Patent Publication Number: US-2021175375-A1

Title: Metal-foil-assisted fabrication of thin-silicon solar cell

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 15/722,480, filed on Oct. 2, 2017, which is a continuation of U.S. application Ser. No. 13/725,580, filed on Dec. 21, 2012, now U.S. Pat. No. 9,812,592, all of which are incorporated herein by reference in their entirety. 
    
    
     TECHNICAL FIELD 
     Embodiments of the subject matter described herein relate generally to solar cells. More particularly, embodiments of the subject matter relate to solar cell fabrication processes and structures. 
     BACKGROUND 
     Solar cells are well known devices for converting solar radiation to electrical energy. A solar cell has a front side that faces the sun during normal operation to collect solar radiation and a backside opposite the front side. Solar radiation impinging on the solar cell creates electrical charges that may be harnessed to power an external electrical circuit, such as a load. 
     Solar cell fabrication processes typically include numerous steps involving masking, etching, deposition, diffusion, and other steps. Embodiments of the present invention provide advantageous solar cell processes. 
     BRIEF SUMMARY 
     One embodiment relates to a method of fabricating a solar cell. A silicon lamina is cleaved from the silicon substrate. The backside of the silicon lamina includes the P-type and N-type doped regions. A metal foil is attached to the backside of the silicon lamina. The metal foil may be used advantageously as a built-in carrier for handling the silicon lamina during processing of a frontside of the silicon lamina. 
     Another embodiment relates to a solar cell that includes a silicon lamina having P-type and N-type doped regions on the backside. A metal foil is adhered to the backside of the lamina, and there are contacts formed between the metal foil and the doped regions. 
     Another embodiment relates to a method of fabricating a solar cell that involves adhering a metal foil to a backside of a silicon substrate. A silicon lamina may then be separated from the backside of the silicon substrate. The metal foil is used as a built-in carrier for handling the silicon lamina during processing of a frontside of the silicon lamina. 
     These embodiments and other embodiments, aspects, and features of the present invention will be readily apparent to persons of ordinary skill in the art upon reading the entirety of this disclosure, which includes the accompanying drawings and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete understanding of the subject matter may be derived by referring to the detailed description and claims when considered in conjunction with the following figures, wherein like reference numbers refer to similar elements throughout the figures. The figures are not drawn to scale. 
         FIGS. 1-6  are cross-sectional views schematically illustrating fabrication of a solar cell in accordance with an embodiment of the present invention. 
         FIG. 7  is a flow diagram of a method of fabricating a solar cell in accordance with an embodiment of the present invention. 
         FIG. 8  is a flow diagram of a method of fabricating a solar cell in accordance with an alternate embodiment of the present invention. 
         FIG. 9  is a cross-sectional view of a fabricated solar cell as fabricated in accordance with the method of  FIG. 8 . 
         FIG. 10  is a planar view of a metal foil over the backside of a silicon lamina in accordance with an embodiment of the present invention. 
         FIG. 11  is a flow diagram of a method of fabricating a thin-silicon solar cell in accordance with another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     In the present disclosure, numerous specific details are provided, such as examples of apparatus, structures, materials, and methods, to provide a thorough understanding of embodiments of the invention. Persons of ordinary skill in the art will recognize, however, that the invention can be practiced without one or more of the specific details. In other instances, well-known details are not shown or described to avoid obscuring aspects of the invention. 
     The present disclosure provides techniques for forming thin-silicon solar cells using a metal foil. Advantageously, the metal foil may be used as a built-in carrier for handling the otherwise fragile silicon lamina during processing of a frontside of the lamina. Subsequently, the metal foil may be re-used to form metal fingers and contacts to the P-type and N-type emitters on the backside of the lamina. 
       FIGS. 1-6  are cross-sectional views schematically illustrating fabrication of a thin-silicon solar cell in accordance with an embodiment of the present invention. Shown in  FIG. 1  is a silicon substrate  102  having formed on it a P-type doped (P+) region  104  and an N-type doped (N+) region  106  formed on the backside of the substrate  102 . The P+ and N+ doped regions may be referred to as P-type and N-type emitters in the context of the solar cell being fabricated. In the backside contact solar cell, which is shown in  FIG. 1 , the emitters and corresponding contacts are on the backside of the solar cell. The doped regions may be formed, for example, by diffusing dopants from dopant sources. 
     A thin dielectric layer  108  may be formed over the P+ and N+ regions on the backside for electrical insulation, passivation, and/or other purposes. The dielectric layer  108  may comprise, for example, silicon oxide and/or silicon nitride. Alternatively, the emitter surface may be passivated by means other than forming the dielectric layer  108 , such as by chemical passivation, for example. 
     The solar cell structure of  FIG. 1  may be placed in an ion implantation tool, which is also referred to as an “ion implanter.” The implanter may be used to implant ions at a predetermined implant depth  202 , as depicted in  FIG. 2 . The ions may be hydrogen ions (i.e. protons). In alternate embodiments, other ions may be implanted or co-implanted with the hydrogen. For example, helium ions may be implanted instead of hydrogen ions, or may be co-implanted with hydrogen ions. The dose of the implantation induces defects at the implant depth so that the planar lamina of silicon above the implant depth may be separated or exfoliated from the remainder of the silicon substrate below the implant depth. The energy of the implantation controls the implant depth and so controls the thickness of the thin silicon substrate after the exfoliation. For example, the energy of the implantation may be calibrated to cleave a thin lamina with a thickness within a range from 10 microns to 100 microns. The exfoliation may be accomplished by heating the substrate at an elevated temperature. 
     As depicted in  FIG. 3 , a metal foil  306  may be adhered to the dielectric layer  108  on the backside of the silicon lamina  302 . The metal foil  306  may be an aluminum foil. To facilitate the foil being used as a carrier for the lamina, an extended area (handling area) of the metal foil  306  may extend beyond a perimeter of the silicon lamina  302 . In an exemplary implementation, the composition of the metal foil  306  may be Al-1% Si (99% aluminum and 1% silicon), or more generally Al-x % Si, where x % is from 0% to 3%. Other compositions for aluminum foil may be used. It is also possible to use metal foils other than aluminum, such as silver foil, for example. 
     In one embodiment, an adhesive layer  304  may be used to adhere the metal foil  306  to the backside of the silicon lamina  302 . The adhesive layer  304  may be a thin layer of epoxy, silicone, ethelyne vinyl acetate (EVA) or other encapsulant material which is applied to the backside of the substrate. In one implementation, the adhesive layer may be a coating pre-applied to the metal foil prior to the adhesion. 
     In an alternate embodiment, the metal foil  306  may be adhered to the backside of the substrate using an array of contact spots between the metal foil  306  and the backside of the substrate. The contact spots may be formed by spot melting of the metal foil using a pulsed laser, for example. In this embodiment, the adhesion layer  304  is not needed. Air gaps beneath the foil between the contact spots may be removed by flattening the foil. 
     As depicted in  FIG. 4 , using the foil as a built-in or integrated carrier to support the lamina, the surface  402  at the front-side of the lamina  302  may then be textured and passivated. The surface texturing serves to increase the capacity of the silicon surface to absorb light, and the surface passivation serves to reduce charge recombination at the surface. The surface texturing may be accomplished using a wet surface etching process, for example. The surface passivation may be accomplished by chemical passivation or by other means. 
     Thereafter, a glass encapsulation process may be performed on the frontside of the silicon lamina  302 .  FIG. 5  shows the resultant glass layer  502  which is attached to the frontside using encapsulant material  503 . 
     As shown in  FIG. 6 , further steps may then be performed on the backside of the silicon lamina  302 . These steps include forming metal contacts  604  and  606  in contact holes to electrically couple to corresponding P+ regions  104  and N+ regions  106 , respectively. A first set of metal contacts  604  may be from the metal foil  306  to the P+ region  104 , and a second set of metal contacts  606  may be formed from the metal foil  306  to the N+ region  106 . In one embodiment, the metal contacts  604  and  606  may be formed using a laser-based contact formation process. In such a process, a laser scanner may controllably scan a pulsed laser beam across the backside of the solar cell being fabricated. The pulsed laser beam may form the contact openings through the adhesive layer  304  and the dielectric layer  108 , and the contact openings may be filled by melted metal from the foil  306 . 
     In addition, a finger separation  608  pattern may be formed on the foil area to electrically separate the first set of metal contacts  604  from the second set of metal contacts  606 . The finger separation  608  may be configured so that the fingers of the foil that lead to the contacts are interdigitated. 
       FIG. 7  is a flow diagram of an exemplary method  700  of fabricating a thin-silicon solar cell in accordance with an embodiment of the present invention. In the exemplary method  700  of  FIG. 7 , emitter regions may be first formed on a silicon wafer per block  702 . The silicon wafer may be of a thickness of several hundred microns or more and may be referred to as a thick handle wafer. The emitter regions include both P-doped and N-doped regions and may be formed on the backside of the wafer as shown in  FIG. 1 . 
     Per block  704 , a thin silicon lamina may be cleaved from the silicon wafer. For example, the silicon lamina may be of a thickness between 10 microns to 100 microns. In one implementation, the cleaving may be performed using ion implantation and exfoliation as described above in relation to  FIG. 2 . Alternatively, the cleaving may be performed by spalling or etching a sacrificial layer from the frontside of the wafer. 
     In block  706 , metal foil may be adhered to the silicon lamina, as described above in relation to  FIG. 3 . In particular, the metal foil may be adhered to the backside surface of the silicon lamina. The metal foil may be of a thickness between 50 microns and 1 millimeter so as to provide mechanical support for the thin silicon lamina. To facilitate the foil being used as a carrier for the lamina, an extended area (handling area) of the metal foil may extend beyond a perimeter of the silicon lamina. In one implementation, the adhesion may be accomplished by using a laser to fire contacts between the metal foil and the silicon lamina. In another implementation, the adhesion may be accomplished using a thin adhesive layer coated on the metal foil. 
     Per block  708 , the metal foil may be used as an integrated carrier for handling the silicon lamina so that the frontside surface of the silicon lamina may be processed. The frontside surface processing may include texturing and passivation, as described above in relation to  FIG. 4 . The surface texturing and passivation may be accomplished, for example, by dipping the lamina into chemical solutions to etch and passivate the frontside surface. Subsequently, the metal-foil-supported silicon lamina may have its frontside processed with a glass lamination procedure, as described above in relation to  FIG. 5 . Subsequent to the frontside processing, the extended area (handling area) of the metal foil may be trimmed. 
     Per block  710 , contacts may be formed from the metal foil to the emitter regions. As described above in relation to  FIG. 6 , the contacts formed may include a first set of contacts  604  to P-doped emitter regions  104  and a second set of contacts  606  to N-doped emitter regions  106 . In addition, a finger separation  608  pattern may be formed on the foil to electrically separate the first set and the second set of contacts. 
     In an alternate embodiment, instead of adhering a continuous metal foil layer to the backside and subsequently creating the finger separation pattern while the foil is attached to the backside, the finger separation pattern may be pre-formed in the metal foil before the metal foil is applied to the backside of the silicon lamina.  FIG. 8  is a flow diagram of an alternate method  800  of fabricating a thin-silicon solar cell which uses such a pre-patterned metal foil in accordance with an embodiment of the present invention. 
     As shown in  FIG. 8 , after the thin silicon lamina is cleaved from the wafer per block  704 , a pre-patterned metal foil may be sandwiched  806  between the backside of the silicon lamina and a secondary substrate. The patterning of the metal foil achieves the finger separation between the P-type and N-type contacts. The secondary substrate may be transparent such that laser light may be transmitted through it. The secondary substrate may be, for example, a stiff polymer layer, such as a polyethylene terephthalate (PET) layer or a fluoropolymer layer. Thereafter, per block  807 , the contacts may be formed between the metal foil and the emitter regions. The formation of the contacts may be accomplished, for example, using a pulsed laser which is transmitted through the secondary substrate to create the contact openings and flow melted metal from the foil into those openings. Per block  708 , the front surface may then be processed, as described above in relation to  FIG. 7 . Subsequent to the frontside processing, the extended area (handling area) of the metal foil may be trimmed. 
       FIG. 9  is a cross-sectional view of a fabricated thin-silicon solar cell as fabricated in accordance with the method  800  of  FIG. 8 . As depicted in  FIG. 9 , the metal foil  306  with the pre-patterned finger separation  908  is sandwiched between the secondary substrate  902  and the backside of the silicon lamina  302 . In addition, a P-type contact  904  and an N-type contact  906  are shown. As described above, these contacts may be formed by transmission of a pulsed laser through the transparent secondary substrate  902 . 
       FIG. 10  is a planar view of a metal foil over the backside of a silicon lamina in accordance with an embodiment of the present invention. The view of  FIG. 10  shows a portion  1004  of the foil over the backside of the lamina and an extended area  1006  of the foil which extends beyond a perimeter  1002  of the lamina. Note that the extended area  1006  may extend over one or more sides of the perimeter and need not necessarily extend over all sides of the perimeter. 
       FIG. 11  is a flow diagram of a method  1100  of fabricating a thin-silicon solar cell in accordance with another embodiment of the present invention. In the exemplary method  1100  of  FIG. 11 , a sacrificial layer may be formed on a silicon substrate per block  1102 . 
     The sacrificial layer may be composed of porous silicon, such as formed in a HF bath with bias. Alternatively, the sacrificial layer may be silicon with, for example, germanium doping and/or a carbon doping, either of which can be formed by epitaxial deposition or a chemical vapor deposition (CVD) process. The sacrificial layer may be thin, on the order of approximately 700 micrometers, although it may be slightly or significantly larger or smaller, as desired for a particular embodiment to perform the functions described herein. For example, in certain embodiments, the sacrificial layer may be as thin as 10 micrometers. Smaller thicknesses may also be used in certain instances. 
     An epitaxial layer of silicon may then be grown over the sacrificial layer per block  1104 . The emitter regions may be formed in the epitaxial layer per block  1106 , and a dielectric layer may be formed over the emitter regions per block  1108 . 
     A metal foil may then be adhered over the emitter regions per block  1110 . Subsequently, epitaxial lift-off per block  1112  may be performed by selective wet etching or otherwise removing the sacrificial layer. After lift-off, the epitaxial layer becomes the silicon lamina of the solar cell. A cross-sectional view of the structure at this point in the process corresponds to the view shown in  FIG. 3 . As disclosed herein, the metal foil provides structural support and an integrated carrier functionality to the silicon lamina. 
     Subsequently, the front surface may be processed per block  708 . The contacts between the metal foil and the emitter regions may then be formed per block  710 . In other words, after the epitaxial lift-off per block  1110 , the processing may proceed as described above in relation to  FIGS. 4-6 . 
     Techniques for forming thin-silicon solar cells using a metal foil have been disclosed. Advantageously, the metal foil may be used as a built-in carrier for handling the otherwise fragile silicon lamina during processing of a frontside of the lamina. Subsequently, the metal foil may be re-used to form the P-type and N-type emitter contacts and metal fingers on the backside of the lamina. 
     While specific embodiments of the present invention have been provided, it is to be understood that these embodiments are for illustration purposes and not limiting. Many additional embodiments will be apparent to persons of ordinary skill in the art reading this disclosure.