Abstract:
The present invention provides an encapsulant material with a modified index of refraction for increasing the acceptance angle of a concentrated photovoltaic system. The encapsulant material may include filler material of a higher index of refraction than the encapsulant. The filler material may be particulates that are smaller than the wavelength of light converted to electricity by a solar cell.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    A concentrating photovoltaic (CPV) is a solar energy device which utilizes one or more optical elements to concentrate incoming light onto a solar cell. This concentrated light, which may exhibit a power per unit area of 500 or more suns, requires an optical system that can withstand such intensity over an operational lifetime and efficiently deliver this light to a solar cell. The optical elements in a concentrated solar energy device are integral components of the device and require optimization in order to utilize the maximum amount of available solar radiation. In some CPV systems, an optical component such as a non-imaging concentrator may be utilized to assist in transmitting concentrated light to the solar cell. Often an encapsulant material is used to reduce transmission losses that can occur at the interface between the non-imaging concentrator, or other optical component, and solar cell. 
         [0002]    Limitations of available materials having the necessary optical properties and durability to withstand the intense conditions of a CPV may constrain the overall performance that can be achieved by a system. In particular, a mismatch of refractive indices between materials, such as a non-imaging concentrator and solar cell, can result in undesirable optical losses. Thus, there exists a need for an improved optical system for use in a solar concentrator in order to minimize differences in refractive index of the light path to a solar cell. Such a system may improve the acceptance angle of rays entering the system and thereby enable greater electrical energy to be produced from a CPV system. 
       SUMMARY OF THE INVENTION 
       [0003]    The present invention is directed to a concentrated photovoltaic (CPV) system in which the refractive index of an encapsulant material is modified to approach or substantially match that of an optical element to which it is coupled. The encapsulant may also be coupled to a solar cell that converts solar radiation into usable electrical energy. In one embodiment the encapsulant may be silicone combined with a filler material. The filler material may have a higher index of refraction than silicone, for example titania particles, resulting in a composite encapsulant, or “effective medium,” with higher index of refraction than silicone alone. In one aspect, the particle size of the filler material may be smaller than the least wavelength of light converted into energy by the solar cell (e.g., 200 nm diameter particles). In another aspect, the amount of filler may be adjusted, such as to approximately 20% of the encapsulant volume, to achieve the desired composite refractive index. 
         [0004]    The invention also provides a method for increasing the acceptance angle of a CPV system which includes applying an encapsulant with a refractive index that closely matches that of an optical element, such as a non-imaging concentrator, and coupling that to a solar cell. The encapsulant may be prepared by mixing the encapsulant with a filler material of a different refractive index. The refractive index of the filler/encapsulant system may then be altered—typically raised—to approximate the refractive index of the non-imaging concentrator. In one embodiment, the encapsulant may be combined with titania (e.g., TiO 2 ) spheres, and the spheres may be smaller than the wavelength of light that is converted by the solar cell. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0005]      FIG. 1  shows a schematic view of a current optical system for a CPV showing an optical device and an encapsulant with unmatched refractive indices. 
           [0006]      FIG. 2  shows a schematic view of an ideal optical system having matched refractive indices. 
           [0007]      FIG. 3  shows a schematic view of an embodiment of the present invention, showing an optical system incorporating an encapsulant with filler particles. 
           [0008]      FIG. 4  shows a graph of the calculated percent transmission at the interface of an optical element and an encapsulant for a variety of optical systems. 
       
    
    
     DETAILED DESCRIPTION 
       [0009]    The present invention provides an optical system in which an encapsulant is modified to have a refractive index that approaches or substantially matches that of an optical component to which it is coupled. The present invention may be used in conjunction with a CPV system in order to increase the acceptance angle of light reaching the solar cell. Thus a CPV system of this invention may provide usable electrical energy from solar radiation collected at broader range of angles than a CPV system with a narrower range of acceptance angles. This may result in increased performance of the CPV system, as more light is converted into usable electrical energy. In one embodiment a filler material may be used to modify the index of refraction of the encapsulant. 
         [0010]      FIG. 1  is a schematic diagram of an exemplary optical system  100  used in the art to provide electrical energy from solar radiation. In optical system  100 , an encapsulant  120  serves as an interface between an optical element (e.g., non-imaging concentrator)  110  and a solar cell  130 , which is generally assembled in a leadframe package  140 . Solar cell  130  may comprise a III-V solar cell, a II-VI solar cell, a silicon solar cell, or any other type of solar cell that is or becomes known. Solar cell  130  may comprise any number of active, dielectric and metallization layers, and may be fabricated using any suitable methods that are or become known. 
         [0011]    In  FIG. 1 , an optical device in the form of an optical element  110 , which may be, for example, a lightguide or a truncated prism, is shown. Note that while a truncated prism is depicted in this disclosure, other optical elements such as a refractive element or Fresnel lens may be substituted for optical element  110  while still remaining within the scope of this invention. Optical element  110  enables incoming solar radiation to be delivered to solar cell  130  from a range of incident angles represented by rays  150  and  160 . Materials for encapsulant  120  which are deemed suitable for CPV applications are typically limited to silicones having a narrow range of refractive indices near 1.4. Optical element  110  typically requires glass, such as BK7 with a refractive index of 1.52, to meet the desired design parameters such as UV shielding and cost. Because of this transition from a higher refraction index material (e.g., BK7 glass) to a lower index material (e.g., silicone) at the interface between optical element  110  and encapsulant  120 , a total internal reflection (TIR) condition may occur for a portion of incoming light rays. As demonstrated by ray  160 , incoming light having an angle of incidence (measured from vertical) greater than the critical TIR angle at the bottom surface of optical element  110  will be reflected back into optical element  110  rather than being refracted through encapsulant  120 . For encapsulant thicknesses that are substantially greater than the wavelength of light, this TIR cutoff serves to limit the potential acceptance angle which could otherwise be achieved. Rays reaching the glass/encapsulant interface that are at angles larger than the TIR limit will be rejected. Consequently, a portion of usable solar energy will not be converted to electrical energy by solar cell  130  due to the mismatch of refractive indices between optical element  110  and encapsulant  120 . 
         [0012]    In contrast to  FIG. 1 ,  FIG. 2  depicts a cross sectional view of an ideal optical system  100 . In  FIG. 2 , the refractive indices of optical element  110  and encapsulant  120  are matched, as represented by the identical shading of the two components. For example, optical element  110  may be silicone or any optically transparent material, and encapsulant  120  may be silicone or other polymers whose index of refraction matches that of the optical element  110 . Because the refraction indices of optical element  110  and encapsulant  120  match, ray  160  is transmitted through encapsulant  120  rather than being completely reflected as in  FIG. 1 . Solar radiation incident on the optical element  110  at a wide range of angles may be directed to the solar cell  130 . 
         [0013]    Because limited material choices exist in the industry to address the issues described in relation to  FIG. 1 , and because new materials to approach the ideal situation of  FIG. 2  are not readily available, the present invention beneficially modifies encapsulant  120  to have a refractive index more closely matching that of the optical element  110 .  FIG. 3  depicts a cross sectional view of an exemplary optical system  100  of this invention which includes a filler material  125  in the encapsulant  120 . In one embodiment the filler material  125  may be a material with a higher index of refraction than the encapsulant  120 . The combined filler/encapsulant system may have a combined index of refraction which reduces the difference or substantially matches that of an optical element  110  or any optical device. In one embodiment, the filler material may be any dielectric material such as oxides (e.g., silica, zirconia, tantala or titania), nitrides (e.g., silicon nitride, alumina nitride), carbides (e.g., silicon carbide, diamond), or silicates (e.g., zircon) which when combined with the encapsulant material modifies the index of refraction of the system. In one embodiment the choice and amount of filler material may modify the refractive index of an encapsulant to within 5% of the refractive index of an optical element. In another embodiment the choice and amount of filler material may modify the refractive index of an encapsulant to within 1% of the refractive index of an optical element. In a particular embodiment, the filler material may TiO 2  spheres which have an index of refraction of 2.25. 
         [0014]    In one aspect of designing a combined filler/encapsulant, or effective medium, with the desired performance characteristics, the particle size of filler material  125  may be considered. The particle size of filler material  125  may be chosen to be smaller than the shortest wavelength of light converted by the solar cell in a CPV system in order to reduce significant transmission loss due to Rayleigh scatter. In one embodiment, the filler material may be titania (e.g., TiO 2 ) spheres with a particle size of about 100-300 nm. In an exemplary embodiment, titanium dioxide TiO 2  spheres of 200 nm in diameter in a host encapsulant with a refractive index of 1.4 (e.g,. silicone) is estimated to result in a scatter of less than 1.4% for 400 nm light. In another embodiment of the invention, the filler material may be coated with an insulating material, (e.g., SiO 2 ) to prevent potential electrical transfer via the filler particles. 
         [0015]    In another aspect of modifying the encapsulant  120 , the volume fraction of filler material  125  may be chosen based on the desired change in encapsulant refractive index, as well as the refractive indices of the cured encapsulant and the filler material. If the particle size of filler material  125  is substantially sub-wavelength, the effective medium index can be computed for any index of refraction desired according to the following formula, for which n is the number of dimensions, “σ” stands for the dielectric constant (refractive index squared for optical frequencies), the “i” subscript is the filler material, and the “e” subscript is the effective medium formed of the mixture of filler and base material. 
         [0000]    
       
         
           
             
               
                 ∑ 
                 i 
               
                
               
                 
                   δ 
                   i 
                 
                  
                 
                   
                     
                       σ 
                       i 
                     
                     - 
                     
                       σ 
                       e 
                     
                   
                   
                     
                       σ 
                       i 
                     
                     + 
                     
                       
                         ( 
                         
                           n 
                           - 
                           1 
                         
                         ) 
                       
                        
                       
                         σ 
                         e 
                       
                     
                   
                 
               
             
             = 
             0 
           
         
       
     
         [0016]    In one embodiment of the present invention, the volume of titania (e.g. TiO 2 ) filler material  125  in a silicone encapsulant  125  may be 10-20% to achieve an effective medium index of approximately 1.52. In a particular embodiment the volume of titania filler material in a silicone encapsulant may be 16%. 
         [0017]    Filler material  125  may be added as a suspension to the encapsulant  120  and dispensed in colloidal suspension or by any other method known in the art for mixing and applying an encapsulant. The combined filler/encapsulant material may be applied to a surface of the optical element  110  in the colloidal suspension, subsequently cured and then disposed on a surface of the solar cell  130 . In one embodiment a portion filler/encapsulant material may be cured on a surface of the optical element  110  and placed onto an uncured portion of filler/encapsulant material disposed on the surface of the solar cell  130 . After the curing of both portions of the filler/encapsulant material, a solid optical flow path for incoming radiation with a matched index of refraction may be formed between the optical element  110  and the solar cell  130 . 
         [0018]      FIG. 3  shows a further optional feature of the present invention. An anti-reflective (AR) layer  135  may be disposed on the surface of the solar cell  130 . The anti-reflective layer  135  may increase the amount of incident light that reaches the solar cell  130  by reducing Fresnel reflections between the solar cell  130  and encapsulating material  120 . The solar cell  130  and AR layer  135  may be incorporated into a leadframe package  140 . In another embodiment (not shown), a passivation layer may be disposed between the AR layer  135  and the solar cell  130  which may provide environmental protection for the solar cell. The material composition of the AR and passivation layers may also be optimized for effective refractive indices in order to maximize the angle at which solar radiation may reach the solar cell  130 . 
         [0019]    In a yet another embodiment of the present invention, the acceptance angle of an optical system may be improved by using a higher index material for both the encapsulant and the optical element, which may referred to more generally as the immersion material. The maximum possible light concentration in a CPV system for a given acceptance angle depends on the index of refraction of the immersion material. If the index of the immersing medium increases, the potential acceptance angle increases. The formula relating acceptance angle θ accept , immersing index n, and geometric concentration C of an optical device (e.g. non-imaging concentrator) is shown below. 
         [0000]    
       
         
           
             
               θ 
               accept 
             
             = 
             
               
                 arcsin 
                 ( 
                 
                   ( 
                   
                     n 
                     * 
                     
                       sin 
                        
                       
                         ( 
                         
                           θ 
                           cell 
                         
                         ) 
                       
                     
                   
                   ) 
                 
               
               
                 
                   [ 
                   C 
                   ] 
                 
               
             
           
         
       
     
         [0020]    In one embodiment of the invention, the acceptance angle of a CPV system may be raised by starting with a high refractive index glass (e.g. 1.8) or other material and coupling that with a filler/encapsulant system that would match this index of refraction. For instance, an encapsulant of silicone mixed with a 50% volume fill of titania (e.g. TiO 2 ) spheres may provide a refractive index substantially matching 1.8. For a geometric concentration of 850, this could increase the acceptance angle to 3.54 degrees from the 3.00 degrees potential associated with index 1.52. 
       ADDITIONAL EXAMPLES 
       [0021]    The simulation shown in the graph of  FIG. 4  depicts the calculated transmission of light at the interface between an optical element and a series of encapsulants with varying indices of refraction. The transmission is calculated as a function of incidence angle of incoming light. For all cases, the index of refraction of the optical element is 1.52. The scenarios are depicted by curves that are shown in  FIG. 4 . In can be seen from curves corresponding to indices of refraction 1.40 (--▪--), 1.45 (--♦--), and 1.51 (--▴--) that as the index of refraction of the encapsulant approaches that of the optical element, the transmission of light increases dramatically at larger angles of incidence. When the index of refraction of the encapsulant is 1.51, the transmission of light is greater than 90% for light approaching at 83°. This represents an improvement of over 15° from light approaching an interface with an encapsulant that has an index of refraction of 1.40. It can also be seen that an encapsulant with an index of refraction just above (1.53 -□-) that of the optical element shows the best performance as 90% transmission levels may be achieved for incoming light above 86°. Curves representing encapsulants with higher indices of refraction 1.60 (-⋄-), and 1.64 (-∘-) which demonstrate that encapsulants exceeding the index of refraction of the optical element may result in a decrease in acceptance angle are also shown. An encapsulant with an index of refraction that matches that of the optical element would provide ideal transmission at all angles of incidence. 
         [0022]    Taken together, the curves demonstrate that the impact of total internal reflection (TIR) and Fresnel reflectivity may be reduced when the encapsulant index is adjusted to match that of the optical device, as was described in relation to  FIG. 3 . 
         [0023]    While the specification has been described in detail with respect to specific embodiments of the invention, it will be appreciated that those skilled in the art, upon attaining an understanding of the foregoing, may readily conceive of alterations to, variations of, and equivalents to these embodiments. These and other modifications and variations to the present invention may be practiced by those of ordinary skill in the art, without departing from the spirit and scope of the present invention, which is more particularly set forth in the appended claims. Furthermore, those of ordinary skill in the art will appreciate that the foregoing description is by way of example only, and is not intended to limit the invention. Thus, it is intended that the present subject matter covers such modifications and variations as come within the scope of the appended claims and their equivalents.