Ohmic copper-cuprous oxide contacts
BACKGROUND OF THE INVENTION 
     This invention relates to the production of novel ohmic contacts involving copper metal and thin films of semiconducting cuprous oxide. 
     Photovoltaic generation of electric current has become a subject of great interest as increased attention has been given to means for the utilization of solar energy, specifically the generation of electricity from sunlight. Photovoltaic effects depend upon the development of semi-conductor PN junctions, which in turn depend upon the creation and stabilizing of electron holes in the selected lattice structure. In a commonly employed system, silicon crystals are doped with boron and with phosphorus atoms to produce, respectively, P-type material and N-type material. P-type material, due to an electron deficiency, attracts electrons and thus appears to act like a positive material. Conversely, N-type material, having a supply of free electrons, acts like a negative material. When P- and N-type materials are fused together, free electrons from the N-type material flow into the holes in the P-type material. The resulting PN junction provides the fundamental unit for the modern solar cell. Photovoltaic processes are not limited to single-crystal or polycrystalline silicon. Other common systems employ germanium, copper-cadmium sulfide, or gallium arsenide. Less common, but effective, systems employ cuprous oxide in contact with metallic copper. The cuprous oxide-copper system is an attractive one because of the lower materials cost but its use in solar cells has been limited because of poor efficiency. 
     In U.S. Pat. No. 3,398,028, Scott reported the formation of a cuprous oxide coating on copper by heating a copper base alloy to a temperature in the range 600°-1050° C. in an inert atmosphere and then cooling in an atmosphere containing a very limited proportion (about 0.0001%) of oxygen. Barnett, et al., U.S. Pat. No. 4,239,553 relates to solar cells comprising a copper sulfide absorber layer protected by a thin copper oxide layer, the entire system being encapsulated in glass for increased durability and maintenance of efficiency. In another solar cell, Rothwarf U.S. Pat. No. 4,267,398 provides a layer of copper oxide adjacent to the absorber material. 
     Photovoltaic properties of single-crystal and polycrystalline cuprous oxide-copper contacts were studied extensively by Assimos et al., J. Appl. Phys., vol. 44, pp. 1,687-1,693 (1973). Single-crystal cuprous oxide was reduced to metallic copper employing atomic hydrogen at room temperature. Polycrystalline contacts were prepared by partial oxidation of copper plate in air at 1000° C. The copper plate was 2 mm. thick and the resulting oxide layer was 150 microns in thickness. Herion, Appl. Phys. Lett., vol. 34, pp. 599-601 (1979) oxidized a copper sheet (0.5 mm. thick) in air at 1050° C. for 1 minute to obtain a cuprous oxide layer (20 microns thick) having very good adherence. Fortin et al., Can. J. Phys., vol. 60, pp. 901-908 (1982), prepared a 1-micron layer of cuprous oxide on a copper film (1 mm. thick) by employing a high oxygen pressure (50 Torr.) at 960° C. for less than  1 second exposure time. 
     The process of this invention, employing very thin films of copper foil, surprisingly has produced not only unusually large photovoltaic effects but also ohmic copper-cuprous oxide contacts, under process conditions not suggested by the prior art. 
     SUMMARY OF THE INVENTION 
     The process of this invention relates to the production of ohmic contacts integral to a film of cuprous oxide upon metallic copper by heating a very thin metallic copper foil in an oxygen-containing atmosphere at an elevated temperature. Thereafter the oxidized foil is screen printed with a hydrocarbon-containing paste, heat treated, and quenched to ambient temperature to provide increases in photovoltaic effects as well as novel ohmic contacts observed at the cuprous oxide-paste junction. These results are achieved when employing copper foil having a thickness no greater than about 0.02 inch, desirably about 0.002 inch, to provide a cuprous oxide film preferably having a thickness of about 20 microns. Isolated portions of the cuprous oxide film surface are then painted with a reducing component prior to a critical heat-treating step. The critical heat treatment is effected in a substantially oxygen-free atmosphere, usually for at least about 15 minutes at a temperature ranging from about 300° to about 700° C., preferably at about 500° C. Subsequent quenching of the oxidized foil is conveniently effected with water. This treatment leaves the cuprous oxide film in surprisingly good optical condition such that no etching, polishing or grinding is required to optimize the optical properties of the film. Accordingly, electrical and optical contacts may be made directly. The reduced isolated portions of the oxide film layer provide discrete, very thin metallic copper plates which exhibit the desired ohmic properties. 
     The process of this invention further relates to the continuous production of a succession of ohmic contacts embodied in a film of cuprous oxide on a thin metallic copper backing strip under conditions substantially as set forth above. 
     Such ohmic contacts are particularly useful in various configurations devised for solar cells and related energy conservation devices. 
     DESCRIPTION OF THE INVENTION 
     This invention relates to a novel and improved process for the production of ohmic contacts between a film of cuprous oxide and copper metal. Although the formation of such films on various copper surfaces is old, the process of this invention provides an increase of one to two magnitudes in the observed photovoltaic effects. In this novel process, selected portions of a thin film of cuprous oxide (on thin copper foil) are subjected to selected reducing conditions and finally heat treated and quenched to achieve this novel ohmic contact. 
     Photovoltaic films of cuprous oxide on copper are attractive because of the abundance and low cost of the required materials. The cuprous oxide film adheres well to copper which provides a stable support for the fragile film. 
     Prior art studies of cuprous oxide-copper systems have concluded that defect concentration is inherently at a low level and that heat treatment would have no effect. Contrary to this teaching, it is believed that the conductivity results achieved by this novel process are due to induced variations in the concentration of vacancies in the copper atoms of the cuprous oxide crystal system, an intrinsic P-type conductor, and to the relief of stress in the cuprous oxide film by leaving a minimum amount of copper metal (the cause of the stress) after oxidation. 
     It has been found that in the process of this invention, conductivity levels increase regularly with heat treatment at increasing temperatures in the range from 300° to 500° C. As the heat treating temperature is increased further, the conductivity level decreases although the improved performance of this invention is still observed at 700° C., suggesting the thermally activated formation of free copper defects from clusters of defects and, at the higher temperatures, the destruction of such defects at convenient cuprous oxide surfaces to provide progressively fewer effective defect sites. 
     The novel process of this invention has been found to be effective only with relatively thin copper sheet or foil, having a thickness no greater than about 0.02 inch, preferably from about 0.001 to about 0.005 inch, and most preferably about 0.002 inch. Progressively poorer results have been observed with greater thicknesses of copper foil such that the cuprous oxide film (ca. 20 microns) cracked and peeled when developed on a foil that was 0.02 inch thick, thus showing the stress induced in the cuprous oxide film and the preference for thinner foils. Prior art studies have been limited to the use of copper plates whose thickness was at least 0.04 inch. 
     The metallic copper surface may be cleaned in any conventional manner and subjected to oxidation at substantially atmospheric pressure in the presence of an oxygen-containing gas, generally air, or from about 0.1 to about 10.0 vol. % oxygen in argon, preferably about 1.0 vol. % oxygen in argon, and at a temperature within the range from about 900° to about 1100° C., preferably about 1000° C. The oxidation time under the aforesaid conditions should be chosen to provide a cuprous oxide film, on the surface of the metallic copper, whose thickness is desirably within the range from about 10 to about 25 microns, preferably about 20 microns. The oxidation time is generally found to be within the range from about 1 to about 5 minutes, preferably about 2 minutes. 
     Following the oxidation step of the process of the invention, the cuprous oxide coated metallic copper foil is cooled, at least to a temperature in the subsequent heat-treating temperature range, and, when desired, all the way to ambient temperature. After cooling to the desired degree, selected portions of the cuprous oxide film surface are treated with a reducing component, as by painting, doping, and the like, to provide, for example, a pattern, or multiplicity, of dots arrayed upon the oxide film. The reducing component must be capable of reducing cuprous oxide to metallic copper. It has been observed that various hydrocarbons, aldehydes, alcohols, ketones, ketals, and acetals are effective for this reduction. Other hydrocarbon-based organic compounds should be similarly effective. These reducing materials may additionally include colloidal carbon which serves to keep the hydrocarbon vapors in place during the subsequent heat treatment. 
     The novel heat treatment of this invention may be effectively conducted at any temperature within the range from about 300° to about 700° C. best conductivity results have been achieved at or near the preferred heat treating temperature of about 500° C. It is essential that the heat treatment be conducted in the absence of oxygen. The required oxygen-free gas atmosphere is conveniently provided by an inert gas such as argon, nitrogen, or neon, or mixtures thereof, although other inert gases may be employed. A preferred gaseous medium is argon. The heat treatment at the selected temperature should be applied for an effective period of time which has usually been found to be about 15 minutes, although a treating time within the range from about 10 to about 60 minutes has been observed to provide the surprisingly enhanced conductivity described herein. 
     The heat treatment serves to provide an annealing effect, to rearrange the defect structure, and to provide a patterned array or grid of very thin metallic copper areas integral in the cuprous oxide surface. These provide convenient contacts although other metals provide effective contacts with the cuprous oxide surface. 
     The quenching step is intended to reduce the temperature of the cuprous oxide-copper system to substantially ambient temperature. This may preferably be effected with water, as, for example, a spray or mist, although any suitable quenching agent may be employed. 
     The resulting cooled film may be stored in any conventional manner for use as desired. The optical condition of the film is unusually good, so that there is no need to etch, polish or grind the surface to optimize the optical properties. In consequence of this surprising result, electrical and optical contacts can be made to the film without further preparation. One attractive use of the films made by the process of this invention is in a battery system powered by solar energy. 
     In another attractive aspect of this invention, the process described above is adapted to the continuous production of a photovoltaic film of cuprous oxide on copper on an elongated, rolled strip of copper foil. The foil strip is led through successive zones for oxidation, heat treatment, reduction treatment, and quenching to occur under conditions of time, temperature, and medium, as previously described. After drying, the product strip is taken up on a storage roll for subsequent use. 
    
    
     The following examples are illustrative, without limitation, of the process of this invention. 
     EXAMPLE I 
     A square of copper foil, 1/2 in. by 1/2 in., having a thickness of 0.002 in., was placed on the end of a thermocouple tube, serving as a sample holder, and then introduced into a quartz sample tube. While still at room temperature the sample tube was continuously flushed with pure titanium-gettered argon gas to remove all oxygen and thus prevent seeding of cuprous oxide prior to the oxidation step. The sample tube was then inserted into a quartz furnace tube, within a furnace maintained at 900° C., by means of a programmed stepper motor. 
     Once the copper foil had gradually reached a temperature of 900° C., gas flow in the sample tube was switched to argon containing 1 vol. % oxygen for 2 minutes, to effect formation of cuprous oxide. Thereafter the sample tube was flushed with argon gas, containing up to about 1 p.p.m. oxygen (with no apparent effect on the photovoltaic properties of the prepared optical surface film of cuprous oxide), while withdrawing the sample tube from the furnace by means of the programmed stepper motor. The sample was then permitted to cool to room temperature. No cupric oxide was observed in the cuprous oxide layer. 
     The cooled, oxidized copper foil square was treated to provide an array of nine carbon paste dots (in a 3×3 arrangement) screen-printed on the cuprous oxide surface. The carbon paste comprised a colloidal suspension of carbon in a hydrocarbon fluid. 
     Heat treatment was then effected by suspending the oxidized square of copper foil, now having a dotted cuprous oxide film thereon, within a vertical furnace that had been preheated to 500° C. The square, while continuously flushed with substantially oxygen-free argon gas, was maintained at 500° C. for 15 minutes. The square was then quenched by dropping directly into a container of water at room temperature. 
     The residual copper dots, either clean or covered with dried carbon paste, gave good ohmic contact. This is in remarkable contrast to the Schottky barrier typically existing at the junction of the copper foil base and the cuprous oxide film. 
     EXAMPLE II 
     The procedure of Example I was repeated with a square of copper foil having a thickness of 0.02 in. Upon quenching the carbon dot fell off, leaving a copper dot integral with the cuprous oxide film which could not be wiped off. The copper dots gave good ohmic contact.