Patent Application
20120006687
1. Technical Field
The present invention relates to a method of forming a CIGS thin film.
2. Description of the Related Art
In the last several years, a I-III-VI group semiconductor compound represented by CuInSe2 has been highlighted as a raw material of a novel high-efficiency solar cell which can replace a currently-used crystal silicon solar cell. Here, a thin-film solar cell containing the semiconductor compound CuInSe2 is referred to as a CuInSe2 thin-film solar cell. CuInSe2 has a chalcopyrite structure.
Meanwhile, the CuInSe2 thin-film solar cell has a bandgap of 1.04 eV, which is smaller than the bandgap of 1.4 eV at which solar light is ideally absorbed. In order to enlarge the bandgap of the CuInSe2 thin-film solar cell, a part of the indium (In) in the CuInSe2 is replaced by gallium (Ga), and a part of the selenium (Se) in the CuInSe2 may be replaced by sulfur (S). As such, a five-component compound whose component has been replaced by a different component is represented by CIGSS {Cu(InxGa1-x)(SeyS1-y)2}, and is generally referred to as a CIGS material together with the quaternary compound Cu(InxGa1-x)Se2.
CIGS material has a very high absorption coefficient of 1×105 cm−1 in a semiconductor because it has a direct transition type of energy bandgap. The fact that the absorbance of a light-absorbing layer of a solar cell is high means that it is possible to manufacture a high-efficiency solar cell using a thin film having a thickness of 1˜2 μm. When CIGS material is used to manufacture a thin-film solar cell, there are economical advantages.
According to the documents reported to date, a CIGS thin-film solar cell has a higher efficiency than do thin-film solar cells made of other materials in a module as well as a laboratory unit. Further, the CIGS thin-film solar cell compares favorably with a crystal silicon solar cell. In the case of a laboratory unit, the National Renewable Energy Laboratory (NREL) of U.S.A. reported that a CIGS thin-film solar cell has an efficiency of 19.9%. The efficiency of the CIGS thin-film solar cell is higher than those of other commercial thin-film solar cells such as thin-film solar cells made of amorphous silicon or CdTe, and approximates to 20.3% which is the maximum efficiency of conventional polycrystalline silicon solar cells.
A process of forming a CIGS thin film is very complicated because CIGS is a multi-component compound. Physical thin film forming methods include co-evaporation, sputtering, and the like. Electrochemical thin film forming methods include electrodeposition and the like. Among these methods, co-evaporation and sputtering are already being used commercially. Co-evaporation is a technology used to form a CIGS thin film by putting components into a vacuum chamber, resistance-heating the components and then vacuum-depositing the heated components on a substrate. Co-evaporation has been widely used in laboratories for a long time because the structure of the apparatus used in co-evaporation is simple. However, co-evaporation is problematic in that it is difficult to form a thin film having a large area, and in that it is not easy to form a high-quality thin film because the inner part of a vacuum apparatus becomes seriously contaminated. Currently, in the case of the CIGS thin-film solar cell having an energy conversion efficiency of 19.9% reported by the NREL, since a light-absorbing layer was formed by co-evaporation, it is determined that CIGS-based solar cells can be technically advanced in terms of efficiency. Meanwhile, sputtering has also been widely used to form a CIGS thin film for the purpose of research and production because metals or insulating materials can be easily deposited by a relatively simple apparatus. In particular, sputtering is effectively used to form an oxide thin film or a nitride thin film because compounds accompanying reactions can be deposited using inert gas such as argon gas and the like. Further, sputtering is considered to be a technology suitable for commercializing a CIGS thin-film solar cell because it can form a large-area thin film. Sputtering is advantageous in that a large-area thin film can be easily formed. However, sputtering is problematic in that it is difficult to form a high-quality thin film, so that the maximum energy conversion efficiency of the thin film obtained by sputtering does not reach that of the thin film obtained by co-evaporation.
Therefore, a method of forming a CIGS thin film, employing the advantages of both co-evaporation and sputtering, is required.
Accordingly, the present invention has been devised to solve the above-mentioned problems, and an object of the present invention is to provide a method of forming a CIGS thin film, which can form a large-area thin film at room temperature and normal pressure without using a vacuum chamber.
Another object of the present invention is to provide a method of forming a CIGS thin film, which can form a simple and cheap CIGS thin film having high light efficiency.
Still another object of the present invention is to provide a method of forming a CIGS thin film, which can improve the crystallinity of a CIGS thin film.
In order to accomplish the above objects, an aspect of the present invention provides a method of forming a CIGS thin film, comprising the steps of immersing a substrate comprising an electrode into an electrolyte solution comprising Na2SO4, a water-soluble copper (Cu) precursor, a water-soluble indium (In) precursor, a water-soluble gallium (Ga) precursor, and a water-soluble selenium (Se) precursor; performing electrodeposition in such a way as to apply a direct current (DC) voltage of −0.95V˜−0.85V to the electrolyte solution at room temperature and normal pressure for 10˜120 minutes to form a preliminary CIGS thin film; and heat-treating the preliminary CIGS thin film at 230˜270° C. to form a CIGS thin film.
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:
The electrolyte solution does not additionally comprise an additive and a buffer solution. Therefore, the electrodeposition of the present invention can be conducted more simply and efficiently than a conventional electrodeposition.
The water-soluble copper (Cu) precursor may be selected from the group consisting of Cu(NO3)2, CuSO4; and hydrates thereof. The water-soluble indium (In) precursor may be selected from the group consisting of In(NO3)3, In2(SO4)3, and hydrates thereof. The water-soluble gallium (Ga) precursor may be selected from the group consisting of Ga(NO3)3, Ga2(SO4)3, and hydrates thereof. The water-soluble selenium (Se) precursor may be selected from the group consisting of SeO2, H2SeO3, and hydrates thereof. The water-soluble precursors of the present invention are environment-friendly because they do not include chloride (Cl−).
The atom ratio of copper, indium, gallium and selenium in the formed preliminary CIGS thin film may be changed depending on pH of an electrolyte solution, the status of the surface of an electrode, temperature, the concentration of each element, the existence and nonexistence of an additive, the kind of additive, the electrodeposition potential, the electrodeposition time and the like. The atom ratio of copper, indium, gallium, and selenium in the electrolyte solution used in the present invention may be 0.8˜1.2:0.8˜1.2:1.8˜2.2:2.8˜3.2, preferably, 1:1:2:3. When the atom ratio thereof falls within the above range, the bandgap of a CIGS thin film is increased, thus forming a high-quality CIGS thin film.
Further, the pH of the electrolyte solution may be 2˜3. Only when the pH thereof falls in this range, it is easy to form a CIGS thin film.
The method of forming a CIGS thin film according to the present invention is comprised of the step of performing electrodeposition in such a way as to apply a direct current (DC) voltage of −0.95V˜−0.85V to the electrolyte solution at morn temperature and normal pressure for 10˜120 minutes to form a preliminary CIGS thin film.
When the voltage is applied to the electrolyte solution under the above condition, the simultaneous growth of four kinds of metal particles of the CIGS thin film can be explained by the reduction reactions attributable to the movement of electrons from the back electrode toward to the metal ions of the precursors in the electrolyte solution. A standard reduction potential of each oldie metal ions is as follows (vs. NHE <Normal Hydrogen Electrode>, an aqueous acidic to solution in the case of Se).
Cu
In
Ga
Se + 3H20
Meanwhile, when CIGS is used as a raw material of a solar cell, it is known that it is greatly effective to increase the efficiency of a solar cell when a CuInSe2 phase having a bandgap of 1.04 eV and a CuGaSe2 phase having a bandgap of 1.7 eV are mixed with each other to maximize the solar light absorption of a light-absorbing layer or the solar cell. In this case, the atom ratio of the two phases, Ga/(In+Ga), may be 0.2˜0.4, more preferably, 0.3. When the atomic ratio thereof falls within the above range, the bandgap increases, so that the bandgap approximates an ideal bandgap, thereby increasing the solar light absorption of the light-absorbing layer of the solar cell. The present inventors examined the optimal electrodeposition potential for setting the correlation ratio of In and Ga, and, as a result, they found that the direct current (DC) voltage of −0.95V˜−0.85V is the most suitable for the optimal electroposition potential. Further, the electrodeposition of the CIGS thin film depends on various experimental variables, such as the surface resistance of the molybdenum electrode deposited on glass, etc., and the characteristics of the layer that is electrodeposited depending on the above experimental variables is not strictly defined.
The method of forming a CIGS thin film according to the present invention is comprised of the step of heat-treating the preliminary CIGS thin film at 230˜270° C. to form a CIGS thin film.
When heat treatment is performed within the above range, the crystallinity of the CIGS thin film becomes high, and the consumption of energy is not high because the heat treatment temperature is relatively low, so that the method of forming a CIGS thin film according to the present invention is economical. Further, when the treatment is performed within the above range, the phenomenon in which selenium of the CIGS thin film volatilizes and thus disappears does not occur. Therefore, an additional process for replenishing selenium is not required. The melting points of the elements included in the preliminary CIGS thin film are 1083° C. (Cu), 156° C. (In), 29.78° C. (Ga), and 217° C. (Se), respectively. However, the elements included in the preliminary CIGS thin film formed by electrodeposition exist in the form of compounds rather than independently. For this reason, the preliminary CIGS thin film is comparatively stable during the low-temperature heat treatment thereof. Due to the heat treatment of the preliminary CIGS thin film, various phases of compounds including two kinds of elements, three kinds of elements and four kinds of elements can be produced. The present invention is to provide a method and condition of forming a CIGS thin film having a suitable composition by heat-treating the electrodeposited preliminary CIGS thin film in an atmosphere at low temperature.
The method of forming CIGS thin film according to the present invention is advantageous in that a large-area thin film can be formed at room temperature and normal pressure without using a vacuum chamber. Further, the method of forming CIGS thin film according to the present invention is advantageous in that a simple and cheap CIGS thin film having high light efficiency can be formed. Furthermore, the method of forming a CIGS thin film according to the present invention is advantageous in that the crystallinity of a CIGS thin film can be improved upon.
Hereinafter, the present invention will be described in more detail with reference to the following Examples. These Examples are set forth only to illustrate the present invention, and it is considered obvious by those skilled in the art that the scope of the present invention is not limited to these Examples.
A substrate, in which molybdenum (Mo) was deposited on soda-lime glass to a thickness of about 1 μm in the form of a two-layer structure by sputtering, was provided. Molybdenum had a specific resistance of about 15 μΩ·cm. The surface of the formed molybdenum layer was a uniform mirror surface.
Meanwhile, water-soluble precursors, a buffer solution and an additive were mixed with a 0.5M Na2SO4 electrolyte aqueous solution (here, distilled water Milli-Q was used as a solvent) as shown in Table 1 below to prepare an electrolyte solution. In this case, the pH of the electrolyte solution was adjusted using 2 mM of hydrochloric acid, and the pH thereof was about 2.7. Here, the contents of each of the water-soluble precursors, the buffer solution and the additive mean the contents thereof in the electrolyte solution.
Subsequently, the substrate was immersed into the electrolyte solution. Then, an electric potential was applied between the electrodes in the electrolyte solution at room temperature and normal pressure during the time that electrolytic deposition was carried out as given in Table 2 below to form a preliminary CIGS thin film. Here, a potentiostat/galvanostat (model M 270, manufactured by EG&G Corp.) was used as a potentiostat, Ag/AgCl was used as a reference electrode, and a platinum (Pt) wire was used as a counter electrode.
Subsequently, the preliminary CIGS thin film was heat-treated at the temperature of heat treatment and for the duration of the heat treatment as given in Table 2 below to complete a CIGS thin film.
Referring to
Hereinafter, the present invention will be described in detail with reference to the attached drawings.
The method of forming a CIGS thin film according to the present invention is comprised of the step of immersing a substrate comprising an electrode into an electrolyte solution comprising Na2SO4, a water-soluble copper (Cu) precursor, a water-soluble indium (In) precursor, a water-soluble gallium (Ga) precursor and a water-soluble selenium (Se) precursor.
The substrate is not particularly limited as long as it is a substrate for a solar cell. However, a substrate comprising a molybdenum electrode or a substrate comprising a silicon electrode may be used as the substrate. Preferably, a substrate comprising a molybdenum electrode may be used as the substrate. The reason why the substrate comprising a molybdenum electrode is preferably used is because molybdenum has high electroconductivity and because the ohmic contact between molybdenum and a CIGS layer is excellent. Molybdenum is stable in a high-temperature selenium atmosphere. Even when a CIGS solar cell comprising a back electrode made of molybdenum is being operated for a long time, its energy conversion efficiency does not fall. Further, since molybdenum is relatively cheap, it is economical for the mass production of solar cells.
Here, the substrate comprising a silicon electrode may be a silicon wafer used in a semiconductor process.
The electrolyte solution may comprise a water-soluble copper (Cu) precursor, a water-thin film which was electrodeposited but not heat-treated yet. The CIGS thin film (b) means a CIGS thin film which was electrodeposited and heat-treated.
Referring to
In the CIGS thin film of Example 1, (112) and (220) diffraction peaks of CIS chalcopyrite were observed, and traces of CIGS were observed in the vicinity of a 2θ value of 45 degrees. In the case of CIS (112), it was observed that its XRD pattern was very sharp and its intensity was increased. Therefore, it is determined that CIS chalcopyrite is considerably crystallized by heat treatment at 250° C.
That is, according to the present invention, since a CIGS thin film of line particles is formed only by electrodeposition at room temperature and normal pressure, it is predicted that the CIGS thin film formed by the electrodeposition can be crystallized at a lower temperature than the heat treatment temperature of 500° C. or more which is required by sputtering.
Referring to Table 5, it is observed that selenium (Se) does not remain in the CIGS thin film after heat treatment. Therefore, it is determined that Comparative Example 1, unlike the present invention, needs an additional process of replenishing selenium (Se).
Table 7 shows that the atom ratio of Ga/(In+Ga) is 0.27. However, the CIGS thin film of Comparative Example 2 is relatively non-economical compared to the CIGS thin film of Example 1 because it includes a buffer and SDS.
Further, Table 7 shows the change of the compositions of CIGS thin films of Comparative Examples 2, 3 and 4 before and after heat treatment. In Table 7, the unit of each atom is atomic percent (at %).
Further, comparing Table 9 with Table 3, it is shown that the ratio of the gallium (Ga) in the CIGS thin film is sensitive to changes depending on the composition ratio of Cu:In:Ga:Se in the electrolyte. The ratio of (Ga) in the electrolyte solution of Comparative Example 6 is relatively small compared to that of gallium (Ga) in the electrolyte solution of Example 1, whereas the ratio of gallium (Ga) in the CIGS thin film of Comparative Example 6 is relatively large compared to that of gallium (Ga) in the preliminary CIGS thin film of Example 1. It is determined that these results are attributable to electrode reaction kinetic characteristics occurring in a multi-component system, like CIGS.
The composition of the CIGS thin film of Comparative Example 5 and that of the CIGS thin film of Comparative Example 6 are compared in Table 10 below.
Referring to Table 11, a large amount of oxygen is discovered. Comparing the compositions of the CIGS thin films in Table 8 and Table 11, which were produced by using the electrolyte solution of the same composition of the four elements with the different potentials applied, a considerable amount of sulfur (S) is discovered in Table 8, whereas a large amount of oxygen (O) is discovered in Table 11. It is determined that these results are due to the fact that the voltages applied between the electrodes to the electrolyte solutions of Comparative Example 5 and Comparative Example 7 are −0.91 and −0.90 V, respectively. Further, it is determined that the applied voltage, compared to the composition of the electrolyte solution, is an experimental factor greatly influencing the composition of the preliminary CIGS thin film formed on the surface of a molybdenum (Mo) electrode. The fact that Sulfur (S) was not discovered at all in the CIGS thin film of Comparative Example 7, unlike Comparative Example 5, also shows that the composition of a CIGS thin film can be changed by controlling the applied voltage regardless of the existence of SDS in the electrolyte solution.
Referring to Table 12, a large amount of oxygen (O) was discovered as in Table 11 above. It is analyzed that these results are due to the fact that the voltages applied to the electrolyte solutions of Comparative Example 7 and Comparative Example 8 are equal to each other. It is determined that oxygen (O) serves to produce oxides of Cu, In, Ga or Se by reducing oxygen molecules or hydroxide ions at the time of forming a preliminary CIGS thin film. Therefore, it is not preferred that large amount of oxygen (O) be discovered.
Table 13 shows that oxygen (O) or sulfur (S) was not discovered. Comparative Example 9 is greatly different from other Comparative Examples in that a large amount of molybdenum (Mo) was discovered. This result verifies that an additive does not greatly influence the formation of a CIGS thin film. In Comparative Example 9, unlike Comparative Example 7, oxygen (O) was not discovered although the same voltage of −0.90 V was applied. This result verifies that buffer influences an electrode reaction rate in addition to adjusting the pH of an electrolyte solution.
As described above, the method of forming a CIGS thin film according to the present invention is advantageous in that a large-area thin film can be formed at room temperature and normal pressure without using a vacuum chamber. Further, the method of trimming a CIGS thin film according to the present invention is advantageous in that a simple and cheap CIGS thin film having high light efficiency can be formed. Furthermore, the method of forming a CIGS thin film according to the present invention is advantageous in that the crystallinity of a CIGS thin film can be improved.
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.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10-2010-0065066 | Jul 2010 | KR | national |
