RELATIVE DOPANT CONCENTRATION LEVELS IN SOLAR CELLS

1 . A solar cell, comprising:

a substrate, the substrate including a front side facing the sun to receive solar radiation during normal operation and a backside opposite the front side; and

a butting PN junction formed over the backside of the substrate between a P-type diffusion region and an N-type diffusion region, wherein the P-type diffusion region is formed from a P-type doped region including a first dopant source having a first dopant concentration level and wherein the N-type diffusion region is formed from an N-type doped region including a second dopant source having a second dopant concentration level greater than the first dopant concentration level.

2 . The solar cell of claim 1 , further comprising:

polysilicon formed over the backside of the substrate, wherein the P-type diffusion region and the N-type diffusion region are formed in the polysilicon.

3 . The solar cell of claim 1 , further comprising:

a passivation region at a boundary region of the butting PN junction.

4 . The solar cell of claim 1 , wherein the P-type diffusion region comprises boron having a dopant concentration level less than approximately 5E17/cm3.

5 . The solar cell of claim 4 , wherein the P-type diffusion region is doped at a dopant concentration level that reduces recombination at the butting PN junction to an extent that a resulting device efficiency is greater than 20%.

6 . The solar cell of claim 4 , wherein the N-type diffusion region comprises phosphorus having a dopant concentration level greater than approximately 10% of 1E20/cm3.

7 . The solar cell of claim 1 , further comprising:

a first metal contact finger coupled to the P-type diffusion region formed from the P-type doped region on the backside of the substrate; and

a second metal contact finger coupled to the N-type diffusion region formed from the N-type doped region on the backside of the substrate.

8 . The solar cell of claim 1 , wherein the P-type doped region and the N-type doped region are disposed over a dielectric layer over the substrate.

9 . A method of fabricating a solar cell, the method comprising:

forming a P-type diffusion region over a substrate from a P-type doped region including a first dopant source having a first dopant concentration level; and

forming an N-type diffusion region over the substrate and adjacent to the P-type diffusion region from an N-type doped region including a second dopant source having a second dopant concentration level to provide a butting PN junction between the P-type diffusion region and the N-type diffusion region such that the first dopant concentration level is less than the second dopant concentration level.

10 . The method of claim 9 , wherein forming a butting PN junction further comprises:

forming a layer of polysilicon over a backside of the substrate, the substrate having a front side facing the sun to receive solar radiation during normal operation, the backside opposite the front side;

forming the P-type doped region on the layer of polysilicon; and

forming the N-type doped region on the layer of polysilicon.

11 . The method of claim 9 , further comprising:

diffusing dopants from the P-type doped region to form the P-type diffusion region on the substrate;

diffusing dopants from the N-type doped region to form the N-type diffusion region on the substrate; and

forming the P-type and N-type diffusion regions external to the substrate and over a dielectric layer.

12 . The method of claim 9 , further comprising:

passivating a boundary region of the butting PN junction using Hydrogen.

13 . The method of claim 9 , wherein diffusing dopants from the P-type doped region further comprises:

using boron as a P-type dopant source at a dopant concentration level less than 1E17/cm3.

14 . The method of claim 13 , wherein diffusing dopants from the N-type doped region further comprises:

using phosphorus as an N-type dopant source at a dopant concentration level greater than 1E20/cm3.

15 . The method of claim 9 , further comprising:

printing the P-type and N-type doped regions using a printable ink

16 . The method of claim 10 , further comprising:

electrically coupling a first metal contact finger to the P-type diffusion region on the backside of the substrate; and

electrically coupling a second metal contact finger to the N-type diffusion region on the backside of the substrate.

17 . The method of claim 9 , further comprising:

depositing in situ doped P-type polysilicon to form the P-type diffusion region; and

forming the N-type diffusion region by counter doping dopants from the second dopant source with a masked N-type diffusion.

18 . A solar cell, comprising:

a substrate, the substrate including a front side facing the sun to receive solar radiation during normal operation and a backside opposite the front side; and

a polysilicon layer formed over the backside of the substrate; and

a P-type diffusion region and an N-type diffusion region formed in the polysilicon layer, wherein a butting PN junction is formed between the P-type diffusion region and the N-type diffusion region, wherein the P-type diffusion region has a first dopant concentration level and the N-type diffusion region has a second dopant concentration level greater than the first dopant concentration level.

19 . The solar cell of claim 1 , wherein the first dopant concentration level of the P-type diffusion region is less than approximately 5E17/cm3.

20 . The solar cell of claim 1 , wherein a concentration ratio from a P-type dopant source used to form the P-type diffusion region to an N-type dopant source used to form the N-Type diffusion region is approximately 1:100.