LOW-COST MULTI-JUNCTION SOLAR CELLS AND METHODS FOR THEIR PRODUCTION
Methods for fabricating solar cells without the need to perform gasification of metallurgical-grade silicon are disclosed. Consequently, the costs and health and environmental hazards involved in fabricating the solar or silicon grade silicon are being avoided. A solar cell structure comprises a metallurgical grade doped silicon substrate and a thin-film structure formed over the substrate to form a p-i-n junction with the substrate. The substrate may be doped p-type, and the thin film structure may be an intrinsic amorphous layer formed over the substrate and an n-type amorphous layer formed over the intrinsic layer.
1 . A method for producing a solar cell comprising:
obtaining a multi-crystalline substrate consisting essentially of doped metallurgical grade silicon of purity of 99.9%-99.999% doped as one of a p-type or n-type and having an upper side and an underside, the upper side for facing the sun;
forming a first thin-film structure comprising an intrinsic silicon layer formed directly over and in direct contact with the upper side of the substrate and a first doped layer of opposite polarity as that of the substrate and formed directly over and in direct contact with the intrinsic layer, to thereby form a first p-i-n structure with the substrate;
forming a second thin-film structure over the first thin film structure;
forming a top conductive contact above the second thin-film structure;
forming a bottom conductive contact on the underside of the substrate.
2 . The method of claim 1 , further comprising wet chemical etching the substrate so as to texture the substrate prior to forming a first thin-film structure.
3 . The method of claim 1 , wherein forming a first thin-film structure comprises forming at least one of the first intrinsic layer and the first doped layer as an amorphous silicon layer.
4 . The method of claim 3 , wherein in forming a first thin-film structure at least one of the first intrinsic layer and the first doped layer are formed to comprise hydrogen atoms dispersed within a silicon layer.
5 . The method of claim 4 , wherein forming a bottom conductive contact comprises forming a doped conductive layer, of the same polarity as the substrate, on the underside of the substrate and forming a metal layer over the doped conductive layer.
6 . The method of claim 5 , wherein forming the top conductive contact comprises forming a transparent conductor over the first doped layer.
7 . The method of claim 6 , further comprising forming conductive electrodes over the transparent conductor.
8 . The method of claim 5 , further comprising forming an amorphous intrinsic layer on the underside of the substrate such that it is positioned between the underside of the substrate and the doped conductive layer.
9 . The method of claim 1 , wherein obtaining a multi-crystalline substrate comprises slowly solidifying metallurgical silicon melt into a cylinder with large silicon grains and resistivity of about 1 Ohmcm.
10 . The method of claim 1 , wherein forming a the second thin-film structure comprises forming a first doped thin-film over the first thin-film structure, forming an intrinsic thin-film over the first doped thin-film, and forming a second doped thin-film over the intrinsic thin-film, to thereby form a second thin-film p-i-n structure over the first p-i-n structure.
11 . The method of claim 1 , wherein the first doped layer is an n-type layer formed over and in contact with the intrinsic layer, and wherein forming the second thin-film structure comprises forming a p-type layer over and in contact with the first n-type layer, forming a second intrinsic layer over the p-type layer, and forming a second n-type layer.
12 . The method of claim 11 , further comprising forming a diffusion layer in the metallurgical grade substrate, the diffusion layer being one of an n-type or p-type.
13 . The method of claim 1 , wherein obtaining a multi-crystalline substrate comprises comprising obtaining silicon wafers consisting essentially of a p-type metallurgical grade silicon of resistivity of about 1.0 Ωcm.
14 . The method of claim 1 , wherein obtaining a multi-crystalline substrate comprises fabricating multi-crystalline wafers doped with both boron and phosphorous and having p-type polarity.
15 . A method of fabricating a solar cell, comprising:
obtaining a multi-crystalline substrate consisting essentially of doped metallurgical grade silicon of purity of 99.9%-99.999% doped to a first type of conductance;
forming a diffusion layer on top surface of the substrate such that the diffusion layer has opposite conductance of the first type; and,
forming a first thin-film structure over and in contact with the diffusion layer.
16 . The method of claim 15 , further comprising forming a second thin-film structure over the first thin-film structure.
17 . The method of claim 16 , wherein forming a first thin film structure comprises:
forming a first intrinsic layer over and in contact with the diffusion layer; and,
forming a first doped layer of same conductance of the first type over and in contact with the intrinsic layer.
18 . The method of claim 16 , wherein forming a second thin-film structure over the first thin-film structure comprises forming a thin-film p-i-n junction.