Buried insulator isolation for solar cell contacts
The present invention relates to methods and apparatuses for providing a buried insulator isolation for solar cell contacts. According to certain aspects, the invention places a buried oxide under the emitter of a polysilicon emitter solar cell. The oxide provides an excellent passivation layer over most of the surface. Holes in the oxide provide contact areas, increasing the current density to enhance efficiency. The oxide isolates the contacts from the substrate, achieving the advantage of a selective emitter structure without requiring deep diffusions. The oxide further enables use of screen printing on advanced shallow emitter cells. Positioning of the grid lines close to the openings also enables use of a very thin emitter to maximize blue response.
1. A solar cell comprising:
a silicon substrate having a front surface and a back surface, wherein the back surface of the silicon substrate includes a contact structure;
a doped polysilicon emitter layer formed on the front surface of the silicon substrate, the silicon substrate and the doped polysilicon emitter layer having opposite conductivity types thereby forming a semiconductor junction at the front surface;
a contact structure formed on the doped polysilicon emitter layer;
an insulating layer between the doped polysilicon emitter layer and the front surface of the silicon substrate;
contact holes patterned in the insulating layer, wherein the contact holes concentrate current at the semiconductor junction, and allow the concentrated current flow to the contact structure on the doped polysilicon emitter layer; and
a tunnel oxide between the doped polysilicon emitter layer and the silicon substrate, wherein the tunnel oxide is formed in the contact holes and is thin enough to allow current therethrough.
2. The solar cell according to claim 1 , wherein the contact structure on the doped polysilicon emitter layer is offset a non-zero lateral distance from the contact holes.
3. The solar cell according to claim 1 , wherein the back surface of the silicon substrate is textured to expose a <111> plane of the silicon substrate.
4. The solar cell according to claim 1 , wherein the insulating layer comprises silicon dioxide.
5. The solar cell according to claim 4 , wherein the silicon dioxide is thermally grown.
6. The solar cell according to claim 5 , wherein the insulating layer is less than about 20 Å thick.
7. A method of operating a solar cell comprising:
providing a polysilicon emitter layer formed on a front surface of a silicon substrate; the substrate and doped polysilicon emitter layer having opposite conductivity types thereby forming a semiconductor junction;
forming a contact structure on the doped polysilicon emitter layer;
forming a contact structure on a back surface of the silicon substrate;
providing an insulating layer between the doped polysilicon emitter layer and the front surface of the silicon substrate;
patterning contact holes in the insulating layer;
forming a tunnel oxide in the contact holes between the doped polysilicon emitter layer and the silicon substrate; and
concentrating current at the semiconductor junction corresponding to the patterned contact holes and allowing the concentrated current to flow through the tunnel oxide in the contact holes to the contact structure on the doped polysilicon emitter layer.
8. A method of fabricating a solar cell comprising:
preparing a silicon substrate having a front surface and a back surface;
forming an insulating layer on the front surface of the silicon substrate;
patterning contact holes in the insulating layer that allow current flow therethrough;
forming a doped polysilicon emitter layer on the insulating layer, the silicon substrate and the doped polysilicon emitter layer having opposite conductivity types thereby establishing a semiconductor junction at the front surface;
forming a contact structure on the doped polysilicon emitter layer;
forming a contact structure on the back surface of the silicon substrate; and
forming a tunnel oxide in the contact holes between the doped polysilicon emitter layer and the silicon substrate, wherein the tunnel oxide is thin enough to allow current therethrough.
9. The method according to claim 8 , wherein the step of forming the contact structure on the doped polysilicon emitter layer includes offsetting the contact structure on the doped polysilicon emitter layer a non-zero lateral distance from the contact holes.
10. The method according to claim 8 , wherein the insulating layer comprises silicon dioxide and wherein the step of forming the insulating layer includes performing thermal oxidation.
11. The method according to claim 10 , wherein the insulating layer is less than about 20 Å thick.
12. The method according to claim 10 , further comprising annealing the silicon substrate after doped polysilicon emitter layer, the insulating layer and the tunnel oxide have been formed to provide hydrogen passivation.