Patent ID: 12199201

DETAILED DESCRIPTION

The following detailed description is merely illustrative in nature and is not intended to limit the embodiments of the subject matter or the application and uses of such embodiments. As used herein, the word “exemplary” means “serving as an example, instance, or illustration.” Any implementation described herein as exemplary is not necessarily to be construed as preferred or advantageous over other implementations. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, brief summary or the following detailed description.

This specification includes references to “one embodiment” or “an embodiment.” The appearances of the phrases “in one embodiment” or “in an embodiment” do not necessarily refer to the same embodiment. Particular features, structures, or characteristics may be combined in any suitable manner consistent with this disclosure.

Terminology. The following paragraphs provide definitions and/or context for terms found in this disclosure (including the appended claims):

“Comprising.” This term is open-ended. As used in the appended claims, this term does not foreclose additional structure or steps.

“Configured To.” Various units or components may be described or claimed as “configured to” perform a task or tasks. In such contexts, “configured to” is used to connote structure by indicating that the units/components include structure that performs those task or tasks during operation. As such, the unit/component can be said to be configured to perform the task even when the specified unit/component is not currently operational (e.g., is not on/active). Reciting that a unit/circuit/component is “configured to” perform one or more tasks is expressly intended not to invoke 35 U.S.C. § 112, sixth paragraph, for that unit/component.

“First,” “Second,” etc. As used herein, these terms are used as labels for nouns that they precede, and do not imply any type of ordering (e.g., spatial, temporal, logical, etc.). For example, reference to a “first” solar cell does not necessarily imply that this solar cell is the first solar cell in a sequence; instead the term “first” is used to differentiate this solar cell from another solar cell (e.g., a “second” solar cell).

“Coupled”—The following description refers to elements or nodes or features being “coupled” together. As used herein, unless expressly stated otherwise, “coupled” means that one element/node/feature is directly or indirectly joined to (or directly or indirectly communicates with) another element/node/feature, and not necessarily mechanically.

In addition, certain terminology may also be used in the following description for the purpose of reference only, and thus are not intended to be limiting. For example, terms such as “upper”, “lower”, “above”, and “below” refer to directions in the drawings to which reference is made. Terms such as “front”, “back”, “rear”, “side”, “outboard”, and “inboard” describe the orientation and/or location of portions of the component within a consistent but arbitrary frame of reference which is made clear by reference to the text and the associated drawings describing the component under discussion. Such terminology may include the words specifically mentioned above, derivatives thereof, and words of similar import.

“Inhibit”—As used herein, inhibit is used to describe a reducing or minimizing effect. When a component or feature is described as inhibiting an action, motion, or condition it may completely prevent the result or outcome or future state completely. Additionally, “inhibit” can also refer to a reduction or lessening of the outcome, performance, and/or effect which might otherwise occur. Accordingly, when a component, element, or feature is referred to as inhibiting a result or state, it need not completely prevent or eliminate the result or state.

Thermocompression bonding approaches for foil-based metallization of non-metal surfaces of solar cells, and the resulting solar cells, are described herein. In the following description, numerous specific details are set forth, such as specific process flow operations, in order to provide a thorough understanding of embodiments of the present disclosure. It will be apparent to one skilled in the art that embodiments of the present disclosure may be practiced without these specific details. In other instances, well-known fabrication techniques, such as emitter region fabrication techniques, are not described in detail in order to not unnecessarily obscure embodiments of the present disclosure. Furthermore, it is to be understood that the various embodiments shown in the figures are illustrative representations and are not necessarily drawn to scale.

Disclosed herein are solar cells. In one embodiment, a solar cell includes a substrate and a plurality of alternating N-type and P-type semiconductor regions disposed in or above the substrate. A plurality of conductive contact structures is electrically connected to the plurality of alternating N-type and P-type semiconductor regions. Each conductive contact structure includes a metal foil portion disposed in direct contact with a corresponding one of the alternating N-type and P-type semiconductor regions.

In another embodiment, a solar cell includes a substrate and a plurality of alternating N-type and P-type semiconductor regions disposed in or above the substrate. A plurality of conductive contact structures is electrically connected to the plurality of alternating N-type and P-type semiconductor regions. Each conductive contact structure includes a metal foil portion disposed above and in alignment with a corresponding one of the alternating N-type and P-type semiconductor regions. The metal foil portion has a texturized surface proximate to the corresponding one of the alternating N-type and P-type semiconductor regions.

Also disclosed herein are methods of fabricating solar cells. In one embodiment, a method of fabricating a solar cell includes texturizing a surface of a metal foil. The method also includes locating the texturized surface of the metal foil over a non-metalized surface of a wafer of the solar cell. The method also includes, subsequent to the locating, electrically connecting the metal foil with the non-metalized surface of the wafer by thermocompression bonding.

One or more embodiments described herein provides a technique for thermocompression bonding of a metal foil (such as an aluminum foil) to a solar cell. In an embodiment, bonding of an aluminum foil is performed directly to polycrystalline silicon regions of the solar cell and a bottom anti-reflective coating (BARC) layer exposing portions of the polycrystalline silicon regions. The resulting structure may be described as a seed-free thermocompression bonded solar cell, where a seed layer is an otherwise intervening metal layer. Specific embodiments described herein involve thermocompression bonding of an aluminum foil to non-metal surfaces together with foil texturing for implemented for improved thermocompression bonding.

To provide context, process approaches described herein may be motivated by a need for cost and operation reduction in a cell metallization process. Earlier attempts at reducing cost associated with use of an intervening metal seed layer included the use of a printed seed layer, which was limited by compatibility with plating and series resistance. On the other hand, thermocompression bonding of an aluminum foil has previously also required the use of a sputtered metal “seed” layer that is deposited on the cell to enable bonding of the metal foil to semiconductor and insulating materials. By contrast, embodiments described herein do not involve use of a plating process or use of an intervening metal seed layer.

Addressing one or more of the above issues, in accordance with an embodiment of the present disclosure, electrical contact is made directly from a metal foil to a silicon wafer through the contact openings without the need for a metal seed layer. The aluminum foil can be sufficiently adhered to a BARC layer enabling direct contact between the metal foil and exposed semiconductor regions on or in the substrate. Particular embodiments that may be implemented to enhance such direct bonding may include one or more of use of a roller tool for boding, foil cleaning before bonding, higher temperature and pressure for bonding, etc. Regardless, in at least some of the embodiments described below, use of an intervening metal seed layer is omitted.

To exemplify the concepts at hand,FIG.1illustrates a cross-sectional view of a solar cell including a metal seed layer. Referring toFIG.1, a solar cell100includes a substrate102. A plurality of alternating N-type and P-type semiconductor regions104is disposed in or above the substrate102. A bottom-anti-reflective coating (BARC) layer106is disposed over and exposes portions of the plurality of alternating N-type and P-type semiconductor regions104. A metal seed layer108is disposed on the BARC layer106and on the exposed portions of the plurality of alternating N-type and P-type semiconductor regions104. A metal foil110is disposed on the metal seed layer108.

In contrast toFIG.1,FIG.2illustrates a cross-sectional view of a solar cell omitting an intervening metal seed layer, in accordance with an embodiment of the present disclosure. Referring toFIG.2, a solar cell200includes a substrate202. A plurality of alternating N-type and P-type semiconductor regions204is disposed in or above the substrate202. A bottom-anti-reflective coating (BARC) layer206is disposed over and exposes portions of the plurality of alternating N-type and P-type semiconductor regions204. A metal foil210is disposed on the BARC layer206and on the exposed portions of the plurality of alternating N-type and P-type semiconductor regions204. The metal foil210is in direct contact with the exposed portions of the plurality of alternating N-type and P-type semiconductor regions204. In an embodiment, the metal foil210is bonded to the alternating N-type and P-type semiconductor regions204by thermocompression bonding. It is to be appreciated thatFIG.2may represent a partially completed solar cell, as further processing may include patterning of the metal foil210, exemplary embodiments of which are described in greater detail below. It is to be appreciated that reference to a BARC layer throughout may be used to more generally refer to a dielectric layer, where the dielectric layer may be anti-reflective or reflective depending on specific implementations. Nonetheless, use of the term “BARC layer” is consistent with general usage of such a dielectric layer in the art.

It is to be appreciated that openings in the BARC layer206that accommodate direct contact of the metal foil210to the plurality of alternating N-type and P-type semiconductor regions204may be formed prior to metallization or during metallization. For example in one embodiment, openings are formed in the BARC layer206prior to metallization, e.g., by patterning with laser ablation of a lithography and etch process. In another embodiment, metallization to form a direct contact of the metal foil210to the plurality of alternating N-type and P-type semiconductor regions204may be performed through the BARC layer206, e.g., by laser or other spot welding, to effectively create openings in the BARC layer206that surround such spot welds.

In accordance with another embodiment of the present disclosure, to enable successful bonding of a metal foil directly to semiconductor regions of a solar cell, texturized foil is used or a foil texturizing operation is performed. As an example,FIG.3Aillustrates a cross-sectional and magnified view of an initial interface between a texturized metal foil and a BARC layer of a solar cell, in accordance with an embodiment of the present disclosure. Referring toFIG.3A, a metal foil300has a texturized surface302in contact with a BARC layer304of a solar cell. The BARC layer may be used to partially cover an underlying semiconductor region (not shown). In one embodiment, the texturized surface302is described as including valleys306.

Not to be bound by theory, it is understood that the valleys306may provide a place for metal to flow to during a bonding process. As a first example,FIG.3Billustrates a cross-sectional and magnified view of bonding interface between a texturized metal foil and a BARC layer of a solar cell where the bonding is performed using a roller, in accordance with an embodiment of the present disclosure. Referring toFIG.3B, a roller320is rolled in the direction322across the surface of metal foil300. A metal flow region324is formed between the texturized surface302and the BARC layer304. In one embodiment, the metal flow region324fills in and at least partially closes the valleys306, providing a strong bond between the metal foil300and the BARC layer304.

In a second example,FIG.3Cillustrates a cross-sectional and magnified view of bonding interface between a texturized metal foil and a BARC layer of a solar cell where the bonding is performed using a pressure plate, in accordance with an embodiment of the present disclosure. Referring toFIG.3C, a pressure plate340is pressed down on the surface of metal foil300. A metal flow region344is formed between the texturized surface302and the BARC layer304. In one embodiment, the metal flow region344fills in and at least partially closes the valleys306, providing a strong bond between the metal foil300and the BARC layer304.

FIG.4is a magnified optical image400of a texturized aluminum metal foil in accordance with an embodiment of the present disclosure. Referring toFIG.4, a surface402of a metal foil has texture404therein. In one embodiment, the texture404is created by a wire brush406, such as is depicted in the inset of the image400. In one such embodiment, the brushed foil texture404is created by first cleaning and then texturizing with a wire brush in two directions. The process may provide a fresh oxide surface and higher bond pressure on raised portions of the metal foil. In other embodiments, embossing or etching is used to texturize the metal foil.

As described above, inclusion of a foil texturizing process for solar cell manufacture can enable adhesion of a metal foil to a solar cell.FIG.5is a flowchart500representing various operations in a method of fabricating a solar cell, in accordance with an embodiment of the present disclosure.

Referring to operation502ofFIG.5, a method of fabricating a solar cell includes texturizing a surface of a metal foil, examples of which were described in association withFIGS.3A and4. In one embodiment, texturizing the surface of the metal foil includes using a technique selected from the group consisting of brushing, embossing and etching.

Referring to operation504ofFIG.5, the method also includes locating the texturized surface of metal foil over a non-metalized surface of a wafer of the solar cell, e.g., by foil and cell alignment. In one embodiment, locating the metal foil with the non-metalized surface of the wafer includes performing a tacking process. In a specific such embodiment, the tacking process involves first forming an array of point or spot welds. The array of point or spot welds may be formed by thermocompression bonding, e.g., using spikes, a spiked roller, a porcupine roller, or a bed of nails. Alternatively the locating may be performed using a laser welding process.

Not to be bound by theory, it is understood that a tacking process may involve breaking through portions of one or more metal oxide layers at an interface between a metal foil and a non-metalized surface of a solar cell to effectively form a plurality of spot welds. In an embodiment, the plurality of spot welds provides channels between the metal foil and the non-metalized surface for subsequent removal of air from between the metal foil and the non-metalized surface.

Referring to operation506ofFIG.5, the method also includes, subsequent to the locating, bonding the metal foil with the non-metalized surface of the wafer by thermocompression bonding, examples of which were described in association withFIGS.3B and3C. In an embodiment, a portion of the non-metallized surface of the solar cell is a semiconductor region exposed by a BARC layer, and the thermocompression bonding electrically connects the metal foil to the semiconductor region. In an embodiment, a relatively high bonding force is used for the thermocompression bonding, e.g., an approximately 50 kg force may be applied using an approximately on 9.5 mm diameter roller. In an embodiment, a relatively high bonding temperature is used, e.g., bonding temperature of approximately 440 degrees Celsius. In one specific embodiment, the thermocompression bonding involves applying a shear force to the metal foil, an example of which is the roller process ofFIG.3B. In another specific embodiment, the thermocompression bonding involves applying a normal force to the metal foil, an example of which is the pressure plate process ofFIG.3C.

As exemplified inFIG.3Babove, in an embodiment, a shear thermocompression process is used to bond the metal foil to the wafer of the solar cell. Other approaches for generating such a shear force may include pressing a graphite puck into the metal foil over the center of the wafer and moving it toward the outside of the wafer in a spiral motion so as to expel the air from between the foil and the wafer, while still pressing the puck downwards on the metal foil. In another approach, a set of graphite paddles or squeegees are used to bond down the metal foil. One possible sequence is to use two paddles to swipe left and right from the center to bond a center strip, followed by up and down motions from the center strip to complete the bonding. It is to be appreciated that other swipe sequences may also be suitable. In one embodiment, a vacuum fixture is implemented to evacuate the air from between the metal foil and the wafer during thermocompression bonding.

A metal foil and solar cell pairing approach involving thermocompression bonding may be implemented using a non-metallized surface of a wafer of the solar cell. As an example,FIGS.6A and6Billustrate angled views of various stages in the fabrication of a solar cell using foil-based metallization. Referring toFIG.6A, a metal foil608is placed over a wafer602having a plurality of emitter regions604(which may include non-metallized polycrystalline silicon regions) disposed on or above a substrate606. InFIG.6B, the metal foil is fit-up with the substrate606, which may include a tacking process.

Upon fitting up of the metal foil608and the substrate606, the metal foil is thermocompression bonded to the plurality of emitter regions604. In an embodiment, a shear force is applied during the thermocompression bonding. In another embodiment, a normal force is applied during the thermocompression bonding. The thermocompression bonding may electrically connect a substantial portion of the metal foil608with a non-metalized plurality of emitter regions604.

In an embodiment, at the time of joining the metal foil608and the substrate602, the metal foil608has a surface area substantially larger than a surface area of the wafer602of the solar cell. In one such embodiment, subsequent to electrically contacting the metal foil608to the non-metalized plurality of emitter regions604, the metal foil is cut to provide the metal foil608having a surface area substantially the same as the surface area of the wafer602of the solar cell. In another embodiment, however, prior to placing the metal foil608over the non-metalized plurality of emitter regions604of the solar cell, a large sheet of foil is cut to provide the metal foil608having a surface area substantially the same as a surface area of the wafer602of the solar cell, as is depicted inFIG.6A.

In an embodiment, the resulting structures from the process described above in association withFIGS.6A and6Bare subjected to a contact patterning process. As an example,FIGS.7A-7Cillustrate cross-sectional views of various stages in the fabrication of a solar cell using foil-based metallization, in accordance with an embodiment of the present disclosure.

Referring toFIG.7A, a plurality of alternating N-type and P-type semiconductor regions are disposed above a substrate. In particular, a substrate700has disposed there above N-type semiconductor regions704and P-type semiconductor regions706disposed on a thin dielectric material702as an intervening material between the N-type semiconductor regions704or P-type semiconductor regions706, respectively, and the substrate700. The substrate700has a light-receiving surface701opposite a back surface above which the N-type semiconductor regions704and P-type semiconductor regions706are formed.

In an embodiment, the substrate700is a monocrystalline silicon substrate, such as a bulk single crystalline N-type doped silicon substrate. It is to be appreciated, however, that substrate700may be a layer, such as a multi-crystalline silicon layer, disposed on a global solar cell substrate. In an embodiment, the thin dielectric layer702is a tunneling silicon oxide layer having a thickness of approximately 2 nanometers or less. In one such embodiment, the term “tunneling dielectric layer” refers to a very thin dielectric layer, through which electrical conduction can be achieved. The conduction may be due to quantum tunneling and/or the presence of small regions of direct physical connection through thin spots in the dielectric layer. In one embodiment, the tunneling dielectric layer is or includes a thin silicon oxide layer.

In an embodiment, the alternating N-type and P-type semiconductor regions704and706, respectively, are formed from polycrystalline silicon formed by, e.g., using a plasma-enhanced chemical vapor deposition (PECVD) process. In one such embodiment, the N-type polycrystalline silicon emitter regions704are doped with an N-type impurity, such as phosphorus. The P-type polycrystalline silicon emitter regions706are doped with a P-type impurity, such as boron. As is depicted inFIG.7A, the alternating N-type and P-type semiconductor regions704and706may have trenches708formed there between, the trenches708extending partially into the substrate700. Additionally, in one embodiment, a bottom anti-reflective coating (BARC) material710or other protective layer (such as a layer amorphous silicon) is formed on the alternating N-type and P-type semiconductor regions704and706, exposing only portions of the N-type and P-type semiconductor regions704and706, as is depicted inFIG.7A. In one such embodiment, the metal foil is in direct contact with, and possibly thermocompression bonded to, the BARC material layer710. In an embodiment, BARC layer includes a silicon-rich silicon nitride layer. It is to be appreciated that reference to a BARC layer throughout may be used to more generally refer to a dielectric layer, where the dielectric layer may be anti-reflective or reflective depending on specific implementations. Nonetheless, use of the term “BARC layer” is consistent with general usage of such a dielectric layer in the art.

In an embodiment, the light receiving surface701is a texturized light-receiving surface, as is depicted inFIG.7A. In one embodiment, a hydroxide-based wet etchant is employed to texturize the light receiving surface701of the substrate700and, possibly, the trench708surfaces as is also depicted inFIG.7A. It is to be appreciated that the timing of the texturizing of the light receiving surface may vary. For example, the texturizing may be performed before or after the formation of the thin dielectric layer702. In an embodiment, a texturized surface may be one which has a regular or an irregular shaped surface for scattering incoming light, decreasing the amount of light reflected off of the light receiving surface701of the solar cell. Referring again toFIG.7A, additional embodiments can include formation of a passivation and/or anti-reflective coating (ARC) layers (shown collectively as layer712) on the light-receiving surface701. It is to be appreciated that the timing of the formation of passivation and/or ARC layers may also vary. It is also to be appreciated that while N-type and P-type semiconductor regions704and706are depicted and described as regions discrete from substrate700, in another embodiment, semiconductor regions are fabricated using diffusion regions formed in the substrate700.

Referring again toFIG.7A, a metal foil718is adhered to the alternating N-type and P-type semiconductor regions704and706by directly coupling portions of the metal foil718with a corresponding portion of each of the alternating N-type and P-type semiconductor regions704and706. In one such embodiment, the direct coupling of portions of the metal foil718with a corresponding portion of each of the alternating N-type and P-type semiconductor regions704and706involves thermocompression bonding that may involve formation of a metal flow region714at each of such locations, as is depicted inFIG.7A. In relation to embodiments described herein, such a metal flow region714is considered part of the metal foil718.

In an embodiment, the metal foil718is an aluminum (Al) foil having a thickness approximately in the range of 5-100 microns. In one embodiment, the Al foil is an aluminum alloy foil including aluminum and second element such as, but not limited to, copper, manganese, silicon, magnesium, zinc, tin, lithium, or combinations thereof. In one embodiment, the Al foil is a temper grade foil such as, but not limited to, F-grade (as fabricated), O-grade (full soft), H-grade (strain hardened) or T-grade (heat treated). In one embodiment, the aluminum foil is an anodized aluminum foil.

FIG.7Billustrates the structure ofFIG.7Afollowing formation of laser grooves in the metal foil. Referring toFIG.7B, the metal foil718is laser ablated through only a portion of the metal foil718at regions corresponding to locations between the alternating N-type and P-type semiconductor regions704and706, e.g., above trench708locations as is depicted inFIG.7B. The laser ablating forms grooves730that extend partially into, but not entirely through, the metal foil718. In an embodiment, forming laser grooves730involves laser ablating a thickness of the metal foil718approximately in the range of 80-99% of an entire thickness of the metal foil718. That is, in one embodiment, it is critical that the lower portion of the metal foil718is not penetrated, such that metal foil718protects the underlying emitter structures. In an alternative embodiment, an indentation approach may be used in place of a laser ablation approach.

The grooves730ofFIG.7Bmay then be used to isolate conductive regions740as metallization structures for the underlying emitter regions. For example, referring toFIG.7C, the grooves730are extended to provide gaps732between conductive regions740. In an embodiment, the patterned metal foil718is etched to isolate portions740of the metal foil718. In one such embodiment, the structure ofFIG.7Bis exposed to a wet etchant. Although the wet etchant etches all exposed portions of the metal foil718, a carefully timed etch process is used to break through the bottoms of the laser grooves730without significantly reducing the thickness of the non-grooved regions740of the metal foil718, as is depicted inFIG.7C. In a particular embodiment, a hydroxide based etchant is used, such as, but not limited to, potassium hydroxide (KOH) or tetramethylammonium hydroxide (TMAH).

In another embodiment (not shown), the remaining metal foil718ofFIG.7Bis subsequently anodized at exposed surfaces thereof to isolate regions740of the remaining metal foil718corresponding to the alternating N-type and P-type semiconductor regions704and706. In particular, the exposed surfaces of the metal foil718, including the surfaces of the grooves730, are anodized to form an oxide coating. At locations corresponding to the alternating N-type and P-type semiconductor regions704and706, e.g., in the grooves730at locations above the trenches708, the entire remaining thickness of the metal foil718is anodized there through to isolate regions740of metal foil718remaining above each of the N-type and P-type semiconductor regions704and706.

Referring again toFIG.7C, a solar cell750includes a substrate700and a plurality of alternating N-type and P-type semiconductor regions704and706disposed in or above the substrate700. A plurality of conductive contact structures740is electrically connected to the plurality of alternating N-type and P-type semiconductor regions704and706. Each conductive contact structure includes a metal foil portion740disposed in direct contact with a corresponding one of the alternating N-type and P-type semiconductor regions704and706.

In accordance with an embodiment of the present disclosure, the metal foil portions740each have a texturized surface proximate to the corresponding one of the alternating N-type and P-type semiconductor regions704and706. Examples of such a texturized surface are described in association withFIG.3A-3C and4. In one such embodiment, the texturized surface of the metal foil portion is in direct contact with the corresponding one of the alternating N-type and P-type semiconductor regions704and706. However, although embodiments described herein are largely directed to seedless arrangements, a texturized surface for a metal foil may enhance arrangements where a seed layer is still used. Accordingly, in an alternative embodiment, although not depicted, the texturized surface of the metal foil portions740is disposed on a metal seed layer disposed on the corresponding one of the alternating N-type and P-type semiconductor regions704and706.

Although certain materials are described specifically with reference to above described embodiments, some materials may be readily substituted with others with such embodiments remaining within the spirit and scope of embodiments of the present disclosure. For example, in an embodiment, a different material substrate, such as a group III-V material substrate, can be used instead of a silicon substrate. Additionally, although reference is made significantly to back contact solar cell arrangements, it is to be appreciated that approaches described herein may have application to front contact solar cells as well. In other embodiments, the above described approaches can be applicable to manufacturing of other than solar cells. For example, manufacturing of light emitting diode (LEDs) may benefit from approaches described herein.

Thus, thermocompression bonding approaches for foil-based metallization of non-metal surfaces of solar cells, and the resulting solar cells, have been disclosed.

Although specific embodiments have been described above, these embodiments are not intended to limit the scope of the present disclosure, even where only a single embodiment is described with respect to a particular feature. Examples of features provided in the disclosure are intended to be illustrative rather than restrictive unless stated otherwise. The above description is intended to cover such alternatives, modifications, and equivalents as would be apparent to a person skilled in the art having the benefit of this disclosure.

The scope of the present disclosure includes any feature or combination of features disclosed herein (either explicitly or implicitly), or any generalization thereof, whether or not it mitigates any or all of the problems addressed herein. Accordingly, new claims may be formulated during prosecution of this application (or an application claiming priority thereto) to any such combination of features. In particular, with reference to the appended claims, features from dependent claims may be combined with those of the independent claims and features from respective independent claims may be combined in any appropriate manner and not merely in the specific combinations enumerated in the appended claims.