JP5709652B2 - CZTS thin film solar cell manufacturing method - Google Patents

Description

  The present invention relates to a method for producing a CZTS-based thin film solar cell, and more particularly to a method for producing a CZTS-based thin film solar cell having high photoelectric conversion efficiency.

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. The present invention has been made with respect to this point, and in particular, by forming a p-type CZTS light absorption layer having excellent crystal quality, it is possible to manufacture a CZTS thin film solar cell having high photoelectric conversion efficiency. Is an issue.

In order to solve the above-mentioned problem, in an aspect of the present invention, a metal back electrode layer is formed on a substrate, a p-type CZTS light absorption layer is formed on the metal back electrode layer, and the p-type CZTS light absorption is formed. the n-type transparent conductive film is formed on the layer, which during a 30 minute anneal over 90 minutes or less at less than ambient temperature 100 ° C. or higher 300 ° C. in the n-type transparent conductive coating atmosphere in the substrate formed with the respective steps A method for producing a CZTS-based thin film solar cell is provided.

Furthermore, annealing may be performed at a temperature set to 200 ° C. or higher and 250 ° C. or lower .

  The p-type CZTS light absorption layer may be formed by forming a metal precursor film containing at least Cu, Zn, and Sn on the metal back electrode layer, and sulfiding and / or selenizing the metal precursor film. good.

  In the CZTS-based thin film solar cell including the p-type CZTS-based light absorption layer, the p-type CZTS-based light absorption layer includes many crystal defects derived from Group VI elements, which reduce the function as the light absorption layer, As a result, the photoelectric conversion efficiency of the CZTS thin film solar cell is lowered. In the present invention, a solar cell substrate having a p-type CZTS light absorption layer formed on a metal back electrode layer and an n-type transparent conductive film formed thereon is annealed in an oxygen-containing atmosphere.

  By this annealing, oxygen contained in the annealing atmosphere passes through the n-type transparent conductive film, reaches the p-type CZTS light absorption layer, and is captured by crystal defects present in the layer. As a result, crystal defects derived from the group VI element in the p-type CZTS light absorption layer are inactivated and the crystal quality of the light absorption layer is improved, so that the photoelectric conversion efficiency of the CZTS thin film solar cell is improved. By performing the annealing at an atmospheric temperature of 200 ° C. or higher, oxygen in the atmosphere is more effectively taken into the p-type CZTS light absorption layer, and the photoelectric conversion efficiency is further improved.

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. The figure for demonstrating the manufacturing method of the CZTS type thin film solar cell which concerns on one Embodiment of this invention.

  Various embodiments of the present invention will be described below with reference to the drawings, but these embodiments are merely examples and do not limit the present invention. Moreover, since the same code | symbol shows the same or similar component through all the drawings, the overlapping description is not performed. Further, each drawing is for the purpose of illustrating the present invention only, and therefore 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 manufactured by the method of the present invention. In FIG. 1, 1 is a glass substrate, 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 an n-type transparent conductive film. Indicates. The p-type CZTS light absorption layer 3 is formed in a hydrogen sulfide and / or hydrogen selenide atmosphere at 500 ° C. to 650 ° C. after a metal precursor film containing Cu, Zn, and Sn is formed on the metal back electrode layer 2. It is formed by sulfurization and / or selenization.

By sulfiding a metal precursor film containing at least Cu, Zn, and Sn in a hydrogen sulfide atmosphere, the p-type CZTS light absorption layer 3 made of Cu ₂ ZnSnS ₄ is formed. On the other hand, the metal precursor film is selenized in a hydrogen selenide atmosphere to form a p-type CZTS light absorption layer 3 made of Cu ₂ ZnSnSe ₄ . Alternatively, the same metal precursor film is selenized and sulfided to form the p-type CZTS light absorption layer 3 made of Cu ₂ ZnSn (S, Se) ₄ .

When the p-type CZTS light absorption layer 3 is thus formed, the n-type high-resistance buffer layer 4 and the n-type transparent conductive film 5 are formed thereon, thereby completing the 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 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 a CZTS-based thin film solar cell completed through the above steps, usually a very high photoelectric conversion efficiency cannot be obtained. The present inventors considered that the cause may be the crystal quality of the p-type CZTS-based light absorption layer 3. In the p-type CZTS light absorption layer 3, many crystal defects due to group VI elements (S, Se, etc.) are generated due to sulfidation and / or selenization of the metal precursor film. The crystal quality of the light absorption layer is lowered.

  Such crystal defects contained in the light absorption layer capture electrons generated by light incident on this layer, and as a result, reduce the power generation performance of the light absorption layer. Therefore, if the crystal defects due to the group VI element formed in the p-type CZTS light absorption layer 3 can be effectively deactivated, the crystal quality of the p-type CZTS light absorption layer 3 is improved. Therefore, it is considered that the power generation performance is improved and the photoelectric conversion efficiency of the solar cell product is improved. The present invention proposes the following method based on such knowledge.

  (A)-(c) of Drawing 2 is a figure showing the manufacturing method of the CZTS type thin film solar cell concerning one embodiment of the present invention. In this embodiment, first, as shown in FIG. 2A, a metal back electrode layer 2 of Mo is formed on a substrate 1 by, for example, sputtering, and then a metal precursor film containing at least Cu, Zn, and Sn. And p-type CZTS-based light absorption layer 3 is formed by sulfidation and / or selenization. At this time, in the p-type CZTS-based light absorption layer 3, many crystal defects derived from the group VI element are generated due to sulfurization and / or selenization. Thereafter, an n-type high-resistance buffer layer 4 and a transparent conductive film 5 are formed on the p-type CZTS-based light absorption layer 3 to form a solar cell semi-finished product.

  Thus, when the solar cell semi-finished product provided with the p-type CZTS light absorption layer 3 containing many crystal defects is formed, as shown in FIG. Annealing inside. By this annealing, oxygen gas contained in the atmosphere passes through the transparent conductive film 5 and the n-type high-resistance buffer layer 4 and reaches the p-type CZTS light absorption layer 3 to cause crystal defects caused by the VI group element. Captured and deactivates these defects as shown in FIG. 2 (c).

  As a result, the crystal quality of the p-type CZTS light absorption layer 3 is improved and the power generation function is improved. As a result, a CZTS thin film solar cell product having high photoelectric conversion efficiency can be obtained. The annealing atmosphere temperature and annealing time may be selected so as to maximize the photoelectric conversion efficiency of the final solar cell product. Note that the n-type high-resistance buffer layer 4 is not an essential component for manufacturing a CZTS-based thin film solar cell. Therefore, the n-type high-resistance buffer layer 4 is directly formed on the p-type CZTS-based light absorption layer 3 without forming the layer 4. The type transparent conductive film 5 may be formed.

In order to clarify the effect of the above manufacturing method, the present inventors conducted experiments to manufacture a plurality of CZTS-based thin film solar cells by varying the annealing temperature and annealing time and to measure the photoelectric conversion efficiency. . Table 1 below shows experimental result 1. Experimental result 1 was manufactured through the same composition and the same manufacturing process (experimental condition 1), but the sample was annealed with the conventional example 1 sample in which the annealing process was not performed after the n-type transparent conductive film 5 was formed. The photoelectric conversion efficiency (Eff) was measured for each of Experiment 1 sample to Experiment 4 sample.

  The compositions and manufacturing methods of the conventional example 1 sample, the experiment 1 sample to the experiment 4 sample are summarized in Table 2 described later. Furthermore, in Experiment 1 Sample to Experiment 4 Sample, CZTS-based thin film solar cells were manufactured by changing the annealing temperature. Any annealing is performed in the atmosphere. That is, the sample of Experiment 1 was annealed at an ambient temperature of 100 ° C. for 30 minutes, the sample of Experiment 2 was annealed at 150 ° C., the sample of Experiment 3 was 200 ° C., and the sample of Experiment 4 was annealed at 250 ° C. for 30 minutes.

  As is clear from Table 1, in the experiment 1 sample to the experiment 4 sample in which the annealing was performed, the photoelectric conversion efficiency of the solar cell product was increased as compared with the conventional example 1 sample. In order to normalize the rate of increase, Table 1 shows the rate of increase in photoelectric conversion efficiency of each experimental sample when the sample of the conventional example 1 is set to 100. As shown in Table 1, in the experiment 1 sample that was annealed at a temperature of 100 ° C. for 30 minutes, the photoelectric conversion efficiency was 107, which is an improvement of about 7%, compared to the case where the conventional example 1 sample was 100. Similarly, the experiment 2 sample shows an improvement of about 13%, but the experiment 3 sample with an annealing temperature of 200 ° C. and the experiment 4 sample with an annealing temperature of 250 ° C. show an improvement of 60% or more compared to the conventional example 1 sample. I understand.

  From this, even if it is comparatively low temperature (about 100 degreeC), the improvement of the photoelectric conversion efficiency by annealing is confirmed, and also remarkable improvement can be confirmed by annealing more than the temperature of 200 degreeC.

Table 2 below summarizes the composition and manufacturing process of the conventional example 1 sample and the experiment 1 sample to the experiment 4 sample.

Table 3 below shows the experimental result 2. Each sample of the experimental result 2 shows the photoelectric conversion efficiency of a CZTS-based thin film solar cell formed of the same material and the same process as the experimental result 1 (that is, formed as shown in Table 2). However, the composition ratio of Cu, Zn, and Sn when forming the p-type CZTS-based light absorption layer 3 is different from that of each sample of the experimental result 1. As a result, the photoelectric conversion efficiency of each sample is considerably improved as compared with each sample of the experimental result 1 as an actual measurement value. However, when annealing is performed and when annealing is not performed, the photoelectric conversion efficiency is improved by about 30%, and the effect of the method of the present invention can be confirmed. In the experimental result 2, the annealing time as well as the annealing temperature is changed between the samples.

  According to Table 3, in the experiment 5 sample in which annealing was performed at a temperature of 150 ° C. for 90 minutes, the photoelectric conversion efficiency was improved by about 11% as compared with the conventional example 2 sample in which annealing was not performed. An improvement of 29% is seen in the 6 experimental samples that were annealed at a temperature of 250 ° C. for 30 minutes. On the other hand, the experimental 7 samples that were annealed at a temperature of 250 ° C. for 60 minutes and the experimental 8 samples that were annealed at a temperature of 250 ° C. for 90 minutes were the same or somewhat lower than the experimental 6 samples that were annealed for 30 minutes It is an improvement rate. Moreover, in the experiment 9 sample which annealed for 30 minutes at the temperature of 300 degreeC, the improvement rate of photoelectric conversion efficiency became 19%, and compared with the experiment 6 sample-experiment 8 sample, it has fallen.

  There is a big difference in the measured value of photoelectric conversion efficiency between the sample group shown in Table 1 and the sample group shown in Table 3. Therefore, a simple comparison of the photoelectric conversion efficiency between the samples does not clearly show the effect of annealing according to the present invention. However, the effect of the present invention becomes more conspicuous by referring to the rate of increase in conversion efficiency that represents how much the photoelectric conversion efficiency is improved with respect to the conventional sample.

Table 4 below shows the results of sorting the samples of Experimental Result 1 and Experimental Result 2 based on the rate of increase in photoelectric conversion efficiency.

  From Table 4, in Experiment 1 sample to Experiment 9 sample in which annealing was performed, the photoelectric conversion efficiency of the final product was improved from the conventional example sample (Conventional example 1 sample or Conventional example 2 sample). The effect of annealing according to the present invention can be confirmed. Furthermore, it can be understood that a significant improvement in photoelectric conversion efficiency (an improvement of 20% or more) is observed when the annealing temperature is 200 ° C. or higher, and that the annealing time is not so different if it is, for example, 30 minutes or more.

  It should be noted that although the experiment 4 sample and the experiment 6 sample are both annealed at an ambient temperature of 250 ° C. for 30 minutes, the rate of increase in photoelectric conversion efficiency is greatly different. This is considered to be due to the fact that the photoelectric conversion efficiency of the sample of Experiment 1 was greatly improved by annealing because the photoelectric conversion efficiency of the sample of Conventional Example 1 was low due to the composition ratio of the p-type CZTS light absorption layer. The same can be said for the experimental 3 samples.

  Note that FIG. 2B shows a state in which oxygen in the atmosphere is directly introduced into the p-type light absorption layer 3 by annealing. A mechanism in which oxygen in the resistance buffer layer 5 is substituted and the substituted oxygen passes through the thin n-type high resistance buffer layer 4 and reaches the p-type light absorption layer 3 is also conceivable.

Each of the above samples has the p-type CZTS light absorbing layer 3 having a composition of Cu ₂ ZnSnS _4. However, the present invention is not limited to this example, and Cu ₂ ZnSnSe ₄ or Cu ₂ ZnSn (S, Se It is clear that the same effect can be obtained even with ₄ . Furthermore, the metal precursor film for forming the p-type CZTS light absorption layer 3 may also use Zn instead of ZnS shown in Table 3, or SnS 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.

  Further, 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 3. 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 have a film thickness of about 0.05 to 2.5 μm by using a material having n-type conductivity, wide forbidden band width, transparent 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 p-type CZTS-based light absorption layer on the metal back electrode layer;
Forming an n-type transparent conductive film on the p-type CZTS-based light absorption layer;
To During a 30 minute anneal over 90 minutes or less at less than ambient temperature 100 ° C. or higher 300 ° C. In the n-type transparent conductive coating atmosphere in the substrate formed with,
A method for producing a CZTS-based thin film solar cell, comprising each step. 2. The method according to claim 1 , wherein the p-type CZTS-based light absorption layer forms a metal precursor film containing at least Cu, Zn, and Sn on the metal back electrode layer, and the metal precursor film is sulfided and / or Alternatively, a method for producing a CZTS-based thin-film solar cell, wherein the p-type CZTS-based light absorption layer is formed by selenization. 3. The method according to claim 1, wherein the n-type transparent conductive film is a ZnO film doped with a group III element. 4. The method according to claim 1, wherein the annealing temperature is 200 ° C. or higher and 250 ° C. or lower. 5.

Images