Metallization method for silicon solar cells
A method of forming contacts on a surface emitter of a silicon solar cell is provided. In the method an n-type diffusion of a surface is performed to form a doped emitter surface layer that has a sheet resistance of 10-40 Ω/□. The emitter surface layer is then etched back to increase the sheet resistance of the emitter surface layer. Finally the surface is selectively plated. A method of fabrication of a silicon solar cell includes performing a front surface emitter diffusion of n-type dopant and then performing a dielectric deposition on the front surface by PECVD. The dielectric deposition comprises: a. growth of a thin silicon oxide; b. PECVD deposition of silicon nitride to achieve a silicon nitride. The silicon is then annealed to drive hydrogen from the silicon nitride layer into the silicon to passivate the silicon.
1. A method of forming emitter contacts on a surface emitter of a silicon solar cell comprising a substrate of silicon material, the method comprising:
(i) forming the surface emitter by performing an n-type diffusion of a surface of the substrate to form an emitter surface layer;
(ii) etching back the surface of the substrate to achieve a dopant concentration at the surface of the substrate of less than 10 20 atoms/cm 3 ;
(iii) depositing a surface dielectric layer or layers onto the etched back surface of the substrate;
(iv) patterning the dielectric layer or layers in localized areas to expose regions of a silicon surface of the substrate where contacts are to be formed; and
(v) selectively plating the silicon surface of the substrate in the exposed regions where contacts are to be formed to make an electrical connection to the emitter surface layer.
2. The method of claim 1 wherein the emitter surface layer is formed with a sheet resistance in the range of 10-40 Ω/square.
3. The method of claim 1 wherein depositing the surface dielectric layer or layers onto the etched back surface of the substrate comprises:
a. growing a thin 5-30 nm silicon oxide layer; and
b. PECVD deposition of an Anti-Reflection Coating (ARC).
4. The method of claim 3 wherein the thin silicon oxide layer is grown to a thickness of 10-20 nm.
5. The method as claimed in claim 3 wherein the ARC layer has a thickness of 65-75 nm and a refractive index of 2.0-2.1.
6. The method as claimed in claim 3 wherein the ARC layer is a single hydrogen rich layer.
7. The method as claimed in claim 3 wherein the ARC layer comprises at least two layers as follows:
i. 80-120 angstroms of hydrogen rich material; and
ii. 550-650 angstroms of material of refractive index 2.0-2.1.
8. The method as claimed in claim 3 wherein the ARC comprises a layer or layers of silicon nitride, aluminium oxide, titanium oxide, or a combination of these.
9. The method as claimed in claim 8 wherein the ARC comprises a layer or layers of silicon nitride.
10. The method of claim 1 wherein the surface dielectric layer or layers comprises a layer of hydrogen rich material.
11. The method of claim 10 further comprising annealing the substrate to drive hydrogen from the layer of hydrogen rich material into the substrate to passivate the silicon material.
12. The method as claimed in claim 1 wherein annealing is performed at a temperature in the range of 350 to 720° C. for 0.5 to 20 mins.
13. The method as claimed in claim 12 wherein annealing is performed at a temperature in the range of 350 to 500° C. for 5 to 20 mins.
14. The method as claimed in claim 12 wherein annealing is performed at a temperature in the range of 380 to 420° C. for 8to 12 mins.
15. The method of claim 12 wherein annealing is performed at a temperature in the range of 500 to 720° C. for 0.5 to 8 mins.
16. The method of claim 10 wherein the hydrogen rich material is silicon nitride.
17. The method of claim 1 wherein patterning the dielectric layer or layers comprises performing a localised laser melting of the silicon substrate surface through the surface dielectric in the presence of a dopant containing source to simultaneously melt and dope the surface of the substrate, creating doped silicon n+ regions, which are more heavily doped than the emitter surface layer, in the regions where the front surface metal contacts are to be formed, while destroying the overlying dielectric layer or layers to expose a surface of the more heavily doped silicon n+ regions.
18. The method of claim 17 wherein selectively plating comprises plating a layer of nickel over the more heavily doped silicon n+ regions.
19. A method of forming emitter contacts on a surface emitter of a silicon solar cell comprising a substrate of silicon material, the method comprising:
(i) forming the surface emitter by performing an n-type diffusion of a surface of the substrate to form an emitter surface layer with a pn junction depth of at least 1.5 μm;
(ii) etching back the surface of the substrate to achieve a dopant concentration at the surface of the substrate to less than 10 20 atoms/cm 3 and to increase a sheet resistance of the emitter surface layer to 100-160 Ω/square;
(iii) depositing a surface dielectric layer or layers onto the etched back surface of the substrate;
(iv) patterning the dielectric layer in localized areas to expose regions of a silicon surface of the substrate where contacts are to be formed;
(v) doping the surface of the substrate in the exposed regions where contacts are to be formed whereby a dopant concentration at the surface of the exposed regions is greater than the dopant concentration at the surface of the substrate over a remainder of the surface of the substrate by a factor of at least two; and
(vi) selectively plating the silicon surface of the substrate in the exposed regions where contacts are to be formed to make an electrical connection the emitter surface layer.