Apparatus and method for optimizing the efficiency of germanium junctions in multi-junction solar cells
View Patent ↗Apparatus and Method for Optimizing the Efficiency of Germanium Junctions in Multi-Junction Solar Cells. In a preferred embodiment, an indium gallium phosphide (InGaP) nucleation layer is disposed between the germanium (Ge) substrate and the overlying dual-junction epilayers for controlling the diffusion depth of the n-doping in the germanium junction. Specifically, by acting as a diffusion barrier to arsenic (As) contained in the overlying epilayers and as a source of n-type dopant for forming the germanium junction, the nucleation layer enables the growth time and temperature in the epilayer device process to be minimized without compromising the integrity of the dual-junction epilayer structure. This in turn allows the arsenic diffusion into the germanium substrate to be optimally controlled by varying the thickness of the nucleation layer. An active germanium junction formed in accordance with the present invention has a typical diffused junction depth that is ⅕ to ½ of that achievable in prior art devices. Furthermore, triple-junction solar cells incorporating a shallow n-p germanium junction of the present invention can attain 1 sun AM0 efficiencies in excess of 26%.
1. A solar cell comprising:
a germanium substrate;
a layer of material selected from the group consisting of InP and InGaP disposed directly on the germanium substrate; and
a diffused photoactive germanium junction in the substrate.
2. A solar cell as defined in claim 1 , wherein the layer of material is InGaP.
3. A solar cell as defined in claim 1 , further comprising a top solar subcell formed from InGaP, a middle solar subcell formed from GaAs, and a lower solar subcell formed in the germanium substrate.
4. A solar cell as defined in claim 1 , wherein the diffused junction is formed by the diffusion of arsenic into the germanium substrate.
5. A solar cell as defined in claim 1 , wherein the layer of material has a lattice parameter substantially equal to the lattice parameter of the germanium substrate.
6. A solar cell as defined in claim 1 , wherein the layer has a thickness equal to 350 Angstroms or less.
7. A solar cell defined in claim 1 , wherein the cell is capable of photoactively converting radiation ranging from approximately ultraviolet (UV) radiation to radiation having a wavelength of approximately 1800 nm.
8. A solar cell defined in claim 4 , wherein the junction in the germanium substrate layer is located between 0.3 μm and 0.7 μm from the top surface of the germanium substrate.
9. A solar cell as defined in claim 1 , wherein the diffused germanium substrate forms a first cell layer and has a dopant diffusion profile that optimizes the current and voltage generated therefrom.
10. A solar cell as defined in claim 1 , wherein the cell has 1 sun AMO efficiencies in excess of 26%.
11. A solar cell comprising:
a germanium substrate;
a solar subcell layer overlying said substrate and composed of GaAs;
a barrier layer directly overlying and contacting said substrate and underneath said solar subcell layer and functioning to inhibit the diffusion of arsenic into the germanium substrate; and
a two step diffusion profile in the germanium substrate with two different dopants.
12. A solar cell as defined in claim 11 , further comprising a solar subcell formed in the germanium substrate.
13. A solar cell as defined in claim 12 , wherein the subcell formed in the germanium substrate is formed from an n-type germanium layer overlying a p-type germanium substrate.
14. A solar cell as defined in claim 13 , wherein the n-type germanium layer is formed by diffusion of arsenic into the germanium substrate.
15. A solar cell a defined in claim 13 , wherein the n-type germanium layer is formed by diffusion of phosphorous into the germanium substrate.
16. A solar cell as defined in claim 13 , wherein the n-type germanium layer is formed by diffusion of both arsenic and phosphorous into the germanium substrate.
17. A solar cell as defined in claim 11 , wherein the barrier layer is composed of InGaP, InP, or GaP.
18. A solar cell as defined in claim 11 , wherein the barrier layer has a thickness less than 350 Angstroms.
19. A triple-junction solar cell comprising:
a dual-junction structure comprising a first junction and a second junction;
a third junction having a p-type substrate, wherein the third junction is doped with arsenic (As) and phosphorus (P), wherein the p-type substrate includes first and second diffusion sublayers, wherein P atoms have higher concentration compared to As atoms in the first diffusion sublayer and As atoms have a higher concentration compared to P atoms in the second diffusion sublayer; and
a nucleation layer disposed between the dual-junction structure and the third junction and comprising a material that shares a substantially similar lattice parameter with the p-type substrate of the third junction, wherein the nucleation layer serves to control the diffusion of arsenic atoms into the substrate.
20. The triple-junction solar cell as recited in claim 19 wherein the p-type substrate of the third junction is germanium (Ge) and the nucleation layer comprises indium gallium phosphide (InGaP).
21. The triple-junction solar cell as recited in claim 19 wherein the nucleation layer has a thickness substantially equal to 350 Å or less.
22. The triple-junction solar cell as recited in claim 19 wherein the junction depth of the third junction is substantially between 0.3 μm and 0.7 μm.
23. The triple-junction solar cell as recited in claim 19 wherein the third junction comprises a two-step diffusion profile capable of optimizing current and voltage generated from the third junction.
24. The triple-junction solar cell as recited in claim 19 having 1 sun AM0 efficiencies in excess of 26%.
25. The triple-junction solar cell as recited in claim 19 capable of absorbing radiation ranging from approximately ultraviolet (UV) radiation to radiation having a wavelength of approximately 1800 nm.
26. A multi-junction solar cell comprising:
a p-type germanium (Ge) substrate having a first surface, wherein the p-type Ge substrate further includes a diffusion portion having a first diffusion region situated adjacent to the first surface of the p-type Ge substrate and a second diffusion region, which includes a part of the first diffusion region, wherein the second diffusion region diffuses deeper into the Ge substrate than the first diffusion region;
a phosphorus (P) containing nucleation layer disposed over the fist surface of the p-type Ge substrate, wherein the P containing nucleation layer provides n-type P atoms to the first diffusion region; and
an arsenic (As) containing buffer layer disposed over the P containing nucleation layer, wherein the As containing buffer layer provides n-type As atoms to the second diffusion region in response to the thickness of the P containing nucleation layer, wherein the second diffusion region has a higher concentration of As atoms than P atoms.
27. The multi-junction solar cell of claim 26 , further comprising a second surface situated at the bottom of the multi-junction solar cell.
28. The multi-junction solar cell of claim 26 , wherein the first diffusion region has a higher concentration of P atoms than As atoms.
29. A multi-junction solar cell comprising:
a germanium (Ge) substrate having a first diffusion region and a second diffusion region, wherein the second diffusion region diffuses deeper into the Ge substrate than the first diffusion region;
a phosphide nucleation layer disposed over the first surface of the substrate, wherein the phosphide nucleation layer provides diffusion dopants of phosphorus (P) atoms to the first diffusion region; and
an arsenide layer disposed over the phosphide nucleation layer, wherein the arsenide layer provides diffusion dopants of arsenic (As) atoms into the second diffusion region in response to the thickness of the phosphide nucleation layer, wherein the first diffusion region has a higher concentration of P atoms than As atoms.
30. The multi-junction solar cell of claim 29 , further comprising a second surface situated at the bottom of the multi-junction solar cell.
31. The multi-junction solar cell of claim 29 , wherein the second diffusion region has a higher concentration of As atoms than P atoms.