed sloped shapes. 
     In an aspect, the method includes selecting a separation of the mask and the substrate to control a slope of the sloped nanostructures. In an aspect, the method includes separating the mask and the substrate by about 1 (micrometer) μm to about 2 μm. in an aspect, the method includes separating the mask and the substrate by about 0.5 μm to about 1 μm. 
     In an aspect, the method includes forming tips at edges of the openings in the mask to enhance clogging during deposition. In an aspect, the mask includes round openings to form conical nanostructures. In an aspect, the mask includes square and triangular openings to form pyramidal nanostructures. In an aspect, the mask includes slits to form ridges. 
     In an aspect, the method includes vaporizing silicon nitride to form the sloped nanostructures. 
     In an aspect, a solar cell is the substrate. In an aspect, a back panel of a liquid crystal display is the substrate. 
     Another embodiment described in examples herein provides a solar cell. The solar cell includes an antireflective coating including sloped nanostructures formed in a vapor deposition process. 
     In an aspect, the sloped nanostructures include conical nanostructures. In an aspect, the sloped nanostructures include pyramidal nanostructures. In an aspect, the sloped nanostructures include ridges. 
     In an aspect, the sloped nanostructures include an angle of less than 45° along a cross-section through a tip of the sloped nanostructures. In an aspect, the sloped nanostructures include an angle of greater than 45° along a cross-section through a tip of the sloped nanostructures. 
     In an aspect, the sloped nanostructures are disposed at about 500 nm of separation. In an aspect, the sloped nanostructures are between about 200 nm and about 450 nm in height. In an aspect, the sloped nanostructures are between about 550 nm and about 750 nm in height. In an aspect, the sloped nanostructures overlap to form a base layer over the solar cell. In an aspect, the sloped nanostructures include silicon nitride. 
     Other implementations are also within the scope of the following claims.