Abstract:
Mechanically stacked multijunction solar cells are provided. In one embodiment, a mechanically stacked, multijunction solar cell comprises: a first solar cell having a first bandgap; a second solar cell having a second bandgap; and a plurality of spaced apart metal pillars sandwiched between the first solar cell and the second solar cell.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of and priority to U.S. Provisional Application No. 61/528,668, filed on Aug. 29, 2012 and entitled “MECHANICALLY STACKED MULTIJUNCTION SOLAR CELLS”, which is incorporated herein by reference in its entirety. 
     
    
     GOVERNMENT LICENSE RIGHTS 
       [0002]    The United States Government has rights in this invention under Contract No. DE-AC36-08GO28308 between the United States Department of Energy and the Alliance for Sustainable Energy, LLC, the Manager and Operator of the National Renewable Energy Laboratory. 
     
    
     BACKGROUND 
       [0003]    High-efficiency multijunction solar cells are fabricated from materials with different band gaps. In a typical multijunction solar cell, individual single-junction cells with different energy band gaps (Eg) are stacked on top of each other. Sunlight falls first on the material having the largest band gap, and the highest energy photons are absorbed. Photons not absorbed in the first or top cell are transmitted to the second cell, which absorbs the higher energy portion of the remaining solar radiation, while remaining transparent to the lower energy photons. In theory, any number of cells can be used in multijunction devices. There is a desire to make multijunctions solar cells with four or more cells. However, to date, only two or three cells have been functionally designed. 
         [0004]    Multijunction solar cells may be made in one of two ways, monolithically or mechanically stacked. Monolithic multijunction solar cells are typically made by sequentially growing all the necessary layers of materials for two or more cells and the necessary interconnection between the cells. Ideally these materials can be grown epitaxially, but for some material combinations, this is impossible or undesirable. Growing four solar cell junctions on the same substrate requires lattice-mismatched epitaxy, and the associated dislocations can degrade the performance of the fourth solar cell, such that the resulting device performs more poorly than existing three junction devices. 
         [0005]    Another approach is to spectrally split the light and send the spectrally split light to different junctions grown on different substrates. This approach is inherently complex, and optical losses may reduce the device efficiency to below the level of existing three junction solar cell devices. 
         [0006]    A third option is direct semiconductor bonding used to bond together solar cells that have been grown on different substrates. To date, bonds with adequate electrical conductivity and mechanical integrity for concentrated photovoltaics (CPV) applications do not exist. 
         [0007]    Yet another solution is to mechanically stack sub-cells in such a manner that the entire stack of sub-cells converts incident light into electricity. Many different combinations of solar cells have been created using mechanical stacks. However, most mechanically stacked multijunction solar cells have poor thermal conductivity and optical coupling between the upper and lower subcells. In principle, this approach enables the use of a wide range of materials and therefore, very high conversion efficiencies. In practice, it is important to minimize the electrical resistivity and optical reflectivity losses at each bonded interface in the mechanical stack. For most applications, it is also important that heat from the upper solar cells can easily pass through the bonded interface and lower solar cells to reach a heat sink beneath the lower cells. 
         [0008]    The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    Exemplary embodiments presented in this disclosure can be more easily understood and further advantages and uses thereof more readily apparent, when considered in view of the description of the following figures in which: 
           [0010]      FIG. 1  shows a top view of a solar cell with an interfacial metallization grid having substantially parallel lines of metallization; 
           [0011]      FIG. 2  illustrates a side cut-away view of a solar cell with an interfacial metallization grid having substantially parallel lines of metallization sandwiched between two solar cells; 
           [0012]      FIG. 3  shows a top view of a solar cell with an interfacial metallization pattern of spaced-apart pillars; 
           [0013]      FIG. 4  shows a side cut-away view of a solar cell with an interfacial metallization pattern of spaced-apart pillars sandwiched between two solar cells; 
           [0014]      FIG. 5  shows a side cut-away view of a solar cell with an interfacial metallization pattern of spaced-apart pillars sandwiched between two solar cells, including an optically transparent bonding material; 
           [0015]      FIG. 6  shows a side cut-away view of a solar cell with an interfacial metallization pattern of spaced-apart pillars sandwiched between two solar cells, including layers of optically transparent material; 
           [0016]      FIG. 7  shows a side cut-away view of a solar cell with an interfacial metallization pattern of spaced-apart pillars sandwiched between two solar cells, including an index-matched semiconductor material as an optical coupling material with an air gap; and 
           [0017]      FIGS. 8   a - j  illustrate a fabrication sequence for fabricating a solar cell with an interfacial metallization pattern of spaced-apart pillars sandwiched between two solar cells. 
       
    
    
       [0018]    In accordance with common practice, the various described features are not drawn to scale but are drawn to emphasize features relevant to the presented embodiments. Reference characters denote like elements throughout figures and text. 
       DETAILED DESCRIPTION 
       [0019]      FIG. 1  shows a partial top view of a mechanically stacked multijunction solar cell  100  with an interfacial metallization grid having substantially parallel lines of metallization  130  that intersect with bus bars  110  and  120  at or near the edges of solar cell  100 .  FIG. 2  illustrates a side cut-away view of a mechanically stacked multijunction solar cell  100  with an interfacial metallization grid having substantially parallel lines of metallization  130  sandwiched between an upper solar cell  150  and a lower solar cell  160 . Arrows  140  show example of potential current movement in this embodiment of a mechanically stacked multijunction solar cell  100  with interfacial metallization grid having substantially parallel lines of metallization  130 . One of the issues of this embodiment is to minimize optical obscuration of the metallization lines  130 . In principal, narrow metal lines or fingers at the interface could be places in the shadow of the fingers on the top surface of the top cell stack, giving good electrical conductivity with no additional shadow loss, beyond that of the top surface grid fingers. In practice, the optical obscuration footprint of the interfacial metal fingers or lines  130  can be much wider than that of the overlying top-surface grid fingers. 
         [0020]      FIG. 3  shows a partial top view of a mechanically stacked, multijunction solar cell  200  with an interfacial metallization pattern of spaced-apart pillars  230 .  FIG. 4  shows a partial side, cut-away view of the mechanically stacked, multijunction solar cell  200  of  FIG. 3  with an interfacial metallization pattern of spaced-apart pillars  230  sandwiched between an upper solar cell  250  and a lower solar cell  260 . This mechanically stacked solar cell  200  arrangement with an array of metal pillars  230  may reduce the optical losses for two-terminal configurations, in which external current-collecting contacts to a load are only made to the very top and bottom of the mechanical stack  200 , and no external current-collecting contact is made to the bonded interface layer. The array of metal pillars  230  provides an improved compromise between minimal shadow loss and minimal electrical resistivity. The advantages of an array of metal pillars  230  may be even greater for the non-normal light paths inherent to concentrating photovoltaic (CPV) applications. In a two-terminal device, lateral current conduction by the metal (parallel to the interface) is unnecessary, and providing for it may incur unnecessary optical obscuration for the non-normal light paths inherent to concentrating photovoltaics (CPV) applications. 
         [0021]    In the array-of-metal-pillars arrangement  230 , each pillar may carry current (shown as arrows  240 ) collected from a small portion of the total area. As the spacing between pillars  230  is decreased, the total amount of current collected by each pillar decreases. Because of current-crowding, perimeter length of pillars affects R series. Therefore, the optimal shape may be a rectangular cross section, as shown. However, the pillars  230  may be any shape, such as circular, oval, triangular, discontinuous line segments, etc. 
         [0022]    An interfacial grid line array (such as shown at  100 ) may appear to be optimal, because it maximizes the amount of metal at the interface with no apparent shadow loss, assuming a perfect geometry with no alignment or lithography related losses and substantially perfect normal-incident light. However, inclusion of shadow losses, and therefore, loss of light and subsequent current to bottom cell(s), due to lithography and alignment errors may favor an interfacial pillar geometry (such as shown at  200 ). 
         [0023]    Specifically, a pillar arrangement has a similar or lower shadow loss than a grid line arrangement. For example, a 20×20 μm pillar is significantly less sensitive to alignment and fabrication errors than a 5 μm wide grid line. In particular, the sum of the errors may raise the effective shadow loss of each grid line significantly (from 5 μm to 8-11 μm in the above example). For a concentrator grid with a shadow loss of 4% in the top cell(s), the shadow loss of the bottom cell(s) may be in the order of 6 to 8.8%, for normal incidence light. For non-normal light (as from a lens), the shadow loss for the bottom cell may be much higher. Also, a 1 μm mis-alignment of grid lines reduces bonding area by 1 μm from 5 μm to 4 μm, which may result in a 20% reduction. However, for a 20×20 μm pillar, a 1 μm mis-alignment may have less shadow losses and maintain a good bonding area. Accordingly, the pillar arrangement will have a greater metal-to-metal overlap contact area for bonding. The shadow loss for non-normal light should be less for pillars than for grid lines under non-normal light conditions, such as from a lens. Furthermore, the 5 μm wide grid lines may be unrealistic. If 10 μm grid lines are required, then pillars will have a significantly smaller shadow loss. 
         [0024]    Although most of the above summary concerns light in a normal-incidence geometry, it may be noted that non-normal light, as from a lens, will likely favor a pillar arrangement. Specifically, given substantially equal shadow loss for normal incidence, pillars should have lower shadow loss for off-normal incidence. At high concentrations, the range of angles can be large, up to approximately 42° for glancing incidence light. This embodiment may minimize electrical and optical losses for a configuration in which metal interconnects are used to carry electrical current from an upper cell(s) across a bonded interface to a lower cell(s). 
         [0025]      FIG. 5  shows a side cut-away view of a mechanically stacked, multijunction solar cell  300  with an interfacial metallization pattern of spaced-apart pillars  330  and  331  sandwiched between an upper solar cell  350  and a lower solar cell  360 , including an optically transparent bonding material  380 . In this embodiment, the metal-to-metal bonds  335  of pillars  330  and  331  are for strength and current conduction, while the optically transparent bonding material  380  supports optical coupling within the mechanically stacked, multijunction solar cell  300 . The optically transparent bonding material  380  may be a single material for optical coupling, such as SiO 2 , SiN, TiO 2 , etc. This embodiment attempts to fill the voids between the metal-to-metal pillar interconnects  330  and  331  with a material that provides optical and thermal coupling across the bonded interface. 
         [0026]      FIG. 6  shows a side cut-away view of a mechanically stacked, multijunction solar cell  400  with an interfacial metallization pattern of spaced-apart pillars  430  and  431  sandwiched between a top solar cell  450  and a bottom solar cell  460 , including layers  481 ,  482 ,  483  of optically transparent material  480 . The layers  481 ,  482 ,  483  may be a stack of materials optimized for maximizing optical transmission of light exiting the upper solar cell  450  to the lower solar cell  460  for absorption and conversion to electricity. The optically transparent bonding material  480  may include a very slight air gap, which may reflect unusable light. This embodiment may utilize epitaxially grown filler material  480 , such as a semiconductor material, to fill the space between the metal-to-metal pillars  430  and  431 . The filler material  480  may be grown on the bottom surface of the top solar cell  450  and/or on the top surface of the bottom solar cells  460 . The filler material  480  may be etched, such as with photolithography, to create vias into which the metal contacts to both the upper solar cell  450  and the lower solar cells  460  may be deposited. The upper solar cell  450  and the lower solar cell  460  may then be brought together and bonded. 
         [0027]      FIG. 7  shows a side cut-away view of a mechanically stacked, multijunction solar cell  500  with an interfacial metallization pattern of spaced-apart metal on thin metal pillars  530  and  531  sandwiched between an upper solar cell  550  and a lower solar cell  560 , including an index-matched semiconductor material  580  as an optical coupling material that may include an air gap  570 . This embodiment may simplify lithography, eliminate the need for growing optical coupling materials or stacks, and may give good optical transmission for very thin air gaps. The thickness of the thin metal pillars  530  and  531  can be tuned during fabrication. The index-matched semiconductor material  580  may be grown during epitaxial growth or during fabrication. 
         [0028]      FIGS. 8   a - j  illustrate a fabrication sequence for fabricating a mechanically stacked, multijunction solar cell  600  with an interfacial metallization pattern of spaced-apart, metal-to-metal pillars  630  and  631 , sandwiched between an upper solar cell  650  and a lower solar cell  660 , including an optical coupling material  680  that may include a small air gap  670 . During fabrication, a layer of photoresist  690  may be added to an optical coupling layer  680  and a top solar cell  650 , as shown in  FIG. 8   a . It should be noted that the optical coupling layer  680  may be grown epitaxially, such as on the top solar cell  650 . The photoresist  690  may be selectively removed at predetermined locations  695  for receiving metal pillars, as shown in  FIG. 8   b . The optical coupling layer  680  is then selectively removed by any known method, such as by etching with photolithography to create vias onto which metal contacts to the upper solar cell  650  may be deposited, as shown in  FIG. 8   c . Metal  630  is then deposited into the vias  695 , as shown in  FIG. 8   d . The photoresist is then removed, as shown in  FIG. 8   e . With respect to the bottom solar cell  660 , a photoresist layer  691  is deposited, as shown in  FIG. 8   f . The photoresist is selectively removed to form vias  696 , as shown in  FIG. 8   g . Metal  631  is deposited in the vias  696 , as shown in  FIG. 8   h . The photoresist layer  691  is then removed, as shown in  FIG. 8   i . The upper solar cell  650  and the lower solar cell  660  are then brought together and bonded, typically by heating or annealing, as shown in  FIG. 8   j . It should be noted that element h, shown in  FIGS. 8   e  and  8   h , may be adjusted to help with height mismatch in the fabrication process. Another method is to add in a small gap between the optically transparent material and the lower solar cell, as shown in  FIGS. 7 and 8   j.    
         [0029]    The geometry and dimensions are such that metal-to-metal bonds are made between the upper and lower contacts, and a filler-to-semiconductor or filler-to-filler bond is made over the rest of the interface. Because the metal-to-metal bonds carry electrical current between the upper and lower solar cells, the filler material does not need to perform this function. The filler material, and bonds to it, must, however, be optically transparent to light used by the lower solar cell(s) and have excellent thermal conductivity. In order to accomplish excellent thermal conductivity, the filler material must be in physical contact to the material above and below it. However, physical contact is sufficient, and a strong bond is not necessary. Also, to assist with fabrication limitations, a small air gap is tolerable optically. However, for good thermal conductivity between the solar cells, physical contact between the optical transparent materials and the upper and lower solar cells may be an improvement over an air gap. 
         [0030]    While a number of exemplary aspects and embodiments have been discussed above, those of skill in the art will recognize certain modifications, permutations, additions and sub combinations thereof. For example, the metal pillars do not necessarily have to be metal. They can be of any material which can be bonded together with excellent electrical conductivity. It is therefore intended that the following appended claims and claims hereafter introduced are interpreted to include all such modifications, permutations, additions and sub-combinations as are within their true spirit and scope. 
         [0031]    Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement, which is calculated to achieve the same purpose, may be substituted for the specific embodiment shown. This application is intended to cover any adaptations or variations of the embodiments described herein. Therefore, it is manifestly intended that this invention be limited only by the claims and the equivalents thereof.