High efficiency solar photovoltaic cells produced with inexpensive materials by processes suitable for large volume production

Abstract

A solar energy device comprising: a substrate; a photovoltaic layer on said substrate; a back conductor in contact with said substrate; a grid conductor in contact with said substrate; said photovoltaic layer being of a material selected from the class consisting of: monoclinic zinc diphosphide (also referred to as beta zinc diphosphide and indicated by β-ZnP 2 ); copper diphosphide (CuP 2 ); magnesium tetraphosphide (MgP 4 ); γ-iron tetraphosphide (γ-FeP4) and mixed crystals formed from these four materials.

1 . A photovoltaic cell comprising: 
 (a) a thin film photovoltaic generating layer, having a pn junction within which photogenerated free carriers are generated, by incident light;    (b) a back conductor in electrical contact with said photovoltaic layer; and    (c) a front transparent conductor in electrical contact with said photovoltaic layer;    wherein said thin film photovoltaic generating layer is selected from the class consisting of: β-zinc diphosphide; copper diphosphide; magnesium tetraphosphide; y-iron tetraphosphide; and, a mixed crystal of composition Zn p Mg q Fe r —Cu s P 2t , where t≧p+q+r+s, whereby said photovoltaic cell exhibits a greatly improved average cost per Watt-Hour over a useful life of this photovoltaic cell.    
     
     
         2 . A photovoltaic cell as in  claim 1  wherein said thin film photovoltaic generating layer has a thickness on the order of 1-3 microns, whereby this layer absorbs a high fraction of incident sunlight and yet has surfaces that are sufficiently separated to avoid significant dopant diffusion along crystallite boundaries extending between these surfaces.  
     
     
         3 . A photovoltaic cell as in  claim 1  wherein said pn junction is a homojunction buried sufficiently below a front surface of said thin film photovoltaic generating layer that degradation of cell photovoltaic conversion efficiency by outdiffusion from this layer or by indiffusion of ambient chemicals is substantially eliminated.  
     
     
         4 . A photovoltaic cell as in  claim 1  wherein said pn junction is a heterojunction.  
     
     
         5 . A photovoltaic cell as in  claim 4  wherein said photovoltaic layer is comprised of a layer of β-ZnP 2  and a layer of ZnAs 2  and wherein said pn junction is formed at an interface between the ZnP 2  and ZnAs 2  layers.  
     
     
         6 . A photovoltaic cell as in  claim 1  further comprising a passivation layer between an exterior surface of said cell and a front surface of said thin film photovoltaic generating layer, where incident light passes to said photovoltaic generating layer.  
     
     
         7 . A photovoltaic cell as in  claim 6  wherein said passivation layer is selected from the class consisting of BP, B 6 P, AlPO 4 , BPO 4  and ZnPO X .  
     
     
         8 . A photovoltaic cell as in  claim 7  wherein said passivation layer is BP, whereby said passivation layer is particularly hard and inert and can be grown from vapor in either p-doped or n-doped compositions.  
     
     
         9 . A photovoltaic cell as in  claim 8  wherein said thin film photovoltaic layer is β-ZnP 2 , whereby said BP passivation layer is physically and chemically compatible with said β-ZnP 2 photovoltaic layer.  
     
     
         10 . A photovoltaic cell as in  claim 9  wherein said β-ZnP 2  photovoltaic layer and said BP passivation layer are in direct contact thereby forming said pn junction at a heterojunction between these two layers.  
     
     
         11 . A photovoltaic cell as in  claim 10  wherein said β-ZnP 2 , photovoltaic layer is doped n-type with boron dopant and said BP is doped p-type with Zn dopant, thereby utilizing the fact that boron is an n-dopant in β-ZnP 2 , and that Zn is a p-dopant in BP.  
     
     
         12 . A photovoltaic cell as in  claim 1  further comprising on a front surface 
 of said β-ZnP 2  layer, a layer of degenerately-doped zinc phosphate (ZnPO X ), whereby this zinc phosphate layer functions as a passivation layer that prevents transport of atmospheric oxygen and moisture to said diode junction, that prevents the transport of the initial decomposition products from said β-ZnP 2 , layer and that functions as a transparent conductor.  
 
     
     
         13 . A photovoltaic cell as in  claim 1  wherein said thin film photovoltaic generating layer is β-ZnP 2 .  
     
     
         14 . A photovoltaic cell as in  claim 13  wherein said β-ZnP 2  layer is doped with Cu, which exhibits a low mobility in the β-ZnP 2  layer.  
     
     
         15 . A photovoltaic cell as in  claim 13  wherein said front conductor is degenerately n-doped boron phosphide (BP), which thereby functions as a transparent conductor as well as a passivation layer.  
     
     
         16 . A photovoltaic cell as in  claim 13  wherein said front conductor is of 
 a material selected from the class consisting of tin oxide and indium tin oxide.  
 
     
     
         17 . A photovoltaic cell as in  claim 13  wherein said front conductor is zinc oxide.  
     
     
         18 . A photovoltaic cell as in  claim 13  wherein said front conductor is of 
 a material selected from the class consisting of doped aluminum phosphate, zinc phosphate, boron phosphide and antimony tin oxide.  
 
     
     
         19 . A photovoltaic cell as in  claim 13  wherein said front conductor is zinc metaphosphate and which functions as a transparent conductor, whereby a very stable interface is formed between said β-zinc diphosphide layer and said zinc metaphosphate layer because the ratio of zinc to phosphorus is the same in both layers.  
     
     
         20 . A photovoltaic cell as in  claim 13  wherein said front conductor is polycrystalline zinc orthophosphate, which is advantageous in functioning as a transparent conductor, in being stable and in being easy to grow on ZnP 2 .  
     
     
         21 . A photovoltaic cell as in  claim 1  further comprising an antireflection layer on said front conductor through which incident light is to pass to said photovoltaic layer.  
     
     
         22 . A photovoltaic cell as in  claim 21  wherein said antireflection layer is B 6 P, whereby this antireflection layer is particularly hard and inert and exhibits a low absorption for ultraviolet light.  
     
     
         23 . A photovoltaic cell as in  claim 1  wherein said front conductor is augmented by a grid conductor.  
     
     
         24 . A photovoltaic cell as in  claim 1  wherein said back conductor is made 
 of a material selected from the class consisting of copper, aluminum, molybdenum and any alloy containing predominantly one or more of these three materials, whereby this layer forms a low resistance contact to the photovoltaic layer.  
 
     
     
         25 . A photovoltaic cell as in  claim 1  wherein said back conductor is copper.  
     
     
         26 . A photovoltaic cell as in  claim 1  wherein said back conductor is a copper alloy selected from the class consisting of: brass, phosphocopper alloy, cupronickel and bronze.  
     
     
         27 . A photovoltaic cell as in  claim 1  wherein said back conductor is thick enough that it also functions as a substrate, thereby avoiding the need to form a separate substrate layer.  
     
     
         28 . A photovoltaic cell as in  claim 27  wherein said substrate is a foil selected from the class consisting of: copper, aluminum, molybdenum and any alloy containing predominantly one or more of these three materials.  
     
     
         29 . A photovoltaic cell as in  claim 27  wherein said back substrate has a thickness in the range 40-100 microns.  
     
     
         30 . A photovoltaic cell as in  claim 1 , further comprising between said photovoltaic layer and said back conductor an interlayer selected from the class consisting of molybdenum, copper, silver, lithium, gold, platinum, palladium, titanium, vanadium, chromium, manganese, tungsten, tantalum, iron, copper and nickel  
     
     
         31 . A photovoltaic cell as in  claim 1  in which a p-type conductivity of said photovoltaic layer has been produced by forming this layer with a phosphorus-rich stoichiometry.  
     
     
         32 . A photovoltaic cell as in  claim 1  in which an n-type conductivity of said photovoltaic layer has been produced by forming this layer with a metal-rich stoichiometry to produce an n-type layer.  
     
     
         33 . A photovoltaic cell as in  claim 1  wherein said thin film photovoltaic generating layer is CuP 2 , which is a material that uniquely has both direct and indirect bandgap states with high solar response.  
     
     
         34 . A photovoltaic cell as in  claim 1  wherein said thin film photovoltaic layer is MgP 4 .  
     
     
         35 . A photovoltaic cell as in  claim 1  wherein said thin film photovoltaic layer is γ-FeP 4 .  
     
     
         36 . A photovoltaic cell as in  claim 35  further comprising on a photovoltaic layer of said γ-FeP 4  layer and a layer of α-FeP 4 , thereby forming a heterojunction pn junction at an interface between these two layers.  
     
     
         37 . A photovoltaic cell as in  claim 1  wherein said thin film photovoltaic layer is a mixed crystal of composition Zn p Mg q Fe p Cu x P 2t .  
     
     
         38 . A photovoltaic cell as in  claim 1  wherein said thin film photovoltaic layer was deposited by a chemical vapor deposition (CVD) process, whereby a high quality thin film is produced.  
     
     
         39 . A photovoltaic cell as in  claim 38  wherein said CVD process is an organometallic CVD process, whereby a high quality thin film photovoltaic layer is produced.  
     
     
         40 . A photovoltaic cell as in  claim 39  wherein said photovoltaic layer was deposited in a plasma-enhanced environment, thereby enabling deposition at reduced temperatures, making this process suitable for deposition onto high temperature stable plastic substrates.  
     
     
         41 . A photovoltaic cell as in  claim 38  wherein said CVD process is a microwave plasma-enhanced, CVD process, at a total chamber pressure of 1-100 torr, whereby it can be operated at temperatures that are low enough to enable a high temperature, stable to be used as a substrate.  
     
     
         42 . A photovoltaic cell as in  claim 38  wherein said CVD process is operated at a near-atmospheric pressure, thereby enabling this process to form said photovoltaic layer on a substrate that is fed through an open-ended CVD chamber, whereby a high volume fabrication process can be implemented.  
     
     
         43 . A photovoltaic cell as in  claim 38  wherein process parameters of said CVD process are selected to lie in a single phase process region, whereby a high quality photovoltaic layer can be formed.  
     
     
         44 . A photovoltaic cell as in  claim 38  wherein said CVD process uses white phosphorus as a phosphorus source, whereby this low-cost form of phosphorus produces very high quality photovoltaic films.  
     
     
         45 . A photovoltaic cell as in  claim 1 , further comprising a back surface field formed near an interface between said back conductor and said photovoltaic layer.  
     
     
         46 . A photovoltaic cell comprising: 
 (a) a film photovoltaic generating layer of γFeP 4 , having a pn junction within which photogenerated free carriers are generated by incident light;    (b) a back conductor in electrical contact with said photovoltaic layer; and    (c) a transparent front conductor in electrical contact with said photovoltaic layer.    
     
     
         47 . A photovoltaic cell comprising: 
 (a) a film photovoltaic generating layer of MgP 4 , having a pn junction within which photogenerated free carriers are generated by incident light;    (b) a back conductor in electrical contact with said photovoltaic layer; and    (c) a transparent front conductor in electrical contact with said photovoltaic layer.    
     
     
         48 . A method of depositing, onto a substrate, a photovoltaic layer of a material selected from the set consisting of β-ZnP 2 , CuP 2 , MgP 4 , γ-FeP 4  and mixed crystals formed from these four materials, said method comprising the steps of: 
 (a) supplying, to a chemical vapor deposition (CVD) chamber, a source of metal selected from a set consisting of Zn, Cu, Mg, Fe and a material containing at least two of these four metals;  
 (b) supplying a source of phosphorus to said CVD chamber; and  
 (c) controlling the temperature of the substrate and the partial pressure of gases within this reactor to lie within a phase region which deposits a layer that includes at least one material selected from a set consisting of β-ZnP 2 , CuP 2 , MgP 4 , γ-FeP 4  and mixed crystals formed from at least two of these four metals and a number of phosphorus atoms that is at least twice the total number of metal atoms.  
 
     
     
         49 . A method of providing phosphorus to a chemical vapor deposition (CVD) chamber, comprising the steps of: 
 (a) vaporizing white phosphorus to form gaseous phosphorus; and    (b) supplying this gaseous phosphorus to a location within said CVD chamber at which chemical vapor deposition is to be implemented, whereby this gaseous phosphorus is inexpensive and pure.    
     
     
         50 . A method as in  claim 49  wherein step (a) comprises: 
 supplying white phosphorus to a bubbler, within which this white phosphorus is vaporized; and  
 flowing a carrier gas through said bubbler to carry said vaporized white phosphorus to a reaction site within said CVD chamber.  
 
     
     
         51 . A method as in  claim 49  wherein step (a) comprises: 
 feeding a block of material containing solid white phosphorus into a hot zone within said CVD chamber; and  
 heating a surface of said block of material to a temperature at which it vaporizes.  
 
     
     
         52 . A method as in  claim 49  wherein step (a) comprises: 
 feeding a block of material containing solid white phosphorus into a hot zone remote from said CVD chamber;  
 heating a surface of said block of material to a temperature at which it vaporizes; and  
 flowing a carrier gas through said remote chamber to carry said vaporized white phosphorus to a reaction site within said CVD chamber.  
 
     
     
         53 . A method as in  claim 51  wherein said block of white phosphorus contains at least one other reactant in a concentration selected to produce a preselected chemical composition within this CVD chamber.  
     
     
         54 . A method as in  claim 49 , further comprising the steps of: 
 heating said substrate to at least 300° C. to decompose gaseous phosphorus, making it available for reaction.    
     
     
         55 . A method as in  claim 49  further comprising the step of: 
 heating any nonsubstrate surfaces, within said CVD chamber, with which said vaporized white phosphorus can come into contact to a temperature sufficient to prevent deposition of said white phosphorus on such surfaces.  
 
     
     
         56 . A method as in  claim 55  wherein said temperature of the nonsubstrate surfaces is less than 200° C., whereby conventional O-rings can be used in said CVD chamber.  
     
     
         57 . A method as in  claim 49  wherein a total pressure within said chamber is near atmospheric pressure and wherein this chamber has an open-ended configuration, said method further comprising: 
 feeding an elongated substrate through this chamber to deposit a coat onto said substrate.  
 
     
     
         58 . A method as in  claim 49  wherein the total pressure of gases within said chamber during growth of said photovoltaic layer is less than 100 Torr, thereby improving uniformity of a resulting thin film.  
     
     
         59 . A method as in  claim 58 , wherein a plasma is produced within said reactor to enhance quality of deposited films and reduce substrate temperature.  
     
     
         60 . A method as in  claim 59 , wherein a microwave source provides energy that excites said plasma, thereby enabling production of a plasma at a higher pressure than is obtainable by an RF source.  
     
     
         61 . A method as in  claim 49  further comprising a step of supplying an organometallic material containing a metal selected from the set consisting of: Zn, Cu, Mg, Fe and a mixture containing at least two of these four metals.  
     
     
         62 . A method of manufacturing a photovoltaic cell comprising the steps of: (a) supplying a source of phosphorus to a chemical vapor deposition (CVD) chamber; 
 (b) supplying a source of a metal selected from the set consisting of: Zn, Cu, Mg, Fe, and mixtures of these materials; and    (c) controlling temperature of the substrate and partial pressures of the gases within said CVD chamber to produce on a conductive substrate a photovoltaic thin film selected from the set consisting of: β-ZnP 2 , CuP 2 , MgP 4 , γ-FeP 4  and a mixed crystal containing at least two of the metals selected from the set consisting of Mg, Fe, Cu, and Zn, wherein said conductive substrate can be a single layer of conductive material or a layer of conductive material formed on a support layer.    
     
     
         63 . A method as in  claim 62 , wherein a temperature and partial pressure of the reactants are selected to lie within a double-phase process region, wherein a large excess of phosphorus is used to produce single phase growth.  
     
     
         64 . A method as in  claim 63  wherein a temperature and partial pressures of reactants are selected to lie within a single phase process region.  
     
     
         65 . A method as in  claim 64  wherein, after growth of a layer by process conditions within a single phase process region, said layer is maintained in a phosphorus gas environment at a pressure sufficient to prevent decomposition of said layer that was grown in said single phase process region, until this layer is cooled substantially to room temperature.  
     
     
         66 . A method as in  claim 62  wherein the ratio of phosphorus to metal in the gaseous species within this chamber is at a minimum value, that will permit growth, at which substantially all of the phosphorus gas is utilized in forming a deposited layer.  
     
     
         67 . A method as in  claim 62  wherein a total pressure within said CVD chamber is substantially atmospheric pressure.  
     
     
         68 . A method as in  claim 67 , further comprising a step (d) of supplying an inert diluent to process gases to produce a substantially atmospheric total pressure within said reactor.  
     
     
         69 . A method as in  claim 67  wherein an elongated substrate that has a length, along a direction of feed, that is much longer than a width of this substrate, enters an input end of the chamber, has layers formed thereon and exits through an output end of the reactor.  
     
     
         70 . A method as in  claim 67  wherein said substrate has a width on the order of or larger than a meter, whereby high volume fabrication is possible for providing the large volume of photovoltaic cells required for providing electrical power.  
     
     
         71 . A method as in  claim 62  wherein said substrate is heated to an above ambient temperature selected to improve deposition quality.  
     
     
         72 . A method as in  claim 62  wherein the total pressure of gases within said chamber during growth of said photovoltaic layer is less than 100 Torr, thereby improving uniformity of a resulting thin film.  
     
     
         73 . A method as in  claim 72  wherein a plasma is produced within said reactor to enhance quality of deposited films and reduce substrate temperature.  
     
     
         74 . A method as in  claim 73 , wherein a microwave source provides energy that excites said plasma, thereby enabling production of a plasma at a higher pressure than is obtainable by an RF source.  
     
     
         75 . A method as in  claim 73  wherein said substrate has a temperature on the order of or less than 375° C., whereby this method is compatible with high temperature, stable plastic substrates.  
     
     
         76 . A method as in  claim 73 , wherein said plasma dissociates hydrogen within said reactor, whereby this hydrogen will react with carbon within said reactor to produce gaseous products that are exhausted from this reactor, thereby prevent incorporation of carbon into a film being deposited within this reactor, whereby FeP 4  films can be produced without harmful incorporation of unwanted stable iron carbide.  
     
     
         77 . A method as in  claim 73 , wherein halides are added to said plasma to catalyze nucleation and growth of a film.  
     
     
         78 . A method as in  claim 73 , wherein noble gases are added to said plasma to activate growth of a film by transferring energy from said plasma to said reactants used to grow a film.  
     
     
         79 . A method as in  claim 73 , wherein organic materials containing methyl, ethyl and phenyl groups are added to said plasma to reduce a rate of growth and/or to remove or etch these films after growth.  
     
     
         80 . A method as in  claim 62  wherein said source of metal is an organometallic compound.  
     
     
         81 . A method as in  claim 62  wherein said source of phosphorus is phosphine, t-butyl phosphine, bisphosphinoethane, trimethyl phosphine and triethyl phosphine.  
     
     
         82 . A method as in  claim 62  wherein phosphorus is supplied to said reactor as white phosphorus, said method further comprising the step of heating said phosphorus to a temperature selected to achieve a desired phosphorus partial pressure.  
     
     
         83 . A method as in  claim 82  wherein said white phosphorus is heated in 
 a bubbler and a carrier gas carries vaporized phosphorus to a substrate on which deposition is to take place.  
 
     
     
         84 . A method as in  claim 82 , wherein solid white phosphorus is used as said phosphorus source by introduction directly into said chamber or by introduction indirectly by generation at a remote site followed by transport to said substrate.  
     
     
         85 . A method as in  claim 82  further comprising the step of heating any nonsubstrate surface of said CVD chamber that is exposed to said vaporized phosphorus to prevent deposition and subsequent flaking of phosphorus.  
     
     
         86 . A method as in  claim 85  wherein a temperature of a reactor wall is less than 200° C., whereby conventional O-rings can be used to sear said reactor.  
     
     
         87 . A method as in  claim 62  wherein doping elements are introduced into said vapor above said substrate during growth as a hydride or organometallic compound of said doping elements.  
     
     
         88 . A method as in  claim 87 , wherein said doping element is copper, producing a p-type conductivity.  
     
     
         89 . A method as in  claim 87 , wherein said doping element is lithium, producing p-type conductivity.  
     
     
         90 . A method as in  claim 87 , wherein said doping element is boron, producing n-type conductivity.  
     
     
         91 . A method as in  claim 87 , wherein said doping element is selected from a set consisting of aluminum, gallium, indium and tin, which, when substituted for a metal atom yields n-type conductivity.  
     
     
         92 . A method as in  claim 87 , wherein said doping element is selected from a set consisting of sulfur, selenium and tellurium, which, when substituted for a phosphorus, yields n-type conductivity.  
     
     
         93 . A method as in  claim 61  wherein a film of material selected from the set consisting of β-ZnP 2 , CuP 2 , MgP 4 , γ-FeP 4  and a mixed crystal, containing at least two of the metals selected from the set consisting of Mg, Fe, Cu, and Zn, is deposited on said substrate.  
     
     
         94 . A method as in  claim 62  wherein said photovoltaic thin film is β-ZnP 2 .  
     
     
         95 . A method as in  claim 94  further comprising the step of: 
 doping said β-ZnP 2  layer with a dopant selected from the set consisting of copper and lithium, producing p-type carriers.  
 
     
     
         96 . A method as in  claim 95  wherein said dopant is copper, which is a preferred dopant because of its low mobility in β-ZnP 2 .  
     
     
         97 . A method as in  claim 94 , comprising a step of forming 'a ZnPO x  passivation layer between a front surface of said photovoltaic cell and a front surface of said thin film photovoltaic layer.  
     
     
         98 . A method as in  claim 97  wherein said ZnPO X  passivation layer is strongly n-doped with a dopant selected from the set consisting of Al, Ga, In and Sn.  
     
     
         99 . A method as in  claim 97  wherein said passivation layer is sufficiently strongly doped that it functions as a transparent conductor.  
     
     
         100 . A method of depositing, onto a substrate, layer of ZnPO X , said method comprising the steps of: 
 (a) supplying, to a chemical vapor deposition (CVD) chamber, a source Zn;    (b) supplying a source of phosphorus to said CVD chamber;    (c) supplying a source of oxygen; and    (c) controlling the temperature of the substrate and the partial pressure of gases within this reactor to deposit a layer of material having the composition xZnO: yP 2 O 5 ,:aSiO 2 :bM 2 O 3 :cSnO 2 , where x and y are integers and x/y is approximately 1, 2 or 3 and a, b and c are small (or zero) and where M is a Group III element.    
     
     
         101 . A method as in  claim 100 , wherein said source of zinc is selected from the set consisting of diethylzinc and dimethylzinc and wherein said source of phosphorus is selected from the set consisting of: liquid P 4  and P 2 O 3 .  
     
     
         102 . A method as in  claim 100 , wherein said sources of zinc and phosphorus are both solid sources, wherein said source of zinc is selected from the set consisting of: 
 diphenylzinc, bis(cyclopentadienyl)zinc, bis(methylcyclopentadienyl)zinc bis-(pentamethylcyclopentadienyl)zinc; and    wherein said source of phosphorus is selected from the set consisting of: solid white phosphorus, solid P 2 O 3  and organophosphates.    
     
     
         103 . A method as in  claim 100 , wherein said film is produced by a two-stage process in which a film is deposited without being fully oxidized, followed by an oxygen treatment process.  
     
     
         104 . A method as in  claim 103 , wherein said oxygen treatment, involves plasma-enhanced oxygen species.  
     
     
         105 . A method as in  claim 94  further comprising the step of: 
 forming on said ZnP 2  layer a zinc oxide conductive layer, whereby this conductive oxide layer is fabricated from inexpensive, nonpolluting materials that are readily available and are compatible with fabrication of said ZnP 2  layer.  
 
     
     
         106 . A method as in  claim 94  further comprising the step of: 
 forming on said ZnP 2  layer a boron phosphide layer, which provides an especially hard protective layer from low cost components.  
 
     
     
         107 . A method as in  claim 94  wherein said ZnP 2  layer is grown at a substrate temperature in the range from 500-600° C., in a gas mixture having a phosphorus-to-zinc partial pressure ratio in the range from 10-20 and having a sum of the zinc and phosphorus partial pressures in the range from 1-100 Torr.  
     
     
         108 . A method as in  claim 107 , wherein said gas mixture includes an inert gas component that raises the total pressure within the reactor, during growth of said ZnP 2  layer, to substantially atmospheric pressure.  
     
     
         109 . A method as in  claim 107  wherein said source of Zn is selected from the set consisting of diethylzinc and dimethylzinc.  
     
     
         110 . A method, as in  claim 94 , wherein said Zn and phosphorus reactants are supplied as a rod containing white phosphorus and at least one of the following zinc sources: diphenylzinc, bis(cyclopentadienyl)zinc, bis(methylcyclopentadienyl)zinc and bis(pentamethylcyclopentadienyl)zinc.  
     
     
         111 . A method, as in  claim 48 , wherein said photovoltaic thin film is CuP 2 .  
     
     
         112 . A method as in  claim 111  wherein said CuP 2  photovoltaic layer is formed in a plasma-enhanced environment at a substrate temperature in the 
 range from 400-600° C., at a phosphorus-to-copper ratio of 10-20, in a total pressure of 1-10 Torr.  
 
     
     
         113 . A method as in  claim 112  wherein said metal source is selected from 
 the set consisting of: copper phenyl acetylide, copper hexafluoracetylacetonate, copper trifluoracetylacetonate and cyclopentyldienylcopper triethylphosphine.  
 
     
     
         114 . A method, as in  claim 48 , wherein said photovoltaic thin film is MgP 4 .  
     
     
         115 . A method, as in  claim 114 , wherein said MgP 4  photovoltaic layer is formed at a substrate temperature in the range from 400-550° C., at a phosphorus-to-magnesium ratio of 15-25, in a total pressure of 1-50 Torr.  
     
     
         116 . A method, as in  claim 115 , wherein said metal source is selected from the set consisting of: bis(cyclopentadienyl) magnesium, bis(pentamethylcyclopentadienyl) magnesium and bis(methylcyclopentadienyl) magnesium.  
     
     
         117 . A method, as in  claim 116 , wherein said chamber wall is heated to about 150° C.  
     
     
         118 . A method, as in  claim 115 , wherein said MgP 4  layer is formed in a plasma-enhanced environment.  
     
     
         119 . A method, as in  claim 48 , wherein said photovoltaic thin film is γ-FeP 4 .  
     
     
         120 . A method, as in  claim 119 , wherein said γ-FeP 4  photovoltaic layer is formed at a substrate temperature in the range from 400-550° C., at a phosphorus-to-iron ratio of 15-25 and at a total pressure of 1-50 Torr.  
     
     
         121 . A method, as in  claim 120 , wherein said metal source is selected from 
 the set consisting of: bis(cyclopentadienyl) iron, pentacarbonyl iron and bis(diphenylphosphino) ferrocene.    
     
     
         122 . A method, as in  claim 121 , wherein said chamber wall is heated to about 150° C.  
     
     
         123 . A method as in  claim 119  wherein layer growth is nucleated with small amounts of nucleants selected from the set consisting of Mg and Cu.  
     
     
         124 . A method, as in  claim 48 , wherein said photovoltaic thin film is a mixed crystal having a composition of Zn p Mg q Fe r Cu s P 2t , where t≧p+q+r+s, whereby said photovoltaic cell exhibits a greatly improved average cost per Watt-Hour over a useful life of this photovoltaic cell.  
     
     
         125 . A method as in  claim 124 , wherein said mixed crystal photovoltaic layer is formed in a plasma-enhanced environment at a substrate temperature in the range from 400-550° C., at a phosphorus-to-metal ratio of 15-25, in a total pressure of 1-50 Torr.  
     
     
         126 . A method, as in  claim 125 , wherein said chamber wall is heated to about 150° C.  
     
     
         127 . A method as in  claim 62 , wherein said substrate is plastic, whereby a lightweight photovoltaic device is produced.  
     
     
         128 . A method as in  claim 62  wherein said substrate is an alloy of: iron, copper, molybdenum, aluminum or a combination of these elements.  
     
     
         129 . A method for forming a diode junction, wherein a transparent conducting layer that is heavily doped with n-type dopant is deposited upon a p-type photovoltaic layer without forming a buried diode junction; and 
 subjecting this structure to subsequent heat treatment causes the dopant to outdiffuse from the transparent conductor into a photovoltaic layer, creating a shallow diode junction.    
     
     
         130 . A method for forming a back surface field, wherein a p-type photovoltaic layer is deposited upon a back conducting layer that is heavily doped with a p-type dopant without forming a back surface field; and 
 subjecting this structure to subsequent heat treatment which causes the dopant to outdiffuse from the back conducting layer into a photovoltaic layer, creating a shallow back surface field.    
     
     
         131 . A method for forming a diode junction, wherein a transparent conducting layer that is heavily doped with n-type dopant is deposited upon a p-type photovoltaic layer without forming a buried diode junction; 
 wherein a p-type photovoltaic layer is deposited upon a back conducting layer that is heavily doped with a p-type dopant without forming a back surface field; and    subjecting this structure to subsequent heat treatment which causes the dopant to outdiffuse from the back conducting layer into a photovoltaic layer, creating a shallow back surface field and a diode junction, concurrently.