METHOD OF PASSIVATING AN IRON DISULFIDE SURFACE VIA ENCAPSULATION IN A ZINC SULFIDE MATRIX

Abstract

A method for passivating the surface of crystalline iron disulfide (FeS.sub.2) by encapsulating it within an epitaxial zinc sulfide (ZnS) matrix. Also disclosed is the related product comprising FeS.sub.2 encapsulated by a ZnS matrix in which the sulfur atoms at the FeS.sub.2 surfaces are passivated. Additionally disclosed is a photovoltaic (PV) device incorporating FeS.sub.2 encapsulated by a ZnS matrix.

Claims

1. A method for passivating iron disulfide crystallites, comprising: forming iron disulfide crystallites comprising crystal surfaces; and encapsulating the iron disulfide crystallites within an epitaxial zinc sulfide matrix; 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.

2. The method of claim 1, further comprising placing the zinc sulfide matrix comprising encapsulated iron disulfide crystallites on a substrate,

3. The method of claim 2, wherein the substrate comprises a rigid material.

4. The method of claim 2, wherein the substrate comprises a flexible material.

5. The method of claim 4, wherein the epitaxial zinc sulfide matrix and encapsulated iron disulfide crystallites form a film that flexes along with the flexible substrate.

6. The method of claim 1, wherein the iron disulfide crystallites range in size from 1 nm to 10 m.

7. The method of claim 1, wherein a layer of the epitaxial zinc sulfide matrix at least one monolayer thick separates the iron disulfide crystallites.

8. The method of claim 1, wherein the surface defects in the iron disulfide crystallites 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.

9. The method of claim 1, wherein the crystal surfaces of the iron disulfide crystallites and the epitaxial zinc sulfide matrix form a lattice match.

10. The method of claim 1, wherein the epitaxial zinc sulfide matrix comprises crystal surfaces having a lattice constant of about 5.411 .

11. The method of claim 1, wherein the crystal surfaces of the iron disulfide crystallites have a lattice constant of about 5.417 .

12. The method of claim 1, wherein the epitaxial zinc sulfide matrix is deposited by physical vapor deposition.

13. The method of claim 1, wherein the epitaxial zinc sulfide matrix is deposited by chemical vapor deposition.

14. The method of claim 13, wherein the chemical vapor deposition is atomic layer deposition.

15. The method of claim 1, wherein the iron disulfide crystallites are formed by physical vapor deposition.

16. The method of claim 1, wherein the iron disulfide crystallites are formed by chemical vapor deposition.

17. The method of claim 16, wherein the chemical vapor deposition is atomic layer deposition.

18. The method of claim 1, further comprising incorporating the passivated iron disulfide crystallites into a photovoltaic device.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] FIG. 1 shows a polycrystalline FeS.sub.2 film encapsulated by coating it with a ZnS capping layer.

[0012] 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.

[0013] FIG. 3 shows the atomic structure of an FeS.sub.2 nanocrystal embedded in a ZnS matrix, based on DFT results.

[0014] 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.

[0015] FIG. 5 shows FeS.sub.2 crystallites encapsulated within a ZnS matrix being employed in a PV device.

DETAILED DESCRIPTION OF THE INVENTION

[0016] 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., Ts=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.

[0017] 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.

[0018] 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.

[0019] 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.

[0020] 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.

[0021] 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.

[0022] 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.

[0023] 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.

[0024] 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%.

[0025] 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.