JP5709662B2 - CZTS thin film solar cell manufacturing method - Google Patents

Description

  The present invention relates to a method for producing a CZTS thin film solar cell with high photoelectric conversion efficiency and a CZTS thin film solar cell produced by the method.

In recent years, a thin film solar cell using a chalcogenide-based compound semiconductor, generally called CZTS, has attracted attention as a p-type light absorption layer. Since this type of solar cell is relatively inexpensive and has a band gap energy suitable for sunlight, it is expected that a highly efficient solar cell can be manufactured at low cost. CZTS is an I ₂ -II-IV-VI ₄ group compound semiconductor containing Cu, Zn, Sn, and S. Typical examples include Cu ₂ ZnSnS ₄ .

  In a CZTS thin film solar cell, a metal back electrode layer is formed on a substrate, a p-type CZTS light absorption layer is formed thereon, and an n-type high resistance buffer layer and an n-type transparent conductive film are sequentially laminated. Formed. As the metal back electrode layer material, high corrosion resistance and high melting point metal such as molybdenum (Mo), titanium (Ti), or chromium (Cr) is used. The p-type CZTS light absorption layer is formed by, for example, forming a Cu—Zn—Sn or Cu—Zn—Sn—S precursor film on a substrate on which a molybdenum (Mo) metal back electrode layer is formed by sputtering or the like. This is formed by sulfiding in a hydrogen sulfide atmosphere (see, for example, Patent Document 1).

JP 2010-215497 A

  As described above, CZTS-based thin-film solar cells have high potential, but at present, products with high photoelectric conversion efficiency that can withstand practical use have not been obtained, and further progress in manufacturing technology is required. It has been. This invention is made | formed regarding the point which concerns, and makes it a subject to provide the manufacturing method of the CZTS type thin film solar cell which has high photoelectric conversion efficiency, and the CZTS type thin film solar cell manufactured by the method.

  In order to solve the above-mentioned problems, in the first aspect of the present invention, a metal back electrode layer is formed on a substrate, and a metal precursor film containing at least Cu, Zn, and Sn is formed on the metal back electrode layer. The gas on which the metal precursor film is formed is heat treated in a furnace filled with a selenium-containing gas, thereby selenizing the metal precursor film, and after the selenization, a gas containing sulfur or a sulfur compound in the furnace To form a p-type CZTS light absorption layer containing selenium and sulfur, and to form an n-type transparent conductive film on the p-type CZTS light absorption layer. A method of manufacturing a CZTS-based thin film solar cell including each step to be formed is provided.

  In the first aspect, the sulfurization may be performed by introducing a gas containing sulfur or a sulfur compound into the furnace in which the gas after selenization is present.

  The metal precursor film formed on the metal back electrode layer may further contain sulfur.

  Further, after the metal precursor film is formed on the metal back electrode and before the selenization, the substrate on which the metal precursor film is formed is placed in a furnace filled with a gas containing sulfur or a sulfur compound. It may be sulfurized by heat treatment.

  In order to solve the above-described problems of the present invention, in the second aspect of the present invention, a metal back electrode layer formed on a substrate, and a p-type CZTS light absorbing layer formed on the metal back electrode layer, An n-type transparent conductive film formed on the p-type CZTS light absorption layer, and the sulfur concentration at the upper part of the p-type CZTS light absorption layer on the n-type transparent conductive film side is the CZTS-based light. Provided is a CZTS-based thin-film solar cell, characterized in that it is higher than the central portion of the absorption layer in the layer thickness direction.

  In the CZTS-based thin film solar cell of the above aspect, the sulfur concentration in the lower part on the metal back electrode layer side of the p-type CZTS-based light absorption layer is further made higher than the central portion in the layer thickness direction of the CZTS-based light absorption layer. May be.

  In the method of the present invention, both selenization and sulfidation of the metal precursor film are performed. Therefore, it is possible to control the chalcogen ratio (ratio of sulfur and selenium) in the p-type CZTS-based light absorption layer. By this control, it is possible to control the band gap of the p-type CZTS-based light absorption layer. Therefore, it is possible to select a band gap that increases the photoelectric conversion efficiency.

  In addition, after the metal precursor film is heat-treated in a selenium-containing gas to be selenized, and further sulfurized, the sulfur concentration in the formed p-type CZTS-based light absorption layer is reduced on the light-receiving surface side. A concentration distribution is formed which increases and decreases toward the center of the p-type CZTS light absorption layer. As a result, the band gap of the p-type CZTS light absorption layer on the light receiving surface side is increased, and the open circuit voltage of the solar cell product is improved. Furthermore, when the sulfur concentration on the light-receiving surface side is increased, the sulfur concentration is relatively lowered at portions other than the light-receiving surface side, the band gap is reduced, and the short-circuit current of the solar cell product is increased.

  Thus, by forming a gradient of the sulfur concentration that increases on the light-receiving surface side in the p-type CZTS-based light absorption layer, it is possible to achieve both a high open-circuit voltage and a large short-circuit current. Photoelectric conversion efficiency is improved. In addition, in sulfurization after selenization, Sn is released from the selenized p-type precursor film by introducing a gas containing sulfur or a sulfur compound without exhausting the selenium-containing gas in the furnace by evacuation. Is prevented. As a result, an increase in crystal defects due to Sn loss is suppressed, crystal quality of the p-type CZTS light absorption layer after selenization / sulfurization is improved, and photoelectric conversion efficiency is improved.

  Further, if sulfur is contained in the metal precursor film before selenization by forming a metal precursor film containing sulfur or by sulfidizing the metal precursor film before selenization, the subsequent selenium The sulfur contained in advance is pushed from the light receiving surface side to the metal back electrode side, and the sulfur concentration is increased on the metal back electrode side. This increases the band gap in the vicinity of the interface with the metal back electrode in the p-type CZTS-based light absorption layer, and suppresses carrier recombination in this portion, thereby increasing the short-circuit current and increasing the photoelectric conversion efficiency. improves.

Schematic which shows the cross-section of the CZTS type thin film solar cell formed by the manufacturing method concerning one embodiment of the present invention. Process drawing for demonstrating the manufacturing method which concerns on one Embodiment of this invention. Schematic which shows the concentration distribution of the sulfur in a p-type CZTS type light absorption layer.

  Embodiments of the present invention will be described below with reference to the drawings, but the following embodiments are merely examples and do not limit the present invention. Further, the drawings are only for the purpose of illustrating the present invention, so the size of each layer on the drawing does not correspond to the actual scale.

  FIG. 1 is a schematic cross-sectional view showing the structure of a CZTS-based thin film solar cell according to an embodiment of the present invention, and FIG. 2 is a diagram for explaining a process for manufacturing the CZTS-based thin film solar cell shown in FIG. FIG. In FIG. 1, 1 is a substrate such as glass, 2 is a metal back electrode layer made of a metal such as Mo, 3 is a p-type CZTS light absorption layer, 4 is an n-type high-resistance buffer layer, and 5 is n-type transparent. A conductive film is shown.

  Next, with reference to FIG. 2, the manufacturing method of the CZTS type thin film solar cell which concerns on one Embodiment of this invention is demonstrated. First, in step S1, a metal back electrode layer 2 such as Mo is formed on the substrate 1 by, for example, DC sputtering. The film thickness is, for example, 200 to 500 nm. In the next step S2, a metal precursor film containing at least Cu, Zn, and Sn is formed on the metal back electrode layer 2 by an electron beam evaporation method or the like. On the metal back electrode layer 2 of Mo, ZnS, Sn, and Cu may be sequentially formed by electron beam evaporation to form a metal precursor film. In this case, the metal precursor film contains Cu, Zn, Sn, and S.

When the metal precursor film is thus formed on the metal back electrode layer 2, in step S3, the substrate 1 is transferred into a furnace filled with a gas containing selenium and heat treated at 450 ° C. to 600 ° C. By doing so, the metal precursor film is selenized. The selenization time is about 30 to 120 minutes. Then, in step S4, without exhausting the inside of the furnace to make a vacuum, a gas containing sulfur (S) or a sulfur compound is introduced into the furnace in which the gas after selenization is present, and 450 ° C. to 600 ° C. By performing a heat treatment for about 30 to 120 minutes, the selenized metal precursor film is sulfided. As a result, in step S4, the p-type CZTS light absorption layer 3 of Cu ₂ ZnSn (S, Se) ₄ is formed.

Next, in steps S5 and S6, an n-type high-resistance buffer layer 4 and an n-type transparent conductive film 5 are formed on the p-type CZTS-based light absorption layer 3 to complete a CZTS-based thin film solar cell. The n-type high resistance buffer layer 4 is, for example, a thin film (thickness of about 3 nm to 50 nm) of a compound containing Cd, Zn, and In, typically CdS, ZnO, ZnS, Zn (OH) ₂ , In _2. It is formed of O ₃ , In ₂ S ₃ , or Zn (O, S, OH) which is a mixed crystal thereof. This layer is generally formed by a solution growth method (CBD method), but a metal organic chemical vapor deposition method (MOCVD method) or an atomic layer deposition method (ALD method) can also be applied as a dry process.

  In the CBD method, a thin film is deposited on a base material by immersing the base material in a solution containing a chemical species that serves as a precursor and causing a heterogeneous reaction between the solution and the base material surface. The n-type high-resistance buffer layer 4 is not necessarily essential for producing a CZTS-based thin film solar cell, and a solar cell can be produced without providing this layer.

  The n-type transparent conductive film 5 is formed with a film thickness of about 0.05 to 2.5 μm by a transparent material with n-type conductivity, wide forbidden band width, and low resistance. Typically, there is a zinc oxide thin film (ZnO) or an ITO thin film. In the case of a ZnO film, a low resistance film can be formed by adding a group III element (eg, Al, Ga, B) as a dopant. The n-type transparent conductive film 5 can also be formed by sputtering (DC, RF) or the like other than MOCVD.

  In this method, as described above, both selenization and sulfidation are performed on the metal precursor film. In this method, the composition ratio of sulfur (S) and selenium (Se) in the p-type CZTS light absorption layer 3 formed by controlling the concentration of the atmospheric gas and / or the time for selenization / sulfurization ( The chalcogen ratio) can be arbitrarily controlled. When the chalcogen ratio is changed, the band gap of the p-type CZTS light absorption layer 3 is changed, so that the band gap of the p-type CZTS light absorption layer 3 can be controlled.

  Furthermore, by performing sulfurization after selenization, the following effects are produced.

  FIGS. 3A and 3B are diagrams conceptually showing the gradient of the sulfur concentration in the p-type CZTS-based light absorption layer 3 formed as described above. FIG. 3A shows a gradient of sulfur concentration when a Cu—Zn—Sn metal precursor film is sulfided after selenization. When the metal precursor film containing Cu—Zn—Sn is sulfurized after being selenized, the concentration of sulfur introduced into the p-type CZTS-based light absorption layer 3 by sulfidation is as shown in FIG. Higher on the side, that is, on the n-type high resistance buffer layer 3 and n-type transparent conductive film 4 side, and lower toward the center in the layer thickness direction.

  When the sulfur concentration is high, the band gap becomes large. Therefore, in the p-type CZTS light absorbing layer 3 that is selenized and then sulfurized, the light receiving surface side, that is, between the n-type high-resistance buffer layer 4 and the n-type transparent conductive film 5 is used. The band gap of the p-type CZTS-based light absorption layer 3 at the pn junction interface increases, and as a result, the open circuit voltage of the solar cell product increases. Furthermore, when the sulfur concentration on the light-receiving surface side is increased, the sulfur concentration in the portion other than the light-receiving surface is relatively lowered, and the band gap of the portion is decreased. As a result, the short circuit current is increased.

  Thus, by performing sulfurization after selenization, the p-type CZTS-based light absorption layer 3 has a concentration gradient as shown in FIG. 3A, thereby achieving both a high open circuit voltage and a large short-circuit current. As a result, the photoelectric conversion efficiency of the solar cell product is improved.

  FIG. 3B shows a case where the metal precursor film is made of Cu—Zn—Sn—S, or when the Cu—Zn—Sn metal precursor film is once sulfided and then selenized and sulfided. The sulfur concentration gradient in the p-type CZTS type light absorption layer 3 is shown. Prior to selenization, a metal precursor film containing sulfur (S) is formed, or sulfur is contained by sulfurization, so that these sulfur are converted into metal by thermal diffusion of selenium during the subsequent selenization. Since it is pushed to the back electrode side or sulfur is replaced by selenium from the light receiving surface side, the sulfur concentration is high on the metal back electrode layer side as shown in FIG. As a result, the band gap on the metal back electrode side in the p-type CZTS-based light absorption layer 3 is increased, and carrier recombination at this portion is suppressed. As a result, the short circuit current is increased and the photoelectric conversion efficiency is improved.

Table 1 below shows a CZTS thin film solar cell manufactured by a method according to an embodiment of the present invention (described as “Example” in Table 1) and a CZTS thin film solar cell manufactured by a conventional method. The photoelectric conversion efficiency (Eff) of the battery (described as “Comparative Example 1” and “Comparative Example 2”), the open circuit voltage Voc, and the short circuit current Jsc are shown. The production methods of the “Examples” are summarized in Table 2. “Comparative Example 1” is basically manufactured using the same materials and processes as in “Example”, but does not have a selenization process, and only the sulfurization process is performed. “Comparative Example 2” is basically manufactured using the same materials and processes as those in “Example”, but only the selenization process was performed without the sulfurization process.

  As can be seen from Table 1, the solar cell of “Example” in which sulfidation was performed after selenization of the metal precursor film according to the method of the present invention was “Comparative Example 1” with only sulfidation, and “Comparative Example 2 with only selenization”. It has a photoelectric conversion efficiency Eff superior to '. Similarly, the open circuit voltage Voc and the short circuit current Jsc show excellent values. Therefore, the effectiveness of the method of the present invention can be confirmed from this result.

  In addition, in the manufacturing process of the “Example” in Table 1, as described above, in the sulfurization process after selenization, sulfur or a sulfur compound is not used in a state where the furnace for exhausting selenium-containing gas from the processing furnace is not evacuated. Sulfurization is carried out by introducing a gas containing it into the furnace. In this case, the escape of Sn from the metal precursor film due to evacuation of the furnace is suppressed, and the composition ratio between the Group I element, Group II element and Group IV element in the p-type CZTS light absorption layer after film formation is desired. It is possible to control to maintain the value, that is, the value after forming the precursor film. That is, it becomes possible to maintain a composition ratio selected in advance so as to achieve high photoelectric conversion efficiency.

  However, the sulfidation step without vacuuming the furnace as described above is not an essential component of the present invention. In the present invention, a sulfur concentration gradient is formed in the formed p-type CZTS light absorption layer by performing a sulfurization step after selenization of the metal precursor film, thereby realizing high photoelectric conversion efficiency. Is.

  There are no particular restrictions on the selenization time and temperature shown in Table 2. For example, the time may be 10 to 150 minutes, and the temperature may be in the range of 400 to 600 ° C. Furthermore, the volume concentration of hydrogen selenide and the pressure in the furnace are not limited to the values shown in Table 1. Similarly, the sulfurization time may be 5 to 150 minutes, and the temperature may be in the range of 500 ° C to 650 ° C. The volume concentration of hydrogen sulfide and the pressure in the furnace are not limited to the values shown in Table 2.

  The metal precursor film for forming the p-type CZTS-based light absorption layer 3 may also use Zn or ZnSe instead of ZnS shown in Table 2, or SnS or SnSe instead of Sn. Further, in addition to the sequential deposition of Zn, Sn, and Cu, an evaporation source in which Zn and Sn are alloyed in advance may be used. As a film forming method, a sputtering method may be used in addition to EB vapor deposition.

  The substrate 1, the metal back electrode layer 2, the n-type high resistance buffer layer 4, and the n-type transparent conductive film 5 are not limited to the examples described in Table 2. For example, as the substrate 1, in addition to a glass substrate such as blue plate glass or low alkali glass, a metal substrate such as a stainless plate, a polyimide resin substrate, or the like can be used. As a method for forming the metal back electrode layer 2, there are an electron beam evaporation method, an electron layer deposition method (ALD method) and the like in addition to the DC sputtering method described in Table 2. As a material for the metal back electrode layer 2, a high corrosion resistance and high melting point metal such as chromium (Cr), titanium (Ti), or the like may be used.

  The n-type transparent conductive film 5 is formed to a thickness of 0.05 to 2.5 μm using a transparent material with n-type conductivity, wide forbidden band width, and low resistance. Typically, there is a zinc oxide thin film (ZnO) or an ITO thin film. In the case of a ZnO film, a low resistance film is formed by adding a group III element (for example, Al, Ga, B) as a dopant. The n-type transparent conductive film 5 can also be formed by sputtering (DC, RF) or the like other than MOCVD.

DESCRIPTION OF SYMBOLS 1 Glass substrate 2 Metal back electrode layer 3 p-type CZTS type light absorption layer 4 n-type high resistance buffer layer 5 n-type transparent conductive film

Claims (4)

Forming a metal back electrode layer on the substrate;
Forming a metal precursor film containing at least Cu, Zn, Sn on the metal back electrode layer;
By heat-treating the substrate on which the metal precursor film is formed in a furnace filled with a selenium-containing gas, the metal precursor film is selenized,
After the selenization, a gas containing sulfur or a sulfur compound is introduced into the furnace to sulfidize the selenized metal precursor film, thereby forming a p-type CZTS light absorption layer containing selenium and sulfur,
Forming an n-type transparent conductive film on the p-type CZTS-based light absorption layer;
A method for producing a CZTS-based thin-film solar cell , comprising each step , wherein the sulfurization is performed by introducing a gas containing the sulfur or a sulfur compound into the furnace in which the gas after selenization is present . The method according to claim 1 , wherein the metal precursor film formed on the metal back electrode layer further contains sulfur. 2. The method according to claim 1 , wherein the substrate on which the metal precursor film is formed after the formation of the metal precursor film on the metal back electrode and before the selenization includes sulfur or a sulfur compound. A method for producing a CZTS-based thin-film solar cell, characterized by sulfiding by heat treatment in a furnace filled with gas. The method according to claim 1, wherein the sulfidation is performed without evacuating the selenium-containing gas in the furnace by evacuation.

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