Method of manufacturing solar cell
A solar cell that is readily manufactured using processing techniques which are less expensive than microelectronic circuit processing. In preferred embodiments, printing techniques are utilized in selectively forming masks for use in etching of silicon oxide and diffusing dopants and in forming metal contacts to diffused regions. In a preferred embodiment, p-doped regions and n-doped regions are alternately formed in a surface of the wafer through use of masking and etching techniques. Metal contacts are made to the p-regions and n-regions by first forming a seed layer stack that comprises a first layer such as aluminum that contacts silicon and functions as an infrared reflector, second layer such titanium tungsten that acts as diffusion barrier, and a third layer functions as a plating base. A thick conductive layer such as copper is then plated over the seed layer, and the seed layer between plated lines is removed. A front surface of the wafer is preferably textured by etching or mechanical abrasion with an IR reflection layer provided over the textured surface. A field layer can be provided in the textured surface with the combined effect being a very low surface recombination velocity.
1. A method of fabricating a solar cell comprising the steps of:
a) providing a silicon substrate having a front surface and a back surface,
b) forming a first doped region of first conductivity type in the back surface and a thermally grown silicon oxide layer over the doped region,
c) printing a patterned etch resist over the back surface whereby a plurality of rows across the back surface are not masked,
d) removing exposed silicon oxide from the back surface,
e) removing the patterned etch resist from the back surface,
f) removing exposed silicon from the back surface extending through the first doped region,
g) diffusing a dopant of second conductivity type into the back surface where silicon oxide was removed in step d) thereby forming a plurality of regions of second conductivity type between regions of first conductivity type and a thermally grown oxide over the back surface, and
h) forming first and second interdigitated metal contacts respectively ohmically contacting the regions of first conductivity type and the regions of second conductivity type, including:
h1) printing a patterned etch resist over the thermally grown oxide from step g) with arrays of small contact openings over the regions of first conductivity type and the regions of second conductivity type,
h2) removing the thermally grown oxide from the arrays of small contact openings,
h3) removing the patterned etch resist, and
h4) forming the first and second interdigitated metal contacts respectively ohmically contacting the regions of first conductivity type and the regions of second conductivity type.
2. The method as defined by claim 1 , wherein step h4) comprises:
i) forming a first metal layer on the back surface contacting the regions of first conductivity type and the regions of second conductivity type through the thermally grown oxide and serving as an infrared reflector,
ii) forming a barrier metal layer over the first metal layer,
iii) forming a base metal layer over the barrier layer that functions as a base to initiate metal plating,
iv) printing a plating resist over the base metal layer to thereby define the geometries of the first and second metal contacts,
v) plating an electrically conductive metal layer over the base metal layer,
vi) stripping the plating resist, and
vii) etching the exposed base metal layer.
3. The method as defined by claim 1 wherein steps vii) and vi) precede step v).
4. The method as defined by claim 2 , wherein a solderable layer is plated over the electrically conductive metal layer after step v).
5. The method as defined by claim 4 where the solderable layer is resistant to the etchant used in step vii).
6. The method as defined by claim 2 , and further including the steps of:
i) forming a textured surface on the front surface, and
j) forming an antireflection coating over the textured surface.
7. The method as defined by claim 6 , wherein step i) includes etching the front surface.
8. The method as defined by claim 6 , wherein step i) includes mechanical abrasion.
9. The method as defined by claim 6 , wherein step j) includes forming a nitride coating over the textured surface.
10. The method as defined by claim 6 , wherein step j) includes forming a titanium oxide coating over the textured surface.
11. The method as defined by claim 6 , and further including after step i) the step of forming a front surface diffusion in the textured surface and a silicon oxide layer over the textured surface.
12. The method as defined by claim 1 , and further including the steps of:
i) forming a textured surface on the front surface, and
j) forming an antireflection coating over the textured surface.
13. The method as defined by claim 12 , wherein step i) includes etching the front surface.
14. The method as defined by claim 12 , wherein step i) includes mechanical abrasion.
15. The method as defined by claim 12 , wherein step j) includes forming a nitride coating over the textured surface.
16. The method as defined by claim 12 , wherein step j) includes forming a titanium oxide coating over the textured surface.
17. The method as defined by claim 12 , and further including after step i) the step of forming a front surface diffusion in the textured surface and a silicon oxide layer over the textured surface.
18. The method as defined by claim 2 , wherein the contact metal layer comprises aluminum, the barrier metal layer comprises a titanium tungsten, and the plating base and electrically conductive layers comprise copper.