Abstract:
A method for fabrication of a multijunction photovoltaic (PV) cell includes providing a stack comprising a plurality of junctions on a substrate, each of the plurality of junctions having a respective bandgap, wherein the plurality of junctions are ordered from the junction having the smallest bandgap being located on the substrate to the junction having the largest bandgap being located on top of the stack; forming a top metal layer, the top metal layer having a tensile stress, on top of the junction having the largest bandgap; adhering a top flexible substrate to the metal layer; and spalling a semiconductor layer from the substrate at a fracture in the substrate, wherein the fracture is formed in response to the tensile stress in the top metal layer.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/185,247, filed Jun. 9, 2009. This application is also related to attorney docket numbers YOR920100056US1, YOR920100058US1, FIS920100005US1, and FIS920100006US1, each assigned to International Business Machines Corporation (IBM) and filed on the same day as the instant application, all of which are herein incorporated by reference in their entirety. 
     
    
     FIELD 
       [0002]    This disclosure relates generally to the field of multijunction photovoltaic cell fabrication. 
       DESCRIPTION OF RELATED ART 
       [0003]    Multijunction III-V based photovoltaic (PV) cells, or tandem cells, are comprised of multiple p-n junctions, each junction comprising a different bandgap material. A multijunction PV cell is relatively efficient, and may absorb a large portion of the solar spectrum. The multijunction cell may be epitaxially grown, with the larger bandgap junctions on top of the lower bandgap junctions. Conversion efficiencies for commercially available 3-junction III-V based photovoltaic structures may be about 30% to 40%. A III-V substrate based triple junction PV cell may be about 200 microns thick range, a major portion of the thickness being contributed by a bottom layer of a substrate, which may also serve as a junction. The relative thickness of the substrate may cause the substrate layer to be relatively inflexible, rendering the PV cell inflexible. 
       SUMMARY 
       [0004]    In one aspect, a method for fabrication of a multijunction photovoltaic (PV) cell includes providing a stack comprising a plurality of junctions on a substrate, each of the plurality of junctions having a respective bandgap, wherein the plurality of junctions are ordered from the junction having the smallest bandgap being located on the substrate to the junction having the largest bandgap being located on top of the stack; forming a top metal layer, the top metal layer having a tensile stress, on top of the junction having the largest bandgap; adhering a top flexible substrate to the metal layer; and spalling a semiconductor layer from the substrate at a fracture in the substrate, wherein the fracture is formed in response to the tensile stress in the top metal layer. 
         [0005]    In one aspect, a multijunction photovoltaic (PV) cell includes a bottom flexible substrate; a bottom metal layer located on the bottom flexible substrate; a semiconductor layer located on the bottom metal layer; and a stack comprising a plurality of junctions located on the semiconductor layer, each of the plurality of junctions having a respective bandgap, wherein the plurality of junctions are ordered from the junction having the smallest bandgap being located on the substrate to the junction having the largest bandgap being located on top of the stack. 
         [0006]    Additional features are realized through the techniques of the present exemplary embodiment. Other embodiments are described in detail herein and are considered a part of what is claimed. For a better understanding of the features of the exemplary embodiment, refer to the description and to the drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0007]    Referring now to the drawings wherein like elements are numbered alike in the several FIGURES: 
           [0008]      FIG. 1  illustrates an embodiment of a method for fabricating a multijunction PV cell. 
           [0009]      FIG. 2  illustrates an embodiment of a multijunction PV cell on a substrate. 
           [0010]      FIG. 3  illustrates an embodiment of a multijunction PV cell after formation of a top metal layer. 
           [0011]      FIG. 4  illustrates an embodiment of a multijunction PV cell after adhering a top flexible substrate to the top metal layer. 
           [0012]      FIG. 5  illustrates an embodiment of a multijunction PV cell after spalling the substrate. 
           [0013]      FIG. 6  illustrates an embodiment of a multijunction PV cell after formation of a bottom metal layer. 
           [0014]      FIG. 7  illustrates an embodiment of a multijunction PV cell after formation of a bottom metal layer. 
           [0015]      FIG. 7  illustrates an embodiment of a multijunction PV cell after adhering a bottom flexible substrate to the bottom metal layer. 
           [0016]      FIG. 8  illustrates an embodiment of a multijunction PV cell after removing the top flexible substrate. 
           [0017]      FIG. 9  illustrates an embodiment of a multijunction PV cell after top-of-cell processing. 
       
    
    
     DETAILED DESCRIPTION 
       [0018]    Embodiments of a systems and methods for multijunction PV cell fabrication are provided, with exemplary embodiments being discussed below in detail. Spalling may be used to create a thin semiconductor film for use in fabrication of a flexible PV cell. Spalling allows for the controlled removal of a relatively thin semiconductor layer from a wafer or ingot of a semiconductor substrate using a layer of tensile stressed metal. The thin semiconductor layer may be transferred onto a mechanically flexible support substrate, such as a polymer, or may be left as a free-standing layered-transferred structure. Once the thin semiconductor layer is spalled, the tensile stressed metal used for the spalling process remains on one side of the thin semiconductor layer. The tensile stressed metal may block the illumination of the solar cell. Therefore, a flipping process may be necessary after spalling to achieve an operational PV cell. This is particularly important for III-V multijunction cells, in which the order of the various junctions comprising the cell is crucial for proper cell operation. This spalling may be applied to a single region of a surface of a semiconductor substrate, or to a plurality of localized regions, allowing for selected-area use of the semiconductor substrate. The plurality of localized regions may comprise less than one-hundred percent of the original substrate surface area in some embodiments. 
         [0019]      FIG. 1  illustrates an embodiment of a method  100  for fabricating a multijunction PV cell.  FIG. 1  is discussed with reference to  FIGS. 2-9 . In block  101 , a multijunction PV cell  200  as shown in  FIG. 2  is provided. The multijunction PV cell may be formed by any appropriate growth method, such as molecular beam epitaxy (MBE) or metalorganic vapor phase epitaxy (MOCVD). Junction  202  is formed on substrate  201 , junction  203  is formed on junction  202 , and junction  204  is then formed on junction  203 . Substrate  201  may comprise a III-V substrate, such as gallium arsenide (GaAs), or germanium (Ge) in some embodiments. The bandgap of junction  202  is less than the bandgap of junction  203 , and the bandgap of junction  203  is less than the bandgap of junction  204 . The largest bandgap p-n junction  204  is grown last, such that that after spalling is performed (discussed below with respect to block  103 ), junction  204  will be located adjacent to a back metal contact of the multijunction cell. In some embodiments, junction  204  comprises any appropriate relatively large band-gap p/n material, such as a GaInP 2 -based material; junction  202  comprises any appropriate relatively small bandgap material, such as a GaAs or Ge material; and junction  203  comprises any appropriate material having a bandgap between that of junctions  202  and  204 . Junctions  202 - 204  are shown for illustrative purposes only; cell  200  may be grown with any desired number of junctions, ordered from the junction having the smallest bandgap being located on the substrate  201  to the junction having the largest bandgap located at the top of the stack. 
         [0020]    In block  102 , a top metal layer  301  is formed on junction  204 , as is shown in  FIG. 3 . Top metal layer  301  comprises a tensile stressed metal layer, and may comprise nickel (Ni) in some embodiments. Formation of top metal layer  301  may optionally include formation of a striking layer comprising a metal such as titanium (Ti) on junction  204  before formation of top metal layer  301 . The striking layer may act as an adhesion promoter for top metal layer  301 . Top metal layer  301  may be about 5-6 microns thick in some embodiments. In block  103 , a top flexible substrate  401  is adhered to metal layer  301 , as is shown in  FIG. 4 . Top flexible substrate  401  may comprise polyimide (e.g., Kapton tape) in some embodiments. 
         [0021]    In block  104 , semiconductor layer  501  is separated from substrate  201  at fracture  502 , as is shown in  FIG. 5 . Top flexible substrate  401  may serve as a mechanical handle once the spalling of semiconductor layer  501  and junctions  202 - 204  is initiated. The tensile stress in metal layer  301  encourages formation of fracture  502 . Semiconductor layer  501  may be less than about 10 microns thick in some embodiments. In some embodiments, a compressively strained cleave layer may be formed in substrate  201  to weaken the substrate  201  at a pre-determined physical depth or region, allowing precision in the location of fracture  502 . The cleave layer may comprise a layer that is preferentially hydrogenated, or may comprise a layer having a lower melting point than substrate  201 , such as germanium tin (GeSn) or any material having a stoichiometry that may be preferentially weakened by a physio-chemical means. A temperature gradient (for example, a physical gradient or quenching) or etching may also be used to help induce spalling of semiconductor layer  501  from substrate  201 . 
         [0022]    In block  105 , a bottom surface of semiconductor layer  501  may be planarized, and bottom metal layer  601  deposited on semiconductor layer  501 , as shown in  FIG. 6 . Bottom metal layer  601  may comprise a back electrical contact for the multijunction PV cell, and may comprise a metal such as germanium gold (GeAu), Ni, or gold (Au) in some embodiments. Any other necessary back of cell processing may also be performed in block  105 , such as back surface field creation, texturing, or patterning. An acid- and temperature-resistant epoxy, wax, or polymer may also be applied to cover the back and protect the sides of the structure  600 . In block  106 , bottom flexible substrate  701  is adhered to bottom metal layer  601 , as shown in  FIG. 7 . Bottom flexible substrate  701  allows electrical contact to bottom metal layer  601 , and may comprise polyimide (e.g., Kapton tape) in some embodiments. 
         [0023]    In block  107 , top flexible substrate  401  is removed, as is shown in  FIG. 8 . Removal of top flexible substrate  401  may be performed by placing structure  700  shown in  FIG. 7  on a relatively hot surface, or may be detach by a chemical or physical means, which enables the adhesive of top flexible substrate  401  to be weakened and subsequently removed, resulting in structure  800  shown in  FIG. 8 . The previously applied acid- and temperature-resistant epoxy may protect the structure  700  during removal of top flexible substrate  401 . 
         [0024]    In block  108 , top-of-cell processing is performed to form finished multijunction PV cell  900 . Some or all of metal layer  301  may be removed, as shown in  FIG. 9 . Top metal layer  301  may be removed by any appropriate etching method. In some embodiments, top metal layer  301  may be etched to form metal electrodes  902   a - c . In other embodiments, metal electrodes  902   a - c  may be separately deposited on junction  204  after removal of top metal layer  301 . Metal electrodes  902   a - c  are shown for illustrative purposes only; any appropriate top of cell circuitry may be formed on junction  204  to complete the multijunction PV cell  900 . In embodiments comprising a striking layer, the etch of metal layer  301  may be selective to the striking layer material (for example, Ti). Top of cell processing may further comprise formation of an antireflective coating  901   a - b  on the exposed top surface of junction  204 . In embodiments comprising a striking layer, the striking layer may be oxidized to create antireflective coating  901   a - b  on the surface of junction  204 . The antireflective coating  901   a - b  may provide enhanced light trapping in the multijunction PV cell  900  and enhance cell performance. A total thickness of semiconductor layer  501  and junctions  202 - 204  may be less than about 15 microns in some embodiments. 
         [0025]    Due to the tensile stress in metal layers  301  and  601 , the semiconductor layer  501  and junctions  202 - 204  may possess residual compressive strain after spalling in some embodiments. The magnitude of the strain contained in semiconductor layer  501  and junctions  202 - 204  may be controlled by varying the thickness and/or stress of the metal layers  301  and  601 , either before or after spalling. The optical properties of multijunction PV cell  900 , which is built using semiconductor layer  501  and junctions  202 - 204 , may be tuned by adjusting the amount of strain in semiconductor layer  501  and/or junctions  202 - 204 . 
         [0026]    The technical effects and benefits of exemplary embodiments include a relatively cost-effective method of fabricating a flexible, efficient multijunction PV cell. 
         [0027]    The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an”, and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
         [0028]    The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. The embodiment was chosen and described in order to best explain the principles of the invention and the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated.