Abstract:
A solar cell and method for producing same is disclosed. The solar cell includes a multijunction solar cell structure and a notch filter designed to reflect solar energy that does not contribute to the current output of the multijunction solar cell. By reflecting unused solar energy, the notch filter allows the solar cell to run cooler (and thus more efficiently) yet it still allows all junctions to fully realize their electrical current production capability.

Description:
STATEMENT OF RIGHTS OWNED 
     This invention was made with Government support under contract. The Government has certain rights in this invention. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to systems and methods for generating electricity from solar radiation, and in particular, to a method and apparatus for notch filtering for triple junction solar cells. 
     2. Description of the Related Art 
     Solar panels are typically used to generate power in spacecraft. These solar panels comprise a plurality of solar cells typically arranged in a planar matrix of multiple layers. Solar cell current is produced by photons causing electrons to jump energy states within solar cell junctions. 
     Increasing a solar cell&#39;s absorption of incident solar energy increases the solar energy available for the solar cell to convert into electricity. However, it can also increase the temperature of the solar cell, and solar cells operate with reduced efficiency (η) at elevated temperatures. 
     Early technology solar cells (including those using double junction or silicon technology) respond to relatively limited wavelength bands of solar energy, and often used reflection filters to reflect some of the solar energy at unneeded wavelengths to reduce their temperatures. 
     Newer technology solar cells include three or more junctions. Such solar cells are discussed in U.S. Pat. No. 6,380,601, issued to Ermer et al and “1-eV GaInAs Solar Cells for Ultrahigh-Efficiency Multijunction Devices” by D. J. Friedman, J. F. Geisz, S. R. Kurtz, and J. M. Olson, published July 1998 and Presented at the 2 nd  World Conference and Exhibition on Photovoltaic Solar Energy Conversion, 34% Efficient InGaP/GaAs/GaSb Cell-Interconnected Circuits for Line-Focus Concentrator Arrays,” Munich Conference, 2001, by L. M. Fraas et al., all of which are hereby incorporated by reference. 
     Unfortunately, filters designed for two junction solar cells are inappropriate for use with triple junction cells because the solar cells respond to a wider wavelength band than are passed by those filters. What is needed is a filter design that maximizes the current output of solar cells with three or more junctions. The present invention satisfies that need. 
     SUMMARY OF THE INVENTION 
     To address the requirements described above, the present invention discloses a solar device and a method for making a solar device. In one embodiment, the solar device comprises a germanium substrate, a multijunction solar cell structure having at least first, second and third subcells disposed over the substrate; and a notch filter disposed over the multijunction solar cell structure. The notch filter comprises a repeating pattern of layers of materials formed of materials H, M and L, the repeating pattern comprising (LMHHML) x , materials H and L having respective properties of high and low indices of refraction, material M having properties between those of materials H and L. The method comprises the steps of depositing a multijunction solar cell structure having at least first, second and third subcells on a substrate; and depositing a notch filter, comprising a repeating pattern of layers of materials formed of materials H, M and L on the multijunction solar cell structure, the repeating pattern comprising (LMHHML) x , materials H and L having respective properties of high and low indices of refraction, material M having properties between those of materials H and L. In another embodiment, the method comprises the steps of depositing a multijunction solar cell structure having at least first, second and third subcells on a substrate, and depositing a notch filter, comprising a repeating pattern of layers of materials formed of materials H, M and L a first side of a coverglass, adhering the coverglass to the multijunction solar cell structure, the repeating pattern comprising (LMHHML) x , materials H and L having respective properties of high and low indices of refraction, material M having properties between those of materials H and L. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Referring now to the drawings in which like reference numbers represent corresponding parts throughout: 
         FIG. 1  is a plot illustrating the spectral characteristics of solar radiation; 
         FIG. 2  is a diagram of a three-junction solar cell; 
         FIG. 3  is a plot comparing spectral characteristics of the solar radiation with that of the photon count; 
         FIG. 4  is a plot of the quantum efficiency as for the Ge, GaAs and GaInP solar cell junctions; 
         FIG. 5  is a diagram illustrates the theoretical current for the first, second, and third layer; 
         FIG. 6  is a diagram illustrating one embodiment of a solar device having a notch filter; 
         FIG. 7  is a diagram illustrating another embodiment of a solar device which further comprises a coverglass and an anti-reflection coating on the outside surface of the coverglass and in which the notch filter is formed on the coverglass and affixed to the solar cell via a coverglass adhesive; 
         FIG. 8  is a plot of the spectral response of a notch filter; 
         FIG. 9  is a diagram showing the theoretical current from the triple junction solar cell; 
         FIG. 10  presents a table showing the theoretical current capacity per projected solar area for each of the solar cell junctions for a variety of incidence angles; and 
         FIG. 11  presents a table showing the solar absorptance (α) and solar cell equilibrium temperature. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     In the following description, reference is made to the accompanying drawings which form a part hereof, and which is shown, by way of illustration, several embodiments of the present invention. It is understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the present invention. 
       FIG. 1  is a plot illustrating the spectral characteristics of solar radiation. Plot  102  plots the spectral irradiance as a function of wavelength of the solar spectrum. Plot  104  shows the integrated fraction of plot  102 . 
       FIG. 2  is a diagram of a three-junction solar cell  200 . The three junction solar cell  200  comprises a first layer  202 , a second layer  204  and a third layer  206  disposed on a substrate  201 , with each layer representing a subcell of the cell  200 . In the illustrated embodiment, the first layer  202  comprises germanium (Ge), the second layer  204  comprises gallium arsenide (GaAs), and the third layer  206  comprises gallium-indium-phosphide (GaInP). Each of the layers has a respective junction  207 ,  208 , and  210  which produces electrical current from incident photons within a particular frequency (or wavelength) band. This current is produced by photons causing electrons to jump energy states, or to have electron-hole pairs generated within the cell junctions. 
       FIG. 3  is a plot comparing spectral characteristics of the solar radiation with that of the photon count. Plot  302  shows the same information as plot  102  of  FIG. 1 , while plot  304  shows the photon count at the same respective wavelengths. As is shown in area  306  of plot  304 , the photon count drops off slower as the wavelength goes up. This is due to the reduced energy per photon in these wavelength ranges. 
     A photon reaching a solar cell junction will produce an electron according to its quantum efficiency.  FIG. 4  is a plot of the equivalent quantum efficiency as for the Ge solar cell junction  207  (plot  406 ), the GaAs solar cell junction  208  (plot  404 ) and the GaInP solar cell junction  210  (plot  402 ), and the total quantum efficiency for a three layer solar cell  200  using the foregoing layers (plot  408 ). 
     Combining the foregoing information regarding the photon distribution of solar energy ( FIG. 2 ) and the quantum efficiency of each layer shown in  FIG. 4  results in the theoretical current production (in electrons/cm 2 -sec-μm) for each of the three layers/subcells.  FIG. 5  is a diagram that illustrates the theoretical current for the first, second, and third layer in plots  506 ,  504 , and  502 , respectively. Integrating under the curves defined by plots  502 ,  504 , and  506  produces a theoretical current capacity for each solar cell junction. 
     Table I illustrates the theoretical current (mA/cm 2 ) for solar cell junctions in each of the layers described above: 
     
       
         
               
               
             
               
               
               
               
             
           
               
                 TABLE I 
               
             
             
               
                   
               
               
                   
                 Solar Cell Junction 
               
             
          
           
               
                   
                 GaInP 
                 GaAs 
                 Ge 
               
               
                   
               
               
                 Wavelength Band 
                 (0.35-0.69 μm) 
                 (0.6-0.89 μm) 
                 (0.88-1.76 μm) 
               
               
                 Theoretical Current 
                 17.0 
                 17.6 
                 28.5 
               
               
                   
               
             
          
         
       
     
     Inspection of Table I reveals that the theoretical current capacity from the Ge solar cell junction  207  is substantially higher than that of the GaAs solar cell junction  208  and the GaInP solar cell junction  210 . However, since the junctions  208 - 212  are electrically connected in series, the current produced will be limited by the lowest of the three. Hence, the theoretical current capacity of the GaInP solar cell junction  210  limits the total theoretical current capacity for the other solar cell junctions as well. As a consequence, the Ge solar cell junction&#39;s  207  excess current capacity is wasted as heat. 
     As described above, solar cells produce energy with reduced efficiency at elevated temperatures. Hence, the efficiency of the triple junction solar cell  200  can be increased if photons in the Ge wavelength band can be reflected to reduce heating (thus avoiding the resulting loss in solar efficiency η) while providing sufficient current (in an amount so that the theoretical current from the Ge junction  207  is about equal to that of the remaining solar cell junctions). At the same time, it is important not to reduce the current output from the remaining series-coupled junctions  208 ,  210 . As discussed above, the shorter wavelength photons carry more energy than the longer wavelength photos, so reflecting the shorter wavelength photons reflects more energy. To decrease the temperature of the solar cell  200  to increase solar cell efficiency while not negatively impacting the current output solar cell, the applicants have devised a notch filter, having a bandwidth and center frequency such that the theoretical current capacity from the Ge solar cell junction  207  is reduced to approximately that of the remaining solar cell junctions  208 ,  210 , while not appreciably impacting the theoretical current capacity of those junctions. 
       FIG. 6  is a diagram illustrating one embodiment of a solar device  600  having a notch filter  602 . This notch filter  602  does not appreciably reduce the current producing capacity of the GaAs solar junction or the GaInP solar junction  208 ,  210  (there is &lt;1% loss), and provides an appreciable reduction in solar α (e.g. the difference between the solar α with and without the filter Δα is 0.1 or greater). 
     In the embodiment shown in  FIG. 6 , this is accomplished by use of a periodic multi-layer construction (LMHHML) x  of materials of different indicies of reflection wherein materials H and L having respective properties of high and low indices of refraction, material M having properties between those of materials H and L. The shorthand notation (ABA) x  is shorthand for a periodic series of a thickness of the material A, layered over a thickness of the material B, layered over a thickness of the material A, e.g. (ABA), repeated x times. For example, the periodic series (ABAABAABAABAABA) can be written in shorthand notation as (ABA) 5 . Repeating periods of LMHHML are used in series to sharpen the filter&#39;s notch through redundant filtering, but the invention can be practiced with as few as period of LMHHML, albeit with reduced efficiency. 
     In one embodiment, the L material is ¼ wavelength (quarter wave optical thickness at the design wavelength) silicon dioxide (SiO 2 ), the H material is ¼ wavelength hafnium dioxide (HfO 2 ) material, and the M material is ¼ wavelength of a mix of about 60% SiO 2  and 40% HfO 2 . Where the layers of the notch filter  601  comprise adjacent layers of the same material (e.g. HH or LL), the a single layer of double-thickness material (e.g. ½ wavelength instead of ¼ wavelength) can be used. 
       FIG. 6  shows an embodiment where a single layer of double thickness material is used for the HH and LL layers. As illustrated, the notch filter  602  comprises a first period  602 A of LMHHML material comprising a first layer  604 A of ¼ wavelength L material disposed on the multijunction solar cell  200 , a second layer  604 B of ¼ wavelength M material disposed on the first layer  604 A, a third layer  604 C of ½ wavelength H material disposed on the second layer  604 B (a dashed line is used to indicate that the ½ wavelength H material layer  604 C may be two ¼ wavelength layers), a fourth layer  604 D of ¼ wavelength M material disposed on the third layer  604 C, and a fifth layer  604 E of ¼ wavelength L material disposed on the fourth layer  604 D. A second period of LMHHML material  602 B (with elements indicated as  604 A′- 604 E′) is disposed on the first period of LMHHML material  602 A. This process is repeated as necessary until the desired bandpass characteristics are realized. The dashed line between the last L material layer  604 E of the first period  602 A and the first L material layer  604 A′ of the second period  602 B is again used to indicate that although ½ wavelength L layer material is used, L material layer this can be implemented by two ¼ wavelength layers. 
     The solar device  600  shown in  FIG. 6  can be made by suitable deposition on the appropriate structure. Such deposition techniques can include, for example, growing via molecular beam or other epitaxial growth methods, chemical vapor deposition, drive and diffusion techniques, sputtering, and other standard semiconductor growth techniques. In one embodiment, the solar device  600  is produced by depositing a multijunction solar cell structure having at least first  202 , second  204  and third  206  subcells on a substrate, and depositing the repeating pattern of layers of materials formed of materials H, M and L on the multifunction solar cell structure  200 . U.S. Pat. No. 6,107,564, issued to Aguilera et al., which is hereby incorporated by reference herein, discloses further information regarding how the notch filter described above may be fabricated. 
       FIG. 7  is a diagram illustrating another embodiment of a solar device  700  which further comprises a coverglass  702  and an anti-reflection coating  704  on the outside surface of the coverglass  702  and in which the notch filter  602  is formed on the coverglass  702  and affixed to the solar cell  200  via a coverglass adhesive  706 . In this embodiment, the coverglass  702  itself can become the uppermost L material layer of the notch filter  602 , as can an anti-reflection coating  704 . If desired, a cerium-doped microsheet can be used between (or in front of) the notch filter  602  and the solar cell  200  to block energy in the ultraviolet spectrum. 
     Table II below provides one embodiment of the anti-reflection coating  704  where alternating layers of HfO 2  and SiO 2  are used. Layer # 1  is disposed adjacent the coverglass. 
     
       
         
               
               
               
             
           
               
                 TABLE II 
               
               
                   
               
               
                 Layer # 
                 Layer Formula 
                 Layer Thickness (nm) 
               
               
                   
               
             
             
               
                 1 
                 HfO 2   
                 7.4-7.8 
               
               
                 2 
                 SiO 2   
                 50.6-53.6 
               
               
                 3 
                 HfO 2   
                 26.5-28.1 
               
               
                 4 
                 SiO 2   
                 23.4-24.8 
               
               
                 5 
                 HfO 2   
                 78.7-83.5 
               
               
                 6 
                 SiO 2   
                 10.8-11.4 
               
               
                 7 
                 HfO 2   
                 36.1-38.3 
               
               
                 8 
                 SiO 2   
                 90.3-95.9 
               
               
                   
               
             
          
         
       
     
     Alternating layers of SiO 2 , 60% SiO 2  and 40% HfO 2  (approximate volumetric blend ratios), and HfO 2  can be used to produce a notch filter  602 , as shown in Table III. In this embodiments, the anti-reflection coating  704  and notch filter  602  coatings are applied to sides of the coverglass  702 , and the notch filter side can affixed to the solar cell  200  with an adhesive. In the configuration, the adhesive  706  and the coverglass  702  provide the L material for the first and last layers. 
     
       
         
               
               
               
               
               
             
           
               
                 TABLE III 
               
               
                   
               
               
                   
                   
                   
                 Layer Thickness 
                 Layer 
               
               
                   
                 Layer # 
                 Layer Formula 
                 (nm) 
                 Type 
               
               
                   
               
             
             
               
                   
                  2 
                 60% SiO 2  40% HfO 2   
                 75.9-80.5 
                 M 
               
               
                   
                  3, 4 
                 HfO 2   
                 121.2-128.6 
                 HH 
               
               
                   
                  5 
                 60% SiO 2  40% HfO 2   
                 89.0-94.4 
                 M 
               
               
                   
                  6, 7 
                 SiO 2   
                 39.3-41.7 
                 LL 
               
               
                   
                  8 
                 60% SiO 2  40% HfO 2   
                 85.9-91.3 
                 M 
               
               
                   
                  9, 10 
                 HfO 2   
                 110.7-117.5 
                 HH 
               
               
                   
                 11 
                 60% SiO 2  40% HfO 2   
                 62.1-58.5 
                 M 
               
               
                   
                 12, 13 
                 SiO2 
                 138.8-147.4 
                 LL 
               
               
                   
                 14 
                 60% SiO 2  40% HfO 2   
                 58.5-62.1 
                 M 
               
               
                   
                 15, 16 
                 HfO 2   
                  97.6-103.6 
                 HH 
               
               
                   
                 17 
                 60% SiO 2  40% HfO 2   
                 58.5-62.1 
                 M 
               
               
                   
                 18, 19 
                 SiO2 
                 138.8-147.4 
                 LL 
               
               
                   
                 20 
                 60% SiO 2  40% HfO 2   
                 58.5-62.1 
                 M 
               
               
                   
                 21, 22 
                 HfO 2   
                  97.6-103.6 
                 HH 
               
               
                   
                 23 
                 60% SiO 2  40% HfO 2   
                 58.5-62.1 
                 M 
               
               
                   
                 24, 25 
                 SiO2 
                 138.8-147.4 
                 LL 
               
               
                   
                 26 
                 60% SiO 2  40% HfO 2   
                 62.1-58.5 
                 M 
               
               
                   
                 27, 28 
                 HfO 2   
                 110.7-117.5 
                 HH 
               
               
                   
                 29 
                 60% SiO 2  40% HfO 2   
                 85.9-91.3 
                 M 
               
               
                   
                 30, 31 
                 SiO2 
                 39.3-41.7 
                 LL 
               
               
                   
                 32 
                 60% SiO 2  40% HfO 2   
                 89.0-94.4 
                 M 
               
               
                   
                 33, 34 
                 HfO 2   
                 121.2-128.6 
                 HH 
               
               
                   
                 35 
                 60% SiO 2  40% HfO 2   
                 75.9-80.5 
                 M 
               
               
                   
               
             
          
         
       
     
     Thin-film designs rely on index of refraction differences or the ratio of the index of refraction between two adjacent materials, and the resulting optical thickness (physical thickness*index of refraction) to define their characteristics when deposited. Accordingly, the number, composition, and thickness of the layers described above are selected to achieve the appropriate bandpass characteristics of the notch filter. In the embodiment shown in Table III, the layer materials and thicknesses are symmetric around layers  18  and  19 . This redundancy provides a more robust method for accommodating layer-to-layer optical thickness variations that can arise in the manufacturing process. 
     Different layer formula compositions and layer thicknesses can also be used. For example, as the thickness of the layers is increased, the spectral characteristics of the notch filter also change, generally moving up in wavelength. Further, the index of refraction of the “M” material may be selected at different values, according to the relationship i M =√{square root over (i H i L )}. Techniques for selecting the appropriate number, composition, and thicknesses of the layers are set forth more fully in U.S. Pat. No. 6,107,564, U.S. Pat. No. 3,423,147, U.S. Pat. No. 3,914,023, U.S. Pat. No. 4,229,066, U.S. Pat. No. 5,449,413, and the paper “Multilayer Films with Wide Transmission Bands,” J. Opt. Soc. Am 53, 1266, by Thelan, all of which are hereby incorporated by reference herein. 
     The periodic multi-layer construction (LMHHML) x  may also include other intervening layers, so long as such additional layers do not significantly impact the spectral characteristics of the notch filter  602 . 
     The solar device  700  shown in  FIG. 7  can be made by suitable deposition on the appropriate structure. Such deposition techniques can include, for example, growing via molecular beam or other epitaxial growth methods, chemical vapor deposition, drive and diffusion techniques, sputtering, and other standard semiconductor growth techniques. In one embodiment, the solar device  700  is produced by depositing a multijunction solar cell structure having at least first  202 , second  204  and third  206  subcells on a substrate, and depositing the repeating pattern of layers of materials formed of materials H, M and L, depositing a notch filter, comprising a repeating pattern of layers of materials formed of materials H, M and L a first side of a coverglass, and adhering the coverglass to the multijunction solar cell structure, the repeating pattern comprising (LMHHML) x , materials H and L having respective properties of high and low indices of refraction, material M having properties between those of materials H and L. 
       FIG. 8  is a plot of the spectral response of the notch filter  602  described above. Plot  802  shows the transparency of the notch filter  602  as a function of wavelength, while plot  804  shows the transparency of a simple magnesium flouride (MgF 2 ) filter. As shown, the notch filter  602  provides a reflectance band  806  in the wavelengths of interest. 
       FIG. 9  is a diagram showing the theoretical current from the triple junction solar cell  200 . Plot  902  shows the theoretical current from the GaInP layer  206 , plot  904  shows the theoretical current from the GaAs layer  204 , while plots  906 A and  906 B show the theoretical current from the Ge layer. Note that the notch filter substantially reduces the theoretical current in a band of wavelengths from 0.96 to 1.59 μm. Integrating the area under these curves results in the result shown in Table IV below: 
     
       
         
               
               
             
               
               
               
               
             
           
               
                 TABLE IV 
               
             
             
               
                   
               
               
                 Theoretical Currents 
                 Solar Cell Junction 
               
             
          
           
               
                 (mA/cm 2 ) 
                 GaInP 
                 GaAs 
                 Ge 
               
               
                   
               
               
                   
                 (0.35-0.69 μm) 
                 (0.6-0.89 μm) 
                 (0.88-1.76 μm) 
               
               
                 MgF 2  Filter 
                 17.0 
                 17.6 
                 28.5 
               
               
                 Notch Filter 602 
                 17.0 
                 17.7 
                 17.9 
               
               
                   
               
             
          
         
       
     
     It is known that the spectral performance of the filters described above vary with incidence angle θ, in terms of the center wavelength and the depth of the notch provided by the notch filter  602 . Typically, the notch  808  shifts to shorter wavelengths as the incidence angle θ diverges from 90 degrees (normal to the surface). Since the notch filter  602  preferably does not compromise the performance of the remaining solar cell junctions  208 - 210 , the notch filter  602  may be designed to avoid the shift of the notch within the active band of the GaAs solar cell junction  208  by selecting the location of the notch filter&#39;s notch to longer wavelengths. 
       FIG. 10  presents a table showing the theoretical current capacity per projected solar area for each of the solar cell junctions for a variety of incidence angles. Very steep incidence angles (70 degrees and above) are insignificant contributors to the electrical power system. The cell total columns refer to the cell total obtained from the current limiting solar cell junction (in all cases, the GaInP junction)  210 . 
       FIG. 11  presents a table showing the solar absorptance (α) and solar cell equilibrium temperature. Note that in incidence angles of interest, solar cell temperature are reduced by about 10 degrees Celsius, providing an increase in cell conversion efficiency of in the order of one percent. While this improvement may seem small, this translates into a 3-4% increase in power. 
     A notch filter  602  with the spectral characteristics shown in  FIG. 8  provides an optimal amount of solar energy from the Ge junction  207  wavelength band without impacting the GaInP or GaAs junctions  210 ,  208  for all ranges of incidence angles Θ. However, other less optimal designs may be used. For example, the transparency of the notch filter  602  in the GaInP and/or GaAs junction  210 ,  208  wavelengths can be reduced, thereby decreasing the electrical current output and the overall solar cell  602  efficiency. The depth and/or the width of the reflecting notch  808  could also be reduced or increased by altering the number, composition, and thicknesses of the notch filter  602  layers. If the bandwidth of the notch  806  is reduced, then a less than optimal quantity of sunlight is reflected, leading to an increase in the solar cell temperature and reduced solar cell efficiency. If the bandwidth of the notch  806  is increased, then too much sunlight will be reflected, causing the Ge junction  207  to become the current limiter of the triple junction solar cell, significantly reducing the solar cell efficiency. Similarly, if the notch  806  is shifted to a longer wavelength, less solar energy is reflected, leading to an increased solar cell temperature. If the notch  806  is shifted to a shorter wavelength, the notch filter  602  will reflect sunlight from the GaAs junction  208  wavelengths, and make the GaAs junction  208  even more of a current limiter, particularly for off-normal incidence angles Θ. 
     CONCLUSION 
     This concludes the description of the preferred embodiments of the present invention. The foregoing description of the preferred embodiment of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto. The above specification, examples and data provide a complete description of the manufacture and use of the composition of the invention. Since many embodiments of the invention can be made without departing from the spirit and scope of the invention, the invention resides in the claims hereinafter appended.