METHOD OF FORMING CHALCOGEN COMPOUND LIGHT-ABSORPTION LAYER THIN FILM FOR SOLAR CELL

Abstract

Disclosed is a method of forming a chalcogen compound thin film suitable for use in a light-absorption layer of a solar cell. The method includes manufacturing a precursor liquid including an Sn precursor material and an S precursor material, applying the precursor liquid to form a precursor film, and heat-treating the precursor film. The Sn precursor material and the S precursor material are liquid materials. The present invention provides a method of forming a chalcogen compound thin film using a liquid precursor material without a sulfurization process, thereby forming a high-quality SnS thin film at low cost using a process which is suitable for mass production. Further, the light-absorption layer is formed using a process which is suitable for mass production, thus enabling the manufacture of a solar cell including the chalcogen compound thin film at low cost.

Claims

1. A method of forming a chalcogen compound thin film, the method comprising: manufacturing a precursor liquid including an Sn precursor material and an S precursor material; applying the precursor liquid to form a precursor film; and heat-treating the precursor film without drying the formed precursor film, wherein the Sn precursor material and the S precursor material are liquid materials.

2. The method of claim 1, wherein an Sn/S ratio of Sn and S, which are included in the precursor liquid, is in a range of 0.8 to 1.5.

3. The method of claim 1, wherein the heat-treating is performed at a temperature in a range of 450 to 520 C.

4. The method of claim 1, wherein the Sn precursor material is a liquid tin-organic complex.

5. The method of claim 4, wherein the tin-organic complex is Sn-ethyl hexanoate.

6. The method of claim 1, wherein the S precursor material is DMSO (dimethyl sulfoxide).

7. The method of claim 1, wherein the heat-treating is performed in an inert gas atmosphere.

8. A chalcogen compound thin film formed using the method of claim 1, wherein a precursor liquid, which includes a liquid Sn precursor material and a liquid S precursor material, is applied and then heat-treated to prevent Sn.sub.2S.sub.3 and SnS.sub.2 from being formed.

9. The chalcogen compound thin film of claim 8, wherein the chalcogen compound thin film is used in a light-absorption layer of a solar cell.

10. A solar cell comprising: a chalcogen compound thin film as a light-absorption layer, wherein a precursor liquid, which includes a liquid Sn precursor material and a liquid S precursor material, is applied and then heat-treated to prevent Sn.sub.2S.sub.3 and SnS.sub.2 from being formed in the chalcogen compound thin film.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] The above and other objects, features and advantages of the present invention will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

[0029] FIG. 1 shows the result of XRD analysis of an SnS thin film manufactured in an Example of the present invention and a thin film manufactured in a Comparative Example;

[0030] FIG. 2 is an electron microscopic picture showing the surface of the SnS thin film manufactured in the Example of the present invention;

[0031] FIG. 3 is an electron microscopic picture showing the section of the SnS thin film manufactured in the Example of the present invention;

[0032] FIG. 4 is an electron microscopic picture showing the surface of the SnS thin film manufactured in the Comparative Example; and

[0033] FIG. 5 is an electron microscopic picture showing the section of the SnS thin film manufactured in the Comparative Example.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0034] A detailed description will be given of an Example of the present invention with reference to the appended drawings.

[0035] A method of forming an SnS thin film according to the Example of the present invention includes manufacturing a precursor liquid, which includes a precursor material, applying the precursor liquid to form a film, and performing heat treatment.

[0036] In order to manufacture the precursor liquid, Sn-ethyl hexanoate, which is an Sn precursor material in a liquid phase, and DMSO (dimethyl sulfoxide), which is an S precursor material in a liquid phase, are mixed.

[0037] Since both the Sn precursor material and the S precursor material used in the present Example are in a liquid phase, the Sn precursor material and the S precursor material are mixed without the use of an additional solvent, thereby manufacturing the precursor liquid. Sn-ethyl hexanoate and DMSO are mixed at a molar ratio of 1:1 to manufacture the precursor liquid.

[0038] Next, the precursor liquid is applied using spin coating at 2000 rpm for 20 sec and spread to thus form a precursor film. The precursor liquid manufactured in the present Example is obtained by mixing the precursor materials in a liquid phase and has high viscosity, and accordingly, the precursor film may be formed to a sufficient thickness using only a single spin coating process for 20 sec without a separate drying process.

[0039] Finally, the precursor film, which is formed using spin coating, is heat-treated to synthesize SnS. The heat treatment process of the present Example is performed in a nitrogen gas atmosphere at a temperature of 500 C. for 30 min. Since the precursor liquid of the present Example includes both the Sn precursor material and the S precursor material, no additional material is required to synthesize SnS, and the SnS thin film may be formed using only heat treatment in the nitrogen atmosphere. First, a vacuum of 10.sup.4 to 10.sup.5 torr is created in a chamber, the chamber is filled with nitrogen gas until pressure reaches 1.02 atm, and the temperature is increased to 500 C. and maintained for 30 min.

[0040] A conventional process was used to form the SnS thin film as the Comparative Example to confirm the effect of the Example of the present invention. The method of forming the SnS thin film according to the Comparative Example is similar to that of the present invention in that the precursor liquid is manufactured, applied using spin coating, and heat-treated, but is different from that of the present invention in terms of the specific content of each step.

[0041] First, Sn acetate (Sn(Ac).sub.2), which is the Sn precursor in a powder phase, is dissolved in pyridine, which is a solvent, according to a typical process to manufacture a precursor solution. Sn acetate and pyridine are mixed at a weight ratio of 1:10.

[0042] Next, the manufactured precursor solution is spread using spin coating to form a precursor film. Unlike the precursor liquid of the present Example, the precursor solution of the Comparative Example is obtained by dissolving the Sn precursor in a solid phase in the solvent and has low viscosity. Accordingly, spin coating at 2000 rpm for 20 sec and drying at a temperature of 200 C. for 4 min are repeated five times to form the precursor film. Since the drying process is included, the processing time is increased, thus lengthening the entire process compared to the above Example. Further, a typical drying process is simply performed in the atmosphere. There is a drawback in that oxygen in the atmosphere may react with the precursor during the drying process.

[0043] Finally, the precursor film is sulfurized. Since only the Sn precursor is included in the precursor solution of the Comparative Example, S is supplied during the heat treatment process to perform sulfurization, during which Sn included in the precursor film is reacted with S. First, a vacuum of 10.sup.4 to 10.sup.5 torr is created in the chamber, the chamber is filled with a nitrogen gas until pressure reaches 1.02 atm, and the temperature is increased to 650 C. while H.sub.2S gas flows, thereby performing sulfurization for 30 min. Heat treatment is performed at a temperature higher than the heat treatment temperature of the Example due to sulfurization. The H.sub.2S and nitrogen gases are supplied in amounts of 100 sccm and 30 scan, respectively, during the heat treatment process.

[0044] Hereinafter, the characteristics of the thin films, manufactured in the Example according to the present invention and the Comparative Example, will be described.

[0045] FIG. 1 shows the result of XRD analysis of the SnS thin film manufactured in the Example of the present invention and the thin film manufactured in the Comparative Example.

[0046] As shown in the drawing, no peak of a secondary phase, other than SnS, is visible for the SnS thin film formed in the present Example, but strong peaks of Sn.sub.2S.sub.3 and SnS.sub.2 are exhibited, in addition to the peak of SnS, for the thin film manufactured in the Comparative Example.

[0047] The process of forming the precursor film using the solution, which includes the precursor material in a powder phase, and then performing sulfurization (or selenization) to form the chalcogen compound thin film as in the Comparative Example, is a typical process which is used to manufacture the thin film such as CIS or CIGS. The quality of the thin film formed using the aforementioned process is inferior to that of a vacuum process, which is a costly process. However, a non-vacuum type forming process is used due to the high economic profitability of the non-vacuum process based on the low processing cost. However, as seen in the result of the Comparative Example, an excessive amount of the secondary phase is generated together with SnS, in addition to the problem of the low quality when the SnS thin film is formed. Accordingly, it is known that the non-vacuum process is not used to form an SnS thin film.

[0048] The present invention relates to a novel non-vacuum process for forming an SnS thin film that does not include the secondary phase and is shown in FIG. 1, using the precursor liquid including the liquid Sn precursor material and the liquid S precursor material mixed therein, so as to overcome the problem of the precursor solution having powder dissolved therein and the drawback of the sulfurization process.

[0049] FIGS. 2 and 3 are electron microscopic pictures showing the surface and the section of the SnS thin film manufactured in the Example of the present invention, and FIGS. 4 and 5 are electron microscopic pictures showing the surface and the section of the SnS thin film manufactured in the Comparative Example.

[0050] As shown in the drawings, the thin films formed in the present Example and the Comparative Example are different from each other in terms of the fine structures of the surfaces and the sections thereof. The difference depends on whether or not sulfurization is performed and on whether or not the secondary phase is formed in the thin film.

[0051] It can be confirmed that the surface and the section of the thin film formed in the present Example are regular. The thin film formed in the Comparative Example includes the secondary phase, thus exhibiting an irregular fine structure.

[0052] From the aforementioned description, it can be confirmed that a high-quality SnS thin film that does not include a secondary phase is formed using the non-vacuum process according to the present invention, which is a very inexpensive process. The SnS thin film may be applied to the light-absorption layer of a solar cell, and the process of forming the thin film according to the present invention may be applied as a process for forming the light-absorption layer to manufacture the solar cell.

[0053] A solar cell according to another aspect of the present invention may be obtained by applying the constitution of a solar cell of a typical chalcogen compound thin film without any modification, except that the light-absorption layer is included and the process of forming the SnS thin film is applied as the process of manufacturing the light-absorption layer. The specific constitution of the solar cell may include a soda lime glass substrate and a molybdenum lower electrode, which are known to improve the performance of the chalcogen compound light-absorption layer, and other specific descriptions will be omitted.

[0054] The manufacturing cost of the solar cell manufactured using the aforementioned process may be significantly reduced due to the low cost of the process of forming the light-absorption layer.

[0055] Although the preferred embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.