Iron pyrite nanocrystal film as a copper-free back contact for polycrystalline CdTe thin film solar cells
The invention discloses nanocrystalline (NC) FeS 2 thin films as the back contact for CdTe solar cells. In one example, the FeS 2 NC layer is prepared from a solution directly on the CdTe surface using spin-casting and chemical treatment at ambient temperature and pressure, without a thermal treatment step. Solar cells prepared by applying the NC FeS 2 back contact onto CdTe yield efficiencies of about 95% to 100% that of standard Cu/Au back contact devices. In another example, FeS 2 is interposed between Cu and Au to form a Cu/FeS 2 NC/Au back contact configuration yielding an efficiency improvement of 5 to 9 percent higher than standard Cu/Au devices.
1. A photovoltaic cell comprising:
a front window comprising an n-type semiconductor material;
a semiconductor layer comprising a p-type semiconductor material; and
a back contact layer configured as an ohmic contact having an FeS 2 portion;
wherein the FeS 2 portion is a nanocrystalline portion comprising a film of FeS 2 in a cubic phase; and
wherein the FeS 2 portion comprises nanocrystals having an average size in a range of from about 70 nm to about 151 nm.
2. The photovoltaic cell of claim 1 wherein the FeS 2 portion is in direct contact with the semiconductor layer.
3. The photovoltaic cell of claim 1 wherein the semiconductor layer comprises CdTe and the front window comprises CdS.
4. The photovoltaic cell of claim 1 wherein a conductive material forms a portion of the back contact layer.
5. The photovoltaic cell of claim 4 wherein the FeS 2 portion is disposed between the semiconductor layer and the conductive material portion of the back contact layer.
6. The photovoltaic cell of claim 5 wherein the conductive material is one of Au, Cu, Sb, Hg, Bi-telluride, and graphene.
7. The photovoltaic cell of claim 4 wherein the conductive material is a first conductive material disposed between the semiconductor layer and the FeS 2 portion, and a second conductive material is disposed on an opposite side of the FeS 2 portion from the first conductive material.
8. The photovoltaic cell of claim 7 wherein the FeS 2 portion has a thickness in a range of about 5 nanometers to about 1500 nanometers.
9. The photovoltaic cell of claim 7 wherein one of the first and second conductive materials is Cu and other of the first and second conductive materials is Au.
10. The photovoltaic cell of claim 4 wherein the semiconductor layer includes at least one pinhole that permits a charge carrier dysfunction in an interface region of the semiconductor layer and the FeS 2 portion blocks the charge carrier dysfunction.
11. The photovoltaic cell of claim 10 wherein the conductive material is one of Au, Cu, Sb, Hg, Bi-telluride, and graphene.
12. The photovoltaic cell of claim 1 wherein the semiconductor layer includes one of CdTe, copper indium gallium di-selenide (CIGS), copper zinc tin sulfide (CZTS), copper zinc tin sulfur selenium alloy (CZTSSe), copper antimony sulfide, and tin sulfide.
13. The photovoltaic cell of claim 12 wherein a conductive material forms a portion of the back contact layer.
14. The photovoltaic cell of claim 13 wherein the conductive material is one of Au, Cu, Sb, Hg, Bi-telluride, and graphene.
15. The photovoltaic cell of claim 14 wherein the FeS 2 portion directly contacts the semiconductor layer, the semiconductor layer including at least one pinhole that permits a shunting effect at an interface region, wherein the FeS 2 portion blocks the shunting effect at the interface region.
16. A photovoltaic cell comprising:
a transparent conductive layer permitting a portion of visible light to pass through and forming a first ohmic contact;
a first active semiconductor layer having an n-type characteristic;
a second active semiconductor layer having a p-type characteristic; and
a back contact layer configured as a second ohmic contact having an FeS 2 portion, the back contact layer applied onto the second active semiconductor layer;
wherein the FeS 2 portion is a nanocrystalline portion comprising a film of FeS 2 in a cubic phase; and
wherein the FeS 2 portion comprises nanocrystals having an average size in a range of from about 70 nm to about 151 nm.
17. The photovoltaic cell of claim 16 wherein the transparent conductive layer includes a resistive coating layer, the first active semiconductor layer is a CdS layer, the second active semiconductor layer is a CdTe layer, and the back contact layer includes a conductive material.
18. The photovoltaic cell of claim 17 wherein the conductive material is one of Au, Cu, Sb, Hg, Bi-telluride, and graphene.
19. A photovoltaic cell comprising:
a transparent conductive layer permitting a portion of visible light to pass through and forming a first ohmic contact;
a CdS semiconductor layer having an n-type characteristic;
a CdTe semiconductor layer having a p-type characteristic; and
a back contact layer configured as a second ohmic contact, the back contact layer including a first conductive material applied onto the CdTe semiconductor layer, an FeS 2 nanocrystalline layer applied onto the first conductive material, and a second conductive material applied onto the FeS 2 nanocrystalline layer;
wherein the FeS 2 nanocrystalline layer comprises a film of FeS 2 in a cubic phase; and
wherein the FeS 2 nanocrystalline layer comprises nanocrystals having an average size in a range of from about 70 nm to about 151 nm.
20. The photovoltaic cell of claim 19 wherein the FeS 2 nanocrystalline layer is a ligand-free layer.
21. The photovoltaic cell of claim 1 , wherein the film of FeS 2 has a thickness of greater than 1.5 microns.