Method for making a photovoltaic cell comprising contact regions doped through a lamina
View Patent ↗In aspects of the present invention, a method is disclosed to form a lamina having opposing first and second surfaces. Heavily doped contact regions extend from the first surface to the second surface. Generally the lamina is formed by affixing a semiconductor donor body to a receiver element, then cleaving the lamina from the semiconductor donor body wherein the lamina remains affixed to the receiver element. In the present invention, the heavily doped contact regions are formed by doping the semiconductor donor body before cleaving of the lamina. A photovoltaic cell comprising the lamina is then fabricated. By forming the heavily doped contact regions before bonding to the receiver element and cleaving, post-bonding high-temperature steps can be avoided, which may be advantageous.
1. A method for forming a photovoltaic cell, the method comprising:
forming heavily doped contact regions in a donor wafer having a first surface, the heavily doped contact regions extending from the first surface into the donor wafer to a first depth from the first surface;
defining a cleave plane in the donor wafer at a second depth from the first surface;
affixing the donor wafer to a receiver element at the first surface;
cleaving a lamina from the donor wafer at the cleave plane, wherein the lamina remains affixed to the receiver element, wherein the cleaving step creates a second surface of the lamina opposite the first surface;
treating the second surface so that, after the treating step, the heavily doped contact regions are exposed at the second surface; and
forming a photovoltaic cell comprising the semiconductor lamina.
2. The method of claim 1 wherein the thickness of the lamina, between the first and second surfaces, is between about 0.2 and about 10 microns.
3. The method of claim 2 wherein the thickness of the lamina, between the first and second surfaces, is between about 0.5 and about 6 microns.
4. The method of claim 1 wherein the second depth is within about 0.5 microns of the first depth.
5. The method of claim 1 wherein, after the treating step, the heavily doped contact regions define stripes or isolated spots at the second surface, the stripes or spots having a surface width no greater than about 120 microns.
6. The method of claim 5 wherein the donor wafer is lightly doped to a first conductivity type, the heavily doped contact regions are heavily doped to the first conductivity type,
and further comprising, before the affixing step, forming a heavily doped junction region at the first surface, the heavily doped junction region doped to a second conductivity type opposite the first conductivity type.
7. The method of claim 6 wherein the heavily doped contact regions and the heavily doped junction region are doped in the same thermal step.
8. The method of claim 6 wherein the heavily doped contact regions and the heavily doped junction region are doped in at least two separate thermal steps.
9. The method of claim 6 wherein the step of forming the heavily doped junction region comprises forming doped glass on the first surface and annealing to dope portions of the donor wafer at or near the first surface.
10. The method of claim 9 wherein the doped glass is boron-doped.
11. The method of claim 9 wherein the doped glass is arsenic-doped.
12. The method of claim 1 wherein the donor wafer is a silicon wafer.
13. The method of claim 12 wherein the donor wafer is a monocrystalline silicon wafer.
14. The method of claim 1 wherein the receiver element is glass, metal, or a polymer.
15. The method of claim 1 wherein a conductive layer intervenes between the receiver element and the lamina.
16. The method of claim 15 wherein the conductive layer is a reflective metal or metal alloy.
17. The method of claim 15 wherein the conductive layer is a transparent conductive oxide.
18. The method of claim 1 wherein the step of defining a cleave plane comprises implanting hydrogen and/or helium ions into the donor wafer through the first surface.
19. The method of claim 18 wherein the step of cleaving the lamina from the donor wafer at the cleave plane comprises heating the donor wafer to an exfoliation temperature to exfoliate the lamina.
20. The method of claim 1 wherein, in the completed cell, the lamina comprises a base region and an emitter region of the photovoltaic cell.