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

Description:
PRIORITY CLAIM 
     The present application is a non-provisional application claiming the benefit of U.S. Provisional Application No. 61/954,703, filed on Mar. 18, 2014 by Jesse A. Frantz et al., entitled “Method of Passivating an Iron Disulfide Surface via Encapsulation in Zinc Sulfide,” the entire contents of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to passivating the surface of crystalline iron disulfide (FeS 2 ) by encapsulating it in crystalline zinc sulfide (ZnS). It also relates to FeS 2  encapsulated by ZnS in which the sulfur atoms at the FeS 2  surfaces are passivated. Additionally, this invention relates to a photovoltaic (PV) device incorporating FeS 2  encapsulated by ZnS. 
     Description of the Prior Art 
     Iron disulfide has great promise as an Earth-abundant material for PV applications. In a recent survey of 23 known semiconductor systems with potential as PV absorbers, FeS 2  ranked highest in potential annual electricity production based on known reserves and had the lowest extraction cost. Wadia et al., “Materials availability expands the opportunity for large-scale photovoltaics deployment,” Environ. Sci. Technol., 43, pp. 2072-2077 (2009). Its bandgap is 0.95eV, high enough to result in a potential solar to electricity conversion efficiency similar to that of Si, but unlike Si, it has an exceptionally high absorption coefficient of α=6×10 5  cm −1  resulting in a required thickness of &lt;40 nm for &gt;90% absorption (Ennaoui et al., “Iron disulfide for solar energy conversion,” Sol. Energ. Mat. Sol. Cells, 29, pp. 289-370 (1993)) compared to typical thicknesses &gt;100 μm for Si. 
     Despite these apparent advantages, FeS 2  PV devices have not yet lived up to their potential. Efficiency has been limited to approximately 3% due to open circuit voltage (V OC ) of &lt;200 mV, only ˜20% of the bandgap. Ennaoui et al., “Iron disulfide for solar energy conversion,” Sol. Energ. Mat. Sol. Cells, 29, pp. 289-370 (1993) and Wilcoxon et al., “Strong quantum confinement effects in semiconductors: FeS 2  nanoclusters,” Sol. State Comm., 98, pp. 581-585(1996). These limits have been shown to be the direct result of surface termination of FeS 2  crystals. Bulk FeS 2  crystallizes in the cubic pyrite structure, and its sulfur atoms are paired in an S-S bond (S 2   2− ). Crystal surfaces, however, are typically terminated by S monomers (S 1− ) that may convert to S 2− through a redox reaction. Bi et al., “Air stable, photosensitive, phase pure iron pyrite nanocrystal thin films for photovoltaic application,” Nano Lett., 11, pp. 4953-4957 (2011) and Zhang et al., “Effect of surface stoichiometry on the band gap of the pyrite FeS 2 (100) surface,” Phys. Rev. B, 85, art. 085314 (2012). The resulting surface states exhibit a high density of defects within the FeS 2  bandgap and shows properties similar to the iron monosulfide phase, with a bandgap of approximately 0.3 eV. In PV devices, these surface states at films&#39; surfaces and grain boundaries lead to high dark current and low V OC− . 
     BRIEF SUMMARY OF THE INVENTION 
     The aforementioned problems are overcome in the present invention which provides a method of passivating the surface of crystalline iron disulfide (FeS 2 ) by encapsulating it in crystalline zinc sulfide (ZnS), a product comprising FeS 2  encapsulated by ZnS in which the sulfur atoms at the FeS 2  surfaces are passivated, and a photovoltaic (PV) device incorporating FeS 2  encapsulated by ZnS. 
     To move beyond its current performance bottleneck, FeS 2  requires passivation of its surface states. The present invention provides a method of passivating surface defects in FeS 2  by encapsulating it in ZnS. A density-functional theory (DFT) study indicates that ZnS can create a defect-free interface with FeS 2 . Experimental results indicate that surface defects in polycrystalline FeS 2  films are indeed passivated by encapsulation in ZnS. 
     The present invention has many advantages. It results in passivation of surface states in crystalline FeS 2 , which is a feature that has not been shown with any other encapsulant. Using the method of the present invention, the FeS 2 /ZnS interface can be made free of mid-gap states that are typically associated with S monomers at the FeS 2  surface. Also, this invention can be incorporated into a rigid or flexible PV device, making it possible to build efficient solar cells from an Earth-abundant material. 
     These and other features and advantages of the invention, as well as the invention itself, will become better understood by reference to the following detailed description, appended claims, and accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a polycrystalline FeS 2  film encapsulated by coating it with a ZnS capping layer. 
         FIG. 2  shows XPS results comparing the S 2P peak.  FIG. 2 a    is for FeS 2  encapsulated by a ZnS capping layer.  FIG. 2 b    is for FeS 2  encapsulated by a ZnO capping layer.  FIG. 2 c    is for FeS 2  encapsulated by a SiO 2  capping layer. 
         FIG. 3  shows the atomic structure of an FeS 2  nanocrystal embedded in a ZnS matrix, based on DFT results. 
         FIG. 4  shows FeS 2  crystallites encapsulated within a ZnS matrix. In  FIG. 4 a   , the substrate is a rigid material. In  FIG. 4 b   , the substrate is a flexible material. 
         FIG. 5  shows FeS 2  crystallites encapsulated within a ZnS matrix being employed in a PV device. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In one embodiment, FeS 2  is sputtered at room temperature from a single target in a partial pressure (1×10−5 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 ZnS was deposited by thermal evaporation. A sketch of this sample is shown in  FIG. 1 . Transferring the sample between deposition chambers under vacuum avoids oxidation and contamination. Alternatively, high substrate temperature deposition of FeS 2  may be carried out by sputtering from a multi-component target to a high temperature substrate (T s  =400° C.), and sulfurdizing under flowing H 2 S at 500° C., for 5 hours. All vacuum-deposited fabrication results in high quality films. 
     Initial X-ray photoelectron spectroscopy (XPS) results for this sample were obtained and compared to results for bare FeS 2  and films with ZnS, ZnO and SiO 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 doublets of 400 nm thick FeS 2  films with 40 nm thick ZnS, ZnO, and SiO 2  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  3/2 component of the doublet and is associated with these surface defects. For both ZnO and SiO 2 , this peak is stronger than the peak associated with the bulk states, indicating a larger concentration of surface defects, presumably S 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 2  surface defects by a ZnS capping layer. 
     To obtain an atomic scale understanding of the bonding between FeS 2  and ZnS, DFT calculations were carried out. The FeS 2  and ZnS have a nearly perfect lattice match, with lattice spacings of 5.417 Åand 5.411 Å, respectively, and form an epitaxial layer. Because of this the two materials can form a nearly defect-free interface. An illustration of an FeS 2 nanocrystal encapsulated in ZnS, based on DFT, is shown in  FIG. 3 . 
     Several other embodiments of the invention are shown in  FIG. 4 . In these cases, FeS 2  crystallites are encapsulated within a ZnS matrix. The FeS 2  crystallites may vary in size from 1 nm to 10 μm, and the ZnS separating FeS 2  crystallites is at least one monolayer thick.  FIG. 4 a    shows an embodiment in which the substrate is a rigid material such as rigid glass or a semiconductor wafer; and  FIG. 4 b    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 2  and ZnS matrix constitute a film that may flex along with the substrate. 
     In another embodiment, the film comprising FeS 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 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 2  is typically an n-type semiconductor, so in this architecture, the transparent p-type layer is used in conjunction with the FeS 2 /ZnS layer to form a p-n junction. Any other suitable PV device architecture, such as a Schottky junction device, could be used. 
     The FeS 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 2  may be a natural or synthetic bulk sample. 
     The FeS 2  may be 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. 
     The ZnS 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. The S content in FeS 2  could vary by up to ±20% from stoichiometry. 
     The Fe in FeS 2  could be partially substituted by Si with a ratio of up to 50%, i.e. Fe 1−x Si x S 2  where x&lt;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%. 
     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.