Process for the Production of Sintered Films of Cd.sub.1-x Zn.sub.x S

1. Field of the Invention

The present invention relates to a process for the production of sintered films of Cd.sub.1-x Zn.sub.x S (0.ltoreq.x<1) having high optical transmittance which may be used for window layers in solar cells. More particularly, the invention relates to a process for preparing sintered films under a ZnCl.sub.2 atmosphere. The process may include the step of resintering films which have first been sintered under a ZnCl.sub.2 atmosphere.

2. Description of the Prior Art

It is well known in the art of film type solar energy cells, such as CdTe, CuInSe.sub.2, Cu.sub.2 S and the like to use different semiconductor films having an energy gap (band gap) greater than that of the light absorbing layer (CdTe, etc.) and that improved optical transmittance considerably enhances the performances of such cells. (References 1-3) Cd.sub.1-x Zn.sub.x S is useful as a window material, and its energy gap and lattice constant may vary depending upon the composition thereof, that is, at different values of x. (Reference 4) Many methods for obtaining sintered films x is 0 (i.e., CdS) have been devised and proposed. (References 5,6) However, these methods do not apply to the production of the sintered films of Cd.sub.1-x Zn.sub.x S having high optical transmission wherein x is greater than 0. (Reference 7)

The above references 1-7 are incorporated into this specification as a part of the present invention. These references are identified in the detailed description of the preferred Embodiments.

In general, the sintered film of CdS (x=0) is prepared by adding a suitable binder to a mixture of CdS power and CdCl.sub.2 powder to form a paste composition, and applying the paste onto a proper substrate, followed by sintering thereof at a high temperature. However, when a sintered film of Cd.sub.1-x Zn.sub.x S is prepared by adding ZnS powder at the stage of the initial compounding of the paste in the same manner as in the method of producing the sintered film of CdS, an undesirable microscopic structure is created and, thus, it is difficult to obtain sintered films with high optical transmittance.

The object of the invention is to provide a process for the production of a sintered film of Cd.sub.1-x Zn.sub.x S having high optical transmittance by means of a sintering method.

This and other objects of the invention can be attained by adding powdered CdCl.sub.2 to powdered CdS or to a mixture of powdered CdS and ZnS to give a paste, applying the paste onto an appropriate substrate and sintering the paste on the substrate under an inert gas atmosphere containing ZnCl.sub.2 vapor. Alternatively, the sintered material may be resintered under a nitrogen atmosphere, if desired.

The present invention will be understood more fully from the detailed description given hereinbelow and the accompanying drawings of the preferred embodiment of the invention.

FIG. 1 is a device for use in the production of sintered films of Cd.sub.1-x Zn.sub.x S; and

FIG. 2 is a graph showing a comparison of the optical transmittance of the sintered films of Cd.sub.1-x Zn.sub.x S produced by a prior art method and those prepared according to the invention.

A mixture of CdS and appropriate amounts (depending upon the desired composition of the sintered film) of ZnS and CdCl.sub.2 is first prepared and then a suitable binder such as propylene glycol is added to the mixture to produce a paste. This paste is applied onto the surface of a substrate such as borosilicate glass and is then dried to remove the binder. The resulting specimen is placed into a specially designed device as shown in FIG. 1 and heated to a temperature of 500.degree.-700.degree. C. Subsequently, the specimen is sintered under an inert gas atmosphere by adjusting the temperature of ZnCl.sub.2 to 400.degree. C. or higher but below that of the specimen. At this stage, the ZnCl.sub.2 is heated and its vapor is applied to the specimen.

Further, when the temperature of the ZnCl.sub.2 is altered, its vapor pressure is also changed. Accordingly, the reaction rate and direction are altered as shown below and, thus, it is quite possible to control the concentration of the constituents of the sintered film (i.e., the concentration of Zn) by means of adjusting the temperature of the ZnCl.sub.2.

In addition, the amount of the CdCl.sub.2 which remains is determined by the amount of ZnS added and the vapor pressure of the ZnCl.sub.2. Therefore, the higher the vapor pressure of the ZnCl.sub.2, the greath the amount CdCl.sub.2 remains and thereby the worse the microscopic structure of the spcimen is. In such a case, the specimen must further be sintered at a temperature of 500.degree.-700.degree. C. under an inert gas atmosphere to endow the specimen with high optical transmissivity.

The invention will be described below in greater detail by way of the following non-limiting examples.

Ten percent by weight of powdered CdCl.sub.2 was added to 90% by weight of highly pure powdered CdS. Propylene glycol was then mixed with these compounds as a binder to obtain a paste. This paste was then applied onto a borosilicate glass substrate, which was then dried in air. The resulting specimen was sintered for an hour with the temperature of the ZnCl.sub.2 maintained at 550.degree. C. and that of the specimen at 600.degree. C., to give a sintered film of Cd.sub.0.9 Zn.sub.0.1 S. This film has an average optical transmittance of 87% to long wavelength light of 600-1,000 nm and was 16 .mu.m in thickness.

A paste having the same composition as that of Example 1 was prepared and the procedures in Example 1 were repeated, except that the resulting specimen was sintered under a nitrogen atmosphere without the ZnCl.sub.2 vapor. A sintered film of CdS having a thickness of 18.5 .mu.m and an average optical transmittance of 80% to long wave-length lights was obtained.

Example 1 was repeated except that 10 mol% of ZnS was added to the initial paste. The resulting sintered Cd.sub.0.85 Zn.sub.0.15 S film had a thickness of 18.5 .mu.m and an average transmittance of 80% to long wavelength lights.

Example 1 was repeated except that 10 mol% after sintering of ZnS was added to the initial paste. The resulting Cd.sub.0.97 Zn.sub.0.03 S has a thickness of 19.5 .mu.m and an average optical transmittance of 70% to long wavelength lights. The addition of 25 mol% of ZnS gave a sintered film of Cd.sub.0.89 Zn.sub.0.11 S having a thickness of 30 .mu.m and an average transmittance of 57% to long wavelength lights. The reason that the film thickness of these specimens is greater than that of Example 1 is not considered to be due to the differences in the thickness thereof before sintering, but to the differences in density after the sintering.

A specimen A was obtained by applying a paste containing 10 mol% of ZnS onto the same substrate used in Example 1 and a specimen B was obtained by applying a paste containing 25 mol% of ZnS onto the substrate. These specimens were each sintered in the presence of the ZnCl.sub.2 maintained at a temperature of 560.degree. C. During the processing, each of the specimens was maintained at 600.degree. C., and was then further sintered for 2 hours at 600.degree. C. in nitrogen.

The resulting sintered films of Cd.sub.0.87 Zn.sub.0.1 S (A) and Cd.sub.0.79 Zn.sub.0.21 S (B) had a thickness of 18 .mu.m and transmittances of 87% (A) and 82% (B) respectively.

As can be seen from Examples 1 to 3 and Comparative Examples 1 and 2 above, when the specimens processed in accordance with the present invention are compared with those sintered conventionally under an inert gas atmosphere such as nitrogen, the Cd.sub.1-x Zn.sub.x S film sintered under an atmosphere containing ZnCl.sub.2 vapor produces a film of higher optical transmittance. In addition, the film sintered under a ZnCl.sub.2 atmosphere may be resintered under a nitrogen atmosphere in order to increase further the optical transmittance. In practice, when the layers with higher optical transmittance are used in solar cells, the efficiency of the solar cells is increased.

FIG. 2 shows a graph demonstrating how much sunlight of 1.1 eV or more (in percent) is transmitted through each of the following three specimens: first, a specimen sintered under an inert gas atmosphere in accordance with the prior art (Curve I), second, a specimen sintered under a ZnCl.sub.2 atmosphere in accordance with the invention (Curve II), and third, a specimen resintered under a nitrogen atmosphere after having been sintered under a ZnCl.sub.2 atmosphere in accordance with the invention (Curve III). From these data, it is evident that both the specimen sintered under a ZnCl.sub.2 atmosphere and the specimen resintered under a nitrogen atmosphere after having been sintered under a ZnCl.sub.2 atmosphere are capable of transmitting a greates amount of sunlight than the specimen sintered under an inert gas atmosphere.

1. A. L. Fahrenbruch and R. H. Bube, "Fundamentals of Solar Cells," Academic Press, New York--London, (1983) pp. 483-496.

2. R. A. Mickelsen, W. S. Chen, Y. R. Hsiao, and V. E. Lowe, "Polycrystalline Thin-Film CuInSe.sub.2 /CdZnS Solar Cells" IEE Trans. ED-31, pp. 542-546, 1984.

3. R. B. Hall, R. W. Birkmire, J. E. Phillips, and J. D. Meakin, "Thin-film Polycrystalline Cu.sub.2 S/Cd.sub.1-x Zn.sub.x S Solar Cells of 10% Efficiency." Appl. Phys. Letter, 38, pp. 925-926, 1981.

4. W. M. Kane, J. P. Spratt, L. W. Hershinger, and I. H. Khan, "Structural and Optical Properties of Thin Films of Cd.sub.1-x Zn.sub.x S," J. Electrochem. Soc., 113, pp. 136-138, 1966.

5. H. Uda, H. Matsumoto, K. Kuribayashi, Y. Komatsu, A. Nakano, and S. Ikegami, "Effect of Resistivity of CdS Sintered Film on Photovoltaic Properties of Screen-Printed CdS/CdTe Solar Cells," Jpn. J. Appl. Phys., 22, pp. 1832-1836, 1983.

6. H. G. Yang and H. B. Im, "Preparation and Properties of Sintered CdS Films," J. Electrochem. Soc., 133, pp. 479-485, 1986.

7. Y. S. Seol and H. B. Im, "Sintering Behaviors and Electrical Properties of Cd.sub.1-x Zn.sub.x S Films Containing CdCl.sub.2," J. Mater. Sci. (in printing).