Patent Application
20140179056
Embodiments of the present invention are in the field of renewable energy and, in particular, laser absorbing seed layers for solar cell conductive contacts and methods of forming solar cell conductive contacts.
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 invention allow for increased solar cell manufacture efficiency by providing novel processes for fabricating solar cell structures. Some embodiments of the present invention allow for increased solar cell efficiency by providing novel solar cell structures.
Laser-absorbing seed layers for solar cell conductive contacts and methods of forming solar cell conductive contacts 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 invention. It will be apparent to one skilled in the art that embodiments of the present invention 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 invention. 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 methods of fabricating conductive contacts for solar cells. In an embodiment, a method of fabricating a solar cell includes forming a metal seed paste disposed above a substrate. The metal seed paste includes a laser-absorbing species. The metal seed paste is irradiated with a laser to form a metal seed layer. The irradiating includes exciting the laser-absorbing species. A conductive contact for the solar cell is then formed from the metal seed layer where it is in proximity of the silicon. In another embodiment, a method of fabricating a solar cell includes forming a metal seed paste on an emitter region disposed above a substrate. The metal seed paste includes a laser-absorbing species, and the emitter region includes a doped polycrystalline silicon layer. The metal seed paste is irradiated with a laser to form a metal seed layer. The irradiating includes exciting the laser-absorbing species. A conductive back-contact is then formed from the metal seed layer for the emitter region of the solar cell. In yet another embodiment, a method of fabricating a solar cell includes forming a metal seed paste on a surface of an N-type or P-type doped region of an N-type bulk crystalline silicon substrate. The metal seed paste includes a laser-absorbing species. The metal seed paste is irradiated with a laser to form a metal seed layer. The irradiating includes exciting the laser-absorbing species. A conductive back-contact is formed for the N-type or P-type doped region of the substrate from the metal seed layer. Also disclosed herein are compositions for seed layers for forming contacts on solar cells. In an embodiment, a composition includes aluminum/silicon (Al/Si) particles, binders and frit, a solvent, and a laser-absorbing species.
One or more embodiments of the present invention are directed to laser annealing of a printed seed layer having a laser absorber species therein. For example, an economical way perform a module level metallization process flow includes printing a metal seed layer following formation of an encapsulant material. Such an approach can be used whether or not a plating step or a back sheet was applied next. An issue with applying a metal ink or paste on an encapsulant, however, is that performing a thermal anneal to reduce the resistance of the paste may not be possible without degradation of the encapsulant material. In a specific example, an encapsulant material decomposes at approximately 150 degrees Celsius, while a metal paste may need thermal annealing at a temperature approximately in the range of 200-500 degrees Celsius.
To address the above, one or more embodiments involve annealing a metal paste with a laser. To enhance the laser annealing, absorbers can be included in the metal paste to enhance absorbing of the laser energy. By enhancing the absorbing ability of the paste, an annealing may be able to be performed more quickly, since the heating would occur faster. And, possibly, damage to underlying, overlying or adjacent materials can be avoided since the laser radiation would be attenuated or prevented from reaching the material.
In an exemplary embodiment, following opening up of contact holes in an insulating layer (such as a bottom anti-reflective coating layer) on a back side of a solar cell, a plurality of cells can be disposed on glass and encapsulant layers without use of a back sheet. A low cost metal material (e.g., referred to as a paste) is deposited on the back of the cells and over the encapsulant to ultimately fabricate contacts or interconnects for the cells. The process can be performed with a single tool, leading to decreased aggregate processing time and tool cost. In a specific embodiment, the paste includes an absorber to enhance absorption of light at a particular wavelength of light which is matched to a subsequent laser annealing operation.
In the above exemplary embodiment, the laser annealing operation involves scanning a laser over the metal paste lines, annealing the metal paste such that resistivity drops in the paste. The resulting annealed paste can be used as a metal seed layer for plating (as described in greater detail below) or, possibly, to carry current to regional collection points of a metallized back sheet. In the latter case, for example, lower metal densities may be enabled by channeling the cell current to specific points in the module.
As a general overview of an application of embodiments described herein,
Referring to
In an embodiment, the metal seed paste 104 is composed aluminum/silicon (Al/Si) particles, binders, frit, a solvent, and the laser-absorbing species 106. In one such embodiment, the laser-absorbing species 106 is not bound to the Al/Si particles. In another embodiment, however, the laser-absorbing species 106 is bound to the Al/Si particles. In one embodiment, the Al/Si particles are composed of less than approximately 25% Si, with the remainder of the composition made up by Al. In one embodiment, if included, the binders can be composed of zinc oxide (ZnO), tin oxide (SnO), or both, and the frit can be composed of glass particles.
The metal seed paste 104 can be formed in a global deposition or as a patterned deposition, as depicted in
Referring to
In an embodiment, the irradiating 108 involves exciting the laser-absorbing species 106 of the metal seed paste 104. In one such embodiment, during the irradiating process 108, the metal seed paste 104 is annealed by localized heating generated from exciting the laser-absorbing species 106. In a specific such embodiment, the laser absorbing species 106 is volatilized during the annealing and, thus, substantially not present in the resulting metal seed layer 110. In another specific embodiment, however, following the annealing, the laser absorbing species 106 is substantially present in the resulting metal seed layer 110. In an embodiment, the irradiating 108 involves matching a wavelength of a laser with an absorbance maxima peak of the laser-absorbing species 106. For example, in a specific embodiment, the laser-absorbing species 106 are light-absorbing nano-particles, and exciting the laser-absorbing species 106 involves pulsing the laser while scanning the laser across the light-absorbing nano-particles. The laser has a wavelength that is matched with an absorbance maxima peak of the light-absorbing nano-particles.
The irradiating 108 can be performed adjacent to, or in the presence of, materials having otherwise low decomposition or transition temperatures. The localized heating of the laser process ensures that such materials are not overheated, e.g., as can be the case with a more globalized heating process such as thermal annealing. For example, in an embodiment, the irradiating 108 of the metal seed paste 104 with the laser involves performing the irradiating adjacent a material having a decomposition or melting temperature of approximately 150 degrees Celsius. In a specific such embodiment, the material is an encapsulant layer disposed adjacent to the metal seed paste 104, an example of which is described below in association with
Referring to
In an exemplary embodiment, referring again to
As described briefly above, in a first aspect, a metal seed paste that includes a laser-absorbing species can be used to ultimately fabricate contacts, such as back-side contacts, for a solar cell having emitter regions formed above a substrate of the solar cell. For example,
Referring to
In an embodiment, referring again to
In an embodiment, the metal seed layer 230 has a total composition including approximately 10-30% binders and frit, with the remainder the Al/Si particles and, possibly, and remaining laser-absorbing species. In one such embodiment, the binders are composed of zinc oxide (ZnO), tin oxide (SnO), or both, and the frit includes glass particles. It is to be understood that, when initially applied, a seed layer (e.g., an as-applied layer 230) further includes a solvent. However, the solvent can be removed upon annealing the seed layer, leaving essentially the binders, frit and Al/Si particles (and any remaining laser-absorbing species) disposed on the plurality of n-type doped polysilicon regions 320 and the plurality of p-type doped polysilicon regions 322.
In an embodiment (not shown), the metal seed layer 230 has a thickness greater than approximately 50 microns, and the conductive contact 328 fabricated there from is a back contact of the solar cell composed essentially of only the metal seed layer 230. In such an embodiment, the seed layer is not actually a seeding layer used for subsequent plating, as is the case in other embodiments. However, in another embodiment, the metal seed layer 230 has a thickness of approximately 0.5-50 microns. In that embodiment, the conductive contact 328 is a back contact of the solar cell and is composed of the metal seed layer 230, an electroless plated nickel (Ni) layer 232 disposed on the metal seed layer 230, and an electroplated copper (Cu) layer 234 disposed on the Ni layer, as depicted in
As also described briefly above, in a second aspect, a metal seed paste that includes a laser-absorbing species can be used to ultimately fabricate contacts, such as back-side contacts, for a solar cell having emitter regions formed in a substrate of the solar cell. For example,
Referring to
In an embodiment, referring again to
In an embodiment, the metal seed layer 230 has a total composition including approximately 10-30% binders and frit, with the remainder the Al/Si particles and, possibly, and remaining laser-absorbing species. In one such embodiment, the binders are composed of zinc oxide (ZnO), tin oxide (SnO), or both, and the frit is composed of glass particles. It is to be understood that, when initially applied, a seed layer (e.g., an as-applied layer 230) further includes a solvent. However, the solvent can be removed upon annealing the seed layer, leaving essentially the binders, frit (optional) and Al/Si particles (and any remaining laser-absorbing species) disposed on the diffusion regions 220 and 222.
In an embodiment (not shown), the metal seed layer 230 has a thickness greater than approximately 50 microns, and the conductive contact 228 fabricated there from is a back contact of the solar cell composed essentially of only the metal seed layer 230. As mentioned above, in such an example, the seed layer is not actually used for subsequent seeding during an electroplating process, as is the case for other embodiments. However, in another embodiment, the metal seed layer 230 has a thickness of approximately 0.5-50 microns. In that embodiment, the conductive contact 228 is a back contact of the solar cell and is composed of the metal seed layer 230, an electroless plated nickel (Ni) layer 232 disposed on the metal seed layer 230, and an electroplated copper (Cu) layer 234 disposed on the Ni layer, as depicted in
Referring again to
The use of a metal seed paste that includes a laser-absorbing species can allow for localized annealing of the metal seed paste layer such that an adjacent, underlying or overlying temperature sensitive film does is not impacted or degraded during annealing of the metal seed paste. For example, in a first embodiment, referring to
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 invention. 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. In another embodiment, silver (Ag) particles or the like can be used in a seed paste instead of, or in addition to, Al particles. In another embodiment, plated or like-deposited cobalt (Co) can be used instead of or in addition to the plated Ni described above. Similarly, a tungsten (W) layer disposed over the substrate can provide the same function.
Furthermore, the formed contacts need not be formed directly on a bulk substrate, as was described in
Referring to
In an embodiment, the thin dielectric layer 302 is composed of silicon dioxide and has a thickness approximately in the range of 5-50 Angstroms. In one embodiment, the thin dielectric layer 302 ultimately performs as a tunneling oxide layer in a functioning solar cell. In an embodiment, substrate 300 is a bulk single-crystal substrate, such as an n-type doped single crystalline silicon substrate. However, in an alternative embodiment, substrate 300 includes a polycrystalline silicon layer disposed on a global solar cell substrate.
Referring again to
Referring again to
Referring to
Referring to
Thus, in an embodiment, conductive contacts 328 are formed on or above a surface of a bulk N-type silicon substrate 300 opposing a light receiving surface 301 of the bulk N-type silicon substrate 300. In a specific embodiment, the conductive contacts are formed on regions (322/320) above the surface of the substrate 300, as depicted in
In an embodiment, forming the metal seed layer includes printing a paste on a bulk N-type silicon substrate or on a polysilicon layer formed above such as substrate. The paste can be composed of a solvent, aluminum/silicon (Al/Si) alloy particles, and laser-absorbing species. The printing includes using a technique such as, but not limited to, screen printing or ink-jet printing. Additionally, one or more embodiments described herein are directed to approaches to, and structures resulting from, reducing the contact resistance of printed Al/Si seed formed on a silicon substrate by incorporating the electroless-plated Ni therein. More specifically, one or more embodiments are directed to contact formation starting with an Al-based paste seed layer. Annealing by laser is performed after seed printing to form contact between Al from the paste and an underlying silicon substrate or layer. Then Ni is deposited by electroless plating on top of the resulting Al-based seed layer. Since the Al-based seed layer can have a porous structure, in an embodiment, the Ni forms not only above, but also on the outside of the Al particles, and fills up at least a portion of the empty space. In a specific such embodiment, a zincate activation operation is also used in this process. The Ni may be graded in that more Ni may form on upper portions of the Al (away from the Si). Nonetheless, the Ni on the outside of the Al particles can be utilized to reduce the contact resistance of a contact ultimately formed there from. In particular, if the thickness of the Al-based seed layer is generally reduced, more Ni can accumulate at the Al to silicon interface. Compared to conventional approaches, the contacts formed can have a greater surface area of actual metal to silicon contact within a given region of the contact structure formation. As a result, the contact resistance can be lowered relative to conventional contacts.
Thus, laser-absorbing seed layers for solar cell conductive contacts and methods of forming solar cell conductive contacts have been disclosed. In accordance with an embodiment of the present invention, a method of fabricating a solar cell includes forming a metal seed paste above a substrate. The metal seed paste includes a laser-absorbing species. The metal seed paste is irradiated with a laser to form a metal seed layer. The irradiating includes exciting the laser-absorbing species. A conductive contact for the solar cell is then formed from the metal seed layer. In one embodiment, the method further includes, subsequent to irradiating the metal seed paste with the laser, electroless plating a nickel (Ni) layer on the metal seed layer, and electroplating a copper (Cu) layer on the Ni layer.
