LOW-COST 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 fabricating solar cells, comprising:
obtaining multi-crystalline silicon wafers consisting essentially of a metallurgical grade silicon of impurity of 99.9%-99.999% doped as one of a p-type or n-type;
texturing front surface of the wafers;
depositing an intrinsic layer directly over and in contact with the front surface of the wafers;
depositing a doped layer of opposite polarity of the wafers over and in direct contact with the intrinsic layer;
forming a top conductive contact above the doped layer;
forming a bottom conductive contact on the underside of the wafers.
2 . The method of claim 1 , wherein at least one of forming the intrinsic layer and forming the doped layer comprises forming an amorphous layer.
3 . The method of claim 1 , wherein at least one of forming the intrinsic layer and forming the doped layer comprises forming a hydrogenated layer.
4 . The method of claim 1 , wherein forming the bottom conductive contact comprises forming a doped conductive layer of same polarity as the wafers over the underside of the wafers and forming a metal layer over the doped conductive layer.
5 . The method of claim 1 , wherein forming the top conductive contact comprises forming a transparent conductor over the doped layer.
6 . The method of claim 4 , further comprising forming an amorphous intrinsic layer between the underside of the wafers and the doped conductive layer.
7 . The method of claim 4 , wherein forming a metal layer comprises sputtering an aluminum layer.
8 . The method of claim 1 , wherein the wafers are p-type and the doped layer is n-type.
9 . The method of claim 8 , forming a transparent conductor comprises forming an indium-tin-oxide (ITO) layer.
10 . The method of claim 9 , further comprising fabricating top electrodes over the ITO.
11 . The method of claim 10 , wherein fabricating top electrodes comprises silk-screening silver paste.
12 . The method of claim 1 , wherein texturing front surface comprises etching the front surface.
13 . The method of claim 1 , wherein wafer are p-type with resistivity in the range 0.1-1 ohmcm.
14 . The method of claim 1 , wherein forming the intrinsic layer comprises processing the wafers in plasma enhanced chemical vapor deposition (PECVD) chamber and maintaining plasma using silane gas (SiH4) mixed with hydrogen gas (H2).
15 . The method of claim 1 , wherein forming the doped layer comprises processing the wafers in a plasma enhanced chemical vapor deposition (PECVD) chamber and maintaining plasma using silane, hydrogen, and phosphine gas (PH3).
16 . The method of claim 4 , wherein forming the doped conductive layer comprises processing the wafers in a plasma enhanced chemical vapor deposition (PECVD) chamber and maintaining plasma using silane, hydrogen, and diborane gas (B2H6).
17 . The method of claim 1 , wherein:
forming the intrinsic layer comprises forming an amorphous hydrogenated layer intrinsic layer;
forming the doped layer comprise forming an amorphous hydrogenated n-type layer;
forming the bottom conductive contact comprises forming a p-type layer over the underside of the wafers and forming a metal layer over the doped conductive layer; and
forming the top conductive contact comprises forming a transparent conductor layer over the doped layer and forming top electrodes over the transparent conductor layer.
18 . The method of claim 17 , wherein fabricating top electrodes comprises silk-screening silver paste.
19 . The method of claim 17 , wherein forming a metal layer comprises sputtering an aluminum layer.
20 . The method of claim 17 , wherein texturing front surface comprises etching the front surface.