Metal foil metallization for backplane-attached solar cells and modules
A back contact solar cell is described which includes a semiconductor light absorbing layer; a first-level metal layer (M1), the M1 metal layer on a back side of the light absorbing layer, the back side being opposite from a front side of the light absorbing layer designed to receive incident light; an electrically insulating backplane sheet backside of said solar cell with the M1 layer, the backplane sheet comprising a plurality of via holes that expose portions of the M1 layer beneath the backplane sheet; and an M2 layer in contact with the backplane sheet, the M2 layer made of a sheet of pre-fabricated metal foil material comprising a thickness of between 5-250 μm, the M2 layer electrically connected to the M1 layer through the via holes in the backplane sheet.
1. A method for fabricating a back contact solar cell comprising:
preparing a semiconductor light absorbing layer;
forming a first-level metal (Ml) layer, the Ml layer on a back side of the light absorbing layer, the back side being opposite from a front side of the light absorbing layer designed to receive incident light;
laminating an electrically insulating backplane sheet on the Ml layer, the backplane sheet comprising a plurality of via holes that expose portions of the Ml layer beneath the backplane sheet; and bringing an M2 layer in permanent contact with the backplane sheet, the M2 layer containing a sheet of metal foil material comprising a thickness of between 5-250 micrometers (um), the M2 layer electrically connected to the Ml layer through the via holes in the backplane sheet; wherein the method further comprises:
cleaning, prior to lamination of the M2 layer, the exposed surface of the Ml layer in the via holes to remove oxides and organic residue formed thereon;
depositing, prior to lamination of the M2 layer, a seed layer which makes electrical contact with the exposed cleaned Ml layer in the via holes, and with a solderable surface comprising a solder compatible with the metal foil; and
wherein laminating the M2 layer on the backplane sheet further comprises heating the M2 layer to cause the solderable seed layer surface and the metal foil to form an alloy or metallic compound across the cell area that physically and electrically connects the Ml and M2 layers.
2. The method of claim 1 wherein the laminating the M2 layer comprises placing the metal foil on the backplane sheet under vacuum and heating the M2 layer to a temperature between 180-400° Celsius for a duration of time under a pressure of between 10-300 pounds per square inch (psi).
3. The method of claim 2 wherein the M2 layer further comprises a pre-coated solder compatible with the metal foil, and wherein the heating of the M2 layer causes the pre-coated solder and the metal foil to form a metallic compound in the via holes that physically and electrically connect the Ml and M2 layers.
4. The method of claim 1 further comprising:
placing the metal foil on the backplane sheet;
irradiating the M2 layer over the via holes with a plurality of pulses of a laser thereby causing the metal foil to fuse with the Ml layer.
5. The method of claim 1 wherein preparing the light absorbing layer comprises: performing a saw damage removal on a semiconductor wafer; and cleaning the semiconductor wafer.
6. The method of claim 1 wherein preparing the light absorbing layer comprises: forming a porous semiconductor epitaxial seed layer on a semiconductor wafer; and depositing a layer doped epitaxial semiconductor material to grow the light absorbing layer.
7. The method of claim 1 wherein preparing the light absorbing layer comprises: implanting ions into the back side of the light absorbing layer, the implanted ions comprise emitter dopants and base dopants, the emitter and base dopants located in separate locations across the light absorbing layer.
8. The method of claim 1 wherein forming the Ml layer comprises: patterning the Ml layer to create interdigitated patterns of fingers, the fingers associated with either an emitter contact or a base contact.
9. The method of claim 1 wherein laminating the backplane sheet on the M1 layer comprises:
laminating the backplane sheet on the Ml layer without the via holes; and
drilling the via holes into the backplane sheet.
10. The method of claim 1 further comprising:
forming a passivation layer on the front side of the light absorbing layer;
adding an anti-reflection coating to the front side of the light absorbing layer.
11. The method of claim 1 further comprising:
laminating the front side of the light absorbing layer with at least one from the group consisting a flexible fluoropolymer covering or a glass covering.
12. The method of claim 1 wherein the step of cleaning, prior to lamination of the M2 layer, the exposed surface of the Ml layer in the via holes to remove oxides and organic residue formed thereon, comprises using a plasma sputter etch process.
13. The method of claim 1 , wherein the step of depositing, prior to lamination of the M2 layer, a seed layer further comprises: depositing a first layer of highly conductive material, which is followed by depositing a second layer of material, and which is followed by depositing a third layer of solder.
14. The method of claim 13 , wherein the first layer comprises aluminum, and the second layer comprises Nickel-Vanadium (NiV).