Patent Application
20160284881
Embodiments of the present disclosure are in the field of renewable energy and, in particular, solar cells having epitaxial passivation layers.
Photovoltaic cells, commonly known as solar cells, are well known devices for direct conversion of solar radiation into electrical energy. Generally, solar cells are fabricated on a semiconductor wafer or substrate using semiconductor processing techniques to form a p-n junction near a surface of the substrate. Solar radiation impinging on the surface of, and entering into, the substrate creates electron and hole pairs in the bulk of the substrate. The electron and hole pairs migrate to p-doped and n-doped regions in the substrate, thereby generating a voltage differential between the doped regions. The doped regions are connected to conductive regions on the solar cell to direct an electrical current from the cell to an external circuit coupled thereto.
Efficiency is an important characteristic of a solar cell as it is directly related to the capability of the solar cell to generate power. Likewise, efficiency in producing solar cells is directly related to the cost effectiveness of such solar cells. Accordingly, techniques for increasing the efficiency of solar cells, or techniques for increasing the efficiency in the manufacture of solar cells, are generally desirable. Some embodiments of the present disclosure allow for increased solar cell manufacture efficiency by providing novel processes for fabricating solar cell structures. Some embodiments of the present disclosure allow for increased solar cell efficiency by providing novel solar cell structures.
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.
Solar cells having epitaxial passivation layers are described herein. In the following description, numerous specific details are set forth, such as specific material compositions, 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 lithography and patterning 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 an embodiment, a solar cell includes a crystalline substrate. An epitaxial passivation layer is disposed directly on the crystalline substrate. A plurality of alternating N-type and P-type emitter regions is disposed on the epitaxial passivation layer. In one such embodiment, the plurality of alternating N-type and P-type emitter regions is a plurality of alternating N-type and P-type semiconductor material regions. In another such embodiment, the plurality of alternating N-type and P-type emitter regions is a plurality of alternating N-type and P-type metal regions.
In another embodiment, a solar cell includes a crystalline substrate. A first plurality of emitter regions of a first conductivity type is disposed on a surface of the crystalline substrate. Each of the first plurality of emitter regions includes a doped polycrystalline silicon region disposed on an amorphous dielectric layer. A second plurality of emitter regions of a second conductivity type is disposed on the surface of the crystalline substrate and alternates with the first plurality of emitter regions. Each of the second plurality of emitter regions includes a doped epitaxial material layer disposed directly on the crystalline substrate.
In another embodiment, a solar cell includes a crystalline substrate. An epitaxial passivation layer is disposed directly on the crystalline substrate. A plurality of alternating N-type and P-type emitter regions is disposed in the epitaxial passivation layer.
One or more embodiments described herein are directed to solar cells incorporating III-V material epitaxial passivation. Utilization of epitaxially grown materials can provide better passivation and improve emitter properties as compared with a standard tunnel oxide/polycrystalline silicon structure. Epitaxial growth may allow for improved passivation versus thermal oxide passivation. In addition, by tailoring the properties of the epitaxial material, the tunneling effects and collection properties of the associated emitter regions can be improved. Described herein are at least four structural types applicable to one or both of interdigitated back contact (IBC) or front contact solar cells.
To provide context, a passivated contact strategy typically requires that a principal surface passivation be carried out by thermal oxide formation. However, such an oxide must be thin (e.g., less than approximately 15 Angstroms) to allow a tunneling regime for collection of the respective carriers. The characteristics for such a material can suffer due to processing drifts and errors which manifest in decreased efficiency and reliability. In addition, the passivation can be limited due to imperfect oxidation and lack of barrier abilities to the dopant materials. There is need for improved passivation, improved tunneling properties, improved barrier properties, and improved emitter functionality which, in an embodiment, may be realized using tailored epitaxial materials on Si.
In accordance with one or more embodiments described herein, employing epitiaxial growth such as GaP, AlGaP, GaAs, InGaAs, or other III-V materials on Si(100) (or 110 or 111) can be used in a number of different structures to improve IBC solar cell efficiency and performance. As described below, there are at least four major structures which can be improved by epitaxial passivated growth that can improve and simplify process flows. For example, in an embodiment, a thin epitaxial layer is deposited as a tunneling layer prior to a process flow for fabricating polysilicon emitters. Such an approach can allow for better passivation by minimizing dangling bonds on the device/Si interface as well as may allow for improved dopant barrier. Improved tunneling effects may also be achieved by minimizing minority carrier barrier in the valence band allowing for a more robust process flow as thickness of the epitaxial layer can be increased. In another embodiment, an epitaxial material is tailored as an intrinsic layer and metal contacts are used to adjust Fermi levels to determine polarity collection. Such an approach may allow for a drastically reduced process flow and improved collection efficiency. In another embodiment, a doped epitaxial material is used as an N-type emitter and surface passivation layer in a hybrid material architecture process flow. In another embodiment, doped and/or in-situ doped epitaxial material is used as a passivation material and an emitter material. Such an epitaxial grown material may be doped N-type and P-type, respectively, to fabricate the emitter features on the device. Barrier width can be controlled by dopant incorporation allowing tunneling property control.
In a first aspect, an amorphous tunnel oxide layer of a conventional interdigitated back contact solar cell can effectively be replaced by an epitaxial passivation layer. As an example,
Referring to
In an embodiment, the crystalline substrate 102 is a monocrystalline silicon substrate, such as a bulk single crystalline N-type doped silicon substrate. The global crystal orientation at the surface of the substrate 102 may be, e.g., (100), (110) or (111). It is to be appreciated, however, that crystalline substrate 102 may be a layer, such as a multi-crystalline silicon layer, disposed on a global solar cell substrate.
In an embodiment, the epitaxial passivation layer 104 is an epitaxial III-V material layer. In one such embodiment, the epitaxial III-V material layer is a material such as, but not limited to, gallium phosphide (GaP), aluminum gallium phosphide (AlGaP), gallium arsenide (GaAs), indium gallium arsenide (InGaAs), gallium nitride (GaN) or aluminum gallium nitride (AlGaN). In an embodiment, the epitaxial passivation layer 104 is a continuous layer across a global surface (e.g., across surface 103) of the crystalline substrate 102, as is depicted in
In an embodiment, the epitaxial passivation layer 104 is a mono- or single-crystalline layer. In other embodiments, however, the epitaxial passivation layer 104 is a multi- or poly-crystalline epitaxial layer. In an embodiment, the epitaxial layer 104 has a thickness of approximately three times the thickness of a corresponding tunnel dielectric layer, e.g., the epitaxial passivation layer 104 has a thickness of approximately 50 Angstroms. In an embodiment, the epitaxial passivation layer 104 is an undoped or intrinsic layer.
In an embodiment, the plurality of alternating N-type 106 and P-type 108 semiconductor material regions is a plurality of alternating phosphorous-doped polycrystalline silicon and boron-doped polycrystalline silicon regions. In one such embodiment, the alternating N-type and P-type polycrystalline silicon regions are formed by, e.g., using a plasma-enhanced chemical vapor deposition (PECVD) process.
Referring again to
It is to be appreciated that additional layers may be formed above the structure of
Referring again to
Energy band diagrams may be used to represent the effect of including an epitaxial passivation layer in the structure of
In line with the above first aspect, metal Fermi level tailoring to form emitter regions is described. As an example,
Referring to
In an embodiment, the crystalline substrate 202 is a monocrystalline silicon substrate, such as a bulk single crystalline N-type doped silicon substrate. The global crystal orientation at the surface of the substrate 202 may be, e.g., (100), (110) or (111). It is to be appreciated, however, that crystalline substrate 202 may be a layer, such as a multi-crystalline silicon layer, disposed on a global solar cell substrate.
In an embodiment, the epitaxial passivation layer 204 is an epitaxial III-V material layer. In one such embodiment, the epitaxial III-V material layer is a material such as, but not limited to, gallium phosphide (GaP), aluminum gallium phosphide (AlGaP), gallium arsenide (GaAs), indium gallium arsenide (InGaAs), gallium nitride (GaN) or aluminum gallium nitride (AlGaN). In an embodiment, the epitaxial passivation layer 204 is a continuous layer across a global surface (e.g., across surface 203) of the crystalline substrate 202, as is depicted in
In an embodiment, the epitaxial passivation layer 204 is a mono- or single-crystalline layer. In other embodiments, however, the epitaxial passivation layer 204 is a multi- or poly-crystalline epitaxial layer. In an embodiment, the epitaxial layer 204 has a thickness of approximately three times the thickness of a corresponding tunnel dielectric layer, e.g., the epitaxial passivation layer 204 has a thickness of approximately 50 Angstroms. In an embodiment, the epitaxial passivation layer 204 is an undoped or intrinsic layer.
In an embodiment, the plurality of alternating N-type 206 and P-type 208 metal regions is a plurality of alternating aluminum (Al) and nickel (Ni) regions. In another embodiment, the plurality of alternating N-type 206 and P-type 208 metal regions is a plurality of alternating aluminum (Al) and platinum (Pt) or nickel (Ni) regions.
Referring again to
It is to be appreciated that additional layers may be formed above the structure of
Referring again to
Energy band diagrams may be used to represent the effect of including an epitaxial passivation layer in the structure of
In a second aspect, a solar cell having different structural types for N-type and P-type emitter regions may be fabricated to include an epitaxial passivation layer. As an example,
Referring to
In an embodiment, the doped polycrystalline silicon region 308 of each of the first plurality of emitter regions includes P-type doped polycrystalline silicon, and the doped epitaxial material layer 304 is an N-type doped epitaxial material layer. In another embodiment, the doped polycrystalline silicon region 308 of each of the first plurality of emitter regions includes N-type doped polycrystalline silicon, and the doped epitaxial material layer 304 is a P-type doped epitaxial material layer. Referring again to
In an embodiment, the doped epitaxial material layer 304 is a doped epitaxial III-V material layer. In one such embodiment, the doped epitaxial III-V material layer is a doped material such as, but not limited to, a doped GaP, AlGaP, GaAs, InGaAs, GaN or AlGaN layer. In an embodiment, the doped epitaxial material layer 304 is N-type doped with phosphorous. In another embodiment, the doped epitaxial material layer 304 is P-type doped with boron. In an embodiment, the doped epitaxial passivation layer 304 is formed by chemical vapor deposition (CVD), physical vapor deposition (PVD) or molecular beam epitaxy (MBE). In an embodiment, the doped epitaxial passivation layer 304 is a mono- or single-crystalline layer. In other embodiments, however, the doped epitaxial passivation layer 304 is a multi- or poly-crystalline epitaxial layer. In an embodiment, the doped epitaxial layer 304 has a thickness of approximately 50-1000 Angstroms.
In an embodiment, the thin dielectric layer 320 is 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.
Referring again to
It is to be appreciated that additional layers may be formed above the structure of
Referring again to
Energy band diagrams may be used to represent the effect of including an epitaxial passivation layer in the structure of
In a third aspect, a selectively doped epitaxial material passivation layer is incorporated to provide emitter regions for a solar cell. As an example,
Referring to
In an embodiment, the epitaxial material layer 404 is an epitaxial III-V material layer. In one such embodiment, the epitaxial III-V material layer is a material such as, but not limited to, a GaP, AlGaP, GaAs, InGaAs, GaN or AlGaN layer. In an embodiment, the doped N-type portions 404A of the epitaxial material layer 404 are N-type doped with phosphorous, and the doped P-type portions 404B of the epitaxial material layer 404 are P-type doped with boron. It is to be appreciated, however, that other dopants such as Si (for N-Type) or Mg (for P-type) may be used. In an embodiment, the epitaxial passivation layer 404 is formed by chemical vapor deposition (CVD), physical vapor deposition (PVD) or molecular beam epitaxy (MBE). Regions of the epitaxial passivation layer 404 may then be doped, e.g., with a patterned PSG layer 422 and a patterned BSG layer 424 to form N-type and P-type regions, respectively, as is depicted in
Referring again to
It is to be appreciated that additional layers may be formed above the structure of
Referring again to
Energy band diagrams may be used to represent the effect of including an epitaxial passivation layer in the structure of
Overall, although certain materials are described specifically above, some materials may be readily substituted with others with other such embodiments remaining within the spirit and scope of embodiments of the present disclosure. For example, in an embodiment, a different substrate material ultimately provides a solar cell substrate. In one such embodiment, a group III-V material substrate ultimately provides a solar cell substrate. Furthermore, it is to be appreciated that, where N+ and P+ type doping is described specifically, other embodiments contemplated include the opposite conductivity type, e.g., P+ and N+ type doping, respectively. 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, solar cells having epitaxial passivation layers 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.
