Abstract:
According to an embodiment, a method of manufacturing a solar cell includes depositing a sequence of layers of semiconductor material forming at least one solar cell on a first substrate; temporarily bonding a flexible film to a support second substrate; permanently bonding the sequence of layers of semiconductor material to the flexible film so that the flexible film is interposed between the first and second substrates; thinning the first substrate while bonded to the support substrate to expose the sequence of layers of semiconductor material; and subsequently removing the support substrate from the flexible film.

Description:
[0001]    This is a continuation of U.S. patent application Ser. No. 12/401,137, filed Mar. 10, 2009 (pending), and a continuation-in-part of U.S. patent application Ser. No. 11/860,142, filed Sep. 24, 2007 (pending), which are all incorporated herein by reference in their entireties. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present application is directed to solar cell manufacturing and, more particularly, to mounting of solar cells on a flexible substrate. 
       BACKGROUND 
       [0003]    Thin solar cells are fabricated by depositing layers of light absorbing semiconductor material on the surface of a semiconductor wafer and then removing the wafer. The solar cell layer stack is typically bonded to a carrier to provide support during certain manufacturing steps, including removal of the semiconductor wafer. Additional processing can be performed after removal of the semiconductor wafer such as depositing metal wiring and cutting the thin layers of solar cell material into individual solar cell chips. Each solar cell chip can be removed from the support carrier and attached to a solar array device such as a solar panel, collector, etc. The solar cell chips can be made thin enough so that they flex when attached to curved surfaces. 
         [0004]    The layers of solar cell material are typically attached to a carrier support using an adhesive or solder. It is difficult to remove the thin solar cell chips from the support carrier after the semiconductor wafer is removed and processing of the cells is completed. The thin solar cell chips are often damaged during the support substrate removal process, which can require excessively high temperatures and/or mechanical/chemical forces to break the bond formed between the solar cells and the support carrier. Damaging solar cells during the support substrate removal process significantly reduces conventional thin film solar cell manufacturing yields. 
       SUMMARY 
       [0005]    According to one embodiment, a method of manufacturing a solar cell includes depositing a sequence of layers of semiconductor material forming at least one solar cell on a first substrate; temporarily bonding a flexible film to a support second substrate; permanently bonding the sequence of layers of semiconductor material to the flexible film so that the flexible film is interposed between the first and second substrates; thinning the first substrate while bonded to the support substrate to expose the sequence of layers of semiconductor material; and subsequently removing the support substrate from the flexible film. 
         [0006]    According to another embodiment, a method of manufacturing a solar cell includes depositing a sequence of layers of semiconductor material forming at least one solar cell on a first substrate; attaching a flexible film to a support second substrate with a temporary adhesive; attaching the sequence of layers of semiconductor material to the flexible film with a permanent adhesive so that the flexible film is interposed between the first and second substrates; thinning the first substrate while bonded to the support substrate to expose the sequence of layers of semiconductor material; and subsequently applying an adhesive remover to holes formed through the support substrate to dissolve the temporary adhesive and remove the support substrate from the flexible film. 
         [0007]    According to yet another embodiment, a method of manufacturing a solar cell includes depositing a sequence of layers of semiconductor material forming at least one inverted metamorphic multifunction solar cell on a first substrate; temporarily bonding a flexible film to a support second substrate; permanently bonding the sequence of layers of semiconductor material to the flexible film so that the flexible film is interposed between the first and second substrates; thinning the first substrate while bonded to the support substrate to expose the sequence of layers of semiconductor material; and subsequently removing the support substrate from the flexible film. 
         [0008]    Of course, the present invention is not limited to the above features and advantages. Those skilled in the art will recognize additional features and advantages upon reading the following detailed description, and upon viewing the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  illustrates an exploded perspective view of a solar cell structure temporarily attached to a support substrate according to an embodiment of the present invention. 
           [0010]      FIGS. 2-6  illustrate cross-sectional views of the solar cell structure shown in  FIG. 1  being temporarily attached to the support substrate according to an embodiment of the present invention. 
           [0011]      FIGS. 7-9  illustrate cross-sectional views of the solar cell structure shown in  FIG. 1  being processing after attachment to the support substrate according to an embodiment of the present invention. 
           [0012]      FIG. 10  illustrates a side perspective view of individual solar cell chips manufactured according to an embodiment of the present invention attached to a surface. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    Details of the present invention will now be described including exemplary aspects and embodiments thereof. Referring to the drawings and the following description, like reference numbers are used to identify like or functionally similar elements, and are intended to illustrate major features of exemplary embodiments in a highly simplified diagrammatic mariner. Moreover, the drawings are not intended to depict every feature of the actual embodiment nor the relative dimensions of the depicted elements, and are not drawn to scale. 
         [0014]    With this in mind, the present application is directed to permanently bonding (i.e., bonding with a permanent adhesive) a thin solar cell formed on a growth substrate to one side of a flexible film and temporarily bonding (i.e., bonding with a temporary adhesive) the other side of the flexible film to a support substrate so that the support substrate can be easily removed from the flexible film after processing of the thin solar cell is complete. As used herein, a “temporary adhesive” is an adhesive in which the temporarily bonded layers can be readily separated upon treatment of the temporary adhesive with an organic solvent under conditions that do not damage the semiconductor material. Such conditions typically soften or dissolve the temporary adhesive. In contrast, a “permanent adhesive” as used herein, is an adhesive in which the permanently bonded layers cannot be readily separated upon treatment of the permanent adhesive with a solvent under typical processing conditions for separation of temporarily bonded layers without damaging the semiconductor material. Thin solar cells manufactured in accordance with the embodiments described herein weigh less and are thus well suited for applications where weight is a concern such as space applications. In addition, the solar cells are relatively thin and thus can be readily attached to curved surfaces. Still other advantages of having thin solar cells attached to a flexible film will become readily apparent in view of the detailed description below. 
         [0015]      FIG. 1  shows an exploded perspective view of an embodiment of a sequence of layers of semiconductor material  100  temporarily bonded to a support substrate  110 . The sequence of layers of semiconductor material  100  forms at least one solar cell and is deposited on a growth substrate  120  such as a GaAs wafer, Ge wafer, etc. A flexible film  130  is interposed between the support substrate  110  and the growth substrate  120 . In one embodiment, the flexible film  130  is a polyimide film such as Kapton (manufactured by DuPont.). One side  132  of the flexible film  130  is permanently bonded to the sequence of layers of semiconductor material  100  using a permanent adhesive  140  so that the flexible film  130  cannot be easily removed from the sequence of layers of semiconductor material  100 . The other side  134  of the flexible film  130  is temporarily bonded to the support substrate  110  using a temporary adhesive  150  so that the support substrate  110  can be easily removed from the flexible film  130  without causing damage to the sequence of layers of semiconductor material  100 . 
         [0016]    The support substrate  110  provides support to the sequence of layers of semiconductor material  100  during subsequent processing step(s). This way, the growth substrate  120  on which the sequence of layers of semiconductor material  100  is deposited can be removed after attachment to the support substrate  110 . The sequence of layers of semiconductor material  100  can also be segmented into individual solar cell chips (not shown in  FIG. 1 ) when attached to the support substrate  110  without causing damage to the chips. After completing the desired processing step(s), the support substrate  110  is removed from the flexible film  130 . In one embodiment, the support substrate  110  has holes  112  which extend from one surface  114  of the support substrate  110  to the opposing surface  116  as indicated by the dashed lines in the Figures. The support substrate  110  may comprise any suitable material such as sapphire or any other material having suitable chemical and temperature stability and strength. In one embodiment, the support substrate  110  has a thickness of about 40 mils. In one embodiment, the support substrate  110  is removed from the flexible film  130  by applying an adhesive remover to the holes  112  which dissolves the temporary adhesive  150 , leaving the sequence of layers of semiconductor material  100  permanently bonded to the flexible film  130 . 
         [0017]      FIG. 2  shows a cross-sectional view of the growth substrate  120  after the sequence of layers of semiconductor material  100  is deposited on the substrate  120 , e.g. via epitaxial growth. The sequence of layers of semiconductor material  100  can include any number and type of layers of semiconductor material for generating current in response to incident light. In one embodiment, the layers  100  form at least one inverted metamorphic multifunction (IMM) solar cell, e.g., as described in U.S. Patent Application Pub. No. 2010/0122724 A1 (Cornfeld et al.), the contents of which is incorporated herein by reference in its entirety. 
         [0018]    According to one embodiment, the sequence of layers of semiconductor material  100  is deposited on the growth substrate  120  by forming a first solar subcell on the growth substrate  110  having a first band gap and forming a second solar subcell over the first solar subcell having a second band gap smaller than the first band gap. A grading interlayer is formed over the second solar subcell having a third band gap larger than the second band gap. A third solar subcell having a fourth band gap smaller than the second band gap is formed such that the third solar subcell is lattice mismatched with respect to the second solar subcell. In one embodiment, the first solar subcell is composed of an InGaAlP emitter region and an InGaAlP base region and the second solar subcell is composed of an InGaP emitter region and an InGaAs base region. The grading interlayer can be composed of InGaAlAs. Alternatively, the grading interlayer can be composed of a plurality of layers with a monotonically increasing lattice constant. Yet other layers of semiconductor material can be deposited on the growth substrate  120  to form a solar cells which is now ready for attachment to the support substrate  110 . 
         [0019]      FIG. 3  shows a cross-sectional view of the support substrate  110  during bonding to the flexible film  130 . The support substrate  110  is bonded to the flexible film  130  using a temporary adhesive  150  such as Wafer Bond (manufactured by Brewer Science, Inc. of Rolla, Mo.) or any other type of suitable polymer that can be applied by spin coating and has suitable chemical and temperature stability and relatively low curing temperature to produce a temporary bond which can be easily broken without causing damage to the sequence of layers of semiconductor material  100  temporarily attached to the support substrate  110 . 
         [0020]    In one embodiment, the flexible film  130  is vacuum sealed to a chuck (not shown) and the temporary adhesive  150  spun onto the film  130 . The support substrate  110  is then mated with the flexible film  130  while on the chuck. Alternatively, the temporary adhesive  150  can be spun onto the support substrate  110 . According to this embodiment, the holes  112  formed in the support substrate  110  are temporarily plugged so that the adhesive  150  does not escape through the holes  112 . The holes  112  can be plugged by placing tape (not shown) over the side  116  of the support substrate  110  not being bonded to the flexible film  130 . The tape can be removed after the support substrate  110  and flexible film  130  are brought into contact. The support substrate  110  and the flexible film  130  are then bonded together via the temporary adhesive  150  under appropriate heat and/or pressure conditions for curing the temporary adhesive  150 . The growth substrate  120  with the sequence of layers of semiconductor material  100  is also prepared for bonding to the flexible film  130 . 
         [0021]      FIG. 4  shows a cross-sectional view of the growth substrate  120  after the sequence of layers of semiconductor material  100  is deposited thereon. According to one embodiment, the sequence of layers of semiconductor material  100  has a metallized surface  160 . Alternatively, the sequence of layers of semiconductor material  100  does not have a metallized surface. In either case, a permanent adhesive  140  such as benzocyclobutene (BCB) or SU-8 is applied to the surface of the sequence of layers of semiconductor material  100  facing away from the growth substrate  120 . The permanent adhesive  140  can also be applied to the surface  132  of the flexible film  130  not bonded to the support substrate  120  for increased adhesion. 
         [0022]      FIG. 5  shows a cross-sectional view of the two substrates  110 ,  120  during the substrate attachment process. The substrates  110 ,  120  are brought into contact so that the sequence of layers of semiconductor material  100  can be permanently bonded to one surface  132  of the flexible film  130  via the permanent adhesive  140  and the support substrate  110  can be temporarily bonded to the other surface  134  of the flexible film  130  via the temporary adhesive  150 . In one embodiment, the substrates  110 ,  120  are brought into contact under vacuum to prevent air voids in the adhesives  140 ,  150 . An appropriate temperature and/or pressure are applied to the substrates  110 ,  120  for curing the permanent adhesive  140 . Alternatively, the flexible film  130  can be permanently bonded to the sequence of layers of semiconductor material  100  and then temporarily bonded to the support substrate  110 . In either case, the support substrate  110  is temporarily bonded to the sequence of layers of semiconductor material  100 . 
         [0023]      FIG. 6  shows a cross-sectional view of the two substrates  110 ,  120  after the substrates  110 ,  120  are bonded together. At this point, the support substrate  110  can be used to support the sequence of layers of semiconductor material  100  during subsequent processing step(s). 
         [0024]      FIG. 7  shows a cross-sectional view of the bonded structure after the growth substrate  120  is removed, leaving only the sequence of layers of semiconductor material  100  and the flexible film  130  bonded to the support substrate  110 . The growth substrate  120  can be removed by grinding, lapping and/or etching. The support substrate  110  prevents the thin sequence of layers of semiconductor material  100  from being damaged during the substrate removal process. Additional processing can be done to the sequence of layers of semiconductor material  100  while temporarily attached to the support substrate  110 . 
         [0025]      FIG. 8  is a cross-sectional view of the bonded structure after the growth substrate  120  is removed and after a metal grid  200  is formed on the exposed surface of the sequence of layers of semiconductor material  100 . The metal grid  200  collects current from across the surface of the cell, and also can be contacted to bring current to the outside world, interconnect adjacent cells, etc. In one embodiment, the metal grid  200  is formed by evaporation and lithographic patterning. 
         [0026]      FIG. 9  is a cross-sectional view of the bonded structure after the sequence of layers of semiconductor material  100  is cut into a plurality of thin solar cell chips  210 . The flexible film  130  interposed between the layers of semiconductor material  100  and the support substrate  110  can also be cut so that each solar cell chip  210  can be easily separated from the support substrate  110  and still have a portion of the flexible film  130  permanently attached thereto. In one embodiment, the solar cell chips  210  are removed from the support substrate  110  by applying an adhesive remover to the holes  112  formed through the support substrate  110 . The adhesive remover travels through the holes  112  and dissolves the temporary adhesive  150 , freeing the solar cell chips  210  and the flexible film  130  from the support substrate  110  without damaging the chips  210 . 
         [0027]    In another embodiment, the support substrate  110  does not have holes  112  formed therein and the temporary adhesive  150  is dissolved by heating the adhesive  150  to a temperature which breaks the temporary bond between the support substrate  110  and the flexible film  130 . The individual solar cell chips  210  each with a layer of the flexible film  130  permanently bonded thereto can then be attached to any type of desirable surface. The solar cell chips  210  are thin and flexible and can be readily attached to flat or curved surfaces. Cover glasses (not shown) and interconnects  200  can be applied to solar cell chips  210  either before or after demounting from the support substrate  110  since the flexible film  130  provides ample support to the chips  210  during this type of processing. The flexible film  130  permanently bonded to the solar cell chips  210  can be sucked down with a vacuum to make the film  130  flat to do cover glassing and welding or soldering. 
         [0028]      FIG. 10  shows a side-view of an embodiment of a solar panel  300  having a curved surface  302  to which the solar cell chips  210  can be attached. The solar cell chips  210  can be permanently or temporarily attached to the solar panel  300 , e.g. via an appropriate type of adhesive. 
         [0029]    Spatially relative terms such as “under”, “below”, “lower”, “over”, “upper”, and the like, are used for ease of description to explain the positioning of one element relative to a second element. These terms are intended to encompass different orientations of the device in addition to different orientations than those depicted in the figures. Further, terms such as “first”, “second”, and the like, are also used to describe various elements, regions, sections, etc and are also not intended to be limiting. Like terms refer to like elements throughout the description. 
         [0030]    As used herein, the terms “having”, “containing”, “including”, “comprising” and the like are open ended terms that indicate the presence of stated elements or features, but do not preclude additional elements or features. The articles “a”, “an” and “the” are intended to include the plural as well as the singular, unless the context clearly indicates otherwise. 
         [0031]    The present invention may be carried out in other specific ways than those herein set forth without departing from the scope and essential characteristics of the invention. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive, and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.