FIELD OF THE INVENTION

This disclosure relates generally to the field of multijunction photovoltaic cell fabrication. DESCRIPTION OF RELATED ART

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

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.

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.

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

Referring now to the drawings wherein like elements are numbered alike in the several FIGURES:

FIG. 1 illustrates an embodiment of a method for fabricating a multijunction PV cell.

FIG. 2 illustrates an embodiment of a multijunction PV cell on a substrate.

FIG. 3 illustrates an embodiment of a multijunction PV cell after formation of a top metal layer. FIG. 4 illustrates an embodiment of a multijunction PV cell after adhering a top flexible substrate to the top metal layer.

FIG. 5 illustrates an embodiment of a multijunction PV cell after spalling the substrate.

FIG. 6 illustrates an embodiment of a multijunction PV cell after formation of a bottom metal layer. FIG. 7 illustrates an embodiment of a multijunction PV cell after formation of a bottom metal layer, and after adhering a bottom flexible substrate to the bottom metal layer.

FIG. 8 illustrates an embodiment of a multijunction PV cell after removing the top flexible substrate.

FIG. 9 illustrates an embodiment of a multijunction PV cell after top-of-cell processing. DETAILED DESCRIPTION

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.

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.

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.

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 compressive ly strained cleave layer may be formed in substrate 201 to weaken the substrate 201 at a predetermined 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.

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.

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.

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 902a-c. In other embodiments, metal electrodes 902a-c may be separately deposited on junction 204 after removal of top metal layer 301. Metal electrodes 902a-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 901a-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 901a-b on the surface of junction 204. The antireflective coating 901a- 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.

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.

The technical effects and benefits of exemplary embodiments include a relatively cost- effective method of fabricating a flexible, efficient multijunction PV cell.

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.

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.