Photovoltaic devices and method of manufacturing
Disclosed are methods for the surface cleaning and passivation of PV absorbers, such as CdTe substrates usable in solar cells, and devices made by such methods. In some embodiments, the method involves an anode layer ion source (ALIS) plasma discharge process to clean and oxidize a CdTe surface to produce a thin oxide layer between the CdTe layer and subsequent back contact layer(s).
1. A process for manufacturing a photovoltaic device having a front contact layer stack and a semiconductorer stack, the process comprising:
plasma cleaning an exposed surface of the semiconductor stack by exposing it to a plasma of ionized gases, wherein the plasma cleaning step removes from about 5 to about 500 angstroms at the surface of the exposed surface;
exposing the exposed surface of the semiconductor stack to an atmosphere that contains from about 1% to about 60% oxygen in an otherwise inert atmosphere to form an oxide layer on the exposed surface; and
forming a back contact layer stack on the oxide layer.
2. The process of claim 1 wherein the plasma cleaning and exposing steps are performed sequentially in a two-stage process.
3. The process of claim 1 wherein the plasma cleaning and exposing steps are performed simultaneously in a one-stage process.
4. The process of claim 3 wherein the one-stage process further comprises exposing the exposed surface of the semiconductor stack to a plasma emitted from a discharge channel of an anode layer ion source instrument; wherein the plasma is generated by flowing an ionizable gas through the discharge channel in the presence of crossed magnetic and electric fields to become ionized, the ionizable gas comprising from about 1% to about 60% oxygen in an otherwise inert gas, thereby forming the oxide layer on the exposed surface, prior to forming the back contact layer.
5. The process of claim 4 wherein the inert gas is selected from neon and argon.
6. The process of claim 1 wherein the plasma cleaning step is performed between about 25° C. and about 400° C.
7. The process of claim 1 wherein the exposing step is performed between about 25° C. and about 400° C.
8. A process for manufacturing a photovoltaic device having a front contact layer stack and a semiconductor stack, the process comprising:
plasma cleaning an exposed surface of the semiconductor stack by exposing it to a plasma of ionized gases;
exposing the exposed surface of the semiconductor stack to an atmosphere that contains from about 1% to about 60% oxygen in an otherwise inert atmosphere to form an oxide layer on the exposed surface, wherein the plasma cleaning step is performed in a linear anode layer ion source (ALIS) instrument having an ion discharge channel, and wherein the exposing step is performed by flowing oxygen through the ion discharge channel; and
forming a back contact layer stack on the oxide layer.
9. The process of claim 1 wherein the oxide layer serves as a barrier layer to a dopant or as a tunneling layer or as a passivation layer, or a combination of these.
10. The process of claim 1 wherein the plasma cleaning step is performed under vacuum and wherein the back contact layer stack is formed on the oxide layer in situ without removal from the vacuum.
11. The process of claim 1 , wherein the oxide layer is about 2 Å to about 50 Å thick.
12. The process of claim 11 , wherein the back contact layer stack is formed on the oxide layer in situ.
13. The process of claim 1 , wherein the semiconductor stack comprises an absorber layer, and wherein the plasma cleaning step removes carbon residues and contaminants from a surface of the absorber layer.
14. The process of claim 13 wherein the plasma cleaning step exposes a bulk portion of the absorber layer that is stoichiometric.
15. The process of claim 1 , wherein the semiconductor stack comprises an absorber layer, and wherein the plasma cleaning step removes defects in a surface of the absorber layer.
16. The process of claim 15 wherein the plasma cleaning step exposes a bulk portion of the absorber layer that is stoichiometric.
17. The process of claim 1 wherein the plasma cleaning and exposing steps are performed simultaneously in a one-stage process, and wherein the atmosphere contains from about 3% to about 40% oxygen by volume.
18. The process of claim 1 wherein the plasma cleaning step is performed for a duration between about 0.2 minutes to about 10 minutes.
19. The process of claim 1 , wherein the oxide layer is about 2 Å to about 30 Å thick.
20. The process of claim 1 , wherein the wherein the back contact layer comprises zinc and tellurium.
21. The process of claim 1 , wherein the wherein the semiconductor stack comprises cadmium and tellurium and at least one of sulfur, selenium, and zinc.
22. The process of claim 8 , wherein the plasma cleaning and exposing steps are performed simultaneously in a one-stage process, and wherein the atmosphere contains from about 2% to about 55% oxygen by volume.
23. The process of claim 8 , wherein the plasma cleaning step removes from about 5 to about 500 angstroms at the surface of the exposed surface.