Low-cost multi-junction solar cells and methods for their production
Solar cells fabricated without gasification of metallurgical-grade silicon. The substrates are prepared by: melting metallurgical grade silicon in a furnace; solidifying the melted metallurgical grade silicon into an ingot; slicing the ingot to obtain a plurality of wafers; polishing and cleaning each wafer; depositing aluminum layer on backside of each wafer; depositing a layer of hydrogenated silicon nitride on front surface of each wafer; annealing the wafers at elevated temperature; removing the hydrogenated silicon nitride; and, removing the aluminum layer. The front surface may be textured prior to forming the solar cell. The 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 preparing substrates using metallurgical grade silicon, comprising:
melting metallurgical grade silicon in a furnace;
solidifying the melted metallurgical grade silicon of 99.9% to 99.999% purity into an ingot;
slicing the ingot to obtain a plurality of wafers;
cleaning each wafer;
depositing aluminum layer on backside of each wafer;
depositing a layer of hydrogenated silicon nitride on front surface of each wafer;
annealing the wafers at elevated temperature wherein the aluminum layer is maintained on the backside to confine hydrogen during the annealing;
removing the hydrogenated silicon nitride from the front surface of each wafer after the annealing;
depositing a layer of SiO2 or Si on the front surface of the wafers;
etching the deposited layer such that after etching the deposited layer partially remains at grain boundaries of the metallurgical grade silicon on the front surface of the wafers;
depositing a layer of intrinsic amorphous silicon directly on the front surface of the wafer and depositing an amorphous n-doped layer over the intrinsic amorphous silicon layer.
2. The method of claim 1 , further comprising cutting peripheral sections of the ingot prior to slicing the ingot.
3. The method of claim 1 , further comprising dry etching the front surface of the wafer prior to depositing the layer of SiO2 or Si.
4. The method of claim 1 , further comprising removing the aluminum layer before depositing a thin-film silicon layer on the front surface of each wafer.
5. The method of claim 1 , further comprising depositing an n-doped layer over the intrinsic amorphous silicon layer.
6. The method of claim 5 , further comprising forming a layer of ITO over the n-doped layer.
7. The method of claim 1 , further comprising forming a layer of amorphous silicon on the backside prior to depositing aluminum layer.
8. A method for making solar cells using metallurgical grade silicon, comprising:
obtaining multi-grain wafers consisting essentially of metallurgical grade silicon of 99.9% to 99.999% purity;
neutralizing grain boundaries of metallurgical grade silicon at the front surface of each wafer;
depositing a layer of intrinsic amorphous silicon directly on the front surface of the wafer and depositing an amorphous n-doped layer over the intrinsic amorphous silicon layer;
wherein neutralizing the grain boundaries comprises:
depositing a layer of SiO2 or Si on the front surface of each wafer; and,
etching the deposited layer such that after etching the deposited layer partially remains at the grain boundaries of the metallurgical grade silicon at the front surface of the wafer.
9. The method of claim 8 , wherein the layer of intrinsic amorphous silicon and amorphous n-doped layer are hydrogenated.
10. A method for making a solar cell using metallurgical grade silicon, comprising:
obtaining a metallurgical grade silicon wafer consisting essentially of metallurgical grade silicon of 99.9% to 99.999% purity;
neutralizing grain boundaries at the front surface of the metallurgical grade silicon wafer by depositing a silicon layer on the front surface of the metallurgical grade silicon wafer and thereafter etching the silicon layer such that after etching part of the silicon layer partially remains at the grain boundaries of the metallurgical grade silicon wafer;
depositing a layer of intrinsic amorphous silicon directly on the front surface of the wafer and depositing an amorphous n-doped layer over the intrinsic amorphous silicon layer.