Photoactive pyrite layer and process for making and using same
FIELD AND BACKGROUND OF THE INVENTION 
     The invention relates to photoactive pyrite layers, processes for their preparation, and the use of such pyrite layers. 
     The semiconductors heretofore principally used for solar cells are silicon, cadmium sulfide, gallium arsenide, copper-indium selenide, and indium phosphide. Semiconductors made from silicon were developed over three decades ago. However, it is known that the costs of making solar cells are still too high for wide commercialization. For single-crystal and polycrystalline silicon, the high requirements in terms of purity of the material and the energy consumption for their preparation are the most important cost factors. For amorphous silicon, adequate stability of the material is not yet assured, so that its use for solar cells has thus far been justified only in special cases. 
     The other solar cell materials which have been developed intensively are, as a rule, even more expensive than silicon (e.g. GaAs, InP), or they contain less common and toxic elements (e.g. cadmium in cadmium sulfide). 
     At present it cannot yet be forseen which semiconductor material will prove industrially successful. The development of silicon is generally assigned a high degree of importance. Also, most research work is concerned with this material for solar cells and optoelectronic components. However, for a broad, economical use of silicon as a solar cell material, the costs of preparation must be capable of reduction at least to 10% of present costs. 
     The fundamental possibility of using pyrite as a solar cell material is known from the Journal of Applied Electrochemistry, 13, 743-750 (1983). However, the detailed knowledge required cannot be gleaned from this source. 
     SUMMARY OF THE INVENTION 
     The objective of the present invention is to make pyrite material suitable for technological applications, in particular for conversion of solar energy to electrical or chemical energy under favorable cost conditions. 
     Accordingly, the inventive photoactive pyrite layer is characterized by deviating from stoichiometry of pyrite material by the formula FeS 2 ±x, wherein x≦0.05, with an impurity concentration of less than 10 20  per cm 3 , and a doping of about 10 16  to 10 19  per cm 3  of one of the materials selected from the group consisting of manganese (Mn), arsenic (As), cobalt (Co), and chlorine (Cl). 
     According to another object of the invention, the photoactive pyrite layer has a conversion yield of at least 1%, a quantum yield of up to 90%, a photovoltage of more than 0.25 volts, an effective charge carrier density of more than 10 15  per cm 3 , and a derived charge carrier mobility of about 200 cm 2  /Vs. 
     Depending on the desired type of conductivity, i.e. intrinsic conductivity, p- or n-conductivity, doping should be carried out with one of the elements of Group VA (e.g. As) or Group VIIB (e.g. Mn) for the p type; or with one of the elements of Group VIIA (e.g. Cl) or Group VIII (e.g. Co) for the n type. Since the pyrite exhibits n-conductivity character by means of, for example, vacant sulfur sites, the corresponding inherent conductivity character can be achieved in this way, i.e. a compensation can be effected. 
     The following should be stated with regard to the pyrite material and its characteristics. The necessary starting materials exist in nature in large amounts. Owing to the extremely high absorption coefficients, i.e. a very small depth of penetration of light, even ultrathin layers of pyrite are sufficient, and one can be satisfied with a relatively low conversion yield. 
     Further, the high quantum yield which can easily be achieved is surprising. If the photovoltage is allowed to rise to the theoretically achievable maximum of 0.5 V (half band gap), the conversion yield increases sharply, e.g. over 10%. Finally, the extremely small depth of penetration of exactly 160 Å (=0.016 μm) means that the purity requirement for the pyrite material need not be as high as in the case of silicon, for example. Moreover, the flexibility of the ultrathin layers and their low weight are of considerable advantage for manipulation and transport. 
     Experiments with pyrite (FeS 2  =iron pyrite) have shown that even the pyrite which occurs in nature is suitable for solar cells and optoelectronic components. This material is also sometimes referred to as iron pyrite, pyrite or, in popular usage, &#34;fool&#39;s gold&#34;. The raw material supply is no problem, and the established technological experience of the iron industry can also be relied upon. Moreover, the compound itself, as well as the starting elements, are environmentally compatible. 
     Up to now over 100 different compounds with the pyrite structure have been investigated, all of which exhibit similar lattice constants and therefore permit the preparation of heterojunctions, Schottky barriers and multijunctions with pyrite (FeS 2 ). Among these are compounds such as 
     MX 2 , MXX&#39;, MXY, M x  M y  X 2 , MY 2 , and MYY&#39;; 
     where 
     M and M&#39;=Mn, Ni, Zn, Cd, Co, Cu, Ru, Os, Rh, etc.; 
     X and X&#39;=S, Se, Te; and 
     Y and Y&#39;=P, As, Sb, Bi. 
     The compounds are either semiconducting with a large variation in energy gaps, ionization energies, and workfunctions, or they are metallic with widely varying workfunctions. 
     In comparison with silicon, which is the semiconductor material mostly used up to now, the energy requirement for the production of pyrite is appreciably lower. Thus FeS 2  can be made from Fe 2  O 3  and/or Fe 3  O 4  and sulfur. The technological know-how of the iron industry is usable here. Furthermore, natural pyrite can be used with thermal treatment, for purification and improvement, at relatively low temperatures (e.g. ion purification, annealing). Finally, pyrite is formed under natural conditions by geological-biological mechanisms which are not yet sufficiently understood. A synthesis oriented in such a direction would be extremely advantageous from the energy viewpoint. 
     Pyrite and other transition metal chalcogenides of similar electronic structure (valence and conductance band with high contribution of metal d-states) show slight sensitivity with respect to corrosion and photocorrosion. This results from the excitation of electron-hole pairs in quasi-non-binding electronic states. This makes possible a highly promising application in photoelectrochemical solar cells and optoelectronic components. 
     It is also essential for the invention to indicate preferred possibilities for the preparation of photoactive pyrite layers. As already mentioned above, the starting material for these can be natural pyrite as well as compounds which contain iron and sulfur. In each case the treatment of the natural or synthetic production of the pyrite material is followed by a surface processing of the photoactive layer. The processing chiefly involves polishing and etching operations and the like. 
     PROCESSES OF THE INVENTION 
     The present invention is also directed to particular processes for production and indicates the procedures to be carried out. These are classified according to the various methods for the preparation of photosensitive pyrite: 
     A. From Naturally Occurring Pyrite 
     It has been shown that natural pyrite crystals themselves often show a small photoeffect, whose magnitude is highly dependent on the site where they are found. Relatively high photocurrents were obtained, for example, with pyrite crystals from Peru (found at Huanzala). An improvement in photosensitivity was achieved by thermal treatment of the material. Here the natural pyrite was annealed in an evacuated quartz ampoule for one week at 500° C. As a result, the photocurrent in some samples was increased by a factor of 100. 
     B. By Crystal Growth 
     Various processes exist for preparing single crystal and polycrystalline pyrite (bromide or PbCl 2  melt). One process is the preparation by means of gas transport reactions with halogens as the transporting agent. Iodine and chlorine have already been used as transporting agents. Polycrystalline and single crystal pyrite with high photosensitivity have been obtained in this way in laboratory experiments: 
     B.1. Preparation of Polycrystalline Pyrite as Starting Material: 
     For this purpose, high-purity sulfur (t5N5) was melted under high vacuum (10 -5  bar) in a quartz crucible for 30 minutes in order to degas it. After cooling, the sulfur remained under vacuum and was manipulated under a protective gas for removal of material. High-purity iron in the form of a foil (t4N, thickness=0.25 mm) or powder (m5N, 60 mesh) was heated in a dried H 2  /Ar stream (ratio 1:5) for 2 hours at 800 C. for reduction of the surface. 
     Stoichiometric quantities of iron and sulfur (10 g) were caused to react in evacuated (10 -5  bar), sealed quartz ampoules (diameter 20 mm, length 200 mm) at ca. 600° C. (in an earlier experiment, at ca. 650° C.). To guard against the thermal dissociation of pyrite into pyrrhotite (FeS) and sulfur, the synthesis was carried out with a small excess of sulfur (650° C., ampoule volume=50 ml, excess of sulfur=15 mg). In the presence of a small amount of a halogen (less than 1 mg/cm 3 ), iodine in the earlier experiment (0.5 mg/cm 3 ), the reaction was complete after 100 hours. 
     B.2. Preparation of Polycrystalline Pyrite Layers from Pyrrhotite (FeS) and Sulfur (S) Arsenic (As) Doped: 
     A quartz ampoule having external diameter 20 mm and a length of 300 mm was charged with 2 g FeS 2  (pyrite) in powder form and 5 mg As, and evacuated to 10 -5  bar. After addition of bromine (0.5 mg/cm 3 ), the ampoule was sealed off. The substances, placed at one end of the ampoule (see also FIG. 1), were heated to 800° C. for 10 days in a resistance furnace. The free end of the ampoule remained at a temperature of 550° C. By the decomposition of the pyrite powder and by local transport with bromine, pyrrhotite crystals having an edge length of 10 mm and 2 mm thickness were formed (I→II). The magnetic crystals aligned themselves along the lines of the magnetic field of the resistance furnace (I=6 amp, E=220 V; diameter of heater winding 50 mm, slope of winding 2 mm, number of turns 30). 
     After reversal of the temperature gradient for 10 days longer, the pyrrhotite crystals were transformed, with retention of their external shape, back into statistically oriented pyrite crystals (5-20 μm) (III). Thus polycrystalline pyrite layers were produced, having 1 cm 2  area, which show photoactive properties. FIG. 1 shows the arrangement of the ampoules in the temperature gradient of the furnace. FIG. 2 shows scanning electron microscopic photographs of these polycrystalline layers. 
     B.3. Preparation of Single Crystals by Chemical Transport through the Gas Phase with ICl 3  : 
     A quartz ampoule containing a 2 g FeS 2  pyrite in powder form was evacuated to 10 -5  bar (external diameter of the quartz ampoule 22 mm, length 125 mm). After addition of ICl 3  (0.5 mg/cm 3 ), the ampoule was sealed off and the FeS 2  was transported in the tube furnace from 630° C. to 550° C. (in an earlier experiment, from 650° C. to 600° C.) After 10 days, polyhedric crystals of pyrite ((100) and (111) faces) formed, having edge lengths of ≦5 mm. The crystals are photoactive. 
     B.4. Preparation of Single Crystals by Chemical Transport through the Gas Phase with Br 2  : 
     A quartz ampoule (external diameter 22 mm, length 340 mm) containing 2 g FeS 2  (pyrite) in powder form and 1 mg As was evacuated to 10 -5  and 0.5 mg/cm 3  of bromine was added. After the ampoule was sealed off, the contents were transported in the tube furnace from 650° to 600° C. After 11 days, crystals ((100 and (111) faces) had formed, with edge lengths ≦1 mm. The photoactive properties of these crystals are better than in the examples showing the preparation of polycrystalline pyrite layers. (FIG. 3 shows a scanning electron microscope photograph of synthetic photoactive single crystals of FeS 2  prepared in this manner.) 
     B.5. Thin Layer Techniques: 
     The possibility also exists of preparing pyrite in thin layers for thin-layer solar cells. The prerequisite for this application is a high absorptivity of pyrite for light in the visible wavelength region. The investigations have shown that pyrite possesses a high absorption coefficient of more than 2×10 4  cm -1  for visible light. Values of 6.5×10 5  cm -1  have even been measured. An energy-converting pyrite layer must therefore be no thicker than 1μ or a few μ, and, as already mentioned above, can be as low as 0.016μ, and can be made by the usual technological thin layer methods. Among these methods, which have already proven themselves, are: 
     1. Epitaxy; 
     2. Sputtering techniques; 
     3. Electrochemical deposition; 
     4. Chemical vapor phase transport, especially CVD (chemical vapor deposition); and 
     5. Plasma deposition. 
     Interesting and especially preferred starting compounds for expitaxy, for CVD techniques, and plasma deposition are, for example, iron halides and sulfur-hydrogen compounds or organic iron compounds, e.g. carbonyls such as pentacarbonyliron (Fe(CO) 5 ) or nonacarbonyldiiron (Fe 2  (CO) 9 ) and sulfur. Besides the preferred compounds named here, other iron-halogen and iron-organic compounds can be considered. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Several figures have already been referred to in the above disclosure. The following description of the figures refers to preferred embodiments of the invention. 
     FIG. 1 is a schematic representation of a process for preparing polycrystalline, photoactive pyrite; 
     FIG. 2 shows a scanning electron microscope photograph of a polycrystalline, photoactive layer of FeS 2  ; 
     FIGS. 3 to 7 show several scanning electron microscope photographs of synthetic photoactive single crystals of FeS 2  ; 
     FIG. 8 is a schematic diagram of a photoelectrochemical solar cell using pyrite. 
     FIG. 9 is a graph of the performance character of a photoelectrochemical solar cell using monocrystalline FeS 2 . 
     FIG. 10 is a graph of the performance character of a photoelectrochemical FeS 2  solar cell under periodic illumination. 
     FIGS. 11 to 13 are graphs of the relative spectral sensitivity or the absorption spectra of FeS 2 . 
     FIGS. 14 and 15 are graphs of the performance characteristic of electrochemical solar cells using polycrystalline FeS 2 . 
     FIG. 16 is a perspective representation of the construction possibilities of solid-state solar cells based on pyrite. 
     FIG. 17 is a graph of the performance characteristic of a photovoltaic solar cell (Schottky barrier). 
     FIG. 18 shows three graphs on the same scale of the time-dependent photoconductivity after a 15 ns laser pulse (microwave measurement) using: 
     a. natural pyrite, annealed at 300° C.; 
     b. synthetic pyrite; and 
     c. natural pyrite, annealed at 400° C. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The single crystals of FeS 2  shown in FIGS. 4 to 7 were prepared in a manner similar to that described in B.4 above, but with the addition of 1 mg of Mn in place of As. The temperature for crystal growth was 580° C. These crystals exhibited edge lengths of up to 6 mm. A half-width of 4 angular seconds was measured from rocking curves for the crystals in FIGS. 4 and 5. This signifies a high degree of perfection of the crystallinity. 
     Photoelectrochemical Solar Cells 
     In the arrangement shown in FIG. 8, pyrite is used as a photosensitive electrode in an electrochemical solar cell. The photoactive electrode 1 consists of an electrically contacted pyrite layer. A metal screen or a carbon rod serves as the counter electrode 2. Both electrodes are in contact with an aqueous or organic electrolyte 3, which contains an O/R redox system, such as I -  /I 2 , as an additive, and are inside a vessel 4. When light source 5 is switched on, the system converts the light into electrical energy, which can be measured at voltmeter 6. The oxidation and reduction processes which take place at the two electrodes are indicated schematically. 
     The performance curve for such a photoelectrochemical FeS 2  solar cell using single-crystal FeS 2  and aqueous electrolytes (pH 3.5) with I -  /I 2  (3M KI, 10 -2  M I 2 ) is represented in FIG. 9. 
     In the photocurrent-voltage curve shown in FIG. 10, an aqueous electrolyte solution comprising 4M HI, 0.05M I 2  and 1M CaI 2  was used. The illumination was carried out with a power of 4.5 W. The periodic illumination clearly shows the effect of light on the differences between photo- and dark currents, as well as on the decay of the photocurrent resulting from a depletion of the reduced species I -  at the surface of the FeS 2  electrode. 
     The spectral dependence of the light sensitivity is shown in FIG. 11, wherein curve a is the photocurrent spectrum of polycrystalline FeS 2  (aqueous electrolyte, 3M KI, 0.05M I 2 ); and b,c is the absorption coefficient. 
     FIG. 12 shows the relative spectral sensitivity in a broader energy range under short-circuit conditions. The quantum yield in the high-energy portion of the sensitivity spectrum is dependent on the surface treatment of the pyrite. 
     A complete absorption spectrum of pyrite, measured on a crystal plate 8 μm thick and 50 mm 2  in area is presented in FIG. 13. 
     The performance characteristic of a first electrochemical solar cell using polycrystalline FeS 2  (aqueous electrolyte, 3M KI, 0.05M I 2 ) is shown in FIG. 14, while FIG. 15 shows a version which is considerably improved by surface treatment. 
     The electrode surfaces can be altered and the conversion yield increased by pretreatment of the surfaces, i.e. etching procedures, immersion procedures, and the like. The photoelectrochemical characteristics of pyrite can be improved, e.g. by immersion of the material into HF (e.g. 40%, for 40 sec). It has been found particularly effective, after a polishing of the surface, to treat the material in a concentrated solution of HF/CH 3  COOH/HNO 3  in the volume ratio 1:1:2 for 60 sec, followed by a rinsing with deionized water and then immersion under anodic polarization in a concentrated solution of H 2  O 2  /H 2  SO 4  in the volume ratio 1:1 for an additional 60 sec. 
     Among the possibilities for optimization which can lead to an increase in the energy conversion yield is variation of the electrolytes. An improvement can also be achieved by coating the surface of the FeS 2  with other compounds which likewise crystallize in the same structure. Here a utilization for for photoelectrolysis is likewise conceivable (e.g. RuS 2  deposited on FeS 2 ). 
     Solid State Solar Cells 
     Various possibilities exist for utilizing pyrite as a semiconductor material in solid-state solar cells. FIG. 16 is a schematic representation of the construction of solid-state solar cells based on pyrite. Thus, in the case of n-p homojunction, for example, A=n-FeS 2  and B=p-FeS 2 . In the case of a Schottky solar cell, A=M and B=FeS 2 . For a heterojunction, A=MX 2  and B=FeS 2 . The light is assumed to fall on the structure at arrows hv. 
     The conversion of light energy has already been shown on a laboratory scale with an FeS 2  /Ni and an FeS 2  /Au Schottky barrier. 
     Here FeS 2  crystals made by the preparation of polycrystalline pyrite layers from pyrrhotite were coated with Ni or Au (500 Å) without pretreatment and measured. The system acted as a photovoltaic solar cell. The performance characteristic is represented in FIG. 17. The conversion yield here is quite modest, which is attributable in part to the large proportion of impurities at the FeS 2  surface (with XPS the following could be detected; FeO, Fe 2  O 3 , SO x  species, C compounds). Considerable improvement in the yields, which were expected by treatment of the surfaces as well as by identification of more suitable contact materials (e.g metallically conducting MX 2  compounds having the pyrite structure), have already been effected to some extent. 
     P-N Junctions 
     Pyrite has the advantage, as a solar cell material, that it can be produced as both an n- and a p-conductive material. The n-conductivity can be achieved e.g. by doping with Co or Ni, and the p-conductivity by doping with As. Therefore p-n junctions can be made in a simple manner by diffusion of suitable dopants into n- or p-conductive FeS 2 . 
     As an example of this embodiment, a crystal of FeS 2  with n-conductivity was coated with a 500 Å thick Cu layer. Cu was then diffused in for 24 hours at 200° C. An n-p homojunction was formed. After contacting the n and p regions (Cu doped), a photovoltage of 40 mV was measured on illumination. This photovoltage can be further improved by defined doping. 
     Heterojunctions 
     The great number of available compounds having the pyrite structure (MX 2 , MXX&#39;, MXY, M x  M y  X 2 , MY 2 , MYY&#39;) permits the development of different heterojunctions. Chemical immersion methods (metal ion exchange), electrochemical deposition, growth from the gas phase and the like can be used as simple preparative methods. 
     Multijunctions 
     Multijunctions can also be prepared by similar methods. Here too the large number of known compounds having the pyrite structure (MX 2 , MXX&#39;, MXY, M x  M y  X 2 , MY 2 , MYY&#39;) simplifies the selection of adaptable materials. The theoretical requirements in terms of the positions of the energy bands and the energy gaps are fundamentally known. In this way it is possible to achieve higher yields than with an individual photoactive FeS 2  junction. 
     Application of FeS 2  in Optoelectronic Components 
     The high sensitivity to light which has been determined in suitably treated pyrite permits it to be used in detectors and optoelectronic systems for information transfer. The lifetime of the light-induced charge carriers, which is the determining factor for the time behavior of pyrite as a detector, depends on the nature of the preparation or treatment of the material. Three examples of signals measured as light-inducted microwave absorption as responses to a 15 ns laser pulse are represented in FIG. 18, namely: (a) natural pyrite, annealed at 300° C.; (b) synthetic pyrite; and (c) natural pyrite, annealed at 400° C. The sides of the signal in (a) are so steep, i.e. the half-width of the electrical signal, about 25 ns, is so narrow, that bit rates of about 40 Mbit/s can be directly processed with it. 
     Reports on the work of the inventors which forms the basis for the present invention will or have appeared in publications in the scientific and technical literature, in particular in the July 1985 issue of the Journal of the Electrochemical Society, by Ennaoui et al., &#34;Photoactive Synthetic Polycrystalline Pyrite (FeS 2 )&#34; (Vol. 132, pp 1579-1582) and by Ennaoui et al., expected to appear in an issue of the same journal in the near future, &#34;Photoelectrochemistry of Highly Quantum Efficient Single Crystalline n-FeS 2  (Pyrite)&#34; (Vol. 133, pp 97-106, January 1986). 
     While specific embodiments of the invention have been shown and described in detail to illustrate the application of the principles of the invention, it will be understood that the invention may be embodied otherwise without departing from such principles.