Passivated iron disulfide surface encapsulated in zinc sulfide

Abstract

A passivated iron disulfide (FeS.sub.2) surface encapsulated by an epitaxial zinc sulfide (ZnS) capping layer or matrix is provided. Also disclosed are methods for passivating the surface of crystalline iron disulfide by encapsulating it with an epitaxial zinc sulfide capping layer or matrix. Additionally disclosed is a photovoltaic (PV) device incorporating FeS.sub.2 encapsulated by ZnS.

Claims

1. A passivated iron disulfide film, comprising: a crystalline iron disulfide layer having a surface comprising crystal surfaces; a substrate; and an epitaxial zinc sulfide capping layer encapsulating the crystal surfaces of the crystalline iron disulfide layer, wherein the epitaxial zinc sulfide capping layer passivates sulfur atoms present on the crystal surfaces on the surface of the crystalline iron disulfide layer, thereby reducing surface defects as compared to a crystalline iron disulfide layer not capped with an epitaxial zinc sulfide layer.

2. The iron disulfide film of claim 1, wherein the substrate is a rigid material.

3. The iron disulfide film of claim 1, wherein the substrate is a flexible material.

4. The iron disulfide film of claim 1, wherein the crystalline iron disulfide comprises crystallites ranging in size from 1 nm to 10 m.

5. A photovoltaic device comprising the iron disulfide film of claim 1.

6. The photovoltaic device of claim 5, wherein the substrate is a rigid material.

7. The photovoltaic device of claim 5, wherein the substrate is a flexible material.

8. The photovoltaic device of claim 5, wherein the crystalline iron disulfide comprises crystallites ranging in size from 1 nm to 10 m.

9. The iron disulfide film of claim 1, wherein the surface defects in the crystalline iron disulfide layer are assessed by comparing an X-ray photoelectron spectroscopy scan of S 2p doublets associated with surface defects with an X-ray photoelectric spectroscopy scan of S 2p doublets associated with the bulk state.

10. The iron disulfide film of claim 1, wherein the crystal surfaces on the surface of the layer of crystalline iron disulfide and the capping layer of epitaxial zinc sulfide form a lattice match.

11. The iron disulfide film of claim 1, wherein the capping layer of epitaxial zinc sulfide has a lattice constant of about 5.411 .

12. The iron disulfide film of claim 1, wherein the crystal surfaces on the surface of layer of crystalline iron disulfide have a lattice constant of about 5.417 .

13. The iron disulfide film of claim 1, wherein the capping layer of epitaxial zinc sulfide is deposited by physical vapor depostion.

14. The iron disulfide film of claim 1, wherein the capping layer of epitaxial zinc sulfide is deposited by chemical vapor deposition.

15. The iron disulfide film of claim 14, wherein the chemical vapor deposition is atomic layer deposition.

16. The iron disulfide film of claim 1, wherein the layer of crystalline iron disulfide is deposited by physical vapor depostion.

17. The iron disulfide film of claim 1, wherein the layer of crystalline iron disulfide is deposited by chemical vapor deposition.

18. The iron disulfide film of claim 17, wherein the chemical vapor deposition is atomic layer deposition.

19. Passivated iron disulfide crystallites, comprising: iron disulfide crystallites comprising crystal surfaces; an epitaxial zinc sulfide matrix encapsulating the iron disulfide crystallites; and wherein the epitaxial zinc sulfide matrix passivates sulfur atoms present on the crystal surfaces of the iron disulfide crystallites, thereby reducing surface defects as compared to iron disulfide crystallites not encapsulated by an epitaxial zinc sulfide matrix.

20. A photovoltaic device comprising the passivated iron disulfide crystallites of claim 19.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a polycrystalline FeS.sub.2 film encapsulated by coating it with a ZnS capping layer.

(2) FIG. 2 shows XPS results comparing the S 2P peak. FIG. 2a is for FeS.sub.2 encapsulated by a ZnS capping layer. FIG. 2b is for FeS.sub.2 encapsulated by a ZnO capping layer. FIG. 2c is for FeS.sub.2 encapsulated by a SiO.sub.2 capping layer.

(3) FIG. 3 shows the atomic structure of an FeS.sub.2 nanocrystal embedded in a ZnS matrix, based on DFT results.

(4) FIG. 4 shows FeS.sub.2 crystallites encapsulated within a ZnS matrix. In FIG. 4a, the substrate is a rigid material. In FIG. 4b, the substrate is a flexible material.

(5) FIG. 5 shows FeS.sub.2 crystallites encapsulated within a ZnS matrix being employed in a PV device.

DETAILED DESCRIPTION OF THE INVENTION

(6) In one embodiment, FeS.sub.2 is sputtered at room temperature from a single target in a partial pressure (110.sup.5 T) of sulfur onto a glass substrate. The film was 200 nm thick and polycrystalline. It exhibited the expected cubic pyrite crystal structure as indicated by X-ray diffractometry. The sample was transferred to an evaporation chamber without removal to atmosphere, and a 40 nm thick layer of epitaxial ZnS was deposited by thermal evaporation. A sketch of this sample is shown in FIG. 1. Transferring the sample between deposition chambers under vacuum, or without removal to air, avoids oxidation and contamination of the FeS.sub.2 surface. In other embodiments, high substrate temperature deposition of FeS.sub.2 may be carried out by sputtering from a multi-component target to a high temperature substrate (e.g., T.sub.s=400 C.), and sulfurdizing under flowing H.sub.2S (e.g., at 500 C., for 5 hours). All vacuum-deposited fabrication is preferred according to some aspects of the invention.

(7) Initial X-ray photoelectron spectroscopy (XPS) results for this sample were obtained and compared to results for bare FeS.sub.2 and films with ZnO and SiO.sub.2 encapsulation layers. The encapsulation layers were removed in steps inside an ultra-high vacuum chamber with an ion beam, and XPS scans were carried out after each removal step. The results, shown in FIG. 2, compare the S 2p doublet of a bare film to those with capping layers. In each case a combination of S 2p doublets associated with both the bulk states and surface defects is present. The peak with lowest binding energy (near 161 eV) is the S 2p.sub.3/2 component of the doublet and is associated with these surface defects. For both ZnO and SiO.sub.2, this peak is stronger than the peak associated with the bulk states, indicating a larger concentration of surface defects, presumably S.sup.2-. For the ZnS-capped sample, however, the defect peak is smaller relative to the bulk peak, indicating that the surface defects have been partially passivated. This is the first demonstration of passivation of FeS.sub.2 surface defects by a ZnS capping layer.

(8) To obtain an atomic scale understanding of the bonding between FeS.sub.2 and ZnS, DFT calculations were carried out. The FeS.sub.2 and ZnS have a nearly perfect lattice match, with lattice spacings of 5.417 and 5.411 , respectively. Because of this the two materials can form a nearly defect-free interface. An illustration of an FeS.sub.2 nanocrystal encapsulated in ZnS, based on DFT, is shown in FIG. 3.

(9) Several other embodiments of the invention are shown in FIG. 4. In these cases, FeS.sub.2 crystallites are encapsulated within a ZnS matrix. The FeS.sub.2 crystallites may vary in size from 1 nm to 10 m, and the ZnS separating FeS.sub.2 crystallites is at least one monolayer thick. FIG. 4a shows an embodiment in which the substrate is a rigid material such as rigid glass or a semiconductor wafer; and FIG. 4b shows an embodiment in which the substrate is a flexible material such as polymer, flexible glass, or metal foil. In the latter case, the FeS.sub.2 and ZnS matrix constitute a film that may flex along with the substrate.

(10) In another embodiment, the film comprising FeS.sub.2 crystallites encapsulated within a ZnS matrix is employed as the absorber in a PV device. One example of a suitable device architecture is shown in FIG. 5. In this example, the device comprises a substrate, a conductive bottom contact, the FeS.sub.2 crystallites encapsulated within a ZnS matrix, a transparent p-type layer, a transparent conductive to serve as the top contact, and a metal grid that aids efficient charge collection. FeS.sub.2 is typically an n-type semiconductor, so in this architecture, the transparent p-type layer is used in conjunction with the FeS.sub.2/ZnS layer to form a p-n junction. Any other suitable PV device architecture, such as a Schottky junction device, could be used.

(11) The FeS.sub.2 crystallite size may vary from 1 nm to 10 cm. Individual crystallites may be in contact, as is the case in polycrystalline bulk samples or thin films, or crystallites may be separated with each entirely encapsulated in ZnS. The FeS.sub.2 may be a natural or synthetic bulk sample.

(12) The FeS.sub.2 may be a film deposited by any suitable deposition technique. This technique may be any physical vapor, chemical vapor deposition, atomic layer deposition, or other suitable deposition process.

(13) The ZnS may be a film deposited by any suitable deposition technique. This technique may be any physical vapor, chemical vapor deposition, or other suitable deposition process. The S content in FeS.sub.2 could vary by up to 20% from stoichiometry.

(14) The Fe in FeS.sub.2 could be partially substituted by Si with a ratio of up to 50%, i.e. Fe.sub.1-xSi.sub.xS.sub.2 where x<0.5. The Zn in ZnS could be partially substituted by another metal including Ni, Mn, Cu, Ag, or Pb with a ratio of up to 50%. The S in ZnS could be partially substituted by Se or O with a ratio of up to 50%.

(15) The above descriptions are those of the preferred embodiments of the invention. Various modifications and variations are possible in light of the above teachings without departing from the spirit and broader aspects of the invention. It is therefore to be understood that the claimed invention may be practiced otherwise than as specifically described. Any references to claim elements in the singular, for example, using the articles a, an, the, or said, is not to be construed as limiting the element to the singular.