Patent Application
20160190373
The present invention relates to a CZTS-based thin film solar cell and a method of production of the same, more particularly relates to a high photovoltaic conversion efficiency CZTS-based thin film solar cell and a method for producing the same.
In recent years, thin film solar cells which use p-type light absorption layers constituted by chalcogenide-based compound semiconductors generally called “CZTS” have come under the spotlight. This type of solar cell is made from relatively inexpensive materials and has a band gap energy which is suitable for sunlight, so holds the promise of inexpensive production of high efficiency solar cells. CZTS is a Group I2-II-IV-VI4 compound semiconductor which includes Cu, Zn, Sn, and S. As typical types, there are Cu2ZnSnS4 etc.
A CZTS-based thin film solar cell is formed by forming a metal back surface electrode layer on a substrate, forming on top of that a p-type CZTS-based light absorption layer, and further successively stacking an n-type high resistance buffer layer and n-type transparent conductive film. As the metal back surface electrode layer material, molybdenum (Mo) or titanium (Ti), chrome (Cr), or another high corrosion resistance and high melting point metal is used. A p-type CZTS-based light absorption layer is, for example, formed by forming a Cu—Zn—Sn or Cu—Zn—Sn—S precursor film by the sputter method etc. on the substrate on which the molybdenum (Mo) metal back surface electrode layer has been formed and by sulfurizing this in a hydrogen sulfide atmosphere (for example, see PLT 1).
Here, to improve a CZTS-based thin film solar cell in photovoltaic conversion efficiency, optimization of the ratio of composition of the elements which form the p-type CZTS-based light absorption layer, that is, Cu, Zn, Sn, and S (sulfur or selenium), in particular the ratio of composition of Cu, Zn, and Sn, is important. Regarding this point, in the above PLT 1, the Cu—Zn—Sn composition ratio (atomic ratio) is expressed as the Cu/(Zn+Sn) ratio and the Zn/Sn ratio. It is reported that a high photovoltaic conversion efficiency CZTS-based thin film solar cell is obtained when the Cu/(Zn+Sn) ratio is 0.78 to 0.90 and the Zn/Sn ratio is 1.18 to 1.32.
PLT 1: Japanese Patent Publication No. 2010-215497A
In the above PLT 1, the Cu—Zn—Sn composition ratio at the p-type CZTS-based light absorption layer is specified to obtain a CZTS-based thin film solar cell which has a high photovoltaic conversion efficiency. In this case, as the n-type high resistance buffer layer which is formed on the p-type CZTS-based light absorption layer, mainly CdS is used. As is well known, Cd (cadmium) is highly toxic and has a large impact on the environment, so a Cd-free solar cell is desired. In PLT 1, several Cd-free zinc-based compounds are proposed as buffer layers, but CdS is considered particularly suitable as a buffer layer.
The present invention has as its object the provision of a CZTS-based thin film solar cell which does not use CdS as an n-type high resistance buffer layer and which has a high photovoltaic conversion efficiency and the provision of a method of production of the same.
To solve the above problem, in a first aspect of the present invention, there is provided a CZTS-based thin film solar cell which is provided with a metal back surface electrode layer which is formed on a substrate, a p-type CZTS-based light absorption layer which is formed on the metal back surface electrode layer, an n-type high resistance buffer layer which uses a zinc compound as a material and which is formed on the p-type CZTS-based light absorption layer, and an n-type transparent conductive film which is formed on the n-type high resistance buffer layer, wherein when expressing a Cu—Zn—Sn composition ratio (atomic ratio) of the p-type CZTS-based light absorption layer by coordinates using the
Cu/(Zn+Sn) ratio as the abscissa and the Zn/Sn ratio as the ordinate, it is within a region connecting a point A (0.825, 1.108), a point B (1.004, 0.905), a point C (1.004, 1.108), a point E (0.75, 1.6), and a point D (0.65, 1.5) and wherein further the Zn/Sn ratio of the surface of the p-type CZTS-based light absorption layer at the side which faces the n-type high resistance buffer layer is made 1.11 or less.
In the above aspect, the zinc compound may be Zn(S, O, OH). Further, the region of the surface of the p-type CZTS-based light absorption layer where the Zn/Sn ratio is 1.11 or less may be made a 30 nm range from the interface of the n-type high resistance buffer layer.
To solve the above problem, in a second aspect of the present invention, there is provided a method of production of a CZTS-based thin film solar cell which comprises forming a metal back surface electrode layer on a substrate, forming on the metal back surface electrode layer a metal precursor film which includes at least Cu, Zn, and Sn which is selected so that, when expressed by coordinates using a Cu/(Zn+Sn) ratio as the abscissa and a Zn/Sn ratio as the ordinate, a Cu—Zn—Sn composition ratio (atomic ratio) falls in a region connecting a point A (0.825, 1.108), a point B (1.004, 0.905), a point C (1.004, 1.108), a point E (0.75, 1.6), and a point D (0.65, 1.5), sulfurizing and/or selenizing the metal precursor film to form a p-type CZTS-based light absorption layer, forming on the p-type CZTS-based light absorption layer an n-type high resistance buffer layer of a zinc compound, and forming on the n-type high resistance buffer layer an n-type transparent conductive film, wherein when the metal precursor film has a Zn/Sn ratio over 1.11, after formation of the p-type CZTS-based light absorption layer and before formation of the n-type high resistance buffer layer, the method performs treatment to add Sn to the surface of the p-type CZTS-based light absorption layer on the n-type high resistance buffer layer side so as to form a region with a Zn/Sn ratio of 1.11 or less, then form the n-type transparent conductive film.
In the second aspect, the treatment to add Sn may be dipping the p-type CZTS-based light absorption layer in an SnCl aqueous solution, then annealing it. Further, the zinc compound may be Zn(S, O, OH). Furthermore, the metal precursor film may be formed by successively sputtering ZnS, Sn, and Cu in that order on the metal back surface electrodes.
When expressing a Cu—Zn—Sn composition ratio in a p-type CZTS-based light absorption layer or a metal precursor film by coordinates using a Cu/(Zn+Sn) ratio as the abscissa and a Zn/Sn ratio as the ordinate, it is possible to select the ratio so that it falls in the region which connects a point A (0.825, 1.108), a point B (1.004, 0.905), a point C (1.004, 1.108), a point E (0.75, 1.6), and a point D (0.65, 1.5) and make the Zn/Sn ratio of the surface of the p-type CZTS-based light absorption layer at the side which faces the n-type high resistance buffer layer 1.11 or less so that it is possible to obtain a CZTS-based thin film solar cell which achieves a high photovoltaic conversion efficiency (Eff) even if forming an n-type high resistance buffer layer by a zinc compound. As a result, it is possible to provide a CZTS-based thin film solar cell which does not contain Cd with its detrimental effect on the environment and which is suitable for practical use.
Below, various embodiments of the present invention will be explained with reference to the drawings, but these embodiments are merely examples and do not limit the present invention. Further, the structure which is shown in
Below, a CZTS-based thin film solar cell according to a first embodiment of the present invention and a method of production of the same will be explained.
In the CZTS-based thin film solar cell which is shown in
To obtain a high photovoltaic conversion efficiency (Eff) in a CZTS-based thin film solar cell after manufacture, the composition ratio of the Cu—Zn—Sn in the p-type CZTS-based light absorption layer 3 has to be optimized. The above-mentioned PLT 1 proposes an optimum composition ratio relating to this point, so the inventors selected several points among them to prepare a Cd-free CZTS-based thin film solar cell which has an n-type high resistance buffer layer which is formed by a zinc compound, but could not obtain the high photovoltaic conversion efficiency (Eff) which is described in PLT 1.
PLT 1 indicates that an n-type high resistance buffer layer 4 is formed by CdS. As opposed to this, the present application forms an n-type high resistance buffer layer 4 by a zinc compound. The inventors believed that the p-type CZTS-based light absorption layer 3 might change in optimum composition ratio depending on the material of the n-type high resistance buffer layer 4. Based on this, the inventors selected a plurality of composition ratios which greatly exceeded the range of the optimum composition ratio which was pointed out in PLT 1, prepared CZTS-based thin film solar cells by using zinc compounds to form n-type high resistance buffer layers, and measured the photovoltaic conversion efficiency (Eff).
Here, × indicates a sample with a photovoltaic conversion efficiency (Eff) of 0.0% to less than 1.0%, * indicates a sample with a photovoltaic conversion efficiency (Eff) of 1.0% to less than 2.0%, Δ indicates a sample with a photovoltaic conversion efficiency (Eff) of 2.0% to less than 3.0%, □ indicates a sample with a photovoltaic conversion efficiency (Eff) of 3.0% to less than 4.0%, indicates a sample with a photovoltaic conversion efficiency (Eff) of 4.0% to less than 5.0%, and ♦ indicates a sample with a photovoltaic conversion efficiency (Eff) of 5.0% or more.
In
In general, in a CZTS-based thin film solar cell, when, in the Cu—Zn—Sn composition ratio, Cu is poor with respect to (Zn+Sn), that is, (Cu/(Zn+Sn)<1), and Zn is greater than Sn, that is, (Zn/Sn>1), it is said that a relatively high photovoltaic conversion efficiency (Eff) is exhibited. In experiments which the inventors ran, the optimal composition region X reaches the lower region than the region Y which is considered the optimal composition ratio in PLT 1, that is, the region where the Cu/(Zn+Sn) ratio reaches 1 or more, and the ratio of Zn and Sn approaches 1 more than the region Y.
This fact suggests that the optimal composition region Y which is shown in PLT 1 and the optimal composition region X found by the experiment which was conducted by the inventors are based on different mechanisms. The solar cell samples which are shown in
Note that the composition ratio in the p-type CZTS-based light absorption layer is determined in PLT 1 by fluorescent X-ray analysis of a CZTS-based thin film solar cell product. In the experiment of the present embodiment, it is determined after the formation of the precursor film by inductively coupled plasma spectrometry (ICP). Therefore, the timing of measurement of the composition ratio and the measurement method differ between the two. This difference is also believed to have an effect on the difference in the optimum composition ratio regions. However, regarding this point, the inventors confirmed that there is almost no change in the composition ratio in a p-type CZTS-based light absorption layer between one measured at the time of formation of the precursor film and one measured after completion of the CZTS-based thin film solar cell product.
Furthermore, the inventors used the same method as the method for producing the CZTS-based thin film solar cell of
The sample of the experimental data 1 had a composition ratio at the p-type CZTS-based light absorption layer of Zn/Sn=1.04 and Cu/(Zn+Sn)=0.96. This was positioned outside of the region Y of
In this way, in the CdS:CZTS-based thin film solar cell which was produced by a method similar to the solar cell samples which are shown in
Accordingly, in the CZTS-based thin film solar cell which is shown in
The region X which is shown by the broken line in
However, the area below the line which connects the point A and the point B is the region where Zn becomes smaller compared with Sn. It is considered that there is little possibility of a high photovoltaic conversion efficiency (Eff) being obtained, so this part is eliminated. As a result, it is possible to rationally designate the triangular region having a point C which is defined by the Zn/Sn ratio (=1.108) of the Sample No. 22 and the Zn/Sn value (=0.905) of the Sample No. 23 as a vertex (shown by broken line in
Therefore, according to a first embodiment of the present invention, in a CZTS-based thin film solar cell which uses a zinc compound for the n-type high resistance buffer layer, when expressing the composition ratio by the value of Cu/(Zn+Sn) and the value of Zn/Sn, by setting the value to a value in the region X connecting a point A (0.825, 1.108), a point B (1.004, 0.905), and a point C (1.004, 1.108), it is possible to obtain a Cd-free CZTS-based thin film solar cell which has a high photovoltaic conversion efficiency (Eff).
The following Table 2 summarizes the compositions and methods of production of the CZTS-based thin film solar cell samples which are shown in
Note that, the composition ratio of the p-type CZTS-based light absorption layer 3 can be controlled when forming the precursor film by adjusting the amounts of film formation of ZnS, Sn, and Cu. The precursor film is sulfurized in a hydrogen sulfide atmosphere whereby a p-type CZTS-based light absorption layer is formed.
The compositions, manufacturing conditions, etc. which are shown in Table 2 are ones used for obtaining samples of the solar cells which are shown in
Further, as the method of forming a metal precursor film, instead of the ZnS which is shown in Table 2, Zn or ZnSe may also be used, while instead of the Sn, SnS or SnSe may also be used. Further, other than successively forming Zn, Sn, and Cu films, it is possible to use a vapor deposition source comprised of Zn and Sn alloyed in advance. As the film forming method, in addition to EB deposition, the sputter method may be used as well.
The n-type high resistance buffer layer 4 is generally formed by a chemical bath deposition method (CBD method), but as dry processes, the metal organic chemical vapor deposition method (MOCVD method) and the atomic layer deposition method (ALD method) may also be applied. The CBD method dips a base material in a solution which contains chemical species which form a precursor and causes an uneven reaction to progress between the solution and the surface of the base material so as to cause a thin film to precipitate on the base material.
The n-type transparent conductive film 5 is formed to a thickness of 0.05 to 2.5 μm by using a material which has n-type conductivity, has a broad band gap, and is transparent and low in resistance. Typically, there is a zinc oxide-based thin film (ZnO) or ITO thin film. In the case of a ZnO film, a Group III element (for example, Al, Ga, B) is added as a dopant to obtain a low resistance film. The n-type transparent conductive film 5 may also be formed by the sputter method (DC, RF) etc. in addition to the MOCVD method. Further, the n-type transparent conductive film 5 of the present embodiment has an intrinsic ZnO film (i-ZnO) of a thickness of 0.1 to 0.2 μm to which no dopant of a Group III element is added at a part adjoining the n-type high resistance buffer layer 4. In the present embodiment, an i-ZnO film is continuously formed by the same MOCVD method as the above low resistance film to which the Group III element is added as a dopant. Note that the i-ZnO film can be formed by the sputter method etc. other than the MOCVD. Furthermore, in a CZTS-based thin film solar cell, the i-ZnO film is not an essential constituent and may be omitted.
In the above embodiments, the optimum region X (see
The inventors thought that in Sample Nos. 22 to 29 which are shown in
As the method for making the Cu—Zn—Sn composition ratio of the p-type CZTS-based light absorption layer 3 change from the light receiving surface side (n-type high resistance buffer layer 4 side) toward the back surface side (metal back surface electrode layer 2 side), for example, there is the simultaneous vapor deposition method.
Below, a CZTS-based thin film solar cell according to a second embodiment of the present invention and a method of production of the same will be explained.
As suggested in the section on the first embodiment, when making the Cu—Zn—Sn composition ratio of the surface of the p-type CZTS-based light absorption layer 3 at the light receiving surface side a value in the region X of
The Sn which was added by dipping in an SnCl solution is not easily dispersed into the p-type CZTS-based light absorption layer 3 by annealing. If relatively most of it remains near the light receiving surface, the concentration of Sn near the light receiving surface will rise and the Zn/Sn ratio can be kept low. The inventors thought that by utilizing this, even if shifting the Zn/Sn ratio and Cu/(Zn+Sn) ratio of the p-type CZTS-based light absorption layer 3 as a whole in the direction of the region Y or the direction exceeding that, a low Zn/Sn ratio could be maintained at the interface of the light absorption layer and the Zn-based buffer layer and as a result a CZTS-based thin film solar cell which has a high photovoltaic conversion efficiency can be obtained. Therefore, four types of solar cell samples were prepared for p-type CZTS-based light absorption layers 3 which were prepared by the same compositions and methods of production, that is, one without SnCl treatment, one with dipping in an SnCl solution for a time of 1 minute, one for 5 minutes, and one for 15 minutes. The individual samples were measured for distributions of concentration of Sn and Zn (profiles in thickness direction).
The following Table 3 shows the results of ICP spectrometry of four samples which were prepared in this way. Note that the concentration of the SnCl aqueous solution in the SnCl treatment was 0.1 mol/liter, the solution temperature was room temperature (about 25° C.), and the annealing after dipping was performed at 130° C. in the air atmosphere for 30 minutes. The Zn/Sn ratio in the 30 nm range from the light receiving surface was found by calculation based on the results of analysis of the samples by the GD method (glow discharge spectrometry) (shown in
The graph (c) of
By comparing the graph (a) with the graphs (b) to (d) of
Based on the above experimental results, Samples 30 to 37 of solar cells were prepared to give Cu—Zn—Sn composition ratios which were shifted to the region Y of
The Zn/Sn ratio and the Cu/(Zn+Sn) ratio of Table 4 show the composition ratio of the p-type CZTS-based light absorption layer as a whole (layer 3+ layer 3′ of
Samples 30 to 37 have Cu—Zn—Sn composition ratios at the time of forming the p-type CZTS-based light absorption layers 3 which are selected so as to give Zn/Sn ratios of 1.25 to 1.53 and Cu/(Zn+Sn) ratios of 0.70 to 0.81. This region is beyond the region X which is shown in
In the above way, the optimum composition ratio region X which is derived from the first embodiment and the optimum composition ratio region Z which is derived from the second embodiment greatly differ in the Cu—Zn—Sn composition ratio of the light absorption layer as a whole. However, in the samples of the second embodiment, SnCl treatment is performed to make the Zn/Sn ratio at the interface between the p-type CZTS-based light absorption layer 3 and the Zn-based buffer 4 greatly fall compared with the Zn/Sn ratio of the light absorption layer as a whole. On the other hand, the Zn/Sn ratio of the samples which achieve a high photovoltaic conversion efficiency in the first embodiment was about 1.11 or less in each case. From this, as a condition common to the samples of the first embodiment and the second embodiment, making the Zn/Sn ratio at the surface of the p-type CZTS-based light absorption layer at the Zn-based buffer layer side about 1.11 or less may be mentioned.
By adding the results of the samples according to the second embodiment under conditions making the Zn/Sn ratio of the surface of the light absorption layer 1.11 or less (Conditions R1), it is possible to set such an optimum composition ratio region R2 for a CZTS-based thin film solar cell. This region R2 is formed by connecting a point A which is specified from the samples of the first embodiment with a point D which can be specified from the samples of the second embodiment and connect a point C and a point E. The point D is one where the Cu/(Zn+Sn) ratio is about 0.65 and the Zn/Sn ratio is about 1.5, while a point E is one where the Cu/(Zn+Sn) ratio is about 0.75 and the Zn/Sn ratio is about 1.6.
Therefore, according to the present invention, in a CZTS-based thin film solar cell using a zinc compound for the n-type high resistance buffer layer, when expressing the Cu—Zn—Sn composition ratio by the value of Cu/(Zn+Sn) and the value of Zn/Sn, by setting this composition to a value in the region R2 which connects a point A (0.825, 1.108), a point B (1.004, 0.905), a point C (1.004, 1.108), a point E (0.75, 1.6), and a point D (0.65, 1.5) and, further, making the Zn/Sn ratio near the surface of the p-type CZTS-based light absorption layer at the Zn-based buffer layer side 1.11 or less, it is possible to obtain a Cd-free CZTS-based thin film solar cell which has a high photovoltaic conversion efficiency (Eff).
Note that, in the above second embodiment, as the method of adding Sn on the light receiving surface of the p-type CZTS-based light absorption layer, SnCl treatment is employed, but as the method for adding Sn to form a low Zn/Sn ratio region, there is the method of depositing Sn on the p-type CZTS-based light absorption layer 3 by vapor deposition, the method of depositing Sn by the ALD method, etc.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2011-134446 | Jun 2011 | JP | national |
This application is a divisional of U.S. application Ser. No. 14/126,237, filed Dec. 13, 2013, which is the national stage application under 35 USC 371 of International Application No. PCT/JP2012/064182, filed May 31, 2012, which claims the priority of Japanese Patent Application No. 2011-134446, filed Jun. 16, 2011, the entire contents of which are incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 14126237 | Dec 2013 | US |
| Child | 15060105 | US |
