PHOTOVOLTAIC CELL WITH THIN CdS LAYER

Abstract: The photovoltaic panel of the present invention is initially formed with a CdS and CdTe layers in overlying relationship on a glass substrate and the effective thickness of the CdS layer is reduced during a two-step annealing process to both form larger diameter CdTe crystals (36A, 36B, 36C, 36D) and substantially reduced thickness CdS layer (32) by diffusion into the CdTe layer (44) such that majority of sunlight having a wavelength less than 520 nm passes through the CdS layer (32) to the photovoltaic junction. The p-type CdTe material (44) is first heat treated in an oxidizing environment to oxidize impurities and thereby substantially enhance the desired electrical properties of the p-type CdTe material. The heat treated material is then annealed in a substantially inert environment to increase crystal sizes and thereby enhance photovoltaic efficiency. During the heat treating step, an oxidation gradient may be maintained through the thickness of the p-type CdTe layer (44).

Description: PHOTOVOLTAIC CELL WITH THIN CdS LAYER FIELD OF THE INVENTION The present invention relates to low cost photovoltaic devices and, more particularly, relates to improvements in polycrystalline photovoltaic cells and methods of manufacturing such cells which enable the n-type layer of the photovoltaic junction to be highly transmissive of low wavelength energy, therefore increasing the efficiency of the photovoltaic device. The present invention further relates to improved techniques for forming thin film photovoltaic layers and, more particularly, for forming highly reproducible and uniform p-type polycrystalline layers having controlled electrical properties and improved morphology. The techniques of the present invention are well suited for manufacturing a relatively low cost CdS/CdTe photovoltaic panel by first heat treating the CdTe material in an oxygen containing environment to achieve uniform electrical properties and dictate the morphology of the p-type material and second recrystallizing the heat treated film in an inert atmosphere to obtain the desired morphology and electronic character in the thin film CdTe layer. BACKGROUND OF THE INVENTION Those skilled in alternative energy technologies, e.g., other than hydrocarbon based sources, have long recognized the potential of photovoltaics to satisfy the need for generating electrical power both for remote applications and for comparatively large scale power distribution. The desired low manufacturing costs for reasonably efficient photovoltaic cells can be obtained utilizing polycrystalline thin film layers arranged on a substrate to form a panel. While promising efficiencies have been generated in laboratory work conducted on small cell areas, significant improvements are required to improve manufacturing operations for large scale polycrystalline photovoltaic panel production to become a reality. The present invention is directed to such polycrystalline photovoltaic techniques, and particularly to improved techniques which maximize photovoltaic efficiency, enhance manufacturing yield and reproducibility, and reduce manufacturing costs for polycrystalline photovoltaic panels. Photovoltaic cells capable of converting sunlight directly to electrical energy have been commonly used for decades. Most present-day photovoltaic devices employ single cells, which are satisfactory for low energy consumption devices, but inherently have a high cost/output watt ratio. Polycrystalline photovoltaic cells can be manufactured at a much lower cost than single crystal cells, but have generally reduced efficiencies. Nevertheless, a great deal of research has been and is continuing with respect to the development of higher efficiency polycrystalline photovoltaic products which utilize thin layers of particular chemical compositives to form the n-type material and the p-type material of the photovoltaic junction. Much of this technology has detracted from the advantages of low cost in favor of increased efficiency. While high efficiency is obviously desired, low material and manufacturing costs are critical if photovoltaic technology is to have increased practical significance. Low cost polycrystalline cells can currently provide relatively low cost electrical power at remote locations, such as telecommunication stations, agricultural water pumping sites, remote villages and portable housing facilities. Improvements in such technology may well result in future photovoltaic power plants which compete with conventional hydrocarbon consuming plants. The present invention is directed to such polycrystalline photovoltaic technology and, most importantly, is directed to improvements in photovoltaic technology which will result in increased efficiency but do not significantly increase the material or manufacturing costs of the photovoltaic cells. A low cost photovoltaic cell with a p-type Cu χ S layer and an n-type CdS layer is disclosed in U.S. Patents 3,902,920 and 4,086,101. The term "p-type material" as used herein means material not having as many electrons as holes. The term "n-type material" as used herein means a material not having as many holes as electrons. Such a cell may be formed on a glass substrate according to the techniques disclosed in U.S. Patent 3,959,565. Improved CdS film for such a cell is the subject of U.S. Patent 4,095,006, and residual chlorides in a CdS layer are disclosed in U.S. Patent 4,178,395. The n-type and/or the p-type polycrystalline layers of the photovoltaic cell may be regrown as disclosed in U.S. Patent 4,362,896. Improved equipment for forming such a low cost cell is disclosed in U.S. Patents 4,224,355, 4,239,809, 4,338,078 and 4,414,252. Instead of utilizing a spray pyrolysis technique, the n- type polycrystalline layer or the p-type polycrystalline layer may be formed according to a technique which utilizes compression preceding regrowth, as disclosed in U.S. Patent 4,735,909. After the various polycrystalline layers have been formed on a substrate, individual cells may be formed and interconnected in a series arrangement according to U.S. Patents 4,243,432 and 4,313,022. Various cell or panel encapsulation techniques have been devised, and a low cost yet reliable panel encapsulation technique is disclosed in U.S. Patent 5,022,930. The series connected cells on a glass substrate form a photovoltaic panel, and various photovoltaic panels may be assembled in a module according to the techniques of U.S. Patents 4,233,085. Various materials have been suggested for forming the junction layers of the polycrystalline photovoltaic cell. A great deal of research has been expended with respect to polycrystalline silicon photovoltaic cells, in part because early tests resulted in reasonably high efficiency for these cells. Copper indium diselenide polycrystalline cells have been developed, and show some promise of improved efficiencies. Cadmium telluride cells, and particularly such cells wherein the cadmium telluride is the p-type material, nevertheless are generally considered to offer the lowest cost production potential. It must be recognized, of course, that the commercial cost of a photovoltaic cell does not only consist of the material and manufacturing cost of active junction layers which convert sunlight into electrical energy, since the material and manufacturing costs of the substrate, the appropriate electrode configuration of the cells and for interconnecting the cells, and the encapsulation mechanism must all be considered in analyzing the overall cost of the photovoltaic product. The ideal solution to one problem, i.e., high efficiency and low cost junction formation, must also be compatible with the technology used to achieve a desired overall photovoltaic product at the desired cost/ output ratio. Cadmium telluride photovoltaic cells offer an advantage of relatively low costs. Moreover, cadmium telluride cells may be manufactured with low-cost deposition equipment for applying the CdTe film layer, as described in the previously-referenced patents, and this cell does not require extremely close quality control to obtain reasonable efficiency. Various materials have been proposed for the n-type layer for forming the photovoltaic junction with the cadmium telluride layer. Cadmium sulfide has been considered a suitable n-type material for such a cell, and particularly for a low cost cadmium telluride cell, since it also has a relatively low manufacturing cost and may be deposited at atmospheric pressure utilizing low- cost deposition equipment. U.S. Patent 4,568,792 discloses various types of cadmium telluride cells, and notes that CdS is an advantageous n-type material because of its wide band gap. Various materials have been suggested for "doping" the p-type cadmium telluride layer. U.S. Patent 4,705,911 discloses a CdS/CdTe solar cell, wherein an oxygen-releasing agent is provided to minimize reduction of the p-type characteristics of the cadmium telluride during life in the field. In spite of its low-cost per watt of useful power output, CdS/CdTe cells have not been widely accepted because of continuing relatively poor efficiency. One long-recognized reason for such poor efficiency relates to the construction of the cells and the optical band gap of the CdS layer. To produce electrical energy, light must reach the junction of the cell, i.e., the CdS/CdTe interface. The CdTe layer generally serves as the light absorber, and in a typical structure the CdS serves as a heterojunction partner and an optically transmissive layer. This design may result in a "backwall" cell, wherein light passes through a CdS layer which is deposited on the CdTe layer, which was previously deposited on an opaque substrate. Alternatively, an "inverted backwall" design may be used, wherein light first passes through a highly- transmissive substrate (glass) then through the CdS layer which was deposited over the substrate, to reach the junction formed when the CdTe layer is deposited on the CdS layer. A "front wall" cell may be formed utilizing a -1- CdS/CdTe design, wherein the CdTe layer is deposited over a CdS layer, which was previously deposited on an opaque substrate, on an "inverted front wall" cell formed by depositing the CdTe layer on a glass substrate, with the CdS layer then applied on the CdTe layer. While the backwall or inverted backwall design of a CdS/CdTe cell is preferred, a significant quantity of energy is lost in the CdS layer, since cadmium sulfide does not pass optical energy with a wavelength shorter than approximately 520 nm unless the film is very thin. The CdS layer must be continuous to provide the desired junction and, most importantly, to prevent shorts to the electrode layer. To utilize the desired low-cost deposition equipment, the CdS layer has necessarily been of a thickness such that very little of the optical energy with wavelengths less than 520 nm reached the junction and produced useful energy. U.S. Patent 4,251,286 discloses utilizing a zinc sulfide blocking layer to prevent electrical shorts, but this technique is expensive and introduces additional complexities which are believed to have a significant adverse affect on the life of the cell. U.S. Patent 4,598,306 also discloses the use of a barrier layer for preventing electrical shorts between the active photovoltaic layers and an electrode. The barrier layer operates as a series resistor to limit current flow through the otherwise short circuit current path. U.S. Patent 4,544,797 discloses another technique for preventing short circuits by passivating areas of a first conductive electrical contact which are not covered by the adjoining n-type or p-type material. This passivating step may be performed by immersing, the device in ammonium sulfide to convert a silver metallic layer at the location of pinholes to an n-conductive Ag 2 S material. This procedure is similarly costly and again introduces additional chemicals into the cell formation process which are not desired. An article entitled "Properties of the Screen-Printed and Sintered CdTe Film Formed on a CdS Sintered Film" in Technical Digest of the International PVSEC f Vol. B-III, p. 5 suggests that screen-printed CdS/CdTe cells may have improved longer wavelength sensitivity due to the formation of mixed CdS χ Te_,. χ crystals during sintering of the CdTe. The goal of obtaining high efficiency photovoltaic devices must be balanced against the practical need to obtain uniform and reproducible film layers utilizing commercial manufacturing processes to realize low cost photovoltaic modules. Moreover, the reaction of components within any layer of the photovoltaic device and the reaction of components between layers can have an adverse affect on the useful life of the photovoltaic panel. The film thickness and its morphology, the dimensions and the spacing of the crystals within each film layer, and the electrical properties of the film layer components must all be considered to obtain a high efficiency photovoltaic device. While various techniques have been proposed to achieve a cell which maximizes one or more of the above film layer properties, such techniques are frequently not commercially practical, and/or adversely affect other properties or other film layers of the photovoltaic cell. The present invention is particularly concerned with optimizing the characteristics of a polycrystalline p-type layer of a photovoltaic cell. In an exemplary embodiment of the invention, this layer is the CdTe layer which serves as a heterojunction with the n-type CdS layer. In general, it has been recognized that annealing of this CdTe layer in an oxygen containing environment is desired for enhancing the desired p-type electrical properties of this layer. Annealing the CdTe layer thus promotes growth of crystals within this layer and simultaneously improves the electrical properties of this layer. A description of a prior art CdS/CdTe cell formation process is contained in an article by Woodcock et. al. entitled "Thin Film Solar Cells Based on Electrodeposited CdTe". An alternative screen printed solar cell manufacturing process is disclosed in a 1982 article by Uda et. al. entitled "All Screen Printed CdS/CdTe Solar Cell". Prior art techniques for forming the p-type layer of a photovoltaic cell have several significant drawbacks which have limited the commercial manufacturing efficiency and thus the cost-effectiveness of polycrystalline thin film photovoltaic devices. As previously noted, high uniformity and reproducibility of a low-cost film deposition process, in combination with the desired electrical properties for the film components required to achieve high photovoltaic efficiency, have yet to be optimally achieved. Laboratory techniques involving the evaporation of two or more alternating Te and Cd layers have resulted in cells with a reasonable photovoltaic efficiency in the 6% to 14% range. During scale-up of this technique toward manufacturing, however, reproducible and uniform CdTe layers were not realized. While revised control equipment and methods might alleviate the lack of stoichiometry causing this non-reproducibility and non- uniformity, the initial cost and maintenance expense of sophisticated equipment significantly detracts from the economics of achieving a cost-effective photovoltaic device. To maintain the desired local stoichiometry between the Cd and Te, already formed CdTe has been deposited on the CdS layer, then the deposited films annealed to increase the film crystal size and enhance electrical properties. This annealing process had been proposed with the glass substrates on which the film layers are formed arranged in a convection oven so that the CdTe layers are face-to-face. This face-to-face arrangement presumably serves a purpose similar to the"covered boat" design utilized by some manufacturers of polycrystalline photovoltaic cells, namely to control the concentration of cadmium chloride in the film layers. Cadmium chloride is a flux. As used herein, "flux" refers to any substance, whether a solid, liquid, or gas that lowers the recrystallization temperature of a material. Further information with respect to annealing of the CdS material and the role of cadmium chloride as a flux is disclosed in a 1988 IEEE article entitled "Electrical and Microstructural Properties of Screenprinted CdS Layers for CdS-CdTe Solar Cells." The face-to-face annealing technique did not result in good uniformity by itself due to excessive oxidation of CdTe, particularly along the edges of the panels. This was inevitable since the regrowth was accomplished in an atmosphere containing some oxygen. The oxygen was considered necessary to achieve the desired electrical properties of the p-type layer. At the high regrowth temperatures desired, controlling excessive oxidation while allowing the oxygen to permeate fully and uniformly between the face-to-face film layers is extremely difficult to achieve. Suggestions to control excessive oxidation frequently resulted in an increase in the time constant for oxygen to permeate the entire space between the face-to- face panels, thus undesirably increasing manufacturing costs (longer times) and, more importantly, only partially avoiding panel edge to center non-uniformity. As a consequence, non-uniformity of either crystal sizes and/or electrical properties of the regrown CdTe layer between the center and edges of the panels continued to be a significant problem for photovoltaic cells produced utilizing the face-to-face configuration alone. The disadvantages of the prior art are overcome by the present invention, and an improved photovoltaic cell and method of forming a photovoltaic cell are hereinafter disclosed. A photovoltaic panel comprising a plurality of cells formed according to the techniques of the present invention has the desirable benefit of low material and manufacturing costs, yet produces a considerable increase in photovoltaic conversion efficiency compared to prior art devices which have not included this technology, thereby resulting in a photovoltaic panel having relatively low overall cost per watt of useful power output. SUMMARY OF THE INVENTION A high efficiency CdS/CdTe photovoltaic cell may be formed according to the present invention. A continuous relatively thick CdS layer is initially formed on a substrate, but its thickness is reduced during regrowth of the active photovoltaic layers. At the regrowth temperature, CdS diffuses or migrates into either the space between the formed large CdTe crystals or into the crystals themselves, resulting in a continuous CdS layer, yet a CdS layer having a reduced effective thickness. The effective thickness of the resulting CdS layer may be in the range of approximately 100 A to approximately 500 A, so that a high percentage of optical energy less than 520 nm passes through the CdS layer to the junction, thereby increasing efficiency. The term "effective thickness," as used herein, is intended to mean the apparent thickness of the layer as defined by the layer's transmissivity of short wavelength sunlight, i.e., wavelengths below 520 nm. As an example, using an absorptivity coefficient of CdS of 10 5 /cm, a CdS layer having an actual uniform thickness of 3,500 A may pass less than approximately 3% of sunlight having a wavelength less than 520 nm, while a CdS layer having an actual thickness of approximately 200 A may pass more than 92% of this low wavelength energy. Special precautions are taken to ensure that the CdTe layer does not provide a significant shunting path to the conductive layer adjoining the CdS layer, while at the same time ensuring that the conductive layer is highly transmissive of light. This conductive layer preferably comprises one highly-conductive layer of tin oxide (approximately 10 20 carrier per cm 3 ), and another substantially lower conductivity layer adjoining the CdS layer. This second lower conductivity tin oxide layer has its carrier concentration adjusted so that when in contact with CdTe (where there are flaws in the CdS layer) , a voltage and current are produced and shunting is avoided. The highly conductive tin oxide layer provides the desired low resistance path for the transmission of electrical energy within and between cells. High continuity of the low conductivity tin oxide layer is essential if there are a significant number of flaws in the CdS layer, and the desired uniformity of this tin oxide layer may be obtained by spraying a low molarity tin oxide solution for a long enough time so that a large number of droplets provide uniform statistical coverage of the high conductivity tin oxide layer. The conductivity of tin oxide may be easily varied on the range of at least seven orders of magnitude. According to one embodiment of the present invention, a high efficiency CdS/CdTe backwall photovoltaic panel is formed by first spraying a high conductivity tin oxide layer on a glass substrate utilizing spray pyrolysis to obtain a layer with a specific conductivity in the range of from 1000 to 5000 mho/cm. The low conductivity second tin oxide layer is then applied by the same technique using a low molarity solution suitably doped to obtain a desired carrier concentration in the resulting film. In an exemplary case, a conductivity of approximately 0.10 mho/cm with a carrier concentration of 4 x 10 17 carriers/cm 3 is used, with a CdTe layer having a carrier concentration of 5 x 10 15 carriers/cm 3 to produce a satisfactory photovoltaic cell. In this case, cadmium was used as the tin oxide dopant, but zinc or other metals may be used. It should be pointed out that the carrier concentration, while related to conductivity, is not a direct function of it but is related in part to the dopant used and resulting mobility. Conductivity measurements are expressed in this application because carrier concentration is difficult to measure. The CdS layer may then be applied by spray pyrolysis (or other suitable technique) to an initial thickness in the range of from 2,000 to 12,000 A, and a substantially thick CdTe layer applied by an atmospheric temperature spraying and compression technique (or similar method) . The panel is then heat treated and the active photovoltaic layers regrown at approximately 540"C to form large CdTe crystals, while the CdS diffuses or migrates between or into these crystals and forms a layer having an effective thickness in the range of approximately l/20th of its initial thickness. The panel is then divided into cells and the cells series interconnected, and the completed panel encapsulated. The active areas of a photovoltaic panel have a comparatively high efficiency up to 18% over a long cell life, in large part due to the increased efficiency attributable to the effectively thin CdS layer. The quantum efficiency of the cell in the spectral band at wavelengths shorter than 520 nm may surprisingly be approximately 90%. The tin oxide layer, with an appropriate carrier concentration, in combination with a suitable p-type material layer, will produce voltage and current when exposed to sunlight where there are flaws in the CdS layer. According to the present invention, a two-step process is utilized to achieve the desired electrical and crystal morphology (through control of nucleation) of the p-type layer of a photovoltaic cell, while overcoming the problems of the prior art. In the first step, an impurity getting step, the p-type material is heat treated in an oxidizing environment to adjust the electronic properties of the material. The terms "oxidation" or "oxidize" as used herein have a chemical definition, and traditionally mean the loss of electrons by atomic or molecular species, including the formation of another species with a different oxidation state and/or the formation of a new compound. If a donor species is oxidized or passivated by forming an oxidized or chemically stable species by compounding with an oxidant, then electronic changes can occur by chemical means. As used herein, a "donor" is a species which has additional available electrons compared to a number of electrons which the host semiconductor matrix can easily accept. As used herein, "oxidant" refers to any chemical compound that oxidizes a material. Oxidants include, without limitation, oxygen, sulphates, phosphates, oxides such as sulfur dioxide and carbon monoxide, and peroxides. The impurity gettering step controls the desired semi- conductor characteristics of the material by deactivating donor impurities which otherwise cause the p-type material to be either n-type or poorly p-type. As used herein, a "donor impurity" refers to excess donor species, including without limitation impurities or native defects, present within a material rendering the material less p-type, insulating, or n-type, depending on the concentration of the excess donor species. Oxidation by heat treatment in the impurity gettering step has the general effect of rendering the oxidized species non-donating, which would tend to make a semiconductor more p-type after the heat treating step. The ratio of holes to electrons thus partly defines the semiconductor characteristics of the material. The remaining acceptors take over to make the material desirably p-type. Oxygen itself may also act as an acceptor in the p-type layers. The impurity gettering step may occur either before, during, or after deposition of the p-type material on the substrate. In either case, in the p-type layer donor impurities are deactivated by oxidation and the acceptor concentration forced to dominate. Significant crystal growth during this step is not required. During the second step, or the recrystallization step, the p-type film layer is recrystallized in an environment substantially free of oxidants to obtain proper crystal growth. As used herein, an "inert substance" is any substance that does not cause oxidation of the film layer components. The term "recrystallization" as used herein refers to any process wherein a number of relatively small diameter crystals are converted into fewer crystals of a larger size. Recrystallization occurs at elevated temperatures, and preferably is flux actuated or flux accelerated to produce crystals having a large mean diameter. The film layers are preferably arranged face-to- face during the second recrystallization step to increase the concentration of flux vapors from a dopant added to the deposited film layers. By separately adjusting the electrical properties and the morphology of the crystals in the p-type layer, reproducible and uniform film layers may be reliably obtained for highly efficient photovoltaic cells. In an exemplary embodiment, the techniques of the invention may be used for achieving an improved CdS/CdTe photovoltaic cell. The CdTe layer desirably has an excellent uniformity and good manufacturing reproducibility. By having the ability to separately control the p-type material electrical properties and the size and morphology of crystals in this layer, a high degree of process reliability is obtained. The CdTe film may be deposited on the CdS film, which was previously deposited on a transparent electrically conductive film on a glass substrate. During the impurity gettering step, CdTe film layers are positioned in an oven maintained at a temperature of from about 380"C to 500°C for a relatively short period of time, e.g., from about 5 seconds to 30 minutes. The substrate panels are arranged so that the CdTe films are open faced to obtain uniform oxidation over the length and width of each panel. An oxidation gradient through the p-type film thickness desirably may be established to achieve the final morphology desired and to achieve a good n-type/p-type metallurgical interface with the window material. In the preferred embodiment, crystals adjacent the opposing or upper surface of the CdTe layer will thus have a thicker oxidized coating than the lower crystals adjacent the CdS layer, thereby forming an oxidation gradient through the film thickness. The panels after the impurity gettering step may then be annealed in the recrystallization step with the CdTe layers arranged face-to-face within a substantially inert atmosphere. Any substantially inert gas will suffice including nitrogen, argon, methane and helium. Preferably, the inert atmosphere is substantially composed of nitrogen. The temperature during the recrystallization step may be maintained within the range of from about 400°C to about 700°C, more preferably from about 480°C to 560°C, and occurs for a sufficient time of from about 10 minutes to 60 minutes to obtain the desired crystal growth and morphology. Due to the oxidation gradient through the film thickness obtained during the impurity gettering step, the recrystallization flux is variably retarded within the film and does not act uniformly on all crystals spaced throughout the film thickness during the recrystallization step, so that large CdTe crystals are formed adjacent the top CdS surface. The flux may be any halide, including without limitation, CdCl 2 , Cdl, CdBr, HC1, and NH 4 C1. Preferably, the flux is CdCl 2 . The resulting size of the crystals and the size distribution through the p-type film thickness may be modified to form an optimum metallurgical interface by varying the magnitude and/or the slope of the oxidation gradient, by adjusting the temperature and temperature rise rate of the recrystallization step, and by varying the amount or type of flux, porosity of the p-type layer, and the substrate orientation and placement in the recrystallization step. According to these improved techniques, excessive crystal growth is prevented, and the desired crystal grain volume to grain surface area is obtainable. By controlling the oxidation gradient through the film layer in the impurity gettering step, the flux released during the recrystallization step is retarded in its efficacy to induce recrystallization as a function of oxidation coating thickness, thereby causing nucleation cites to be initiated at or closely adjacent the interface between the p-type layer and the window material. The result can be a high performance metallurgical interface between the p-type layer and the n-type window layer primarily attributable to large p-type crystals each having a base surface substantially planar with the n-type material surface. If this high performance metallurgical interface is not formed, (e.g., the lower surfaces of crystals do not form a planar interface with the n-type window layer) , the reflectivity of this interface due to air n-type layer or air p-type layer reflection would significantly diminish photovoltaic efficiency. Alternatively, a monolayer of p- type material crystals could be formed in the recrystallization step from a p-type material which previously had been gettered in an oxidizing heat treatment step to obtain desired p-type properties. The oxidized coating will be left on or removed along with the impurity. Again, the lower surfaces of the crystals within this monolayer could form a planer interface with the n-type layer. The techniques of the present invention recognize that a small excess of one of the p-type layer components, such as, for example, an excess of either Cd or Te in a CdTe layer as a result of non-equivalent Cd and Te metal ratios, change the susceptibility of the material to oxidation. Since the final morphology of the recrystallized CdTe film is primarily affected by the oxidation gradient through the film thickness, precaution is taken to assure a desired ratio of Cd to Te in the deposited CdTe film layer. Cd and Te may be thoroughly mixed and the mixture heated in an inert atmosphere until nearly complete reaction has occurred to form CdTe. If this mixture is then heated in an inert atmosphere in an open system, the Cd or Te will separate from the reacted material to a desired extent due to its higher volatility. Use of a CdTe material with consistent Cd and Te ratios is practically required if the improved electrical properties and desired oxidation gradient through the CdTe layer is to be obtained as a result of the above described two-step process. The optimum performance might be achieved in a CdTe starting material having non-equivalent Cd and Te ratios to enhance the growth of oxide at grain surfaces without over oxidizing the CdTe bulk grain. It is an object of the present invention to provide a substantially high efficiency polycrystalline photovoltaic cell, wherein the n-type material layer of the photovoltaic junction is made highly transmissive to short wavelength energy by reducing the effective thickness of the n-type material layer by diffusing or interdiffusing into or with the p-type layer a substantial portion of this layer. It is a further object of the present invention to reduce the effective thickness of the n-type material layer in a photovoltaic cell to approximately 1/20th of its applied effective thickness, thereby forming a substantially continuous, reduced thickness layer for increased short wavelength light transmissivity. A major object of this invention is to provide a polycrystalline cell with a relatively thin yet continuous n-type material layer, and with a relatively low conductivity and doped layer adjacent the n-type material layer, and a high conductivity conductor layer adjacent to the low conductivity layer and formed from substantially the same base material as the low conductivity layer, such that any flaws in the n-type material layer do not allow a short between the p-type material layer and the low conductivity layer, but rather actually produce energy in the area of any flaws in the n-type layer by the proper adjustment of tile relative carrier density of the low conductivity layer and the p-type material layer. It is a feature of the present invention to provide a high efficiency CdS/CdTe photovoltaic cell, wherein the effective thickness of the applied CdS layer is significantly reduced by diffusion or migration into the CdTe layer, thereby reducing the effective thickness of the CdS layer to allow a high percentage of short wavelength light to reach the CdS/CdTe junction. It is a further feature of this invention to utilize low cost techniques to apply a uniform and continuous CdS layer, and to substantially reduce the thickness of the applied layer to obtain a desired relatively thin CdS layer. Yet another feature of the present invention is to regrow the CdS/CdTe photovoltaic layers at a temperature in excess of approximately 400°C, such that relatively large CdTe crystals are obtained while a significant portion of the CdS layer migrates or diffuses between or into the CdTe crystals to substantially reduce the effective thickness of the CdS layer. Still another feature of this invention is to provide a CdS/CdTe photovoltaic cell with a tin oxide conductive layer comprising a relatively low conductivity tin oxide layer adjoining the CdS layer, and a high conductivity tin oxide layer adjoining the low conductivity layer. It is still another feature of the invention that the carrier density of the high resistivity tin oxide layer is adjusted to be compatible with the CdTe layer to form an energy producing junction in areas where the CdS layer contains flaws. It is a significant feature of the present invention that the material and equipment costs of applying the required layers of the photovoltaic cell are substantially reduced compared to prior art manufacturing techniques resulting in high efficiency photovoltaic devices. It is an advantage of the present invention that the techniques for effectively reducing the desired effective thickness of the CdS layer and for minimizing or avoiding shunts and/or dead areas between the CdTe layer and the conductive layer adjoining the CdS layer may be used for various photovoltaic cell arrangements. It is a further advantage of the present invention that various techniques may be used for applying the layers of the photovoltaic cell according to the present invention, thereby rendering the present invention suitable to spray pyrolysis, dip coating, gas deposition, and similar processes for applying relatively thin polycrystalline layers. It is a significant advantage of the present invention that improved techniques are provided for modifying both the electrical properties and the morphology of the CdTe layer for a photovoltaic cell without a significant increase in equipment or material cost. The techniques of the present invention are particularly well suited for reliably controlling and enhancing the regrowth of crystals and the electrical properties of the polycrystalline p-type layer, e.g., the CdTe layer of a cadmium sulfide/cadmium telluride photovoltaic panel. The present invention also maintains highly controlled local stoichiometry of the materials forming the p-type layer. A photovoltaic panel comprising a plurality of cells formed according to the techniques of the present invention has the desired benefit of low material and manufacturing costs, yet produces relatively high photovoltaic conversion efficiency at a low overall cost per watt of output power. The techniques of the present invention will make a significant contribution to the manufacture of comparatively high efficiency photovoltaic devices having a relatively low manufacturing cost, and should be readily adaptable to photovoltaic products of various material constructions. These and further objects, features, and advantages of the present invention will become apparent from the following detailed description, wherein reference is made to the figures in the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a pictorial view, partially in cross- section, of one embodiment of a photovoltaic panel formed according to the present invention which is not encapsulated. Fig. 2 is a cross-sectional view of a panel depicting various layers applied over a glass substrate according to the present invention prior to regrowth of the active photovoltaic layers. Fig. 3 is a cross-sectional view of the panel as shown in Fig. 2 subsequent to regrowth and prior to dividing and series interconnection. Fig. 4 is a cross-sectional view of an alternate embodiment photovoltaic panel subsequent to regrowth and prior to dividing and series interconnection. Fig. 5 is a cross-sectional view of a portion of a photovoltaic device according to the present invention, illustrating the deposited layers on a glass substrate. Fig. 6 is a cross-sectional view of a conceptualized CdTe layer subsequent to oxidation and prior to compression. Fig. 7 is a cross-sectional view of a conceptualized CdTe layer after compression. Fig. 8 is a cross-sectional view of a conceptualized CdTe layer after the impurity gettering and recrystallization steps to enhance crystal growth. Fig. 9 is a pictorial view, partially in cross- section, of photovoltaic panels arranged face-to-face during the recrystallization step and after the impurity gettering step. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Fig. 1 generally depicts a photovoltaic panel 10 including a plurality of photovoltaic cells 12 according to the present invention. The panel consists of relatively thin chemical layers 18 formed on a common vitreous substrate 14. Glass is a preferred substrate due to its relatively low cost and its ideal flat surface for applying thin layers, and for subsequently heating those layers due to the low thermal expansion of the substrate. It should be understood that each of these applied layers, which are particularly shown in Fig. 3, are relatively thin to reduce material costs, and together will typically be less than 20 microns, so that the panel as shown in Fig. 1 visually appears to be little more than a rectangular sheet of glass with a thin painted coating. Also, it should be understood that the panel as shown in Fig. 1 may be complete in the sense that it is capable of producing electrical energy directly from sunlight. To maintain any form of reasonable efficiency, however, the panel needs to be encapsulated to prevent water vapor-related degradation of the cells. Fig. 1 depicts a backwall cell configuration, i.e., sunlight passes first through the glass substrate and then to the junction formed by the active photovoltaic layers. The panel as shown in Fig. 1 is thus turned over during typical outdoor use, so that the glass substrate 14 is above the layers 18. Nevertheless, the panel is formed with the glass as the base or substrate, and accordingly, the terms "top" or "upper" when referring to the layers discussed subsequently should be understood with respect to the substrate being beneath these layers. The techniques of the present invention are also applicable to manufacturing front wall photovoltaic cells, wherein sunlight is absorbed by the junction formed by a cadmium telluride absorber layer on top of the CdS window layer, so that light rays do not pass through the CdS layer. The photovoltaic panel shown in Fig. 1 may be formed by successively applying thin, continuous layers of chemicals on a glass substrate. Individual cells are formed by making a series of elongate parallel cuts 20 through at least some of these layers to divide the panel into an elongate strips of cells according to the disclosure of U.S. Patent 4,243,432. A layer or layers applied over the active photovoltaic layers also preferably provide the series interconnection of these cells to form the desired output voltage, and series interconnection strips 16 for accomplishing this purpose are generally depicted in Fig. 1. A suitable series interconnection configuration for electrically connecting cells is discussed subsequently. A suitable panel according to the present invention may thus be approximately 60 cm by 60 cm, and is capable of producing an output of approximately 50 watts with an incident isolation of approximately 1000 watts per square meter. For many commercial purposes, the desired electrical output is obtained by housing a plurality of panels in a module similar to the arrangement disclosed in U.S. Patent 4,233,085. Fig. 2 depicts in cross-section a panel prior to regrowth (crystallization) of the active photovoltaic layers. It should be understood that the thickness of each of the applied layers as shown in Figs. 2 and 3 is approximately to scale with respect to the other depicted layers, but not with respect to the glass substrate 14. During the first step of constructing the photovoltaic panel, a bottom electrode may be formed on the substrate, with the electrode itself consisting of a bottom continuous electrode layer 24 having a high conductivity, and a top electrode layer 26 having a comparatively low conductivity. Each of these layers may be formed from a tin oxide solution utilizing spray pyrolysis techniques disclosed in the previously-cited prior art. The desired conductivity of these layers may be varied and, within reasonable limits, easily controlled by adjusting the amount of zinc or cadmium which is added to the tin oxide spray solution. Since the panel particularly described herein is of a backwall configuration, light must pass both through the glass substrate and the layers 24, 26 before reaching the junction formed by the active photovoltaic layers. High transmissivity of these tin oxide layers can nevertheless be maintained while changing the conductivity of these layers by a factor of approximately seven orders of magnitude. The bottom tin oxide layer 24 serves the primary purpose of passing electrical energy through the cell and interconnecting the cell in a series arrangement, and thus should have a high conductivity. Preferably the layer 24 has a conductivity of more than 1000 mho/cm, and most preferably more than 2200 mho/cm. Using prior art techniques, a highly conductive yet highly transmissive tin oxide layer has been generated having a specific conductivity of 3700 mho/cm, so that obtaining a tin oxide layer with a specific conductivity of approximately 2200 mho/cm is feasible on a commercial basis. For reasons explained subsequently, the top tin oxide layer 26 must have low conductivity, should be thin, and also must have a high continuity (few pinholes or flaws in layer 26 are permissible) . High continuity of this layer may be obtained utilizing a low molarity tin oxide spray solution for the spray pyrolysis process. The specific conductivity of the upper tin oxide layer preferably is in the range from approximately 1.25 x 10 '3 to 100 mho/cm. The thickness of the lower tin oxide layer is not particularly critical, but is preferably in the range 0.4 to 1.0 microns. Specific conductivity is a parameter that may be measured and, as such, is employed simply as a means to estimate the carrier concentration of the layer. The thickness of the upper tin oxide layer may be from approximately 0.1 to approximately 1.0 microns and, as previously noted, is doped with a suitable metal such as cadmium or zinc to produce low carrier concentration. Once the tin oxide layers have been formed, a relatively continuous layer 28 of CdS may be applied utilizing spray pyrolysis. The thickness of the deposited CdS layer may be in the range of from approximately 2,000 A to 12,000 A (0.2 to 1.2 microns), and this layer also has high continuity (few pinholes) . A fairly thick CdTe layer 30 may then be applied on the CdS layer, with the thickness of layer 30 being substantially greater than that of the CdS layer 28. The CdTe layer may be economically formed using the atmospheric temperature spray and compression technique disclosed in U.S. Patent 4,375,909. It should be understood that various techniques may be used for applying any of the layers 24, 26, 28 and 30, including spray pyrolysis, dip coating, or gas deposition. The preferred deposition technique has low cost deposition equipment and thus low manufacturing costs, and will result in continuous, thin, and thus relatively inexpensive layers. As shown in Fig. 2, the layers 28, 30 as applied have relatively small crystals (not shown) , which is undesirable for high photovoltaic efficiency. To increase the conversion efficiency, these layers are regrown at a temperature in excess of 400°C, and preferably from approximately 520°C to approximately 550°C, to form substantially large crystals as shown in Fig. 3. Individual lower crystals 36A, 36B, and 36C in the regrown CdTe layer have dimensions approximating 2 microns, while the total thickness of the regrown layer 44 is typically approximately 6 microns. The smaller CdTe crystals are generally toward the upper portion of layer 44. Alternatively, the CdTe crystals could have a thickness approximating that of the layer itself, which is likely an achievable goal. Regrowth occurs at a temperature and during a time interval sufficient to cause substantial interdiffusion between the CdS and the CdTe layers and the CdS is also believed to migrate into the CdTe layer, and particularly in the interstices between the large diameter CdTe crystals. The photovoltaic cell formed according to the techniques of the present invention has an efficiency with respect to such short wavelength energy which would be equivalent to a device having a very thin CdS layer, and accordingly it is convenient to discuss the effective thickness of the CdS layer. As exemplified in Fig. 3, it is believed that the actual thickness of portion 32 of the CdS layer between the bottom of a CdTe crystal and the top of the low conductivity tin oxide layer 26 is substantially minimized, and it is also believed that the CdS tends to diffuse away from the tin oxide layer and partially enters the cadmium telluride crystals, occupies a portion of the voids between CdTe crystals, and deposits on CdTe grain surfaces. This diffusion or migration of the CdS is generally in the lower portion of the CdTe layer, and some voids 48 generally will still exist between CdTe crystals, with diffused CdS "surrounding" these voids and adjoining the CdTe grain boundaries. The effective thickness of the CdS layer and the tin oxide layer is substantially reduced by the interdiffusion of CdTe and CdS and the "transfer" of CdS onto CdTe grains. Fig. 3 thus represents the presumed flow of CdS material during regrowth, with some of the CdS material migrating upward to deposit on CdTe crystal surfaces or form irregular upwardly extending walls 34 of CdS material between CdTe crystals, while some of the CdS material may form a relatively thin, generally planar layer 32 between the bottom of individual CdTe crystals and the top of the layer 26. During regrowth, some of the CdS material also may diffuse into the CdTe crystals and some CdTe may likewise diffuse into the CdS material. This diffusion and/or interdiffusion also may result in the desired substantial reduction in the effective thickness of the CdS layer. The significant reduction of the effective thickness of the CdS layer during the regrowth of the active photovoltaic layers can be exemplified by noting that the CdS layer 28 in Fig. 2 prior to regrowth has an effective thickness of from 2,000 to 10,000 A, while the effective thickness of the CdS layer 32 after regrowth as shown in Fig. 3 is preferably in the range of from approximately 100 to approximately 500 A. Accordingly, the effective thickness of the CdS layer has been reduced during regrowth so that its thickness subsequent to regrowth is approximately 1/2Oth or less of its thickness prior to regrowth. This substantial reduction in effective thickness of the CdS layer is thus a primary reason for the substantial increase in photovoltaic efficiency, since the thin CdS layer is able to pass short wavelength light (less than 520 nm) to the junction, while the comparatively thick CdS layer absorbed and wasted that short wavelength energy as heat. In the middle of the blue response region of photovoltaic devices, e.g., 450 nm wavelength, cells using relatively thick CdS layers (with absorptivities of 10 5 /cm) may have less than 3% quantum efficiency at that wavelength. Cells according to the present invention, however, have a quantum efficiency at the same wavelength of from 60 up to 80% at that wavelength. While an extremely thin CdS layer is desired to pass this low wavelength energy, the average effective thickness of the CdS layer must be sufficient to minimize the number of pinholes or flaws in this layer, and must also be sufficient to form a reasonable junction with the CdTe crystals. According to the present invention, the effective thickness of the CdS layer may be uniformly reduced to less than 500 A by diffusing a great deal of the material from this layer into the CdTe layer 36 during regrowth, and at least some of this material migrates to enter the gaps between the CdTe crystals. If the CdS is deposited by spray pyrolysis, the quantum efficiency of the regrown cell decreases slightly with an increase in as-deposited CdS effective thickness greater than 2,000 A. In one experimental program, an as- deposited CdS layer effective thickness from 5,000 to 6,000 A nevertheless results in quantum efficiency in excess of 70% at 450 nm on a finished device. While the effective thickness of the CdS layer subsequent to regrowth may thus be reduced to approximately 1/20th of its as- applied thickness, a slightly greater or slightly less effective thickness reduction may occur. In any event, however, the effective thickness of the CdS layer will be significantly reduced during regrowth, and preferably will be reduced to an effective thickness less than approximately 10% of its as-applied thickness, and most preferably will be reduced to approximately 500 A or less. The addition of cadmium chloride as a flux in the CdTe layer during regrowth may be important for large diameter crystal formation. It is also noted that generation of hot halogen-containing gases during regrowth is important for acceleration of the interdiffusion of the CdS and CdTe. Due in large part to the substantially reduced effective thickness of the CdS layer, it is possible that some pinholes or other flaws in the applied CdS layer may occur. If a conventional conductor layer adjoins the CdS layer, a pinhole in the CdS layer will result in a short between the CdTe layer and the conductor layer, thereby destroying the energy producing effect of the cell. According to the present invention, such shorting is avoided by applying two dissimilar tin oxide layers, with the layer adjoining the CdS layer being the relatively low conductivity layer. Due to the special nature of this layer, that is adjustment of the layer's carrier concentration as a function of the carrier concentration of the p-type layer, a pinhole in the CdS layer will not result in shorting of the cell, but rather, it will actually produce power due to the heterojunction formed between the cadmium telluride and the low conductivity tin oxide. Since this low conductivity (high resistivity) tin oxide layer is thin, preferably less than about 8000 A, and since current passes in a direction normal or perpendicular to the plane of this layer, the low conductivity of this layer adds little series resistance to the overall panel. Current flow in the high conductivity tin oxide layer moves in a direction generally parallel to the plane of this layer, and high conductivity for the bottom tin oxide layer is essential to achieve high efficiency. It has been determined that, by properly doping the top tin oxide layer with zinc or other suitable metal, the electron carrier density of this level may be adjusted to be compatible with the presumed electron carrier density of the p-type layer, in this case the CdTe layer. Adjusting the electron carrier density of the upper tin oxide layer by adding a suitable metal also affects the resistivity of this layer, and to a much lesser amount affects transmissivity. Accordingly, a reasonable tradeoff must be made between the desire to achieve the desired carrier density for this layer, at the same time ensuring that this layer has high transmissivity and is less than a maximum allowable resistivity. Nevertheless, it is possible to obtain high transmissivity for this layer, achieve the desired specific conductivity in the range of from 1.25 x 10 3 to 100 mho/cm, and simultaneously obtain an electron carrier density for this layer which preferably is adjusted to be within the range of approximately two to three orders of magnitude of the known or presumed hole carrier density for the CdTe layer. By making the carrier density of the top tin oxide layer compatible with the p- type layer (within at least two or three orders of magnitude) , any reasonable flaws in the CdS layer would not result in short-circuiting of the cell for reasons previously noted, but rather an energy-producing junction is formed by the CdTe layer and the top tin oxide layer. If the adjusted electron carrier density of the top tin oxide layer is too low relative to the carrier density of the CdTe layer, the junction formed with the tin oxide layer is undesirably shallow within the CdTe crystals, thereby resulting in low open-circuit voltage for that small contact area between the CdTe and the high resistivity tin oxide layer. On the other hand, if the electron carrier density of the tin oxide layer is undesirably high relative to that of the CdS layer, the junction formed in the CdTe crystals is too deep, thereby resulting in low short circuit current and an unsatisfactory junction. Nevertheless, a reasonably efficient junction may be obtained between the CdTe layer and a tin oxide layer doped with a suitable metal. While it is preferred to form a cell according to the present invention so that a very thin CdS layer capable of passing low wavelength sunlight is obtained, flaws in such a CdS layer will not destroy the cell for the reasons noted above, and accordingly extreme quality control procedures for applying and for regrowing crystals in this layer are not required. Also, it should be understood that it is possible to entirely eliminate the CdS layer and form a suitable photovoltaic cell, so that the bottom tin oxide layer acts as a conductor, the top tin oxide layer acts as the n-type layer (heterojunction partner) , and cadmium telluride or other suitable material acts as the p-type layer. As a further explanation of the latter embodiment described above, it is recognized that the cost of producing photovoltaic modules has been a major limitation to large-scale terrestrial use of photovoltaics as an energy source. The low-cost production of photovoltaics is primarily a function of reducing the cost of. materials used, reducing the cost of deposition equipment for applying the active film layers, the simplicity of design of the active layers and the module itself, and a lack of sensitivity to process variations as a function of the materials and the design utilized. Each additional material present in the photovoltaic device adds to the complexity and therefore the cost of the device. A number of materials can conceivably be matched to a given absorber layer (typically the p-type layer) by adjusting the relative ratio of electron or hole carrier concentration contained in each of the p-type and n-type layers. The function of practically adjusting the carrier density of the junction partner layer (typically the n-type layer) often includes complexities which add significantly to the cost of the deposition. According to this invention, the method of adjusting the carrier concentration of a tin oxide layer is disclosed. The carrier concentration is related to the specific resistivity which may be adjusted by over seven orders in magnitude by changing the amount and type of dopant added to the tin oxide solution, which may be sprayed on top of the heated substrate (glass) . The reliability of manufacturing according to this technique is simplified since the base material for each of the various tin oxide layers need not be significantly changed and the deposition technique need not change. The carrier concentration of the tin oxide layer may thus be matched to any of various p-type absorber layers, thus eliminating the requirement of a separate CdS layer, and thus reducing the cost of the photovoltaic modules. Fig. 4 illustrates in cross-section of a photovoltaic cell according to this latter invention. The glass substrate and conductive layers 24 and 26 are as previously described. Preferably substantially the same conductive material composition is used to form both layers 24 and 26, with the selected material forming a transparent, conducting layer with suitable characteristics, and preferably being from a group consisting of tin oxide, zinc oxide, indium tin oxide, and cadmium stannate. More particularly, this selected material for the layer 24 should be highly conductive, while at the same time this material can be doped to form a layer 26 which acts as a heterojunction partner and a window layer for the photovoltaic cell. For purposes of explanation, this discussion assumes that the material for the p-type layer will be cadmium telluride, although various compounds may be used to form the p-type layer, and exemplary alternatives are discussed below. Also for purposes of explanation, the layer 26 may be doped with cadmium or zinc to reduce the conductivity of this layer and result in the desired n-type material, although lead, mercury, selenium, sulfur, sodium, cesium, mercury, boron, and chromium may be alternative doping materials. Preferably, substantially all of the dopants in the conductive layer 24 differ from substantially all of the dopants in the conductive layer 26. The photovoltaic panel as shown in Fig. 4 thus comprises a substantially continuous conductive layer 24 of tin oxide for electrically interconnecting the plurality of cells, and a polycrystalline tin oxide layer 26 being applied on layer 24 and doped with a selected amount of a desired dopant. A polycrystalline p-type layer 46 is then formed on the layer 26 with the cadmium telluride crystals of layer 46 being of the form and size previously described. The top electrode layer 38 as shown in Fig. 3 is applied over the cadmium telluride layer, and may migrate down into the porous CdTe layer 26 with no significant adverse affect. A spacing between the lowermost migrated material of layer 38 and the uppermost migrated n-type material is preferred, as shown in Fig. 3, with this spacing being occupied by CdTe crystals and voids between the crystals. For the embodiment as shown in Fig. 4, the migrating material of layer 38 does not contact the tin oxide layer 26, and again voids occur between the CdTe crystals and between this migrating material and the tin oxide layer 26. Sunlight thus passes through the substrate 14 and the layer 24 to reach the junction formed by the n-type layer 26 and the p-type layer 46. Preferably both the layers 24 and 26 are deposited by the same process to reduce manufacturing costs and improve quality control, and spray pyrolysis is one suitable process for depositing both the conductive tin oxide layer 24 and the doped tin oxide layer 26. Gas deposition is clearly envisioned as an economical and controllable deposition process. According to the method of this invention, a photovoltaic panel including a plurality of photovoltaic cells may be formed on a common substrate by selecting material for the conductive polycrystalline film layer 24, then depositing this selected material by spray pyrolysis or another technique on to the substrate to form a substantially continuous optically transmissive conductive layer for electrically interconnecting the plurality of photovoltaic cells. The p-type material for forming the photovoltaic heterojunction is selected, and the presumed carrier density or approximate carrier density of the p- type layer is known. Substantially the same material used to form the layer 24 may then be selectively doped to form the n-type layer, with, the amount of dopant being a function of the presumed carrier density of the p-type photovoltaic layer. The high conductivity layer 24 may consist of tin oxide and a small amount of fluorine, which may be added to the tin oxide material for layer 24 to enhance conductivity. Although a different dopant is added to the tin oxide material for layer 26, the material for forming both layers 24 and 26 prior to adding the dopant is substantially the same, e.g., tin oxide. In many deposition techniques, temperature also affects the carrier concentration and conductivity of the resulting film. This doped material may then be deposited by spray pyrolysis on the conductive thin film layer 24 to form a n-type polycrystalline thin film layer for the photovoltaic cells, and finally the selected p-type material may be deposited on the n-type material to form a p-type photovoltaic layer and the junction with the n-type layer. Once a panel as shown in Fig. 4 is formed by this technique, the large photovoltaic cell may be divided to form a plurality of photovoltaic cells in the manner described in the prior art, and the divided photovoltaic cells then interconnected to form a photovoltaic panel. Tin oxide is a preferred material for achieving the above purposes due to its high transparency and dopability to achieve a wide range of resistivity. Other materials may, however, be used. Zinc oxide is one alternative and, although its extinction coefficient is smaller than that for tin oxide, its specific resistivity is considerably higher. The large electron density of tin oxide, generally greater than about 10 20 /cm 3 , prevents its use as a junction material with every potential semi-conductor partner layer. Cadmium telluride may form the p-type layer for such a photovoltaic cell, since its carrier density can be approximately I0 16 /cm 3 . The cell formed from these layers can thus be expected to have a low open-circuit voltage and/or unsatisfactory short-circuit current. By doping the tin oxide layer, however, a junction may be produced with the cadmium telluride layer which results in a reasonable voltage, current, and fill factor. The uniform continuity of the low carrier concentration density tin oxide layer is necessary to avoid shorts, while at the same time this tin oxide layer must remain thin, and preferably less than about 8,000 A, to prevent unnecessary optical absorption. These objectives can be achieved by applying this layer with spray pyrolysis utilizing a large number of low molarity droplets to consistently and completely cover the low resistance tin oxide layer and thereby prevent low resistance shunt paths. It should also be understood that the p-material layer may be formed from materials other than CdTe. According to the above technique, an inexpensive copper indium diselenide cell may be formed with the n-type material being a doped tin oxide layer formed by spray pyrolysis. A similar cell may be formed utilizing copper sulfide, copper indium disulfide or copper indium diselenide as the p-type material. Other cells which may be formed according to this invention include cells having a p-type semi- conductor layer of either polycrystalline silicon, aluminum antiminide, gallium arsenide, or indium phosphide. According to the method of the present invention, the effective thickness of the n-type material layer is reduced so that at least a substantial amount of sunlight, i.e., at least 25%, and preferably at least 50%, having a wavelength with an energy higher than the bandgap of the n-type material for this layer (short wavelength light) passes through this reduced effective thickness n-type layer to be absorbed by the photovoltaic heterojunction. Since the n- type layer must be physically thin, flaws or holes in this otherwise continuous layer can be expected. According to a preferred embodiment of this invention, the p-type layer forms a desired photovoltaic junction with this thin n-type layer, and also forms a photovoltaic junction with the conductive layer 26 where flaws occur in the n-type layer. This latter junction has a reasonably high efficiency due to the doping at this layer 26, so that its electron carrier density is within at least three orders of magnitude of the known or presumed carrier density of the p-type material. In order to prevent shorts in the cell, the specific conductivity of this layer 26 is also maintained within the range previously described. Once a panel has been regrown in a manner which results in layers 24, 26, 32 and 36 as shown in Fig. 3, or 24, 26 and 46 as shown in Fig. 4, the panel may be divided and series interconnected according to prior art techniques. Using either a mechanical cutter or a laser, a thin strip of applied layers may be removed down to the glass substrate, and a portion of the thin elongate gap formed by this operation filled with a suitable insulating film to cut the electrical connection between the bottom electrode layers 24 of adjacent cells. A desired electrode strip may be formed on an edge portion of the bottom tin oxide layer. Either prior or subsequent to this operation, a top electrode layer 38 as shown in Fig. 3 may be deposited over the CdTe layer, with some of this layer 38 optionally filling the upper portion of gaps between the CdTe crystals. Accordingly, the top electrode layer 38 may include downwardly projecting walls 40 which preferably do not come into contact with the upwardly projecting walls 34 of the CdS layer. Various materials may be used for the layer 38, and it is presently preferred that the layer be formed by a graphite paste process, thereby achieving relatively low material and deposition cost. The series interconnection of the cells may then be formed by depositing a conductive electrode layer on top of layer 38, with the conductive electrode layer filling part of the gap formed by the cell division technique and forming a reliable electrical connection between the top electrode of one cell and the bottom electrode of an adjacent cell. The completed cell may then be encapsulated according to techniques described in patents previously noted. Any of the layers 24, 26 or 46 may thus include a narrow elongate cut to divide the panel into individual cells and to series interconnect the cells. Each of these layers is nevertheless deposited as and remains a substantially continuous layer, thereby resulting in comparatively low manufacturing costs. While tin oxide is a preferred material for each of the high conductivity and low conductivity layers of a cell according to the present invention due to its high transmissivity and ability to easily adjust its carrier concentration, other materials may be used to form this bottom electrode. A zinc oxide layer may form this conductor layer, and is particularly well suited for forming the low conductivity layer. The material used to dope the conductor and adjust its carrier density and resistivity should not substantially affect the high transmissivity of this layer, and zinc, indium, gallium, and aluminum are a suitable doping metal for this purpose. Other dopants may also be used. While the tin oxide layers may be formed according to spray pyrolysis techniques, the invention is not limited to using spray pyrolysis to form the conductor layers. Also, it should be understood that while the bottom conductor layer as described herein consists of the bottom high conductivity tin oxide layer and a substantially increased resistivity top tin oxide layer, the conductivity of the conductor layer may change gradually from the bottom to the top of the conductor layer, so that two distinct layers are not formed but rather a gradual change in conductivity of the tin oxide layer occurs as one moves up through the thickness of the layer. The prospect of gradually changing the conductivity of this layer is not difficult to obtain when panels are formed on a mass production basis, since the glass substrate may be moved over a series of deposition areas each having an increasing or decreasing amount of added metal as a dopant. As previously noted, high continuity of the tin oxide layer adjoining the CdS layer is essential, and this high continuity can be obtained by reducing the molarity of the tin oxide solution which form this upper layer, and increasing the deposition time and thus the number of droplets reaching the substrate. While no precise molarity for forming this uppermost tin oxide layer is critical, the molarity of the solution forming the uppermost tin oxide layer typically will be less than .5 moles/liter, and frequently in the range of about .2 moles/liter or less. While the techniques of the present invention are particularly well-suited for forming a high efficiency CdS/CdTe photovoltaic cell, it should be understood that the concepts of the present invention are not limited to use of these chemical layers for either the p-type or the n-type material. In particular, it should be understood that substantially increased efficiency of a photovoltaic device is formed according to the present invention by obtaining an n-type material layer which has a substantially reduced effective thickness compared to its as-deposited thickness, and that this n-type layer is obtained by simultaneously heating the p-type material and the n-type material layer to substantially increase the size of the crystals in each of these layers while simultaneously diffusing and interdiffusing the n-type and the p-type layers. A complete understanding of the mechanical and chemical functions occurring during the substantial reduction in the effective thickness of the CdS layer (or other n-type layer highly transmissive of sunlight) is not yet fully understood. This reduction in effective thickness is primarily believed to be due to diffusion, interdiffusion, and/or migration of CdS material into the CdTe (or other p-type material) layer, with CdS material entering the voids between the regrown CdTe crystals and depositing on the available CdTe surfaces. The term "diffusion" as used herein with respect to the action occurring during the reduction in the effective thickness of the n-type layer should be understood to encompass conventional diffusion, as well as interdiffusion and migration into the p-type layer. Also, those skilled in the, art will appreciate that while diffusion of the n-type layer into the p-type layer will reduce the effective thickness of the n-type layer, as explained above, this action can also be similarly described as diffusion of the p-type layer into the n-type layer. The key is the desired effective reduction in the thickness of the n-type layer which occurs during heating, not the specific mechanical and/or chemical function which causes this reduction. Another benefit of this CdS "diffusion" action is that the n-type material tends to cover a substantially higher portion of the CdTe grain surfaces than would occur during a planar interface of two layers. Also, it should be understood that the benefit of passing short wavelength light to the junction will occur regardless of the selected n-type material, and that the earlier reference to passing light less than 520 nm is based on the bandgap of CdS, which is approximately 520 nm. The bandgap of various materials suitable for forming the n-type layer is well known, and energy bandgaps for various materials can be easily calculated from minimum room temperature energy gap values published in available handbooks, such as CRC HANDBOOK OF CHEMISTRY & PHYSICS. 58th Edition. The present invention thus envisions the significant reduction in the effective thickness of the n-type material layer by "diffusion" into the p-type layer, such that a majority of sunlight energy having a wavelength with an energy higher than the bandgap of the selected n-type material passes through the reduced thickness n-type layer to react with the photovoltaic junction. Fig. 5 depicts in cross-section components of a photovoltaic cell 80 according to the present invention. The depicted cell comprises film layers 84, 86, 88 and 90 each formed successively on a glass substrate 82. Those skilled in the art will appreciate that the relative thickness of film layers with respect to each other is shown, but that the glass substrate 82 is in reality much thicker than that depicted with respect to the film layers. Also, those skilled in the art will appreciate that the cell shown in Fig. 5 has not yet been encapsulated or series interconnected, and that these operations may occur in the manner as explained in the prior art in order to produce a useful photovoltaic panel comprised of a plurality of series connected photovoltaic cells. The cell depicted in Fig. 5 is of a back wall configuration, so that sun light passes first through the glass substrate and then to the junction formed by the active photovoltaic layers. A panel formed according to the Fig. 5 construction is thus turned over during typical outdoor use, so that the glass substrate 82 is above the active layers discussed subsequently. Nevertheless, the cell is formed with the glass as a base or substrate, and accordingly the terms "top" or "upper" when referring to the layers discussed subsequently should be understood with respect to the substrate being beneath these layers. It should be understood that the techniques of the present invention may also be applicable to manufacturing frontwall photovoltaic cells, wherein sun light is absorbed by the junction formed by a p-type layer on top of an n-type layer, so that light rays do not pass through the n-type layer, and to photovoltaic devices structures having layers deposited on an opaque rather than a transparent substrate. The vitreous substrate 82 has a substantially flat upper surface 83 on which is formed an electrode layer. The electrode may be any electrically conductive material that has a high transmissivity to maintain reasonably high photovoltaic efficiency since sun light must pass through these layers to reach the photovoltaic heterojunction discussed subsequently. The electrode may be composed of tin oxide, zinc oxide, indium tin oxide, cadmium stannate or metal containing grid. For the embodiment depicted in Fig. 5, this electrode layer consists of a bottom tin oxide layer 84 and an optional top tin oxide layer 86. The bottom electrode layer 84 has a comparatively high conductivity for transmitting energy between interconnected cells with low current flow resistance. The top electrode layer 86 has a comparatively low conductivity and has high continuity and serves as a backup n-type heterojunction material to produce energy in an area where pin-holes appear in the n-type layer 88. The lower tin oxide layer 84 will typically have a thickness in the range of from about 0.4 to 1.0 microns, and the thickness of the upper tin oxide layer 86 will typically be in the range of from about 0.1 to 1.0 microns. Deposited on top of the tin oxide layer 86 is a continuous or semicontinuous layer 88 of an n-type material. For the embodiment discussed below, the n-type layer 88 is a cadmium sulfide layer which forms a photovoltaic heterojunction with the p-type layer 90, which may be cadmium telluride. The CdS layer 88 has an effective thickness subsequent to regrowth which is preferably in the range of from approximately 100 A to approximately 500 A, and its effective thickness may have been substantially reduced during regrowth. The present invention is primarily concerned with improved techniques for forming the p-type layer 90 and, for the embodiment discussed below and shown in Fig. 5, the p-type material will be cadmium telluride. Some semiconductor materials, such as CdTe, achieve desired conduction characteristics due to a combination of residual impurities in the material and/or non-equivalence of the ratios of the material components. Accordingly, such impurities or non-equivalence conditions are necessary for certain semiconductor applications. On the other hand, semiconductor materials may form insulators or low or very low conductivity semi-conductors if prepared such that components are equivalent. The desired properties of a semiconductor material forming a layer of a thin film photovoltaic cell can be markedly different than the properties desired for numerous other semiconductor applications. The active layers of a thin film cell are desirably p-type. These film layers are polycrystalline, and generally film layers are recrystallized at elevated temperatures thereby increasing the size of crystals and modifying their morphology to enhance the photovoltaic conversion efficiency. A low cost and highly reproducible technique for forming a uniform layer 90 having high photovoltaic efficiency will now be described. A key feature of this technique is the separation of steps involving heat treating of this p-type material in an oxidizing environment to adjust and improve its electronic properties, and the subsequent regrowth of the p-type crystals during recrystallization in a substantially inert environment. While both steps are in a sense "heat treating" or "annealing" steps, it is preferable for purposes of this explanation to refer to the first step as an impurity gettering step, and the second step as a recrystallization step, thereby allowing the steps to be more easily distinguished. The technique of this invention allows for forming a p-type film of a desired thickness and morphology, with this film layer having desired electronic properties and crystal dimensions and morphology which will effectively result in a reproducible and highly efficient cell. The CdTe film may be deposited at substantially ambient or room temperature by spraying a cadmium telluride powder mixed with a suitable carrier, such as propylene glycol or terpineol, to form a slurry. The sprayed layer may be dried at room temperature or at a slightly elevated temperature of less than about 250°C to evaporate the carrier. The formed layer at this stage will be similar to layer 90A depicted conceptually in Fig. 6. The individual crystals 94, 96 within this layer have a mean diameter in the range of less than about 0.6 microns, and the total thickness of a typical layer 90A may be approximately 68 microns. Once the carrier has evaporated, substantial voids 98 are present between crystals, and the volume of the crystals within the layer 90A compared to the total volume of the layer 90A will be about 25% (75% porous) . Fig. 6 depicts a conceptualized CdTe layer at this stage. Although edge surfaces of crystals are in contact with each other, this contact is not completely apparent in cross- section in view of the high porosity of this layer. Also, many of the apparently smaller crystals shown in Fig. 6 will have a larger mean diameter than depicted, since the cross-section of some crystals is through a corner portion of the crystal. Similarly, the edge surfaces of the crystals subsequently described and shown in Fig. 7 and 8 are in contact with neighboring crystals, and some of the crystals shown in these figures also have a larger diameter than is shown in the cross-sectional views. The cell construction substantially as shown in Fig. 6 may then be subjected to an impurity gettering step which is a heat treating process in an oxidizing environment. The preferred oxidizing environment for low cost processing is air. The impurity gettering step may occur in a belt oven maintained at a temperature of from about 300°C to 500°C, and typically requires a relatively short time period of from about 5 seconds to 30 minutes, and preferably from about 2 minutes to 10 minutes at oxidizing temperatures. During this impurity gettering step, the CdTe layer 90A as shown in Fig. 6 is open faced, so that both oxidants and heat are uniformly available over the entire CdTe layer surface on the panel, thereby achieving high uniformity. Water content affects oxidation rate, and the water vapor in the air during this step should be controlled, preferably to less than 50 grains per pound of dry air or at least to a reproducible value. Preferably, the oxidizing environment contains at least 1.0% by volume oxidants and more preferably at least 20% by volume oxidants. The oxidants may include halogens, oxygen, sulphate, phosphates, oxides such as sulfur dioxide and carbon monoxide, and peroxides. The preferred oxidants are oxygen, sulfur dioxide and carbon monoxide with the most preferred oxidant being oxygen. The impurity gettering step achieves the desired uniform electrical properties of the CdTe film, in that donor impurities in the film layer, which otherwise render the film layer 90A n-type or poorly p-type, are "bound-up" or deactivated, so that residual acceptors are dominant and cause the CdTe film to be desirably p-type in its electronic characteristics. It may also be that oxygen acts as an impurity acceptor to further enhance the p-type character of the CdTe. Regardless of the actual mechanism, the result is that a p-type CdTe is formed with satisfactory p-type characterization. It is not believed that any morphological changes occur during the impurity gettering step, though some chemical changes and physical property changes presumably do occur. Since the heat treated film is open faced, oxidation of the crystals is highly uniform at different spaced planar locations of the CdTe film. Even though layer 90A is highly porous at this stage, an oxidation gradient through the thickness of the CdTe layer is established during this impurity gettering step, so that crystals in the lower portion of this layer (adjacent the CdS layer 88) are less oxidized than crystals in the upper portion of the layer 90A. In a hypothetical embodiment, for example, crystals adjacent the top of the layer 90A may have an oxidized coating about 70 Angstroms thick on the exterior surface of each crystal, while CdTe crystals spaced approximately in the center of the CdTe film thickness may have an oxidized coating approximately 10 Angstroms thick on the crystal exterior surfaces. The preferred oxidized coating is formed of oxides. The bottom crystals closely adjacent the CdS layer will be the least oxidized, with an exemplary average of about 2 Angstroms of an oxidized coating thickness on the exterior surface of these crystals. The degree of oxidation and slope of the oxidation gradient is a function of the concentration of oxidants in the oxidizing environment, the relative concentrations of the p-layer components, and the temperature and duration of the impurity gettering step at oxidizing temperatures. The sharpest oxidation gradient may be obtained by heating the cell in an inert atmosphere and, when the system is at the desired temperature, purging oxidant(s) into the atmosphere for a controlled time period. The magnitude of the oxidation gradient is increased over other techniques as the cell is not heated in an oxidizing atmosphere to the desired temperature level. In other words, the time for oxidation in the purged system is substantially less than that for the other techniques. Once the desired electrical properties have been obtained to render the layer 90A properly p-type and the desired oxidation gradient has been established through the thickness of this layer, the CdTe layer may be compressed in a compression step by the application of a substantial mechanical force to reduce the porosity of the layer. The layer is about 75% void space and 25% solids before compression and about 60% void space and 40% solids after compression. This application of mechanical force may occur utilizing equipment and techniques disclosed in U.S. Patent 4,735,909, hereby incorporated by reference. During this compressive step, the CdTe film thickness is preferably reduced to approximately 65% of its original thickness, and accordingly the exemplary compressed layer 90B as shown in Fig. 7 has a thickness of about 12 microns. During this compression step, no chemical or electrical changes in the CdTe crystals occur, and compression serves to reduce the air spaces between crystals to enhance the efficacy of the flux resulting in the desired crystal growth during the subsequent recrystallization step. Subsequent to the compression step, the CdTe film layer may be annealed in an inert atmosphere in a recrystallization step to obtain desired crystal growth. During the recrystallization step, the CdTe layers deposited on a pair of panels are preferably arranged in a face-to-face relationship, or at least against a non- contaminating barrier material to restrict flux movement to a desired area. Any other arrangement resulting in a controlled confinement of the flux may be utilized. The oxidant content of the annealing atmosphere is maintained at less than 5% by volume, and preferably less than about .05% by volume to minimize or avoid non-uniformity which may occur, even if the face-to-face configuration is used due to excessive oxidation at the edges. Preferably, the oxidant is oxygen and the annealing atmosphere has less than 3% oxygen by weight and more preferably less than 0.5% by weight. The temperature of the annealing oven is preferably maintained in the range of from about 400°C to about 700°C, more preferably from about 480°C to 560°C, and during a time interval of from about 10 minutes to 60 minutes, and preferably between about 40 and 60 minutes. This recrystallization process further reduces the CdTe film thickness to a typical range of from about 4 to 8 microns. The benefit of the establishment of an oxidation gradient during the heat treating step, as described above, relates to the ability to form a desired metallurgical interface between the CdS layer and the CdTe layer, and the related formation of substantially large crystals having substantially planar interfaces adjacent the CdS layer and smaller crystals above these large crystals. The rate of temperature rise during the recrystallization step in the temperature range where the flux is active (e.g., above 450"C and preferably less than 600°C), affects the result and is an important factor. Preferably, the temperature rise rate is above 3"C/min in the active flux range, and preferably less than about 25°C/min. If the temperature rise rate is too high, there may be too much crystal growth due to a high flux density; if too low, there may be too little crystal growth due to a low flux density. If the CdTe layer is annealed in a face-to-face manner but an oxidized coating variation and/or gradient has not been properly established through the thickness of the CdTe layer and the surfaces of CdTe crystals adjacent the CdS layer are either not oxidized or oxidized to substantially the same extent as the CdTe crystals adjacent the top of the CdTe layer or if the layer is not oxidized in an impurity gettering step, annealing in the recrystallization step may result in more uniformly sized CdTe crystals throughout the thickness of the layer. This morphology still may be satisfactory with respect to electronic characteristics since the crystal grain boundary surface area to crystal grain volume is minimized. Formation of such crystals in this manner, however, results in a poor metallurgical interface between CdS layer and the CdTe layer. Large crystals throughout the CdTe layer result in substantial interstitial voids between crystals, and accordingly there is poor planer coverage between the top of the CdS layer and the bottom surface of adjoining CdTe crystals. Since the contact area between the CdS crystals and the CdTe crystals is poor, and since the CdS-air and CdTe-air interfaces are reflective, the reflective losses to cell performance can be significant if there is poor planar coverage. This reflective loss can be as high as 10% or even 30% depending on the severity of the interface condition, and this reflective loss inherently reduces the photovoltaic efficiency of the cell since reflected light cannot be absorbed in the CdTe layer. By establishing the oxidation gradient through the thickness of the CdTe layer as described above, the size and morphology of the CdTe crystal grains through the layer may be controlled or adjusted. The CdTe crystals adjacent the CdS layer are only slightly oxidized compared to the top CdTe crystals. Since the thicker oxidized layer on the crystals toward the top of the CdTe film retards the effect of the flux, which is preferably cadmium chloride, recrystallization will necessarily be initiated in the lower planar portion of the CdTe layer during the recrystallization step. In this manner, nucleation sites are initiated (and growth is more rapid) near the top of the CdS film, thus resulting in substantially large crystals 106, 108, 110, 112 and 113 as shown in Fig. 8 which properly grow in a manner such that the lowermost surface of these crystals obtain substantially desired planar metallurgical interface with the top surface of the CdS film. More particularly, recrystallization is initiated at sites within the CdTe layer within roughly 1 micron of the top CdS surface, and preferably within roughly 1/2 micron of the CdS surface. This lower planar layer consists essentially of relatively large crystals having a mean diameter of about 1.5 to 3 microns, and this layer has a residual porosity of less than 30%, and preferably about 15% or less. The relatively large crystals are preferably 2 to 1,000 times, more preferably, 3 to 30 times, and most preferably 10 times their as deposited crystal size. The crystals 114, 116 in the upper layer portion of the regrown CdTe film layer 90C are significantly smaller than the lower crystals, but this is not significant to photovoltaic efficiency, since only the lower crystals are critical to both establishing the desired metallurgical interface for high efficiency devices and minimizing surface recombination to insure good electronic properties and exhibiting satisfactory diffusion lengths. The crystals in the upper planar portion of the CdTe layer (above the lower large crystal layers) may have a mean diameter of from about 0.5 to 2 microns, and typically the majority of crystals in this upper planar portion will have a mean diameter of about 1 micron or less. The porosity of this upper layer portion is substantially greater, and typically in the range of about 20% to 45%. The overall porosity of the regrown CdTe layer will thus be in the range of from about 15% to 30%, and typically about 25%. While not wishing to be bound by any theory, it must be noted that the porous layer (the top 2-4 microns of the CdTe layer) is non-essential for proper device function. It is only the result of insuring that an improved interface exists at the CdTe/window layer metallurgical interface. As a variation, a multiple film deposition may be utilized to allow both the metallurgical interface to be satisfactory and the porous CdTe layer to be further modified in order to achieve its optimal morphological and electronic characteristics. Fig. 9 depicts two panels 120, 122 arranged in a face- to-face relationship for the recrystallization step. These panels are maintained in the desired relationship by channel-shape clips 124. A number of other methods for retaining the panels in place are envisioned. During the compression step as referred to above, very thin gaps of about 0.03 inches are intentionally formed between elongate areas 126 each compressed by a roller and having a width of about 0.40 inches. These gaps result in slightly raised and uncompressed portions 128, which do not form active areas of the photovoltaic cells, and instead are subsequently removed to form the series interconnection. Similarly, the edges of each of the panels are not compressed, thereby establishing a non-compressed border around each panel comprising raised border portions 130, 132, 134 and 136. By intentionally not compressing these portions, the panels 120, 122 may be placed in a face-to- face relationship, with the opposing non-compressed portions contacting each other to slightly space the active compressed areas apart (typically at least 10 to 20 microns) during the recrystallization step and to form a "seal" around the periphery. As an alternative to using non-compressed areas to provide the desired spacing between the active film layers during this regrowth, corner spacers of a desired uniform thickness may be used between the panels to separate the films during this face-to-face regrowth operation. The spacers may be placed about the perimeter of the face-to- face panels, and may be sufficiently solid to substantially seal the interior spacing between the opposing CdTe layers. Alternatively, if so desired, the spacers may have one or more holes uniformly or randomly spaced therethrough, so that the interior space is in greater fluid communication with the atmosphere in the oven exterior of the face-to- face panels. As yet another alternative, corner or intermediate space may be used to uniformly separate the CdTe layers, and elongate substantially solid clips or clips with holes may be used for establishing the desired sealing/partial "breathing" relationship during the recrystallization step. A modification of the two-step procedure described above includes an additional impurity gettering step after the recrystallization step. The second impurity gettering step effectively results in at least three heat treatment or annealing steps, and may be added to the two-step process to further substantially minimize the number of impurities in the recrystallized p-type material. Impurities bound up during the second impurity gettering step may include residual impurities which were not oxidized during the first impurity gettering step, and impurities or contaminants which were introduced as a result of or evolved from the handling or process steps subsequent to the first impurity gettering step. This second impurity gettering step preferably occurs with the substrate panels open faced in the same manner as the first impurity gettering step. This second impurity gettering step may occur in an atmosphere containing oxidants, such as air or other oxygen containing gas, and at a temperature of about 150°C to 340°C, and preferably between about 150°C to 250°C, and for a time interval of about 5 to 60 minutes. The second impurity gettering step may both enhance the p- type character of the CdTe film, and result in an improved CdTe surface layer for forming a better mechanical and/or electrical connection to the top electrode layer. The second impurity gettering step may occur prior to, subsequent to, or prior to and subsequent to, etching the CdTe layer for the electrode connection. The technique described earlier establishes a lower CdTe layer comprising large mean diameter crystals adjacent and forming a planar interface with the CdS layer, and an upper, more porous CdTe layer having relatively smaller crystals. Alternative techniques may be used for forming a cell comprising two or more distinct layers which are each part of the p-type layer, with one dense layer including large crystals forming a planar interface with the n-type material layer, and a second layer or layers comprising crystals having dissimilar p-type characteristics. The first layer may be formed from a CdTe material which is heat treated to getter impurities in a manner discussed above. This first layer may be approximately 1-1/2 microns thick, so that the first layer, once recrystallized in an inert atmosphere, is substantially a monolayer of large CdTe crystals with desired electrical properties attributable to oxidation. The optimal second or upper layer, which may be several microns thick, may be applied subsequent to heat treatment and may not be heat treated, or may be heat treated to form a material which is more p-type than it was prior to heat treatment, and significantly more p-type than the first layer. For a cell having high photovoltaic efficiency, the first layer should have a low conductivity (for cadmium telluride, from about 0.5 to 5 x 10 15 holes/cm) since this layer adjoins the n-type window layer and forms a heterojunction therewith. The second layer may have a higher conductivity for easier contactability and/or lower series resistance losses and thus a higher hole concentration than the first layer, since it is spaced from the n-type heterojunction partner. The second layer may even be an alloy of CdTe. Another technique for forming such a CdTe layer of two dissimilar p-type layer characteristics is to utilize the additional impurity gettering step discussed above to adjust the p-type characteristics of either the second p- type layer, or both the first and second p-type layers. This second layer may be formed from a material slightly different than the first layer, or could be formed according to a slightly modified process, as described subsequently. Thus, the second layer may have substantially more carriers after recrystallization than the first layer, and post oxidizing this CdTe layer may be used to desirably achieve the p-type characteristics for both the first and second layers. The second layer may also act as a buffer layer to allow improved electrical isolation of the device electrodes, one of which is subsequently applied. The second layer may optionally be excluded, however, and the electrode layer applied directly to the 1-1/2 micron thick CdTe monolayer. A significant benefit of face-to-face recrystallization procedure, and particularly one which obtains substantial sealing of the spacing between opposing CdTe layers, is that the desired cadmium chloride flux level is maintained during the recrystallization operation to enhance crystal growth. Cadmium chloride may be added to the CdS material to form a desired flux to assist the formation of large CdTe crystals or to help modify the CdTe/CdS interface. The face-to-face recrystallization as disclosed above also substantially reduces the sensitivity of the heat treated p-type layer to contamination. By providing face-to-face recrystallization, the time for exchange of gases between the opposing CdTe layers is slow, and generally the half-life of gas exchange will be of the order of at least thirty minutes. This enables the materials for construction of the recrystallization oven to be fabricated from less expensive stainless steel alloys rather than ceramic or expensive alloys, thereby reducing manufacturing costs. Once annealed, the CdTe layer may be chemically etched, a top electrode layer applied (preferably forming a series interconnection between divided cells) , and the completed and interconnected cells encapsulated. It should be understood that the CdTe raw material composition has a significant affect upon obtaining the desired CdTe layer. The desired CdTe material may be formed by placing cadmium metal and tellurium metal into evacuated quartz ampules and the mixture processed at about 1200°C with little or no allowable contamination. For this process, the cadmium and tellurium are weighed out in equal molar quantities, and true stoichiometry requires that the cadmium and tellurium metals are present within approximately 0.01 atomic percentage of each other. Any initial metal weight that exists as an oxide will not form CdTe, or an hydroxide or other material that converts to an oxide at high temperatures and will additionally push the CdTe ratio away from true stoichiometry. Extreme care must thus be used to produce CdTe that contains a consistent amounts of excess cadmium or tellurium that is not bound together as CdTe. An off-stoichiometry condition changes the susceptibility of the material to oxidation during the p-type oxidation or heat treating step, which can affect (either adversely or beneficially) achieving the desired morphology of the CdTe film. According to a second technique for forming a CdTe compound, powders of cadmium and tellurium are mixed thoroughly together, then heated in an inert atmosphere from about 500°C to 700°C until completely reacted. Excess amount of cadmium or tellurium will be more volatile than the reacted CdTe, and will thus evaporate to leave the desired stoichiometry. Regardless of the technique used to form the CdTe compound, it will be recognized that impurities present within the formed cadmium telluride will affect the oxidation susceptibility during the heat treating step, and that effective control of oxidized and oxidizable impurities during this step is necessary for achieving a consistent (and/or optimum) CdTe stoichiometry. The separate impurity gettering and recrystallization steps as discussed above produce a uniform CdTe layer utilizing a reproducible process wherein the electrical properties and morphology of the CdTe may be easily adjusted. The produced result is surprising since the electronic property adjustment achieved during the impurity gettering step occurs prior to the recrystallization step, and it would be presumed that electronic characteristics of the CdTe film would change dramatically during the recrystallization step. It is also fortuitous that the oxidation gradient desirable to retard the flux and define the nucleation and growth of the CdTe during the recrystallization step may be simultaneously obtained during the impurity gettering step which achieves the adjustment of the electrical properties of the CdTe film to result in high photovoltaic efficiencies. The fact that electronic adjustment of the p-type material occurs prior to re-crystallization allows this two-step technique to be used in alternative processes wherein the electronic properties are adjusted prior to or during deposition of the CdTe film on the CdS layer. In an alternative process, the CdTe could be oxidized during deposition by heating the substrate to a desired temperature in each zone of a multiple zone oxidation oven, while the CdTe is sprayed on the substrate as it passes through these zones. By providing multiple spray nozzles and by varying the oxidant concentration in the multiple heated zones, the desired oxidation gradient may be established as the CdTe layer is built up through spraying. CdTe material may be heat treated in an oxidizing environment and, once its electronic properties are adjusted, the p-type oxidized film then sprayed as a slurry on the CdS layer. By pre-oxidizing the CdTe material before it is deposited on the substrate, the necessity of a belt-type furnace to heat the entire substrate is avoided, and a batch-type oven may be used for uniformly heating a relatively small amount of CdTe raw material having only a small fraction of the heat capacity of the film/substrate combination. This technique thus has an advantage of a substantial savings in utility cost compared to the previously described technique. A disadvantage of this technique, however, is that the desired oxidation gradient through the CdTe film is not established, since raw material CdTe is uniformly preoxidized then deposited on the substrate. A combination of this alternative with the previous or following methods may also be used as a viable process. Techniques could also be used to oxidize CdTe raw material by varying amounts, then deposit the less oxidized material on the CdS layer, and more oxidized material on the previously deposited CdTe material. This alternative technique may result in a superior optimization, but also may complicate the CdTe deposition process, making it difficult to maintain high quality and uniformity. As yet another alternative, the CdTe raw material may be slightly pre-oxidized prior to being deposited on the CdS material, so that all the deposited CdTe crystals throughout the thickness of the layer were, for example, oxidized to 10% of full or complete oxidation. An impurity gettering step may then be used to establish the desired gradient throughout the film thickness, so that the top CdTe crystals were significantly more oxidized, while the crystals adjacent the CdS layer would be oxidized only to substantially the extent which the raw material was oxidized during the preoxidizing step. Those skilled in the art will appreciate that techniques other than spraying may be used to deposit the CdTe film on the n-type layer. One alternative to the spraying technique described above is to deposit a layer of cadmium chloride having a thickness of from about 75 A to 200 A on top of the CdS layer. As noted in the prior art section, subsequent layers of tellurium, cadmium, and tellurium may be deposited on the cadmium chloride layer. The disadvantage of this alternative technique is the difficulty with maintaining the desired true stoichiometry between the Cd and Te, particularly on a local basis. Electrodeposition or screen printing techniques could also be used to deposit the p-type layer. The techniques of the present invention will be applicable to forming p-type photovoltaic layers from various semiconductor materials. When cadmium telluride is the semiconductor material, aluminum, indium, and cadmium ions are all donor species which can be oxidized during the heat treating step so that they no longer act as a donor to the matrix, thereby causing the CdTe semiconductor material to become more p-type. Cadmium telluride is typically an n-type material due to naturally occurring impurities or, in some cases is, "intrinsic" with the number of holes approximating the number of electrons. The desired reduction in the number of donors to render the material desirably p-type will thus depend on the condition of the material prior to the heat treating step. Cadmium telluride has theoretically about 1.5 x 10 22 CdTe molecules/cm 3 , and an excess of about 1 x 10 u holes/cm 3 is generally required to render this material desirably p- type. Hole concentration more optimally is from about 5 x I0 14 /cm 3 to 5 x I0 16 /cm 3 for a desirable p-type CdTe material. A 1 ppm, 100% active donor impurity level corresponds to approximately 1.5 x 10 16 holes/cm 3 . If the hole concentration level is l x lθ 15 / c m 3 , then a reduction of donors by more than 15 times is required during the heat treating step to obtain the desired result. The techniques of this invention can be used with other semiconductor materials to enhance their p-type properties and the desired crystal growth and morphology of the p-type layer. If copper indium diselenide ("CIS") is used as the p-type material, for example, indium ++ ions can be converted to indium +++ ions through oxidation in an air heat treatment, thereby resulting in a conversion from an n-type, insulating, or poorly p-type material to a quality p-type material. In this CIS cell construction, copper species may also be neutralized by oxidation, resulting in a more p-type material compared to the material prior to the heat treating step. If gallium arsenide is the semiconductor material, gallium, and to a larger extent, arsenic, may exist in other oxidation states. Heat treating and annealing according to the present invention should also improve the p-type characteristics of this cell and thus enhance the photovoltaic efficiency of such a cell construction. The techniques of the present invention are thus applicable to various photovoltaic devices based on polycrystalline II-VI, I-III-VI, or III-V P-type semiconductor materials given certain sequences of processing. While a back wall photovoltaic cell construction on a glass substrate has been described in detail above, it should be understood that the concepts of the present invention are applicable to back wall and front wall cells formed on either a vitreous or opaque substrate. The techniques of the present invention may thus be used to form a cell construction comprising a conductive substrate, a first CdTe layer heat treated in an oxidizing environment to oxidize impurities, and a second CdTe layer which is not heat treated or heat treated to oxidize fewer impurities than in the first CdTe layer. The two CdTe layers at this stage may be recrystallized in an inert environment to substantially increase crystal size and, depending on the manner in which the CdTe layers is applied, this recrystallization step may be preceded by a compression step. Subsequent to recrystallization of the CdTe layers, an n-type material such as CdS may then be deposited on the CdTe layer, and finally a conductive transparent layer applied on the n-type material layer. The concepts of the present may be particularly well suited for forming this type of cell construction on an opaque substrate. While the invention has thus been described in terms of specific embodiments which are set forth in detail, it should be understood that this discussion and the drawings which form a part of this disclosure should not be understood as limiting this invention. Various alternative embodiments and operating techniques will become apparent to those skilled in the art in view of this disclosure. The invention should thus be understood to include various embodiments not described herein, and the invention is limited only by reasonable construction of the claims attached hereto in view of this disclosure.

Claims: What is Claimed: 1. A method of manufacturing a photovoltaic panel including a plurality of polycrystalline photovoltaic cells each formed on a common substrate, the method comprising: forming a substantially continuous and optically transmissive conductive layer for electrically interconnecting the plurality of photovoltaic cells while passing sunlight through the conductive layer to a photovoltaic junction; forming a substantially continuous n-type semiconductor polycrystalline layer of a first effective thickness adjacent to the conductive layer; forming a substantially continuous p-type semiconductor polycrystalline layer adjoining the n-type polycrystalline layer; and simultaneously heating the n-type polycrystalline layer and the adjoining p-type polycrystalline layer to a temperature and for a time interval sufficient to significantly increase the size of the crystals within the P~tγpe polycrystalline layer while substantially reducing the effective thickness of the n-type layer to a second effective thickness substantially less than the first effective thickness by diffusion into the p-type layer, such that a majority of sunlight energy having an energy higher than the bandgap of the n-type layer passes through the reduced second effective thickness of the n-type layer to be absorbed by the photovoltaic junction. 2. The method of manufacturing a photovoltaic panel as defined in Claim 1, wherein the step of heating occurs at a temperature and during a time interval sufficient to reduce the effective thickness of the n-type layer such that the second effective thickness is less than 1/1Oth of the first effective thickness. 3. The method of manufacturing a photovoltaic panel as defined in Claim 2, wherein the step of heating occurs at a temperature and during a time interval sufficient to reduce the effective thickness of the n-type layer such that the second thickness is approximately 1/2Oth of the first effective thickness. 4. The method of manufacturing a photovoltaic panel as defined in Claim 1, wherein the n-type layer consists essentially of a CdS material, the p-type layer consists essentially of a CdTe material, and the step of heating occurs at a temperature in excess of 400°C. 5. The method of manufacturing a photovoltaic panel as defined in Claim 1, wherein the conductive layer is formed on a relatively flat surface of a vitreous substrate, the n-type layer is deposited on the conductive layer, and the p-type layer is deposited on the n-type layer. 6. The method of manufacturing a photovoltaic panel as defined in Claim 5, wherein the step of forming the conductive layer comprises: forming a first substantially continuous high conductivity conductive layer on the vitreous substrate having a specific conductivity more than approximately 1000 mho/cm; and subsequently forming a second substantially continuous low conductivity conductive layer adjoining the first conductive layer, the second conductive layer having a specific conductivity in the range of from 1.25 x 10 "3 to 100 mho/cm. 7. The method of manufacturing a photovoltaic panel as defined in Claim 6, further comprising: adjusting the electron density of the second conductive layer to a level which results in a magnitude within three orders of a presumed electron density value of the p-type layer, such that an energy-producing junction is formed by the p-type layer and the second conductive layer in an area of any flaws in the n-type layer. 8. The method of manufacturing a photovoltaic panel as defined in Claim 6, wherein: each of the first and second conductive layers are selected from a group consisting of tin oxide or zinc oxide. 9. The method of manufacturing a photovoltaic panel as defined in Claim 1, wherein the n-type layer comprises CdS, and the step of heating occurs at a temperature and during a time interval sufficient to reduce the second effective thickness of the n-type material layer to a level such that a majority of the sunlight energy having a wavelength less than 520 nm passes through the n-type material layer to be absorbed by the photovoltaic junction. 10. The method of manufacturing a polycrystalline photovoltaic panel as defined in Claim l, wherein the step of heating occurs at a temperature and during a time interval sufficient to reduce the second effective thickness of the n-type material layer to less than approximately 500 A. 11. The method of Claim 10, wherein said first effective thickness is from about 2,000 to 12,000 angstroms. 12. The method of Claim 6, wherein said second substantially continuous high conductivity conductive layer consists essentially of tin oxide and includes a dopant selected from the group consisting essentially of calcium, zinc, lead, mercury, selenium, sulfur, sodium, cesium, mercury, boron, and chromium and said first substantially continuous low conductivity conductive layer consists essentially of tin oxide and includes a dopant selected from the group consisting essentially of fluorine. 13. The method of Claim 6, wherein substantially all of the dopants in said first substantially continuous high conductivity conductive layer differ from substantially all of the dopants in said second substantially continuous high conductivity conductive layer. 14. The method of Claim 1, wherein at least 50% of sunlight energy having an energy higher than the bandgap of the n-type layer passes through the reduced second effective thickness of the n-type layer. 15. A method of manufacturing a polycrystalline photovoltaic cell comprising: forming a substantially continuous and optically transmissive first conductive layer having a specific conductivity more than approximately 1000 mho/cm for electrically transmitting current through the photovoltaic cell; forming a substantially continuous and optically transmissive second conductive layer having a specific conductivity in the range from 1.25 x 10 3 to 100 mho/cm, and having an electron density within three orders of magnitude of a presumed electron density value of a p-type material of the photovoltaic cell; forming a substantially continuous n-type semiconductor polycrystalline layer adjoining the second conductive layer, the n-type layer having an effective thickness such that at least a substantial amount of sunlight energy having an energy higher than the bandgap of the n-type layer passes through the n-type layer; and forming a substantially continuous p-type semi- conductive polycrystalline layer adjoining the n-type layer and forming a photovoltaic junction with the n-type layer and also with the second conductive layer in areas of any flaws in the n-type layer to convert sunlight to electrical energy. 16. The method of manufacturing a polycrystalline photovoltaic cell as defined in Claim 15, further comprising: simultaneously heating the n-type polycrystalline layer and the adjoining p-type polycrystalline layer to a temperature sufficient to significantly increase the size of the crystals within the p-type layer and to reduce the effective thickness of the n-type layer by diffusion into the p-type layer and between the p-type layer crystals. 17. The method of forming a polycrystalline photovoltaic cell as defined in Claim 16, wherein the n- type layer consists essentially of a CdS material, the p- type layer consists essentially of a CdTe material, and the step of heating occurs at a temperature and during a time interval sufficient to reduce the effective thickness of the n-type layer to less than 1/lOth of its effective thickness prior to the step of heating. 18. The method of manufacturing a polycrystalline photovoltaic cell as defined in Claim 16, wherein the n- type layer consists essentially of a CdS material, and the step of heating occurs at a temperature and during a time interval sufficient to reduce the effective thickness of the n-type material layer to a level such that at least 50% of sunlight energy having a wavelength less than 520 nm passes through the n-type material layer and to the photovoltaic junction. 19. The method of manufacturing a polycrystalline photovoltaic cell as defined in Claim 15, wherein the step of forming the second conductive layer further comprises: selectively doping a material prior to deposition for forming the second conductive layer, such that the selected amount of dopant achieves the desired electron carrier density of the second conductive layer. 20. A photovoltaic panel including a plurality of polycrystalline photovoltaic cells formed on a common vitreous substrate, comprising: a substantially continuous and optically transmissive conductive layer adjoining the vitreous substrate for electrically interconnecting the plurality of photovoltaic cells; a substantially continuous n-type polycrystalline layer adjoining the conductive layer and having an effective thickness such that a majority of sunlight having an energy higher than the bandgap of the n-type layer passes through the conductive layer and the n-type layer to be absorbed by a photovoltaic junction; and a substantially continuous p-type polycrystalline layer adjoining the n-type layer and consisting essentially of relatively large p-type crystals and n-type material diffused into the p-type layer. 21. A photovoltaic panel as defined in Claim 20, wherein: the n-type layer consists essentially of CdS; and the p-type layer consists essentially of CdTe. 22. A photovoltaic panel as defined in Claim 21, wherein: the effective thickness of the CdS layer is less than approximately 500 A. 23. A photovoltaic panel as defined in Claim 20, wherein: the conductive layer includes a first substantially continuous highly conductive layer having a specific conductivity more than approximately 1000 mho/cm, and a second conductive layer adjoining both the first conductor layer and the n-type layer and have a specific conductivity in the range of from 1.25 x 10 43 to 100 mho/cm. 24. A photovoltaic panel as defined in Claim 23, wherein: the first and second conductive layers are each tin oxide layers; and the electron density of a second conductive layer is within three orders of magnitude of a presumed electron density value of the p-type layer, such that an energy- producing junction is formed in areas of flaws in the n- type layer by the p-type layer and the second conductive layer. 25. A photovoltaic panel including a plurality of polycrystalline photovoltaic cells, comprising: a substantially continuous and optically transmissive first conductive layer having a specific conductivity more than 1000 mho/cm for electrically interconnecting the plurality of photovoltaic cells; a substantially continuous and optically transmissive second conductive layer adjoining the first conductive layer, the second conductive layer having a specific conductivity in the range of from 1.25 x 10 3 to 100 mho/cm and having an electron carrier density within three orders of magnitude of a presumed electron carrier density of a p- type material of the photovoltaic cell; a substantially continuous n-type polycrystalline layer adjoining the second conductive layer and having an effective thickness such that at least a substantial amount of sunlight having a wavelength with an energy higher than the bandgap of the n-type layer passes through the first and second conductive layers and the n-type layer to be absorbed by a photovoltaic junction; and a substantially continuous p-type polycrystalline layer adjoining the n-type layer and forming a photovoltaic junction with both the n-type layer and with the second conductive layer where flaws occur in the n-type layer to convert sunlight to electrical energy. 26. A photovoltaic panel as defined in Claim 25, further comprising: th.e p—type layer including relatively large p— ype crystals and n-type material diffused between the p-type crystals. 27. A photovoltaic panel as defined in Claim 25, wherein: the effective thickness of the n-type layer is less than approximately 500 A. 28. A photovoltaic panel as defined in Claim 25, wherein: the n-type layer consists essentially of CdS; the p-type layer consists essentially of CdTe; and the first and second conductive layers are each tin oxide layers. 29. A photovoltaic panel as defined in Claim 25, wherein the second conductive layer includes an amount of dopant selected as a function of the presumed electron carrier density of the p-type material. 30. A method of forming a photovoltaic device including a plurality of photovoltaic cells each on a common substrate for electrical interconnection to obtain a photovoltaic panel, the method comprising: forming an electrically conductive film layer on the common substrate; depositing at least one substantially continuous n- type material film layer on the electrically conductive film layer for forming the n-type material layer for each of the photovoltaic cells; selecting a p-type material for forming a p-type film layer for each of the photovoltaic cells; heat treating the p-type material in an oxidizing environment to oxidize impurities in the selected p-type material and thereby substantially enhance the desired electrical properties of the selected p-type material; and thereafter recrystallizing the heat treated p-type material in an environment substantially free of oxidants to substantially increase crystal size within the p-type film layer compared to the heat treated p-type material and thereby substantially enhancing conversion efficiency of the photovoltaic cells. 31. The method of Claim 30, wherein said heat treating step occurs subsequent to deposition of the selected p-type material on the common substrate. 32. The method of Claim 31, further comprising: establishing an oxidation gradient through the deposited p-type film layer during said heat treating step, such that p-type crystals adjacent a surface of the p-type layer adjoining the n-type layer are less oxidized than p- type crystals adjacent an opposing surface of the p-type layer. 33. The method of Claim 32, wherein the step of establishing an oxidation gradient through the deposited p- type film layer comprises: forming an oxidized coating having a thickness on p- type crystals adjoining the surface the n-type layer less than the oxidized coating on remaining crystals spaced throughout the p-type layer, such that after said recrystallizmg step the portion of said p-type layer adjacent said n-type layer has crystals of a larger mean crystal size than crystals in other portions of said p-type layer. 34. The method of Claim 32, wherein the p-type layer is arranged open faced within the oxidizing environment to uniformly oxidize the p-type layer across an area of the substrate supporting the plurality of photovoltaic cells. 35. The method of Claim 30, wherein at least a portion of the step of heat treating the p-type material is performed prior to deposition of the selected p-type material on the n-type layer. 36. The method of Claim 30, wherein at least a portion of the step of heat treating the p-type material is performed during deposition of the selected p-type material on the n-type layer. 37. The method of Claim 30, wherein the step of heat treating the p-type material comprises: initially heat treating the p-type material in an oxidizing environment to oxidize some of the impurities in the selected p-type material; thereafter depositing the initially heat treated p- type material in a film layer on the n-type layer; subsequently heat treating the deposited p-type material in an oxidizing environment to oxidize additional impurities in the deposited p-type material and thereby substantially enhance the desired electrical properties of the p-type material. 38. The method of Claim 30, further comprising: adding a flux to the selected n-type material to enhance crystal growth of the p-type material; placing a pair of p-type film layers in face-to-face relationship during the recrystallization step. 39. The method of Claim 30, further comprising: controlling the oxygen content of said environment in said recrystallizmg step to less than 0.5% oxygen by weight. 40. The method of Claim 30, further comprising: re-heat treating the annealed p-type film layer in an oxidizing environment to oxidize additional impurities in the p-type material layer. 41. The method of Claim 40, wherein said heat treating step has a duration of about 5 to 60 minutes and a temperature of about 150°C to 340°C. 42. The method of Claim 30, wherein: the common substrate is substantially transparent for passing sun light; and the electrically conductive film layer is substantially transparent for transmitting sun light, through the conductive film layer and to a photovoltaic junction formed by the n-type layer and the p-type layer. 43. The method of Claim 30, further comprising: the step of recrystallizmg the heat treated p-type material occurs prior to the step of depositing the n-type material layer; and another electrically conductive film layer is formed on the n-type material layer and is a substantially transparent film layer. 44. The method of Claim 30, wherein the environment in said heat treating step has at least 1.0% by volume oxidants. 45. The method of Claim 30, wherein the environment in said recrystallizmg step has no more than .05% by volume oxidants. 46. The method of Claim 30, wherein the oxidants are selected from the group consisting of halogens, oxygen, oxides, sulphate, phosphates and peroxides. 47. The method of Claim 33, wherein the rate of crystal growth in said recrystallizmg step is related to the thickness of said oxidized coating. 48. The method of Claim 32, wherein the size and size distribution of said p-type crystals after said recrystallizing step is a function of the magnitude and slope of said oxidation gradient, the temperature and temperature rise rate in said recrystallization step, the amount and type of flux, porosity of the p-type layer, and substrate orientation and placement in said recrystallizmg step. 49. The method of Claim 30, wherein said impurities include aluminum, indium, copper, gallium, arsenic, and cadmium ions. 50. The method of Claim 30, wherein the duration of said heat treating step is a function of the temperature of said p-type material in said heat treating step and the concentration of oxidants in said oxidizing atmosphere. 51. The method of Claim 30, wherein the duration of said heat treating step is about 5 seconds to about 30 minutes and the temperature in said heat treating step is about 380°C to about 500°C. 52. The method of Claim 30, wherein the duration of said recrystallizmg step is about 10 minutes to about 60 minutes and the temperature in said recrystallizmg step is about 400°C to about 700°C. 53. The method of Claim 32, wherein the magnitude and slope of said oxidation gradient is a function of the concentration and type of oxidants in said heating step, the temperature in said heating step, the duration of said heating step, and the relative concentrations of the components of said p-type crystals. 54. The method of Claim 32, wherein said oxidation gradient is formed by heating said p-type material in a substantially inert atmosphere to a selected temperature and contacting said p-type material with an oxidant for a time and at said selected temperature to produce said oxidation gradient. 55. The method of Claim 30, wherein reflective losses at the interface between said n-type and p-type films are less than 10%. 56. The method of Claim 53, wherein said, p-type material is CdTe and said magnitude and slope of said oxidation gradient is a function of the ratio of Cd to Te in said p-type material. 57. The method of Claim 30, wherein the mean crystal size of said p-type material after said recrystallizmg step is 2 to 1,000 times the mean crystal size of said p- type material before said heat treating step. 58. A method of forming a polycrystalline p-type CdTe film layer of a photovoltaic cell including an n-type material layer, the method comprising: depositing one or more CdTe materials in a film layer on the n-type material layer; thereafter heat treating the deposited CdTe film layer in an oxidizing environment to oxidize impurities in the CdTe material and thereby substantially enhance the desired electrical properties of the CdTe material; establishing an oxidation gradient through the deposited CdTe film layer during the heat treating step, such that CdTe crystals adjacent a surface of the CdTe layer adjoining the n-type layer are less oxidized than CdTe crystals adjacent an opposing surface of the CdTe layer; and thereafter recrystallizmg the heat treated CdTe material in a substantially environment to substantially increase crystal size within the CdTe film layer compared to the crystals within heat treated CdTe layer and thereby substantially enhance photovoltaic conversion efficiency of the photovoltaic cell. 59. The method of Claim 58, wherein the step of establishing an oxidation gradient through the CdTe film layer comprises: forming an oxide coating on CdTe crystals adjoining the surface of the n-type layer less than the oxide coating on remaining CdTe crystals spaced throughout the CdTe layer, such that the oxide coating acts to retard a recrystallization flux and initiate crystal growth during the step of recrystallizmg the heat treated CdTe material substantially adjacent the interface of the n-type layer and the CdTe layer. 60. The method of Claim 58, further comprising: initially heat treating CdTe material in an oxidizing environment to oxidize some of the impurities in the CdTe material prior to depositing the CdTe in a film layer. 61. The method of Claim 58, further comprising: depositing CdTe material under substantially ambient temperature conditions on the n-type layer; mechanically compressing at least a substantial portion of the deposited p-type material subsequent to the heat treating step and prior to the annealing step; and placing a pair of cells in face-to-face relationship during the annealing step. 62. The method of Claim 61, further comprising: selecting border areas of photovoltaic cells as mechanically uncompressed areas; and positioning the selected border areas of one photovoltaic cell in physical engagement with selected border areas of another photovoltaic cell during. the recrystallizmg step, such that the border areas substantially limit gas exchange between a cavity between the face-to-face CdTe layers and an environment exterior of the face-to-face of photovoltaic cells while physically separating the mechanically compressed p-type material. 63. A method of controlling the morphology of a polycrystalline p-type film layer of a photovoltaic cell including an n-type material layer, the method comprising: establishing an oxidation gradient through the p-type film layer, such that crystals within the polycrystalline layer adjacent a surface of the p-type film layer adjacent the n-type layer have a thinner oxide coating than crystals within the polycrystalline layer adjacent an opposing surface of the p-type film layer; and thereafter recrystallizmg the p-type film layer in an environment containing a recrystallization flux, such that the oxide coating acts to retard the recrystallization flux and initiate crystal growth within the p-type film layer adjacent the n-type layer to form a substantially planar interface between the p-type film layer and the n-type film layer. 64. The method of Claim 63, wherein the step of establishing an oxidation gradient through the p-type film layer further comprises: heat treating the material forming p-type film layer in an oxidizing environment to oxidize impurities and to enhance the desired electrical properties of the p-type film layer. 65. The method of Claim 64, further comprising: heat treating the p-type material in an oxidizing environment to oxidize some of the impurities in the p-type material; and thereafter depositing the heat treated p-type material in a film layer. 66. The method of Claim 63, further comprising: adding a flux to the n-type material layer for forming the environment containing the recrystallization flux; and placing a pair of p-type film layers in a face-to-face relationship during the recrystallizmg step. 67. A plurality of photovoltaic cells each formed on a common substrate, each of the plurality of cells comprising: an n-type polycrystalline layer; a p-type polycrystalline layer adjoining the n-type polycrystalline layer and forming a photovoltaic junction therewith, the p-type polycrystalline layer including at least a planar layer portion consisting essentially of relatively large crystals adjoining the n-type polycrystalline layer; the planar layer portion including oxidized impurities to obtain p-type electrical properties for the planar layer portion; and each of the relatively large crystals having a planar surface in substantially planar engagement with an interface surface of the n-type polycrystalline layer. 68. A photovoltaic cell of Claim 67, wherein the p- type polycrystalline layer further includes a porous layer portion opposing the n-type layer with respect to the planar layer portion, the porous layer portion consisting essentially of crystals having a mean diameter smaller than the relatively large crystals within the planar layer portion, and the porous layer portion having a porosity greater than the porosity of the planar layer portion. 69. The photovoltaic cell of Claim 68, wherein the planar layer portion is formed by an oxidation gradient through the thickness of the p-type layer which retards a recrystallization flux during annealing to initiate nucleation during recrystallization within the planar layer portion of the p-type layer. 70. The photovoltaic cell of Claim 67, wherein the p- type layer consists essentially of cadmium telluride crystals, and the relatively large crystals within the planar layer portion have a mean diameter of from about 1.5 to 3.0 microns. 71. The photovoltaic cell of Claim 67, wherein the porosity of the planar layer portion is less than about 30%. 72. The photovoltaic cell of Claim 68, wherein said porous layer comprises cadmium telluride crystals having a mean diameter of from about 0.5 to 2 microns. 73. The photovoltaic cell of Claim 68, wherein the porosity of said porous portion is from about 20% to about 45%. 74. The photovoltaic cell of claim 67, wherein said p-type polycrystalline layer includes at least a first portion adjacent said planar layer portion having dissimilar p-type characteristics to said planar layer portion. 75. The photovoltaic cell of Claim 74, wherein said first portion has a higher conductivity than said planar layer portion. 76. The photovoltaic cell of Claim 74, wherein said first portion has more carriers than said planar portion.

Inventors: ALBRIGHT SCOTT P, CHAMBERLIN RHODES, JORDAN JOHN F, ALBRIGHT, SCOTT, P., CHAMBERLIN, RHODES, JORDAN, JOHN, F.
Applicants: PHOTON ENERGY INC, PHOTON ENERGY, INC.
Application year: 1993
Main classification: H01L31
IPC codes: H01L27, H01L31