Method for Production of Copper Indium Diselenide

1. Field of Technology

This invention relates to a method for producing copper indium diselenide (CuInSe.sub.2) which can find application in solar cells and other photovoltaic devices such as photosensors. The object of the invention is to provide an improved production process for CuInSe.sub.2 which can be easily carried into practice on a high production basis.

2. Description of the Prior Art

Single-crystal CuInSe.sub.2 has heretofore been produced by weighing Cu, In and Se in stoichiometric proportions into a quartz tube, evacuating the quartz tube to about 10.sup.31 6 mmHg and, after sealing, heating the tube in an electric furnace at a temperature in the range of 1,050 to 1,100.degree. C. to cause a reaction to proceed in the melt phase (for example, P. Migliorato et al., J. Appl. phys., 46 (1975), 1777-1782.) As the contents of the quartz tube are then cooled gradually, CuInSe.sub.2 crystals separate out. CuInSe.sub.2 films have been manufactured by vacuum deposition technology such as vacuum vapor deposition, magnetron sputtering or the like using elementary Cu, In and Se (for example, SERI Photovoltaic Advanced Research & Development, An Overview, SERI/SP-281-2236).

However, the CuInSe.sub.2 produced by the former method, involving a reaction in melt phase, is expensive and, moreover, the manufacturing process itself has the disadvantage that it does not lend itself well to mass production. The latter method, i.e. vacuum deposition technology, has the disadvantage that it does not yield a sufficiently thick film or a film having a sufficiently large area.

Having been accomplished to solve the above-mentioned problems, this invention provides a mass production process by which copper indium diselenide can be manufactured in large quantities and at low cost. According to this invention, respective powders of Cu, In and Se are ground together in a liquid cooling medium to give a finely ground powder including CuInSe.sub.2 of low crystallinity and, then, sintering the same at a temperature below the melting point of polycrystalline CuInSe.sub.2 so as to synthesize CuInSe.sub.2.

In accordance with this invention, the energy of said grinding in a liquid cooling medium not only induces a low-temperature synthesis of CuInSe.sub.2, though of predominantly low crystallinity, but also comminutes the respective component powders to render them so reactive that they can be sintered together at a temperature below the melting point of polycrystalline CuInSe.sub.2. Furthermore, by adding a binding agent to the ground powder mixture thus obtained to give a paste and printing a substrate with the paste, a sintered film of virtually any optional configuration can be manufactured. In addition, by using the screen printing technique, for instance, a CuInSe.sub.2 film having an increased area can by easily obtained. Thus, this invention provides copper indium diselenide at low cost on a mass production scale.

FIG. 1 is a sectional elevation showing a solar cell fabricated using a polycrystalline CuInSe.sub.2 manufactured by the method of this invention;

FIG. 2 is a schematic sectional view showing an agitated ball-mill used in the manufacturing method of this invention;

FIG. 3 shows x-ray diffraction patterns of powders ground for (a) 2 hours, (b) 6 hours and (c) 14 hours, respectively;

FIG. 4 shows the grinding time-dependence of CuInSe.sub.2 synthesis and median value of grain size in Example 1 of this invention; and

FIG. 5 is an x-ray diffraction pattern of the sintered film fabricated in Example 1.

Preferred embodiments of this invention are described in detail below, with particular reference to CdS/CuInSe.sub.2 solar cells fabricated using the copper indium diselenide produced by the method of the invention. In the description, reference is sometimes made to the accompanying drawings for a better understanding of this invention.

To a CdS powder was added 10 weight percent, based on the weight of said powder, of CdCl.sub.2 as a flux and then an appropriate amount of propylene glycol was added as a binding agent. The mixture was kneaded well to give a paste. Using this paste, a glass substrate 1 was printed as illustrated in FIG. 1 and fired at 690.degree. C. in a nitrogen gas atmosphere for 60 minutes, whereby an n-type sintered CdS film 2 was produced. On top of this sintered CdS film 2 was formed a p-type sintered CuInSe.sub.2 film 3 by the process described hereinafter. Then, a Mo film 4, intended to serve as an electrode, was deposited on top of said p-type sintered CuInSe.sub.2 film 3, while an ohmic electrode 5 was formed on top of the sintered CdS film 2, both by the per se conventional procedure. A lead wire 6 was connected to each of the electrodes. Indicated at 7 are incident light rays.

The sintered CuInSe.sub.2 film was formed by the following technique.

First, 360 g of Cu powder, 680 g of In powder and 950 g of Se powder, each having a purity not less than 99.99% and having been previously comminuted to a grain size not exceeding 500 .mu.m were provided. To a mixture of these powders was added 3 kg of water or ethylene glycol monophenyl ether and the resulting composition was ground in an agitated ball-mill as illustrated in FIG. 2. Referring to FIG. 2, balls 10 are made of alumina and each measures 5 mm in diameter. The total weight of these balls is 12 kg. The wall of a chamber 8, as well as stirring rods 9, are made of stainless steel. The chamber 8 is provided with a coolant jacket means 11 for a coolant which serves to control the temperature buildup during the grinding operation. It has been found that even when the balls, chamber and stirring rods were made of any zirconia, alumina and silicon carbide, instead of the above-mentioned materials, there occurred no reaction with the powder materials, nor was found any other trouble. The assembly of stirring rods 9 is driven by an external drive source in the counterclockwise direction indicated by arrowmark 12 in FIG. 2. FIG. 3 shows the x-ray diffraction patterns of powders ground for (a) 1 hour, (b) 6 hours and (c) 14 hours. It is clear that with the progress of grinding time, the peaks of Cu and In elements were diminished, while the peak of CuInSe.sub.2 increased. Taking the diffraction intensity ratio of the (112) plane of CuInSe.sub.2 to the (111) plane of Cu as the degree of CuInSe.sub.2 synthesis, the dependency of CuInSe.sub.2 synthesis on grinding time was investigated. The results are shown in FIG. 4. It is apparent that the degree of CuInSe.sub.2 synthesis increases substantially in proportion with grinding time. On the other hand, the relationship between the median value of grain size of ground powder and the grinding time is indicated by a solid line in FIG. 4.

It is seen that whereas the comminution of powder progresses rapidly during an initial phase of 6 hours, it then tends to reach a saturation point of about 1.5 .mu.m. Based on the above results, a ground powder having degree of CuInSe.sub.2 synthesis in the neighborhood of 0.8 and a median grain size of 2.0 .mu.m was prepared by carrying out the grinding for 3.5 hours.

To the resulting ground powder including CuInSe.sub.2 of low crystallinity was added a suitable amount of propylene glycol as a binding agent and the resulting paste was printed on the aforementioned n-type sintered CdS film 2 and dried at 120.degree. C. for 60 minutes. The dried film was then sintered in a N.sub.2 atmosphere at 650.degree. C. for 60 minutes to give a sintered CuInSe.sub.2 film 3. The x-ray diffraction pattern of the resulting sintered CuInSe.sub.2 film is shown in FIG. 5. Table 1 shows these x-ray diffraction data in comparison with the JCPDS (Joint Committee on Powder Diffraction Standards) card. The presence of diffraction peaks corresponding to the (101), (103), (211), (213), (105) and (301) planes indicates that this CuInSe.sub.2 film is of the chalcopyrrhotite structure. Furthermore, based on the agreement in values of d, the sintered CuInSe.sub.2 film was found to contain In.sub.2 O.sub.3. This impurity was probably attributable to the grinding in water and it can be seen from Table 1 that the formation of In.sub.2 O.sub.3 is significantly inhibited by performing the grinding operation in ethylene glycol monophenyl ether.

More or less similar results were obtained when perfluorobutyltetrahydrofuran or propylene glycol was used in lieu of ethylene glycol monophenyl ether.

Then, using a DC magnetron sputter, a Mo film 4 was formed on the sintered CuInSe.sub.2 film as an electrode on the CuInSe.sub.2 film 3. Thereafter, an ohmic electrode 5 was provided on top of the sintered CdS film 2 and a lead wire 6 was connected to each of the electrodes 4 and 5. In this manner, a heterojunction solar cell having a photovoltaic function between the n-type sintered CdS film and the p-type sintered CuInSe.sub.2 film was fabricated. The radiation 7 was incident on the sintered CdS film 2 side.

Table 2 shows the relationship between the compounding ratio of material Cu, In and Se powders and the conversion efficiency of a CdS/CuInSe.sub.2 solar cell fabricated using the materials. A solar cell having the highest conversion efficiency could be obtained when the compounding ratio of Cu, In and Se was 24:25:51. When the Cu content was increased, reflections due to Cu element were detected in the x-ray diffraction pattern of the sintered film.

When the degree of CuInSe.sub.2 synthesis relative to the Cu content after grinding was less than 0.1, an acceptable sintered film could not be obtained at any temperature below 700.degree. C.

When the degree of CuInSe.sub.2 synthesis exceeded 1.5, sintering produced CuInSe.sub.2 but caused a drastic decrease in adhesion to the underlying CdS film.

Furthermore, when the diameter of balls used in grinding was either smaller than 1.0 mm or larger than 10.0 mm, or when the ratio of the total weight of material Cu, In and Se powders to the total weight of the balls was either less than 0.05 or more than 0.5, the degree of CuInSe.sub.2 synthesis became lower than 0.1 or higher than 1.5 or the median grain size of ground powder became smaller than 1.0 or larger than 5.0 .mu.m. As a result, the paste prepared from such ground powder was extremely poor in printability and the conversion efficiency of the solar cell fabricated therefrom was extremely low.

In the same manner as Example 1, an n-type sintered CdS film 2 was first formed as illustrated in FIG. 1. Then, Cu.sub.2 Se and In.sub.2 Se.sub.3 powders, each having a purity not less than 99.99% and having been preliminarily comminuted to a grain size not exceeding 500 .mu.m, were taken in a mole ratio of 1:1. To a mixture of these powders was added the same weight of water and the whole composition was ground in an agitated ball-mill as illustrated in FIG. 2. Quantitative x-ray diffraction analysis of the ground powder revealed that the total proportion of polycrystalline CuInSe.sub.2 and CuInSe.sub.2 of low crystallinity was about 50% by weight. The median grain size of this powder was 2.0 .mu.m. To this ground powder including polycrystalline CuInSe.sub.2 and CuInSe.sub.2 of low crystallinity was added propylene glycol, a binding agent, to prepare a paste. This paste was printed on the aforementioned n-type sintered CdS film 2, dried and sintered in a nitrogen atmosphere at 600.degree. C. for 60 minutes, whereby a sintered CuInSe.sub.2 film 3 was provided. The formation of the sintered CuInSe.sub.2 film 3 was confirmed by x-ray diffraction analysis. Then, using a DC magnetron sputter, a 1.5 .mu.m-thick Mo film was formed on top of the sintered CuInSe.sub.2 film 3 to serve as an electrode on the CuInSe.sub.2 side. Then, an ohmic electrode 5 was provided on top of the sintered CdS film 2, and a lead wire 6 was connected to each of the electrodes 4 and 5. In this manner, a heterojunction solar cell having a photovoltaic effect between the n-type sintered CdS film 2 and the p-type sintered CuInSe.sub.2 film 3 was provided. The radiation 7 was incident on the sintered CdS film side. The conversion efficiency at AM 1.5, 100 mW/cm.sup.2 was 6.0%.

When the proportion of CuInSe.sub.2 after grinding was less than 10% by weight, the sintered film was extremely fragile due to insufficient intergrain binding. On the other hand, when said proportion was in excess of 70% by weight, a sintered film could not be obtained at a temperature below 600.degree. C. Similar changes were observed when the grain size of ground powder was less than 0.5 .mu.m, while the paste was poor in printability when the grain size exceeded 5 .mu.m.

On the other hand, the equivalent conversion efficiency was obtained when CuSe and InSe, instead of the corresponding elements, were used in a mole ratio of 1:1 and processed in the same manner as above to provide a CuInSe.sub.2 film. Thus, copper indium diselenide could be mass produced at low cost by the above-described steps of grinding a mixture of Cu, In and Se powders or Cu-Se and In-Se compounds in a liquid cooling medium to give a finely ground powder including CuInSe.sub.2 of low crystallinity, preparing a paste thereof, coating a substrate with the paste, and sintering the coat at a temperature below the melting point of CuInSe.sub.2.